Interaction of calcineurin and type-A GABA receptor gamma 2 subunits produces long-term depression at CA1 inhibitory synapses

NMDA receptors regulate GABAA receptor lateral mobility and clustering at inhibitory synapses through serine 327 on the γ2 subunit

Modification of the number of GABA(A) receptors (GABA(A)Rs) clustered at inhibitory synapses can regulate inhibitory synapse strength with important implications for information processing and nervous system plasticity and pathology. Currently, however, the mechanisms that regulate the number of GABA(A)Rs at synapses remain poorly understood. By imaging superecliptic pHluorin tagged GABA(A)R subunits we show that synaptic GABA(A)R clusters are normally stable, but that increased neuronal activity upon glutamate receptor (GluR) activation results in their rapid and reversible dispersal. This dispersal correlates with increases in the mobility of single GABA(A)Rs within the clusters as determined using single-particle tracking of GABA(A)Rs labeled with quantum dots. GluR-dependent dispersal of GABA(A)R clusters requires Ca(2+) influx via NMDA receptors (NMDARs) and activation of the phosphatase calcineurin. Moreover, the dispersal of GABA(A)R clusters and increased mobility of individual GABA(A)Rs are dependent on serine 327 within the intracellular loop of the GABA(A)R γ2 subunit. Thus, NMDAR signaling, via calcineurin and a key GABA(A)R phosphorylation site, controls the stability of synaptic GABA(A)Rs, with important implications for activity-dependent control of synaptic inhibition and neuronal plasticity.

Glutamate and GABA in the balance: convergent pathways sustain seizures during status epilepticus

Seizures rapidly become self-sustaining and pharmacoresistant to benzodiazepines during status epilepticus (SE). A decrease in the number of postsynaptic gamma-aminobutyric acid (GABA)(A) receptors with SE causes a loss of synaptic inhibition, whereas increases in postsynaptic glutamatergic receptors further upset the balance between excitation and inhibition. Although extracellular GABA levels may increase during SE and contribute to postsynaptic GABA(A) receptor desensitization, other pathways involving glutamatergic activation ultimately may be responsible for the persistent down-regulation of postsynaptic GABA(A) receptors and erosion of synaptic inhibition.

Regulated lysosomal trafficking as a mechanism for regulating GABAA receptor abundance at synapses in Caenorhabditis elegans

GABA(A) receptor plasticity is important for both normal brain function and disease progression. We are studying GABA(A) receptor plasticity in Caenorhabditis elegans using a genetic approach. Acute exposure of worms to the GABA(A) agonist muscimol hyperpolarizes postsynaptic cells, causing paralysis. Worms adapt after several hours, but show uncoordinated locomotion consistent with decreased GABA signaling. Using patch-clamp and immunofluorescence approaches, we show that GABA(A) receptors are selectively removed from synapses during adaptation. Subunit mRNA levels were unchanged, suggesting a post-transcriptional mechanism. Mutants with defective lysosome function (cup-5) show elevated GABA(A) receptor levels at synapses prior to muscimol exposure. During adaptation, these receptors are removed more slowly, and accumulate in intracellular organelles positive for the late endosome marker GFP-RAB-7. These findings suggest that chronic agonist exposure increases endocytosis and lysosomal trafficking of GABA(A) receptors, leading to reduced levels of synaptic GABA(A) receptors and reduced postsynaptic GABA sensitivity.

DAPK1 interaction with NMDA receptor NR2B subunits mediates brain damage in stroke

N-methyl-D-aspartate (NMDA) receptors constitute a major subtype of glutamate receptors at extrasynaptic sites that link multiple intracellular catabolic processes responsible for irreversible neuronal death. Here, we report that cerebral ischemia recruits death-associated protein kinase 1 (DAPK1) into the NMDA receptor NR2B protein complex in the cortex of adult mice. DAPK1 directly binds with the NMDA receptor NR2B C-terminal tail consisting of amino acid 1292-1304 (NR2B(CT)). A constitutively active DAPK1 phosphorylates NR2B subunit at Ser-1303 and in turn enhances the NR1/NR2B receptor channel conductance. Genetic deletion of DAPK1 or administration of NR2B(CT) that uncouples an activated DAPK1 from an NMDA receptor NR2B subunit in vivo in mice blocks injurious Ca(2+) influx through NMDA receptor channels at extrasynaptic sites and protects neurons against cerebral ischemic insults. Thus, DAPK1 physically and functionally interacts with the NMDA receptor NR2B subunit at extrasynaptic sites and this interaction acts as a central mediator for stroke damage.

Molecules, signaling, and schizophrenia

Schizophrenia is one of the most common psychiatric disorders, but despite some progress in identifying the genetic factors implicated in its development, the molecular mechanisms underlying its etiology and pathogenesis remain poorly understood. However, accumulating evidence suggests that regardless of the underlying genetic complexity, the mechanisms of the disease may impact a small number of common signaling pathways. In this review, we discuss the evidence for a role of schizophrenia susceptibility genes in intracellular signaling cascades by focusing on three prominent candidate genes: AKT, PPP3CC (calcineurin), and DISC1. We describe the regulation of a number of signaling cascades by AKT and calcineurin through protein phosphorylation and dephosphorylation, and the recently uncovered functions of DISC1 in cAMP and GSK3beta signaling. In addition, we present independent evidence for the involvement of their downstream signaling pathways in schizophrenia. Finally, we discuss evidence supporting an impact of these susceptibility genes on common intracellular signaling pathways and the convergence of their effects on neuronal processes implicated in schizophrenia.

Mutation I810N in the alpha3 isoform of Na+,K+-ATPase causes impairments in the sodium pump and hyperexcitability in the CNS

In a mouse mutagenesis screen, we isolated a mutant, Myshkin (Myk), with autosomal dominant complex partial and secondarily generalized seizures, a greatly reduced threshold for hippocampal seizures in vitro, posttetanic hyperexcitability of the CA3-CA1 hippocampal pathway, and neuronal degeneration in the hippocampus. Positional cloning and functional analysis revealed that Myk/+ mice carry a mutation (I810N) which renders the normally expressed Na(+),K(+)-ATPase alpha3 isoform inactive. Total Na(+),K(+)-ATPase activity was reduced by 42% in Myk/+ brain. The epilepsy in Myk/+ mice and in vitro hyperexcitability could be prevented by delivery of additional copies of wild-type Na(+),K(+)-ATPase alpha3 by transgenesis, which also rescued Na(+),K(+)-ATPase activity. Our findings reveal the functional significance of the Na(+),K(+)-ATPase alpha3 isoform in the control of epileptiform activity and seizure behavior.

