Organization of Peptidergic and Catecholaminergic Efferents from the Nucleus of the Solitary Tract to the Rat Amygdala

Previous studies have focused on the role of the central nucleus of the amygdala (CeA) in cardiovascular and other amygdaloid functions. The combined retrograde tracing/immunohistochemical method was used to test for the presence of enkephalin, neurotensin, neuropeptide Y, and catecholamine neurons within the nucleus of the solitary tract that send efferents to the CeA. After injections of retrograde tracer into the CeA, retrogradely labeled neurons were observed within the caudal, medial nucleus of the solitary tract. Most CeA-projecting neurons were located ipsilaterally within the medial nucleus of the solitary tract at the level of the area postrema. Retrogradely labeled enkephalin- and neurotensin-immunoreactive neurons were found within the medial nucleus of the solitary tract at this level, while retrogradely labeled neuropeptide Y-immunoreactive neurons were found within the medial nucleus of the solitary tract rostral to the area postrema. About 60-74% of CeA-projecting cells were also immunoreactive for tyrosine hydroxylase. Approximately 9% of retrogradely neurons were phenylethanolamine-N-methyltransferase immunoreactive. The results provide evidence that within the nucleus of the solitary tract, peptidergic CeA-projecting neurons have a topographic distribution. In addition, noradrenergic neurons within the A2 group, rather than adrenergic neurons of the C2 group, provide the bulk of catecholaminergic input to the CeA from the nucleus of the solitary tract. Cell counts indicate that each of these peptides may be colocalized (to varying extents) within catecholamine-producing neurons. Also the catecholaminergic and enkephalinergic contribution to the ascending pathway from the nucleus of the solitary tract to the CeA distinguishes it neurochemically from the descending pathway. Thus, although there are afferent and efferent connections between the nucleus of the solitary tract and CeA, their peptidergic/neurotransmitter connections are not necessarily reciprocal. Input from nucleus of the solitary tract peptidergic and catecholaminergic neurons to the CeA may be important in the etiology of a number of pathophysiological conditions including hypertension, gastric ulcers, and schizophrenia.