Modulation of GABAergic receptor binding by activation of calcium and calmodulin-dependent kinase II membrane phosphorylation

GABAA Receptor Subunit Transcriptional Regulation, Expression Organization, and Mediated Calmodulin Signaling in Prefrontal Cortex of Rats Showing Testosterone-Mediated Impulsive Behavior

Testosterone can induce impulsivity, a behavioral impairment associated with various psychiatric illnesses. The molecular mechanisms associated with testosterone-induced impulsivity are unclear. Our earlier studies showed that supraphysiological doses of testosterone to rats induced impulsive behavior, impacted hypothalamic-pituitary-adrenal axis (HPA) and hypothalamic-pituitary-gonadal axis interactions, and altered α_2A adrenergic receptors in prefrontal cortex (PFC). Owing to the importance of GABAergic system in impulsivity and memory, the present study examines whether testosterone-mediated impulsivity is associated with changes in the expression of Gamma-Aminobutyric Acid (GABA) A and B receptor subunit transcripts (Gabra1, Gabra2, Gabra2 transcript variant 2, Gabra3, Gabra4, Gabra5, Gabra6, Gabrb1, Gabrb2, Gabrb3, Gabrg1, Gabrg2, Gabrg3, Gabbr1, Gabbr2) in rat PFC, and whether testosterone influences GABA_A receptor subunit organization. We studied GABA receptor functions by examining GABA receptor-mediated calcium/calmodulin-dependent kinase signaling genes (Calm1, Calm2, Calm3, Camk2a, Camk2b, Camk2g, Camk2d, Camk4) in the testosterone-induced impulsivity model. Rats were left untreated as controls (C), gonadectomized (GDX), or GDX and injected with supraphysiological doses of testosterone (T). Impulsive behavior was examined using the go/no-go paradigm. Gene expression was studied using qRT-PCR and GABA_A subunit reorganization using cross correlation. Our findings show that expressions of select GABA_A receptor subunits (Gabra3, Gabra5, Gabra6) were significantly upregulated in PFC of T group compared to GDX or C groups. GABA_A receptor subunit organization was different in C, T, and GDX groups. Additionally, Camk4 expression was significantly downregulated in T compared to C group. Our findings suggest that specific GABA_A receptor subunit expression, their reorganization, and Camk4-mediated functions may be associated with testosterone-mediated impulsivity.

Inflammatory mediators potentiate high affinity GABA(A) currents in rat dorsal root ganglion neurons

Following acute tissue injury action potentials may be initiated in afferent processes terminating in the dorsal horn of the spinal cord that are propagated back out to the periphery, a process referred to as a dorsal root reflex (DRR). The DRR is dependent on the activation of GABA(A) receptors. The prevailing hypothesis is that DRR is due to a depolarizing shift in the chloride equilibrium potential (E(Cl)) following an injury-induced activation of the Na(+)-K(+)-Cl(-)-cotransporter. Because inflammatory mediators (IM), such as prostaglandin E(2) are also released in the spinal cord following tissue injury, as well as evidence that E(Cl) is already depolarized in primary afferents, an alternative hypothesis is that an IM-induced increase in GABA(A) receptor mediated current (I(GABA)) could underlie the injury-induced increase in DRR. To test this hypothesis, we explored the impact of IM (prostaglandin E(2) (1 μM), bradykinin (10 μM), and histamine (1 μM)) on I(GABA) in dissociated rat dorsal root ganglion (DRG) neurons with standard whole cell patch clamp techniques. IM potentiated I(GABA) in a subpopulation of medium to large diameter capsaicin insensitive DRG neurons. This effect was dependent on the concentration of GABA, manifest only at low concentrations (<10 μM). THIP evoked current were also potentiated by IM and GABA (1 μM) induced tonic currents enhanced by IM were resistant to gabazine (20 μM). The present data are consistent with the hypothesis that an acute increase in I(GABA) contributes to the emergence of injury-induced DRR.

The anticholinergic and antiglutamatergic drug caramiphen reduces seizure duration in soman-exposed rats: synergism with the benzodiazepine diazepam

Therapy of seizure activity following exposure to the nerve agent soman (GD) includes treatment with the anticonvulsant diazepam (DZP), an allosteric modulator of γ-aminobutyric acid A (GABA(A)) receptors. However, seizure activity itself causes the endocytosis of GABA(A) receptors and diminishes the inhibitory effects of GABA, thereby reducing the efficacy of DZP. Treatment with an N-methyl-d-aspartic acid (NMDA) receptor antagonist prevents this reduction in GABAergic inhibition. We examined the efficacy of the NMDA receptor antagonist caramiphen edisylate (CED; 20mg/kg, im) and DZP (10mg/kg, sc), administered both separately and in combination, at 10, 20 or 30min following seizure onset for attenuation of the deleterious effects associated with GD exposure (1.2 LD(50); 132μg/kg, sc) in rats. Outcomes evaluated were seizure duration, neuropathology, acetylcholinesterase (AChE) activity, body weight, and temperature. We also examined the use of the reversible AChE inhibitor physostigmine (PHY; 0.2mg/kg, im) as a therapy for GD exposure. We found that the combination of CED and DZP yielded a synergistic effect, shortening seizure durations and reducing neuropathology compared to DZP alone, when treatment was delayed 20-30min after seizure onset. PHY reduced the number of animals that developed seizures, protected a fraction of AChE from GD inhibition, and attenuated post-exposure body weight and temperature loss independent of CED and/or DZP treatment. We conclude that: 1) CED and DZP treatment offers considerable protection against the effects of GD and 2) PHY is a potential therapeutic option following GD exposure, albeit with a limited window of opportunity.

