Protein phosphatase-mediated regulation of protein kinase C during long-term depression in the adult hippocampus in vivo

Luteolin and its antidepressant properties: From mechanism of action to potential therapeutic application

Luteolin is a natural flavonoid compound exists in various fruits and vegetables. Recent studies have indicated that luteolin has variety pharmacological effects, including a wide range of antidepressant properties. Here, we systematically review the preclinical studies and limited clinical evidence on the antidepressant and neuroprotective effects of luteolin to fully explore its antidepressant power. Network pharmacology and molecular docking analyses contribute to a better understanding of the preclinical models of depression and antidepressant properties of luteolin. Seventeen preclinical studies were included that combined network pharmacology and molecular docking analyses to clarify the antidepressant mechanism of luteolin and its antidepressant targets. The antidepressant effects of luteolin may involve promoting intracellular noradrenaline (NE) uptake; inhibiting 5-hydroxytryptamine (5-HT) reuptake; upregulating the expression of synaptophysin, postsynaptic density protein 95, brain-derived neurotrophic factor, B cell lymphoma protein-2, superoxide dismutase, and glutathione S-transferase; and decreasing the expression of malondialdehyde, caspase-3, and amyloid-beta peptides. The antidepressant effects of luteolin are mediated by various mechanisms, including anti-oxidative stress, anti-apoptosis, anti-inflammation, anti-endoplasmic reticulum stress, dopamine transport, synaptic protection, hypothalamic-pituitary-adrenal axis regulation, and 5-HT metabolism. Additionally, we identified insulin-like growth factor 1 receptor (IGF1R), AKT serine/threonine kinase 1 (AKT1), prostaglandin-endoperoxide synthase 2 (PTGS2), estrogen receptor alpha (ESR1), and epidermal growth factor receptor (EGFR) as potential targets, luteolin has an ideal affinity for these targets, suggesting that it may play a positive role in depression through multiple targets, mechanisms, and pathways. However, the clinical efficacy of luteolin and its potential direct targets must be confirmed in further multicenter clinical case-control and molecular targeting studies.

The location, physiology, pathology of hippocampus Melatonin MT2 receptor and MT2-selective modulators

Melatonin, a neurohormone secreted by the pineal gland and regulated by the suprachiasmatic nucleus (SCN) of the hypothalamus, is synthesized and directly released into the cerebrospinal fluid (CSF) of the third ventricle (3rdv), where it undergoes rapid absorption by surrounding tissues to exert its physiological function. The hippocampus, a vital structure in the limbic system adjacent to the ventricles, plays a pivotal role in emotional response and memory formation. Melatonin MT1 and MT2 receptors are G protein-coupled receptors (GPCRs) that primarily mediate melatonin's receptor-dependent effects. In comparison to the MT1 receptor, the widely expressed MT2 receptor is crucial for mediating melatonin's biological functions within the hippocampus. Specifically, MT2 receptor is implicated in hippocampal synaptic plasticity and memory processes, as well as neurogenesis and axogenesis. Numerous studies have demonstrated the involvement of MT2 receptors in the pathophysiology and pharmacology of Alzheimer's disease, depression, and epilepsy. This review focuses on the anatomical localization of MT2 receptor in the hippocampus, their physiological function in this region, and their signal transduction and pharmacological roles in neurological disorders. Additionally, we conducted a comprehensive review of MT2 receptor ligands used in psychopharmacology and other MT2-selective ligands over recent years. Ultimately, we provide an outlook on future research for selective MT2 receptor drug candidates.

Memory corticalization triggered by REM sleep: mechanisms of cellular and systems consolidation

Once viewed as a passive physiological state, sleep is a heterogeneous and complex sequence of brain states with essential effects on synaptic plasticity and neuronal functioning. Rapid-eye-movement (REM) sleep has been shown to promote calcium-dependent plasticity in principal neurons of the cerebral cortex, both during memory consolidation in adults and during post-natal development. This article reviews the plasticity mechanisms triggered by REM sleep, with a focus on the emerging role of kinases and immediate-early genes for the progressive corticalization of hippocampus-dependent memories. The body of evidence suggests that memory corticalization triggered by REM sleep is a systemic phenomenon with cellular and molecular causes.

Withania somnifera leaf alleviates cognitive dysfunction by enhancing hippocampal plasticity in high fat diet induced obesity model

Background

Sedentary lifestyle, psychological stress and labor saving devices in this current society often disrupts the energy gain and expenditure balance leading to obesity. High caloric diet is associated with the high prevalence of cognitive dysfunction and neuropsychiatric disorders in addition to cardiovascular and metabolic abnormalities. The present study was aimed to elucidate the potential beneficial effect of dry leaf powder of Withania somnifera (Ashwagandha) in preventing the cognitive decline associated with diet induced obesity.

Methods

Experiments were performed on four groups of young adult female rats: [Low fat diet (LFD) rats fed on regular low fat chow, High fat diet (HFD) rats on feed containing 30% fat by weight, Low fat diet extract (LFDE) rats given regular chow and dry leaf powder of Ashwagandha 1 mg/g of body weight (ASH) and high fat diet extract (HFDE) rats fed on diet containing high fat and dry leaf powder of ASH. All the rats were kept on their respective diet regimen for 12 weeks.

