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Are 5-hydroxytryptamine and noradrenaline inhibitory transmitters in the medial geniculate nucleus?
780
SHORT COMMUNICATIONS
5 KOZAK, W., MCFARLANE,M. B., AND WESTERMAN, R., Long lasting reversible changes in the reflex responses of chronic spinal cats to touch, heat and cold, Nature (Lond.), 193 0962) 171-173. 6 KUGELaERG,E., Facial reflexes, Brain, 75 (1952) 385-396. 7 KUGELBERG,E., Polysynaptic reflexes of clinical importance, Electroenceph. clin. Neurol., Suppl.
22 (1962) 103-111. 8 NESMEINOVA,T. N., The inhibition of the motor reflex in spinal dogs under conditions of chronic experimentation, Sechenovphysiol. J. U.S.S.R., 43 0957) 281-288.
9 PROSSER,C. L., ANDHUNTER,W. S., The extinction of startle responses and spinal reflexes in the white rat, Amer. J. Physiol., 117 (1936) 61D-618. lO SPENCER, W. A., THOMPSON,R. F., AND NEILSON,D. R., Response decrement of the flexion reflex in the acute spinal cat and transient restoration by strong stimuli, J. Neurophysiol., 29 (1966) 221-239. 11 SPENCER,W. A., THOMPSON,R.. F., ANDNEILSON,D. R., Alterations in responsiveness of ascending and reflex pathways by iterated cutaneous afferent volleys, J. NeurophysioL, 29 (1966) 240-252. (Accepted October 16th, 1967) Brain Research, 6 (1967) 777-780
Are S-hydroxytryptamine and noradrenaline inhibitory transmitters in the medial geniculate nucleus? Evidence is accumulating that the two monoamines 5-hydroxytryptamine (5-HT) ~hd noradrenaline (NA) may be synaptic transmitter substances in several areas of the mammalian central nervous system ~,4,°,1°,1a. The possibility that they may be transmitters in the medial geniculate nucleus ( M G N ) is suggested by the findings of Fuxe 5, Andrn et al. 1, Hillarp et al. 7 and Shute and LewislL These authors found by means of a fluorescence histochemical technique that the M G N of rats contains numerous fine 5-HT and NA containing nerve terminals which originate from the lower brain stem. In the present investigation, pharmacological studies on the feline M G N , using the technique of microiontophoresis, support the suggestion that 5-HT and N A may be inhibitory transmitters in this structure. Monoamines and related compounds were tested on 258 cells in 10 adult cats under nitrous oxide and halothane (Fluothane, ICI) anaesthesia. Methods of recording and applying drugs have previously been described in detail 1°. The cerebral cortex was removed by suction to expose the dorsal hippocampus. Five- and 9-barrelled micropipettes were stereotaxically aligned and driven through the hippocampus and dorsal lateral geniculate nucleus into the M G N . L-Glutamic acid was applied at regular intervals to neurones and the extracellularly recorded cell firing frequencies monitored continuously with an electronic counter (Hewlett Packard 5214L) and ink recorder. M G N units were identified by evoking them with clicks into both ears e,11, or by confirming their locations in the M G N histologically, using 'acid lesions 's. A concentric bipolar electrode was used to stimulate the brain stem (BS) at the stereotaxic co-ordinates A3, L3, D - - 1 (mesencephalic reticular formation). The position of this electrode tip was also verified histologically. The most c o m m o n effect of brain stem stimulation was a depression of Lglutamate induced firing, although it also excited some cells and had no effect on Brain Research, 6 (1967) 780-782
SHORT COMMUNICATIONS
781
others. 5-HT bimaleinate was tested on 136 M G N units. Its most c o m m o n effect was a depression followed by a rapid recovery (108 cells) although it clearly excited a small proportion of cells (7) and had no effect on others (21). L-Noradrenaline bitartrate was tested on 114 cells. This drug also depressed most cells (71) but recovery often took longer than that with 5-HT. NA also excited some neurones (12) and was without effect on others (31). Strychnine has been reported to have a specific blocking action upon some inhibitory synaptic processes in the feline central nervous system 3. It was employed in the present investigation to determine whether it has a similar effect in the M G N . Iontophoretically applied strychnine usually had a marked depressant action on L-glutamate-, DL-homocysteic acid- and spontaneous-firing (84/92 cells) and this effect generally took several minutes to wear off. Fig. 1 illustrates the effects of iontophoretically applied strychnine on a neurone in which the actions of NA and BS stimulation were depressant. L-Glutamate (50 nA) was applied for 15 sec periods at 25 sec intervals. NA (50 nA) almost completely abolished L-glutamate induced firing. Recovery lasted for 1 min. BS stimulation (9 V at 30/sec) also markedly depressed L-glutamate induced firing, followed by a slow
NA 50
HA SO
BS stirn
BS stim ,200
ixl
z z
U O I/D
-1-
.100
>.
