Journal of Comparative Physiology. A

J. Comp. Physiol. 138, 167-172 (1980)

9 by Springer-Verlag 1980

Sugar Reception by an Insect Water Receptor* Helmut Wieczorek Institut ffir Zoologie, Universitfit Regensburg, D-8400 Regensburg, Federal Republic of Germany Accepted April 2, 1980

Summary. The water receptor in the labellar taste hairs of Protophormia terraenovae (Diptera, Calliphoridae) was examined electrophysiologically. On the basis of previous findings (Wieczorek and K6ppl, 1978) which indicated remarkable similarities between the "sugar site" of the water receptor and the furanose site of the classical sugar receptor, the present investigation of the water receptor's sugar receptor properties led to the following results: 1. The furanose component of D-fructose is the efficient molecular conformation also in the water receptor, as can be concluded from experiments with freshly dissolved and equilibrium solutions of D-fructose. 2. The reactions of the water receptor to solutions of sugars in the absence of inhibiting salts vary for different sugars. Of three sugars tested D-trehalose inhibits with increasing concentrations more than Dglucose, while D-fucose is at high concentrations an even better stimulus than water. 3. There seems to be a correlation between the sugar site of the water receptor and its ability of water reception: In all labellar taste hairs which possess a water receptor, the water receptor is equipped with a sugar site.

Introduction Of all contact chemoreceptors in insects the sugar receptor in the labellar taste hairs of the fly has been studied the most intensively. Its properties are typical for a receptor cell which is specialized to recognize certain chemical stimuli; the dendritic membrane of the sugar receptor contains at least three types of receptor sites, each reacting in a highly specific man*

Supported by the Deutsche Forschungsgemeinschaft SFB 4/C1

ner to a limited number of compounds (see Shimada and Isono, 1978): The pyranose site is affected by glucose-like monosaccharides and by oligosaccharides, the furanose site is stimulated by D-fructose, D-fucose, D-galactose, and by some aromatic amino acids, and the third site reacts to aliphatic amino acids and to carboxylic acids. For a long time it was thought that the water receptor functioned more or less like a simple osmometer. After the discovery that pure water stimulates a receptor cell (Evans and Mellon, 1962), Rees investigated its properties with respect to the effects on it of salts and several nonelectrolytes (1970, 1972), Divalent cations inhibit water receptor activity at concentrations of about 0.01 M, monovalent cations at about 0.1 M, and nonelectrolytes at concentrations greater than 1 M. The inhibition by non-electrolytes seemed to be unspecific and dependent only on their osmotic values. Later Dethier and Goldrich-Rachman (1976) found multiple actions of anesthetics on water receptor activity: They described a stimulating effect of vaporous and liquid halothane. Wieczorek and K6ppl (1978) demonstrated that the water receptor also possesses sugar specificity. In the presence of an inhibiting concentration of salt certain sugars stimulate the water receptor if they are added to the salt solution: the water receptor is ~ reactivated ". Of sixteen sugars tested mainly three sugars, D-fructose, D-fucose, and D-galactose have a relatively strong reactivating effect, while other monosaccharides, oligosaccharides and glucosides are less or not at all effective. Two questions arose from these observations. 1. What is the nature of the '" sugar site"? Wieczorek and K6ppl (1978) mentioned a remarkable similarity to the furanose site of the sugar receptor with regard to specificity and pharmacological sensitivity. 2. Is the specificity of reactivation the same as that of inhibition if diverse sugars are presented in high concentration but without the addi-

0340-7594/80/0138/0167/$01.20

168

tion of an inhibiting salt? It seemed plausible to assume that sugars like D-fructose or D-fucose, which fully reactivate the salt-inhibited water receptor, inhibit to a smaller extent than sugars like D-glucose or D-trehalose which have only little or no reactivating effect in the presence of salts. The results obtained show new aspects of the water receptor. It will be demonstrated that the water receptor reacts like a typical chemoreceptor and not merely like an osmometer.

