Journal of Evolutionary Biochemistry and Physiology, Vol. 38, No. 1, 2002, pp. 1—15. Translated from Zhurnal Evolyutsionnoi Biokhimii i Fiziologii, Vol. 38, No. 1, 2002, pp. 3—13. Original Russian Text Copyright © 2002 by Maksimovich.

REVIEWS

Structure and Function of the Vertebrate Pineal Gland A. A. Maksimovich Institute of Marine Biology, Far East Branch of the Russian Academy of Sciences, Vladivostok, Russia Received March 4, 1999

Abstract—This review presents data from the literature on structure and function of the pineal gland. Discussed are the histological and ultrastructural characteristics of the gland, its function according to novel results, peculiarity of synthesis and secretion of melatonin and its function, as well as the role of the pineal gland in circadian organization of organisms. The problems of evolution of the pineal function in the row of vertebrates are considered.

INTRODUCTION A small cone-shaped structure, the pineal body, is located in most vertebrates on the top of the midbrain and appears in embryos as a cerebral process. The parenchyma of the pineal gland that is also called epiphysis consists of large, light, cytoplasm-rich cells with the large nucleus (these are the chief cells, pinealocytes) and of small cells with the dark nucleus and a narrow stripe of cytoplasm. Pinealocytes produce and secrete melatonin, serotonin, and other physiologically active substances. The pineal body has both nervous and endocrine properties; in primitive vertebrates, such as the lamprey, it is attached to the brain with a stalk near the aperture in the skull and functions as a photosensitive organ. Apart from nerve connections, epiphysis has an abundant blood supply from the carotid artery system and contacts directly with the cerebrospinal fluid. The photoreceptor pinealocytes still persist in vertebrates, such as reptiles and even some bird species. In mammals, the pineal body is insensitive to light, but the nerve connection between the eye retina and the gland is preserved. Thus, functions of the pineal body in animals depend on the light intensity in the environment. In humans, this structure is developing

by the age of 7, when it acquires the size somewhat larger than the pea; subsequently, during the life, small mineral particles, specifically, calcium salts are deposited in the pineal body. The understanding of the pineal body functions was essentially expanded owing to isolation of the hormone melatonin by Lerner and co-authors [1]. Studies on animals show that almost the entire 24-h dose of melatonin is synthesized and secreted by the gland in the night time, whereas this gland function is ceased in the day time. The pineal and its hormone melatonin are commonly accepted to be important components of the “circadian system,” as they participate directly in control of many biological rhythms [1]. In addition to its role in the circadian system, melatonin also participates in controlling many other biological functions, such as growth, sex maturation, reproductive cycle, aging, and cellular immunity [2]. Melatonin is able to modulate functions of other endocrine organs, such as the thyroid, adrenal cortex, and gonads. Changes of melatonin concentration in the body of seasonally breeding animals affect their reproductive cycle, whereas a decrease of melatonin concentration, produced by an artificial illumination, can prolong the reproductive activity. Some Russian researchers specify 4 periods in the

0022-0930/02/3801-0001$27.00 © 2002 MAIK “Nauka/Interperiodica”

2

MAKSIMOVICH

history of study of the pineal gland and its main hormone melatonin: the 1st period (1958–1965)—discovery of melatonin, establishment of its physicalchemical structure and of pathways of synthesis and metabolism; the 2nd period (1966–1971)—physiological studies dealing with effects of melatonin on various systems of the organism, first of all, on the reproductive one; the 3rd period (1972–1979)—detection of extra-pineal sites of melatonin synthesis in the organism, study of the role of other hormones and biogenic amines in regulation of the melatonin synthesis, development of diagnostic methods of determination of the melatonin level in the blood and cerebrospinal fluid [3]. Since 1980, the 4th period has began, when the attention of researchers has been drawn to the role of melatonin in the appearance and development of pathological processes, in regulation of homeostasis and its participation in neuroendocrine regulation of immunity [3, 4]. Besides, for the recent years, the investigations have appeared on a wide range of problems, including description of the structure (from the anatomical to electron microscopic one) of the pineal gland in different animal species and humans, of peculiarities of its innervation and blood supply, dependence of the melatonin biosynthesis and metabolism on the time of day and their light regime [5] in representatives of different vertebrate classes [6], elucidation of the role of melatonin in mammals and humans [2, 7], actions of electromagnetic fields, light, and temperature on melatonin synthesis, effect of products of the pineal gland secretion on the sex development, behavior, immunity, development of malignant tumors and aging, as well as on clinical application of melatonin. In the Russian literature, advances in study of the structure and functions of the pineal gland are presented in several reviews. Evtushenko [3] describes history of discovery of the pineal gland and experience of the first interpretations of its role in the organism, anatomy and embryonal development in humans, the ways of nerve propagation of the light signal from the retina to the pineal gland. In the late 1870s, it was first suggested that the pineal gland was a peculiar neuroendocrine organ transforming the signal of the nerve type into the signal-hormone [3]. This suggestion was later developed into a conception of the sensory-hormonal system, in which the detected environmental signals are transformed directly into secretion of hormones [8]. In the1970–1980s, it was

believed that only the pineal gland was the source of melatonin. However, even at that time, the data appeared to argue in favor of the presence of the enzyme hydroxyindole-O-methyltransferase, the most important for melatonin synthesis not only in the pineal gland but also in the retina (in some species of birds, reptiles, and fish), and even in the brain (in amphibians). At that time, the endogenous melatonin was not reveled in the mammalian brain (cerebral cortex, hypothalamus, midbrain, basal ganglia) to be found only in peripheral nerves of the humans, monkey, and cow [3]. As to the ways of melatonin secretion, the pineal gland was believed to release melatonin into the cerebrospinal fluid, rather than into the blood. From the cerebrospinal fluid, melatonin seemed to enter the brain. At that time, a probable way of circulation and degradation of the pineal melatonin was also elucidated: the pineal gland → cerebrospinal fluid of brain ventricles → brain (first of all, hypothalamus and midbrain) → blood → urine. It was suggested that melatonin might also enter the blood from the pineal gland; however, its content in blood was very low. Also determined were the most important physiological effects of melatonin: effect on pigment metabolism, regulation of 24-h and seasonal rhythms, antigonadotropic action, inhibition of cell proliferation and cell division, certain anticancerogenic properties. Melatonin also increased oxygen consumption and carbon dioxide excretion, glucose consumption by tissues, their concentrations of ATP and creatine phosphoric acid, and promoted deposit of glycogen in tissues [3]. Some authors believe that, in the same way as serotonin has some hormonal properties, melatonin may be considered a neurotransmitter or modulator of physiological processes [6]. In Arendt’s monograph [5], it is claimed that the nocturnal melatonin secretion both in mammals that have a nocturnal or daytime behavior and in protozoans is evolutionarily rather a conservative phenomenon. Differences in duration of the daylight period provide the known seasonal differences in the levels of synthesis and secretion of melatonin in different animal species as well as in humans. The author presents data on dependence of the pineal function on intensity and spectrum of the visible light as well as on ability of electromagnetic fields to suppress the melatonin production. Also discussed are peculiarities of the pineal gland function in blind humans and the role of social signals (the time of morning getting-

