Glutamate-induced loss of Ca2+/calmodulin-dependent protein kinase II activity in cultured rat hippocampal neurons

Calcium/Calmodulin-Dependent Kinases in the Hypothalamus, Pituitary, and Pineal Gland: An Overview

We review the literature on the little-known roles of specific CaMKs in regulating endocrine functions of the pineal gland, the pituitary gland, and the hypothalamus. Melatonin activates hippocampal CaMKII, which then influences dendritogenesis. In the pituitary gland, the signal pathways activated by the CaMK in lower vertebrates, such as fishes, differ from those of mammals. In the teleost anterior pituitary, the activation of CaMKII induces the expression of somatolactin by glucagon b. In rats and humans, CaMKIVs have been associated with gonadotropes and thyrotropes and CaMKII with several types of human tumor cells and with a specific signaling pathway. Neuropeptides such as vasopressin and endothelin are also involved in the CaMKII signaling chain, as is the CaMKIIδ isoform which participates in generating the circadian rhythms of the suprachiasmatic nucleus. What arises from this review is that most of the hypothalamic CaMKs are involved in activities of the endocrine brain. Furthermore, among the CaMKs, type II occurs with the highest frequency followed by CaMKIV and CaMKI.

Neuroprotective hypothermia - Why keep your head cool during ischemia and reperfusion

BACKGROUND

Targeted temperature management (TTM) is the induced cooling of the entire body or specific organs to help prevent ischemia and reperfusion (I/R) injury, as may occur during major surgery, cardiac resuscitation, traumatic brain injury and stroke. Ischemia and reperfusion induce neuronal damage by mitochondrial dysfunction and oxidative injury, ER stress, neuronal excitotoxicity, and a neuroinflammatory response, which may lead to activation of apoptosis pathways.

SCOPE OF REVIEW

The aim of the current review is to discuss TTM targets that convey neuroprotection and to identify potential novel pharmacological intervention strategies for the prevention of cerebral ischemia and reperfusion injury.

MAJOR CONCLUSIONS

TTM precludes I/R injury by reducing glutamate release and oxidative stress and inhibiting release of pro-inflammatory factors and thereby counteracts mitochondrial induced apoptosis, neuronal excitotoxicity, and neuroinflammation. Moreover, TTM promotes regulation of the unfolded protein response and induces SUMOylation and the production of cold shock proteins. These advantageous effects of TTM seem to depend on the clinical setting, as well as type and extent of the injury. Therefore, future aims should be to refine hypothermia management in order to optimize TTM utilization and to search for pharmacological agents mimicking the cellular effects of TTM.

GENERAL SIGNIFICANCE

Bundling knowledge about TTM in the experimental, translational and clinical setting may result in better approaches for diminishing I/R damage. While application of TTM in the clinical setting has some disadvantages, targeting its putative protective pathways may be useful to prevent I/R injury and reduce neurological complications.

Differential expression of CaMKII isoforms and overall kinase activity in rat dorsal root ganglia after injury

Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) decodes neuronal activity by translating cytoplasmic Ca(2+) signals into kinase activity that regulates neuronal functions including excitability, gene expression, and synaptic transmission. Four genes lead to developmental and differential expression of CaMKII isoforms (α, β, γ, δ). We determined mRNA levels of these isoforms in the dorsal root ganglia (DRG) of adult rats with and without nerve injury in order to determine if differential expression of CaMKII isoforms may contribute to functional differences that follow injury. DRG neurons express mRNA for all four isoforms, and the relative abundance of CaMKII isoforms was γ>α>β=δ, based on the CT values. Following ligation of the 5th lumbar (L5) spinal nerve (SNL), the β isoform did not change, but mRNA levels of both the γ and α isoforms were reduced in the directly injured L5 neurons, and the α isoform was reduced in L4 neurons, compared to their contemporary controls. In contrast, expression of the δ isoform mRNA increased in L5 neurons. CaMKII protein decreased following nerve injury in both L4 and L5 populations. Total CaMKII activity measured under saturating Ca(2+)/CaM conditions was decreased in both L4 and L5 populations, while autonomous CaMKII activity determined in the absence of Ca(2+) was selectively reduced in axotomized L5 neurons 21days after injury. Thus, loss of CaMKII signaling in sensory neurons after peripheral nerve injury may contribute to neuronal dysfunction and pain.

Involvement of AMPA receptors in CSD-induced impairment of LTP in the hippocampus

OBJECTIVE

To investigate the alteration of hippocampal long-term plasticity and basal synaptic transmission induced by repetitive cortical spreading depressions (CSDs).

BACKGROUND

There is a relationship between migraine aura and amnesia attack. CSD, a state underlying migraine attacks, may be responsible for hippocampus-related symptoms. However, the precise role of CSD on hippocampal activity has not been investigated.

METHODS

Male Wistar rats were divided into CSD and control groups. Repetitive CSDs were induced in vivo by topical application of solid KCl. Forty-five minutes later, the ipsilateral hippocampus was removed, and hippocampal slices were prepared for a series of electrophysiological studies.

RESULTS

Repetitive CSDs led to a decrease in the magnitude of long-term potentiation in the hippocampus. CSD also reduced hippocampal synaptic efficacy, as shown by a reduction in post-synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor responses. In contrast, the post-synaptic N-methyl-d-aspartate receptor responses remained unchanged. In addition, there were no changes in paired-pulse profiles between the groups, indicating that CSD did not induce any presynaptic alterations.

CONCLUSION

These findings suggest that a reduction of post-synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor responses is the mechanism responsible for impaired hippocampal long-term potentiation induced by CSD.

Excitotoxic neuroprotection and vulnerability with CaMKII inhibition

Aberrant calcium signaling is a common feature of ischemia and multiple neurodegenerative diseases. While activation of calcium-calmodulin (CaM)-dependent protein kinase II (CaMKII) is a key event in calcium signaling, its role in excitotoxicity is controversial. Our findings demonstrate neuroprotection in neuronal cultures treated with the small molecule (KN-93) and peptide (tat-AIP and tat-CN21) inhibitors of CaMKII immediately prior to excitotoxic glutamate/glycine insult. Unlike KN-93 which blocks CaMKII activation, but not constitutively active forms of CaMKII, tat-CN21 and tat-AIP significantly reduced excitotoxicity in cultured neurons when applied post-insult. We observed that the neuroprotective effects of tat-CN21 are greatest when applied before the toxic glutamate challenge and diminish with time, with the neuroprotection associated with CaMKII inhibition diminishing back to control 3h post glutamate insult. Mechanistically, tat-CN21 inhibition of CaMKII resulted in an increase in CaMKII activity and the percentage of soluble αCaMKII observed in neuronal lysates 24h following glutamate stimulation. To address the impact of prolonged CaMKII inhibition prior to excitotoxic insult, neuronal cultures were treated with CaMKII inhibitors overnight and then subjected to a sub-maximal excitotoxic insult. In this model, CaMKII inhibition prior to insult exacerbated neuronal death, suggesting that a loss of CaMKII enhances neuronal vulnerability to glutamate. Although changes in αCaMKII or NR2B protein levels are not responsible for this enhanced glutamate vulnerability, this process is blocked by the protein translation inhibitor cycloheximide. In total, the neuroprotection afforded by CaMKII inhibition can be seen as neuroprotective immediately surrounding the excitotoxic insult, whereas sustained CaMKII inhibition produced by excitotoxicity leads to neuronal death by enhancing neuronal vulnerability to glutamate.

