Phototaxis and thermotaxis by Dictyostelium discoideum slugs on water agar were impaired by the presence in the agar of adenosine, which is a cyclic AMP receptor antagonist in aggregating amoebae. Caffeine, and presumably its analogue theophylline, inhibit cyclic AMP signalling in aggregating amoebae of D. discoideum. Both compounds perturbed slug behaviour in a similar manner to adenosine, as did both ammonium and sulphate ions. (NH4)3SO4 is known to perturb cyclic AMP binding to its receptor, and ammonia is an inhibitor of cyclic AMP signalling in aggregating amoebae. The receptor agonist, cyclic AMPS, disrupted slug organization and impaired phototaxis when present at concentrations high enough to saturate cyclic AMP receptors and compete effectively with endogenous cyclic AMP signals of similar magnitude to those observed during aggregation. Taken together with the considerable circumstantial evidence for cyclic AMP signalling in slugs, these results support a role for cyclic AMP signalling in slug behaviour.

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