Facilitation and depression at different branches of the same motor axon: evidence for presynaptic differences in release

This study provides evidence that a neuron can exhibit differences in activity-dependent transmitter release at two synaptic sites due to variations in the properties of its presynaptic terminals. Two muscles in the stomatogastric system of the lobster Homarus americanus are innervated by a single motor neuron but respond differently to that motor neuron's input, resulting in two different movements evoked by one motor neuron. During continued motor neuron stimulation, the gm8 muscle contracts slowly and maintains contraction, while the gm9 muscle contracts rapidly and then relaxes. These different muscle responses can be accounted for, in large part, by the properties of the respective neuromuscular synapses: the excitatory junctional potentials recorded in gm8 are initially small but summate and facilitate with repeated stimulation, while those in gm9 are initially large but depress with repeated stimulation. Presynaptic differences in neurotransmitter release contribute strongly to the divergent responses; reduction of the excitatory junction potential amplitude by partial postsynaptic receptor blockade or by desensitization does not change the amount of depression at gm9. However, reduction of neurotransmitter release with low-Ca2+, high-Mg2+ saline removes gm9 synaptic depression and reveals that both neuromuscular junctions exhibit frequency-dependent homosynaptic facilitation. Postsynaptic differences in muscle input resistance and muscle composition may enhance the effects of the divergent release properties, but are not responsible for the activity-dependent changes. Ultrastructural features of the nerve terminals on the two muscles are consistent with the differential output of the terminals; the synapses on gm9 are larger and have more presynaptic dense bars than their counterparts on gm8. These data suggest that the basis for the differences in transmitter release between the two muscles may be a higher density of release sites in the gm9 synapses that leads to a higher output of neurotransmitter, rapid depletion of transmitter stores, and synaptic depression.