GABA(A) receptors containing alpha 5 subunits in the CA1 and CA3 hippocampal fields regulate ethanol-motivated behaviors: An extended ethanol reward circuitry

GABA receptors within the mesolimbic circuitry have been proposed to play a role in regulating alcohol-seeking behaviors in the alcohol-preferring (P) rat. However, the precise GABA(A) receptor subunit(s) mediating the reinfo rcing properties of EtOH remains unknown. We examined the capacity of intra hippocampal infusions of an alpha5 subunit-selective (similar to 75-fold) b enzodiazepine (BDZ) inverse agonist [i.e., RY 023 (RY) (tert-butyl 8-(trime thylsilyl) acetylene-5,6-dihydro-5-methyl-6-oxo-4H-imidazo [1,5a] [1,4] ben zodiazepine-3-carboxylate)] to alter lever pressing maintained by concurren t presentation of EtOH (10% v/v) and a saccharin solution (0.05% w/v). Bila teral (1.5-20 mug) and unilateral (0.01-40 mug) RY dose-dependently reduced EtOH-maintained responding, with saccharin-maintained responding being red uced only with the highest doses (e.g., 20 and 40 mug). The competitive BDZ antagonist ZK 93426 (ZK) (7 mug) reversed the RY-induced suppression on Et OH-maintained responding, confirming that the effect was mediated via the B DZ site on the GABA(A) receptor complex. Intrahippocampal modulation of the EtOH-maintained responding was site-specific; no antagonism by RY after in tra-accumbens [nucleus accumbens (NACC)] and intraventral tegmental [ventra l tegmental area (VTA)] infusions was observed. Because the VTA and NACC co ntain very high densities of alpha1 and alpha2 subunits, respectively, we d etermined whether RY exhibited a "negative" or "neutral" pharmacological pr ofile at recombinant alpha1 beta3 gamma2, alpha2 beta3 gamma2, and alpha5 b eta3 gamma2 receptors expressed in Xenopus oocytes. RY produced "classic" i nverse agonism at all alpha receptor subtypes; thus, a neutral efficacy was not sufficient to explain the failure of RY to alter EtOH responding in th e NACC or VTA. The results provide the first demonstration that the alpha5- containing GABA(A) receptors in the hippocampus play an important role in r egulating EtOH-seeking behaviors.