Results

ASH treated rats showed significant improvement in their working memory and locomotor coordination during behavioral studies as compared to HFD rats. At the molecular level, ASH treatment was observed to restore the levels of BDNF and its receptor TRKB as well as the expression of other synaptic regulators, which are highly implicated in synaptic plasticity. Further, ASH triggered the activation of PI3/AKT pathway of cell survival and plasticity by enhancing the levels of phosphorylated Akt-1 and immediate early genes viz. c-Jun and c-fos.

Conclusions

ASH could be a key regulator in maintaining the synaptic plasticity in HFD induced obesity and can serve as a nootropic candidate against obesity induced cognitive impairments.

Diacylglycerol Kinases in the Coordination of Synaptic Plasticity

Synaptic plasticity is activity-dependent modification of the efficacy of synaptic transmission. Although, detailed mechanisms underlying synaptic plasticity are diverse and vary at different types of synapses, diacylglycerol (DAG)-associated signaling has been considered as an important regulator of many forms of synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD). Recent evidences indicate that DAG kinases (DGKs), which phosphorylate DAG to phosphatidic acid to terminate DAG signaling, are important regulators of LTP and LTD, as supported by the results from mice lacking specific DGK isoforms. This review will summarize these studies and discuss how specific DGK isoforms distinctly regulate different forms of synaptic plasticity at pre- and postsynaptic sites. In addition, we propose a general role of DGKs as coordinators of synaptic plasticity that make local synaptic environments more permissive for synaptic plasticity by regulating DAG concentration and interacting with other synaptic proteins.

Experience-dependent upregulation of multiple plasticity factors in the hippocampus during early REM sleep

Sleep is beneficial to learning, but the underlying mechanisms remain controversial. The synaptic homeostasis hypothesis (SHY) proposes that the cognitive function of sleep is related to a generalized rescaling of synaptic weights to intermediate levels, due to a passive downregulation of plasticity mechanisms. A competing hypothesis proposes that the active upscaling and downscaling of synaptic weights during sleep embosses memories in circuits respectively activated or deactivated during prior waking experience, leading to memory changes beyond rescaling. Both theories have empirical support but the experimental designs underlying the conflicting studies are not congruent, therefore a consensus is yet to be reached. To advance this issue, we used real-time PCR and electrophysiological recordings to assess gene expression related to synaptic plasticity in the hippocampus and primary somatosensory cortex of rats exposed to novel objects, then kept awake (WK) for 60 min and finally killed after a 30 min period rich in WK, slow-wave sleep (SWS) or rapid-eye-movement sleep (REM). Animals similarly treated but not exposed to novel objects were used as controls. We found that the mRNA levels of Arc, Egr1, Fos, Ppp2ca and Ppp2r2d were significantly increased in the hippocampus of exposed animals allowed to enter REM, in comparison with control animals. Experience-dependent changes during sleep were not significant in the hippocampus for Bdnf, Camk4, Creb1, and Nr4a1, and no differences were detected between exposed and control SWS groups for any of the genes tested. No significant changes in gene expression were detected in the primary somatosensory cortex during sleep, in contrast with previous studies using longer post-stimulation intervals (>180 min). The experience-dependent induction of multiple plasticity-related genes in the hippocampus during early REM adds experimental support to the synaptic embossing theory.

LTP- and LTD-inducing stimulations cause opposite changes in arc/arg3.1 mRNA level in hippocampal area CA1 in vivo

Immediate early genes (IEGs) typically are the first genetic responders to a variety of cellular activations. The IEG that encodes activity-regulated cytoskeleton-associated protein (arc/arg3.1) has attracted much interest because its mRNA is transported to and translated near activated synapses. Moreover, arc has been implicated in both long-term potentiation (LTP) and long-term depression (LTD). However, little is known about the time course of altered arc expression during LTP and LTD. Here we characterized arc mRNA levels in area CA1 of the adult rat hippocampus in vivo after LTP- and LTD-inducing stimulations that were identical, except for the temporal patterning of the stimulation pulses. We observed a persistent increase in arc mRNA level during LTP. In contrast, during LTD, arc mRNA level first was decreased and then transiently increased relative to control level. These findings demonstrate that arc mRNA is regulated differently during LTP and LTD, and they provide evidence for stimulation-induced downregulation of mRNA availability during LTD. Findings of abbreviated LTD when transcription was inhibited indicate that the prolonged maintenance of the type of N-methyl-D-aspartate receptor-dependent LTD studied here requires de novo transcription. Furthermore, lack of evidence for a LTD-associated change in the mRNA level of the IEG zif268 demonstrates that the decrease in arc mRNA during LTD is not a general genetic response. Thus, the regulation of arc expression not only differs between LTP and LTD, but also diverges from that of other IEGs implicated in activity-dependent synaptic plasticity.

