Neuroexcitatory and depressant effects of penicillin at the cat soleus neuromuscular junction.

Penicillin administered intraarterially in a dose of 200 units per kg produced high frequency (ca 300 Hz) repetitive discharges originating in motor nerve endings in response to single stimuli applies to the cat soleus nerve. The high frequency repetitive discharges were antidromically conducted and recorded on ventral root filaments. Simultaneously each repetitive burst was transmitted to the muscle, producing an increase in contractile strength by converting a twitch into a brief tetanic contraction. Posttetanic potentiation in this system, which is mediated by repetitive discharges originating in nerve terminals after high frequency stimulation, is augmented by penicillin; larger doses depressed posttetanic potentiation, and still larger doses produced varying degrees of neuromuscular block. Events similar to those observed after intraarterial injections were recorded when penicillin was administered intravenously in larger doses. These data suggest that in the presence of penicillin the motor nerve terminals remain depolarized for a prolonged period after excitation by the stimulus, thus providing a current sink for the parent axon. Under the influence of this constant cathodal focus the axon fires repetitively at the high frequency.