Characterization of the sympatho-facilitative area postrema pathway.

1. The mechanism by which the area postrema augments central sympathetic drive during electrical stimulation is presently unknown. This pathway may involve either direct facilitation of brain-stem vasomotor neurons or inhibition of the sympatho-inhibitory baroreceptor relay in the nucleus tractus solitarii. 2. The present study employed selective lesions within the solitary tract nucleus to assess the participation of the primary baroreceptor relay in the pressor response during electrical stimulation of the area postrema. 3. The magnitude of the pressor response was unchanged after destruction of the solitary tract and lateral solitary nucleus which centrally interrupted the baroreceptor reflex. However, microknife cuts through the medial solitary nucleus, which spared the carotid sinus reflex, significantly reduced the magnitude of the area postrema pressor response. 4. Previous anatomical studies support these results and confirm that, although the area postrema pressor pathway traverses the most medial portion of the solitary complex, it does not produce augmented sympathetic outflow by inhibition of the primary baroreflex relay.