The mitogen-activated protein kinase cascade couples PKA and PKC to cAMP response element binding protein phosphorylation in area CA1 of hippocampus

Enhanced ERK dependent CREB activation reduces apoptosis in staurosporine-treated human neuroblastoma SK-N-BE(2)C cells

Activation of cAMP response element binding protein (CREB) is implicated in neuronal survival. The mitogen-activated protein kinase/extracellular signal regulated kinase (MAPK/ERK) activates a transcription factor CREB. Previously, we reported that N-acetyl-O-methyldopamine (NAMDA) protects neurons from ischemia via enhancing ERK dependent CREB phosphorylation. To investigate whether NAMDA induces endogenous survival pathways in apoptotic conditions and whether the neuroprotectant enhances a preexisting survival pathway, we determined the degree of ERK-CREB activation and resistance to apoptosis in staurosporine-treated SK-N-BE(2)C neurons. Compared to forskolin-treated apoptotic cultures, NAMDA-treated cultures induced a minimum activation on ERK (pERK) or CREB (pCREB). However, NAMDA enhanced the activation of ERK and CREB in the presence of forskolin (1.7-fold increase for pCREB, 2.1-fold increase for pERK2, p<0.05 from forskolin). The effect was completely blocked by a specific MEK inhibitor U0126, suggesting the involvement of ERK dependent CREB signaling. Cleavage of caspase-3 and poly-(ADP-ribose)-polymerase was additively reduced in cultures treated with NAMDA and forskolin simultaneously, but not in the presence of U0126. The data showed that NAMDA enhances forskolin-induced ERK-CREB activation and potentiates forskolin-induced resistance to apoptosis. The study indicates that enhancing endogenous survival pathways by NAMDA combined with other neuroprotective measure(s) might be a useful strategy to reduce apoptosis.

Xenon preconditioning reduces brain damage from neonatal asphyxia in rats

Xenon attenuates on-going neuronal injury in both in vitro and in vivo models of hypoxic-ischaemic injury when administered during and after the insult. In the present study, we sought to investigate whether the neuroprotective efficacy of xenon can be observed when administered before an insult, referred to as 'preconditioning'. In a neuronal-glial cell coculture, preexposure to xenon for 2 h caused a concentration-dependent reduction of lactate dehydrogenase release from cells deprived of oxygen and glucose 24 h later; xenon's preconditioning effect was abolished by cycloheximide, a protein synthesis inhibitor. Preconditioning with xenon decreased propidium iodide staining in a hippocampal slice culture model subjected to oxygen and glucose deprivation. In an in vivo model of neonatal asphyxia involving hypoxic-ischaemic injury to 7-day-old rats, preconditioning with xenon reduced infarction size when assessed 7 days after injury. Furthermore, a sustained improvement in neurologic function was also evident 30 days after injury. Phosphorylated cAMP (cyclic adenosine 3',5'-monophosphate)-response element binding protein (pCREB) was increased by xenon exposure. Also, the prosurvival proteins Bcl-2 and brain-derived neurotrophic factor were upregulated by xenon treatment. These studies provide evidence for xenon's preconditioning effect, which might be caused by a pCREB-regulated synthesis of proteins that promote survival against neuronal injury.

Differential roles for group 1 mGluR subtypes in induction and expression of chemically induced hippocampal long-term depression

Although metabotropic glutamate receptors (mGluRs) mGluR1 and mGluR5 are often found to have similar functions, there is considerable evidence that the two receptors also serve distinct functions in neurons. In hippocampal area CA1, mGluR5 has been most strongly implicated in long-term synaptic depression (LTD), whereas mGluR1 has been thought to have little or no role. Here we show that simultaneous pharmacological blockade of mGluR1 and mGluR5 is required to block induction of LTD by the group 1 mGluR agonist, (RS)-3,5-dihydroxyphenylglycine (DHPG). Blockade of mGluR1 or mGluR5 alone has no effect on LTD induction, suggesting that activation of either receptor can fully induce LTD. Consistent with this conclusion, mGluR1 and mGluR5 both contribute to activation of extracellular signal-regulated kinase (ERK), which has previously been shown to be required for LTD induction. In contrast, selective blockade of mGluR1, but not mGluR5, reduces the expression of LTD and the associated decreases in AMPA surface expression. LTD is also reduced in mGluR1 knockout mice confirming the involvement of mGluR1. This shows a novel role for mGluR1 in long-term synaptic plasticity in CA1 pyramidal neurons. In contrast to DHPG-induced LTD, synaptically induced LTD with paired-pulse low-frequency stimulation persists in the pharmacological blockade of group 1 mGluRs and in mGluR1 or mGluR5 knockout mice. This suggests different receptors and/or upstream mechanisms for chemically and synaptically induced LTD.

Is there a role for the endocannabinoid system in the etiology and treatment of melancholic depression?

With advances in basic and clinical neuroscience, many gaps have appeared in the traditional monoamine theory of depression that have led to reformulation of the hypotheses concerning the neurobiology of depression. The more recent hypotheses suggest that melancholic depression is characterized by central glucocorticoid resistance that results in hypercortisolemia, which in turn leads to down-regulation of neurotrophins and subsequent neurodegeneration. Examining the neurobiology of depression from this perspective suggests that the endocannabinoid system may play a role in the etiology of melancholic depression. Specifically, pharmacological and genetic blockade of the cannabinoid CB1 receptor induces a phenotypic state that is analogous to melancholic depression, including symptoms such as reduced food intake, heightened anxiety, increased arousal and wakefulness, deficits in extinction of aversive memories and supersensitivity to stress. These similarities between melancholic depression and an endocannabinoid deficiency become more interesting in light of recent findings that endocannabinoid activity is down-regulated by chronic stress and possibly increased by some antidepressant regimens. We propose that an endocannabinoid deficiency may underlie some of the symptoms of melancholic depression, and that enhancement of this system may ultimately be a novel form of pharmacotherapy for treatment-resistant depression.

Long-Term Exposure to the Atypical Antipsychotic Olanzapine Differently Up-Regulates Extracellular Signal-Regulated Kinases 1 and 2 Phosphorylation in Subcellular Compartments of Rat Prefrontal Cortex

Antipsychotics are the drugs of choice for the treatment of schizophrenia. Besides blocking monoamine receptors, these molecules affect intracellular signaling mechanisms, resulting in long-term synaptic alterations. Western blot analysis was used to investigate the effect of long-term administration (14 days) with the typical antipsychotic haloperidol and the atypical olanzapine on the expression and phosphorylation state of extracellular signal-related kinases (ERKs) 1 and 2 (ERK1/2), proteins involved in the regulation of multiple intracellular signaling cascades. A single injection of both drugs produced an overall decrease in ERK1/2 phosphorylation in different subcellular compartments. Conversely, long-term treatment with olanzapine, but not haloperidol, increased ERK1/2 phosphorylation in the prefrontal cortex in a compartment-specific and time-dependent fashion. In fact, ERK1/2 phosphorylation was elevated in the nuclear and cytosolic fractions 2 h after the last drug administration, whereas it was enhanced only in the membrane fraction when the animals were killed 24 h after the last injection. This effect might be the result of an activation of the mitogen-activated protein kinase pathway, because the phosphorylation of extracellular signal-regulated kinase kinase 1/2 was also increased by long-term olanzapine administration. Our data demonstrate that long-term exposure to olanzapine dynamically regulates ERK1/2 phosphorylation in different subcellular compartments, revealing a novel mechanism of action for this atypical agent and pointing to temporally separated locations of signaling events mediated by these kinases after long-term olanzapine administration.

