CORRELATED MORPHOLOGICAL AND NEUROCHEMICAL FEATURES IDENTIFY DIFFERENT SUBSETS OF VASOACTIVE INTESTINAL POLYPEPTIDE-IMMUNOREACTIVE INTERNEURONS IN RAT HIPPOCAMPUS

Vasoactive intestinal polypeptide-immunoreactive interneurons have bee n classified according to their axonal and dendritic patterns and neur ochemical features in the hippocampus of the rat. A correlation of the se characteristics unravelled three distinct types of vasoactive intes tinal polypeptide-containing cells. Interneurons forming a dense axona l plexus at the border of stratum oriens and alveus always contain the calcium binding protein, calretinin, but lack the neuropeptide cholec ystokinin. The axon of another type of vasoactive intestinal polypepti de-positive interneuron surrounds pyramidal cell bodies in a basket-li ke manner, and co-localizes cholecystokinin but not calretinin. Vasoac tive intestinal polypeptide-containing cells projecting to stratum rad iatum form two subsets distinguished by dendritic morphology. Those wi th dendrites restricted to stratum lacunosum-moleculare lack both calr etinin and cholecystokinin, whereas the other subtype with dendrites s panning all layers contains calretinin in 40% of the cases and occasio nally also cholecystokin. GABA was shown to be present, and the calciu m binding proteins calbindin D-28k and parvalbumin absent from all thr ee types of vasoactive intestinal polypeptide-positive interneurons. T he specific dendritic and axonal arbours imply different input and out put properties for the three interneuron types. The correlation of the se features with the content of neurochemical markers strongly suggest s that they are specialized for distinct inhibitory functions in the h ippocampal network. Copyright (C) 1996 IBRO. Published by Elsevier Sci ence Ltd.