Involvement of hippocampal PKCbetaI isoform in the early phase of memory formation of an inhibitory avoidance learning

Involvement of kinases in memory consolidation of inhibitory avoidance training

The inhibitory avoidance (IA) task is a paradigm widely used to investigate the molecular and cellular mechanisms involved in the formation of long-term memory of aversive experiences. In this review, we discuss studies on different brain structures in rats associated with memory consolidation, such as the hippocampus, striatum, and amygdala, as well as some cortical areas, including the insular, cingulate, entorhinal, parietal and prefrontal cortex. These studies have shown that IA training triggers the release of neurotransmitters, hormones, growth factors, etc., that activate intracellular signaling pathways related to protein kinases, which induce intracellular non-genomic changes or transcriptional mechanisms in the nucleus, leading to the synthesis of proteins. We have summarized the temporal dynamics and crosstalk among protein kinase A, protein kinase C, mitogen activated protein kinase, extracellular-signal-regulated kinase, and Ca2+/calmodulin-dependent protein kinase II described in the hippocampus. Protein kinase activity has been associated with structural changes and synaptic strengthening, resulting in memory storage. However, little is known about the molecular mechanisms involved in intense IA training, which protects memory from typical amnestic treatments, such as protein synthesis inhibitors, and induces increased spinogenesis, suggesting an unexplored mechanism independent of the genomic pathway. This highly emotional experience causes an extinction-resistant memory, as has been observed in some pathological states such as post-traumatic stress disorder. We propose that the changes in spinogenesis observed after intense IA training could be generated by protein kinases via non-genomic pathways.

Hippocampal CaMKII inhibition induces reactivation-dependent amnesia for extinction memory and causes fear relapse

Hippocampal GluN2B subunit-containing NMDAR (GluN2B-NMDAR) activation during recall destabilizes fear extinction memory, which must undergo brain-derived neurotrophic factor (BDNF)-dependent reconsolidation to persist. Ca2+/calmodulin-dependent protein kinase II (CaMKII) is a Ser/Thr protein kinase essential for hippocampus-dependent memory processing that acts downstream GluN2B-NMDAR and controls BDNF expression, but its participation in fear extinction memory reconsolidation has not yet been studied. Using a combination of pharmacological and behavioral tools, we found that in adult male Wistar rats, intra dorsal-CA1 administration of the CaMKII inhibitors autocamtide-2-related inhibitory peptide (AIP) and KN-93, but not of their inactive analogs scrambled AIP and KN-92, after fear extinction memory recall impaired extinction and caused GluN2B-NMDAR-dependent recovery of fear. Our results indicate that hippocampal CaMKII is necessary for fear extinction reconsolidation, and suggest that modulation of its activity around the time of recall controls the inhibition that extinction exerts on learned fear.

NMDARs control object recognition memory destabilization and reconsolidation

Object recognition memory (ORM) allows identification of previously encountered items and is therefore crucial for remembering episodic information. In rodents, reactivation during recall in the presence of a novel object destabilizes ORM and initiates a Zif268 and protein synthesis-dependent reconsolidation process in the hippocampus that links the memory of this object to the reactivated recognition trace. Hippocampal NMDA receptors (NMDARs) modulate Zif268 expression and protein synthesis and regulate memory stability but their possible involvement in the ORM destabilization/reconsolidation cycle has yet to be analyzed in detail. We found that, in adult male Wistar rats, intra dorsal-CA1 administration of the non-subunit selective NMDAR antagonist AP5, or of the GluN2A subunit-containing NMDAR antagonist TCN201, 5min after an ORM reactivation session in the presence of a novel object carried out 24h post-training impaired retention 24h later. In contrast, pre-reactivation administration of the GluN2B subunit-containing NMDAR antagonist RO25-6981 had no effect on ORM recall or retention but impeded the amnesia caused by Zif268 silencing and protein synthesis inhibition in dorsal CA1. Our results indicate that GluN2B-containing hippocampal NMDARs are necessary for ORM destabilization whereas GluN2A-containing NMDARs are involved in ORM reconsolidation, and suggest that modulation of the relative activity of these receptor subtypes during recall regulates ORM persistence.

On the effect of hippocampal c-Jun N-terminal kinase inhibition on object recognition memory

c-Jun N-terminal kinase (JNK) phosphorylates the transcription factor c-Jun in response to stress stimuli and contributes to both hippocampal synaptic plasticity and memory processing in mammals. Object recognition memory (ORM) is essential for remembering facts and events. In rodents, ORM consolidation and reconsolidation require a functional hippocampus. However, the possible involvement of hippocampal JNK on ORM processing has not yet been studied. Here we show that when injected into dorsal CA1 5 min, but not 6 h, after training adult male rats in the novel object recognition learning task, the JNK inhibitor SP600125 impaired ORM for at least 7 days without affecting exploratory activity, short-term ORM retention, or the functional integrity of the hippocampus. SP600125 did not hinder ORM retention when given in CA1 after a memory reactivation session carried out 24 h post-training in the presence of the same two objects presented during the training session, but caused time-dependent amnesia when one of the objects presented at training was replaced by a different but behaviorally equivalent novel one. Taken together, our results indicate that hippocampal JNK activity is necessary for ORM consolidation and reconsolidation but not for ORM recall or short-term retention.

