DIFFERENT POPULATIONS OF VASOACTIVE INTESTINAL POLYPEPTIDE-IMMUNOREACTIVE INTERNEURONS ARE SPECIALIZED TO CONTROL PYRAMIDAL CELLS OR INTERNEURONS IN THE HIPPOCAMPUS

The postsynaptic targets of three vasoactive intestinal polypeptide-co ntaining GABAergic interneuron types were examined in the rat hippocam pus. Two of them showed remarkable target selectivity for other GABAer gic neurons, while the third contacted the somata and proximal dendrit es of pyramidal cells. Vasoactive intestinal polypeptide-positive inte rneurons innervating the stratum oriens/alveus border in the CA1 regio n were shown to establish multiple contacts with horizontal GABAergic interneurons immunoreactive for type 1 metabotropic glutamate receptor . Similarly, identified axons of vasoactive intestinal polypeptide-pos itive interneurons projecting to stratum radiatum were found to establ ish symmetrical synapses largely on GABAergic dendrites. The majority of these postsynaptic GABAergic neurons were shown to contain calbindi n or vasoactive intestinal polypeptide. In contrast to the first two v asoactive intestinal polypeptide-containing cell populations, vasoacti ve intestinal polypeptide-positive interneurons arborizing in stratum pyramidale formed baskets around pyramidal cells. These results reveal ed a new element in cortical microcircuits, interneurons which are spe cialized to innervate other GABAergic interneurons. The role of this n ew component may be the synchronization of dendritic inhibition, or an input-specific disinhibition of pyramidal cells in various dendritic domains. In contrast, vasoactive intestinal polypeptide-containing bas ket cells are likely to be involved in perisomatic inhibition of pyram idal neurons, and represents a new basket cell type different from tha t containing parvalbumin. Copyright (C) 1996 Published by Elsevier Sci ence.