MODULATION OF BURST FREQUENCY BY CALCIUM-DEPENDENT POTASSIUM CHANNELSIN THE LAMPREY LOCOMOTOR SYSTEM - DEPENDENCE OF THE ACTIVITY LEVEL

It is crucial to determine the effects on the network level of a modul ation of intrinsic membrane properties. The role calcium-dependent pot assium channels, K-Ca, in the lamprey locomotor system has been invest igated extensively. Earlier experimental studies have shown that apami n, which affects one type of K-Ca, increases the cycle duration of the locomotor network, due to effects on the burst termination. The effec ts of apamin were here larger when the network had a low level of acti vity (burst frequency 0.5 to 1 Hz) as compared to a higher rate (>2 Hz ). By using a previously developed simulation model based on the lampr ey locomotor network, we show that the model could account for the fre quency dependence of the apamin modulation, if only the K-Ca conductan ce activated by Ca2+ entering during the action potential was altered and not the Kc, conductance activated by Ca2+ entering through NMDA ch annels. The present simulation model of the spinal network in the lamp rey can thus account for earlier experimental results with apamin on t he network and cellular level that previously appeared enigmatic.