Spinal Synthesis of Estrogen and Concomitant Signaling by Membrane Estrogen Receptors Regulate Spinal κ- and μ-Opioid Receptor Heterodimerization and Female-Specific Spinal Morphine Antinociception

Abstract

We previously demonstrated that the spinal cord κ-opioid receptor (KOR) and μ-opioid receptor (MOR) form heterodimers (KOR/MOR). KOR/MOR formation and the associated KOR dependency of spinal morphine antinociception are most robust during proestrus. Using Sprague Dawley rats, we now demonstrate that (1) spinal synthesis of estrogen is critical to these processes, and (2) blockade of either estrogen receptor (ER) α-, β-, or G-protein-coupled ER1 or progesterone receptor (PR) substantially reduces KOR/MOR and eliminates mediation by KOR of spinal morphine antinociception. Effects of blocking ERs were manifest within 15 min, whereas those of PR blockade were manifest after 18 h, indicating the requirement for rapid signaling by estrogen and transcriptional effects of progesterone. Individual or combined blockade of ERs produced the same magnitude of effect, suggesting that they work in tandem as part of a macromolecular complex to regulate KOR/MOR formation. Consistent with this inference, we found that KOR and MOR were coexpressed with ERα and G-protein-coupled ER1 in the spinal dorsal horn. Reduction of KOR/MOR by ER or PR blockade or spinal aromatase inhibition shifts spinal morphine antinociception from KOR dependent to KOR independent. This indicates a sex steroid-dependent plasticity of spinal KOR functionality, which could explain the greater analgesic potency of KOR agonists in women versus men. We suggest that KOR/MOR is a molecular switch that shifts the function of KOR and thereby endogenous dynorphin from pronociceptive to antinociceptive. KOR/MOR could thus serve as a novel molecular target for pain management in women.

Estrogen Facilitates Spinal Cord Synaptic Transmission via Membrane-bound Estrogen Receptors

“…In addition to their nuclear localization, classical ERs (ER␣ and ER␤) can also be trafficked to the plasma membrane, where they may directly bind to G proteins or link to other G protein-coupled receptors (e.g. mGluRs), which then induce signaling pathways (4,59,60,61). Through these signaling pathways, estrogen can not only rapidly modulate the synaptic transmission of dorsal horn neurons but also can activate transcription factors, such as cAMP-response element binding protein, to affect gene expression, leading to long term effects, including LTP.…”

Interaction of CRF and Kappa Opioid Systems on GABAergic Neurotransmission in the Mouse Central Amygdala

“…Female C57BL/6 mice (60-90 days old; Charles River Laboratories, Raleigh, NC), and homozygous KOR knockout (KO) mice and wild-type (WT) littermate mice (120-180 days old, shipped from the Scripps Research Institute, La Jolla, CA) were used for this study. We exclusively used female mice, as it has been reported that female animals showed a greater or selective sensitivity to KORmediated behaviors such as antinociception, motivation, mood-related disorders, and social interaction (Chang et al, 2000;Chakrabarti et al, 2010;Lawson et al, 2010;Liu et al, 2011;Robles et al, 2014;Russell et al, 2014). The genetic background of KOR KO and WT mice was a hybrid C57BL/6Orl  129/SV strain.…”

Sex differences in pain and pain inhibition: multiple explanations of a controversial phenomenon

“…Although most of the research has been carried out in mice, for κ-opioid analgesia this sex difference is seen in humans as well, with genetic dysfunction of MC1R leading to increased pentazocine analgesia in women but not men 39 . Studies have documented that the neural circuitry subserving analgesia from sex steroids and morphine in the rat spinal cord show profound sex divergence 60,61,92 , and recent work suggests that the core of the sex difference stems from a bias in μ-and/or κ-opioid heterodimer expression 59,93 . These heterodimers -the formation of which is under regulation by spinal oestrogen synthesis 93 -are vastly more prevalent in the female rat spinal cord than in the male spinal cord and are activated by endogenous dynorphin 1-17, which itself can be released by intrathecal injection of morphine, producing effects that are not seen in monomeric κ-opioid Box 2 | Are there sex differences in laboratory pain sensitivity?…”