Activity-dependent tuning of inhibitory neurotransmission based on GABAAR diffusion dynamics

An activity-dependent change in synaptic efficacy is a central tenet in learning, memory, and pathological states of neuronal excitability. The lateral diffusion dynamics of neurotransmitter receptors are one of the important parameters regulating synaptic efficacy. We report here that neuronal activity modifies diffusion properties of type-A GABA receptors (GABA(A)R) in cultured hippocampal neurons: enhanced excitatory synaptic activity decreases the cluster size of GABA(A)Rs and reduces GABAergic mIPSC. Single-particle tracking of the GABA(A)R gamma2 subunit labeled with quantum dots reveals that the diffusion coefficient and the synaptic confinement domain size of GABA(A)R increases in parallel with neuronal activity, depending on Ca(2+) influx and calcineurin activity. These results indicate that GABA(A)R diffusion dynamics are directly linked to rapid and plastic modifications of inhibitory synaptic transmission in response to changes in intracellular Ca(2+) concentration. This transient activity-dependent reduction of inhibition would favor the onset of LTP during conditioning.

Molecular responses of the Ts65Dn and Ts1Cje mouse models of Down syndrome to MK-801

Down syndrome (DS), caused by trisomy of human chromosome 21 (chr21), is the most common genetic cause of intellectual disability. The Ts65Dn mouse model of DS is trisomic for orthologs of 94 chr21-encoded, confirmed protein-coding genes and displays a number of behavioral deficits. Recently, Ts65Dn mice were shown to be hypersensitive to the locomotor stimulatory effects of the high-affinity N-methyl-d-aspartate (NMDA) receptor (NMDAR) channel blocker, MK-801. This is consistent with the functions of several chr21 proteins that are predicted directly or indirectly to impact NMDAR function or NMDAR-mediated signaling. In this study, we show that a second mouse model of DS, the Ts1Cje, which is trisomic for 70 protein-coding genes, is also hypersensitive to MK-801. To investigate the molecular basis for the responses to MK-801, we have measured levels of a subset of chr21 and phosphorylated non-chr21 proteins, in the cortex and hippocampus of Ts65Dn and Ts1Cje mice and euploid controls, with and without treatment with MK-801. We show that in euploid mice, the chr21-encoded proteins, TIAM1 and DYRK1A, and phosphorylation of AKT, ERK1/2 and the transcription factor ELK are involved in the MK-801 response. However, in both Ts65Dn and Ts1Cje mice, levels of phosphorylation are constitutively elevated in naïve, unstimulated mice, and the MK-801-induced changes in TIAM1 and DYRK1A and in phosphorylation are either absent or abnormal, with both genotype and brain-region-specific patterns. These results emphasize the complexities of the pathway perturbations that arise with segmental trisomy.

Post-hypoxic changes in rat cortical neuron GABA A receptor function require L-type voltage-gated calcium channel activation

Hypoxia modifies GABA(A) receptor (GABA(A)R) function and can cause seizures, encephalopathy or myoclonus. To characterize the effects of hypoxia on neuronal GABA(A)Rs, we subjected rat cortical neurons to 1% O2 for 2, 4 or 8h, followed by recovery times of 0-96h, and used whole-cell and perforated patch-clamp recording to assess GABA(A)R currents and pharmacology. Hypoxic exposure for 4h caused downregulation of maximal GABA current immediately following hypoxia and after 48h recovery without changing the EC50 for GABA. Two- and eight-hour hypoxic exposures had inconsistent effects on GABA(A)R currents. Maximal diazepam potentiation was increased immediately following 4h hypoxia, while potentiation by zolpidem was increased after 48h recovery. Pentobarbital enhancement and zinc inhibition of GABA currents were unchanged. Hypoxia also caused a depolarizing shift in the reversal potential of GABA-induced Cl(-) currents after 24h recovery. The L-type voltage-gated calcium channel (L-VGCC) blocker, nitrendipine, during hypoxia or control treatment prevented the reduction in GABA(A)R currents, and increased control currents over baseline. Nitrendipine also prevented the increase in zolpidem potentiation 48h after hypoxia, and blocked the depolarizing shift in Cl(-) reversal potential 24h after hypoxia. The effects of hypoxia on maximal GABA(A)R currents, zolpidem pharmacology and Cl(-) reversal potential thus require depolarization-induced calcium entry via L-VGCCs, and constitutive L-VGCC activity appears to reduce maximal GABA(A)R currents in control neurons via a calcium-dependent process. Calcium-dependent modulation of GABA(A)R currents via L-VGCCs may be a fundamental regulatory mechanism for GABA receptor function.

Regulation of inhibitory synaptic transmission by a conserved atypical interaction of GABA(A) receptor beta- and gamma-subunits with the clathrin AP2 adaptor

The number of surface and synaptic GABA(A) receptors is an important determinant of inhibitory synapse strength. Surface receptor number is in part controlled by removal of receptors from the membrane by interaction with the clathrin adaptor AP2. Here we demonstrate that there are two binding sites for AP2 in the gamma2-subunit: a Yxxvarphi type motif specific to gamma2-subunits and a basic patch AP2 binding motif, that is also found in GABA(A) receptor beta-subunits. Blocking GABA(A) receptor-AP2 interactions using a peptide that inhibits AP2 binding to GABA(A) receptors via the conserved basic patch mechanism increases synaptic responses within minutes, whereas simultaneously blocking both binding mechanisms has an additive effect. These data suggest that multiple AP2 internalization signals control the levels of surface and synaptic GABA(A) receptors to regulate synaptic inhibition.