Signaling networks from Gβ1 subunit to transcription factors and actin remodeling via a membrane-located ERβ-related protein in the rapid action of daidzein in osteoblasts

Although estrogen replacement has been the main therapy to prevent and treat osteoporosis, there are concerns about its safety. Phytoestrogens have attracted attention to their potential impacts in osteoporosis prevention and treatment. Among phytoestrogens, the isoflavone daidzein (Dz) acts on transcription via the intracellular estrogen receptors (ER), mainly ERbeta, in osteoblasts, but mimics only part of the estrogen effects. Since estradiol also exerts rapid effects in osteoblasts, we investigated the multistep processes involved in the rapid actions of low (1-100 pM) doses of daidzein. Dz bound to a membrane moiety, related to ERbeta since the calcium response to Dz was blocked by an anti-ERbeta antibody directed against the C-terminus, but not by a double-stranded siRNA specific for ERbeta. This protein was coupled to a pertussis toxin (PTX)-sensitive Gbeta1 subunit whose transducer was PLC-beta2, which triggered a rapid (5 sec) mobilization of calcium from the endoplasmic reticulum. Dz phosphorylated within 15 sec ERK1/2 whose phosphorylation involved two routes: Gbeta1/PLC-beta2/PKC/c-Raf-1/MEK1/2 and Gbeta1/PI3K/cSrc/c-Raf-1/MEK1/2 as shown using several inhibitors. Dz induced rapid (1 min) changes in the actin cytoskeleton via the two routes. The rapid (20 sec) phosphorylation of Elk-1 and CREB by Dz involved Gbeta1 and ERK1/2. All the processes were insensitive to the estradiol antagonist ICI 182,780. In conclusion, the rapid effects of Dz seem to be biologically relevant for the function of osteoblast in bone since the isoflavone activates transcription factors linked to early genes controlling cellular proliferation and differentiation, and modulates actin cytoskeleton which controls cell adhesion, division, or secretion.

Signal transduction mechanisms in memory disorders

This chapter explores some of the molecular events contributing to memory formation and how, when these events malfunction, disturbances in memory occur. After a brief discussion of signaling in the hippocampus, we will explore the topics of human mental retardation syndromes that involve disruption of these processes, including Angelman syndrome (AS), Neurofibromatosis 1 (NF1)-associated learning disorders, Coffin-Lowry syndrome (CLS), Rubinstein-Taybi syndrome (RTS), and Rett syndrome (RTT).

Age-related disturbance of memory and CREB phosphorylation in CA1 area of hippocampus of rats

In the early process of long-term memory formation, cyclic AMP response element-binding protein (CREB), a transcription factor on which multiple signal transduction pathways converge, has been implicated. We examined whether the age difference in the performance of contextual fear conditioning (CFC) is associated with a change in activation of CREB in the hippocampus which is an important neural structure for long-term memory. The activation of CREB in the hippocampus in young (15 weeks old) and old (120 weeks old) male rats was determined immunohistochemically with an antibody that specifically recognizes the phosphorylated form of CREB (pCREB). Young rats exhibited better performance than old rats with respect to the freezing time in CFC. Phosphorylation of CREB as revealed by the ratio of the pCREB-immunoreactive cell number to the CREB-immunoreactive cell number was increased in the CA1 region, but not in other hippocampal regions following training for CFC. The close relationship between behavioral performance and CREB phosphorylation in the CA1 region suggests that hippocampal CREB is involved in age-related decline of learning and memory.

Long-term potentiation persists in an occult state following mGluR-dependent depotentiation

Depotentiation, the reversal of long-term potentiation (LTP), can be induced by activation of metabotropic glutamate receptors (mGluRs) or NMDA receptors (NMDARs). Although NMDAR-dependent depotentiation is due to a protein phosphatase-dependent erasure of LTP, the notion that mGluR-dependent depotentiation also involves LTP erasure is controversial. To address this issue we used electrophysiological and biochemical approaches to investigate mGluR-dependent depotentiation in hippocampal slices. Activating group I mGluRs with (R,S)-3,5-dihydroxyphenylglycine (DHPG) induced robust depotentiation in both the CA1 and CA3 regions of hippocampal slices. Western immunoblotting of samples prepared from DHPG-treated slices revealed, however, that activation of group I mGluRs causes a transient increase in phosphorylation of AMPA receptor GluR1 subunits at sites crucial for LTP and under some conditions causes persistent activation of alphaCamKII. The paradoxical ability of DHPG to induce depotentiation while at the same time activating signaling pathways involved in LTP suggests that LTP might not be erased by mGluR-dependent depotentiation. Consistent with this, DHPG-induced depotentiation did not restore the ability of high-frequency stimulation to induce LTP at synapses that had previously undergone saturating levels of LTP. In addition, blocking the expression of DHPG-induced LTD revealed hidden LTP at depotentiated synapses. Our results indicate that LTP and mGluR-dependent LTD can co-exist at excitatory synapses.

Differential translation and fragile X syndrome

Fragile X syndrome (FXS) is caused by the transcriptional silencing of the Fmr1 gene, which encodes a protein (FMRP) that can act as a translational suppressor in dendrites, and is characterized by a preponderance of abnormally long, thin and tortuous dendritic spines. According to a current theory of FXS, the loss of FMRP expression leads to an exaggeration of translation responses linked to group I metabotropic glutamate receptors. Such responses are involved in the consolidation of a form of long-term depression that is enhanced in Fmr1 knockout mice and in the elongation of dendritic spines, resembling synaptic phenotypes over-represented in fragile X brain. These observations place fragile X research at the heart of a long-standing issue in neuroscience. The consolidation of memory, and several distinct forms of synaptic plasticity considered to be substrates of memory, requires mRNA translation and is associated with changes in spine morphology. A recent convergence of research on FXS and on the involvement of translation in various forms of synaptic plasticity has been very informative on this issue and on mechanisms underlying FXS. Evidence suggests a general relationship in which the receptors that induce distinct forms of efficacy change differentially regulate translation to produce unique spine shapes involved in their consolidation. We discuss several potential mechanisms for differential translation and the notion that FXS represents an exaggeration of one 'channel' in a set of translation-dependent consolidation responses.

ERK phosphorylation is required for retention of trace fear memory

The extracellular signal-regulated kinase (ERK) has been previously associated with long-term memory formation. Earlier studies have demonstrated a role for phospho-ERK in delay fear conditioning and it has been shown to disrupt trace fear memory when inhibited after training. cAMP response element binding protein (CREB) is a key transcription factor that has been implicated in long-term memory formation across different species. It has also been shown to be modulated by ERK. In our study, we used the drug SL327 to prevent ERK phosphorylation. Two groups of Fischer 344 male rats (2-4 months) were injected intraperitoneally with 100% DMSO (2 ml/kg) or SL327 (100 mg/kg/2 ml dissolved in DMSO) 45 min before 10 trials of trace fear conditioning. Each trial consisted of a tone paired with a footshock with a 30-s interval separating the stimuli. Twenty-four hours later, rats were tested for fear to the tone. Our results showed that SL327-treated rats displayed memory deficits 24 h after training. Western blot analyses of total hippocampal protein revealed a significant increase in phosphorylated ERK immediately after training. There were also decreases in phosphorylated ERK at 45 and 90 min post-injection of SL327-treated rats as compared to DMSO-treated rats, but levels of phosphorylated CREB remained the same. These findings indicate that ERK phosphorylation is increased immediately after trace fear conditioning and inhibiting this increase is correlated with memory deficits in trace fear conditioning 24 h later. These findings support a role for ERK phosphorylation in the formation of trace fear memories.

Activation of MAPK is necessary for long-term memory consolidation following food-reward conditioning

Although an important role for the mitogen-activated protein kinase (MAPK) has been established for memory consolidation in a variety of learning paradigms, it is not known if this pathway is also involved in appetitive classical conditioning. We address this question by using a single-trial food-reward conditioning paradigm in the freshwater snail Lymnaea stagnalis. This learning paradigm induces protein synthesis-dependent long-term memory formation. Inhibition of MAPK phosphorylation blocked long-term memory consolidation without affecting the sensory and motor abilities of the snails. Thirty minutes after conditioning, levels of MAPK phosphorylation were increased in extracts from the buccal and cerebral ganglia. These ganglia are involved in the generation, modulation, and plasticity of the feeding behavior. We also detected an increase in levels of MAPK phosphorylation in the peripheral tissue around the mouth of the snails where chemoreceptors are located. Although an increase in MAPK phosphorylation was shown to be essential for food-reward conditioning, it was also detected in snails that were exposed to the conditioned stimulus (CS) or the unconditioned stimulus (US) alone, suggesting that phosphorylation of MAPK is necessary but not sufficient for learning to occur.

