Abstracts

Rationale: Electrical recordings of partial complex seizures in pediatric patients often exhibit bursting activity in neocortex. Here we test a mechanism by which small input oscillations may be amplified into network oscillations, a seizure. We hypothesize that in cortical neurons, an elevated NMDA receptor conductance can impart a nonlinear N-shaped region to the whole cell I-V curve. Neurons possessing this region would have a disproportionate response to voltage fluctuations, effectively amplifying small oscillations and possibly generating periodic bursting behavior. Behavior of this sort could then push the network into a state of oscillation. If verified, the action of the NMDA receptor would constitute a crucial link in the seizure initiation chain. We evaluate how NMDA activation changes cellular firing behaviors and how these firing behaviors are correlated to network oscillation by examining the phase and frequency relationships of both bursting and non-bursting neurons with the network behavior. Methods: The membrane potential and current of layer V pyramidal neurons in mouse frontal cortex (P8-11) are recorded in whole cell patch clamp with simultaneous recording of extracellular activity. To understand the cellular response to NMDA activation, the occurrence of bursting behavior, amplification of input as well as the presence of a nonlinear region in the whole cell I-V curve is assessed in control conditions and bath applied NMDA (5-10 μM). TTX (1.5 μM) is also used to isolate the neuron from the network and record the steady state current-voltage relationship of the cell. These experiments are also performed in aCSF with 0 mM [Mg2+] to determine the role of the NMDA receptor’s voltage dependence on these behaviors. During NMDA application, the network often exhibits seizure-like oscillations with simultaneous activity in both neurons that produce NMDA-dependent intrinsic bursts (IB) and non-bursting neurons (NB). To understand how these neurons interact to produce the network oscillation, we evaluate whether IB neurons have a different phase or frequency synchronization relationship with the network oscillation compared to the NB neurons using Fourier analysis, network-burst triggered spike averaging, and phase analysis. Results: A subset of layer V pyramidal neurons exhibits intrinsic oscillation as well as amplification of input when NMDA receptors are activated. These behaviors are correlated to a nonlinear whole cell I-V curve with a RNSC (Fig.1). Washing out Mg2+ terminates intrinsic oscillation and results in a linear I-V curve. In addition, these IB neurons exhibit an increase in phase synchronization with the network oscillation compared to NB neurons (Fig.2). Conclusions: The voltage dependence of NMDA receptors is necessary for amplification of input and promotes intrinsic bursting in a population of layer V pyramidal neurons. These neurons also have an increase in phase synchronization metrics with the network oscillation suggesting an important role in initiating and sustaining seizure-like activity in neocortex. We thank the American Epilepsy Society and the Falk Foundation.