Modulation of neuropeptide effects by achatin-I, an Achatina endogenous tetrapeptide.

Achatin-I (Gly-D-Phe-L-Ala-L-Asp), an endogenous tetrapeptide in the ganglia of Achatina fulica Férussac, at 3 x 10(-6) M suppressed both the inward current (Iin) of an Achatina giant neurone, PON (periodically oscillating neurone), caused by locally ejected oxytocin, and the outward current (Iout) of v-RCDN (ventral-right cerebral distinct neurone) induced by APGW-amide. Dose (pressure duration)-response studies with oxytocin and APGW-amide showed that their ED50 values were not affected by achatin-I, whereas the Emax values were suppressed. The v-RCDN Iout caused by FMRF-amide was enhanced by achatin-I, but the same current induced by [Ser2]Mytilus inhibitory peptide ([Ser2]MIP) was not affected. Achatin-I suppressed the Iout of TAN (tonically autoactive neurone) caused by acetylcholine, but did not affect the Iin of v-RCDN elicited by acetylcholine. The Iout caused by gamma-aminobutyric acid, threo-beta-hydroxy-L-glutamic acid and dopamine was not affected by achatin-I. It is considered that achatin-I acts as a neuromodulator and has both suppressing and enhancing actions on the effects of various neurotransmitters, including peptides, in Achatina giant neurones.