Abstracts

RATIONALE: Recently, we have demonstrated that principal neurons of the entorhinal cortex (EC) display a TTX-sensitive voltage-dependent persistent sodium current (I[sub]NaP[/sub]), which activates at subthreshold potentials and has very slow inactivation. I[sub]NaP[/sub] has been proposed to contribute to subthreshold oscillations, excitability and epileptogenesis. In this study, we investigated whether I[sub]NaP[/sub] displays plasticity during the development of an epileptic condition by comparing persistent sodium currents in EC Layer II neurons of age-matched control rats and pilocarpine epileptic rats, both at 2 weeks and at 9 weeks post-pilocarpine injection. We also investigated I[sub]NaP[/sub] in entorhinal cortex obtained from selective amygdalohippocampectomies of human patients with refractory temporal lobe epilepsy. In addition, the role of the pharmacological agent ethosuximide was examined.
METHODS: Male Long-Evans rats were made epileptic following a standard scopolamine methlybromide (1 mg/kg) and pilocarpine (380 mg/kg) treatment protocol, with post-application of diazepam (1 mg/kg) one hour after status epilepticus (SE). Only animals that displayed SE were selected for the epileptic population. Control animals were treated identically except with the administration of saline instead of pilocarpine. Horizontal brain slices (400 [mu]) were prepared, EC Layer II was dissected out and its neurons were acutely dissociated for voltage-clamp studies. Only rats at 2 and 9 weeks post-pilocarpine and their age-matched controls were studied. Voltage-step and ramp protocols were applied to explore transient and persistent sodium currents. TTX (1 [mu]M) was used to block sodium currents and isolate I[sub]NaP[/sub]. For human TLE tissue, neurons were acutely dissociated and investigated following similar procedures. Ethosuximide (5 [mu]M to 1.6 mM) was used to examine pharmacological effects on transient and persistent currents.
RESULTS: The 2-week control and epileptic groups displayed a similar peak I[sub]NaP[/sub] (pA/pF) of 5.9[plusminus]0.7 ([italic]n[/italic]=13) and 6.5[plusminus]1.0 ([italic]n[/italic]=18), respectively. At ~9-weeks, the epileptic group displayed a significantly larger I[sub]NaP[/sub] (12.7[plusminus]1.6; [italic]n[/italic]=12) than the control group (7.2[plusminus]0.8; [italic]n[/italic]=6). We found no significant differences in the voltage dependence of activation between groups. Persistent sodium channel activity was also found to be present in the surgically resected TLE human entorhinal cortical principal neurons. Ethosuximide showed evidence for blocking transient and persistent sodium currents in entorhinal neurons.
CONCLUSIONS: These data indicate that epileptic-like activity is correlated with an upregulation of I[sub]NaP[/sub]. The increase in plasticity of the persistent sodium current may contribute to neuronal hyperexcitability in the temporal lobe. Pharmacological
agents such as ethosuximide may be used to control this activity.
Support: the Canadian Institutes for Health Research, the Medical Research Council, and the Savoy Foundation. Newton Agrawal was supported by a fellowship from the American Epilepsy Society/Epilepsy Foundation and is currently supported by a fellowship from the Savoy Foundation.