Voltage-clamp analysis of the potentiation of the slow Ca2+-activated K+ current in hippocampal pyramidal neurons

The subthreshold, transient A-type K+ current is a vital regulator of the excitability of neurons throughout the brain. In mammalian hippocampal pyramidal neurons, this current is carried primarily by ion channels comprising Kv4.2 α-subunits. These channels occupy the somatodendritic domains of these principle excitatory neurons and thus regulate membrane voltage relevant to the input–output efficacy of these cells. Owing to their robust control of membrane excitability and ubiquitous expression in the hippocampus, their dysfunction can alter network stability in a manner that manifests in recurrent seizures. Indeed, growing evidence implicates these channels in intractable epilepsies of the temporal lobe, which underscores the importance of determining the molecular mechanisms underlying their regulation and contribution to pathologies. Here, we describe the role of p38 kinase phosphorylation of a C-terminal motif in Kv4.2 in modulating hippocampal neuronal excitability and behavioral seizure strength. Using a combination of biochemical, single-cell electrophysiology, and in vivo seizure techniques, we show that kainic acid-induced seizure induces p38-mediated phosphorylation of Thr607 in Kv4.2 in a time-dependent manner. The pharmacological and genetic disruption of this process reduces neuronal excitability and dampens seizure intensity, illuminating a cellular cascade that may be targeted for therapeutic intervention to mitigate seizure intensity and progression.

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Calcium-dependent potassium currents in neurons from cat sensorimotor cortex

Journal of Neurophysiology ◽

10.1152/jn.1992.67.1.216 ◽

1992 ◽

Vol 67 (1) ◽

pp. 216-226 ◽

Cited By ~ 73

Author(s):

P. C. Schwindt ◽

W. J. Spain ◽

W. E. Crill

Keyword(s):

Membrane Potential ◽

Voltage Clamp ◽

Time Constant ◽

Sensorimotor Cortex ◽

Pyramidal Neurons ◽

Outward Current ◽

Tail Current ◽

K Current ◽

The Difference

1. Ca(2+)-dependent K+ currents were studied in large pyramidal neurons (Betz cells) from layer V of cat sensorimotor cortex by use of an in vitro brain slice and single microelectrode voltage clamp. The Ca(2+)-dependent outward current was taken as the difference current obtained before and after blockade of Ca2+ influx. During step depolarizations in the presence of tetrodotoxin (TTX), this current exhibited a fast onset of variable amplitude and a prominent slowly developing component. 2. The Ca(2+)-dependent outward current first appeared when membrane potential was stepped positive to -40 mV. Downsteps from a holding potential of -40 mV revealed little or no time-, voltage-, or Ca(2+)-dependent current. When membrane potential was stepped positive to -40 mV, a prolonged Ca(2+)-dependent outward tail current followed repolarization. The decay of this tail current at -40 mV was best described by a single exponential function having a time constant of 275 +/- 75 (SD) ms. The tail current reversed at 96 +/- 5 mV in 3 mM extracellular K+ concentration ([K+]o) and at more positive potentials when [K+]o was raised, suggesting that it was carried predominantly by K+. 3. The Ca(2+)-dependent K+ current consisted of two pharmacologically separable components. The slowly developing current was insensitive to 1 mM tetraethylammonium (TEA), but a substantial portion was reduced by 100 nM apamin. Most of the remaining current was blocked by the addition of isoproterenol (20-50 microM) or muscarine (10-20 microM). 4. The time courses of the apamin- and transmitter-sensitive components were similar when activated by step depolarizations in voltage clamp, but they were quite different when activated by a train of action potentials. Applying the voltage clamp at the end of a train of 90 spikes (evoked at 100-200 Hz) resulted in an Ca(2+)-dependent K+ current with a prominent rapidly decaying portion (time constant approximately 50 ms at -64 mV) and a smaller slowly decaying portion (time constant approximately 500 ms at -64 mV). The rapidly decaying portion was blocked by apamin (50-200 nM), and the slowly decaying portion was blocked by isoproterenol (20-50 microM). 5. When recorded with microelectrodes containing 2 mM dimethyl-bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (dimethyl-BAPTA), which causes prolonged afterhyperpolarizations, the Ca(2+)-dependent K+ current evoked by step depolarizations had an extremely slow onset and decay. The current recorded after a train of evoked spikes had a similar slow decay.(ABSTRACT TRUNCATED AT 400 WORDS)

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