A macromolecular synthesis-dependent late phase of long-term potentiation requiring cAMP in the medial perforant pathway of rat hippocampal slices

Hippocampal long term potentiation in rats under different regimens of vitamin D: an in vivo study

Evidence indicates that vitamin D involves in development of brain as well as its function. This study assesses occurrence of long term potentiation (LTP), as an experimental form of synaptic plasticity, in adult rats under the normal regimen (CON), and the regimens without vitamin D (CON-D) or with a supplement of 1,25(OH)2D3 (CON+D). Stimulating the Schaffer collaterals pre- and post-tetanus excitatory postsynaptic potentials (EPSPs) were recorded in the CA1 area of hippocampus in anesthetized animals. Amplitude change of the EPSPs was considered for comparisons. Our results indicated that the basic EPSPs were similar in the three groups. Tetanization elicited a considerable LTP in both the CON and CON+D rats but a moderate potentiation in the CON-D group. We concluded that optimal level of vitamin D is required for induction of LTP.

Modulation of fear memory by retrieval and extinction: a clue for memory deconsolidation

Memories are fragile and easily forgotten at first, but after a consolidation period of hours to weeks, are inscribed in our brains as stable traces, no longer vulnerable to conventional amnesic treatments. Retrieval of a memory renders it labile, akin to the early stages of consolidation. This phenomenon has been explored as memory reactivation, in the sense that the memory is temporarily 'deconsolidated', allowing a short time window for amnesic intervention. This window closes again after reconsolidation, which restores the stability of the memory. In contrast to this 'transient deconsolidation' and the short-spanned amnesic effects of consolidation blockers, some specific treatments can disrupt even consolidated memory, leading to apparent amnesia. We propose the term 'amnesic deconsolidation' to describe such processes that lead to disruption of consolidated memory and/or consolidated memory traces. We review studies of these 'amnesic deconsolidation' treatments that enhance memory extinction, alleviate relapse, and reverse learning-induced plasticity. The transient deconsolidation that memory retrieval induces and the amnesic deconsolidation that these regimes induce both seem to dislodge a component that stabilizes consolidated memory. Characterizing this component, at both molecular and network levels, will provide a key to developing clinical treatments for memory-related disorders and to defining the consolidated memory trace.

Amyloid β-peptide oligomers stimulate RyR-mediated Ca2+ release inducing mitochondrial fragmentation in hippocampal neurons and prevent RyR-mediated dendritic spine remodeling produced by BDNF

Soluble amyloid β-peptide oligomers (AβOs), increasingly recognized as causative agents of Alzheimer's disease (AD), disrupt neuronal Ca(2+) homeostasis and synaptic function. Here, we report that AβOs at sublethal concentrations generate prolonged Ca(2+) signals in primary hippocampal neurons; incubation in Ca(2+)-free solutions, inhibition of ryanodine receptors (RyRs) or N-methyl-d-aspartate receptors (NMDARs), or preincubation with N-acetyl-l-cysteine abolished these signals. AβOs decreased (6 h) RyR2 and RyR3 mRNA and RyR2 protein, and promoted mitochondrial fragmentation after 24 h. NMDAR inhibition abolished the RyR2 decrease, whereas RyR inhibition prevented significantly the RyR2 protein decrease and mitochondrial fragmentation induced by AβOs. Incubation with AβOs (6 h) eliminated the RyR2 increase induced by brain-derived nerve factor (BDNF) and the dendritic spine remodeling induced within minutes by BDNF or the RyR agonist caffeine. Addition of BDNF to neurons incubated with AβOs for 24 h, which had RyR2 similar to and slightly higher RyR3 protein content than those of controls, induced dendritic spine growth but at slower rates than in controls. These combined effects of sublethal AβOs concentrations (which include redox-sensitive stimulation of RyR-mediated Ca(2+) release, decreased RyR2 protein expression, mitochondrial fragmentation, and prevention of RyR-mediated spine remodeling) may contribute to impairing the synaptic plasticity in AD.

The mechanisms of the strong inhibitory modulation of long-term potentiation in the rat dentate gyrus

The hippocampus is essential for the formation of certain types of memory, and synaptic plasticity such as long-term potentiation (LTP) is widely accepted as a cellular basis of hippocampus-dependent memory. Although LTP in both perforant path-dentate gyrus (DG) granule cell and CA3-CA1 pyramidal cell synapses is similarly dependent on activation of postsynaptic N-methyl-D-aspartate receptors, several reports suggest that modulation of LTP by γ-aminobutyric acid (GABA) receptor-mediated inhibitory inputs is stronger in perforant path-DG granule cell synapses. However, little is known about how different the mechanism and physiological relevance of the GABAergic modulation of LTP induction are among different brain regions. We confirmed that the action of GABA(A) receptor antagonists on LTP was more prominent in the DG, and explored the mechanism introducing such difference by examining two types of GABA(A) receptor-mediated inhibition, i.e. synaptic and tonic inhibition. As synaptic inhibition, we compared inhibitory vs. excitatory monosynaptic responses and their summation during an LTP-inducing stimulus, and found that the balance of the summated postsynaptic currents was biased toward inhibition in the DG. As tonic inhibition, or sustained activation of extrasynaptic GABA(A) receptors by ambient GABA, we measured the change in holding currents of the postsynaptic cells induced by GABA(A) receptor antagonists, and found that the tonic inhibition was significantly stronger in the DG. Furthermore, we found that tonic inhibition was associated with LTP modulation. Our results suggest that both the larger tonic inhibition and the larger inhibitory/excitatory summation balance during conditioning are involved in the stronger inhibitory modulation of LTP in the DG.

'Silent' priming of translation-dependent LTP by ß-adrenergic receptors involves phosphorylation and recruitment of AMPA receptors

The capacity for long-term changes in synaptic efficacy can be altered by prior synaptic activity, a process known as "metaplasticity." Activation of receptors for modulatory neurotransmitters can trigger downstream signaling cascades that persist beyond initial receptor activation and may thus have metaplastic effects. Because activation of β-adrenergic receptors (β-ARs) strongly enhances the induction of long-term potentiation (LTP) in the hippocampal CA1 region, we examined whether activation of these receptors also had metaplastic effects on LTP induction. Our results show that activation of β-ARs induces a protein synthesis-dependent form of metaplasticity that primes the future induction of late-phase LTP by a subthreshold stimulus. β-AR activation also induced a long-lasting increase in phosphorylation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) GluA1 subunits at a protein kinase A (PKA) site (S845) and transiently activated extracellular signal-regulated kinase (ERK). Consistent with this, inhibitors of PKA and ERK blocked the metaplastic effects of β-AR activation. β-AR activation also induced a prolonged, translation-dependent increase in cell surface levels of GluA1 subunit-containing AMPA receptors. Our results indicate that β-ARs can modulate hippocampal synaptic plasticity by priming synapses for the future induction of late-phase LTP through up-regulation of translational processes, one consequence of which is the trafficking of AMPARs to the cell surface.