oc
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Fig. 1. Effect of iontophoretic strychnine on depression caused by NA and BS stimulation. Unit recorded 2450 # below dorsal surface of MGN. Horizontal bars below trace represent L-glutamate (50 hA) applications. Strychnine (60 nA) was applied for 1 min (represented by gap in trace). The trace following the gap was recorded 2 min after the end of strychnine application. Both, the depressant effects of NA (50 nA) and of BS stimulation (9 V at 30/sec), were reduced after strychnine. recovery. Strychnine (60 nA for 1 min) caused an even more pronounced depression and the glutamate firing reached control level 5 min after this drug had been applied. After strychnine both N A (50 nA) and BS stimulation (30/sec) had only slight depressant effects, followed by a rapid recovery. Similar results have been obtained with 5-HT except that recovery after 5-HT induced depression was generally rapid. The blocking action of iontophoretically applied strychnine on depression caused by 5-HT, NA and BS stimulation has been observed on more than 20 cells. In 2 cats intravenous strychnine (0.5-1.0 mg/kg) had similar effects but these were not as potent as those with iontophoretic strychnine. 5-HT and N A induced depressions were blocked more readily than that from BS stimulation. The latter was usually reduced after repeated applications but strychnine was without effect on synaptic activation in some cells. Strychnine blocked 5-HT Brain Research, 6 (1967) 780-782
782
SHORT COMMUNICATIONS
and N A induced depressions to the same extent in most cells tested (90 %) but on some cells an initial strychnine application blocked one m o n o a m i n e but not the other. It was observed that not all m o n o a m i n e induced depressions were blocked with strychnine. Excitatory effects o f 5-HT and N A were never blocked with strychnine. Thus, the depressant effects of 5-HT and N A on some cells in the M G N , and the depressant effects o f synaptic stimulation of the brain stem, can be blocked with strychnine. It is difficult to evaluate the significance of the effects of 5-HT and N A in the M G N . If strychnine is a specific blocker of postsynaptic inhibition, the above results suggest that 5-HT and N A are inhibitory transmitters in the M G N . However, if it has a general inhibitory action on depression, no oonclusion can be drawn. Studies are continuing to establish whether strychnine has a specific or a genera. blocking action. This investigation was supported in part by a grant from the National Health and Medical Research Council o f Australia. Department of Physiology, Monash University, Clayton, Victoria (Australia)
A. K. TEBE-CIS
REFERENCES 1 ANDI~N,N.-E., DAHLSTR•M, A., FUXE, K., LARSSON, K., OLSON, L., AND UNGERSTEDT,U., Ascending monoamine neurons to the telencephalon and diencephalon, Acta physiol, scand., 67 (1966) 313-326. 2 BRADLEY,P. B., AND WOLSTENCROF'r,J. H., Actions of drugs on single neurons in the brain stem, Brit. reed. Bull., 21 (1965) 15-18. 3 CURTIS, D. R., The pharmacology of central and peripheral inhibition, PharmacoL Rev., 15 (1963) 333-364. 4 ENGBERG, I., AND RYALL, R. W., The inhibitory action of noradrenaline and other monoamines on spinal neurones, J. Physiol. (Lond.), 185 (1966) 298-322. 5 FuxE, K., Evidence for the existence of monoamine neurons in the central nervous system. IV Distribution of monoamine terminals in the central nervous system, Acta physiol, scand., 64 (1965) Suppl. 247, 37-85. 6 GALAMBOS,g., ROSE, J. E., BROM1LEY,R. B., AND HUGHES, J., Microelectrode studies on medial geniculate body of cat. II. Response to clicks, J. Neurophysiol., 15 (1952) 359- 380. 