NEUROCALCIN-IMMUNOREACTIVE CELLS IN THE RAT HIPPOCAMPUS ARE GABAERGICINTERNEURONS

Neurocalcin (NC) is a recently described calcium-binding protein isola ted and characterized from bovine brain. NC belongs to the neural calc ium-sensor proteins defined by the photoreceptor cell-specific protein recoverin that have been proposed to be involved in the regulation of calcium-dependent phosphorylation in signal transduction pathways. We analyzed the distribution and morphology of the NC-immunoreactive (IR ) neurons in the rat dorsal hippocampus and the coexistence of NC with GABA and different neurochemical markers which label perisomatic inhi bitory cells [parvalbumin (PV) and cholecystokinin (CCK)], mid-proxima l dendritic inhibitory cells [calbindin D28k (CB)], distal dendritic i nhibitory cells [somatostatin (SOM) and neuropeptide Y (NPY)], and int erneurons specialized to innervate other interneurons [calretinin (CR) and vasoactive intestinal polypeptide (VIP)]. NC-IR cells were presen t in all layers of the dentate gyrus and hippocampal fields. In the de ntate gyrus, NC-IR cells were concentrated in the granule cell layer, especially in the hilar border, whereas in the CA fields they were mos t frequently found in the stratum radiatum. NC-IR cells were morpholog ically heterogeneous and exhibited distinctive features of non-princip al cells. In the dentate gyrus, pyramidal-like, multipolar and fusifor m (horizontal and vertical) cells were found. In the CA3 region most N C-IR cells were multipolar, but vertical and horizontal fusiform cells also appeared. in the CA1 region, where NC-IR cells showed most frequ ently vertically arranged dendrites, multipolar, bitufted and fusiform (vertical and horizontal) cells could be distinguished. All the NC-IR cells were found to he GABA-IR in all hippocampal layers and regions, and they represented about 19% of the GABA-positive cells. NC/CB, NC/ CR and NC/VIP double-labeled cells were found in ail hippocampal regio ns, and represented 29%, 24% and 18% of the NC-IR cells, respectively. NC and CCK did not coexist in the dentate gyrus; however, 9% of the N C-IR cells in the CA fields also contained CCK. No coexistence of NC w ith PV SOM or NPY was found in any hippocampal region. We conclude tha t NC is exclusively expressed by interneurons in the rat hippocampus. MC-IR cells are a morphologically and neurochemically heterogeneous su bset of GABAergic non-principal cells, which, on the basis of the know n termination pattern of the colocalizing markers, are also functional ly heterogeneous and are mainly involved in feed-forward dendritic inh ibition in the commissural-associational and Schaffer collateral termi nation zones (CB containing cells), in innervation of other interneuro ns (CR-and VIP-containing cells), and in perisomatic inhibition (CCK-c ontaining cells). NC is never present in perisomatic inhibitory PV-con taining cells, or in feed-back distal dendritic inhibitory SOM/NPY-con taining cells. (C) 1998 Wiley-Liss, Inc.