Nucleus ambiguus projections to cardiac ganglia of rat atria: An anterograde tracing study

We injected the anterograde fluorescent tracer 1,1'-dioleyl-3,3,3',3'-tetra methylindocarbocyanine bocyanine methanesulfonate (DiL) into the nucleus am biguus (NA) and used confocal microscopy to inventory NA fibers and axon te rminals in whole-mounts of rat atrial tissues. Both the axons projecting to cardiac ganglia and the innervated principal neurons (PNs) were counted. R ats were injected unilaterally in the NA with DiI, either at four sites (be tween 600 mu m rostral and 600 mu m caudal to the obex) or at nine sites (1 ,600 mu m rostral to 1,600 mu m caudal). Fluoro-Gold was administered intra peritoneally to retrogradely label neurons of the dorsal motor nucleus of t he vagus (DmnX), NA, and cardiac ganglia. To verify that the DiI-labeled fi bers examined in the atria originated exclusively from the NA, neurons of t he DmnX and the nodose ganglia were surveyed for DiI labeling. Our observat ions established that (1) NA fibers in the cardiac branches of the vagus nu mbered in the range of 82-151, left; or 60-122, right. (2) Both left and ri ght NA supplied substantial numbers of fibers to each of the three major ca rdiac ganglionic plexuses. (3) NA axons terminated in dense basket, or caly x, endings around individual PNs. (4) By issuing divergent collaterals, ind ividual NA fibers supplied numerous PNs with these calyx endings. (5) Label ed axons innervated 2,248 (left vagus) and 1,784 (right), or at least 56% a nd 45%, of the cardiac PNs. (6) Divergence (i.e., NA axons:PNs innervated) averaged between 1:27 (left vagus) and 1:30 (right vagus). Several features of these NA projections to cardiac ganglia contrasted sharply with those o f DmnX projections that we have recently characterized with the same tracin g protocol: (1) NA fibers did not innervate small intensely fluorescent cel l clusters in cardiac ganglia, whereas DmnX axons did. (2) NA efferent fasc icles contained more large fibers (presumably B-type), whereas the DmnX iss ued more fine caliber fibers (presumably C-type). (3) NA fibers diverged ab out three times as extensively as did DmnX axons. Taken together, our data strongly suggest that vagal control of the heart involves the convergence a nd integration of distinct NA and DmnX projections within the cardiac plexu ses. (C) 2000 Wiley-Liss, Inc.