Abstracts

RATIONALE: Epilepsy occurs after brain injury with some delay. While abberant axonal sprouting and defective synaptic inhibition have received widespread attention, relatively little is known about the postsynaptic changes that contribute to chronic hyperexcitability. METHODS: CA1 pyramidal cells in 14-day old organotypic hippocampal slice cultures were deafferented by making wedge-shaped lesions with a razor blade. After 1-10 days, whole-cell recordings were made from CA1 cells within the wedge, and glutamate or NMDA were photoreleased in a 30 m diameter spot at a distance of 100 m from the cell body with brief laser pulses (2-25 ms) in the presence of TTX, glycine, and 1 mM Mg2+. RESULTS: Photolysis of caged NMDA (200 M) produced inward currents of 20-150 pA lasting 2-3 sec at -60 mV. When voltage ramps from -60 to 0 mV (1 sec duration) were applied during the NMDA-activated inward current, a large, slow spike-like response could be elicited when the membrane potential reached -30 to -10 mV. Neither the depolarizing ramp nor NMDA application alone were sufficient to generate these 'NMDA spikes'. Partially deafferented cells required a 4.4-fold shorter laser pulse to trigger NMDA-spikes in conjunction with the ramp command than control cells (4.4 b0.3 ms vs. 19.6 b1.8 ms; n=13, 17). Thresholds for the NMDA spikes were comparable in apical and basal dendrites. At -60 mV, the threshold laser pulse duration for triggering NMDA-spikes elicited inward currents that were 2.8-fold smaller in deafferented cells than control cells. The threshold laser pulse durations for NMDA-spikes in CA1 cells examined T6 days postlesion were comparable to those in control cells. CONCLUSIONS: We conclude that the excitability of CA1 cells increases with a delay after they have been deafferented as a consequence of injury. This hyperexcitability is thus concomittant with a marked loss of dendritic spines. Use of photoreleased NMDA allows us to attribute this hyperexcitability to postsynaptic dendrites. Changes in postsynaptic excitability, coupled with the sprouting of presynaptic CA3 cell axons, are likely to play a prominent role in the generation of posttraumatic epilepsy.