Abstracts

Recent experimental study using simultaneous multiple whole-cell current clamp recordings revealed that inhibitory networks initiate seizures and inhibition transiently fails during the body of the seizure (Ziburkus et al, 2004). The goal of the present study is to describe the dynamics of excitatory and inhibitory synaptic conductances in interneurons and pyramidal cells before, during, and after [italic]in vitro[/italic] seizures. Simultaneous dual voltage clamp whole-cell and extracellular recordings were performed in the CA1 region of rat transverse hippocampal slice preparations (P16-P30). Inhibitory interneurons from CA1 oriens layer were distinguished from pyramidal cells with differential IR microscopy. To induce seizures, slices were bathed in 100-200[mu]M 4-Aminopyridine. Voltage clamp solution contained blockers for voltage-activated conductances. Excitatory conductances (Ge) were recorded at -80mV and inhibitory (Gi) at 0mV. Reversal potentials for interictal bursts and seizures were measured by varying holding potential from -80mV to +40mV. [italic]Post-hoc,[/italic] the cells were stained with fluorescence conjugated antibody cocktails (neurobiotin, somatostatin, parvalbumin, and mGLUR1). Following immunocytochemical testing, the sections were processed for peroxidase-based biocytin visualization used for morphological reconstructions of the recorded cells. Voltage clamp recordings showed the interplay of Ge and Gi during spontaneous seizures. Ge/Gi ratios were compared before, during, and after the seizures. Interneurons and pyramidal cells contained stronger Gi during initiation and termination phases of the seizures. This ratio was inverted during the body of the seizure. The average total synaptic currents reversed at slightly positive (0 to +5mV) potentials confirming high levels of excitatory inputs invading the cells during the body of the seizure (Fig.1C). Despite these strong excitatory synaptic inputs, inhibitory conductances also increased which partially corrected for the transient Ge/Gi imbalance. Inhibitory scaling maybe a necessary mechanism for the network to reproduce recurrent seizure events. We report, to our knowledge, the first comparative interneuron-principal cell conductance study in a spontaneous seizure model. Unlike physiological UP states in normal brains, these seizures are characterized by an imbalance in excitatory and inhibitory conductances which appear to have distinct temporal time scales.[figure1] (Supported by Epilepsy Foundation Fellowship and NIH grants: K02MH01493, and RO1MH50006.)