ANATOMICAL EVIDENCE FOR INPUTS TO VENTROLATERAL MEDULLARY CATECHOLAMINERGIC NEURONS FROM THE MIDBRAIN PERIAQUEDUCTAL GRAY OF THE RAT

Previous studies have shown that the midbrain periaqueductal gray (FAG ) projects to the ventrolateral medulla (VLM). Here, we studied FAG pr ojections to the area of A1/C1 neurons in the VLM in the rat using pha seolus vulgaris leucoagglutinin (PHA-L) anterograde tracing combined w ith immunocytochemistry for tyrosine hydroxylase (TH) or phenylethanol amine N-methyl transferase (PNMT). Following FAG injections, PHA-L lab eled fibers and terminals were intermingled among TH-immunoreactive (T H-ir) neurons in the VLM. High-power light microscopic examination rev ealed that some of the PHA-L labeled varicose fibers and boutons were in close contiguity with TH-ir elements. Such apparent appositions app eared more frequently on TH-ir elements in the A1 area than on TH-ir o r PNMT-ir neurons in the C1 area. These results indicate that some FAG inputs to the VLM may directly innervate A1/C1 neurons.