1. 1.

    Dopamine, a cardioexcitor in decapod crustaceans, increased the frequency and/or duration of bursts of action potentials in the semi-isolated cardiac ganglia of two species of crabs. The number of motoneurone action potentials in each burst was increased, which in the intact heart would increase the force and amplitude of heart contraction.

  2. 2.

    The effects were concentration-dependent, with a threshold concentration of 10−8M or lower when dopamine was applied by continuous perfusion. At 5×10−6M, dopamine increased burst frequency by 200%.

  3. 3.

    The main site of dopamine action was the group of four posterior small interneurones which normally function as the pacemaker for the cardiac ganglion system. Effects on the five large motoneurones occurred at higher concentrations. This regional difference in sensitivity was demonstrated by selective applications of dopamine to different parts of the cardiac ganglion and by the use of preparations in which the two ends of the ganglion had been functionally separated by a ligature around the ganglionic trunk.

  4. 4.

    In the small neurones, dopamine was found to stimulate the slow tetrodotoxin-resistant regenerative depolarizations known as driver potentials. The effects on driver potential frequency and train duration were concentration dependent. In one of the two species of crabs, in which electrotonic connections between small and large neurones are strong, large neurone driver potentials were indirectly induced by dopamine.

  5. 5.

    In the tetrodotoxin-treated large motoneurones, dopamine, at a concentration about ten-fold higher than needed to activate the small neurones, decreased the threshold for current-induced driver potentials, and slightly reduced membrane resistance.

  6. 6.

    We suggest that the excitatory action of dopamine on the untreated cardiac ganglion can in large part be accounted for by its action on driver potential production in the small neurones.

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