GluN2B and GluN2A-containing NMDAR are differentially involved in extinction memory destabilization and restabilization during reconsolidation

Extinction memory destabilized by recall is restabilized through mTOR-dependent reconsolidation in the hippocampus, but the upstream pathways controlling these processes remain unknown. Hippocampal NMDARs drive local protein synthesis via mTOR signaling and may control active memory maintenance. We found that in adult male Wistar rats, intra dorsal-CA1 administration of the non-subunit selective NMDAR antagonist AP5 or of the GluN2A subunit-containing NMDAR antagonist TCN201 after step down inhibitory avoidance (SDIA) extinction memory recall impaired extinction memory retention and caused SDIA memory recovery. On the contrary, pre-recall administration of AP5 or of the GluN2B subunit-containing NMDAR antagonist RO25-6981 had no effect on extinction memory recall or retention per se but hindered the recovery of the avoidance response induced by post-recall intra-CA1 infusion of the mTOR inhibitor rapamycin. Our results indicate that GluN2B-containing NMDARs are necessary for extinction memory destabilization whereas GluN2A-containing NMDARs are involved in its restabilization, and suggest that pharmacological modulation of the relative activation state of these receptor subtypes around the moment of extinction memory recall may regulate the dominance of extinction memory over the original memory trace.

mTOR inhibition impairs extinction memory reconsolidation

Fear-motivated avoidance extinction memory is prone to hippocampal brain-derived neurotrophic factor (BDNF)-dependent reconsolidation upon recall. Here, we show that extinction memory recall activates mammalian target of rapamycin (mTOR) in dorsal CA1, and that post-recall inhibition of this kinase hinders avoidance extinction memory persistence and recovers the learned aversive response. Importantly, coadministration of recombinant BDNF impedes the behavioral effect of hippocampal mTOR inhibition. Our results demonstrate that mTOR signaling is necessary for fear-motivated avoidance extinction memory reconsolidation and suggests that BDNF acts downstream mTOR in a protein synthesis-independent manner to maintain the reactivated extinction memory trace.

Arsenic-Induced Neurotoxicity by Dysfunctioning Cholinergic and Dopaminergic System in Brain of Developing Rats

Chronic exposure to arsenic via drinking water throughout the globe is assumed to cause a developmental neurotoxicity. Here, we investigated the effect of perinatal arsenic exposure on the neurobehavioral and neurochemical changes in the corpus striatum, frontal cortex, and hippocampus that is critically involved in motor and cognition functions. In continuation of previous studies, this study demonstrates that perinatal exposures (GD6-PD21) to arsenic (2 or 4 mg/kg body weight, p.o.) cause hypo-activity in arsenic-exposed rats on PD22. The hypo-activity was found to be linked with a decrease in the mRNA and protein expression of the DA-D2 receptor. Further, a protein expression of tyrosine hydroxylase (TH), levels of dopamine, and its metabolites were also significantly impaired in corpus striatum. The arsenic-exposed groups showed spatial learning and memory significantly below the average in a dose-dependent manner for the controls. Here, we evaluated the declined expression of CHRM2 receptor gene and protein expression of ChAT, PKCβ-1 in the frontal cortex and hippocampus, which are critically involved in cognition functions including learning and memory. A trend of recovery was found in the cholinergic and dopaminergic system of the brain, but changes remained persisted even after the withdrawal of arsenic exposure on PD45. Taken together, our results indicate that perinatal arsenic exposure appears to be critical and vulnerable as the development of cholinergic and dopaminergic system continues during this period.

PKMζ Inhibition Disrupts Reconsolidation and Erases Object Recognition Memory

Object recognition memory (ORM) confers the ability to discriminate the familiarity of previously encountered items. Reconsolidation is the process by which reactivated memories become labile and susceptible to modifications. The hippocampus is specifically engaged in reconsolidation to integrate new information into the original ORM through a mechanism involving activation of brain-derived neurotrophic factor (BDNF) signaling and induction of LTP. It is known that BDNF can control LTP maintenance through protein kinase Mζ (PKMζ), an atypical protein kinase C isoform that is thought to sustain memory storage by modulating glutamatergic neurotransmission. However, the potential involvement of PKMζ in ORM reconsolidation has never been studied. Using a novel ORM task combined with pharmacological, biochemical, and electrophysiological tools, we found that hippocampal PKMζ is essential to update ORM through reconsolidation, but not to maintain the inactive recognition memory trace stored over time, in adult male Wistar rats. Our results also indicate that hippocampal PKMζ acts downstream of BDNF and controls AMPAR synaptic insertion to elicit reconsolidation and suggest that blocking PKMζ activity during this process deletes active ORM.SIGNIFICANCE STATEMENT Object recognition memory (ORM) is essential to remember facts and events. Reconsolidation integrates new information into ORM through changes in hippocampal plasticity and brain-derived neurotrophic factor (BDNF) signaling. In turn, BDNF enhances synaptic efficacy through protein kinase Mζ (PKMζ), which might preserve memory. Here, we present evidence that hippocampal PKMζ acts downstream of BDNF to regulate AMPAR recycling during ORM reconsolidation and show that this kinase is essential to update the reactivated recognition memory trace, but not to consolidate or maintain an inactive ORM. We also demonstrate that the amnesia provoked by disrupting ORM reconsolidation through PKMζ inhibition is due to memory erasure and not to retrieval failure.

The PKC-β selective inhibitor, Enzastaurin, impairs memory in middle-aged rats

Enzastaurin is a Protein Kinase C-β selective inhibitor that was developed to treat cancers. Protein Kinase C-β is an important enzyme for a variety of neuronal functions; in particular, previous rodent studies have reported deficits in spatial and fear-conditioned learning and memory with lower levels of Protein Kinase C-β. Due to Enzastaurin's mechanism of action, the present study investigated the consequences of Enzastaurin exposure on learning and memory in 12-month-old Fischer-344 male rats. Rats were treated daily with subcutaneous injections of either vehicle or Enzastaurin, and behaviorally tested using the spatial reference memory Morris Water Maze. Rats treated with Enzastaurin exhibited decreased overnight retention and poorer performance on the latter testing day, indicating a mild, but significant, memory impairment. There were no differences during the probe trial indicating that all animals were able to spatially localize the platform to the proper quadrant by the end of testing. RNA isolated from the hippocampus was analyzed using Next Generation Sequencing (Illumina). No statistically significant transcriptional differences were noted. Our findings suggest that acute Enzastaurin treatment can impair hippocampal-based learning and memory performance, with no effects on transcription in the hippocampus. We propose that care should be taken in future clinical trials that utilize Protein Kinase C-ß inhibitors, to monitor for possible cognitive effects, future research should examine if these effects are fully reversible.

