Age-related enhancement of a protein synthesis-dependent late phase of LTP induced by low frequency paired-pulse stimulation in hippocampus

The Dostoyevsky effect: epileptogenesis and memory enhancement after kindling stimulation in the primate basolateral amygdala

Kindling is an electrical stimulation technique used to lower the threshold for epileptogenic activity in the brain. It can also be used as a tool to investigate electrophysiologic alterations that occur as a result of seizures. Epileptiform activity, like seizures and after-discharges (AD; evoked epileptiform activity), commonly cause memory impairment but rarely, can elicit vivid memory retrieval. We kindled the basolateral amygdala of a non-human primate (NHP) once weekly and had him perform a spatial memory task in a 3D virtual environment before, during and after kindling. AD were associated with an initial average performance increase of 46.6%. The enhancement which followed AD persisted up to 2 days. Memory task performance enhancement was accompanied by significant resetting of hippocampal theta oscillations and robust hippocampal potentiation as measured by field evoked potentials. However, neither lasted throughout the duration of performance enhancement. Sharp-wave ripples (SWR), a local field event that supports episodic memory, were generated more often throughout the period of enhancement. SWR rate increased from 14.38 SWR per min before kindling to 24.22 SWR per min after kindling on average. Our results show that kindling can be associated with improved memory. Memory function appears to depend on the particular induction circuit and the resultant excitation/inhibition ratio of the mesial temporal lobe network. Investigating the electrophysiologic underpinnings of this observed memory enhancement is an important step towards understanding the network alterations that occur after seizures and stimulation.Clinical Relevance- Our findings provide new insight into the effects of kindling stimulation in the primate brain. Kindling can cause increase MTL synchrony and the frequency of spontaneous seizures in a primate. This work highlights important considerations for therapeutic deep brain stimulation.

Rho-associated kinases contribute to the regulation of tau phosphorylation and amyloid metabolism during neuronal plasticity

BACKGROUND

Neural plasticity under physiological condition develops together with normal tau phosphorylation and amyloid precursor protein (APP) processing. Since restoration of PI3-kinase signaling has therapeutic potential in Alzheimer's disease, we investigated plasticity-related changes in tau and APP metabolism by the selective Rho-kinase inhibitor fasudil.

METHODS

Field potentials composed of a field excitatory post-synaptic potential (fEPSP) and a population spike (PS) were recorded from a granule cell layer of the dentate gyrus. Plasticity of synaptic strength and neuronal function was induced by strong tetanic stimulation (HFS) and low-frequency stimulation (LFS) patterns. Infusions of saline or fasudil were given for 1 h starting from the application of the induction protocols. Total and phosphorylated tau levels and soluble APPα levels were measured in the hippocampus, which was removed after at least 1 h post-induction period.

RESULTS

Fasudil infusion resulted in attenuation of fEPSP slope and PS amplitude in response to both HFS and LFS. Fasudil reduced total tau and phosphorylated tau at residue Thr¹⁸¹ in the HFS-stimulated hippocampus, while Thr²³¹ phosphorylation was reduced by fasudil treatment in the LFS-stimulated hippocampus. Ser⁴¹⁶ phosphorylation was increased by fasudil treatment in both HFS- and LFS-stimulated hippocampus. Fasudil significantly increased soluble APPα in LFS-stimulated hippocampus, but not in HFS-stimulated hippocampus.

CONCLUSION

In light of our findings, we suggest that increased activity of Rho kinase could trigger a mechanism that goes awry during synaptic plasticity which is reversed by a Rho-kinase inhibitor. Thus, Rho-kinase inhibition might be a therapeutic target in cognitive disorders.

The aging mouse brain: cognition, connectivity and calcium

Aging is a complex process that differentially impacts multiple cognitive, sensory, neuronal and molecular processes. Technological innovations now allow for parallel investigation of neuronal circuit function, structure and molecular composition in the brain of awake behaving adult mice. Thus, mice have become a critical tool to better understand how aging impacts the brain. However, a more granular systems-based approach, which considers the impact of age on key features relating to neural processing, is required. Here, we review evidence probing the impact of age on the mouse brain. We focus on a range of processes relating to neuronal function, including cognitive abilities, sensory systems, synaptic plasticity and calcium regulation. Across many systems, we find evidence for prominent age-related dysregulation even before 12 months of age, suggesting that emerging age-related alterations can manifest by late adulthood. However, we also find reports suggesting that some processes are remarkably resilient to aging. The evidence suggests that aging does not drive a parallel, linear dysregulation of all systems, but instead impacts some processes earlier, and more severely, than others. We propose that capturing the more fine-scale emerging features of age-related vulnerability and resilience may provide better opportunities for the rejuvenation of the aged brain.

