Dopaminergic acceleration and GABAergic inhibition in extrinsic neural control of the hermit crab heart

The acceleratory and inhibitory cardio-regulatory nerves of hermit crabs (Aniculus aniculus, Dardanus crassimanus) were studied using histochemical, immunocytochemical and pharmacological tests. Glyoxylic acid-induced fluorescence was observed in two of three axons of the dorsal cardiac nerve. One axon of the nerve showed gamma-aminobutyric acid-like immunoreactivity. Effects of stimulation of cardio-acceleratory axons were blocked by the dopaminergic antagonists, haloperidol and chlorpromazine, but not by cholinergic, adrenergic or serotonergic blockers, suggesting that dopamine is the primary potential candidate for the neurotransmitter of cardio-accelerator neurons. Picrotoxin antagonized inhibition of the cardiac ganglion induced by gammaam-inobutyric acid and by cardio-inhibitory axons. Both small and large ganglionic cells may receive dopaminergic and GABAergic extrinsic neural control.Abbreviations ACh acetylcholine - CA cardio-accelerator - CA1 and CA2 first and second cardio-accelerators - CI cardio-inhibitor - EJP excitatory junction potential - GABA gamma-aminobutyric acid - EPSP excitatory postsynaptic potential - IPSP inhibitory postsynaptic potential - LGC large ganglionic cell - SGC small ganglionic cell - 5-HT serotonin