Influence of muscle cells on the development of calcium currents in Xenopus spinal neurons

The influence of muscle cells on the development of voltage-dependent Ca 2+ currents was investigated in Xenopus spinal neurons grown in neuron-muscle co-cultures or in muscle-free cultures. Whole-cell currents were separated into low- and high-voltage-activated currents. Developmental changes were assessed by comparing the results obtained at two different periods after plating: 5–10 h (young neurons) and 2–30 h (mature neurons). Our results show a drop in the incidence of low-voltage-activated Ca 2+ current with time in both environments: the fraction of young versus mature neurons expressing this current was 67% and 36% in neuron muscle co-cultures, and 69% and 23% in muscle-free cultures. In both neuron-muscle and muscle-free cultures, the density of low-voltage-activated Ca 2+ current (when expressed) did not change during the development. In contrast, the density of high-voltage-activated Ca 2+ currents increased more than two-fold during the first 30 h in neuron-muscle co-cultures, but remained unchanged in muscle-free cultures. This difference was not related to neuronal growth since the increase in neuronal membrane capacitance with time was similar in the two environments. In addition, direct cellmell interaction through the establishment of functional neuron-muscle synaptic contacts did not further modify the overall expression of high-voltage-activated Ca 2+ currents. In conclusion, these results suggest the presence of diffusible factors in neuron-muscle co-cultures which up-regulate the expression of high-voltage-activated Ca 2+ currents during neuronal development, but do not have any effect on low-voltage-activated Ca 2+ currents.