Long-term facilitation and low-frequency depression in a crayfish phasic motor axon

1. Stimulation of the fast axon to the closer muscle of the isolated crayfish claw preparation at 0.1 Hz leads to low-frequency depression (LFD) of the excitatory postsynaptic potentials (EPSPs) recorded from single muscle fibers. EPSP amplitude is reduced by 50% after only 30 stimuli at 0.1 Hz, and declines during maintained 0.1 Hz stimulation with a time constant of 10-15 min. 2. LFD is not affected by temperature changes ranging from 10 °C to 25 °C. However, ouabain (10^-4M-10^-3M) in the extracellular solution slightly reduces the rate of LFD. 3. When 0.1 Hz stimulation is delivered following a period of conditioning stimulation (5-10 Hz) which causes depression, a facilitated EPSP appears. Facilitation persists for 1/2 h or more after induction. This long-term facilitation (LTF) has some features in common with that seen after stimulation of a tonic motoneuron in crayfish and crabs. 4. The amplitude of LTF is directly related to the number of conditioning impulses. However, the effect is pattern-sensitive: the same number of impulses delivered in a bursting pattern enhances the expression of LTF. 5. LTF is enhanced by lowering the temperature or applying ouabain (10^-3M) during conditioning. 6. The results implicate sodium loading of phasic axon nerve terminals during stimulation as a factor in enhancing EPSP amplitude. This suggests that the mechanism for induction of LTF at phasic neuromuscular synapses is similar to that in crustacean tonic neuromuscular synapses.