Abstracts

RATIONALE: Small-conductance Ca²⁺-activated K⁺ channels (SK channels) underlie the prolonged postspike afterhyperpolarization (AHP) observed in many central neurons and modulate firing frequency and pattern. Neuronal hyperactivity such as occurs during seizures is associated with a rise in intracellular Ca²⁺. Levetiracetam (LEV; KEPPRA[reg]) has been reported to suppress a non-GABA[sub]A[/sub]-mediated epileptogenic effect of bicuculline (Margineanu et al. [italic]Br. J. Pharmacol.[/italic] 1997;122:1146) and we reported that bicuculline blocks SK channels (Seutin and Johnson [italic]TIPS[/italic] 1999;20:268). This suggests a putative interaction with the SK channel blocking properties of this compound. Thus, the possibility that LEV could interfere with Ca²⁺ signaling and facilitate SK channel activation, thereby reducing neuronal excitability, has been explored in this work.
METHODS: Intracellular recordings of CA1 hippocampus pyramidal cells were performed in rat brain slices. These cells possess both I[sub]AHP[/sub] and sI[sub]AHP[/sub] currents underlying a medium and a slow AHP, respectively, and involved in setting frequency of discharge and spike frequency adaptation. Cell excitability was measured by giving depolarizing pulses of increasing amplitude and counting the number of evoked action potentials (APs). Reference compounds used for validation purposes were apamin (I[sub]AHP[/sub] blocker), isoproterenol (indirect sI[sub]AHP[/sub] blocker) and EBIO (1-ethyl-2-benzimidazolinone), a stabilizer of the Ca²⁺-calmodulin-SK channel interaction.
RESULTS: Validation experiments showed that: 1) apamin increased the number of APs evoked in CA1 neurons without modifying spike frequency adaptation; 2) isoproterenol increased the number of APs and suppressed spike frequency adaptation; 3) EBIO induced a marked and reversible decrease of the excitability of CA1 neurons, an effect which persisted in the presence of either apamin or isoproterenol. These results are in agreement with published data (Pedarzani et al. [italic]J. Biol. Chem.[/italic] 2001;276:9762). LEV (10 to 100 [mu]M) failed to modify the excitability of CA1 neurons when applied alone or in the presence of apamin or isoproterenol. Furthermore no effect of LEV was observed on AP parameters, input resistance or resting membrane potential.
CONCLUSIONS: The present results do not support the hypothesis that a modulation of SK channels is involved in the anti-seizure activity of LEV. The lack of effect on AP parameters is in agreement with previous data showing that LEV does not block voltage-dependent Na⁺ channels and the lack of action on basal input resistance and membrane potential demonstrate that LEV does not modulate the activity of the leak channels expressed by these neurons.
[Supported by: UCB S.A. Pharma Sector]; (Disclosure: Grant - UCB S.A.)