Abstract
1. 1. Stimulation of the cardio-regulator nerves of Mercenaria mercenaria normally causes inhibition of the heart. Stimulation of the nerves following treatment of the heart with benzoquinonium, an antagonist of acetylcholine, causes excitation. 2. 2. This excitation is prevented by 1-methyl- d-lysergic acid butanolamide, a drug which also decreases or blocks the excitatory action of 5-hydroxytryptamine on the heart. 3. 3. Stimulation of the cardio-regulator nerves of benzoquinonium-treated preparations which were made tachyphylactic to 5-hydroxytryptamine fails to elicit an excitatory response. 4. 4. Excitation does not occur in preparations which were pre-treated with reserpine. This alkaloid was found to decrease endogenous 5-hydroxytryptamine concentration in the ganglia of Mercenaria. 5. 5. The experimental evidence indicates that 5-hydroxytryptamine is a normal mediator of excitation in the heart of Mercenaria.
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