Diurnal changes in the plasma concentrations of LH and hypothalamic contents of LHRH-I and LHRH-II in the domestic hen

Diurnal changes of LH secretion in sexually immature hens of 9, 11, 13 and 15 weeks of age consisted of 25-40% increases in the mean concentrations of LH in plasma between 15.00 and 18.00 h, i.e. between 2 h before and 1 h after the onset of darkness. During this time there was a tendency for the mean contents of LHRH-I in the anterior hypothalamus and posterior hypothalamus to increase by 21-74% and 20-56% respectively. In hens of 9 and 15 weeks, diurnal changes in the plasma concentration of LH closely paralleled those of LHRH-I content in the posterior hypothalamus. In contrast, the diurnal rhythm of LH secretion in hens of 11 and 13 weeks was more marked and plasma concentrations of LH continued to rise steeply between 18.00 and 2 1.00 h, i.e. between 1 and 4 h after the onset of darkness. At 11 weeks, this was associated with a reduction (P < 0.01) in the contents of LHRH-I and LHRH-II, particularly in the anterior hypothalamus. In laying hens, a diurnal decline (P < 0.01) in the plasma concentration of LH between 1 and 4 h after the onset of darkness was preceded by a fall (P < 0.05) in the content of LHRH-I in the posterior hypothalamus and in the total hypothalamic content of LHRH-II (P < 0.01). In all groups of hens, irrespective of the times of day at which tissue was taken, significant (P < 0.05 - < 0.001) correlations between the contents of LHRH-I and LHRH-II in the anterior hypothalamus were observed. It is concluded that a diurnal rhythm of release of LHRH-L may drive the diurnal rhythm of LH secretion. Thus, in sexually immature hens of 9 and 15 weeks and laying hens in which diurnal changes in plasma LH were small there were parallel changes in the content of LHRH-L in the posterior hypothalamus. However, where the plasma concentration of LH was increased substantially, as at 11 weeks, there was a decline in the hypothalamic contents of LHRH-I. A simultaneous fall in the hypothalamic content of LHRH-II raises the possibility of a causal relationship between the activities of LHRH-II, LHRH-L and the release of LH.