EFFECTS OF CYTOCHALASIN TREATMENT ON SHORT-TERM SYNAPTIC PLASTICITY AT DEVELOPING NEUROMUSCULAR-JUNCTIONS IN FROGS

1. The role of actin microfilaments in synaptic transmission was teste d by monitoring spontaneous and evoked transmitter release from develo ping neuromuscular synapses in Xenopus nerve-muscle cultures, using wh ole-cell recording of synaptic currents in the absence and presence of microfilament-disrupting agents cytochalasins B and D. 2. Treatment w ith cytochalasins resulted in disruption of microfilament networks in the growth cone and the presynaptic nerve terminal of spinal neurons i n Xenopus nerve-muscle cultures, as revealed by rhodamine-phalloidin s taining. 3. The same cytochalasin treatment did not significantly affe ct the spontaneous or evoked synaptic currents during low-frequency st imulation at 0.05 Hz in these Xenopus cultures. Synaptic depression in duced by high-frequency (5 Hz) stimulation, however, was reduced by th is treatment. Paired-pulse facilitation for short interpulse intervals was also increased by the treatment. 4. These results indicate that d isruption of microfilaments alters short-term changes in transmitter r elease induced by repetitive activity, without affecting normal synapt ic transmission at low frequency. 5. Our results support the notion th at actin microfilaments impose a barrier for mobilization of synaptic vesicles from the reserve pool, but do not affect the exocytosis of im mediately available synaptic vesicles at the active zone.