Naloxone’s Effect on Activity-Induced Phase Shifts of Circadian Rhythms

Vigorous running in a novel exercise wheel by hamsters during the subjective day results in large (about 3 hr) phase advances of their circadian rhythms. We administered the opiate receptor antagonist naloxone at 10 mg/kg and 20 mg/kg (i.p.) and found that at 20 mg/kg it significantly reduced phase shifts. This dose of naloxone also reduced the amount of running shown by hamsters in the novel wheel. At the 20 mg/kg dose, the reduced amount of running resulted in smaller phase shifts. The results are consistent with the hypothesis that nonphotic phase shifts are linked to exercise and arousal in hamsters. However, because high amounts of running were accompanied by phase shifting in animals whose opiate receptors were blocked, the results provide no evidence that the rewarding nature of running in rodents is an important causal factor in nonphotic phase shifting beyond its role in promoting running.