Kainate-induced calcium overload of cortical neurons in vitro: Dependence on expression of AMPAR GluA2-subunit and down-regulation by subnanomolar ouabain

Excitotoxic glutamate levels cause the secretion of resident endoplasmic reticulum proteins

Dysregulation of synaptic glutamate levels can lead to excitotoxicity such as that observed in stroke, traumatic brain injury, and epilepsy. The role of increased intracellular calcium (Ca2+ ) in the development of excitotoxicity is well established. However, less is known regarding the impact of glutamate on endoplasmic reticulum (ER)-Ca2+ -mediated processes such as proteostasis. To investigate this, we expressed a secreted ER Ca2+ modulated protein (SERCaMP) in primary cortical neurons to monitor exodosis, a phenomenon whereby ER calcium depletion causes the secretion of ER-resident proteins that perform essential functions to the ER and the cell. Activation of glutamatergic receptors (GluRs) led to an increase in SERCaMP secretion indicating that normally ER-resident proteins are being secreted in a manner consistent with ER Ca2+ depletion. Antagonism of ER Ca2+ channels attenuated the effects of glutamate and GluR agonists on SERCaMP release. We also demonstrate that endogenous proteins containing an ER retention/retrieval sequence (ERS) are secreted in response to GluR activation supporting that neuronal activation by glutamate promotes ER exodosis. Ectopic expression of KDEL receptors attenuated the secretion of ERS-containing proteins caused by GluR agonists. Taken together, our data indicate that excessive GluR activation causes disruption of neuronal proteostasis by triggering the secretion of ER-resident proteins through ER Ca2+ depletion and describes a new facet of excitotoxicity.

Unveiling the Role of Cholesterol in Subnanomolar Ouabain Rescue of Cortical Neurons from Calcium Overload Caused by Excitotoxic Insults

Na/K-ATPase maintains transmembrane ionic gradients and acts as a signal transducer when bound to endogenous cardiotonic steroids. At subnanomolar concentrations, ouabain induces neuroprotection against calcium overload and apoptosis of neurons during excitotoxic stress. Here, the role of lipid rafts in interactions between Na/K-ATPase, sodium-calcium exchanger (NCX), and N-methy-D-aspartate receptors (NMDARs) was investigated. We analyzed 0.5-1-nanometer ouabain's effects on calcium responses and miniature post-synaptic current (mEPSCs) frequencies of cortical neurons during the action of NMDA in rat primary culture and brain slices. In both objects, ouabain attenuated NMDA-evoked calcium responses and prevented an increase in mEPSC frequency, while the cholesterol extraction by methyl-β-cyclodextrin prevented the effects. The data support the conclusions that (i) ouabain-induced inhibition of NMDA-elicited calcium response involves both pre- and post-synapse, (ii) the presence of astrocytes in the tripartite synapse is not critical for the ouabain effects, which are found to be similar in cell cultures and brain slices, and (iii) ouabain action requires the integrity of cholesterol-rich membrane microdomains in which the colocalization and functional interaction of NMDAR-transferred calcium influx, calcium extrusion by NCX, and Na/K-ATPase modulation of the exchanger occur. This regulation of the molecules by cardiotonic steroids may influence synaptic transmission, prevent excitotoxic neuronal death, and interfere with the pharmacological actions of neurological medicines.

The Janus face of ouabain in Na+ /K+ -ATPase and calcium signalling in neurons

Na⁺ /K⁺ -ATPase, a transmembrane protein essential for maintaining the electrochemical gradient across the plasma membrane, acts as a receptor for cardiotonic steroids such as ouabain. Cardiotonic steroids binding to Na⁺ /K⁺ -ATPase triggers signalling pathways or inhibits Na⁺ /K⁺ -ATPas activity in a concentration-dependent manner, resulting in a modulation of Ca²⁺ levels, which are essential for homeostasis in neurons. However, most of the pharmacological strategies for avoiding neuronal death do not target Na⁺ /K⁺ -ATPase activity due to its complexity and the poor understanding of the mechanisms involved in Na⁺ /K⁺ -ATPase modulation. The present review aims to discuss two points regarding the interplay between Na⁺ /K⁺ -ATPase and Ca²⁺ signalling in the brain. One, Na⁺ /K⁺ -ATPase impairment causing illness and neuronal death due to Ca²⁺ signalling and two, benefits to the brain by modulating Na⁺ /K⁺ -ATPase activity. These interactions play an essential role in neuronal cell fate determination and are relevant to find new targets for the treatment of neurodegenerative diseases.

