Organization of medullary adrenergic and noradrenergic projections to the periaqueductal gray matter in the rat

The periaqueductal or midbrain central gray matter (CG) in the rat contains a dense network of adrenergic and noradrenergic fibers. We examined the origin of this innervation by using retrograde and anterograde axonal tracers combined with immunohistochemistry for the catecholamine biosynthetic enzymes tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), and phenylethanolamine N-methyltransferase (PNMT). Following injections of the fluorescent tracers Fast Blue or Fluorogold into the CG, double-labeled neurons in the medulla were identified mainly in the noradrenergic A1 group in the caudal ventrolateral medulla (VLM) and A2 group in the medial part of the nucleus of the solitary tract (NTS); and in the adrenergic C1 group in the rostral ventrolateral medulla and C3 group in the rostral dorsomedial medulla. Injections of Phaseolus vulgaris-leucoagglutinin (PHA-L) into these cell groups resulted in a distinct pattern of axonal labeling in various subdivisions of the CG. Anterogradely labeled fibers originating in the medial NTS were predominantly found in the lateral portion of the dorsal raphe nucleus and in the adjacent part of the lateroventral CG (CGlv). Following PHA-L injections into the C3 region the anterogradely labeled fibers were diffusely distributed in the CGlv and the dorsal raphe nucleus at caudal levels, but rostrally tended to be located laterally in the CGlv. In contrast, ascending fibers from the caudal and rostral VLM terminated in the rostral dorsal part of the CGlv and in the dorsal nucleus of the CG, whereas ventral parts of the CG, including the dorsal raphe nucleus, contained few afferent fibers. Double-label studies with antisera against DBH and PNMT confirmed that noradrenergic neurons in the A1 and A2 groups and adrenergic neurons in the C1 and C3 groups contributed to these innervation patterns in the CGlv. Noradrenergic and adrenergic projections from the medulla to the CG may play an important role in a variety of autonomic, sensory and behavioral processes.