Abstract
Microdialysis was employed to investigate whether γ-aminobutyric acid (GABA) receptor mechanisms are involved in the regulation of noradrenaline (NA) release in the median preoptic nucleus (MnPO) in awake, freely moving rats. Perfusion with the GABA receptor antagonists as well as agonists was performed in the region of the MnPO through a microdialysis probe and dialysate levels of NA were measured. Perfusion with either bicuculline (10 and 50 μM), a GABA A receptor antagonist, or phaclofen (10 and 50 μM), a GABA B receptor antagonist, enhanced the release of NA in the MnPO area. Higher-dose perfusion with the GABA A agonist muscimol (50 μM) or the GABA B agonist baclofen (250 μM) decreased dialysate NA in the MnPO area. An iso-osmotic reduction of fluid volume following subcutaneous treatment with polyethylene glycol (PEG, 30%, 5 ml) significantly increased the NA level in the MnPO area. The increased levels of NA caused by the PEG treatment were attenuated by perfusion with muscimol (10 μM), but not by baclofen (50 μM). These results show the participation of both GABA A and GABA B receptors in the modulation of the release of NA in the MnPO area, and imply that the GABA A receptor mechanism may play an important role in the noradrenergic regulatory system of body fluid balance.
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