Analysis of kinase gene expression in the frontal cortex of suicide victims: implications of fear and stress

Suicide is a serious public health issue that results from an interaction between multiple risk factors including individual vulnerabilities to complex feelings of hopelessness, fear, and stress. Although kinase genes have been implicated in fear and stress, including the consolidation and extinction of fearful memories, expression profiles of those genes in the brain of suicide victims are less clear. Using gene expression microarray data from the Online Stanley Genomics Database and a quantitative PCR, we investigated the expression profiles of multiple kinase genes including the calcium calmodulin-dependent kinase (CAMK), the cyclin-dependent kinase, the mitogen-activated protein kinase (MAPK), and the protein kinase C (PKC) in the prefrontal cortex (PFC) of mood disorder patients died with suicide (N = 45) and without suicide (N = 38). We also investigated the expression pattern of the same genes in the PFC of developing humans ranging in age from birth to 49 year (N = 46). The expression levels of CAMK2B, CDK5, MAPK9, and PRKCI were increased in the PFC of suicide victims as compared to non-suicide controls (false discovery rate, FDR-adjusted p < 0.05, fold change >1.1). Those genes also showed changes in expression pattern during the postnatal development (FDR-adjusted p < 0.05). These results suggest that multiple kinase genes undergo age-dependent changes in normal brains as well as pathological changes in suicide brains. These findings may provide an important link to protein kinases known to be important for the development of fear memory, stress associated neural plasticity, and up-regulation in the PFC of suicide victims. More research is needed to better understand the functional role of these kinase genes that may be associated with the pathophysiology of suicide.

Mass spectrometric analysis of GABAA receptor subtypes and phosphorylations from mouse hippocampus

The brain GABA(A) receptor (GABA(A) R) is a key element of signaling and neural transmission in health and disease. Recently, complete sequence analysis of the recombinant GABA(A) R has been reported, separation and mass spectrometrical (MS) characterisation from tissue, however, has not been published so far. Hippocampi were homogenised, put on a sucrose gradient 10-69% and the layer from 10 to 20% was used for extraction of membrane proteins by a solution of Triton X-100, 1.5 M aminocaproic acid in the presence of 0.3 M Bis-Tris. This mixture was subsequently loaded onto blue native PAGE (BN-PAGE) with subsequent analysis on denaturing gel systems. Spots from the 3-DE electrophoretic run were stained with Colloidal Coomassie Brilliant Blue, and spots with an apparent molecular weight between 40 and 60 kDa were picked and in-gel digested with trypsin, chymotrypsin and subtilisin. The resulting peptides were analysed by nano-LC-ESI-MS/MS (ion trap) and protein identification was carried out using MASCOT searches. In addition, known GABA(A) R-specific MS information taken from own previous studies was used for searches of GABA(A) R subunits. β-1, β-2 and β-3, θ and ρ-1 subunits were detected and six novel phosphorylation sites were observed and verified by phosphatase treatment. The method used herein enables identification of several GABA(A) R subunits from mouse hippocampus along with phosphorylations of β-1 (T227, Y230), β-2 (Y215, T439) and β-3 (T282, S406) subunits. The procedure forms the basis for GABA(A) R studies at the protein chemical rather than at the immunochemical level in health and disease.

Wnt-5a modulates recycling of functional GABAA receptors on hippocampal neurons

GABA(A) receptors (GABA(A)-Rs) play a significant role in mediating fast synaptic inhibition and it is the main inhibitory receptor in the CNS. The role of Wnt signaling in coordinating synapse structure and function in the mature CNS is poorly understood. In previous studies we found that Wnt ligands can modulate excitatory synapses through remodeling both presynaptic and postsynaptic regions. In this current study we provide evidence for the effect of Wnt-5a on postsynaptic GABA(A)-Rs. We observed that Wnt-5a induces surface expression and maintenance of this receptor in the neuronal membrane. The evoked IPSC recordings in rat hippocampal slice indicate that Wnt-5a can regulates postsynaptically the hippocampal inhibitory synapses. We found also that Wnt-5a: (a) induces the insertion and clustering of GABA(A)-Rs in the membrane; (b) increases the amplitude of GABA-currents due exclusively to postsynaptic mechanisms; (c) does not affect the endocytic process, but increases the receptor recycling. Finally, all these effects on the GABA(A)-Rs are mediated by the activation of calcium/calmodulin-dependent kinase II (CaMKII). Therefore, we postulate that Wnt-5a, by activation of CaMKII, induces the recycling of functional GABA(A)-Rs on the mature hippocampal neurons.