In vivo NMDA receptor activation accelerates motor unit maturation, protects spinal motor neurons, and enhances SMN2 gene expression in severe spinal muscular atrophy mice

Spinal muscular atrophy (SMA), a lethal neurodegenerative disease that occurs in childhood, is caused by the misexpression of the survival of motor neuron (SMN) protein in motor neurons. It is still unclear whether activating motor units in SMA corrects the delay in the postnatal maturation of the motor unit resulting in an enhanced neuroprotection. In the present work, we demonstrate that an adequate NMDA receptor activation in a type 2 SMA mouse model significantly accelerated motor unit postnatal maturation, counteracted apoptosis in the spinal cord, and induced a marked increase of SMN expression resulting from a modification of SMN2 gene transcription pattern. These beneficial effects were dependent on the level of NMDA receptor activation since a treatment with high doses of NMDA led to an acceleration of the motor unit maturation but favored the apoptotic process and decreased SMN expression. In addition, these results suggest that the NMDA-induced acceleration of motor unit postnatal maturation occurred independently of SMN. The NMDA receptor activating treatment strongly extended the life span in two different mouse models of severe SMA. The analysis of the intracellular signaling cascade that lay downstream the activated NMDA receptor revealed an unexpected reactivation of the CaMKII/AKT/CREB (cAMP response element-binding protein) pathway that induced an enhanced SMN expression. Therefore, pharmacological activation of spinal NMDA receptors could constitute a useful strategy for both increasing SMN expression and limiting motor neuron death in SMA spinal cord.

Nobiletin improves brain ischemia-induced learning and memory deficits through stimulation of CaMKII and CREB phosphorylation

Decreased cerebral blood flow causes cognitive impairments and neuronal injury in the progressive age-related neurodegenerative disorders such as Alzheimer's disease (AD) and vascular dementia. In the present study, we for the first time found that nobiletin, a novel leading compound for AD therapy, improved cerebral ischemia-induced memory deficits in vivo. Treatment with 50 mg/kg of nobiletin (i.p.) for the consecutive 7 days before and after brain ischemia significantly inhibited delayed neuronal death in the hippocampal CA1 neurons in a 20-min bilateral common carotid arteries occlusion (BCCAO) ischemia. However, the contextual memory assessed by passive avoidance task was not improved. On the other hand, a 5-min BCCAO-induced contextual memory deficit was significantly improved by the nobiletin treatment. In the 5-min BCCAO mice, Western blot analysis evidently showed that the levels of synaptic proteins, including calcium/calmodulin-dependent protein kinase II (CaMKII), microtubule-associated protein 2 (MAP2) and glutamate receptor 1 (GluR1), significantly decreased in the hippocampal CA1 region. The nobiletin treatment prevented the reduction in CaMKII, MAP2 and GluR1 protein levels in the hippocampal CA1 region, accompanied by restoration of both ERK and CREB phosphorylation and CaMKII autophosphorylation. Consistent with the restored CaMKII and ERK phosphorylation, an electrophysiological study showed that the impaired hippocampal long-term potentiation (LTP) observed in the 5-min ischemic mice was significantly improved by the nobiletin treatment. These findings suggest that the activation of CaMKII and ERK signaling in part mediates improvement of ischemia-induced learning and memory deficits by nobiletin.

The relationship between neurotrophic factors and CaMKII in the death and survival of retinal ganglion cells

The scientific discourse relating to the causes and treatments for glaucoma are becoming reflective of the need to protect and preserve retinal neurons from degenerative changes, which result from the injurious environment associated with this disease. Knowledge, in particular, of the signal transduction pathways which affect death and survival of the retinal ganglion cells is critical to this discourse and to the development of a suitable neurotherapeutic strategy for this disease. The goal of this chapter is to review what is known of the chief suspects involved in initiating the cell death/survival pathways in these cells, and what still remains to be uncovered. The least controversial aspect of the subject relates to the potential role of neurotrophic factors in the protection of the retinal ganglion cells. On the other hand, the postulated triggers for signaling cell death in glaucoma remain controversial. Certainly, the restricted flow of neurotrophic factors has been cited as one possible trigger. However, the connections between glaucoma and other factors present in the retina, such as glutamate, long held to be a prospective culprit in retinal ganglion cell death are still being questioned. Whatever the outcome of this particular debate, it is clear that the downstream intersections between the cell death and survival pathways should provide important foci for future studies whose goal is to protect retinal neurons, situated as they are, in the stressful environment of a cell destroying disease. The evidence for CaMKII being one of these intersecting points is discussed.

Leptin facilitates learning and memory performance and enhances hippocampal CA1 long-term potentiation and CaMK II phosphorylation in rats

Leptin, an adipocytokine encoded by an obesity gene and expressed in adipose tissue, affects feeding behavior, thermogenesis, and neuroendocrine status via leptin receptors distributed in the brain, especially in the hypothalamus. Leptin may also modulate the synaptic plasticity and behavioral performance related to learning and memory since: leptin receptors are found in the hippocampus, and both leptin and its receptor share structural and functional similarities with the interleukin-6 family of cytokines that modulate long-term potentiation (LTP) in the hippocampus. We therefore examined the effect of leptin on (1) behavioral performance in emotional and spatial learning tasks, (2) LTP at Schaffer collateral-CA1 synapses, (3) presynaptic and postsynaptic activities in hippocampal CA1 neurons, (4) the intracellular Ca(2+) concentration ([Ca(2+)](i)) in CA1 neurons, and (5) the activity of Ca(2+)/calmodulin protein kinase II (CaMK II) in the hippocampal CA1 tissue that exhibits LTP. Intravenous injection of 5 and/or 50mug/kg, but not of 500mug/kg leptin, facilitated behavioral performance in passive avoidance and Morris water-maze tasks. Bath application of 10(-12)M leptin in slice experiments enhanced LTP and increased the presynaptic transmitter release, whereas 10(-10)M leptin suppressed LTP and reduced the postsynaptic receptor sensitivity to N-methyl-d-aspartic acid. The increase in the [Ca(2+)](i) induced by 10(-10)M leptin was two times greater than that induced by 10(-12)M leptin. In addition, the facilitation (10(-12)M) and suppression (10(-10)M) of LTP by leptin was closely associated with an increase and decrease in Ca(2+)-independent activity of CaMK II. Our results show that leptin not only affects hypothalamic functions (such as feeding, thermogenesis, and neuroendocrine status), but also modulates higher nervous functions, such as the behavioral performance related to learning and memory and hippocampal synaptic plasticity.

Enhanced activation of Ca2+/calmodulin-dependent protein kinase II upon downregulation of cyclin-dependent kinase 5-p35

Cyclin-dependent kinase 5 (Cdk5)-p35 is downregulated in cultured neurons by N-methyl-D-aspartate (NMDA) via the proteasomal degradation of p35. However, it is not known where in neurons this downregulation occurs or the physiologic meaning of the reaction. We show the enrichment of Cdk5 and p35 in the postsynaptic density and the NMDA-induced degradation of postsynaptic p35 using brain slices and cultured neurons. To evaluate the role of this downregulation, we examined the relationship between Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) activation and Cdk5 downregulation, as events downstream from NMDA stimulation. Glutamate or NMDA stimulation induced CaMKII autophosphorylation over a time course that mirrored the time course of p35 degradation. To simulate the downregulation of postsynaptic Cdk5 in invitro experiments, we used the Cdk5 inhibitor roscovitine. The inhibition of Cdk5 activity by roscovitine enhanced CaMKII autophosphorylation and activation in cultured neurons, and in an isolated postsynaptic-density-enriched fraction. These results suggest that Cdk5 activity suppresses CaMKII activation, and that the downregulation of Cdk5 activity after treatment withNMDA facilitates CaMKII activation, leading to the easier induction of long-term potentiation.

