Decreased norepinephrine content in the medulla oblongata in severely hypertensive rats.

1. To clarify possible abnormalities in catecholamines in the medulla oblongata in relation to severe hypertension, the authors measured changes in catecholamine levels in the medulla oblongata of malignant stroke-prone spontaneously hypertensive rats (M-SHRSP). Effects of the adrenal medullae and peripheral nerves were ruled out by adrenal demedullation and chemical sympathectomy. 2. The level of norepinephrine in the medulla oblongata was significantly lower in untreated M-SHRSP than in untreated WKY (control) rats at 10 weeks of age. Further, it was significantly lower in treated M-SHRSP than in the treated WKY group at both 6 and 10 weeks of age. The level of epinephrine in 6 week old treated M-SHRSP was significantly higher than that in age-matched treated WKY, but no other differences were observed in terms of epinephrine content. There were no age- or treatment-related differences in dopamine levels in the medullar oblongata. 3. Since norepinephrine has an inhibitory effect on blood pressure elevation in the nucleus tractus solitarii (NTS) in the medulla oblongata, the suppression of negative feedback due to a decrease in the activity of inhibitory neurons in the medulla oblongata appears to be involved in the development and progression of severe hypertension in M-SHRSP.