Projections from the periaqueductal gray (PAG) to the periambigual area: Relation to cholinergic vago-cardiac neurons
Activation of the midbrain periaqueductal gray (PAG) elicits species specific defense reactions and marked autonomic adjustments. In agreement with recent studies by Bandler and colleagues in the cat, we found that activation of rostrocaudally-oriented, longitudinally organized columns in PAG elicit differential cardiovascular and behavioral responses. Activation of dorsolateral/lateral PAG produces aggressive/flight behaviors accompanied by pressor responses while stimulation of ventrolateral PAG produces immobility and depressor responses. The pathways mediating these selective behavioral and autonomic responses are poorly understood.The projection from PAG to the sympathoexcitatory zone in the rostral ventrolateral medulla is a likely substrate for PAG-evoked pressor responses. However, circuits mediating PAG-evoked depressor responses are not known. The present studies have identified a robust, focal projection from PAG to the nucleus ambiguus (NA), a potent depressor area in the ventral medulla. In addition, we have used tract tracing techniques and immunocytochemistry to examine the organization of PAG projections in relation to cholinergic preganglionic parasympathetic NA neurons that innervate the heart.