title: Rhythmic nature of naloxone-induced aversions and nociception in rats.
abstract: Taste conditioning procedures revealed a rhythmic fluctuation in the aversiveness of naloxone in opiate-naive rats. This rhythm exhibited two peaks during each 24-hour period. One peak was evident in the light phase and the second, which was more prominent occurred in the dark phase. Analgesia produced by morphine showed an essentially similar rhythm with heat and pressure as noxious stimuli. Although control nociceptive thresholds for heat also showed this rhythm, those for pressure showed only one broad peak through most of the dark phase. With noxious heat the hyperalgesic effect of naloxone also showed two peaks of activity but, as in control responding, this was not true for pressure. Opioid systems mediating nociception of different stimuli may show differential diurnal fluctuations.
mesh_terms: D000698:Analgesia; D000818:Animals; D002940:Circadian Rhythm; D003214:Conditioning, Classical; D008297:Male; D009020:Morphine; D009270:Naloxone; D010146:Pain / Q000503:physiopathology*; D051381:Rats; D012684:Sensory Thresholds; D013652:Taste Threshold
chemical_list: D009270:Naloxone; D009020:Morphine