H. Wieczorek: Sugar Reception by an Insect Water Receptor

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Material and Methods Two-six days old male adults of Protophormia terraenovae from a laboratory culture were used for all investigations. Prior to the experiments the flies were food but not water-deprived for 20 24 h. Recordings were taken from intact flies, severed heads, or from proboscis-preparations. No difference with respect to influences on receptor reactions among the three methods was observed; the preparation with the severed proboscis allowed reproducible receptor reactions for up to two hours. All experiments were performed at ambient temperatures of 25 + 1 ~ The recording technique employed was that of Hodgson and Roeder (1956). In order to reduce variations in concentration due to evaporation at the tip of the capillary the stimulating solution was renewed about 1-2 s before touching a taste hair. Duration of stimulation never exceeded 1.5 s, a five-minute period between two stimulations was sufficient to exclude adaptation effects. According to the results of Morita (1969), response magnitude was first defined as the number of impulses during a period of 0.2 s beginning 0.15 s after the onset of the stimulus. According to own observations the analysis of an interval of 0.3 s beginning 0.05 s after the onset of the stimulus leads to identical results ; later during the experiments this period was usually used. In most experiments (exceptions see text) the " l a r g e " labellar hairs were selected for stimulation (nomenclature according to Wilczek, 1967). The electronic equipment was similar to that described by Wieczorek (1976). For polarimetric measurement of the mutarotation of D-fructose a Perkin-Elmer-241 polarimeter was used at a wavelength of 589 nm. The temperature was kept constant at 25+_0.1~ All chemicals had a purity of at least 99% and were supplied by the following firms: Fluka (D-fucose, D-trehalose, D-fructose, D-glucose), Serva (D-fructose), Merck (D-fructose, NaC1, CaC12, trisodium citrate).

Results The Nature of the Sugar Site Hanamori et al. (1974) showed in a very elegant experiment that in the sugar receptor of the fly the furanose form of D-fructose is the stimulating molecular conformation. They used the time-dependent mutarotation of D-fructose and observed that a freshly dissolved D-fructose solution consisting predominantly of fructopyranose does not stimulate the sugar receptor, whereas an equilibrium solution of D-fructose containing about 30% furanose has a stimulating effect. Therefore, the mentioned site has been called " f u r a n o s e " site.

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Fig. 1. Time course of mutarotation of a freshly dissolved 0.2 M D-fructose solution in 0.02 M CaCI2; only the concentration change of the furanose conformation is shown. Points represent the mean values of 6 experiments, the 95%-probabilities of the means are smaller than the diameters of the points. Mutarotation in water leads to identical results

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Fig. 2. Influence of CaC12-concentration (e) on the response of the water receptor and reactivation of the CaC12-inhibited water receptor by D-fructose ([], 0.5 M D-fructose/0.02 M CaC12). The values are normalized relatively to the response induced by 0.005 M NaC1. Points represent means and the 95%-probabilities of the means for 6 11 receptors (0.02 M CaClz: 21 receptors)

The same type of experiment was used to investigate the sugar site of the water receptor: It was assumed that an equilibrium solution of D-fructose should have a better reactivating effect in the saltinhibited water receptor than a freshly dissolved solution if the site reacting to sugars is a " f u r a n o s e " site. Tri-sodium citrate, used in earlier experiments as inhibiting salt, could not be employed in this case, because the mutarotation of D-fructose, which is strongly dependent on the pH (Isbell and Pigman, 1969, own observations), occurs too quickly: The pH of 0.05 M tri-sodium citrate is about 7.7. Therefore CaC12 was chosen, as the speed of mutarotation of D-fructose in CaC12 is the same as that in water. The time course of mutarotation is shown in Fig. 1 ; velocity constants coincide well with those observed by Andersen and Degn (1962). Increasing concentrations of CaC12 inhibit the water receptor (Fig. 2). In addition, D-fructose dissolved in 0.02 M

H. Wieczorek: Sugar Reception by an Insect Water Receptor

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Fig. 3. Effects of 0.02 M CaCI2, a freshly dissolved solution of 0.2 M D-fructose/0.02 M CaC12 (t=70 s after dissolution), and an equilibrium solution of 0.2 M D-fructose/0.02 M CaC12 (t > 20 rain after dissolution) on the water receptor activity. The deviations marked by small bars indicate the 95%-probabilities of the means; eleven taste hairs were tested. Two typical recording examples from one taste hair are shown (horizontal bars, beginning with the onset of stimulus: 0.1 s). Small spikes: water receptor; large spikes: sugar receptor. Each taste hair was stimulated in the following way: CaCl2~equilibrium fructose--,freshly dissolved fructose~equilibrium fructose--+CaClz. The response elicited by freshly dissolved D-fructose is normalized relatively to the mean response to the enveloping two stimuli with equilibrium fructose. In a similar manner each experiment mentioned in this paper was carried out and analyzed, if experimental values are normalized relatively to a response to a special stimulus (e.g. to the response to 0.005 M NaC1, as shown in Fig. 2)

