Introduction
Coordinated motor patterns are generated by neural circuits, the activity of which depends on the intrinsic properties of single neurons and their synaptic interactions. Although a few neural circuits underlying rhythmic motor patterns have been identified and characterized in some detail, our understanding about their function remains incomplete. Fundamental understanding of how neural circuits generate and control behavior requires knowledge about the role of different subclasses of ion channels for the function of network neurons and thereby also how neurons regulate information processing, synaptic interactions and the operation of the whole network.
Methods
In this study, the lamprey spinal cord was used as a model system. Patch-clamp recordings from isolated spinal cord neurons in culture were used to characterize potassium channels. Intra and extracellular recordings from the intact spinal cord were made to test the importance of these channels in the generation of the locomotor rhythmic pattern.
Results
Whole cell patch clamp recordings from spinal neurons in culture show that they possess a transient A-type and a sustained potassium current. The A-type current is blocked specifically by catechol. This current activates a membrane potential above the spiking threshold and displays a rapid activation and inactivation kinetics. At the single neuron level, blockade of this current reduces the repetitive firing of neurons. The A-type potassium current allows neurons to maintain a sustained repetitive firing by facilitating sodium channels recovery from inactivation. At the synaptic level, blockade of the A-type potassium current results in an increased synaptic transmission through increased calcium influx. During fictive locomotion induced by NMDA, application of the specific blocker of A-type potassium current catechol increases the frequency of the locomotor ventral roots bursts and decreases the firing of neurons.
Conclusions
In conclusion, we have characterized an A-type potassium current activated by supra-threshold depolarization with fast kinetics. The biophysical properties of this current facilitate sodium channels recovery from inactivation during repetitive firing. In addition, this current also limits calcium influx in the presynaptic axons. The cellular and synaptic effects of blockade of A-type potassium current account for the increase in the locomotor burst frequency.
References