General anaesthetics and the acetylcholine-sensitivity of cortical neurons

Cholinergic modulation of vibrissal receptive fields in trigeminal nuclei

In sensory systems, it is usually considered that mesopontine cholinergic neurons exert their modulatory action in the thalamus by enhancing the relay of sensory messages during states of neural network desynchronization. Here, we report a projection heretofore unknown of these cholinergic cells to the interpolar division of the brainstem trigeminal complex in rats. After FluoroGold injection in the interpolar nucleus, a number of retrogradely labeled cells were found bilaterally in the pedunculopontine tegmental nucleus, and immunostaining revealed that the vast majority of these cells were also positive for choline acetyltransferase. Immunostaining for the acetylcholine vesicular transporter confirmed the presence of cholinergic terminals in the interpolar nucleus, where electron microscopy showed that they make symmetric and asymmetric synaptic contacts with dendrites and axon terminals. In agreement with these anatomical data, recordings in slices showed that the cholinergic agonist carbachol depolarizes large-sized interpolaris cells and increases their excitability. Local application of carbachol in vivo enhances responses to adjacent whiskers, whereas systemic administration of the cholinergic antagonist scopolamine produces an opposite effect. Together, these results show that mesopontine cholinergic neurons exert a direct, effective control over receptive field size at the very first relay stations of the vibrissal system in rodents. As far as receptive field synthesis in the lemniscal pathway relies on intersubnuclear projections from the spinal complex, it follows that cholinergic modulation of sensory transmission in the interpolar nucleus will have a direct bearing on the type of messages that is forwarded to the thalamus and cerebral cortex.

Influence of pentobarbital-Na on stimulation-evoked catecholamine secretion in the perfused rat adrenal gland

OBJECTIVES

The present study was attempted to investigate the effects of pentobarbital-Na, one of the barbiturates which are known to depress excitatory synaptic transmission in the central nervous system at concentrations similar to those required for the induction and maintenance of anesthesia, on catecholamines (CA) secretion evoked by cholinergic stimulation and membrane-depolarization from the isolated perfused rat adrenal gland, and to clarify the mechanism of its action.

METHODS

Mature male Sprague-Dawley rats were anesthetized with thiopenal-Na (40 mg/kg, s.c.). The adrenal gland was isolated by the methods of Wakade. A cannula used for perfusion of the adrenal gland was inserted into the distal end of the renal vein. The adrenal gland was carefully removed from the animal and placed on a platform of a leucite chamber.

RESULTS

The perfusion of pentobarbital-Na(30-300 uM) into an adrenal vein for 20 min produced relatively dose-dependent inhibition in CA secretion evoked by ACh(5.32 mM), DMPP(100 uM for 1 min), McN-A-343(200 uM for 2 min), Bay-K-8644(10 uM) and high potassium(56 mM), while it did not affect the CA secretion of cyclopiazonic acid(10 uM). Also, in the presence of thiopental-Na (100 uM), CA secretory responses evoked by ACh, DMPP, McN-A-343 and high K+ were markedly depressed. Moreover, in adrenal glands preloaded with ketamine(100 uM for 20 min), which is known to be a dissociative anesthetic, CA secretion evoked by ACh, DMPP, McN-A-343 and high K+ were significantly attenuated.

CONCLUSION

Taken together, these experimental results suggest that pentobarbital-Na depresses CA release evoked by both cholinergic stimulation and membrane-depolarization from the isolated rat adrenal medulla and that this inhibitory activity may be due to the result of the direct inhibition of Ca++ influx into the chromaffin cells without any effect on the calcium mobilization from the intracellular store.

[Prevention of respiratory complications after abdominal surgery]