Protective Role of Quercetin in Cadmium-Induced Cholinergic Dysfunctions in Rat Brain by Modulating Mitochondrial Integrity and MAP Kinase Signaling

With the increasing evidences of cadmium-induced cognitive deficits associated with brain cholinergic dysfunctions, the present study aimed to decipher molecular mechanisms involved in the neuroprotective efficacy of quercetin in rats. A decrease in the binding of cholinergic-muscarinic receptors and mRNA expression of cholinergic receptor genes (M1, M2, and M4) was observed in the frontal cortex and hippocampus on exposure of rats to cadmium (5.0 mg/kg body weight, p.o.) for 28 days compared to controls. Cadmium exposure resulted to decrease mRNA and protein expressions of choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) and enhance reactive oxygen species (ROS) generation associated with mitochondrial dysfunctions, ultrastructural changes, and learning deficits. Enhanced apoptosis, as evidenced by alterations in key proteins involved in the pro- and anti-apoptotic pathway and mitogen-activated protein (MAP) kinase signaling, was evident on cadmium exposure. Simultaneous treatment with quercetin (25 mg/kg body weight, p.o.) resulted to protect cadmium-induced alterations in cholinergic-muscarinic receptors, mRNA expression of genes (M1, M2, and M4), and expression of ChAT and AChE. The protective effect on brain cholinergic targets was attributed to the antioxidant potential of quercetin, which reduced ROS generation and protected mitochondrial integrity by modulating proteins involved in apoptosis and MAP kinase signaling. The results exhibit that quercetin may modulate molecular targets involved in brain cholinergic signaling and attenuate cadmium neurotoxicity.

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.

Brain cholinergic alterations in rats subjected to repeated immobilization or forced swim stress on lambda-cyhalothrin exposure

Role of immobilization stress (IMS), a psychological stressor and forced swim stress (FSS), a physical stressor was investigated on the neurobehavioral toxicity of lambda-cyhalothrin (LCT), a new generation type-II synthetic pyrethroid. Pre-exposure of rats to IMS (15 min/day) or FSS (3 min/day) for 28 days on LCT (3.0 mg/kg body weight, p.o.) treatment for 3 days resulted to decrease spatial learning and memory and muscle strength associated with cholinergic-muscarinic receptors in frontal cortex and hippocampus as compared to those exposed to IMS or FSS or LCT alone. Decrease in acetylcholinesterase activity, protein expression of ChAT and PKC-β1 associated with decreased mRNA expression of CHRM2, AChE and ChAT in frontal cortex and hippocampus was also evident in rats pre-exposed to IMS or FSS on LCT treatment, compared to rats exposed to IMS or FSS or LCT alone. Interestingly, changes both in behavioral and neurochemical endpoints were marginal in rats subjected to IMS or FSS for 28 days or those exposed to LCT for 3 days alone, compared to controls. The results suggest that stress is an important contributor in LCT induced cholinergic deficits.

PKC in developmental hypothyroid rat brain

Thyroid hormone (TH) is essential for the proper development of mammalian central nervous system. TH deficiency during the critical period of brain development results in permanent cognitive and neurological impairments. Members of the protein kinase C (PKC) family play a key role in the regulation of cellular functions in the nervous system. Alteration of PKC can be involved in the pathogenesis of neuronal disorders. This review details recent progress made in determining the roles played by PKC isoforms in developing hypothyroid rat brain. Evidence indicates that hippocampus down-regulation of PKCβ and PKCγ may be related to impaired learning and memory observed in perinatal hypothyroid rats. Enhanced PKCα activity in neonatal hypothyroid brain may bring about oxidative stress and cause brain damage. The activated pro-apoptotic PKCs including PKCδ can cause extensive apoptosis in the hypothyroid rat brain.

Pharmacological alterations that could underlie radiation-induced changes in associative memory and anxiety

It is widely known that ionizing radiation is a physical agent broadly used to kill tumor cells during human cancer therapy. Unfortunately, adjacent normal tissues can concurrently undergo undesirable cell injury. Previous data of our laboratory demonstrated that exposure of developing rats to ionizing radiations induced a variety of behavioral differences respect to controls, including changes in associative memory and in anxiety state. However, there is a lack of data concerning modifications in different related pharmacological intermediaries. Therefore, the aim of the present study was to investigate whether the behavioral differences observed in young animals irradiated at birth might be underlain by early changes in PKCß1 levels which, in turn, could lead to changes in hippocampal GABAergic neurotransmission. Male Wistar rats were irradiated with 5Gy of X rays between 24 and 48 h after birth. Different pharmacological markers related to the affected behavioral tasks were assessed in control and irradiated hippocampus at 15 and 30 days, namely GABAA receptor, GAD65-67, ROS and PKCß1. Results showed that all measured parameters were increased in the hippocampus of 30-days-old irradiated animals. In contrast, in the hippocampus of 15-days-old irradiated animals only the levels of PKCß1 were decreased. These data suggest that PKCß1 might constitute a primary target for neonatal radiation damage on the hippocampus. Therefore, it could be hypothesized that an initial decrease in the levels of this protein can trigger a subsequent compensatory increase that, in turn, could be responsible for the plethora of biochemical changes that might underlie the previously observed behavioral alterations.

