Brain stem area with C1 epinephrine neurons mediates baroreflex vasodepressor responses.

In anesthetized, paralyzed rats, bilateral electrolytic lesions of rostral ventrolateral medulla (RVL) containing epinephrine neurons of the C1 group 1) abolished the reflex hypotension and bradycardia elicited by electrical stimulation of the left vagus nerve or stretch of the left carotid sinus and 2) reduced arterial pressure (AP) and heart rate (HR) to values comparable to spinal cord transection. Combined lesions of the right nucleus tractus solitarii (NTS) and left or right C1 area did not alter AP. However, lesions of right NTS combined with lesions or microinjection of kainic acid into the left, but not right, C1 area abolished vasodepressor reflexes. Vasodepressor reflexes were unchanged by midcollicular decerebration, bilateral lesions of the parabrachial complex of the pons, or (after right NTS lesions) by lesions of the raphe or other reticular areas of the left half of the medulla. We conclude that neurons of the RVL, possibly belonging to the C1 epinephrine group, mediate vasodepressor responses from arterial baro- and other cardiopulmonary receptors, the pathway may be via a direct NTS-RVL projection, neurons in the C1 area of RVL are necessary for maintaining resting levels of arterial pressure.