B-50/GAP-43 phosphorylation and PKC activity are increased in rat hippocampal synaptosomal membranes after an inhibitory avoidance training

Proximodistal Heterogeneity in Learning-promoted Pathway-specific Plasticity at Dorsal CA1 Synapses

Contextual learning requires the delivery of AMPA receptors to CA1 synapses in the dorsal hippocampus. However, proximodistal heterogeneity of pathway-specific plasticity remains unclear. Here, we examined the proximodistal heterogeneity in learning-induced plasticity at the CA1 synapses with inputs from the entorhinal cortex layer III (ECIII) or from CA3. We subjected male rats to an inhibitory avoidance task and prepared acute hippocampal slices for whole-cell patch clamp experiments, where we stimulated ECIII-CA1 or CA3-CA1 input fibers to analyze evoked excitatory postsynaptic currents (EPSCs). Compared to untrained controls, trained rats exhibited higher AMPA/NMDA current ratios at CA3-CA1 synapses of proximal and intermediate, but not distal CA1 neurons, which suggested that region-specific plasticity occurred after learning. Moreover, trained rats exhibited higher AMPA/NMDA current ratios at ECIII-CA1 synapses of intermediate and distal, but not proximal CA1 neurons. These findings suggested the presence of proximodistal heterogeneity in pathway-specific postsynaptic plasticity. Regarding presynaptic plasticity, training slightly, but significantly increased the paired-pulse ratios of CA3-CA1 synapses of proximal and intermediate, but not distal CA1 neurons. Moreover, trained rats exhibited higher paired-pulse ratios at ECIII-CA1 synapses of intermediate and distal, but not proximal CA1 neurons, which suggested region-specific presynaptic plasticity. Finally, learning was clearly prevented by the bilateral microinjection of a plasticity blocker in the proximal or intermediate, but not distal CA1 subfields, which suggested functional heterogeneity along the proximodistal axis. Understanding region- and pathway-specific plasticity at dorsal CA1 synapses could aid in controlling encoded memory.

Clocking In Time to Gate Memory Processes: The Circadian Clock Is Part of the Ins and Outs of Memory

Learning, memory consolidation, and retrieval are processes known to be modulated by the circadian (circa: about; dies: day) system. The circadian regulation of memory performance is evolutionarily conserved, independent of the type and complexity of the learning paradigm tested, and not specific to crepuscular, nocturnal, or diurnal organisms. In mammals, long-term memory (LTM) formation is tightly coupled to de novo gene expression of plasticity-related proteins and posttranslational modifications and relies on intact cAMP/protein kinase A (PKA)/protein kinase C (PKC)/mitogen-activated protein kinase (MAPK)/cyclic adenosine monophosphate response element-binding protein (CREB) signaling. These memory-essential signaling components cycle rhythmically in the hippocampus across the day and night and are clearly molded by an intricate interplay between the circadian system and memory. Important components of the circadian timing mechanism and its plasticity are members of the Period clock gene family (Per1, Per2). Interestingly, Per1 is rhythmically expressed in mouse hippocampus. Observations suggest important and largely unexplored roles of the clock gene protein PER1 in synaptic plasticity and in the daytime-dependent modulation of learning and memory. Here, we review the latest findings on the role of the clock gene Period 1 (Per1) as a candidate molecular and mechanistic blueprint for gating the daytime dependency of memory processing.

Modulation of learning and memory by natural polyamines

Spermine and spermidine are natural polyamines that are produced mainly via decarboxylation of l-ornithine and the sequential transfer of aminopropyl groups from S-adenosylmethionine to putrescine by spermidine synthase and spermine synthase. Spermine and spermidine interact with intracellular and extracellular acidic residues of different nature, including nucleic acids, phospholipids, acidic proteins, carboxyl- and sulfate-containing polysaccharides. Therefore, multiple actions have been suggested for these polycations, including modulation of the activity of ionic channels, protein synthesis, protein kinases, and cell proliferation/death, within others. In this review we summarize these neurochemical/neurophysiological/morphological findings, particularly those that have been implicated in the improving and deleterious effects of spermine and spermidine on learning and memory of naïve animals in shock-motivated and nonshock-motivated tasks, from a historical perspective. The interaction with the opioid system, the facilitation and disruption of morphine-induced reward and the effect of polyamines and putative polyamine antagonists on animal models of cognitive diseases, such as Alzheimer's, Huntington, acute neuroinflammation and brain trauma are also reviewed and discussed. The increased production of polyamines in Alzheimer's disease and the biphasic nature of the effects of polyamines on memory and on the NMDA receptor are also considered. In light of the current literature on polyamines, which include the description of an inborn error of the metabolism characterized by mild-to moderate mental retardation and polyamine metabolism alterations in suicide completers, we can anticipate that polyamine targets may be important for the development of novel strategies and approaches for understanding the etiopathogenesis of important central disorders and their pharmacological treatment.

