Abstracts

NMDA receptor-mediated synaptic transmission is increased in the hippocampi of epileptic rats and in humans. However, the precise level at which this increase is mediated remains unknown because of the difficulties inherent to recordings of NMDA-receptor mediated spontaneous synaptic events. Here we describe the basic properties of pharmacologically isolated spontaneous and evoked NMDA receptor mediated synaptic events in dentate gyrus granule cells of control and kindled rats.
Male Wistar rats were surgically implanted with bipolar stimulating electrodes in the hippocampal commissures and were stimulated once daily, five days a week until the occurrence of at least ten consecutive grade 5 seizures. Horizontal brain slices were prepared 24-48 hrs after the last seizure for whole-cell [ldquo]blind[rdquo] voltage-clamp recordings of dentate gyrus granule cells. Recordings were performed at 34 [deg]C in modified ACSF containing 5 [micro]M Mg, 30 [micro]M picrotoxin, 10 [micro]M DNQX and 10 [micro]M D-serine. A small stimulus (40 [micro]s) was given every 30 s in the perforant path to compare spontaneous and evoked event characteristics. Ifenprodil (10 [micro]M), or D-APV (50 [micro]M) or D-CPP (25 [micro]M) was perfused after 10 min recording of basal activity. Alternatively, D-APV or D-CPP was perfused 10 min after ifenprodil. Recordings were digitized at 8 kHz, filtered at 2 kHz and analyzed using the EVAN Labview-based software.
Average peaks and 10-90% rise times (RT) of spontaneous NMDA receptor mediated events were not significantly different between control (n = 17) and kindled cells (n = 9). The decay time constants of the events were significantly increased in kindled cells (60.1 [plusmn] 4.9 ms, Mean [plusmn] S.E.M.) compared to 41.1 [plusmn] 2.9 ms in controls. Kindling significantly increased the baseline variance (10.8 [plusmn] 1.5 pA² vs 7.5 [plusmn] 1.0 pA² in controls) and a tonic NMDA-receptor mediated conductance, measured as the decrease in holding current upon perfusion of D-APV or D-CPP (46.5 [plusmn] 7.3 pA vs. 18.8 [plusmn] 3.0 pA in controls). In control cells, the NR2B antagonist ifenprodil (10 [micro]M) had no effect on the kinetic properties of spontaneous events but significantly reduced their frequency. It also reduced the peak of the stimulus-evoked events. In sharp contrast, ifenprodil had no significant effects on any of the properties of the spontaneous or stimulus-evoked events in the kindled rats.
Granule cells of kindled rats display an increase in the charge carried by spontaneous NMDA receptor-mediated synaptic events, mainly due to an increase in the decay time constant. Furthermore, the much increased baseline variance and effect of D-APV in kindled rats suggest that the activity of most likely extrasynaptically localted NMDA receptors is also increased. Finally, the lack of ifenprodil sensitivity in kindled rats points to an altered NMDA receptor composition in the granule cells themselves or in the neurons of the circuitry providing excitatory input to the dentate gyrus.
[Supported by: NIH/NINDS Grant NS 02808 and the Coelho Endowment to I.M.]