Amygdala kindling activates the phosphorylation of Ca2+/calmodulin-dependent protein kinase II in rat hippocampus

Voltage-Dependent Calcium Channels, Calcium Binding Proteins, and Their Interaction in the Pathological Process of Epilepsy

As an important second messenger, the calcium ion (Ca²⁺) plays a vital role in normal brain function and in the pathophysiological process of different neurodegenerative diseases including Alzheimer’s disease (AD), Parkinson’s disease (PD), and epilepsy. Ca²⁺ takes part in the regulation of neuronal excitability, and the imbalance of intracellular Ca²⁺ is a trigger factor for the occurrence of epilepsy. Several anti-epileptic drugs target voltage-dependent calcium channels (VDCCs). Intracellular Ca²⁺ levels are mainly controlled by VDCCs located in the plasma membrane, the calcium-binding proteins (CBPs) inside the cytoplasm, calcium channels located on the intracellular calcium store (particular the endoplasmic reticulum/sarcoplasmic reticulum), and the Ca²⁺-pumps located in the plasma membrane and intracellular calcium store. So far, while many studies have established the relationship between calcium control factors and epilepsy, the mechanism of various Ca²⁺ regulatory factors in epileptogenesis is still unknown. In this paper, we reviewed the function, distribution, and alteration of VDCCs and CBPs in the central nervous system in the pathological process of epilepsy. The interaction of VDCCs with CBPs in the pathological process of epilepsy was also summarized. We hope this review can provide some clues for better understanding the mechanism of epileptogenesis, and for the development of new anti-epileptic drugs targeting on VDCCs and CBPs.

Analysis of pThr286-CaMKII and CaMKII immunohistochemistry in the hippocampus of patients with temporal lobe epilepsy

Ca2+/calmodulin-dependent protein kinase II (CaMKII) phosphorylates a variety of neuronal proteins, thereby, coordinating responses to changes of intracellular Ca2+ concentrations. Autophosphorylation at threonine286 generates an autonomously active form of CaMKII (pThr286-CaMKII), thus prolonging responses to transient increases in Ca2+. Our previous studies in hippocampi of temporal lobe epilepsy (TLE) patients revealed a significant up-regulation of CaMKII in dentate granule cells (DGCs) of specimens with Ammon's horn sclerosis (AHS). However, the functional status of the up-regulated enzyme remained unclear. Therefore, we performed double immunofluorescence staining for CaMKII and pThr286-CaMKII in hippocampi of TLE patients and controls. Furthermore, we analyzed the ratio of the relative fluorescence intensities pThr286-CaMKII: CaMKII in DGCs. CaMKII immunoreactivity was significantly increased in DGC bodies and their proximal dendrites in AHS. In contrast, immunostaining for pThr286-CaMKII was localized to the DGC bodies, revealing similar labeling intensities in all TLE and control specimens, and was not observed in the dendritic compartment of DGCs. Analysis of the ratio of the relative fluorescence intensities pThr286-CaMKII:CaMKII in DGC bodies revealed a significantly reduced ratio in AHS compared to lesion-associated TLE and controls. Thus, up-regulation of total CaMKII in DGCs of AHS specimens is not paralleled by an increase of its autonomously active form.

Amygdala kindling induces upregulation of mRNA for NKCC1, a Na(+), K(+)-2Cl(-) cotransporter, in the rat piriform cortex

GABA, the main inhibitory neurotransmitter in the brain, elicits a hyperpolarizing response by activation of the GABA(A)-receptor/chloride-channel complex under conditions of normal Cl(-) homeostasis. Thus the pathogenesis of epilepsy could involve an impairment of GABA(A)-receptor-mediated inhibition due to a collapse of the Cl(-) gradient. We examined the expression patterns of Cl(-) transporters and a Cl(-) channel in a rat amygdala-kindling model. Activity-dependent increases were observed in the mRNA for NKCC1, an inwardly-directed Cl(-) transporter, in the piriform cortex. This suggests that an increase in [Cl(-)](i) and a resultant reduction in GABAergic inhibition may occur in the kindled piriform cortex.

Dendritic targeting of mRNAs for plasticity genes in experimental models of temporal lobe epilepsy

PURPOSE

To analyze whether the subcellular localization of the messenger RNAs (mRNAs) coding for the neurotrophin brain-derived neurotrophic factor (BDNF), its receptor TrkB, and the alpha and beta subunits of calcium-calmodulin-dependent kinase II (CaMKII) are modified after pilocarpine and kindled seizures.