Properties and mechanisms of olfactory learning and memory

Memories are dynamic physical phenomena with psychometric forms as well as characteristic timescales. Most of our understanding of the cellular mechanisms underlying the neurophysiology of memory, however, derives from one-trial learning paradigms that, while powerful, do not fully embody the gradual, representational, and statistical aspects of cumulative learning. The early olfactory system—particularly olfactory bulb—comprises a reasonably well-understood and experimentally accessible neuronal network with intrinsic plasticity that underlies both one-trial (adult aversive, neonatal) and cumulative (adult appetitive) odor learning. These olfactory circuits employ many of the same molecular and structural mechanisms of memory as, for example, hippocampal circuits following inhibitory avoidance conditioning, but the temporal sequences of post-conditioning molecular events are likely to differ owing to the need to incorporate new information from ongoing learning events into the evolving memory trace. Moreover, the shapes of acquired odor representations, and their gradual transformation over the course of cumulative learning, also can be directly measured, adding an additional representational dimension to the traditional metrics of memory strength and persistence. In this review, we describe some established molecular and structural mechanisms of memory with a focus on the timecourses of post-conditioning molecular processes. We describe the properties of odor learning intrinsic to the olfactory bulb and review the utility of the olfactory system of adult rodents as a memory system in which to study the cellular mechanisms of cumulative learning.

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.

Recurrent HERV-H-mediated 3q13.2-q13.31 deletions cause a syndrome of hypotonia and motor, language, and cognitive delays

We describe the molecular and clinical characterization of nine individuals with recurrent, 3.4-Mb, de novo deletions of 3q13.2-q13.31 detected by chromosomal microarray analysis. All individuals have hypotonia and language and motor delays; they variably express mild to moderate cognitive delays (8/9), abnormal behavior (7/9), and autism spectrum disorders (3/9). Common facial features include downslanting palpebral fissures with epicanthal folds, a slightly bulbous nose, and relative macrocephaly. Twenty-eight genes map to the deleted region, including four strong candidate genes, DRD3, ZBTB20, GAP43, and BOC, with important roles in neural and/or muscular development. Analysis of the breakpoint regions based on array data revealed directly oriented human endogenous retrovirus (HERV-H) elements of ~5 kb in size and of >95% DNA sequence identity flanking the deletion. Subsequent DNA sequencing revealed different deletion breakpoints and suggested nonallelic homologous recombination (NAHR) between HERV-H elements as a mechanism of deletion formation, analogous to HERV-I-flanked and NAHR-mediated AZFa deletions. We propose that similar HERV elements may also mediate other recurrent deletion and duplication events on a genome-wide scale. Observation of rare recurrent chromosomal events such as these deletions helps to further the understanding of mechanisms behind naturally occurring variation in the human genome and its contribution to genetic disease.

Neural functions of calcineurin in synaptic plasticity and memory

Major brain functions depend on neuronal processes that favor the plasticity of neuronal circuits while at the same time maintaining their stability. The mechanisms that regulate brain plasticity are complex and engage multiple cascades of molecular components that modulate synaptic efficacy. Protein kinases (PKs) and phosphatases (PPs) are among the most important of these components that act as positive and negative regulators of neuronal signaling and plasticity, respectively. In these cascades, the PP protein phosphatase 2B or calcineurin (CaN) is of particular interest because it is the only Ca(2+)-activated PP in the brain and a major regulator of key proteins essential for synaptic transmission and neuronal excitability. This review describes the primary properties of CaN and illustrates its functions and modes of action by focusing on several representative targets, in particular glutamate receptors, striatal enriched protein phosphatase (STEP), and neuromodulin (GAP43), and their functional significance for synaptic plasticity and memory.

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.