CaC12 nearly totally reactivates the CaC12-inhibited water receptor. Figure 3 shows the results of the mutarotation experiments : 0.02 M CaC12 inhibits the water receptor to a great extent. A freshly dissolved solution of 0.2 M D-fructose in 0.02 M CaC12 elicits a slightly higher response, and an equilibrium solution of 0.2 M D-fructose in 0.02 M CaC12 induces the highest response. The difference between the effects of equilibrium fructose and freshly dissolved fructose is highly significant ( P ~ 0.001). The stronger stimulating effect of equilibrium fructose in the sugar receptor described by Hanamori et al. (1974) can also be observed in the recording examples.

Inhibition oJ" the Water Receptor by Sugars In a short paper Wieczorek (1978) reported that in the absence of salts the water receptor is inhibited by D-glucose with an apparent inhibition constant (defined as concentration of half maximal inhibition) of approximately 0.7 M, and that D-fructose has no inhibitory effect up to 1 M. At present it cannot be excluded that electrical activities in different receptor

Fig. 4. Dose response curves of the sugar receptor for D-fucose (e), D-glucose (A), and D-trehalose (m), all dissolved in 0.005 M NaCI. Responses are normalized relatively to the response induced by 1 M D-glucose. Points represent means of 8 18 receptors; bars: 95%-probabilities of the means uJ

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Fig. 5. Dose response curves of the water receptor for D-fucose (e), D-glucose (A), and D-trehalose ( i ) , all dissolved in 0.005 M NaCh Values originate from the same recordings as those in Fig. 4. Responses are normalized relatively to the response induced by 0.005 M NaCI. At a concentration of 1 M, 38 receptors were tested with D-fucose, 29 with D-glucose, and 36 with D-trehalose

cells of the same sensillum influence each other. Therefore, the comparison of the responses of the water receptor to the two sugars is not ideal: Although stimulating already at lower concentrations, D-fructose induces only about 50% maximal response in the sugar receptor, compared to D-glucose. For this reason three sugars were chosen which differed only slightly in their stimulating properties for the sugar receptor: D-fucose, D-glucose, and Dtrehalose (Fig. 4). Figure 5 demonstrates clearly that the three sugars act differently on the water receptor. D-trehalose is the most inhibiting sugar, while for D-glucose higher concentrations are needed to obtain the respective degrees of inhibition. Both sugars may

170

H. Wieczorek: Sugar Reception by an Insect Water Receptor

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by "osmolality". While increasing concentrations of D-trehalose and D-glucose inhibit the water receptor D-fucose activates it depending on its concentration and induces a maximal response of more than 1.2 compared to the effectiveness of 0.005 M NaC1, which itself does not inhibit the water receptor at this low concentration. The difference between the uninfluenced water receptor response and that to 1 M D-fucose is highly significant (P~0.001).

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Fig. 6. Pattern of taste hair responses to l M D-fucose/0.05 M tri-sodium citrate for five types of labellar hairs. White columns: one-cell-response. Black columns : two-cell-response. LL, "largest" hairs ; LA, '~ hairs ; IM, "intermediate" hairs ; BM, "big marginal" hairs; LM, "little marginal" hairs

Water Receptors in Other Types of Labellar Hairs An earlier paper (Wieczorek and K6ppl, 1978) reported that firing of two receptor cells in response to D-fructose or D-fucose can also be observed in the "largest", the "intermediate", and a part of the "marginal" hairs. In addition to Wilczek's (1967) nomenclature the "marginal" hairs can be further divided into two groups: On the basis of their lengths "big marginal" and "little marginal" hairs are distinguishable. The "big marginal" hairs are approximately 120 gm long, and the "little marginal" hairs have a length of about 60 ~tm. In Fig. 6 the five hair types are compared with regard to the pattern of receptor activity induced by 1 M D-fucose. While in t h e " largest", "large", "intermediate", and "little marginal" hairs two receptor cells per taste hair respond, in the "big marginal" hairs only one receptor cell per taste hair is active. Figure 7 demonstrates that the single cell in a "big marginal" hair stimulated by 1 M D-fucose is the classical sugar receptor. The water receptor is absent, as is indicated by the lack of response to 0.005 M NaC1. In a "large" hair of the same fly the water receptor responds to 0.005 M NaC1, and 1 M D-fucose elicits responses in the water receptor and in the sugar receptor.

