Intrinsic differences in sensitivity to 5-HT between high- and low-output terminals innervating the same target

The differential action of neuromodulators on synapses of various efficacy provides additional fine tuning of synaptic regulation beyond frequency induced plasticity. We used the well-characterized high- and low-output motor nerve terminals, of the tonic and phasic neuromuscular junctions (NMJs) in the walking leg extensor muscle of the crayfish, to investigate differential actions of serotonin (5-HT) since both terminals innervate the same target. The excitatory postsynaptic potentials of the tonic NMJ are enhanced to a greater extent than for the phasic NMJs during exposure to 5-HT (100 nM). Macropatch current recordings at identified sites along the motor nerve terminals and quantal analysis indicate that mean quantal content is substantially increased by 5-HT. The overall probability of vesicular release increases to a greater extent at tonic terminals than at phasic terminals when exposed to 100 nM 5-HT. Measures in the area (i.e. charge) of spontaneous quantal currents indicate no difference in postsynaptic receptivity to the glutamatergic synaptic transmission upon exposure to 5-HT. The results provide new details concerning differential modulation of low- and high-output synapses present on the same target tissue.