CHANGES

IN V I S U A L E V O K E D

INJECTION GENICULATE

OF T E T A N U S

POTENTIALS

TOXIN INTO THE

AFTER LATERAL

BODY UDC 612.825.263 ~*612.826

B . A. K o n n i k o v , M, B . R e k h t m a n , and G. N. K r y z h a n o v s k i i

Changes in visual evoked potentials were studied in rats at different stages of formation of experimental photogenic epilepsy induced by injection of tetanus to, tin into the lateral genAculate body. The greatest change in evoked potentials in the lateral genie~flate body consisted of the appearance of an aditional component in the series of waves of the p r i m a r y response. Meanwhile in the ipsflateral visual cortex the amplitude of the first negative component of the evoked potential was considerably increased. Correlation Was found between the changes in the amplitude of this component in the visual cortex and the change in steepness of the additiomal component of the evoked potential in the genieulate body, reflecting functional reorganization of that nucleus. The results are evidence of significant disturbances of the relay function of the lateral geniculate body when a generator of pathologically enhanced excitation is formed in it. INTRODUCTION It was shown previously that after injection of tetanus toxin (TT) into the lateraI genieulate body (LGB) a specific neuropathological syndrome of photogenic epilepsy is formed in the experimental animal [2]. Chm~ges in evoked potentials (EP) in the visual centers of the brain have been shown to be among the characteristics of the different phases of formation of the syndrome [1, 2]. However, changes in global electrogenssis in the structures of the visual system are not only an indication of the stage of development of the neuropathological syndrome, but they may also serve as an informative criterion for the analysis of the mechanisms of its formation. The object of this investigation was to study the dynamics of the visual EP at different periods of experimental photogenic epilepsy induced by injection of TT into LGB and by the formation of a generator of pathologically enhanced excitation in that nucleus. EXPERIMENTAL

METHOD

Experiments were c a r r i e d out on 12 male albino rats weighing 250-300 g. Under hexobarbital anesthesia electrodes were implanted chronically into the p r i m a r y visual cortex of the rats (6 m m caudally to the coronal suture and 4 r a m from the midline of the skull) and into LGB (eontralaterally to the side of subsequent injection of TT). Stereotaxic insertion of the electrodes [14] was checked by reference ¢0 EP arisi}~g ill respo~,se to photie stimulation. During the two weeks after the operation the rats were accustomed to the experimental situation. To r e s t r i c t movements of the animals during recording they were fixed in a special hammock; the head, limbs, andtail remained free. Two weeks after the first o p e r ~ i o n a micropipet soldered to a niehrome electrode, glazed except at the tip, was introduced under hexobarbital anesthesia into the opposite LGB and TT was injected through it in a dose of 20 MLD for mice in a volume of 4 °10-~ ~lo The nichrome electrode was fixed to t h e skull by cement. Phctic stimulation of the retina was carried out by flashes applied from a distance of 25-30 cm from the animal' s eyes. The intensify of illumination of the flash could be modified by means of filters from 3 to 45 Ix. During the experiment the wi~h of the animal Ts pupils was fixed by application of a 0.1% solution of atropine to the cornea. Before and after injectionof TT into LGB, evoked responses to photic stimulation were recorded by a monopolar technique: in the cortex by means of silver bali electrodes (dlame~er 0.5 ram), in LGB by means of glazed nichrome electrodes (diameter 150 p) with tips uninsulated for a distance of 0.4-0.5 ram. The indifferent electrode consisted of a segment of uninsuIated silver wire inserted on the boundary between the frontal and temporal lobes. After injection of TT into LGB, EP L~ the visual cortex and LGB were recorded every 2 h. Summation of 50 responses to photic stimulation with a frequency of 0.3/see was carried out. After the end of the experiment the position of the tips of +.he subcortical eIectrode was Institute of General Pathologyand Pathological Physiology., Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, V,ol. 10, No. 2, pp. t42-149, March-April, 1978. Original article submitted June 27, 1977. 0090-2977/78/1002-0095507,50 © 1978 Plenum Publishing Corporation