Molecular pharmacodynamics, clinical therapeutics, and pharmacokinetics of topiramate

Topiramate (TPM; TOPAMAX) is a broad-spectrum antiepileptic drug (AED) that is approved in many world markets for preventing or reducing the frequency of epileptic seizures (as monotherapy or adjunctive therapy), and for the prophylaxis of migraine. TPM, a sulfamate derivative of the naturally occurring sugar D-fructose, possesses several pharmacodynamic properties that may contribute to its clinically useful attributes, and to its observed adverse effects. The sulfamate moiety is essential, but not sufficient, for its pharmacodynamic properties. In this review, we discuss the known pharmacodynamic and pharmacokinetic properties of TPM, as well as its various clinically beneficial and adverse effects.

Protein kinase C epsilon regulates gamma-aminobutyrate type A receptor sensitivity to ethanol and benzodiazepines through phosphorylation of gamma2 subunits

Ethanol enhances gamma-aminobutyrate (GABA) signaling in the brain, but its actions are inconsistent at GABA(A) receptors, especially at low concentrations achieved during social drinking. We postulated that the epsilon isoform of protein kinase C (PKCepsilon) regulates the ethanol sensitivity of GABA(A) receptors, as mice lacking PKCepsilon show an increased behavioral response to ethanol. Here we developed an ATP analog-sensitive PKCepsilon mutant to selectively inhibit the catalytic activity of PKCepsilon. We used this mutant and PKCepsilon(-/-) mice to determine that PKCepsilon phosphorylates gamma2 subunits at serine 327 and that reduced phosphorylation of this site enhances the actions of ethanol and benzodiazepines at alpha1beta2gamma2 receptors, which is the most abundant GABA(A) receptor subtype in the brain. Our findings indicate that PKCepsilon phosphorylation of gamma2 regulates the response of GABA(A) receptors to specific allosteric modulators, and, in particular, PKCepsilon inhibition renders these receptors sensitive to low intoxicating concentrations of ethanol.

Induction of cerebellar long-term depression requires activation of calcineurin in Purkinje cells

Cerebellar long-term depression (LTD) is an activity-dependent depression of synaptic transmission from parallel fibers to Purkinje cells underlying certain forms of motor learning. LTD is induced by the conjunctive stimulation of parallel fibers and climbing fibers, both of which supply excitatory inputs to Purkinje cells. The conjunctive stimulation induces a large increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) in Purkinje cells. Although the increase in [Ca(2+)](i) is essential for LTD induction, the downstream signal transduction mechanism remains elusive. In this study, we show that LTD induction requires the activation of the Ca(2+)/calmodulin-dependent protein phosphatase 2B calcineurin. In acute cerebellar slices of mice, the LTD amplitude was significantly reduced in the presence of calcineurin inhibitors (cyclosporin A or FK506), whereas the basic electrophysiological properties of the parallel fiber-Purkinje cell synaptic transmission remained constant. Furthermore, a calcineurin autoinhibitory peptide perfused into Purkinje cells completely blocked LTD induction. On the other hand, microcystin LR, an inhibitor of protein phosphatase 1 and 2A, did not affect the induction of LTD. These results indicate that calcineurin activation is essential for LTD induction downstream of the conjunctive-stimulation-induced Ca(2+) signal in Purkinje cells.

Effects of enriched environment on gene expression and signal pathways in cortex of hippocampal CA1 specific NMDAR1 knockout mice

N-methyl-D-aspartate glutamate receptor 1 (NMDAR1) plays a pivotal role in different forms of memory. Indeed, hippocampal CA1 region specific knockout (KO) of NMDAR1 in mice showed memory impairment. Recently, it has been reported that environmental enrichment enhanced memory and rescued the memory deficits of the NMDAR1-KO mice. It is well known that cortex has synaptic connections with hippocampus and is the storage region of the brain for long-term memory. To understand the molecular mechanisms of the memory impairments in the NMDAR1-KO mice, we have examined gene expression profiles in cortex from the receptor KO mice compared to wild type mice. Furthermore, since memory deficits were rescued after exposure of the NMDAR1-KO mice to enriched environment, we also analyzed the gene expression in the cortex of the KO mice after 3 hours, 2 days and 2 weeks enrichment. We found that the expression levels of 104 genes were altered in the cortex of NMDAR1-KO mice. Environmental enrichment for 3 hours, 2 days and 2 weeks affected the expression of 45, 34 and 56 genes, respectively. Genes involved in multiple signal pathways were regulated in the NMDAR1-KO mice, such as neurotransmission, structure, transcription, protein synthesis and protein processing. It is not surprising that since enriched environment rescued the memory decline in the NMDAR1-KO mice, the expression changes of a number of genes involved in these signal pathways were recovered or even reversed after enrichment. Our results further demonstrated that reelin and Notch signal pathways could be involved in the enrichment effects on memory improvement in the KO mice.

Protein serine/threonine phosphatases in neuronal plasticity and disorders of learning and memory

Phosphorylation and dephosphorylation of cellular proteins by protein kinases and phosphatases represent important mechanisms for controlling major biological events. In the nervous system, protein phosphatases are contained in highly dynamic complexes localized within specialized subcellular compartments and they ensure timely dephosphorylation of multiple neuronal phosphoproteins. This modulates the responsiveness of individual synapses to neural activity and controls synaptic plasticity. These enzymes in turn play a key role in many forms of learning and memory, and their dysfunction contributes to cognitive deficits associated with aging and dementias or neurodegenerative diseases. Here, we review key modes of regulation of neuronal protein serine/threonine phosphatases and their contribution to disorders of learning and memory.

