- Email: [email protected]
Effects of monoamines and amino acids on medial geniculate neurones of the cat
Neuropharmacology, 1970, 9, 381-390 Pergamon Press. Printed in Gt. Britain.
EFFECTS OF MONOAMINES AND AMINO ACIDS ON MEDIAL GENICULATE NEURONES OF THE CAT A. K. TEBECIS* Department of Physiology, Monash University, Clayton, Victoria, Australia
(Accepted 12 December 1969) Summary--A study was made of the actions of microelectrophoretically administered monoamines and amino acids on neurones of the feline M G nucleus. The most common effect of the monoamines was depression. Depression by 5-HT and D A was generally rapid in onset and recovery whereas depression by N A was more often of a slower time course. The monoamines also excited some neurones, and 5-HT and N A had both excitant and depressant ("dual") actions o n a small proportion. The excitant actions of N A and adrenaline were usually of a slow time course and prone to desensitization. Although the effects of the monoamines were often similar when compared on the same neurone, there were several instances when one compound caused depression and the other/s, a n excitation, a "dual" action or no effect. The results suggest that characteristic differences exist between the actions of 5-HT, N A and D A on M G neurones. GABA, fl-alanine and glycine depressed almost all M G neurones tested, the descending order of relative potency being GABA, fl-alanine and glycine. Their depressant actions were invariably rapid in onset and recovery. Electrophoretically administered strychnine readily blocked the depressant action of glycine but not that of GABA. Higher concentrations of strychnine also reduced or blocked the depressant actions of 5-HT, N A and D A on some neurones.
THE DIENCEPHALONof the cat contains 5-hydroxytryptamine (5-HT), noradrenaline (NA), dopamine (DA), dihydroxyphenylalanine (DOPA and 5-hydroxytryptophan) decarboxylase and monoamine oxidase (BOGDANSKI et al., 1957; BERTLERand ROSENGREN, 1959; MCGEER et al., 1963). Fluorescent histochemical studies of rats indicate that monoamines are localized in nerve terminals in the medial geniculate (MG) nucleus (FuXE, 1965) and that these terminals are derived from ascending noradrenergic and 5-hydroxytryptaminergic projections from the brain stem to the diencephalon (AND~N et al., 1966). These observations suggest the possibility that one or more monoamines may have a synaptic function in the mammalian MG nucleus. For this reason a study was made of the actions of microelectrophoretically administered monoamines and related compounds on M G neurones. Some monocarboxylic amino acids were also tested, as recent evidence suggests that glycine and 7-aminobutyric acid (GABA) are probably inhibitory transmitters in certain areas of the feline central nervous system (see review by CURTIS and CRAWFORD,1969). The action of strychnine on the depressant effects of various compounds was also investigated. Preliminary reports of a part of this work have been published (TEBP.CIS, 1967, 1969).
*Present address: Department of Neurophysiology, Neurological Clinic, University of Basle, Socinstrasse 55, Basle, Switzerland. 381
382
A.K. TEB~CIS
METHODS These studies were performed on 22 cats. The majority of the results were obtained from quiescent neurones which were fired by ejections of L-glutamate; few spontaneously firing cells were studied. A detailed description of the methods has been given in a recent paper (TEB~:CIS, 1969b). The animals were initially anaesthetized with ethyl chloride and ether, or with an intravenous injection of sodium thiopentone (30-50 mg/kg). After tracheotomy, anaesthesia was maintained by a mixture of nitrous oxide (60-65 ~), oxygen (35-40 ~) and halothane (1-2 ~), (Fluothane, I.C.I.) or methoxyflurane (1-2 ~) (Penthrane, Abbott). The cerebral cortex and dorsal hippocampus overlying the lateral geniculate nucleus were removed, and 5-7-barrel glass micropipettes were driven through the lateral geniculate nucleus into the MG nucleus between the stereotaxic co-ordinates A3-6 and L8-11 (SNIDER and NIEMER,1961). Single cells in the MG nucleus were identified by click stimulation and by stimulation of the auditory cortex and inferior colliculus. Electrode positions were verified by H + induced lesions and subsequent histology (MCCANCEand PHILLIS,1965). Evoked field and action potentials were recorded through the sodium chloride (2M)filled central barrel of the multi-barrel micropipettes, the tip diameters of which usually ranged between 5 and 8 microns. One barrel was routinely filled with sodium L-glutamate (0.2-2M, pH 8 with NaOH) for activating quiescent neurones. L-Glutamate could be applied with pulses of constant duration at regular intervals by means of a timing circuit. The other barrels contained aqueous solutions of the drugs to be tested. 5-Hydroxytryptamine creatinine sulphate (May and Baker; 0.1M), 5-hydroxytryptamine bimaleinate (KochLight Laboratories; 0.2M), dopamine hydrochloride (Calbiochem and Koch-Light Laboratories; 0.5-1.0 M), L-noradrenaline bitartrate (British Drug Houses; 0.5-1.0 M), L-adrenaline bitartrate (British Drug Houses; 0.5-1.0 M), DL-3, 4-dihydroxyphenylalanine(Calbiochem ; 0.5-1.0 M) and isoprenaline sulphate (Burroughs Wellcome and Co.; 0.5-1.0 M) were adjusted to pH 3-4 with HC1 and ejected as cations. Glycine,/3-alanine and GABA (British Drug Houses) were used as 0.5 M solutions (pH 3-4 with HC1). Strychnine hydrochloride (British Drug Houses) was made up either as a 10 mM solution in 200 mM NaCI, or as 50-60 mM in distilled water. Action potentials were displayed on a cathode ray oscilloscope and counted after conversion into pulses by means of an electronic counter, the analogue output of which was displayed on an ink recorder (TEBECIS,1970).
RESULTS The effects of 5-HT and some catecholamines on MG neurones are summarized in Table 1. The predominant effect of each monoamine was depression, which varied in degree and duration. It was useful to classify the depressions into two broad (arbitrary) groups-those with a duration of less than 30 sec ("rapid recovery") and those with a duration of greater than 30 sec ("slow recovery") after termination of the electrophoretic current. The effects of each compound will be described separately.
5-Hydroxytryptamine 5-HT depressed 78 ~ of the neurones tested. Typically, the depressant effects of this compound were of a rapid onset, followed by a rapid recovery, as illustrated in Fig. 1. Bilateral click stimulation (1/sec) evoked a repetitive discharge from this neurone with a
383
Pharmacology of medial geniculate neurones TABLE 1. SUMMARYOF THE EFFECTSOr SOMEMONOAMINESON NEURONESOF THE MG NUCLEUS Compound
Total
5-HT NA DA DL-DOPA Adrenaline Isoprenaline
198 218 70 24 35 19
Excitation
Depression
12 25 1 0 3 1
155 131 66 13 22 14
(6%) (12%) (IX) (0 ~ ) (9 %) (5~)
(78~) (60~o) (94~o) (54 %) (63 %) (74~)
Dual
No effect
(5~) (6~) (0K) (0 ~ ) (0 %) (0~)
21 49 3 11 10 4
10 13 0 0 0 0
(11 ~ ) (22Yo) (5%) (46 ~) (28 ~) (21%)
Figures outside brackets refer to number of neurones. Those inside brackets represent proportions (to the nearest whole percent). 'Dual' represents the cells which were both excited and depressed by the monoamines.
