Median eminence and anterior pituitary degradation of luteinizing hormone releasing hormone in hens undergoing changes in luteinizing hormone secretion

Abstract

Studies described in this report provide physiological evidence for a possible involvement of median eminence (ME) and anterior pituitary (AP) luteinizing hormone releasing hormone degrading activity (LHRH-DA) in the genesis of the hen's preovulatory surge of luteinizing hormone (LH). Serum LH and progesterone (P4), ME LHRH content and LHRH-DA, and AP LHRH-DA were determined in laying hens in the following reproductive conditions: 1) a "spontaneous" preovulatory LH surge; 2) a "premature" preovulatory LH surge; and 3) an ovulatory failure induced by feed withdrawal. The premature preovulatory surge of LH occurred 3 h after P4 administration and was preceded by an increase in both ME and AP LHRH-DA and by a decrease in ME LHRH content 1 h after P4 administration. However, the premature preovulatory LH surge was associated with a decrease in LHRH-DA back to control levels as ME LHRH content increased, even though LHRH was presumably being released from the ME at this time to maintain the preovulatory surge of LH. Although similar changes in LHRH-DA (an increase in both ME and AP LHRH-DA, followed by a decrease) also preceded the spontaneous preovulatory surge of LH, its profile was significantly blunted and no changes in ME LHRH content were associated with this LH surge. In contrast, ovulatory failure was correlated with a decrease in ME LHRH content but no changes in LHRH-DA. Therefore, in the hen, ME and AP enzymatic degradation of LHRH I might be involved in the genesis of a premature preovulatory surge of LH.