PROTEIN-KINASE-A MAINTAINS CELLULAR TOLERANCE TO MU-OPIOID RECEPTOR AGONISTS IN HYPOTHALAMIC NEUROSECRETORY-CELLS WITH CHRONIC MORPHINE TREATMENT - CONVERGENCE ON A COMMON PATHWAY WITH ESTROGEN IN MODULATING MU-OPIOID RECEPTOR EFFECTOR COUPLING/

The present study examined protein kinase A (PKA) and protein kinase C (PKC) involvement in the maintenance of cellular tolerance to mu opio id receptor agonists resulting from chronic opiate exposure in neurose cretory cells of the hypothalamic arcuate nucleus (ARC). The possibili ty that the diminution of mu opioid receptor/effector coupling produce d by acute 17 beta-estradiol or chronic opiate exposures is mediated b y a common kinase pathway also was investigated. Intracellular recordi ngs were made in hypothalamic slices prepared from ovariectomized fema le guinea pigs. The mu opioid receptor agonist D-Ala(2), N-Me-Phe(4), Gly-ol(5)-enkephalin (DAMGO) produced dose-dependent hyperpolarization s of ARC neurons. Chronic morphine treatment for 4 days reduced DAMGO potency 2.5-fold with no change in the maximal response. This effect w as mimicked by a 20-min bath application of the PKA activator cAMP, Sp -isomer, or the PKC activator phorbol-12,13-dibutyrate. A 30-min bath application of the broad-spectrum protein kinase inhibitor staurospori ne completely abolished the reduced DAMGO potency seen in morphine-tol erant neurosecretory cells, including those immunopositive for gonadot ropin-releasing hormone. The effect of staurosporine was mimicked by t he PKA inhibitor cAMP, Rp-isomer, but not by the PKC inhibitor calphos tin C. Finally, a 20-min bath application of 17 beta-estradiol did not further reduce DAMGO potency in morphine-tolerant ARC neurons. Theref ore, increased PKA activity maintains cellular tolerance to mu opioid receptor agonists in ARC neurosecretory cells caused by chronic morphi ne treatment. Furthermore, acute 17 beta-estradiol and chronic opiate treatments attenuate mu opioid receptor-mediated responses via a commo n PKA pathway.