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 projections to the area of A1/C1 neurons in the VLM in the rat using phaseolus vulgaris leucoagglutinin (PHA-L) anterograde tracing combined with immunocytochemistry for tyrosine hydroxylase (TH) or phenylethanolamine N-methyl transferase (PNMT). Following FAG injections, PHA-L labeled fibers and terminals were intermingled among TH-immunoreactive (TH-ir) neurons in the VLM. High-power light microscopic examination revealed that some of the PHA-L labeled varicose fibers and boutons were in close contiguity with TH-ir elements. Such apparent appositions appeared more frequently on TH-ir elements in the A1 area than on TH-ir or PNMT-ir neurons in the C1 area. These results indicate that some FAG inputs to the VLM may directly innervate A1/C1 neurons.