br Estrogen receptors activate mGluR signaling pathways In
Estrogen receptors activate mGluR signaling pathways In order to distinguish the opposing effects of estradiol on CREB phosphorylation, we turned to a “bottom-up” approach, working backwards from CREB phosphorylation to pharmacologically isolate the two signaling pathways (Boulware et al., 2005). (The signaling pathway described below is summarized in Fig. 1) We first focused on proteins known to influence MAPK, and found that inhibition of PKC and IP3 receptors decreased estradiol-induced CREB phosphorylation. PLC inhibition similarly blocked the enhancement, without affecting estradiol-induced inhibition of L-type calcium channel effects. Because PLC, PKC, and IP3Rs are activated by Gq-coupled GPCRs (Gutkind, 2000), we hypothesized that estradiol may act through a known Gq-linked GPCR to enhance CREB phosphorylation. Our first step in addressing this hypothesis was to simply power through the catalog of pharmacological agents that act as antagonists for individual Gq-coupled receptors. With a bit of luck, the second drug we tried was LY367385, an antagonist for mGluR1a, a group I mGluR. Pretreatment of AXL1717 with this drug blocked estradiol enhancement of CREB phosphorylation without altering estradiol attenuation of L-type calcium channel signaling. These results were confirmed with a second mGluR1 antagonist. In addition, direct pharmacological activation of mGluR1a increased CREB phosphorylation, and occluded any further enhancement by estradiol. The mGluR1a agonist DHPG elicited CREB phosphorylation regardless of sex, suggesting that the sex difference in estradiol signaling lies upstream of the mGluR. This also supported the idea that estradiol was not acting directly on the mGluR, but rather relies on the ability of an estrogen receptor to activate its signaling. Indeed, we found that the ERα agonist PPT increased CREB phosphorylation, while the ER blocker ICI 182,780 eliminated the effect of estradiol. These data indicated that ERα was able to solicit mGluR1a signaling in female hippocampal neurons. We later went on to repeat these experiments in female striatal neurons, finding that the same type of ER/mGluR signaling occurs, but ERα pairs with mGluR5 instead of mGluR1a (Grove-Strawser et al., 2010). Having demonstrated that Gq-coupled mGluR1a was necessary for estradiol-induced enhancement of CREB phosphorylation, we hypothesized that activation of a separate G-protein signaling pathway could explain the effect of estradiol on L-type calcium channel signaling (Boulware et al., 2005). Indeed, inhibition of group II mGluRs, which couple to Gi/o proteins to inhibit adenylyl cyclase, eliminated the effect of estradiol. Furthermore, activation of either ERα or ERβ triggered group II mGluR signaling (Fig. 1). This work demonstrated specific bidirectional effects of estradiol within the same system, wherein the magnitude of concurrent excitatory input will dictate the outcome of estradiol exposure. Since these initial studies, a greater appreciation has emerged for how estradiol can influence neuronal systems via interactions with group I mGluRs. For example, group I mGluRs can activate a variety of cellular responses, including the release of endogenous cannabinoids (Alger and Kim, 2011; Wilson and Nicoll, 2002). Estradiol has also been known to influence this system (Maccarrone et al., 2002; Scorticati et al., 2004), and these lines of evidence have recently converged. Specifically, it has been demonstrated that in female hippocampal neurons, ERα activates mGluR1a leading to endocannabinoid signaling, subsequently decreasing presynaptic GABA neurotransmission (Huang and Woolley, 2012; Tabatadze et al., 2015). This example illustrates the potential impact of ER/mGluR interaction and the importance of thoroughly understanding of the signaling mechanism.
Estradiol activation of ER/mGluR signaling underlies sex differences in addiction