Here, we investigated the mechanism of the antihyperalgesic effect of capsaicin cream in the nerve injury-induced neuropathic pain model in mice. In naive mice, application of capsaicin cream onto footpad caused no significant changes in the thermal latency in contrast to the severe thermal hyperalgesia induced by a capsaicin ointment. On the other hand, application of the cream 3 h before test concentration dependently reversed both thermal and mechanical hyperalgesia observed after partial sciatic nerve injury in mice. In algogenic-induced nociceptive flexion (ANF) test, application of 0.1% capsaicin cream in naive mice blocked intraplantar (i.pl.) nociceptin-and ATP-induced flexion responses, whereas prostaglandin I 2 (PGI 2 ) agonist-induced responses were unaffected. After nerve injury PGI 2 agonist-induced flexion responses were hypersensitized, and capsaicin cream concentration dependently blocked these hyperalgesic responses. Intraplantar injection of capsaicin solution in ANF test also produced potent flexion responses in naive mice that were lost after neonatal capsaicintreatment. Partial sciatic nerve injury in neonatal capsaicintreated mice caused reappearance of i.pl. capsaicin-induced flexion responses, suggesting novel expression of capsaicin receptors due to injury. The PGI 2 agonist-induced responses were also hypersensitized in such injured mice. Capsaicin cream completely reversed both i.pl. capsaicin-or i.pl. PGI 2 agonist-induced hyperalgesia in neonatal capsaicin-treated injured mice. Finally, novel expression of VR1 receptors on neonatal capsaicin-insensitive neurons after nerve injury was confirmed by immunohistochemistry. The newly expressed VR1 receptors after nerve injury were mainly confined to A-fibers. Together, our results suggest that novel expression of capsaicin receptors in neuropathic condition contributes to the analgesic effects of the capsaicin cream.

Nociceptin/orphanin FQ (N/OFQ), nocistatin, and prepro-N/OFQ 160 -187 (C-peptide) are all derived from the same precursor protein. We examine the pharmacological mechanisms of nocistatin-and C-peptide-induced pronociceptive responses in a novel algogenic-induced nociceptive flexion test in mice. The intraplantar (i.pl.) injection of nocistatin-and C-peptide induced pronociceptive responses in a range of 0.01 to 10 or 1 pmol, respectively, which showed 100-to 1000-fold less potent effects than the N/OFQ. The nociceptive effects of both peptides were not affected by 1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H-benzimidazole-2-one (J-113397) (i.pl.), an N/OFQ receptor antagonist, indicating that they are mediated by a novel mechanism independent of activation of N/OFQ receptor. Like N/OFQ, nocistatin-induced nociception was abolished by i.pl. injection of pertussis toxin, phospholipase C inhibitor, or CP-99994, a neurokinin 1 receptor antagonist, indicating that nocistatin may elicit nociception through a substance P release from nociceptor endings via activation of G i/o and phospholipase C. The nociception was abolished by neonatal pretreatment (s.c.) with capsaicin or by i.t. pretreatment with CP-99994, but not , an N-methyl-D-aspartate receptor antagonist. In contrast, C-peptide-induced nociception was attenuated by the pretreatment with antisense oligodeoxynucleotide for G␣ s (i.t.) and with KT-5720 (i.pl.), a cyclic AMP-dependent protein kinase inhibitor, but not with pertussis toxin. The nociception was neither attenuated by neonatal capsaicin nor by i.t. injection with CP-99994, but it was attenuated by i.t. injection with MK-801. These results suggest that nocistatin and C-peptide derived from prepro-N/OFQ stimulate distinct nociceptive fibers through different in vivo signaling mechanisms.Nociceptin/orphanin FQ (N/OFQ), discovered as the endogenous peptide ligand for N/OFQ receptor/NOP (Meunier et al., 1995;Reinscheid et al., 1995), is generated from a larger precursor protein, prepro-N/OFQ (Saito et al., 1995;Mollereau et al., 1996;Nothacker et al., 1996). N/OFQ has been seen as active at multiple sites of nociceptive transmission, ranging from peripheral nociceptors (Inoue et al., 1998) to nociceptive centers in the brain (Morgan et al., 1997). Pharmacologically, the actions of N/OFQ are complex and seem to be contradictory; intracerebroventricular or intrathecal (i.t.) administration of this peptide exerts pronociceptive (or hyperalgesia) and/or analgesia (for reviews, see Calo, 2000;Mogil and Pasternak, 2001). We also previously reported that N/OFQ (i.t.) exerts nocifensive actions in a femtomolar dose range through a substance P release from primary substance P fibers, whereas analgesic actions in a nanomolar dose range come through an inhibition of substance P actions on the second-order neuron in the spinal cord (Inoue et al., 1999). These findings suggest that the opposing actions of N/OFQ may depend on the dose administered.However, nocistatin, another...

Because nociceptin/orphanin FQ (N/OFQ) has both pronociceptive (hyperalgesia) and antinociceptive actions in pharmacological experiments, and there is no significant difference in the nociceptive responses between NOP Ϫ/Ϫ mice and their wild-type (NOP ϩ/ϩ ) littermates, the physiological role of N/OFQ in pain regulation remains to be determined. Under the hypothesis that the use of molecularly distinct nociception test may reveal the pain modality-specific role of N/OFQ, we attempted to examine the physiological role of N/OFQ in pain transmission by using newly developed algogenic-induced nociceptive flexion test in NOP Ϫ/Ϫ and NOP ϩ/ϩ mice or NOP antagonisttreated mice. The nociceptive flexor responses upon intraplantar injection of bradykinin or substance P, which stimulates polymodal substance P-ergic fibers, were markedly potentiated in NOP Ϫ/Ϫ mice, compared with those in its NOP ϩ/ϩ mice. However, there were no significant changes in NOP Ϫ/Ϫ mice with adenosine triphosphate or prostaglandin I 2 agonist, which stimulates glutamatergic but not substance P-ergic fibers. The nocifensive responses induced by substance P (i.t.) were also potentiated in NOP Ϫ/Ϫ mice. On the other hand, there were no significant differences in NK1-like immunoreactivity, [3 H]substance P binding, or NK1 gene expression in the dorsal horn of the spinal cord between NOP Ϫ/Ϫ and NOP ϩ/ϩ mice. In addition, NOP antagonists decreased the threshold in nociception tests driving spinal substance P neurotransmission. All these findings suggest that the N/OFQ-ergic neuron may play an in vivo inhibitory role on the second-order neurons for primary polymodal substance P-ergic fibers in the spinal cord.

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