Chronic Lesion of Rostral Ventrolateral Medulla in Spontaneously Hypertensive Rats

We studied the effects of chronic selective neuronal lesion of rostral ventrolateral medulla on mean arterial pressure, heart rate, and neurogenic tone in conscious, unrestrained spontaneously hypertensive rats. The lesions were placed via bilateral microinjections of 30 nmol/200 nl N-methyl-D-aspartic acid. The restimulation of this area with N-methyl-D-aspartic acid 15 days postlesion failed to produce a pressor response. One day postlesion, the resting mean arterial pressure was significantly decreased in lesioned rats when compared with sham rats (100 +/- 7 versus 173 +/- 4 mm Hg, p less than 0.05). Fifteen days later, the lesioned group still showed values significantly lower than the sham group (150 +/- 6 versus 167 +/- 5 mm Hg, p less than 0.05). No significant heart rate differences were observed between the sham and lesioned groups. The ganglionic blocker trimethaphan (5 mg/kg i.v.) caused similar reductions in mean arterial pressure in both lesioned and sham groups. The trimethaphan-induced hypotension was accompanied by a significant bradycardia in lesioned rats (-32 +/- 13 beats per minute) but a tachycardia in sham rats (+33 +/- 12 beats per minute) 1 day postlesion. Therefore, rostral ventrolateral medulla neurons appear to play a significant role in maintaining hypertension in conscious spontaneously hypertensive rats. Spinal or suprabulbar structures could be responsible for the gradual recovery of the hypertension in the lesioned rats.