GABAERGIC CELLS ARE THE MAJOR POSTSYNAPTIC TARGETS OF MOSSY FIBERS INTHE RAT HIPPOCAMPUS

Dentate granule cells communicate with their postsynaptic targets by t hree 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 f ibers by combining in vivo intracellular labeling of granule cells, im munocytochemistry, and electron microscopy. Single granule cells forme d 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 mGluR1 a-receptor, were selectively innervated by very thin (filopodial) exte nsions of the mossy terminals and by small en passant boutons in both the hilar and CA3 regions. These terminals formed single, often perfor ated, 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. The se findings show that in contrast to cortical pyramidal neurons, (1) g ranule cells developed distinct types of terminals to affect interneur ons and pyramidal cells and (2) they innervated more inhibitory than e xcitatory cells. These findings may explain the physiological observat ions that increased activity of granule cells suppresses the overall e xcitability of the CA3 recurrent system and may form the structural ba sis of the target-dependent regulation of glutamate release in the mos sy fiber system.