Third ventricular injection of alpha-bungarotoxin decreases pulsatile luteinizing hormone secretion in the...

The injection of purified alpha-bungarotoxin (alpha-BTX), an antagonist of the muscle nicotinic cholinergic receptors, into the third ventricle (3rd V) of ovariectomized rats decreased the frequency of pulses of luteinizing hormone (LH) but had no significant effect on the amplitude or nadir of LH secretion. Approximately 24 h after the 3rd V injection of alpha-BTX, the pulsatile secretion rate of LH was increased and thus the alpha-BTX effect was reversible. The 3rd V injection of the two nicotinic cholinergic agents, cytisine (10 micrograms) and dihydro-beta-erythroidine, also decreased the frequency of pulses of LH. Cytisine and dihydro-beta-erythroidine inhibited the binding of (125I)alpha-BTX to rat hypothalamic synaptosomes (P2B) and also to frozen coronal sections of the hypothalamus as studied by film autoradiography. Injection of a lower dose of cytisine (1 micrograms) had no effect on LH secretion, but when administered with alpha-BTX into the 3rd V, cytisine significantly reduced the alpha-BTX-induced decreased LH secretion. The injection of cytisine with (125I)alpha-BTX into the 3rd V also decreased in vivo labeling of the (125I)alpha-BTX-binding sites in the hypothalamus of ovariectomized rats. The results suggest that alpha-BTX can inhibit the pulsatile secretion of LH in ovariectomized rats and this inhibition may be mediated by specific subclass of a CNS (central nervous system) nicotinic acetylcholine receptor.