Convergence of excitatory and inhibitory inputs onto CCK-containing basket cells in the CA1 area of the rat hippocampus

The number and distribution of excitatory and inhibitory inputs affect the integrative properties of neurons. These parameters have been studied recently for several hippocampal neuron populations. Besides parvalbumin- (PV) containing cells that include basket and axo-axonic cells, cholecystokinin (CCK)-containing interneurons also form a basket cell population with several properties distinct from PV cells. Here, at the light microscopic level, we reconstructed the entire dendritic tree of CCK-immunoreactive (IR) basket cells to describe their geometry, the total length and laminar distribution of their dendrites. This was followed by an electron microscopic analysis of serial ultrathin sections immunostained against gamma-aminobutyric acid, to estimate the density of excitatory and inhibitory synapses on their somata, axon initial segments and different subclasses of dendrites. The dendritic tree of CCK-IR basket cells has an average length of 6300 microm and penetrates all layers. At the electron microscopic level, CCK basket cells receive dendritic inputs with a density of 80-230 per 100 microm. The ratio of inhibitory inputs is relatively high (35%) and increases towards the soma (83%). The total numbers of excitatory and inhibitory synapses converging onto CCK-IR cells are approximately 8200. Comparison of the two, neurochemically distinct basket cells reveals that CCK-containing basket cells receive much less synaptic input than PV cells; however, the relative weight of inhibition is higher on CCK cells. Additional differences in their anatomical and physiological properties predict that CCK basket cells are under a more diverse, elaborate control than PV basket cells, and thus the function of the two populations must be different.