A
CLICKS
o
,
.
0
.
.
.
.
.
.
E~
.
.
.
,'...
. . . .
IOmsec
COLLICULUS
.
.
.
.
.
.
.
.
.
.
msec
FIG. 1. Depressant effects of 5-HT on two different cells (A and B). AI ; response to bilateral click stimulation (1/sec). A2; 4 sec after the beginning of a 10-sec ejection of 5-HT (60nA). A3; 6 sec after terminating the 5-HT ejecting current. B1 ; response to 1/sec stimulation of the ipsilateral inferior colliculus. (The first deflection is the stimulus artefact). B2; 12 sec after the beginning of a 20-sec ejection of 5-HT (80nA). B3; 15 sec after the ejection had been terminated. The vertical calibration represents 0-2mV (A) and 0.1mV (B). latency o f 20-30 msec a n d a d u r a t i o n o f up to 200 msec (Fig. 1A). A d m i n i s t r a t i o n o f 5 - H T (60nA) c o m p l e t e l y suppressed the s y n a p t i c response within 4 sec (A,2) a n d recovery was c o m p l e t e 6 sec after the c u r r e n t ejecting 5 - H T h a d been t e r m i n a t e d (A,3). Similarly, the s y n a p t i c spike o f a n o t h e r cell e v o k e d by 1/see s t i m u l a t i o n o f the inferior colliculus (B,1) was b l o c k e d within 12 sec b y 5 - H T (80nA) (B,2) a n d recovery t o o k 15 see (B,3). Since equal currents ejecting N a + h a d no effect on the o r t h o d r o m i c responses shown in A a n d B, it is unlikely t h a t the o b s e r v e d d e p r e s s i o n was merely the result o f c u r r e n t flow. O f a t o t a l o f 155 cells depressed b y 5-HT, 88 ~ were c h a r a c t e r i z e d by a r a p i d recovery a n d 12 ~ b y a slow recovery. A n o t h e r characteristic o f 5 - H T was the low p r o p o r t i o n (11 ~ ) o f cells unaffected by this i n d o l e a m i n e .
384
A.K. TEB~-ClS
5-HT exhibited either excitant, or a combination of both excitant and depressant effects on some neurones. Two examples of the 'dual' effects of 5-HT are shown in Fig. 2. The response of one of these cells to L-glutamate was depressed by repetitive click stimulation (Fig. 2A). 5-HT (40nA), administered twice, depressed L-glutamate-induced firing but later caused an increase in the direct firing. The neurone illustrated by Fig. 2B fired in response to acetylcholine (60nA). 5-HT (60hA) depressed this acetylcholine-induced firing but also appeared to excite the cell. A
CLICKS IO/sec 5-HT40
GSO
5-HT4.0
i
I 60sec
B
5-HT60
IOO
o G60 ACH60
FIG. 2. Two examples (A and B) of the 'dual' excitant-depressant effects of 5-HT. In this and subsequent figures the firing frequency (in spikes per sec) is shown on the vertical scale. The durations of the ejecting currents are represented by horizontal bars above and below the tracings. In A, the duration of repetitive click stimulation (10/sec) is also shown. L-Glutamate, 50nA (G50) was administered at periodic intervals. In B, L-glutamate, 60nA (G60) was administered initially and then acetylcholine, 60nA (ACH 60) was administered at intervals until the end of the trace.
Noradrenaline N A depressed 60 ~ of the cells tested, and although this depression was usually of short duration (72 ~ of cells depressed), N A caused a long-duration depression approximately twice as often as did 5-HT. Long-duration depression by N A occurred even though the firing frequency of the neurones was not markedly reduced. N A was without effect on approximately twice as many cells as was 5-HT. The proportion of cells excited by N A was double that excited by 5-HT. These excitant actions were usually of long duration. A long-duration effect of N A is illustrated in Fig. 3. The first record in Fig. 3A is of a field potential evoked in the M G nucleus by stimulating
Pharmacology of medial geniculate neurones
385
A
O.ImV L I
B
2msec I
NA._50
200 f lO0 LO L-GLUT 6(
i
i
30 sec
FIG. 3. An example of the excitant action of NA. A; responses evoked by stimulation of the inferior colliculus (1/sec). The first was recorded before the administration of NA. The second was obtained 6 sec after the beginning of administering NA (80hA). The third, 6 min after the current ejecting N A had been terminated. B; chart recording of the firing frequency of the same cell, showing the facilitatory effect of a subsequent ejection of N A (50hA) on firing induced by repeated applications of L-glutamate, 60hA (L-GLUT 60).
the inferior colliculus at 1/sec. Six seconds after administering NA (80nA), another potential was superimposed on the field potential (second record). The third record shows that this potential was still present 6 min after the administration of NA had been terminated. It persisted for a further 9 min, after which a field potential comparable to that initially recorded was observed. The chart tracing in Fig. 3B was recorded from the same cell and confirmed the observation that NA had a long-lasting facilitatory effect on the cell. The excitant effects of NA on some neurones were susceptible to desensitization (tachyphylaxis). This was manifested as a progressive reduction in sensitivity of the cell to repeated applications of NA. After waiting for 10-20 min, an application of NA caused an excitation comparable to that observed initially. NA both excited and depressed some cells, as shown in Fig. 4. NA (60nA) caused an initial depression followed by a long-duration facilitation of glutamate-firing of the neurone illustrated by Fig. 4A. On another cell (Fig. 4B) NA (80nA) caused direct firing as well as a depression of glutamate-firing. 5-HT (80hA) caused only a depression of this cell.
Dopamine and dihydroxyphenylalanine The effects of DA were the most consistent of all the monoamines studied. DA depressed 94 ~o of the cells to which it was applied and 83 ~o of these depressions were followed by a rapid recovery. The remainder were long-duration depressions, as shown in Fig. 5A. DA was without effect on only 5 ~o of neurones. It had an excitant action on only 1 cell (Fig. 5B). This record shows that DA (40nA) produced a slowly-developing, long-lasting increase in the direct firing frequency of the cell. DA (40nA) facilitated glutamate-firing
386
A . K . TEB~CtS
,~,
NA..._~
0J 5-HT80 N~O
B
30 sec OJ FIG. 4. Two examples of 'dual' excitant-depressant effects of NA. L-Glutamate was ejected with currents of 50nA (A) and 40hA (B), indicated by lower horizontal lines.
A
DOPAMINE 40
I00
J G~O
B
DOPAMINE40
G6(
G60
30sec F~G. 5. Effects of DA on two different cells (A and B). A; D A (40hA) depressed L-glutamate, NInA (1360) firing for up to 3 min. B; D A (40hA) produced a slowly-developing excitation of this cell.
