Subcortical afferent connections of the amygdala in the monkey

Abstract The cells of origin of the afferent connection of the amygdala in the rhesus and squirrel monkeys were analyzed by means of the retrograde axonal transport of the enzyme horseradish peroxidase (HRP) injected into various quadrants of the amygdala. Tabulation of the relative numbers of HRP labeled cells found in each brain section through various nuclei in a series of injected brains revealed several patterns of ipsilateral subcortical nuclear connections with the amygdala. The dorsal thalamic nuclei most consistently exhibiting large numbers of labeled cells in all experiments were the ipsilateral halves of the midline nucleus paraventricularis thalami and both the parvo‐ and magnocellular parts of the nucleus subparafascicularis. All of the subdivisions of the midline nucleus centralis complex (Olszewski, '52) exhibit HRP labeled cells in most cases. A cell group corresponding to the nucleus reuniens ventralis (Kuhlenbeck, '54) and one identified as the nucleus interventralis of Aronson and Papez ('34) also were found to contain labeled cells. The largest populations of HRP labeled cells in the hypothalamus appear chiefly in the middle and posterior parts of the ipsilateral, lateral hypothalamic area in all cases and densely in the ventromedial hypothalamic nucleus only in cases with medial amygdalar quadrant injections. Scattered HRP positive cells frequently can be found in the supramammillary and dorsomedial nuclei and the posterior hypothalamic area. A limited but consistent number of labeled cells can be located in Tsai's ventral tegmental area. In the midbrain some labeled cells can be demonstrated in the rostral and caudal subdivisions of the nucleus linearis, and scattered groups of HRP positive cells can be localized in the dorsal raphe nucleus, chiefly on the side ipsilateral to the injection. The most conspicuous subdiencephalic source of amygdalar afferent connections found in the present study is the pars lateralis of the nucleus parabrachialis located in the dorsolateral pontine tegmentum. Cells in this nucleus are heavily labeled in all cases, while only a few HRP positive cells are occasionally found in the pars medialis. A few labeled cells can be differentiated from pigmented cells in the locus coeruleus, but the labeling of cells scattered throughout the caudal half of the nucleus of the tractus solitarius and in a paravagal cell group is accepted with reservations because both cell groups frequently display endogenously pigmented cells in normal, non‐injected monkeys.