Extracellular Zn2+-Dependent Amyloid-β1-42 Neurotoxicity in Alzheimer's Disease Pathogenesis

The basal level of extracellular Zn²⁺ is in the range of low nanomolar (~ 10 nM) in the hippocampus. However, extracellular Zn²⁺ dynamics plays a key role for not only cognitive activity but also cognitive decline. Extracellular Zn²⁺ dynamics is modified by glutamatergic synapse excitation and the presence of amyloid-β_1-42 (Aβ_1-42), a causative peptide in Alzheimer's disease (AD). When human Aβ_1-42 reaches high picomolar (> 100 pM) in the extracellular compartment of the rat dentate gyrus, Zn-Aβ_1-42 complexes are readily formed and taken up into dentate granule cells, followed by Aβ_1-42-induced cognitive decline that is linked with Zn²⁺ released from intracellular Zn-Aβ_1-42 complexes. Aβ_1-42-induced intracellular Zn²⁺ toxicity is accelerated with aging because of age-related increase in extracellular Zn²⁺. The recent findings suggest that Aβ_1-42 secreted continuously from neuron terminals causes age-related cognitive decline and neurodegeneration via intracellular Zn²⁺ dysregulation. On the other hand, metallothioneins (MTs), zinc-binding proteins, quickly serve for intracellular Zn²⁺-buffering under acute intracellular Zn²⁺ dysregulation. On the basis of the idea that the defense strategy against Aβ_1-42-induced pathogenesis leads to preventing the AD development, this review deals with extracellular Zn²⁺-dependent Aβ_1-42 neurotoxicity, which is accelerated with aging.

Calcium Export from Neurons and Multi-Kinase Signaling Cascades Contribute to Ouabain Neuroprotection in Hyperhomocysteinemia

Pathological homocysteine (HCY) accumulation in the human plasma, known as hyperhomocysteinemia, exacerbates neurodegenerative diseases because, in the brain, this amino acid acts as a persistent N-methyl-d-aspartate receptor agonist. We studied the effects of 0.1–1 nM ouabain on intracellular Ca²⁺ signaling, mitochondrial inner membrane voltage (φ_mit), and cell viability in primary cultures of rat cortical neurons in glutamate and HCY neurotoxic insults. In addition, apoptosis-related protein expression and the involvement of some kinases in ouabain-mediated effects were evaluated. In short insults, HCY was less potent than glutamate as a neurotoxic agent and induced a 20% loss of φ_mit, whereas glutamate caused a 70% decrease of this value. Subnanomolar ouabain exhibited immediate and postponed neuroprotective effects on neurons. (1) Ouabain rapidly reduced the Ca²⁺ overload of neurons and loss of φ_mit evoked by glutamate and HCY that rescued neurons in short insults. (2) In prolonged 24 h excitotoxic insults, ouabain prevented neuronal apoptosis, triggering proteinkinase A and proteinkinase C dependent intracellular neuroprotective cascades for HCY, but not for glutamate. We, therefore, demonstrated here the role of PKC and PKA involving pathways in neuronal survival caused by ouabain in hyperhomocysteinemia, which suggests existence of different appropriate pharmacological treatment for hyperhomocysteinemia and glutamate excitotoxicity.

