Patterns of pituitary LH release during sustained stimulation by synthetic LED in cycling female rats
Lu, K.H.; Simpkins, J.W.; Chen, H.T.; Meites, J.
Materia Medica Polona. Polish Journal of Medicine and Pharmacy 12(1-2): 63-69
1980
ISSN/ISBN: 0025-5246 PMID: 7035764 Document Number: 166288
In an attempt to mimic the pattern of pituitary LH release during the pro-oestrous surge, rats were given 5 or 6 consecutive subcutaneous injections of LHRH, the 1st of 100 and the rest of 50 ng/100 g body weight, 30 or 35 min apart, on the early afternoon of pro-oestrus and/or at other stages of the cycle. Females showed the greatest LH response when oestrogen secretion was maximal on the day of pro-oestrus. On day 2 of di-oestrus, rats showed less LH response than on the day of pro-oestrus, but significantly more than on di-oestrous day 1. In contrast, on the day of oestrus rats showed only a small LH response to LHRH injections. The role of oestrogen in the pituitary LH response to LHRH was also demonstrated in neonatally androgenised females , which showed a similar or identical pattern of LH release to that of cycling females on the day of pro-oestrus. The 2nd and 3rd injections of LHRH produced much greater increases in serum LH than the 1st injection in pro-oestrous rats or on di-oestrous days 1 and 2, but not on oestrus. Although the precise mechanism of the self-priming effect of LHRH on LH release is not readily apparent, results indicate that initial stimulation by LHRH changed intra-pituitary dynamics so that more LH was released in response to subsequent LHRH stimulation. These results indicate that the expression of the priming effect of LHRH is mainly determined by the amount of releasable LH in the pituitary subsequent to oestrogen stimulation during the oestrous cycle. Rats on the early afternoon of pro-oestrus showed great increases in serum LH in response to the 1st, 2nd and 3rd injections of LHRH, but LH release declined progressively after the 4th or 5th injection. Acute ovariectomy only slightly changed the pattern of LH response to LHRH, but did not prevent the fall in LH release in pro-oestrous rats. Sustained LH release by consecutive injections of LHRH on the early afternoon of pro-oestrus resulted in a 33% reduction of total LH content in the pituitary. A similar decrease in LH content was also seen in rats after the spontaneous LH surge on the late afternoon of pro-oestrus. These results suggest that the decline in LH release during sustained stimulation by LHRH is due mainly to a decrease in releasable LH in the pituitary.