Molecular signaling mechanisms underlying epileptogenesis

Epilepsy, a disorder of recurrent seizures, is a common and frequently devastating neurological condition. Available therapy is only symptomatic and often ineffective. Understanding epileptogenesis, the process by which a normal brain becomes epileptic, may help identify molecular targets for drugs that could prevent epilepsy. A number of acquired and genetic causes of this disorder have been identified, and various in vivo and in vitro models of epileptogenesis have been established. Here, we review current insights into the molecular signaling mechanisms underlying epileptogenesis, focusing on limbic epileptogenesis. Study of different models reveals that activation of various receptors on the surface of neurons can promote epileptogenesis; these receptors include ionotropic and metabotropic glutamate receptors as well as the TrkB neurotrophin receptor. These receptors are all found in the membrane of a discrete signaling domain within a particular type of cortical neuron--the dendritic spine of principal neurons. Activation of any of these receptors results in an increase Ca2+ concentration within the spine. Various Ca2+-regulated enzymes found in spines have been implicated in epileptogenesis; these include the nonreceptor protein tyrosine kinases Src and Fyn and a serine-threonine kinase [Ca2+-calmodulin-dependent protein kinase II (CaMKII)] and phosphatase (calcineurin). Cross-talk between astrocytes and neurons promotes increased dendritic Ca2+ and synchronous firing of neurons, a hallmark of epileptiform activity. The hypothesis is proposed that limbic epilepsy is a maladaptive consequence of homeostatic responses to increases of Ca2+ concentration within dendritic spines induced by abnormal neuronal activity.

ADAR2-dependent RNA editing of AMPA receptor subunit GluR2 determines vulnerability of neurons in forebrain ischemia

ADAR2 is a nuclear enzyme essential for GluR2 pre-mRNA editing at Q/R site-607, which gates Ca2+ entry through AMPA receptor channels. Here, we show that forebrain ischemia in adult rats selectively reduces expression of ADAR2 enzyme and, hence, disrupts RNA Q/R site editing of GluR2 subunit in vulnerable neurons. Recovery of GluR2 Q/R site editing by expression of exogenous ADAR2b gene or a constitutively active CREB, VP16-CREB, which induces expression of endogenous ADAR2, protects vulnerable neurons in the rat hippocampus from forebrain ischemic insult. Generation of a stable ADAR2 gene silencing by delivering small interfering RNA (siRNA) inhibits GluR2 Q/R site editing, leading to degeneration of ischemia-insensitive neurons. Direct introduction of the Q/R site edited GluR2 gene, GluR2(R607), rescues ADAR2 degeneration. Thus, ADAR2-dependent GluR2 Q/R site editing determines vulnerability of neurons in the rat hippocampus to forebrain ischemia.

Nicotinic activity stabilizes convergence of nicotinic and GABAergic synapses on filopodia of hippocampal interneurons

Nicotinic acetylcholine receptors containing alpha7 subunits occupy pre- and postsynaptic sites in the adult hippocampus. We find that embryonic hippocampal slices in culture display the receptors most prominently on interneurons where they form clusters localized in part on filopodia. The receptors often co-distribute specifically with GABAA receptors. In septal-hippocampal co-cultures, the filopodia become co-innervated by cholinergic and GABAergic terminals abutting the receptor clusters. Nicotinic transmission appears to stabilize the cholinergic contacts: pharmacological blockade of the alpha7-containing nicotinic receptors increases the rate of filopodia movement and decreases the incidence of the clusters being adjacent to cholinergic terminals. Immunostaining fresh hippocampal slices from neonatal rat pups confirms that cholinergic and GABAergic terminals contact alpha7-containing nicotinic receptor clusters in vivo, and the clusters appear to include filopodial sites. The results indicate a convergence of nicotinic and GABAergic input at specific sites on developing hippocampal interneurons and suggest that synaptic activity helps stabilize the nicotinic contribution.

Enhanced neuronal damage after ischemic insults in mice lacking Kir6.2-containing ATP-sensitive K+ channels

Adenosine triphosphate (ATP)-sensitive potassium (KATP) channels, incorporating Kir6.x and sulfonylurea receptor subunits, are weak inward rectifiers that are thought to play a role in neuronal protection from ischemic insults. However, the involvement of Kir6.2-containing KATP channel in hippocampus and neocortex has not been tested directly. To delineate the physiological roles of Kir6.2 channels in the CNS, we used knockout (KO) mice that do not express Kir6.2. Immunocytochemical staining demonstrated that Kir6.2 protein was expressed robustly in hippocampal neurons of the wild-type (WT) mice and absent in the KO. To examine neuronal sensitivity to metabolic stress in vitro, and to ischemia in vivo, we 1) exposed hippocampal slices to transient oxygen and glucose deprivation (OGD) and 2) produced focal cerebral ischemia by middle cerebral artery occlusion (MCAO). Both slice and whole animal studies showed that neurons from the KO mice were severely damaged after anoxia or ischemia, whereas few injured neurons were observed in the WT, suggesting that Kir6.2 channels are necessary to protect neurons from ischemic insults. Membrane potential recordings from the WT CA1 pyramidal neurons showed a biphasic response to OGD; a brief hyperpolarization was followed by a small depolarization during OGD, with complete recovery within 30 min after returning to normoxic conditions. By contrast, CA1 pyramidal neurons from the KO mice were irreversibly depolarized by OGD exposure, without any preceding hyperpolarization. These data suggest that expression of Kir6.2 channels prevents prolonged depolarization of neurons resulting from acute hypoxic or ischemic insults, and thus protects these central neurons from the injury.

Modulation of GABAergic transmission by activity via postsynaptic Ca2+-dependent regulation of KCC2 function

Activity-induced modification of GABAergic transmission contributes to the plasticity of neural circuits. In the present work we found that prolonged postsynaptic spiking of hippocampal neurons led to a shift in the reversal potential of GABA-induced Cl- currents (E(Cl)) toward positive levels in a duration- and frequency-dependent manner. This effect was abolished by blocking cytosolic Ca2+ elevation and mimicked by releasing Ca2+ from internal stores. Activity- and Ca2+-induced E(Cl) shifts were larger in mature neurons, which express the K-Cl cotransporter KCC2 at high levels, and inhibition of KCC2 occluded the shifts. Overexpression of KCC2 in young cultured neurons, which express lower levels of KCC2 and have a more positive E(Cl), resulted in hyperpolarized E(Cl) similar to that of mature cells. Importantly, these young KCC2-expressing neurons became responsive to neuronal spiking and Ca2+ elevation by showing positive E(Cl) shifts. Thus, repetitive postsynaptic spiking reduces the inhibitory action of GABA through a Ca2+-dependent downregulation of KCC2 function.

