In mammals, the "master clock" controlling circadian rhythmicity is located in the hypothalamic suprachiasmatic nuclei (SCN). Until now, no comparable structure has been unambiguously described in the brain of any nonmammalian vertebrate. In birds, early anatomical and lesioning studies described a SCN located in the anterior hypothalamus, but whether birds possess a nucleus equivalent to the mammalian SCN remained controversial. By reviewing the existing literature it became evident that confusion in delineation and nomenclature of hypothalamic cell groups may be one of the major reasons that no coherent picture of the avian hypothalamus exists. In this review, we attempt to clarify certain aspects of the organization of the avian hypothalamus by summarizing anatomical and functional studies and comparing them to immunocytochemical results from our laboratory. There is no single cell group in the avian. hypothalamus that combines the morphological and neurochemical features of the mammalian SCN. Instead, certain aspects of anatomy and morphology suggest that at least two anatomically distinct cell groups, the SCN and the lateral hypothalamic nucleus (LHN), bear some of the characteristics of the mammalian SCN.