Calcium/calmodulin-dependent kinase IV contributes to translation-dependent early synaptic potentiation in the anterior cingulate cortex of adult mice

Calcium/calmodulin-dependent kinase IV (CaMKIV) phosphorylates the major transcription factor, cyclic AMP-responsive element binding protein (CREB), which plays key roles in synaptic plasticity and memory consolidation. Our previous study showed that long-term potentiation (LTP) in the anterior cingulate cortex (ACC) was significantly enhanced in transgenic mice overexpressing CaMKIV. Considering that the CaMKIV-CREB pathway plays a central role in the protein synthesis-dependent LTP, it is possible that upregulation of CaMKIV contributes to enhancement of LTP by promoting protein synthesis. To test this possibility, we examined the effects of transcription and translation inhibitors on synaptic potentiation induced by pairing of synaptic activity with postsynaptic depolarization (paired training) in ACC pyramidal neurons of wild-type and CaMKIV transgenic mice. We found that synaptic potentiation induced by paired training was partially inhibited by transcription or translation inhibitors both in wild-type and CaMKIV transgenic mice; the extent of inhibition was markedly larger in the CaMKIV transgenic mice than in the wild-type mice. Biochemical and immunohistochemical studies revealed that CaMKIV was distributed in the membrane, cytosol and nucleus of ACC neurons. Our results reveal in the first time a transcription- and translation-dependent component of early synaptic LTP in adult ACC synapses, and demonstrate that CaMKIV enhances early synaptic potentiation by activating new protein synthesis.

Pyridoxal-5'-phosphate phosphatase/chronophin inhibits long-term potentiation induction in the rat dentate gyrus

Pyridoxal-5'-phosphate (PLP)-phosphatase/chronophin (PLPP/CIN) directly dephosphorylates actin-depolymerizing factor (ADF)/cofilin as well as PLP. Although PLPP/CIN plays a role in the regulation of F-actin and vitamin B(6) metabolism, there is no direct evidence to support a correlation between PLPP/CIN and F-actin polymerization during long-term potentiation (LTP) induction. In this study, we investigated whether the expression of PLPP/CIN is altered following LTP induction, and whether Tat-PLPP/CIN transduction affects LTP induction in the rat dentate gyrus (DG). PLPP/CIN immunoreactivity was markedly decreased in dentate granule cells after the induction of LTP. Tat-PLPP/CIN transduction (20 and 200 microg/kg) decreased the efficiency of high frequency stimulus-induced potentiation of populations spike amplitude as compared to saline or Tat-protein-treated animals. The PLPP/CIN protein level showed an inverse correlation with phosphorylated ADF/cofilin levels and F-actin content. These findings suggest that PLPP/CIN-mediated actin dynamics may play an important role in the changes of morphological properties (dendritic spine reorganization) of the hippocampus in LTP.

Memory impairments in healthy aging: Role of aging-induced microglial sensitization

Healthy aged individuals are more likely to suffer profound memory impairments following a challenging life event such as a severe bacterial infection, surgery, or an intense psychological stressor, than are younger adults. These peripheral challenges are capable of producing a neuroinflammatory response, (e.g., increased pro-inflammatory cytokines), and in the healthy aged brain this response is exaggerated and prolonged. Normal aging primes or sensitizes microglia and this appears to be the source of this amplified response. Among the outcomes of this exaggerated neuroinflammatory response is an impairment in synaptic plasticity, and a reduction in key downstream mediators such as Arc and BDNF. Each of these mechanisms is important for long-term memory formation, and is compromised by elevated pro-inflammatory cytokines. Pharmacological, dietary and physical interventions are discussed as potential therapies to abrogate the challenge-induced neuroinflammatory response, thereby preventing or reducing memory deficits in aged subjects.

Attention enhances the retrieval and stability of visuospatial and olfactory representations in the dorsal hippocampus

Attention enhances the encoding and retrieval of olfactory and visuospatial representations by modulating place field stability, firing rate, and neuronal synchronization of pyramidal cells in the hippocampus.

A key question in the analysis of hippocampal memory relates to how attention modulates the encoding and long-term retrieval of spatial and nonspatial representations in this region. To address this question, we recorded from single cells over a period of 5 days in the CA1 region of the dorsal hippocampus while mice acquired one of two goal-oriented tasks. These tasks required the animals to find a hidden food reward by attending to either the visuospatial environment or a particular odor presented in shifting spatial locations. Attention to the visuospatial environment increased the stability of visuospatial representations and phase locking to gamma oscillations—a form of neuronal synchronization thought to underlie the attentional mechanism necessary for processing task-relevant information. Attention to a spatially shifting olfactory cue compromised the stability of place fields and increased the stability of reward-associated odor representations, which were most consistently retrieved during periods of sniffing and digging when animals were restricted to the cup locations. Together, these results suggest that attention selectively modulates the encoding and retrieval of hippocampal representations by enhancing physiological responses to task-relevant information.

Attention modulates the encoding and retrieval of memories, but the physiological basis of this interaction has largely been unexplored. The formation of memories which depend on the hippocampus involves the conscious recall of events that occur in specific spatial contexts, a form of memory known as episodic. To investigate the physiological consequences of the interaction between attention and memory in the hippocampus, we recorded single-cell activity and local field potentials — the local rhythmic oscillatory activity of neurons — from the same cells over several days while animals learned one of two goal-oriented tasks. In the visuospatial version of the task, mice had to associate a specific spatial location with a reward, independent of an odor cue. In the nonspatial, olfactory version, mice had to associate a specific odor with the food reward, independent of spatial location. We found that, during periods of navigation, only neurons in the visuospatially trained animals displayed long-term stable representations of space, and neuronal synchronization to so-called gamma oscillations, a mechanism of signal amplification that has been proposed to underlie attentional processes. Conversely, when animals were sniffing the odors in fixed spatial locations, only neurons in the olfactory-trained group displayed a stable increase in firing rate in response to the reward-associated odor. Our data suggest that attention modulates what is encoded and retrieved by hippocampal cells and that neuronal synchronization to gamma oscillations may underlie the mechanism whereby attention leads to stable spatial memory retrieval during navigation.

Mechanisms of chronic central neuropathic pain after spinal cord injury

Not all spinal contusions result in mechanical allodynia, in which non-noxious stimuli become noxious. The studies presented use the NYU impactor at 12.5 mm drop or the Infinite Horizons Impactor (150 kdyn, 1 s dwell) devices to model spinal cord injury (SCI). Both of these devices and injury parameters, if done correctly, will result in animals with above level (forelimb), at level (trunk) and below level (hindlimb) mechanical allodynia that model the changes in evoked somatosensation experienced by the majority of people with SCI. The sections are as follows: 1) Mechanisms of remote microglial activation and pain signaling in "below-level" central pain 2) Intracellular signaling mechanisms in central sensitization in "at-level" pain 3) Peripheral sensitization contributes to "above level" injury pain following spinal cord injury and 4) Role of reactive oxygen species in central sensitization in regional neuropathic pain following SCI. To summarize, differential regional mechanisms contribute to the regional chronic pain states. We propose the importance of understanding the mechanisms in the differential regional pain syndromes after SCI in the chronic condition. Targeting regional mechanisms will be of enormous benefit to the SCI population that suffer chronic pain, and will contribute to better treatment strategies for other chronic pain syndromes.