Calcineurin regulates enteric muscle contraction through EXP-1, excitatory GABA-gated channel, in C. elegans

The enteric muscle contraction (EMC) is the last step of the defecation behavior which occurs every 50 s in Caenorhabditis elegans. This EMC is regulated by intestinal and anal depressor muscles, which are innervated by GABA motor neurons. Our data show that calcineurin (tax-6) is expressed in intestinal muscle and anal depressor muscle, and the gain-of-function mutant of calcineurin, tax-6(jh107), shows defects in enteric muscle contractions. In addition, the intracellular region of EXP-1, an excitatory GABA receptor, specifically binds to calcineurin A. This interaction between TAX-6 and EXP-1 appears to be independent of both calcium and CNB, which is the calcium-binding regulatory subunit. Genetic evidence of epistasis between cnb-1(jh103) and exp-1(sa6) suggests that calcineurin functions as a negative regulator of excitatory GABA receptor in GABA signaling in C.elegans.

A persistent change in subcellular distribution of calcineurin following fluid percussion injury in the rat

Calcineurin, a neuronally enriched, calcium-stimulated phosphatase, is an important modulator of many neuronal processes, including several that are physiologically related to the pathology of traumatic brain injury. The effect of moderate, central fluid percussion injury on the subcellular distribution of this important neuronal enzyme was examined. Animals were sacrificed at several time points post-injury and calcineurin distribution in subcellular fractions was assayed by Western blot analysis and immunohistochemistry. A persistent increase in calcineurin concentration was observed in crude synaptoplasmic membrane-containing fractions. In cortical fractions, calcineurin immunoreactivity remained persistently increased for 2 weeks post-injury. In hippocampal homogenates, calcineurin immunoreactivity remained increased for up to 4 weeks. Finally, immunohistochemical analysis of hippocampal slices revealed increased staining in the apical dendrites of CA1 neurons. The increased staining was greatest in magnitude 24 h post-injury; however, staining was still more intense than control 4 weeks post-injury. The data support the conclusion that fluid percussion injury results in redistribution of the enzyme in the rat forebrain. These changes have broad physiological implications, possibly resulting in altered cellular excitability or a greater likelihood of neuronal cell death.

GABA synapses and the rapid loss of inhibition to dentate gyrus granule cells after brief perforant-path stimulation

PURPOSE

To study the pharmacologic and synaptic basis for the early loss of paired-pulse inhibition that occurs in the perforant-path stimulation model of status epilepticus.

METHODS

Hippocampal slices were prepared from male Wistar rats. Test paired pulses (20- to 50-ms interstimulus interval) of the perforant path were used before and after an abbreviated period of perforant-path stimulation (1-5 min; 2-Hz continuous with 20 Hz of 10 s/min pulses) while either recording field potentials from the dentate gyrus granule cell layer or directly measuring whole-cell patch-clamp currents from granule cells. Paired-pulse field recordings also were obtained during perfusion of the gamma-aminobutyric acid (GABA)(A) antagonist bicuculline.

RESULTS

Prolonged loss of paired-pulse inhibition occurs after brief (< 5 min) perforant-path stimulation in vitro (similar to results in vivo) with the paired-pulse population spike amplitude ratio (P2/P1) increasing from a baseline of 0.53 +/- 0.29 to 1.17 +/- 0.09 after perforant-path stimulation (p < 0.05). After perfusion with the GABA(A) antagonist, bicuculline, the P2/P1 ratio also increased from a baseline of 0.52 +/- 0.16 to 1.15 +/- 0.26 (p < 0.05). After 1-2 min of perforant-path stimulation, a 22 +/- 6% (p < 0.05) decrease occurred in the P2/P1 amplitude ratio of paired-pulse evoked inhibitory postsynaptic currents.

CONCLUSIONS

Similar to in vivo, loss of paired-pulse inhibition occurs with brief perforant-path stimulation in vitro. GABA(A) antagonism causes a similar loss of paired-pulse inhibition, and the effects of perforant-path stimulation on postsynaptic inhibitory currents also are consistent with the involvement of GABA(A) synaptic receptors. The findings suggest that loss of inhibition at GABA synapses may be an important early event in the initiation of status epilepticus.

Contribution of GABAergic inhibition to synaptic responses and LTD early in postnatal development in the rat superior colliculus

We studied the development of optic tract evoked field potentials (FP) in the rodent superior colliculus (SC) and the effect of GABA antagonists upon their development and upon induction of long-term depression (LTD). Brain slices were cut from Lister Hooded rats. The optic tract was stimulated while recording from the superficial grey layer. GABAergic inhibition was assessed by adding 100 microm picrotoxin and 3 microm CGP55845 antagonists to block GABA A,B,C receptors. LTD was induced with a 50 Hz, 20 s tetanus. At age P2, the FP consisted only of a presynaptic spike. The GABA antagonists had no effect. By P4, the FP consisted of a presynaptic spike, a longer latency population spike, and a field excitatory postsynaptic potential (fEPSP). The fEPSP was slightly prolonged by the GABA antagonists at this age. By P7-P14, a prominent FP with trailing fEPSP was recorded. The GABA antagonists usually had a large effect, with the fEPSP increasing in both amplitude and duration. A mature FP was usually recorded in P15-P23 slices where the GABA antagonist effect remained substantial. LTD could be induced in 17 of 30 control slices from rats aged P4-P26. The average fEPSP amplitude after tetanus was 77.9% of control. Pre-treatment with GABA antagonists produced a short-term potentiation (average 114.0%), rather than LTD, in 14 of 19 cases. This STP was followed by a more prolonged potentiation in 12 of the 14 cases. We conclude that GABAergic inhibitory circuits mature before eye opening and that GABA contributes to induction of LTD in the developing SC.

Aspects of the homeostaic plasticity of GABAA receptor-mediated inhibition

Plasticity of ligand-gated ion channels plays a critical role in nervous system development, circuit formation and refinement, and pathological processes. Recent advances have mainly focused on the plasticity of channels gated by excitatory amino acids, including their acclaimed role in learning and memory. These receptors, together with voltage-gated ion channels, have also been known to be subjected to a homeostatic form of plasticity that prevents destabilization of the neurone's function and that of the network during various physiological processes. To date, the plasticity of GABA(A) receptors has been examined mainly from a developmental and a pathological point of view. Little is known about homeostatic mechanisms governing their plasticity. This review summarizes some of the findings on the homeostatic plasticity of tonic and phasic inhibitory activity.

