Recurrent excitation of granule cells with basal dendrites and low interneuron density and inhibitory postsynaptic current frequency in the dentate gyrus of macaque monkeys

Temporal lobe epilepsy is often associated with pathological changes in the dentate gyrus, and such changes may be more common in humans than in some nonprimate species. To examine species-specific characteristics that might predispose the dentate gyrus to epileptogenic damage, we evaluated recurrent excitation of granule cells with and without basal dendrites in macaque monkeys, measured miniature inhibitory postsynaptic currents (mIPSCs) of granule cells in macaque monkeys and compared them to rats, and estimated the granule cell-to-interneuron ratio in macaque monkeys and rats. In hippocampal slices from monkeys, whole-cell patch recording revealed antidromically evoked excitatory PSCs that were four times larger and inhibitory PSCs that were over two times larger in granule cells with basal dendrites than without. These findings suggest that granule cells with basal dendrites receive more recurrent excitation and, to a lesser degree, more recurrent inhibition. Miniature IPSC amplitude was slightly larger in monkey granule cells with basal dendrites than in those without, but mIPSC frequency was similar and only 26% of that reported for rats. In situ hybridization for glutamic acid decarboxylase and immunocytochemistry for somatostatin, parvalbumin, and neuronal nuclei revealed interneuron proportions and distributions in monkeys that were similar to those reported for rats. However, the interneuron-to-granule cell ratio was lower in monkeys (1:28) than in rats (1:11). These findings suggest that in the primate dentate gyrus, recurrent excitation is enhanced and inhibition is reduced compared with rodents. These primate characteristics may contribute to the susceptibility of the human dentate gyrus to epileptogenic injuries.