THE PHARMACOLOGY OF THE SYNAPTIC AND ACETYLCHOLINE-INDUCED EXCITATION OF VENTROBASAL THALAMIC NEURONES

The responses of thalamic neurons to iontophoretically applied monoamines

1. The effects of noradrenaline (NA), adrenaline, dopamine (DA) 5-hydroxytryptamine (5-HT) and a number of related drugs were tested on the extracellularly recorded responses of neurones in the feline thalamus. Substances were applied iontophoretically and tested on synaptically, antidromically and chemically evoked neuronal activity.2. NA, adrenaline, isoprenaline and 5-HT had a variety of effects, depressing some cells, exciting others and not affecting the responses of a third group. DA depressed most of the cells tested; excitation was not observed with this compound.3. The magnitude of depressant actions and their duration varied considerably. The more sensitive cells responded to extremely small amounts of catecholamine or 5-HT and recovery often took several minutes. Recovery after DA was always rapid. Neurones in the dorsal thalamus were generally more susceptible to depression than those in the ventro-basal complex.4. Excitatory responses were most marked in the ventro-basal complex of the thalamus. Desensitization occurred if NA or adrenaline was applied repeatedly and this tachyphylaxis lasted for several minutes. After desensitization to the excitatory effects, some of these cells were depressed by catecholamines. These findings suggest the presence of at least two types of membrane receptor.5. beta-adrenergic antagonists (alderlin, D-INPEA and MJ 1999) and alpha-antagonists (phentolamine, dibenzyline and chlorpromazine) had pronounced depressant actions on some thalamic neurones. With the exceptions of D-INPEA and MJ 1999 they also excited cells that were excited by the catecholamines. Alderlin and phentolamine had both excitatory and inhibitory effects on some cells.6. The monoamine oxidase inhibitor, iproniazid, depressed neurones which were sensitive to NA depression. It did not appear to potentiate the effects of NA on most of the cells tested.7. Reticular formation stimulation depressed some neurones in the thalamus and excited others. The depressant effects of NA and reticular formation stimulation were reduced or abolished by an intravenous injection of picrotoxin (1 mg/kg).8. It is suggested that NA and 5-HT may be inhibitory transmitters in the thalamus, released at the terminals of ascending pathways from the brain stem that have been defined by fluorescence microscopy. The excitatory actions of these compounds may also be related to a synaptic role.

The inhibitory action of monoamines on lateral geniculate neurones

1. L-Noradrenaline (NA), dopamine, 5-hydroxytryptamine (5-HT) and lysergide were administered iontophoretically to neurones in the dorsal lateral geniculate nucleus of the cat and the responses to these drugs recorded.2. Many neurones were depressed by the monoamines and lysergide.3. This depression was manifested by a reduction or abolition of the effects of optic nerve or visual stimulation, by a failure in some instances of an antidromically propagating spike to invade the cell soma, and by a depression of the excitant effects of L-glutamate and acetylcholine.4. Although there was considerable variation in the magnitude of the depressant effects of the monoamines, dopamine was found to be slightly more potent than 5-HT and NA. Neurones which were inhibited by catecholamines did not always respond to 5-HT and lysergide and vice versa.5. The inhibitory actions of monoamines on lateral geniculate neurones are comparable with those that have been recorded in other structures of the central nervous system.6. Lysergide did not antagonize the action of 5-HT.7. Catecholamines and 5-HT are present in nerve terminals in the lateral geniculate nucleus and the findings of this study suggest that they act as inhibitory transmitters.

Cholinergic mechanisms in the caudate nucleus

1. Acetylcholine (ACh), and a number of cholinomimetic and cholinolytic agents, have been applied to single neurones of the caudate nucleus, and the effects compared with those produced by stimulation of nucleus ventralis anterior thalami (VA).2. Neurones responding by excitation following ACh and VA stimulation, and others depressed by both ACh and VA stimulation have been observed. The two types of cell are located in different regions of the nucleus.3. Both types of ACh effect, and the responses to VA stimulation, are prevented by atropine.4. Taken with the earlier observation that the release of ACh from the caudate can be enhanced by VA stimulation, it has been suggested that the present results indicate a final cholinergic link in the pathway from VA to caudate.

Discharge patterns of principal cells and interneurones in lateral geniculate nucleus of rat

1. The cells of the lateral geniculate nucleus of the rat are classified into one of two distinct groups on the basis of their responses to a single shock to optic nerve or visual cortex. The distinctive difference between these two groups is that the P cells (87% of all cells) responded with short bursts of 1-5 spikes, whereas the I cells (13%) responded with long bursts of about 10 spikes. Both groups give recurring bursts at intervals of 100 msec or more for a total duration of up to about 5 sec.2. The majority of P cells (75%) have a lower threshold for late firing than for early firing. The majority of I cells (62%) have about the same threshold and all I cells have very low thresholds.3. To stimulation of the optic nerve the mean latency of the first spikes of all P cells is 4.4 msec. The mean latency of the first spikes of all I cells is 5.8 msec. The latency of the earliest spike in an I cell is 0.9 msec longer than the earliest spike in a P cell.4. To stimulation of the visual cortex the latency of the earliest spike in an I cell is 1.0 msec longer than the earliest spike in a P cell. Most P cells (78%) respond with either very short latency (less than 1.7 msec) or with a wave form characteristic of antidromic invasion or both. No I cell has either of these characterstics.5. It is concluded that P cells are principal cells projecting to visual cortex and that I cells are interneurones.

