Excitatory actions of GABA and of acetylcholine in sea urchin tube feet

Abstract

1. 1. The musculature of isolated tube feet of several species of sea urchins was found to contract in response to GABA and to acetylcholine (ACh). The action of GABA but not that of ACh is blocked by bicuculline and picrotoxin. 2. 2. The action of GABA but not that of ACh requires the presence of Na +; it is reduced when the external Cl concentration is diminished. 3. 3. The following cholinomimetics were found to be effective: acetyl-β-methylcholine, carbamylcholine, butyrylcholine and nicotine. The contractile effect of these agents, but also that of GABA was prevented by the cholinolytics atropine, banthine, decamethonium, flaxedil, hexamethonium and mytolon. 4. 4. The action of ACh and of acetyl-β-methylcholine, but also that of GABA, was greatly enhanced by anticholinesterases (eserine, neostigmine). 5. 5. Both ACh and GABA were detected in tube foot extracts. ACh is specifically localized in those parts that contain nerve tissue, with concentrations up to 75 μg/g wet wt. 6. 6. It is concluded that GABA causes excitation of cholinergic motoneurones and does not affect the muscle fibers directly. The cholinoreceptors of the muscle fibers have both muscarinic and nicotinic properties.