CaMKII phosphorylation of the GABA(A) receptor: receptor subtype- and synapse-specific modulation

As a major inhibitory neurotransmitter, GABA plays a vital role in the brain by controlling the extent of neuronal excitation. This widespread role is reflected by the ubiquitous distribution of GABA(A) receptors throughout the central nervous system. To regulate the level of neuronal inhibition requires some endogenous control over the release of GABA and/or its postsynaptic response. In this context, Ca(2+) ions are often used as primary or secondary messengers frequently resulting in the activation of protein kinases and phosphatases. One such kinase, Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), can target the GABA(A) receptor to cause its phosphorylation. Evidence is now emerging, which is reviewed here, that GABA(A) receptors are indeed substrates for CaMKII and that this covalent modification alters the expression of cell surface receptors and their function. This type of regulation can also feature at inhibitory synapses leading to long-term inhibitory synaptic plasticity. Most recently, CaMKII has now been proposed to differentially phosphorylate particular isoforms of GABA(A) receptors in a synapse-specific context.

GABA(B) receptor activation mediates frequency-dependent plasticity of developing GABAergic synapses

Activity-induced long-term modification of glutamatergic synapses depends on the frequency of synaptic activation. We found that long-term modification of developing rat hippocampal GABAergic synapses that was induced by repetitive coincident pre- and postsynaptic spiking was also frequency dependent. Spiking at 20-50 Hz resulted in synaptic potentiation, whereas spiking at 5 Hz led to synaptic depression. The potentiation was abolished by blocking GABA(B) receptors (GABA(B)Rs), whereas the depression was independent of GABA(B)R activation and could be converted to potentiation by elevating GABA(B)R activity. The potentiation could be attributed to a local postsynaptic increase in Na(+)/K(+)/2Cl(-) co-transporter activity near activated synapses. The activity of postsynaptic Ca(2+)/calmodulin-dependent protein kinase II was necessary for long-term potentiation of these developing GABAergic synapses and its phosphorylation at Thr286 could be enhanced by activating GABA(B)Rs with baclofen. Together with our finding that activation of GABA(B)Rs is frequency dependent, these results indicate that postsynaptic GABA(B)R activation mediates frequency-dependent potentiation of developing GABAergic synapses.

A combination of ketamine and diazepam synergistically controls refractory status epilepticus induced by cholinergic stimulation

PURPOSE

New treatments are needed for status epilepticus (SE) that is refractory to drugs modulating GABA(A) receptors, and NMDA receptor antagonists are candidate drugs.

METHODS

Clinically available NMDA receptor antagonist ketamine was tested for effectiveness in terminating prolonged SE induced by a combination of lithium and pilocarpine. Animals were treated 10 min after first grade 5 behavioral seizure (Racine scoring scale) by intraperitoneal administration of ketamine, diazepam, or saline. Seizure termination was determined by electroencephalogram (EEG) recordings from the hippocampus and the cortex.

RESULTS

Animals treated with normal saline or either 20 mg/kg diazepam, or 50 mg/kg ketamine continued in SE for the next 300 min. However, combined treatment with diazepam and ketamine rapidly terminated prolonged cholinergic stimulation-induced SE. Detailed study of dose response relationships demonstrated that diazepam enhanced efficacy and potency of ketamine in terminating SE.

DISCUSSION

This study demonstrated synergistic action of diazepam and ketamine in terminating SE. It suggests that a ketamine-diazepam combination might be a clinically useful therapeutic option for the treatment of refractory SE.

Identification of the sites for CaMK-II-dependent phosphorylation of GABA(A) receptors

Phosphorylation can affect both the function and trafficking of GABA(A) receptors with significant consequences for neuronal excitability. Serine/threonine kinases can phosphorylate the intracellular loops between M3-4 of GABA(A) receptor beta and gamma subunits thereby modulating receptor function in heterologous expression systems and in neurons (1, 2). Specifically, CaMK-II has been demonstrated to phosphorylate the M3-4 loop of GABA(A) receptor subunits expressed as GST fusion proteins (3, 4). It also increases the amplitude of GABA(A) receptor-mediated currents in a number of neuronal cell types (5-7). To identify which substrate sites CaMK-II might phosphorylate and the consequent functional effects, we expressed recombinant GABA(A) receptors in NG108-15 cells, which have previously been shown to support CaMK-II modulation of GABA(A) receptors containing the beta3 subunit (8). We now demonstrate that CaMK-II mediates its effects on alpha1beta3 receptors via phosphorylation of Ser(383) within the M3-4 domain of the beta subunit. Ablation of beta3 subunit phosphorylation sites for CaMK-II revealed that for alphabetagamma receptors, CaMK-II has a residual effect on GABA currents that is not mediated by previously identified sites of CaMK-II phosphorylation. This residual effect is abolished by mutation of tyrosine phosphorylation sites, Tyr(365) and Tyr(367), on the gamma2S subunit, and by the tyrosine kinase inhibitor genistein. These results suggested that CaMK-II is capable of directly phosphorylating GABA(A) receptors and activating endogenous tyrosine kinases to phosphorylate the gamma2 subunit in NG108-15 cells. These findings were confirmed in a neuronal environment by expressing recombinant GABA(A) receptors in cerebellar granule neurons.

