Luteinizing hormone releasing activity in the hypothalamus of anestrous and cyclic ewes.

Abstract

To investigate the anestrous phase of the ewe correlated with an increasing photoperiod the level of luteinizing hormone releasing factor (LRF) of the hypothalamus of anestrous and estrous ewes was compared. LRF content of the extracts of the stalk median eminence (SME) was measured by quantitating the release of LH from rat anterior pituitary glands in vitro. Experiments were conducted in a balanced incomplete block design. LH was assayed by the 00 radioimmunoassay for rat LH. In one experiment to test relationship between the dose of hypathalamic extract and in vitro release of LH extracts from the SME of 2 ewes killed on Day 14 of the estrous cycle 22 and 44 mg equivalents per incubation flask gave significantly higher responses the control and 11 mg eq (p<.05 in each case). The specificity of the SME extract was demonstrated by the lack of response elicited by equivalent weights of cerebral cortex from anestrous ewes and by synthetic vasopressin. Levels of LRF activity (25 mg eq SME) were low during the early part of the estrous cycle rose to a high in midcycle on Day 9 or 11 then at proestrous (3.20 plus or minus .26 mcg LH). Levels during all months of anestrous were significantly higher (p<.01) than on Day 13 of the estrous cycle. Levels during July and August were higher than on Day 11 and levels during May and June were no different than on Day 11. Variation in levels of LRF on Days 5.9 and 11 may be related to pooling of only a few SME extracts in some cases from different ewes. The absence of signals for release of LRF is indicated.