Abstracts

RATIONALE: Many studies have focused on understanding the role of NMDA-receptors in mediating synaptic transmission, but little is known about their role in modulating voltage-dependent membrane properties. The objective of the present study is to investigate how NMDA alters voltage-dependent membrane properties in cortical neurons as a potential mechanism for the generation of epileptic seizures.
METHODS: Experiments were performed on male and female mice (P8 [ndash] P13) that were deeply anesthetized with ether. The cortex was isolated in ice-cold artificial CSF containing (in mM): 118 NaCl, 3 KCl, 1.5 CaCl[sub]2[/sub], 1 MgCl[sub]2[/sub], 25 NaHCO[sub]3[/sub], 1 NaH[sub]2[/sub]PO[sub]4[/sub], and 30 D-glucose at a pH of 7.4 bubbled with carbogen (95% oxygen and 5% CO2). The cerebral hemispheres were separated at the midline. Slices (500 [mu]m thick) were sectioned 1500[mu]m from the frontal pole, and were immediately transferred into a recording chamber, which was maintained at a temperature of 29[degree]C. After 30 min the potassium concentration was raised from 3 to 5 mM to obtain spontaneous rhythmic activity. Population activity recordings were obtained with suction electrodes positioned onto the surface of cortical layers 4 and 5. Intracellular whole-cell patch-clamp recordings were obtained from cortical neurons using the blind-patch technique. Cell layer and cell type were identified by staining each neuron with biocytin.
RESULTS: Intracellular and extracellular recordings were simultaneously obtained from the motor cortex. The majority of slices spontaneously generated population activity occurring in slow ([lt] 1 Hz) recurrent oscillations, which changed into higher frequency ([gt]3Hz) epileptiform population activity following the bath application of NMDA (5-10 [mu]M). Associated with the induction of epileptiform activity was a change in the intrinsic membrane properties of individual cortical neurons. Under control conditions depolarizing current injections evoked tonic regular spiking activity, which linearly increased in frequency when increasing the amplitude of current injections. The same neurons generated rhythmic burst activity in the presence of NMDA. Brief current pulses reset the rhythmic burst activity, long depolarizing current injections increased and hyperpolarizing current injections decreased the frequency of intrinsically generated pacemaker activity. This bursting activity persisted in the absence of extracellular population activity, which was eliminated by blocking calcium currents and synaptic transmission with Cd++ (200 [mu]M). The NMDA-induced bursts depended on the activation of the persistent sodium current and were blocked by Riluzole ( 20 [mu]M).
CONCLUSIONS: We conclude that NMDA induces in a subpopulation of cortical neurons intrinsic bursting properties, which depend on the activation of the persistent sodium current. This change in the intrinsic membrane properties could be a possible cellular mechanism that leads at the network level to the transformation from slow recurrent oscillations into high frequency epileptiform activity.
[Supported by: Falk Foundation (CJM, WvD, KEH)
Rett Syndrome Research Foundation (JMR)
NIH HL60120 (JMR)]