IMMUNOSTAINING FOR SUBSTANCE-P RECEPTOR LABELS GABAERGIC CELLS WITH DISTINCT TERMINATION PATTERNS IN THE HIPPOCAMPUS

A specific antiserum against substance P receptor (SPR) labels nonprin cipal neurons in the cerebral cortex of the rat (T. Kaneko et al. [199 4], Neuroscience 60:199-211; Y. Nakaya et al. [1994], J. Comp. Neurol. 347:249-274). In the present study, we aimed to identify the types of SPR-immunoreactive neurons in the hippocampus according to their cont ent of neurochemical markers, which label interneuron populations with distinct termination patterns. Markers for perisomatic inhibitory cel ls, parvalbumin and cholecystokinin (CCK), colocalized with SPR in pyr amidallike basket cells in the dentate gyrus and in large multipolar o r bitufted cells within all hippocampal subfields respectively. A dens e meshwork of SPR-immunoreactive spiny dendrites in the hilus and stra tum lucidum of the CA3 region belonged largely to inhibitory cells ter minating in the distal dendritic region of granule cells as indicated by the somatostatin and neuropeptide Y (NPY) content. In addition, SPR and NPY were colocalized in numerous multipolar interneurons with den drites branching close to the soma. Twenty-five percent of the SPR-imm unoreactive cells overlapped with calretinin-positive neurons in all h ippocampal subfields, showing that interneurons specialized to contact other gamma-aminobutyric acid-ergic cells may also contain SPR. On th e basis of the known termination pattern of the colocalized markers, w e conclude that SPR-positive interneurons are functionally heterogeneo us and participate in different inhibitory processes: (1) perisomatic inhibition of principal cells (CCK-containing cells, and parvalbumin-p ositive cells in the dentate gyrus), (2) feedback dendritic inhibition in the entorhinal termination zone (somatostatin and NPY-containing c ells), and (3) innervation of other interneurons (calretinin-containin g cells). (C) 1997 Wiley-Liss, Inc.