Protein phosphatases 1 and 2A are both required for long-term depression and associated dephosphorylation of cAMP response element binding protein in hippocampal area CA1 in vivo

Evidence shows that the serine/threonine protein phosphatase 1 (PP1) plays a critical role in synaptic plasticity and memory. Little is known about the contribution of the serine/threonine phosphatase 1 (PP2A) to synaptic plasticity. Both protein phosphatases can target the transcription factor cAMP response element binding protein (CREB), whose phosphorylation at Ser133, we previously found, was downregulated during long-term depression (LTD) of glutamatergic transmission in area CA1 of the adult hippocampus in vivo. Other work from our group showed that the activity of PP2A, as well as that of PP1, is increased after LTD induction in area CA1 in vivo. We therefore investigated here whether both protein phosphatases are necessary for LTD in area CA1, and whether they both are involved in the LTD-associated modification of CREB. We found that inhibition of either PP1 or PP2A interferes with the establishment of LTD. Furthermore, inhibition of either enzyme alone abrogated the LTD-associated dephosphorylation of CREB. Interestingly, inhibition of PP1 disrupted CREB dephosphosphorylation rapidly after LTD-inducing stimulation, whereas inhibition of PP2A did not blunt the CREB modification until a later time point. Thus, both PP1 and PP2A regulate CREB during LTD in area CA1, although possibly through different signaling pathways. Our results demonstrate that PP2A, similar to PP1, plays an essential role in the molecular events that underlie LTD at glutamatergic synapses in hippocampal area CA1 in vivo. We propose that one of the mechanisms through which these protein phosphatases may contribute to the prolonged maintenance of LTD is through the regulation of CREB.

Involvement of reactive oxygen species in long-term potentiation in the spinal cord dorsal horn

Recent studies suggest that reactive oxygen species (ROS) are functional messenger molecules in central sensitization, an underlying mechanism of persistent pain. Because spinal cord long-term potentiation (LTP) is the electrophysiological basis of central sensitization, this study investigates the effects of the increased or decreased spinal ROS levels on spinal cord LTP. Spinal cord LTP is induced by either brief, high-frequency stimulation (HFS) of a dorsal root at C-fiber intensity or superfusion of a ROS donor, tert-butyl hydroperoxide (t-BOOH), onto rat spinal cord slice preparations. Field excitatory postsynaptic potentials (fEPSPs) evoked by dorsal root stimulations with either Abeta- or C-fiber intensity are recorded from the superficial dorsal horn. HFS significantly increases the slope of both Abeta- and C-fiber evoked fEPSPs, thus suggesting LTP development. The induction, not the maintenance, of HFS-induced LTP is blocked by a N-methyl-D-aspartate (NMDA) receptor antagonist, D-2-amino-5-phosphonopentanoic acid (D-AP5). Both the induction and maintenance of LTP of Abeta-fiber-evoked fEPSPs are inhibited by a ROS scavenger, either N-tert-butyl-alpha-phenylnitrone or 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl. A ROS donor, t-BOOH-induced LTP is inhibited by N-tert-butyl-alpha-phenylnitrone but not by D-AP5. Furthermore, HFS-induced LTP and t-BOOH-induced LTP occlude each other. The data suggest that elevated ROS is a downstream event of NMDA receptor activation and an essential step for potentiation of synaptic excitability in the spinal dorsal horn.

Calcineurin and cytoskeleton in low-frequency depression

Transmitter release at high probability phasic synapses of crayfish neuromuscular junctions depresses by over 50% in 60 min when stimulated at 0.2 Hz. Inhibition of the protein phosphatase calcineurin by intracellular pre-synaptic injection of autoinhibitory peptide inhibited low-frequency depression (LFD) and resulted in facilitation of transmitter release. Since this inhibitor had no major effects when injected into the post-synaptic cell, only pre-synaptic calcineurin activity is necessary for LFD. To examine changes in phosphoproteins during LFD we performed a phosphoproteomic screen on proteins extracted from motor axons and nerve terminals after LFD induction or treatment with various drugs that affect kinase and phosphatase activity. Proteins separated by PAGE were stained with phospho-specific/total protein ratio stains (Pro-Q Diamond/SYPRO Ruby) to identify protein bands for analysis by mass spectrometry. Phosphorylation of actin and tubulin decreased during LFD, but increased when calcineurin was blocked. Tubulin and phosphoactin immunoreactivity in pre-synaptic terminals were also reduced after LFD. The actin depolymerizing drugs cytochalasin and latrunculin and the microtubule stabilizer taxol inhibited LFD. Therefore, dephosphorylation of pre-synaptic actin and tubulin and consequent changes in the cytoskeleton may regulate LFD. LFD is unlike long-term depression found in mammalian synapses because the latter requires in most instances post-synaptic calcineurin activity.Thus, this simpler invertebrate synapse discloses a novel pre-synaptic depression mechanism.

Subcellular distribution of the Rho-GEF Lfc in primate prefrontal cortex: effect of neuronal activation

The strength of synaptic connections in the brain varies with activity, and this plasticity depends on remodeling of the actin cytoskeleton in dendritic spines. Critical to this are the Rho family GTPases, whose activity is controlled by various modulatory proteins, including the Rho-GEF Lfc. In cultured neurons and nonneuronal cells, Lfc has been shown both to bind to microtubules and to regulate the actin cytoskeleton. Significantly, Lfc was found to be concentrated in the dendritic shafts of cultured hippocampal neurons under control conditions but then translocated into spines when neural activity was stimulated. In this study, we used immunohistochemistry and electron microscopy to examine activity-dependent changes in the distribution of Lfc in the neuropil of monkey prefrontal cortex. We found that, although Lfc was concentrated in dendrites, it also had a complex distribution in the neuropil, including being present in spines, axons, terminals, and glial processes. Moreover, Lfc distribution varied in different layers of cortex. By using an in vitro slice preparation of monkey prefrontal cortex, we demonstrated an activity-dependent translocation of Lfc from dendritic shafts to spines. The results of this study support a role for Lfc in activity-dependent spine plasticity and demonstrate the feasibility of studying activity-dependent changes in protein localization in tissue slices.

