Summary

Aggregation of Dictyostelium discoideum amoebae is effected by relayed cyclic AMP (cAMP) signals. The period of this wave propagation drops abruptly at the transition from aggregative to post-aggregative development. In this paper I demonstrate that the light-scattering response induced by a low concentration pulse of cAMP evolves from one lasting 5 min to one lasting 2 min.The definitive establishment of the 2-min response coincides with the beginning of post-aggregative gene expression. Amoebae at the aggregative stage are unable to respond to a second cAMP pulse delivered less than 4 min after the first, whereas at the post-aggregative stage they can respond to 2 pulses separated by 1 min or longer. Addition of cAMP phosphodiesterase to amoebae at the aggregative stage can in part mimic the change in excitable behaviour: the response is shortened and the amoebae can respond more frequently. However, the shortened response is not post-aggregative in shape and the prolonged presence of cyclic nucleotides induces different responses at the aggregative and post-aggregative stage. Both these results suggest that the rate of destruction of the relayed signal is not solely responsible for the change in excitability.