The regulation of compensatory dopamine synthesis in the arcuate nucleus of rats

Dopamine that is synthesized by neurons of the arcuate nucleus plays a key role in the regulation of the reproductive function of the body by inhibition of prolactin secretion. Earlier we showed that dopamine in the arcuate nucleus is synthesised not only by dopaminergic neurons that have enzymes for this but also neurons that express only one of the complementary enzymes, tyrosine hydroxylase or decarboxylase of aromatic L-amino acids. Here, we examined the original hypothesis that dopamine synthesis in mono-enzymatic neurons is under inhibitory control of noradrenergic afferents. We performed a comparison of the secretory activity of dopamine-synthesizing neurons of the arcuate nucleus in two groups of rats. In one group, we pharmacologically induced degeneration of the larger part of the dopaminergic neurons but saved noradrenergic afferentation of the arcuate nucleus. In another group, we induced degeneration of the same group of neurons and destroyed noradrenergic afferents. The data we obtained suggest that (i) noradrenergic innervation in the arctuate nucleus receive predominantly neurons that contain only one enzyme of dopamine synthesis, tyrosine hydroxylase and (ii) noradrenaline inhibits expression of tyrosine hydroxylase in mono-enzymatic neurons and the first stage of dopamine synthesis, i.e., conversion of L-tyrosine into L-dihydroxyphenylalanine in these neurons.