Electrophysiologic correlates of steroid modulation of luteinizing hormone release

In both ovariectomized (OVX) and steroid-primed OVX freely moving rats, attempts were made to correlate the effects of intraventricular norepinephrine (NE) on multiunit activity (MUA) of different brain regions with NE-induced alterations in blood LH levels. MUA-recording electrodes were implanted in the diagonal band of Broca (DBB), medial preoptic area (MPOA), arcuate nucleus (ARC) and/or ventromedial hypothalamic nucleus (VMH). Steroid priming included 50 micrograms estradiol benzoate (EB) and 25 mg progesterone (P) 3 days prior to experiment. The unanesthetized animals were bled via indwelling atrial cannulas before and after intraventricular infusion of NE (10 micrograms in 2 microliters over 2 min). In OVX-primed rats NE lengthened the interval between episodic LH peaks and decreased mean blood LH levels. In contrast, in OVX-EBP-primed rats, NE stimulated an LH surge. Concurrent recording of MUA revealed that, in OVX-unprimed rats, NE dramatically depressed MUA in both DBB-MPO and ARC-VMH neurons. However, in OVX-EBP-primed rats, while still markedly inhibiting ARC-VMH units, NE failed to depress MUA recorded in DBB-MPO sites (some units were actually excited by NE), perhaps reflecting the higher ratio of LHRH neurons/inhibitory neurons in DBB-MPO.