In-vitro pulsatile luteinizing hormone-releasing hormone output is dependent on hypothalamic region and the stage of the estrous cycle

In the following experiments, the role of the preoptic-suprachiasmatic area (POA-SCN) in the control of luteinizing hormone-releasing (LHRH) release was examined by in vitro superfusion of either mediobasal hypothalamus (MBH) or MBH-POA-SCN fragments obtained from cycling rats killed on various days of the estrous cycle. The lowest level of LHRH output occurred during estrus, highest levels during diestrus, and intermediate levels on proestrus in the MBH-POA-SCN preparation. As expected, significant decreases in LHRH output from the MBH alone occurred during both days of diestrus and on proestrus, as compared to output from the MBH-POA-SCN tissue, since this structure contains most of the LHRH perikarya. However, similar LHRH secretion patterns were detected in estrus from both preparations. The average period of the LHRH pulses for the estrous cycle in the MBH-POA-SCN was 30.9 +/- 1.2 min compared to 97.7 +/- 25.1 min in the MBH, with significant differences occurring on diestrus 2. The increase and extreme variability of the period of LHRH pulses in the MBH region, compared to the MBH-POA-SCN region, suggests that it is the latter region that contains the neural circuits that control the LHRH pulse generator. The LHRH pulse amplitudes from both hypothalamic regions were similar during all phases of the estrous cycle, except diestrus 2, when the LHRH pulse amplitude from the MBH region was significantly lower than the LHRH pulse amplitude from MBH-POA-SCN. The percentage of LHRH released in the MBH did not vary with the estrous cycle, however, in the MBH-POA-SCN significant changes were observed.(ABSTRACT TRUNCATED AT 250 WORDS)