Pre- and postsynaptic actions of valproic acid at the frog neuromuscular junction

The effect of valproic acid (VPA) on synaptic transmission was investigated at the frog neuromuscular junction using electrophysiological methods. The drug caused a dose-dependent decrease in the quantal content of the endplate potential (EPP) and a significant reduction in the input resistance of the postjunctional membrane. There were no apparent changes in the resting membrane potential or the frequency of the miniature endplate potential (MEPP). These results suggest that VPA has pre- as well as postsynaptic actions, both of which could lead to suppression of synaptic transmission.