Novel Players in the Aging Synapse: Impact on Cognition

While neuronal loss has long been considered as the main contributor to age-related cognitive decline, these alterations are currently attributed to gradual synaptic dysfunction driven by calcium dyshomeostasis and alterations in ionotropic/metabotropic receptors. Given the key role of the hippocampus in encoding, storage, and retrieval of memory, the morpho- and electrophysiological alterations that occur in the major synapse of this network-the glutamatergic-deserve special attention. We guide you through the hippocampal anatomy, circuitry, and function in physiological context and focus on alterations in neuronal morphology, calcium dynamics, and plasticity induced by aging and Alzheimer's disease (AD). We provide state-of-the art knowledge on glutamatergic transmission and discuss implications of these novel players for intervention. A link between regular consumption of caffeine-an adenosine receptor blocker-to decreased risk of AD in humans is well established, while the mechanisms responsible have only now been uncovered. We review compelling evidence from humans and animal models that implicate adenosine A2A receptors (A2AR) upsurge as a crucial mediator of age-related synaptic dysfunction. The relevance of this mechanism in patients was very recently demonstrated in the form of a significant association of the A2AR-encoding gene with hippocampal volume (synaptic loss) in mild cognitive impairment and AD. Novel pathways implicate A2AR in the control of mGluR5-dependent NMDAR activation and subsequent Ca2+ dysfunction upon aging. The nature of this receptor makes it particularly suited for long-term therapies, as an alternative for regulating aberrant mGluR5/NMDAR signaling in aging and disease, without disrupting their crucial constitutive activity.

Astrocyte HIF-2α supports learning in a passive avoidance paradigm under hypoxic stress

BACKGROUND

The brain is extensively vascularized, useŝ20% of the body's oxygen, and is highly sensitive to changes in oxygen. While synaptic plasticity and memory are impaired in healthy individuals by exposure to mild hypoxia, aged individuals appear to be even more sensitive. Aging is associated with progressive failure in pulmonary and cardiovascular systems, exposing the aged to both chronic and superimposed acute hypoxia. The HIF proteins, the "master regulators" of the cellular response to hypoxia, are robustly expressed in neurons and astrocytes. Astrocytes support neurons and synaptic plasticity via complex metabolic and trophic mechanisms. The activity of HIF proteins in the brain is diminished with aging, and the increased exposure to chronic and acute hypoxia with aging combined with diminished HIF activity may impair synaptic plasticity.

PURPOSE

Herein, we test the hypothesis that astrocyte HIF supports synaptic plasticity and learning upon hypoxia.

MATERIALS AND METHODS

An Astrocyte-specific HIF loss-of-function model was employed, where knock-out of HIF-1α or HIF-2α in GFAP expressing cells was accomplished by cre-mediated recombination. Animals were tested for behavioral (open field and rotarod), learning (passive avoidance paradigm), and electrophysiological (long term potentiation) responses to mild hypoxic challenge.

RESULTS

In an astrocyte-specific HIF loss-of-function model followed by mild hypoxia, we identified that the depletion of HIF-2α resulted in an impaired passive avoidance learning performance. This was accompanied by an attenuated response to induction in long-term potentiation (LTP), suggesting that the hippocampal circuitry was perturbed upon hypoxic exposure following HIF-2α loss in astrocytes, and not due to hippocampal cell death. We investigated HIF-regulated trophic and metabolic target genes and found that they were not regulated by HIF-2α, suggesting that these specific targets may not be involved in mediating the phenotypes observed.

CONCLUSION

Together, these results point to a role for HIF-2α in the astrocyte's regulatory role in synaptic plasticity and learning under hypoxia and suggest that even mild, acute hypoxic challenges can impair cognitive performance in the aged population who harbor impaired HIF function.

Altered function of neuronal L-type calcium channels in ageing and neuroinflammation: Implications in age-related synaptic dysfunction and cognitive decline

The rapid developments in science have led to an increase in human life expectancy and thus, ageing and age-related disorders/diseases have become one of the greatest concerns in the 21st century. Cognitive abilities tend to decline as we get older. This age-related cognitive decline is mainly attributed to aberrant changes in synaptic plasticity and neuronal connections. Recent studies show that alterations in Ca²⁺ homeostasis underlie the increased vulnerability of neurons to age-related processes like cognitive decline and synaptic dysfunctions. Dysregulation of Ca²⁺ can lead to dramatic changes in neuronal functions. We discuss in this review, the recent advances on the potential role of dysregulated Ca²⁺ homeostasis through altered function of L-type voltage gated Ca²⁺ channels (LTCC) in ageing, with an emphasis on cognitive decline. This review therefore focuses on age-related changes mainly in the hippocampus, and with mention of other brain areas, that are important for learning and memory. This review also highlights age-related memory deficits via synaptic alterations and neuroinflammation. An understanding of these mechanisms will help us formulate strategies to reverse or ameliorate age-related disorders like cognitive decline.