Diversity of inhibitory neurotransmission through GABAA receptors

In the brain, highly connected and heterogeneous GABAergic cells are crucial in controling the activity of neuronal networks. They accomplish this task by communicating through remarkably diverse sets of inhibitory processes, the complexity of which is reflected by the variety of interneuron classification schemes proposed in recent years. It is now becoming clear that the subcellular localization and intrinsic properties of heteropentameric GABA(A) receptors themselves also constitute major sources of diversity in GABA-mediated signaling. This review summarizes some of the factors underlying this diversity, including GABA(A) receptor subunit composition, localization, activation, number and phosphorylation states, variance of GABA concentration in the synaptic cleft, and some of the presynaptic factors regulating GABA release.

Preferential accumulation of GABAA receptor gamma 2L, not gamma 2S, cytoplasmic loops at rat spinal cord inhibitory synapses

Alternative splicing generates two variants of the GABAAR gamma2-subunit, gamma2S and gamma2L, which differ by insertion of the amino acid sequence LLRMFSFK into the large cytoplasmic loop between transmembrane domains 3 and 4. This additional sequence within the GABAAR gamma2L-subunit contains the potential protein kinase C (PKC) phosphorylation site serine 343 (Ser343). In the present study we intended to determine the capacity of these two splice variants to accumulate at inhibitory synaptic terminals and to colocalize with gephyrin, and to find out whether phosphorylation of Ser343 has any effect on GABAAR distribution. Green fluorescent protein (GFP)-tagged large cytoplasmic loops of GABAAR gamma2S and gamma2L (GFP::gamma2S/L) were used as surrogates for full-length receptors to study the function of the individual gamma2S and gamma2L peptides in transfected spinal cord neurones (SCNs) and COS-7 cells. It was found that GFP::gamma2L displayed a significantly higher capacity to accumulate at inhibitory synapses than GFP::gamma2S. GABAAR GFP::gamma2S accumulation at inhibitory postsynaptic sites was suppressed to the extent that GFP::gamma2S assumed a diffuse cytosolic distribution. PKC activation facilitated the postsynaptic clustering of GFP::gamma2L but not of GFP::gamma2S. This required the Ser343 residue, since substituting Ala343 for Ser343 produced a diffuse cytosolic localization pattern, like that of GFP::gamma2S. Furthermore, upon PKC activation Discosoma Red2-tagged GABAAR gamma2L (DsRed 2::gamma2L) colocalized with gephyrin in transfected COS-7 cells. These results support the idea that alternative splicing regulates the access of GABAARs to inhibitory postsynaptic sites in a Ser343 phosphorylation-regulated way.

A four PDZ domain-containing splice variant form of GRIP1 is localized in GABAergic and glutamatergic synapses in the brain

We have isolated, from a rat brain cDNA library, a clone corresponding to a 2779-bp cDNA encoding a novel splice form of the glutamate receptor interacting protein-1 (GRIP1). We call this 696-amino acid splice form GRIP1c 4-7 to differentiate it from longer splice forms of GRIP1a/b containing seven PDZ domains. The four PDZ domains of GRIP1c 4-7 are identical to PDZ domains 4-7 of GRIP1a/b. GRIP1c 4-7 also contains 35 amino acids at the N terminus and 12 amino acids at the C terminus that are different from GRIP1a/b. In transfected HEK293 cells, a majority of GRIP1c 4-7 was associated with the plasma membrane. GRIP1c 4-7 interacted with GluR2/3 subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptor. In low density hippocampal cultures, GRIP1c 4-7 clusters colocalized with GABAergic (where GABA is gamma-aminobutyric acid) and glutamatergic synapses, although a higher percentage of GRIP1c 4-7 clusters colocalized with gamma-aminobutyric acid, type A, receptor (GABA(A)R) clusters than with alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptor clusters. Transfection of hippocampal neurons with hemagglutinin-tagged GRIP1c 4-7 showed that it could target to the postsynaptic complex of GABAergic synapses colocalizing with GABA(A)R clusters. GRIP1c 4-7-specific antibodies, which did not recognize previously described splice forms of GRIP1, recognized a 75-kDa protein that is enriched in a postsynaptic density fraction isolated from rat brain. EM immunocytochemistry experiments showed that in intact brain GRIP1c 4-7 concentrates at postsynaptic complexes of both type I glutamatergic and type II GABAergic synapses although it is also presynaptically localized. These results indicate that GRIP1c 4-7 plays a role not only in glutamatergic synapses but also in GABAergic synapses.

Stress-facilitated LTD induces output plasticity through synchronized-spikes and spontaneous unitary discharges in the CA1 region of the hippocampus

Long-term potentiation (LTP) and long-term depression (LTD) of the excitatory synaptic inputs plasticity in the hippocampus is believed to underlie certain types of learning and memory. Especially, stressful experiences, well known to produce long-lasting strong memories of the event themselves, enable LTD by low frequency stimulation (LFS, 3 Hz) but block LTP induction by high frequency stimulation (HFS, 200 Hz). However, it is unknown whether stress-affected synaptic plasticity has an impact on the output plasticity. Thus, we have simultaneously studied the effects of stress on synaptic plasticity and neuronal output in the hippocampal CA1 region of anesthetized Wistar rats. Our results revealed that stress increased basal power spectrum of the evoked synchronized-spikes and enabled LTD induction by LFS. The induction of stress-facilitated LTD but not LFS induced persistent decreases of the power spectrum of the synchronized-spikes and the frequency of the spontaneous unitary discharges; However, HFS induced LTP in non-stressed animals and increased the power spectrum of the synchronized-spikes, without affecting the frequency of the spontaneous unitary discharges, but HFS failed to induce LTP in stressed animals without affecting the power spectrum of the synchronized-spikes and the frequency of the spontaneous unitary discharges. These observations that stress-facilitated LTD induces the output plasticity through the synchronized-spikes and spontaneous unitary discharges suggest that these types of stress-related plasticity may play significant roles in distribution, amplification and integration of encoded information to other brain structures under stressful conditions.

