The pharmacology and roles of two K+ channels in motor pattern generation in the Xenopus embryo

Abstract

The spinal neurons of the Xenopus embryo that participate in the swimming motor pattern possess two kinetically distinct sets of potassium currents: the fast I(Kf) and sodium-dependent I(KNa), which together constitute ~80% of the outward current; and the stow I(Ks), which constitutes the remainder. To study their respective roles in cell excitability and the swimming pattern, we have characterized their pharmacological properties. Catechol selectively blocked the fast potassium currents (IC50, ~10 μM). The block was voltage-dependent, with partial unblocking occurring at positive voltages. α-Dendrotoxin and dendrotoxin-1 selectively blocked the slow potassium current. Catechol and the dendrotoxins had different effects on membrane excitability: catechol caused spike broadening but had little effect on repetitive firing, whereas both dendrotoxins makedly increased repetitive firing without affecting spike width. By applying these agents to the whole embryo, we tested the role of the fast and slow currents in motor pattern generation. Catechol had little effect on fictive swimming, suggesting that the fast K+ currents are not critical to circuit operation. However, dendrotoxin disrupted swimming early in the episode and increased the duration of ventral root bursts. The slow K+ current, which is a minor component of the total outward current, thus appears to play an important role in motor pattern generation.