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Characterization of Na⁺-Activated K⁺ Currents in Larval Lamprey Spinal Cord Neurons

Nobel Institute for Neurophysiology, Department of Neuroscience, Karolinska Institutet, Stockholm, Sweden

Submitted 18 July 2006; accepted in final form 15 February 2007

Potassium channels play an important role in controlling neuronal firing and synaptic interactions. Na⁺-activated K⁺ (K_Na) channels have been shown to exist in neurons in different regions of the CNS, but their physiological function has been difficult to assess. In this study, we have examined if neurons in the spinal cord possess K_Na currents. We used whole cell recordings from isolated spinal cord neurons in lamprey. These neurons display two different K_Na currents. The first was transient and activated by the Na⁺ influx during the action potentials, and it was abolished when Na⁺ channels were blocked by tetrodotoxin. The second K_Na current was sustained and persisted in tetrodotoxin. Both K_Na currents were abolished when Na⁺ was substituted with choline or N-methyl-D-glucamine, indicating that they are indeed dependent on Na⁺ influx into neurons. When Na⁺ was substituted with Li⁺, the amplitude of the inward current was unchanged, whereas the transient K_Na current was reduced but not abolished. This suggests that the transient K_Na current is partially activated by Li⁺. These two K_Na currents have different roles in controlling the action potential waveform. The transient K_Na appears to act as a negative feedback mechanism sensing the Na⁺ influx underlying the action potential and may thus be critical for setting the amplitude and duration of the action potential. The sustained K_Na current has a slow kinetic of activation and may underlie the slow Ca²⁺-independent afterhyperpolarization mediated by repetitive firing in lamprey spinal cord neurons.

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