Estradiol Rapidly Attenuates ORL-1 Receptor-Mediated Inhibition of Proopiomelanocortin Neurons via G_q-Coupled, Membrane-Initiated Signaling

Estradiol rapidly regulates the activity of arcuate nucleus (ARH) proopiomelanocortin (POMC) neurons that project to the medial preoptic nucleus (MPN) to regulate lordosis. Orphanin FQ/nociceptin (OFQ/N) acts via opioid receptor-like (ORL)-1 receptors to inhibit these POMC neurons. Therefore, we tested the hypothesis that estradiol excites POMC neurons by rapidly attenuating inhibitory ORL-1 signaling in these cells. Hypothalamic slices through the ARH were prepared from ovariectomized rats injected with Fluorogold into the MPN. Electrophysiological recordings were generated in ARH neurons held at or near -60 mV, and neuronal phenotype was determined post hoc by immunohistofluorescence. OFQ/N application induced robust outward currents and hyperpolarizations via G protein-gated, inwardly rectifying K⁺ (GIRK) channels that were attenuated by pretreatment with either 17-β estradiol (E₂) or E₂ conjugated to bovine serum albumin. This was blocked by the estrogen receptor (ER) antagonist ICI 182,780 and mimicked by the G_q-coupled membrane ER (G_q-mER) ligand STX and the ERα agonist PPT. Inhibiting phosphatidylinositol-3-kinase (PI3K) blocked the estrogenic attenuation of ORL-1/GIRK currents. Antagonizing either phospholipase C (PLC), protein kinase C (PKC), protein kinase A (PKA) or neuronal nitric oxide synthase (nNOS) also abrogated E₂ inhibition of ORL-1/GIRK currents, whereas activation of PKC, PKA, protein kinase B (Akt) and nNOS substrate L-arginine all attenuated the OFQ/N response. This was observed in 92 MPN-projecting, POMC-positive ARH neurons. Thus, ORL-1 receptor-mediated inhibition of POMC neurons is rapidly and negatively modulated by E₂, an effect which is stereoselective and membrane initiated via G_q-mER and ERα activation that signals through PLC, PKC, PKA, PI3K and nNOS.