Cellular and molecular pathways of ischemic neuronal death

Three routes have been identified triggering neuronal death under physiological and pathological conditions. Excess activation of ionotropic glutamate receptors cause influx and accumulation of Ca2+ and Na+ that result in rapid swelling and subsequent neuronal death within a few hours. The second route is caused by oxidative stress due to accumulation of reactive oxygen and nitrogen species. Apoptosis or programmed cell death that often occurs during developmental process has been coined as additional route to pathological neuronal death in the mature nervous system. Evidence is being accumulated that excitotoxicity, oxidative stress, and apoptosis propagate through distinctive and mutually exclusive signal transduction pathway and contribute to neuronal loss following hypoxic-ischemic brain injury. Thus, the therapeutic intervention of hypoxic-ischemic neuronal injury should be aimed to prevent excitotoxicity, oxidative stress, and apoptosis in a concerted way.

Phosphorylation of proteins involved in activity-dependent forms of synaptic plasticity is altered in hippocampal slices maintained in vitro

The acute hippocampal slice preparation has been widely used to study the cellular mechanisms underlying activity-dependent forms of synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD). Although protein phosphorylation has a key role in LTP and LTD, little is known about how protein phosphorylation might be altered in hippocampal slices maintained in vitro. To begin to address this issue, we examined the effects of slicing and in vitro maintenance on phosphorylation of six proteins involved in LTP and/or LTD. We found that AMPA receptor (AMPAR) glutamate receptor 1 (GluR1) subunits are persistently dephosphorylated in slices maintained in vitro for up to 8 h. alpha calcium/calmodulin-dependent kinase II (alphaCamKII) was also strongly dephosphorylated during the first 3 h in vitro but thereafter recovered to near control levels. In contrast, phosphorylation of the extracellular signal-regulated kinase ERK2, the ERK kinase MEK, proline-rich tyrosine kinase 2 (Pyk2), and Src family kinases was significantly, but transiently, increased. Electrophysiological experiments revealed that the induction of LTD by low-frequency synaptic stimulation was sensitive to time in vitro. These findings indicate that phosphorylation of proteins involved in N-methyl-D-aspartate (NMDA) receptor-dependent forms of synaptic plasticity is altered in hippocampal slices and suggest that some of these changes can significantly influence the induction of LTD.

Reduced basal CAMKII levels in hippocampal CA1 region: Possible cause of stress-induced impairment of LTP in chronically stressed rats

Chronic psychosocial stress markedly reduces the expression of high-frequency stimulation (HFS)-evoked early long-term potentiation (LTP) in the CA1 region of the hippocampus of anesthetized rats. Immunoblotting was performed to determine changes in molecular levels of key signaling proteins that might be responsible for this inhibitory effect. Western blot analysis of the CA1 region demonstrates that chronic psychosocial stress decreases basal levels of calcium calmodulin kinase II (CaMKII), phosphorylated (P)-CaMKII, calmodulin, and protein kinase C (PKCgamma) while markedly increasing protein phosphatase 2B (calcineurin) levels. The decrease of basal levels of P-CaMKII may be triggered primarily by excessive dephosphorylation resulting from enhanced basal levels of calcineurin. The decline in the basal levels of the upstream molecules, PKCgamma and calmodulin may be a consequence of the diminished basal P-CaMKII levels. Analysis of signaling molecules in CA1 region of chronically stressed rat subjected to HFS in vivo showed only one difference compared to similarly stimulated control rats; no increase in P-CaMKII levels. Our results suggest that decreased P-CaMKII levels may be primarily responsible for the stress-induced reduction in LTP expression.

Chronic psychosocial stress decreases calcineurin in the dentate gyrus: a possible mechanism for preservation of early ltp

Chronic psychosocial stress impairs early long-term potentiation (LTP) in the hippocampal CA1 region but not in the dentate gyrus of anesthetized rats. Analysis of putative signaling molecules involved in the expression of LTP was performed to determine the possible reason(s) for the apparent resistance of the LTP of the dentate gyrus to chronic psychosocial stress. Immunoblotting was used to determine possible changes in the basal levels of various fractions of calcium-dependent calmodulin kinase II (CaMKII), phosphorylated CaMKII (P-CaMKII), calmodulin, protein kinase C gamma (PKCgamma) and calcineurin in the dentate gyrus of chronically stressed rats. Western blot analysis revealed that chronic stress significantly decreased the levels of the total CaMKII without affecting P-CaMKII levels. No significant change was detected in the levels of the upstream effectors, calmodulin and PKCgamma. However, chronic stress produced a significant decrease in calcineurin levels. The data suggest that the dentate gyrus of chronically stressed rats may have developed a compensatory mechanism whereby calcineurin levels are reduced to maintain normal P-CaMKII levels, which may be responsible for the normal early LTP of the dentate gyrus of chronically stressed rats. The results of this work will increase understanding of why certain brain regions are more resistant to deleterious effects of conditions that deteriorate learning and memory.

Hyperexcitability and changes in activities of Ca2+/calmodulin-dependent kinase II and mitogen-activated protein kinase in the hippocampus of rats exposed to 1-bromopropane

Chronic inhalation of 1-bromopropane (1-BP), a substitute of ozone-depleting chlorofluorocarbons, has been suspected of having central neurotoxicity (Clinical Neurology and Neurosurgery 101 (1999) 199; Journal of Occupational Health 44 (2002) 1) for humans. In animal experiments, 1-BP inhalation (1500 ppm) caused hyperexcitability in the CA1 and the dentate gyrus (DG) [Journal of Occupational Health 42 (2000) 149, Journal of Occupational Health 44 (2002) 156]. We studied whether the hyperexcitability is associated with changes of Ca2+/calmodulin-dependent kinase II (CaMKII), mitogen-activated protein kinase (MAPK), and protein kinase C (PKC). Male Wistar rats were exposed to 1-BP for 6 hours in a day in an exposure chamber with a concentration of 700 ppm for 8 weeks. After the inhalation, paired-pulse ratios of field excitatory postsynaptic potentials and population spikes (PSs) were analyzed in the CA1 and DG of hippocampal slices. Control rats were then given fresh air in the inhalation chamber. Semiquantitative immunoblotting analyses of protein kinases using antibodies against active and conventional protein kinases were done using the whole hippocampus. A paired-pulse ratio of PS was increased at the 5 ms interpulse interval in the CA1 and at the 10-20 ms interpulse intervals in the DG. The amount of active MAPK and total amount of CaMKIIalpha and beta were significantly increased by 28, 29, and 46% compared to control, respectively, without any change in PKC activity. In contrast, the amount of active CaMKIIbeta was decreased to 78%. These results suggest that modifications of intracellular signaling cascades are associated with hyperexcitability that occurred in the hippocampal formation of rats exposed to the chronic inhalation of 1-BP.

