MHC CLASS-II EXPRESSION BY MICROGLIA IN TETANUS TOXIN-INDUCED EXPERIMENTAL EPILEPSY IN THE RAT

Minute amounts of tetanus toxin injected into the hippocampus of rats results in an epileptiform syndrome. When the toxin injection is made unilaterally or bilaterally into the ventral hippocampus, about one-th ird of animals with seizures show bilateral neuronal loss in dorsal CA 1 of the hippocampus after 1 week. In animals with seizures, microglia in hippocampus are found to be activated. The present work shows that during the acute phase, microglia in the substantia nigra become acti vated and express MHC class II antigens in the majority of animals wit h seizures. After the animals have recovered from the acute phase at 8 weeks, the MHC class LT expression has largely disappeared from the s ubstantia nigra but MHC class II-expressing microglia are found in the dorsal hippocampus of those rats with loss of cells from CA1. These r esults show that microglia are responsive to abnormal electrical activ ity in the central nervous system in the absence of degenerative chang es. Further studies are required to determine how microglia may contri bute to the neuropathology of epilepsy.