JOURNAL OF EVOLUTIONARY BIOCHEMISTRY AND PHYSIOLOGY Vol. 38 No. 1 2002

STRUCTURE AND FUNCTION OF THE VERTEBRATE PINEAL GLAND

up, the food consumption, the street noise, etc.) in regulation of the 24-h rhythm of synthesis and secretion of melatonin. Of great interest is the role of the pineal gland in the process of aging. The data are exposed about the lifetime shortening, development of hypertension and diabetes-like states in pinealectomized rats, as well as the evidence for predisposition of such animals to development of cirrhosis and malignant tumors. The author considers skeptically the data that melatonin may increase the lifetime of mice and, on the whole, the idea that melatonin is a natural geriaprotector [5]; however, some recent observations (1993–1995) argued more certainly in favor of geriaprotective properties of melatonin. Arendt summarized the evidence for the role of the pineal gland in human physiology and pathology, particularly, for the rhythms of melatonin synthesis in people with a natural regime of the day–night change and in those who often have to change this regime for professional reason (for instance, during the night-time work, in flights over several time zones, etc.) [5]. A review of the data is also presented about development of the melatonin secretion rhythm in sex maturation and about a decrease of its production in aging, as well as about the role of melatonin in the human immune system. There are discussed possibilities of therapeutic use of melatonin in the circadian rhythm disturbances (the night-time work, overseas flying, blindness, old age), insomnia, disturbances of reproductive function, in oncology, and in some other processes. The lack of melatonin toxicity as well as of severe side effects and contraindications is emphasized [5]. In her review on the role of melatonin in humans, Malinovskaya [7] refers to the works arguing that the circadian rhythm of melatonin production by the pineal gland is set up by hypothalamic suprachiasmatic nuclei (SCN). Impulses from SCN transmit the circadian rhythm to the pineal gland by regulating activity of noradrenergic neurons of upper cervical ganglia, whose processes reach pinealocytes. It can be claimed that melatonin is one of the main messengers of endogenous rhythms generated by SCN and, at the same time, a corrector of the endogenous rhythms relative to the exogenous ones. In mediating the melatonin rhythmogenic effects the role is played not only by its level in circulation but also by duration of its nocturnal production. The role of melatonin as a regulator of biological rhythms is universal for all living organisms, which is indicated by the fact

3

of the presence of melatonin and by the circadian rhythm of its production in all known animals, beginning from unicellular organisms, as well as in plants. Under the conditions of polar latitudes, with no changes of the photoperiod for a long time, some people (who are not indigenes here) can acquire the appearance of freely running rhythms of the melatonin production in spite of the presence of social time sensors. These facts indicate the secondary character, relative to the photoperiod, of the social time sensors and other circadian rhythm regulators, such as oscillations of the Earth electromagnetic field, temperature, and humidity [7]. Thus, melatonin represents a hormone that has unique adaptive possibilities. The disturbance of its quantitative production and of the production rhythm of the latter is a trigger signal that leads, at initial stages, to the appearance of desynchronosis, with the subsequent appearance of an organic pathology. Hence, the very fact of the melatonin production disturbance may became the cause of various diseases Despite significant experimental and theoretical evidence accumulated so far, it is only at present that the pineal gland begins to disclose its functions. Further in this review there will be discussed the data about the structure and function of the pineal gland, which have been reported predominantly in the works of 1995–2000. HISTOLOGICAL AND ULTRASTRUCTURAL COMPOSITION OF THE PINEAL GLAND In teleosts, the pineal organ consists of the nervous tissue resembling the retinal tissue. It contains photoreceptor cells, axons with synaptic buttons, dendrites, and perikarya of secondary pineal neurons. As early as in embryos before hatching, both the pineal and retinal photoreceptors show well-developed outer segments and synaptic terminals. The distal part of the pineal organ is differentiated earlier than its proximal stem. The retinal differentiation begins from the center, but the caudal and dorsal retinas are differentiated earlier than the rostral and ventral ones. In the end of the larval period, the lateral retina is not yet differentiated [9]. In fish, the pineal gland contains extensively fenestrated capillaries and lacks the blood–brain barrier. The pineal epithelial cells of the teleost ayu Plecoglossus altivelis is located on the unusually thick and

JOURNAL OF EVOLUTIONARY BIOCHEMISTRY AND PHYSIOLOGY Vol. 38 No. 1 2002

4

MAKSIMOVICH

coiled basal membrane (2.2–2.4 µm thick) that is visible even under light microscope [10]. The basal membrane was studied in scanning and transmission electron microscopes, and its detailed structure and interrelation with the fenestrated capillaries and perivascular space were analyzed. Since the basal membrane had from 3 to 8 layers of the basal plate, which were inserted between laminae lucidae, the authors called this structure the “multi-layer basal membrane” and suggested that such multi-layer membrane to be able to prevent from penetration of foreign substances into the pineal epithelium [10]. The fine structure of the parietal eye of lizards and location of the excitatory amino acids, glutamate and aspartate, were studied using the method of immune electron microscopy [11]. The parietal eye contains conic photoreceptor cells, secondary neurons, and ependyma, as well as the cells similar to the crystalline lens cells of the lateral eye. The photoreceptors form long inner and outer segments; some of the photoreceptors are paired (like the “double photoreceptors” of zonulae adherentes). The perikarya of secondary neurons had sensory cilia (containing 9 × 2 + 0 pairs of tubules) protruding into the intercellular space. In the parietal eye epithelium, no neuroendocrine hypothalamic terminals were revealed. In the photoreceptors and secondary neurons of the parietal eye, the glutamate immunoreactivity was higher than the aspartate one. The immune glutamate label was more intensive in axons of the photoreceptors and neurons, as well as in most nerve fibers of the parietal nerve running to the brain stem. In cells of ependyma and crystalline lens, it was possible to detect only a negligible immunoreactivity of aspartate and glutamate. In the same animals, in the photoreceptor cells, secondary neurons and ependymal glial elements of the pineal organ, and in the lateral eye retina, a similar distribution of immunoreactive amino acids was found. The immunoreactive glutamate was accumulated in the photoreceptor cell axons and in the secondary neurons of the parietal eye; this permitted the authors to suggest that glutamate performed a transmitter role in efferent nerves of this organ. Thus, the efferent light-conducting pathway of the parietal eye is similar to that of the pineal organ and retina of the lateral eye. Like retinal Muller cells, the cells of ependyma and lens of the parietal eye and ependymal-glial cells of the pineal organ can participate in metabolism and/or elimination of excitatory amino

acids released by photoreceptor cells [11]. In rats, the ultrastructural interrelation between the pineal gland and the testicles was evaluated [12]. The Wistar rats were divided into 6 groups. Groups I and II were composed of sham testiculectomized and testiculectomized rats, respectively. The rats in group III were testiculoectomized and injected daily with testosterone propionate (TP) for 1 month. Groups IV and I included sham pinealectomized and pinealectomized rats, respectively. The animals in group VI were pinealectomized and administered daily with melatonin for 2 months. All animals were anesthetized with ketamine for fixation of the tissues by the method of vascular perfusion. The pineal glands in the animals from groups I, II, and III and the testicles of the animals from groups IV, V, and VI were dissected out and weighed. All samples were examined under an electron microscope. The testiculoectomy produced an increase of lipid droplets, cytoplasmic dense bodies, and lysosomes. Rough endoplasmic reticulum, Golgi apparatus, and mitochondria were large in the cytoplasm. The administration of TP to testiculectomized rats resulted in formation of smaller lipid droplets and mitochondria. In pinealectomized rats, the Golgi complex, mitochondria, and enlarged smooth endoplasmic reticulum were extensive in the Leydig cell cytoplasm. Formation of cytoplasmic secretory granules and osmiophilic bodies was observed. The testicle mass was increased as compared with group IV. Melatonin decreased the testicle mass in comparison with group V and prevented ultrastructural changes. Pinealectomy and testiculoectomy caused hyperfunction of Leydig cells and pinealocytes, respectively, which permitted suggesting the existence of interrelations between the pineal gland and the testis in rats [12]. Also studied were interrelations between collagen fibrils and calcinated concretions that appear exclusively in the pineal gland of adult aging rats. The lanthanum precipitates that replace calcium were distributed along collagen fibrils with a periodicity of approximately 70 nm. Calcium was detected histochemically between the collagen fibrils surrounding extracellular concretions, as well as by the method of the X-ray microanalysis. Calcium was bound to phosphorus. These data allow suggesting that the collagen fibrils participate in genesis and growth of extracellular structures located in the connective tissue surrounding the pineal gland of aging rats [13].

