GABAergic Cells Are the Major Postsynaptic Targets of Mossy Fibers in the Rat Hippocampus

Abstract

Dentate granule cells communicate with their postsynaptic targets by three distinct terminal types. These include the large mossy terminals, filopodial extensions of the mossy terminals, and smaller en passant synaptic varicosities. We examined the postsynaptic targets of mossy fibers by combining in vivo intracellular labeling of granule cells, immunocytochemistry, and electron microscopy. Single granule cells formed large, complex "mossy" synapses on 11-15 CA3 pyramidal cells and 7-12 hilar mossy cells. In contrast, GABAergic interneurons, identified with immunostaining for substance P-receptor, parvalbumin, and mGluR1a-receptor, were selectively innervated by very thin (filopodial) extensions of the mossy terminals and by small en passant boutons in both the hilar and CA3 regions. These terminals formed single, often perforated, asymmetric synapses on the cell bodies, dendrites, and spines of GABAergic interneurons. The number of filopodial extensions and small terminals was 10 times larger than the number of mossy terminals. These findings show that in contrast to cortical pyramidal neurons, (1) granule cells developed distinct types of terminals to affect interneurons and pyramidal cells and (2) they innervated more inhibitory than excitatory cells. These findings may explain the physiological observations that increased activity of granule cells suppresses the overall excitability of the CA3 recurrent system and may form the structural basis of the target-dependent regulation of glutamate release in the mossy fiber system.

Timing and efficacy of transmitter release at mossy fiber synapses in the hippocampal network

“…They project to the CA3 region, mainly traveling within a narrow band termed "stratum lucidum." Several (∼15) large "giant" boutons (∼3-5-μm diameter) emerge from a single mossy fiber axon, either arranged in an en passant manner or attached to the main axon via a short perpendicular axonal branch [1,8,29,31] (Fig. 1c,d).…”

“…(1) Large boutons emerging from hilar collaterals (∼15 per mossy fiber axon) form synapses on glutamatergic hilar mossy cells [1]. Mossy cells, in turn, establish an excitatory feedback circuit by forming synapses on granule cells of the ipsilateral dentate gyrus and on granule cells of the contralateral side.…”

“…Mossy cells, in turn, establish an excitatory feedback circuit by forming synapses on granule cells of the ipsilateral dentate gyrus and on granule cells of the contralateral side. (2) Filopodial extensions emerging from large mossy fiber boutons (∼2.5 filopodia per large bouton, [1]) form synapses on GABAergic interneurons in the stratum lucidum [1,81] (Fig. 1d).…”

“…In addition, large mossy fiber boutons can contact the dendritic shafts of GABAergic interneurons in the stratum lucidum directly [29]. (3) Small boutons emerging from the main axon (∼15) or from hilar collaterals (∼150 per mossy fiber axon) form synapses on GABAergic interneurons in the stratum lucidum and the hilus [1]. Thus, the total number of synapses formed on GABAergic interneurons (in CA3 and hilus) via filopodial extensions and small boutons greatly exceeds the total number of synapses formed on glutamatergic neurons (CA3 pyramidal neurons and mossy cells) via large boutons [1].…”

“…Arrows indicate mossy fiber axons, arrowheads show filopodial extensions, which contact interneuron dendrites. Illustration from Acsády et al [1], with permission of the Society for Neuroscience, copyright (1998) vertical direction, resulting in an improved preservation of superficial tissue layers [34], although mossy fiber terminal recordings with commercially available tissue slicers were also reported [72]; (3) the use of sucrose-based cutting solutions with reduced Na + and Ca 2+ concentrations, which further improves tissue preservation during cutting and maintenance of slices [34]. Collectively, these refinements made it possible to routinely obtain patch-clamp recordings from large hippocampal mossy fiber boutons in the stratum lucidum [6,27,33,37].…”