Long-term adaptation of crayfish neurons depends on the frequency and number of impulses.

Increasing the impulse activity of crustacean neurons for a few days causes long-lasting changes in transmitter release, which are termed 'long-term adaptation' (LTA) in previous studies. Both the amount of transmitter released at the beginning of a stimulus train, and synaptic fatigue during repetitive stimulation, are reduced. The present study examines the dependence of these synaptic changes on the frequency and number of impulses used to elicit LTA. Fatigue resistance develops consistently when crayfish phasic motor neurons are stimulated for 3 days with as few as 9,000 impulses per day, and occurs in response either to low frequency stimulation (0.2 or 0.5 Hz), or to stimulation in short bursts at a moderate average frequency (2.5 Hz). In contrast, the reduction in initial transmitter release does not appear consistently when the frequency and number of impulses are both low (9,000 impulses per day delivered at 0.2 Hz), but does occur at the moderate stimulus frequency (2.5 Hz) and when a larger number of impulses (18,000) are delivered at a low frequency (0.5 Hz). The data suggest that the two changes in synaptic transmission that comprise LTA have different stimulus requirements.