Abstract
Though sex differences in pain and analgesia are known, underlying mechanisms remain elusive. This study addresses the selective contribution of membrane estrogen receptors (mERs) and mER-initiated non-genomic signaling mechanisms in our previously reported estrogen-induced attenuation of α2-adrenoceptor-mediated antinociception. By selectively targeting spinal mERs in ovariectomized female rats using β-estradiol 6-(O-carboxy-methyl)oxime bovine serum albumin (E2BSA) (membrane impermeant estradiol analog), and ERα selective agonist 4,4',4″-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol (PPT), ERβ selective agonist 2,3-bis(4-hydroxyphenyl)-propionitrile (DPN), G-protein-coupled estrogen receptor 30 (GPR30) agonist G1 and Gq-coupled mER (Gq-mER) agonist STX, we provide strong evidence that Gq-mER activation may solely contribute to suppressing clonidine (an α2-adrenoceptor agonist)-induced antinociception, using the nociceptive tail-flick test. Increased tail-flick latencies (TFLs) by intrathecal (i.t.) clonidine were not significantly altered by i.t. PPT, DPN, or G1. In contrast, E2BSA or STX rapidly and dose-dependently attenuated clonidine-induced increase in TFL. ICI 182,780, the ER antagonist, blocked this effect. Consistent with findings with the lack of effect of ERα and ERβ agonists that modulate receptor-regulated transcription, inhibition of de novo protein synthesis using anisomycin also failed to alter the effect of E2BSA or STX, arguing against a contribution of genomic mechanisms. Immunoblotting of spinal tissue revealed that mER activation increased levels of phosphorylated extracellular signal-regulated kinase (ERK) but not of protein kinase A (PKA) or C (PKC). In vivo inhibition of ERK with U0126 blocked the effect of STX and restored clonidine antinociception. Although estrogen-induced delayed genomic mechanisms may still exist, data presented here indicate that Gq-mER may solely mediate estradiol-induced attenuation of clonidine antinociception via a rapid, reversible, and ERK-dependent, non-genomic mechanism, suggesting that Gq-mER blockade might provide improved analgesia in females.