Projections from the periamygdaloid cortex to the amygdaloid complex, the hippocampal formation, and the parahippocampal region: a PHA-L study in the rat.

The periamygdaloid cortex, an amygdaloid region that processes olfactory information, projects to the hippocampal formation and parahippocampal region. To elucidate the topographic details of these projections, pathways were anterogradely traced using Phaseolus vulgaris leukoagglutinin (PHA-L) in 14 rats. First, we investigated the intradivisional, interdivisional, and intra-amygdaloid connections of various subfields [periamygdaloid subfield (PAC), medial subfield (PACm), sulcal subfield (PACs)] of the periamygdaloid cortex. Thereafter, we focused on projections to the hippocampal formation (dentate gyrus, hippocampus proper, subiculum) and to the parahippocampal region (presubiculum, parasubiculum, entorhinal, and perirhinal and postrhinal cortices). The PACm had the heaviest intradivisional projections and it also originated light interdivisional projections to other periamygdaloid subfields. Projections from the other subfields converged in the PACs. All subfields provided substantial intra-amygdaloid projections to the medial and posterior cortical nuclei. In addition, the PAC subfield projected to the ventrolateral and medial divisions of the lateral nucleus. The heaviest periamygdalohippocampal projections originated in the PACm and PACs, which projected moderately to the temporal end of the stratum lacunosum moleculare of the CA1 subfield and to the molecular layer of the ventral subiculum. The PACm also projected moderately to the temporal CA3 subfield. The heaviest projections to the entorhinal cortex originated in the PACs and terminated in the amygdalo-entorhinal, ventral intermediate, and medial subfields. Area 35 of the perirhinal cortex was lightly innervated by the PAC subfield. Thus, these connections might allow for olfactory information entering the amygdala to become associated with signals from other sensory modalities that enter the amygdala via other nuclei. Further, the periamygdalohippocampal pathways might form one route by which the amygdala modulates memory formation and retrieval in the medial temporal lobe memory system. These pathways can also facilitate the spread of seizure activity from the amygdala to the hippocampal and parahippocampal regions in temporal lobe epilepsy.