The substance P innervation of the rat hippocampal region

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Abstract
The distribution of substance P (SP) immunoreactive nerve cell bodies and preterminal processes was studied in the rat brain by using several anti-SP-antibodies in combination with immunohistochemical techniques. In normal rats and in rats pretreated with colchicine, SP immunoreactive preterminal processes were found in the hippocampal region, but SP positive cellbodies could be detected only after colchicine pretreatment. Medium-sized to large, multipolar cells immunoreactive for SP were found in stratum oriens of the hippocampal subfield CA3 and in the hilus of the area dentata. Medium-sized to small, round or fusiform cells were detected in the pyramidal layer of the ventral subiculum and in layers III–VI of the ventral entorhinal area. The SP stained preterminal processes were of two types. Numerous fine, varicose axons were stained in different parts of Ammon's horn, while in the retrohippocampal structures, the SP immunoreactivity was present in small distinctly stained puncta. These frequently formed pericellular arrangements around unstained cells, indicative of axosomatic contacts between SP terminals and cells in the hipocampus. In Ammon's horn, the densest SP innervation was found in strata oriens, radiatum and moleculare of subfields CA3a and CA2. Scattered fibers were also present in the stratum oriens of CA3a-c and in the hilus, in particular at ventral levels. In retrohippocampal structures, the SP innervation predominated in the deep pyramidal layer of the subiculum, the second layer of the presubiculum and in layers VI and IV of the medial and lateral entorhinal area. Many of these terminals may arise from local interneurons as well as from sources outside the hippocampal region. Taken together, these studies demonstrate a far more extensive innervation by SP, or a closely related peptide, of the rat hippocampal region than was previously recognized. This suggests that SP may play an important role in neurotransmission within the hippocampal region.

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