METHODS

Epilepsy models: pilocarpine and kindling. Analysis of mRNA levels in the dendrites: high-resolution, nonradioactive in situ hybridization.

RESULTS

Nonstimulated rats: BDNF, TrkB, and CaMKII-beta mRNAs localized in the soma and in the proximal dendrites of hippocampal pyramidal cells, and in the soma only of dentate gyrus (DG) granule cells; CaMKII-alpha mRNA localized throughout the dendritic length in neurons of all hippocampal subfields. Pilocarpine seizures: increased staining levels of CaMKII-alpha mRNA throughout the whole dendritic length in all hippocampal subfields; induction of CaMKII-beta, BDNF, and TrkB mRNAs dendritic targeting in CA1, CA3, and DG neurons. Class 2 kindled seizures: increase in dendritic staining intensity for CaMKII-alpha in CA1, CA3, and DG neurons; induction of dendritic localization of CaMKII-beta, BDNF, and TrkB mRNAs in CA3 neurons. Fully kindled seizures: no change in the subcellular distribution of BDNF, TrkB and CaMKII-beta mRNAs; reduction of CaMKII-alpha mRNA dendritic staining, as compared with unstimulated kindled animals.

CONCLUSIONS

Data provide evidence that BDNF, TrkB, and CaMKII-alpha and -beta mRNAs are accumulated in the dendrites of specific hippocampal neurons during pilocarpine seizures and kindling development. The dendritic targeting of these genes may be causally involved in epileptogenesis and thus may represent a new therapeutic target for some forms of partial epilepsy.

Regulation of neuronal excitability in Drosophila by constitutively active CaMKII

The ability of calcium/calmodulin-dependent protein kinase II (CaMKII) to become calcium independent after autophosphorylation makes this enzyme a temporal marker of neuronal activity. Here we show that the calcium-independent form of CaMKII has unique effects on larval viability, locomotion, and neuronal excitability in Drosophila. Expression of constitutively active T287D, but not calcium-dependent T287A, mutant CaMKII in Drosophila neurons resulted in decreased viability, behavioral defects, and failure of action potential propagation. The actions of T287D may be mediated, at least in part, by increased potassium conductances. Expression of T287D CaMKII also stimulated an increase in the number of boutons at the larval neuromuscular junction, but did not affect the mechanics of release. This study defines a role for autophosphorylation of CaMKII in the regulation of multiple neuronal functions including the intrinsic properties of neurons.

Epileptiform activity and EPSP-spike potentiation induced in rat hippocampal CA1 slices by repeated high-K(+): involvement of ionotropic glutamate receptors and Ca(2+)/calmodulin-dependent protein kinase II

We have previously demonstrated that repeated brief increases in extracellular K(+) (K(+)(o)) induce a hyperexcitability in CA1 pyramidal cells that persists for a long time after the final application of K(+) [Neurosci. Lett. 223 (1997) 177; Epilepsy Research (2000) 75]. This epileptiform activity, which was associated with a lasting excitatory postsynaptic potential (EPSP)-spike potentiation, presented some of the characteristic features of traditional in vivo kindling. We have also found that Ca(2+) influx through L-type voltage-sensitive Ca(2+) channels is essential for the development of both in vitro kindling and EPSP-spike potentiation. The aims of this study were to investigate the involvement of ionotropic glutamate receptors, especially those of the NMDA subtype, and the requirement for Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) in these phenomena. Field EPSPs with presynaptic fibre volleys from the stratum radiatum, and population spikes from the stratum pyramidale, were recorded in the CA1 area of rat hippocampal slices in response to electrical stimulation of the Schaffer collateral/commissural fibres. Repeated (three episodes) brief (30 s) increases in extracellular K(+) induced a sustained decrease in the threshold for development of evoked epileptiform discharges (i.e. an in vitro kindling-like state) and a lasting potentiation of the EPSP-spike transfer in CA1 pyramidal neurons (EPSP-spike potentiation). The selective antagonist of NMDA receptors, APV (50 microM), blocked the EPSP-spike potentiation, depressed the induction phase of the in vitro kindling-like state, and blocked the maintenance phase of this state. In contrast to APV, the blockade of AMPA/kainate receptors by CNQX (10 microM) had no effect. Like APV, KN62 (3 microM), a selective membrane permeable inhibitor of CaMKII, blocked the EPSP-spike potentiation and the maintenance phase of the in vitro kindling-like state. Our previous and present results therefore demonstrate that Ca(2+) influxes through L-type voltage-dependent-and NMDA receptor-dependent-Ca(2+) channels contribute differentially to the development of an in vitro kindling-like state, and both induce EPSP-spike potentiation in CA1 hippocampal pyramidal cells in response to repeated brief increases in K(+)(o). It is suggested that these effects of intracellular Ca(2+) on the maintenance phase of the in vitro kindling-like state and EPSP-spike potentiation are mediated by CaMKII-dependent mechanisms.

