Circadian locomotor rhythms in the desert iguana. II. Effects of electrolytic lesions to the hypothalamus.

Desert iguanas, Dipsosaurus dorsalis, displaying freerunning circadian locomotor rhythms in conditions of constant darkness and temperature received electrolytic lesions to the hypothalamus. The locomotor activity of those lizards (N = 9) which sustained 80% or more damage to the suprachiasmatic nucleus (SCN) became arrhythmic whereas all animals that sustained less than 35% damage to the SCN remained rhythmic, even though they sustained significant damage to nearby regions of the hypothalamus and preoptic area. These results suggest strongly that the SCN plays a role in the regulation of circadian rhythms in the desert iguana. Taken together with other evidence, they support the view that this structure is homologous to the mammalian SCN, which acts as a pacemaker in the circadian system.