Effects of neonatal cocaine treatment and gender on opioid agonist-stimulated [S]GTPγS binding in the striatum and nucleus accumbens

Prenatal cocaine exposure increases μ-opioid receptor binding in dopaminergic terminal areas and enhances behavioral responsiveness to μ-opioid agonists. We investigated the influence of early postnatal cocaine treatment on in vitro μ- and δ-opioid receptor activation in male and female weanling rats. Pups received subcutaneous injections of either 20 mg/kg cocaine HCl or saline once daily on postnatal days 1 through 5. On postnatal day 25, animals were decapitated and their brains were removed and frozen for later sectioning. Opioid receptor activation was assessed in the striatum and the shell of the nucleus accumbens by autoradiographic analysis of agonist-stimulated [³⁵S]GTPγS binding. Brain sections were incubated in the presence of [³⁵S]GTPγS, GDP, and either the μ-opioid agonist [-Ala²-N-MePhe⁴-Gly⁵-ol]enkephalin (DAMGO) or the δ-opioid agonist -Pen²-D-Pen⁵-enkephalin (DPDPE). Baseline binding was assessed in the absence of agonist, and nonspecific binding was determined by the addition of unlabeled GTPγS. Film images were quantified using brain mash-calibrated [¹⁴C] standards. Neonatal cocaine treatment had no effect on either baseline or agonist-stimulated [³⁵S]GTPγS binding. However, males exhibited significantly greater activation than females of δ-opioid receptors in both striatum and accumbens shell, regardless of neonatal treatment. These findings indicate a gender difference in δ-opioid receptor function that could mediate behavioral differences in response to opioid agonists.