Regulation of the GABA(A) receptor: Effects of chronic GABA, benzodiazepines and methylxanthines

GABA is one of the major inhibitory neurotransmitters in the brain. It exerts many of its effects through the hetero-oligomeric GABA{dollar}sb{lcub}rm A{rcub}{dollar} receptor. This receptor complex contains an integral chloride channel, a GABA recognition site and a number of allosteric modulator sites through which compounds such as benzodiazepines (BZDs), barbiturates, cage convulsants and steroids act to alter GABAergic transmission. In addition, while the mechanism is unknown, methylxanthines (MXs) have also been shown to alter GABAergic activity and inhibit several central effects of the BZDs.; Using {dollar}sp3{dollar}H-flunitrazepam (a BZD) as an indicator for the GABA{dollar}sb{lcub}rm A{rcub}{dollar} receptor and the allosteric interactions between GABA and BZD binding sites I show that chronic GABA exposure causes a down-regulation of the GABA{dollar}sb{lcub}rm A{rcub}{dollar} receptor that is both dose and time dependent. This effect is reversible and blocked by the specific GABA{dollar}sb{lcub}rm A{rcub}{dollar} antagonist SR-95531. In addition to chronic GABA-induced down-regulation, allosteric interactions between GABA and BZD recognition sites are uncoupled. The half time and pharmacology for chronic GABA induced uncoupling is indistinguishable from that for GABA induced down-regulation.; In contrast to the effects of chronic GABA exposure, chronic BZD; and MXs elicit no change in the number of BZD binding sites or the affinity of benzodiazepine binding in the absence of GABA but do cause an uncoupling of GABA/BZD binding site interactions. Ro15-1788 (a low efficacy BZD) inhibits uncoupling caused by flurazepam (a BZD) but not that caused by theophylline (a MX). Moreover, chloroadenosine (an adenosine agonist) inhibits theoplylline-induced but not flurazepam-induced uncoupling. Thus, while BZDs induce uncoupling through the BZD site, MXs appear to induce uncoupling through a site that resembles an adenosine receptor. These results suggest that the characteristics of GABA{dollar}sb{lcub}rm A{rcub}{dollar} receptor regulation are site dependent and also raise the possibility that the GABA{dollar}sb{lcub}rm A{rcub}{dollar} receptor is subject to trans-receptor regulation.