The effects of pentobarbitone sodium on acetylcholine excitation and noradrenaline inhibition of thalamic neurons

Cholinergic excitation and inhibition in the visual thalamus of the cat--influences of cortical inactivation and barbiturate anesthesia

Neuronal responses to microiontophoretically applied pulses of acetylcholine (ACh) were recorded in the perigeniculate nucleus (PGN) and dorsal lateral geniculate nucleus (dLGN) of the cat. In the PGN, ACh inhibited 95% of the cells under N2O/O2-halothane anesthesia. Systemic application of sodium pentobarbitone did not abolish ACh inhibition. In the dLGN with N2O-halothane, ACh increased firing rates of neurons before (88%) and after (78%) interruption of their direct retinal inputs by photocoagulation of the receptive field area on the retina. Functional inactivation of the corticogeniculate loop by cooling of the ipsilateral visual cortex did not selectively influence ACh sensitivity in the retinally deafferented dLGN. It is concluded that ACh exerts a direct, excitatory action on geniculate neurons. Infusion of sodium pentobarbitone resulted in reduction of ACh firing rates to about 30% of the control values in 35% of the normally innervated cells (65% of that sample not being excited by ACh), whereas excitatory ACh responses after retinal deafferentation were completely blocked by the barbiturate. It is concluded that pentobarbitone influences direct ACh excitation, but not cholinergic disinhibition in the dLGN or ACh inhibition in the PGN. Possible differential effects of pentobarbitone on nicotinic and muscarinic types of ACh responses are discussed.

Molecular mechanisms of general anaesthesia

This review aims to give a balanced view of the various mechanisms which have been proposed to explain the phenomenon of general anaesthesia on both a molecular and whole animal level. An attempt is made to interrelate these and produce one cohesive model.

Acetylcholinesterase activity and acetylcholine effects in the cerebello-rubro-thalamic pathway of the cat

Unilateral transections of the brachium conjunctivum (BC) of cats resulted, after 2-3 weeks, in marked loss of acetylcholinesterase (AChE) activity from the contralateral red nucleus (RN) and ventral tier nuclei of the thalamus (VA-VL). Significant changes in activity were not observed in other locations. Sensitivity of RN neurons to iontophoretically applied acetylcholine (ACh) was studied under conditions which should maximize ACh sensitivity, including AChE inhibition, but ACh was found to have only a weak depressant effect on excitability or no effect at all. Intravenous physostigmine usually increased spontaneous activity of RN neurons, and sometimes increased potentials evoked by electrical stimulation of cerebellar nuclei, to a small extent. Anticholinergic drugs were found not to influence such evoked responses, except to reverse the effects of physostigmine. It is concluded that ACh is not a major transmitter in the excitatory cerebello-rubral tract in spite of the relationship of AChE to this pathway.

CNS depressants: effects on post-synaptic pharmacology

(1) The effects of 5 anesthetics (chloralose, chloroform, ethanol, pentobarbital and urethane) and one anticonvulsant (diphenylhydantoin) were studied on the membrane properties and post-synaptic responses of crustacean neuromuscular junction preparations and molluscan neurons to putative transmitters and peptides. (2) In crustacean preparations pentobarbital selectively depressed, in a dose-dependent, reversible manner, post-synaptic, Na+-dependent, depolarizing responses to the putative transmitter glutamate without altering post-synaptic, Cl(-)-dependent inhibitory responses to the putative transmitter gamma-aminobutyric acid. (3) The effects of all the agents on post-synaptic pharmacology of a molluscan neurosecretory cell were studied either by causing the cell to hyperpolarize to about--100mV through repeated application of acetylcholine (ACh) in a K+-free, Ca++-containing solution or by hyperpolarization through injection of intracellular current in a K+-free solution. Effects of these agents on post-synaptic responses on other molluscan neurons were studied using intracellular current injection to manipulate membrane potential. (4) All of the agents tested selectively depressed the depolarizing Na+-K+-dependent post-synaptic responses of the neurosecretory cell to ACh in a dose-dependent reversible manner without appreciably altering the membrane properties of the cell (over the potential range of the ACh responses). (5) Pentobarbital did not alter the inversion potential of the ACh response. (6) Reciprocal plot analysis of all of the agents tested revealed that the antagonism of the ACh response was primarily non-competitive. (7) None of the agents tested altered hyperpolarizing, K+-dependent responses to dopamine and glutamate on the neurosecretory cell, nor did they affect either the induction or enhancement of BPP activity by the vertebrate peptide vasopressin on this cell.

Modification of the responses of brain stem neurones to transmitter substances by anaesthetic agents

1. The effects of microiontophoretic applications of acetylcholine (ACh), (-)-noradrenaline ((-)-NA) and 5-hydroxytryptamine (5-HT) have been investigated on spontaneously active brain stem neurones in decerebrate unanaesthetized rats and in rats anaesthetized with either tribromoethanol, urethane or pentobarbitone.2. Four types of responses to both (-)-NA and 5-HT were seen. These were: simple excitation; excitation preceded by a short-lasting inhibition; short-lasting inhibition and prolonged inhibition. Three types of responses to ACh were seen: an excitation with long latency of onset; excitation with short latency of onset, resembling the response to an excitant amino acid, and a short-lasting inhibitory response.3. The types of responses to microiontophoretically applied ACh, (-)-NA or 5-HT in anaesthetized and unanaesthetized animals were similar.4. The number of ACh excitatory responses with short latency of onset were significantly reduced in the pentobarbitone-anaesthetized group and a small but significant increase in the number of 5-HT inhibitory effects were observed in each anaesthetized group of animals.5. A significantly greater proportion of slower firing neurones (less than 10 spikes/s) were found in the pentobarbitone-anaesthetized animals.6. The effects of microiontophoretically applied and i.v. administered pentobarbitone were studied on spontaneously active neurones which responded consistently to ACh and a control agonist.7. Pentobarbitone administered by either route reduced the firing rate of most neurones studied and was shown to antagonize specifically the excitation of neurones by exogenously applied ACh.8. It is suggested that postsynaptic antagonism of endogenously released ACh may be a contributing factor in the mechanism of action of pentobarbitone.