Long-term treatment with standardized extract of Ginkgo biloba L. enhances the conditioned suppression of licking in rats by the modulation of neuronal and glial cell function in the dorsal hippocampus and central amygdala

Our group previously demonstrated that short-term treatment with a standardized extract of Ginkgo biloba (EGb) changed fear-conditioned memory by modulating gene expression in the hippocampus, amygdaloid complex and prefrontal cortex. Although there are few controlled studies that support the long-term use of EGb for the prevention and/or treatment of memory impairment, the chronic use of Ginkgo is common. This study evaluated the effects of chronic treatment with EGb on the conditioned emotional response, assessed by the suppression of ongoing behavior and in the modulation of gene and protein expression. Male adult Wistar rats were treated over 28days and assigned to five groups (n=10) as follows: positive control (4mgkg(-1) Diazepam), negative control (12% Tween 80), EGb groups (0.5 and 1.0gkg(-1)) and the naïve group. The suppression of the licking response was calculated for each rat in six trials. Our results provide further evidence for the efficacy of EGb on memory. For the first time, we show that long-term treatment with the highest dose of EGb improves the fear memory and suggests that increased cAMP-responsive element-binding protein (CREB)-1 and glial fibrillary acidic protein (GFAP) mRNA and protein (P<0.001) in the dorsal hippocampus and amygdaloid complex and reduced growth and plasticity-associated protein 43 (GAP-43) (P<0.01) in the hippocampus are involved in this process. The fear memory/treatment-dependent changes observed in our study suggest that EGb might be effective for memory enhancement through its effect on the dorsal hippocampus and amygdaloid complex.

Ectopic growth of hippocampal mossy fibers in a mutated GAP-43 transgenic mouse with impaired spatial memory retention

In a previous study, it was shown that transgenic mice, designated G-NonP, forget the location of a water maze hidden platform when tested 7 days after the last training day (Holahan and Routtenberg (2008) Hippocampus 18:1099-1102). The memory loss in G-NonP mice might be related to altered hippocampal architecture suggested by the fact that in the rat, 7 days after water maze training, there is discernible mossy fiber (MF) growth (Holahan et al. (2006) Hippocampus 16:560-570; Rekart et al. (2007) Learn Mem 14:416-421). In the present report, we studied the distribution of the MF system within the hippocampus of naïve, untrained, G-NonP mouse. In WT mice, the MF projection was restricted to the stratum lucidum of CA3 with no detectable MF innervation in distal stratum oriens (dSO). In G-NonP mice, in contrast, there was an ectopic projection terminating in the CA3 dSO. Unexpectedly, there was nearly a complete loss of immunostaining for the axonal marker Tau1 in the G-NonP transgenic mice in the MF terminal fields indicating that transgenesis itself leads to off-target consequences (Routtenberg (1996) Trends Neurosci 19:471-472). Because transgenic mice overexpressing nonmutated, wild type GAP-43 do not show this ectopic growth (Rekart et al., in press) and the G-NonP mice overexpress a mutated form of GAP-43 precluding its phosphorylation by protein kinase C (PKC), the possibility exists that permanently dephosphorylated GAP-43 disrupts normal axonal fasciculation which gives rise to the ectopic growth into dSO.

Effects of intrahippocampal administration of the phosphatase inhibitor okadaic acid: Dual effects on memory formation

Protein phosphorylation mediated by serine-threonine kinases in the hippocampus is crucial to the synaptic modifications believed to underlie memory formation. The role of phosphatases has been the focus of comparatively little study.

Different effect of chronic stress on learning and memory in BALB/c and C57BL/6 inbred mice: Involvement of hippocampal NO production and PKC activity

Nitric oxide (NO) has been involved in many pathophysiological brain processes. Recently, we showed that neuronal nitric oxide synthase (nNOS)-mediated decrease in NO production is involved in memory impairment induced by chronic mild stress (CMS) in BALB/c mice. Two genetically different inbred murine strains, C57BL/6 and BALB/c, show distinct behavioral responses, neurodevelopmental and neurochemical parameters. Here, we perform a comparative study on CMS effects upon learning and memory in both strains, analyzing the role of NO production and its regulation by protein kinase C (PKC). Stressed BALB/c, but not C57Bl/6 mice, showed a poor learning performance in both the open field and passive avoidance inhibitory tasks. Also, CMS induced a diminished NO production by nNOS, associated with an increment in gamma and zeta PKC isoenzymes in BALB/c mice. In C57BL/6 mice, CMS had no effect on NO production, but increased delta and decreased betaI PKC isoforms. In vivo administration of a NOS inhibitor induced behavioral alterations in both strains. These results suggest a differential effect of stress, with BALB/c being more vulnerable to stress than C57BL/6 mice. This effect could be related to a differential regulation of NOS and PKC isoenzymes, pointing to an important role of NO in learning and memory.

