SUBSTANCE-P INNERVATION OF THE RAT HIPPOCAMPAL-FORMATION

Light and electron microscopic substance P (SP) immunostaining was per formed on hippocampal sections of colchicine-pretreated, control, untr eated fimbria-fornix-transected (5 days), as well as perforant path-st imulated Sprague-Dawley rats fixed in 5% acrolein. Numerous SP-immunor eactive neurons could be observed in the stratum oriens of the Ammon's horn and subiculum, fewer were seen in the dentate hilar area and str atum radiatum of CA2 and CA3, and even fewer were seen at the border b etween the CA1 strata radiatum and the lacunosum moleculare of CA1 sub field. A higher dose of colchicine resulted in SP immunoreactivity in a large population of granule cells and messy axon terminals. The enti re CA2 region, the stratum oriens of CA1, CA3, and the subiculum were densely innervated by SP-containing axon terminals. A homogeneous SP i nnervation was found in the stratum radiatum of CA1. Only a few SP fib ers were seen adjacent to the granule cell layer. Symmetric axosomatic contacts were seen between SP-containing boutons and somata in the st ratum oriens of the Ammon's horn. However, throughout the hippocampal formation, the majority of SP-containing axons formed axodendritic sym metric synapses. A dense population of SP-immunoreactive boutons that formed axodendritic asymmetric synapses was observed in the strata ori ens and radiatum of the CA3a and CA2 regions, and a few were found in the supragranular and subgranular layers of the dentate gyrus. Fimbria -fornix transection resulted in a marked loss of SP fibers in the stra ta oriens, pyramidale, and radiatum of the CA3a and CA2 subfields. In perforant pathway-stimulated animals, a population of granule cells an d a large number of messy axon terminals were immunoreactive for SP. T hese observations suggest two sources of SP innervation to the hippoca mpal formation: one arising from intrinsic sources (interneurons and g ranule cells) and one arising from extrinsic sources, most likely the supramammillary region. (C) 1997 Wiley-Liss, Inc.