Cholinergic and non-cholinergic transmission in the medial geniculate nucleus of the cat

1. Studies involving the electrophoretic administration of antagonists of ACh (atropine, DHbetaE) and cholinesterase inhibitors (neostigmine, physostigmine) to MGN neurones indicate that ACh is an excitatory transmitter in the feline MGN, most probably released from fibres which originate in or traverse the mesencephalon.2. Auditory afferents to the MGN, cortico-geniculate fibres and the excitatory fibres which mediate ;spontaneous' firing of MGN neurones are unlikely to be cholinergic.3. Almost all geniculo-cortical relay cells are excited by ACh, this excitation being mediated by receptors which have both muscarinic and nicotinic properties. The excitation of relay cells by ACh is sometimes preceded or followed by a depression of firing which is resistant to atropine and DHbetaE, but the significance of this depression is unknown.4. The firing of many unidentified MGN neurones is depressed by ACh in the absence of any excitation, and this depression is blocked by both atropine and DHbetaE, and potentiated by anticholinesterases. This type of depression by ACh may be related to cholinergic inhibition, but this possibility has yet to be investigated.

Properties of cholinoceptive neurones in the medial geniculate nucleus

1. Acetylcholine (ACh), other cholinomimetics, cholinesterase inhibitors and cholinergic antagonists were administered iontophoretically to medial geniculate (MG) neurones and their effects on chemically or neurally evoked responses recorded extracellularly.2. Acetylcholine had excitant actions on 45% of the neurones tested. Most of these were of a slow time course. Desensitization to the excitant effects was frequently observed.3. Acetylcholine excited 91% of neurones activated antidromically by stimulation of the auditory cortex, 71% of neurones activated synaptically from the auditory cortex, 74% of neurones activated from the inferior colliculus and 100% of geniculo-cortical relay neurones.4. Acetylcholine had depressant effects, which were generally of a rapid time course, on 29% of MG neurones. No desensitization to the depressant effects was observed.5. On 4% of neurones, ACh had both excitant and depressant effects. Such "dual" effects were manifested either as an initial excitation followed by a depression, or as a depression followed by an excitation.6. Eserine, neostigmine and edrophonium potentiated both excitant and depressant actions of ACh on many cells. Neostigmine and edrophonium occasionally antagonized the effects of ACh.7. Atropine, hyoscine, dihydro-beta-erythroidine, hexamethonium and (+)-tubocurarine antagonized both excitant and depressant effects of ACh. The muscarinic blocking agents were usually more effective than the nicotinic agents.8. Carbamylcholine, acetyl-beta-methylcholine, nicotine, butyrylcholine, arecoline and pilocarpine had excitant, depressant or no effects on MG neurones. Generally, carbamylcholine was more potent than acetyl-beta-methylcholine and ACh, which were more potent than nicotine. Butyrylcholine, arecoline and pilocarpine were even less potent, often having no effect.9. The cholinomimetics generally had similar effects to those of ACh on the same neurones, but sometimes were quite different. Carbamylcholine, acetyl-beta-methylcholine and nicotine antagonized the effects of ACh on some neurones.10. The results suggest that cholinoceptive receptors on MG neurones are not homogeneous. Although there are possibly some purely muscarinic and purely nicotinic receptors, the majority appear to be of intermediate muscarinic-nicotinic type. These mediate either excitation or inhibition.

The responses of cortical neurones to monoamines under differing anaesthetic conditions

1. Noradrenaline, isoprenaline, 5-hydroxytryptamine and acetylcholine have been applied into the environment of single neurones in the cat cerebral cortex by micro-iontophoresis. The influence of anaesthesia on the neuronal responses to these drugs was studied.2. In general, excitation of neurones by noradrenaline occurred commonly in both unanaesthetized encéphale isolé (48%) and N(2)O-halothane anaesthetized preparations (57%) while depressions were less frequent (24 and 20% respectively). The picture differed markedly in barbiturate anaesthetized animals where excitation was uncommon (12%) and the majority of cells (59%) was depressed by noradrenaline. Although fewer cells were studied, similar differences were obtained with isoprenaline and 5-hydroxytryptamine for the three types of preparation. In sharp contrast, the vast majority of cells was excited by acetylcholine in each of the preparations: encéphale isolé 84%; N(2)O-halothane 92%; barbiturate anaesthetized preparations 85%.3. The differing neuronal responses observed in these experimental situations were not the result of variations in the depth of anaesthesia.4. Although the depths at which neurones were encountered within the cortex did not differ in N(2)O-halothane and encéphale isolé preparations, significantly more cells were found in deeper layers of cortex in barbiturate preparations. The proportion of cells excited or depressed by noradrenaline was generally similar at each depth in the three preparations used.5. The distribution of rates of cell firing was strikingly similar in each preparation, and most of the cells studied had frequencies below 10/sec.6. The direction in which a cell responds to noradrenaline is mainly determined by the type of anaesthetic used, and not by the depth of anaesthesia, the rate of cell firing, or cell depth within the cortex. This suggests important differences in the central pharmacology of halothane and barbiturates.

Acetylcholine-sensitivity of thalamic neurones: its relationship to synaptic transmission

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