Abstracts

Rationale: The entorhinal cortex (EC) has been implicated in the generation of seizures in temporal lobe epilepsy (TLE). Layer II neurons of the EC form the major excitatory input into the hippocampus and consist of stellate and non-stellate neurons. In TLE, layer II neurons are spared and become hyper-excitable due to synaptic reorganization (Kumar et al., 2007: J Neurosci 27, 1239-1246). Here we report that EC layer II neurons are also intrinsically hyper-excitable in TLE, attributed in part to changes in sodium (Na) channel gating parameters. Methods: TLE was induced by electrical stimulation of the hippocampus for 90 mins to induce status epilepticus. Only rats documented as having two or more spontaneous seizures per day, 3 months after status epilepticus were used in the study. Brain slices were prepared and AP recorded from EC layer II neruons. Na channel currents were recorded from isolated layer II neurons using the whole cell patch clamp technique. Results: At a step of 470 pA, AP firing frequencies were significantly increased in TLE neurons compared to control (TLE: 34.9 ± 2.9 Hz; n=4; control: 19.5 ± 3.0 Hz; n=7: P<0.001). In the presence of synaptic inhibitors (APV (30 uM), NBQX (10 uM), picrotoxin (50 uM) and strychnine (50 uM)), firing frequency was reduced in both conditions, yet was still significantly increased in TLE (TLE: 13.3 ± 2.3 Hz; n=4; control: 6.2 ± 0.9 Hz; n=7: P<0.01). In a similar manner, non-stellate neurons were also intrinsically hyper-excitable. AP firing frequencies were increased in TLE as compared to control (TLE: 34.9 ± 4.4 Hz; n=3: control: 20.0 ± 6.2 Hz; n=5: P<0.001). In the presence of synaptic inhibitors the firing frequency was reduced in both populations, but TLE frequency was still significantly greater than control (TLE: 16.7 ± 3.6 Hz; n=3; control: 8.3 ± 2.1 Hz; n=5: P<0.01). Na channel currents were recorded from isolated stellate and non-stellate neurons. Non-stellate conductance curves were more depolarized in TLE than control (control: V1/2 = -36.4 ± 1.6 mV: n = 7; TLE: V1/2 = -29.7 ± 1.2 mV; n = 7: P<0.05). Steady state-inactivation curves for TLE neurons were also shifted to more depolarizing potentials (from V1/2 = -69.8 ± 0.7 mV; n = 7 in control to V1/2 = -61.4 ± 1.8 mV; n = 6, in TLE: P<0.001). Slope factors were also increased for TLE neurons (control k = 5.9 ± 0.3 mV; TLE k = 8.5 ± 1.0 mV: P<0.05). These shifts resulted in a 10 fold increase in the Na channel window current in TLE neurons compared to control. Activation parameters for stellate neurons were not different in TLE (control: V1/2 = -29.3 ± 1.7 mV, k = -6.3 ± 0.4 mV; n = 8; TLE: V1/2 = -29.7 ± 2.3 mV, k = -7.0 ± 0.5 mV; n = 5). However, steady-state inactivation curves were shifted to depolarized potentials in TLE (control: V1/2 = -68.2 ± 1.9 mV; n = 5; TLE: V1/2 = -60.0 ± 2.0 mV; n = 4), causing a two-fold increase in the window current of TLE neurons.