REGIONAL VARIABILITY AND POSTSYNAPTIC TARGETS OF CHANDELIER CELLS IN THE HIPPOCAMPAL-FORMATION OF THE RAT

Chandelier cells are specialized cortical GABAergic neurons that estab lish synaptic contacts exclusively with the axon initial segments of p rincipal neurons. They are found in all regions of the hippocampal for mation. Here we describe their morphological features in the hilus and in regions CA1 and CA3 by using Golgi/electron microscopy. Attempts w ere also made to identify the target neurons of chandelier cells in th e hilus and entorhinal cortex. Golgi-impregnated chandelier cells disp lay a complex axonal arbor in CA1, with many collaterals forming strin gs of boutons. The axon plexuses of such cells are less developed in C A3, whereas those in the hilus cover the entire region, although singl e collaterals are rather simple, with only a few boutons. The dendrite s of chandelier cells in CA1 and CA3 have an orientation similar to th at of pyramidal cell dendrites and are thus likely to be activated by the same afferent fiber systems. The hilar chandelier cells do not giv e rise to dendrites invading the molecular layer. Thus, these cells ma y not receive a dense input from the entorhinal cortex but may be driv en by the abundant messy fiber collaterals in the hilar region. In the CA1 and CA3 regions, the axons of chandelier cells contact the axon i nitial segments of pyramidal cells. In the hilar region, gold-toned bo utons were found to impinge on the initial segments of neurons display ing characteristics of mossy cells. This notion was substantiated by e lectronmicroscopic analysis of messy cells identified by intracellular injection of Lucifer yellow. Those cells regularly showed numerous sy mmetric synapses on their axon initial segments. Entorhinohippocampal projection cells, identified by injection of horseradish peroxidase in to the hippocampus, were found to be preferential targets of chandelie r cells in the entorhinal cortex. Our data point to regional variation s in chandelier cell morphology and connectivity and indicate that cha ndelier cells are a principal component of inhibitory mechanisms in al l stations of the main excitatory pathway of the hippocampal formation . (C) 1996 Wiley-Liss, Inc.