γ-Aminobutyric acid receptor subtype antagonists differentially alter opioid-induced feeding in the shell region of the nucleus accumbens in rats

Food intake is significantly increased by administration of μ-selective opioid agonists into the nucleus accumbens, particularly its shell region. Pretreatment with either opioid (μ, δ 1, δ 2 or κ 1) or dopaminergic (D 1) receptor antagonists in the nucleus accumbens shell reduce μ opioid agonist-induced feeding. Selective GABA A (muscimol) and GABA B (baclofen) agonists administered into the nucleus accumbens shell each stimulate feeding which is respectively and selectively blocked by GABA A (bicuculline) and GABA B (saclofen) antagonists. The present study investigated whether feeding elicited by the μ-selective opioid agonist, [ d-Ala 2,NMe 4,Gly-ol 5]-enkephalin in the nucleus accumbens shell was decreased by intra-accumbens pretreatment with an equimolar dose range of either GABA A or GABA B antagonists, and further, whether general opioid or selective GABA antagonists decreased feeding elicited by GABA A or GABA B agonists in the nucleus accumbens shell. Feeding elicited by the μ-selective opioid agonist was dose-dependently increased following intra-accumbens pretreatment with GABA A (bicuculline) antagonism; this enhancement was significantly blocked by pretreatment with general or μ-selective opioid antagonists. In contrast, μ opioid agonist-induced feeding elicited from the nucleus accumbens shell was dose-dependently decreased by GABA B (saclofen) antagonism. Neither bicuculline nor saclofen in the nucleus accumbens shell altered baseline food intake. Whereas muscimol-induced feeding elicited from the nucleus accumbens shell was reduced by bicuculline and naltrexone, but not saclofen pretreatment, baclofen-induced feeding elicited from the nucleus accumbens shell was reduced by saclofen, but not by bicuculline or naltrexone. These data indicate that GABA A and GABA B receptor subtype antagonists differentially affect feeding elicited by μ opioid receptor agonists within the nucleus accumbens shell in rats.