Pulsatile luteinizing hormone (LH) patterns in ovariectomized rats: involvement of norepinephrine and dopamine in the release of LH-releasing hormone and LH.

Cannulated ovariectomized (OVX) rats were bled every 10 min for 2 h to characterize their individual patterns of LH release. After the last sample, all rats were decapitated, and their brains were removed for analysis of LHRH, norepinephrine (NE), and dopamine (DA) levels in median eminence (ME), arcuate-ventromedial, and suprachiasmatic-medial preoptic region (Sch-PO) to determine if changes in NE, DA, and LHRH levels in any of those areas could be observed at different points during the pulsatile release of LH. The results showed that when LH levels started to increase, NE levels peaked in the Sch-PO, whereas a sharp drop in DA and LHRH levels in the ME was detected, which may reflect an acute release in the ME of both DA and LHRH. When LH levels reached peak values, NE levels in the Sch-PO returned to lower values, and DA and LHRH in the ME rose to higher levels. Inhibition of NE synthesis with diethyldithiocarbamate resulted in the suppression of LH pulses. L-Dopa administered after diethyldithiocarbamate induced an increased release of LH in both OVX and OVX, estrogen-primed rats with a simultaneous inhibition of PRL release. Peak levels of LH after L-dopa treatment coincided with increased DA levels in the ME, no change in NE, and a clear drop in LHRH. The results suggest that both NE and DA are involved in the pulsatile release of LH in OVX rats.