ERK/MAPK pathway regulates GABAA receptors

The GABAA receptor is a ligand-gated ion channel whose function and activity can be regulated by ligand binding or alternatively may be influenced indirectly through the phosphorylation of specific subunits that comprise the GABAA receptor pentamer. With respect to phosphorylation, most studies have focused on either beta or gamma subunits, whereas the role of the alpha subunit as a relevant target of signaling kinases is largely unknown. Interestingly, we found a putative phosphorylation site for extracellular-signal regulated kinase (ERK), a key effector of the MAPK pathway, in almost all known alpha subunits of the GABAA receptor, including the ubiquitously expressed alpha1 subunit. To determine whether this putative ERK phosphorylation site was functionally relevant, we evaluated if ERK inhibition (through pharmacological inhibition of its upstream kinase, MEK) altered GABA-gated currents. Using HEK293 cells stably transfected with the alpha1beta2gamma2 form of the GABAA receptor, we found that UO126 reduced basal ERK phosphorylation and resulted in an enhancement of GABA-induced peak current amplitudes. Further, the enhancement of GABA-gated currents required an intact intracellular environment as it was robust in perforated patch recordings (which preserves the intracellular milieu), but absent in conventional whole-cell recordings (which dialyzes the cytosolic contents), supporting the involvement of an intracellular signaling pathway. Finally, mutation of the ERK phosphorylation site (T375-->A) prevented the UO126-induced enhancement of GABA-gated currents. Collectively, our results implicate the MAPK pathway as a negative modulator of GABAA receptor function, whose influence on GABA-gated currents may be mediated by phosphorylation of the alpha subunit.

Erratum to "Cellular mechanisms underlying acquired epilepsy: the calcium hypothesis of the induction and maintenance of epilepsy." [Pharmacol. Ther. 105(3) (2005) 229-266]

Epilepsy is one of the most common neurological disorders. Although epilepsy can be idiopathic, it is estimated that up to 50% of all epilepsy cases are initiated by neurological insults and are called acquired epilepsy (AE). AE develops in 3 phases: (1) the injury [central nervous system (CNS) insult]. (2) epileptogenesis (latency), and (3) the chronic epileptic (spontaneous recurrent seizure) phases. Status epilepticus (SE), stroke, and traumatic brain injury (TBI) are 3 major examples of common brain injuries that can lead to the development of AE. It is especially important to understand the molecular mechanisms that cause AE because it may lead to innovative strategies to prevent or cure this common condition. Recent studies have offered new insights into the cause of AE and indicate that injury-induced alterations in intracellular calcium concentration levels ([Ca(2+)](i)) and calcium homeostatic mechanisms play a role in the development and maintenance of AE. The injuries that cause AE are different, but the share a common molecular mechanism for producing brain damage--an increase in extracellular glutamate and are exposed to increased [Ca(2+)](i) are the cellular substrates to develop epilepsy because dead cells do not seize. The neurons that survive injury sustain permanent long-term plasticity changes in [Ca(2+)](i) and calcium homeostatic mechanisms that are permanent and are a prominent feature of the epileptic phenotype. In the last several years, evidence has accumulated indicating that the prolonged alteration in neuronal calcium dynamics plays an important role in the induction and maintenance of the prolonged neuroplasticity changes underlying the epileptic phenotype. Understanding the role of calcium as a second messenger in the induction and maintenance of epilepsy may provide novel insights into therapeutic advances that will prevent and even cure AE.

CaMK-II modulation of GABA(A) receptors expressed in HEK293, NG108-15 and rat cerebellar granule neurons