95

Fig. 1. Evoked potentials in lateral geniculate body in response to photic stimulation with intensity of 3 Ix at different periods after injection of tetanus

toxin (TT): 1) 2.5, 2) 8, and 3) 12 h after injection of TT. Each record obtained by averaging 50 responses. Calibration: 150 #V, 100 msec. verified. During analysis of the EP the latent period of the maximum of each component was measured from the time of the flash to the maximum of the potential. The amplitude of the components was measured from the peak of the preceding component to the peak of the given component. EXPERIMENTAL

RESULTS

Changes in E P in L G B after Injection of T.T. As a result of injection of T T into L G B signs of the formation of photogenic epilepsy gradually appeared in the rats [2]. This process could be represented as three basic phases: latent, preparoxysmal, and paroxysmal. During the first 4-6 h after injection of T T into L G B (latent phase) E P in the rats were indistinguishable from those under normal conditions (Fig. I, I). Before injection of T T into L G B and during the latent phase, in response to presentation of a single flash a potential arose in L G B in which primary (Ni, PI) and secondary (N2, P~) components could be distinguished, and these were followed by a long positive wave (Fig. I, I). At the end of the latent phase an over-all increase in the negativity of all components of E P in L G B was observed, in the form of a shift of all the waves upward relative to the zero line. One of the significant changes in E P under these conditions was the appearance and gradual increase of an additional negative wave N~ (Fig. I :2, 3) in the series of waves of the primary response. In the initial period of the preparoxysmal phase, withlow intensities of illumination this additional wave was recorded together with the intact first negative component N I (the m e a n latent period of the m a x i m u m of which was 20~$ ~3.6 msee) (Fig. Ib), but with an increase in the intensity of illumination wave Nlq0eeame indistinguishable (Fig. 2). In the late stages of the preparoxysmal phase, withdifferentbrightnsss of the flashes, the main negative component of the response was NIc (with a m e a n latent period of its m a x i m u m of 44.8 • 5.7 msec) (Fig. Ic; Fig. 2). To estimate changes in the parameters of the new component of E P in L G B (NIc) arising as the result of TT, the dependence of the various parameters of this wave from the time elapsing after injection was studied. A n example of the changes in the amplitude of the ascending and descending phases of wave NIc (A I, A2) and the waxing time of the aseending and waning time of the descending phase (&T I, AT 2) with time is illustrated in Fig. 2. As the graphs show, none of these values change consistently with time. On the basis of the values mentioned above the rate of waxing and waning of the wave NIc or the steepness of its ascending and descending phases also was calculated. These characteristics were determined as the tangents of the angles of slope of the corresponding fronts: tan ~ = At/AT I and tan ~ = A2/A T 2. Characteristlcs such as these, as we know, can serve as an important index of temporal arid amplitude transformation of the visual afferent flow and the degree of its synchronization at this particular level of the visual system [4]. Besides values characterizing the steepness of the ascending and descending phases of the components of NIe for the different periods of time after injection of TT, the characteristics of the general "taper" of the l~l wave - the multipllcative slope - also was investigated. The numerical value of this parameter was obtained by multiplying the tangents of the angles of slope of the ascending and descending phases (tan ~-tan ~). At the end of the preparoxysmal phase tan ~ ceased to rise steadily whereas tan ~ at the beginning of this phase in-