Molecular Substrates for Retrieval and Reconsolidation of Cocaine-Associated Contextual Memory

Relapse into drug taking among addicts often depends on learned associations between drug-paired cues and the rewarding effects of these drugs, such as cocaine (COC). Memory for drug-paired cues resists extinction and contributes to the high rate of relapse; however, the molecular mechanisms underlying these associations are not understood. We show that COC-conditioned place preference (CPP) activates ERK, CREB, Elk-1, and Fos in the nucleus accumbens core (AcbC) but not shell. Intra-AcbC infusions of U0126, an inhibitor of the ERK kinase MEK, prevent both the activation of ERK, CREB, Elk-1, and Fos and retrieval of COC-CPP. When tested again 24 hr or 14 days after intra-AcbC infusions of U0126 or another MEK inhibitor, PD98059, CPP retrieval and concomitant protein activation were significantly attenuated. Together, these findings indicate the necessity of the AcbC ERK signaling pathway for drug-paired contextual cue memories and suggest that these strong memories can become susceptible to disruption by therapeutic agents.

Inhibition of protein phosphatase 2B upregulates serine phosphorylation of N-methyl-d-aspartate receptor NR1 subunits in striatal neurons in vivo

This study investigated the role of protein phosphatase 2B (calcineurin) in regulating phosphorylation of N-methyl-D-aspartate receptor (NMDAR) NR1 subunits and other phosphoproteins in the rat striatum in vivo. In chronically cannulated rats, microinjection of the calcineurin selective inhibitor cyclosporin A increased phosphorylation of NMDAR NR1 subunits at serine 896 and serine 897 in the injected dorsal striatum. The increase in NMDAR NR1 phosphorylation was dose-dependent in a dose range surveyed (0.005, 0.05, and 0.5 nmol). Parallel with increased serine phosphorylation of NR1 subunits, cyclosporin A dose-dependently increased phosphorylation of a Ca2+-sensitive protein kinase, extracellular signal-regulated protein kinase 1/2 (ERK1/2), and a Ca2+/cAMP-sensitive transcription factor, cAMP response element-binding protein (CREB), in the dorsal striatum. Using an immediate early gene product Fos as a reporter of inducible gene expression, cyclosporin A was found to upregulate Fos expression in the dorsal striatum. These results indicate that calcineurin plays an important role in the tonic dephosphorylation of NMDAR NR1 subunits and other two key cytoplasmic and nuclear signaling proteins (ERK1/2 and CREB) in striatal neurons.

Contributions of the mitogen-activated protein kinase and protein kinase C cascades in spatial learning and memory mediated by the nucleus accumbens

Several studies have reported a role for the nucleus accumbens (NAcc) in learning and memory. Specifically, NAcc seems to function as a neural bridge for the translation of corticolimbic information to the motor system mediating locomotor learning, but the signaling mechanisms involved in this striatal learning await further investigation. The present experiments investigated the role of the mitogen-activated protein kinase (MAPK) and protein kinase C (PKC) cascades within the NAcc of Long-Evans rats in a food-search spatial learning task (FSSLT). First, we used immunoblotting to examine changes in MAPK p42/p44 phosphorylation within the NAcc in the acquisition phase of the FSSLT. Second, we examined the effect on the acquisition and retention phases in the FSSLT of pretraining intra-accumbal microinjections of the MAPK [U0126; 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophynyltio)butadiene, 1 microg/side] or PKC [GF109203X; bisindolylmaleimide or 1-(3-dimethylaminopropyl)-indol-3-yl]-3-(indol-3-yl) maleimide, 0.5 ng/side] inhibitors (four training sessions; one session/day). Third, the potential coupling of PKC and MAPK signaling pathways in the NAcc in spatial learning was studied using microinjections of GF109203X, radioactive activity assays, and immunoblotting. Results showed that 1) MAPK p42/p44 phosphorylation is augmented within the NAcc after spatial learning, 2) MAPK and PKC inhibition caused differential deficits in the acquisition and formation of spatial memories, and 3) inhibition of PKC activity by GF109203X caused a reduction in MAPKs phosphorylation in the NAcc in an early stage of the acquisition phase. Overall, these findings suggest that NAcc-PKC and -MAPK play important roles in spatial learning and that MAPKs phosphorylation seems to be mediated through the activation of the PKC signaling pathway.

Group I metabotropic glutamate receptors, mGlu1a and mGlu5a, couple to cyclic AMP response element binding protein (CREB) through a common Ca2+ - and protein kinase C-dependent pathway

Coupling of the group I metabotropic glutamate receptors, mGlu1a and mGlu5a, to the cAMP response element binding protein (CREB) has been studied in Chinese hamster ovary cell lines where receptor expression is under the control of an inducible promoter. Both receptors stimulate CREB phosphorylation with similar time courses, and agonist potency was also comparable between the two receptors. Stimulation of cells in Ca(2+)-free medium containing EGTA (100 microm), with or without the additional depletion of intracellular stores, caused marked decreases in agonist-mediated responses in both cell lines. Down-regulation of protein kinase C (PKC) activity by phorbol ester treatment, or treatment with the broad spectrum PKC inhibitor Ro 31-8220, partially attenuated both mGlu1a and mGlu5a receptor-mediated responses. Furthermore, stimulation of cells in the absence of extracellular Ca(2+) following prior PKC down-regulation resulted in additive inhibitory effects. The involvement of extracellular signal-regulated kinases (ERK1/2), Ca(2+)/calmodulin or Ca(2+)/calmodulin-dependent protein kinases was assessed using pharmacological inhibitors. Results indicated that coupling of the group I mGlu receptors to CREB phosphorylation occurs independently of these pathways. Thus, although the [Ca(2+)](i) signatures activated by these mGlu receptors differ, they couple to CREB with comparable potency and recruit similar downstream components to execute CREB phosphorylation.

Dissociation of the effect of spatial behaviors on the phosphorylation of cAMP-response element binding protein (CREB) within the nucleus accumbens

Several studies have reported a role for the nucleus accumbens (NAcc) in learning and synaptic plasticity. Many of them suggest that the NAcc is involved in translating cortico-limbic information to the motor system mediating spatial learning and memory processes. Previous studies from our laboratory have shown that protein kinase C is activated following training in a food search spatial learning task. The present study further characterizes the molecular substrates associated with NAcc-dependent spatial behavior. The cyclic AMP-response element binding protein (CREB), a transcription factor implicated in the formation of long-term memory, was studied in the NAcc following spatial training in a food search spatial learning task. Western blots were performed to detect phosphorylated (activated) and total CREB protein levels. Our results show that CREB is significantly phosphorylated in the NAcc 48 h after habituation and at 5 min and 1 h after the first spatial training session in comparison with the naive animals that remained in their home cages. Since published data show that NAcc plays a role in novelty detection and reactivity, we conducted further experiments in order to dissociate the effect on CREB phosphorylation and expression of spatial novelty (single exposure), exploration, and spatial learning in the food search apparatus. Results show that CREB phosphorylation is significantly increased 48 h after exposure to a novel environment. The present study suggests that CREB phosphorylation observed in the NAcc during habituation and spatial training may be mainly triggered by detection of spatial novelty.