Kinase-dependent modification of dendritic excitability after long-term potentiation

Patterns of presynaptic activity properly timed with postsynaptic action potential output can not only increase the strength of synaptic inputs but can also increase the excitability of dendritic branches of adult CA1 pyramidal neurons. Here, we examined the role of protein kinase A (PKA) and mitogen-activated protein kinase (MAPK) in the enhancement of dendritic excitability that occurs during theta-burst pairing of presynaptic and postsynaptic firing activity. Using dendritic and somatic whole-cell recordings in rat hippocampal slices, we measured the increase in the amplitude of back-propagating action potentials in the apical dendrite that occurs in parallel with long-term potentiation (LTP) of synaptic inputs. We found that inhibition of the MAPK pathway prevents this enhancement of dendritic excitability using either a weak or strong LTP induction protocol, while synaptic LTP can still be induced by the strong protocol. Both forms of plasticity are blocked by inhibition of PKA and occluded by interfering with cAMP degradation, consistent with a PKA-mediated increase in MAPK activity following induction of LTP. This provides a signalling mechanism for plasticity of dendritic excitability that occurs during neuronal activity and demonstrates the necessity of MAPK activation. Furthermore, this study uncovers an additional contribution of kinase activation to plasticity that may occur during learning.

Activation of Fyn tyrosine kinase in the mouse dorsal hippocampus is essential for contextual fear conditioning

Fyn-tyrosine-kinase-deficient mice exhibit defects in the Morris water maze test and long-term potentiation (LTP) induction in the hippocampus, and given that LTP has been postulated as the neural basis for memory formation, Fyn may be required for hippocampus-dependent memory formation. However, how Fyn is involved in the process of memory formation is unclear. To investigate the role of Fyn in hippocampal memory formation, we first tested the behavior of Fyn-deficient mice by contextual fear conditioning. A mouse was placed in a context and a foot shock was delivered, so that the mouse associated the context with the shock. We found that the freezing response of Fyn-deficient mice to the context was impaired at 24 h after conditioning. We then measured freezing at 1 h after conditioning, and found that their short-term contextual fear memory was also impaired. We used Western blotting to examine the mode of Fyn activation in dorsal hippocampal tissue following contextual fear conditioning. Fyn activation peaked as early as 5-10 min after contextual fear conditioning and persisted for at least 40 min. Concomitant increases in tyrosine phosphorylation of several proteins, including NR2B, were also observed, but no increases in tyrosine phosphorylation were observed in Fyn-deficient mice. Thus, both short-term and long-term (24-h) contextual fear memory were impaired in Fyn-deficient mice, and Fyn activation in the dorsal hippocampus transiently increased after contextual fear conditioning. These findings strongly suggest that activation of the Fyn signaling pathway is involved in hippocampus-dependent formation of contextual fear memory.

Activation of exchange protein activated by cyclic-AMP enhances long-lasting synaptic potentiation in the hippocampus

cAMP is a critical second messenger implicated in synaptic plasticity and memory in the mammalian brain. Substantial evidence links increases in intracellular cAMP to activation of cAMP-dependent protein kinase (PKA) and subsequent phosphorylation of downstream effectors (transcription factors, receptors, protein kinases) necessary for long-term potentiation (LTP) of synaptic strength. However, cAMP may also initiate signaling via a guanine nucleotide exchange protein directly activated by cAMP (Epac). The role of Epac in hippocampal synaptic plasticity is unknown. We found that in area CA1 of mouse hippocampal slices, activation of Epac enhances maintenance of LTP without affecting basal synaptic transmission. The persistence of this form of LTP requires extracellular signal-regulated protein kinase (ERK) and new protein synthesis, but not transcription. Because ERK is involved in translational control of long-lasting plasticity and memory, our data suggest that Epac is a crucial link between cAMP and ERK during some forms of protein synthesis-dependent LTP. Activation of Epac represents a novel signaling pathway for rapid regulation of the stability of enduring forms of LTP and, perhaps, of hippocampus- dependent long-term memories.

Donepezil markedly potentiates memantine neurotoxicity in the adult rat brain

The NMDA antagonist, memantine (Namenda), and the cholinesterase inhibitor, donepezil (Aricept), are currently being used widely, either individually or in combination, for treatment of Alzheimer's disease (AD). NMDA antagonists have both neuroprotective and neurotoxic properties; the latter is augmented by drugs, such as pilocarpine, that increase cholinergic activity. Whether donepezil, by increasing cholinergic activity, might augment memantine's neurotoxic potential has not been investigated. In the present study, we determined that a dose of memantine (20mg/kg, i.p.), considered to be in the therapeutic (neuroprotective) range for rats, causes a mild neurotoxic reaction in the adult rat brain. Co-administration of memantine (20 or 30 mg/kg) with donepezil (2.5-10mg/kg) markedly potentiated this neurotoxic reaction, causing neuronal injury at lower doses of memantine, and causing the toxic reaction to become disseminated and lethal to neurons throughout many brain regions. These findings raise questions about using this drug combination in AD, especially in the absence of evidence that the combination is beneficial, or that either drug arrests or reverses the disease process.

Long-term down-regulation of BDNF mRNA in rat hippocampal CA1 subregion correlates with PTSD-like behavioural stress response

Brain-derived neurotrophic factor (BDNF) and its intracellular kinase-activating receptor TrkB, have been implicated in the neurobiological mechanisms underlying the clinical manifestations of PTSD, especially those related to synaptic efficacy and neural plasticity. BDNF interacts with components of the stress response such as corticosterone, and plays an important role in growth, maintenance and functioning of several neuronal systems. This study employed an animal model of PTSD to investigate the relationship between prevalence rates of distinct patterns of behavioural responses to predator stress, circulating levels of corticosterone and local levels of mRNA for BDNF, TrkB and two other neurotrophic factors in selected brain areas. Animals whose behaviour was extremely disrupted by exposure selectively displayed significant down-regulation of mRNA for BDNF and up-regulation of TrkB mRNA in the CA1 subregion of the hippocampus, compared to animals whose behaviour was minimally or partially affected and to unexposed controls. The response was consistent throughout the entire study only in CA1. The consistent long-term the BDNF down-regulation and TrkB up-regulation associated with extreme behavioural compromise may be associated with chronic stress-induced psychopathological processes, especially in the hippocampus. The corresponding changes in neural plasticity and synaptic functioning may mediate clinical manifestations of PTSD.

Polysialic acid–neural cell adhesion molecule in brain plasticity: From synapses to integration of new neurons

Isoforms of the neuronal cell adhesion molecule (NCAM) carrying the linear homopolymer of alpha 2,8-linked sialic acid (polysialic acid, PSA) have emerged as particularly attractive candidates for promoting plasticity in the nervous system. The large negatively charged PSA chain of NCAM is postulated to be a spacer that reduces adhesion forces between cells allowing dynamic changes in membrane contacts. Accumulating evidence also suggests that PSA-NCAM-mediated interactions lead to activation of intracellular signaling cascades that are fundamental to the biological functions of the molecule. An important role of PSA-NCAM appears to be during development, when its expression level is high and where it contributes to the regulation of cell shape, growth or migration. However, PSA-NCAM does persist in adult brain structures such as the hippocampus that display a high degree of plasticity where it is involved in activity-induced synaptic plasticity. Recent advances in the field of PSA-NCAM research have not only consolidated the importance of this molecule in plasticity processes but also suggest a role for PSA-NCAM in the regulation of higher cognitive functions and psychiatric disorders. In this review, we discuss the role and mode of actions of PSA-NCAM in structural plasticity as well as its potential link to cognitive processes.

