Effects of drugs which modify catecholamine activity on amplification of LH release induced by locus coeruleus...

Norepinephrine may be the neurotransmitter responsible for triggering the surge release of luteinizing hormone (LH) in proestrous and steroid-treated ovariectomized rats. In a previous study, we electrically stimulated (ES) the locus coeruleus (LC), a norepinephrine cell body region from which some axons project to the hypothalamus. These studies were performed in estradiol-treated rats in which the medial preoptic nucleus (MPN) initially had been electrochemically stimulated (ECS) to partially depolarize luteinizing hormone-releasing hormone (LH-RH) neurons. We observed that LC-ES markedly amplified the amount of LH released after preoptic-ECS. The purpose of the present study was to determine if these amplifying effects of LC-ES were due to the release of a catecholamine and if so which neurotransmitter was involved in this response. All rats in this study were ovariectomized and 7 days later (day 0) they were treated with estradiol for 2 days (day 2). On day 2, all animals were anesthetized with chloral hydrate. In control rats, the MPN was electrochemically stimulated (100 microA/60 s DC) and 30 min later the LC was electrically stimulated for 15 min. MPN-ECS significantly increased plasma concentrations of LH and LC-ES markedly amplified these plasma LH levels. Thereafter, 3 studies were performed and the following drugs were given: (1) alpha-methyl-p-tyrosine (MPT, 300 mg/kg i.p.); (2) phenoxybenzamine (PX, 15 mg/kg i.p.) and (3) propranolol (PROP, 15 mg/kg i.p.). MPT and PROP were administered 15 min prior to MPN-ECS and PX was given 90 min before MPN-ECS. None of these drugs affected the amount of LH released after preoptic stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)