Periaqueductal gray matter projection to vagal preganglionic neurons and the nucleus tractus solitarius.

The periaqueductal gray matter (PAG) has been implicated in a variety of different functions, including autonomic regulation. Chemical stimulation of the lateral PAG produces hypertension and tachycardia while activation of the ventrolateral PAG produces the opposite effect. While these effects are the result of alterations in sympathetic activity, little is known about whether the PAG can modulate vagal functions as well. The anterograde axonal tracing method using the plant lectin Phaseolus vulgaris leucoagglutinin (PHA-L) was used to determine whether both of the lateral and ventrolateral PAG columns project to vagal preganglionic neurons and/or to the nucleus tractus solitarius (NTS). Highly restricted PHA-L injections were made in all four PAG columns throughout their rostrocaudal extent in rats. Labeled fibers were visualized by immunohistochemistry and studied in relationship with choline acetyltransferase (ChAT) immunostained parasympathetic preganglionic neurons of the dorsal motor vagal nucleus (DMV) and nucleus ambiguous (NA). The lateral PAG projects to the lateral DMV and to the caudal part of the external NA. The ventrolateral PAG innervates the same regions and also projects to the rostral part of the external NA -- a site that contains cardiac parasympathetic preganglionic neurons. Both the lateral and ventrolateral PAG project to the NTS in a similar fashion innervating the medial, ventrolateral and commissural subnuclei. In summary, the lateral and ventrolateral PAG have similar patterns of innervation of the NTS and DMV, but their projection to the NA is different: the rostral external NA receives innervation only from the ventrolateral PAG and the lateral PAG innervates the caudal part.