Luteinizing Hormone-Releasing Hormone Peptidase Activities in Discrete Hypothalamic Regions and Anterior Pituitary of the Rat: Apparent Regulation during the Prepubertal Period and First Estrous Cycle at Puberty*

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Abstract
Presently, two peptidase activities degrading LHRH have been observed in rat hypothalamic tissue: an endopeptidase and postproline cleaving enzyme (PPCE). Since no clear evidence for their physiological role in the onset of puberty has been presented, we examined the relationship between the content and degradation of LHRH by the median eminence (ME) and that by the ventromedial preoptic area (POA) during the prepubertal period and during the first estrous cycle at puberty. LHRH degradation by anterior pituitary during this period was also determined. Using kinetically optimized conditions, total LHRH degradation was measured by high performance liquid chromatographic estimation of the loss of synthetic LHRH catalyzed by low speed tissue supernatants, and PPCE activity was measured using a fluorogenic substrate (carbobenzoxy-1-glycl-1-prolyl-4- methylcourmarine-7-amide). The ME and POA LHRH content and serum levels of LH were estimated by RIA. During the prepubertal period, in animals killed between 22 and 30 days of age, the total LHRH-degrading activity in the ME remained constant (50 pmol LHRH degraded/μg protein- 30 min), while the LHRH content increased steadily. During the pubertal period (first estrous cycle), a high correlation between LHRH degradation and LHRH content in the ME was observed (r = 0.95). In animals killed between 34 and 38 days of age, total LHRH degradation increased from basal levels in anestrous animals to higher levels in early proestrous rats, reaching the highest value on the morning of late proestrous (300% increase). After the morning of late proestrus, LHRH degradation decreased (by 1300 h; P < 0.01) to reach prepubertal values on the first estrus, the following day. On diestrous day 1, both total LHRH degradation and LHRH content returned to early pubertal values. Cleavage at Pro9-Gly10 of LHRH (PPCE) was unchanged throughout the period studied. Both total LHRH degradation and postproline degrading activity on LHRH showed only minor fluctuations during the first estrous cycle in POA and anterior pituitary samples. The data suggest that LHRH degradation may contribute to the regulation of LHRH levels appropriate for gonadotropin release during the onset of puberty.