Potential role of cAMP response element-binding protein in ethanol-induced N-methyl-D-aspartate receptor 2B subunit gene transcription in fetal mouse cortical cells

We have shown previously that long-term ethanol treatment causes an up-regulation of N-methyl-D-aspartate (NMDA) receptor 2B subunit (NR2B) number and function in cultured fetal mouse cortical neurons. To examine the intracellular signaling pathways involved in this NR2B gene transcription, we have subjected fetal cortical neurons to long-term treatment with ethanol and studied its effect on cAMP response element-binding protein (CREB) and extracellular signal-regulated kinase (ERK) levels by Western blot and enzyme-linked immunosorbent assay. We find a significant increase in phosphorylated CREB, without change in total CREB protein, in cells treated with ethanol for 5 days. Long-term ethanol treatment did not increase levels of both total and phospho-ERK in serum-free medium, whereas it did increase ERK phosphorylation in medium containing serum, without affecting total ERK levels. CREB phosphorylation was increased by ethanol treatment in both media, irrespective of the presence of serum. Electrophoretic mobility shift assay, using a 25-base pair (bp) double-stranded DNA fragment containing the cyclic AMP response element (CRE)-like sequence of the NR2B promoter as (32)P-labeled probe, showed an increase in specific CRE binding to nuclear proteins isolated from cells undergoing long-term ethanol treatment. A 467-bp DNA fragment of the NR2B promoter containing the CRE sequence cloned into the luciferase vector exhibited high reporter activity in transient cotransfection assay of mouse cortical neurons, and ethanol treatment increased this activity. Introducing site-directed mutation in the CRE sequence significantly reduced the reporter activity relative to the wild-type construct, and it also abolished the stimulatory effect by ethanol. Our results indicate that CREB is probably involved in mediating ethanol-induced up-regulation of NR2B gene.

Impaired SynGAP expression and long-term spatial learning and memory in hippocampal CA1 area from rats previously exposed to perinatal hypoxia-induced insults: beneficial effects of A68930

Hypoxic encephalopathy is a common cause of neonatal seizures and long-term neurological cognitive deficits. In rats at postnatal days 10-12 (P10-P12), global hypoxia induced spontaneous seizures and chronic brain injury, mimicking clinical aspects of neonatal hypoxia. Synaptic Ras-GTPase activating protein (SynGAP) has important roles in RAS/MAPK-dependent synaptic plasticity and mammalian learning. We investigated possible alterations of SynGAP expression occurring in memory-impaired animals previously exposed to perinatal hypoxia insults. We also evaluated the therapeutic efficacy of A68930, a selective agonist of dopamine D1/D5 receptors, on perinatal hypoxia insults. In the hippocampal CA1 region, perinatal hypoxia insults (P10) led to a reduction in SynGAP expression associated with impairment in long-term spatial learning and memory performance at P45. The use of A68930 (at a dose of 1, 2, 3mg/kg, P17-P23) effectively attenuated the deleterious effects as described above. Our results may indicate the involvement of SynGAP in certain forms of brain injury, leading to long-term learning and memory deficits. A68930 may have clinical potential as a therapeutic agent for alleviation of long-term cognitive deficits in rats and other animal models.

Kainate mediates nuclear factor-kappa B activation in hippocampus via phosphatidylinositol-3 kinase and extracellular signal-regulated protein kinase

The transcription factor nuclear factor-kappa B (NF-kappaB) is an inducible regulator of genes that plays a crucial role in the nervous system. Glutamate receptor stimulation is one well-described mechanism for NF-kappaB activation. In the studies presented here we used the glutamate analog, kainate to investigate the signaling mechanisms that couple to NF-kappaB activation in hippocampus. Kainate (250 nM) application to hippocampal slices elicited a time-dependent increase in nuclear NF-kappaB levels in areas CA3 and CA1, but not dentate, compared with controls. Further analysis focused on hippocampal area CA3, revealed increased NF-kappaB DNA binding activity in response to kainate stimulation. Supershift electrophoretic mobility shift assay indicated that the kainate-mediated NF-kappaB complex binding DNA was composed of p65, p50, and c-Rel subunits. Through inhibition studies we found that extracellular signal-regulated protein kinase (ERK) and phosphatidylinositol-3 kinase (PI3K) couple to basal and kainate-mediated NF-kappaB DNA binding activity in area CA3. Kainate elicited decreased total and increased phospho-inhibitor kappa B alpha (IkappaBalpha), suggesting that kainate-mediated activation of NF-kappaB is via the classical IkappaB kinase pathway. Interestingly, inhibition of ERK but not PI3K blocked the kainate-mediated increase in phospho-IkappaBalpha. Thus, our findings support a role for the ERK and PI3K pathways in kainate-mediated NF-kappaB activation in hippocampal area CA3, but these kinases may target the NF-kappaB pathway at different loci.

Increased expression of G11α in osteoblastic cells enhances parathyroid hormone activation of phospholipase C and AP-1 regulation of matrix metalloproteinase-13 mRNA

In osteoblasts parathyroid hormone (PTH) stimulates the PTH/PTH-related peptide (PTHrP) receptor (PTH1R) that couples via G(s) to adenylyl cyclase stimulation and via G(11) to phospholipase C (PLC) stimulation. We have investigated the effect of increasing G(11)alpha levels in UMR 106-01 osteoblastic cells by transient transfection with cDNA encoding G(11)alpha on PTH stimulation of PLC and protein kinase C (PKC) as well as PTH regulation of mRNA encoding matrix metalloproteinase-13 (MMP-13). Transfection with G(11)alpha cDNA resulted in a 5-fold increase in PTH-stimulated PLC activity with no change in PTH-stimulated adenylyl cyclase. PTH-induced translocation of PKC-betaI, -delta, and -zeta to the cell membrane and PKC-zeta to the nucleus was also increased. Increased G(11)alpha protein resulted in increased stimulation of MMP-13 mRNA levels at all doses of PTH. There was a 2.5 +/- 0.35 fold increase in maximal PTH-stimulation of c-jun mRNA and smaller but significant increases in c-fos accompanied by increased basal and PTH-stimulated AP-1 binding in cells expressing increased G(11)alpha. Runx-2 mRNA and protein levels were not significantly increased by increased G(11)alpha expression. The increase in PTH stimulation of c-jun, c-fos, and MMP-13 in G(11)alpha-transfected cells were all blocked by bisindolylmaleimide I, a selective inhibitor of PKC. These results demonstrate that regulation of the PLC pathway through the PTH1R is significantly increased by elevating expression of G(11)alpha in osteoblastic cells. This leads to increased PTH stimulation of MMP-13 expression by increased stimulation of AP-1 factors c-jun and c-fos.

Regulation of extracellular signal-regulated kinase by homocysteine in hippocampus

In several neurological disorders including hyperhomocysteinemia, homocysteine (Hcy) accumulates in the brain, and acts as a potent neurotoxin. However, the molecular mechanisms induced by increased levels of Hcy in brain are not well understood. Here we show an activation of the extracellular signal-regulated kinases (ERK1 and ERK2) and the downstream nuclear targets Elk-1 and calcium/cAMP response element binding protein, in the hippocampus of cystathionine beta synthase deficient mice, a murine model of hyperhomocysteinemia. An ex vivo model of hippocampal slices allowed us to reproduce Hcy -induced ERK activation and to unravel the mechanisms responsible of this activation. Of interest, N-methyl-d-aspartate (NMDA), non-NMDA and metabotropic glutamate receptor antagonists all blocked Hcy -induced ERK activation. Moreover, the ERK activation was blocked in the presence of Na+-channel blocker tetrodotoxin, indicating the existence of a trans-synaptic activity in ERK activation by Hcy in hippocampal slices. The effects of Hcy on ERK cascade activation were also dependent on calcium influx, CaMK-II, PKC as well as PKA activation. Thus, altogether these data support a role of Hcy on ERK activation, via complex mechanisms, starting with a control of glutamate release, which in turn activates ionotropic and metabotropic receptor subtypes and produces increases in intracellular calcium levels.

