Dopamine-induced muscle contractions and modulation of neuromuscular transmission in Aplysia

Although there is a considerable body of evidence supporting a role for dopamine (DA) as a neurotransmitter in both vertebrates and invertebrates, the cellular mechanisms whereby DA exerts its effects are not completely understood. DA application to some tissues has been shown to result in changes in membrane voltage and conductance 4,7. However, often in the same tissue, DA has the ability to stimulate the production of cyclic AMP 9. We report here initial results from a preparation that may serve as a model system for studying possible modes of action of DA at the cellular level. DA application to this preparation has at least 3 effects. It produces muscle contractions, modulates contractions induced by the selective firing of identified motor neurons, and (in results not presented here) stimulates cyclic AMP production 6. We have studied neuromuscular transmission in the gill of Aplysia. The gill contains a high concentration of DA (3 #g/g)~. Moreover, DA has been identified histochemically in nerve processes in the gilP °. The neural control of gill movements has been studied by several laboratories, and the physiological properties of two major gill motor neurons used in these experiments, L7 and LDG1, have been describedl,S, 11. Fig. l a shows the semi-intact gill preparation used in these experiments. Fig. 1 also shows drawings of the contractions produced by DA perfusion (d) and contractions induced by L7 stimulation before (c), immediately after (e), and 15 rain after DA perfusion (f). Asynchronous movements of efferent vessel trunklets, pinnule longitudinal muscle, and the afferent vessel are induced by DA. At higher concentrations of DA (not shown), tonic contractions of these muscle groups occur. These contractions appear to be due to a direct action of DA on gill muscle fibers, since they are unaltered by perfusion of the preparation with high-Mg 2~ seawater (100 m M added to 50 mM in normal seawater) which effectively blocks neuromuscular transmission in this preparation (our unpublished observations). The contractions caused by discharge of Lv are very similar to those induced by