LATERAL HYPOTHALAMIC REGULATION OF CIRCADIAN-RHYTHM PHASE

The suprachiasmatic nuclei generate a circadian rhythm which can be described by period, phase and amplitude variables. Evidence is accumulating that the three descriptors of circadian rhythmicity can be modulated independently by several brain structures. This report describes the effects of lateral hypothalamic (LHA) damage on control of period, phase and amplitude of the hamster locomotor rhythm. Adult male hamsters received bilateral electrolytic lesions of the far lateral LHA. These lesions had no effect on the circadian period in constant dim, but significantly advanced the onset of nocturnal wheel running and lengthened the duration of the activity phase. Rate of reentrainment after a 6-h phase advance or delay was not affected by the lesion. Rhythm amplitude, as indicated by the number of wheel revolutions per day, was not affected by the lesions. The results support the view that different brain regions can exert independent modulatory control over the basic circadian rhythm generated by the suprachiasmatic nucleus.