Morphine-induced activity attenuates phase shifts to light in C57BL/6J mice

Circadian rhythms can be phase shifted and entrained by daily schedules of light and by non-photic stimuli such as locomotor activity. Relatively litt le is known of how photic and non-photic stimuli interact to regulate circa dian phase. Morphine injections were used to examine the effects of locomot or activity on phase shifts to light pulses in mice free-running in constan t dark. Morphine injections scheduled early or late in the active period (s ubjective night) induced hyperactivity, but did not induce phase shifts. Li ght pulses late in the subjective night induced phase advance shifts that w ere significantly attenuated (63% smaller, p < 0.01) by pretreatment with m orphine. This inhibitory effect of morphine on light-induced phase advances was blocked by preventing mice from running for 6 h after the injections. Light pulses early in the subjective night induced phase delay shifts that were only weakly attenuated (15% smaller, p = 0.06) by morphine. These resu lts demonstrate behavioral inhibition of light-induced phase resetting of c ircadian rhythms in mice, and suggest that the strength of this effect may be phase dependent, although other interpretations are possible. (C) 1999 E lsevier Science B.V. All rights reserved.