Upregulation of neurotrophic factors selectively in frontal cortex in response to olfactory discrimination learning

We have previously shown that olfactory discrimination learning is accompanied by several forms of long-term enhancement in synaptic connections between layer II pyramidal neurons selectively in the piriform cortex. This study sought to examine whether the previously demonstrated olfactory-learning-task-induced modifications are preceded by suitable changes in the expression of mRNA for neurotrophic factors and in which brain areas this occurs. Rats were trained to discriminate positive cues in pair of odors for a water reward. The relationship between the learning task and local levels of mRNA for brain-derived neurotrophic factor, tyrosine kinase B, nerve growth factor, and neurotrophin-3 in the frontal cortex, hippocampal subregions, and other regions were assessed 24 hours post olfactory learning. The olfactory discrimination learning activated production of endogenous neurotrophic factors and induced their signal transduction in the frontal cortex, but not in other brain areas. These findings suggest that different brain areas may be preferentially involved in different learning/memory tasks.

PREGS induces LTP in the hippocampal dentate gyrus of adult rats via the tyrosine phosphorylation of NR2B coupled to ERK/CREB [corrected] signaling

An acute application of neurosteroid pregnenolone sulfate (PREGS) to hippocampal slices from adult rats induced a long-lasting potentiation (LLP PREGS) at the perforant path-granule cell synapse. As a partial mechanism of the LLP PREGS, we previously revealed that PREGS transiently increases the probability of presynaptic glutamate release via a sensitization of alpha7-nicotinic acetylcholine receptor (alpha7nAChR). We herein demonstrate that the LLP PREGS could be separated into two independent processes: the above-mentioned early presynaptic-origin short-term potentiation (STP PREGS) and a delayed postsynaptic N-methyl-d-aspartate receptor (NMDAr)-dependent long-term potentiation termed LTP(PREGS). This study focused on the analysis of the signaling mechanism underlying the LTP PREGS. PREGS increased the tyrosine phosphorylation of NR2B, a subunit of NMDAr, and the NMDAr-mediated Ca2+ influx in the granule cells. The enhanced Ca2+ influx was largely attenuated by the NR2B subunit inhibitor ifenprodil and the Src kinase family inhibitor PP2. PREGS also triggered a persistent phosphorylation of extracellular signal-regulated kinase 2 (ERK2) followed by an ERK-dependent phosphorylation of cAMP-response element-binding protein (CREB), which was crucial for the LTP PREGS induction and was sensitive to ifenprodil. These results suggest that PREGS induces an acute increase in the NR2B tyrosine phosphorylation which enhances the Ca2+ influx through NMDAr, followed by an activation of the ERK/CREB signaling cascade that leads to LTP PREGS.

Impaired hippocampal LTP in inbred mouse strains can be rescued by β-adrenergic receptor activation

Long-term potentiation (LTP), an activity-dependent enhancement of synaptic strength, and memory can be influenced by neuromodulatory transmitters such as norepinephrine (NE) and also by genetic background. beta-Adrenergic receptor activation can facilitate the expression of hippocampal CA1 LTP induced by weak stimulus patterns, but its influence on LTP induced by stronger stimulus patterns is unclear. We examined neural NE and dopamine (DA) levels, beta-adrenergic receptor expression and hippocampal LTP in genetically diverse inbred mouse strains. Brain tissue levels of NE were significantly lower in strains 129S1/SvImJ (129), BALB/cByJ (BALB) and C3H/HeJ (C3H) than in C57BL/6NCrlBR (B6). Western blot analysis showed that hippocampal beta(1)-adrenergic receptor expression was similar in strains B6, 129 and C3H, but was increased in BALB. LTP was induced in area CA1 of hippocampal slices by four trains of high-frequency stimulation (HFS) of the Schaeffer collaterals in the four inbred strains. Two hours after induction, LTP was significantly reduced in strains 129, BALB and C3H compared to B6, correlating with neural NE levels. We rescued hippocampal LTP in strains 129, BALB and C3H to levels seen in B6 by bath application of 1 microm isoproterenol, a beta-adrenergic receptor agonist, during HFS. Propranolol, a beta-adrenergic receptor antagonist, blocked this rescue in 129, BALB and C3H but did not affect LTP in strain B6. Thus, although this form of multitrain LTP does not rely on beta-adrenergic receptor activation, our data show that pharmacological activation of beta-adrenergic receptors during multiple trains of HFS can rescue CA1 LTP in genetically diverse strains with impaired LTP.

Electrochemical evaluation of self-disassociation of PKA upon activation by cAMP

The allosteric reaction of protein kinase A (PKA) upon binding of cyclic AMP (cAMP) is revealed with an electrochemical technique through the redox current change of an electrochemically active marker. The different effect of cAMP's regulation at a distinct concentration level is obtained in this system. The influence of structural analogues is also examined with respect to the affinity and special selectivity. This study presents an electrochemical approach to the rapid and sensitive investigation of the protein-ligand interaction in the signal transduction networks.

Brain-derived neurotrophic factor regulates the expression and synaptic delivery of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor subunits in hippocampal neurons

Brain-derived neurotrophic factor (BDNF) plays an important role in synaptic plasticity in the hippocampus, but the mechanisms involved are not fully understood. The neurotrophin couples synaptic activation to changes in gene expression underlying long term potentiation and short term plasticity. Here we show that BDNF acutely up-regulates GluR1, GluR2, and GluR3 alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor subunits in 7-day in vitro cultured hippocampal neurons. The increase in GluR1 and GluR2 protein levels in developing cultures was impaired by K252a, a tropomyosin-related [corrected] kinase (Trk) inhibitor, and by translation (emetine and anisomycin) and transcription (alpha-amanitine and actinomycin D) inhibitors [corrected] The increase in GluR1 and GluR2 protein levels in developing cultures was impaired by K252a, a Trk inhibitor, and by translation (emetine and anisomycin) and transcription (alpha-amanitine and actinomycin D) inhibitors. Accordingly, BDNF increased the mRNA levels for GluR1 and GluR2 subunits. Biotinylation studies showed that stimulation with BDNF for 30 min selectively increased the amount of GluR1 associated with the plasma membrane, and this effect was abrogated by emetine. Under the same conditions, BDNF induced GluR1 phosphorylation on Ser-831 through activation of protein kinase C and Ca(2+)-calmodulin-dependent protein kinase II. Chelation of endogenous extracellular BDNF with TrkB-IgG selectively decreased GluR1 protein levels in 14-day in vitro cultures of hippocampal neurons. Moreover, BDNF promoted synaptic delivery of homomeric GluR1 AMPA receptors in cultured organotypic slices, by a mechanism independent of NMDA receptor activation. Taken together, the results indicate that BDNF up-regulates the protein levels of AMPA receptor subunits in hippocampal neurons and induces the delivery of AMPA receptors to the synapse.

