LHRH injected into the medial preoptic area potentiates LH secretion in ovariectomized estrogen-primed and proestrous rats

To determine if luteinizing hormone releasing hormone (LHRH) might act within the central nervous system to modulate its own secretion, microinjection of LHRH into the medial preoptic area (MPO) was performed at 13:00 h in both the ovariectomized estrogen-primed and proestrous rat, and the effects on the afternoon rise in the serum concentration of luteinizing hormone (LH) were studied. Microinjection of 5 ng LHRH into the MPO in the ovariectomized estrogen-primed rat resulted in a significant increase in serum LH from 16:00 to 19:00 h compared to the value in the saline-injected rat, whereas the injection at a dose of 2 ng did not induce a significant change. The injection of 5 ng LHRH into the third ventricle in the ovariectomized estrogen-primed rat induced a transient increase in serum LH immediately after the injection, followed by recovery to the preinjection concentration by 1 h, indicating that the potentiating effect of 5 ng LHRH in the MPO was not due to the LHRH leaked into the third ventricle. Finally, microinjection of 5 ng LHRH in the proestrous rat induced an early increase in serum LH, resulting in an advancement of the peak of the afternoon rise by 1 h. It is suggested that LHRH secreted in the MPO may, directly or indirectly, stimulate its own secretion in the median eminence to produce the surge of LH secretion.