EPILEPTIC ACTIVITY PREVENTS SYNAPSE FORMATION OF HIPPOCAMPAL MOSSY FIBERS VIA L-TYPE CALCIUM-CHANNEL ACTIVATION IN-VITRO

Hippocampal slice from early postnatal rat was used to elucidate the i nfluence of epileptic activity elicited by picrotoxin on synapse forma tion of messy fibers. Neurite reelongation and synaptogenesis of messy fibers transected at 8 days in vitro were confirmed by staining with Dil, a fluorescent membrane dye used as a neuronal tracer, and by reco rding field excitatory postsynaptic potentials (fEPSP) in the CA3 regi on evoked by stimulation of the dentate gyrus. Picrotoxin (50 mu M), w hich evoked spontaneous epileptiform firing in the CA3 region that was occluded by tetrodotoxin (1 mu M), hindered development of fEPSP ampl itude after a lesion of messy fibers. Furthermore, observations using a Timm method, a histochemical technique that preferentially labels sy naptic terminals of messy fibers, revealed that picrotoxin prevented s ynaptogenesis in the CA3 region. This inhibitory effect of picrotoxin was completely abolished by tetrodotoxin or nicardipine (10 mu M), a L -type calcium channel blocker, but not by 2-amino-5-phosphonopentanoic acid (50 mu M), a N-methyl-D-aspartate receptor antagonist, suggestin g that influx of calcium ion via L-type calcium channels during epilep tic bursts mediated the disturbance of appropriate synapse formation o f messy fibers.