Abstract
The possible role of cyclic AMP in mediating opposite modulatory effects of serotonin (5-HT) on Aplysia buccal mass muscles E1 and E2 was examined. Serotonin enhances E1 contractions and inhibits E2 contractions. Adenylate cyclase in membranes of both E1 and E2 is stimulated approximately 180% by 10(-6) M 5-HT and 300% by 10(-3) M 5-HT. Dibutyryl cyclic AMP and 8-benzylthio cyclic AMP mimicked the effect of 5-HT on E1 but had no effect on E2. Theophylline (Th) and isobutylmethylxanthine (IBMX) mimicked the effect of 5-HT on E1 at high concentrations. Concentrations of Th and IBMX low enough not to have any direct effect on contraction increased both the magnitude and duration of the effect of 5-HT on E1 contraction. Neither Th nor IBMX had a direct effect on E2 contraction, although Th produced a small increase in the effect of 5-HT on E2. These data are consistent with the hypothesis that cyclic AMP mediates the enhancement effect of 5-HT on E1 contraction. Other mechanisms probably mediate the effect of 5-HT on E2 contraction.
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