Effects of two volatile anesthetics and a volatile convulsant on the excitatory and inhibitory amino acid responses in dissociated CNS neurons of the rat

1 A common anaesthetic endpoint, prevention of withdrawal from a noxious stimulus, is determined primarily in spinal cord, where glycine is an important inhibitory transmitter. To de®ne pre-and postsynaptic anaesthetic actions at glycinergic snyapses, the e ects of volatile anaesthetic agents on spontaneous and evoked glycinergic currents in spinal cord motor neurons from 6 ± 14-day old rats was investigated. 2 The volatile anaesthetic agents en¯urane, iso¯urane and halothane signi®cantly increased the frequency of glycinergic mIPSCs, en¯urane to 190.4% of control+22.0 (mean+s.e.m., n=7, P50.01), iso¯urane to 199.0%+28.8 (n=7, P50.05) and halothane to 198.2%+19.5 (n=7, P50.01). However without TTX, iso¯urane and halothane had no signi®cant e ect and en¯urane decreased sIPSC frequency to 42.5% of control+12.4 (n=6, P50.01). All the anaesthetics prolonged the decay time constant (t) of both spontaneous and glycine-evoked currents without increasing amplitude. With TTX total charge transfer was increased; without TTX charge transfer was unchanged (iso¯urane and halothane) or decreased (en¯urane). 3 En¯urane-induced mIPSC frequency increases were not signi®cantly a ected by Cd 2+ (50 mM), thapsigargin (1 ± 5 mM), or KB-R7943 (5 mM). KB-R7943 and thapsigargin together abolished the en¯urane-induced increase in mIPSC frequency. 4 There are opposing facilitatory and inhibitory actions of volatile anaesthetics on glycine release dependent on calcium homeostatic mechanisms and sodium channels respectively. Under normal conditions (no TTX) the absolute amount of glycinergic inhibition does not increase. The contribution of glycinergic inhibition to anaesthesia may depend on its duration rather than its absolute magnitude.

Section: Anaesthetic Actions: Postsynapticcontrasting

confidence: 56%

Pre‐ and postsynaptic volatile anaesthetic actions on glycinergic transmission to spinal cord motor neurons

Gong

Kendig

2002

“…Con-versely, the loreclezole site is affected only by the P subunit, being dependent on the presence of fl2 or 113 . Other anaesthetic compounds such as propofol, halothane and enflurane as well as the neuroactive steroids such as alphaxalone have been shown to act by enhancing the GABA-induced chloride current (Tanelian et al, 1993;Keane & Biziere, 1987;Wakamori et al, 1991;Lin et al, 1992). Some of these compounds, propofol and neurosteroids for example, also produce direct activation of the receptor at high concentrations (Tanelian et al, 1983;Keane & Biziere, 1987;Hara et al, 1993).…”

Section: Discussionmentioning

confidence: 99%

Barbiturate interactions at the human GABA_A receptor: dependence on receptor subunit combination

Thompson

Whiting

Wafford

1996

170

114

1 Human GABAA receptors containing different and subunits with a y2s subunit were expressed in Xenopus oocytes and the effects of pentobarbitone on these subunit combinations were examined by electrophysiological recording of GABA currents with the two-electrode voltage-clamp method. 2 Pentobarbitone has previously been shown to have three actions on GABAA receptors: a potentiation of GABA responses, a direct activation of GABAA receptors and, at high concentrations, a block of the GABA chloride channel. In this study pentobarbitone activity consisted of the above mentioned three components on all the subunit combinations tested. However, the affinities and efficacies varied with receptor subtype. 3 Potentiation of GABA by pentobarbitone occurred over the same concentration-range for all the subunits with affinities in the range of 20-35 tiM. The degree of potentiation obtained, however, varied from 236% of GABA EC20 on al/#2y2s to 536% on a6fl2y2s.4 Examination of the direct effect of pentobarbitone revealed that the type of a subunit present determines both the degree of affinity and efficacy obtained. Receptors containing an cc6 subunit produced maximum direct responses to pentobarbitone larger than that obtainable with maximum GABA (150% to 170% of maximum GABA). The maximum direct pentobarbitone response obtainable with other a subunits ranged between 45% of maximum GABA for ac5f2y2s to 82% for a2/32y2s. The affinity of the direct action of pentobarbitone on a6/32y2s was 58 tiM compared to affinities for the other subunits ranging from 139 pM on a2fl2y2s to 528 gM on ac5f2y2s.5 The type of subunit present did not influence the direct action of pentobarbitone to the same extent as the a subunit. There were no significant differences between affinity or efficacy on oocytes expressing a6 and y2s with /31, /32 or P3. Affinities and efficacies on oocytes expressing al and y2s with PI, /2 or /3 were significantly different with pentobarbitone having a higher affinity and efficacy on al/33y2s followed by al/32y2s and then al/3ly2s. 6 The direct effect of pentobarbitone was blocked by picrotoxin but not by competitive antagonists, such as bicuculline or SR95531, indicating that the direct agonist activity of pentobarbitone was not mediated via the GABA binding site. 7 For the first time the influence of the various a and subunits on the effects of pentobarbitone were demonstrated. The results indicate that GABAA receptors containing a6 subunits have both a higher affinity and efficacy for direct activation by pentobarbitone, and reveal that pentobarbitone binds to more than one site on the GABAA receptor, and these are dependent on receptor subunit composition.