Neuromodulatory property of standardized extract Ginkgo biloba L. (EGb 761) on memory: behavioral and molecular evidence

Although it has been suggested that the standardized Ginkgo biloba leaf extract (Egb 761) may have a beneficial effect on memory, the cellular and molecular changes that underlie this process are not yet well defined. The present study evaluated the effects of acute (one dose) or subacute treatments (one daily dose/seven days) with EGb 761 (0.5 g kg(-1) and 1.0 g kg(-1)) on rats submitted to a conditioned emotional response (CER) in comparison with positive (4 mg kg(-1) Diazepam) and negative (12%Tween 80) control groups. To this end, eighty (n=10/group) adult, male, Wistar rats (+/-250-300 g) were used in an off-baseline CER procedure. We here observed that the rats submitted to an acute and subacute EGb 761 treatments had acquisition of fear conditioning. Additionally, we investigate if the expression of genes previously associated with classical conditioning (CREB-1 and GAP-43) and new candidate genes (GFAP) are modulated following EGb 761 acute treatment. CREB-1, GAP-43 and GFAP mRNA and protein expressions were evaluated using both quantitative PCR (qPCR) and immunohistochemical analysis, respectively. We here show, for the first time, that EGb 761 modulated GAP-43, CREB-1 and GFAP expression in the prefrontal cortex, amygdala and hippocampus. We observed an underexpression of GAP-43 in all structures evaluated and over-expression of GFAP in the amygdala and hippocampus following acute G. biloba treatment when compared to control group (Tween; p<0.01). GAP-43 expression was decreased in prefrontal cortex and hippocampus in the subacute treatment with EGb 761. Subacute treatment with EGb 761 lead to a decreased CREB-1 in mPFC (p<0.001) and increased in the hippocampus to 1.0 g kg(-1)G. biloba group (p<0.001). The results obtained from immunohistochemical analysis support our aforementioned findings and revealed that the changes in expression occurred within specific regions in the areas evaluated. All together, our findings not only provide new evidence for a role of EGb 761 on memory but also identify molecular changes that underlie the fear memory consolidation.

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.

Conditioning training and retrieval increase phospholipase A(2) activity in the cerebral cortex of rats

In rats, phospholipase A(2) (PLA(2)) activity was found to be increased in the hippocampus immediately after training and retrieval of a contextual fear conditioning paradigm (step-down inhibitory avoidance [IA] task). In the present study we investigated whether PLA(2) is also activated in the cerebral cortex of rats in association with contextual fear learning and retrieval. We observed that IA training induces a rapid (immediately after training) and long-lasting (3 h after training) activation of PLA(2) in both frontal and parietal cortices. However, immediately after retrieval (measured 24 h after training), PLA(2) activity was increased just in the parietal cortex. These findings suggest that PLA(2) activity is differentially required in the frontal and parietal cortices for the mechanisms of contextual learning and retrieval. Because reduced brain PLA(2) activity has been reported in Alzheimer disease, our results suggest that stimulation of PLA(2) activity may offer new treatment strategies for this disease.

The evidence for hippocampal long-term potentiation as a basis of memory for simple tasks

Long-term potentiation (LTP) is the enhancement of postsynaptic responses for hours, days or weeks following the brief repetitive afferent stimulation of presynaptic afferents. It has been proposed many times over the last 30 years to be the basis of long-term memory. Several recent findings finally supported this hypothesis: a) memory formation of one-trial avoidance learning depends on a series of molecular steps in the CA1 region of the hippocampus almost identical to those of LTP in the same region; b)hippocampal LTP in this region accompanies memory formation of that task and of another similar task. However, CA1 LTP and the accompanying memory processes can be dissociated, and in addition plastic events in several other brain regions(amygdala, entorhinal cortex, parietal cortex) are also necessary for memory formation of the one-trial task, and perhaps of many others.

Expression of PKC substrate proteins, GAP-43 and neurogranin, is downregulated by cAMP signaling and alterations in synaptic activity

Growth-associated protein 43 (GAP-43) and neurogranin are protein kinase C substrate proteins that are thought to play an important role in synaptic plasticity, but little is currently known about the mechanisms that may regulate their function at the synapse. In this study, we show that long-term elevation of intracellular cAMP levels in rat primary cortical cultures results in a persistent downregulation of GAP-43 and neurogranin, most likely at the transcriptional level. This effect may be at least partially mediated by protein kinase A, but is independent of protein kinase C activation. Moreover, it is mimicked and occluded by manipulations that alter the levels of spontaneous synaptic activity in primary cultures, such as bicuculline and tetrodotoxin. These data suggest that levels of GAP-43 and neurogranin are regulated by factors known to modulate synaptic strength, thus providing a potential mechanism by which protein kinase C signaling pathways and their substrates might contribute to synaptic function and/or plasticity.

