SEX DIFFERENCES IN MORPHINE ANALGESIA AND THE ROLE OF MICROGLIA IN THE PERIAQUEDUCTAL GRAY OF THE RAT

Morphine has been and continues to be one of the most potent and widely used drugs for the treatment of pain. Clinical and animal models investigating sex differences in pain and analgesia demonstrate that morphine is a more potent analgesic in males than in females; indeed, we report the effective dose of morphine for female rats is twice that of male rats. In addition to binding to the neuronal mu opioid receptor, morphine binds to the innate immune receptor toll-like receptor 4 (TLR4) on microglia. Morphine action at TLR4 initiates a neuroinflammatory response and directly opposes morphine analgesia. Our recent studies demonstrate that administration of chronic morphine activates microglia within the ventrolateral periaqueductal gray (vlPAG), a critical brain region for the antinociceptive effects of morphine, while blockade of vlPAG microglia increases morphine analgesia and suppresses the development of tolerance in male rats. Despite increasing evidence of the involvement of microglia in altering morphine efficacy, no studies have examined sex differences in microglia within the PAG. The present experiments seek to characterize the distribution and activity of vlPAG microglia in males and females using behavioral, immunohistochemical and molecular techniques, while demonstrating the sufficiency and necessity of vlPAG microglia to produce sex differences in morphine analgesia using site-specific pharmacological manipulation of TLR4. We also investigate a novel pharmacokinetic mechanism underlying the sexually dimorphic effects of morphine administration on microglial activity. Here, we address a fundamental gap in our current understanding of sex differences in morphine analgesia and establish a mechanistic understanding of how the activation of vlPAG microglia sex-specifically influences morphine analgesia