Repeated applications of high potassium elicit long-term changes in a motor circuit from the crab, .

We examined the effects of altered extracellular potassium concentration on the output of the well-studied pyloric circuit in the crab, . Pyloric neurons initially become quiescent, then recover spiking and bursting activity in high potassium saline (2.5x[K]). These changes in circuit robustness are maintained after the perturbation is removed; pyloric neurons are more robust to subsequent potassium perturbations even after several hours of wash in control saline. Despite this long-term "memory" of the stimulus history, we found no differences in neuronal activity in control saline. The circuit's adaptation is erased by both low potassium saline (0.4x[K]) and direct hyperpolarizing current. Initial sensitivity of PD neurons to high potassium saline also varies seasonally, indicating that changes in robustness may reflect natural changes in circuit states. Thus, perturbation, followed by recovery of normal activity, can hide cryptic changes in neuronal properties that are only revealed by subsequent challenges.