title: Fos expression in the spinal cord is suppressed in rats displaying conditioned hypoalgesia.
abstract: Two experiments used c-fos expression as a marker of spinal nociceptive processing to study the neural correlates of hypoalgesic responses to conditioned stimuli (CSs) paired with an aversive event. Immunoreactive (ir) neuronal labeling of Fos, the nuclear protein encoded by the c-fos gene, was examined in the spinal cords of rats killed 2 hr after injection of dilute formalin into a hind paw. Compared with control rats either not conditioned or conditioned in one environment but tested elsewhere, there were significantly fewer Fos-ir neurons in the spinal cords of rats displaying hypoalgesic responses when tested in the presence of aversive CSs. Naloxone abolished hypoalgesic responses and reinstated spinal Fos expression, indicating that aversive CSs activated opioid-based antinociceptive mechanisms. The results confirm that aversive CSs produce hypoalgesia by inhibiting the transmission of ascending nociceptive information.
authors.affiliation: School of Psychology, University of New South Wales, Sydney, Australia.
authors.forename: J A
authors.initials: JA
authors.lastname: Harris
authors.forename: R F
authors.initials: RF
authors.lastname: Westbrook
authors.forename: T Q
authors.initials: TQ
authors.lastname: Duffield
authors.forename: M
authors.initials: M
authors.lastname: Bentivoglio
mesh_terms: D000818:Animals; D001245:Association Learning / Q000502:physiology; D001362:Avoidance Learning / Q000502:physiology; D003214:Conditioning, Classical / Q000502:physiology*; D015870:Gene Expression / Q000502:physiology; D007124:Immunoenzyme Techniques; D008297:Male; D009433:Neural Inhibition / Q000235:genetics / Q000502:physiology; D009619:Nociceptors / Q000502:physiology*; D017288:Pain Threshold / Q000502:physiology*; D016760:Proto-Oncogene Proteins c-fos / Q000235:genetics*; D051381:Rats; D017208:Rats, Wistar; D011957:Receptors, Opioid / Q000235:genetics / Q000502:physiology; D013116:Spinal Cord / Q000502:physiology*; D013697:Thermosensing / Q000502:physiology
chemical_list: D016760:Proto-Oncogene Proteins c-fos; D011957:Receptors, Opioid