Afferent connections to the amygdaloid complex of the rat with some observations in the cat. III. Afferents from the lower brain stem.

The projections from the medulla oblongata, pons, and mesencephalon to each nucleus of the amygdaloid complex of the rat were investigated by the use of retrograde transport of horseradish peroxidase (HRP). The enzyme was injected stereotactically by microiontophoresis using four different approaches. The findings indicate that the majority of the ascending fibers terminate in the central and medial amygdalar nuclei. Injections in the central nucleus label neurons at the dorsal aspect of substantia nigra, para compacta, and in the adjacent ventral tegmental area and peripeduncular nucleus. At more caudal levels, reactive neurons are found in the periaqueductal gray substance, various raphe nuclei, the locus coeruleus, the parabrachial nucleus, the nucleus of the solitary tract, and the mesencephalic and bulbar reticular formation. Injections in the medical nucleus lead to labeling of neurons in the peripeduncular nucleus, the dorsal raphe and superior central nuclei, the parabrachial nucleus, and in the dorsomedial extreme of the dorsal nucleus of the lateral lemniscus. The parabrachial nucleus is the most important lower brain stem source of amygdalopetal fibers. This nucleus projects to the ipsilateral as well as the contralateral amygdala in a topographical manner. Most of the lower brain stem structures found to project to the amygdala in the rat are identified as sources of amygdalopetal fibers in the cat as well.