Impairment of long-term potentiation and spatial memory in leptin receptor-deficient rodents

Leptin is well known to be involved in the control of feeding, reproduction and neuroendocrine functions through its action on the hypothalamus. However, leptin receptors are found in brain regions other than the hypothalamus (including the hippocampus and cerebral cortex) suggesting extrahypothalamic functions. We investigated hippocampal long-term potentiation (LTP) and long-term depression (LTD), and the spatial-memory function in two leptin receptor-deficient rodents (Zucker rats and db/db mice). In brain slices, the CA1 hippocampal region of both strains showed impairments of LTP and LTD; leptin (10(-12) M) did not improve these impairments in either strain. These strains also showed lower basal levels of Ca(2+)/calmodulin-dependent protein kinase II activity in the CA1 region than the respective controls, and the levels did not respond to tetanic stimulation. These strains also showed impaired spatial memory in the Morris water-maze test (i.e. longer swim-path lengths during training sessions and less frequent crossings of the platform's original location in the probe test. From these results we suggest that the leptin receptor-deficient animals show impaired LTP in CA1 and poor spatial memory due, at least in part, to a deficiency in leptin receptors in the hippocampus.

Phosphorylation of neuronal nitric oxide synthase at Ser847 by CaM-KII in the hippocampus of rat brain after transient forebrain ischemia

The authors previously demonstrated that Ca2+/calmodulin (CaM)-dependent protein kinase IIalpha (CaM-KIIalpha) can phosphorylate neuronal nitric oxide synthase (nNOS) at Ser847 and attenuate NOS activity in neuronal cells. In the present study, they established that forebrain ischemia causes an increase in the phosphorylation of nNOS at Ser847 in the hippocampus. This nNOS phosphorylation appeared to be catalyzed by CaM-KII: (1) it correlated with the autophosphorylation of CaM-KIIalpha; (2) it was blocked by the CaM-KII inhibitor, KN-93; and (3) nNOS and CaM-KIIalpha were found to coexist in the hippocampus. Examination of the spatial relation between nNOS and CaM-KIIalpha in the brain revealed coexistence in the hippocampus but not in the cortex during reperfusion, with a concomitant increase in autophosphorylation of CaM-KIIalpha. The phosphorylation of nNOS at Ser847 probably takes place in nonpyramidal hippocampal neurons, which increased after 30 minutes of reperfusion in the hippocampus, whereas no significant increase was detected in the cortex. An intraventricular injection of KN-93 significantly decreased the phosphorylation of nNOS in the hippocampus. These results point to CaM-KII as a protein kinase, which by its colocalization may attenuate the activity of nNOS through its Ser847 phosphorylation, and may thus contribute to promotion of tolerance to postischemic damage in hippocampal neurons.

Modulation of synaptic plasticity by stress and antidepressants

Recent preclinical and clinical studies have shown that mechanisms underlying neuronal plasticity and survival are involved in both the outcome of stressful experiences and the action of antidepressants. Whereas most antidepressants predominantly affect the brain levels of monoamine neurotransmitters, it is increasingly appreciated that they also modulate neurotransmission at synapses using the neurotransmitter glutamate (the most abundant in the brain). In the hippocampus, a main area of the limbic system involved in cognitive functions as well as attention and affect, specific molecules enriched at glutamatergic synapses mediate major changes in synaptic plasticity induced by stress paradigms or antidepressant treatments. We analyze here the modifications induced by stress or antidepressants in the strength of synaptic transmission in hippocampus, and the molecular modifications induced by antidepressants in two main mediators of synaptic plasticity: the N-methyl-D-aspartate (NMDA) receptor complex for glutamate and the Ca2+/calmodulin-dependent protein kinase II (CaM kinase II). Both stress and antidepressants induce alterations in long-term potentiation of hippocampal glutamatergic synapses, which may be partly accounted for by the influence of environmental or drug-induced stimulation of monoaminergic pathways projecting to the hippocampus. In the course of antidepressant treatments significant changes have been described in both the NMDA receptor and CaM kinase II, which may account for the physiological changes observed. A central role in these synaptic changes is exerted by brain-derived neurotrophic factor (BDNF), which modulates both synaptic plasticity and its molecular mediators, as well as inducing morphological synaptic changes. The role of these molecular effectors in synaptic plasticity is discussed in relation to the action of antidepressants and the search for new molecular targets of drug action in the therapy of mood disorders.

Requirement of appropriate glutamate concentrations in the synaptic cleft for hippocampal LTP induction

Although glutamate transporters maintain low extracellular levels of the excitatory neurotransmitter glutamate in the nervous system, little is known about their roles in synaptic plasticity. Here, using knockout mice lacking GLT-1, that is the most abundant glial subtype of glutamate transporters, we showed that long-term potentiation (LTP) induced by tetanic stimulation in mutant mice was impaired in the hippocampal CA1 region. When tetanic stimulation was applied in the presence of low concentrations of an N-methyl-D-aspartate (NMDA) receptor antagonist, the impairment was overcome. Consistent with these results, the increased glutamate in the synaptic cleft of mutant mice preferentially activated NMDA receptors. Furthermore, analyses of mutant mice revealed that the magnitude of NMDA receptor-dependent transient synaptic potentiation during low-frequency stimulation depended on the concentration of glutamate in the synaptic cleft. These findings suggest that GLT-1 plays critical roles in LTP induction, as well as in short-term potentiation, through regulation of extracellular levels of glutamate, which enables appropriate NMDA receptor activation.

Neuroprotective effect of AIP on N-methyl-D-aspartate-induced cell death in retinal neurons

Excessive activation of glutamate receptors mediates neuronal death, but the intracellular signaling pathways that mediate this type of neuronal death are only partly understood. Previously, we have demonstrated that calcium/calmodulin-dependent protein kinase II-alpha(B) (CaMKII-alpha(B)) containing a nuclear localizing signal but not CaMKII-alpha is altered in retinal neurons exposed to N-methyl-D-aspartate (NMDA). The present study describes a prospective function of CaMKII-alpha(B) in signal transduction leading to apoptosis. The terminal deoxyribonucleotidyl transferase (TdT)-mediated biotin-16-dUTP nick-end labelling (TUNEL) method was used to detect fragmented DNA in fixed tissue sections of rat retina. The TUNEL assay confirmed that cell death occurs in the inner nuclear and ganglion cell layers following injection of 4 mM NMDA. A specific AIP (myristoylated autocamtide-2-related inhibitory peptide) with proven cell permeability inhibits CaMKII activity in vivo. Neuroprotection achieved by 500 microM AIP was complete when administered 2 h before and coincident with the NMDA application. Additionally, 100 microM of AIP protects only partially against the NMDA-induced excitotoxicity. The conformationally active fragment of caspase-3 (17 kDa), known to be involved in neuronal apoptosis was apparent within 30 min and at 2 h postinjection with NMDA. This activation was inhibited by 500 microM AIP when administered 2 h before and coincident with the NMDA application. The results suggest that CaMKII-alpha(B) isoform plays a role in excitotoxicity-induced neuronal apoptosis.

