Developmental changes in the diurnal rhythm of secretion of corticosterone and LH in the domestic hen.

When diurnal changes in the plasma concentrations of corticosterone and LH were compared at various times during sexual development a diurnal rhythm in the secretion of corticosterone was most pronounced at 4.5 weeks of age in hens maintained on a schedule of 8 h light: 16 h darkness (8L: 16D) and at 8.5 weeks of age in hens maintained on 16L: 8D. At these ages the phase-angle of the rhythm in relation to the onset of darkness was 3 h earlier than had been observed in previous studies of the adult hen. The characteristics of the corticosterone rhythm changed during sexual development and diurnal changes were no longer evident in hens at 17 weeks of age at a time when prepubertal plasma concentrations of LH were maximal. However, there was a tendency for plasma concentrations of corticosterone to fluctuate diurnally in hens studied at less than 10 days before the onset of lay although not to the same extent as has previously been observed in the adult hen. This period of development was associated with a pronounced increase in the plasma concentration of progesterone. An increase in the plasma concentration of LH at the onset of darkness was observed at all ages in hens maintained on either photoschedule . In hens of 4.5-15 weeks of age maintained on 16L: 8D or 8L: 16D this increase in the plasma concentration of LH was sustained during the first 5 or 8-11 h of darkness respectively but at 17 weeks of age only a transient increase in LH was seen at the onset of darkness. Plasma concentrations of LH in samples of plasma taken at intervals of between 3 days and 2 weeks throughout sexual development were, until 18-19 weeks, consistently higher in hens maintained on a photoschedule of 16L: 8D than on 8L: 16D.