Unit activity in nucleus paragigantocellularis lateralis during cerebral ischemia in the rat

In urethane-anesthetized, paralyzed and respirated rats with both vertebral arteries ligated between atlas and axis, bilateral occlusion of the common carotids produces a 19-85 mm Hg rise in mean arterial pressure. This pressor response is entirely mediated by the activation of the sympathetic system, is unaffected by brainstem transection at midpontine levels and is completely eliminated by bilateral lesions of the anterior ventrolateral medulla (including nucleus paragigantocellularis lateralis or PGCL). The magnitude of the response is unaltered following the bilateral section of the 9th-12th cranial nerves. The pressor response is preceded by a rise (25-250%) in the rate of discharge of a specific group of 'vasomotor' neurons present in the pressor area of the PGCL which exhibit the following characteristics: axonal projection to the thoracic spinal cord, spontaneous discharges at low blood pressure, pulse-synchronous activity, discharge rate inversely related to mean arterial pressure and total inactivity above 140-160 mm Hg. It is suggested that the same population of vasomotor neurons relays the sympathoexcitation produced by cerebral ischemia to the cord and also mediates baroreceptor reflexes and generates the basal sympathetic tone.