Pharmacological studies on feline Betz cells

1. A study has been made of the sensitivity of single neurones in the pericruciate cortex of anaesthetized and unanaesthetized cats to cholinomimetics administered electrophoretically from multibarrel micropipettes.2. A high proportion of deep pyramidal cells, including Betz cells, were excited by these substances, and the receptors involved have muscarinic characteristics.3. Atropine specifically reduced the sensitivity of cortical neurones to acetylcholine, but no such action could be demonstrated for dihydro-beta-erythroidine, gallamine or general anaesthetics.4. The significance of these results is discussed in relation to the possible synaptic or non-synaptic action of acetylcholine upon cortical neurones.

Acetylcholine sensitivity of cerebellar neurones in the cat

1. Cholinomimetics, acetylcholine antagonists and some other compounds of pharmacological interest were administered electrophoretically near neurones within the vermal cerebellar cortex of anaesthetized (pentobarbitone) and unanaesthetized (cerveau isolé) cats.2. The neurones were identified by position within the cortex, spontaneous activity, and the responses to afferent and antidromic stimulation.3. Purkinje cells, but neither granule nor basket cells, were excited by cholinomimetics, and the acetylcholine receptors had muscarinic properties. Excitation was often preceded by depression of the spontaneous firing.4. Intravenously administered atropine and dihydro-beta-erythroidine did not depress the synaptic excitation of cerebellar neurones evoked by impulses in mossy, climbing or parallel fibres.5. Acetylcholine is thus unlikely to be an excitatory transmitter within the feline cerebellum, particularly at mossy fibre-granule cell synapses, despite the presence of relatively high levels of acetylcholinesterase within mossy fibre terminals.

Pharmacological properties of cholinoceptive neurones in the medulla and pons of the cat

1. Iontophoretic application of acetylcholine to neurones in the medulla and pons of the unanaesthetized decerebrate cat excited 35.5% and inhibited 22%. This was not a representative sample of lower brain stem neurones, since most penetrations were made medially. Some cholinoceptive neurones were in the medial reticular formation.2. The time course of the response to different doses has been described.3. Nicotine and nicotinic agents 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) and choline phenyl ether (TM1) excited neurones excited by acetylcholine but, with one exception, did not inhibit neurones inhibited by acetylcholine.4. Muscarinic agents mimicked both the excitatory and inhibitory responses to acetylcholine. (+/-)-muscarine and bethanechol (beta-methylcholine carbamate) had a prolonged and powerful inhibitory action.5. Oxotremorine [1-(2-oxopyrrolidino)-4-pyrrolidino-butyne-2] had a long-lasting excitatory action on neurones excited by acetylcholine.6. Eserine and neostigmine potentiated the action of acetylcholine but in addition they had a strong excitatory effect on many brain stem neurones, independently of the action of acetylcholine.7. Excitatory and inhibitory responses to acetylcholine could be antagonized by gallamine, hexamethonium and atropine. Dihydro-beta-erythroidine (DHbetaE) antagonized only excitatory responses. (+)-Tubocurarine was a weak antagonist.8. It is concluded that the excitatory responses have both nicotinic and muscarinic properties but the inhibitory responses are only muscarinic. Since the pharmacology of these two types of response is different, it is concluded that the inhibitory action is not due to an indirect effect from excitation of a neighbouring inhibitory neurone.

Noradrenaline and inhibition of Renshaw cells

Although postsynaptic inhibition of Renshaw cells in the spine is blocked by strychnine the depression of these neurons by electrophoretically administered noradrenaline is resistant to this alkaloid. This finding raises doubts as to whether noradrenaline is an inhibitory transmitter in the spinal cord of the cat.

Micro-electrophoretic studies of neurones in the cat hippocampus

1. Drugs have been applied micro-electrophoretically to units in the hippocampal cortex of the anaesthetized cat, and their effects on cell firing were recorded simultaneously.2. L-Glutamate rapidly and powerfully excited hippocampal units, an effect which was quickly reversed on stopping the expelling current. The local application of L-glutamate also excited a fast seizure discharge at 15-50/sec. Both these effects of L-glutamate were strongly depressed by fimbrial stimulation.3. gamma-Aminobutyric acid had a strong depressant action on all the units on which it was tested; the time course of this effect was rapid.4. ACh excited half the units to which it was applied. Characteristically this excitation developed slowly over many seconds and persisted after stopping the expelling current. Most cholinoceptive units were found to be concentrated in the superficial layer of the cortex corresponding to the hippocampal pyramidal cells and their main dendritic processes.5. Atropine selectively blocked the excitation of cholinoceptive units by ACh, but not the excitation by L-glutamate. No cholinoceptive units were blocked by dihydro-beta-erythroidine, though several were selectively blocked by dimethyl (+)-tubocurarine.6. The most usual effect seen with 5-HT was depression, though several units were found to be excited. Some of the units tested with 3-hydroxytyramine (dopamine) or noradrenaline were found to be depressed.