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

Abstract

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 <smlcap>L</smlcap>-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.

Estradiol Membrane-Initiated Signaling in the Brain Mediates Reproduction

“…Estradiol signaling rapidly induces neurotransmitter release [45, 86–88] and regulates neurotransmission [89]. Indeed, estradiol (and steroid hormones in general) signaling through membrane receptors resembles GPCR neurotransmitter signaling [90].…”

“…On the other hand, steroid treatments that induce sexual receptivity (estradiol, STX and PPT, an ERα agonist) have robust GIRK-1 currents, inhibiting β-end neurotransmission [108]. The decoupling of ORL-1 from GIRK-1 is mediated by activation of PLC/PKC/PKA and the phosphatidylinositol-3-kinase (PI3K)/neuronal nitric oxide synthase (nNOS) pathways [89; Fig 4]. …”

“…Activation of ORL-1 increases potassium (K + ) currents through G protein-coupled inwardly-rectifying potassium (GIRK) channels that inhibit β-END transmission, allowing lordosis to proceed. Modified from [67, 89, 94]. …”

Testosterone Rapidly Augments Retrograde Endocannabinoid Signaling in Proopiomelanocortin Neurons to Suppress Glutamatergic Input from Steroidogenic Factor 1 Neurons via Upregulation of Diacylglycerol Lipase-α

“…This is important in light of the fact that the cannabinoid regulation of energy homeostasis is sexually differentiated, with males being more sensitive to the hyperphagic and hypothermic effects than females. Moreover, estradiol further dampens the cannabinoid-induced hyperphagia, hypothermia, and presynaptic inhibition of glutamatergic input onto POMC neurons via the activation of estrogen receptor-α and the putative, G q -coupled membrane estrogen receptor, as well as the subsequent activation of phosphatidylinositol 3-kinase/neuronal nitric oxide synthase and phospholipase C/protein kinase C/protein kinase A pathways, respectively [for a review see 38, 39]. The hypothetical role of DAGL in the androgen-induced hyperphagia was presently affirmed by the fact that the DAGL inhibitor orlistat blocked the increase in energy intake caused by TP.…”

17α-Estradiol down-regulates glutathione synthesis in serum deprived PC-12 cells

“…Interestingly, a truncated ER receptor variant, ER-α36 located on the plasma membrane discovered in 2005 (Wang et al 2005), is also reported to be involved in estrogen neuroprotection through rapid signaling of MAPK/ERK and phosphatidylinositol-3kinase (Han et al 2015). In addition, a Gq-coupled membrane ER (Gq-mER) exists which has a role in sexual behavior, and like ERα, rapid membrane initiated signaling triggers a signal transduction cascade involving phospholipase C, protein kinase C, protein kinase A, as well as phosphatidylinositol-3-kinase (Conde et al 2016). Moreover, crosstalk with classical ERs and GPER within the cell is postulated to modulate signaling in five different scenarios which complicates estrogen-meditated signaling even further (Hadjimarkou and Vasudevan 2017).…”