Physiological and anatomical correlates of the human dentate gyrus: consequences or causes of epilepsy.

The presence of paroxysmal discharges in the epileptic human dentate gyrus provides a physiologic basis for hyperexcitability that may initiate seizure discharges during the development of epilepsy. Although these responses can occur with single orthodromic stimulation, data obtained under conditions that weaken synaptic inhibition (e.g., 1 Hz stimulation or bicuculline disinhibition) suggest that paroxysmal discharges may be a more common feature of tissue from temporal lobe epileptic patients than has been reported previously. Hilar cell loss and weakened synaptic inhibition may provide conditions favorable for the activation of N-methyl-D-aspartate acid (NMDA) receptors that would allow triggering of paroxysmal discharges that normally never are evoked in dentate granule cells in nonepileptic humans. As the dentate gyrus in normal animal tissue is not susceptible to intrinsic bursting behavior and is characterized by a relatively short duration excitatory postsynaptic potential even under pharmacologic disinhibition, paroxysmal discharges in the epileptic human dentate gyrus may provide an important clue to understanding the prerequisite conditions for seizure discharge.