Synthesis of achatin-I (Gly-D-Phe-l-Ala-l-Asp) analogs having dehydrophenylalanine or aminoisobutyric acid residue at position 2, and their effects on Achatina giant neurons

Abstract

1. 1. Achatin-I (Gly-D-Phe-L-Ala-L-Asp), a neuroactive tetrapeptide having a D-phenyl-alanine residue, has been proposed to be an excitatory neurotransmitter of Achatina giant neurons. It was revealed that the D-Phe² residue is essential for bioactivity of achatin-I, which seems to adopt β-turn conformation. In the present study, in order to investigate the structure-activity relationships of achatin-I and its derivatives, the two highly constrained analogs of achatin-I, [Δ^ZPhe²]achatin-I (Gly-Δ^ZPhe-L-Ala-L-Asp) (Δ^ZPhe: (Z)-α,β-dehydrophenylalanine) and [Aib²]achatin-I (Gly-Aib-L-Ala-L-Asp) (Aib: α-aminoisobutyric acid), were synthesized, and their effects on the two identifiable Achatina giant neuron types, PON (periodically oscillating neuron) and v-RCDN (ventral-right cerebral distinct neuron), were examined in comparison with those of achatin-I under voltage clamp. 2. 2. Achatin-I (n = 6), ejected onto the neurone by brief pneumatic pressure (2 kg/cm², 400 ms, 10^-3 M, at 10-min intervals), produced an inward current (I_i) on PON. The I_i value (mean ± SEM) was 0.44±0.03 nA. The interval between the achatin-I ejection and the I_i peak was 14.74±3.15 s (n=6). [Δ^ZPhe²]achatin-I (n=6) and [Aib²]achatin-I (n=6) had no effect on this neuron type. 3. 3. On the other hand, achatin-I (n=10) and [Δ^ZPhe²]-achatin-I (n=10), ejected by brief pressure, produced an I_in on v-RCDN. The I_in values were 0.85±0.07 nA for achatin-I and 0.48±0.05 nA (p<0.01, compared with that of achatin-I by Student's t-test for paired data) for [Δ^ZPhe²]achatin-I. The intervals between the compound ejection and the I_in peak were 5.95±0.33 s for achatin-I and 8.70±0.81 s (p<0.05, compared with that of achatin-I) for [Δ^ZPhe²]achatin-I. [Aib²]achatin-I (n=10) had no effect on this neuron type.