Fentanyl, a mu-opioid receptor agonist, phase shifts the hamster circadianpacemaker

The phase-shifting effects of the mu-opioid receptor agonist fentanyl on th e circadian timing system were investigated in the hamster. Fentanyl inject ions during the mid-subjective day induced phase advances of the hamsters' wheel-running activity rhythm. The shifts were not accompanied by an increa se in locomotor activity but instead a decrease of activity was often obser ved. A dose-response curve indicated that with increasing dosage, the respo nse probability increased, while the magnitude of the induced shift remaine d stable. The present data suggest that there is some role for opioid regul ation of the circadian system. (C) 2000 Elsevier Science B.V. All rights re served.