A Method for Assessing the Conributions of Different Types of Ionotropic Receptors to Postsynaptic Responses during Epileptiform Discharges In Vitro

Abstract

Epileptiform activity in living brain slices is the result of the complex integration of excitatory and inhibitory synaptic signals. The time pattern of activation of different synaptic conductances during interictal discharges provides a description of the patterns of operation of neural networks in epileptiform states and a route to analysis of the effects of different pharmacological agents on it. Existing methods of assessing synaptic conductances generally make the assumption that the voltage-current relationships (VCR) of neuron responses are linear and provide for assessment of the contributions of two types of conductance to the postsynaptic response, i.e., excitatory and inhibitory, without discriminating the contributions of specific types of synaptic receptors. We describe here the theoretical basis of a method for simultaneous assessment of the postsynaptic conductances of three types of ionotropic receptors (AMPA NMDA, and GABAA), which undergo activation during interictal discharges in the entorhinal cortex. The proposed algorithm utilizes the different types of VCR of these receptors to assess their contributions to the postsynaptic responses of a neuron and proposes use of the patch clamp method in the whole cell configuration in voltage clamping mode. Use of this method was demonstrated using various models of epileptiform activity as examples. The time courses of synaptic conductances during interictal discharges generated with full preservation of GABAA receptor-mediated conductance were compared using the antibiotic cefepime or bicuculline. Despite the fact that cefepime is a GABAA receptor antagonist, total conductance at the late stages of the interictal discharge was significantly greater as compared with a model in which this type of receptor was not blocked or on use of another competitive antagonist, bicuculline. Thus, the method proposed here identifies fine differences in the time courses of three types of synaptic conductance and allows differences in the proepileptic effects of GABAA receptor blockers to be demonstrated