Cooperative synthesis of dopamine in rat mediobasal hypothalamus as a compensatory mechanism in hyperprolactinemia

Abstract

Dopamine (DA), synthesized in the mediobasal hypothalamus by dopaminergic neurons containing two enzymes of DA synthesis - tyrosine hydroxylase and decarboxylase of aromatic L-amino acids, or by monoenzymatic non-dopaminergic neurons containing one DA synthesis enzyme in cooperation, is known to have an inhibitory effect on prolactin secretion. Deterioration of this inhibitory control leads to an increase in prolactin concentration in the blood and to the development of hyperprolactinemia syndrome. In a rat model of hyperprolactinemia induced by administration of a neurotoxin causing degeneration of dopaminergic and noradrenergic neurons, the level of DA first decreases, leading to an increase in prolactin level (decompensation stage), while later both levels are restored to normal (compensation stage). However, the mechanism of such compensation is still not clear. The aim of the present study was to analyze whether the increase in cooperative synthesis of DA by monoenzymatic neurons during hyperprolactinemia is a manifestation of a compensatory mechanism representing a particular case of neuroplasticity. The level of cooperative synthesis in the hyperprolactinemia model and in the control was estimated as the level of synthesis of DA and L-dihydroxyphenylalanine (L-DOPA) - an intermediate product of DA synthesis, when L-DOPA transfer from neurons containing tyrosine hydroxylase into neurons containing aromatic L-amino acid decarboxylase is inhibited. The level of DA synthesis during the decompensation stage was not changed, while during the compensation stage it was lower than the control. Along with a reduction in DA level, during the compensation stage an increase in the extracellular L-DOPA level in the medium was detected. Thus, the compensation of DA deficiency after degeneration of dopaminergic neurons in the mediobasal hypothalamus is due to the increase in cooperative synthesis of DA by monoenzymatic neurons containing one of the complementary enzymes of the DA synthesis pathway.

Tyrosine Hydroxylase Neurons Regulate Growth Hormone Secretion via Short-Loop Negative Feedback

“…However, some TH neurons in the hypothalamus possibly produce L-DOPA as an end-product rather than dopamine (Meister et al, 1988;Komori et al, 1991). Although their function is not well understood, there is evidence of cooperative synthesis of dopamine by different neurons that express complementary enzymes of the pathway for dopamine biosynthesis (Ugrumov et al, 2002;Kurina et al, 2017). Thus, the exact neurotransmitters used by these TH 1 / DAT À /DBH À neurons to regulate GH secretion still need to be identified.…”

Efficacy of Combined Cabergoline and Metformin Compared to Metformin Alone on Cycle Regularity in Patients with Polycystic Ovarian Disease with Hyperprolactinemia: A Randomized Clinical Trial

“…Dopamine released from the hypothalamus inhibits prolactin secretion, and it also affects the secretion of gonadotropins. When this inhibitory effect of dopamine is reduced, prolactin secretion will increase in addition to abnormalities in gonadotropins including luteinizing hormone (LH) [3].…”