Physical exercise can reverse the deficit in fear memory induced by maternal deprivation

Maternal deprivation during the first 10 days of life induces significant behavioral alterations in rodents which persist through adulthood. Physical exercise reduces the cognitive deficits associated with pharmacologic and pathological conditions. Here we investigated whether forced physical exercise alters memory deficits caused by postnatal maternal deprivation. Male rats were divided into four groups: (1) control, (2) deprived, (3) exercised, and (4) deprived+exercised. In groups 2 and 4, pups were deprived from their mothers for 3h/day during the first 10 days post-birth. In groups 3 and 4, from postnatal day 45 (PND-45) on, animals were submitted to forced treadmill exercise. At adulthood, animals were submitted to four different behavioral tasks: open field, Morris water maze (MWM), object recognition (OR) and inhibitory avoidance (IA). Maternal deprivation had no effect on open field behavior, but disrupted memory in the three other tasks. Physical exercise alone had no effect, except for a slight enhancement of MWM learning. Importantly, physical exercise reversed the deficit of IA and reduced the deficit of spatial memory but not that of OR seen in deprived animals. It is possible that physical exercise may counteract the influence of maternal deprivation on neurohumoral or hormonal memory modulatory systems related to stress. Indeed, the decreasing order of the effect of exercise on the memory disturbances induced by deprivation roughly follows the descending degree of stress associated with each task (IA>MWM>OR). Maternal deprivation is known to hinder hormonal mechanisms involved in coping with stress.

Phospholipase A2 activation as a therapeutic approach for cognitive enhancement in early-stage Alzheimer disease

RATIONALE

Alzheimer disease (AD) is the leading cause of dementia in the elderly and has no known cure. Evidence suggests that reduced activity of specific subtypes of intracellular phospholipases A2 (cPLA2 and iPLA2) is an early event in AD and may contribute to memory impairment and neuropathology in the disease.

OBJECTIVE

The objective of this study was to review the literature focusing on the therapeutic role of PLA2 stimulation by cognitive training and positive modulators, or of supplementation with arachidonic acid (PLA2 product) in facilitating memory function and synaptic transmission and plasticity in either research animals or human subjects.

METHODS

MEDLINE database was searched (no date restrictions) for published articles using the keywords Alzheimer disease (mild, moderate, severe), mild cognitive impairment, healthy elderly, rats, mice, phospholipase A(2), phospholipid metabolism, phosphatidylcholine, arachidonic acid, cognitive training, learning, memory, long-term potentiation, protein kinases, dietary lipid compounds, cell proliferation, neurogenesis, and neuritogenesis. Reference lists of the identified articles were checked to select additional studies of interest.

RESULTS

Overall, the data suggest that PLA2 activation is induced in the healthy brain during learning and memory. Furthermore, learning seems to regulate endogenous neurogenesis, which has been observed in AD brains. Finally, PLA2 appears to be implicated in homeostatic processes related to neurite outgrowth and differentiation in both neurodevelopmental processes and response to neuronal injury.

CONCLUSION

The use of positive modulators of PLA2 (especially of cPLA2 and iPLA2) or supplementation with dietary lipid compounds (e.g., arachidonic acid) in combination with cognitive training could be a valuable therapeutic strategy for cognitive enhancement in early-stage AD.

Early activation of extracellular signal-regulated kinase signaling pathway in the hippocampus is required for short-term memory formation of a fear-motivated learning

1. According to its duration there are, at least, two major forms of memory in mammals: short term memory (STM) which develops in a few seconds and lasts several hours and long-term memory (LTM) lasting days, weeks and even a lifetime. In contrast to LTM, very little is known about the neural, cellular and molecular requirements for mammalian STM formation. 2. Here we show that early activation of extracellular signal-regulated kinases 1/2 (ERK1/2) in the hippocampus is required for the establishment of STM for a one-trial inhibitory avoidance task in the rat. Immediate posttraining infusion of U0126 (a selective inhibitor of ERK kinase) into the CA1 region of the dorsal hippocampus blocked STM formation. 3. Reversible inactivation of the entorhinal cortex through muscimol infusion produced deficits in STM and a selective and rapid decrease in hippocampal ERK2 activation.4. Together with our previous findings showing a rapid decrease in ERK2 activation and impaired STM after blocking BDNF function, the present results strongly suggest that ERK2 signaling in the hippocampus is a critical step in STM processing.

Prenatal restraint stress impairs learning and memory and hippocampal PKCbeta1 expression and translocation in offspring rats

Prenatal stress results in various learning, behavioral and emotional alterations observed in later life. However, the mechanisms underlying these effects of prenatal stress are not fully understood. In the present study we examined the impact of prenatal stress (an unpredictable restraint stress) during gestational days 13 to 20 on the performance in Morris water maze and passive avoidance training in 1- and 3-month-old rat offspring. The expression and translocation/activation of protein kinase C (PKC) beta1 in the hippocampus of prenatally stressed offspring were also investigated. One-month-old female and male and 3-month-old female prenatally stressed offspring showed longer latency to find the platform and used the inefficient search strategy in the water maze task and showed lower memory score in the passive avoidance training compared with controls. The expression of PKCbeta1 protein and mRNA in the hippocampus of prenatally stressed offspring was dramatically weakened. In the control offspring hippocampus, passive avoidance training induced the PKCbeta1 translocation from the cytosol to the membrane, which, however, was not observed in prenatally stressed offspring. Our results suggest that deficient signal transduction of PKCbeta1 in the hippocampus resulting from prenatal restraint stress may play an important role in the impairment of learning and memory abilities of offspring.

Effect of Radicicol Infusion on the Src Tyrosine Kinase Activity of Rat Hippocampus before and after Training in an Inhibitory Avoidance Task

The participation of protein serine/threonine kinases in memory formation and retrieval is well established. In contrast, relatively little is known on the role of protein tyrosine kinases (PTKs). Previous work showed that intra-hippocampal infusion of the Src-PTK inhibitor radicicol inhibits memory acquisition, consolidation, and retrieval of one-trial step-down inhibitory avoidance task. In this study, we investigated the possible interaction between levels of Src-PTK activity in hippocampus and memory acquisition, formation, and retrieval of this task. Radicicol (0.5 microg/ml) was infused into the CA1 region of the hippocampus of rats trained in a one-trial step-down inhibitory avoidance task. Radicicol infused 15 min before training decreased Src-PTK activity, as measured 0, 1.5, and 24 h after training, and impaired memory acquisition of the task. When given immediately after training, there was a decrease in Src-PTK activity 1.5 h, but not 0 or 24 h after training. This treatment depressed memory consolidation. Radicicol infused into CA1 10 min prior to retrieval testing inhibited hippocampal Src-PTK activity, as measured immediately after the test session. The results suggest that Src-PTKs participate in memory acquisition, consolidation, and retrieval processes, but the timing of the role of the enzyme is different in each case.