96

Phases ~ H o u m

LGB-TT

N o r m a l ~ -~ __

LGB-C _ _

parox-j 8

Parox-

14

[ysmal

msec L ~Tt 40

pV

6 8 IOlfl~

6 8 /OR I~

J

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~

6 8 /Ol2l~h

Fig. 2. Dynamics of evoked potentials (EP) in l a t e r a l geniculate body (LGB) of both hemispheres after unilateral injection of tetanus toxin (TT) into LGB. LGB-TT) nucleus into which TT was injected, LGB-C) contralateral nucleus. Each r e c o r d obtained by averaging 50 responses. Calibration: 150 #V, 100 m s e c . Records obtained in response to photic stimulation with an insensibly of 45 Ix. Graphs below show dependence of p a r a m e t e r s of steepness of ascending and descending phases of component Nie of E P in LGB on side of injection of TT: amplitudes of astending (Al) and descending (A2) phases, waxing time of ascending (AT1) and waning time of descending (AT2) phase, tangent of angle of slope of ascending (tan ~) and descending (tan ~) phases, and muitiplicative gradient (tan a "tan ~) on time elapsing after injection of TT. The method of measuring the corresponding p a r a m e t e r s is shown on the enlarged fragment of the potential (in the f r a m e consisting of broken lines). Values of tangents of above-mentioned angles calculated by formula: tan ~ = A~/~T1; tan fi = A2/&T 2. c r e a s e d nonmonotonically; their product - the multiplicative steepness - of the wave N1c as a whole, moreover, was found to be a monotonically increasing function of the time elapsing after injection of TT into LGB. At the time when significant changes in E P in response to flashes were observed in LGB on the side of injection of TT, in the contralateral LGB the shape and p a r a m e t e r s of EP remained virtually unchanged untfl the beginning of the paroxysmal phase (Fig. 2}. Chmmges in E P in the Visual Cortex (VC) after Injection of TT into LGB. The typical E19 in VC of normal r a t s consisted of a biphasic p o s i t i v e - n e g a t i v e p r i m a r y response (Pi, NI), a triphasic secondary respor~e (P~, N2, P 3 ) , and a slow negative wave (N3) (Figs. 3 and 4). As a result of injection of TT into LGB, besides the changes in local electrogenesls in that nucleus mentioned above, variation of EP in the ipsilateral VC also was observed. When these changes are defined it should be noted that the p a r a m e t e r s of the f i r s t positive wave remained virtually constant throughout the period of formation of photogenic epilepsy. Meanwhile, beth the amplitude of the f i r s t negative componer~ (NI) and the latent period of the maximum and the duration of that wave increased gradually.

97

phases b

VC-I

VC-C

[.aten: 3

~arox~ 8 ysmal~

ysmal ,

I

pv

Fig. 3. Changes in evoked potentials (EP) in visual cortex (VC) at various times after injection of tetanus toxin (TT) into lateral geniculate body (LGB). VC-1) Ipsilateral, VC-C) contralateral regions of VC relative to LGB into which TT was injected. Each r e c o r d obtained by averaging 50 responses. Calibration: 150 #V, 100 msec. Records obtained in response to flashes with intensity of 45 Ix. Graph showing amplitude of component N1 in ipsilateral (1) and contralateral (2) regions of VC under normal conditions (N) and at various times after injection of TT into LGB shown below r e c o r d s of averaged EP.

Meanwhile fusion of the components P2 and P3 and an increase in amplitude of the combined wave were observed (Fig. 3). In the contralateral VC the p a r a m e t e r s of component Pl also remained unchanged. The amplitude of N1 at the beginning of the preparoxysmal phase could equal or even exceed the amplitude of N1 in the ipsilateral VC, but later, during the formation of photogenic epilepsy, it was somewhat reduced. Under these circumstances a marked decrease in amplitude of the P2 wave was observed and component N2 appeared (Fig.3). Examples of averaged EP before and at various times after injection of TT into LGB are shown in Fig. 3 for the ipsilateral (VC-I) and contralateral (VC-C) VC. Beginning usually 8-10 h after the time of injection of TT into LGB, the amplitude Of wave N1 in EP of the ipsilateral VC was significantly g r e a t e r than the c o r r e sponding normal value (P < 0.01). The amplitude of component N1 at these times also significantly exceeded the amplitude of the corresponding wave in the contralateral VC (P < 0.01). DISCUSSION EP were recorded in that p a r t of the p r i m a r y visual cortex where they were expressed maximally. The shape and p a r a m e t e r s of E1) in VC were stable, readily reproducible, and they corresponded to the analogous data obtained by other workers [9, 10, 18, 19]. Components of EP in LGB were identified p r i m a r i l y from their correspondence to definite waves of EP in VC o F o r instance, the appearance of a p r i m a r y negative- positive complex of potentials (Nl-Pi) corresponded