Age-dependent changes in brain hydration and synaptic plasticity

Aging in humans and animals is associated with gradual and variable changes in some cognitive functions, but what causes them and explains individual variations remains unclear. Hydration decreases with aging but whether dehydration contributes to cognitive dysfunction is not known. The brain hydration of aging mice was determined by colloidosmotic-pressure titration. Dehydration increased with age from ∼76 mmHg at 6 weeks to ∼105 mmHg at 40 weeks, or a progressive ∼10 percent loss of brain water but seemed to level off afterward. When we adjusted dehydration in hippocampal slices of <8-week-old mice to the levels seen in mice 40 weeks and older, their basal synaptic responses were amplified at all stimulus voltages tested, but induction of late-phase long-term potentiation was impaired. Our results document progressive brain dehydration with age in inbred mice to levels at which in vitro synaptic plasticity appears dysregulated. They also suggest that dehydration contributes to some of the changes in synaptic plasticity observed with aging, possibly due to adjustments in neuronal excitation mechanisms.

Enhanced LTP in aged rats: Detrimental or compensatory?

Age-dependent memory deterioration has been well documented and yet an increase in rat hippocampal LTP upon aging has been reported. This poses the question of whether the enhanced LTP is a cause or an attempt to compensate the memory deficits described in aged rats. Hippocampal slices from young, adult and aged Wistar rats were pre-incubated, with an NMDA receptor (NMDAR) antagonist, memantine (1 μM, 4 h), and hippocampal LTP was evaluated. The results show that memantine significantly decreases the larger LTP magnitude recorded in hippocampal slices from aged rats without compromising LTP recorded in slices from young and adult animals. To unveil the impact of in vivo administration of memantine, different doses (1, 5 and 10 mg/kg/day) or saline vehicle solution were intraperitoneally administered, for 15-20 days, to both young and aged animals. Memantine did not significantly affect neither the place learning of young animals, evaluated by Morris Water Maze, nor LTP recorded from hippocampal slices from the same group of animals. However, memantine (5 and 10 mg/kg/day) significantly decreased the large LTP recorded in hippocampal slices from aged animals. Moreover, aged animals treated with memantine (10 mg/kg/day) showed a significantly compromised place learning when compared to aged control animals. Overall, these results suggest that the larger LTP observed in aged animals is a compensatory phenomenon, rather than pathological. The finding that age-dependent blockade of LTP by a NMDAR antagonist leads to learning deficits, implies that the increased LTP observed upon aging may be playing an important role in the learning process.

Metabotropic glutamate receptor, mGlu5, mediates enhancements of hippocampal long-term potentiation after environmental enrichment in young and old mice

The metabotropic glutamate (mGlu) receptor, mGlu5, is of particular relevance for hippocampal function. It is critically required for the expression of long-term potentiation (LTP) and long-term depression (LTD), regulates neuronal oscillations, maintains the stability of place fields and is required for hippocampus-dependent memory. MGlu5-dysfunctions are associated with profound cognitive deficits in humans, and mGlu5 has been targeted as a putative cognitive enhancer. Cognitive enhancement, by means of environmental enrichment (EE) in rodents, results in improved hippocampal synaptic plasticity and memory. Here, we explored whether mGlu5 contributes to these enhancements. MGlu5-antagonism dose-dependently impaired the early phase of LTP (>4 h) in the CA1 region of young(3-4 month old) mice. Late-LTP (>24 h) was also impaired. LTP (>24 h) elicited in old (10-14 month old) mice displayed reduced sensitivity to mGlu5 antagonism. Short-term potentiation (STP, < 2 h) that was elicited by weaker afferent stimulation was unaffected by mGlu5-antagonism in both age-groups. EE significantly amplified STP (<2 h) in old and young animals, but did not increase the duration of synaptic potentiation, or promote induction of LTP. The improvement in STP was prevented by mGlu5-antagonism, in both young and old animals. These results indicate that modifications of the synapse that underlie improvements of LTP by EE require the contribution of mGlu5. Strikingly, although LTP in old mice does not critically depend on mGlu5, improvements in synaptic potentiation resulting from EE are mGlu5-dependent in old mice. Regarded in light of the known role for mGlu5 in hippocampal function and pathophysiology, these data suggest that mGlu5 regulation of synaptic information storage is pivotal to optimal hippocampal function. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

Frontal cortex and hippocampus neurotransmitter receptor complex level parallels spatial memory performance in the radial arm maze

