Effect of group I metabotropic glutamate transmission on in vitro interictal- and ictal-like epileptiform activity and characterization of a new in vitro model of epileptogenesis.

Abstract

Co-application of the convulsant 4-aminopyridine (4-AP) and the GABAB receptor antagonist CGP 55845 to adult guinea pig hippocampal slices elicits giant GABA-mediated postsynaptic potentials (GPSPs) and interictal-like epileptiform discharges. In the first part of the thesis, I tested the effects of the group I metabotropic glutamate receptor (mGluR) subtype-selective antagonists on this synchronous activity. Electrophysiological field recordings were performed in the CA3 region of hippocampal slices from adult guinea pigs. The mGlu5 receptor antagonists MPEP and MTEP increased GPSP rate, but the mGlu1 receptor antagonist LY 367385 did not. Simultaneous block of both subtypes of group I mGluR did not decrease pre-existing epileptiform activity and also did not prevent the emergence of epileptiform activity. Thus, in the 4-AP/CGP 55845 model, enough glutamate was released to activate group I mGluRs and affect GPSP rate via mGlu5 receptors; however, this group I mGluR activation was not required for the generation of the epileptiform activity. Application of 4-AP (100 μM) in a solution containing reduced divalent cation concentrations (0.6mM Mg2+ and 1.2 mM Ca2+) to hippocampal-entorhinal-perirhinal slices of adult rat brain induced ictal-like epileptiform activity in entorhinal and perirhinal cortices as revealed by electrophysiological field potential recordings. The most exciting finding of the second part of my thesis was that epileptiform activity, including ictal-like activity, persisted after washing out the 4-AP. This persistence indicated that a change had occurred in the slice so that it was now “epileptic” in the absence of the convulsant 4-AP. The maintenance of the persistent ictal-like epileptiform activity required ionotropic glutamate-mediated synaptic transmission but not group I mGluR activation. The presence of hippocampus was not required for the induction of persistent ictal-like events in entorhinal cortex. The induction of persistent ictal-like epileptiform activity was dependent upon both NMDA and AMPA/kainate ionotropic glutamate receptor activation but not upon mGluR activation. Application of the protein synthesis inhibitor cycloheximide did not prevent the induction of persistent ictal-like activity. In conclusion, I have demonstrated both interictal-like and ictal-like activity that is not dependent upon group I mGluR activation and have characterized a new in vitro model of epileptogenesis that is clearly distinct from the group I metabotropic glutamate-dependent model.