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.