7 HILLARP,N.-t~., FUXE, K., AND DAHLSTR6M,A., c. Demonstrating and mapping of central neurons containing dopamine, noradrenaline, and 5-hydroxytryptamine and their reactions to psychopharmaca, Pharmacol. Rev., 18 (1966) 72%741. 8 MCCANCE, I., AND PHILLIS, J. W., The location of microelectrode tips in nervous tissue, Experientia (Basel), 21 (1965) 108-109. 9 PHILLIS, J. W., AND TEBF.CIS, A. K., The responses of thalamic neurones to iontophoretically applied monoamines, J. Physiol. (Lond.), In press. 10 PHILLIS,J. W., TEBF.CIS,A. K., AND YORK, D. H., The inhibitory action of monoamines on lateral geniculate neurones, J. Physiol. (Lond.), 190 (1967) 563-581. 11 ROSE, J. E., AND GALAMBOS,R., Microelectrode studies on medial geniculate body of cat. I. Thalamic region activated by click stimuli, J. NeurophysioL, 15 (1952) 343-357. 12 SAUTE,C. C. D., A m LEWIS, P. R., Cholinergic and monoaminergic systems of the brain, Nature (Lond.), 212 (1966) 710-711. 13 WEIGHT, F. F., AND SALMOIRAGHI,G. C., Responses of spinal cord interneurons to acetylcholine, norepinephrine and serotonin administered by microiontophoresis, J. Pharmaeol. exp. Ther., 153 (1966) 420-427. (Accepted August 22nd, 1967)
Brain Research, 6 (1967) 780-782
SHORT COMMUNICATIONS
5 KOZAK, W., MCFARLANE,M. B., AND WESTERMAN, R., Long lasting reversible changes in the reflex responses of chronic spinal cats to touch, heat and cold, Nature (Lond.), 193 0962) 171-173. 6 KUGELaERG,E., Facial reflexes, Brain, 75 (1952) 385-396. 7 KUGELBERG,E., Polysynaptic reflexes of clinical importance, Electroenceph. clin. Neurol., Suppl.
22 (1962) 103-111. 8 NESMEINOVA,T. N., The inhibition of the motor reflex in spinal dogs under conditions of chronic experimentation, Sechenovphysiol. J. U.S.S.R., 43 0957) 281-288.
9 PROSSER,C. L., ANDHUNTER,W. S., The extinction of startle responses and spinal reflexes in the white rat, Amer. J. Physiol., 117 (1936) 61D-618. lO SPENCER, W. A., THOMPSON,R. F., AND NEILSON,D. R., Response decrement of the flexion reflex in the acute spinal cat and transient restoration by strong stimuli, J. Neurophysiol., 29 (1966) 221-239. 11 SPENCER,W. A., THOMPSON,R.. F., ANDNEILSON,D. R., Alterations in responsiveness of ascending and reflex pathways by iterated cutaneous afferent volleys, J. NeurophysioL, 29 (1966) 240-252. (Accepted October 16th, 1967) Brain Research, 6 (1967) 777-780
Are S-hydroxytryptamine and noradrenaline inhibitory transmitters in the medial geniculate nucleus? Evidence is accumulating that the two monoamines 5-hydroxytryptamine (5-HT) ~hd noradrenaline (NA) may be synaptic transmitter substances in several areas of the mammalian central nervous system ~,4,°,1°,1a. The possibility that they may be transmitters in the medial geniculate nucleus ( M G N ) is suggested by the findings of Fuxe 5, Andrn et al. 1, Hillarp et al. 7 and Shute and LewislL These authors found by means of a fluorescence histochemical technique that the M G N of rats contains numerous fine 5-HT and NA containing nerve terminals which originate from the lower brain stem. In the present investigation, pharmacological studies on the feline M G N , using the technique of microiontophoresis, support the suggestion that 5-HT and N A may be inhibitory transmitters in this structure. Monoamines and related compounds were tested on 258 cells in 10 adult cats under nitrous oxide and halothane (Fluothane, ICI) anaesthesia. Methods of recording and applying drugs have previously been described in detail 1°. The cerebral cortex was removed by suction to expose the