TEMPORAL DYNAMICS OF CONVERGENT MODULATION AT A CRUSTACEAN NEUROMUSCULAR-JUNCTION

At least 10 different substances modulate the amplitude of nerve-evoke d contractions of the gastric mill 4 (gm4) muscle of the crab, Cancer borealis. Serotonin, dopamine, octopamine, proctolin, red pigment conc entrating hormone, crustacean cardioactive peptide, TNRNFLRFamide, and SDRNFLRFamide increased and D-allatostatin-3 and histamine decreased the amplitude of nerve-evoked contractions. Modulator efficacy was fre quency dependent; TNRNFLRFamide, proctolin, and D-allatostatin-3 were more effective when the motor neuron was stimulated at 10 Hz than at 4 0 Hz, whereas the reverse was true for dopamine and serotonin. The mod ulators that were most effective at high stimulus frequencies produced a significant decrease in muscle relaxation time; those that were mos t effective at low stimulus frequencies produced modest increases in r elaxation time. Thus modulator actions that appear redundant when exam ined only at one stimulus frequency are differentiated when a range of stimulus dynamics is studied. The effects of TNRNFLRFamide, serotonin , proctolin, dopamine, and D-allatostatin-3 on the amplitude and facil itation of nerve-evoked excitatory junctional potentials (EJPs) in the gm4 and gastric mill 6 (gm6) muscles were compared. The EJPs in gm4 h ave a large initial amplitude and show relatively little facilitation, whereas the EJPs in gm6 have a small initial amplitude and show consi derable facilitation. Modulators that enhanced contractions also enhan ced EJP amplitude; D-allatostatin-3 reduced EJP amplitude. The effects of these modulators on EJP amplitude were modest and showed no signif icant frequency dependence. This suggests that the frequency dependenc e of modulator action on contraction results from effects on excitatio n-contraction coupling. The modulators affected facilitation at these junctions in a manner consistent with a change in release probability. They produced a change in facilitation that is inversely related to t heir action on EJP amplitude.