The gamma-aminobutyric acid type A (GABA(A)) receptor is a pentameric ligand-gated ion channel responsible for fast synaptic inhibition in the brain. Phosphorylation of the GABA(A) receptor by serine/threonine protein kinases, at residues located in the intracellular loop between the third and fourth transmembrane domains of each subunit, can dynamically modulate receptor trafficking and function. In this study, we have assessed the effect that Ca(2+)-calmodulin-dependent protein kinase-II (CaMK-II) has on GABA(A) receptors. The intracellular application of preactivated CaMK-II failed to modulate the function of alphabeta and alphabetagamma subunit GABA(A) receptors heterologously expressed in human embryonic kidney (HEK)293 cells. However, application of similarly preactivated alpha-CaMK-II significantly potentiated the amplitudes of whole-cell GABA currents recorded from rat cultured cerebellar granule neurons and from recombinant GABA(A) receptors expressed in neuroblastoma, NG108-15, cells. The modulation by alpha-CaMK-II of current amplitude depended upon the subunit composition of GABA(A) receptors. alpha-CaMK-II potentiated GABA currents recorded from alpha1beta3 and alpha1beta3gamma2 GABA(A) receptors, but was unable to functionally modulate beta2 subunit-containing receptors. Similar results were obtained from beta2 -/- mouse cerebellar granule cell cultures and from rat granule cell cultures overexpressing recombinant alpha1beta2 or alpha1beta3 GABA(A) receptors. alpha-CaMK-II had a greater effect on the modulation of GABA responses mediated by alpha1beta3gamma2 compared with alpha1beta3 receptors, indicating a possible role for the gamma2 subunit in CaMK-II-mediated phosphorylation. In conclusion, CaMK-II can upregulate the function of GABA(A) receptors expressed in neurons or a neuronal cell line that is dependent on the beta subunit co-assembled into the receptor complex.

Heat-enhanced production of chitosanase from Streptomyces griseus in the presence of liposome

The effects of heat stress and liposome treatment on the growth of Streptomyces griseus cells and chitosanase production were investigated on the basis of using the designed strategy of a stress-mediated bioprocess. The effective conditions for increasing the interaction between chitosanase and the 1-palmitoyl-2-oleoyl-3-phosphocholine (POPC) liposome under heat stress condition were determined on the basis of the results of circular dichroism (CD) and dielectric dispersion analysis (DDA). Under these effective conditions, S. griseus cells were cultivated for the effective production of chitosanase. The results obtained from both CD spectra and DDA showed that heat stress enhances the interaction of the POPC liposomes and chitosanase. The strongest interaction between them could be obtained in the specific temperature range of 40-45 degrees C. The enhancement of the target chitosanase production was conducted under heat stress at 41 degrees C in the presence and absence of the POPC liposomes. The growth rates of S. griseus cells in the cases of heat (41 degrees C) and heat (41 degrees C)/POPC treatments were respectively 1.2 and 1.4 times higher than that obtained under the control condition. In the heat (41 degrees C) and heat (41 degrees C)/POPC treatments, chitosanase activity increased to 1.8 and 2 times, respectively, higher than that obtained under the control condition. Heat stress and the addition of the POPC liposomes could therefore be utilized to induce the potential functions of bacterial cells for the enhancement of the final target production.

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.

Modulation of CaM Kinase II Activity Is Coincident with Induction of Status Epilepticus in the Rat Pilocarpine Model

PURPOSE

This study was conducted to characterize the early cellular changes in CaM kinase II activity that occur during the induction of status epilepticus (SE).

METHODS

The pilocarpine model of SE was characterized both behaviorally and electrographically. At specific time points after the first discrete seizure, specific brain regions were isolated for biochemical study. Phosphate incorporation into a CaM kinase II-specific substrate, autocamtide III, was used to determine kinase activity.

RESULTS

After the development of SE, the data show an immediate inhibition of both cortical and hippocampal CaM kinase II activity in homogenate, but a delayed inhibition in synaptic kinase activity. The maintenance of synaptic kinase activity was due to a translocation of CaM kinase II protein to the synapse. However, despite the translocation of functional kinase, CaM kinase II activity was not maintained, membrane potential was not restored, and the newly translocated CaM kinase II did not terminate the SE event. Unlike the homogenate samples, in the crude synaptoplasmic membrane (SPM) subcellular fractions, a positive correlation is found between the duration of SE and the inhibition of CaM kinase II activity in both the cortex and hippocampus.

CONCLUSIONS

The data support the hypothesis that alterations of CaM kinase II activity are involved in the early events of SE pathology.

Age dependence of pilocarpine-induced status epilepticus and inhibition of CaM kinase II activity in the rat

This study was conducted to characterize the post-pubertal developmental aspects on seizure susceptibility and severity as well as calcium/calmodulin protein kinase type II (CaM kinase II) activity in status epilepticus (SE). Thirty- to ninety-day-old rats, in 10-day increments, were studied. This corresponds to a developmental age group that has not received thorough attention. The pilocarpine model of SE was characterized both behaviorally and electrographically. Seven criteria were analyzed for electrographical characterization: seizure severity, SE susceptibility, the average number of discrete seizures, average time until first seizure, average time to SE, average time from first discrete seizure to SE, and death. After 1 h of SE, specific brain regions were isolated for biochemical study. Phosphate incorporation into a CaM kinase II-specific substrate, autocamtide III, was used to determine kinase activity. There was no developmental effect on the average number of discrete seizures, average time until first seizure, average time to SE, average time from first discrete seizure to SE, and death; however, there was a significant effect on SE probability and seizure severity. Once SE was expressed, all animals showed a decrease in both cortical and hippocampal CaM kinase II activities. Conversely, seizure activity in the absence of SE did not result in a decrease in CaM kinase II activity. The data suggest that there is a gradual age-dependent modulation of SE susceptibility and seizure severity within the developmental stages studied. Additionally, once status epilepticus is observed at any age, there is a corresponding SE-induced inhibition of CaM kinase II.

