BRIEF COMMUNICATION Conditioned Stimulus Control in the Rat Circadian System Depends on Clock Resetting During Conditioning

The authors examined the ability of a conditioned stimulus (CS; mild air disturbance) previously paired with an entraining light pulse to reset the circadian pacemaker in rats. Rats were entrained to a single 30-min light stimulus delivered every 25 hr or 24 hr (T cycle). Each daily light presentation was paired with the CS. After at least 20 days of stable entrainment to each of the T cycles, the rats were allowed to free run and were then presented with the CS at circadian time 15. CS-induced phase shifts in wheel-running activity rhythms were taken as evidence for conditioning. For the most part, conditioning occurred after CS-light pairings on the 25-hr but not 24-hr T cycle. The results suggest that CS control of the circadian clock phase depends on the effect that the entraining light pulse has on the clock during conditioning. In mammals, light is essential for stable entrainment of circadian rhythms, providing the critical stimulus for daily resetting of the circadian clock located in the suprachias-matic nucleus (SCN) of the hypothalamus (Morin, 1994; Roenneberg & Foster, 1997; Weaver, 1998). Entrainment is accomplished through small daily shifts in the phase of th