POSTNATAL-DEVELOPMENT OF THE SUBSTANCE-P-CONTAINING, NEUROPEPTIDE-Y-CONTAINING AND SEROTONIN-CONTAINING FIBERS IN THE RAT SUPRACHIASMATIC NUCLEUS IN RELATION TO DEVELOPMENT OF THE RETINO-HYPOTHALAMIC PROJECTION

The suprachiasmatic nucleus (SCN) in the hypothalamus controls many of the circadian rhythms in mammalian species. In the present study, we investigated the development of substance P (SP)-, neuropeptide Y (NPY )- and serotonin (5-hydroxytryptamine, 5-HT)-immunoreactive fibers in the rat SCN and the development of the retino-hypothalamic tract using cholera toxin P subunits (CTB), in order to understand which parts of the SCN participate in diurnal rhythm regulation and entrainment. In newborn rats, SP-, NPY- and 5-HT-immunoreactive fibers were scarcely d etected in the SCN. The number of SP-immunoreactive fibers gradually i ncreased between postnatal days (P) 15 and 30. At P30, the distributio n pattern of SP-immunoreactive fibers in the SCN was similar to that i n the adult rat. The number of NPY- and 5-HT-immunoreactive fibers inc reased greatly between P10 and P15, and the increase in NPY- and 5-HT- immunoreactive fibers continued until P20. CTB was injected into the u nilateral eyeball of the rat at various postnatal stages. In neonates, several labeled retinal fibers already existed in the ventral part an d ventro-lateral edge of the SCN. The number and density of labeled re tinal fibers in the SCN gradually increased between P10 and P20. Betwe en P20 and P30, a decrease in the labeling was observed in the dorsola teral part of the SCN. The adult pattern of labeled retinal fibers was achieved between P20 and P30. The development of SP-immunoreactive fi bers was delayed about 10 days relative to that of NPY-, 5-HT-immunore active fibers and retinal fibers.