Rapid effects of estrogen to modulate G protein-coupled receptors via activation of protein kinase A and protein kinase C pathways

17β-Estradiol (E₂) rapidly (<20 min) attenuates the ability of μ- opioids to hyperpolarize guinea pig hypothalamic neurons. We have used intracellular recordings from female guinea pig hypothalamic slices to characterize the receptor and intracellular pathway(s) mediating E₂'s rapid effects. E₂ acts stereospecifically with physiologically relevant concentration-dependence (EC₅₀ = 8 nM) to cause a fourfold reduction in the potency of the μ-opioid agonist (D-Ala²-N-Me-Phe⁴-Gly⁵-ol)-enkephalin and the GABA(B) agonist baclofen to activate an inwardly rectifying K⁺ conductance in hypothalamic neurons. Both the nonsteroidal estrogen diethylstilbestrol and the anti-estrogen ICI 164,384 blocked E₂ actions to uncouple μ-opioid receptors. Using a pharmacological Schild analysis, we found that ICI 164,384 competed for this E₂ receptor with a K(e) of approximately 0.3 nM. The protein synthesis inhibitor cycloheximide did not block the estrogenic uncoupling of the μ-opioid receptor from its K⁺ channel, implying a rapid, nongenomic mechanism of E₂ action. The effects of E₂ were mimicked by the bath application of the protein kinase A (PKA) activators, forskolin and Sp-cAMP, and the protein kinase C (PKC) activator phorbol-12,13-dibutyrate. Furthermore, the selective PKA antagonists Rp-cAMP and KT5720, which have different chemical structures and modes of action, both blocked the effects of E₂. In addition, the actions of E₂ were blocked by the selective PKC inhibitor Calphostin C. Therefore, it appears that E₂ can activate both PKA and PKC to cause a heterologous desensitization of both μ-opioid and GABA(B) receptors, which has the potential to alter synaptic transmission in many regions of the CNS.