Functional organization of the catecholaminergic neural systems inhibiting luteinizing hormone secretion in anestrous ewes.

Both dopaminergic (DA) and noradrenergic (NA) neural systems contribute to the suppression of luteinizing hormone (LH) pulse frequency in intact anestrous ewes. In this study, we explored the functional relationship between these two inhibitory neural systems by determining if DA or NA antagonists could block the inhibitory actions of DA and NA agonists in ovariectomized anestrous ewes. If these systems are linked 'in series' with one (e.g. NA) exerting its effects by stimulating the other (e.g. DA), then one antagonist (e.g. DA) should block the inhibitory actions of both agonists. If they are organized 'in parallel', then each antagonist should only block the actions of the homologous agonist. In the first experiment, the DA agonist, apomorphine, suppressed LH pulse frequency and this action was blocked by the DA antagonist, pimozide, but not by the NA antagonist, phenoxybenzamine. Similarly, in experiment 2, the ability of the NA agonist, clonidine, to suppress LH pulse frequency was blocked by the NA antagonist. However, the DA antagonist also partially blocked the inhibitory effects of this NA agonist. We next repeated experiment 2 using a lower dose of the NA agonist. The lower dose of clonidine again inhibited LH pulse frequency and this effect was completely blocked by the DA antagonist. These results suggest that the NA agonist suppressed LH pulse frequency by stimulating a set of inhibitory DA neurons and are thus compatible with the hypothesis that the catecholaminergic neurons controlling LH secretion in anestrous ewes are organized 'in series' with the NA neurons stimulating a DA neural system that directly suppresses GnRH release.