New insight into Parkinson's disease pathogenesis from reactive oxygen species-mediated extracellular Zn2+ influx

BACKGROUND

Parkinson's disease (PD) is the common neurodegenerative disorder in the elderly characterized by motor symptoms such as tremors, which is caused by selective loss of nigral dopaminergic neurons. Oxidative stress induced by the auto-oxidation of dopamine has been implicated as a key cause of the selective loss of dopaminergic neurons.

METHODS

To understand the selective loss of nigral dopaminergic neurons, the PD pathogenesis is reviewed focused on paraquat (PQ) and 6-hydroxydopamine (6-OHDA)-induced PD in rats.

RESULTS

Reactive oxygen species (ROS), which are produced by PQ and 6-OHDA, are retrogradely transported to presynaptic glutamatergic neuron terminals. ROS activate presynaptic transient receptor potential melastatin 2 (TRPM2) cation channels and induce extracellular glutamate accumulation in the substantia nigra pars compacta (SNpc), followed by age-related intracellular Zn2+ dysregulation. Loss of nigral dopaminergic neurons is accelerated by age-related intracellular Zn2+ dysregulation in the SNpc of rat PD models. The intracellular Zn2+ dysregulation in nigral dopaminergic neurons is linked with the rapid influx of extracellular Zn2+ via postsynaptic AMPA receptor activation, suggesting that PQ- and 6-OHDA-induced pathogenesis is linked with age-related intracellular Zn2+ dysregulation in the SNpc. Postsynaptic TRPM2 channels may be also involved in intracellular Zn2+ dysregulation in the SNpc.

CONCLUSION

A novel mechanism of nigral dopaminergic degeneration, in which ROS induce rapid intracellular Zn2+ dysregulation, figures out the PD pathogenesis induced by PQ and 6-OHDA in rats. This review deals with new insight into PD pathogenesis from ROS-mediated extracellular Zn2+ influx and its proposed defense strategy.

DR-region of Na+/K+ ATPase is a target to treat excitotoxicity and stroke

Na⁺/K⁺ ATPase (NKA) is important in maintaining cellular functions. We found that loss of NKA activities in NKAα1^+/− mice is associated with increased susceptibility to ischemic injuries following transient middle cerebral artery occlusion (tMCAO). This is corroborated by the neuroprotective effects of an antibody raised against an extracellular DR region (⁸⁹⁷DVEDSYGQQWTYEQR⁹¹¹, sequence number as in rat) of NKAα subunit (DR-Ab) in both preventive and therapeutic settings. DR-Ab protects cortical neurons against glutamate-induced toxicity by stimulating activities of NKA and Na⁺/Ca²⁺ exchanger (NCX), which resulted in accelerated Ca²⁺ extrusion. DR-Ab also enhanced the association between NKA and GluR2 and therefore reduced the internalization of both proteins from membrane induced by glutamate toxicity. The mechanism appears to involve suppression of GluR2 phosphorylation through PKCα/PICK pathway. Our data indicate that DR-region of NKA may be a novel therapeutic target for drug development for the treatment of ischemic stroke.

Calcium-Dependent Desensitization of NMDA Receptors

Glutamate receptors play the key role in excitatory synaptic transmission in the central nervous system (CNS). N-methyl-D-aspartate-activated glutamate receptors (NMDARs) are ion channels permeable to sodium, potassium, and calcium ions that localize to the pre- and postsynaptic membranes, as well as extrasynaptic neuronal membrane. Calcium entry into dendritic spines is essential for long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission. Both LTP and LTD represent morphological and functional changes occurring in the process of memory formation. NMDAR dysfunction is associated with epilepsy, schizophrenia, migraine, dementia, and neurodegenerative diseases. Prolonged activation of extrasynaptic NMDARs causes calcium overload and apoptosis of neurons. Here, we review recent findings on the molecular mechanisms of calcium-dependent NMDAR desensitization that ensures fast modulation of NMDAR conductance in the CNS and limits calcium entry into the cells under pathological conditions. We present the data on molecular determinants related to calcium-dependent NMDAR desensitization and functional interaction of NMDARs with other ion channels and transporters. We also describe association of NMDARs with lipid membrane microdomains.