Inhibition of acetylcholine-induced activation of extracellular regulated protein kinase prevents the encoding of an inhibitory avoidance response in the rat

It has been demonstrated that the forebrain cholinergic system and the extracellular regulated kinase signal transduction pathway are involved in the mechanisms of learning, encoding, and storage of information. We investigated the involvement of the cholinergic and glutamatergic systems projecting to the medial prefrontal cortex and ventral hippocampus and of the extracellular regulated kinase signal transduction pathway in the acquisition and recall of the step-down inhibitory avoidance response in the rat, a relatively simple behavioral test acquired in a one-trial session. To this aim we studied by microdialysis the release of acetylcholine and glutamate, and by immunohistochemistry the activation of extracellular regulated kinase during acquisition, encoding and recall of the behavior. Cholinergic, but not glutamatergic, neurons projecting to the medial prefrontal cortex and ventral hippocampus were activated during acquisition of the task, as shown by increase in cortical and hippocampal acetylcholine release. Released acetylcholine in turn activated extracellular regulated kinase in neurons located in the target structures, since the muscarinic receptor antagonist scopolamine blocked extracellular regulated kinase activation. Both increased acetylcholine release and extracellular regulated kinase activation were necessary for memory formation, as administration of scopolamine and of extracellular regulated kinase inhibitors was followed by blockade of extracellular regulated kinase activation and amnesia. Our data indicate that a critical function of the learning-associated increase in acetylcholine release is to promote the activation of the extracellular regulated kinase signal transduction pathway and help understanding the role of these systems in the encoding of an inhibitory avoidance memory.

Regulation of the CREB signaling cascade in the visual cortex by visual experience and neuronal activity

The cAMP-responsive element (CRE) regulatory pathway has been studied as a model of signal-regulated transcription and is critical for some forms of learning and adaptation. In cell culture systems, the extracellular-regulated kinase (ERK) and ribosomal S6 kinase (RSK) couple synaptic signals to CRE-mediated gene expression by modulating CRE-binding protein (CREB) phosphorylation. However, it is not known whether sensory experience regulates gene expression in the brain by this mechanism. In this study, we ask: Are activated forms of ERK, RSK, and CREB colocalized in the cortex and are they coordinately regulated by synaptic signals? We find that these three signaling components are regulated in distinct ways. First, cells that show CRE-lacZ reporter expression, primarily excitatory neurons, do not colocalize with cells containing phospho-ERK. Second, while phosphorylation of ERK and RSK are modulated by visual experience, phosphorylation of CREB at serines 133, 142, or 143 is detected constitutively and is unaffected by experience. This finding suggests that neural activity might not regulate CREB phosphorylation in vivo. To test this hypothesis, we blocked action potentials by injection of tetrodotoxin and found no effect on CREB phosphorylation. These in vivo data show that, in contrast to cell culture systems, cortical synaptic activity controls CRE-mediated gene expression without affecting CREB phosphorylation, possibly by modification of RSK and CREB-associated coregulators.

8-Cl-cAMP and its metabolite, 8-Cl-adenosine induce growth inhibition in mouse fibroblast DT cells through the same pathways: protein kinase C activation and cyclin B down-regulation

8-Chloro-cyclic AMP (8-Cl-cAMP) is known to be most effective in inducing growth inhibition and differentiation of a number of cancer cells. Also, its cellular metabolite, 8-Cl-adenosine was shown to induce growth inhibition in a variety of cell lines. However, the signaling mechanism that governs the effects of 8-Cl-cAMP and/or 8-Cl-adenosine is still uncertain and it is not even sure which of the two is the key molecule that induces growth inhibition. In this study using mouse fibroblast DT cells, it was found that adenosine kinase inhibitor and adenosine deaminase could reverse cellular growth inhibition induced by 8-Cl-cAMP and 8-Cl-adenosine. And 8-Cl-cAMP could not induce growth inhibition in the presence of phosphodiesterase (PDE) inhibitor, but 8-Cl-adenosine could. We also found that protein kinase C (PKC) inhibitor could restore this growth inhibition, and both the 8-Cl-cAMP and 8-Cl-adenosine could activate the enzymatic activity of PKC. Besides, after 8-Cl-cAMP and 8-Cl-adenosine treatment, cyclin B was down-regulated and a CDK inhibitor, p27 was up-regulated in a time-dependent manner. These results suggest that it is not 8-Cl-cAMP but 8-Cl-adenosine which induces growth inhibition, and 8-Cl-cAMP must be metabolized to exert this effect. Furthermore, there might exist signaling cascade such as PKC activation and cyclin B down-regulation after 8-Cl-cAMP and 8-Cl-adenosine treatment.

Serotonin regulates the secretion and autocrine action of a neuropeptide to activate MAPK required for long-term facilitation in Aplysia

In Aplysia, long-term facilitation (LTF) of sensory neuron synapses requires activation of both protein kinase A (PKA) and mitogen-activated protein kinase (MAPK). We find that 5-HT through activation of PKA regulates secretion of the sensory neuron-specific neuropeptide sensorin, which binds autoreceptors to activate MAPK. Anti-sensorin antibody blocked LTF and MAPK activation produced by 5-HT and LTF produced by medium containing sensorin that was secreted from sensory neurons after 5-HT treatment. A single application of 5-HT followed by a 2 hr incubation with sensorin produced protein synthesis-dependent LTF, growth of new presynaptic varicosities, and activation of MAPK and its translocation into sensory neuron nuclei. Inhibiting PKA during 5-HT applications and inhibiting receptor tyrosine kinase or MAPK during sensorin application blocked both LTF and MAPK activation and translocation. Thus, long-term synaptic plasticity is produced when stimuli activate kinases in a specific sequence by regulating the secretion and autocrine action of a neuropeptide.

A role for ERKII in synaptic pattern selectivity on the time-scale of minutes

Stimulus reinforcement strengthens learning. Intervals between reinforcement affect both the kind of learning that occurs and the amount of learning. Stimuli spaced by a few minutes result in more effective learning than when massed together. There are several synaptic correlates of repeated stimuli, such as different kinds of plasticity and the amplitude of synaptic change. Here we study the role of signalling pathways in the synapse on this selectivity for spaced stimuli. Using the in vitro hippocampal slice technique we monitored long-term potentiation (LTP) amplitude in CA1 for repeated 100-Hz, 1-s tetani. We observe the highest LTP levels when the inter-tetanus interval is 5-10 min. We tested biochemical activity in the slice following the same stimuli, and found that extracellular signal-regulated kinase type II (ERKII) but not CaMKII exhibits a peak at about 10 min. When calcium influx into the slice is buffered using AM-ester calcium dyes, amplitude of the physiological and biochemical response is reduced, but the timing is not shifted. We have previously used computer simulations of synaptic signalling to predict such temporal tuning from signalling pathways. In the current study we consider feedback and feedforward models that exhibit temporal tuning consistent with our experiments. We find that a model incorporating post-stimulus build-up of PKM zeta acting upstream of mitogen-activated protein kinase is sufficient to explain the observed temporal tuning. On the basis of these combined experimental and modelling results we propose that the dynamics of PKM activation and ERKII signalling may provide a mechanism for functionally important forms of synaptic pattern selectivity.

Tyrosine hydroxylase induction by basic fibroblast growth factor and cyclic AMP analogs in striatal neural stem cells: role of ERK1/ERK2 mitogen-activated protein kinase and protein kinase C

Neural stem cells (NSC) with self-renewal and multilineage potential are considered good candidates for cell replacement of damaged nervous tissue. In vitro experimental conditions can differentiate these cells into specific neuronal phenotypes. In the present study, we describe the combined effect of basic fibroblast growth factor (bFGF) and dibutyryladenosine 3',5'-cyclic monophosphate (dbcAMP) on the differentiation of fetal rat striatal NSC into tyrosine hydroxylase-positive cells. Tyrosine hydroxylase induction was accompanied by the activation of ERK1/ERK2 mitogen-activated protein kinase and was inhibited by the ERK1/ERK2 pathway blocker PD98059, suggesting that ERK activation may be important for this process. In addition, protein kinase C (PKC) was shown to be required for tyrosine hydroxylase protein expression. The inhibition of PKC by staurosporin, as well as its downregulation, decreased the ability of bFGF+dbcAMP to generate tyrosine hydroxylase-positive cells. Moreover, the PKC activator phorbol 12-myristate 13-acetate (PMA) together with bFGF and dbcAMP led to a significant increase in phospho-ERK1/ERK2 levels, and the percentage of beta-tubulin III-positive cells that expressed tyrosine hydroxylase increased by 3.5-fold. PMA also promoted the phosphorylation of the cyclic AMP response element binding protein that might contribute to the increase in tyrosine hydroxylase-positive cells observed in bFGF+dbcAMP+PMA-treated cultures. From these results, we conclude that the manipulation in vitro of NSC from rat fetal striatum with bFGF, cyclic AMP analogs, and PKC activators promotes the generation of tyrosine hydroxylase-positive neurons.

