The integrated role of ACh, ERK and mTOR in the mechanisms of hippocampal inhibitory avoidance memory

Acute psychoactive and toxic effects of D. metel on mice explained by 1H NMR based metabolomics approach

Datura metel L. (D. metel) is one well-known folk medical herb with wide application and also the most abused plants all over the world, mainly for spiritual or religious purpose, over-dosing of which often produces poisonous effects. In this study, mice were orally administered with the extract of D. metel once a day at doses for 10 mg/kg and 40 mg/kg for consecutive 4 days, ¹H NMR based metabolomics approach aided with histopathological inspection and biochemical assays were used for the first time to study the psychoactive and toxic effects of D. metel. Histopathological inspection revealed obvious hypertrophy of hepatocytes, karyolysis and karyorrhexis in livers as well as distinct nerve cell edema, chromatolysis and lower nuclear density in brains. The increased tissue level of methane dicarboxylic aldehyde (MDA) and superoxide dismutase (SOD), decreased tissue level of glutathione (GSH) along with increased serum level of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) suggested brain and liver injury induced by D. metel. Orthogonal signal correction-partial least squares-discriminant analysis (OSC-PLS-DA) of NMR profiles supplemented with correlation network analysis revealed significant altered metabolites and related pathway that contributed to oxidative stress, energy metabolism disturbances, neurotransmitter imbalance and amino acid metabolism disorders.

Cotinine improves visual recognition memory and decreases cortical Tau phosphorylation in the Tg6799 mice

Alzheimer's disease (AD) is associated with the progressive aggregation of hyperphosphorylated forms of the microtubule associated protein Tau in the central nervous system. Cotinine, the main metabolite of nicotine, reduced working memory deficits, synaptic loss, and amyloid β peptide aggregation into oligomers and plaques as well as inhibited the cerebral Tau kinase, glycogen synthase 3β (GSK3β) in the transgenic (Tg)6799 (5XFAD) mice. In this study, the effect of cotinine on visual recognition memory and cortical Tau phosphorylation at the GSK3β sites Serine (Ser)-396/Ser-404 and phospho-CREB were investigated in the Tg6799 and non-transgenic (NT) littermate mice. Tg mice showed short-term visual recognition memory impairment in the novel object recognition test, and higher levels of Tau phosphorylation when compared to NT mice. Cotinine significantly improved visual recognition memory performance increased CREB phosphorylation and reduced cortical Tau phosphorylation. Potential mechanisms underlying theses beneficial effects are discussed.

Dammarane Sapogenins Ameliorates Neurocognitive Functional Impairment Induced by Simulated Long-Duration Spaceflight

Increasing evidence indicates the occurrence of cognitive impairment in astronauts under spaceflight compound conditions, but the underlying mechanisms and countermeasures need to be explored. In this study, we found that learning and memory abilities were significantly reduced in rats under a simulated long-duration spaceflight environment (SLSE), which includes microgravity, isolation confinement, noises, and altered circadian rhythms. Dammarane sapogenins (DS), alkaline hydrolyzed products of ginsenosides, can enhance cognition function by regulating brain neurotransmitter levels and inhibiting SLSE-induced neuronal injury. Bioinformatics combined with experimental verification identified that the PI3K-Akt-mTOR pathway was inhibited and the MAPK pathway was activated during SLSE-induced cognition dysfunction, whereas DS substantially ameliorated the changes in brain. These findings defined the characteristics of SLSE-induced cognitive decline and the mechanisms by which DS improves it. The results provide an effective candidate for improving cognitive function in spaceflight missions.

The role of sleep in regulating structural plasticity and synaptic strength: Implications for memory and cognitive function

Dendritic spines are the major sites of synaptic transmission in the central nervous system. Alterations in the strength of synaptic connections directly affect the neuronal communication, which is crucial for brain function as well as the processing and storage of information. Sleep and sleep loss bidirectionally alter structural plasticity, by affecting spine numbers and morphology, which ultimately can affect the functional output of the brain in terms of alertness, cognition, and mood. Experimental data from studies in rodents suggest that sleep deprivation may impact structural plasticity in different ways. One of the current views, referred to as the synaptic homeostasis hypothesis, suggests that wake promotes synaptic potentiation whereas sleep facilitates synaptic downscaling. On the other hand, several studies have now shown that sleep deprivation can reduce spine density and attenuate synaptic efficacy in the hippocampus. These data are the basis for the view that sleep promotes hippocampal structural plasticity critical for memory formation. Altogether, the impact of sleep and sleep loss may vary between regions of the brain. A better understanding of the role that sleep plays in regulating structural plasticity may ultimately lead to novel therapeutic approaches for brain disorders that are accompanied by sleep disturbances and sleep loss.