Increased levels of mRNA for beta- but not alpha-subunit of calmodulin kinase II following kindled seizures

We studied levels of mRNA for the alpha- and beta-subunits of calmodulin (CaM) kinase II using the amygdaloid kindling model of epilepsy. There were significant increases in mRNA for the beta-subunit of CaM kinase II in the hippocampus 4-24 h after stage 5-kindled seizures. Moreover, this mRNA was significantly increased by 20.0-26.5% in the bilateral dentate gyrus 8 to 24 h after kindled seizures. The beta-subunit mRNA was also significantly increased by 13.5-19.0% in the CA3 on the side ipsilateral to the stimulation, 4 to 8 h after kindled seizures. mRNA for the alpha-subunit of CaM kinase II was not significantly changed in the regions examined for up to 24 h after the kindled seizures. These results suggest that CaM kinase II mediates the molecular processes that follow kindled seizures. It is possible that increases in CaM kinase II-dependent protein phosphorylation are associated with the plastic changes in kindling.

Decreased expression of the alpha subunit of Ca2+/ calmodulin-dependent protein kinase type II mRNA in the adult rat CNS following recurrent limbic seizures

Calcium/calmodulin-dependent protein kinase type II (CamKII) is a ubiquitous brain enzyme implicated in a wide variety of neuronal processes. Understanding CamKII has become increasingly complicated with the recent identification of multiple gene transcripts coding for separate subunits. Previous studies have shown that mRNA for the alpha subunit of CamKII can be increased by reduction of afferent input. In this study we have examined the regulation of alpha CamKII mRNA following increased activity due to seizures. Using in situ hybridization with a cRNA probe against the rat alpha CamKII sequence we found reduced levels of hybridization following limbic seizures induced by lesions of the hilus of the dentate gyrus. Hybridization was most dramatically reduced in the granule cells of the dentate gyrus and the pyramidal cells of hippocampal region CA1. There were also significant reductions in hybridization in the superficial layers of neocortex and piriform cortex. In each of these region hybridization was decreased in the molecular layers which is consistent with the reported dendritic localization of alpha CamKII mRNA. All changes in mRNA content were transient, with maximal reductions at 24 h following lesion placement and a return to control levels by 96 h. These findings demonstrate the negative regulation of alpha CamKII mRNA by seizure activity and raise the possibility that synthesis of this kinase may be regulated by normal physiological activity.

Expression of GABAA receptor subunit mRNAs in hippocampal pyramidal and granular neurons in the kindling model of epileptogenesis: an in situ hybridization study

To investigate the molecular changes underlying kindling epileptogenesis in the rat hippocampus, the expression levels of the genes encoding for 13 different gamma-aminobutyric acid type-A receptor (GABAAR) subunits were measured in hippocampal principal neurons using in situ hybridization techniques and semi-quantitative analysis of the autoradiograms. Schaffer collateral-commissural pathway kindled rats were investigated at three different stages of kindling acquisition, at 24 h after the last seizure and at long-term (28 days) after termination of kindling stimulations. Changes were distinct for the different subunits in the three analyzed regions (CA1, CA3, fascia dentata) and also different for the various kindling stages. In all hippocampal areas at the early phases of kindling epileptogenesis, before the appearance of generalized seizures, an increase was found of those transcripts that constituted the majority of the expressed variants in control animals (alpha 1, alpha 2, alpha 4, beta 1, beta 2, beta 3, gamma 2/gamma 2L mRNA). In these stages, the increased levels of different variants in the granular neurons of the fascia dentata were more pronounced when compared to the pattern of changes in pyramidal cells of CA1 and CA3. In fully kindled animals, the expression levels of several subunits returned to control levels, whereas beta 3 and gamma 2/gamma 2L mRNA expression was still significantly enhanced in all areas. At long-term, few changes were encountered. The long-splice variant of gamma 2 was decreased within pyramidal and granular neurons while the total level of gamma 2 mRNA was not different from controls. The increased GABAAR subunit expression in the fascia dentata may underly the reported increased GABAAR ligand binding and the increased GABA mediated inhibition. However, the decreased GABAAR binding and the attenuation of GABAergic inhibition in CA1, could not be explained by a decrement of receptor subunit expression.