Spontaneous changes in LHRH release during the rat estrous cycle, as measured with repetitive push-pull perfusions of the pituitary gland in the same female rats

In the present study, we examined the in vivo release profiles of luteinizing hormone-releasing hormone (LHRH) during the rat estrous cycle by utilizing repetitive push-pull perfusions of the anterior pituitary in the same freely behaving animal. Throughout the estrous cycle, LHRH levels were higher than those reported from previous work using a push-pull cannula placed in the mediobasal hypothalamus. A significant increase in LHRH input to the anterior pituitary was obtained in proestrus (Pro), though the magnitude of increment was different among animals. This LHRH increment in Pro was attributable to a significant increase in the amplitude of LHRH pulses. Interestingly, no changes were observed in the frequency of LHRH pulses (about 1 pulse/50 min) throughout the estrous cycle. The data demonstrate that the anterior pituitary of female rats receives an increased amount of LHRH during Pro due to changes in the amplitude of the LHRH signal without changes in the frequency.