Downregulation of calcium-dependent NMDA receptor desensitization by sodium-calcium exchangers: a role of membrane cholesterol

BACKGROUND

The plasma membrane Na+/Ca2+-exchanger (NCX) has recently been shown to regulate Ca2+-dependent N-methyl-D-aspartate receptor (NMDAR) desensitization, suggesting a tight interaction of NCXs and NMDARs in lipid nanoclasters or "rafts". To evaluate possible role of this interaction we studied effects of Li+ on NMDA-elicited whole-cell currents and Ca2+ responses of rat cortical neurons in vitro before and after cholesterol extraction by methyl-β-cyclodextrin (MβCD).

RESULTS

Substitution Li+ for Na+ in the external solution caused a concentration-dependent decrease of steady-state NMDAR currents from 440 ± 71 pA to 111 ± 29 pA in 140 mM Na+ and 140 mM Li+, respectively. The Li+ inhibition of NMDAR currents disappeared in the absence of Ca2+ in the external solution (Ca2+-free), suggesting that Li+ enhanced Ca2+-dependent NMDAR desensitization. Whereas the cholesterol extraction with MβCD induced a decrease of NMDAR currents to 136 ± 32 pA in 140 mM Na+ and 46 ± 15 pA in 140 mM Li+, the IC50 values for the Li+ inhibition were similar (about 44 mM Li+) before and after this procedure. In the Ca2+-free Na+ solution the steady-state NMDAR currents after the cholesterol extraction were 47 ± 6% of control values. Apparently this amplitude decrease was not Ca2+-dependent. In the Na+ solution containing 1 mM Ca2+ the Ca2+-dependent NMDAR desensitization was greater when cholesterol was extracted. Obviously, this procedure promoted its development. In agreement, Li+ and KB-R7943, an inhibitor of NCX, both considerably reduced NMDA-activated Ca2+ responses. The cholesterol extraction itself caused a decrease of NMDA-activated Ca2+ responses and, in addition, abolished the effects of Li+ and KB-R7943. The cholesterol loading into the plasma membrane caused a recovery of the KB-R7943 effects.

CONCLUSIONS

Taken together our data suggest that NCXs downregulate the Ca2+-dependent NMDAR desensitization. Most likely, this is determined by a tight functional interaction of NCX and NMDAR molecules because of their co-localization in membrane lipid rafts. The destruction of these rafts is accompanied by an enhancement of NMDAR desensitization and a loss of NCX-selective agent effects on NMDARs.

The Impact of Synaptic Zn2+ Dynamics on Cognition and Its Decline

The basal levels of extracellular Zn²⁺ are in the range of low nanomolar concentrations and less attention has been paid to Zn²⁺, compared to Ca²⁺, for synaptic activity. However, extracellular Zn²⁺ is necessary for synaptic activity. The basal levels of extracellular zinc are age-dependently increased in the rat hippocampus, implying that the basal levels of extracellular Zn²⁺ are also increased age-dependently and that extracellular Zn²⁺ dynamics are linked with age-related cognitive function and dysfunction. In the hippocampus, the influx of extracellular Zn²⁺ into postsynaptic neurons, which is often linked with Zn²⁺ release from neuron terminals, is critical for cognitive activity via long-term potentiation (LTP). In contrast, the excess influx of extracellular Zn²⁺ into postsynaptic neurons induces cognitive decline. Interestingly, the excess influx of extracellular Zn²⁺ more readily occurs in aged dentate granule cells and intracellular Zn²⁺-buffering, which is assessed with ZnAF-2DA, is weakened in the aged dentate granule cells. Characteristics (easiness) of extracellular Zn²⁺ influx seem to be linked with the weakened intracellular Zn²⁺-buffering in the aged dentate gyrus. This paper deals with the impact of synaptic Zn²⁺ signaling on cognition and its decline in comparison with synaptic Ca²⁺ signaling.

