CHRONIC PENTOBARBITAL ADMINISTRATION ALTERS GAMMA-AMINOBUTYRIC ACID(A) RECEPTOR ALPHA(6)-SUBUNIT MESSENGER-RNA LEVELS AND DIAZEPAM-INSENSITIVE [H-3] RO15-4513 BINDING

In order to study the chronic effects of pentobarbital, a positive GAB A(A) receptor modulator, on the inverse agonist binding of the benzodi azepine site, binding of [H-3]Ro15-4513 and levels of GABA(A) receptor alpha(6)-subunit mRNA were investigated in the brains of pentobarbita l-tolerant/dependent animals, using receptor autoradiography and in si tu hybridization histochemistry in consecutive brain sections. Pentoba rbital was administered to rats either 60 mg/kg, i.p., once, for acute treatment, or 300 mu g/10 mu l/h i.c.v. continuously for 6 days via o smotic minipumps to render rats tolerant to pentobarbital. Rats assign ed to the dependent group were sacrificed 24 h after discontinuance of pentobarbital infusion, while those assigned to the tolerant group we re sacrificed at the end of infusion. The alpha(6) subunit mRNA was in creased in the tolerant group only. Diazepam-insensitive [H-3]Ro15-451 3 binding was increased in the cerebellar granule layer of pentobarbit al-tolerant and -dependent rats. No alterations in these parameters we re observed in acutely treated animals. These data suggest that chroni c pentobarbital treatment induced expression of alpha(6)-subunit mRNA. This was in contrast to alpha(1)- and gamma(2)-subunit mRNA, which in tolerant animals are unchanged, but for which withdrawal triggers a s urge in levels. Because the alpha(6)-subunit is a major component of t he diazepam-insensitive [H-3]Ro15-4513 binding site, the increased dia zepam-insensitive [H-3]Ro15-4513 binding implied de novo synthesis of the receptor subunit protein. (C) 1996 Wiley-Liss, Inc.