RESUMEN
The effects of penicillin were investigated in lumbar motoneurones of isolated spinal cord preparation of the frog (Rana ridibunda). Spinal root discharges were recorded and single cell activity was studied with intracellular electrodes. Bath application of 500 IU/ml of penicillin G induced in motoneurones prolonged depolarization shifts, followed by repeated transient depolarizations lasting several hours. In all motoneurones studied, this bursting activity was synchronized with discharges recorded from the ventral root. Blockade of N-methyl-D-aspartate (NMDA) receptors by D,L-2-amino-5-phosphonovaleric acid (50-100 microM) completely eliminated the bursting activity. It is suggested that the spinal cord may be an important locus of the anticonvulsant effect of NMDA antagonists.