Long-term Depression at the Crayfish Fast Extensor Neuromuscular Junction Has Presynaptic and Postsynaptic Components

Abstract

We used measurements of excitatory junction potential (EJP) amplitude variance to explore whether long-term depression (LTD) induced by low-frequency stimulation at the crayfish superficial fast extensor muscle was pre- or post-synaptically expressed. LTD was induced and measured by intermittent low frequency (0.5 Hz) stimulation over the course of several hours. This LTD appeared to have three stages. During the first stage (the first 15 minutes of 0.5 Hz stimulation), the EJPs depressed to around 60% of their original amplitude, an effect that appeared to involve a decrease in quantal content. During the second stage (15-100 minutes of intermittent 0.5 Hz stimulation) the EJP continued to depress slowly, which seems to have involved a decrease in release probability and an increase in quantal content. During the final stage, (100-205 minutes of intermittent 0.5 Hz stimulation) EJP amplitude plateaued with an apparent increase in the probability of release and decrease in the quantal content. It should be noted that this analysis was performed under the assumption that the releasable neurotransmitter vesicle pool size remained constant over the course of the experiment.