Regulation of norepinephrine release from the rat bed nucleus of the stria terminalis: in vivo microdialysis studies.

The microdialysis technique was used to study the in vivo extracellular levels of norepinephrine in the bed nucleus of the stria terminalis. A basal level of 2.34 +/-0.25 fmol/microl of norepinephrine was observed. Desipramine (2 and 10 microM), a norepinephrine uptake blocker, significantly increased extracellular levels of norepinephrine. Reversed perfusion with high potassium in the presence of 2 microM desipramine induced the release of norepinephrine. Instead, in the presence of 10 microM desipramine, a significant decrease in the induced release of norepinephrine was observed. Clonidine, an alpha2-adrenergic agonist, significantly decreased basal extracellular levels of norepinephrine and the K+-induced release of norepinephrine. In contrast, yohimbine and RX821002, two alpha2-adrenergic antagonists, significantly increased basal extracellular levels of norepinephrine but not the release of norepinephrine induced by 70 mM K+. Perfusion of tetrodotoxin through the probe located in the bed nucleus of the stria terminalis significantly decreased both the basal extracellular level and the K+-induced release of norepinephrine. Furthermore, perfusion of tetrodotoxin through a microdialysis probe implanted in the medial forebrain bundle also decreased basal extracellular levels of norepinephrine in the bed nucleus of the stria terminalis. The results show that in vivo there is a significant noradrenergic tonic activity in the bed nucleus of the stria terminalis. This tonic activity depends on the impulse flow through medial forebrain bundle nerve fibers. Under these conditions, extracellular levels of norepinephrine in the bed nucleus of the stria terminalis are regulated by the magnitude of norepinephrine uptake and by presynaptic alpha2-adrenergic receptors.