Activity-related calcium dynamics in lamprey motoneurons as revealed by video-rate confocal microscopy

In lamprey spinal cord, intracellular calcium ([Ca2+]i) plays a key role in mechanisms regulating neuronal activity in the segmental network for locomotion. In this report, measurements of [Ca2+]i with fluo-3 in various regions of motoneurons in the intact spinal cord were obtained on a high speed confocal microscope following electrical stimulation. Likewise, rhythmic calcium fluctuations within dendrites and axons were seen during "fictive swimming" and were directly correlated with electrical activity. Antidromic stimulation of motoneuron axons induced large calcium transients and revealed spatially restricted "hot spots," both of which required external calcium and were blocked by nickel, but not by known calcium channel antagonists. These results suggest that lamprey spinal cord axons may possess a pharmacologically novel class of calcium channel.