Organization of substance P fibers within the hippocampal formation demonstrated with a biotin-avidin immunoperoxidase technique.

The distribution of substance P-containing fibers within the hippocampal formation of the cat was examined using an immunohistochemical approach. A new, indirect immunoperoxidase method based on the high affinity binding of vitamin H (biotin) by avidin was developed to demonstrate substance P-like immunoreactive fibers. The neocortex contained only occasional single substance P fibers. In contrast, the archicortical structures contained a well organized substance P innervation. The entorhinal area of the parahippocampal gyrus contained a delicate network of varicose axons which appeared to ascend from the subcortical white matter to terminate predominantly in layer II. The subiculum contained a network of positive varicosities throughout the neuropil of both the pyramidal and the molecular layers. These substance P terminals appeared to arise, at least in part, from fibers entering laterally across the angular bundle. Within the ventral hippocampus the substance P innervation was predominantly in the pyramidal cell layer, and was heaviest in field CA3 and weakest in CA1. Some fibers were also observed in strata oriens, lucidum, and radiatum. Occasional varicose fibers could be seen entering the hippocampus from the fimbria. Within the dentate gyrus the substance P fibers were most concentrated in the supragranular layer and among the more superficial of the granule cells. A moderately dense plexus of fibers also occurred in the neuropil of the hilus among the polymorphic cells. In addition, a distinct, narrow band of varicosities was found at the outer edge of the inner molecular layer, while the outer molecular layer did not contain any positive elements. It is concluded that the hippocampal formation receives a distinct, well organized substance P innervation. Substance P fibers appear to enter the hippocampus both laterally across the angular bundle and ventrally through the fimbria. From this detailed examination of the distribution of the substance P fibers within the hippocampus in relation to the other known inputs to this area it is suggested that many of the substance P fibers arise in or, more probably, pass through the septal area to innervate the hippocampus.