Interconnectivity between the amygdaloid complex and the amygdalostriatal transition area: a PHA-L study in rat

The amygdala orchestrates the formation of behavioral responses to emotiona lly arousing stimuli. Many of these responses are initiated by the central nucleus, which converges information from other amygdaloid nuclei. Recently , we observed substantial projections from the amygdala to the amygdalostri atal transition area, which is located dorsal to the central nucleus. These projections led us to question whether the amygdalostriatal transition are a has a role in the initiation of behavioral responses in emotionally arous ing circumstances. To explore this anatomically, me traced the interconnect ions between the amygdalostriatal transition area and the amygdaloid comple x using the anterograde tracer Phaseolus vulgaris-leucoagglutinin. The late ral (the medial division and the caudal portion of the dorsolateral divisio n) and the accessory basal nuclei (the parvicellular division) provide mode rate-to-heavy projections to the amygdalostriatal transition area. Projecti ons back to the amygdala are light and are composed of thin, faintly staine d varicose fibers that resemble the labeling of cholinergic terminals. The extra-amygdaloid outputs of the amygdalostriatal transition area are sparse and include moderate projections to the caudoventral globus pallidus, the ansa lenticularis, and the substantia nigra pars lateralis. These data sugg est that the amygdalostriatal transition area is one of the major targets f or projections originating in the lateral and accessory basal nuclei of the amygdala. Via these pathways, emotionally significant stimuli can evoke be havioral responses that are different from those initiated via projections from the amygdala to the central nucleus. One such candidate response is th e orienting response (i.e., saccadic eye movements and head direction) in a pathway that includes a projection from the lateral/accessory basal nucleu s of the amygdala to the amygdalostriatal transition area, and from there t o the substantia nigra, pars lateralis. J. Comp. Neurol. 431:39-58, 2001. ( C) 2001 Wiley-Liss. Inc.