Effects of hexachlorocyclohexanes on gamma-aminobutyric acid receptors expressed in Xenopus oocytes by RNA from mammalian brain and retina.
Poly(A)+ RNA from rat cerebral cortex expresses gamma-aminobutyric acid (GABA)-activated membrane current responses in Xenopus oocytes, mediated by GABAA receptors (IG-Actx). In contrast, RNA from bovine retina expresses GABA responses composed of two pharmacologically distinct Cl- currents, one mediated by GABAA receptors (IG-Aret) and the other by atypical GABA receptors that are resistant to bicuculline and are not activated by baclofen (IG-BR). The pharmacology of the bicuculline/baclofen-insensitive GABA receptors was further investigated by comparing actions of hexachlorocyclohexane (HCH) enantiomers on GABA-activated membrane currents expressed in oocytes by brain and retina RNA. gamma-HCH (lindane) was a potent inhibitor of IG-Actx, with suppression of currents detectable at concentrations as low as 50 nM. The IC50 for gamma-HCH, calculated from inhibitory effects on maximum IG-Actx (current elicited by 3 mM GABA), was 7.3 +/- 3 microM. Inhibitory effects of gamma-HCH on IG-Aret were qualitatively similar to those described for IG-Actx. In contrast, alpha-HCH and delta-HCH induced clear positive modulation of IG-Actx elicited by low (e.g., 10 microM) concentrations of GABA. Thresholds for the modulatory effects of alpha-HCH and delta-HCH were between 100 and 300 nM, with maximum levels of potentiation (5-7-fold) between 20-50 microM. Potentiation of IG-Actx by alpha- and delta-HCH was reversible and largely insensitive to the benzodiazepine antagonist flumazenil (1 microM). Assays on maximum IG-Actx indicated that alpha-HCH (10-100 microM) caused only marginal reductions in response (less than or equal to 15%), whereas delta-HCH had stronger inhibitory effects (IC50, 20-30 microM). At concentrations between 0.1 and 50 microM, beta-HCH induced only 10-25% facilitation of IG-Actx elicited by 10 microM GABA and had no clear effects on maximum responses. IG-BR was also potently inhibited by gamma-HCH. Thresholds for detecting reductions in current were approximately 20 nM, and the IC50 calculated from effects on maximum responses was 5.8 +/- 2 microM. However, neither alpha-HCH nor delta-HCH (1-100 microM) induced any potentiation of IG-BR. alpha-HCH had some weak inhibitory effects that were largely surmountable, whereas delta-HCH and beta-HCH were essentially inactive. These experiments raise the possibility that alpha- and delta-HCH constitute a novel class of GABAA receptor modulators, which might prove to be useful for investigating the mechanisms underlying regulation of GABAA receptors.(ABSTRACT TRUNCATED AT 400 WORDS)
Woodward
R M
RM
Department of Psychobiology, University of California, Irvine 92717-4550.
Polenzani
L
L
Miledi
R
R
D000818:Animals; D001640:Bicuculline / Q000494:pharmacology; D001921:Brain / Q000502:physiology* / Q000648:ultrastructure; D002417:Cattle; D002540:Cerebral Cortex / Q000502:physiology; D005260:Female; D001556:Hexachlorocyclohexane / Q000494:pharmacology*; D007536:Isomerism; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009865:Oocytes / Q000187:drug effects / Q000502:physiology / Q000648:ultrastructure*; D011061:Poly A / Q000235:genetics; D012313:RNA / Q000235:genetics*; D051381:Rats; D011963:Receptors, GABA-A / Q000187:drug effects* / Q000235:genetics / Q000502:physiology; D012160:Retina / Q000502:physiology* / Q000648:ultrastructure; D013237:Stereoisomerism; D014981:Xenopus
D011963:Receptors, GABA-A; D011061:Poly A; C040534:alpha-hexachlorocyclohexane; D001556:Hexachlorocyclohexane; D012313:RNA; D001640:Bicuculline; C023888:beta-hexachlorocyclohexane