Korean J Physiol Pharmacol.  2014 Apr;18(2):135-141. 10.4196/kjpp.2014.18.2.135.

The Downregulation of Somatic A-Type K+ Channels Requires the Activation of Synaptic NMDA Receptors in Young Hippocampal Neurons of Rats

Affiliations
  • 1Department of Physiology, School of Medicine, Jeju National University, Jeju 690-756, Korea. jungsc@jejunu.ac.kr
  • 2Institute of Medical Science, Jeju National University, Jeju 690-756, Korea.

Abstract

The downregulation of A-type K+ channels (IA channels) accompanying enhanced somatic excitability can mediate epileptogenic conditions in mammalian central nervous system. As IA channels are dominantly targeted by dendritic and postsynaptic processings during synaptic plasticity, it is presumable that they may act as cellular linkers between synaptic responses and somatic processings under various excitable conditions. In the present study, we electrophysiologically tested if the downregulation of somatic IA channels was sensitive to synaptic activities in young hippocampal neurons. In primarily cultured hippocampal neurons (DIV 6~9), the peak of IA recorded by a whole-cell patch was significantly reduced by high KCl or exogenous glutamate treatment to enhance synaptic activities. However, the pretreatment of MK801 to block synaptic NMDA receptors abolished the glutamate-induced reduction of the IA peak, indicating the necessity of synaptic activation for the reduction of somatic IA. This was again confirmed by glycine treatment, showing a significant reduction of the somatic IA peak. Additionally, the gating property of IA channels was also sensitive to the activation of synaptic NMDA receptors, showing the hyperpolarizing shift in inactivation kinetics. These results suggest that synaptic LTP possibly potentiates somatic excitability via downregulating IA channels in expression and gating kinetics. The consequential changes of somatic excitability following the activity-dependent modulation of synaptic responses may be a series of processings for neuronal functions to determine outputs in memory mechanisms or pathogenic conditions.

Keyword

A-type K+ channel; Glutamate; Intrinsic excitability; Long-term potentiation; NMDA receptors

MeSH Terms

Animals
Central Nervous System
Dizocilpine Maleate
Down-Regulation*
Glutamic Acid
Glycine
Kinetics
Long-Term Potentiation
Memory
N-Methylaspartate*
Neurons*
Plastics
Rats*
Receptors, N-Methyl-D-Aspartate*
Dizocilpine Maleate
Glutamic Acid
Glycine
N-Methylaspartate
Plastics
Receptors, N-Methyl-D-Aspartate

Figure

  • Fig. 1 The enhancement of neuronal activities reduces the peak amplitude of somatic IA. (A) Example traces of transient outward K+ currents (i.e. IA) recorded by whole-cell patch in cultured hippocampal neurons. KCl (KCl, 20 mM) or glutamate (Glu, 5 µM) for 24 hours was added to culture media before recording. Scale bars: 500 pA, 100 ms. (B) Example traces showing that APV abolishes the effect of KCl or glutamate. APV (100 µM) with KCl or glutamate was added to culture media for 24 hours before recording. Scale bars: 500 pA, 100 ms. (C) A trace showing the effect of MK801 on the reduction of IA peak. KCl and MK801 (25 µM) were added to culture media for 24 hours. After this, long exposure (24 hours) to exogenous glutamate (KCl+MK801/Glu) did not reduce the peak of somatic IA. Scale bars: 500 pA, 100 ms. Dotted lines indicate the averaged IA peak of control neurons. (D) Summarized changes of current density of IA with individual values of neurons. The statistical significance was set to p<0.05 or 0.01 (*compared with control; †compared with KCl or Glu). Square boxes indicate the mean value of each group and error bars represent SEM.

  • Fig. 2 Blocking synaptic NMDA receptors abolishes the effect of glutamate on IA. (A) Example traces showing the effects of acute treatment of glutamate (acuteGlu, 5 µM, 5 min) on somatic IA under either Mg2+-included (Mg2+(+)) or Mg2+-free (Mg2+(-)) recording condition. Scale bars: 500 pA, 100 ms. (B) A trace of IA showing that blocking synaptic NMDA receptors by KCl and MK801 pretreatment abolishes the effect of acute glutamate treatment under Mg2+-free recording condition (KCl+MK801/acuteGlu). (C) Example traces showing the effects of glycine (Glycine, 200 µM, 5 min) and MK801 (Gly+MK801/acuteGlu, 25 µM) under Mg2+-free recording condition. Scale bars: 500 pA, 100 ms. Dotted line indicates the averaged IA peak of control neurons. (D) Summarized changes of current density of IA showing the effects of acute glutamate treatment. The statistical significance was set to p<0.05 or 0.01 (*compared with control; †compared with acuteGlu[Mg2+(-)]). Square boxes indicate the mean value of each group and error bars represent SEM.

  • Fig. 3 The changes of inactivation and activation properties of IA channels under enhanced neuronal activities. (A) Changes of Vh (thick dotted lines in B). Square bars indicate averaged values. (B) Boltzmann-fitted gating kinetics of IA channels. The left-shifted inactivation curves were observed in neurons treated with either KCl (KCl, 20 mM, 24 hours, to culture media) or acute glutamate (acuteGlu[Mg2+(-)], 5 µM, 5 min, to recording solution), indicating the possible involvement of the kinetic downregulation of somatic IA channels for enhancing neuronal excitabilities.


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