Differential distribution of amygdaloid input across rostral solitary nucleus subdivisions in rat.

The orosensory nucleus of the solitary tract (NST) receives input from the amygdala, a key node in the forebrain feeding-related network. Despite numerous studies documenting the existence of this pathway, however, too little is known about the input organization to the gustatory brainstem to allow definitive conclusions about its functional role. Therefore, towards the long-term goal of characterizing such descending regulatory pathways, the purpose of the present study was to describe the distribution of input arising from the amygdala. The anterograde tracer, biotinylated dextran, was injected into the central amygdala based on stereotaxic coordinates in seven adult male rats. Following a 2-week survival time, the animals were sacrificed. Transverse sections of the brains were processed to visualize transported tracer and NST anatomical topography. Labeled fibers were differentially distributed among subdivisions throughout the rostrocaudal extent of NST. Within the rostral NST, the medial (M) subdivision had the highest density of terminal-like endings and swellings (30% of total density), followed by the ventral half of rostral central (vRC, 29%), ventral (V, 25%), dorsal half of rostral central (dRC, 12%) and rostral lateral (RL, 4%). In conclusion, it appears that amygdalar input preferentially overlaps with NST subdivisions (M, V, vRC) containing neurons with local efferent projections to the caudal NST and reticular nuclei that are implicated in medullary reflex circuits, rather than with subdivisions (dRC, RL) receiving primary orosensory afferent input and containing neurons having ascending efferent projections to the parabrachial nucleus. Thus, descending feeding-related pathways may be positioned to act as regulatory substrates controlling the output gain of brainstem circuits which may serve to modulate sensorimotor and autonomic reflexes in response to ingestive behaviors.