CLONIDINE SHORTENS CIRCADIAN PERIOD IN BOTH CONSTANT LIGHT AND CONSTANT DARKNESS

Chronic treatment of rats with clonidine, an alpha-adrenoceptor agonis t, alters the period and amplitude of free-running circadian activity rhythms, as well as the level of home cage locomotor activity. Agents that alter free-running period are presumed to act directly on the cir cadian pacemaker or on its input pathways. Because shortening of free- running period during clonidine treatment has been observed consistent ly under constant light but not under constant darkness, and because i ncreasing light intensity itself lengthens free-running period, this a gent may influence the circadian pacemaker by modulating light-evoked activity in the photic input pathway. The present study reexamined the possible dependence of clonidine-induced alterations of free-running circadian activity rhythms on lighting conditions. Similar effects wer e seen in both constant light and constant darkness, indicating that t he effects of clonidine on the circadian pacemaker are not due to bloc kade of light-evoked activity in the photic input pathway. Instead, cl onidine may act directly on the circadian pacemaker, or on other unspe cified mechanisms influencing free-running period.