Different molecular cascades in different sites of the brain control memory consolidation

To understand cognition, it is important to understand how a learned response becomes a long-lasting memory. This process of memory consolidation has been modeled extensively using one-trial avoidance learning, in which animals (or humans) establish a conditioned response by learning to avoid danger in just one trial. This relies on molecular events in the CA1 region of the hippocampus that resemble those involved in CA1 long-term potentiation (LTP), and it also requires equivalent events to occur with different timings in the basolateral amygdala and the entorhinal, parietal and cingulate cortex. Many of these steps are modulated by monoaminergic pathways related to the perception of and reaction to emotion, which at least partly explains why strong and resistant consolidation is typical of emotion-laden memories. Thus memory consolidation involves a complex network of brain systems and serial and parallel molecular events, even for a task as deceptively simple as one-trial avoidance. We propose that these molecular events might also be involved in many other memory types in animals and humans.

Glutamate uptake is stimulated by extracellular S100B in hippocampal astrocytes

1.S100B is a calcium-binding protein expressed and secreted by astrocytes, which has been implicated in glial-neuronal communication. Extracellular S100B appears to protect hippocampal neurons against toxic concentrations of glutamate. Here we investigated a possible autocrine role of S100B in glutamate uptake activity. 2. Astrocyte cultures were prepared of hippocampi from neonate Wistar rats. [(3)H] Glutamate uptake was measured after addition of S100B protein, antibody anti-S100B or TRTK-12, a peptide that blocks S100B activity mediated by the C-terminal region. 3.Antibody anti-S100B addition decreased glutamate uptake measured 30 min after medium replacement, without affecting cell integrity or viability. Moreover, low levels of S100B (less than 0.1 ng/mL) stimulated glutamate uptake measured immediately after medium replacement. 4. This finding reinforces the importance of astrocytes in the glutamatergic transmission, particularly the role of S100B neuroprotection against excitotoxic damage.

Nerve Ending “Signal” Proteins GAP‐43, MARCKS, and BASP1

Mechanisms of growth cone pathfinding in the course of neuronal net formation as well as mechanisms of learning and memory have been under intense investigation for the past 20 years, but many aspects of these phenomena remain unresolved and even mysterious. "Signal" proteins accumulated mainly in the axon endings (growth cones and the presynaptic area of synapses) participate in the main brain processes. These proteins are similar in several essential structural and functional properties. The most prominent similarities are N-terminal fatty acylation and the presence of an "effector domain" (ED) that dynamically binds to the plasma membrane, to calmodulin, and to actin fibrils. Reversible phosphorylation of ED by protein kinase C modulates these interactions. However, together with similarities, there are significant differences among the proteins, such as different conditions (Ca2+ contents) for calmodulin binding and different modes of interaction with the actin cytoskeleton. In light of these facts, we consider GAP-43, MARCKS, and BASP1 both separately and in conjunction. Special attention is devoted to a discussion of apparent inconsistencies in results and opinions of different authors concerning specific questions about the structure of proteins and their interactions.

One-trial aversive learning induces late changes in hippocampal CaMKIIα, Homer 1a, Syntaxin 1a and ERK2 protein levels

Most studies regarding altered gene expression after learning are performed using multi-trial tasks, which do not allow a clear discrimination of memory acquisition, consolidation and retrieval. We screened for candidate memory-modulated genes in the hippocampus at 3 and 24 h after one-trial inhibitory avoidance (IA) training, using a cDNA array containing 1176 genes. While 33 genes were modulated by training (respect to shocked-only animals), most of them were upregulated (27 genes) and only 6 were downregulated. To confirm and extend these findings, we performed RT-PCRs and analyzed differences in protein levels in rat hippocampus using immunoblot assays. We found several proteins upregulated 24 h after training: extracellular signal-regulated kinase ERK2, Ca2+/calmodulin-dependent protein kinase II alpha (CaMKIIalpha), Syntaxin 1a, c-fos and Homer 1a. The total level of none of these proteins were found to be altered when measured 3-h post-training. Several of the mRNAs corresponding to the upregulated proteins were changed at 3 h but not 24 h. Additionally, a number of other candidates were identified for the first time as modulated by learning. The results presented here suggest that single-trial tasks can expose previously unseen differences in dynamic regulation of gene expression after behavioral manipulations, both at the transcriptional and translational levels, and reveal a diversity of gene products modulated by this task, allowing deeper understanding of the molecular basis of memory formation.

