Abstracts

Rationale: Exposure to the group I metabotropic glutamate receptor (mGluR) agonist dihyrdroxyphenylglycine (DHPG) produces long-lasting changes in excitability and spontaneously occurring epileptiform activity in the CA3 region of rat hippocampal slices. The epileptiform activity consists of interictal discharges and long lasting (>2s) synchronous activity that resembles ictal discharges. We evaluated the afterhyperpolarization (AHP) that follows neuronal firing in neurons exposed to DHPG and related the change to the pattern of epileptiform activity. Methods: Rat hippocampal slices were prepared from 7-40 day old animals and exposed to 50 μM S- or 100 μM R,S-DHPG for 90-120 min. After exposure they were transferred to control artificial cerebrospinal fluid. Patterns of spontaneously occurring epileptiform activity were assessed. In the slices from older animals (30-40 days), sharp electrode recordings were made to characterize the AHP that followed repetitive firing produced by a depolarizing current pulse. The slices from younger animals (7-20) were used to perform whole-cell voltage-clamp recordings to assess the current responsible for the AHP. Results: In slices that demonstrated ictal patterns, the AHP that followed depolarization and action potential generation was not prominent compared to control slices (peak amplitude in control 9.0 ± 1.1 vs. 1.3 ± 0.6 mV, p < 0.01). There was no difference in the resting membrane potential (-61.0 ± 1.4 for control and -59.1 ± 1.2 mV for DHPG-exposed). Control slices demonstrated a peak outward current of 173 ± 13 pA compared to DHPG-exposed neurons which had a peak current of 17 ± 5 pA (p < 0.01). This included a slow as well as a medium AHP current. Bath application of 1-EBIO (1 mM), a compound that enhances both the slow and medium AHP, led to a suppression of ictal discharges. Whole-cell voltage-clamp recordings demonstrated the return of the AHP current in DHPG-exposed neurons when 1-EBIO was bath-applied (12 ± 2 pA vs. 107 ± 11 pA, p < 0.01). DCEBIO (100 μM), a more selective compound for medium AHP also converted ictal slices to an interictal pattern. Conclusions: The suppression of the AHP produced by DHPG exposure was associated with ictal patterns of epileptiform activity. Drugs that brought back the medium or slow AHP suppressed ictal discharges. These results support the hypothesis that the AHP can prevent the transition to ictal activity and loss of the AHP favors long lasting ictal activity. Supported by VA research