Food intake is significantly increased following administration of mu-selective
opioid agonists into the ventral tegmental area (VTA) region acting through multiple local
opioid receptor subtypes. Since
GABA receptor agonists in the VTA region are capable of eliciting feeding, the present study investigated whether feeding elicited by the mu-selective
opioid agonist [D-Ala(2), NMe(4), Gly-ol(5)]-
enkephalin (
DAMGO) in the VTA region was altered by pretreatment into the same site with equimolar doses of either
GABA(A) (
bicuculline) or
GABA(B) (
saclofen) antagonists, and further, whether pretreatment with either general
opioid or selective
GABA receptor antagonists decreased feeding elicited by
GABA(A) (
muscimol) or
GABA(B) (
baclofen) agonists in the VTA region.
DAMGO-induced feeding in the VTA region was dose-dependently decreased following pretreatment with either
GABA(A) or
GABA(B) antagonists in the absence of significant alterations in food intake by the antagonists per se. However, the presence of short-lived
seizures following
bicuculline in the VTA region suggests that this ingestive effect was caused by nonspecific actions. In contrast,
GABA(B) receptors are involved in the full expression of mu-
opioid agonist-induced feeding in this region since
saclofen failed to elicit either seizure activity or a conditioned taste aversion. Pretreatment with
naltrexone in the VTA region reduced intake elicited by
baclofen, but not
muscimol. Finally,
baclofen-induced feeding was significantly reduced by
saclofen, but not
bicuculline, pretreatment in the VTA region. Therefore, possible coregulation between
GABA(B) and
opioid receptors in the VTA region, as suggested by immunocytochemical evidence, is supported by these behavioral effects upon ingestion.