Cellular mechanisms underlying acquired epilepsy: the calcium hypothesis of the induction and maintainance of epilepsy

Epilepsy is one of the most common neurological disorders. Although epilepsy can be idiopathic, it is estimated that up to 50% of all epilepsy cases are initiated by neurological insults and are called acquired epilepsy (AE). AE develops in 3 phases: (1) the injury (central nervous system [CNS] insult), (2) epileptogenesis (latency), and (3) the chronic epileptic (spontaneous recurrent seizure) phases. Status epilepticus (SE), stroke, and traumatic brain injury (TBI) are 3 major examples of common brain injuries that can lead to the development of AE. It is especially important to understand the molecular mechanisms that cause AE because it may lead to innovative strategies to prevent or cure this common condition. Recent studies have offered new insights into the cause of AE and indicate that injury-induced alterations in intracellular calcium concentration levels [Ca(2+)](i) and calcium homeostatic mechanisms play a role in the development and maintenance of AE. The injuries that cause AE are different, but they share a common molecular mechanism for producing brain damage-an increase in extracellular glutamate concentration that causes increased intracellular neuronal calcium, leading to neuronal injury and/or death. Neurons that survive the injury induced by glutamate and are exposed to increased [Ca(2+)](i) are the cellular substrates to develop epilepsy because dead cells do not seize. The neurons that survive injury sustain permanent long-term plasticity changes in [Ca(2+)](i) and calcium homeostatic mechanisms that are permanent and are a prominent feature of the epileptic phenotype. In the last several years, evidence has accumulated indicating that the prolonged alteration in neuronal calcium dynamics plays an important role in the induction and maintenance of the prolonged neuroplasticity changes underlying the epileptic phenotype. Understanding the role of calcium as a second messenger in the induction and maintenance of epilepsy may provide novel insights into therapeutic advances that will prevent and even cure AE.

Increased expression of calcium/calmodulin-dependent protein kinase IIβ in frontal cortex in schizophrenia and depression

In searching for genes dysregulated in schizophrenia, we measured the expression of the two splice variants of calcium/calmodulin-dependent protein kinase II (CaMKIIalpha and CaMKIIbeta) in postmortem frontal cerebral cortex tissues from patients who had died with schizophrenia, bipolar disorder, or severe depression. The mRNA levels of expression of these two splice variants were measured by real-time Quantitative PCR, using an Mx4000 instrument. The values for the expression of CaMKIIalpha and CaMKIIbeta were normalized by the expression of beta-glucuronidase in the tissues. The expression of CaMKIIalpha was significantly elevated in the depression tissues by 29%. The expression of CaMKIIbeta was significantly elevated in the schizophrenia tissues by 27%, and in the depression tissues by 36%. Because CaMKIIbeta influences the expression of many neuroreceptors and influences neural outgrowth and pruning, its altered expression in the cerebral cortex in schizophrenia or depression may contribute to these diseases.

Ethanol regulation of γ-aminobutyric acidA receptors: genomic and nongenomic mechanisms

gamma-Aminobutyric acid(A) (GABA(A)) receptors are ligand-gated ion channels that, predominantly, mediate inhibitory synaptic transmission in the CNS. These receptors are pentameric complexes that are comprised of subunits from several classes (alpha, beta, gamma, delta, ), with each class consisting of several isoforms. Chronic ethanol consumption alters GABA(A) receptor function producing cellular tolerance to GABA and ethanol, cross-tolerance to benzodiazepines and barbiturates, and sensitization to inverse agonists. Recent studies have clearly demonstrated that GABA(A) receptors play an important role in ethanol dependence and functional properties of GABA(A) receptor are altered following chronic ethanol administration. However, the exact mechanisms that account for alterations in GABA(A) receptor function following chronic ethanol administration have not been resolved. The mechanisms responsible for adaptation of GABA(A) receptors to chronic ethanol exposure may involve ethanol-induced changes in cell surface expression, subcellular localization, synaptic localization, receptor phosphorylation, neurosteroids, and/or changes in GABA(A) receptor subunit composition. In this review, we provide an overview of recent data pertaining to mechanisms that could be responsible for altered properties and expression of GABA(A) receptors following chronic ethanol administration.