Place preference induced by nucleus accumbens amphetamine is impaired by antagonists of ERK or p38 MAP kinases in rats

The nucleus accumbens (NAc) plays a role in conditioned place preference (CPP). The authors tested the hypothesis that inhibition of mitogen-activated protein kinases (MAPKs) would inhibit NAc-amphetamine-produced CPP. Results confirmed that NAc amphetamine increased levels of the MAPK extracellular signal-regulated kinase (ERK). In CPP studies, NAc injections (0.5 microl per side) of the ERK inhibitor PD98059 (1.0-2.5 microg) or the p38 kinase inhibitor SB203580 (15-500 ng) dose dependently impaired CPP. The c-Jun-N-terminal kinase (JNK) inhibitor SP600125 (1.0-2.5 microg) failed to block the CPP effect. The drugs did not block amphetamine-induced motor activity. Results suggest that ERK and p38, but not JNK, MAPKs may be necessary for the establishment of NAc amphetamine-produced CPP and may also mediate other forms of reward-related learning dependent on NAc.

Aprendizado e memória

A memória é dividida de duas grandes formas: explícita e implícita. O hipocampo é necessário para a formação das memórias explícitas, ao passo que várias outras regiões do cérebro, incluindo o estriado, a amígdala e o nucleus accumbens, estão envolvidos na formação das memórias implícitas. A formação de todas as memórias requer alterações morfológicas nas sinapses: novas sinapses devem ser formadas ou antigas precisam ser fortalecidas. Considera-se que essas alterações reflitam a base celular subjacente das memórias persistentes. Consideráveis avanços têm ocorrido na última década em relação a nossa compreensão sobre as bases moleculares da formação dessas memórias. Um regulador-chave da plasticidade sináptica é uma via de sinalização que inclui a proteína-quinase ativada por mitógenos (MAP). Como essa via é necessária para a memória e o aprendizado normais, não é surpreendente que as mutações nos membros dessa via levem a prejuízos no aprendizado. A neurofibromatose, a síndrome de Coffin-Lowry e a de Rubinstein-Taybi são três exemplos de transtornos de desenvolvimento que apresentam mutações em componentes-chave na via de sinalização da proteína-quinase MAP.

Heterosynaptic co-activation of glutamatergic and dopaminergic afferents is required to induce persistent long-term potentiation

The persistence of protein synthesis-dependent long-term potentiation (late-LTP) is thought to require heterosynaptic activation of both glutamate and neuromodulatory receptors in the hippocampus. The present series of experiments contrasts two alternative accounts of heterosynaptic activation. The original version of the synaptic-tag hypothesis of the variable persistence of LTP implied that neuromodulatory and glutamatergic activation could occur independently, albeit within a critical time-window; an alternative view is that there needs to be simultaneous co-activation of both receptors to trigger the up-regulation of relevant protein synthesis (Neuron 34 (2002) 235). Our findings include a replication, over 6 h post-LTP-induction, of earlier findings showing heterosynaptic influences on LTP persistence. Specifically, 'strong' tetanisation with multiple trains of stimulation of one input pathway in a conventional hippocampal slice preparation induces a D1/D5 receptor-dependent form of late-LTP that enables 'weak' tetanic stimulation to induce late-LTP on an independent pathway. However, we also observed that when the first pathway was tetanised in the presence of AP5, not only was no LTP observed on that pathway, but there was also no rescue of late-LTP on the second pathway. Thus, it appears that DA receptors must be co-activated with NMDA receptors in a common pool of neurons to enable LTP persistence, although late-LTP can still be induced by selective activation of glutamatergic synapses if this occurs at time periods shortly before or shortly after this essential coactivation.

Inhibition of the phosphodiesterase 4 (PDE4) enzyme reverses memory deficits produced by infusion of the MEK inhibitor U0126 into the CA1 subregion of the rat hippocampus

Cyclic AMP-specific phosphodiesterase 4 (PDE4), which is an integral component of NMDA receptor-mediated cAMP signaling, is involved in the mediation of memory processes. Given that NMDA receptors also mediate MEK/mitogen-activated protein kinase (MAPK, ERK) signaling, which is involved in synaptic plasticity, and that some PDE4 subtypes are phosphorylated and regulated by ERK, it was of interest to determine if PDE4 is involved in MEK/ERK signaling-mediated memory. It was found that rolipram, a PDE4-selective inhibitor, reversed the amnesic effect in the radial-arm maze test of the MEK inhibitor U0126 administered into the CA1 subregion of the rat hippocampus. Consistent with this, rolipram, either by peripheral administration or direct intra-CA1 infusion, enhanced the retrieval of long-term memory impaired by intra-CA1 infusion of U0126 using the step-through inhibitory avoidance test. The same dose of rolipram did not affect U0126-induced reduction of phospho-ERK1/2 levels in the CA1 subregion. However, in primary cultures of rat cerebral cortical neurons, pretreatment with U0126 increased PDE4 activity; this was correlated with the U0126-induced reduction of phospho-ERK1/2 levels. These results suggest that MEK/ERK signaling plays an inhibitory role in regulating PDE4 activity in the brain; this may be a novel mechanism by which MEK/ERK signaling mediates memory. PDE4 is likely to be an important link between the cAMP/PKA and MEK/ERK signaling pathways in the mediation of memory.

Depolarization of skeletal muscle cells induces phosphorylation of cAMP response element binding protein via calcium and protein kinase Calpha

Membrane depolarization of skeletal muscle cells induces slow inositol trisphosphate-mediated calcium signals that regulate the activity of transcription factors such as the cAMP-response element-binding protein (CREB), jun, and fos. Here we investigated whether such signals regulate CREB phosphorylation via protein kinase C (PKC)-dependent pathways. Western blot analysis revealed the presence of seven isoforms (PKCalpha, -betaI, -betaII, -delta, -epsilon, -, and -zeta) in rat primary myotubes. The PKC inhibitors bisindolymaleimide I and Gö6976, blocked CREB phosphorylation. Chronic exposure to phorbol ester triggered complete down-regulation of several isoforms, but reduced PKCalpha levels to only 40%, and did not prevent CREB phosphorylation upon myotube depolarization. Immunocytochemical analysis revealed selective and rapid PKCalpha translocation to the nucleus following depolarization, which was blocked by 2-amino-ethoxydiphenyl borate, an inositol trisphosphate receptor inhibitor, and by the phospholipase C inhibitor U73122. In C2C12 cells, which expressed PKCalpha,-epsilon, and -zeta, CREB phosphorylation also depended on PKCalpha. These results strongly implicate nuclear PKCalpha translocation in CREB phosphorylation induced by skeletal muscle membrane depolarization.

Chronic food restriction increases D-1 dopamine receptor agonist-induced phosphorylation of extracellular signal-regulated kinase 1/2 and cyclic AMP response element-binding protein in caudate-putamen and nucleus accumbens

Results of behavioral and c-fos immunohistochemical studies have suggested that chronic food restriction and maintenance of animals at 75-80% of free-feeding body weight may increase d-1 dopamine (DA) receptor function. The purpose of the present study was to determine whether D-1 DA receptor binding and/or mitogen-activated protein kinase (MAPK) signaling in caudate-putamen (CPu) and nucleus accumbens (NAc) are increased in food-restricted subjects. In the first experiment, saturation binding of the D-1 DA receptor antagonist [3H]SCH-23390 indicated no difference between food-restricted and ad libitum fed rats with regard to density or affinity of d-1 binding sites in CPu or NAc. In the second experiment, activation of extracellular signal-regulated kinases (ERK1/2) and cyclic AMP response element-binding protein (CREB) by i.c.v. injection of the D-1 DA receptor agonist SKF-82958 (20 microg) were markedly greater in food-restricted than ad libitum fed rats. Given a prior finding that SKF-82958 does not differentially stimulate adenylyl cyclase in CPu or NAc of food-restricted versus ad libitum fed subjects, the present results suggest that increased D-1 DA receptor-mediated ERK1/2 MAP kinase signaling may mediate the enhanced downstream activation of CREB, c-fos, and behavioral responses in food-restricted subjects. It is of interest that food restriction also increased the activation of c-Jun N-terminal protein kinase/stress-activated protein kinase, but this effect was no greater in rats injected with SKF-82958 than in those injected with saline vehicle. This represents additional evidence of increased striatal cell signaling in food-restricted subjects, presumably in response to the i.c.v. injection procedure, although the underlying receptor mechanisms remain to be determined. There were no differences between feeding groups in protein levels of the major phosphatases, MKP-2 and PP1. The upregulation of striatal MAP kinase signaling in food-restricted animals may adaptively serve to facilitate associative learning but, at the same time, increase vulnerability to the rewarding and addictive properties of abused drugs.