Effect of acute and chronic administration of methamphetamine on calcium-calmodulin dependent protein kinase II activity in the rat brain

Several lines of evidence have implicated Ca2+/calmodulin (CaM)-dependent protein kinase II (CaM-kinase II), a multifunctional protein kinase, in the regulation of signal transduction after chronic administration of psychostimulants. CaM-Kinase II activities were decreased in discrete brain regions after a single methamphetamine (METH) injection to rats. Pretreatment with either SCH 23390 (a dopamine D1 receptor antagonist) or NMK-801 (an N-methyl-D-aspartate receptor antagonist) prevented the acute METH-induced decrease in CaM-kinase II activity in the parietal cortex, nucleus accumbens, and substantia nigra/ventral tegmental area (SN/VTA). Striatal CaM-kinase II activity was significantly lower than that of the chronic saline-treated controls after a one-week, but not a four-week, abstinence from chronic administration of METH. A METH challenge after a four-week abstinence period decreased CaM-kinase II activity in rats chronically injected with METH to a greater extent than in rats chronically injected with saline. Western blot analysis revealed that protein amount of CaM-kinase II was not altered after a single METH injection or after chronic METH injections, as compared with saline-treated controls. However, amounts of phosphorylated (Thr286) CaM-kinase II in the parietal cortex, striatum, and SN/VTA were significantly decreased at 3 h after an acute METH injection compared with saline-treated controls. It is suggested that dephosphorylation of CaM-kinase II may contribute to the decreased enzyme activities induced by acute METH administration, and that chronic treatment with METH leads to an enhanced capacity of METH to decrease CaM-kinase II activity after an extended withdrawal period.

Inhibition of neuronal nitric oxide synthase activity by 3-[2-[4-(3-chloro-2-methylphenyl)- 1-piperazinyl]ethyl]-5, 6-dimethoxy-1-(4-imidazolylmethyl)-1H-indazole dihydrochloride 3.5 hydrate (DY-9760e), a novel neuroprotective agent, in vitro and in cultured neuroblastoma cells in situ

DY-9760e, 3-[2-[4-(3-chloro-2-methylphenyl)-1-piperazinyl]ethyl]-5, 6-dimethoxy-1-(4-imidazolylmethyl)-1H-indazole dihydrochloride 3.5 hydrate, a novel calmodulin (CaM) antagonist, possesses neuroprotective activity. In the current study, we examined the effects of DY-9760e on nitric oxide synthase (NOS) activities in vitro and on calcium ionophore-induced NO production in situ. DY-9760e inhibited both neuronal NOS and endothelial NOS activities without affecting inducible NOS activity. It also inhibited purified neuronal NOS activity with a potency similar to that seen for purified CaM kinase II activity in vitro. Furthermore, DY-9760e significantly inhibited Ca(2+) ionophore (A23187)-induced NO production in mouse N1E-115 neuroblastoma cells, at a concentration of less than 1 microM. In contrast, no apparent inhibitory effect on Ca(2+)/CaM-dependent protein kinase II activity was observed in cultured hippocampal neurons up to 5 microM. These results suggest that the inhibitory effect of DY-9760e on CaM-dependent NOS activities underlies neuroprotective effects of the agent.

Methamphetamine decreases calcium-calmodulin dependent protein kinase II activity in discrete rat brain regions

A Ca(2+)/calmodulin-dependent signaling cascade has been implicated in the regulation of dopaminergic neurotransmission after chronic administration of amphetamine and methamphetamine (METH). We found a decrease in Ca(2+)/calmodulin-dependent protein kinase II (CaM-kinase II) activity in five regions of the rat brain (parietal cortex; frontal cortex; hippocampus; striatum; and nucleus accumbens) after a single injection of METH. Pretreatment with the selective dopamine D1 receptor antagonist SCH 23390 prevented the acute METH-induced decrease in CaM-kinase II activity in the parietal cortex, striatum, nucleus accumbens, and substantia nigra/ventral tegmental area (SN/VTA). Pretreatment with the N-methyl-D-aspartate receptor antagonist MK-801 significantly restored the acute METH-induced decrease in CaM-kinase II activity in the parietal cortex, nucleus accumbens, and SN/VTA. Striatal CaM-kinase II activity was still significantly lower than that of the chronic saline-treated controls after a 1-week, but not a 4-week, abstinence from chronic administration of METH. A METH challenge after a 4-week abstinence period induced a more pronounced decrease in CaM-kinase II activity in rats chronically injected with METH than in rats chronically injected with saline. Western blot analysis revealed that the amount of CaM-kinase II protein was not altered after a single METH injection or after chronic METH injections, compared with saline-treated controls. However, amounts of phosphorylated (Thr(286)) CaM-kinase II in the parietal cortex, striatum and SN/VTA were significantly decreased at 3 h after an acute METH injection compared with saline-treated controls. These results suggest that dephosphorylation of CaM-kinase II may contribute to the decreased enzyme activities induced by acute METH administration, and that chronic treatment with METH leads to an enhanced capacity of METH to decrease CaM-kinase II activity after an extended withdrawal period.

Calcium/calmodulin-dependent protein kinase II containing a nuclear localizing signal is altered in retinal neurons exposed to N-methyl-D-aspartate

This study investigated N-methyl-D-aspartate (NMDA) mediated cell death and its possible regulation by calcium/calmodulin-dependent protein kinase II (CaMKII) in the adult rat retina. To investigate cell death, the terminal deoxyribonucleotidyltransferase (TdT)-mediated biotin-16-dUTP nick-end labelling (TUNEL) method was used to detect fragmented DNA in fixed tissue sections of rat retina. The TUNEL assay confirmed that apoptosis occurs in the inner nuclear layer (INL) and ganglion cell layer (GCL) following NMDA injection. The level of antibody binding to CaMKII-alpha, the activity of CaMKII, and the mRNA level for the alpha(B) subunit of CaMKII were found to be elevated for short time periods (30 min, 2 h) after a single intravitreal injection of NMDA. In contrast to this, there was a decrease in CaMKII activity and in the CaMKII-alpha(B) mRNA levels at longer time periods (24 h) following injection of NMDA. These effects were specific for the mRNA for the alpha(B) subunit, an alternatively spliced product of the CaMKII-alpha gene, that contains a nuclear localizing signal (NLS) known to target this protein to the nucleus. It is suggested that regulated expression of CaMKII-alpha(B) could be involved in the NMDA-mediated cell death in retinal neurons.

Altered mRNA expression for brain-derived neurotrophic factor and type II calcium/calmodulin-dependent protein kinase in the hippocampus of patients with intractable temporal lobe epilepsy

The expression of brain-derived neurotrophic factor and the alpha subunit of calcium/calmodulin-dependent protein kinase II mRNA in hippocampi obtained during surgical resections for intractable temporal lobe epilepsy were examined. Both calcium/calmodulin-dependent protein kinase II and brain-derived neurotrophic factor are localized heavily within the hippocampus and have been implicated in regulating hippocampal activity (Kang and Schuman [1995] Science 267:1658-1662; Suzuki [1994] Intl J Biochem 26:735-744). Also, the autocrine and paracrine actions of brain-derived neurotrophic factor within the central nervous system make it a likely candidate for mediating morphologic changes typically seen in the epileptic hippocampus. Quantitative assessments of mRNA levels in epileptic hippocampi relative to autopsy controls were made by using normalized densitometric analysis of in situ hybridization. In addition, correlations between clinical data and mRNA levels were studied. Relative to autopsy control tissue, decreased hybridization to mRNA of the alpha subunit of calcium/calmodulin-dependent protein kinase II and increased hybridization to brain-derived neurotrophic factor mRNA were found throughout the granule cells of the epileptic hippocampus. There also was a significant negative correlation between the duration of epilepsy and the expression of mRNA for brain-derived neurotrophic factor. These results are similar qualitatively to those found in animal models of epilepsy and suggest that chronic seizure activity in humans leads to persistent alterations in gene expression. Furthermore, these alterations in gene expression may play a role in the etiology of the epileptic condition.

