Abstracts

RATIONALE: Potassium homeostasis plays an important role in the control of neuronal excitability. Glial cells have been implicated in this ionic regulatory process by supporting the homeostatic regulation of the neuronal environment. In our previous in vitro studies of CNS glia in hippocampal slices, we found that ERG (ether-a-go-go) potassium channels are selectively localized in astrocytes. Electrophysiological characterization of ERG channels suggested that they contribute to the modulation of extracellular potassium levels following neuronal activation, and thus may play a critical role in controlling excitability and epileptogenicity. We now examine long-term changes resulting from ERG channel blockade, to assess their role in the chronic modulation of excitability.
METHODS: Hippocampal organotypic cultures were prepared from 4-7 days old neonatal Sprague Dawley rats and maintained for 14 days before being used in the experiments. Recordings from hippocampal cultures were performed with a Biocell Interface multi-electrodes recording system. This system allows simultaneous stimulation and recording at multiple sites (up to 8 different locations). The recording can be conducted over days, so that electrical activity within the hippocampal circuitry can be followed closely for long-term studies. CA1 field potentials evoked by stimulation of the Schaffer collaterals were recorded before, during, and after injection of 100 [mu]M dofetilide (a selective ERG channel blocker), on Day 1 of the experiment. Additional recordings were collected, with the stimulation protocols repeated without drug administration, on Day 2 and Day 3. A group of cultures treated with ACSF on Day1 was used as control. Paired pulses (PP) (0.1Hz) were delivered to assess hippocampal network inhibition/facilitation using inter-stimulus intervals (ISIs) of 30, 70, and 150 ms. Tests were repeated for three consecutive days for long-term observation.
RESULTS: Baseline recordings showed the appearance of episodes of spontaneous bursting activity in the dofetilide treated cultures on Day 1; spontaneous burst activity was observed also on Days 2 and 3. In contrast, the control group never showed any spontaneous field discharge. Population spike (PS) amplitude in CA1 increased significantly (p[lt]0.005, n=9) on Day 1 during and after the treatment with dofetilide and in comparison to saline controls. PS amplitude remained elevated on Days 2 and 3 post-dofetilide treatment (p[lt]0.05, n=4). While the PP ratio at the 30ms ISI did not show significant changes on Day1 and Day2, a significant increase in PP inhibition (p[lt]0.001) was observed on Day 3. A slight but non-significant increase in facilitation was observed at the 70 and 150ms ISIs on Day 1. A significantly (P[lt]0.001) enhanced facilitation was observed on Day 3 at 150ms ISIs.
CONCLUSIONS: Our results support the view that ERG channels contribute to the regulation of neuronal activation, presumably by their regulation of potassium homeostasis. These data also show long-lasting changes of the electrical activity following ERG channels blockade. Additional experiments are needed to explain the nature of these long-lasting changes. Because ERG channels are specific to glia, these data provide additional evidence for the critical role played by glia in controlling excitability of the neuronal population.
[Supported by: CURE]