Calcium/calmodulin-dependent kinase II phosphorylation of the GABAA receptor alpha1 subunit modulates benzodiazepine binding

gamma-Aminobutyric acid (GABA) is the primary neurotransmitter that is responsible for the fast inhibitory synaptic transmission in the central nervous system. A major post-translational mechanism that can rapidly regulate GABAAR function is receptor phosphorylation. This study was designed to test the effect of endogenous calcium and calmodulin-dependent kinase II (CaM kinase II) activation on both allosteric modulator binding and GABAA receptor subunit phosphorylation. Endogenous CaM kinase II activity was stimulated, and GABAA receptors were subsequently analyzed for bothallosteric modulator binding properties and immunoprecipitated and analyzed for subunit phosphorylation levels. A significant increase in allosteric-modulator binding of the GABAAR was observed under conditions maximal for CaM kinase II activation. In addition, CaM kinase II activation resulted in a direct increase in phosphorylation of the GABAA receptor alpha1 subunit. The data suggest that the CaM kinase II-dependent phosphorylation of the GABAA receptor alpha1 subunit modulated allosteric modulator binding to the GABAA receptor.

Ca2+/calmodulin-dependent protein kinase regulates GABA-activated Cl- current in cockroach dorsal unpaired median neurons

We studied gamma-aminobutyric acid (GABA)-mediated currents in short-term cultured dorsal unpaired median (DUM) neurons of cockroach Periplaneta americana using the whole cell patch-clamp technique in symmetrical chloride solutions. All DUM neurons voltage-clamped at -50 mV displayed inward currents (I(GABA)) when 10(-4) M of GABA was applied by pneumatic pressure-ejection pulses. The semi-logarithmic curve of I(GABA) amplitude versus the ejection time yielded a Hill coefficient of 4.0. I(GABA) was chloride (Cl-) because the reversal potential given by the current-voltage (I-V) curve varied according to the value predicted by the Nernst equation for Cl- dependence. In addition, I(GABA) was almost completely blocked by bath application of the chloride channel blockers picrotoxin (PTX) or 3,3-bis(trifluoromethyl)bicyclo-[2,2,1]heptane-2,2-diacarbonitrile (BIDN). The I-V curve for I(GABA) displayed a unexpected biphasic aspect and was best fitted by two linear regressions giving two slope conductances of 35.6 +/- 2.1 and 80.9 +/- 4.1 nS for potentials ranging from 0 to -30 and -30 to -70 mV, respectively. At -50 mV, the current amplitude was decreased by cadmium chloride (CdCl2, 10(-3) M) and calcium-free solution. The semi-logarithmic curve for CdCl2-resistant I(GABA) gave a Hill coefficient of 2.4. Hyperpolarizing voltage step from -50 to -80 mV was known to increase calcium influx through calcium-resting channels. According to this protocol, a significant increase of I(GABA) amplitude was observed. However, this effect was never obtained when the same protocol was applied on cell body pretreated with CdCl2. When the calmodulin blocker N-(6-aminohexyl)-5-chloro-1-naphtalene-sulfonamide or the calcium-calmodulin-dependent protein kinase blocker 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine (KN-62) was added in the pipette solution, I(GABA) amplitude was decreased. Pressure ejection application of the cis-4-aminocrotonic acid (CACA) on DUM neuron cell body held at -50 mV, evoked a Cl- inward current which was insensitive to CdCl2. The Hill plot yielded a Hill coefficient of 2.3, and the I-V curve was always linear in the negative potential range with a slope conductance of 32.4 +/- 1.1 nS. These results, similar to those obtained with GABA in the presence of CdCl2 and KN-62, indicated that CACA activated one subtype of GABA receptor. Our study demonstrated that at least two distinct subtypes of Cl--dependent GABA receptors were expressed in DUM neurons, one of which is regulated by an intracellular Ca2+-dependent mechanism via a calcium-dependent protein kinase. The consequences of the modulatory action of Ca2+ in GABA receptors function and their sensitivity to insecticide are discussed.

Mechanism of the inhibitory effect of histamine on amygdaloid-kindled seizures in rats

PURPOSE

The mechanism of the inhibitory effect of histamine on amygdaloid-kindled seizures was investigated in rats.

METHODS

Under pentobarbital anesthesia, rats were fixed to a stereotaxic apparatus, and bipolar electrodes were implanted into the right amygdala. A guide cannula made of stainless steel tubing was implanted into the right lateral ventricle. Electrodes were connected to a miniature receptacle, which was embedded in the skull with dental cement. EEG was recorded with an electroencephalograph; stimulation of the amygdala was applied bipolarly every day by a constant-current stimulator and continued until a generalized convulsion was obtained.

RESULTS

Intracerebroventricular (i.c.v.) injection of histamine at doses of 2-10 microg resulted in a dose-related inhibition of amygdaloid-kindled seizures. I.c.v. injection of calcium chloride at doses of 10-50 microg and A23187 at doses of 2-10 microg also caused dose-dependent inhibition of amygdaloid-kindled seizures. Calcium chloride at a dose of 10 microg, which showed no significant effect on amygdaloid-kindled seizures when used alone, significantly potentiated the effect of histamine. Similar findings were observed with A23187 at a dose of 2 microg. In addition, EGTA and EGTA/AM antagonized the inhibition of kindled seizures induced by histamine. Moreover, the inhibition of kindled seizures induced by histamine was antagonized by KN62. However, calphostin C did not antagonize the inhibitory effect of histamine.