Effect of chronic stress or nicotine on hypothyroidism-induced enhancement of LTD: electrophysiological and molecular studies

We have shown recently that either hypothyroidism or chronic psychosocial stress enhances the expression of LTD, which is reversed by chronic nicotine treatment. In this study, we investigated the effect of combining chronic psychosocial stress with hypothyroidism on LTD. We have also investigated the levels of signaling molecules important for LTD in hypothyroid, stressed-hypothyroid and nicotine-treated hypothyroid rats. Following paired pulse stimulation, LTD was evoked in the CA1 region of anesthetized rats. Combining chronic psychosocial stress with hypothyroidism does not further enhance LTD magnitude compared to either alone. Western blot analysis conducted 1 h after induction of LTD, showed that the levels of calcineurin and P-CaMKII were increased in hypothyroid and stressed-hypothyroid groups compared to that of the control group. However, the levels of calcineurin and P-CaMKII after paired pulsed stimulation were not further increased in stressed-hypothyroid group compared to the hypothyroid only group. In addition, these levels were normalized by chronic nicotine treatment. No change was detected in any of the groups in the levels of calmodulin, PKCgamma, and BDNF after paired pulse stimulation. Our results indicate that changes in the levels of calcineurin and P-CaMKII during expression of LTD in the CA1 region may explain the enhanced magnitude of LTD in hypothyroid rats, and its reversal by chronic nicotine treatment.

Learning-induced glutamate receptor phosphorylation resembles that induced by long term potentiation

Long term potentiation and long term depression of synaptic responses in the hippocampus are thought to be critical for certain forms of learning and memory, although until recently it has been difficult to demonstrate that long term potentiation or long term depression occurs during hippocampus-dependent learning. Induction of long term potentiation or long term depression in hippocampal slices in vitro modulates phosphorylation of the alpha-amino-3-hydrozy-5-methylisoxazole-4-propionic acid subtype of glutamate receptor subunit GluR1 at distinct phosphorylation sites. In long term potentiation, GluR1 phosphorylation is increased at the Ca2+/calmodulin-dependent protein kinase and protein kinase C site serine 831, whereas in long term depression, phosphorylation of the protein kinase A site serine 845 is decreased. Indeed, phosphorylation of one or both of these sites is required for long term synaptic plasticity and for certain forms of learning and memory. Here we demonstrate that training in a hippocampus-dependent learning task, contextual fear conditioning is associated with increased phosphorylation of GluR1 at serine 831 in the hippocampal formation. This increased phosphorylation is specific to learning, has a similar time course to that in long term potentiation, and like memory and long term potentiation, is dependent on N-methyl-D-aspartate receptor activation during training. Furthermore, the learning-induced increase in serine 831 phosphorylation is present at synapses and is in heteromeric complexes with the glutamate receptor subunit GluR2. These data indicate that a biochemical correlate of long term potentiation occurs at synapses in receptor complexes in a final, downstream, postsynaptic effector of long term potentiation during learning in vivo, further strengthening the link between long term potentiation and memory.

Adult-onset hypothyroidism facilitates and enhances LTD: Reversal by chronic nicotine treatment

Chronic nicotine treatment reverses hypothyroidism-induced impairment of hippocampus-dependent spatial memory and long-term potentiation (LTP). We investigated the effect of hypothyroidism on long-term depression (LTD) and possible protection by nicotine. Following paired pulse stimulation, LTD was expressed in hippocampal area CA1 of anesthetized thyroidectomized, euthyroid (sham control), nicotine-treated and nicotine-treated thyroidectomized (hypothyroid) rats. In hypothyroid rats, a significantly higher LTD magnitude was seen compared with that in control rats. A brief train of stimuli (5 pulses at 100 Hz), which did not affect synaptic transmission in control rats, induced a robust LTD in hypothyroid rats suggesting facilitation of LTD expression in these rats. Chronic nicotine treatment (1 mg/kg, 2x day) of hypothyroid rats reversed hypothyroidism-induced enhancement and facilitation of LTD. Western blot analysis of the NMDA receptor subunits in the membranous fractions of hippocampal area CA1 neurons revealed that hypothyroidism reduced NR1 and increased NR2B without affecting NR2A protein levels. These changes in NMDA receptors in hypothyroid rats were reversed by chronic nicotine treatment. Hypothyroidism did not alter BDNF or nicotinic acetylcholine receptor (nAChR) levels. However, nicotine treatment increased protein levels of these molecules in both euthyroid and hypothyroid rats. Our results suggest that alterations in the levels of NMDA receptor subunits may account for the facilitation and enhancement of LTD in hypothyroidism.

Spatial learning induces predominant downregulation of cytosolic proteins in the rat hippocampus

Spatial learning is known to depend on protein synthesis in the hippocampus. Whereas the role of the hippocampus in spatial memory is established, the biochemical and molecular mechanisms underlying this process are poorly understood. To comprehend the complex pattern of protein expression induced by spatial learning, we analyzed alterations in the rat hippocampus proteome after 7 days of spatial learning in the Morris water maze. Forty Wistar rats were randomized into two groups. Animals of group A learned to localize a hidden platform in the water maze. Animals of group B served as controls and spent exactly the same time in the water maze as animals of group A. However, no platform was used in this test and the rats could not learn to localize the target. After the last trial, hydrophilic proteins from the hippocampus were isolated. A proteome-wide study was performed, based on two-dimensional gel electrophoresis and mass spectrometry. Compared with non-learning animals, 53 (70%) proteins were downregulated and 23 (30%) proteins were upregulated after 7 days in rats with spatial learning. The overall changes in protein expression, as quantified by the induction factor, ranged from -1.62 (downregulation to 62%) to 2.10 (upregulation by 110%) compared with controls (100%). Most identified proteins exhibit known functions in vesicle transport, cytoskeletal architecture, and metabolism as well as neurogenesis. These findings indicate that learning in the Morris water maze has a morphological correlate on the proteome level in the hippocampus.

