TARGET SELECTIVITY AND NEUROCHEMICAL CHARACTERISTICS OF VIP-IMMUNOREACTIVE INTERNEURONS IN THE RAT DENTATE GYRUS

Vasoactive intestinal polypeptide (VIP) has been shown to be present i n a morphologically heterogeneous subpopulation of interneurons in the dentate gyrus, but the relationship between their input and output ch aracteristics and neurochemical features has not been established. Thr ee types of VIP-immunoreactive cells have been identified on the basis of these criteria: (i) cells forming a dense axonal plexus in the hil us have always coexisted with the calcium binding protein calretinin ( CR), but never with the neuropeptide cholecystokinin (CCK). The postsy naptic targets of these VIP-positive cells were neurons visualized by immunostaining for substance P receptor, which is known to label diffe rent hilar non-principal cells. (ii) VIP-immunoreactive basket cells, innervating predominantly the somata and proximal dendrites of granule cells, were found in the stratum moleculare and stratum granulosum. T hey contained CCK, but not CR. (iii) Cells projecting to the stratum m oleculare were found to have dendrites and axons restricted to this la yer. In 75% of these cells VIP coexisted with CR but not with CCK, and they established multiple contacts largely with non-principal cells. GABA was shown to be present but the calcium-binding proteins calbindi n D28K and parvalbumin were absent in all three types of VIP-containin g interneuron. On the basis of these observations we conclude that thr ee different types of VIP-positive neuron are present in this area, an d are likely to subserve different inhibitory functions: cells with a hilar projection as well as those projecting to the stratum moleculare may synchronize the activity of hilar and other interneurons, or disi nhibit granule cells by specific interneuron-to-interneuron connection s. In contrast, basket cells control the activity of granule cells dir ectly, via perisomatic inhibition.