Abdominal surgery, especially upper abdominal surgical procedures are known to adversely affect pulmonary function. Pulmonary complications are the most frequent cause of postoperative morbidity and mortality. This review article aimed to analyse the incidence and risk factors for postoperative pulmonary morbidity and their prevention. The most important means for preoperative assessment is the clinical examination; pulmonary function tests (spirometry) are not reliably predictive for postoperative pulmonary complications. Age, type of surgical procedure, smoking and nutritional state have all been identified as potential predictors for postoperative complications. However, usually there is not enough preoperative time available to obtain beneficial effects of stopping smoking and improvement of nutritional state. In patients with COPD, a preoperative multidisciplinary evaluation including the primary care physician, pulmonologist/intensivist, anesthesiologist and surgeon is required. Consensus as to preoperative physiologic state, therapeutic preparation, and postoperative management is essential. Simple spirometry and arterial blood gas analysis are indicated in patients exhibiting symptoms of obstructive airway disease. There are no values that contra-indicate an essential surgical procedure. Smoking should stop at least 8 weeks preoperatively. Preoperative therapy for elective surgery with antibiotics, beta2-agonist, or anticholinergic bronchodilator aerosols, as well as training in cough and lung expansion techniques should begin at least 24 to 48 hours preoperatively. Postoperative therapy should be continued for 3 to 5 days. Usually, anaesthesia is responsible for early complications, whereas surgical procedures are often associated with delayed morbidity. Laparoscopic procedures are recommended, as postoperative morbidity and hospital stay seem reduced in patients without COPD. Regional anaesthesia is given as having less adverse effects on pulmonary function than general anaesthesia. However, for unknown reasons these benefits are not associated with a decrease in postoperative respiratory complications. Moreover, the quality or the type of postoperative analgesia does not influence postoperative respiratory morbidity. Postoperatively, oxygen administration increases SaO2, but cannot abolish desaturation due to obstructive apnea. The various techniques of physiotherapy (chest physiotherapy, incentive spirometry, continuous positive airway pressure breathing) seem to be equivalent in efficacy; but intermittent positive pressure breathing has no advantages, compared with the other treatments and could even be deleterious. Chest physiotherapy and incentive spirometry are the most practical methods available for decreasing secretion contents of airways, whereas continuous positive airway pressure breathing is efficient on atelectasis. In stage II or III COPD patients, admission in a intensive therapy unit and prolonged mechanical ventilation may be required.

Ketamine-induced anesthesia involves the N-methyl-D-aspartate receptor-channel complex in mice

The role of the N-methyl-D-aspartate (NMDA) receptor-channel complex in ketamine-induced anesthesia was examined in mice. General anesthetic potencies were evaluated on a rating scale, which provided the data for anesthetic scores, loss of righting reflex, sleeping time and recovery time. All drugs were administered intraperitoneally. NMDA (60-300 mg/kg), an NMDA receptor agonist, dose-dependently antagonized the general anesthetic potencies of ketamine at a dose of 100 mg/kg which produced loss of righting reflex in more than 90% of the mice. On the other hand, a high dose of N-methyl-L-aspartate (400 mg/kg), a stereoisomer of NMDA, did not. A dose of 300 mg/kg of NMDA significantly shifted the dose-response curve of ketamine for loss of righting reflex to the right. A high dose of D-cycloserine (200 mg/kg), an agonist at the glycine site on the NMDA receptor complex, slightly but significantly shortened the sleeping time caused by ketamine (100 mg/kg). However, neither a critical subconvulsive dose of kainate (15 mg/kg), a kainate receptor agonist, nor a subconvulsive dose of quisqualate (120 mg/kg), an alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor agonist, reversed general anesthesia induced by 100 mg/kg of ketamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Action of general anaesthetics on unclamped Ca(2+)-mediated currents in unmyelinated axons of rat olfactory cortex

Na+ and Ca2+ currents were monitored using a suction electrode in unclamped presynaptic axons of rat olfactory cortex pretreated with 0.1 mM 3,4-diaminopyridine and 5 mM tetraethylammonium. The effects of anaesthetics on these currents were compared with tetrodotoxin or cadmium. Ketamine (0.1-1 mM), ether (20-200 mM), diisopropylphenol (0.01-0.5 mM) and lignocaine (0.01-0.2 mmol/l) all depressed both the initial Na+ component and the Ca(2+)-mediated tail of the response. Urethane (5-100 mM), halothane (1-5 mM) and pentobarbitone (0.1-2 mM) showed slight selectivity for the axonal Ca2+ tail. Diisopropylphenol apparently enhanced the Ca2+ tail at low concentrations. The alphaxalone (1-50 microM) depression was very weak. In a few cases the depression may contribute to anaesthesia but with others, high concentrations may contribute to the toxicity of the substances in vivo.

