Plasticity of Signaling by Spinal Estrogen Receptor α, κ-Opioid Receptor, and Metabotropic Glutamate Receptors over the Rat Reproductive Cycle Regulates Spinal Endomorphin 2 Antinociception: Relevance of Endogenous-Biased Agonism

Abstract

We previously showed that intrathecal application of endomorphin 2 [EM2; the highly specific endogenous μ-opioid receptor (MOR) ligand] induces antinociception that varies with stage of the rat estrous cycle: minimal during diestrus and prominent during proestrus. Earlier studies, however, did not identify proestrus-activated signaling strategies that enable spinal EM2 antinociception. We now report that in female rats, increased spinal dynorphin release and κ-opioid receptor (KOR) signaling, as well as the emergence of glutamate-activated metabotropic glutamate receptor 1 (mGluR) signaling, are critical to the transition from an EM2 nonresponsive state (during diestrus) to an analgesically responsive state (during proestrus). Differential signaling by mGluR, depending on its activation by membrane estrogen receptor α (mERα; during diestrus) versus glutamate (during proestrus), concomitant with the ebb and flow of spinal dynorphin/KOR signaling, functions as a switch, preventing or promoting, respectively, spinal EM2 antinociception. Importantly, EM2 and glutamate-containing varicosities appose spinal neurons that express MOR along with mGluRs and mERα, suggesting that signaling mechanisms regulating analgesic effectiveness of intrathecally applied EM2 also pertain to endogenous EM2. Regulation of spinal EM2 antinociception by both the nature of the endogenous mGluR activator (i.e., endogenous biased agonism at mGluR) and changes in spinal dynorphin/KOR signaling represent a novel mechanism for modulating analgesic responsiveness to endogenous EM2 (and perhaps other opioids). This points the way for developing noncanonical pharmacological approaches to pain management by harnessing endogenous opioids for pain relief. The current prescription opioid abuse epidemic underscores the urgency to develop alternative pharmacotherapies for managing pain. We find that the magnitude of spinal endomorphin 2 (EM2) antinociception not only varies with stage of reproductive cycle, but is also differentially regulated during diestrus and proestrus. This finding highlights the need for sex-specific and cycle-specific approaches to pain management. Additionally, our finding that spinal EM2 antinociception in female rats is regulated by both the ebb and flow of spinal dynorphin/κ-opioid receptor signaling over the estrous cycle, as well as the nature of the endogenous mGluR activator, could encourage noncanonical pharmacological approaches to pain management, such as harnessing endogenous opioids for pain relief.

Pharmacological Modulation of Endogenous Opioid Activity to Attenuate Neuropathic Pain in Rats

“…Our findings reveal that compromised spinal estrogenic and mGluR 1 activity, which unleashes antinociceptive responsiveness to i.t. EM2 in intact diestrous rats 46,47 reduced spinal opioid antiallodynia in spinal nerve ligated rats during diestrous. Conversely, compromised mGluR 1 signaling, which minimizes i.t.…”

“…This includes increased release of spinal dynorphin, activation of spinal kappa-opioid receptors (KOR), and a switch in the endogenous agonist of mGluR 1 from mERα in diestrus to glutamate in proestrus. 46 It is likely that signaling mechanisms regulating analgesic effectiveness of intrathecally applied EM2 also pertain to endogenous EM2 antinociception. An anatomical organization is present in spinal cord that enables endogenous interactions among presumed regulatory components of intrathecal (i.t.)…”

“…Additionally, spinal dynorphin-expressing neurons that coexpress mGluR 1 and ERα are apposed by terminals expressing vesicular glutamate transporters (glutamatergic terminals). 46 This cellular organization not only provides basis for cycling between ERα-activated and glutamate-activated mGluR 1 signaling over the estrous cycle, but also enables glutamate modulation of spinal dynorphin release, thereby coordinating glutamate activation of mGluR 1 with dynorphin release, both of which are prerequisites for spinal EM2 antinociception during proestrus. 46 The identification of signaling molecules that gate analgesic responsiveness to spinal EM2 suggests them to be potential targets for tapping into the spinal EM2 analgesic system for pain management.…”

“…46 This cellular organization not only provides basis for cycling between ERα-activated and glutamate-activated mGluR 1 signaling over the estrous cycle, but also enables glutamate modulation of spinal dynorphin release, thereby coordinating glutamate activation of mGluR 1 with dynorphin release, both of which are prerequisites for spinal EM2 antinociception during proestrus. 46 The identification of signaling molecules that gate analgesic responsiveness to spinal EM2 suggests them to be potential targets for tapping into the spinal EM2 analgesic system for pain management. However, under resting physiological conditions, in the absence of acute or chronic pain, endogenous opioid systems are quiescent, i.e., naloxone does not alter basal pain thresholds in laboratory animals 4,24 and humans.…”

“…treatments that unleash the analgesic properties of spinal EM2 during diestrus will also be antiallodynic, in the absence of exogenous opioids, in a spinal nerve ligation (SNL) chronic pain model during diestrus. We also tested the hypothesis that spinal treatments that impair spinal EM2 antinociception during proestrus 46 will exacerbate SNL-induced mechanical allodynia during this estrous cycle stage.…”