BEHAVIORAL AND NEURAL BIOLOGY 25, 545--554 (1979) Entrainment of Circadian Rhythms by Feeding Schedules in Rats with Suprachiasmatic Lesions FRIEDRICH K. STEPHAN, JENNIFER M. SWANN, AND CHERYL L. SISK t Department of Psychology, Florida State University, Tallahassee, Florida 32306 Rats with lesions of the suprachiasmatic nucleus (SCN) and controls were maintained in constant light and exposed to a restricted feeding schedule at 23- and 24-hr intervals, as well as to a 12-hr phase shift in the feeding schedule. Despite the absence of circadian periodicity in acitivity or drinking in ad lib. conditions, rats with SCN lesions showed anticipatory wheel running to both feeding schedules, comparable to sham-operated rats. Following the 12-hr phase shift, transients qualitatively similar to those seen following phase shifts in the light-dark cycle were observed. During a 3-day period of total food deprivation following prolonged entrainment to a 24-hr feeding schedule, wheel running per- sisted with a near 24-hr periodicity while return to ad lib. conditions resulted in a rapid desynchronization of activity. These results indicate that anticipatory wheel running in rats with SCN lesions is based on endogenous circadian oscillators which are entrainable by feeding schedules in the circadian range. Apparently such oscillators free run under certain conditions (food deprivation) but become rapidly desynchronized in others (ad lib. feeding). The evidence strongly supports a multioscillator model of the circadian system in mammals. Rats exposed to restricted feeding schedules at 24-hr intervals substan- tially increase wheel running activity several hours prior to food availability (e.g., Richter, 1927; Bolles & Stokes, 1965; Edmonds & Adler, 1977a). Since rats appear to be unable to anticipate 19- or 29-hr feeding schedules, it seems reasonable to assume that anticipatory wheel running is based on an endogenous circadian rhythm (Bolles & Stokes, 1965). Because of the importance of the suprachiasmatic nucleus (SCN) in the generation and entrainment of circadian rhythms (for review see Zucker, Rusak, & King, 1976; Block & Page, 1978; Menaker, Takahashi, & Eskin, 1978), we have recently studied the ability of rats with SCN lesions to anticipate 18- and 24-hr feeding schedules in constant light (Stephan, Swann, & Sisk, 1979). Rats with SCN lesions were able to anticipate 24-hr schedules as well as controls but both groups were unable 1 This research was supported in part by Psychobiology Training Grant MH-I1218 from the National Institute of Mental Health to Dr. D. R. Kenshalo, Florida State Univer- sity. 545 0163-1047/79/040545-10502.00/0 Copyright © 1979 by Academic Press, Inc. All rights of reproduction in any form reserved.