Long-term adaptation (LTA), a phenomenon previously studied in the crayfish claw, was examined in one of the motoneurones innervating the phasic abdominal extensor muscles. The motoneurone was conditioned by electrically stimulating the second root of the third abdominal ganglion in situ for 4 h per day, using trains of stimuli with an average impulse frequency of 2.5 Hz. In juvenile crayfish, 3 days of conditioning produced a marked (81%) reduction in EPSP amplitude, which recovered only slightly during the succeeding 7 days. The quantal content of synaptic currents also decreased (by an average of 65%). Estimated values of the binomial parameter p were lower for conditioned neurones than for controls, suggesting that the observed decrease in transmitter release involves a decrease in release probability. Conditioned neurones also displayed less synaptic depression than controls during repetitive stimulation at 5 Hz.

In adult crayfish, conditioning for 7 days also produced a marked (74%) reduction in EPSP amplitude and resistance to synaptic depression. These results differ from those of previous work with the phasic axon of the claw closer muscle, which shows virtually no synaptic changes in adults after conditioning for 2 weeks. The ability to exhibit LTA, therefore, is not lost with age in all neurones.

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