Role for ventral pallidal GABAergic mechanisms in the regulation of ethanol self-administration

The striatopallidal medium spiny neurons have been viewed as a final common path for drug reward, and the ventral pallidum (VP) as a convergent point for hedonic and motivational signaling. The medium spiny neurons are GABAergic, but they colocalize enkephalin. The present study investigated the role of the GABAergic mechanisms of the VP in ethanol consumption. The effects of bilateral microinjections of GABA(A) and GABA(B) receptor agonists and antagonists into the VP on voluntary ethanol consumption were monitored in alcohol-preferring Alko alcohol rats given 90 min limited access to ethanol in their home cages every other day. The influences of coadministration of GABA and opioid receptor modulators were also studied. The GABA(A) receptor agonist muscimol (1-10 ng/site) decreased ethanol intake dose-dependently, while administration of the GABA(A) receptor antagonist bicuculline (10-100 ng) had an opposite effect. The GABA(B) receptor agonist baclofen (3-30 ng) also suppressed ethanol intake, but the GABA(B) receptor antagonist saclofen (0.3-3 mu g) failed to modify it. Animals coadministered with bicuculline (30 ng) and baclofen (30 ng) consumed ethanol significantly less than those treated with bicuculline alone. Coadministration of the mu-receptor agonist D-Ala2,N-Me-Phe4,Glyol5-enkephalin (DAMGO, 0.1 mu g) with bicuculline counteracted, whereas the mu-receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP, 1 mu g) enhanced the bicuculline-induced increase of ethanol intake. When given alone, DAMGO decreased while CTOP increased ethanol intake. The study provides evidence for the ventral pallidal GABAergic mechanisms participating in the regulation of ethanol consumption and supports earlier work suggesting a role for pallidal opioidergic transmission in ethanol reward.