Transient receptor potential vanilloid 1 (TRPV1) plays a major role in hyperalgesia and allodynia and is expressed both in the peripheral and central nervous systems (CNS). However, few studies have evaluated mechanisms by which CNS TRPV1 mediates hyperalgesia and allodynia after injury. We hypothesized that activation of spinal cord systems releases endogenous TRPV1 agonists that evoke the development of mechanical allodynia by this receptor. Using in vitro superfusion, the depolarization of spinal cord triggered the release of oxidized linoleic acid metabolites, such as 9-hydroxyoctadecadienoic acid (9-HODE) that potently activated spinal TRPV1, leading to the development of mechanical allodynia. Subsequent calcium imaging and electrophysiology studies demonstrated that synthetic oxidized linoleic acid metabolites, including 9-HODE, 13-HODE, and 9 and 13-oxoODE, comprise a family of endogenous TRPV1 agonists. In vivo studies demonstrated that intrathecal application of these oxidized linoleic acid metabolites rapidly evokes mechanical allodynia. Finally, intrathecal neutralization of 9-and 13-HODE by antibodies blocks CFA-evoked mechanical allodynia. These data collectively reveal a mechanism by which an endogenous family of lipids activates TRPV1 in the spinal cord, leading to the development of inflammatory hyperalgesia. These findings may integrate many pain disorders and provide an approach for developing analgesic drugs.inflammation ͉ pain T ransient receptor potential vanilloid 1 (TRPV1) plays a pivotal role in many pain models, leading to the development of hyperalgesia and/or allodynia (1-2). TRPV1 is expressed in the peripheral as well as central nervous systems (CNS) including several areas involved in nociceptive transmission (3). Numerous studies have demonstrated that peripheral TRPV1 is activated by noxious heat and protons and is regulated by endogenous ligands (2), contributing to peripheral mechanisms of heat hyperalgesia (4). However, TRPV1 is also expressed in the CNS, where comparatively little is known about the role of this receptor in mediating central pain mechanisms.Recent studies have used TRPV1 antagonists to evaluate the role of CNS TRPV1 in inflammatory hyperalgesia. Interestingly, the systemic/spinal administration of TRPV1 antagonists blocks inflammation-induced heat hyperalgesia, as well as mechanical allodynia (5-6). These findings were unexpected since TRPV1 is not activated by mechanical stimuli, suggesting that spinal TRPV1 activation leads to central sensitization to both thermal and mechanical stimuli (7-8). Based upon these observations, we hypothesized that the afferent barrage resulting from peripheral tissue injury leads to generation of endogenous TRPV1 ligands in the spinal cord that activate TRPV1 in the CNS resulting in central sensitization. Results Depolarization of Spinal Cord Results in the Release of EndogenousTRPV1 Ligands. To evaluate the hypothesis that endogenous TRPV1 agonists are released from spinal cord neurons, freshly isolated male rat spinal cords w...

Prolactin (PRL) is a hormone and a neuromodulator. PRL sensitizes TRPV1 responses in sensory neurons, but it is not clear whether peripheral inflammation results in the release of endogenous PRL, or whether endogenous PRL is capable of acting as an inflammatory mediator in a sex-dependent manner. To address these questions, we examined inflammation-induced release of endogenous PRL, and its regulation of thermal hyperalgesia in female and male rats. PRL is expressed in several types of peripheral neuronal and non-neuronal cells, including TRPV1-positive nerve fibers, preadipocytes and activated macrophages/monocytes localized in the vicinity of nerves. Evaluation of PRL levels in hindpaws and plasma indicated that complete Freund’s adjuvant (CFA) stimulates release of peripheral, but not systemic PRL within 6–48h in both ovariectomized females with estradiol replacement (OVX-E) and male rats. The time course of release varies in OVX-E and male rats. We next employed the prolactin receptor (PRL-R) antagonist, Δ1-9-G129R-hPRL to assess the role of locally-produced PRL in nociception. Applied at a ratio of 1:1 (PRL:Δ1-9-G129R-hPRL; 40nM each), this antagonist was able to nearly (≈80%) reverse PRL-induced sensitization of capsaicin responses in rat sensory neurons. CFA-induced inflammatory thermal hyperalgesia in OVX-E rat hindpaws was significantly reduced in a dose-dependent manner by the PRL-R antagonist at the 6h, but not the 24h time point. In contrast, PRL contributed to inflammatory thermal hyperalgesia in male rats at 24h, but not 6h. In summary, these findings indicate that inflammation leads to accumulation of endogenous PRL in female and male rats. Further, PRL acts as an inflammatory mediator at different time points for female and male rats.

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