The distribution of substance P in the cerebral cortex and hippocampal formation: an immunohistochemical study in the monkey and rat

The distribution of substance P-containing fibers in the cerebral cortex and the hippocampal formation of the Japanese monkey (Macaca fuscata fuscata) was studied by immunohistochemistry using a monoclonal antibody raised against substance P. The results were compared with the distribution in homologous regions of the rat brain. Substance P-containing fibers and cell bodies were observed in all regions of the cerebral cortex. In deep layers of the neocortex (IV-VI), substance P-immunoreactive fibers formed arrays that ran perpendicular to the surface. These immunoreactive fibers tended to branch as they approached the cortical surface in layers II and III, at which point they were oriented in many directions. The molecular layer (I) of the monkey neocortex contained many granular, substance P-immunoreactive structures, resembling terminal boutons. In contrast to the monkey, rat cortical areas contained substantially fewer substance P-containing fibers. The immunoreactive profiles, mostly fine dot-like structures, were seen uniformly in layers II and IV of the rat neocortex, although in the medial prefrontal cortex many thick, varicose fibers were also observed. Substance P-containing fibers were seen throughout the hippocampal formation of the monkey, including the subiculum and the parahippocampal regions. The regional distribution of immunoreactive fibers was most dense in the molecular layers of dentate gyrus, in the stratum moleculare of the CA1 region, and in the stratum pyramidalis of the CA2 region. In the rat, the hippocampus and dentate gyrus contained fewer immunoreactive fibers. Moderate densities were observed in the rat subiculum and entorhinal cortex.(ABSTRACT TRUNCATED AT 250 WORDS)