The present study examined
protein kinase A (PKA) and
protein kinase C (PKC) involvement in the maintenance of cellular tolerance to
mu opioid receptor agonists resulting from chronic
opiate exposure in neurosecretory cells of the hypothalamic arcuate nucleus (
ARC). The possibility that the diminution of
mu opioid receptor/effector coupling produced by acute 17beta-estradiol or chronic
opiate exposures is mediated by a common
kinase pathway also was investigated. Intracellular recordings were made in hypothalamic slices prepared from ovariectomized female guinea pigs. The
mu opioid receptor agonist D-Ala2, N-Me-Phe4, Gly-ol5-enkephalin (
DAMGO) produced dose-dependent hyperpolarizations of
ARC neurons. Chronic
morphine treatment for 4 days reduced
DAMGO potency 2.5-fold with no change in the maximal response. This effect was 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 staurosporine completely abolished the reduced
DAMGO potency seen in
morphine-tolerant neurosecretory cells, including those immunopositive for
gonadotropin-releasing hormone. The effect of
staurosporine was mimicked by the
PKA inhibitor cAMP, Rp-isomer, but not by the PKC inhibitor
calphostin C. Finally, a 20-min bath application of 17beta-estradiol did not further reduce
DAMGO potency in
morphine-tolerant
ARC neurons. Therefore, increased PKA activity maintains cellular tolerance to
mu opioid receptor agonists in
ARC neurosecretory cells caused by chronic
morphine treatment. Furthermore, acute 17beta-estradiol and chronic
opiate treatments attenuate
mu opioid receptor-mediated responses via a common PKA pathway.