Analysis of anesthetic action on the potassium channels of the Shaker mutant of Drosophila

Recent evidence suggest that exposure to volatile anesthetic agents causes a change in conductance through an undelineated potassium channel. With recently developed genetic and molecular techniques the Drosophila melanogaster (D.m.) genome can be manipulated to study the role that potassium ion channel function plays in anesthetic action. The IA potassium channel is encoded by the Shaker (Sh) locus on the X chromosome of D.m. Because this channel may be one of those involved in volatile anesthetic action, we tested the sensitivity to isoflurane in three Shaker strains with different degrees of dysfunctional IA conductance (Shnull greater than ShKS133 greater than Sh5). Anesthetic sensitivity was also examined in mutant strains of D.m. which express abnormalities either in other potassium channel conductances (eag, slo) or other ion conductances (para). The normally conducting wild type served as the control. Two-day-old adult D.m. were stimulated with a heat source during exposure to the volatile anesthetic isoflurane, and the number moving in response to the noxious stimulus was noted. After testing the Shaker and control strains at no fewer than 13 concentrations, the IC50S (isoflurane concentration in percent vol/vol at which 50% of the flies did not respond to the heat stimulus) were derived. The IC50 values for Sh 5 (0.89), Sh133 (1.29), and Shnull (1.37) were significantly different from the wild type (0.56). The rank order of insensitivity of these Shaker mutants corresponded to the extent of the alteration in IA conductance as established by previous studies in these mutants. Neither eag (0.66), para (0.48), nor slo (0.63) differed significantly from the wild type. These data suggest that the IA potassium channel plays a role in volatile anesthetic action.

Anaesthetic suppression of transmitter actions in neocortex

1. The effects of general anaesthetics were investigated on neuronal sensitivities to transmitter substances, which were determined by iontophoretic applications of acetylcholine, glutamate, N-methyl-D-aspartate (NMDA) and gamma-aminobutyrate (GABA) during intracellular recording in in vitro slice preparations of neocortex (guinea-pig). 2. In most of the 65 neurones studied, perfusion of isoflurane (0.5-2.5 minimum alveolar concentration (MAC)) or Althesin (25-200 microM) and, in some cases, halothane (0.5-2 MAC), markedly reduced the depolarizing responses and associated membrane conductance changes evoked by dendritic applications of acetylcholine, glutamate, NMDA and GABA. 3. The order of depression was acetylcholine greater than glutamate or NMDA much greater than GABA. This selectivity could also be assessed from the EC50 for the isoflurane-induced depression of the just-maximal responses to acetylcholine, which was 0.9 MAC compared with an EC50 = 1.9 MAC for the suppression of glutamate responses. The selectivity was less pronounced in the case of the actions of Althesin, where the EC50s were 75 microM for the depression of acetylcholine responses and 90 microM for the depression of glutamate responses. 4. The hyperpolarizing responses observed when GABA was applied near the perikaryon in 7 neurones, were slightly reduced (approximately 15%) in 4, and unchanged in 3 neurones during anaesthetic application. 5. The pronounced depression of the responsiveness to the putative arousal transmitters and an observed blockade of acetylcholine-induced potentiation of glutamate actions suggest that anaesthetics produce unconsciousness, at least in part, by interfering with subsynaptic mechanisms of neocortical activation.

Diethyl ether effects on muscarinic acetylcholine receptor complexes in rat brainstem

The influence of diethyl ether on muscarinic acetylcholine receptor-G protein interactions was studied using membranes isolated from rat brainstem. Membranes were equilibrated with diethyl ether (0.5 to 10%) for 20 min before, and then during, the binding assay. The affinity, but not the number, of [3H]N-methylscopolamine [( 3H]MS) binding sites was increased in the presence of diethyl ether (KD in air = 0.41 nM, KD in 2% diethyl ether = 0.21 nM). This increase in affinity reflected a decrease in the rapid dissociation rate constant (air k-1 = 13 X 10(-3) min-1, 2% diethyl ether k-1 = 7 X 10(-4) min-1) rather than a change in the association rate constant. Diethyl ether had no effect on the binding affinity of the muscarinic agonist carbamylcholine. However, the binding of a radiolabeled muscarinic agonist, [3H]oxotremorine-M [( 3H]Oxo-M), to high affinity binding sites decreased about 25% in the presence of 2% diethyl ether. The ability of a guanine nucleotide to depress the high affinity binding of both carbamylcholine and [3H]Oxo-M was decreased or eliminated by diethyl ether. Diethyl ether appears to interfere with muscarinic receptor-G protein interactions, perhaps by stabilizing receptor-G protein complexes or inhibiting the binding of guanine nucleotides.

