Neuromuscular modulation in Limulus by both octopamine and proctolin.

Both octopamine and proctolin potentiate nerve-evoked skeletal muscle contractions in the horseshoe crab, Limulus. The threshold concentration for octopamine was 10(-9) to 10(-8)M, while for proctolin it was 3 X 10(-9)M. Norepinephrine and dopamine produced effects similar to octopamine but at higher thresholds; tyramine and serotonin were ineffective. Octopamine caused significant increases in amplitudes of excitatory postsynaptic potentials (epsps) of muscle fibers, but had little effect on muscle fiber input resistance or membrane potential. Also, octopamine did not affect depolarization of muscle fibers and subsequent contraction due to the direct action of exogenously applied glutamate. These results suggest that octopamine potentiates nerve-evoked contractions primarily by facilitating release of neuromuscular transmitter. At concentrations above 10(-7)M, however, octopamine sometimes caused muscle spikes in response to motoneuron stimulation, a finding that suggests that octopamine may also have some postsynaptic action. Proctolin potentiated the muscle contractions evoked by glutamate but had little effect on glutamate-evoked muscle fiber depolarization, muscle fiber input resistance, or membrane potential. Thus, proctolin appears to act directly on skeletal muscle to enhance contractility. The proctolin-induced potentiations of contraction were sometimes accompanied by modest increases in epsp amplitude, so that unlike lobster skeletal and Limulus cardiac neuromuscular preparations, proctolin may have a secondary direct synaptic effect. Both octopamine and proctolin have been found in Limulus cardiac ganglion. This potential access to the hemolymph and the relatively low threshold concentrations needed for physiological action suggest that octopamine and proctolin could function as hormonal modulators of neuromuscular function in Limulus.