Pharmacological characterization of cholinoreceptors of cardiac ganglion cells of crustaceans

Abstract

1. 1. Acetylcholine applied to neurons of the heart ganglion of Squilla mantis causes slight depolarization, an increased slope and frequency of pacemaker potentials and an increased number of action potentials in each burst. The result is an increased rate and amplitude of heart beats. 2. 2. Isolated and opened hearts of lobsters ( Homarus americanus) continue to beat. ACh applied to the heart muscle is without effect. When applied to the heart ganglion directly, it causes an immediate increase in rate and amplitude of heart beats. 3. 3. The nature of the cholinoreceptors on heart ganglion cells was explored in the crayfish, Astacus leptodactylus, by perfusing the semi-isolated heart and recording its contractions. Supplementary experiments were performed with a similar technique on hearts of the crab, Eriphia spinifrons. 4. 4. ACh and muscarinic agonists are powerful excitatory agents; of these methacholine is at least as effective as ACh; methylfurmethide, muscarine and carbachol are nearly as effective. Nicotine, on the other hand causes excitation only at high concentration and is four orders of magnitude less effective than muscarine. A number of other agents were also tested. 5. 5. Muscarinic antagonists are effective blockers; nicotinic blockers, on the other hand, have a very low potency. This is especially true for bis-quarternary compounds. 6. 6. The significance of the presence of typical muscarinic cholinoreceptors in crustacean ganglion cells is discussed, especially in the light of Michelson's concept of the evolution of cholinoreceptor structure.