Peptidase Activity in the Hypothalamus and Pituitary of the Rat: Fluctuations and Possible Regulatory Role of Luteinizing Hormone Releasing Hormone-Degrading Activity During the Estrous Cycle

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Abstract
Peptidase activity capable of inactivating luteinizing hormone (LHRH) may have a physiological role in partially determining hypothalamic LHRH levels as well as LHRH levels at the gonadotrope. Use of the synthetic substrate leucine-p-nitroanilide (Leu-p-NA) to assay LHRH-degradative activity was validated previously by several methods. Peptidase activity in the hypothalamus and pituitary throughout the rat 4-day estrous cycle was monitored. Activity in both tissues was significantly decreased during proestrus and diestrus I. The proestrous reduction in peptidase activity represents a permissive period necessary for the induction of the LHRH and LH surges. The decreased degradative activity in the pituitary on diestrus I may be involved in inducing the pituitary LHRH receptors which are reportedly synthesized prior to proestrus. The peptidase exhibits positive cooperativity with Leu-p-NA, and the degree of this cooperativity also fluctuates during the estrous cycle. Estradiol and progesterone given alone or in combination to prepubertal castrate animals increased the activity of the hypothalamic peptidase in vitro. The degree of positive cooperativity with which the enzyme functioned was also apparently altered by these gonadal steroids.