DISCRETE HYPOTHALAMIC DISTRIBUTION OF LUTEINIZING-HORMONE-RELEASING HORMONE (LHRH) CONTENT AND OF LHRH-DEGRADING ACTIVITY IN LAYING AND NONLAYING HENS

Hypothalamic enzymatic LHRH-degrading activity (LHRH-DA) may play a physiologic role in the neuroendocrine control of LHRH in mammals. The present study analyzes the existence and possible physiologic role of LHRH-DA in birds. The LHRH content in discrete hypothalamic samples of laying and nonlaying hens was correlated to their LHRH-DA. Degrading activity was assessed by high-performance liquid chromatography (HPLC) of chicken LHRH and of its degradation fragments. LHRH content was estimated by radioimmunoassy. LHRH content of discrete medial preoptic, infundibulum, and arcuate samples, as well as serum LH [luteinizing hormone] and progesterone levels, were higher (P < 0.05) in laying than in nonlaying hens. The LHRH content of these hypothalamic areas was also higher (P < 0.05) than those of immediately adjacent areas, in both animal groups. LHRH-degrading activity, which generates LHRH1-5 as the main degradation fragment, was higher (P < 0.01) in the infundibulum of laying than in nonlaying hens. It was also higher (P < 0.01) in samples from the infundibulum and medial preoptic area than in immediately adjacent lateral samples. Finally, LHRH-DA, showing a similar HPLC profile of degradation fragments; was also present in areas of low or undetectable LHRH content. A LHRH-DA generating LHRH1-5 as its main degradation product is present throughout the hypothalamus of the hen. However, a physiologically relevant portion of this LHRH-DA may be located within the LHRH hypophysiotropic pathway. This suggests an anatomic locus for a possible physiologic interaction between LHRH and LHRH-DA.