Discriminative stimulus effects of gamma-hydroxybutyrate (GHB) in rats discriminating GHB from baclofen and diazepam.

Gamma-hydroxybutyrate (GHB) is a drug of abuse with actions at GHB and GABA receptors. This study tried to increase the selectivity of the discriminative stimulus effects of GHB by training animals to discriminate GHB from compounds that share pharmacological mechanisms with GHB. In comparison with a previous GHB versus saline discrimination (group 1), rats were trained to discriminate GHB (200 mg/kg) either from saline and the GABA(B) agonist baclofen (3.2 mg/kg) (group 2) or from saline, baclofen, and the positive GABA(A) modulator diazepam (1 mg/kg) (group 3). In all groups, GHB produced more than 80% GHB-appropriate responding. Baclofen produced 84% GHB-appropriate responding in group 1 but less than 30% in groups 2 and 3. Diazepam produced 68% GHB-appropriate responding in group 1, 30% in group 2, and only 5% in group 3. The GABA(B) receptor antagonists CGP35348 [3-[aminopropyl(diethoxymethyl)phosphinic acid] and CGP52432 [3-[[[((3,4-dichlorophenyl)methyl]amino]propyl]diethoxymethyl)phosphinic acid] attenuated the discriminative stimulus effects of GHB; CGP35348 did so with similar potency in all groups, but CGP52432 was significantly less potent in groups 2 and 3 than in group 1. In all groups, the GHB antagonist NCS-382 [(2E)-(5-hydroxy-5,7,8,9-tetrahydro-6H-benzo[a][7]annulen-6-ylidene ethanoic acid] partially attenuated the discriminative stimulus effects of GHB. The selective GHB receptor ligand UMB86 (4-hydroxy-4-napthylbutanoic acid sodium) tended to attenuate the discriminative stimulus effects of GHB more in group 3 than in the other groups. The finding that animals can discriminate GHB from baclofen is further evidence that the effects of GHB and baclofen are not identical. Effects that GHB does not share with baclofen may involve GHB receptors or differential interactions with GABA(B) receptors.