The pump, the exchanger, and the holy spirit: origins and 40-year evolution of ideas about the ouabain-Na+ pump endocrine system

Two prescient 1953 publications set the stage for the elucidation of a novel endocrine system: Schatzmann's report that cardiotonic steroids (CTSs) are all Na⁺ pump inhibitors, and Szent-Gyorgi's suggestion that there is an endogenous "missing screw" in heart failure that CTSs like digoxin may replace. In 1977 I postulated that an endogenous Na⁺ pump inhibitor acts as a natriuretic hormone and simultaneously elevates blood pressure (BP) in salt-dependent hypertension. This hypothesis was based on the idea that excess renal salt retention promoted the secretion of a CTS-like hormone that inhibits renal Na⁺ pumps and salt reabsorption. The hormone also inhibits arterial Na⁺ pumps, elevates myocyte Na⁺ and promotes Na/Ca exchanger-mediated Ca²⁺ gain. This enhances vasoconstriction and arterial tone-the hallmark of hypertension. Here I describe how those ideas led to the discovery that the CTS-like hormone is endogenous ouabain (EO), a key factor in the pathogenesis of hypertension and heart failure. Seminal observations that underlie the still-emerging picture of the EO-Na⁺ pump endocrine system in the physiology and pathophysiology of multiple organ systems are summarized. Milestones include: 1) cloning the Na⁺ pump isoforms and physiological studies of mutated pumps in mice; 2) discovery that Na⁺ pumps are also EO-triggered signaling molecules; 3) demonstration that ouabain, but not digoxin, is hypertensinogenic; 4) elucidation of EO's roles in kidney development and cardiovascular and renal physiology and pathophysiology; 5) discovery of "brain ouabain", a component of a novel hypothalamic neuromodulatory pathway; and 6) finding that EO and its brain receptors modulate behavior and learning.

In vitro and in vivo physiology of low nanomolar concentrations of Zn2+ in artificial cerebrospinal fluid

Artificial cerebrospinal fluid (ACSF), i.e., brain extracellular medium, which includes Ca²⁺ and Mg²⁺, but not other divalent cations such as Zn²⁺, has been used for in vitro and in vivo experiments. The present study deals with the physiological significance of extracellular Zn²⁺ in ACSF. Spontaneous presynaptic activity is suppressed in the stratum lucidum of brain slices from young rats bathed in ACSF containing 10 nM ZnCl₂, indicating that extracellular Zn²⁺ modifies hippocampal presynaptic activity. To examine the in vivo action of 10 nM ZnCl₂ on long-term potentiation (LTP), the recording region was perfused using a recording electrode attached to a microdialysis probe. The magnitude of LTP was not modified in young rats by perfusion with ACSF containing 10 nM ZnCl₂, compared to perfusion with ACSF without Zn²⁺, but attenuated by perfusion with ACSF containing 100 nM ZnCl₂. Interestingly, the magnitude of LTP was not modified in aged rats even by perfusion with ACSF containing 100 nM ZnCl₂, but enhanced by perfusion with ACSF containing 10 mM CaEDTA, an extracellular Zn²⁺ chelator. The present study indicates that the basal levels of extracellular Zn²⁺, which are in the range of low nanomolar concentrations, are critical for synaptic activity and perhaps increased age-dependently.

Downregulation of Purkinje Cell Activity by Modulators of Small Conductance Calcium-Activated Potassium Channels In Rat Cerebellum

Small-conductance calcium-activated potassium channels (SK channels) are widely expressed in CNS tissues. Their functions, however, have not been well studied. Participation of SK channels in Purkinje cell (PC) pacemaker activity has been studied predominantly in vitro. Here we studied for the first time the effects of SK channel activation by NS309 or CyPPA on the PC simple spike frequency in vivo in adult (3 - 6 months) and aged (22 - 28 months) rats using extracellular microelectrode recordings. Both pharmacological agents caused a statistically significant decrease in the PC simple spike frequency. The maximum value of the decrease in the simple spike frequency did not depend on age, whereas a statistically significant inhibition of the spike frequency was achieved faster in aged animals than in adult ones. In experiments on cultured neurons PCs were identified by the expression of calbindin as the PC-specific marker. Registration of transmembrane currents in cerebellar neurons revealed the direct action of NS309 and CyPPA on the SK channels of PC consisted in the enhancement of outward potassium currents and action potential after-hyperpolarization. Thus, SK channel activators can compensate for age-related changes of the autorhythmic functions of the cerebellum.

