The Role of Prolactin/Prolactin Receptor System in Nociceptor Sensitization and Pain in Females

Women have higher pain sensitivity and are over-represented as pain patients. However, the underlying biological mechanisms for sexual dimorphism in pain remain unclear. A puzzling question is if there is sexual dimorphism in peripheral nociceptors, the fundamental building blocks of pain. Prolactin (PRL) is an endocrine hormone regulated by estrogen. Previous reports suggest that PRL can regulate neuronal excitability in a female-selective manner. The action of PRL is transduced by prolactin receptor (PRLR) including short and long isoforms (PRLR-S and PRLR-L). It is hypothesized that PRL promotes sensitization of sensory neurons through PRLR-S, which is mutually inhibited by PRLR-L. PRL levels are significantly higher in women and PRLR is more abundant in women. Nevertheless, women do not normally live in pain. We therefore hypothesized that expression of PRLR-L might be a physiological mechanism protecting women from circulating PRL-promoted pain. Here, we found that circulating PRL was significantly higher in female, than male, mice. PRLR-L expression was significantly greater in the dorsal root ganglia (DRG) in female mice compared to male mice. Both PRLR isoforms were not detected in either female or male mice in spinal cord dorsal horn tissues. Remarkably, CRISPR-mediated disruption of PRLR-L expression in DRGs resulted in allodynia in naïve and uninjured female, but not male, mice suggesting PRLR-L is protective from PRL-promoted pain selectively in females. Then, we determined whether the imbalance of PRLR-L and PRLR-S signaling could be the underlying mechanism of pathological pain conditions. Injury induces the local release of PRL and PRL has been demonstrated to be critical for postoperative pain and inflammatory pain selectively in female rodents. We therefore studied if PRL/PRLR system contributes to pain caused by trauma-induced neuropathic pain in a the spared nerve injury (SNI) model. We found that trauma-induced nerve injury did not affect PRLR isoform expression in DRGs of female mice and CRISPR-mediated deletion of PRLR did not prevent or reverse the hypersensitivity in the SNI female mice. Thus, PRL/PRLR signaling is not critical for the development of trauma-induced neuropathic pain in female mice. Next, we determined the role of PRL/PRLR in injury-free pain conditions. While opioids are used to treat pain, paradoxically these drugs have also been associated with nociceptor sensitization and allodynia commonly referred to as opioid-induced hyperalgesia (OIH). An additional, well-known side-effect of opioids is disruption of hormonal function that includes hyperprolactinemia and galactorrhea most often observed in females. We therefore determined whether the PRL/PRLR system might be disrupted by opioid administration in the absence of injury. We found that PRLR-L expression in DRGs could be negatively regulated by circulating PRL. Treatment with opioids promoted nociceptor sensitization and allodynia through down-regulation of PRLR-L. Upregulation of PRLR-L by genetic over-expression strategy or by lowering circulating PRL by dopamine D2 agonist, cabergoline, rescued OIH selectively in females. Thus, our studies demonstrate that circulating PRL/PRLR system is a female-selective mechanism of OIH. Opioids are also strongly associated with the development of medication overuse headache (MOH). MOH is a secondary headache due to excessive intake of acute headache treatments. The most common trigger of migraine attacks is stress, which induces PRL release via kappa opioid receptor (KOR) signaling in the brain. We therefore investigated the relationship of stress-induced PRL/PRLR signaling and MOH development. We chronically treated the mice with opioids to mimic overuse of opioids as headache treatments and then triggered the expression of allodynia by stress as a model of MOH. We demonstrated that blockade of KOR signaling prevented stress-induced allodynia as well as PRL release. Dural injection of PRL induced both periorbital and hindpaw allodynia only in morphine-primed female mice at a low dose. Down-regulation of PRLR-L in DRGs was sustained after the resolution of OIH. CRISPR-mediated deletion of total PRLR prevented stress-induced allodynia in morphine-primed female mice. Our data therefore demonstrate that PRL/PRLR system in morphine primed nociceptors is critical for MOH attacks triggered by stress in female mice. In summary, our studies reveal that circulating PRL/PRLR promotes nociceptor sensitization, injury-free pain, which might suggest novel approaches for pain control in women