MICROINJECTION OF LHRH AND ITS ANTAGONISTIC ANALOG INTO THE MEDIAL PREOPTIC AREA DOES NOT AFFECT PULSATILE SECRETION OF LH IN OVARIECTOMIZED RATS

To gain a better understanding of the existence of an ultra-short feed back mechanism controlling LHRH secretion in the medial preoptic area (MPO), we examined the effects of microinjections of LHRH or the LHRH antagonist [D-pGlu(1), D-Phe(2), D-Try(3,6)] into the MPO on pulsatile secretion of LH in ovariectomized rats and on the surge of LH secreti on in proestrous rats. Neither the injection of 10 ng LHRH nor 100 ng its antagonist into the MPO had any effect on pulse frequency or the m ean LH concentration in ovariectomized rats. The injection of 100 ng L HRH antagonist or 10 ng LHRH delayed or advanced, respectively, the LH surge in proestrous rats. Taking these results together with our prev ious report, the present study indicates that 1) endogenous LHRH in th e MPO is involved in the ultra-short feedback regulation of LHRH relea se and 2) the ultra-short positive feedback mechanism in the MPO acts at the time of the proestrous LH surge but not under a hormonal milieu as in ovariectomized rats.