Circulatory Effects of Clonidine after Pre‐hypothalamic Section in the Rat

Clonidine 15 μg/kg intravenously was administered to unanaesthetized rats during recording of blood pressure and heart rate. The rats were either sham operated or sectioned at a level rostral to the hypothalamus. The basal blood pressure and heart rate of the sectioned rats was significantly higher than that of the control rats. The blood pressure increase seen after clonidine in sham operated rats was absent in the rats with a pre‐hypothalamic section and after 5–10 min. hypotension was observed. The decrease in heart frequency was more pronounced in the rats with a diencephalic section than in the sham operated rats. The results suggest that the hypertensive effect of clonidine is dependent on intact structures rostral to the hypothalamus.