Blockade of the release of LH induced by pharmacological suppression of adrenergic inflow into the locus coeruleus.

Previous studies have indicated that stimulation of the locus coeruleus (LC) produced inhibition of the release of LH induced by stimulation of the medial preoptic area (mPOA) in ovariectomized, estrogen-primed rats. A similar response following the application of drugs into the LC was taken as an index of activation of the LC neurons. The injection of the phenylethanolamine N-methyltransferase (PNMT) blocking-agent, 2,3- dichloromethylbenzylamine ( DCMB , 0.5 micrograms) into the LC, 2 and 1 h before starting the mPOA stimulation greatly attenuated the induced release of LH. Injection of saline had no effect. The reduced release of LH in DCMB -treated rats was restored to normal by injecting clonidine (0.5 micrograms) into the LC immediately before the mPOA stimulus started, but not by injecting phenylephrine (0.5 micrograms) or saline (0.5 microliter). The release of LH induced by mPOA stimulation was also blocked in rats in which the alpha 2-antagonist, piperoxane (0.5 micrograms) was injected into the LC but not in those injected with the alpha 1-antagonist, phenoxybenzamine (0.5 micrograms) or the beta-antagonist, propranolol (0.5 micrograms). It is concluded that adrenergic afferents into the LC tonically inhibited the activity of LC neurons and that this effect is mediated by alpha 2-adrenoceptors. Interference with the synthesis of epinephrine or blockade of alpha 2-adrenoceptors resulted in activation of LC neurons and thereby in inhibition of LH release.