Ischemic cell death in brain neurons

This review is directed at understanding how neuronal death occurs in two distinct insults, global ischemia and focal ischemia. These are the two principal rodent models for human disease. Cell death occurs by a necrotic pathway characterized by either ischemic/homogenizing cell change or edematous cell change. Death also occurs via an apoptotic-like pathway that is characterized, minimally, by DNA laddering and a dependence on caspase activity and, optimally, by those properties, additional characteristic protein and phospholipid changes, and morphological attributes of apoptosis. Death may also occur by autophagocytosis. The cell death process has four major stages. The first, the induction stage, includes several changes initiated by ischemia and reperfusion that are very likely to play major roles in cell death. These include inhibition (and subsequent reactivation) of electron transport, decreased ATP, decreased pH, increased cell Ca(2+), release of glutamate, increased arachidonic acid, and also gene activation leading to cytokine synthesis, synthesis of enzymes involved in free radical production, and accumulation of leukocytes. These changes lead to the activation of five damaging events, termed perpetrators. These are the damaging actions of free radicals and their product peroxynitrite, the actions of the Ca(2+)-dependent protease calpain, the activity of phospholipases, the activity of poly-ADPribose polymerase (PARP), and the activation of the apoptotic pathway. The second stage of cell death involves the long-term changes in macromolecules or key metabolites that are caused by the perpetrators. The third stage of cell death involves long-term damaging effects of these macromolecular and metabolite changes, and of some of the induction processes, on critical cell functions and structures that lead to the defined end stages of cell damage. These targeted functions and structures include the plasmalemma, the mitochondria, the cytoskeleton, protein synthesis, and kinase activities. The fourth stage is the progression to the morphological and biochemical end stages of cell death. Of these four stages, the last two are the least well understood. Quite little is known of how the perpetrators affect the structures and functions and whether and how each of these changes contribute to cell death. According to this description, the key step in ischemic cell death is adequate activation of the perpetrators, and thus a major unifying thread of the review is a consideration of how the changes occurring during and after ischemia, including gene activation and synthesis of new proteins, conspire to produce damaging levels of free radicals and peroxynitrite, to activate calpain and other Ca(2+)-driven processes that are damaging, and to initiate the apoptotic process. Although it is not fully established for all cases, the major driving force for the necrotic cell death process, and very possibly the other processes, appears to be the generation of free radicals and peroxynitrite. Effects of a large number of damaging changes can be explained on the basis of their ability to generate free radicals in early or late stages of damage. Several important issues are defined for future study. These include determining the triggers for apoptosis and autophagocytosis and establishing greater confidence in most of the cellular changes that are hypothesized to be involved in cell death. A very important outstanding issue is identifying the critical functional and structural changes caused by the perpetrators of cell death. These changes are responsible for cell death, and their identity and mechanisms of action are almost completely unknown.

Increased calcium/calmodulin protein kinase activity in astrocytes chronically exposed to ethanol: influences on glutamate transport

The effects of ethanol exposures on calcium/calmodulin-dependent protein kinase activity as well as its influence on glutamate uptake were determined in astrocytes prepared from neonatal rat cerebral cortex. Acute 15-min exposure to 100 mM ethanol had no effect on Ca2+/CaM-dependent protein kinase activity. However, chronic exposure to 100 mM ethanol for 4 days elicited a significant increase in the activity of this enzyme with no parallel increase in its expression. Ca2+/CaM-independent kinase activity was less than 1% of the Ca2+/CaM-dependent kinase activity and was unaffected by any of the ethanol exposures. Exposure to 100 mM ethanol for four days also resulted in a significant increase in Na+-dependent [3H]glutamate uptake which was reversed when ethanol-exposed astrocytes were co-incubated with KN-93, a specific inhibitor of Ca2+/CaM kinase. These results suggest that the effects of ethanol on glutamate transport may be mediated in part, by the level of Ca2+/CaM kinase activity.

Activities of calcineurin and phosphatase 2A in the hippocampus after transient forebrain ischemia

We investigated the changes in the enzyme activity and immunoreactivity of calcineurin in the rat hippocampus after transient forebrain ischemia. Immediately after 20-min transient forebrain ischemia, calcineurin activity decreased to about 40% of the control in the CA1 region and to about 55% in other regions. Protein phosphatase 2A activity showed no remarkable changes. By 12 h after ischemia, calcineurin activity recovered, more in the CA1 region than in other regions. At 24 h it decreased again, but only in the CA1 region. Immunohistochemical- and immunoblot analyses showed no remarkable change in calcineurin in any region of the hippocampus within 12 h after ischemia. Thus, the activity of calcineurin is dissociated from its immunoreactivity and quantity. Several studies have suggested that unknown inhibitory factor(s) and/or reversible changes in calcineurin act to modify enzyme activity after ischemia. In contrast, phosphatase 2A activity underwent no obvious changes during the post-ischemia period we examined. This unique time course of calcineurin activity may contribute to the mechanism of ischemic neuronal injury.

Potential role of calcineurin for brain ischemia and traumatic injury

Calcineurin belongs to the family of Ca2+/calmodulin-dependent protein phosphatase, protein phosphatase 2B. Calcineurin is the only protein phosphatase which is regulated by a second messenger, Ca2+. Furthermore, calcineurin is highly localized in the central nervous system, especially in those neurons vulnerable to ischemic and traumatic insults. For these reasons, calcineurin is considered to play important roles in neuron-specific functions. Recently, on the basis of the finding that FK506 and cyclosporin A serve as calcineurin-specific inhibitors, this enzyme has become the subject of much study. It is clear that calcineurin is involved in many neuronal (or non-neuronal) functions such as neurotransmitter release, regulation of receptor functions, signal transduction systems, neurite outgrowth, gene expression and neuronal cell death. In this review, we describe the calcineurin functions, functions of the substrates, and the pathogenesis of traumatic and ischemic insults, and we discuss the potential role of calcineurin. There are many similarities in traumatic and ischemic pathogenesis of the brain in which the release of excessive glutamate is followed by an intracellular Ca2+ increase. However, the intracellular cascade which leads to neuronal cell death after the release of excess Ca2+ is unclear. Although calcineurin is thought to be a key toxic enzyme on the basis of studies using immunosuppressants (FK506 or cyclosporin A), many of the functions of the substrates for calcineurin protect against neuronal cell death. We concluded that calcineurin is a bi-directional enzyme for neuronal cell death, having protective and toxic actions, and the balance of the bi-directional effects may be important in ischemic and traumatic pathogenesis.

