Determinants of ictal epileptiform patterns in the hippocampal slice

Purpose: The transition from an interictal to an ictal pattern of epileptiform activity is a strategic target for antiepileptic drug (AED) action. Both the muscarinic agonist pilocarpine and the selective group I metabotropic glutamate receptor (mGluR) agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) produce prolonged synchronous activity in the hippocampal slice that resembles ictal discharges. We evaluated the role of synaptic mechanisms and release of calcium from intracellular stores in the generation of prolonged ictal oscillations. Methods: Pilocarpine (10 muM) in 7.5 mM [K+](o) or DHPG (100 muM) in 5 mM [K+](o) artificial cerebrospinal fluid (ACSF) were bath applied to hippocampal slices, and extracellular recordings were made from the CA3 region. The pattern of activity was characterized as ictal if prolonged oscillations of discharges occurred at >2 Hz lasting for >3 s. The pattern of epileptiform activity was characterized and compared with the pattern observed after bath application of pharmacologic agents. Results: The AMPA/kainic acid (KA) glutamate receptor blocker DNQX (20 muM) dampened and stopped ictal oscillations; however, antagonism of N-methyl-D-aspartate (NMDA) or gamma-aminobutyric acid (GABA(A)) receptors had minimal effects on ictal patterns. Ictal discharges were suppressed by dantrolene (30-100 muM), which blocks release of calcium from intracellular stores, or thapsigargin (1-5 muM), which inhibits the adenosine triphosphatase (ATPase) that maintains intracellular calcium stores. The L-type calcium channel antagonist nifedipine (1 muM) blocked ictal activity produced by pilocarpine or DHPG. Conclusions: Ictal discharges produced by pilocarpine or DHPG depended on intact synaptic transmission mediated by AMPA/KA receptors, release of calcium from intracellular stores, and L-type calcium channel activation. The results suggest that muscarinic and group I mGluRs activate a positive-feedback system that creates calcium oscillations and prolonged neuronal synchronization mediated by recurrent excitatory synaptic connections in the CA3 region of the hippocampus.