The organization of dorsal medullary projections to the central amygdaloid nucleus and parabrachial nuclei in the rabbit

Abstract

The amygdaloid central nucleus and the pontine parabrachial nucleus receive direct, ascending projections from autonomic regulatory nuclei of the dorsal medulla and are recognized as important components of a forebrain system which contributes to autonomic regulation. The present study was designed to provide more detailed information on the anatomical organization of this ascending system in the rabbit by determining (a) the extent to which separate populations of neurons within the solitary complex project to the central nucleus and parabrachial nucleus, (b) the topographical distribution of the projections of the solitary complex within the amygdaloid central nucleus and parabrachial nucleus and (c) the extent to which projections from the solitary complex to the parabrachial nucleus terminate in the region of origin of projections from the parabrachial nucleus to the amygdaloid central nucleus. A fluorescent dye, double retrograde-labeling technique demonstrated that separate populations of neurons in the solitary complex projected to the amygdaloid central nucleus and parabrachial nucleus. Neurons of both populations were more heavily concentrated within the caudal two thirds of nucleus of the solitary tract and were most numerous within the commissural, medial and dorsomedial subnuclei. Labeled neurons were also located within the dorsal motor nucleus of the vagus nerve. Autoradiographic experiments demonstrated that injections of amino acids into the solitary complex resulted in terminal labeling in the central nucleus. This labeling extended rostrally into the adjacent sublenticular substantia innominata and lateral component of the bed nucleus of the stria terminalis. Label was also observed within the lateral, medial, and Kolliker-Fuse regions of the parabrachial nucleus. A particularly dense field was observed overlying cells located within the ventrolateral region of the lateral parabrachial nucleus. This region contained the majority of labeled neurons within the parabrachial nucleus following fluorescent dye injections into the central nucleus. Furthermore, injections of amino acids into this region resulted in terminal labeling within the central nucleus, with a particularly dense area observed within the medial aspect of the nucleus. The results demonstrate that separate populations of neurons within the solitary complex of the rabbit project to the central amygdaloid and parabrachial nuclei and that the majority of these are located within the caudal two-thirds of the complex. Furthermore, the results suggest that the solitary complex projects both directly and indirectly, primarily via the lateral parabrachial nucleus, to the central amygdaloid nucleus. These projections offer an anatomical substrate by which visceral afferent information may influence the limbic forebrain.