Prenatal heroin exposure alters cholinergic receptor stimulated activation of the PKCβII and PKCγ isoforms

Prenatal exposure of mice to heroin (SC injection of 10mg/kg to the dams on gestational days 9-18) resulted at adulthood in behavioral deficits related to septohippocampal cholinergic innervation accompanied with both presynaptic and postsynaptic cholinergic hyperactivity; including an increase membrane PKC activity, and a desensitization of PKC to cholinergic input which were highly correlated with the behavioral performance and were reversed by cholinergic grafting. Therefore, we studied the receptor induced activation of the behaviorally relevant PKCgamma and PKCbetaII isoforms and the less behaviorally relevant PKCalpha isoform. Time course studies revealed peak translocation after 40 min incubation with carbachol for PKCgamma (110% increase from basal, i.e. no carbachol level, P < 0.01), 30 min for phosphorylated PKCbetaII (130%, P < 0.05) and 5 min for non-phosphorylated PKCbetaII (64%, P < 0.05) with no peak for alpha. Prenatal heroin abolished the translocation of PKCgamma and PKCbetaII while PKCalpha remained unaffected. A decrease occurred in basal phosphorylated membrane (-45%, P < 0.01) and cytosol-associated (-29%, P < 0.01) PKCbetaII, in membrane-associated non-phosphorylated PKCbetaII (-32%, P < 0.01) and PKCgamma (-25%, P < 0.01) and in cytosolic PKCalpha (-27%, P < 0.01), while membrane-associated PKCalpha was slightly increased (11%, P < 0.05). The results suggest that prenatal heroin disrupts cholinergic receptor induced PKC translocation and activation with the underlying mechanism of neuroteratogenicity potentially lying in the PKCgamma and PKCbetaII, while PKCalpha remains unaffected.

Hippocampal glutamate receptors in fear memory consolidation

It is thought that activity-dependent changes in synaptic efficacy driven by biochemical pathways responsive to the action of the excitatory neurotransmitter glutamate are critical components of the mechanisms responsible for memory formation. In particular, the early activation of the NMDA (rNMDA) and AMPA (rAMPA) subtypes of ionotropic glutamate receptors has been demonstrated to be a necessary event for the acquisition of several types of memory. In the rat, consolidation of the long-term memory for a one-trial, step-down inhibitory avoidance task is blocked by antagonists of the rNMDA and rAMPA infused into the CA1 region of the dorsal hippocampus early after training and is associated with a rapid and reversible increase in the total number of [3H]AMPA binding sites. The learning-induced increase in [[3H]AMPA is accompanied by translocation of the GluR1 subunit of the rAMPA to the post-synaptic terminal together with its phosphorylation at Ser831. In addition, learning of the mentioned fear-motivated task induces the activation and rNMDA-dependent translocation of CaMKII to the post-synaptic density. Inhibition of this protein kinase as well as blockade of the rNMDA abolishes both the learning-induced translocation of GluR1 and its phosphorylation. Our data suggest that learning of an avoidance task enhances hippocampal rAMPA signaling through rNMDA and CaMKII-dependent mechanisms.

Molecular mechanisms of memory retrieval

Memory retrieval is a fundamental component or stage of memory processing. In fact, retrieval is the only possible measure of memory. The ability to recall past events is a major determinant of survival strategies in all species and is of paramount importance in determining our uniqueness as individuals. Most biological studies of memory using brain lesion and/or gene manipulation techniques cannot distinguish between effects on the molecular mechanisms of the encoding or consolidation of memories and those responsible for their retrieval from storage. Here we examine recent findings indicating the major molecular steps involved in memory retrieval in selected brain regions of the mammalian brain. Together the findings strongly suggest that memory formation and retrieval may share some molecular mechanisms in the hippocampus and that retrieval initiates extinction requiring activation of several signaling cascades and protein synthesis.

Aniracetam improves contextual fear conditioning and increases hippocampal gamma-PKC activation in DBA/2J mice

DBA/2J (D2) mice display poor contextual learning and have less membrane-bound hippocampal protein kinase C (PKC) compared with C57BL/6 (B6) mice. Aniracetam and oxiracetam were previously shown to improve contextual learning in D2 mice and increase PKC activity. This study investigated a possible mechanism for learning enhancement by examining the effects of aniracetam on contextual fear conditioning and activation of the y isoform of PKC (gamma-PKC) in male D2 mice. In comparison to animals treated with vehicle only (10% 2-hydroxypropyl-beta-cyclodextrin), mice treated with aniracetam (100 mg/kg) 30 min prior to fear conditioning training demonstrated significantly improved contextual learning when tested 30 min and 24 h after training. This corresponded with a significant increase in activated, membrane-bound hippocampal gamma-PKC 30 min after training. No increase in learning or gamma-PKC was found 5 min after training. These results suggest an altered time course of activation of gamma-PKC in response to treatment with aniracetam, which improves learning in D2 mice.

Two time periods of hippocampal mRNA synthesis are required for memory consolidation of fear-motivated learning

Information storage in the brain is a temporally graded process involving different memory types or phases. It has been assumed for over a century that one or more short-term memory (STM) processes are involved in processing new information while long-term memory (LTM) is being formed. It has been repeatedly reported that LTM requires de novo RNA synthesis around the time of training. Here we show that LTM formation of a one-trial inhibitory avoidance training in rats, a hippocampal-dependent form of contextual fear conditioning, depends on two consolidation periods requiring synthesis of new mRNAs. By injecting the RNA polymerase II inhibitors 5,6-dichloro-1-beta-d-ribofuranosylbenzimidazole or alpha-amanitin into the CA1 region of the dorsal hippocampus at various times before and after training, we found that hippocampal gene expression is critical in two time windows: around the time of training and 3-6 hr after training. Interestingly, these two periods of sensitivity to transcriptional inhibitors are similar to those observed using the protein synthesis inhibitor anisomycin. These findings underscore the parallel dependence of LTM formation of contextual fear on mRNA and protein synthesis in the hippocampus and suggest that the two time periods of anisomycin-induced amnesia depend at least in part on new mRNA synthesis.

