Circadian rhythms and partial recovery of regulatory drinking in rats after lateral hypothalamic lesions.

Rats that had recovered spontaneous ingestive behavior after lesions of the lateral hypothalamus were challenged with acute injections of hypertonic NaCl administered at different times during the day/night cycle. Following these injections, drinking was observed only during the nighttime. After morning injections the rats frequently waited until nightfall before drinking, whereas animals injected at night showed much shorter delays in the behavioral response; a similar nocturnal predominance of drinking was seen after food deprivation and in the ad-lib situation. Studies in blinded animals suggested that these effects were due to an endogenous circadian rhythm.