Pharmacology of medial geniculate neurones
387
during two successive ejections before this record was obtained. However, the lack of other examples of an excitant action of DA raises the possibility that the increased firing rate in Fig. 5B may have been due to a depressant action of DA on adjacent cells, which exerted a tonic inhibitory bombardment on the cell recorded. It was not possible to discover whether the effects were due to disinhibition or to a direct excitant action of DA. DL-DOPA had slight depressant effects on approximately half the cells tested. These were always less potent than those of DA ejected with equal currents. No excitant action of DL-DOPA was observed.
Adrenaline and isoprenaline The two catecholamines, adrenaline and isoprenaline, depressed the majority of M G neurones tested (Table 1). Recovery was generally rapid, although some long-duration depressions were also observed. Both amines had excitant actions on a small proportion of cells. Desensitization to the excitant action of adrenaline was demonstrated on one cell.
Comparison of monoamines Although 5-HT, NA and DA often had comparable effects on the same cell, there was frequently no correlation between their effects. Thus, on some cells one monoamine caused depression and the others caused either an excitation or a 'dual' effect. Those cells on which the monoamines had comparable effects varied considerably in their sensitivity to a particular monoamine. Such differences in relative potency were found even in cells recorded with the same electrode. Generally, however, DA was a more potent depressant than 5-HT or NA. The monoamines usually depressed glutamate-induced firing of a neurone more readily than a response evoked by neural stimulation, as was observed for the depressant effects of acetylcholine on M G neurones (TEB~CIS, 1970). No desensitization to the depressant actions of the monoamines was observed.
Amino acids and strychnine GABA and fl-alanine depressed all M G neurones tested. Glycine depressed 94~o of neurones and had no effect on the remainder. The depressant actions of all three amino acids were rapid in both onset and recovery, as occurs in other areas of the central nervous system (CURTIS and CRAWFORD, 1969). The relative descending order of potency was GABA, fl-alanine and glycine. GABA and fl-alanine (and sometimes glycine) were more potent than the monoamines in those neurones on which they were compared. Strychnine was administered electrophoretically to more than 100 M G neurones. When administered with currents of 20-80nA from 50-60mM solutions, strychnine usually caused a long-lasting depression of spontaneous or glutamate-induced firing. This was possibly due to blockade of the generation of action potentials, since strychnine has been shown to block the nodal membrane of myelinated nerve fibres (MARUHASHIet al., 1956). The firing frequency of cells often increased after recovery from such strychnine-induced depressions. The firing frequency of some cells was unaffected by strychnine (even when applied with currents of 80-100nA from 50 to 60mM solutions) and 4 neurones were excited by the convulsant. When strychnine was ejected from 10mM solutions (in 200mM NaCI) it generally caused no depression;with currents of 20-30nA there was often a slowly-developing increase in excitability of the cell. Strychnine readily and consistently blocked the depressant action of glycine. When administered with currents up to 20nA from a 10raM solution, strychnine usually blocked
388
A.K. TEB~CB
the effect of glycine within 5 min, and recovery was generally complete within 10 min after the strychnine current had been terminated. The depressant action of GABA was never blocked by strychnine, even when the latter was administered with high currents (80-150nA) from a concentrated (60mM) solution. On 2 cells, however, strychnine slightly reduced the depressant action of GABA. The effects of strychnine on the depressant actions of the monoamines were less consistent than on glycine-induced depression. Although the convulsant had no effect on some monoamine-induced depressions, it clearly reduced or blocked the depressant actions of 5-HT, NA and DA on some cells, In all cases the quantity of strychnine necessary to produce such antagonism was higher than that used to block the depressant action of glycine. The effects of strychnine on depressions produced by 5-HT and NA in the M G nucleus have been briefly reported previously (TEB~CIS, 1967). DISCUSSION The results suggest that there are characteristic differences between the effects of 5-HT, NA and DA on M G neurones fired by glutamate. Thus, DA and 5-HT had mainly depressant, and few excitant, actions. Their depressant actions were usually of rapid onset and recovery. Few cells were unaffected by DA, which usually resembled GABA in the time course of depression produced. NA also depressed most neurones but excited or had no effect more frequently than did 5-HT and DA. The finding that the type of effect observed was often related to the type of monoamine applied suggests that the mechanisms of action of, or receptors for, the monoamines differ. Thus, NA may have long-duration actions because it is bound to the receptor more firmly than either 5-HT or DA, or because it is removed less rapidly from the extracellular environment. Alternatively, different receptors for catechol- and indole-amines may be present. Further studies on the effects of monoamines on physiologically identified neurones may enable a more precise characterization to be made of the types of receptors on M G neurones. For example, it has been shown that NA depresses paramedian reticular nuclear neurones (AvANZINO et al., 1966) and excites Deiters' neurones (YAMAMOTO,1967) and neurones which project from the lateral geniculate nucleus to the visual cortex (SATINSKY,1967). The effects of monoamines on M G neurones are comparable to those on thalamic neurones (PHILLIS and TEB~CIS, 1967). Desensitization to the excitant (but not depressant) effects, as well as 'dual' excitant-depressant effects of monoamines have also been observed in the thalamus (PHILLISand TEB~CIS, 1967), cerebral cortex (ROBERTSand STRAUGHAN, 1967, 1968; PHILLIS et aL, 1968) and brain stem (BOAKESet al., 1968, 1969). It is also noteworthy that cells in the superior cervical ganglion have adrenergic receptors which mediate excitatory and inhibitory effects (DE GROAT and VOLLE, 1966). These observations suggest that excitatory and inhibitory receptors for monoamines on the same neurone are more widespread than had previously been supposed. Glycine, fl-alanine and GABA depressed almost all M G neurones, as has been observed in other areas of the central nervous system (CURTIS and WATKINS,1965; CURTIS and CRAWFORD, 1969). GABA was more potent than glycine, as has also been reported for cerebral cortical neurones (KELLYand KRNJEVI~,1968; CURTIS et al., 1968). This is in contrast to the spinal cord (CURTISe t al., 1968), medullary reticular formation (H6SLI et al., 1969) and red nucleus (DAvis and HUFFMANN,1969), where glycine is usually a more potent depressant than GABA.
