Projections from the amygdalo-piriform transition area to the amygdaloid complex: A PHA-L study in rat

The amygdalo-piriform transition area is a poorly defined region in the tem poral lobe that is heavily connected with the olfactory system. As part of an ongoing project aimed at understanding the neuronal pathways that provid e sensory information to the amygdala, we investigated the cytoarchitectoni c and chemoarchitectonic features of the amygdalo-piriform transition area and its connections to the amygdaloid complex in 13 rats by using the anter ograde tracer, Phaseolus vulgaris-leucoagglutinin. Our analysis indicates t hat the amygdalo-piriform transition area has medial (rostral and caudal po rtions) and lateral parts. The rostromedial part projects heavily to the in termediate and lateral divisions of the central nucleus, whereas the caudom edial part projects mainly to the medial division. The lateral part of the amygdalo-piriform transition area projects heavily to the capsular and late ral divisions of the central nucleus. Electron microscopic analysis reveale d that the projection to the lateral division of the central nucleus forms asymmetric contacts with the spines and shafts of postsynaptic neurons and, therefore, is assumed to be excitatory. The amygdalo-piriform transition a rea also projects moderately to other amygdaloid nuclei, including the parv icellular division of the basal nucleus, the anterior cortical nucleus, and the nucleus of the lateral olfactory tract. The lateral and medial parts o f the amygdalo-piriform transition area also project to the distal temporal CA1 and distal temporal subiculum, respectively. Unlike the adjacent entor hinal cortex, the amygdalo-piriform transition area does not project to the dentate gyrus. These data suggest that the amygdalo-piriform transition ar ea is a region that influences both emotional and memory processing in para llel by means of pathways to the amygdala and the hippocampus, respectively . (C) 2001 Wiley-Liss, Inc.