JOURNAL OF EVOLUTIONARY BIOCHEMISTRY AND PHYSIOLOGY Vol. 38 No. 1 2002

STRUCTURE AND FUNCTION OF THE VERTEBRATE PINEAL GLAND

In transmission electron microscope, the synaptic bands (SB) of the rat pineal gland look like rod-shaped organelles. Their three-dimensional structure is not exactly known. The pineal SB were studied on serial sections of the rat pineal gland, the animals were killed at noon or at midnight. The SB shape was reconstructed, based on the length of SB profiles and on the number of sections containing these profiles. The data obtained have shown that SB are principally plate lamina-like structures with polymorph side edges. The mean size of SB was 300 × 150 × 35 nm, and they were larger by 19.3% at night, than in the daytime. The authors believe the differences in the SB dimensions to indicate a possibility of important, yet unknown differences in synaptic functions at the day and night time [14]. FUNCTIONS OF THE PINEAL GLAND According to classic conceptions, sensory receptors are specialized cells that record specific changes in the environment and propagate nerve impulses for integration, control, and/or regulation of effector organs [8]. Recently, a peculiar class of sensory receptors was proposed, which were called sensory-hormonal cells; they use hormones as appropriate tools of transmission of biological information. Sensoryhormonal cells are able to detect and to transform the environmental signals directly into the hormone secretion. Theoretically, all sensitive receptors can demonstrate direct sensory-hormonal responses. However, studied so far has been only one group of sensoryhormonal cells, photoendocrine cells. Photoendocrine cells are able to discriminate light and darkness and to transform the electromagnetic energy of irradiation into a hormonal response. In general, the light suppresses, while the darkness stimulates synthesis and secretion of melatonin by photoendocrine cells. Contrary to many hormonal systems that using for regulation predominantly the feedback mechanism, the sensory-hormonal cells use most often the mechanism of preceding information. However, other factors also can serve as additional control tools by affecting the system and acting on the signal transformation processes and/or on synthesis and secretion of the hormone. The capability for the sensory-hormonal transduction is believed to be important for survival of the organism and the species; so the sensory-hormonal cells or their equivalent were to appear as soon as at

5

early stages of animal evolution. The sequence of the appearance of melatonin functions in the course of evolution is likely to be as follows: hormone → neuromodulator → neurotransmitter [8]. The pineal gland of teleosts is a direct light-sensitive organ that contains photoreceptor cells similar to retinal photoreceptor cells [15]. It transmits information about the photoperiod to the brain via nerve pathways and releases indoleamines, first of all melatonin, into the blood. The photoreceptor cells respond to changes in environmental illumination by a gradual modulation of the nerve transmission to the second-order neurons that innervate different cerebral centers, as well as by modulation of synthesis of indoleamines. Melatonin is produced rhythmically; its synthesis can be regulated either directly by the photoperiod or by an endogenous circadian oscillator whose frequency is determined by the photoperiod. Under natural conditions, the greatest amount of melatonin is produced at the night time. Although the pineal gland undoubtedly affects different physiological parameters, which was demonstrated by experimental removal of the pineal gland and/or by administration of exogenous indoleamines, its role in many physiological situations not always is clear. The results of any interference in the pineal functions seem to be depend on the season and on the light and temperature regimes of the experiment. There are all grounds to believe that the pineal gland is one of the CNS components, which forms in animals the system responding to the photoperiod, i.e., the system responsible for the correct choice of the time of daily and seasonal physiological rhythms. It is important to consider the pineal gland as a part of this system; it interacts with other light-sensitive structures (the retina as well as, probably, the extraretinal non-pineal visual receptors) and with generators of circadian rhythms [15]. Although the pineal gland is usually implied to take part in the photoperiod-dependent control of seasonal reproduction in some non-mammalian species, the possible physiological role of melatonin is not yet clear [16]. Several studies performed on birds, reptiles, amphibians, fish, and even some invertebrates reported that administration of melatonin could affect (usually by suppression) their reproductive characteristics. However, in most of these studies, melatonin was applied in a non-physiological way, usually as injections and/or as food additions. It is known that most works

JOURNAL OF EVOLUTIONARY BIOCHEMISTRY AND PHYSIOLOGY Vol. 38 No. 1 2002

6

MAKSIMOVICH

that either used implants with constant release of the hormone or tried to imitate the natural 24-hour rhythm of melatonin have failed to reveal any effects (only sometimes restricted) on reproductive processes. It seems that the current data do not confirm the conception on the important physiological role of melatonin in the photoperiodic control of reproduction in non-mammalian species [16]. In mammals, melatonin secreted by the pineal gland at the night time plays an important role in regulation of the reproductive function in seasonally reproducing animals and affects the age of sexual maturation in laboratory rodents [17]. In humans, these interrelations are less clear. The evidence confirming interrelations of melatonin and sex hormones is based on findings of abnormal secretion of melatonin in disturbances of the sexual system and on the data on the pineal gland pathologies that are connected with clinical disturbances of the sex hormone secretion. Normal melatonin rhythms are closely connected with the sex hormone rhythms during the prepubertal period and intercorrelate during sexual maturity. These interconnections are also confirmed by discovery of melatonin receptors in the brain and in the reproductive organs, as well as location of sex hormone receptors in the pineal gland. However, the physiological meaning of these correlations remains so far obscure, as the fact of antigonadal effects of exogenous melatonin is not definitively established [17]. The study of the pineal window in the skull of the adult Atlantic salmon Salma salar (the length to the caudal fin groove measuring 45–55 cm) has shown that it is rather transparent, except for the region of a roof-like bony structure formed by pineal plates (the mean slope is 120 ± 5°) [18]. Each of the sides of the pineal bony plates contains is a white structure that acts like an optic diffuser. In the experiment, the initial scattering of the incident narrow laser light beam by pineal plates was observed using an optic fiber and a diode battery. The scattered light of maximal intensity below the pineal plates turned out to be refracted relative to the angle of incidence, while the half-width of the scattered light intensity amounted to 22°. Besides, the maximal spreading of the light beam with the incidence angle µ relative to the pineal plates depends on µ according to the function (1 + cos µ). Hence, each pineal plate of the laminar roof receives the light from the corresponding hemisphere and then collects and projects the light beams to the pineal

gland at the angle of approximately 30° relative to the vertical axis body plane. The polarization factor that amounts to 0.60 was determined at the level of the pineal gland for linearly polarized light of incidence in spite of the light scattering by the pineal window. It is suggested that the revealed properties of the pineal window developed eventually as a goal-oriented auxiliary tool allowing the fish to be oriented relative to direction of the sun light [18]. SYNTHESIS AND SECRETION OF MELATONIN Melatonin, N-acetyl-5-methoxytryptamine, is an indoleamine with well-known effect on internal biological rhythms, which it performs by transformation of the light information into a chemical signal. The circadian melatonin secretion is a necessary component of the 24-hour and seasonal rhythms in many vertebrate species [19]. As to invertebrates, they have been established to have only seasonal rhythm of melatonin production: in the colonial actinia Rentilla köllikeri, the melatonin level in spring and summer is 4–5-fold higher than in autumn and winter [13]. It is not clear whether other multicellular animals and organisms from other taxons are able to produce melatonin by using homologous pathways of biosynthesis; however, some preliminary results permit suggesting that in vertebrates and insects, production of melatonin is of convergent or parallel phylogeny. The issue of the existence and functions of melatonin in algae and plants deserves a further study, but so far it is not yet solved. As to vertebrates, the role of melatonin in behavioral and systemic physiology can be differentiated into two phylogenetic types. In one of them, the circadian regulation of visual systemic structures, including the hypothalamic suprachiasmatic region, internal retina, and retinoreceptive and integrative visual structures, is evolutionarily an ancient characteristic of vertebrates. In the second type, in mammals, a relative decrease of the role of visual regulation and the presence of melatonin binding in the adenopituitary pars tuberalis is to be ascribed to a later evolutionary acquisition, as these features have been found only in this group [19]. In spite of a pronounced inhibitory effect of the light on the melatonin synthesis by the pineal gland, the daily melatonin rhythm is not usually a passive response to the environment. In mammals and almost