Central respiratory depression induced by acetylcholinesterase inhibition: involvement of anaesthesia

We have studied the effects of anaesthesia on the changes in central respiratory activity following the inhibition of acetylcholinesterase in chronically implanted cats. The organophosphate paraoxon was administered to the brainstem respiratory centres by intracerebroventricular (i.c.v.) injection (3 mg) into the IVth ventricle, thus avoiding peripheral effects such as paralysis of respiratory muscles. Paraoxon had opposite effects on respiratory activity depending on whether the cats were anaesthetized or not: it induced respiratory depression and sometimes respiratory arrest during pentobarbital (30 mg/kg i.v.) or halothane anaesthesia, but in the same animals in the waking state, the same dose of paraoxon always stimulated respiration. These results show a strong interaction between anaesthetics and the effects of acetylcholine (ACh) accumulation on central respiratory activity. This study extends previous results showing an interaction between ACh and pentobarbital on single respiratory neurons and stresses the importance of a 'wakefulness stimulus' for sustaining respiratory activity after organophosphate poisoning.

Effects of halothane on high affinity agonist binding and guanine nucleotide sensitivity of muscarinic acetylcholine receptors from brainstem of rat

The influence of halothane on muscarinic receptors with a high affinity for agonists was studied using [3H]oxotremorine-M. [3H]Oxotremorine-M bound with high affinity (KD = 2.8 nM) to a subpopulation of muscarinic receptors in the brainstem of rat, representing 32% of the total receptor pool. Agonist affinity for binding sites for [3H]oxotremorine-M was not affected by a guanine nucleotide (5'-guanylylimidodidiphosphate; Gpp(NH)p), although the level of binding was decreased, presumably due to the conversion of receptors to lower affinity conformations. However, only 58% of 3 nM binding of [3H]oxotremorine-M was sensitive to Gpp(NH)p. Halothane had two effects on the binding of [3H]oxotremorine-M: halothane (1) decreased the level of binding of [3H]oxotremorine-M without affecting agonist affinity for the surviving sites, and (2) lowered the sensitivity of the binding of [3H]oxotremorine-M to Gpp(NH)p by a factor of 120. The decrease in binding of [3H]oxotremorine-M binding was nonselective with regard to the sensitivity of the receptors to the guanine nucleotide, insofar as Gpp(NH)p inhibited the binding of [3H]oxotremorine-M to the same extent in the presence and absence of halothane. These results suggest that halothane (1) converts both G protein-coupled and -uncoupled muscarinic receptors to states of lower agonist affinity and (2) lowers the affinity of receptor-G protein complexes for guanine nucleotides.

The effects of chronic trichloroethylene exposure on neurobehavioral functioning in the rat

Groups of rats were exposed by inhalation to either clean air (Controls) or trichloroethylene at 500, 1000 or 1500 ppm TCE for 16 hr/day, 5 days/week for 18 weeks. At preselected intervals, animals were evaluated for changes in: spontaneous activity, gripstrength, coordinated hindlimb movement, performance of a discrete-trial operant two-choice visual discrimination task, and peripheral nerve conduction velocity. Compared to Controls, TCE-treated rats showed no significant differences in open field behavior, fore- and hindlimb gripstrength or coordinated movement throughout the exposure period. Peripheral nerve conduction time was also unaffected. In contrast, TCE produced progressively marked changes in the speed and patterning of responding in the two-choice visual discrimination task. Two-choice response latency, for example, demonstrated an approximately four-fold increase (p less than 0.001) in the highest dose group. In addition, a recurrent within-week functional tolerance developed for all TCE-exposed groups. However, tolerance was lost in the TCE 1500 group as exposure became chronic. Finally, following the termination of exposure, there was no carry-over of TCE-related effects on any of the measures and performance quickly returned to baseline levels. This profile of effects argues for a primary involvement of the CNS with chronic TCE exposure and is quite unlike that seen with, e.g., n-hexane and carbon disulfide. Such findings underscore the differences in the effects which can be produced by long-term exposure to organic solvents and emphasize the need for a battery of tests in the evaluation of neurotoxicant-induced changes in nervous system functioning.