Several neurotransmitter receptors have been proposed to be involved in memory formation. However, information on receptor complexes (RCs) in the radial arm maze (RAM) is missing. It was therefore the aim of this study to determine major neurotransmitter RCs levels that are modulated by RAM training because receptors are known to work in homo-or heteromeric assemblies. Immediate early gene Arc expression was determined by immunohistochemistry to show if prefrontal cortices (PFC) and hippocampi were activated following RAM training as these regions are known to be mainly implicated in spatial memory. Twelve rats per group, trained and untrained in the twelve arm RAM were used, frontal cortices and hippocampi were taken, RCs in membrane protein were quantified by blue-native PAGE immunoblotting. RCs components were characterised by co-immunoprecipitation followed by mass spectrometrical analysis and by the use of the proximity ligation assay. Arc expression was significantly higher in PFC of trained as compared to untrained rats whereas it was comparable in hippocampi. Frontal cortical levels of RCs containing AMPA receptors GluA1, GluA2, NMDA receptors GluN1 and GluN2A, dopamine receptor D1, acetylcholine nicotinic receptor alpha 7 (nAChR-α7) and hippocampal levels of RCs containing D1, GluN1, GluN2B and nAChR-α7 were increased in the trained group; phosphorylated dopamine transporter levels were decreased in the trained group. D1 and GluN1 receptors were shown to be in the same complex. Taken together, distinct RCs were paralleling performance in the RAM which is relevant for interpretation of previous and design of future work on RCs in memory studies.

Impact of aging brain circuits on cognition

Brain networks that engage the hippocampus and prefrontal cortex are central for enabling effective interactions with our environment. Some of the cognitive processes that these structures mediate, such as encoding and retrieving episodic experience, wayfinding, working memory and attention are known to be altered across the lifespan. As illustrated by examples given below, there is remarkable consistency across species in the pattern of age-related neural and cognitive change observed in healthy humans and other animals. These include changes in cognitive operations that are known to be dependent on the hippocampus, as well as those requiring intact prefrontal cortical circuits. Certain cognitive constructs that reflect the function of these areas lend themselves to investigation across species, allowing brain mechanisms at different levels of analysis to be studied in greater depth.

Aging of the NMDA receptor: from a mouse's point of view

Our elderly population is growing and declines in cognitive abilities, such as memory, can be costly, because it can interfere with a person's ability to live independently. The NMDA receptor is very important for many different forms of memory and this receptor is negatively affected by aging. This review examines the progress that has been made recently in characterizing selective vulnerabilities of different subunits and splice variants of the NMDA receptor to normal aging in C57BL/6 mice. Evidence is also presented for changes in the relationships of NMDA receptors to plasticity across aging. Recent interventions show that enhancing NMDA receptors in aged individuals is associated with improvements in memory, but mouse models of neurodegenerative diseases suggest that finding the right balance between too little and too much NMDA receptor activity will be the key to enhancing memory without inducing pathology.

A novel form of low-frequency hippocampal mossy fiber plasticity induced by bimodal mGlu1 receptor signaling

Mossy fiber synapses act as the critical mediators of highly dynamic communication between hippocampal granule cells in the dentate gyrus and CA3 pyramidal neurons. Excitatory synaptic strength at mossy fiber to CA3 pyramidal cell synapses is potentiated rapidly and reversibly by brief trains of low-frequency stimulation of mossy fiber axons. We show that slight modifications to the pattern of stimulation convert this short-term potentiation into prolonged synaptic strengthening lasting tens of minutes in rodent hippocampal slices. This low-frequency potentiation of mossy fiber EPSCs requires postsynaptic mGlu1 receptors for induction but is expressed presynaptically as an increased release probability and therefore impacts both AMPA and NMDA components of the mossy fiber EPSC. A nonconventional signaling pathway initiated by mGlu1 receptors contributes to induction of plasticity, because EPSC potentiation was prevented by a tyrosine kinase inhibitor and only partially reduced by guanosine 5'-O-(2-thiodiphosphate). A slowly reversible state of enhanced synaptic efficacy could serve as a mechanism for altering the integrative properties of this synapse within a relatively broad temporal window.

D1/D5 dopamine receptors modulate spatial memory formation

We investigated the effect of the intra-CA1 administration of the D1/D5 receptor antagonist SCH23390 and the D1/D5 receptor agonist SKF38393 on spatial memory in the water maze. When given immediately, but not 3h after training, SCH23390 hindered long-term spatial memory formation without affecting non-spatial memory or the normal functionality of the hippocampus. On the contrary, post-training infusion of SKF38393 enhanced retention and facilitated the spontaneous recovery of the original spatial preference after reversal learning. Our findings demonstrate that hippocampal D1/D5 receptors play an essential role in spatial memory processing.