JOURNAL OF EVOLUTIONARY BIOCHEMISTRY AND PHYSIOLOGY Vol. 38 No. 1 2002

STRUCTURE AND FUNCTION OF THE VERTEBRATE PINEAL GLAND

in all vertebrate species studied by the present time, the rhythm of melatonin is coupled with an endogenous pacemaker, i.e., with circadian clocks. In mammals, the main circadian pacemaker is located in suprachiasmatic nuclei (SCN), bilateral accumulations of neurons in anterior hypothalamus. In rats, it has been recently shown that after a bilateral block of transmission in SCN, using γ-aminobutyric acid (GABA), but with preservation of the vasopressinergic neuronal transmission to this region, during the day time (a subjective period), there occurs an increase of the pineal melatonin level [20]. The fact that the complete ablation of SCN leads to a clearly seen rise of the daytime pineal level of mRNA for arylalkylamino-N-acetyl transferase (AA-NAT), an enzyme limiting the melatonin synthesis rate, demonstrates again the existence of an inhibitory effect of SCN on the circadian rhythm of melatonin [20]. The darkness-induced synthesis of melatonin in the pineal gland of the rainbow trout Oncorhynchus mykiss was inhibited at a deficit of calcium, as well as after treatment with nitrendipine, an antagonist of electrosensitive calcium channels of the L type, and with ω-conotoxin, an agonist of the N-type channels. The load with K-8644, an agonist of L-type channels, did not significantly affect the pineal melatonin synthesis. These data show that the entrance of calcium into pineal photoreceptor cells through electrosensitive calcium channels is important for maintenance of the darkness-induced melatonin synthesis and that the inhibitory effect of the light on the melatonin synthesis may be mediated by closing of these channels. Inhibition of the darkness-induced melatonin synthesis with nitrendipine was eliminated after addition of dibutyryl-cAMP to the nitrendipine-treated pineal glands. This permits suggesting that calcium interferes with action of cAMP in regulation of the melatonin synthesis [21]. The in vitro study of the trout Oncorhynchus mykiss pineal gland showed in some pinealocytes the occurrence of spontaneous fluctuations of the calcium level, although most of these cells had a stable basal concentration of these ions [22]. The melatonin release in the light- and dark-adapted states was eliminated by a decrease of extracellular calcium concentration and an increase of magnesium content. The load with cobalt decreased the melatonin secretion in twilight and dark diapasons of illumination, blocked reversibly spontaneous oscillations of the calcium content,

7

decreased the basal intracellular calcium concentration in pinealocytes, in which the calcium level did not change, and inhibited the KCl-induced elevation of intracellular calcium concentration. Use of glutamate, noradrenaline, isoproterenol, or dopamine did not affect significantly the melatonin secretion. Noradrenaline produced no effect on the calcium concentration in any of the studied trout pinealocytes. Treatment with muscimol, an A-agonist of GABA-receptors, brought about some small single release of melatonin during the twilight and dark diapasons of illumination with no effect on intracellular calcium concentrations. Thus, cobalt and low-calcium/high-magnesium buffer decreased the melatonin release in the trout pinealocytes by affecting calcium concentration, rather than via the synaptic transmission block. The data obtained show that the trout pineal gland synthesizes and releases melatonin, depending on the rhythms of the incident light illumination, whereas the neuronal inputs produce a negligible effect (if at all) on melatonin synthesis [22]. Thus, it has been established that in all the fish species studied so far, except for the trout, rhythmic production of melatonin is controlled by intrapineal oscillators. To find out whether the trout is, indeed, an exception of other fish species, the melatonin secretion was measured in vivo in the pineal bodies in 9 wild fresh-water and 6 sea teleost species. The pineal glands were cultivated at constant temperature, but under different conditions of illumination. The results have shown the pineal glands in all the studied species to maintain the rhythmic melatonin secretion not only in the light–dark cycles but also in absolute darkness. Most fish species, except for the trout, are likely to have endogenous intrapineal generators that act as pacemakers of melatonin secretion [23]. The 24-hour changes of the melatonin content in blood were determined in the juvenile Atlantic salmon Salmo salar were kept at natural and out-of-phase seasonal photoperiods [24]. With the natural daytime duration in autumn, winter, and summer, the blood melatonin levels were inversely proportional to the light intensity: the levels were low during the daytime and high at night. The duration of nocturnal increase of the blood melatonin content was correlated with the length of the dark period of the day, i.e., it was more pronounced in winter than in summer. In simulated seasonal photoperiods, the blood melatonin concentrations measured in August, October, and

JOURNAL OF EVOLUTIONARY BIOCHEMISTRY AND PHYSIOLOGY Vol. 38 No. 1 2002

8

MAKSIMOVICH

December also were elevated in the dark period regardless of whether the photoperiods were synchronized or they did not coincide for 6 months by phase with the natural illumination and temperature cycles. The melatonin circulating in blood allowed fish to distinguish precisely the predominant photoperiod, if they were initially kept in simulated natural photoperiods (which were either synchronized or did not coincide by phase for 6 months with the natural photoperiod) and then, were kept for 3 months at the daytime duration approaching that in the summer solstice. Distinct melatonin rhythms were observed always, regardless of the time of year, photoperiods, and temperature. The amplitude of nocturnal increase of the blood-circulating melatonin was similar in the fish groups that were kept in simulated seasonal photoperiods that did not coincide by phase with each other. Since otherwise the conditions were identical, these results indicate that the day duration in itself does not affect the melatonin rhythm amplitude. The melatonin rhythm amplitude was slightly higher during summer months, probably, because of a possible effect of temperature on the melatonin levels circulating in blood. These results show that the circulating melatonin patterns always reflect the prevalent daytime duration. Hence, there is a possibility to provide fish with an exact information as to the daily and calendar time. This information may be used in determination of the favorable time for their daily and seasonal activity [24]. To determine characteristics of the oscillator located in the pineal gland of the lamprey Lampetra japonica, there was studied effect of temperature and light on the melatonin secretion rhythm, using the pineal gland cell culture [25]. At 20°C, the melatonin rhythm was evident: a low secretion during the daytime and a high one, at night. When the temperature was reduced from 20 to 10°C, the melatonin rhythm disappeared. When the water temperature was returned to the initial level (20°C), the rhythm was rapidly restored. To determine the in vivo melatonin profile at low temperature, the plasma melatonin level was measured in living lampreys kept at 7°C. It was shown that the melatonin secretion under these conditions did not significantly differ during daylight hours and at night. In an uninterrupted light regime, the elevated melatonin secretion observed usually during the subjective night disappeared after 72 h of the constant darkness. When the cycle light–darkness (LD) was shifted by 6 hours (out-of-phase forestalling or retarding cycles),

the melatonin rhythm was shifted and remained in the same phase as the new LD cycle. These results show that in the pinealocyte culture, the lamprey melatonin secretion rhythm is both light- and temperature-dependent and that under in vivo conditions, the melatonin rhythm is not the key factor maintaining the rhythm of the lamprey motor activity [25]. The circadian rhythm of the β1-adrenergic receptor activity has been revealed in human pinealocytes [26]. The light perceived by retina reaches SCN in the form of nerve signals through the non-optic way called the retinohypothalamic tract. By acting the SCN activity, the light suppresses dose-dependently the melatonin secretion in the pineal gland. The nocturnal melatonin production was inhibited by the fullspectrum light with a minimal intensity of 200– 300 lux, while the complete inhibition of the hormone production occurred at the intensity higher than 2000–2500 lux. The light is necessary for synchronization of biological rhythms of the organism, including the melatonin rhythm, with the environmental rhythms, whose period is 24 h. The studies both in vivo and on animals have permitted elucidating mechanisms of regulation of melatonin synthesis by the pineal gland. Tryptophane is captured by the pinealocyte, then is transformed into serotonin, and serotonin is converted into melatonin by a two-stage process that includes consecutive activation of two enzymes: N-acetyltransferase (NAT), an enzyme limiting melatonin synthesis, and hydroxyindole-O-methyltransferase. The melatonin synthesis is initiated by binding of noradrenaline to β1-adrenergic receptors, with subsequent activation of pineal adenylyl cyclase. The inhibition of melatonin production is performed by γ-aminobutyric acid, benzodiazepines, dopamine, glutamate, and ∆-sleep-inhibiting peptide. Melatonin is not accumulated in the pineal gland, but is released at once into circulation from pinealocytes, by passive diffusion. However, based on available data, it is difficult to suggest to where melatonin is predominantly secreted from the pineal—to blood or to cerebrospinal fluid. The existence of the concentration gradient between cerebrospinal fluid and blood may argue in favor of simultaneous melatonin secretion into both fluids. Apart from the pineal gland, the melatonin synthesis is performed by the retina and the ocular ciliary body, as well as by the gastrointestinal tract organs (presumably, by mucosal enterochromaffin cells) [26]. The pineal gland produces about 80%

