Excitatory effect of amino acids on identified neuron R14 of Aplysia. II. Neutral amino acids and structure-activity relationships.

The effects of 20 amino acids on identified neuron R14 of Aplysia kurodai were studied by conventional intracellular recording and voltage-clamp techniques. Neutral alpha-amino acids caused marked dose-dependent depolarizations of the neuron. The effects of several amino acids applied simultaneously were additive. L-isomers were much more effective than the corresponding D-isomers. R14 was at most slightly depolarized by perfusion with 2-aminoisobutyric acid and the non-alpha-amino acids beta-alanine and beta- and gamma-aminobutyric acid. Putative neurotransmitters ACh, 5-HT, dopamine, GABA, and taurine were much less effective in depolarizing R14 than neutral amino acids. Values of the Hill coefficient (nH) and the apparent dissociation constant (KA) were approximately 0.64 and 80 mM, respectively, for glycine. These results suggest that R14 has a general sensitivity to neutral amino acids and that amino acids in the hemolymph may be able to influence the electrical activity of R14.