Enhancement of LTP in aged rats is dependent on endogenous BDNF

Long-term potentiation (LTP), considered the neurophysiological basis for learning and memory, is facilitated by brain-derived neurotrophic factor (BDNF), an action more evident when LTP is evoked by weak θ-burst stimuli and dependent on co-activation of adenosine A(2A) receptors (A(2A)R), which are more expressed in aged rats. As θ-burst stimuli also favor LTP in aged animals, we hypothesized that enhanced LTP in aging could be related to changes in neuromodulation by BDNF. The magnitude of CA1 LTP induced by a weak θ-burst stimuli delivered to the Schaffer collaterals was significantly higher in hippocampal slices taken from 36 to 38 and from 70 to 80-week-old rats, when compared with LTP magnitude in slices from 4 or 10 to 15-week-old rats; this enhancement does not impact in cognitive improvement as aged rats revealed an impairment on hippocampal-dependent learning and memory performance, as assessed by the Morris water maze tests. The scavenger for BDNF, TrkB-Fc, and the inhibitor of Trk phosphorylation, K252a, attenuated LTP in slices from 70 to 80-week-old rats, but not from 10 to 15-week-old rats. When exogenously added, BDNF significantly increased LTP in slices from 4 and 10 to 15-week-old rats, but did not further increased LTP in 36 to 38 or 70 to 80-week-old rats. The effects of exogenous BDNF on LTP were prevented by the A(2A)R antagonist, SCH58261 (7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine). These results indicate that the higher LTP magnitude observed upon aging, which does not translate into improved spatial memory performance, is a consequence of an increase in the tonic action of endogenous BDNF.

Age-Induced Alterations in Hippocampal Function and Metabolism

As the nervous system ages, a variety of changes occur in metabolism supporting glial and neuronal function, resulting in greater susceptibility to disease conditions. Changes with aging in the metabolic unit (i.e., neurons, glial cells and blood vessels) have been reported to include alterations of vascular reactivity, impaired transport of critical substrates underlying metabolism, enhanced reactive oxygen species production and alterations in calcium signaling. Some diseases are focused on the elderly, particularly cerebral ischemia, cognitive limitations, iatrogenic hypoglycemia, malignant brain tumors (i.e., glioblastoma), and Alzheimer's disease, partly due to metabolic alterations with aging. These metabolic changes with aging are discussed in light of primary theories of aging of the brain, which include mitochondrial, calcium dysfunction and enhanced oxidative damage. Here we focus on metabolic changes with aging which can influence the susceptibility of the brain to ischemia and cognitive function. Lastly, we describe treatment possibilities for these abnormal responses to aging, particularly the topic of caloric/dietary restriction, and possible mechanisms underlying this treatment direction.

Neural Protein Synthesis during Aging: Effects on Plasticity and Memory

During aging, many experience a decline in cognitive function that includes memory loss. The encoding of long-term memories depends on new protein synthesis, and this is also reduced during aging. Thus, it is possible that changes in the regulation of protein synthesis contribute to the memory impairments observed in older animals. Several lines of evidence support this hypothesis. For instance, protein synthesis is required for a longer period following learning to establish long-term memory in aged rodents. Also, under some conditions, synaptic activity or pharmacological activation can induce de novo protein synthesis and lasting changes in synaptic transmission in aged, but not young, rodents; the opposite results can be observed in other conditions. These changes in plasticity likely play a role in manifesting the altered place field properties observed in awake and behaving aged rats. The collective evidence suggests a link between memory loss and the regulation of protein synthesis in senescence. In fact, pharmaceuticals that target the signaling pathways required for induction of protein synthesis have improved memory, synaptic plasticity, and place cell properties in aged animals. We suggest that a better understanding of the mechanisms that lead to different protein expression patterns in the neural circuits that change as a function of age will enable the development of more effective therapeutic treatments for memory loss.

Selective Vulnerabilities of N-methyl-D-aspartate (NMDA) Receptors During Brain Aging

N-methyl-D-aspartate (NMDA) receptors are present in high density within the cerebral cortex and hippocampus and play an important role in learning and memory. NMDA receptors are negatively affected by aging, but these effects are not uniform in many different ways. This review discusses the selective age-related vulnerabilities of different binding sites of the NMDA receptor complex, different subunits that comprise the complex, and the expression and functions of the receptor within different brain regions. Spatial reference, passive avoidance, and working memory, as well as place field stability and expansion all involve NMDA receptors. Aged animals show deficiencies in these functions, as compared to young, and some studies have identified an association between age-associated changes in the expression of NMDA receptors and poor memory performance. A number of diet and drug interventions have shown potential for reversing or slowing the effects of aging on the NMDA receptor. On the other hand, there is mounting evidence that the NMDA receptors that remain within aged individuals are not always associated with good cognitive functioning. This may be due to a compensatory response of neurons to the decline in NMDA receptor expression or a change in the subunit composition of the remaining receptors. These studies suggest that developing treatments that are aimed at preventing or reversing the effects of aging on the NMDA receptor may aid in ameliorating the memory declines that are associated with aging. However, we need to be mindful of the possibility that there may also be negative consequences in aged individuals.