The Influence of Na(+), K(+)-ATPase on Glutamate Signaling in Neurodegenerative Diseases and Senescence

Decreased Na(+), K(+)-ATPase (NKA) activity causes energy deficiency, which is commonly observed in neurodegenerative diseases. The NKA is constituted of three subunits: α, β, and γ, with four distinct isoforms of the catalytic α subunit (α1-4). Genetic mutations in the ATP1A2 gene and ATP1A3 gene, encoding the α2 and α3 subunit isoforms, respectively can cause distinct neurological disorders, concurrent to impaired NKA activity. Within the central nervous system (CNS), the α2 isoform is expressed mostly in glial cells and the α3 isoform is neuron-specific. Mutations in ATP1A2 gene can result in familial hemiplegic migraine (FHM2), while mutations in the ATP1A3 gene can cause Rapid-onset dystonia-Parkinsonism (RDP) and alternating hemiplegia of childhood (AHC), as well as the cerebellar ataxia, areflexia, pescavus, optic atrophy and sensorineural hearing loss (CAPOS) syndrome. Data indicates that the central glutamatergic system is affected by mutations in the α2 isoform, however further investigations are required to establish a connection to mutations in the α3 isoform, especially given the diagnostic confusion and overlap with glutamate transporter disease. The age-related decline in brain α2∕3 activity may arise from changes in the cyclic guanosine monophosphate (cGMP) and cGMP-dependent protein kinase (PKG) pathway. Glutamate, through nitric oxide synthase (NOS), cGMP and PKG, stimulates brain α2∕3 activity, with the glutamatergic N-methyl-D-aspartate (NMDA) receptor cascade able to drive an adaptive, neuroprotective response to inflammatory and challenging stimuli, including amyloid-β. Here we review the NKA, both as an ion pump as well as a receptor that interacts with NMDA, including the role of NKA subunits mutations. Failure of the NKA-associated adaptive response mechanisms may render neurons more susceptible to degeneration over the course of aging.

Cardiotonic Steroids as Modulators of Neuroinflammation

Cardiotonic steroids (CTS) are a class of specific ligands of the Na(+), K(+)- ATPase (NKA). NKA is a P-type ATPase that is ubiquitously expressed and although well known to be responsible for the maintenance of the cell electrochemical gradient through active transport, NKA can also act as a signal transducer in the presence of CTS. Inflammation, in addition to importantly driving organism defense and survival mechanisms, can also modulate NKA activity and memory formation, as well as being relevant to many chronic illnesses, neurodegenerative diseases, and mood disorders. The aim of the current review is to highlight the recent advances as to the role of CTS and NKA in inflammatory process, with a particular focus in the central nervous system.

Inhibition of Plasma Membrane Na/Ca-Exchanger by KB-R7943 or Lithium Reveals Its Role in Ca-Dependent N-methyl-d-aspartate Receptor Inactivation

