Characterization of in vitro dopamine synthesis in the median eminence of rats with haloperidol-induced hyperprolactinemia and bromocriptine-induced hypoprolactinemia.

In order to investigate the mechanism of PRL action on dopamine synthesis in tuberoinfundibular dopaminergic (TIDA) neurons, in vitro dopamine synthesis in the median eminence of hypothalamic slices was compared between hyperprolactinemic and hypoprolactinemic rats, Hyper- and hypoprolactinemia were induced in ovariectomized rats by repetitive injections of the dopamine antagonist haloperidol (Halo) and the dopamine agonist bromocriptine (Bromo), respectively. In vitro dopamine synthesis in TIDA neurons was estimated by measuring 3,4-dihydroxyphenylalanine (DOPA) accumulated in the median eminence after incubation of hypothalamic slices with a DOPA decarboxylase inhibitor. Treatment with Halo or Bromo produced increases or decreases, respectively, in the concentration of PRL in serum and in in vivo DOPA accumulation in the median eminence, as compared with vehicle treatment. The basal rate of in vitro DOPA accumulation in the median eminence was increased in Halo-treated rats and was decreased in Bromo-treated rats. The increase in basal DOPA accumulation after Halo treatment was inhibited by Ca2+ removal from medium or tetrodotoxin addition. A CA2+ -dependent increase in DOPA accumulation in the median eminence by depolarization was greater in Halo-treated rats than in Bromo-treated rats. This difference in DOPA accumulation was due to the changes in PRL secretion after Halo and Bromo treatments, since hypophysectomy abolished it. Incubation of hypothalamic slices in Na+-free media to increase the intracellular concentration of Ca2+ through inhibition of Na+-Ca2+ exchange caused an increase in DOPA accumulation. The rate of DOPA accumulation in Na+-free media was increased in Halo-treated rats and was decreased in Bromo-treated rats. On the other hand, neither Halo nor Bromo treatment altered the increase in DOPA accumulation induced by (Bu)2cAMP or forskolin. These results support the view that PRL stimulates dopamine synthesis in TIDA neurons by mechanisms which include an increase in the firing rate of TIDA neurons and increased depolarization-induced synthesis due to an enhanced response of the component that regulates dopamine synthesis to intracellular Ca2+.