Effects of barbiturates on responses evoked by excitatory amino acids in slices of rat olfactory cortex

A study has been made of the effects of ranges of concentrations of phenobarbitone, pentobarbitone and thiopentone on responses evoked by gamma-aminobutyric acid (GABA), L-glutamate, L-aspartate, N-methyl-D-aspartate, kainate and quisqualate in slices of olfactory cortex of the rat. All three barbiturates affected GABA-evoked depolarizations similarly in that responses were potentiated by small doses but markedly inhibited at greater concentrations. Responses to L-aspartate and L-glutamate were little affected except at the largest dose of phenobarbitone tested (10 mM). The responses evoked by the selective agonists of excitatory amino acid receptors were inhibited by the barbiturates, the relative sensitivities being quisqualate greater than or equal to N-methyl-D-aspartate greater than or equal to kainate with phenobarbitone, quisqualate greater than or equal to kainate greater than N-methyl-D-aspartate with pentobarbitone and quisqualate greater than kainate = N-methyl-D-aspartate with thiopentone. The possible significance of these findings is discussed.

Halothane effects on muscarinic acetylcholine receptor complexes in rat brain

Muscarinic acetylcholine receptors in membranes from rat cerebral cortex or brainstem were equilibrated with halothane (0.5 to 5%). Halothane did not affect the number of [3H]methylscopolamine [( 3H]MS) binding sites. [3H]MS binding affinity, however, was increased in the presence of halothane (KD, air = 0.41 nM; KD, 2% halothane = 0.26 nM). This increase reflected a decrease in the dissociation rate constant (from 13 X 10(-3) min-1 to 6.5 X 10(-3) min-1) rather than a change in the bimolecular rate constant of association (1.8 and 1.9 X 10(7) M-1 min-1 in the absence and presence of 2% halothane respectively). Carbamylcholine affinity for brainstem or cortical muscarinic receptors was not affected by halothane. The ability of a guanine nucleotide to lower carbamylcholine affinity for brainstem receptors, however, was eliminated after equilibration with 2% halothane.

The mechanism of steroid anaesthetic (alphaxalone) block of acetylcholine-induced ionic currents

The effects of the steroid anaesthetic alphaxalone on acetylcholine (ACh)-induced ionic channels were studied in voltage clamped 'myoballs' in culture. Alphaxalone produced a reversible blockade of the ACh-evoked inward current, ED50 = 6.0 microM. The ACh reversal potential (-5.0 mV), the single channel conductance (13.5 pS) and mean open time (3.6 ms) were unchanged by the drug. Thus, alphaxalone produced an 'all or none' block of the ionic channel. In double pulse conditioning experiments, alphaxalone produced an additional inhibition with a time constant of recovery (550 ms) much longer than the time constant of recovery of the normal desensitization (250 ms). It was concluded that alphaxalone blocks active (open) ionic channels.

The effects of acetylcholine on bulbar respiratory related neurones. Consequences of anaesthesia by pentobarbital

The effects of cholinergic agonists and antagonists applied by microiontophoresis to bulbar respiratory neurones were determined in different preparations: cats anaesthetized by pentobarbital and immobilized by gallamine triethiodide, intercollicular decerebrate cats either not immobilized or immobilized by gallamine triethiodide. Respiratory neurones located at the bulbar level exhibited a muscarinic cholinergic sensitivity in all preparations. Muscarinic responses were either excitatory or inhibitory. The number of neurones showing excitatory responses was lower under pentobarbital anaesthesia than in decerebrate cats.

M-current in voltage-clamped olfactory cortex neurones

The soma of olfactory cortex neurones in vitro was voltage-clamped by means of a single microelectrode sample-and-hold technique. In most neurones, hyperpolarizing voltage commands from relatively positive holding potentials (-40 to -50 mV) elicited a slow inward current relaxation with voltage-dependent and kinetic properties similar to the non-inactivating K+-current (M-current; IM), first described in amphibian sympathetic neurons. Deactivation of IM at negative potentials probably accounts for the slow sag of the hyperpolarizing electrotonic potential measured during current-clamp experiments. IM was inhibited by the cholinergic agonist muscarine or barium ions.