CONCLUSIONS

These results indicated that histamine-induced inhibition of amygdaloid-kindled seizures may be closely associated with a calcium calmodulin-dependent protein kinase II activation pathway.

Status epilepticus results in an N-methyl-D-aspartate receptor-dependent inhibition of Ca2+/calmodulin-dependent kinase II activity in the rat

Status epilepticus is a major medical emergency that results in significant alteration of neuronal function. Status epilepticus involves seizure activity recurring frequently enough to induce a sustained alteration in brain function. This study was initiated to investigate how status epilepticus affects the activity of calcium and calmodulin-dependent kinase II in the brain. Calcium and calmodulin-dependent kinase II is a neuronally enriched signal transducing system involved in the regulation of neurotransmitter synthesis and release, cytoskeletal function, gene transcription, neurotransmitter receptor function and neuronal excitability. Therefore, alteration of this signal transduction system would have significant physiological effects. Status epilepticus was induced in rats by pilocarpine injection, allowed to progress for 60 min and terminated by repeated diazepam injections. Animals were killed at specific time-points and examined for calcium and calmodulin-dependent kinase II activity. Calcium and calmodulin-dependent kinase II activity was significantly reduced in cerebral cortex and hippocampal homogenates obtained from status epilepticus rats when compared with control animals. Once established, the status epilepticus-induced inhibition of calcium and calmodulin-dependent kinase II activity was observed at all time-points tested following the termination of seizure activity. However, calcium and calmodulin-dependent kinase II activity was not significantly decreased in thalamus and cerebellar homogenates. In addition, status epilepticus-induced inhibition of calcium and calmodulin-dependent kinase II activity was dependent upon activation of N-methyl-D-aspartate subtype of glutamatergic receptors. Thus, status epilepticus induced a significant inhibition of calcium and calmodulin-dependent kinase II activity that involves N-methyl-D-aspartate receptor activation. The data support the hypothesis that inhibition of calcium and calmodulin-dependent kinase II activity may be involved in the alteration of neuronal function following status epilepticus.

Protein kinase and phosphatase modulation of quail brain GABA(A) and non-NMDA receptors co-expressed in Xenopus oocytes

The GABA(A) receptor and the non-NMDA subtype of the ionotropic glutamate receptor were co-expressed in Xenopus oocytes by injection of quail brain mRNA. The oocytes were treated with various protein kinase (PK) and protein phosphatase (PP) activators and inhibitors and the effects on receptor functioning were monitored. Two phorbol esters, 4-beta-phorbol 12-myristate-13-acetate (PMA) and 4-beta-phorbol 12,13-dibutyrate (PDBu); the cGMP-dependent PK activators sodium nitroprusside (SNP) and S-nitrosoglutathione (SNOG); and the PP inhibitor okadaic acid (OA) reduced the amplitude of the GABA-induced currents, whilst the PK inhibitor staurosporine potentiated it. In addition, PMA, PDBu, SNP, and OA reduced the desensitization of the GABA-induced response. Identical treatments generally had similar but less pronounced effects on responses generated by kainate (KA) but the desensitization characteristic of the non-NMDA receptor was not affected. None of the treatments had any effect on the reversal potentials of the induced currents. Immunoblots revealed that the oocytes express endogenous PKG and guanylate cyclase. The results are discussed in terms of the molecular structures of GABA(A) and non-NMDA receptors and the potential functional consequences of phosphorylation/dephosphorylation.

Long-lasting decrease in neuronal Ca2+/calmodulin-dependent protein kinase II activity in a hippocampal neuronal culture model of spontaneous recurrent seizures

Ca2+/calmodulin-dependent protein kinase II (CaM Kinase II) activity was evaluated in a well-characterized in vitro model of epileptiform activity. Long-lasting spontaneous recurrent seizure (SRS) activity was induced in hippocampal neuronal cultures by exposure to low Mg2+ media for 3 h. Analysis of endogenous Ca2+/calmodulin-dependent phosphorylation revealed a significant long-lasting decrease in 32P incorporation into the alpha (50 kDa) and beta (60 kDa) subunits of CaM kinase II in association with the induction of SRS activity in this preparation. Ca2+/calmodulin-dependent substrate phosphorylation of the synthetic peptides, Autocamtide-2 and Syntide II, was also significantly reduced following the induction of SRSs and persisted for the life of the neurons in culture. The decrement in CaM kinase II activity associated with low Mg2+ treatment remained significantly decreased when values were corrected for changes in levels of alpha subunit immunoreactivity and neuronal cell loss. Addition of the protein phosphatase inhibitors, okadaic acid and cyclosporin A, to the phosphorylation reaction did not block the SRS-associated decrease in substrate phosphorylation, indicating that enhanced phosphatase activity was not a contributing factor to the observed decrease in phosphate incorporation. The findings of this study demonstrate that CaM kinase II activity is decreased in association with epileptogenesis observed in these hippocampal cultures and may contribute to the production and maintenance of SRSs in this model.