Unusual target selectivity of perisomatic inhibitory cells in the hilar region of the rat hippocampus

Perisomatic inhibitory innervation of all neuron types profoundly affects t heir firing characteristics and vulnerability. In this study we examined th e postsynaptic targets of perisomatic inhibitory cells in the hilar region of the dentate gyrus where the proportion of potential target cells (excita tory mossy cells and inhibitory interneurons) is approximately equal. Both cholecystokinin (CCK) and parvalbumin-immunoreactive basket cells formed mu ltiple contacts on the somata and proximal dendrites of mossy cells. Unexpe ctedly, however, perisomatic inhibitory terminals arriving from these cell types largely ignored hilar GABAergic cell populations. Eighty-ninety perce nt of various GABAergic neurons including other CCK-containing basket cells received no input from CCK-positive terminals. Parvalbumin-containing cell s sometimes innervated each other but avoided 75% of other GABAergic cells. Overall, a single mossy cell received 40 times more CCK-immunoreactive ter minals and 15 times more parvalbumin-positive terminals onto its soma than the cell body of an average hilar GABAergic cell. In contrast to the pronou nced target selectivity in the hilar region, CCK-and parvalbumin-positive n eurons innervated each other via collaterals in stratum granulosum and mole culare. Our observations indicate that the inhibitory control in the hilar region i s qualitatively different from other cortical areas at both the network lev el and the level of single neurons. The paucity of perisomatic innervation of hilar interneurons should have profound consequences on their action pot ential generation and on their ensemble behavior. These findings may help e xplain the unique physiological patterns observed in the hilus and the sele ctive vulnerability of the hilar cell population in various pathophysiologi cal conditions.