dorsal hippocampus. Five- and 9-barrelled micropipettes were stereotaxically aligned and driven through the hippocampus and dorsal lateral geniculate nucleus into the M G N . L-Glutamic acid was applied at regular intervals to neurones and the extracellularly recorded cell firing frequencies monitored continuously with an electronic counter (Hewlett Packard 5214L) and ink recorder. M G N units were identified by evoking them with clicks into both ears e,11, or by confirming their locations in the M G N histologically, using 'acid lesions 's. A concentric bipolar electrode was used to stimulate the brain stem (BS) at the stereotaxic co-ordinates A3, L3, D - - 1 (mesencephalic reticular formation). The position of this electrode tip was also verified histologically. The most c o m m o n effect of brain stem stimulation was a depression of Lglutamate induced firing, although it also excited some cells and had no effect on Brain Research, 6 (1967) 780-782
SHORT COMMUNICATIONS
781
others. 5-HT bimaleinate was tested on 136 M G N units. Its most c o m m o n effect was a depression followed by a rapid recovery (108 cells) although it clearly excited a small proportion of cells (7) and had no effect on others (21). L-Noradrenaline bitartrate was tested on 114 cells. This drug also depressed most cells (71) but recovery often took longer than that with 5-HT. NA also excited some neurones (12) and was without effect on others (31). Strychnine has been reported to have a specific blocking action upon some inhibitory synaptic processes in the feline central nervous system 3. It was employed in the present investigation to determine whether it has a similar effect in the M G N . Iontophoretically applied strychnine usually had a marked depressant action on L-glutamate-, DL-homocysteic acid- and spontaneous-firing (84/92 cells) and this effect generally took several minutes to wear off. Fig. 1 illustrates the effects of iontophoretically applied strychnine on a neurone in which the actions of NA and BS stimulation were depressant. L-Glutamate (50 nA) was applied for 15 sec periods at 25 sec intervals. NA (50 nA) almost completely abolished L-glutamate induced firing. Recovery lasted for 1 min. BS stimulation (9 V at 30/sec) also markedly depressed L-glutamate induced firing, followed by a slow
NA 50
HA SO
BS stirn
BS stim ,200
ixl
z z
U O I/D
-1-
.100
>.
oc
:0 1 rain
Fig. 1. Effect of iontophoretic strychnine on depression caused by NA and BS stimulation. Unit recorded 2450 # below dorsal surface of MGN. Horizontal bars below trace represent L-glutamate (50 hA) applications. Strychnine (60 nA) was applied for 1 min (represented by gap in trace). The trace following the gap was recorded 2 min after the end of strychnine application. Both, the depressant effects of NA (50 nA) and of BS stimulation (9 V at 30/sec), were reduced after strychnine. recovery. Strychnine (60 nA for 1 min) caused an even more pronounced depression and the glutamate firing reached control level 5 min after this drug had been applied. After strychnine both N A (50 nA) and BS stimulation (30/sec) had only slight depressant effects, followed by a rapid recovery. Similar results have been obtained with 5-HT except that recovery after 5-HT induced depression was generally rapid. The blocking action of iontophoretically applied strychnine on depression caused by 5-HT, NA and BS stimulation has been observed on more than 20 cells. In 2 cats intravenous strychnine (0.5-1.0 mg/kg) had similar effects but these were not as potent as those with iontophoretic strychnine. 5-HT and N A induced depressions were blocked more readily than that from BS stimulation. The latter was usually reduced after repeated applications but strychnine was without effect on synaptic activation in some cells. Strychnine blocked 5-HT Brain Research, 6 (1967) 780-782
