An autoradiographic study of the projections of the central nucleus of the monkey amygdala

The efferent connections of the central nucleus of the monkey amygdala have been studied using the autoradiographic method for tracing axonal projections. Small injections of 3H-amino-acids which are largely confined to the central nucleus lead to the labeling of several brainstem nuclei as far caudally as the spinomedullary junction. Specifically, in the forebrain, the central nucleus projects heavily to the bed nucleus of the stria terminalis, the basal nucleus of Meynert, the nucleus of the horizontal limb of the diagonal band, and more lightly to the substantia innominata and the preoptic area. In the hypothalamus, label is found over the dorsomedial nucleus, the perifornical region, the lateral hypothalamus, the supramammillary area, and most heavily in the paramammillary nucleus. In the thalamus, all components of the nucleus centralis medialis and the nucleus reuniens receive fibers from the central nucleus and there is a light projection to the medial pulvinar nucleus. In the mesencephalon, there is heavy labeling dorsal to the substantia nigra ad over the peripeduncular nucleus and lighter labeling within the substantia nigra pars compacta and the ventral tegmental area; the midbrain central gray is also labeled. More caudally, fibers from the central nucleus travel in the lateral tegmental reticular fields and contribute collaterals to the raphe nuclei, the cuneiform nucleus, and the central gray substance. Perhaps one of the heaviest terminal zones is the parabrachial region of the pons, both the lateral and the medial nuclei of which receive a prominent central nucleus projection. Only the ventral aspect of the adjacent locus coeruleus appears to receive a substantial input, but there is labeling also over the area of the nucleus subcoeruleus. Finally, there is heavy labeling around the dorsal motor nucleus of the vagus and over the parvocellular component of the nucleus of the solitary tract. A number of intra-amygdaloid connections between the basal and lateral nuclei of the amygdala and the central nucleus are also described. The present findings, taken together with recently reported widespread projections from the temporal association cortex to the amygdala, point out a potentially trisynaptic route between neocortical association regions and a variety of brainstem nuclei, many of which are related to autonomic function.