CALRETININ IS PRESENT IN NONPYRAMIDAL CELLS OF THE RAT HIPPOCAMPUS .3. THEIR INPUTS FROM THE MEDIAN RAPHE AND MEDIAL SEPTAL NUCLEI

The subcortical innervation of a recently described subpopulation of non-pyramidal neurons, containing the calcium binding protein, calretinin, was investigated in the rat hippocampus using the anterograde tracer Phaseolus vulgaris-leucoagglutinin and double immunocytochemistry for calretinin and serotonin at the light and electron microscopic levels. Our results show that the GABAergic component of the septohippocampal pathway and the serotonergic raphe afferents establish multiple synaptic contacts with the calretinin-immunoreactive interneurons. The majority of the targets of both pathways were spine-free calretinin neurons known to innervate the dendritic region of the principal cells, but the GABAergic septal pathway was found to terminate also on the spiny neurons of stratum lucidum of the CA3 region and in the dentate hilus. The present results demonstrate that the serotonergic raphe-hippocampal and the GABAergic septohippocampal pathways are able to modulate dendritic inhibition of principal cells via calretinin-containing GABAergic interneurons.