The ubiquitin-proteasome cascade is required for mammalian long-term memory formation

It has been recently demonstrated that ubiquitin-proteasome-mediated proteolysis is required for long-term synaptic facilitation in Aplysia. Here we show that the hippocampal blockade of this proteolytic pathway is also required for the formation of long-term memory in the rat. Bilateral infusion of lactacystin, a specific proteasome inhibitor, to the CA1 region caused full retrograde amnesia for a one-trial inhibitory avoidance learning when given 1, 4 or 7h, but not 10 h, after training. Proteasome inhibitor I produced similar effects. In addition, inhibitory avoidance training resulted in an increased ubiquitination and 26S proteasome proteolytic activity and a decrease in the levels of IkappaB, a substrate of the ubiquitin-proteasome cascade, in hippocampus 4 h after training. Together, these findings indicate that the ubiquitin-proteasome cascade is crucial for the establishment of LTM in the behaving animal.

Behavioural pharmacology and its contribution to the molecular basis of memory consolidation

Recent findings have significantly advanced our understanding the mechanisms of memory formation. Most of these advances stemmed from behavioural pharmacology research involving, particularly, the localized infusion of drugs with specific molecular actions into specific brain regions. This approach has revealed brain structures involved in different memory types and the main neurotransmitter systems and sequence of metabolic cascades that participate in memory consolidation. Biochemical studies and, in several cases, studies of genetically manipulated animals, in which receptors or enzymes affected by the various drugs were absent or overexpressed, have complemented the pharmacological research. Although most studies have concentrated on the involvement of the hippocampus, many have also investigated the entorhinal cortex, other regions of the cortex, and the amygdala. Behavioural pharmacology has been of crucial importance in establishing the major neurohumoral and hormonal systems involved in the modulation of memory formation. These systems act on specific steps of memory formation in the hippocampus and in the entorhinal, parietal, and cingulate cortex. A specialized system mediated by the basolateral amygdaloid nucleus, and involving several neuromodulatory systems, is activated by emotional arousal and serves to regulate memory formation in other brain regions. The core mechanisms involved in the formation of explicit (declarative) memory are in many respects similar to those of long-term potentiation (LTP), particularly in the hippocampus. However, there are also important differences between memory formation and LTP. Memory formation involves numerous modulatory influences, the co-participation of various brain regions other than the hippocampus, and some properties that are specific to memory and absent in LTP (i.e. flexibility of response). We discuss the implications of these similarities and differences for understanding the neural bases of memory.

Molecular signalling pathways in the cerebral cortex are required for retrieval of one-trial avoidance learning in rats

Rats were implanted bilaterally with cannulae in the CA1 region of the dorsal hippocampus, the entorhinal cortex, anterior cingulate cortex, posterior parietal cortex, or the basolateral complex of the amygdala. The animals were trained in one-trial step-down inhibitory avoidance and tested 24 h later. Prior (10 min) to the retention test, through the cannulae, they received 0.5 microl infusions of a vehicle (2% dimethylsulfoxide in saline), or of the following drugs dissolved in the vehicle: the glutamate NMDA receptor blocker, aminophosphonopentanoic acid (AP5, 2.0 or 5.0 microg), the AMPA receptor blocker, 6,7-dinitroquinoxaline-2,3 (1H,4H)dione (DNQX, 0.4 or 1.0 microg), the metabotropic receptor antagonist, methylcarboxyphenylglycine (MCPG, 0.5 or 2.5 microg), the inhibitor of cAMP-dependent protein kinase (PKA), Rp-cAMPs (0.1 or 0.5 microg), the PKA stimulant, Sp-cAMPs (0.5 microg), or the inhibitor of the mitogen-activated protein kinase (MAPK), PD098059 (10 or 50 microM). All these drugs, at the same doses, had been previously found to alter long-term memory formation of this task. Here, retrieval test performance was blocked by DNQX, MCPG, Rp-cAMPs and PD098059 and enhanced by Sp-cAMPs infused into CA1 or the entorhinal cortex. The drugs had similar effects when infused into the parietal or anterior cingulate cortex, except that in these two areas AP5 also blocked retrieval, and in the cingulate cortex DNQX had no effect. Infusions into the basolateral amygdala were ineffective except for DNQX, which hindered retrieval. None of the treatments that affected retrieval had any influence on performance in an open field or in a plus maze; therefore, their effect on retention testing can not be attributed to an influence on locomotion, exploration or anxiety. The results indicate that the four cortical regions studied participate actively in, and are necessary for, retrieval of the one-trial avoidance task. They require metabotropic and/or NMDA glutamate receptors and PKA and MAPK activity. In contrast, the basolateral amygdala appears to participate only through a maintenance of its regular excitatory transmission mediated by glutamate AMPA receptors.

Short- and long-term memory: differential involvement of neurotransmitter systems and signal transduction cascades

Since William James (1890) first distinguished primary from secondary memory, equivalent to short- and long-term memory, respectively, it has been assumed that short-term memory processes are in charge of cognition while long-term memory is being consolidated. From those days a major question has been whether short-term memory is merely a initial phase of long-term memory, or a separate phenomena. Recent experiments have shown that many treatments with specific molecular actions given into the hippocampus and related brain areas after one-trial avoidance learning can effectively cancel short-term memory without affecting long-term memory formation. This shows that short-term memory and long-term memory involve separate mechanisms and are independently processed. Other treatments, however, influence both memory types similarly, suggesting links between both at the receptor and at the post-receptor level, which should not be surprising as they both deal with nearly the same sensorimotor representations. This review examines recent advances in short- and long-term memory mechanisms based on the effect of intra-hippocampal infusion of drugs acting upon neurotransmitter and signal transduction systems on both memory types.

The contribution of pharmacology to research on the mechanisms of memory formation

Pharmacological studies of memory are motivated by the hope or hypothesis that behavioural findings, considered together with knowledge of the mechanisms of drug action, will help to elucidate the neurobiological bases of memory. There is now considerable evidence that this hope is justified.