Stimulation of accumbal GABAA receptors inhibits delta2-, but not delta1-, opioid receptor-mediated dopamine efflux in the nucleus accumbens of freely moving rats

The nucleus accumbens contains delta-opioid receptors that may reduce inhibitory neurotransmission. Reduction in GABAA receptor-mediated inhibition of accumbal dopamine release due to delta-opioid receptor activation should be suppressed by stimulating accumbal GABAA receptors. As delta-opioid receptors are divided into delta2- and delta1-opioid receptors, we analysed the effects of the GABAA receptor agonist muscimol on delta2- and delta1-opioid receptor-mediated accumbal dopamine efflux in freely moving rats using in vivo microdialysis. Drugs were administered intracerebrally through the dialysis probe. Doses of compounds indicate total amount administered (mol) during 25-50 min infusions. The delta2-opioid receptor agonist deltorphin II (25.0 nmol)- and delta1-opioid receptor agonist DPDPE (5.0 nmol)-induced increases in dopamine efflux were inhibited by the delta2-opioid receptor antagonist naltriben (1.5 nmol) and the delta1-opioid receptor antagonist BNTX (150.0 pmol), respectively. Muscimol (250.0 pmol) inhibited deltorphin II (25.0 nmol)-induced dopamine efflux. The GABAA receptor antagonist bicuculline (50.0 pmol), which failed to affect deltorphin II (25.0 nmol)-induced dopamine efflux, counteracted the inhibitory effect of muscimol on deltorphin II-induced dopamine efflux. Neither muscimol (250.0 pmol) nor bicuculline (50.0 and 500.0 pmol) altered DPDPE (5.0 nmol)-induced dopamine efflux. The present results show that reduction in accumbal GABAA receptor-mediated inhibition of dopaminergic activity is necessary to produce delta2-opioid receptor-induced increase in accumbal dopamine efflux. This study indicates that activation of delta2- but not delta1-opioid receptors on the cell bodies and/or terminals of accumbal GABAergic interneurons inhibits GABA release and, accordingly, decreases GABAA receptor-mediated inhibition of dopaminergic terminals, resulting in enhanced accumbal dopamine efflux.