Palmitoylation regulates the clustering and cell surface stability of GABAA receptors

GABAA receptors are the major mediators of fast synaptic inhibition in the brain. These receptors are ionotropic, hetero-pentameric, ligand-gated ion channels, which are predominantly composed of alpha, beta, and gamma2 subunits. Here, we reveal that the gamma2 subunit of neuronal and recombinant GABAA receptors is palmitoylated. We further establish that palymitoylation of the gamma2 subunit occurs on multiple cysteine residues within the major intracellular domain of this receptor subunit. In cultured hippocampal neurons, inhibitors of protein palymitoylation reduced the synaptic clustering of GABAA receptors and steady-state cell surface receptor number. These effects are likely to be mediated by direct palmitoylation of the gamma2 subunit, as mutation of palmitoylation sites within this protein reduces GABAA receptor clustering. Taken together, these results suggest that palmitoylation of GABAA receptors plays an essential role in regulating the clustering of these receptors at synaptic sites.

Regulation of GABAA receptor trafficking, channel activity, and functional plasticity of inhibitory synapses

Neural inhibition in the brain is mainly mediated by ionotropic gamma-aminobutyric acid type A (GABA(A)) receptors. Different subtypes of these receptors, distinguished by their subunit composition, are either concentrated at postsynaptic sites where they mediate phasic inhibition or found at perisynaptic and extrasynaptic locations where they prolong phasic inhibition and mediate tonic inhibition, respectively. Of special interest are mechanisms that modulate the stability and function of postsynaptic GABA(A) receptor subtypes and that are implicated in functional plasticity of inhibitory transmission in the brain. We will summarize recent progress on the classification of synaptic versus extrasynaptic receptors, the molecular composition of the postsynaptic cytoskeleton, the function of receptor-associated proteins in trafficking of GABA(A) receptors to and from synapses, and their role in post-translational signaling mechanisms that modulate the stability, density, and function of GABA(A) receptors in the postsynaptic membrane.

Calcineurin in memory and bidirectional plasticity

The molecular mechanisms of learning and memory, and the underlying bidirectional changes in synaptic plasticity that sustain them largely implicate protein kinases and phosphatases. Specifically, Ca(2+)-dependent kinases and phosphatases actively control neuronal processing by forming a tightly regulated balance in which they oppose each other. In this balance, calcineurin (PP2B) is a critical protein phosphatase whose main function is to negatively modulate learning, memory, and plasticity. It acts by dephosphorylating numerous substrates in different neuronal compartments. This review outlines some of CN neuronal targets and their implication in synaptic functions, and describes the role of CN in the acquisition, storage, retrieval, and extinction of memory, as well as in bidirectional plasticity.

Calcineurin regulation of neuronal plasticity

From the most basic of nervous systems to the intricate circuits found within the human brain, a fundamental requirement of neuronal function is that it be malleable, altering its output based upon experience. A host of cellular proteins are recruited for this purpose, which themselves are regulated by protein phosphorylation. Over the past several decades, research has demonstrated that the Ca(2+) and calmodulin-dependent protein phosphatase calcineurin (protein phosphatase 2B) is a critical regulator of a diverse array of proteins, leading to both short- and long-term effects on neuronal excitability and function. This review describes many of the influences of calcineurin on a variety of proteins, including ion channels, neurotransmitter receptors, enzymes, and transcription factors. Intriguingly, due to the bi-directional influences of Ca(2+) and calmodulin on calcineurin activity, the strength and duration of particular stimulations may cause apparently antagonistic functions of calcineurin to work in concert.

GABAB receptor- and metabotropic glutamate receptor-dependent cooperative long-term potentiation of rat hippocampal GABAA synaptic transmission

Repetitive stimulation of Schaffer collaterals induces activity-dependent changes in the strength of polysynaptic inhibitory postsynaptic potentials (IPSPs) in hippocampal CA1 pyramidal neurons that are dependent on stimulation parameters. In the present study, we investigated the effects of two stimulation patterns, theta-burst stimulation (TBS) and 100 Hz tetani, on pharmacologically isolated monosynaptic GABAergic responses in adult CA1 pyramidal cells. Tetanization with 100 Hz trains transiently depressed both early and late IPSPs, whereas TBS induced long-term potentiation (LTP) of early IPSPs that lasted at least 30 min. Mechanisms mediating this TBS-induced potentiation were examined using whole-cell recordings. The paired-pulse ratio of monosynaptic inhibitory postsynaptic currents (IPSCs) was not affected during LTP, suggesting that presynaptic changes in GABA release are not involved in the potentiation. Bath application of the GABAB receptor antagonist CGP55845 or the group I/II metabotropic glutamate receptor antagonist E4-CPG inhibited IPSC potentiation. Preventing postsynaptic G-protein activation or Ca2+ rise by postsynaptic injection of GDP-beta-S or BAPTA, respectively, abolished LTP, indicating a G-protein- and Ca2+-dependent induction in this LTP. Finally during paired-recordings, activation of individual interneurons by intracellular TBS elicited solely short-term increases in average unitary IPSCs in pyramidal cells. These results indicate that a stimulation paradigm mimicking the endogenous theta rhythm activates cooperative postsynaptic mechanisms dependent on GABABR, mGluR, G-proteins and intracellular Ca2+, which lead to a sustained potentiation of GABAA synaptic transmission in pyramidal cells. GABAergic synapses may therefore contribute to functional synaptic plasticity in adult hippocampus.