“…This is a quite different situation from that in which we have studied potentiation (low GABA concentrations for many seconds). However, there is a connection between the two situations, in that it is well-known (see, for example, Parker et al, 1986, andWakamori et al, 1991) that anaesthetics increase the apparent affinity of GABA for the GABAA receptor (i.e. they shift the GABA dose-response curve to lower GABA concentrations).…”

Section: Gabaa Currentsmentioning

confidence: 99%

“…For example, results from biochemical assays of 36Cl-uptake or release by CNS membrane vesicles have been contradictory, with some workers (Moody et al, 1988) finding inhibition of agonist-induced Cl-flux by volatile agents and others (Huidobro-Toro et al, 1987;Longoni et al, 1993) reporting enhancement. Electrophysiological data on inhibitory postsynaptic currents (i.p.s.cs) and potentials (i.p.s.ps) have also given equivocal results, with volatile anaesthetics being shown both to prolong i.p.s.cs (Gage & Robertson, 1985;Mody et al, 1991) and inhibit i.p.s.ps (Yoshimura et al, 1985;Fujiwara et al, 1988;El-Beheiry & Puil, 1989) (Jones et al, 1992), dorsal root ganglia (Nakahiro et al, 1989) and nucleus tractus solitarius (Wakamori et al, 1991). These recent voltage-clamp results add to the growing evidence (Tanelian et al, 1993;Franks & Lieb, 1994) that the GABAA receptor channel is an important target for volatile general anaesthetics.…”

Section: Introductionmentioning

confidence: 99%

Stereoselective and non‐stereoselective actions of isoflurane on the GABA_A receptor

Hall

Lieb

Franks

1994

108

1 Acutely dissociated cerebellar Purkinje neurones from 8-14 day old rats were studied under voltage clamp in the whole-cell patch-clamp configuration. Cl-currents induced by bath application of Ty-aminobutyric acid (GABA) were measured (using symmetrical Cl-solutions) at both low (2 JM) non-desensitizing and high (300 JM) desensitizing concentrations of GABA.2 At 2 JM GABA, the bicuculline-sensitive Cl-currents were potentiated by racemic isoflurane and both of its optical isomers. Isoflurane had no effect on membrane current in the absence of GABA. The dose-response data for potentiation by racemic isoflurane could be fitted with a Hill equation with an EC50 = 320 ± 20 gM isoflurane and a Hill coefficient of h = 2.7 ± 0.4 (means ± s.e.mean).3 The potentiations produced by the optical isomers of isoflurane at 2 JM GABA were stereoselective at moderate and high anaesthetic concentrations. The maximum stereoselectivity, about two fold, occurred at the ECm concentration for general anaesthesia (310I1M isoflurane), with S( + )-isoflurane being more effective than R(-)-isoflurane. At sub-anaesthetic concentrations, the stereoselectivity was less marked and vanished at the lowest concentration used (77JM isoflurane).4 The sustained residual current remaining after exposure of neurones to a desensitizing concentration of GABA (300JM) was inhibited non-stereoselectively, but only at high concentrations of isoflurane.The ratio of inhibitions by S(+)-and R(-)-isoflurane (mean ± s.e.mean) was 1.14 ± 0.21 at 770 JM isoflurane. At the EC50 concentration for general anaesthesia, however, the inhibition was barely significant. 5 The above results are discussed in relation to the possible role of the GABAA receptor channel in general anaesthesia.