Sources and targets of reactive oxygen species in synaptic plasticity and memory

Increasing evidence suggests that reactive oxygen species (ROS), such as superoxide and hydrogen peroxide, act as necessary signaling molecules in processes underlying cognition. Moreover, ROS have been shown to be necessary in molecular process underlying signal transduction, synaptic plasticity, and memory formation. Research from several laboratories suggests that NADPH oxidase is an important source of superoxide in the brain. Evidence is presented here to show that ROS are in fact important signaling molecules involved in synaptic plasticity and memory formation. Moreover, evidence that the NADPH oxidase complex is a key regulator of ROS generation in synaptic plasticity and memory formation is discussed. Understanding redox signaling in the brain, including the sources and molecular targets of ROS, are important for a full understanding of the signaling pathways that underlie synaptic plasticity and memory. Knowledge of ROS function in the brain also is critical for understanding aging and neurodegenerative diseases of the brain given that several of these disorders, including Alzheimer's disease and Parkinson disease, may be exacerbated by the unregulated generation of ROS.

Comparison of cellular mechanisms of long-term depression of synaptic strength at perforant path-granule cell and Schaffer collateral-CA1 synapses

This chapter compares the cellular mechanisms that have been implicated in the induction and expression of long-term depression (LTD) at Schaffer collateral-CA1 synapses to perforant path-dentate gyrus (DG) synapses. In general, Schaffer collateral LTD and long-term potentiation (LTP) both appear to be a complex combination of many alterations in synaptic transmission that occur at both presynaptic and postsynaptic sites, while at perforant path synapses, most evidence has focused on postsynaptic long-term alterations. Within the DG, the medial perforant path is far more studied than lateral perforant path synapses, where most evidence relates to the induction of heterosynaptic LTD at lateral perforant path synapses when LTP is induced in the medial perforant path. Of course, there remain many other classes of synapses in the DG where synaptic plasticity, including LTD, have been largely neglected. It is clear that a better understanding of the range of DG loci where long-lasting activity-dependent plasticity, both LTD and LTP, are expressed will be essential to improve our understanding of the cognitive roles of such DG plasticity.

Molecular studies on the protective effect of nicotine in adult-onset hypothyroidism-induced impairment of long-term potentiation

We have recently shown that chronic nicotine treatment reverses hypothyroidism-induced learning and memory impairment. Chronic nicotine treatment also reverses the hypothyroidism-induced impairment of long-term potentiation (LTP). Analysis of LTP associated key signaling molecules revealed that chronic nicotine treatment prevented the hypothyroidism-induced reduction of the basal phosphotransferase activity of CaMKII and protein levels of P-CaMKII. In addition, the failure of high frequency stimulation to increase the levels of P-CaMKII in hypothyroid rats was reversed by nicotine treatment, suggesting that the neuroprotective effect of nicotine during hypothyroidism involved activation of CaMKII. Furthermore, chronic nicotine treatment reverses the hypothyroidism-induced elevated phosphatase activity and protein levels of calcineurin, a phosphatase that regulates CaMKII activation. We conclude that the neuroprotective effects of nicotine in adult-onset hypothyroidism may result from restoration of CaMKII and calcineurin activity.

Synaptic plasticity and phosphorylation

A number of neuronal functions, including synaptic plasticity, depend on proper regulation of synaptic proteins, many of which can be rapidly regulated by phosphorylation. Neuronal activity controls the function of these synaptic proteins by exquisitely regulating the balance of various protein kinase and protein phosphatase activity. Recent understanding of synaptic plasticity mechanisms underscores important roles that these synaptic phosphoproteins play in regulating both pre- and post-synaptic functions. This review will focus on key postsynaptic phosphoproteins that have been implicated to play a role in synaptic plasticity.

Modulation of phosphoinositide-protein kinase C signal transduction by omega-3 fatty acids: implications for the pathophysiology and treatment of recurrent neuropsychiatric illness

The phosphoinositide (PI)-protein kinase C (PKC) signal transduction pathway is initiated by pre- and postsynaptic Galphaq-coupled receptors, and regulates several clinically relevant neurochemical events, including neurotransmitter release efficacy, monoamine receptor function and trafficking, monoamine transporter function and trafficking, axonal myelination, and gene expression. Mounting evidence for PI-PKC signaling hyperactivity in the peripheral (platelets) and central (premortem and postmortem brain) tissues of patients with schizophrenia, bipolar disorder, and major depressive disorder, coupled with evidence that PI-PKC signal transduction is down-regulated in rat brain following chronic, but not acute, treatment with antipsychotic, mood-stabilizer, and antidepressant medications, suggest that PI-PKC hyperactivity is central to an underlying pathophysiology. Evidence that membrane omega-3 fatty acids act as endogenous antagonists of the PI-PKC signal transduction pathway, coupled with evidence that omega-3 fatty acid deficiency is observed in peripheral and central tissues of patients with schizophrenia, bipolar disorder, and major depressive disorder, support the hypothesis that omega-3 fatty acid deficiency may contribute to elevated PI-PKC activity in these illnesses. The data reviewed in this paper outline a potential molecular mechanism by which omega-3 fatty acids could contribute to the pathophysiology and treatment of recurrent neuropsychiatric illness.

