The action of acetylcholine antagonists on amino acid responses in the frog spinal cord in vitro.

1 The isolated hemisected frog spinal cord has been used to study the action of acetylcholine antagonists on amino acid responses by means of sucrose gap recording. 2 Primary afferents and motoneurones were shown to contain few, if any, cholinoceptors, since acetylcholine and carbachol responses were essentially abolished when synaptic transmission was blocked with magnesium ions or when action potentials were blocked by tetrodotoxin. 3 Curare antagonized the gamma-aminobutyric acid (GABA) and beta-alanine depolarizations of primary afferents and hyperpolarizing action of these amino acids on motoneurones. Nicotine also antagonized beta-alanine depolarizations and to a small extent GABA depolarizations of primary afferents. These actions are similar to but weaker than those obtained previously with picrotoxin. 4 Atropine selectively antagonized beta-alanine depolarizations of primary afferents and blocked beta-alanine and glycine hyperpolarizations of motoneurones. GABA responses were entirely resistant to the action of atropine. These actions are similar to but 50 times weaker than those obtained previously with strychnine. 5 Dihydro-beta-erythroidine, tetraethylammonium, and gallamine were entirely ineffective in antagonizing amino acid responses. Since these agents are known to block the dorsal root potential elicited by ventral root stimulation but have no effect on the amino acid responses of primary afferents, it is evident that a cholinergic step is involved in this pathway.