Suppressing effects of neuroactive peptides on the inward current caused by achatin-I, an Achatina endogenous peptide.

1. Modulatory effects of the four molluscan neuroactive peptides. FMRFamide (Phe-Met-Arg-Phe-NH2), APGW-amide (Ala-Pro-Gly-Trp-NH2), oxytocin and [SER2]-Mytilus inhibitory peptide ([SER2]-MIP) (Gly-Ser-Pro-Met-Phe-Val-NH2) were examined on the inward current (Iin) caused by achatin-I (Gly-D-Phe-Ala-Asp), which has been isolated from the Achatina ganglia. 2. Two Achatina giant neurone types, v-RCDN (ventral-right cerebral distinct neurone) and PON (periodically oscillating neurone), were used. Achatin-I was applied locally to the neurone tested by brief pneumatic pressure ejection, and the other molluscan neuroactive peptides were perfused around the ganglia. 3. FMRFamide, perfused at 3 microM, suppressed markedly the Iin elicited by the achatin-I of both v-RCDN and PON. APGW-amide at 3 microM also suppressed the Iin of v-RCDN, but did not affect that of PON. Oxytocin at 1 microM suppressed the Iin of PON, but did not affect that of v-RCDN. [Ser2]-MIP at 3 microM did not affect the Iin of v-RCDN. 4. The dose-response curves of FMRFamide, APGW-amide and oxytocin, indicated that their respective suppressive effects on the Iin of achatin-I were dose-dependent, and that APGW-amide was slightly more potent than the other peptides. The dose (pressure duration)-response curves of achatin-I (1 kg/cm2, 10(-3) M, 5 min interval), obtained by varying the duration of the achatin-I pressure ejection, were measured in the presence and absence of each of the three peptides.(ABSTRACT TRUNCATED AT 250 WORDS)