782
SHORT COMMUNICATIONS
and N A induced depressions to the same extent in most cells tested (90 %) but on some cells an initial strychnine application blocked one m o n o a m i n e but not the other. It was observed that not all m o n o a m i n e induced depressions were blocked with strychnine. Excitatory effects o f 5-HT and N A were never blocked with strychnine. Thus, the depressant effects of 5-HT and N A on some cells in the M G N , and the depressant effects o f synaptic stimulation of the brain stem, can be blocked with strychnine. It is difficult to evaluate the significance of the effects of 5-HT and N A in the M G N . If strychnine is a specific blocker of postsynaptic inhibition, the above results suggest that 5-HT and N A are inhibitory transmitters in the M G N . However, if it has a general inhibitory action on depression, no oonclusion can be drawn. Studies are continuing to establish whether strychnine has a specific or a genera. blocking action. This investigation was supported in part by a grant from the National Health and Medical Research Council o f Australia. Department of Physiology, Monash University, Clayton, Victoria (Australia)
A. K. TEBE-CIS
REFERENCES 1 ANDI~N,N.-E., DAHLSTR•M, A., FUXE, K., LARSSON, K., OLSON, L., AND UNGERSTEDT,U., Ascending monoamine neurons to the telencephalon and diencephalon, Acta physiol, scand., 67 (1966) 313-326. 2 BRADLEY,P. B., AND WOLSTENCROF'r,J. H., Actions of drugs on single neurons in the brain stem, Brit. reed. Bull., 21 (1965) 15-18. 3 CURTIS, D. R., The pharmacology of central and peripheral inhibition, PharmacoL Rev., 15 (1963) 333-364. 4 ENGBERG, I., AND RYALL, R. W., The inhibitory action of noradrenaline and other monoamines on spinal neurones, J. Physiol. (Lond.), 185 (1966) 298-322. 5 FuxE, K., Evidence for the existence of monoamine neurons in the central nervous system. IV Distribution of monoamine terminals in the central nervous system, Acta physiol, scand., 64 (1965) Suppl. 247, 37-85. 6 GALAMBOS,g., ROSE, J. E., BROM1LEY,R. B., AND HUGHES, J., Microelectrode studies on medial geniculate body of cat. II. Response to clicks, J. Neurophysiol., 15 (1952) 359- 380. 7 HILLARP,N.-t~., FUXE, K., AND DAHLSTR6M,A., c. Demonstrating and mapping of central neurons containing dopamine, noradrenaline, and 5-hydroxytryptamine and their reactions to psychopharmaca, Pharmacol. Rev., 18 (1966) 72%741. 8 MCCANCE, I., AND PHILLIS, J. W., The location of microelectrode tips in nervous tissue, Experientia (Basel), 21 (1965) 108-109. 9 PHILLIS, J. W., AND TEBF.CIS, A. K., The responses of thalamic neurones to iontophoretically applied monoamines, J. Physiol. (Lond.), In press. 10 PHILLIS,J. W., TEBF.CIS,A. K., AND YORK, D. H., The inhibitory action of monoamines on lateral geniculate neurones, J. Physiol. (Lond.), 190 (1967) 563-581. 11 ROSE, J. E., AND GALAMBOS,R., Microelectrode studies on medial geniculate body of cat. I. Thalamic region activated by click stimuli, J. NeurophysioL, 15 (1952) 343-357. 12 SAUTE,C. C. D., A m LEWIS, P. R., Cholinergic and monoaminergic systems of the brain, Nature (Lond.), 212 (1966) 710-711. 13 WEIGHT, F. F., AND SALMOIRAGHI,G. C., Responses of spinal cord interneurons to acetylcholine, norepinephrine and serotonin administered by microiontophoresis, J. Pharmaeol. exp. Ther., 153 (1966) 420-427. (Accepted August 22nd, 1967)
Brain Research, 6 (1967) 780-782