Bidirectional synaptic plasticity in response to single or paired pulse activation of NMDA receptors

It is still incompletely known how NMDA receptors (NMDA-R) regulate bidirectional synaptic plasticity. We examined this issue by an experimental protocol in which paired pulse stimulation (PPS) with 50ms interstimulus interval and basal frequency of 0.1Hz was applied to CA1 area of rat hippocampal slices during low Mg(2+) perfusion. Under blockade of NMDA-Rs by AP5, PPS for 12-60min led to only a minor depression. In contrast, when PPS was applied in the absence of AP5, there was a prominent short-term potentiation (STP), mainly of AMPA-R mediated responses, with peak at 1min and lasting 10-15min. The STP was followed by a slowly developing long-term depression (LTD). Applying AP5 during the STP, converted it to a stable increase relative to the control pathway. Following peak STP, plasticity was controlled in a composite manner. Whereas the initial decay was counteracted by NMDA-R activation, the following LTD was dependent on such activation. Our data suggest that synaptic changes do not only depend on the instantaneous, NMDA-dependent Ca(2+) concentration in the dendritic spine, but are also influenced by prior induction events. In addition to NMDA-R driven processes, passive relaxation contributes to the synaptic plasticity and in some cases outbalances the active control.

Intrinsic neuronal plasticity in the juxtacapsular nucleus of the bed nuclei of the stria terminalis (jcBNST)

The juxtacapsular nucleus of the anterior division of the BNST (jcBNST) receives robust glutamatergic projections from the basolateral nucleus of the amygdala (BLA), the postpiriform transition area, and the insular cortex as well as dopamine (DA) inputs from the midbrain. In turn the jcBNST sends GABAergic projections to the medial division of the central nucleus of the amygdala (CEAm) as well as other brain regions. We recently described a form of long-term potentiation of the intrinsic excitability (LTP-IE) of neurons of the juxtacapsular nucleus of BNST (jcBNST) in response to high-frequency stimulation (HFS) of the stria terminalis that was impaired during protracted withdrawal from alcohol, cocaine, and heroin and in rats chronically treated with corticotropin-releasing factor (CRF) intracerebroventricularly. Here we show that DAergic neurotransmission is required for the induction of LTP-IE of jcBNST neurons through dopamine (DA) D1 receptors. Thus, activation of the central CRF stress system and altered DAergic neurotransmission during protracted withdrawal from alcohol and drugs of abuse may contribute to the disruption of LTP-IE in the jcBNST. Impairment of this form of intrinsic neuronal plasticity in the jcBNST could result in inadequate neuronal integration and reduced inhibition of the CEA, contributing to the negative affective state that characterizes protracted abstinence in post-dependent individuals. These results provide a novel neurobiological target for vulnerability to alcohol and drug dependence.

Dopamine controls persistence of long-term memory storage

The paradigmatic feature of long-term memory (LTM) is its persistence. However, little is known about the mechanisms that make some LTMs last longer than others. In rats, a long-lasting fear LTM vanished rapidly when the D1 dopamine receptor antagonist SCH23390 was injected into the dorsal hippocampus 12 hours, but not immediately or 9 hours, after the fearful experience. Conversely, intrahippocampal application of the D1 agonist SK38393 at the same critical post-training time converted a rapidly decaying fear LTM into a persistent one. This effect was mediated by brain-derived neurotrophic factor and regulated by the ventral tegmental area (VTA). Thus, the persistence of LTM depends on activation of VTA/hippocampus dopaminergic connections and can be specifically modulated by manipulating this system at definite post-learning time points.

Non-coding RNA transcripts: sensors of neuronal stress, modulators of synaptic plasticity, and agents of change in the onset of Alzheimer's disease

Non-protein-coding RNAs (ncRNAs) play critical roles on many levels of cellular information processing and pervasive expression of ncRNAs in the nervous system could help explain brain complexity. NcRNAs are enriched in the central nervous system and are associated with specific neuroanatomical regions. Additionally, several recent publications have revealed an important role for deregulation of ncRNAs in various human neuropathologies, such as Alzheimer's disease, Parkinson's disease and Fragile X mental retardation. Herein, we summarize reports on functional ncRNA molecules involved in cellular stress response, particularly related to Alzheimer's disease. We conclude that ncRNAs have a prominent role in maintaining precise physiological levels of gene products directly implicated in Alzheimer's disease pathology.

