Role of Central Nervous System-Derived or Circulating y-Aminobutyric Acid on Prolactin Secretion in the Rat

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Abstract
To investigate the respective role in PRL secretion of γ-aminobutyric acid (GABA), either derived from the central nervous system or circulating in plasma, experiments were performed using ethanolamine-O-sulfate (EOS), a specific inhibitor of GABA catabolism. Intracerebroventricular injection of EOS (2 mg/kg) induced in unanesthetized male rats 2-8 h post injection a clear-cut rise in hypothalamic, anterior pituitary (AP), and plasma GABA concentrations. Rises in GABA titers occurred earlier in the hypothalamus and AP (2 h) than in the plasma (4 h). Concomitant to alterations of GABA, there was a striking lowering of plasma PRL evident at 2 h and still present 24 h after EOS administration. In contrast, systemic administration of graded doses of EOS (200–400 mg/kg, iv) did not induce significant changes in plasma GABA concentrations 4 h post injection; only the 600 mg/kg dose of EOS increased GABA concentrations 4 h post injection in the hypothalamo-AP system and decreased plasma PRL concentrations. Finally, in hypophysectomized rats bearing ecotopic pituitaries, despite the occurrence of rises in the hypothalamic GABA after intracerebroventricular or systemic (600 mg/kg) administration of EOS, AP, plasma GABA, and plasma PRL concentrations were not altered. In all, these findings indicate that: 1) changes in plasma PRL are best correlated to variations in the amino acid titers occurring in the hypothalamo-AP system; and 2) circulating GABA does not play a functional role in the control of PRL secretion. Finally, since alterations in blood GABA levels after central or systemic administration of EOS appear to reflect primary changes occurring in the brain concentration of the amino acid, circulating GABA may be a reliable indicator of central nervous system GABAergic function.