To evaluate the possible role of the plasma membrane Na(+)/Ca(2+)-exchanger (NCX) in regulation of N-methyl-d-aspartate (NMDA) receptors (NMDARs), we studied effects of 2-[2-[4-(4-nitrobenzyloxy) phenyl]ethyl]isothiourea methanesulfonate (KB-R7943; KBR) and lithium (inhibitors of NCX) on NMDA-elicited whole-cell currents using the patch-clamp technique on rat cortical neurons and human embryonic kidney 293T cells expressing recombinant NMDARs. KBR inhibited NMDAR currents in a voltage-independent manner with similar potency for receptors of GluN1/2A and GluN1/2B subunit compositions that excludes open-channel block and GluN2B-selective inhibition. The inhibition by KBR depended on glycine (Gly) concentration. At 30 μM NMDA, the KBR IC50 values were 5.3 ± 0.1 and 41.2 ± 8.8 μM for 1 and 300 μM Gly, respectively. Simultaneous application of NMDA + KBR in the absence of Gly induced robust inward NMDAR currents that peaked and then rapidly decreased. KBR, therefore, is an agonist (EC50 is 1.18 ± 0.16 µM) of the GluN1 subunit coagonist binding sites. The decrease of NMDA-elicited currents in the presence of KBR was abolished in Ca(2+)-free solution and was not observed in the presence of extracellular Ca(2+) on 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-loaded neurons, suggesting that Ca(2+) affects NMDARs from the cytosol. In agreement, the substitution of Li(+) for extracellular Na(+) caused a considerable decrease of NMDAR currents, which was not observed in the absence of extracellular Ca(2+). Most likely, the accumulation of intracellular Ca(2+) is caused by the inhibition of Ca(2+) extrusion via NCX. Thus, KBR and Li(+) provoke Ca(2+)-dependent receptor inactivation due to the disruption of Ca(2+) extrusion by the NCX. The data reveal the role of NCX in regulation of Ca(2+)-dependent inactivation of NMDARs.

The role of glia in stress: polyamines and brain disorders

This review focuses on the roles of glia and polyamines (PAs) in brain function and dysfunction, highlighting how PAs are one of the principal differences between glia and neurons. The novel role of PAs, such as putrescine, spermidine, and spermine and their precursors and derivatives, is discussed. However, PAs have not yet been a focus of much glial research. They affect many neuronal and glial receptors, channels, and transporters. They are therefore key elements in the development of many diseases and syndromes, thus forming the rationale for PA-focused and glia-focused therapy for these conditions.

The tricyclic antidepressant desipramine inhibited the neurotoxic, kainate-induced [Ca(2+)]i increases in CA1 pyramidal cells in acute hippocampal slices

Kainate (KA), used for modelling neurodegenerative diseases, evokes excitotoxicity. However, the precise mechanism of KA-evoked [Ca(2+)]i increase is unexplored, especially in acute brain slice preparations. We used [Ca(2+)]i imaging and patch clamp electrophysiology to decipher the mechanism of KA-evoked [Ca(2+)]i rise and its inhibition by the tricyclic antidepressant desipramine (DMI) in CA1 pyramidal cells in rat hippocampal slices and in cultured hippocampal cells. The effect of KA was dose-dependent and relied totally on extracellular Ca(2+). The lack of effect of dl-2-amino-5-phosphonopentanoic acid (AP-5) and abolishment of the response by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) suggested the involvement of non-N-methyl-d-aspartate receptors (non-NMDARs). The predominant role of the Ca(2+)-impermeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) in the initiation of the Ca(2+) response was supported by the inhibitory effect of the selective AMPAR antagonist GYKI 53655 and the ineffectiveness of 1-naphthyl acetylspermine (NASPM), an inhibitor of the Ca(2+)-permeable AMPARs. The voltage-gated Ca(2+) channels (VGCC), blocked by ω-Conotoxin MVIIC+nifedipine+NiCl2, contributed to the [Ca(2+)]i rise. VGCCs were also involved, similarly to AMPAR current, in the KA-evoked depolarisation. Inhibition of voltage-gated Na(+) channels (VGSCs; tetrodotoxin, TTX) did not affect the depolarisation of pyramidal cells but blocked the depolarisation-evoked action potential bursts and reduced the Ca(2+) response. The tricyclic antidepressant DMI inhibited the KA-evoked [Ca(2+)]i rise in a dose-dependent manner. It directly attenuated the AMPA-/KAR current, but its more potent inhibition on the Ca(2+) response supports additional effect on VGCCs, VGSCs and Na(+)/Ca(2+) exchangers. The multitarget action on decisive players of excitotoxicity holds out more promise in clinical therapy of neurodegenerative diseases.