Chronic labile hypertension produced by lesions of the nucleus tractus solitarii in the cat.

Bilateral electrolytic lesions of the nucleus tractus solitarii (NTS) were made at the level of the obex in seven cats. Within 1 hour the mean arterial pressure (MAP) rose to a maximum of 144 mm Hg (141% of control), and by 7 hours heart rate reached a peak of 236 beats/min (148% of control). The baroreceptor reflexes were abolished. After 24 hours the arterial pressure became extremely labile, with variations of 80-100 mm Hg observed. The lability occurred spontaneously and during behaviors that were self-initiated or elicited by environmental stimuli. The MAP in the lesion group was 144 mm Hg (180% of control) during the day, and 96 mm Hg (120% of control) at night. The lability, measured by the standard deviation, during the day in the lesion group was 4 times greater than in the control group and at night there were no differences. The heart rate of the lesion group was always higher than that of the control group but the lability of both groups was the same. We conclude that lesions of the NTS produced labile hypertension, probably by disinhibition of sympathetic activity through central interruption of the baroreceptor reflexes. The higher, more labile arterial pressures during the day may be caused by uninhibited increases in sympathetic activity elicited by environmental stimuli that are present during the day and absent at night. The daily variation of pressure may also be caused by somatomotor activity or by a daily rhythm of sympathetic activity which is unmasked by the lesions.