Identification of the suprachiasmatic nucleus in birds

Circadian rhythms are generated by an internal biological clock. The suprac hiasmatic nucleus (SCN) in the hypothalamus is known to be the dominant bio logical clock regulating circadian rhythms in mammals. In birds, two nuclei , the so-called medial SCN (mSCN) and the visual SCN (vSCN), have both been proposed to be the avian SCN. However, it remains an unsettled question wh ich nuclei are homologous to the mammalian SCN. We have identified circadia n clock genes in Japanese quail and demonstrated that these genes are expre ssed in known circadian oscillators, the pineal and the retina. Here, we re port that these clock genes are expressed in the mSCN but not in the vSCN i n Japanese quail, Java sparrow, chicken, and pigeon. In addition, mSCN lesi ons eliminated or disorganized circadian rhythms of locomotor activity unde r constant dim light, but did not eliminate entrainment under light-dark (L D) cycles in pigeon. However, the lesioned birds became completely arrhythm ic even under LD after the pineal and the eye were removed. These results i ndicate that the mSCN is a circadian oscillator in birds.