Arc/Arg3.1 mediates homeostatic synaptic scaling of AMPA receptors

Homeostatic plasticity may compensate for Hebbian forms of synaptic plasticity, such as long-term potentiation (LTP) and depression (LTD), by scaling neuronal output without changing the relative strength of individual synapses. This delicate balance between neuronal output and distributed synaptic weight may be necessary for maintaining efficient encoding of information across neuronal networks. Here, we demonstrate that Arc/Arg3.1, an immediate-early gene (IEG) that is rapidly induced by neuronal activity associated with information encoding in the brain, mediates homeostatic synaptic scaling of AMPA type glutamate receptors (AMPARs) via its ability to activate a novel and selective AMPAR endocytic pathway. High levels of Arc/Arg3.1 block the homeostatic increases in AMPAR function induced by chronic neuronal inactivity. Conversely, loss of Arc/Arg3.1 results in increased AMPAR function and abolishes homeostatic scaling of AMPARs. These observations, together with evidence that Arc/Arg3.1 is required for memory consolidation, reveal the importance of Arc/Arg3.1's dynamic expression as it exerts continuous and precise control over synaptic strength and cellular excitability.

Arc/Arg3.1 interacts with the endocytic machinery to regulate AMPA receptor trafficking

Arc/Arg3.1 is an immediate-early gene whose mRNA is rapidly transcribed and targeted to dendrites of neurons as they engage in information processing and storage. Moreover, Arc/Arg3.1 is known to be required for durable forms of synaptic plasticity and learning. Despite these intriguing links to plasticity, Arc/Arg3.1's molecular function remains enigmatic. Here, we demonstrate that Arc/Arg3.1 protein interacts with dynamin and specific isoforms of endophilin to enhance receptor endocytosis. Arc/Arg3.1 selectively modulates trafficking of AMPA-type glutamate receptors (AMPARs) in neurons by accelerating endocytosis and reducing surface expression. The Arc/Arg3.1-endocytosis pathway appears to regulate basal AMPAR levels since Arc/Arg3.1 KO neurons exhibit markedly reduced endocytosis and increased steady-state surface levels. These findings reveal a novel molecular pathway that is regulated by Arc/Arg3.1 and likely contributes to late-phase synaptic plasticity and memory consolidation.

Plastic processes in the dentate gyrus: a computational perspective

The dentate gyrus has the capacity for numerous types of synaptic plasticity that use diverse mechanisms and are thought essential for the storage of information in the hippocampus. Here we review the various forms of synaptic plasticity that involve afferents and efferents of the dentate gyrus, and, from a computational perspective, relate how these plastic processes might contribute to sparse, orthogonal encoding, and the selective recall of information within the hippocampus.

Control of synaptic consolidation in the dentate gyrus: mechanisms, functions, and therapeutic implications

Synaptic consolidation refers to the development and stabilization of protein synthesis-dependent modifications of synaptic strength as observed during long-term potentiation (LTP) and long-term depression (LTD). Activity-dependent changes in synaptic strength are thought to underlie memory storage and other adaptive responses of the nervous systems of importance in mood stability, reward behavior, and pain control. This chapter focuses on the mechanisms and functions of synaptic consolidation in the dentate gyrus, a critical structure not only in hippocampal memory function, but also in regulation of stress responses and cognitive aspects of depression. Recent evidence suggests that synaptic consolidation at excitatory medial perforant path-granule cell synapses requires brain-derived neurotrophic factor (BDNF) signaling and induction of the immediate early gene activity-regulated cytoskeleton-associated protein (Arc). Arc mRNA is strongly induced and transported to dendritic processes following high-frequency stimulation (HFS) that induces LTP in the rat dentate gyrus in vivo. Sustained synthesis of Arc during a surprisingly protracted time-window is required for hyperphosphorylation of actin depolymerizing factor/cofilin and local expansion of the actin cytoskeleton in vivo. Furthermore, this process of Arc-dependent synaptic consolidation is activated in response to brief infusion of BDNF. Microarray expression profiling has revealed a panel of BDNF-regulated genes that may cooperate with Arc during synaptic consolidation. In addition to regulating gene expression, BDNF signaling modulates the fine localization and biochemical activation of the translation machinery. By modulating the spatial and temporal translation of newly induced (Arc) and constitutively-expressed mRNA in dendrites, BDNF may effectively control the window of synaptic consolidation. Dysregulation of BDNF synthesis and Arc function, specifically within the dentate gyrus, is linked to behavioral symptoms and cognitive deficits in animal models of depression and Alzheimer's disease. Therapeutics strategies targeting synaptic consolidation hold promise for the future.

Genetic program of neuronal differentiation and growth induced by specific activation of NMDA receptors

Glutamate and its receptors are expressed very early during development and may play important roles in neurogenesis, synapse formation and brain wiring. The levels of glutamate and activity of its receptors can be influenced by exogenous factors, leading to neurodevelopmental disorders. To investigate the role of NMDA receptors on gene regulation in a neuronal model, we used primary neuronal cultures developed from embryonic rat cerebri in serum-free medium. Using Affymetrix Gene Arrays, we found that genes known to be involved in neuronal plasticity were differentially expressed 24 h after a brief activation of NMDA receptors. The upregulation of these genes was accompanied by a sustained induction of CREB phosphorylation, and an increase in synaptophysin immunoreactivity. We conclude that NMDA receptor activation elicits expression of genes whose downstream products are involved in the regulation of early phases of the process leading to synaptogenesis and its consolidation, at least in part through sustained CREB phosphorylation.

Chronic exposure to estrogen and tamoxifen regulates synaptophysin and phosphorylated cAMP response element-binding (CREB) protein expression in CA1 of ovariectomized rat hippocampus

We report here the in vivo effects of estrogen (E2) on modulation of synaptic plasticity and the agonistic (estrogen-like) role of selective estrogen receptor modulator (SERM), tamoxifen (TAM) in the CA1 of the rat hippocampus. Effects on synaptophysin (SYP), a presynaptic vesicular protein, and phosphorylated cyclic AMP responsive element-binding (p-CREB) protein, a signal transduction pathway molecule, were studied using the ovariectomized (OVX) experimental rat model. Bilateral ovariectomy was performed on 40 rats and these were divided into 4 groups based on the treatment they received (at 2 weeks post-ovariectomy, a subcutaneous injection daily for 4 weeks) viz., OVX+E2 (0.1 mg/kg body weight), OVX+TAM (0.05 mg/kg body weight), OVX+vehicle and one group served as OVX control. An additional 10 animals served as the ovary intact control group. At the end of the treatment schedule, five animals/group were used for immunohistochemical staining of SYP and p-CREB using specific antibodies with peroxidase anti-peroxidase technique on paraformaldehyde-fixed cryostat sections. Protein estimation and Western blot analysis coupled with densitometric analysis (using gel-documentation system and image analysis software) were performed on unfixed hippocampus collected from rest of the five animals/group. Serum estradiol levels were estimated with radioimmunoassay prior to sacrifice. The results revealed that ovariectomy reduced SYP and p-CREB expression whereas E2 or TAM administration resulted in their upregulation. Serum estradiol levels of E2 administered animals were comparable with the ovary intact group whereas those of TAM administered group persisted in the range of OVX controls. To conclude, long-term estrogen therapy modulates the synaptic plasticity of hippocampal neurons and presumably, the agonist biocharacter of TAM as observed in the present investigations, may in the long run have a potential in the treatment and prevention of various estrogen-related disorders.

