Decrease in noradrenergic activity in hypothalamic nuclei during the development of spontaneous hypertension

The results of two sets of experiments are reported. In the first set the in vivo accumulation of [ 3 H]noradrenaline from [ 3 H]tyrosine was measured in various brain regions of spontaneously hypertensive rats (SH-rats) and their normotensive controls of the Wistar-Kyoto strain (WK-rats) at the age of 3, 7 and 11 weeks. No differences were observed in [ 3 H]noradrenaline accumulation in any of these regions between WK- and SH-rats. In several brain regions of SH-rats, however, the tyrosine concentration, the amount of [ 3 H]tyrosine taken up and the specific activity of [ 3 H]tyrosine were found to be higher than in the corresponding regions of the brains of WK-rats. In the second set of experiments we measured the noradrenaline concentration and the α-MPT-induced noradrenaline disappearance in several nuclei of the hypothalamus and the medulla oblongata of SH- and WK-rats at 3, 7 and 10 weeks after birth. A decreased α-MPT-induced noradrenaline disappearance was found in the paraventricular nucleus, the periventricular nucleus and the anterior hypothalamic nucleus of 3-week-old SH-rats compared to that of WK-rats of the same age. No significant differences were found in this parameter of 7-week-old SH- and WK-rats. A lowered noradrenaline disappearance was evident in the anterior hypothalamic nucleus and in the nucleus commissuralis of the medulla oblongata of 10-week-old SH-rats. At the age of 10 weeks the noradrenaline concentration in the anterior hypothalamic nucleus of SH-rats was significantly lower than in that of WK-rats. These findings are indicative of a transient decrease in the activity of noradrenergic neurons in the anterior hypothalamus during the onset of the development of the hypertension. This is compatible with the transient increase in sympathetic activity which, according to several authors, plays an important role in the onset of the hypertension in the SH-rats.