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

Abstract

The K+ currents of spinal neurons acutely dissociated from Xenopus larvae were studied and compared with those of neurons dissociated from Xenopus embryos. The density of total outward current in the larval and embryonic neurons remained the same from stage 37/38 to stage 42. Almost all neurons at stage 42 expressed a fast activating Ca2+‐dependent K+ current (IKCa) that was largely absent from embryonic neurons. Whereas IKCa became larger and more prevalent during development, the delayed rectifier K+ currents were down‐regulated. About 53% of IKCa was selectively blocked by iberiotoxin which had no effect on the delayed rectifier K+ currents or the K+ currents of embryonic neurons. The firing properties of neurons isolated from embryos were unchanged by iberiotoxin. However, the toxin greatly increased the frequency of firing in larval neurons. 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 IKCa thus contributed to burst termination. 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.

slo K + channel gene regulation mediates rapid drug tolerance

“…That is, generically, one assumes that increased K ϩ channel activity would hyperpolarize the cell and reduce excitability, whereas reduced K ϩ channel activity would interfere with the ability of the cell to terminate an electrical impulse and thereby enhance electrical excitability. This role of slo channels is well documented (29)(30)(31). However, there is substantial evidence that slo channel activity also can affect neural excitability in the opposite manner.…”

BK channels play a counter-adaptive role in drug tolerance and dependence

“…The role of neural excitant is an unusual one to postulate for a K + channel, as these channels are commonly associated with cell repolarization and suppression of neural activity. Certainly, in some preparations, increased BK channel activity reduces neural excitability (19,20). However, BK channel activity has long been positively correlated with neural excitability (21)(22)(23).…”

Strategies for delineating spinal locomotor rhythm-generating networks and the possible role of Hb9 interneurones in rhythmogenesis

“…Plateau potentials ensure continued action potential firing in the absence of sustained synaptic input (Conway et al, 1988;Russell and Hartline, 1978). Spike frequency adaptation contributes to termination of a burst (el Manira et al, 1994;Sun and Dale, 1998). Rhythm may be generated by either endogenously bursting neurones or by reciprocally inhibitory neurones (Getting, 1989;Satterlie, 1985).…”