98

I I

I I

I" I

I I

I

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__J

L.l ~V

4"1

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V

tg~.tg£

soo ~"

oL't~"~'-8 8......... io~ /2 1,

h

Fig. 4. Evoked potentials (E P} in different parts of visual system of normal rats and after un£lateral injection of tetanus toxin (TT) into lateral gen~culate body (LGB}. 1) Averaged EP in LGB (broken line} and in ipsilateral visual cortex (VC) (continuous line} under normal conditions; 2 and 3) 10 h after injection of TT on eontralateral side and on side of injection of TT respectively; 4) averaged E P in LGB 3 h after injection of TT (co~inuous line) and in contralateral LGB (broken line); 5) the same but 12 h later. Records obtained during photic stimulation with an intensity of 10 lx. Calibratiom I00 ~V, 100 m s e c . 6} Graph showing amplitude of f i r s t negative component of EP in VC (AN~) and of multiplieative steepness of first negative c o m ponent of EP in LGB (tana -tan fi) on side of injection of TT as a function of time elapsing after injection of TT. in time to the appearance of components P~-I~, in VG. The N2, P2, and ~ waves in LGB corresponded in their development to components P2, N2, and P3 in VC. The slow positive potentials (P3) in LGB corresponded to the slow negative wave in VC (N3). Fluctuations of potential in LGB and VC also correlated "~th each other and in the time of appearance o f the visual after-discharge (Fig. 4, 1}. Analysis of the photoreactivity of the various structures of the visual system during the formation of a g e n e r a t o r of pathologically enhanced excitation in LGB after local injection of T T showed that in tl~ds case the EP were specifically altered. One of the most important changes was the appearance of an a c c e s s o r y (N1e} negative wave of potential in LGB. Meanwhile a considerable increase in the amplitude of the negative wave of the p r i m a r y response was observed in VC; this increase corresonded to the dynamics of the change in multiplicative steepness ofthe newly appearing component of E P in LGB a ~ e r injection of TT (Fig. 4, 6}. This fact is evidence that the p r o c e s s e s mentioned above a r e interconnected and that the relay cells of LGB~ with direct afferent connections with the cortex [7], may perhaps participate in the formation of the a c c e s s o r y wave of the combined potential in LGB. The relationship discovered between the r a t e of waxing and waning of the corresponding phases of component Nc in LGB and the amplitude of the wave N1 developing synchronously with this component in VC can be explained by the existence of definite correlation bet-vceen the combined potentials and unit activity in L G ~ i.e., by correspondence of the steepness of the combined EP to the number of responding neurons and their mean discharge frequency [2!]. 99

The passage of a visual signal through LGB during illumination of the retina by a short flash is associated with both excitation and inhibition of cells of the nucleus at different moments of time [3, 4, 7, 8, 20]. It can tentatively be suggested that injection of TT, a neurotropic poison capable of disturbing the discharge of inhibitory mediators (glycine and GABA) from presynaptic terminals in various parts of the CNS [6, I l L and also in this nucleus, is the cause of the disturbance of GABA-ergio inhibitory mechanisms [13]. Gradual masking of the p r i m a r y short-latency wave N1 by the additional delayed component Nc could arise as a result of distrubance of inhibitory mechanisms in the initial period of formation of the response and activation of an accessory group of neurons with a longer latent period of discharge. The fact that with the course of time the descending phase of the short-latency component of N1 tends to disappear with the course of time can be explained by lengthening of the duration of discharge of the "short-latency" neurons (see [1]) responsible for the formation of this component [17], and this also is connected with a disturbance of the inhibitory mechanisms controlling the duration of the spike volley. An important distinguishing feature of EP in LGB, when modified by injection of TT, was the general shift of polarity of all the components toward negativity mentioned above. Besides changes in the early components of the response examined above, however, this shift did not lead to any decrease in the late slow positive wave, and this was particularly noticeable in response to flashes of weak intensity, namely 3 and 10 Ix (Fig. 4, 3, 5). The presence of this component, connected with the duration of inhibition of the relay neurons [7, 20], indicates preservation of process es of late inhibition inthe nucleus. The intensity of the corresponding slow component in the cortex, in the case of weak photic stimulation, also was unchanged. Comparison of these results with those indicating a possible disturbance of inhibiterymeehanisms in the initial period of EP formation suggests that the chemical mechanism of the processes of inhibition in LGB, which is evidently connected with the realization of different mediators [12, 16], may be damaged to a different degree in the course of the poststimulus period under the influence of ST. The dissociated effects of TT on the processes of early and late inhibition also was discovered when its action on the spinal cord [15] and cortex [5] was investigated. LITERATURE 1. 2.

