• Makoto Inoue, Toshiko Kawashima, Hiroshi Takeshima, Girolamo Calo, Atsuko Inoue, Yoshihiro Nakata, and Hiroshi Ueda.
• Division of Molecular Pharmacology and Neuroscience, Nagasaki University Graduate School of Biomedical Sciences, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan. ueda@net.nagasaki-u.ac.jp
• J. Pharmacol. Exp. Ther. 2003 May 1; 305 (2): 495-501.

AbstractBecause 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(-/-) mice and their wild-type (NOP(+/+)) 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(-/-) and NOP(+/+) mice or NOP antagonist-treated 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(-/-) mice, compared with those in its NOP(+/+) mice. However, there were no significant changes in NOP(-/-) 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(-/-) 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(-/-) and NOP(+/+) 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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