Pharmacology of medial geniculate neurones
389
The significance of the results o b t a i n e d with strychnine is difficult to interpret. A l t h o u g h m u c h evidence supports the hypothesis that strychnine competes with the spinal i n h i b i t o r y t r a n s m i t t e r (CURTIS, 1963; CURTIS et al., 1968) there is n o t complete agreement o n this view (LARSON, 1969; ROVER et al., 1969). The observations that strychnine reduces or blocks the depressant actions o f glycine, m o n o a m i n e s a n d acetylcholine on M G n e u r o n e s [present report a n d TEB~CIS (1967, 1968)] suggest that strychnine c a n n o t act as a competitive a n t a g o n ist. However, higher c o n c e n t r a t i o n s of strychnine are needed to block the effects of the m o n o a m i n e s a n d acetylcholine t h a n to block those of glycine. A n t a g o n i s m by strychnine m a y be specific only when the c o n v u l s a n t is administered at low c o n c e n t r a t i o n s (CURl"IS et al., 1969). Studies are c o n t i n u i n g in order to assess the potency of strychnine as a blocking agent of various c o m p o u n d s applied from the same electrode. Acknowledgements I am grateful to Professor A. K. MCINTVREand Dr. J. W. PHILHS for the use of their facilities and valuable discussions. This investigation was supported by a Monash University Research Scholarship. Some of the equipment was provided by the National Health and Medical Research Council of Australia.
REFERENCES
ANDI~N,N.-E., DAHLSTR(JM,A., FUXE,K., LARSSON,K., OLSON,L. and UNGERSTEDT,U. (1966). Ascending monoamine neurons to the telencephalon and diencephalon. Acta physiol, scancl. 67: 313-326. AVANZINO,G. L., BRADLEY,P. B. and WOLSTENCROFT,J. H. (1966). Pharmacological properties of neurones of the paramedian reticular nucleus. Experientia 22:410. BOAKES,R. J., BRADLEY,P. B., BROOKES,N. and WOLSTENCROF~r,J. H. (1968). Characteristics of the response of brain-stem neurones to noradrenaline. Br. J. Pharmac. Chemother. 32: 417P. BOAKES,R. J., BRADLEY,P. B., BROOKES,N., CANDY,J. M. and WOLSTENCROFT,J. H. (1969). Effects of noradrenaline and its analogues on brain stem neurones. J. Physiol., Lond. 201:20-21P. BOGDANSKI,D. F., WEISSBACH,H. and UDENFRIEND,S. (1957). The distribution of serotonin, 5-hydroxytryptophan decarboxylase and monoamine oxidase in brain. J. Neurochem. 1: 272-278. BERTLER,A. and ROSENGREN,E. (1959). Occurrence and distribution of catecholamines in brain. Actaphysiol. scand. 47: 350-361. CURTIS,D. R. (1963). The pharmacology of central and peripheral inhibition. Pharmac. Rev. 15: 333-364. CURTIS,D. R. and CRAWFORD,J. M. (1969). Central synaptic transmission--Microelectrophoretic studies. Ann. Rev. Pharmac. 9: 209-240. CURTIS,D. R., DUGGAN,A. W. and JOHNSTON,G. A. R. (1969). Glycine, strychnine, picrotoxin and spinal inhibition. Brain Res. 14: 759-762. CURTIS,D. R., H6SLI, L., JOHNSTON,G. A. R. and JOHNSTON,I. H. (1968). The hyperpolarization of spinal motoneurones by glycine and related amino acids. Expl Brain Res. 5: 235-258. CURTIS,D. R. and WATKINS,J. C. (1965). The pharmacology of amino acids related to gamma-aminobutyric acid. Pharmac. Rev. 17: 347-391. DAvis, R. and HUFFMAN,R. D. (1969). Pharmacology of the brachium conjunctivum-red nucleus synaptic system in the baboon. Fedn. Proc. 28: 775. DEGROAT,W. C. and VOLLE,R. L. (1966). The actions of the catecholamines on transmission in the superior cervical ganglion of the cat. J. Pharmac. exp. Ther. 154: 1-13. FUXE, K. (1965). Evidence for the existence of monamine neurons in the central nervous system--IV. Distribution of monoamine terminals in the central nervous system. Acta physiol, scand. 64: Suppl. 247, 37-85. H6SLI, L., TEB~CIS,A. K. and FIHAS,N. (1969). Effects of glycine, beta-alanine and GABA, and their interaction with strychnine, on brain stem neurones. Brain Res. 16: 293-295. KELLY, J. S. and KRNJEVI6, K. (1968). Effects of ~,-aminobutyric acid and glycine on cortical neurones. Nature, Lond. 219: 1380--1381. LARSON,M. D. (1969). An analysis of the action of strychnine on the recurrent IPSP and amino acid induced inhibitions in the cat spinal cord. Brain Res. 15: 185-200. MARUHASm,J., OTANI,T., TAgArIASnI,H. and YAMADA,M. (1956). On effects of strychnine upon the myelinated nerve fibers of toads. Jap. J. Physiol. 6: 175-189. MCCANCE, I. and PmLLIS, J. W. (1965). The location of microelectrode tips in nervous tissue. Experientia 21: 108-109.
390
A . K . TEB~.CIS
MCGEER, P. L., McGEER, E. G. and WADA, J. A. (1963). Central aromatic amine levels and behaviour--II. Serotonin and catecholamine levels in various cat brain areas following administration of psychoactive drugs or amine precursors. Archs Neurol. Psychiat., Chicago 9: 81-89. PHILLIS, J. W. and TEB~CIS, A. K. (1967). The responses of thalamic neurones to iontophoretically applied monoamines. J. PhysioL, Lond. 192: 715-745. PHILLIS, J. W., TEB~CIS, A. K. and YORK, D. H. 0968). Histamine and some antihistamines: their actions on cerebral cortical neurones. Br. J. Pharmac. Chemother. 33: 426~140. ROBERTS, M. H. T. and STRAUGHAN, D. W. (1967). Excitation and depression of cortical neurones by 5hydroxytryptamine. J. Physiol., Lond. 193: 269-294. ROBERTS, M. H. T. and STRAUGHAN,D. W. (1968). Actions of noradrenaline and mescaline on cortical neurones. Naunyn-Schmiedebergs Arch. exp. Path. Pharmak. 259: 191-192. ROPER, S., DIAMOND, J. and YASARGIL, G. M. (1969). Does strychnine block inhibition post-synaptically? Nature, Lond. 223: 1168-1169. SATINSKY, D. (1967). Pharmacological responsiveness of lateral geniculate nucleus neurons. Int. J. Neuropharmac. 6: 387-397. SNIDER, R. S. and NIEMER, W. T. (1961). A Stereotaxic Atlas o f the Cat Brain. University of Chicago Press, Chicago. TEB~CIS, A. K. (1967). Are 5-hydroxytryptamine and noradrenaline inhibitory transmitters in the medial geniculate nucleus ? Brain Res. 6: 780-782. TEBECIS, A. K. (1968). Acetylcholine and medial geniculate neurones. Aust. J. exp. Biol. reed. Sci. 46: P3. TEB~CIS, A. K. (1969). Monoamines and medial geniculate neurones. Med. J. Aust. 1: 193. TEB~CIS,A. K. (1970). Properties of cholinoceptive neurones in the medial geniculate nucleus. Br. J. Pharmac. Chemother., 38:117-137. YAMAMOTO, C. (1967). Pharmacologic studies of norepinephrine, acetylcholine and related compounds on neurons in Deiters' nucleus and tile cerebellum. J. Pharmac. exp. Ther. 156: 39-47.
