Noradrenaline inhibits glutamate release in the rat bed nucleus of the stria terminalis: in vivo microdialysis studies.

Abstract

The microdialysis technique was used to simultaneously study the in vivo extracellular levels of noradrenaline, glutamate, and gamma aminobutyric acid (GABA) in the bed nucleus of the stria terminalis in order to assess the regulation that noradrenaline may exert upon the release of amino acid neurotransmitters. Perfusion through the probe with UK14304, a selective alpha2-adrenergic agonist, produced a significant decrease of noradrenaline and glutamate extracellular levels. Perfusion through the probe with RX821002, a selective alpha2-adrenergic antagonist, produced a significant increase of noradrenaline and glutamate basal extracellular levels. Perfusion with prazosine, a selective alpha1-adrenergic antagonist, produced a significant decrease of noradrenaline basal extracellular levels without affecting glutamate levels. Under the same conditions, GABA basal extracellular levels were not changed in the presence of any of the alpha-adrenergic ligands studied. The perfusion of high potassium through the probe induced a significant Ca++dependent release of the three neurotransmitters; however, extracellular noradrenaline returned to normal levels even though potassium was still present. In addition, it was observed that alpha-adrenergic receptor ligands exerted differential effects upon K+-induced release of noradrenaline and glutamate. Perfusion with the nonselective alpha-adrenergic antagonist, phenoxybenzamine, presented a biphasic effect upon K+-induced release of noradrenaline; a significant decrease during the first 5 min of stimulation followed by a significant increase in the next 5 min of stimulation. Perfusion with RX821002 produced a significant increase in K+-induced release of noradrenaline that returned to normal basal values before the end of the stimulation period. In contrast, local perfusion with prazosine caused a significant decrease of K+-induced noradrenaline release. In the case of glutamate, perfusion through the probe with phenoxybenzamine produced a significant increase in K+-induced release of glutamate. In addition, RX821002 and prazosine produced a significant increase in K+-induced release of glutamate. Perfusion through the probe with UK14304 produced a significant decrease of both noradrenaline and glutamate K+-induced release. The present results show that noradrenaline in the bed nucleus of stria terminalis exerts a significant inhibition over its own release through alpha2-adrenergic receptors and over glutamate release mainly through alpha2-adrenergic receptors. Thus, the results suggest that noradrenaline in the bed nucleus of the stria terminalis maintains an inhibitory tone over the information flow mediated by glutamate.