Clonidine shortens circadian period in both constant light and constant darkness.

Chronic treatment of rats with clonidine, an alpha-adrenoceptor agonist, 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 circadian pacemaker or on its input pathways. Because shortening of free-running period during clonidine treatment has been observed consistently under constant light but not under constant darkness, and because increasing light intensity itself lengthens free-running period, this agent 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 were seen in both constant light and constant darkness, indicating that the effects of clonidine on the circadian pacemaker are not due to blockade of light-evoked activity in the photic input pathway. Instead, clonidine may act directly on the circadian pacemaker, or on other unspecified mechanisms influencing free-running period.