Developmental changes in expression of ion currents accompany maturation of locomotor pattern in frog tadpoles

1 The K + currents of spinal neurons acutely dissociated from Xenopus larvae were studied and compared with those of neurons dissociated from Xenopus embryos. 2 The density of total outward current in the larval and embryonic neurons remained the same from stage 37/38 to stage 42. 3 Almost all neurons at stage 42 expressed a fast activating Ca 2+ ‐dependent K + current ( I KCa ) that was largely absent from embryonic neurons. Whereas I KCa became larger and more prevalent during development, the delayed rectifier K + currents were down‐regulated. 4 About 53% of I KCa was selectively blocked by iberiotoxin which had no effect on the delayed rectifier K + currents or the K + currents of embryonic neurons. 5 The firing properties of neurons isolated from embryos were unchanged by iberiotoxin. However, the toxin greatly increased the frequency of firing in larval neurons. 6 Iberiotoxin extended the duration of ventral root bursts during fictive swimming in larvae at stages 41 and 42 but had no effect at stage 40. The progressive expression of I KCa thus contributed to burst termination. 7 We have found that changes in expression of outward current closely correlate with the maturation of the motor pattern during development. At a time when the motor pattern has a need for a burst‐terminating mechanism, the larval neurons express a channel with properties appropriate for such a role.