Nicotine prevents stress-induced enhancement of long-term depression in hippocampal area CA1: electrophysiological and molecular studies

Nicotine treatment prevents chronic psychosocial stress-induced impairment of hippocampus-dependent spatial memory and long-term potentiation (LTP). In this study, we investigated the effect of chronic nicotine treatment on stress-induced enhancement of long-term depression (LTD). After paired-pulse stimulation, LTD was evoked in area CA1 of anesthetized control, stressed, nicotine-treated, and nicotine-treated stressed rats. In stressed rats, a significantly greater LTD magnitude was seen than in control rats. Stress also facilitated the induction of LTD. Nicotine treatment of stressed rats prevented stress-induced enhancement and facilitation of LTD. For chronically stressed rats, we previously reported marked decreases in the basal levels of brain-derived neurotrophic factor (BDNF), CaMKII, P-CaMKII, and calmodulin as well as a significant increase in calcineurin basal levels. Herein, Western blot analysis conducted 1 hr after induction of LTD by paired-pulse stimulation showed that the levels of calcineurin and P-CaMKII were increased in the stressed group compared with the other groups and were normalized by chronic nicotine treatment. Additionally, after paired-pulse stimulation, the levels of total CaMKII were increased in all groups with no change in the levels of BDNF and calmodulin. Therefore, the increase in the levels of calcineurin and P-CaMKII during expression of LTD in area CA1 may explain the enhanced magnitude of LTD in chronically stressed rats.

Levothyroxin restores hypothyroidism-induced impairment of LTP of hippocampal CA1: Electrophysiological and molecular studies

Hypothyroidism impairs synaptic plasticity as well as learning and memory. Clinical reports are conflicting about the ability of thyroid hormone replacement therapy to fully restore the hypothyroidism-induced learning and memory impairment. Recently, we have shown that hypothyroidism impairs LTP and cognition in adult rats. We have studied the effect of thyroxin replacement therapy on hypothyroidism-induced LTP impairment using electrophysiological and molecular approaches. Recording from CA1 region of the hippocampus in anesthetized adult rat indicated that 6 weeks of thyroxin replacement therapy (20 microg/kg/day) fully restored LTP impaired by hypothyroidism. Western blotting showed reduction in phosphorylated (P)-CAMKII, total-CaMKII, neurogranin, and calmodulin basal levels in the CA1 region of the hippocampus of hypothyroid rats. The levels of these molecules were normalized by thyroxin replacement therapy. The hypothyroid-induced elevation of basal calcineurin levels and activity was also normalized by thyroxin treatment. However, thyroxin replacement therapy did not restore hypothyroidism-induced reduction in PKCgamma basal protein levels. Additionally, real-time PCR, showed a reduction in basal neurogranin mRNA level that was normalized by thyroxin replacement therapy. In the sham (control) rats, induction of LTP by high-frequency stimulation increases P-CaMKII, and total CaMKII levels as well as CaMKII phosphotransferase activity. However, in hypothyroid rats, the same stimulation protocol induced an increase only in total-CaMKII. Thyroxin treatment normalized the levels and activity of these molecules. The results demonstrated that thyroxin therapy normalized the electrophysiological and molecular effects of hypothyroidism on the CA1 region and emphasized the critical role P-CaMKII plays in hypothyroidism-induced LTP impairment.

Role of phosphorylated CaMKII and calcineurin in the differential effect of hypothyroidism on LTP of CA1 and dentate gyrus

Hypothyroidism impairs early long-term potentiation (LTP) in the CA1 but not in the dentate gyrus (DG) of hippocampus of anesthetized adult rats. Protein levels and activities of signaling molecules in both the CA1 and DG of surgically thyroidectomized and sham-operated euthyroid rats were measured. Basal levels of total calmodulin kinase II (CaMKII) protein in both the CA1 and DG were decreased in hypothyroidism. Marked reduction of basal P-CaMKII levels and CaMKII activity was seen in CA1, but not in the DG of the same hypothyroid animals. Basal levels of calmodulin and protein kinase Cgamma (PKCgamma) were decreased in CA1 but remained unchanged in the DG of hypothyroid rats. Basal calcineurin levels and activity, although enhanced in CA1, were reduced in the DG of hypothyroid rats. These findings suggest that the DG may possess a compensatory mechanism whereby calcineurin levels are reduced, to allow sufficient CaMKII activity to produce an apparently normal LTP in hypothyroid rats.