3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

I00

CITED

G . N . Kryzhanovskii, M. B. Rekhtman and B. A. Konnikov, "Changes in the functional organization of the lateral geniculate body after injection of tetanus toxin into it," Neirofiziologiya, 10, No. 1, 38 (1978). G . N . Kryzhanovskii, M. B. Rekhtman, B. A. Konnikov, and V. Kh. Petlyuk, "Formation of photogenic epilepsy by a generator of pathologically enhanced excitation located in the lateral geniculate body," Byull. Eksp. Biol. Med., 81., No. 1, 23 (1976). M . B . Rekhtman, B. A. Konnikov, and G. N. Kryzhanovskii, "Analysis of unit activity on the cat lateral geniculate body," Neiroflziologiya, 10, No. 1, 30 (1978). I . A . Shevelev, Dynamics of the Visual Sensory Signal [in Russian], Nauka, Moscow (1971), p. 248. V . B . Brooks and H. Asanuma, ',Pharmacological studies of recurrent inhibition and facilitation," Am. J. Physiol., 208, 674 (1965). V . B . Brooks, D. R. Curtis, and J. G. Eccles, "The action of tetanus toxin on the inhibition of motoneurones," J. Physiol. (London), 135, 655 (1957). W. Burke and A. J. Sefton, "Recovery of responsiveness of cells of lateral geniculate nucleus of rat," J . Physiol. (London), 187, 213 (1966). W. Burke and A. J. Sefton,, Inhibitory mechanisms in lateral geniculate nucleus of rat," J. Physiol. (London), 187, 231 (1966). D . J . Creel, R. E. Dustman, and E. G. Beck, "Differences in visually evoked responses in albino versus hooded rats," Exp. Neurol., 29, 298 (1970). D . J . Creel, R. E. Dustman, and E. C. Beck, "Intensity of flash illumination and the visually evoked potential of rats, guinea pigs, and cats," Vision Res., 14, 725 (1974). D . R . Curtis, D. Felix, C. I. A. Game, and R. M. McGulloch, "Tetanus toxin and the synaptic release of GABA,," Brain Res., 51, 358 (1973). D . R . Curtis and O. A. R. Johnston, "Amino acid transmitters in the mammalian central nervous system," in: Reviews of Physiology, 69, 97 (1974). D . R . Curtis and A. K. Tebecis, "Bicuculline and thalamic inhibition," Exp. Brain Res., 16, 210 (1972). E. Fifkova and J. Marsala, "Atlas of Brain," in: Electrophysiological Methods in Biological Research, Prague (1967), pp. 653-695. G . N . Kryzhanovsky (G. N. Kryzhanovskii) and F. D. Sheikhon" ',Descending supraspinal effects under conditions of disturbance of the inhibitory processes in the nuclei of medulla," Exp. Neuronl., 50, 387 (1976).

16. 17. 18. 19. 20. 21.

