Postnatal development of the circadian adrenocortical rhythm in rats with different neonatal nutrition.

The development of diurnal rhythm of plasma and adrenal corticosterone levels was examined in rats overfed, standardfed and underfed during the suckling period (realised by adjustment of the number of suckling pups per litter to 4, 8, and 14). The first presence of circadian rhythm of plasma corticosterone was observed in 18-day-old overfed and standardfed rats, and slight delay in the manifestation of rhythm was noted in neonatally underfed animals (rhythm present in 21-day-old rats). The distinct circadian rhythm was present in 23- to 30-day-old rats. However in adult animals a significant diurnal rhythm of plasma and adrenal corticosterone content was noted in standardfed and underfed rats, whereas the variation of hormone levels was not significant in the neonatally overfed group. In young animals the peak of plasma corticosterone concentration occurred during the dark period while in adult animals it was shifted to the end of the light period. The values of amplitude of plasma corticosterone variation are increasing during the postnatal period. In weaned and adult rats the mesor values were found to be higher in neonatally underfed rats in comparison to the neonatally overfed group.