AXON ARBORS AND SYNAPTIC CONNECTIONS OF HIPPOCAMPAL MOSSY CELLS IN THE RAT IN-VIVO

The axon collateralization patterns and synaptic connections of intrac ellularly labeled and electrophysiologically identified messy cells we re studied in rat hippocampus. Light microscopic analysis of 11 biocyt in-filled cells showed that messy cell axon arbors extended through an average of 57% of the total septotemporal length of the hippocampus ( summated two-dimensional length, not adjusted for tissue shrinkage). A xon collaterals were densest in distant lamellae rather than in lamell ae near the soma. Most of the axon was concentrated in the inner one-t hird of the molecular layer, with the hilus containing an average of o nly 26% Of total axon length and the granule cell layer containing an average of only 7%. Ultrastructural analysis was carried out on three additional intracellularly stained messy cells, in which axon collater als and synaptic targets were examined in serial sections of chosen ax on segments. In the central and subgranular regions of the hilus, mess y cell axons established a low density of synaptic contacts onto dendr itic shafts, neuronal somata, and occasional dendritic spines. Most hi lar synapses were made relatively close to the messy cell somata. At g reater distances from the labeled messy cell (1-2 mm along the septote mporal axis), the axon collaterals ramified predominantly within the i nner molecular layer and made a high density of asymmetric synaptic co ntacts almost exclusively onto dendritic spines. Quantitative measurem ents indicated that more than 90% of messy cell synaptic contacts in t he ipsilateral hippocampus are onto spines of proximal dendrites of pr esumed granule cells. These results are consistent with a primary mess y cell role in an excitatory associational network with granule cells of the dentate gyrus. (C) 1996 Wiley-Liss, Inc.