Phosphorylation of myristoylated alanine-rich protein kinase C substrate by mitogen-activated protein kinase in cultured rat hippocampal neurons following stimulation of glutamate receptors

Glutamate-induced phosphorylation of myristoylated alanine-rich protein kinase C substrate (MARCKS) was investigated in cultured rat hippocampal neurons. In 32P-labeled hippocampal neurons, exposure to 10 microM glutamate induced a long lasting increase in phosphorylation of MARCKS. The long lasting increase in MARCKS phosphorylation mainly required activation of the N-methyl-D-aspartate receptor. Unexpectatively, the MARCKS phosphorylation after the 10-min incubation with glutamate was not inhibited by treatment with calphostin C, a potent inhibitor for protein kinase C (PKC), or down-regulation of PKC but was largely prevented by PD098059, a selective inhibitor for mitogen-activated protein (MAP) kinase kinase. In contrast, the phosphorylation following the short exposure to glutamate was prevented by a combination of PD098059 and calphostin C. The phosphopeptide mapping and immunoblotting analyses confirmed that PKC-dependent phosphorylation of MARCKS was transient and the MAP kinase-dependent phosphorylation was relatively persistent. Investigations of the functional properties also showed that the MARCKS phosphorylation by MAP kinase regulates its calmodulin-binding ability and its interaction with F-actin as seen in the PKC-dependent phosphorylation. These results suggest that glutamate causes a long lasting increase in MARCKS phosphorylation through activation of the N-methyl-D-aspartate receptor and subsequent activation of MAP kinase in the hippocampal neurons.

Reduced transduction mechanisms in the anterior accumbal interface of an animal model of Attention-Deficit Hyperactivity Disorder

The aim of this study was to map the neural substrates of attention-deficit hyperactivity disorder (ADHD) in the spontaneously hypertensive rat (SHR), which is thought to be a model for ADHD. To this aim, the Ca2+/calmodulin-dependent protein kinase II (CaMKII) and transcription factors (TF) were used as markers. The focus of interest was the nucleus accumbens complex (ACB) which is thought to be an interface between limbic and motor systems. Juvenile, male rats of the SHR line and Wistar-Kyoto (WKY) controls were perfused and the brains processed for immunocytochemistry for CaMKII and the TF peptides of the FOS, JUN-B and ZIF-268 families. The results revealed that: (i) in both groups there were more CaMKII-positive neurones in the shell than in the core of the ACB; (ii) SHR had a reduced number of CaMKII-positive elements in anterior portions of the shell; and (iii) SHR had a lower expression of peptide products of the FOS family (c-FOS, in particular) and ZIF-268. In addition, there was a lower expression of c-FOS and zif-268 in the core of the ACB in the SHR. In contrast, there was an increased basal level of JUN-B in the core of the ACB of SHR. The reduced number of CaMKII and TF-positive elements in the most rostral portions of the accumbal complex of SHR, associated to the higher number of binding sites for the DA D-1/D-5 subtype, appears as a discrete alteration in the prosomeric development of the anterior basal forebrain and could be the key to the understanding of ADHD.

Astrocytes protect neurons from neurotoxic injury by serum glutamate

Serum is used widely for culturing neurons and glial cells, and is thought to provide essential, albeit undefined, factors such as hormones, growth factors, and trace elements that promote the growth of cells in vitro. Moreover, serum can have profound effects on cell proliferation, differentiation, and cell morphology, and may even influence cell fate decisions. Despite the overall growth-promoting influence of serum on cell culture, frequent media changes have been shown to be detrimental to neuronal cultures, significantly reducing the yield of viable neurons. The reason for this loss of neurons by frequent media changes has been puzzling. We demonstrate that bovine and horse sera, the most popular serum complements for CNS cell culture, are a significant source for glutamate, supplying glutamate at concentrations sufficient to kill primary cultured hippocampal neurons. By using the bioluminescence detection method, we determined the glutamate concentration [Glu] in several batches of fetal bovine (calf) sera (FBS) to be close to 1 mM, and that of horse sera to be approximately 0.3 mM. Thus 10% serum supplement to culture media results in [Glu] of 30-100 microM due to serum alone. We subsequently produced glutamate depleted media (GDM) by using primary cultures of hippocampal astrocytes to absorb glutamate from media containing 10% FBS. Within 3 h, astrocytes reduced the [Glu] in the medium from approximately 90 microM to less than 1 microM. Sister cultures of hippocampal neuron that underwent frequent media changes with GDM or GDM + partial untreated media demonstrated that GDM significantly increase neuronal survival (10-fold at 21 DIV). Subsequent exposure to glutamate provided by either untreated serum or by equivalent doses of exogenous glutamate added to GDM led to dose-dependent neuronal cell death. The relative sensitivity of hippocampal neurons to glutamate increased with increasing culture age from initial ED50 values of > 100 microM (< 6 DIV) to approximately 6 microM in cultures maintained for 3 weeks or longer. The relative sensitivity to exogenous glutamate was at least 2-fold higher in neurons cultured in GDM than in sister cultures maintained in media containing untreated serum. The death of neurons exposed to untreated media was blocked by the NMDA receptor antagonist MK-801. These experiments suggest that the vulnerability of neurons to media changes can be solely explained by excitotoxicity resulting from serum-borne glutamate. Moreover, we propose that use of GDM may be advantageous for culturing hippocampal neurons and may eliminate the possible selection for glutamate resistant neurons. The use of GDM could be particularly important for studies of excitotoxicity; our study predicts that the ED50 for neuronal culture with regular serum will be artificially high and may not adequately reflect the in vivo state.

DY-9760e, a novel calmodulin antagonist with cytoprotective action

We report the pharmacological characterization and cytoprotective effect of DY-9760e, 3-[2-[4-(3-chloro-2-methylphenyl)-1-piperazinyl]ethyl]-5,6-dimethoxy-1-( 4-imidazolylmethyl)-1H-indazole dihydrochloride 3.5 hydrate, a novel antagonist of calmodulin. DY-9760e inhibited calmodulin-dependent enzymes, including calmodulin-dependent protein kinase II and IV, calcineurin, [corrected] calmodulin-dependent phosphodiesterase and myosin light chain kinase with Ki values of 1.4, 12, 2.0, 3.8 and 133 microM, respectively. These antagonistic effects of DY-9760e were more potent than those of W-7, N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide, another calmodulin antagonist. This compound showed little or no effect on calmodulin-independent enzymes, such as protein kinase A and C and calpain I and II. Analysis of the hydrophobic interaction of DY-9760e with calmodulin by using 2-p-toluidinylnaphthalene-6-sulfonate and 9-anthroylcholine revealed that, like W-7, DY-9760e bound to the hydrophobic regions of calmodulin. The [14C]DY-9760e binding assay indicated that DY-9760e bound to calmodulin at one class of binding site. Finally, DY-9760e substantially protected N1E-115 neuroblastoma cells from cytotoxicity induced by the Ca2+ ionophore, A23187. These results indicate that DY-9760e, a novel calmodulin antagonist, possesses a cytoprotective action and suggest that calmodulin plays a critical role in mediating some of the biochemical events leading to cell death following Ca2+ overload.