Mediation of BMP7 neuroprotection by MAPK and PKC IN rat primary cortical cultures

We have previously demonstrated that pretreatment with bone morphogenetic protein 7 (BMP7), a trophic factor in the TGFbeta superfamily, reduces ischemia-induced brain infarction induced by middle cerebral artery ligation in rats. Since the mitogen-activated protein kinase (MAPK) pathway is involved in many TGFbeta-mediated responses, we examined the interaction of BMP7 and MAPK in primary cultures obtained from the cerebral cortex of E16-17 rat embryos. Lactate dehydrogenase (LDH) in the media was used as an index of cell death. BMP7 did not alter LDH levels at low concentration (1.25 nM), but exhibited increased cellular toxicity at higher concentration (>12.5 nM). BMP7 at the low concentration significantly attenuated H2O2-induced increases in LDH activity and decreases in neuronal density. Pharmacological interactions were used to examine if MAPK was involved in this response. BMP7-induced protection was antagonized by the p42,44 MAPK kinase inhibitors PD98059 and U0125. The p38 MAPK antagonist SB203580, and their inactive analog SB202474, also attenuated BMP7-induced protection, suggesting that the interaction with p38 MAPK is nonspecific. Previous studies have indicated that SB202474 has inhibitory effects on other protein kinases. We found that the protein kinase C inhibitor chelerythrine antagonized BMP7-induced protection against H2O2. Western blot analysis indicated that BMP7 increased phosphorylation of p42,44 MAPK and PKC. Taken together, our data suggest that BMP7 is neuroprotective at low concentrations in primary cortical cell culture. The protective effects of BMP7 may involve the activation of p42,44 MAPK and PKC.

Mitogen-activated protein kinases in synaptic plasticity and memory

This review highlights five areas of recent discovery concerning the role of extracellular-signal regulated kinases (ERKs) in the hippocampus. First, ERKs have recently been directly implicated in human learning through studies of a human mental retardation syndrome. Second, new models are being formulated for how ERKs contribute to molecular information processing in dendrites. Third, a role of ERKs in stabilizing structural changes in dendritic spines has been defined. Fourth, a crucial role for ERKs in regulating local dendritic protein synthesis is emerging. Fifth, the importance of ERK interactions with scaffolding and structural proteins at the synapse is increasingly apparent. These topics are discussed within the context of an emerging role for ERKs in a wide variety of forms of synaptic plasticity and memory formation in the behaving animal.

4-Aminopyridine-induced epileptogenesis depends on activation of mitogen-activated protein kinase ERK

Extracellular signal-regulated kinases such as ERK1 [p44 mitogen-activated protein kinase (MAPK)] and ERK2 (p42 MAPK) are activated in the CNS under physiological and pathological conditions such as ischemia and epilepsy. Here, we studied the activation state of ERK1/2 in rat hippocampal slices during application of the K(+) channel blocker 4-aminopyridine (4AP, 50 micro m), a procedure that enhances synaptic transmission and leads to the appearance of epileptiform activity. Hippocampal slices superfused with 4AP-containing medium exhibited a marked activation of ERK1/2 phosphorylation that peaked within about 20 min. These effects were not accompanied by changes in the activation state of c-Jun N-terminal kinase (JNK), another member of the MAP kinase superfamily. 4AP-induced ERK1/2 activation was inhibited by the voltage-gated Na(+) channel blocker tetrodotoxin (1 micro m). We also found that application of the ERK pathway inhibitors U0126 (50 micro m) or PD98059 (100 micro m) markedly reduced 4AP-induced epileptiform synchronization, thus abolishing ictal discharges in the CA3 area. The effects induced by U0126 or PD98059 were not associated with changes in the amplitude and latency of the field potentials recorded in the CA3 area following electrical stimuli delivered in the dentate hylus. These data demonstrate that activation of ERK1/2 accompanies the appearance of epileptiform activity induced by 4AP and suggest a cause-effect relationship between the ERK pathway and epileptiform synchronization.

Activity-driven sharpening of the retinotectal projection: the search for retrograde synaptic signaling pathways

Patterned visual activity, acting via NMDA receptors, refines developing retinotectal maps by shaping individual retinal arbors. Because NMDA receptors are postsynaptic but the retinal arbors are presynaptic, there must be retrograde signals generated downstream of Ca(++) entry through NMDA receptors that direct the presynaptic retinal terminals to stabilize and grow or to withdraw. This review defines criteria for retrograde synaptic messengers, and then applies them to the leading candidates: nitric oxide (NO), brain-derived neurotrophic factor (BDNF), and arachidonic acid (AA). NO is not likely to be a general mechanism, as it operates only in selected projections of warm blooded vertebrates to speed up synaptic refinement, but is not essential. BDNF is a neurotrophin with strong growth promoting properties and complex interactions with activity both in its release and receptor signaling, but may modulate rather than mediate the retrograde signaling. AA promotes growth and stabilization of synaptic terminals by tapping into a pre-existing axonal growth-promoting pathway that is utilized by L1, NCAM, N-cadherin, and FGF and acts via PKC, GAP43, and F-actin stabilization, and it shares some overlap with BDNF pathways. The actions of both are consistent with recent demonstrations that activity-driven stabilization includes directed growth of new synaptic contacts. Certain nondiffusible factors (synapse-specific CAMs, ephrins, neurexin/neuroligin, and matrix molecules) may also play a role in activity-driven synapse stabilization. Interactions between these pathways are discussed.

Serotonin-mediated phosphorylation of extracellular regulated kinases in platelets of patients with panic disorder versus controls

Phosphorylation of extracellular signal-regulated kinases (ERK 1/2) represents a converging intracellular signalling pathway which is involved in the modulation of gene transcription and may contribute to the feed-back regulation of neurotransmitter receptor functioning. The purpose of the current study was to investigate the serotonin-mediated phosphorylation of ERK 1/2 in platelets from patients (n = 17) with panic disorder, with respect to healthy volunteers (n = 17). Patients presented a severe symptomatology as assessed by the self-report rating scales for panic-agoraphobic (PAS-SR) and mood (MOOD-SR) spectrum, and by Clinical Global Impression Severity Scale (CGI-S). In platelets from healthy volunteers, serotonin induced a rapid increase of ERK 1/2 phosphorylation with a transient monophasic kinetic. The dose-response curves showed this effect was concentration dependent with an average of the EC(50) value of 22.8 +/- 2.4 microM. Platelet pre-incubation with 5HT(1A) and 5HT(2A) antagonists, pindobind and ritanserin, significantly inhibited serotonin-mediated kinase activation with an EC(50) of 3.2 +/- 0.2 and 1.99 +/- 0.08 nM, respectively, suggesting an involvement of these specific receptor subtypes in serotonin-mediated response. Furthermore, the 5HT(1A) and 5HT(2A) agonists, 8-hydroxy-N,N-dipropyl-aminotetralin (8OH-DPAT) and 1-(2,5-dimethoxy)-4-iodophenyl-2-aminopropane (DOI), were able to modulate ERK 1/2 phosphorylation in a concentration-dependent manner with an EC(50) value of 3.1 +/- 0.2 and 76 +/- 4.5 nM, respectively. ERK 1/2 phosphorylation was not observed after serotonin treatment of platelets from drug-free panic disorder patients, suggesting an alteration in intracellular phosphorylative pathways. Since ERK 1/2 responsiveness to other stimulus, such as collagen and thrombin, was comparable in platelets from healthy volunteers and patients, our results suggested that a specific alteration of serotonergic system occurred in panic disorder. Further studies to investigate 5HT(1A) and 5HT(2A) receptor expression and threonine phosphorylation levels showed that, nevertheless no significant differences in the receptor expression levels were detected, an increase of both 5HT receptor phosphorylation, on threonine residues, occurred in platelet from panic patients with respect to controls, suggesting that a reduction of serotonin receptor functioning was involved in the loss of serotonin responsiveness in panic.