Suppression of hippocampal plasticity-related gene expression by sleep deprivation in rats

Previous work shows that sleep deprivation impairs hippocampal-dependent learning and long-term potentiation (LTP). Brain-derived neurotrophic factor (BDNF), cAMP response-element-binding (CREB) and calcium-calmodulin-dependent protein kinase II (CAMKII) are critical modulators of hippocampal-dependent learning and LTP. In the present study we compared the effects of short- (8 h) and intermediate-term (48 h) sleep deprivation (SD) on the expression of BDNF and its downstream targets, Synapsin I, CREB and CAMKII in the neocortex and the hippocampus. Rats were sleep deprived using an intermittent treadmill system which equated total movement in the SD and control treadmill animals (CT), but permitted sustained periods of rest in CT animals. Animals were divided into SD (treadmill schedule: 3 s on/12 s off) and two treadmill control groups, CT1 (15 min on/60 min off) and CT2 (30 min on/120 min off - permitting more sustained sleep). Real-time Taqman RT-PCR was used to measure changes in mRNA; BDNF protein levels were determined using ELISA. In the hippocampus, 8 h treatments reduced BDNF, Synapsin I, CREB and CAMKII gene expression in both SD and control groups. Following 48 h of experimental procedures, the expression of all these four molecular markers of plasticity was reduced in SD and CT1 groups compared to the CT2 and cage control groups. In the hippocampus, BDNF protein levels after 8 h and 48 h treatments paralleled the changes in mRNA. In neocortex, neither 8 h nor 48 h SD or control treatments had significant effects on BDNF, Synapsin I and CAMKII mRNA levels. Stepwise regression analysis suggested that loss of REM sleep underlies the effects of SD on hippocampal BDNF, Synapsin I and CREB mRNA levels, whereas loss of NREM sleep underlies the effects on CAMKII mRNA.

Long-Term Potentiation Is Mediated by Multiple Kinase Cascades Involving CaMKII or Either PKA or p42/44 MAPK in the Adult Rat Dentate Gyrus In Vitro

The induction of NMDA-receptor–dependent long-term potentiation (LTP) in adult CA1 is contingent on activation of Ca/calmodulin-dependent protein kinase II (CaMKII). However, little is known about kinase mediation of LTP in the dentate gyrus. In the present study, the involvement of the kinases CaMKII, PKA, and MAPK in the induction of LTP was studied in the dentate gyrus of adult rats. Individual application of selective inhibitors of CaMKII, MEK, or PKA did not inhibit induction of LTP. In contrast, coapplication of a CaMKII inhibitor with either a PKA or MEK inhibitor resulted in a strong block of LTP. Induction of LTP was blocked by the coapplication of the inhibitors CaMKII and PKA or MEK, both when they were applied 1 h before the induction stimulus and also when they were applied after the induction stimulus. Thus LTP is mediated by either of two parallel cascades, one involving CaMKII and the other PKA or MAPK. Moreover, these cascades are active for a certain period after the induction stimulus.

Contribution of AMPA and NMDA receptors to early and late phases of LTP in hippocampal slices

Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor mediated responses were investigated in rat hippocampal slices under 4h of long-term potentiation (LTP) expression. A modified medium containing the NMDA receptor antagonist AP5 and low concentration of Mg(2+) was used to monitor isolated AMPA responses. NMDA components were determined from composite excitatory postsynaptic potentials (EPSPs) under brief (15-20 min) wash-out of AP5. LTP was induced in a medium with low concentration of AP5, resulting in an about two-fold larger increase of the AMPA component than of the NMDA component at both 1h and 4h after induction. Similar results were obtained if LTP was induced in "normal Mg(2+)" and the NMDA components were assessed at the end of experiment, from either composite or isolated NMDA EPSPs, with or without blockade of GABAergic inhibition. It is generally believed that LTP undergoes biochemical and/or structural conversions during the first few hours. Our study, however, shows constant expression of LTP, at least in terms of AMPA versus NMDA components, during this time. The data support the notion that LTP initiates as a predominant amplification of AMPA receptors and remains so for at least 4h.

Identification of genes co-upregulated with Arc during BDNF-induced long-term potentiation in adult rat dentate gyrus in vivo

Brain-derived neurotrophic factor (BDNF) is a critical regulator of transcription-dependent adaptive neuronal responses, such as long-term potentiation (LTP). Brief infusion of BDNF into the dentate gyrus of adult anesthetized rats triggers stable LTP at medial perforant path-granule synapses that is transcription-dependent and requires induction of the immediate early gene Arc. Rather than acting alone, Arc is likely to be part of a larger BDNF-induced transcriptional program. Here, we used cDNA microarray expression profiling to search for genes co-upregulated with Arc 3 h after BDNF-LTP induction. Of nine cDNAs encoding for known genes and up-regulated more than four-fold, we selected five genes, Narp, neuritin, ADP-ribosylation factor-like protein-4 (ARL4L), TGF-beta-induced immediate early gene-1 (TIEG1) and CARP, for further validation. Real-time PCR confirmed robust up-regulation of these genes in an independent set of BDNF-LTP experiments, whereas infusion of the control protein cytochrome C had no effect. In situ hybridization histochemistry further revealed up-regulation of all five genes in somata of post-synaptic granule cells following both BDNF-LTP and high-frequency stimulation-induced LTP. While Arc synthesis is critical for local actin polymerization and stable LTP formation, several of the co-upregulated genes have known functions in excitatory synaptogenesis, axon guidance and glutamate receptor clustering. These results provide novel insight into gene expression responses underlying BDNF-induced synaptic consolidation in the adult brain in vivo.

NMDA receptor blockers prevents the facilitatory effects of post-training intra-dorsal hippocampal NMDA and physostigmine on memory retention of passive avoidance learning in rats

In the present study, the effects of post-training intra-dorsal hippocampal (intra-CA1) injection of an N-methyl-D-aspartate (NMDA) receptor agonist and competitive or noncompetitive antagonists, on memory retention of passive avoidance learning was measured in the presence and absence of physostigmine in rats. Intra-CA1 administration of lower doses of the NMDA receptor agonist NMDA (10(-5) and 10(-4) microg/rat) did not affect memory retention, although the higher doses of the drug (10(-3), 10(-2) and 10(-1) microg/rat) increased memory retention. The greatest response was obtained with 10(-1) microg/rat of the drug. The different doses of the competitive NMDA receptor antagonist DL-AP5 (1, 3.2 and 10 microg/rat) and noncompetitive NMDA receptor antagonist MK-801 (0.5, 1 and 2 microg/rat) decreased memory retention in rats dose dependently. Both competitive and noncompetitive NMDA receptor antagonists reduced the effect of NMDA (10(-2) microg/rat). In another series of experiments, intra-CA1 injection of physostigmine (2, 3 and 4 microg/rat) improved memory retention. Post-training co-administration of lower doses of NMDA (10(-5) and 10(-4) microg/rat) and physostigmine (1 microg/rat), doses which were ineffective when given alone, significantly improved the retention latency. The competitive and noncompetitive NMDA receptor antagonists, DL-AP5 and MK-801, decreased the effect of physostigmine (2 microg/rat). Atropine decreased memory retention by itself and potentiated the response to DL-AP5 and MK-801. In conclusion, it seems that both NMDA and cholinergic systems not only play a part in the modulation of memory in the dorsal hippocampus of rats but also have demonstrated a complex interaction as well.