Discussion

Fig. 7. Electrical responses of a "big marginal" hair of the same fly to 1 M D-fucose/0.005 M NaC1 NaC1. Horizontal bars beginning with the onset of 8 "big marginal" hairs in 2 flies were tested for water receptor activity

and a " l a r g e " and to 0.005 M stimulus: 0.1 s. the absence of

really induce different dose response curves of inhibition as is shown by the fact that the water receptor response to 1 M D-glucose differs significantly from that of 1 M D-trehalose (P~0.001). Differences also remain if "molarity" on the abscissa is substituted

The following two facts indicate that the sugar receptor properties of the fly's labellar water receptor are similar to those of the furanose site in the classical sugar receptor: First, as has been shown previously (Wieczorek and K6ppl, 1978), the spectrum of sugars stimulating the salt-inhibited water receptor resembles that of the sugar receptor's furanose site in that as D-fructose, D-fucose, and D-galactose are the most efficient sugars in both systems. Second, as is demonstrated in this paper, the furanose form in the fructose solution seems to be the reactivating molecular conformation. Its concentration changes fourfold from 0.015 M (fresh) to 0.060 M (equilibrium-solution), while the concentration of the pyranose remains rela-

H. Wieczorek: Sugar Reception by an insect Water Receptor tively constant (0.185 M ~ 0 . 1 4 0 M). In this context it should be noted that it makes no difference whether monovalent or divalent cations inhibit the water receptor. In both cases D-fructose reactivates the saltinhibited water receptor. In contrast to the results of Rees (1970) the water receptor in the " l a r g e " hairs is not so easily affected by Ca 2 +_ions; total inhibition takes place at concentrations between 0.05 and 0.1 M, whereas Rees found a value of about 0.01 M in the " l a r g e s t " hairs. Possibly the " l a r g e " hairs are generally less sensitive: Na § inhibits the water receptor in the " l a r g e s t " hairs even at concentrations lower than 0.1 M (Rees, 1970, Fig. 3), while in the " l a r g e " hairs at least 0.1-0.3 M Na + is needed. Even in the labellar sugar receptors of Phormia regina the dose response curve for sucrose is shifted to the right by about a factor of 2 in the " l a r g e " hairs compared to the dose response curve in the " l a r g e s t " hairs (Shiraishi and Tanabe, 1974). Also in the absence of inhibiting salts various sugars influence the water receptor in different ways. The specificity seems to be the same as the sugar specificity of the salt-inhibited water receptor: D-trehalose inhibits to a high extent, and has no reactivating effect, D-glucose behaves in an intermediate manner, and D-fructose and D-fucose reactivate the saltinhibited water receptor totally, and do not inhibit it if no salt is present in the solution. The simplest interpretation is that in addition to distinct regions in the dendritic membrane of the water receptor where water reception takes place and where monovalent and divalent cations as well as non-electrolytes inhibit water reception, an additional site exists, which reacts especially to D-fructose, D-fucose, and D-galactose. In this case a dose response curve of the water receptor for a special sugar represents a mixed-function relationship. For example, D-fucose, which has a high affinity to the sugar site, activates it and inhibits water reception at the same time. Both the activating and the inhibiting function compensate each other to a certain extent, and when the activation exceeds the inhibition, a dose response curve results with a maximal response to D-fucose which is higher than that to pure water. On the other hand D-trehalose, with no affinity to the sugar site, acts only as a non-electrolyte on water reception, and the inhibition curve represents a simple function. The interpretation that there are two distinct receptor sites in the water receptor seems to be plausible. However, there is a close connection between water and sugar-reception in the water receptor: In all hairs which possess a water receptor, the water receptor can be stimulated by D-fucose. If in a hair only the classical sugar receptor responds to D-fucose, the water receptor is absent. In any case it seems

171 premature to discuss site properties of the water receptor in more detail, as the molecular mechanism of water reception is not sufficiently understood.

