Discriminative stimulus effects of the GABA_B receptor-positive modulator rac-BHFF: Comparison with GABA_B receptor agonists and drugs of abuse

GABA_B receptor-positive modulators are thought to have advantages as potential medications for anxiety, depression, and drug addiction. They may have fewer side effects than GABA_B receptor agonists, because selective enhancement of activated receptors could have effects different from nonselective activation of all receptors. To examine this, pigeons were trained to discriminate the GABA_B receptor-positive modulator (R,S)-5,7- di-tert-butyl-3-hydroxy-3-trifluoromethyl-3H-benzofuran-2- one (rac-BHFF) from its vehicle. The discriminative stimulus effects of rac-BHFF were not mimicked by the GABA_B receptor agonists baclofen and g-hydroxybutyrate (GHB), not by diazepam, and not by alcohol, cocaine, and nicotine, whose selfadministration has been reported to be attenuated by GABA_B receptor-positive modulators. The discriminative stimulus effects of rac-BHFF were not antagonized by the GABA_B receptor antagonist 3-aminopropyl (diethoxymethyl)phosphinic acid (CGP35348) but were attenuated by the less efficacious GABAB receptor-positive modulator 2,6-di-tert-butyl-4-(3-hydroxy-2,2- dimethylpropyl)phenol (CGP7930), suggesting the possibility that rac-BHFF produces its discriminative stimulus effects by directly activating GABA^B2 subunits of GABA_B receptors. At a dose 10- fold lower than the training dose, rac-BHFF enhanced the discriminative stimulus effects of baclofen, but not of GHB. This study provides evidence that the effects of GABA_B receptorpositive modulators are not identical to those of GABA_B receptor agonists. In addition, the results suggest that positive modulation of GABA_B receptors does not produce discriminative stimulus effects similar to those of benzodiazepines, alcohol, cocaine, and nicotine. Finally, the finding that rac-BHFF enhanced effects of baclofen but not of GHB is consistent with converging evidence that the populations of GABA_B receptors mediating the effects of baclofen and GHB are not identical.