CRF and urocortin I are not only readily
CRF and urocortin I are not only readily found throughout the spinal cord (Korosi et al., 2007), CRF analogs are clearly able to alter nociceptive signals (Imbe et al., 2010; Lariviere and Melzack, 2000). However, it was unclear whether they are released endogenously and involved in the spinal modulation of musculoskeletal nociception following stress. We addressed this question using the CRF receptor antagonists NBI-35965, a CRF1-selective compound, and astressin 2B, a CRF2-selective compound, to determine the contribution of these receptors to swim stress-induced hyperalgesia. Based on the ability of astressin 2B, but not NBI-35965, to attenuate the decrease in grip force responses after the swim stress, CRF2 receptor activity appears to contribute to stress-induced musculoskeletal hyperalgesia. This is consistent with the CRF2 receptor mediation of hypersensitivity of the urinary Clotrimazole induced by footshock in the rat (Robbins and Ness, 2008). Which CRF analog is released endogenously to produce this hyperalgesic effect cannot be determined from these studies. Although urocortin I binds more readily with CRF2 receptors than CRF, making urocortins good candidates, CRF is found more abundantly in the normal healthy spinal cord than urocortin I (Korosi et al., 2007) providing a concentration of CRF that may be sufficient to activate CRF2 sites. TRPV1 receptors are distributed throughout the body, including the brain, spinal cord (Kim et al., 2012) and peripheral nervous system (Menigoz and Boudes, 2011; Roberts et al., 2004; Szallasi et al., 1995) where they play a vital role in hyperalgesia. Because TRPV1 ligands are crucial to the enhancement of nociception in many models of hyperalgesia (Chung et al., 2011; Fujii et al., 2008; Roberts et al., 2011; Szabo et al., 2005), we questioned whether TRPV1 receptor-expressing neuronal populations mediate stress-induced musculoskeletal hyperalgesia. In our study, RTX delivered either intrathecally or subcutaneously, induced thermal antinociception, consistent with an efficacious dose, but failed to prevent the decrease in grip force induced by a swim stress. Delivered subcutaneously, RTX desensitizes TRPV1 sites on primary afferent C-fibers while intrathecal injections desensitize TRPV1 sites on interneurons (Kim et al., 2012) or postsynaptic neurons in the dorsal horn (Gibson et al., 2008; Zhou et al., 2009). In addition to RTX, SB-366791, a TRPV1 receptor antagonist, injected i.p., i.t., or i.c.v had no effect on swim-induced decreases in grip force responses, in spite of its ability to inhibit the effect of capsaicin. The combined results from RTX-induced desensitization and TRPV1 receptor antagonism indicate that neither central nor peripheral afferent neurons that express TRPV1 receptors are necessary for swim stress-induced hyperalgesia in normal healthy mice. This is consistent with the conclusion that pretreatment with TRPV1 antagonists or RTX (to desensitize TRPV1 sites) have no effect on acute models of mechanical hyperalgesia but attenuate mechanical hyperalgesia (measured using von Frey fibers or Randall-Selitto assays) when induced by models of nerve injury or inflammation (Bishnoi et al., 2011b; Kanai et al., 2007; Tender et al., 2008; Watabiki et al., 2011a, 2011b). No tolerance to the musculoskeletal hyperalgesic effect of stress appears to develop when presented daily based on the persistence of the hyperalgesic response following 15 daily forced swims. This is consistent with similar findings that swim stress-induced hyperalgesia can be elicited daily when measured using chemical sensitivity (formalin assay: (Imbe et al., 2010; Quintero et al., 2011, 2003; Suarez-Roca et al., 2008)). Stress-induced hyperalgesia in rodents induced by repeated exposure to alternating episodes of cold and warm also lasts for several days with no apparent adaptation when measured using the tail flick (Hata et al., 1988; Kita et al., 1979), tail pressure (Hata et al., 1988; Kita et al., 1979), paw pressure (Satoh et al., 1992), acetic acid or phenylquinone assays (Kita et al., 1979). In contrast, when acute stress-induced thermal antinociception is monitored chronically (Grisel et al., 1993; Marek et al., 1993; Mogil et al., 1996; Vaccarino and Clavier, 1997) tolerance develops to antinociception (Imbe et al., 2010). We too demonstrated in the tail flick assay that tolerance develops to the swim stress-induced thermal antinociception during repeated daily exposure to the stress as it is resolved by day 10. Chronic exposure to a stress can even eventually lead to hyperalgesia rather than antinociception along thermal pathways (Quintero et al., 2000; Suarez-Roca et al., 2006a; Suaudeau and Costentin, 2000), perhaps by unmasking a more persistent hyperalgesic component.