Ovariectomy-induced disruption of long-term synaptic depression in the hippocampal CA1 region in vivo is attenuated with chronic estrogen replacement

Endogenous cyclical changes in the levels of estrogen can have marked effects on hippocampal synaptic plasticity. In two experiments, we examined the effect of chronic estrogen loss and replacement following ovariectomy on the induction of bidirectional changes in synaptic plasticity in the CA1 region in vivo. In Experiment 1, ovariectomy carried out either 5 days or 5 weeks before testing impaired the induction of long-term depression (LTD) and but not long-term potentiation (LTP). In Experiment 2, chronic estrogen replacement (0.2 ml of 10 microg injection of 17beta-estradiol every 48 h) over the course of 5 weeks enhanced the magnitude of paired-pulse-induced LTD in the CA1 region but had no effect on the induction of LTP. The results demonstrate that acute and chronic estrogen deprivation disrupted dynamic synaptic plasticity processes in the hippocampal CA1 region and that this disruption was ameliorated by chronic estrogen replacement. The findings are discussed with reference to: (1) the contribution of Ca(2+) regulated synaptic signalling pathways in the CA1 region to estradiol modulation of LTP and LTD and (2) the potential functional significance of ovariectomy-induced changes in synaptic plasticity for learning and memory processes.

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.

Inhibition of protein phosphatase 2A- and protein phosphatase 1-induced tau hyperphosphorylation and impairment of spatial memory retention in rats

Tau hyperphosphorylation leads to formation of paired helical filament/neurofibrillary tangles, the hallmark lesion seen in Alzheimer's disease (AD) brain. An imbalanced regulation in protein kinases and protein phosphatases in the affected neurons is proposed to be a reasonable causative factor to the disease process. To verify the hypothesis, we have injected in the present study calyculin A, a potent and specific inhibitor of protein phosphatase (PP) 2A and PP1, into rat hippocampus bilaterally, thus reproduced an Alzheimer's-like deficiency in dephosphorylation system. It was found that calyculin A-injected rats developed lesions in spatial memory retention in Morris water maze test. At mean time, tau was hyperphosphorylated at Ser396/Ser404 (PHF-1) and Ser-262/Ser-356 (12E8) sites determined both by immunohistochemistry and Western blot. It is implicated that (1) PP2A and PP1 participate in the in vivo regulation of tau phosphorylation, and down-regulation of the two phosphatases will result in tau hyperphosphorylation; (2) hyperphosphorylation of tau at PHF-1 and 12E8 sites might be crucial to affect spatial memory in AD.

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.

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

Long-term depression (LTD) is an activity-dependent weakening of synaptic efficacy at individual inhibitory synapses, a possible cellular model of learning and memory. Here, we show that the induction of LTD of inhibitory transmission recruits activated calcineurin (CaN) to dephosphorylate type-A GABA receptor (GABA(A)Rs) via the direct binding of CaN catalytic domain to the second intracellular domain of the GABA(A)R-gamma(2) subunits. Prevention of the CaN-GABA(A) receptor complex formation by expression of an autoinhibitory domain of CaN in the hippocampus of transgenic mice blocks the induction of LTD. Conversely, genetic expression of the CaN catalytic domain in the hippocampus depresses inhibitory synaptic responses, occluding LTD. Thus, an activity-dependent physical and functional interaction between CaN and GABA(A) receptors is both necessary and sufficient for inducing LTD at CA1 individual inhibitory synapses.

Calcineurin as a potential contributor in estradiol regulation of hippocampal synaptic function

Estradiol influences Ca(2+) regulation and Ca(2+)-dependent synaptic plasticity, suggesting estrogenic effects on Ca(2+)-dependent enzymes that regulate synaptic plasticity may mediate hormonal influences on cognition. In ovariectomized female rats, injections of estradiol benzoate (EB, 10 microg) reduced hippocampal cytosolic activity of serine/threonine protein phosphatases, calcineurin and protein phosphatase 1 (PP1). The decreased activity was rapid and recovered substantially over a 24-h period. Decreased calcineurin activity was associated with a decreased level of calcineurin in the cytosol. In contrast, expression of PP1 was not altered suggesting that the level of calcineurin activity regulated PP1 activity. EB application to hippocampal slices rapidly decreased cytosolic phosphatase activity, which was not blocked by the estrogen receptor antagonist, ICI 182780. Decreased phosphatase activity was associated with an increase in CA3-CA1 synaptic transmission. In addition, EB application shifted synaptic plasticity, blocking the induction of long-term depression and facilitating the establishment of long-term potentiation. The reduction in calcineurin activity and shift in synaptic plasticity were mimicked to a lesser extent by 17-alpha-estradiol. From these results we suggest that EB can act to rapidly influence Ca(2+) signaling pathways including the activity of Ca(2+)-regulated phosphatases involved in synaptic plasticity.

"Silent" metaplasticity of the late phase of long-term potentiation requires protein phosphatases

The late phase of long-term potentiation (L-LTP) is correlated with some types of long-term memory, but the mechanisms by which L-LTP is modulated by prior synaptic activity are undefined. Activation of protein phosphatases by low-frequency stimulation (LFS) given before induction of L-LTP may significantly modify L-LTP. Using cellular electrophysiological recording methods in mouse hippocampal slices, we show that LFS given before induction of L-LTP inhibited L-LTP in an activity-dependent manner without affecting either basal synaptic strength or the early phase of LTP (E-LTP). This anterograde inhibitory effect of LFS was persistent, required N-methyl-D-aspartate (NMDA) receptor activation, and was blocked by inhibitors of protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A). These data indicate that certain patterns of LFS can activate PP1 and/or PP2A, and that long-lasting activation of these phosphatases by prior LFS can suppress the subsequent expression of L-LTP without affecting E-LTP. Because this inhibition of L-LTP is caused by prior synaptic activity that, alone, produced no net effect on synaptic efficacy, we suggest that this is a "silent" form of metaplasticity that may influence long-term information storage by modulating the capacity of synapses to express L-LTP after repeated bouts of activity.

