Retinohypothalamic projection and suprachiasmatic nucleus of the house sparrow, Passer domesticus

The distribution of retinohypothalamic projections and the organization of the suprachiasmatic region of the hypothalamus was investigated in the house sparrow (Passer domesticus). Retinohypothalamic projections (RHT) were studied by two anterograde tracing methods, and hypothalamic organization was investigated immunohistochemically with antisera against a number of substances known to be present in the mammalian suprachiasmatic nucleus (SCN): bombesin (BBS), glutamic acid decarboxylase (GAD), 5‐hydroxytryptamine (5HT), neuropeptide Y (NPY), neurotensin (NT), somatostatin (SS), substance P (SP), vasoactive intestinal polypeptide (VIP), and arginine vasopressin (AVP). Observations from these experiments were analysed within the framework of a cytoarchitectural study using Nissl‐stained material. From this study, we have identified an area in the anterior hypothalamus which we believe is an avian homologue of the mammalian SCN. This area contains a nucleus located in close apposition to the optic chiasm between the dorsal supraoptic decussation (DSD) and the ventral lateral geniculate body (GLv) for much of its rostrocaudal extent. The central portion of this nucleus contains neurons that exhibit GAD‐ and BBS‐like immunoreactivity and is the terminal field for the RHT. For this reason, we term this nucleus the visual SCN. It also contains axon plexuses exhibiting 5HT‐like, SP‐like, and NPY‐like immunoreactivity and is bordered ventrally by AVP‐like, SP‐like, and NT‐like immunoreactive cells and medially by VIP‐like and SS‐like immunoreactive cells. Although it is not established that these cell groups together compose a single suprachiasmatic nucleus, the organization in the avian brain of a nuclear complex with a retinorecipient area surrounded by nonvisual components would be very similar to that of the mammalian SCN.