Development of skeletal muscle sodium and potassium currents in zebrafish sofa potato mutants

In some cells, the development of voltage-gated channels requires synaptic input, while in others it does not. Here we investigate whether the sodium and potassium currents in the skeletal muscle of zebrafish sofa potato (sop(-/-)) mutants develop normally. Zebrafish sop(-/-) mutants do not express nicotinic acetylcholine receptors at neuromuscular junctions, and therefore do not exhibit synaptic activity in muscle. We find that in both red and white muscle fibers, sop(-/-) mutants are able to support normal potassium current development during early stages of development [1-3 days post fertilization (dpf)], but at 6 dpf the potassium current density is significantly smaller than that in their phenotypically wild-type siblings (sop(+/?)). In contrast, sodium current density is unaffected. The steady-state properties of potassium currents are unaltered in the sop(-/-) mutants, but there is a significant difference in the V(50) of inactivation of sodium currents. This is the first study in zebrafish to investigate activity-dependent mechanisms of ion channel development and our results indicate that some aspects of ion current development in skeletal muscle require synaptic activity, whereas others do not.