Long-term depression in the adult hippocampus in vivo involves activation of extracellular signal-regulated kinase and phosphorylation of Elk-1

Protein kinase cascades likely play a critical role in the signaling events that underlie synaptic plasticity and memory. The extracellular signal-regulated kinase (ERK) cascade is suited well for such a role because its targets include regulators of gene expression. Here we report that the ERK cascade is recruited during long-term depression (LTD) of synaptic strength in area CA1 of the adult hippocampus in vivo and selectively impacts on phosphorylation of the nuclear transcription factor Elk-1. Using a combination of in vivo electrophysiology, biochemistry, pharmacology, and immunohistochemistry, we found the following: (1) ERK phosphorylation, including phosphorylation of nuclear ERK, and ERK phosphotransferase activity are increased markedly, albeit transiently, after the induction of NMDA receptor-dependent LTD at the commissural input to area CA1 pyramidal cells in the hippocampus of anesthetized adult rats; (2) LTD-inducing paired-pulse stimulation fails to produce lasting LTD in the presence of the ERK kinase inhibitor SL327, which suggests that ERK activation is necessary for the persistence of LTD; and (3) ERK activation during LTD results in increased phosphorylation of Elk-1 but not of the transcription factor cAMP response element-binding protein. Our findings indicate that the ERK cascade transduces signals from the synapse to the nucleus during LTD in hippocampal area CA1 in vivo, as it does during long-term potentiation in area CA1, but that the pattern of coupling of the ERK cascade to transcriptional regulators differs between the two forms of synaptic plasticity.

Transient translocation of conventional protein kinase C isoforms and persistent downregulation of atypical protein kinase Mzeta in long-term depression

Persistent dephosphorylation has been implicated in the molecular mechanisms of long-term depression (LTD). Dephosphorylation may be due to either a persistent increase in phosphatase activity or a persistent decrease in kinase activity. We have previously found that protein kinase Mzeta (PKMzeta), the autonomously active form of the atypical PKCzeta isozyme that increases in long-term potentiation (LTP), decreases in LTD. This is consistent with the hypothesis that decreased levels of phosphorylation by PKC are important in LTD. Recently, however, increased phosphorylation by PKC has also been implicated in LTD. These contradictory results might be explained, in part, by the multiple isoforms of PKC, which may be independently regulated during the different phases of LTD. We now find that 45 s after low-frequency (3 Hz) stimulation that induces LTD in the CA1 region of hippocampal slices, conventional Ca(2+)/lipid-dependent PKC isoforms translocate from the cytosol to the membrane. This translocation was transient, lasting less than 15 min. In contrast, PKMzeta was persistently decreased through 2 h of LTD maintenance. Therefore, the activation and downregulation of distinct PKC isoforms may participate in the induction and maintenance mechanisms of LTD.

Interaction of the AMPA receptor subunit GluR2/3 with PDZ domains regulates hippocampal long-term depression

The interaction of PDZ domain-containing proteins with the C termini of alpha-amino-3-hydroxy-5-methylisoxazolepropionate (AMPA) receptors has been suggested to be important in the regulation of receptor targeting to excitatory synapses. Recent studies have shown that the rapid internalization of AMPA receptors at synapses may mediate, at least in part, the expression of long-term depression (LTD). We have previously shown that phosphorylation of Ser-880 on the AMPA receptor GluR2 subunit differentially regulated the interaction of GluR2 with the PDZ domain-containing proteins GRIP1 and PICK1. Here, we show that induction of LTD in hippocampal slices increases phosphorylation of Ser-880 within the GluR2 C-terminal PDZ ligand, suggesting that the modulation of GluR2 interaction with GRIP1 and PICK1 may regulate AMPA receptor internalization during LTD. Moreover, postsynaptic intracellular perfusion of GluR2 C-terminal peptides that disrupt GluR2 interaction with PICK1 inhibit the expression of hippocampal LTD. These results suggest that the interaction of GluR2 with PICK1 may play a regulatory role in the expression of LTD in the hippocampus.

A significant increase in both basal and maximal calcineurin activity in the rat pilocarpine model of status epilepticus

This study focused on the effects of status epilepticus on the activity of calcineurin, a neuronally enriched, calcium-dependent phosphatase. Calcineurin is an important modulator of many neuronal processes, including learning and memory, induction of apoptosis, receptor function and neuronal excitability. Therefore, a status epilepticus-induced alteration of the activity of this important phosphatase would have significant physiological implications. Status epilepticus was induced by pilocarpine injection and allowed to continue for 60 min. Brain region homogenates were then assayed for calcineurin activity by dephosphorylation of p-nitrophenol phosphate. A significant status epilepticus-dependent increase in both basal and Mn(2+)-dependent calcineurin activity was observed in homogenates isolated from the cortex and hippocampus, but not the cerebellum. This increase was resistant to 150 nM okadaic acid, but sensitive to 50 microM okadaic acid. The increase in basal activity was also resistant to 100 microM sodium orthovanadate. Both maximal dephosphorylation rate and substrate affinity were increased following status epilepticus. However, the increase in calcineurin activity was not found to be due to an increase in calcineurin enzyme levels. Finally, increase in calcineurin activity was found to be NMDA-receptor activation dependent. The data demonstrate that status epilepticus resulted in a significant increase in both basal and maximal calcineurin activity.