JOURNAL OF EVOLUTIONARY BIOCHEMISTRY AND PHYSIOLOGY Vol. 38 No. 1 2002

STRUCTURE AND FUNCTION OF THE VERTEBRATE PINEAL GLAND

of melatonin circulating in blood. In humans, melatonin produced by the pineal gland in the dark period of the day plays the key role in synchronization of the sleep–awakening cycle [27]. Changes in the sleep–awakening cycle are a distinguishing feature of biological aging. The complaints of difficulties in going to sleep and maintaining the sleep, as well as daylight-hour sleepiness are more often noticed in elderly people, than in persons of other age groups. It has been found that the circulating melatonin level is considerably lower in elderly people suffering from insomnia, than in control persons of corresponding age, while the onset and peak of the hormone secretion occur with a delay. Administration of melatonin to elderly people, whose insomnia was due to a deficit of this hormone, has shown that the substitution therapy may be useful for initiating and maintaining sleep in this category of patients [27]. The pineal gland has been for a long time considered as the only source of melatonin in the organism. However, as soon as the highly specific antibody to indolealkylamines had become accessible to researchers, melatonin, its precursors, and the catalytic enzymes associated with it began to revealed in extrapineal tissues, first in the retina and harderian gland connected anatomically with the optic system. The Russian researchers Raikhlin, Kvetnoy and co-authors [26, 28] were the first to suggest that melatonin can also be produced by gastrointestinal tract enterochromaffin cells (EC-cells) and identified the hormone in the cells. The mathematical analysis performed by these authors allows considering that the total number of EC-cells along the entire intestine is much higher than the possible number of melatonin-producing cells in the pineal gland. Besides, the gastrointestinal tract of birds and mammals has been shown to contain the amount of melatonin at least 400 times higher, than the pineal gland [26]. These facts have permitted the authors to consider the EC-cells the main source of melatonin in the human and animal organism. Nevertheless, they believe that there exist the central and peripheral chains of melatonin-producing cells in the organism. The central chain includes melatonin-producing cells of the pineal gland and optic system (the retina and harderian gland). The melatonin secretion rhythm in these cells coincides with the light–darkness rhythm. Belonging to the peripheral chain are all other hormone-producing cells in other organs, whose hormone production does

9

not seem to depend on illumination. The authors of the work [26] believe that extra-pineal melatonin can be considered the key paracrine signal molecule for local coordination of cellular functions. In several studies, it has already been shown that synthesis of melatonin occurs in retina as well as in other organs of some vertebrate classes, including mammals [29]. Like in the pineal gland, the melatonin synthesis in retina increases at night and falls during the daylight. Since the melatonin receptors are present in retina and the retinal melatonin does not contributes to the blood melatonin level, the retinal melatonin seems to act locally as a neuromodulator. The melatonin synthesis in mammalian retina occurs under control of a circadian oscillator located within the limits of the retina itself. The circadian rhythms of melatonin synthesis and/or release were described in several rodent species. These rhythms revealed under in vivo conditions and preserved under in vitro conditions are initiated by the light and are temperature-compensated. The recent cloning of the gene responsible for synthesis of the enzyme arylalkylamino-N-acetyltransferase should facilitate search for the cellular site of melatonin synthesis in retina and study of the molecular mechanism responsible for the appearance of rhythmicity in the retinal melatonin secretion. Melatonin takes part in many retinal functions, while levels of melatonin and dopamine are likely to regulate the retinal functions connected with adaptation to light and darkness. However, the exact role of retinal melatonin in functioning of the retina so far has not been established [29]. In several fish species, the retina also is able to synthesize melatonin [30, 31]. In most of the species, the melatonin content in the eye imitates the pattern observed in the pineal gland: its low levels in the daytime and high ones at night [32]. However, in the retina of several freshwater fish [34], researchers revealed deviations from the nocturnal pattern of melatonin content. It was also shown that the 24-h profile of the pineal melatonin content was inverted in the brook trout Salvelinus fontinalis, whose low levels of this hormone were observed in the night hours, while high ones, in daytime [35]. A similar inversion was recently described in the sea bass Dicentrarchus labrax [36]. The same authors [35] have found that in the brook trout, the light impulse during a dark adaptation, which is known to cause a fall of melatonin level in the pineal gland and blood plasma, induces peak of

JOURNAL OF EVOLUTIONARY BIOCHEMISTRY AND PHYSIOLOGY Vol. 38 No. 1 2002

10

MAKSIMOVICH

melatonin in the retina. The ocular melatonin rhythms in the goldfish Carassius auratus were studied and compared with those in the pineal gland and plasma [37]. In the light–dark (LD) photoperiod 12h–12h, the melatonin amounts in the eye as well as in the pineal gland and plasma show distinct changes connected with the period of day: higher levels in the middle of the dark period as compared with the middle of the daylight period. However, the ocular melatonin content in the middle of the dark and daylight periods was 100 and 9 times higher, respectively, than in the pineal gland. After pinealectomy, the melatonin content changes connected with the day–night transitions were preserved in the eye, whereas they disappeared in the plasma in the LD photoperiod 12h : 12h. The ocular melatonin content in pinealectomized fish in the middle of the daylight period was considerably higher than its content in the sham operated control animals. In the permanent darkness, the circadian rhythms were generated by the pineal gland for at least 3 days. At the constant illumination, the ocular melatonin content showed considerable fluctuations, with the lower amplitude than in the permanent darkness, whereas the plasma melatonin content remained at a low level. Thus, the ocular melatonin rhythm regulation in the goldfish occurs with participation of LD cycles, circadian clocks, and the pineal gland [37]. FUNCTIONS OF MELATONIN Melatonin performs in biology a unique role as a chemical transmitter of seasonal influences on animal physiology and behavior [29]. Seasonal changes of the night duration produce parallel changes of the melatonin secretion duration in such a way that it is higher in winter and shorter in summer. These changes of nocturnal melatonin secretion, in turn, produce seasonal changes in behavior. The response of retinohypothalamopineal axis (RHP) to light in humans is highly conservative. Like in other animals, melatonin is secreted in humans exclusively at night, and its secretion is interrupted, if at night there is an exposure to light. In many species the RHP is also able to detect changes in the night duration and, correspondingly, to regulate duration of the melatonin night secretion. This was shown both under natural conditions, when summer and winter melatonin levels were compared, and in experimental works that compared

the melatonin patterns after chronic exposure to long and short “artificial nights.” The people who live under modern urban conditions differ by the degree of change in their characteristic melatonin secretion duration (measured under permanent twilight) in response to seasonal changes of the length of solar night. The changes of the characteristic duration of melatonin secretion due to changes in the night length correlate well with the changes in the time of morning cessation of the hormone secretion. On the contrary, only poor correlations with changes in onset of the evening secretion are observed [29]. The effect was studied of frequency and pattern of melatonin signals on sexual development of Siberian hamsters [38]. Sexually immature males were kept under conditions of a short daytime and permanent illumination. To inhibit melatonin secretion, they were administered with exogenous melatonin for 5 h, once or twice a day for 20 days. The melatonin administration with any frequency produced an equivalent increase of the weight of testicles and body, which exceeded that in control animals. Similar changes were revealed in the hamsters that were transferred into long-day conditions. It seems that the reproductive system was maximally stimulated every day by the single short melatonin signal. Other animals kept since the birth at a short dark period were treated in 6 h after beginning of the darkness with DL-propranolol, an antagonist of β-adrenoreceptors, to reduce melatonin secretion at the night of the injection, but not at subsequent nights. This permitted the 4–5-h-long melatonin signals to be interpolated into short nights on the background of prolonged 10–12-h melatonin signals during other nights. The treatment regimes that kept the ratio of short-term to long-term melatonin signals as 1 : 1 for 8 weeks stimulated sexual development. The signal ratio 1 : 2 in each of the three different regimes of treatment was equally ineffective. The number of successful short-term melatonin signals little affected the time interval, at which the successful melatonin signals were summed up to influence the photoperiod characteristics. The neuroendocrine axis looked more sensitive to short-frequency melatonin signals, than to those that served for the summer phenotype development [38]. In the experiment on just-hatched tadpoles of Xenopus laevis under conditions when light did not reach the pineal eye, a model of a centralized swimming generator was initiated; however, the role of the pine-