Theoretical Considerations An attempt to explain how water can stimulate a specific receptor cell is the hypothesis of Rees (1970, 1972) which states that water reception might be governed by electrokinetic phenomena. Rees model postulates that during water stimulation an osmotic gradient causes positive ions in combination with water to flow through negatively lined pores of the dendritic membrane. As a result a streaming potential, the "receptor potential" of water reception, develops. The equations used by Rees are not up to date (e.g. see Korttim, 1972), and they do not allow a quantitative consideration of the height of a developing streaming potential. A new approach to electrokinetic phenomena in thin membranes with narrow pores has been made recently (Rosenberg and Finkelstein, 1978; Finkelstein and Rosenberg, 1979): the investigation of streaming potentials created by osmotic gradients across bilayer membranes containing gramicidin A. Assuming narrow gramicidin A pores ( ~ 0 . 4 n m ) filled with water molecules which cannot pass each other, one can calculate in a first approximation that an osmotic gradient of 1 osmol/kg induces the development of a streaming potential of about 0.5 mV • N (N = number of water molecules per pore). The gramicidin A pore with a length of about 3 nm contains about 6 water molecules; this corresponds to a streaming potential of about 3 mV. The existence of narrow pores is plausible for pores in nerve cell membranes, too. The potassium channel has a diameter of about 0.3 nm (Hille, 1973), and the sodium channel is about 0.4 nm in diameter (Hille, 1975). If the pores in the dendritic membrane of the water receptor are assumed to be three times longer than the gramicidin A pores (this is equivalent to the thickness of a biological membrane), and if an osmotic gradient of 1 osmol/kg is assumed to exist across the dendritic membrane of the water receptor in the water stimulated state (a higher value is not plausible), one can calculate a streaming potential of about 10 mV, corresponding to 20 water molecules per pore. Practically the same value can be calculated if the equation derived from Korttim (1972) is applied: U= AP x CR 1 x F 1 ( U : height of the streaming potential, AP: osmotic gradient between both sides of the dendritic membrane, CR: ionic concentration in the pores of the dendritic membrane, F: Faraday constant).

172 T h e q u e s t i o n is to d e t e r m i n e h o w l a r g e a r e c e p t o r p o t e n t i a l m u s t be to elicit i m p u l s e s in t h e g e n e r a t o r r e g i o n o f t h e s e n s o r y cell, w h i c h is a s s u m e d to be l o c a t e d n e a r t h e h a i r b a s e ( M o r i t a , 1972). It s h o u l d be a s s u m e d t h a t t h e d i s t a n c e b e t w e e n t a s t e h a i r tip a n d g e n e r a t o r r e g i o n is t h e l e n g t h o f a hair, 300 btm. T h e f o l l o w i n g v a l u e s w h i c h w e r e u s e d by R e e s (1968) a n d M a e s (1977) in t h e i r c a l c u l a t i o n s o f e l e c t r i c a l p r o p e r t i e s o f t a s t e h a i r s s h o u l d be a p p l i e d : T h e specific m e m b r a n e r e s i s t a n c e o f t h e d e n d r i t e m a y be 1 000 f~. c m 2, t h e r e s i s t a n c e o f t h e i n n e r c o m p a r t m e n t o f t h e d e n d r i t e 45 f 2 . c m , a n d t h e d e n d r i t e ' s m e a n d i a m e t e r m a y be 0.5 I,tm. I f we i g n o r e t h e m e m b r a n e capacitance, the length constant calculated from these v a l u e s is a b o u t 170 lam. T h a t m e a n s t h a t a d e p o l a r i s a t i o n o f 10 m V at t h e l o c u s o f t h e p r i m a r y p r o c e s s c a u s e s a d e p o l a r i s a t i o n o f < 2 m V at t h e s p i k e g e n e r a t i n g r e g i o n . E v e n if we a s s u m e a t e n f o l d specific membrane resistance of the dendrite, only about 5 mV w o u l d be r e a c h e d ; b o t h v a l u e s are a p p a r e n t l y t o o s m a l l to i n d u c e t h e g e n e r a t i o n o f a c t i o n p o t e n t i a l s . T h e r e f o r e we m a y c o n c l u d e t h a t at t h e m o m e n t t h e r e are n o i n d i s p u t a b l e facts t h a t d e m o n s t r a t e t h e exi s t e n c e o f e l e c t r o k i n e t i c p h e n o m e n a in w a t e r r e c e p tion. I am greatly indebted to Prof. Dr. K. Hansen for valuable discussions and for critical comments on the manuscript. Thanks are also due to Dr. H.-H. Kohler (Lehrstuhl f~r Physikalische Chemie, UniversitS,t Regensburg) for helpful discussions relating electrokinetic problems. Last not least 1 wish to thank Mrs. H. Kiibler for technical assistance.