Dissimilar influences of some injectable anaesthetics on the responses of reticulo-spinal neurones to inhibitory transmitters in the lamprey

1 Intracellular recordings were made from identified bulbar reticulo-spinal neurones in the medulla of lamprey ammocoetes. Responses to iontophoretically applied inhibitory transmitters were measured as changes in membrane potential and input resistance. 2 Dose-dependent alterations in the responses to gamma-aminobutyric acid (GABA) and glycine during bath application of injectable anaesthetic drugs were measured; the compounds used were pentobarbitone, ketamine, metomidate and the steroid mixture alphaxalone/alphadolone (Saffan). 3 GABA responses were potentiated by pentobarbitone (1-3 X 10(-4) M) and prolonged by ketamine (3.7 X 10(-5) M); but depressed by high concentrations (10(-3) M) of all drugs, as well as by anaesthetic concentrations of alphaxalone (1-3 X 10(-5) M). 4 Glycine responses were depressed by alphaxalone (1-3 X 10(-5) M) and by supra-anaesthetic concentrations of ketamine (3.7 X 10(-4) M) and metomidate (1.8 X 10(-3) M). No drug potentiated the glycine responses. 5 In the absence of an effect common to the 4 anaesthetics, it is concluded that neither potentiation nor inhibition of all GABA or glycine responses is an essential feature of anaesthesia. However, effects comparable to those described here may contribute to the overall clinical picture during anaesthesia of higher vertebrates. The findings do not support the notion that all anaesthetic agents act on biological membranes by a single mechanism.

Mechanisms of anaesthesia: a review

Anaesthesia is a drug-induced reversible perturbation of neuronal activity. Since a wide variety of structurally unrelated substances are capable of producing this phenomenon, it has been generally accepted that anaesthetics produce their effects through non-specific hydrophobic interactions. Results of recent studies in whole animal and cellular (membrane) preparations demonstrate that a unitary theory of action does not exist. Anaesthetics can produce a spectrum of activity in the central nervous system, and different agents produce different patterns of activity. At the cellular and membrane level, differential effects have been observed, structural dependent differences occur and optical isomers display very different activities. The perturbation (fluidity change) of membrane components does not appear to be uniform for all anaesthetics. It is concluded that anaesthetics are selective agents, and produce their effects at multiple sites and through a variety of mechanisms.

Anaesthetics depress the sensitivity of cortical neurones to L-glutamate

1 The effects of general anaesthetics on the responses of neurones to iontophoretically applied L-glutamate have been examined in slices of the guinea-pig olfactory cortex in vitro. 2 Concentrations of pentobarbitone, ether, methoxyflurance, trichloroethylene and alphaxalone that are known to depress synaptic transmission in the prepiriform cortex also depressed the sensitivity of prepiriform neurones to L-glutamate. 3 Halothane, in concentrations that depress synaptic transmission (less than 1%) did not alter sensitivity of neurones to glutamate. Higher concentrations (greater than 1% produced a dose-related depression of the glutamate sensitivity of neurones. 4 All four volatile anaesthetics tested caused some cells to alter their glutamate-evoked firing pattern to one in which the spike discharges were more closely grouped. Pentobarbitone and alphaxalone had no such effect. 5 If the sensitivity of the neurones to the endogenous excitatory transmitter is affected by anaesthetics in the same way as the glutamate-sensitivity, these results suggest that halothane depresses synaptic transmission by decreasing the amount of transmitter released from the nerve terminals, whereas the other anaesthetics depress the sensitivity of the post-synaptic membrane to the released transmitter.

General anaesthetics and the acetylcholine-sensitivity of cortical neurons

1The effects of general anaesthetics on neuronal responses to iontophoretically-applied acetylcholine have been examined in slices of guinea-pig olfactory cortex maintained in vitro. 2 Acetylcholine excited 61% of the prepiriform neurones tested. The excitation was blocked by atropine, but not by dihydro-beta-erythroidine or gallamine. 3 Alphaxalone reversibly depressed the acetylcholine-sensitivity of prepiriform neurones. Pentobarbitone did not consistently depress the acetylcholine sensitivity of these cells. 4 Ether, methoxyflurane, trichloroethylene and halothane caused a dose-related augmentation of acetylcholine-induced firing. 5 These results show that general anaesthetics do not necessarily depress the sensitivity of nerve cells to all excitatory substances and that different anaesthetics may affect a particular excitatory process in various ways.