Postnatal development of the substance P-, neuropeptide Y- 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 beta 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 detected in the SCN. The number of SP-immunoreactive fibers gradually increased between postnatal days (P) 15 and 30. At P30, the distribution pattern of SP-immunoreactive fibers in the SCN was similar to that in the adult rat. The number of NPY- and 5-HT-immunoreactive fibers increased greatly between P10 and P15, and the increase in NPY- and 5-HT-immunoreactive fibers continued until P20. CTB was injected into the unilateral eyeball of the rat at various postnatal stages. In neonates, several labeled retinal fibers already existed in the ventral part and ventro-lateral edge of the SCN. The number and density of labeled retinal fibers in the SCN gradually increased between P10 and P20. Between P20 and P30, a decrease in the labeling was observed in the dorsolateral part of the SCN. The adult pattern of labeled retinal fibers was achieved between P20 and P30. The development of SP-immunoreactive fibers was delayed about 10 days relative to that of NPY-, 5-HT-immunoreactive fibers and retinal fibers.