Group I metabotropic glutamate receptors activate the p70S6 kinase via both mammalian target of rapamycin (mTOR) and extracellular signal-regulated kinase (ERK 1/2) signaling pathways in rat striatal and hippocampal synaptoneurosomes

Group I metabotropic glutamate receptors (mGluRs) have been demonstrated to play a role in synaptic plasticity via a rapamycin-sensitive mRNA translation signaling pathway. Various growth factors can stimulate this pathway, leading to the phosphorylation and activation of mammalian target of rapamycin (mTOR), a serine/threonine protein kinase that modulates the activity of several translation regulatory factors, such as p70S6 kinase. However, little is known about the cellular and molecular mechanisms that bring the plastic changes of synaptic transmission after stimulation of group I mGluRs. Here, we investigated the role of the mTOR-p70S6K and the ERK1/2-p70S6K pathways in rat striatal and hippocampal synaptoneurosomes after group I mGluR stimulation. Our findings show that (S)-3,5-dihydroxyphenylglycine (DHPG) increases significantly the activation of mTOR and p70S6K (Thr389, controlled by mTOR) in both brain areas. The mTOR activation is dose-dependent and requires the stimulation of mGluR1 subtype receptors as for the p70S6K activation observed in striatum and hippocampus. In addition, the p70S6K (Thr421/Ser424) activation via the ERK1/2 activation is increased and involved also mGluR1 receptors. These results demonstrate that group I mGluRs are coupled to mTOR-p70S6K and ERK1/2-p70S6K pathways in striatal and hippocampal synaptoneurosomes. The translational factor p70S6K could be involved in the group I mGluRs-modulated synaptic efficacy.

A model of the roles of essential kinases in the induction and expression of late long-term potentiation

The induction of late long-term potentiation (L-LTP) involves complex interactions among second-messenger cascades. To gain insights into these interactions, a mathematical model was developed for L-LTP induction in the CA1 region of the hippocampus. The differential equation-based model represents actions of protein kinase A (PKA), MAP kinase (MAPK), and CaM kinase II (CAMKII) in the vicinity of the synapse, and activation of transcription by CaM kinase IV (CAMKIV) and MAPK. L-LTP is represented by increases in a synaptic weight. Simulations suggest that steep, supralinear stimulus-response relationships between stimuli (e.g., elevations in [Ca(2+)]) and kinase activation are essential for translating brief stimuli into long-lasting gene activation and synaptic weight increases. Convergence of multiple kinase activities to induce L-LTP helps to generate a threshold whereby the amount of L-LTP varies steeply with the number of brief (tetanic) electrical stimuli. The model simulates tetanic, -burst, pairing-induced, and chemical L-LTP, as well as L-LTP due to synaptic tagging. The model also simulates inhibition of L-LTP by inhibition of MAPK, CAMKII, PKA, or CAMKIV. The model predicts results of experiments to delineate mechanisms underlying L-LTP induction and expression. For example, the cAMP antagonist RpcAMPs, which inhibits L-LTP induction, is predicted to inhibit ERK activation. The model also appears useful to clarify similarities and differences between hippocampal L-LTP and long-term synaptic strengthening in other systems.

Protein synthesis-dependent long-term functional plasticity: methods and techniques

There is growing interest in late-LTP and late-LTD, that is, distinct forms of functional plasticity that require somatic functions such as protein synthesis in addition to the transient synaptic processes that are required for short lasting forms. Interestingly, to date only these forms of lasting plastic events could be detected in healthy, freely moving animals and thus, they are considered as physiological cellular models of learning and memory formation. Late-LTP and -LTD are characterized by 'synaptic tagging' or 'capture' and 'synaptic cross-tagging', but there are only a few laboratories that can currently perform experiments studying these properties. In brain slice work, there are many different approaches to investigate these processes using different methodological variations: some allow slices to rest for long periods before the experiment starts, others do not; some run their experiments at near to physiological temperatures, others at lower temperatures; some stimulate frequently, others do not.

The influence of NMDA receptor agonist and antagonist on morphine state-dependent memory of passive avoidance in mice

The measurement of step-down latency in passive avoidance has been used to study memory in laboratory animals. The pre-training injection of 5 mg/kg morphine impaired memory, which was restored when 24 h later the same dose of the drug was administered. To explore the possible involvement of NMDA modulators on morphine-induced memory impairment, we have investigated the effects of intracerebroventricular (i.c.v.) administration of NMDA and the competitive NMDA antagonist, DL-AP5, on morphine-induced memory impairment or recall, on the test day. Morphine (5 mg/kg, s.c.) was administered 30 min before training to induce impairment of memory and 24 h later, 30 min before test to improve it. Pre-test administration of NMDA (0.00001, 0.0001 and 0.001 microg/mouse, i.c.v.) did not alter the retention latency compared to the saline-treated animals. But restored the memory impairment induced by pre-training morphine (5 mg/kg, s.c.). Pre-test administration of DL-AP5 (1, 3.2 and 10 microg/mouse, i.c.v.) by itself decreased the retention latencies. The same doses of DL-AP5 increased pre-training morphine-induced memory impairment. Co-administration of NMDA (0.0001 and 0.001 microg/mouse, i.c.v.) and morphine (5 mg/kg, s.c.) on the test day increased morphine memory improvement. Conversely, DL-AP5 (1, 3.2 and 10 microg/mouse, i.c.v.) inhibited morphine-induced memory recall. It is concluded that NMDA receptors may be involved, at least in part, in morphine state-dependent learning in mice.

The effect of short duration streptozotocin-induced diabetes mellitus on the late phase and threshold of long-term potentiation induction in the rat

Long-term potentiation (LTP) was examined in streptozotocin (STZ)-induced diabetic rats of 8 (DM8) and 20 (DM20) weeks duration of diabetes mellitus (DM). DM8 animals showed significant LTP induction, although the potentiation of the synapses was significantly lower than in the control animals. No significant potentiation of the synapses could be demonstrated in DM20 animals. The different aspects of LTP induction in the DM8 animals were studied. The threshold of LTP induction was measured by stimulating the slices with 100 Hz frequency trains of stimuli containing different number of impulses. The results showed increased threshold for LTP induction in the DM8 animals compared to the controls. The late LTP (L-LTP) phase induction was studied by applying 3 repeated HFSs to the afferent fibers. Diabetic animals (DM8) slices failed to maintain the synaptic potentiation induced by the high frequency stimulations (HFSs) for more than 1 h.