EFFECTS OF MONOAMINES AND AMINO ACIDS ON MEDIAL GENICULATE NEURONES OF THE CAT A. K. TEBECIS* Department of Physiology, Monash University, Clayton, Victoria, Australia
(Accepted 12 December 1969) Summary--A study was made of the actions of microelectrophoretically administered monoamines and amino acids on neurones of the feline M G nucleus. The most common effect of the monoamines was depression. Depression by 5-HT and D A was generally rapid in onset and recovery whereas depression by N A was more often of a slower time course. The monoamines also excited some neurones, and 5-HT and N A had both excitant and depressant ("dual") actions o n a small proportion. The excitant actions of N A and adrenaline were usually of a slow time course and prone to desensitization. Although the effects of the monoamines were often similar when compared on the same neurone, there were several instances when one compound caused depression and the other/s, a n excitation, a "dual" action or no effect. The results suggest that characteristic differences exist between the actions of 5-HT, N A and D A on M G neurones. GABA, fl-alanine and glycine depressed almost all M G neurones tested, the descending order of relative potency being GABA, fl-alanine and glycine. Their depressant actions were invariably rapid in onset and recovery. Electrophoretically administered strychnine readily blocked the depressant action of glycine but not that of GABA. Higher concentrations of strychnine also reduced or blocked the depressant actions of 5-HT, N A and D A on some neurones.
THE DIENCEPHALONof the cat contains 5-hydroxytryptamine (5-HT), noradrenaline (NA), dopamine (DA), dihydroxyphenylalanine (DOPA and 5-hydroxytryptophan) decarboxylase and monoamine oxidase (BOGDANSKI et al., 1957; BERTLERand ROSENGREN, 1959; MCGEER et al., 1963). Fluorescent histochemical studies of rats indicate that monoamines are localized in nerve terminals in the medial geniculate (MG) nucleus (FuXE, 1965) and that these terminals are derived from ascending noradrenergic and 5-hydroxytryptaminergic projections from the brain stem to the diencephalon (AND~N et al., 1966). These observations suggest the possibility that one or more monoamines may have a synaptic function in the mammalian MG nucleus. For this reason a study was made of the actions of microelectrophoretically administered monoamines and related compounds on M G neurones. Some monocarboxylic amino acids were also tested, as recent evidence suggests that glycine and 7-aminobutyric acid (GABA) are probably inhibitory transmitters in certain areas of the feline central nervous system (see review by CURTIS and CRAWFORD,1969). The action of strychnine on the depressant effects of various compounds was also investigated. Preliminary reports of a part of this work have been published (TEBP.CIS, 1967, 1969).
*Present address: Department of Neurophysiology, Neurological Clinic, University of Basle, Socinstrasse 55, Basle, Switzerland. 381
382
A.K. TEB~CIS
METHODS These studies were performed on 22 cats. The majority of the results were obtained from quiescent neurones which were fired by ejections of L-glutamate; few spontaneously firing cells were studied. A detailed description of the methods has been given in a recent paper (TEB~:CIS, 1969b). The animals were initially anaesthetized with ethyl chloride and ether, or with an intravenous injection of sodium thiopentone (30-50 mg/kg). After tracheotomy, anaesthesia was maintained by a mixture of nitrous oxide (60-65 ~), oxygen (35-40 ~) and halothane (1-2 ~), (Fluothane, I.C.I.) or methoxyflurane (1-2 ~) (Penthrane, Abbott). The cerebral cortex and dorsal hippocampus overlying the lateral geniculate nucleus were removed, and 5-7-barrel glass micropipettes were driven through the lateral geniculate nucleus into the MG nucleus between the stereotaxic co-ordinates A3-6 and L8-11 (SNIDER and NIEMER,1961). Single cells in the MG nucleus were identified by click stimulation and by stimulation of the auditory cortex and inferior colliculus. Electrode positions were verified by H + induced lesions and subsequent histology (MCCANCEand PHILLIS,1965). Evoked field and action potentials were recorded through the sodium chloride (2M)filled central barrel of the multi-barrel micropipettes, the tip diameters of which usually ranged between 5 and 8 microns. One barrel was routinely filled with sodium L-glutamate (0.2-2M, pH 8 with NaOH) for activating quiescent neurones. L-Glutamate could be applied with pulses of constant duration at regular intervals by means of a timing circuit. The other barrels contained aqueous solutions of the drugs to be tested. 5-Hydroxytryptamine creatinine sulphate (May and Baker; 0.1M), 5-hydroxytryptamine bimaleinate (KochLight Laboratories; 0.2M), dopamine hydrochloride (Calbiochem and Koch-Light Laboratories; 0.5-1.0 M), L-noradrenaline bitartrate (British Drug Houses; 0.5-1.0 M), L-adrenaline bitartrate (British Drug Houses; 0.5-1.0 M), DL-3, 4-dihydroxyphenylalanine(Calbiochem ; 0.5-1.0 M) and isoprenaline sulphate (Burroughs Wellcome and Co.; 0.5-1.0 M) were adjusted to pH 3-4 with HC1 and ejected as cations. Glycine,/3-alanine and GABA (British Drug Houses) were used as 0.5 M solutions (pH 3-4 with HC1). Strychnine hydrochloride (British Drug Houses) was made up either as a 10 mM solution in 200 mM NaCI, or as 50-60 mM in distilled water. Action potentials were displayed on a cathode ray oscilloscope and counted after conversion into pulses by means of an electronic counter, the analogue output of which was displayed on an ink recorder (TEBECIS,1970).
RESULTS The effects of 5-HT and some catecholamines on MG neurones are summarized in Table 1. The predominant effect of each monoamine was depression, which varied in degree and duration. It was useful to classify the depressions into two broad (arbitrary) groups-those with a duration of less than 30 sec ("rapid recovery") and those with a duration of greater than 30 sec ("slow recovery") after termination of the electrophoretic current. The effects of each compound will be described separately.
5-Hydroxytryptamine 5-HT depressed 78 ~ of the neurones tested. Typically, the depressant effects of this compound were of a rapid onset, followed by a rapid recovery, as illustrated in Fig. 1. Bilateral click stimulation (1/sec) evoked a repetitive discharge from this neurone with a
383
Pharmacology of medial geniculate neurones TABLE 1. SUMMARYOF THE EFFECTSOr SOMEMONOAMINESON NEURONESOF THE MG NUCLEUS Compound
Total
5-HT NA DA DL-DOPA Adrenaline Isoprenaline
198 218 70 24 35 19
Excitation
Depression
12 25 1 0 3 1
155 131 66 13 22 14
(6%) (12%) (IX) (0 ~ ) (9 %) (5~)
(78~) (60~o) (94~o) (54 %) (63 %) (74~)
Dual
No effect
(5~) (6~) (0K) (0 ~ ) (0 %) (0~)
21 49 3 11 10 4
10 13 0 0 0 0
(11 ~ ) (22Yo) (5%) (46 ~) (28 ~) (21%)
Figures outside brackets refer to number of neurones. Those inside brackets represent proportions (to the nearest whole percent). 'Dual' represents the cells which were both excited and depressed by the monoamines.
