CHARACTERIZATION OF TRITIATED NORADRENALINE RELEASE FROM THE RAT PREOPTIC AREA WITH MICRODIALYSIS INVIVO

Present techniques are unable to provide a sensitive and accurate inde x of noradrenergic activity in the rat preoptic area. In this study, w e have examined the brainstem A1 noradrenergic input to the preoptic a rea using a new technique whereby [H-3]noradrenaline is preloaded into the preoptic area and release of radioactivity from this region is me asured subsequently using microdialysis in vivo. Electrical stimulatio n of the ipsilateral A1 area for 20 min at 5, 10, and 15 Hz evoked sig nificant increases in dialysate radioactivity that were repeatable and frequency-dependent. After removal of calcium from the perfusion medi um, basal release of radioactivity was markedly reduced and the effect of A1 stimulation abolished. Changing to a 100 mM K+ medium evoked an increase in the release of radioactivity that was sixfold greater tha n that seen after A1 stimulation. Separation of the dialysate with HPL C showed that 33% of the increase in measured radioactivity after A1 s timulation was directly attributable to [H-3]noradrenaline and the rem ainder to the metabolites vanillylmandelic acid, 3,4-dihydroxymandelic acid, and 3,4-dihydroxyphenylglycol. In contrast, the increase in rad ioactivity after K+ depolarization was due almost completely to [H-3]n oradrenaline. Addition of 10 muM clonidine to the perfusion medium mar kedly reduced basal release of radioactivity, but had no effect on evo ked release following A1 stimulation. Conversely, perfusion with 10 mu M yohimbine had no effect on basal release, but significantly increase d evoked release after A1 stimulation. These results now provide a cha racterization of noradrenergic activity in the preoptic area and indic ate the importance of the A1 noradrenergic input to this region. The t echnique of measuring radioactivity with microdialysis after preloadin g with [H-3]noradrenaline provides a relatively simple, sensitive inde x of noradrenergic activity in vivo with good temporal resolution.