ABSTRACT
Excitotoxicity is the major cause of many neurologic disorders including stroke. Potassium currents modulate neuronal excitability and therefore influence the pathological process. A-type potassium current (I(A)) is one of the major voltage-dependent potassium currents, yet its roles in excitotoxic cell death are not well understood. We report that, following ischemic insults, the I(A) increases significantly in large aspiny (LA) neurons but not medium spiny (MS) neurons in the striatum, which correlates with the higher resistance of LA neurons to ischemia. Activation of protein kinase Cα increases I(A) in LA neurons after ischemia. Cultured neurons from transgenic mice lacking both Kv1.4 and Kv4.2 subunits exhibit an increased vulnerability to ischemic insults. Increase of I(A) by recombinant expression of Kv1.4 or Kv4.2 is sufficient in improving the survival of MS neurons against ischemic insults both in vitro and in vivo. These results, taken together, provide compelling evidence for a protective role of I(A) against ischemia.
Subject(s)
Brain Ischemia/metabolism , Neurons/metabolism , Potassium Channels, Voltage-Gated/genetics , Potassium/metabolism , Up-Regulation , Animals , Brain/cytology , Brain/metabolism , Brain/pathology , Brain Ischemia/genetics , Brain Ischemia/pathology , Cells, Cultured , Female , HEK293 Cells , Humans , Kv1.4 Potassium Channel/genetics , Kv1.4 Potassium Channel/metabolism , Male , Mice , Mice, Knockout , Neurons/cytology , Neurons/pathology , Potassium Channels, Voltage-Gated/metabolism , Protein Kinase C/metabolism , Rats , Rats, Wistar , Shal Potassium Channels/genetics , Shal Potassium Channels/metabolismABSTRACT
Protein kinase C (PKC) plays critical roles in neuronal activity and is widely expressed in striatal neurons. However, it is not clear how PKC activation regulates the excitability of striatal cholinergic interneurons. In the present study, we found that PKC activation significantly inhibited A-type potassium current (I(A)), but had no effect on delayed rectifier potassium currents. Consistently, application of PKC activator caused an increase of firing in response to depolarizing currents in cholinergic interneurons, which was persistent in the presence of both excitatory and inhibitory neurotransmission blockers. These excitatory effects of PKC could be partially mimicked and occluded by blockade of I(A) with potassium channel blocker 4-aminopyridine. In addition, immunostaining demonstrated that PKCalpha, but not PKCgamma and PKCepsilon, was expressed in cholinergic interneurons. Furthermore, activation of group I metabotropic glutamate receptors (mGluRs) led to an inhibition of I(A) through a PKC-dependent pathway. These data indicate that activation of PKC, most likely PKCalpha, increases the neuronal excitability of striatal cholinergic interneurons by down-regulating I(A). Group I mGluR-mediated I(A) inhibition might be important for the glutamatergic regulation of cholinergic tone in the neostriatum.
Subject(s)
Corpus Striatum/physiology , Interneurons/physiology , Protein Kinase C/metabolism , 4-Aminopyridine/pharmacology , Action Potentials/drug effects , Action Potentials/physiology , Animals , Corpus Striatum/drug effects , Delayed Rectifier Potassium Channels/antagonists & inhibitors , Delayed Rectifier Potassium Channels/metabolism , In Vitro Techniques , Interneurons/drug effects , Male , Potassium/metabolism , Potassium Channel Blockers/pharmacology , Potassium Channels, Voltage-Gated/antagonists & inhibitors , Potassium Channels, Voltage-Gated/metabolism , Protein Kinase C-alpha/metabolism , Protein Kinase C-epsilon/metabolism , Rats , Rats, Wistar , Receptors, Metabotropic Glutamate/metabolism , Signal TransductionABSTRACT
Spiny neurons in the neostriatum die within 24 hr after transient global ischemia, whereas large aspiny (LA) neurons remain intact. To reveal the mechanisms of such selective cell death after ischemia, excitatory neurotransmission was studied in LA neurons before and after ischemia. The intrastriatally evoked fast EPSCs in LA neurons were depressed < or =24 hr after ischemia. The concentration-response curves generated by application of exogenous glutamate in these neurons were approximately the same before and after ischemia. A train of five stimuli (100 Hz) induced progressively smaller EPSCs, but the proportion of decrease in EPSC amplitude at 4 hr after ischemia was significantly smaller compared with control and at 24 hr after ischemia. Parallel depression of NMDA receptor and AMPA receptor-mediated EPSCs was also observed after ischemia, supporting the involvement of presynaptic mechanisms. The adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine blocked the inhibition of evoked EPSCs at 4 hr after ischemia but not at 24 hr after ischemia. Electron microscopic studies demonstrated that the most presynaptic terminals in the striatum had a normal appearance at 4 hr after ischemia but showed degenerating signs at 24 hr after ischemia. These results indicated that the excitatory neurotransmission in LA neurons was depressed after ischemia via presynaptic mechanisms. The depression of EPSCs shortly after ischemia might be attributable to the enhanced adenosine A1 receptor function on synaptic transmission, and the depression at late time points might result from the degeneration of presynaptic terminals.
