Neocortical connections with perihippocampal and periamygdalar regions in the hedgehog tenrec.

The perihippocampal fields represent the most important regions connecting the neocortex and the hippocampus in rat, cat and monkey but little is known about their presence and connectivity in species with poorly differentiated brain. Using axonal tracer substances we have recently studied the distribution of cortical cells projecting to the hippocampus in the hedgehog tenrec. In the present study we determined the regions of the paleocortex and the rhinal cortex connected with the neocortex, and provide a tentative view of the site and the extent of the tenrec's entorhinal, perirhinal and postrhinal/parahippocampal fields. It is shown that only the dorsal portions of the posterior rhinal cortex may be considered equivalent to the perirhinal and postrhinal fields of higher mammals, while a considerable expanse of the ventral rhinal cortex may be part of the entorhinal area (its so-called dorsal portion) connected with both the dentate gyrus and the neocortex. A few cells projecting to the neocortex were also noted in the dorsal-most portion of the three-layered paleocortex (ventral entorhinal portion). These cells were linearly arranged and reminiscent of the neocortical projecting cells in the entorhinal layer 4/5 in more differentiated mammals. The main portion of the paleocortex caudal to the corpus callosum remained unlabeled following neocortical and hippocampal tracer injections. Unexpectedly, the area in the most ventral paleocortex adjacent to the amygdala also projected to the neocortex, particularly the tenrec's somatosensorimotor cortex.