Endocrinology, Vol 122, 899-905, Copyright © 1988 by Endocrine Society

ARTICLES

Electrical stimulation of ascending noradrenergic tracts in the midbrain: suppression of steroid-induced luteinizing hormone release

H Bergen and PC Leung
Department of Obstetrics/Gynaecology, University of British Columbia, Vancouver, Canada.

This study examined the effects of electrical stimulation of the dorsal and ventral ascending noradrenergic tracts in the central nervous system [dorsal (DNT) and ventral (VNT) noradrenergic tracts] during an ongoing LH surge induced by ovarian steroids. A bipolar electrode was implanted in the midbrain of ovariectomized rats, in the region of either the DNT or VNT. One week later the rats were given estradiol benzoate (10 micrograms/100 g BW), followed 3 days later by an injection of progesterone (2 mg) at 1100 h. This steroid treatment resulted in a LH surge in the afternoon which peaked at approximately 1800 h. Biphasic electrical stimulation (50-100 microA; 400 musec; 30 Hz) was applied to the conscious unrestrained rats for 1 h beginning at 1600 h. Electrical stimulation of the DNA (n = 10) for 1 h resulted in significantly lower LH levels at 1700 h. Similar electrical stimulation of the VNT (n = 8) also markedly suppressed LH release at 1700 h. Electrical stimulation of brain sites close to but outside the DNT or VNT were ineffective. After the cessation of electrical stimulation, LH levels began to rise again and at 1900 h were not significantly different from levels in the control rats. To determine if the recovering LH surge could be interrupted further, the effects of a second electrical stimulating session, 2 h after the first one, was investigated in other groups of animals. As expected, electrical stimulation of either the DNT or VNT between 1600 and 1700 h resulted in a transient suppression of the steroid-induced LH surge. The mean LH levels of these animals were on the rise between 1700 and 1900 h. Dramatically, a second electrical stimulation applied to the DNT or VNT from 1900-2000 h again inhibited LH release. Thus, the LH release pattern in these twice-stimulated rats was significantly different from that in either the nonstimulated controls or animals that had received only one electrical stimulation session (at 1600 h). Treatment of the rats with an alpha-adrenergic blocker (phenoxybenzamine) reversed the inhibitory effect of DNT activation on the LH surge. Treatment with a beta-adrenergic blocker (propranolol) or a dopamine antagonist (pimozide) was ineffective. These data demonstrate that, similar to intraventricular infusion of norepinephrine, activation of the ascending norepinephrine fibers results in interruption of the steroid- induced LH surge.

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