Estradiol reverses a calcium-related biomarker of brain aging in female rats

An increase in L-type voltage-gated calcium channel (LTCC) current is a prominent biomarker of brain aging and is believed to contribute to cognitive decline and vulnerability to neuropathologies. Studies examining age-related changes in LTCCs have focused primarily on males, although estrogen (17beta-estradiol, E2) affects calcium-dependent activities associated with cognition. Therefore, to better understand brain aging in females, the effects of chronic E2 replacement on LTCC current activity in hippocampal neurons of young and aged ovariectomized rats were determined. The zipper slice preparation was used to expose cornu ammonis 1 (CA1) pyramidal neurons for recording LTCC currents using the cell-attached patch-clamp technique. We found that an age-related increase in LTCC current in neurons from control animals was prevented by E2 treatment. In addition, in situ hybridization revealed that within stratum pyramidale of the CA1 area, mRNA expression of the Ca(v)1.2 LTCC subunit, but not the Ca(v)1.3 subunit, was decreased in aged E2-treated rats. Thus, the reported benefits of E2 on cognition and neuronal health may be attributed, at least in part, to its age-related decrease in LTCC current.

The effects of aging on N-methyl-D-aspartate receptor subunits in the synaptic membrane and relationships to long-term spatial memory

There are declines in the protein expression of the NR2B (mouse epsilon2) and NR1 (mouse zeta1) subunits of the N-methyl-D-aspartate (NMDA) receptor in the cerebral cortex and hippocampus during aging in C57BL/6 mice. This study was designed to determine if there is a greater effect of aging on subunit expression and a stronger relationship between long-term spatial memory and subunit expression within the synaptic membrane than in the cell as a whole. Male, C57BL/6JNIA mice (4, 11 and 26 months old) were tested for long-term spatial memory in the Morris water maze. Frontal cortex, including prefrontal regions, and hippocampus were homogenized and fractionated into light and synaptosomal membrane fractions. Western blots were used to analyze protein expression of NR2B and NR1 subunits of the NMDA receptor. Old mice performed significantly worse than other ages in the spatial task. In the frontal cortex, the protein levels of the NR2B subunit showed a greater decline with aging in the synaptic membrane fraction than in the whole homogenate, while in the hippocampus a similar age-related decline was observed in both fractions. There were no significant effects of aging on the expression of the NR1 subunit. Within the middle-aged mouse group, higher expression of both NR2B and NR1 subunits in the synaptic membrane of the hippocampus was associated with better memory. In the aged mice, however, higher expression of both subunits was associated with poorer memory. These results indicate that aging could be altering the localization of the NR2B subunit to the synaptic membrane within the frontal cortex. The correlational results suggest that NMDA receptor functions, receptor subunit composition, and/or the environment in which the receptor interacted in the hippocampus were not the same in the old animals as in younger mice and this may have contributed to memory declines during aging.

Gene regulation by voltage-dependent calcium channels

Ca2+ is the most widely used second messenger in cell biology and fulfills a plethora of essential cell functions. One of the most exciting findings of the last decades was the involvement of Ca2+ in the regulation of long-term cell adaptation through its ability to control gene expression. This finding provided a link between cell excitation and gene expression. In this review, we chose to focus on the role of voltage-dependent calcium channels in mediating gene expression in response to membrane depolarization. We illustrate the different pathways by which these channels are involved in excitation-transcription coupling, including the most recent Ca2+ ion-independent strategies that highlight the transcription factor role of calcium channels.

Long-term enrichment enhances the cognitive behavior of the aging neurogranin null mice without affecting their hippocampal LTP

Neurogranin (Ng), a PKC substrate, is abundantly expressed in brain regions important for cognitive functions. Deletion of Ng caused severe deficits in spatial learning and LTP in the hippocampal CA1 region of mice. These Ng-/- mice also exhibit deficits in the amplification of their hippocampal signaling pathways critical for learning and memory. A short-term exposure to an enriched environment failed to improve their behavioral performances. Here, we showed that a long-term enrichment protocol for the aging mice was beneficial to the Ng-/- as well as Ng+/+ and Ng+/- mice in preventing age-related cognitive decline. Enrichment also caused an increase in the hippocampal CREB level of all three genotypes and Ng level of Ng+/+ and Ng+/- mice, but not that of alphaCaMKII or ERK. Interestingly, hippocampal slices of these enriched aging Ng-/- mice, unlike those of Ng+/+ and Ng+/- mice, did not show enhancement in the high frequency stimulation (HFS)-induced LTP in the CA1 region. It appears that the learning and memory processes in these enriched aging Ng-/- mice do not correlate with the HFS-induced LTP, which is facilitated by Ng. These results demonstrated that long-term enrichment for the aging Ng-/- mice may improve their cognitive function through an Ng-independent plasticity pathway.

