Catechol blocks the fast outward potassium current in melanotrophs of the rat pituitary

Characterization of a high-voltage-activated IA current with a role in spike timing and locomotor pattern generation

Transient A-type K+ channels (I(A)) in neurons have been implicated in the delay of the spike onset and the decrease in the firing frequency. Here we have characterized biophysically and pharmacologically an I(A) current in lamprey locomotor network neurons that is activated by suprathreshold depolarization and is specifically blocked by catechol at 100 microM. The biophysical properties of this current are similar to the mammalian Kv3.4 channel. The role of the I(A) current both in single neuron firing and in locomotor pattern generation was analyzed. The I(A) current facilitates Na+ channel recovery from inactivation and thus sustains repetitive firing. The role of the I(A) current in motor pattern generation was examined by applying catechol during fictive locomotion induced by N-methyl-d-aspartate. Blockade of this current increased the locomotor burst frequency and decreased the firing of motoneurons. Although an alternating motor pattern could still be generated, the cycle duration was less regular, with ventral roots bursts failing on some cycles. Our results thus provide insights into the contribution of a high-voltage-activated I(A) current to the regulation of firing properties and motor coordination in the lamprey spinal cord.

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

The spinal neurons of the Xenopus embryo that participate in the swimming motor pattern possess two kinetically distinct sets of potassium currents: the fast IKf and sodium-dependent IKNa, which together constitute approximately 80% of the outward current; and the slow IKs, 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, approximately 10 microM). The block was voltage-dependent, with partial unblocking occurring at positive voltages. alpha-Dendrotoxin and dendrotoxin-I 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 markedly 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.

Ion channels and the control of swimming in the Xenopus embryo

The Xenopus embryo has been well studied and the circuitry underlying motor pattern generation largely elucidated. We have extended this analysis by determining the roles of individual voltage- and ligand-gated ion channels in controlling the motor pattern for swimming and two mechanisms that control rundown of this pattern. Xenopus embryo spinal neurons possess at least six classes of ion channel: a fast Na+ channel; a mixture of kinetically similar Ca2+ channels; a fast K+ channel; a slow K+ channel; a Na(+)-dependent K+ channel; and a slowly activating Ca2(+)-dependent K+ channel. The roles of the voltage-gated currents in determining neuronal firing properties and operation of the locomotor circuitry have been examined both pharmacologically and in realistic computer simulations. Model neurons fire repetitively in response to current injection. The Ca2+ current seems essential for repetitive firing. The fast K+ current appears mainly to control spike width, whereas the slow K+ current exerts a powerful influence on repetitive firing. These predictions from the model have been confirmed by the use of specific pharmacological blockers of the fast and slow K+ currents. Both the model network and the real spinal locomotor circuit appear to tolerate a wide variation in the relative strengths of the component synapses but are very sensitive to the magnitudes of the voltage-gated currents. In particular the slow K+ current, despite being a small component of the total outward current, plays a critical role in stabilizing the motor pattern. Like many other rhythmic motor patterns, swimming in the Xenopus embryo is episodic; it undergoes run-down and self-termination even in the absence of sensory inputs. The slow Ca2(+)-dependent K+ current appears to play a role in the self-termination of swimming. However, intrinsic modulation mediated by the release of ATP and production of adenosine in the extracellular space appears to be a very powerful determinant of run-down of the motor pattern.