A
CLICKS
o
,
.
0
.
.
.
.
.
.
E~
.
.
.
,'...
. . . .
IOmsec
COLLICULUS
.
.
.
.
.
.
.
.
.
.
msec
FIG. 1. Depressant effects of 5-HT on two different cells (A and B). AI ; response to bilateral click stimulation (1/sec). A2; 4 sec after the beginning of a 10-sec ejection of 5-HT (60nA). A3; 6 sec after terminating the 5-HT ejecting current. B1 ; response to 1/sec stimulation of the ipsilateral inferior colliculus. (The first deflection is the stimulus artefact). B2; 12 sec after the beginning of a 20-sec ejection of 5-HT (80nA). B3; 15 sec after the ejection had been terminated. The vertical calibration represents 0-2mV (A) and 0.1mV (B). latency o f 20-30 msec a n d a d u r a t i o n o f up to 200 msec (Fig. 1A). A d m i n i s t r a t i o n o f 5 - H T (60nA) c o m p l e t e l y suppressed the s y n a p t i c response within 4 sec (A,2) a n d recovery was c o m p l e t e 6 sec after the c u r r e n t ejecting 5 - H T h a d been t e r m i n a t e d (A,3). Similarly, the s y n a p t i c spike o f a n o t h e r cell e v o k e d by 1/see s t i m u l a t i o n o f the inferior colliculus (B,1) was b l o c k e d within 12 sec b y 5 - H T (80nA) (B,2) a n d recovery t o o k 15 see (B,3). Since equal currents ejecting N a + h a d no effect on the o r t h o d r o m i c responses shown in A a n d B, it is unlikely t h a t the o b s e r v e d d e p r e s s i o n was merely the result o f c u r r e n t flow. O f a t o t a l o f 155 cells depressed b y 5-HT, 88 ~ were c h a r a c t e r i z e d by a r a p i d recovery a n d 12 ~ b y a slow recovery. A n o t h e r characteristic o f 5 - H T was the low p r o p o r t i o n (11 ~ ) o f cells unaffected by this i n d o l e a m i n e .
384
A.K. TEB~-ClS
5-HT exhibited either excitant, or a combination of both excitant and depressant effects on some neurones. Two examples of the 'dual' effects of 5-HT are shown in Fig. 2. The response of one of these cells to L-glutamate was depressed by repetitive click stimulation (Fig. 2A). 5-HT (40nA), administered twice, depressed L-glutamate-induced firing but later caused an increase in the direct firing. The neurone illustrated by Fig. 2B fired in response to acetylcholine (60nA). 5-HT (60hA) depressed this acetylcholine-induced firing but also appeared to excite the cell. A
CLICKS IO/sec 5-HT40
GSO
5-HT4.0
i
I 60sec
B
5-HT60
IOO
o G60 ACH60
FIG. 2. Two examples (A and B) of the 'dual' excitant-depressant effects of 5-HT. In this and subsequent figures the firing frequency (in spikes per sec) is shown on the vertical scale. The durations of the ejecting currents are represented by horizontal bars above and below the tracings. In A, the duration of repetitive click stimulation (10/sec) is also shown. L-Glutamate, 50nA (G50) was administered at periodic intervals. In B, L-glutamate, 60nA (G60) was administered initially and then acetylcholine, 60nA (ACH 60) was administered at intervals until the end of the trace.
Noradrenaline N A depressed 60 ~ of the cells tested, and although this depression was usually of short duration (72 ~ of cells depressed), N A caused a long-duration depression approximately twice as often as did 5-HT. Long-duration depression by N A occurred even though the firing frequency of the neurones was not markedly reduced. N A was without effect on approximately twice as many cells as was 5-HT. The proportion of cells excited by N A was double that excited by 5-HT. These excitant actions were usually of long duration. A long-duration effect of N A is illustrated in Fig. 3. The first record in Fig. 3A is of a field potential evoked in the M G nucleus by stimulating
Pharmacology of medial geniculate neurones
385
A
O.ImV L I
B
2msec I
NA._50
200 f lO0 LO L-GLUT 6(
i
i
30 sec
FIG. 3. An example of the excitant action of NA. A; responses evoked by stimulation of the inferior colliculus (1/sec). The first was recorded before the administration of NA. The second was obtained 6 sec after the beginning of administering NA (80hA). The third, 6 min after the current ejecting N A had been terminated. B; chart recording of the firing frequency of the same cell, showing the facilitatory effect of a subsequent ejection of N A (50hA) on firing induced by repeated applications of L-glutamate, 60hA (L-GLUT 60).
the inferior colliculus at 1/sec. Six seconds after administering NA (80nA), another potential was superimposed on the field potential (second record). The third record shows that this potential was still present 6 min after the administration of NA had been terminated. It persisted for a further 9 min, after which a field potential comparable to that initially recorded was observed. The chart tracing in Fig. 3B was recorded from the same cell and confirmed the observation that NA had a long-lasting facilitatory effect on the cell. The excitant effects of NA on some neurones were susceptible to desensitization (tachyphylaxis). This was manifested as a progressive reduction in sensitivity of the cell to repeated applications of NA. After waiting for 10-20 min, an application of NA caused an excitation comparable to that observed initially. NA both excited and depressed some cells, as shown in Fig. 4. NA (60nA) caused an initial depression followed by a long-duration facilitation of glutamate-firing of the neurone illustrated by Fig. 4A. On another cell (Fig. 4B) NA (80nA) caused direct firing as well as a depression of glutamate-firing. 5-HT (80hA) caused only a depression of this cell.
Dopamine and dihydroxyphenylalanine The effects of DA were the most consistent of all the monoamines studied. DA depressed 94 ~o of the cells to which it was applied and 83 ~o of these depressions were followed by a rapid recovery. The remainder were long-duration depressions, as shown in Fig. 5A. DA was without effect on only 5 ~o of neurones. It had an excitant action on only 1 cell (Fig. 5B). This record shows that DA (40nA) produced a slowly-developing, long-lasting increase in the direct firing frequency of the cell. DA (40nA) facilitated glutamate-firing
386
A . K . TEB~CtS
,~,
NA..._~
0J 5-HT80 N~O
B
30 sec OJ FIG. 4. Two examples of 'dual' excitant-depressant effects of NA. L-Glutamate was ejected with currents of 50nA (A) and 40hA (B), indicated by lower horizontal lines.
A
DOPAMINE 40
I00
J G~O
B
DOPAMINE40
G6(
G60
30sec F~G. 5. Effects of DA on two different cells (A and B). A; D A (40hA) depressed L-glutamate, NInA (1360) firing for up to 3 min. B; D A (40hA) produced a slowly-developing excitation of this cell.
