Endogenous circadian rhythms in rats recovered from lateral hypothalamic lesions

Abstract

Compared with intact controls, rats which have recovered spontaneous ingestion after lateral hypothalamic lesions show exaggerated nocturnal rhythms. Food intake was low and water intake negligible in the light part of a LD 12:12 cycle. The endogenous rhythms persisted in constant dark, and were clearly free running in constant light with periods of 25–26 hr. Animals with relatively asymmetric damage showed strong LL suppression of intake on the first day; animals with symmetric damage showed no LL suppression and the strongest free running rhythms; rats with the largest lesions showed a permanent LL suppression of fluid intake from Day 3 onward. The LL rhythms were rapidly reentrained with a restored LD cycle, with phase shifts of over 90°/24 hr. Recovered laterals which were previously blinded by orbital enucleation showed free running rhythms with periods close to 24 hr. The division of the 24 hr period into active (α) and quiescent (ϱ) phases is much more sharply defined after the lesion. This is especially marked for drinking; feeding and activity rhythms are very similar. It is suggested that the recovered lateral is dependent upon endogenous oscillator(s) subserving motivated behaviors, and implications for the recovery of function and residual impairments are discussed.