AMPK signaling in the nucleus accumbens core mediates cue-induced reinstatement of cocaine seeking

Relapse to drug seeking can be caused by exposure to drug-associated cues, provoking drug craving even after prolonged abstinence. Recent studies demonstrated that AMP-activated protein kinase (AMPK) regulates neuronal morphology and membrane excitability in neurons. Here, we investigated the role of AMPK activity in the nucleus accumbens (NAc) in relapse to cocaine seeking. We found that exposure to drug-related cues reinstated cocaine-seeking behavior and increased AMPK and p70s6k phosphorylation in the NAc core but not shell. Augmenting AMPK activity by intra-NAc core infusions of the AMPK activator 5-amino-1-β-D-ribofuranosyl-imidazole-4-carboxamide (AICAR) or adenovirus expressing constitutively active subunits of AMPK decreased cue-induced reinstatement of cocaine seeking and inhibited the mammalian target of rapamycin complex 1 (mTORC1) and extracellular signal-regulated kinase 1/2 (ERK1/2) pathways. In contrast, inhibition of AMPK activity by intra-NAc core infusions of the AMPK inhibitor compound C or adenovirus expressing dominant-negative subunits of AMPK increased cue-induced reinstatement of cocaine seeking and enhanced mTORC1 and ERK1/2 activity. The regulation of AMPK activity in the NAc shell had no effect on cue-induced cocaine seeking. Altogether, these results indicate that AMPK activity in the NAc core is critical for the cue-induced reinstatement of cocaine seeking, which may be mediated by mTORC1 and ERK1/2 signaling.

Potential of GPCRs to modulate MAPK and mTOR pathways in Alzheimer's disease

Despite efforts to understand the mechanism of neuronal cell death, finding effective therapies for neurodegenerative diseases is still a challenge. Cognitive deficits are often associated with neurodegenerative diseases. Remarkably, in the absence of consensus biomarkers, diagnosis of diseases such as Alzheimer's still relies on cognitive tests. Unfortunately, all efforts to translate findings in animal models to the patients have been unsuccessful. Alzheimer's disease may be addressed from two different points of view, neuroprotection or cognitive enhancement. Based on recent data, the mammalian target of rapamycin (mTOR) pathway arises as a versatile player whose modulation may impact on mechanisms of both neuroprotection and cognition. Whereas direct targeting of mTOR does not seem to constitute a convenient approach in drug discovery, its indirect modulation by other signaling pathways seems promising. In fact, G-protein-coupled receptors (GPCRs) remain the most common 'druggable' targets and as such pharmacological manipulation of GPCRs with selective ligands may modulate phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K), mitogen-activated protein (MAP) kinase and mTOR signaling pathways. Thus, GPCRs become important targets for potential drug treatments in different neurodegenerative disorders including, but not limited to, Alzheimer's disease. GPCR-mediated modulation of mTOR may take advantage of different GPCRs coupled to different G-dependent and G-independent signal transduction routes, of functional selectivity and/or of biased agonism. Signals mediated by GPCRs may act as coincidence detectors to achieve different benefits in different stages of the neurodegenerative disease.

3-iodothyronamine (T1AM), a novel antagonist of muscarinic receptors

3-iodothyronamine (T1AM) is a trace amine suspected to derive from thyroid hormone metabolism. T1AM was described as a ligand of G-protein coupled monoaminergic receptors, including trace amine associated receptors, suggesting the amine may exert a modulatory role on the monoaminergic transmission. Nothing is known on the possibility that T1AM could also modulate the cholinergic transmission interacting with muscarinic receptors. We evaluated whether T1AM (10nM-100μM) was able to i) displace [³H]-NMS (0.20nM) binding to membrane preparations from CHO cells stably transfected with human muscarinic receptor subtypes (M1-M5); ii) modify basal or acetylcholine induced pERK_1/2 levels in CHO expressing the human muscarinic type 3 receptor subtype by Western blot iii) modify basal and carbachol-induced contraction of isolated rat urinary bladder. T1AM fitting within rat muscarinic type 3 receptor was simulated by Docking studies. T1AM recognized all muscarinic receptor subtypes (pKi values in the micromolar range). Interacting at type 3, T1AM reduced acetylcholine-increased pERK_1/2 levels. T1AM reduced carbachol-induced contraction of the rat urinary bladder. The fenoxyl residue and the iodide ion were found essential for establishing contacts with the active site of the rat muscarinic type 3 receptor subtype. Our results indicate that T1AM binds at muscarinic receptors behaving as a weak, not selective, antagonist. This finding adds knowledge on the pharmacodynamics features of T1AM and it may prompt investigation on novel pharmacological effects of T1AM at conditions of hyper-activation of the muscarinic tone including the overactive urinary bladder.

