Effects of N-beta-phenylpropionyl-L-tyrosine and its derivatives on the excitability of an identifiable giant neuron of Achatina fulica férussac.

1. Previously the authors demonstrated the inhibitory effects of the two aromatic amino acid derivatives, N-beta-phenylpropionyl-L-Tyr (critical concentration (c.c.), 3 x 10(-7) - 10(-6) M) and N-beta-phenylpropionyl-L-Trp (c.c., 10(-6) M) on the excitability of an identifiable giant neuron, TAN (tonically autoactive neuron), of Achatina fulica Férussac. The effects of the derivatives of the two inhibitory compounds on the same neuron are examined in the present study. 2. N-beta-Cyclohexylpropionyl-L-Tyr (c.c., 3 x 10(-8) - 10(-7) M) and N-beta-cyclohexylpropionyl-L-Trp (c.c., 10(-6) M) had marked inhibitory effects, whereas N-beta-p-methyl-phenylpropionyl-L-Tyr had none. 3. N-gamma-Phenylbutyroyl-L-Tyr and N-phenylacetyl-L-Tyr, in which the chain length of the phenyl group is different, had no effect. 4. N-beta-Phenylpropionyl-N-methyl-L-Tyr, in which the imino group of the peptide bond is methylated, had no effect. 5. N-beta-Phenylpropionyl-L-Tyr (c.c., 3 x 10(-7) - 10(-6) M) and N-beta-phenylpropionyl-L-Tyr methylester (c.c. 1-3 x 10(-6) M) had marked inhibitory effects, suggesting that their carbonyl group acts as a proton acceptor. 6. N-beta-Phenylpropionyl-L-p-hydroxyphenylglycine, in which the chain length of the hydrogen binding group is shorter, had no effect. 7. N-beta-Phenylpropionyl-L-3,4-dihydroxy-Phe (c.c., 3 x 10(-6) M), N-beta-phenylpropionyl-L-3-nitro-Tyr (c.c., 3 x 10(-5) M) and N-beta-phenylpropionyl-L-p-amino-Phe (c.c., 3 x 10(-5) - 10(-4) M) had inhibitory effects, weaker than that of N-beta-phenylpropionyl-L-Tyr. N-beta-Phenylpropionyl-L-p-chloro-Phe and N-beta-phenylpropionyl-L-p-nitro-Phe showed the same effect only at high concentrations.