JOURNAL OF EVOLUTIONARY BIOCHEMISTRY AND PHYSIOLOGY Vol. 38 No. 1 2002

STRUCTURE AND FUNCTION OF THE VERTEBRATE PINEAL GLAND

al gland in this behavior was unclear. Jemieson and Roberts [39] have shown that the tadpoles spend 99% of their time in hanging under the superficial meniscus or hard objects by using adhesive mucus excreted by a gland on the head. The attachment inhibits swimming, while non-attached tadpoles swim spontaneously. If their pineal eye is not damaged, they are concentrated at the water surface, much more in darkness, than at light, as well as they are attached more often to lower parts of the swimming objects that cast a shadow. The decrease of illumination makes nonattached tadpoles swimming horizontally to turn upwards. Such pineal-dependent responses in the course of swimming are observed up to the 44th developmental stage. Pinealectomy blocks responses to the reduced illumination at all developmental tadpole stages. The recordings obtained in fixed tadpoles show that the decrease of the light intensity stimulates the faster fictive swimming and that during the prolonged decrease of illumination, the duration of the pineal gland activity rises to 20 min. The authors of the work [39] suggest that the increased secretion in the pineal gland during the decreased illumination increases the probability of attachment of tadpoles to the objects that are located higher in the water column and cast a shadow [39]. For the recent years, the possible regulating role of melatonin has been studied for interaction of immunoendocrine and immune systems. The first evidences that melatonin may be an endocrine immunomodulator were obtained in the early works that studied its anti-tumor effect on animals and humans. Since then the data have been accumulated that melatonin, as a molecule well preserved in evolution, indeed, participates in the feedback mechanism between neuroendocrine and immune functions. The presence of melatonin receptor was revealed on membranes of human leukocytes and neutrophils, as well as of thymus and spleen leukocytes, neutrophils, and immunocompetent cells of laboratory and wild animals and T lymphocytes of the rat bone marrow. The close interrelation of melatonin and the immune system is confirmed by the fact of stimulation by γ-interferon of melatonin production in the pineal gland, this fact indicating the existence of the melatonin secretion regulation by the immune system [3, 4]. At present, the molecular mechanisms begin to be revealed, whose participation provides effect of melatonin on cellular functions. Taking into account the

11

diversity of possible direct and indirect cellular interactions, it seems probable that melatonin is able to play a complex physiological role in neuroimmunomodulation [40]. It has been recently shown that the mouse inbred lines have a distinctly shortened circadian rhythm of pineal and serum melatonin. Besides, it has become known that melatonin takes part in many immunoregulatory functions. For instance, melatonin affects hemopoiesis through melatonin-induced opioids to χ-opioid receptors that are present on the bone marrow cells. Conti and co-authors [41] have demonstrated that the bone marrow cells contain melatonin at a high concentration: revealed in the marrow cells are activities of N-acetyltransferase and hydroxy-N-methyltransferase, and on cultivation for a long time, they show high melatonin levels. These result have permitted the authors to suggest that the bone marrow cells of mice and humans are able to synthesize de novo melatonin that can have intracellular and/or paracrine functions [41]. Apart from its action on seasonal changes of the reproductive function, melatonin also seems to produce seasonal changes of the immune function. Melatonin affects the immune function indirectly, by acting via other hormones, as well as directly, by acting on the immune system components. Nelson and Drazen [42] put forward the hypothesis that many of indirect effects of melatonin on the immune function are mediated through glucocorticoids, and melatonin seems to be a part of an integrative system coordinating reproductive, immunological, and other physiological processes to better handle energetic stress-producing factors that appear, for instance, in winter. The direct effects of melatonin on immune function seem to be mediated by melatonin receptors in lymphatic tissue or on the immune blood cells and are connected with the seasons [42]. Melatonin, a universal synchronizer of endogenous biological rhythms as well as their adaptogen, at the same time, is the most powerful of all known endogenous antioxidants [43]. Melatonin also affects the appearance and development of tumors. Cos and Sanchez-Barcelo [44] elucidated the role of the pineal gland and melatonin in genesis of malignant breast tumors both in vivo and in vitro. The early hypotheses on a possible role of the pineal gland have been based on facts indicating that the pineal gland, through its hormone melatonin, reduces the regulatory effect of some pituitary and sex

JOURNAL OF EVOLUTIONARY BIOCHEMISTRY AND PHYSIOLOGY Vol. 38 No. 1 2002

12

MAKSIMOVICH

hormones that control development of breast and are also responsible for growth of its hormone-dependent tumors. Besides, melatonin can act directly on the tumor cells by affecting their proliferation rate. Another possibility follows from the melatonin antitumorigenic effect that may be a consequence of antioxidant immunostimulating properties. The working hypotheses of most researchers consisted in that the pineal gland activation or melatonin administration should have an antitumorigenic response; on the contrary, suppression of the pineal function or the melatonin deficit are to stimulate the appearance of the tumor. The general conclusion that followed from the studies of models of the appearance of tumors in animals in vivo was that the experimental manipulations activated the pineal gland or that the melatonin administration increased the latent period and reduced the scope of action and growth rate of chemically induced mammary tumors, while pinealectomy led usually to opposite effects. The direct effect of melatonin on breast tumors has permitted suggesting that its antitumorigenic effect is accounted for by the ability of melatonin at physiological doses (1 nM) to inhibit in vitro proliferation and invasiveness of the MCF-7 cells of human breast cancer [44]. The fact that most works have been carried out on two models, on the chemically induced rat breast adenocarcinoma (in study in vivo) and on the MCF-7 line tumor cells (in study in vitro), makes generalization of the obtained data rather difficult. However, the character of these effects, particularly, initiation of proliferation and metastases, as well as the doses (in the physiological range), at which the effect occurs, give these results a particular meaning. In this connection, surprising is a small number of clinical studies dealing with a possible use of melatonin in treatment of breast cancer. The review of different publications on medical application of melatonin shows that this field has been developed very intensively for the last few years. There were determined melatonin doses, and studied were interrelations between the melatonin content in the organism and other parameters, such as sleep, circadian rhythm, surgical stress, and anesthesia. Considered are the melatonin properties connected with age, and studied is the hormone role during depression and other psychic disturbances. Finally, similar studies were carried out to substantiate application of melatonin as the tool for treatment of sleep disturbances, in depressions,

time shifts, and as a skin protector in UV radiation [45, 46]. PINEAL GLAND AND CIRCADIAN RHYTHMS The circadian organization is, first, a way by which the entire circadian system above the cellular level is assembled together, and, second, the principle and rule that determine production of physiological and behavioral rhythms [47]. Understanding of the circadian organization and its evolution is of fundamental interest as well as of practical significance. The first main problem confronting researchers is difficulty of understanding the diversity of the results that we obtain in studying the circadian organization of different vertebrates. Some of these results exactly correspond to phylogenetic lines, which permits coming to the following generalizations: (1) in all vertebrates there is a “circadian axis” composed of retina, pineal gland, and SHN; (2) in many vertebrates from all classes (except for mammals), the pineal gland is both photoreceptor and circadian oscillator; (3) in all vertebrates (except for mammals) there are extra-retinal (and extra-pineal) [32, 34] circadian photoreceptors. An interesting explanation of some of these facts, especially of differences between mammals and other vertebrates, may be proposed on the basis of assumption that in ancient times, mammals in the course of evolution passed through “a night bottle neck.”1 On the other hand, the unlimited diversity of the circadian systems in vertebrates does not coincide precisely with the sites of the phylogenetic lines, which correspond to these vertebrates. The task is to understand how to interpret this “phylogenetically inconsistent” diversity and what kinds of new information may help our further understanding of evolution of the vertebrate circadian organization. The following questions are principal in circadian neurobiology: (a) how many oscillators participate in the circadian system? (b) how are their daily oscilla1 In populational genetics, there is a notion “bottle neck of evolution.” It means a considerable random reduction of the population volume due to different unfavorable environmental conditions (earthquakes, floods, eruption of volcanoes, epidemics, wars, etc.). The populations can later restore their usual size but allele frequencies of their genes can change essentially, which affects further evolution of the given species.

