Small-conductance Ca2+-dependent K+ channels are the target of spike-induced Ca2+ release in a feedback regulation of pyramidal cell excitability.
Cooperative regulation of inosiol-1,4,5-trisphosphate receptors (IP(3)Rs) by Ca(2+) and IP(3) has been increasingly recognized, although its functional significance is not clear. The present experiments first confirmed that depolarization-induced Ca(2+) influx triggers an outward current in visual cortex pyramidal cells in normal medium, which was mediated by apamin-sensitive, small-conductance Ca(2+)-dependent K(+) channels (SK channels). With IP(3)-mobilizing neurotransmitters bath-applied, a delayed outward current was evoked in addition to the initial outward current and was mediated again by SK channels. Calcium turnover underlying this biphasic SK channel activation was investigated. By voltage-clamp recording, Ca(2+) influx through voltage-dependent Ca(2+) channels (VDCCs) was shown to be responsible for activating the initial SK current, whereas the IP(3)R blocker heparin abolished the delayed component. High-speed Ca(2+) imaging revealed that a biphasic Ca(2+) elevation indeed underlays this dual activation of SK channels. The first Ca(2+) elevation originated from VDCCs, whereas the delayed phase was attributed to calcium release from IP(3)Rs. Such enhanced SK currents, activated dually by incoming and released calcium, were shown to intensify spike-frequency adaptation. We propose that spike-induced calcium release from IP(3)Rs leads to SK channel activation, thereby fine tuning membrane excitability in central neurons.
Yamada
Shin-Ichiro
S
Department of Geriatric Medicine, Kyoto University Graduate School of Medicine, Kyoto 606-8501, Japan.
Takechi
Hajime
H
Kanchiku
Izumi
I
Kita
Toru
T
Kato
Nobuo
N
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000502:physiology; D020013:Calcium Signaling / Q000502:physiology*; D002217:Carbachol / Q000494:pharmacology; D002614:Chelating Agents / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology; D004594:Electrophysiology; D005246:Feedback / Q000502:physiology*; D007091:Image Processing, Computer-Assisted; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000502:physiology; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D018721:Muscarinic Agonists / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D024681:Potassium Channels, Calcium-Activated / Q000502:physiology*; D017966:Pyramidal Cells / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar; D014793:Visual Cortex / Q000166:cytology / Q000502:physiology
D015220:Calcium Channels; D002614:Chelating Agents; D018721:Muscarinic Agonists; D024681:Potassium Channels, Calcium-Activated; D004533:Egtazic Acid; D015544:Inositol 1,4,5-Trisphosphate; D002217:Carbachol; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D002118:Calcium