R . S . Morgan, A. M. Sillito, and J. H. Soltencroft, "A pharmacological investigation of inhibition in the lateral geniculte nucleus," J. Physiol. (London), 246, 93P (1975). H. Noda and K. Iwama, "Unitary analysis of retino-genleulate response time in rats," Vision Res.,7, 205 (1967). L . E . Rhodes and D. E. Fleming, "Sensory restriction in the albino rat; photically evoked after-discharge correlates," Electroenceph. Clin. Neurophysiol., 29, 488 (1970). J . S . Schwartzbaum, C. J. Kreinick, and J. W. Gustafson, "Cortical evoked potentials and behavioral reactivity to photic stimuli in freely moving cats," Brain Res., 27, 295 (1971). E . F . Vastola, "After-positivity in lateral geniculate body," 5. Neurophysiol., 22, 258 (1959). M. Verzeano, R. C. Dill, E. Vallecalle, P. Groves, and J. Thomas, "Evoked responses and neuronal activity in the lateral gerdculate body," Experientta, 2.4, 696 (1968).

SYNAPTIC

ACTIVATION

BY R E T I C U L O S P I N A L

OF T H O R A C I C

FIBERS

INTERNEURONS

OF T H E L A T E R A L

A . P . G o k i n , A . I. P i l y a v s k i i , a n d I. P a v l a s e k *

FUNICULUS

UDC 612.832 : 612.831

Synaptic responses of different functional groups of interneurons in segments T10 and T l l to stimulation of the ipsilateral and contralateral medullary reticular formation were investigated in anesthetized cats with only the ipsilateral lateral funiculus remaining intact. Activation of reticulospinal fibers of the lateral funiculus with conduction veIocities of 30-100 m / s e c was shown to induce short-latency and, in particular, monosynptic EPSPs in all types of ceils tested: in interneurons excited by group Ia muscle afferents, in cells activated only by high-threshoid cutaneous and muscle afferents (afferents of the flexor reflex), in cells activated mainly by descending systems, and, to a l e s s e r degree, in neurons connected with low-threshold cutaneous afferents. These cell populations are located mainly in the central and lateral parts of Rexed' s lamina VII. Most neurons in laminae I-V of the dorsal horn, except six ceils located in the superficial layers of the dorsal horn, received no reticulofugal influences. The functional organization of connections of the lateral reticnlospinal tract with spinal neurons is discussed and compared with the analogous organization of the medial reticnlospinal tract, and also of the "lateral" (eortico- and rubrospinal) descending systems. INTRODUCTION Control of spinal reflex mechanisms by the reticular formation (RF) is effected by several reticulospinal (RS) systems [8, 12, 13, 24], which differ in their functional organization. In particular, two main direct descending RS systems are distinguished- medial and lateral. The former, fibers of which commence in the pontine nuclei and partly in the gigantocellular nucleus of the bulbar RF, lies in the ventral funiculus (~TF); the latter, Which arises from the bulbar RF, runs bilaterally in the lateral funicnli (LF), m a n l y in their ventral halves [2, 17, 21, 22, 25]. According to electrophysiological investigations the medial system exerts a mono- and disynaptic excitatory action on motoneurons [8, 9] and interneurons excited by group I muscle afferents, and also on neurons activated mainly by descending pathways. Neurons activated by low-threshold cutaneous affererts or only by high-threshold afferents of all types (flexor reflex a f f e r e n t s - FRA) receive no such influences [3]. No similar investigations of the synaptie action of the lateral RS-system have been specially undert~ken. This was the object of the present investigation, in which responses of different groups of thoracic interneurons evoked by stimulation of RF were studied after partial transections of the spinal cord, when LF was the only t r s c t left intact. ~'Institnte of Normal and Pathological Physiology, Slovak Academy of Sciences Bratislava, Czechoslovakia. A. A. Bogomolets Institute of Physiology, Academy of Sciences of the IYicrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 10, No. 2, pp. 150-161, March-April, 1978. Original arficte submitted November 4, 1977. 0090-2977/78/1002-0101507.50

© 1978 Plenum Publishing Corporation

101