Ontogenic changes in the hypothalamic levels of amino acid neurotransmitters in the female rat

Abstract

In order to evaluate the participation of several amino acid neurotransmitters (AANT) in sexual maturation we measured the hypothalamic concentrations of aspartate (Asp), glutamate (Glu), homocysteic acid (HCA), glycine (Gly), taurine (Tau) and γ-aminobutyric acid (GABA) in female rats at different ages of sexual development. Animals (15-, 20-, 25-, 30-, 35- and 40-day-old female rats, as well as adult diestrous rats, n = 8–10/group) were decapitated at noon; each brain was rapidly removed and frozen on dry ice. Preoptic area (POA) and mediobasal hypothalamic (MBH) needle punch samples were obtained from 500–600 μm thick coronal slices, homogenized and centrifuged. AANT concentrations were measured in the supernatants following derivatization with phenyl isothiocyanate and reverse-phase HPLC separation by UV detection. Significant ( P < 0.05) changes in hypothalamic AANT concentrations can be summarized as follows: POA Glu, HCA and Gly levels rose transiently at day 20 and then increased steadily in 35- and 40-day-old rats. Asp concentrations rose in 35- and 40-day-old rats. Tau concentrations decreased markedly from day 30 onwards. GABA levels were lowest in 15-day-old rats, as compared to the other age groups. Asp, Glu, Gly and GABA concentrations were highest in adult diestrous rats. Changes in MBH AANT levels were not as relevant: only Gly concentrations rose significantly at day 40; Asp, Glu, HCA, Gly and GABA concentrations increased in adult diestrous rats and Tau concentrations were lower from day 35 onwards. It is concluded that increased levels of excitatory amino acids (Asp, Glu and HCA) could contribute to sexual maturation as a result of their synaptogenic properties and/or acting directly on the LHRH neurons, stimulating LHRH release. The striking decrease of hypothalamic Tau concentrations in peripubertal rats suggests a possible role of this amino acid in the neuroendocrine events leading to sexual maturation.