Axotomy-induced temporal dissociation of long-term adaptive changes at neuromuscular synapses of a crayfish phasic...

Periodic in situ stimulation of an identified crayfish phasic extensor motoneuron for 3 consecutive days (2 h/day) at 2.5 Hz leads to long-term adaptation (LTA) of its neuromuscular synapses. LTA is characterized by reductions in both initial excitatory postsynaptic potential (EPSP) amplitudes and synaptic depression during repeated stimulation. These adaptive changes were evident 1 day following periodic stimulation. Axotomy of the motoneuron before or after the first day of stimulation of its distal surviving axon abolished both adaptive changes. Axotomy between the second and third stimulation periods abolished only the resistance to synaptic depression. Both adaptive changes were expressed following axotomy after the third day of stimulation. Axotomy alone did not affect neuromuscular transmission in control, unstimulated animals. These results show that axonal continuity between the phasic extensor motoneuron's cell body and its neuromuscular synapses is required at specific times during periodic stimulation for the expression of each of these long-term adaptive changes in neuromuscular transmission. Furthermore, the two adaptive changes in transmission are temporally separable, with the resistance to depression requiring more periodic stimulation to emerge than the reduction in initial EPSP amplitudes. The results also suggest that the molecular components responsible for the expression of these adaptive changes are synthesized in the soma and transported down the axon in response to periodic stimulation of the phasic axon.