GABA-mediated attenuation of noradrenaline release in the rat median preoptic area caused by intravenous injection of metaraminol

Previous studies have shown that the noradrenergic system in the median preoptic nucleus (MnPO) play an important role in the control of the body fluid balance and cardiovascular function and that the release of noradrenaline in the MnPO is regulated by γ-aminobutyric acid (GABA) receptor mechanisms. The present study was carried out to examine whether the GABAergic system is involved in the modulation of the noradrenaline release in the MnPO in response to an elevation in blood pressure using in vivo microdialysis techniques. In urethane-anaesthetised male rats, the rise in arterial pressure caused by intravenous administration of the α-agonist metaraminol significantly decreased dialysate noradrenaline concentration in the MnPO area. The decrease in the noradrenaline level elicited by the metaraminol administration was significantly attenuated by perfusion with either bicuculline (10 μM), a GABAA receptor antagonist, or phaclofen (10 μM), a GABAB receptor antagonist, through a microdialysis probe. The amount of the antagonist-induced attenuation was much greater in the bicuculline-treated group than in the phaclofen-treated group. These results suggest that the release of noradrenaline in the MnPO area may be modulated by neural inputs from the peripheral baroreceptors, and that the neural inputs may be mediated in part through GABAA receptors rather than GABAB receptors in the MnPO area.