Neurobiology and treatment of anxiety: signal transduction and neural plasticity

The stress-dependence and chronic nature of anxiety disorders along with the anxiolytic effectiveness of antidepressant drugs suggests that neuronal plasticity may play a role in the pathophysiology of anxiety. Intracellular signaling pathways are known in many systems to be critical links in the cascades from surface signals to the molecular alterations that result in functional plasticity. Chronic antidepressant treatments can regulate intracellular signaling pathways and can induce molecular, cellular, and structural changes over time. These changes may be important to the anxiolytic effectiveness of these drugs. In addition, the signaling proteins implicated in the actions of chronic antidepressant action, such as cAMP response element binding protein (CREB), have also been implicated in conditioned fear and in anxiety. The cellular mechanisms underlying conditioned fear indicate roles for additional signaling pathways; however, less is known about such mechanisms in anxiety. The challenge to identify intracellular signaling pathways and related molecular and structural changes that are critical to the etiology and treatment of anxiety will further establish the importance of mechanisms of neuronal plasticity in functional outcome and improve treatment strategies.

A Peptide Sequence Controls the Physical Properties of Nanoparticles Formed by Peptide−Polymer Conjugates That Respond to a Protein Kinase A Signal

We previously reported that poly(N-isopropylacrylamide) grafted with Peptide 1 (-GLRRASLG) and poly(ethylene glycol) changed its physical properties in response to an intracellular protein phosphorylation signal, protein kinase A (PKA) (Katayama, Y. et al. (2001) Macromolecules 34, 905). In this study, we investigated the effect of changing peptide structure on the lower critical solution temperature (LCST) of peptide-polymer conjugates, before and after phosphorylation with PKA. For Peptide 2 (Ac-LRRASL-), which has a formal net charge of +2 at physiological pH, the LCST of the conjugate decreased on phosphorylation. In contrast, the LCSTs of the conjugates with Peptide 3 (-ALRRASLE) and Peptide 4 (Ac-DWDALRRASL-), which have neutral net charges, were greatly increased. This suggests that the LCST of the polymer was mainly governed by two factors: the change in hydration around the polymer chain and the interpeptide electrostatic repulsion, resulting from phosphorylation. These polymers have potential for use as drug capsules that respond to cellular conditions.

Removal of G(ialpha1) constraints on adenylyl cyclase in the hippocampus enhances LTP and impairs memory formation

Stimulation of adenylyl cyclase in the hippocampus is critical for memory formation. However, generation of cAMP signals within an optimal range for memory may require a balance between stimulatory and inhibitory mechanisms. The role of adenylyl cyclase inhibitory mechanisms for memory has not been addressed. One of the mechanisms for inhibition of adenylyl cyclase is through activation of G(i)-coupled receptors, a mechanism that could serve as a constraint on memory formation. Here we report that ablation of G(ialpha1) by gene disruption increases hippocampal adenylyl cyclase activity and enhances LTP in area CA1. Furthermore, gene ablation of G(ialpha1) or antisense oligonucleotide-mediated depletion of G(ialpha1) disrupted hippocampus-dependent memory. We conclude that G(ialpha1) provides a critical mechanism for tonic inhibition of adenylyl cyclase activity in the hippocampus. We hypothesize that loss of G(ialpha1) amplifies the responsiveness of CA1 postsynaptic neurons to stimuli that strengthen synaptic efficacy, thereby diminishing synapse-specific plasticity required for new memory formation.

Mouse models of impaired fear memory exhibit deficits in amygdalar LTP

Inbred mouse strains have different genetic backgrounds that can result in impairments of synaptic plasticity and memory. They are valuable models for probing the mechanisms of memory impairments. We examined fear memory in several inbred strains, along with synaptic plasticity that may underlie fear memory. Long-term potentiation (LTP) is a form of activity-dependent synaptic plasticity that is a candidate cellular mechanism for some forms of learning and memory. Strains with impaired contextual or cued fear memory may have selective LTP deficits in different hippocampal subregions, or in the amygdala. We measured fear memory and its extinction in five inbred strains: C57BL/6NCrlBR (B6), A/J, BALB/cByJ (BALB), C57BL/10J (B10), and SM/J (SM). We also measured LTP in the basolateral amygdala and in the hippocampal Schaeffer collateral-commissural (SC) and medial perforant pathways (MPP). All strains exhibited intact contextual fear memory 24 h post-training, but cued fear memory was impaired in strains A/J, BALB, and SM. At 1 h post-training, both contextual and cued fear memory deficits were more widespread: all strains except for B6 and B10 showed impairments of both types of memory. Contextual fear extinction was impaired in BALB and SM. We found that amygdalar LTP was reduced in strains A/J and BALB, but SC LTP was intact in all strains (except for a selective multi-train LTP impairment in BALB). MPPLTP was similar in all five strains. Thus, reduced amygdalar LTP is correlated with impaired cued fear memory in strains A/J and BALB. Also, hippocampal SC LTP is more strongly correlated with 24-h (long-term) than with 1-h (short-term) contextual fear memory. In this first conjoint study of amygdala-dependent memory and amygdalar LTP in inbred mice, we identified specific hippocampal and amygdalar LTP deficits that correlate with fear memory impairments. These deficits should be considered when selecting inbred strains for genetic modification.

Actions of TNF-alpha on glutamatergic synaptic transmission in the central nervous system

Increasing attention is being paid to the role of inflammatory and immune molecules in the modulation of central nervous system (CNS) function. Tumour necrosis factor-alpha (TNF-alpha) is a pro-inflammatory cytokine, the receptors for which are expressed on neurones and glial cells throughout the CNS. Through the action of its two receptors, it has a broad range of actions on neurones which may be either neuroprotective or neurotoxic. It plays a facilitatory role in glutamate excitotoxicity, both directly and indirectly by inhibiting glial glutamate transporters on astrocytes. Additionally, TNF-alpha has direct effects on glutamate transmission, for example increasing expression of AMPA receptors on synapses. TNF-alpha also plays a role in synaptic plasticity, inhibiting long-term potentiation (LTP), a process dependent on p38 mitogen activated kinase (p38 MAP) kinase. In the following review we look at these and other effects of TNF-alpha in the CNS.

BDNF function in adult synaptic plasticity: the synaptic consolidation hypothesis

Interest in BDNF as an activity-dependent modulator of neuronal structure and function in the adult brain has intensified in recent years. Localization of BDNF-TrkB to glutamate synapses makes this system attractive as a dynamic, activity-dependent regulator of excitatory transmission and plasticity. Despite individual breakthroughs, an integrated understanding of BDNF function in synaptic plasticity is lacking. Here, we attempt to distill current knowledge of the molecular mechanisms and function of BDNF in LTP. BDNF activates distinct mechanisms to regulate the induction, early maintenance, and late maintenance phases of LTP. Evidence from genetic and pharmacological approaches is reviewed and tabulated. The specific contribution of BDNF depends on the stimulus pattern used to induce LTP, which impacts the duration and perhaps the subcellular site of BDNF release. Particular attention is given to the role of BDNF as a trigger for protein synthesis-dependent late phase LTP--a process referred to as synaptic consolidation. Recent experiments suggest that BDNF activates synaptic consolidation through transcription and rapid dendritic trafficking of mRNA encoded by the immediate early gene, Arc. A model is proposed in which BDNF signaling at glutamate synapses drives the translation of newly transported (Arc) and locally stored (i.e., alphaCaMKII) mRNA in dendrites. In this model BDNF tags synapses for mRNA capture, while Arc translation defines a critical window for synaptic consolidation. The biochemical mechanisms by which BDNF regulates local translation are also discussed. Elucidation of these mechanisms should shed light on a range of adaptive brain responses including memory and mood resilience.