Sensitization of rat dorsal horn neurons by NGF-induced subthreshold potentials and low-frequency activation. A study employing intracellular recordings in vivo

Intramuscular injection of NGF in human subjects has been reported not to elicit pain, whereas 5% NaCl does. On the other hand, NGF injections induce a long-lasting hyperalgesia. In the present study, the possible neuronal basis of these effects was studied at the spinal level. In anesthetized rats, neurons in the segment L4 were recorded intracellularly before (n=65), during (n=15), and after injections of NGF (n=50) as well as during and after 5% NaCl (during: n=12, after: n=39) into the gastrocnemius-soleus (GS) muscle. The neuronal responses to electrical and mechanical stimuli were tested and possible changes caused by the stimulants recorded. Of those neurons that responded to the NGF injections (7 out of 15), the majority exhibited subthreshold excitatory postsynaptic potentials (EPSPs). Only 3 out of 15 neurons reacted with action potentials (APs) at a low frequency. Already 5 to 30 min after NGF injection, some of these neurons showed signs of a sensitization. In comparison to NGF, hypertonic saline i.m. elicited APs at a higher frequency in a larger number of neurons (9 out of 12). One day after NGF i.m., the proportion of dorsal horn neurons responding with APs to electrical stimulation of the GS nerves had increased significantly from 4.6% to 28.0%. Despite the stronger excitatory effect of 5% NaCl, the sensitization of the dorsal horn neurons after hypertonic saline was less than that after NGF (15.3%). Behavioral experiments showed that NGF injections induced stronger mechanical allodynia and hyperalgesia than hypertonic saline i.m. The data demonstrate that low-frequency activation or even subthreshold potentials in dorsal horn neurons evoked by unmyelinated muscle afferents are an effective means of sensitizing these neurons.

Low-frequency stimulation induces a pathway-specific late phase of LTP in the amygdala that is mediated by PKA and dependent on protein synthesis

Activity-dependent changes in synaptic efficacy are thought to be the key cellular mechanism for the formation and storage of both explicit and implicit memory. Different patterns of stimulation can elicit different changes in the efficiency on excitatory synaptic transmission. Here, we examined the synaptic changes in the amygdala of adult mice produced by low-frequency stimulation (1 Hz, 15 min, LFS). We first compared the synaptic changes induced by LFS in three different synaptic pathways of amygdala: cortical-lateral amygdala, thalamic-lateral amygdala, and lateral-basolateral amygdala pathways. We find that the plastic changes induced by LFS are different between synaptic pathways. Low-frequency stimulation selectively elicits a slow onset and protein synthesis-dependent late-phase LTP in the cortical-lateral amygdala pathway, but not in the thalamic-lateral or lateral-basolateral pathways. We next analyzed LTP induced by LFS in the cortical-lateral amygdala pathway and found that three PKA-coupling neurotransmitter receptors are involved: 5-HT4, Dopamine D1, and beta-adrenergic receptors. Antagonists of these receptors block the LFS L-LTP, but the effects of agonists of these receptors are clearly different. These results indicate that the threshold for the induction of LFS L-LTP is different among these pathways and that the maintenance of LFS L-LTP requires a cross-talk among multiple neurotransmitters.

Induction of long-term memory by exposure to novelty requires protein synthesis: evidence for a behavioral tagging

A behavioral analog of the synaptic tagging and capture process, a key property of synaptic plasticity, has been predicted recently. Here, we demonstrate that weak inhibitory avoidance training, which induces short- but not long-term memory (LTM), can be consolidated into LTM by an exploration to a novel, but not a familiar, environment occurring close in time to the training session. This memory-promoting effect caused by novelty depends on activation of dopamine D1/D5 receptors and requires newly synthesized proteins in the dorsal hippocampus. Thus, our results indicate the existence of a behavioral tagging process in which the exploration to a novel environment provides the plasticity-related proteins to stabilize the inhibitory avoidance memory trace.

Synapse specificity of long-term potentiation breaks down with aging

Memory shows age-related decline. According to the current prevailing theoretical model, encoding of memories relies on modifications in the strength of the synapses connecting the different cells within a neuronal network. The selective increases in synaptic weight are thought to be biologically implemented by long-term potentiation (LTP). Here, we report that tetanic stimulation of afferent fibers in slices from 12-mo-old mice triggers an LTP not restricted to the activated synapses. This phenomenon, which can be anticipated to hinder memory encoding, is suppressed by blocking either L-type Ca(++) channels or Ca(++)-induced Ca(++) release, both well known to become disregulated with aging.