Pro-cognitive action of CART is mediated via ERK in the hippocampus

Although cocaine- and amphetamine-regulated transcript peptide (CART) is detected in several cortical and subcortical areas, its role in higher functions has been largely ignored. We examined the significance of CART in memory formation and tested if the downstream actions of CART involve N-methyl-d-aspartate (NMDA) activated extra-cellular signal-regulated kinase (ERK). Newly formed memory was evaluated using novel object recognition test consisting of familiarization (T1) and choice trials (T2). The choice trials were performed at two time points: 30-min (T230-min ) and 24-h (T224-h ) postacquisition. In choice trial (T230-min ), vehicle control rats explored the novel object for significantly longer duration than the familiar object indicating intact memory formation. However, CART-antibody, U0126 [ERK antagonist, both via intracerebroventricular (icv) or intrahippocampal (ih) route] or MK-801 (NMDA antagonist; intraperitoneal) treated rats spent less time exploring novel objects; CART peptide (icv or ih) was ineffective. During choice trial at T224-h , a significant decrease in novel object exploration time was noticed in vehicle control rats suggesting amnesia. However, treatment with CART, prior to familiarization trial (T1), promoted exploration of the novel object even at T224-h . Pretreatment with U0126 or MK-801 blocked pro-cognitive-like effect of CART suggesting involvement of NMDA-ERK pathway in CART's action. Animals subjected to the object familiarization trial showed a drastic increase in the CART-immunoreactivity in the cells of cornu ammonis 3 and polymorph layer of dentate gyrus, and fibers within ento- (ENT) and peri-rhinal (PRH) cortices. Western blot analysis revealed that CART treatment significantly up-regulated the expression of phospo-ERK1/2 in hippocampus, ENT and PRH. This effect was attenuated following pretreatment with U0126 or MK-801, suggesting the activation of ERK signaling cascade through NMDA receptors. Thus, CART system seems to play an important role in recognition memory and that these effects may be mediated by NMDA receptors-ERK signaling in the ENT/PRH-hippocampal circuit. © 2016 Wiley Periodicals, Inc.

Impairments of spatial learning and memory following intrahippocampal injection in rats of 3-mercaptopropionic acid-modified CdTe quantum dots and molecular mechanisms

With the rapid development of nanotechnology, quantum dots (QDs) as advanced nanotechnology products have been widely used in neuroscience, including basic neurological studies and diagnosis or therapy for neurological disorders, due to their superior optical properties. In recent years, there has been intense concern regarding the toxicity of QDs, with a growing number of studies. However, knowledge of neurotoxic consequences of QDs applied in living organisms is lagging behind their development, even if several studies have attempted to evaluate the toxicity of QDs on neural cells. The aim of this study was to evaluate the adverse effects of intrahippocampal injection in rats of 3-mercaptopropionic acid (MPA)-modified CdTe QDs and underlying mechanisms. First of all, we observed impairments in learning efficiency and spatial memory in the MPA-modified CdTe QD-treated rats by using open-field and Y-maze tests, which could be attributed to pathological changes and disruption of ultrastructure of neurons and synapses in the hippocampus. In order to find the mechanisms causing these effects, transcriptome sequencing (RNA-seq), an advanced technology, was used to gain the potentially molecular targets of MPA-modified CdTe QDs. According to ample data from RNA-seq, we chose the signaling pathways of PI3K–Akt and MPAK–ERK to do a thorough investigation, because they play important roles in synaptic plasticity, long-term potentiation, and spatial memory. The data demonstrated that phosphorylated Akt (p-Akt), p-ERK1/2, and c-FOS signal transductions in the hippocampus of rats were involved in the mechanism underlying spatial learning and memory impairments caused by 3.5 nm MPA-modified CdTe QDs.

Evidence of VTA and LC control of protein synthesis required for the behavioral tagging process

Several works have shown that the formation of different long-term memories relies on a behavioral tagging process. In other words, to establish a lasting memory, at least two parallel processes must occur: the setting of a learning tag (triggered during learning) that defines where a memory could be stored, and the synthesis of proteins, that once captured at tagged sites will effectively allow the consolidation process to occur. This work focused in studying which brain structures are responsible of controlling the synthesis of those proteins at the brain areas where memory is being stored. It combines electrical activation of the ventral tegmental area (VTA) and/or the locus coeruleus (LC), with local pharmacological interventions and weak and strong behavioral trainings in the inhibitory avoidance and spatial object recognition tasks in rats. The results presented here strongly support the idea that the VTA is a brain structure responsible for regulating the consolidation of memories acting through the D1/D5 dopaminergic receptors of the hippocampus to control the synthesis of new proteins required for this process. Moreover, they provide evidence that the LC may be a second structure with a similar role, acting independently and complementary to the VTA, through the β-adrenergic receptors of the hippocampus.