References Andersen, B., Degn, H.: Determination of the optical rotation of fl-fructofuranose by means of kinetic measurements. Acta Chem. Scand. 16, 215-220 (1962) Dethier, V.G., Goldrich-Rachman, N.: Anesthetic stimulation of insect water receptors. Proc. Natl. Acad. Sci. USA 73, 3315 3319 (1976) Evans, D.R., Mellon, D. : Electrophysiological studies of a water receptor associated with the taste sensilla of the blowfly. J. Gen. Physiol. 45, 487-500 (1962) Finkelstein, A., Rosenberg, P.A.: Single-file transport: implications for ion and water movement through gramicidin A channels. In: Membrane transport processes, Vol. 3. Stevens, C.F., Tsien, R.W., (eds.), pp. 73-88. New York: Raven Press 1979

H. Wieczorek: Sugar Reception by an Insect Water Receptor Hanamori, T., Shiraishi, A., Kijima, H., Morita, H.: Structure of effective monosaccharides in stimulation of the sugar receptor of the fly. Chemical Senses and Flavor 1, 147 166 (1974) Hille, B.: Potassium channels in myelinated nerve: selective permeability to small cations. J. Gen. Physiol. 61, 669 686 (1973) Hille, B. : Ionic selectivity, saturation, and block in sodium charinels. A four-barrier model. J. Gen. Physiol. 66, 535-560 (1975) Hodgson, E.S., Roeder, K.D.: Electrophysiological studies of arthropod chemoreception. I. General properties of the labellar chemoreceptor of diptera. J. Cell. Comp. Physiol. 48, 51 76 (1956) Isbell, H.S., Pigman, W. : Mutarotation of sugars in solution: Part II. Catalytic processes, isotope effects, reaction mechanisms, and biochemical aspects. In: Advances in carbohydrate chemistry and biochemistry, Vol. 24. Wolfrom, M.L,, Tipson, R.S. (eds.), pp. 13455. New York, London: Academic Press 1969 Korttim, G.: Lehrbuch der Elektrochemie. Weinheim: Verlag Chemie 1972 Maes, F.W. : Simultaneous chemical and electrical stimulation of labellar taste hairs of the blowfly Calliphora vicina. J. Insect Physiol. 23, 453 460 (1977) Morita, H. : Electrical signs of taste receptor activity. In :III. Internat. Symp. Olfaction and Taste, New York 1968. Pfaffmann, C. (ed.), pp. 370-381. New York: Rockefeller University Press 1969 Morita, H. : Primary processes of insect chemoreception. In: Advances in biophysics, Vol. 3. Kotani, M. (ed.), pp. 161--198. Tokyo: University of Tokyo Press 1972 Rees, C.J.C. : The effect of aqueous solutions of some 1 : 1 electrolytes on the electrical response of the type 1 (" salt') chemoreceptor cell in the labella of Phormia. J. Insect Physiol. 14, 1331-1364 (1968) Rees, C.J,C.: The primary process of reception in the type 3 ("water") receptor cell of the fly, Phormia terraenovae. Proc. R. Soc. (London) Biol. 174, 469-490 (1970) Rees, C.J.C. : Responses of some sensory cells probably associated with the detection of water. In: IV. Intern. Symp. Olfaction and Taste, Starnberg 1971. Schneider, D. (ed.), pp. 88-94. Stuttgart: Wiss. Verlagsges. 1972 Rosenberg, P.A., Finkelstein, A.: Interaction of ions and water in gramicidin A channels: Streaming potentials across lipid bilayer membranes. J. Gen. Physiol. 72, 327 340 (1978) Shimada, I., Isono, K.: The specific receptor site for aliphatic carboxylate anion in the labellar sugar receptor of the fleshfly. J. Insect Physiol. 24, 807-811 (1978) Shiraishi, A., Tanabe, Y.: The proboscis extention response and tarsal and labellar chemosensory hairs in the blowfly. J. Comp. Physiol. 92, 161-179 (1974) Wieczorek, H.: The glycoside receptor of the larvae of Mamestra brassicae L. (Lepidoptera, Noctuidae). J. Comp. Physiol. 106, 153 176 (1976) Wieczorek, H. : Sugar receptor properties of the fly's labellar water receptor. In: Third ECRO Congress. Publ. by: European Chemoreception Research Organization, p. 35 (1978) Wieczorek, H., K6ppl, R. : Effect of sugars on the labellar water receptor of the fly. J. Comp. Physiol. 126, 131-136 (1978) Wilczek, M.: The distribution and neuroanatomy of the labellar sense organs of the blowfly Phormia regina Meigen. J. Morphol. 122, 175-201 (1967)