Pharmacology of medial geniculate neurones
387
during two successive ejections before this record was obtained. However, the lack of other examples of an excitant action of DA raises the possibility that the increased firing rate in Fig. 5B may have been due to a depressant action of DA on adjacent cells, which exerted a tonic inhibitory bombardment on the cell recorded. It was not possible to discover whether the effects were due to disinhibition or to a direct excitant action of DA. DL-DOPA had slight depressant effects on approximately half the cells tested. These were always less potent than those of DA ejected with equal currents. No excitant action of DL-DOPA was observed.
Adrenaline and isoprenaline The two catecholamines, adrenaline and isoprenaline, depressed the majority of M G neurones tested (Table 1). Recovery was generally rapid, although some long-duration depressions were also observed. Both amines had excitant actions on a small proportion of cells. Desensitization to the excitant action of adrenaline was demonstrated on one cell.
Comparison of monoamines Although 5-HT, NA and DA often had comparable effects on the same cell, there was frequently no correlation between their effects. Thus, on some cells one monoamine caused depression and the others caused either an excitation or a 'dual' effect. Those cells on which the monoamines had comparable effects varied considerably in their sensitivity to a particular monoamine. Such differences in relative potency were found even in cells recorded with the same electrode. Generally, however, DA was a more potent depressant than 5-HT or NA. The monoamines usually depressed glutamate-induced firing of a neurone more readily than a response evoked by neural stimulation, as was observed for the depressant effects of acetylcholine on M G neurones (TEB~CIS, 1970). No desensitization to the depressant actions of the monoamines was observed.
Amino acids and strychnine GABA and fl-alanine depressed all M G neurones tested. Glycine depressed 94~o of neurones and had no effect on the remainder. The depressant actions of all three amino acids were rapid in both onset and recovery, as occurs in other areas of the central nervous system (CURTIS and CRAWFORD, 1969). The relative descending order of potency was GABA, fl-alanine and glycine. GABA and fl-alanine (and sometimes glycine) were more potent than the monoamines in those neurones on which they were compared. Strychnine was administered electrophoretically to more than 100 M G neurones. When administered with currents of 20-80nA from 50-60mM solutions, strychnine usually caused a long-lasting depression of spontaneous or glutamate-induced firing. This was possibly due to blockade of the generation of action potentials, since strychnine has been shown to block the nodal membrane of myelinated nerve fibres (MARUHASHIet al., 1956). The firing frequency of cells often increased after recovery from such strychnine-induced depressions. The firing frequency of some cells was unaffected by strychnine (even when applied with currents of 80-100nA from 50 to 60mM solutions) and 4 neurones were excited by the convulsant. When strychnine was ejected from 10mM solutions (in 200mM NaCI) it generally caused no depression;with currents of 20-30nA there was often a slowly-developing increase in excitability of the cell. Strychnine readily and consistently blocked the depressant action of glycine. When administered with currents up to 20nA from a 10raM solution, strychnine usually blocked
388
A.K. TEB~CB
the effect of glycine within 5 min, and recovery was generally complete within 10 min after the strychnine current had been terminated. The depressant action of GABA was never blocked by strychnine, even when the latter was administered with high currents (80-150nA) from a concentrated (60mM) solution. On 2 cells, however, strychnine slightly reduced the depressant action of GABA. The effects of strychnine on the depressant actions of the monoamines were less consistent than on glycine-induced depression. Although the convulsant had no effect on some monoamine-induced depressions, it clearly reduced or blocked the depressant actions of 5-HT, NA and DA on some cells, In all cases the quantity of strychnine necessary to produce such antagonism was higher than that used to block the depressant action of glycine. The effects of strychnine on depressions produced by 5-HT and NA in the M G nucleus have been briefly reported previously (TEB~CIS, 1967). DISCUSSION The results suggest that there are characteristic differences between the effects of 5-HT, NA and DA on M G neurones fired by glutamate. Thus, DA and 5-HT had mainly depressant, and few excitant, actions. Their depressant actions were usually of rapid onset and recovery. Few cells were unaffected by DA, which usually resembled GABA in the time course of depression produced. NA also depressed most neurones but excited or had no effect more frequently than did 5-HT and DA. The finding that the type of effect observed was often related to the type of monoamine applied suggests that the mechanisms of action of, or receptors for, the monoamines differ. Thus, NA may have long-duration actions because it is bound to the receptor more firmly than either 5-HT or DA, or because it is removed less rapidly from the extracellular environment. Alternatively, different receptors for catechol- and indole-amines may be present. Further studies on the effects of monoamines on physiologically identified neurones may enable a more precise characterization to be made of the types of receptors on M G neurones. For example, it has been shown that NA depresses paramedian reticular nuclear neurones (AvANZINO et al., 1966) and excites Deiters' neurones (YAMAMOTO,1967) and neurones which project from the lateral geniculate nucleus to the visual cortex (SATINSKY,1967). The effects of monoamines on M G neurones are comparable to those on thalamic neurones (PHILLIS and TEB~CIS, 1967). Desensitization to the excitant (but not depressant) effects, as well as 'dual' excitant-depressant effects of monoamines have also been observed in the thalamus (PHILLISand TEB~CIS, 1967), cerebral cortex (ROBERTSand STRAUGHAN, 1967, 1968; PHILLIS et aL, 1968) and brain stem (BOAKESet al., 1968, 1969). It is also noteworthy that cells in the superior cervical ganglion have adrenergic receptors which mediate excitatory and inhibitory effects (DE GROAT and VOLLE, 1966). These observations suggest that excitatory and inhibitory receptors for monoamines on the same neurone are more widespread than had previously been supposed. Glycine, fl-alanine and GABA depressed almost all M G neurones, as has been observed in other areas of the central nervous system (CURTIS and WATKINS,1965; CURTIS and CRAWFORD, 1969). GABA was more potent than glycine, as has also been reported for cerebral cortical neurones (KELLYand KRNJEVI~,1968; CURTIS et al., 1968). This is in contrast to the spinal cord (CURTISe t al., 1968), medullary reticular formation (H6SLI et al., 1969) and red nucleus (DAvis and HUFFMANN,1969), where glycine is usually a more potent depressant than GABA.
Pharmacology of medial geniculate neurones
389
The significance of the results o b t a i n e d with strychnine is difficult to interpret. A l t h o u g h m u c h evidence supports the hypothesis that strychnine competes with the spinal i n h i b i t o r y t r a n s m i t t e r (CURTIS, 1963; CURTIS et al., 1968) there is n o t complete agreement o n this view (LARSON, 1969; ROVER et al., 1969). The observations that strychnine reduces or blocks the depressant actions o f glycine, m o n o a m i n e s a n d acetylcholine on M G n e u r o n e s [present report a n d TEB~CIS (1967, 1968)] suggest that strychnine c a n n o t act as a competitive a n t a g o n ist. However, higher c o n c e n t r a t i o n s of strychnine are needed to block the effects of the m o n o a m i n e s a n d acetylcholine t h a n to block those of glycine. A n t a g o n i s m by strychnine m a y be specific only when the c o n v u l s a n t is administered at low c o n c e n t r a t i o n s (CURl"IS et al., 1969). Studies are c o n t i n u i n g in order to assess the potency of strychnine as a blocking agent of various c o m p o u n d s applied from the same electrode. Acknowledgements I am grateful to Professor A. K. MCINTVREand Dr. J. W. PHILHS for the use of their facilities and valuable discussions. This investigation was supported by a Monash University Research Scholarship. Some of the equipment was provided by the National Health and Medical Research Council of Australia.
