Abstract
Experiments were carried out to examine whether γ-aminobutyric acid (GABA) receptor mechanisms participate in the release of serotonin (5-hydroxytryptamine, 5-HT) in the subfornical organ (SFO) using microdialysis methods. Perfusion of the GABA receptor antagonists as well as agonists was performed in the region of the SFO through a microdialysis probe and extracellular concentrations of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) were measured in freely moving rats. Perfusion of the GABAA receptor antagonist bicuculline (10 and 50 μM), but not the GABAB receptor antagonist phaclofen (10 and 50 μM), enhanced dialysate 5-HT and 5-HIAA concentrations in the SFO area, suggesting that the GABAergic system may tonically inhibit the 5-HT release in the SFO area through GABAA receptors. Higher perfusion of the GABAA receptor agonist muscimol (50 μM) or the GABAB receptor agonist baclofen (250 μM) decreased the 5-HT and 5-HIAA concentrations. Hypovolemia caused by subcutaneous injection of polyethylene glycol (PEG, 30%, 5 ml) increased the 5-HT and 5-HIAA concentrations in the SFO area, and the increase in the 5-HT and 5-HIAA levels was reduced by perfusion of muscimol (10 μM), but not by baclofen (50 μM). These results show the involvement of both GABAA and GABAB receptors in the control of 5-HT release in the SFO area, and imply that the GABAA receptor mechanism may be importance for the serotonergic regulatory system of fluid balance. [Jpn J Physiol 54 Suppl:S212 (2004)]