An NMDAR-independent LTP mediated by group II metabotropic glutamate receptors and p42/44 MAP kinase in the dentate gyrus in vitro

The induction of long-term potentiation (LTP) under conditions of blockade of the N-methyl-D-aspartate receptor (NMDAR) was studied in the medial perforant path to granule cell synapse in the dentate gyrus. A small amplitude NMDAR-independent potentiation was induced by a single brief high frequency stimulation (HFS), and a summated larger LTP was induced by repeated spaced HFS. The NMDAR-independent LTP was mediated by activation of group II mGluR as it was inhibited by the group II antagonists EGLU and also low concentrations of LY341495, but not the group I mGluR antagonist MPEP. Perfusion of the group II mGluR agonist DCG-IV induced NMDAR-independent LTP in media containing an NMDAR antagonist. The NMDAR-independent LTP induced by HFS was mediated via activation of p42/44 MAP kinase as it was blocked by the selective inhibitor PD98059.

The protein phosphatase 1/2A inhibitor okadaic acid increases CREB and Elk-1 phosphorylation and c-fos expression in the rat striatum in vivo

Activation of group I metabotropic glutamate receptors (mGluRs) up-regulates transcription factor cyclic AMP response element-binding protein (CREB) and Elk-1 phosphorylation via extracellular signal-regulated kinase 1/2 (ERK1/2) in the striatum in vivo. Protein phosphatase 1/2A further regulates immediate early gene expression by inactivating (dephosphorylating) CREB. In this study, using semi-quantitative immunohistochemical and western blot analyses and in situ hybridization histochemistry, we found that intrastriatal infusion of the protein phosphatase 1/2A inhibitor okadaic acid (0.005, 0.05 and 0.5 nmol) increased CREB and Elk-1 phosphorylation and c-Fos immunoreactivity in the injected dorsal striatum in a dose-dependent manner. In addition, okadaic acid (0.05 and 0.5 nM) increased c-fos mRNA expression in the dorsal striatum in a dose-dependent manner. Intrastriatal infusion of the group I agonist 3,5-dihydroxyphenylglycine (DHPG) at 100 and 250 nM also increased CREB and Elk-1 phosphorylation. Pre-treatment of okadaic acid (0.05 nm) did not alter DHPG-induced increases in the phosphorylation of the two transcription factors. These data suggest that protein phosphatase 1/2A in striatal neurons is tonically active in dephosphorylating CREB and Elk-1 and thus suppressing constitutive c-fos mRNA and protein expression. Inhibition of the phosphatase 1/2A may contribute to the group I mGluR-regulated phosphorylation of these transcription factors and c-fos expression.

In vivo regulation of extracellular signal-regulated protein kinase (ERK) and protein kinase B (Akt) phosphorylation by acute and chronic morphine

In vitro evidence suggests that extracellular signal-regulated protein kinases (ERKs) and Akt (also referred to as protein kinase B) are among the myriad of intracellular signaling molecules regulated by opioid receptors. The present study examined the regulation of ERK and Akt activation in the nucleus accumbens and caudate putamen following acute and chronic morphine administration in the rat. ERK and Akt are activated by phosphorylation, hence the levels of phosphorylated ERK (pERK) and Akt (pAkt) as well as total levels of ERK and Akt protein were measured by Western blot analysis. Male Sprague-Dawley rats received either a single injection of morphine or twice daily injections of morphine for 6 or 10 days. Following acute morphine, pERK levels were significantly decreased in the nucleus accumbens but not in the caudate putamen. Phosphorylated Akt levels in the nucleus accumbens were significantly increased after a single morphine injection. Naltrexone pretreatment prevented both the morphine-induced pERK down-regulation and pAkt up-regulation. Although reductions in pERK levels were evident after 6 days of morphine administration, no differences were observed in pERK levels after 10 days. In contrast to the up-regulation seen after acute morphine, pAkt levels in the nucleus accumbens were significantly decreased after chronic morphine administration. Thus, the differential activation patterns of both ERK and Akt after acute and chronic morphine administration could have important implications for understanding additional pathways mediating opioid signaling in vivo.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Reactivation of developmental programs: the cAMP-response element-binding protein pathway is involved in hydra head regeneration

Hydra regenerate throughout their life. We previously described early modulations in cAMP-response element-binding protein (CREB) DNA-binding activity during regeneration. We now show that the Ser-67 residue located in the P-box is a target for post-translational regulation. The antihydra CREB antiserum detected CREB-positive nuclei distributed in endoderm and ectoderm, whereas the phosphoSer133-CREB antibody detected phospho-CREB-positive nuclei exclusively in endodermal cells. During early regeneration, we observed a dramatic increase in the number of phospho-CREB-positive nuclei in head-regenerating tips, exceeding 80% of the endodermal cells. We identified among CREB-binding kinases the p80 kinase, which showed an enhanced activity and a hyperphosphorylated status during head but not foot regeneration. According to biochemical and immunological evidence, this p80 kinase belongs to the Ribosomal protein S6 kinase family. Exposure to the U0126 mitogen-activated protein kinase kinase inhibitor inhibited head but not foot regeneration, abolished CREB phosphorylation and activation of the early gene HyBra1 in head-regenerating tips. These data support a role for the mitogen-activated protein kinase/ribosomal protein S6 kinase/CREB pathway in hydra head organizer activity.

Central sensitization and LTP: do pain and memory share similar mechanisms?

Synaptic plasticity is fundamental to many neurobiological functions, including memory and pain. Central sensitization refers to the increased synaptic efficacy established in somatosensory neurons in the dorsal horn of the spinal cord following intense peripheral noxious stimuli, tissue injury or nerve damage. This heightened synaptic transmission leads to a reduction in pain threshold, an amplification of pain responses and a spread of pain sensitivity to non-injured areas. In the cortex, LTP - a long-lasting highly localized increase in synaptic strength - is a synaptic substrate for memory and learning. Analysis of the molecular mechanisms underlying the generation and maintenance of central sensitization and LTP indicates that, although there are differences between the synaptic plasticity contributing to memory and pain, there are also striking similarities.

Gene-environment interplay in neurogenesis and neurodegeneration

Factors associated with predisposition and vulnerability to neurodegenerative disorders may be described usefully within the context of gene-environment interplay. There are many identified genetic determinants for so-called genetic disorders, and it is possible to duplicate many elements of recognized human neurodegenerative disorders in either knock-in or knock-out mice. However, there are similarly, many identifiable environmental influences on outcomes of the genetic defects; and the course of a progressive neurodegenerative disorder can be greatly modified by environmental elements. Constituent cellular defense mechanisms responsive to the challenge of increased reactive oxygen species represent only one crossroad whereby environment can influence genetic predisposition. In this paper we highlight some of the major neurodegenerative disorders and discuss possible links of gene-environment interplay. The process of adult neurogenesis in brain is also presented as an additional element that influences gene-environment interplay. And the so-called priming processes (i.e., production of receptor supersensitization by repeated drug dosing), is introduced as yet another process that influences how genes and environment ultimately and co-dependently govern behavioral ontogeny and outcome. In studies attributing the influence of genetic alteration on behavioral phenotypy, it is essential to carefully control environmental influences.