Regulation of luteinizing hormone-releasing hormone and luteinizing hormone secretion by hypothalamic amino acids

RESUMO

1. The present review discusses the proposed roles of the amino acids glutamate and GABA in the central regulation of luteinizing hormone-releasing hormone (LHRH) and in luteinizing hormone (LH) secretion. 2. Descriptions of the mechanisms of action of these neurotransmitters have focused on two diencephalic areas, namely, the preoptic-anterior hypothalamic area where the cell bodies of LHRH neurons are located, and the medial basal hypothalamus which contains the nerve endings of the LHRH system. Increasing endogenous GABA concentration by drugs, GABA agonists, or blockade of glutamatergic neurotransmission by selective antagonists in rats and non-human primates prevents ovulation and pulsatile LH release, and blunts the LH surges induced by estrogen or an estrogen-progesterone combination. In contrast, glutamate and different glutamate agonists such as NMDA, AMPA and kainate, can increase LHRH/LH secretion. 3. The simultaneous enhancement of glutamatergic activity and a decrease of GABAergic tone may positively influence the maturation of the pituitary-gonadal system in rats and non-human primates. Administration of glutamate receptor agonists has been shown to significantly advance the onset of puberty. Conversely, glutamate antagonists or increased endogenous GABA levels may delay the onset of puberty. The physiological regulation of LHRH/LH secretion may thus involve a GABA-glutamate interaction and a cooperative action of the various types of ionotropic glutamate receptors. 4. The inhibitory actions of GABA on LH release and ovulation may be exerted at the level of afferent nerve terminals that regulate LHRH secretion. A likely candidate is noradrenaline, as suggested by the synaptic connections between noradrenergic nerve terminals and GABAergic interneurons in the preoptic area. Recent experiments have provided complementary evidence for the physiological balance between inhibitory and excitatory transmission resulting in modulation of the action of noradrenaline to evoke LHRH release.