Differential effects of gestational buprenorphine, naloxone, and methadone on mesolimbic mu opioid and ORL1 receptor G protein coupling.

In addition to its use for heroin addiction pharmacotherapy in general, buprenorphine has advantages in treating maternal heroin abuse. To examine the gestational effects of buprenorphine on opioid receptor signaling, the [(35)S]-GTP gamma S in situ binding induced by the mu agonist [D-Ala(2),MePhe(4),Gly(5)-ol] enkephalin (DAMGO) or the nociceptin/orphanin FQ (N/OFQ) agonist was measured in mesolimbic structures of pup brains from pregnant rats administered with buprenorphine +/- naloxone, naloxone, or methadone by osmotic minipump. Drug- and gender-based changes in DAMGO- and N/OFQ-induced GTP gamma S binding were discovered in mesolimbic regions of dam, P2, and P7 brains. Buprenorphine and/or methadone gestational treatment attenuated DAMGO-induced GTP gamma S binding in some dam and male P2 mesolimbic regions. Methadone diminished DAMGO-induced GTP gamma S binding in almost all monitored brain regions of the dam but had few effects on their N/OFQ-induced GTP gamma S binding. Naloxone used in combination with buprenorphine blocked the inhibition by buprenorphine alone on DAMGO-induced GTP gamma S binding. In contrast to its inhibitory effects on DAMGO-induced GTP gamma S binding, buprenorphine stimulated N/OFQ-induced GTP gamma S binding in male P2 nucleus accumbens and lateral septum. Brain region-dependent gender differences in DAMGO-induced GTP gamma S binding were seen in P2 pups, and males showed greater sensitivity to buprenorphine and methadone than females. Our findings on mu opioid receptor (MOR) GTP-binding regulatory protein (G protein) coupling and its gender dependency are consistent with our earlier studies on mu receptor binding adaptation induced by buprenorphine in dams and neonatal rats after in utero treatment regimens, and they extend the gestational effects of this opiate to mu and N/OFQ receptor functionality.