The hyperexcitability of dentate granule neurons in organotypic hippocampal slice cultures is due to reorganization of synaptic inputs in vitro

Organotypic hippocampal slice cultures ( OHSC s) provide the experimental flexibility of cell culture while leaving much of the natural neuronal connectivity intact. Previously, it was shown that the functional and morphological features of CA 1 pyramidal neurons in OHSC s resemble, to a surprising extent, those of CA 1 neurons in the acute brain slice preparation. However, the extent to which the characteristics of other principle hippocampal neurons change or are preserved in cultured slices remains to be determined. In the present study, I initially sought to understand whether and how the synaptic inputs and morphology of cultured dentate granule neurons ( GC s) differ from GC s that have developed in vivo. To this end, I compared GC s in OHSC s and GC s in acute slices at two equivalent developmental time points (P14 vs. DIV 7 and P21 vs. DIV 21). The findings suggest that there is considerable reorganization of synaptic input to the organotypic GC s, such that these cells are more susceptible to hyperexcitation than GC s in acute slices after 3 weeks. It appears that this hyperexcitability emerges through an increase in the proportion of mature synapses at proximal dendritic sites and is accompanied by an increase in inhibitory neuron activity. These alterations appear to arise in a coordinated manner such that the substantial increase in excitatory synaptic drive received by the DIV 21 GC s in OHSC s remains local and is not translated into excessive output possibly leading to damage or major morphological alterations of downstream pyramidal neurons.