Amygdaloid and basal forebrain direct connections with the nucleus of the solitary tract and the dorsal motor nucleus

Although the amygdala complex has long been known to exert a profound influence on cardiovascular activity, the neuronal and connectional substrate mediating these influences remains unclear. This paper describes a direct amygdaloid projection to medullary sensory and motor structures involved in cardiovascular regulation, the nucleus of the solitary tract (NTS) and the dorsal motor nucleus (DVN), by the use of autoradiographic anterograde transport and retrograde horseradish peroxidase (HRP) techniques in rabbits. Since all of these structures are highly heterogeneous structurally and functionally, details of the specific areas of the neuronal origin and efferent distribution of the projection were examined in relation to these features and with reference to a cytoarchitecture description of the relevant forebrain regions in the rabbit. Amygdaloid projections to the NTS and DVN, as determined from HRP experiments, arise from an extensive population of neurons concentrated exclusively within the ipsilateral central nucleus and confined to and distributed throughout a large medial subdivision of this nucleus. Projection neurons, however, also distribute without apparent interruption beyond the amygdala dorsomedially into the sublenticular substantia innominata and the lateral part of the bed nucleus of the stria terminalis and thus delineate a single entity of possible anatomical unity across all three structures, extending rostrocaudally within the basal forebrain as a diagonal band. Descending central nucleus connections, based upon autoradiographic experiments, project heavily and extensively to both the NTS and the DVN. Within both nuclei, the projections have a highly specific distribution pattern, appearing to correspond largely to structural subdivisions, including the dorsomedial, medial, ventrolateral, ventral, and commissural NTS, and to cell group "a," a caudally located dorsomedial region, and peripheral regions of the DVN, some of which appear to be involved in cardiovascular regulation. The existence of such an extensive projection system connecting these specific regions is significant evidence in support to its potential for participation in the amygdaloid expression of cardiovascular influences and has important implications for the cellular analysis of the functional role of these influences.