Group I Metabotropic Glutamate Receptors Elicit Epileptiform Discharges in the Hippocampus through PLCβ1 Signaling

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Abstract
Activation of metabotropic glutamate receptors (mGluRs) produces multiple effects in cortical neurons, resulting in the emergence of network activities including epileptiform discharges. The cellular mechanisms underlying such network responses are largely unknown. We examined the properties of group I mGluR-mediated cellular responses in CA3 neurons and attempted to determine their role in the generation of the network activities. Group I mGluR stimulation causes depolarization of hippocampal neurons. This depolarization is primarily mediated by two sets of conductance change: the opening of a voltage-dependent cationic conductance (mediating ImGluR(V)) and the closing of a voltage-independent (background) K+ conductance. ImGluR(V)was no longer elicited by group I mGluR agonists in the presence ofU73122, a phospholipase C (PLC) blocker. Also, the current could not be activated in hippocampal CA3 neurons from PLCβ1 knock-out mice. In contrast, suppression of PLC signaling did not affect the group I mGluR-mediated suppression of background K+conductance. Thus, the suppression of the background K+ conductance occurred upstream to PLC activation, whereas the generation of ImGluR(V) occurred downstream to PLC activation. Group I mGluR agonists normally elicited rhythmic single cell and population burst responses in the CA3 neurons. In the absence of an ImGluR(V) response, CA3 neurons in slices prepared from PLCβ1−/− mutant mice could no longer generate these responses. The results suggest thatImGluR(V) expression in CA3 hippocampal neuron is PLCβ1-dependent and thatImGluR(V) plays a necessary role in the generation of rhythmic single cell bursts and synchronized epileptiform discharges in the CA3 region of the hippocampus.