JOURNAL OF EVOLUTIONARY BIOCHEMISTRY AND PHYSIOLOGY Vol. 38 No. 1 2002

STRUCTURE AND FUNCTION OF THE VERTEBRATE PINEAL GLAND

tions generated and synchronized with the environment? (c) how do the oscillators signal to the organism about the day onset? [48]. It is well established that hypothalamic SHN are the main circadian oscillators in mammals. The total number of the “timeindicating” neurons is about 10 000; they handle evident rhythms that are daily observing in our “physiology” and behavior. These neurons, however, are not the only circadian oscillator, the mechanism of their molecular chronometry is not yet understood, and the ways of their communication with other parts of the brain are likely to be more unusual than it has been believed earlier. When the role of eyes and pineal organ in the circadian system of estimation of the motor activity time was studied in the catfish Silurus asotus in constant darkness and at two different intensities of continuous illumination (light and twilight), it was established that the voluntary active swimming was observed even after removal of the eyes or the pineal gland [49]. It seems that these organs are not signal generators for movement. However, removal of the eyes or the pineal gland produced different effects on the circadian motor activity at different regimes of illumination. These results show that the catfish biological clocks receive light information from different photoreceptors separately and integrally, depending on the stimulus quality [49]. CONCLUSION In conclusion, it is to be accepted that this review deals with the gland that in many aspects is exclusive and even mysterious. One of the researchers [50], when discussing the role and the significance of the pineal gland in the organism, called it “a clock giver” (Zeitgeber). I would risk to somewhat correct this term for a wider generalization and to call the pineal gland “Zeitgeist,” the “time spirit.” In this definition, the most thrilling and unusual thing, as applied to a material organ, is the notion “spirit.” However, somehow there is no desire to object such an obvious contradiction. Indeed, this small gland has absorbed many particular and even sometimes really dramatic properties. On one hand, it is sensitive to electromagnetic fields and provides the organism with the information forestalling the events, while, on the other hand, it is transformed with age into some crystalloid conglomerate with piezoelectric properties [51]. Its numerous functions and qualities permit it to take part

13

in practically all major events occurring in the organism; however, in humans, it undergoes involution by the period of the sexual maturity. It is to mention once more about its function of the “parietal” or the “third” eye, whose photoreceptor cells are thoroughly hidden under the skull roof. The reason of the appearance of such a peculiar aura around the pineal gland seems to be simple and consists in a clear deficit of our knowledge about its functions and intimate mechanisms of its interaction with the environment. There is only one way of overcoming this situation: to get facts and to interpret them timely and successfully. ACKNOWLEDGMENTS The work was supported by the Russian Foundation for Basic Research (project nos. 96-04-48193 and 98-04-63007). REFERENCES 1.

Lerner, A.B., Case, J.D., and Takahashi, Y., Isolation of Melatonin and 5-Methoxyindole-3-Acetic Acid from Bovine Pineal Glands, J. Biol. Chem., 1960, vol. 235, pp. 1992–1997. 2. Melatonin, Biosynthesis, Physiological Effects, and Clinical Application, Yu, H.S. and Reiter, R.J., Eds., Florida: CRC, Boca Ration, 1993. 3. Evtushenko, S.K., Melatonin and Its Role in Experimental and Clinical Neuroimmunology, Zh. Nevrol. Psikhiatr., 1994, vol. 94, no. 3, pp. 93–99. 4. Raikhlin, N.T., Aktual’nye problemy sovremennoi endokrinologii (Actual Problems of Current Endocrinology), Moscow, 1981, pp. 124–140. 5. Arendt, J., Melatonin and Mammalian Pineal Gland, London: Chapman & Hall, 1995. 6. Hastings, M.H., Yance, G., and Maywood, E., Some Reflexions on the Phylogeny and Function of the Pineal Organ, Experientia, 1989, vol. 45, pp. 903–909. 7. Malinovskaya, N.K., Role of Melatonin in Human Organism, Klin. Med., 1998, vol. 76, no. 10, pp. 15–22. 8. Pang, S.F., Lee, P.P.N., and Tang, P.L., Sensory Receptor as a Special Class of Hormonal Cells, Neuroendocrinology, 1991, vol. 53, no. S1, pp. 2–11. 9. Vighteichman, I., Ali, M.A., Czel, A., and Vigh, B., Ultrastructure and Opsin Immunocytochemistry of the Pineal Complex of the Larval Arctic Charr Salvelinus alpinus—A Comparison with the Retina, J. Pineal Res., 1991, vol. 10, no. 4, pp. 196–209. 10. Furukawa, E. and Omura, Y., Scanning and Transmission Electron Microscopic Study on a Multilayered Base-

JOURNAL OF EVOLUTIONARY BIOCHEMISTRY AND PHYSIOLOGY Vol. 38 No. 1 2002

14

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

MAKSIMOVICH ment Membrane in the Pineal Organ of the Ayu Plecoglossus altivelis, Arch. Histol. Cytol., 1997, vol. 60, pp. 511–517. Vigh, B., Debreceni, K., Fejer, Z., and Vighteichmann, I., Immunoreactive Excitatory Amino-Acids in the Parietal Eye of Lizards, a Comparison with the Pineal Organ and Retina, Cell Tissue Res., 1997, vol. 287, pp. 275–283. Kus, I., Sarsilmaz, M., Ogeturk, M., and Yilmaz, B., Ultrastructural Interrelationship between the Pineal Gland and the Testis in the Male Rat, Arch. Andrology, 2000, vol. 45, pp. 119–124. Humbert, W., Cuisinier, F., Voegel, J.C., and Pevet, P., A Possible Role of Collagen Fibrils in the Process of Calcifications Observed in the Capsule of the Pineal-Gland in Aging Rats, Cell Tiss. Res., 1997, vol. 288, pp. 435–439. Jastrow, H., Vonmach, M.A., and Vollrath, L., The Shape of Synaptic Ribbons in the Rat Pineal Gland, Cell Tiss. Res., 1997, vol. 287, pp. 255–261. Ekstrom, P. and Meissl, H., The Pineal Organ of Teleost Fishes, Rev. Fish Biol. Fisheries, 1997, vol. 7, pp. 199– 284. Mayer, I., Bornestaf, C., and Borg, B., Melatonin in Non-Mammalian Vertebrates: Physiological Role in Reproduction?, Comp. Biochem. Physiol., 1997, vol. 118, pp. 515–531. Luboshitzky, R. and Lavie, P., Melatonin and Sex Hormone Interrelationships—A Review, Pediatric Endocrinol. Metabolism, 1999, vol. 12, pp. 355–362. Nordtug, T., Berg, O.K., and Melo, T.B., Directional Light Transmission in the Pineal Window of Atlantic Salmon (Salmo salar L.) May Be Used for Solar Orientation, J. Exper. Zool., 1994, vol. 269, pp. 403–412. Cassone, V.M. and Natesan, A.K., Time and Time Again. The Phylogeny of Melatonin as a Transducer of Biological Time, J. Biol. Rhythms, 1997, vol. 12, pp. 489–497. Kalsbeek, A., Garidou, M.L., and Palm, I.F., Melatonin Sees the Light: Blocking GABA-ergic Transmission in the Paraventricular Nucleus Induces Daytime Secretion of Melatonin, Eur. J. Neuroscience, 2000, vol. 12, pp. 3146–3154. Gasser, P.J. and Gern, W.A., Regulation of Melatonin Synthesis in Rainbow-Trout (Oncorhynchus mykiss) Pineal Organs. Effects of Calcium Depletion and Calcium-Channel Drugs, Gen. Comp. Endocrinol., 1997, vol. 105, pp. 210–217. Meissl, H., Kroeber, S., Yanez, J., and Korf, H.W., Regulation of Melatonin Production and Intracellular Calcium Concentrations in the Trout Pineal Organ, Cell Tissue Res., 1996, vol. 286, pp. 315–323. Bolliet, V., Ali, M.A., Lapointe, F.J., and Falcon, J., Rhythmic Melatonin Secretion in Different Teleost Species—An in vitro Study, J. Compar. Physiol. Biochem. Systemic Environmental Physiol., 1996, vol. 165, pp. 677–

