Demonstration of projections from the lateral nucleus to the basal nucleus of the amygdala: a PHA-L study in the monkey.

Previous studies of the intrinsic connections of the amygdaloid complex in the rat, cat and monkey demonstrated that the lateral nucleus projects prominently to the accessory basal nucleus and periamygdaloid cortex and lightly to several other nuclei of the amygdala. Most previous studies have emphasized the lack of a connection between the lateral and basal nuclei. As part of ongoing studies of the intrinsic circuitry of the monkey amygdala, we have placed discrete, iontophoretic injections of the lectin anterograde tracer Phaseolus vulgaris leucoagglutinin into the lateral nucleus of the Macaca fascicularis monkey. In addition to confirming the well established intrinsic connections of the lateral nucleus, heavy fiber and terminal labeling was also observed in the adjacent basal nucleus. The projection innervated all divisions of the basal nucleus and tended to be somewhat denser rostral to the level of the injection site. This hitherto unreported intrinsic connection provides a means by which the lateral nucleus, which is the principal recipient of afferents from sensory neocortex, can influence cells of the basal nucleus, which originate the major amygdalofugal projection to the neocortex.