The rat's daily pattern of contact with a drinking tube was observed in a light-dark cycle (LD 12:12) and in constant light (LL) after partial isolation of the suprachiasmatic nuclei (SCN) with a wire microknife that transected their lateral, dorsal, and caudal connections or the passage of a wire microknife through the SCN. Animals whose SCN had been partially isolated remained entrained to an LD cycle and phase shifted drinking behavior when the LD cycle was delayed 4 hr. Disintegration of circadian rhytmicity was observed when these rats were exposed to LL. Passage of the knife through the SCN resulted in immediate and persistent disruption of circadian rhythmicity as severe as that seen after electrolytic lesions of the SCN. These data confirm the necessity of the SCN in regulating circadian behavior and suggest that the SCN may mediate entrainment and freerunning rhythms via different neural systems. © 1980, All rights reserved.
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