Abstract
The suprachiasmatic nuclei (SCN) of the hypothalamus are necessary for the persistence of free running circadian activity rhythms in house sparrows. Suprachiasmatic lesions abolish circadian locomotor rhythms in constant darkness. The locomotor activity of lesioned sparrows was analyzed with two different power spectral analysis methods and was found to be arrhythmic. There was a weak correlation between the extent of damage to the SCN and the relative amplitude of the power spectral density in the circadian frequency range. In light-dark cycles (LD 12:12), the locomotor behavior of lesioned sparrows was rhythmic and similar to that of intact birds. However, entrainment was disrupted in SCN-lesioned sparrows exposed to a short photoperiod light cycle (LD 1:24). These results demonstrate that the SCN are crucial for the generation of overt circadian rhythmicity in birds. The fact that SCN lesions abolish circadian rhythms in sparrows and several mammalian species suggests that vertebrate circadian organization may be based on differentially weighted interactions among the pineal, the SCN, and perhaps other brain regions.