Locus coeruleus (LC) stimulation augments LHRH release induced by medial preoptic stimulation. Evidence that the major LC stimulatory component enters contralaterally into the hypothalamus

Abstract

Previous studies by others suggest that mid- and hindbrain noradrenergic projections to the hypothalamus may be stimulatory or inhibitory to luteinizing hormone releasing hormone (LHRH) release depending upon the steroid environment of the rat. In the present study we reevaluated the effects of electrical stimulation of the locus coeruleus (LC) in estrogen-primed ovariectomized (OVX) rats anesthetized with chloral hydrate. This anesthetic agent blocked the spontaneous luteinizing hormone (LH) surges which normally occur in estrogen-treated OVX rats. In such rats, bilateral LC electrical stimulation was ineffective in altering basal LH concentrations. Thereafter, we evaluated the effects of LC electrical stimulation on patterns and concentrations of plasma LH induced by electrochemical stimulation (ECS) of the medial preoptic nuclei (MPN). Bilateral MPN-ECS induced a significant rise in plasma LH. When the LC was bilaterally stimulated for 15 min beginning 30 min after MPN stimulation, peak LH concentrations were significantly augmented and remained elevated throughout the experiment. To learn more of how LC noradrenergic projections reach regions of the hypothalamus which contain LHRH neurons, the right MPN in a group of rats was unilaterally ECS and 30 min later the right LC was unilaterally stimulated for 15 min. Plasma LH levels increased after MPN activation but no further rise or fall in LH occurred after ipsilateral LC stimulation. In the final group of rats, the right MPN was unilaterally electrochemically stimulated and this was followed 30 min later by contralateral electrical stimulation of the left LC. In these animals, peak plasma LH concentrations were significantly elevated above those obtained after only unilateral MPN stimulation. These data demonstrated that LC stimulation augments LH release but only after preliminary depolarization of LHRH neurons by ECS. Moreover, the stimulatory noradrenergic projections from LC decussate to enter contralateral hypothalamic regions containing LHRH neurons.