683. 24. Randall, C.F., Bromage, N.R., Thopre, J.E., Miles, M.S., and Muir, J.S., Melatonin Rhythms in Atlantic Salmon (Salmo salar) Maintained under Natural and Out-of-Phase Photoperiods, Gen. Comp. Endocrinol., 1995, vol. 98, pp. 73–86. 25. Samejima, M., Shavali, S., and Tamotsu, S., Light- and Temperature-Dependence of the Melatonin Secretion Rhythm in the Pineal Organ of the Lamprey Lampetra japonica, Jap. J. Physiol., 2000, vol. 50, pp. 437–442. 26. Kvetnoy, I.M., Raikhlin, N.T., Yuzhakov, V.V., and Ingel’, I.E., Extrapineal Melatonin: Its Place and Role in Endocrine Regulation of Homeostasis, Byul. Eksper. Biol. Med., 1999, vol. 127, pp. 364–370. 27. Haimov, I. and Lavie, P., Melatonin. A Chronobiotic and Soporific Hormone, Arch. Gerontol. Geriatrics, 1997, vol. 24, pp. 167–173. 28. Raikhlin, N.T., Kvetnoy, I.M., and Tolkachev, V.N., Melatonin May Be Synthetized in Enterochromaffin Cells, Nature, 1975, vol. 255, pp. 344–345. 29. Wehr, T.A., Melatonin and Seasonal Rhythms, J. Biol. Rhythms, 1996, vol. 12, pp. 518–527. 30. Cahill, G.M., Grace, M.S., and Besharse, J.C., Rhythmic Regulation of Retinal Melatonin: Metabolic Pathways, Neurochemical Mechanisms, and Ocular Circadian Clock, Cell. Mol. Neurobiol., 1991, vol. 11, pp. 529– 560. 31. Iigo, M., Kezuka, H., Suzuki, T., Tabata, M., and Aida, K., Melatonin Signal Transduction in the Goldfish, Carassius auratus, Neurosci. Biobehav. Rev., 1994, vol. 18, pp. 563–569. 32. Pang, S.F. and Allen, A.E., Extra-Pineal Melatonin in the Retina: Its Regulation and Physiological Function, Pineal Res. Rev., 1986, vol. 4, pp. 55–95. 33. Gern, W.A., Owens, D.W., and Ralph, C.L., Persistence of the Nyctohemeral Rhythm of Melatonin Secretion in Pinealectomized or Optic Tract-Sectioned Trout (Salmo gairnferi), J. Exp. Zool., 1978, vol. 205, pp. 371–376. 34. Serino, I., D’Istria, M., and Monteleone, P., A Comparative Study of Melatonin Production in the Retina, Pineal Gland, and Harderian Gland of Bufo viridis and Rana esculenta, Comp. Biochem. Physiol., 1993, vol. C106, pp. 189–193. 35. Zachmann, A., Kniiff, S.C.M., Ali, M.A., and Anctil, M., Effect of Photoperiod and Different Intensities of Light Exposure on Melatonin Levels in the Blood, Pineal Organ, and Retina of the Brook Trout (Salvenius fontinalis Mitchill), Can. J. Zool., 1992, vol. 70, pp. 25– 29. 36. Sanchez-Vasquez, F.J., Iigo, M., Madrid, J.A., Zamora, S., and Tabata, M., Daily Cycles in Plasma and Ocular Melatonin in Demand-Fed Sea Bass Dicentarchus labrax L., J. Comp. Physiol., 1997, vol. B167, pp. 409– 415.

JOURNAL OF EVOLUTIONARY BIOCHEMISTRY AND PHYSIOLOGY Vol. 38 No. 1 2002

STRUCTURE AND FUNCTION OF THE VERTEBRATE PINEAL GLAND 37. Iigo, M., Furukawa, K., Hattori, A., Ontanikaneko, R., Hara, M., Suzuki, T., Tabata, M., and Aida, K., Ocular Melatonin Rhythms in the Goldfish, Carassius auratus, J. Biol. Rhythms, 1997, vol. 12, pp. 182–192. 38. Flynn, A.K., Freeman, D.A., Zucker, I., and Prendergast, B.J., Testicular Development in Siberian Hamsters Depends on Frequency and Pattern of Melatonin Signals, Am. J. Physiol., 2000, vol. 279, pp. R1182–R1189. 39. Jamieson, D. and Roberts, A., Responses of Young Xenopus laevis Tadpoles to Light Dimming: Possible Roles for the Pineal Eye, J. Exp. Biol., 2000, vol. 203, pp. 1857– 1867. 40. Liebmann, P.M., Wolfer, A., Felsner, P., Hofer, D., and Schauenstein, K., Melatonin and the Immune-System, Intern. Arch. Allergy Immunol., 1997, vol. 112, pp. 203– 211. 41. Conti, A., Conconi, S., and Hertens, E., Evidence for Melatonin Synthesis in Mouse and Human Bone Marrow Cells, J. Pineal Res., 2000, vol. 28, pp. 193–202. 42. Nelson, R.J. and Drazen, D.L., Melatonin Mediates Seasonal Adjustments in Immune Function, Reproduct. Nutr. Develop., 1999, vol. 39, pp. 383–398. 43. Reiter, R.J., Melchiorri, D., and Severynek, E., A Review of the Evidence Supporting Melatonin’s Role as an Antioxidant, J. Pineal Res., 1995, vol. 18, pp. 2–11. 44. Cos, S. and Sanchez-Barcelo, E.J., Melatonin and Mammary Pathological Growth, Front. Neuroendo-

15

crinol., 2000, vol. 21, pp. 133–170. 45. Penev, P.D. and Zee, P.C., Melatonin: A Clinical Perspective, Ann. Neurol., 1997, vol. 42, pp. 545–553. 46. Wetterberg, L., Melatonin and Mammary Pathological Growth, Reproduct. Nutr. Develop., 1999, vol. 39, pp. 367–382. 47. Menaker, M., Moreira, L.F., and Tosini, G., Evolution of Circadian Organization in Vertebrates, Brazil. J. Med. Biol. Res., 1997, vol. 30, pp. 305–313. 48. Haimov, I., Shochat, T., and Lavie, P., Melatonin—A Possible Link between Sleep and Immune Systems, Israel J. Med. Sci., 1997, vol. 33, pp. 246–250. 49. Tabata, M., Minhnyo, M., and Oguri, M., The Role of the Eyes and the Pineal Organ in the Circadian Rhythmicity in the Catfish Silurus asotus, Nippon Suisan Gakkaishi (Bull. Jap. Soc. Sci. Fisheries), 1991, vol. 57, no. 4, pp. 607–612. 50. Falcon, J., Barraud, S., Thibault, C., and Begay, V., Inhibitors of Messenger RNA and Protein Synthesis Affect Differently Serotonin Arylalkylamine N-Acetyl Transferase Activity in Clock-Controlled and NonClock-Controlled Fish Pineal, Brain Res., 1998, vol. 797, pp. 109–117. 51. Lang, S.B., Marino, A.A., Berkovic, G., Fowler, M., and Abreo, K.D., Piesoelectricity in the Human Pineal Gland, Bioelectrochem. Bioenergetics, 1996, vol. 41, no. 2, pp. 191–195.

JOURNAL OF EVOLUTIONARY BIOCHEMISTRY AND PHYSIOLOGY Vol. 38 No. 1 2002