Phosphorylation influences neurosteroid modulation of synaptic GABAA receptors in rat CA1 and dentate gyrus neurones

The neurosteroid 5beta-pregnan-3alpha-ol-20-one (5beta3alpha) is a potent, endogenous, positive allosteric modulator of the GABA(A) receptor. Relatively low concentrations of 5beta3alpha (10-100 nM), thought to occur physiologically, caused a concentration-dependent slowing of the decay of GABA-mediated miniature inhibitory postsynaptic currents (mIPSCs) recorded from hippocampal CA1 pyramidal neurones. However, much greater concentrations of this neurosteroid (> or =300 nM) were required to similarly influence dentate granule cell mIPSCs. By contrast, the allosteric modulators pentobarbitone and flunitrazepam were equi-effective in prolonging mIPSCs in both neuronal types. Hence, the neurosteroid selectively differentiates between the synaptic GABA(A) receptors of these hippocampal neurones. Inhibition of either protein kinase A, or C, greatly reduced the sensitivity of CA1 synaptic GABA(A) receptors to 5beta3alpha, but not pentobarbitone, whereas stimulation of PKC had no effect on steroid sensitivity. However, in dentate gyrus granule cells, activation of PKC made mIPSCs sensitive to a previously ineffective concentration of 5beta3alpha. Collectively, these results suggest that the GABA-modulatory effects of physiological levels of the neurosteroid will not be uniformly experienced throughout the central nervous system, or even within the same brain region such as the hippocampus, but will be neurone-specific and will be dependent on the phosphorylation status of the GABA(A) receptor, or associated proteins.

Cyclothiazide potently inhibits gamma-aminobutyric acid type A receptors in addition to enhancing glutamate responses

Ionotropic glutamate and gamma-aminobutyric acid type A (GABAA) receptors mediate critical excitatory and inhibitory actions in the brain. Cyclothiazide (CTZ) is well known for its effect of enhancing glutamatergic transmission and is widely used as a blocker for alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptor desensitization. Here, we report that in addition to its action on AMPA receptors, CTZ also exerts a powerful but opposite effect on GABAA receptors. We found that CTZ reversibly inhibited both evoked and spontaneous inhibitory postsynaptic currents, as well as GABA application-induced membrane currents, in a dose-dependent manner. Single-channel analyses revealed further that CTZ greatly reduced the open probability of GABAA receptor channels. These results demonstrate that CTZ interacts with both glutamate and GABAA receptors and shifts the excitation-inhibition balance in the brain by two independent mechanisms. Understanding the molecular mechanism of this double-faceted drug-receptor interaction may help in designing new therapies for neurological diseases.

Asymmetric cross-inhibition between GABAA and glycine receptors in rat spinal dorsal horn neurons

Presynaptic nerve terminals of inhibitory synapses in the dorsal horn of the spinal cord and brain stem can release both GABA and glycine, leading to coactivation of postsynaptic GABAA and glycine receptors. In the present study we have analyzed functional interactions between GABAA and glycine receptors in acutely dissociated neurons from rat sacral dorsal commissural nucleus. Although the application of GABA and glycine activates pharmacologically distinct receptors, the current induced by a simultaneous application of these two transmitters was less than the sum of currents induced by applying two transmitters separately. Sequential application of glycine and GABA revealed that the GABA-evoked current is more affected by glycine than glycine-evoked responses by GABA. Activation of glycine receptors decreased the amplitude and accelerated the rate of desensitization of GABA-induced currents. This asymmetric cross-inhibition is reversible, dependent on the agonist concentration applied, but independent of both membrane potential and intracellular calcium concentration or changes in the chloride equilibrium potential. During sequential applications, the asymmetric cross-inhibition was prevented by selective GABAA or glycine receptor antagonists, suggesting that occupation of binding sites did not suffice to induce glycine and GABAA receptors functional interaction, and receptor channel activation is required. Furthermore, inhibition of phosphatase 2B, but not phosphatase 1 or 2A, prevented GABAA receptor inhibition by glycine receptor activation, whereas inhibition of phosphorylation pathways rendered cross-talk irreversible. Taken together, our results demonstrated that there is an asymmetric cross-inhibition between glycine and GABAA receptors and that a selective modulation of the state of phosphorylation of GABAA receptor and/or mediator proteins underlies the asymmetry in the cross-inhibition.

Cdk5 activation induces hippocampal CA1 cell death by directly phosphorylating NMDA receptors

CA1 pyramidal neurons degenerate after transient forebrain ischemia, whereas neurons in other regions of the hippocampus remain intact. Here we show that in rat hippocampal CA1 neurons, forebrain ischemia induces the phosphorylation of the N-methyl-D-aspartate (NMDA) receptor 2A subunit at Ser1232 (phospho-Ser1232). Ser1232 phosphorylation is catalyzed by cyclin-dependent kinase 5 (Cdk5). Inhibiting endogenous Cdk5, or perturbing interactions between Cdk5 and NR2A subunits, abolished NR2A phosphorylation at Ser1232 and protected CA1 pyramidal neurons from ischemic insult. Thus, we conclude that modulation of NMDA receptors by Cdk5 is the primary intracellular event underlying the ischemic injury of CA1 pyramidal neurons.

Neurosteroid modulation of GABAA receptors

Certain metabolites of progesterone and deoxycorticosterone are established as potent and selective positive allosteric modulators of the gamma-aminobutyric acid type A (GABA(A)) receptor. Upon administration these steroids exhibit clear behavioural effects that include anxiolysis, sedation and analgesia, they are anticonvulsant and at high doses induce a state of general anaesthesia, a profile consistent with an action to enhance neuronal inhibition. Physiologically, peripherally synthesised pregnane steroids derived from endocrine glands such as the adrenals and ovaries function as hormones by crossing the blood brain barrier to influence neuronal signalling. However, the demonstration that certain neurons and glial cells within the central nervous system (CNS) can synthesize these steroids either de novo, or from peripherally derived progesterone, has led to the proposal that these steroids (neurosteroids) can additionally function in a paracrine manner, to locally influence GABAergic transmission. Steroid levels are known to change dynamically, for example in stress and during pregnancy. Given that GABA(A) receptors are ubiquitously expressed throughout the central nervous system, such changes in steroid levels would be predicted to cause a global enhancement of inhibitory neurotransmission throughout the brain, a scenario that would seem incompatible with a physiological role as a selective neuromodulator. Here, we will review emerging evidence that the GABA-modulatory actions of the pregnane steroids are highly selective, with their actions being brain region and indeed neuron dependent. Furthermore, the sensitivity of GABA(A) receptors is not static but can dynamically change. The molecular mechanisms underpinning this neuronal specificity will be discussed with particular emphasis being given to the role of GABA(A) receptor isoforms, protein phosphorylation and local steroid metabolism and synthesis.