Actions of anaesthetics and avermectin on GABA A chloride channels in mammalian dorsal root ganglion neurones

1 The actions of GABA and the intravenous general anaesthetic propofol (2,6-diisopropylphenol) on GABAA receptors of self-replicating GT1-7 hypothalamic neurones were investigated by the patch clamp technique. 2 GABA (1 gM-1 mM) dose-dependently activated inward currents with an EC50 of 27 gM, recorded from whole cells voltage-clamped at -60 mV. GABA (100 gM)-activated currents reversed at the Clequilibrium potential. 3 Propofol (0.1-100 tiM) dose-dependently potentiated GABA (100 IM)-evoked currents with an EC50 of 5 gM. 4 In the absence of GABA, propofol (10 UM-1 mM) activated small inward currents with a reversal potential similar to the Cl-equilibrium potential. The peak current amplitudes activated by propofol were only 31% of those activated by GABA in the same cells.5 Like GABA (100 kM)-activated currents, propofol (100 gM)-activated currents were inhibited by the GABAA receptor antagonist, bicuculline (10 gM) and were abolished by Zn2+ (100 pM). 6 Propofol (10, 30 and 100 gM) dose-dependently activated currents in the absence of GABA.However, the peak amplitude of currents activated by propofol declined with concentrations > 100 MM. The cessation of application of a high dose of propofol (1 mM) was associated with a current 'surge'. 7 The surge current, seen after application of propofol (1 mM), had a reversal potential similar to the Cl-equilibrium potential. The ratio between peak current amplitude in the presence of propofol (1 mM) and surge current amplitude after propofol application, were not dependent on holding potential. Thus, it is unlikely that the surge current represents reversal of a voltage-dependent block of GABAA receptors by propofol. 8 The amplitude of the surge current exceeded the amplitude of the initial propofol (1 mM)-evoked current following brief applications, but declined after prolonged applications of the drug. 9 The observed modulatory actions of propofol may be due to separate potentiation, activation and inhibitory sites for this anaesthetic agent on GT1-7 cell GABAA receptors.

Section: Discussionsupporting

confidence: 53%

Section: Discussionmentioning

confidence: 99%

Potentiation, activation and blockade of GABA_A receptors of clonal murine hypothalamic GT1‐7 neurones by propofol

Adodra

Hales

1995

“…This is comparable to another ligand gated ion channel, the nicotinic acetylcholine receptor, which is activated by physostigmine via a site separate from the acetylcholine binding site (Okonjo et al, 1991;Lena & Changeux, 1993). Krishek & Smart (1995) (Akaike et al, 1987;Peters, 1988;Robertson, 1989) have reported on the washout phenomenon observed with high concentrations of pentobarbitone. This effect, which has been termed 'bounce' or 'hump', involves a marked transient increase in current during the washout period of high concentrations of pentobarbitone.…”

Section: Discussionmentioning

confidence: 97%

“…It is now generally accepted that a common feature of general anaesthetic agents is positive modulation of the inhibitory function of the neurotransmitter yaminobutyric acid (GABA) through GABAA receptors (Olsen, 1988;Tanelian et al, 1993;Franks & Lieb, 1994;Zimmerman et al, 1994). Electrophysiological and neurochemical studies have shown that general anaesthetic agents can have three mechanisms of action, namely (i) a potentiation of the GABA response (Evans, 1979;Study & Barker, 1981;Lin et al, 1992), (ii) a direct activation of GABAA receptors (Robertson, 1989;Franks & Lieb, 1994) and (iii) at high concentrations, a block of the GABA chloride channel (Schwartz et al, 1986;Peters et al, 1988;Robertson, 1989). The potentiation of the GABA response has been shown to be due to an increase in the ion channel open time (Mathers & Barker, 1980;Study & Barker, 1981;Jackson et al, 1982;Macdonald et al, 1989).…”

Section: Introductionmentioning

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.

“…In humans and other mammals, behavioural effects which are typical of positive allosteric modulators of GABA A receptors include anxiolysis, cessation of convulsions, sedation and general anaesthesia (Sieghart, 1995), although some of these effects may result from simultaneous action at other receptor types. Some positive GABA A receptor modulators can also act as agonists on the same receptors when tested at higher concentrations (Robertson, 1989;Franks & Lieb, 1994) and this activity may influence the spectrum of clinical effects observed (Sanna et al, 1999). Most GABA A receptor potentiating compounds, with the notable exception of the benzodiazepine clonazepam, also enhance the action of GABA at native and recombinant insect GABA receptors, although they are often less potent than on GABA A receptors and lack the agonist activity observed at some vertebrate GABA A receptors (Belelli et al, 1996;.…”

Section: Introductionmentioning

confidence: 99%

Thymol, a constituent of thyme essential oil, is a positive allosteric modulator of human GABA_A receptors and a homo‐oligomeric GABA receptor from Drosophila melanogaster

Priestley¹,

Williamson²,

Wafford

et al. 2003

454

262

1 The GABA-modulating and GABA-mimetic activities of the monoterpenoid thymol were explored on human GABA A and Drosophila melanogaster homomeric RDL ac GABA receptors expressed in Xenopus laevis oocytes, voltage-clamped at À60 mV. The site of action of thymol was also investigated. 2 Thymol, 1 -100 mM, resulted in a dose-dependent potentiation of the EC 20 GABA response in oocytes injected with either a1b3g2s GABA A subunit cDNAs or the RDL ac subunit RNA. At 100 mM thymol, current amplitudes in response to GABA were 416772 and 715785% of controls, respectively. On both receptors, thymol, 100 mM, elicited small currents in the absence of GABA. 3 The EC 50 for GABA at a1b3g2s GABA A receptors was reduced by 50 mM thymol from 1573 to 471 mM, and the Hill slope changed from 1.3570.14 to 1.0470.16; there was little effect on the maximum GABA response. 4 Thymol (1 -100 mM) potentiation of responses to EC 20 GABA for a1b1g2s, a6b3g2s and a1b3g2s human GABA A receptors was almost identical, arguing against actions at benzodiazepine or loreclezole sites. 5 Neither flumazenil, 3-hydroxymethyl-b-carboline (3-HMC), nor 5a-pregnane-3a, 20a-diol (5a-pregnanediol) affected thymol potentiation of the GABA response at a1b3g2s receptors, providing evidence against actions at the benzodiazepine/b-carboline or steroid sites. Thymol stimulated the agonist actions of pentobarbital and propofol on a1b3g2s receptors, consistent with a mode of action distinct from that of either compound. These data suggest that thymol potentiates GABA A receptors through a previously unidentified binding site.