Inhibition of mammalian target of rapamycin activation in the rostral anterior cingulate cortex attenuates pain-related aversion in rats

Pain is a complex experience that comprises both sensory and affective dimensions. Mammalian target of rapamycin (mTOR) plays an important role in the modulation of neuronal plasticity associated with the pathogenesis of pain sensation. However, the role of mTOR in pain affect is unclear. Using a formalin-induced conditioned place avoidance (F-CPA) test, the current study investigated the effects of the mTOR specific inhibitor rapamycin on noxious stimulation induced aversion in the rostral anterior cingulate cortex (rACC). Intraplantar injection of 5% formalin was associated with significant activation of mTOR, as well as p70 ribosomal S6 protein (p70S6K), its downstream effector, in the rACC. The inhibition of mTOR activation with rapamycin disrupted pain-related aversion; however, this inhibition did not affect formalin-induced spontaneous nociceptive behaviors in rats. These findings demonstrated for the first time that mTOR and its downstream pathway in the rACC contribute to the induction of pain-related negative emotion.

Fear Memory

Fear memory is the best-studied form of memory. It was thoroughly investigated in the past 60 years mostly using two classical conditioning procedures (contextual fear conditioning and fear conditioning to a tone) and one instrumental procedure (one-trial inhibitory avoidance). Fear memory is formed in the hippocampus (contextual conditioning and inhibitory avoidance), in the basolateral amygdala (inhibitory avoidance), and in the lateral amygdala (conditioning to a tone). The circuitry involves, in addition, the pre- and infralimbic ventromedial prefrontal cortex, the central amygdala subnuclei, and the dentate gyrus. Fear learning models, notably inhibitory avoidance, have also been very useful for the analysis of the biochemical mechanisms of memory consolidation as a whole. These studies have capitalized on in vitro observations on long-term potentiation and other kinds of plasticity. The effect of a very large number of drugs on fear learning has been intensively studied, often as a prelude to the investigation of effects on anxiety. The extinction of fear learning involves to an extent a reversal of the flow of information in the mentioned structures and is used in the therapy of posttraumatic stress disorder and fear memories in general.

Distinct cognitive effects and underlying transcriptome changes upon inhibition of individual miRNAs in hippocampal neurons

MicroRNAs (miRNA) are small, non-coding RNAs mediating post-transcriptional regulation of gene expression. miRNAs have recently been implicated in hippocampus-dependent functions such as learning and memory, although the roles of individual miRNAs in these processes remain largely unknown. Here, we achieved stable inhibition using AAV-delivered miRNA sponges of individual, highly expressed and brain-enriched miRNAs; miR-124, miR-9 and miR-34, in hippocampal neurons. Molecular and cognitive studies revealed a role for miR-124 in learning and memory. Inhibition of miR-124 resulted in an enhanced spatial learning and working memory capacity, potentially through altered levels of genes linked to synaptic plasticity and neuronal transmission. In contrast, inhibition of miR-9 or miR-34 led to a decreased capacity of spatial learning and of reference memory, respectively. On a molecular level, miR-9 inhibition resulted in altered expression of genes related to cell adhesion, endocytosis and cell death, while miR-34 inhibition caused transcriptome changes linked to neuroactive ligand-receptor transduction and cell communication. In summary, this study establishes distinct roles for individual miRNAs in hippocampal function.

Environmental enrichment improves learning and memory and long-term potentiation in young adult rats through a mechanism requiring mGluR5 signaling and sustained activation of p70s6k

Previous studies from our lab have demonstrated that mild cognitive impairments identified early in life are predictive of cognitive deficits that develop with age, suggesting that enhancements in cognition at an early age can provide a buffer against age-related cognitive decline. Environmental enrichment has been shown to improve learning and memory in the rodent, but the impact of enrichment on synaptic plasticity and the molecular mechanisms behind enrichment are not completely understood. To address these unresolved issues, we have housed 2-month old rats in environmentally enriched (EE), socially enriched (SE), or standard housing (SC) and conducted tests of learning and memory formation at various time intervals. Here we demonstrate that animals that have been exposed to one month of social or environmental enrichment demonstrate enhanced learning and memory relative to standard housed controls. However, we have found that after 4months EE animals perform better than both SE and SC groups and demonstrate an enhanced hippocampal LTP. Our results demonstrate that this LTP is dependent on mGluR5 signaling, activation of ERK and mTOR signaling cascades, and sustained phosphorylation of p70s6 kinase, thus providing a potential target mechanism for future studies of cognitive enhancement in the rodent.