Cholecystokinin‐, enkephalin‐, and substance P‐like immunoreactivity in the dentate area, hippocampus, and subiculum of the domestic pig

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Abstract
The distribution of cholecystokinin‐like, enkephalin‐like, and substance P‐like immunoreactivities is described in the dentate area, hippocampus, and subiculum of the domestic pig (Sus scrofa domesticus) as a baseline for future experimental studies. The distributions in the pig are compared with previous observations in other species. Cholecystokinin‐like immunoreactive nerve cell bodies were intensely stained and present in large numbers in all subfields studied. Cholecystokinin‐like immunoreactive terminals appeared as stained puncta, whereas fibers were only rarely encounterd. The puncta were mainly seen in the dentate molecular layer and dentate granule cell layer, the pyramidal cell layer of the hippocampal regio inferior, stratum moleculare of the hippocampal regio superior, and in the subiculum. Enkephalin‐like immunoreactive nerve cell bodies were faintly stained and generally present in very small numbers, except for some pyramidal cells in the subicular cell layer. Enkephalin‐like immunoreactive fibers were few in number, whereas stained puncta appeared with variable densities. Puncta of particularly high densities were found in the dentate molecular layer, whereas they appeared of moderate density in the dentate hilus, stratum moleculare of the hippocampal regio superior, and in the subiculum. Substance P‐like immunoreactive nerve cell bodies were few and very faintly stined. They primarily occurred in the dentate hilus, stratum oriens of the hippocampus, and in the subicular cell layer. Stained fibers were few in number, whereas stained puncta were present in abundant numbers corresponding to the mossy fiber projection in the dentate hilus and the layer of mossy fibers of the hippocampal regio inferior, and in moderate numbers in stratum moleculare of the hippocampal regio superior and in the subiculum. For all three neuropeptides there were consistent and very characteristic variations in the distribution of immunoreactivity along the septotemporal axis of the hippocampus. When viewed in a comparative perspective the distribution of enkephalin‐like and substance P‐like terminals in the domestic pig displayed striking differences from the basic pattern observed in other species. This contrasted with the distribution of cholecystokinin‐like neurons and terminals, which resembled more closely these species.