REFERENCES
ANDI~N,N.-E., DAHLSTR(JM,A., FUXE,K., LARSSON,K., OLSON,L. and UNGERSTEDT,U. (1966). Ascending monoamine neurons to the telencephalon and diencephalon. Acta physiol, scancl. 67: 313-326. AVANZINO,G. L., BRADLEY,P. B. and WOLSTENCROFT,J. H. (1966). Pharmacological properties of neurones of the paramedian reticular nucleus. Experientia 22:410. BOAKES,R. J., BRADLEY,P. B., BROOKES,N. and WOLSTENCROF~r,J. H. (1968). Characteristics of the response of brain-stem neurones to noradrenaline. Br. J. Pharmac. Chemother. 32: 417P. BOAKES,R. J., BRADLEY,P. B., BROOKES,N., CANDY,J. M. and WOLSTENCROFT,J. H. (1969). Effects of noradrenaline and its analogues on brain stem neurones. J. Physiol., Lond. 201:20-21P. BOGDANSKI,D. F., WEISSBACH,H. and UDENFRIEND,S. (1957). The distribution of serotonin, 5-hydroxytryptophan decarboxylase and monoamine oxidase in brain. J. Neurochem. 1: 272-278. BERTLER,A. and ROSENGREN,E. (1959). Occurrence and distribution of catecholamines in brain. Actaphysiol. scand. 47: 350-361. CURTIS,D. R. (1963). The pharmacology of central and peripheral inhibition. Pharmac. Rev. 15: 333-364. CURTIS,D. R. and CRAWFORD,J. M. (1969). Central synaptic transmission--Microelectrophoretic studies. Ann. Rev. Pharmac. 9: 209-240. CURTIS,D. R., DUGGAN,A. W. and JOHNSTON,G. A. R. (1969). Glycine, strychnine, picrotoxin and spinal inhibition. Brain Res. 14: 759-762. CURTIS,D. R., H6SLI, L., JOHNSTON,G. A. R. and JOHNSTON,I. H. (1968). The hyperpolarization of spinal motoneurones by glycine and related amino acids. Expl Brain Res. 5: 235-258. CURTIS,D. R. and WATKINS,J. C. (1965). The pharmacology of amino acids related to gamma-aminobutyric acid. Pharmac. Rev. 17: 347-391. DAvis, R. and HUFFMAN,R. D. (1969). Pharmacology of the brachium conjunctivum-red nucleus synaptic system in the baboon. Fedn. Proc. 28: 775. DEGROAT,W. C. and VOLLE,R. L. (1966). The actions of the catecholamines on transmission in the superior cervical ganglion of the cat. J. Pharmac. exp. Ther. 154: 1-13. FUXE, K. (1965). Evidence for the existence of monamine neurons in the central nervous system--IV. Distribution of monoamine terminals in the central nervous system. Acta physiol, scand. 64: Suppl. 247, 37-85. H6SLI, L., TEB~CIS,A. K. and FIHAS,N. (1969). Effects of glycine, beta-alanine and GABA, and their interaction with strychnine, on brain stem neurones. Brain Res. 16: 293-295. KELLY, J. S. and KRNJEVI6, K. (1968). Effects of ~,-aminobutyric acid and glycine on cortical neurones. Nature, Lond. 219: 1380--1381. LARSON,M. D. (1969). An analysis of the action of strychnine on the recurrent IPSP and amino acid induced inhibitions in the cat spinal cord. Brain Res. 15: 185-200. MARUHASm,J., OTANI,T., TAgArIASnI,H. and YAMADA,M. (1956). On effects of strychnine upon the myelinated nerve fibers of toads. Jap. J. Physiol. 6: 175-189. MCCANCE, I. and PmLLIS, J. W. (1965). The location of microelectrode tips in nervous tissue. Experientia 21: 108-109.
390
A . K . TEB~.CIS
MCGEER, P. L., McGEER, E. G. and WADA, J. A. (1963). Central aromatic amine levels and behaviour--II. Serotonin and catecholamine levels in various cat brain areas following administration of psychoactive drugs or amine precursors. Archs Neurol. Psychiat., Chicago 9: 81-89. PHILLIS, J. W. and TEB~CIS, A. K. (1967). The responses of thalamic neurones to iontophoretically applied monoamines. J. PhysioL, Lond. 192: 715-745. PHILLIS, J. W., TEB~CIS, A. K. and YORK, D. H. 0968). Histamine and some antihistamines: their actions on cerebral cortical neurones. Br. J. Pharmac. Chemother. 33: 426~140. ROBERTS, M. H. T. and STRAUGHAN, D. W. (1967). Excitation and depression of cortical neurones by 5hydroxytryptamine. J. Physiol., Lond. 193: 269-294. ROBERTS, M. H. T. and STRAUGHAN,D. W. (1968). Actions of noradrenaline and mescaline on cortical neurones. Naunyn-Schmiedebergs Arch. exp. Path. Pharmak. 259: 191-192. ROPER, S., DIAMOND, J. and YASARGIL, G. M. (1969). Does strychnine block inhibition post-synaptically? Nature, Lond. 223: 1168-1169. SATINSKY, D. (1967). Pharmacological responsiveness of lateral geniculate nucleus neurons. Int. J. Neuropharmac. 6: 387-397. SNIDER, R. S. and NIEMER, W. T. (1961). A Stereotaxic Atlas o f the Cat Brain. University of Chicago Press, Chicago. TEB~CIS, A. K. (1967). Are 5-hydroxytryptamine and noradrenaline inhibitory transmitters in the medial geniculate nucleus ? Brain Res. 6: 780-782. TEBECIS, A. K. (1968). Acetylcholine and medial geniculate neurones. Aust. J. exp. Biol. reed. Sci. 46: P3. TEB~CIS, A. K. (1969). Monoamines and medial geniculate neurones. Med. J. Aust. 1: 193. TEB~CIS,A. K. (1970). Properties of cholinoceptive neurones in the medial geniculate nucleus. Br. J. Pharmac. Chemother., 38:117-137. YAMAMOTO, C. (1967). Pharmacologic studies of norepinephrine, acetylcholine and related compounds on neurons in Deiters' nucleus and tile cerebellum. J. Pharmac. exp. Ther. 156: 39-47.