Long-term adaptation in lobster motor neurons and compensation of transmitter release by synergistic inputs

Earlier studies with crayfish have shown that chronic increases in neural activity, by electrical stimulation, cause a long-lasting reduction in the amount of transmitter released at low stimulus frequencies or at the beginning of a stimulus train. When such chronic stimulation is applied to phasic extensor motor neurons of the lobster abdomen, a similar change in transmitter release is apparent, as indicated by a decrease in excitatory postsynaptic potential (EPSP) size at 0.1 Hz. However, the EPSPs from unstimulated axons which innervate the same target muscle from a different nerve increase in size. Thus, activity-dependent reduction in transmitter release at one set of synapses appears to be compensated for by increased synaptic efficacy from less active synergistic inputs. The mechanism of such compensation is not known.