Modulation of Synaptic Transmission and Excitation-Contraction Coupling in the Opener Muscle of the Crayfish, Astacus Leptodactylus, By 5-Hydroxytryptamine and Octopamine

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Abstract
The modulatory actions of 5-hydroxytryptamine (5-HT) and octopamine (OA) were investigated in the opener nerve-muscle preparation of the crayfish, Astacus leptodactylus. Membrane resistance and resting potential were unaltered by 5-HT and OA at concentrations up to 2·5 × 10−5 M; but EPSP-amplitudes were increased, up to 3-fold by OA and up to 18-fold by 5-HT. The lowest effective concentration was 2·5 × 10−9M; a maximal effect was produced at 2·5 × 10−6M. The effect was reversible only after prolonged washing. The enhancement of EPSPs by 5-HT or OA is due to an increased amplitude of the synaptic current; the current duration is not altered. The facilitation ratio (ratio of amplitudes of a pair of EPSPs) is not significantly affected by 5-HT or OA despite the often enormous increase of the absolute EPSP-amplitudes. The modulatory action also affects the excitation-contraction (e-c) coupling process: the effectiveness of e-c coupling was increased 7·4-fold by 5-HT (2·5 × 10−6M) and 18-7-fold by OA (5 × 10−6M). The threshold potential of e-c coupling was not affected.