Glutamate-dependent phosphorylation of elongation factor-2 and inhibition of protein synthesis in neurons

Postischemic delayed neuronal death is attributed to excitotoxic activation of glutamate receptors. It is preceded by a persistent inhibition of protein synthesis, the molecular basis of which is not known. Here we have examined in cortical neurons in culture the regulation by glutamate of phosphorylation of eukaryotic elongation factor-2 (eEF-2) by eEF-2 kinase, a Ca2+/calmodulin-dependent enzyme. Using a phosphorylation state-specific antibody, we show that glutamate, which triggers a large influx of Ca2+, enhances dramatically the phosphorylation of eEF-2. On the basis of kinetic and pharmacological analysis, we demonstrate a close correlation among the increase in cytosolic Ca2+ concentration, the degree of eEF-2 phosphorylation, and the inhibition of protein synthesis. A 30 min treatment with NMDA induced a transient phosphorylation of eEF-2 and delayed neuronal death. However, pharmacological inhibition of protein translation was not neurotoxic by itself and protected neurons against the toxicity evoked by low concentrations of NMDA. Thus, phosphorylation of eEF-2 and the resulting depression of protein translation may have protective effects against excitotoxicity and open new perspectives for understanding long-term effects of glutamate.

Translocation of autophosphorylated calcium/calmodulin-dependent protein kinase II to the postsynaptic density

Calcium/calmodulin-dependent protein kinase II (CaMKII) undergoes calcium-dependent autophosphorylation, generating a calcium-independent form that may serve as a molecular substrate for memory. Here we show that calcium-independent CaMKII specifically binds to isolated postsynaptic densities (PSDs), leading to enhanced phosphorylation of many PSD proteins including the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)-type glutamate receptor. Furthermore, binding to PSDs changes CaMKII from a substrate for protein phosphatase 2A to a protein phosphatase 1 substrate. Translocation of CaMKII to PSDs occurs in hippocampal slices following treatments that induce CaMKII autophosphorylation and a form of long term potentiation. Thus, synaptic activation leads to accumulation of autophosphorylated, activated CaMKII in the PSD. This increases substrate phosphorylation and affects regulation of the kinase by protein phosphatases, which may contribute to enhancement of synaptic strength.

Postnatal ontogeny of striatal-enriched protein tyrosine phosphatase (STEP) in rat striatum

The present study was undertaken to examine developmental change in expression of striatal-enriched protein tyrosine phosphatase (STEP) in the postnatal striatum of rats. For this purpose, immunohistochemical staining and transimmunoblotting analyses were carried out using a cDNA-generated polyclonal antibody to the STEP with a molecular weight of 46 kDa. Immunostaining showed that in neonatal striatum STEP-immunoreactivity was found in discrete patches composed of many immature cells, which corresponded to the tyrosine hydroxylase-immunopositive "dopamine islands." With development there was an increase in staining intensity and in the number of positively reacting cells. By 4 weeks postnatally, STEP-immunoreactivity was almost homogeneously distributed throughout the striatum, as was seen at the adult stage. Immunoblotting analysis showed that STEP protein expression abruptly increased from 2 to 4 weeks postnatally when it reached the adult level. These findings suggest that STEP is involved in development and maturation of the striatal neurons.

Involvement of N-methyl-D-aspartate receptor in the delayed transneuronal regression of substantia nigra neurons in rats

The substantia nigra pars reticulata (SNr) receives both inhibitory GABAergic and excitatory glutamatergic afferents from diverse origins. Ischemic injury to the striatum and/or the globus pallidus causes delayed transneuronal death of the SNr neurons, in the course of which neuronal disinhibition induced by loss of GABAergic inputs is supposed to trigger a lethal hypermetabolic process. In the in vivo experiment presented herein, we clarified the role of glutamatergic action via the N-methyl-D-aspartate receptor in this cell death process. Continuous intraventricular infusion (0.5 microliter/h) of the N-methyl-D-aspartate receptor antagonist MK-801 (1000 micrograms/ml), or of saline (control group) was initiated 24 h after 2 h of transient middle cerebral artery (MCA) occlusion in rats, by which massive ischemic injury was produced in the striatopallidal regions. The measured rectal temperature was not significantly altered in the MK-801-infused and in the control rats throughout the time period examined. The rats were killed at 15 days after MCA occlusion. The volume of the focal ischemic infarction of the MK-801-infused group did not significantly differ from that of controls. Also, MK-801-infusion did not significantly ameliorate the nigral atrophy subsequent to MCA occlusion. In association with a marked depletion of GABAergic afferent fibers, neuronal cell number in the ipsilateral SNr was significantly decreased in the control group. In contrast, the neuronal cell loss in the nucleus was completely prevented in the MK-801-infusion group. The data suggested that withdrawal of GABAergic inputs may cause a severe imbalance between excitation and inhibition of the SNr neurons and may eventually result in neurotoxicity mediated by the N-methyl-D-aspartate receptor. Suppression of glutamatergic excitatory effects by suitable drugs may be a reasonable therapy for the transneuronal death of the SNr neurons.

CaM kinase II in long-term potentiation

The observation that autophosphorylation converts CaM kinase II from the Ca(2+)-dependent form to the Ca(2+)-independent form has led to speculation that the formation of the Ca(2+)-independent form of the enzyme could encode frequency of synaptic usage and serve as a molecular explanation of "memory". In cultured rat hippocampal neurons, glutamate elevated the Ca(2+)-independent activity of CaM kinase II through autophosphorylation, and this response was blocked by an NMDA receptor antagonist, D-2-amino-5-phosphonopentanoate (AP5). In addition, we confirmed that high, but not low frequency stimulation, applied to two groups of CA1 afferents in the rat hippocampus, resulted in LTP induction with concomitant long-lasting increases in Ca(2+)-independent and total activities of CaM kinase II. In experiments with 32P-labeled hippocampal slices, the LTP induction in the CA1 region was associated with increases in autophosphorylation of both alpha and beta subunits of CaM kinase II 1 h after LTP induction. Significant increases in phosphorylation of endogenous CaM kinase II substrates, synapsin I and microtubule-associated protein 2 (MAP2), which are originally located in presynaptic and postsynaptic regions, respectively, were also observed in the same slice. All these changes were prevented when high frequency stimulation was applied in the presence of AP5 or a calmodulin antagonist, calmidazolium. Furthermore, in vitro phosphorylation of the AMPA receptor by CaM kinase II was reported in the postsynaptic density and infusion of the constitutively active CaM kinase II into the hippocampal neurons enhanced kainate-induced response. These results support the idea that CaM kinase II contributes to the induction of hippocampal LTP in both postsynaptic and presynaptic regions through phosphorylation of target proteins such as the AMPA receptor, MAP2 and synapsin I.

Ischemia-induced neuronal damage: a role for calcium/calmodulin-dependent protein kinase II

Calcium/calmodulin-dependent protein kinase II (CaM-kinase) is a central enzyme in regulating neuronal processes. Imbalances in the activity and distribution of this enzyme have been reported following in vivo ischemia, and sustained decreases in activity correlate with subsequent neuronal death. In this report, mice that had been rendered deficient in the alpha subunit of CaM-kinase using gene knock-out technology were utilized to determine whether this enzyme is causally related to ischemic damage. Using a focal model of cerebral ischemia, we showed that homozygous knock-out mice lacking the alpha subunit exhibited an infarct volume almost twice that of wild-type litter mates. Heterozygous mice exhibited slightly less damage following ischemia than did homozygous mice, but infarct volumes remained significantly larger than those of wild-type litter mates. We conclude that reduced amounts of the alpha subunit of CaM-kinase predisposes neurons to increased damage following ischemia and that any perturbation that decreases the amount or activity of the enzyme will produce enhanced susceptibility to neuronal damage.