Extensive projections from the midbrain periaqueductal gray to the caudal ventrolateral medulla: a retrograde and anterograde tracing study in the rat.

We investigated the innervation of the caudal ventrolateral medulla by the midbrain periaqueductal gray in the rat using retrograde and anterograde tract-tracing. Iontophoretic injection of Fluoro-Gold or cholera toxin B subunit into the caudal ventrolateral medulla resulted in retrogradely labeled neurons in discrete regions of the periaqueductal gray. These labeled cells were observed throughout the rostrocaudal extent of the periaqueductal gray and were distributed (as percentage of total labeled cells) in its lateral (53-67%), ventrolateral (14-28%), ventromedial (7-16%) and dorsomedial aspects (7-10%). About 70-72% of labeled cells were found in the caudal half of the periaqueductal gray and 28-30% in the rostral half. In the ventromedial periaqueductal gray, more labeled cells were seen in the contralateral side (5-13%) than the ipsilateral side (2-3%), whereas for other periaqueductal gray areas labeling was preferentially ipsilateral. Phaseolus vulgaris leucoagglutinin anterograde tracing was used to confirm the retrograde labeling results. Following iontophoretic injection into the periaqueductal gray, labeled fibers and terminals were observed throughout the rostrocaudal extent of the caudal ventrolateral medulla. Injections in the lateral and/or ventrolateral aspect of the periaqueductal gray yielded more anterograde labeling in the ipsilateral than the contralateral caudal ventrolateral medulla, while injections in the ventromedial aspect of the periaqueductal gray produced labeling preferentially in the contralateral caudal ventrolateral medulla. The present study indicates that specific regions of the periaqueductal gray project to the caudal ventrolateral medulla and may regulate cardiovascular and respiratory functions through these connections.