The group imaged calcium dynamics in aspiny dendrites, where synaptic activation created a fast, short-lived, and highly localized calcium influx. However, they saw no morphological structures that could contain the signal. Rather, says Goldberg, “the key to localization is fast kinetics.” This is provided by calcium-permeable glutamate receptors of the AMPA family (CP-AMPA), which turn on and off rapidly. The strong influx of calcium was also quickly purged by the Na+/Ca2+ exchanger. These effects combined to limit calcium diffusion to a space on the order of the size of a synapse.
As expected based on these results, aspiny dendrites tend to have CP-AMPAs rather than the slower NMDA class of glutamate receptors. Goldberg believes the results also show that “any source of calcium can be highly localized without morphological boundaries” as long as the kinetics are right. ▪