GAP-43 gene expression regulates information storage

Previous reports have shown that overexpression of the growth- and plasticity-associated protein GAP-43 improves memory. However, the relation between the levels of this protein to memory enhancement remains unknown. Here, we studied this issue in transgenic mice (G-Phos) overexpressing native, chick GAP-43. These G-Phos mice could be divided at the behavioral level into "spatial bright" and "spatial dull" groups based on their performance on two hidden platform water maze tasks. G-Phos dull mice showed both acquisition and retention deficits on the fixed hidden platform task, but were able to learn a visible platform task. G-Phos bright mice showed memory enhancement relative to wild type on the more difficult movable hidden platform spatial memory task. In the hippocampus, the G-Phos dull group showed a 50% greater transgenic GAP-43 protein level and a twofold elevated transgenic GAP-43 mRNA level than that measured in the G-Phos bright group. Unexpectedly, the dull group also showed an 80% reduction in hippocampal Tau1 staining. The high levels of GAP-43 seen here leading to memory impairment find its histochemical and behavioral parallel in the observation of Rekart et al. (Neuroscience 126: 579-584) who described elevated levels of GAP-43 protein in the hippocampus of Alzheimer's patients. The present data suggest that moderate overexpression of a phosphorylatable plasticity-related protein can enhance memory, while excessive overexpression may produce a "neuroplasticity burden" leading to degenerative and hypertrophic events culminating in memory dysfunction.

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.

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.

Mecp2 deficiency is associated with learning and cognitive deficits and altered gene activity in the hippocampal region of mice

Rett syndrome (RTT) is a debilitating neurological condition associated with mutations in the X-linked MECP2 gene, where apparently normal development is seen prior to the onset of cognitive and motor deterioration at 6-18 months of life. A targeted deletion of the methyl-CpG-binding domain (MBD) coding region and disruption of mRNA splicing was introduced in the mouse, resulting in a complete loss of Mecp2 transcripts and protein. Postnatal comparison of XO and XY mutant Mecp2 allele-containing null mice revealed similar effects on mouse growth and viability, suggesting that phenotypic manifestations are not modulated by the Y-chromosome. Further assessment of Mecp2-null XY mice highlighted cerebellar and hippocampal/amygdala-based learning deficits in addition to reduced motor dexterity and decreased anxiety levels. Brain tissues containing the hippocampal formation of XY Mecp2-null mice also displayed significant changes in genetic activity, which are related to the severity of the mutant phenotype.

Regulation of hippocampal gene expression is conserved in two species subjected to different stressors and antidepressant treatments

BACKGROUND

Chronic stress has significant effects on hippocampal structure and function. We have previously identified nerve growth factor (NGF), membrane glycoprotein 6a (M6a), the guanine nucleotide binding protein (G protein) alpha q polypeptide (GNAQ), and CDC-like kinase 1 (CLK-1) as genes regulated by psychosocial stress and clomipramine treatment in the hippocampus of tree shrews. These genes encode proteins involved in neurite outgrowth.

METHODS

To analyze whether regulation of the above-mentioned genes is conserved between different species, stressors, and antidepressant drugs, we subjected mice to repeated restraint stress and tianeptine treatment and measured hippocampal messenger RNA (mRNA) levels by real time reverse transcription polymerase chain reaction (RT-PCR).

RESULTS

Chronically stressed mice displayed a reduction in transcript levels for NGF, M6a, GNAQ, and CLK-1. In addition, other genes implicated in neuronal plasticity, such as brain-derived neurotrophic factor (BDNF), cyclic adenosine monophosphate (cAMP) response element binding protein (CREB), protein kinase C (PKC), neural cell adhesion molecule (NCAM), and synapsin I were downregulated in stressed mice. Tianeptine treatment reversed the stress effects for the genes analyzed. Alterations in gene expression were dependent on the duration of the stress treatment and, in some cases, were only observed in male mice.

CONCLUSIONS

These results suggest that genes involved in neurite remodeling are one of the main targets for regulation by chronic stress. The finding that this regulation is conserved in different stress models and antidepressant treatments highlights the biological relevance of the genes analyzed and suggests that they might be involved in stress-related disorders.

Relationship between short- and long-term memory and short- and long-term extinction

Both the acquisition and the extinction of memories leave short- and long-term mnemonic traces. Here, we show that in male Wistar rats, the short-term memory for a step-down inhibitory avoidance task (IA) is resistant to extinction, and that its expression does not influence retrieval or extinction of long-term memory. It has been known for some time that short- and long-term inhibitory avoidance memory involve separate and parallel processes. Here we show that, instead, short-term extinction of IA long-term memory is the first step towards its long-term extinction, and that this link requires functional NMDA receptors and protein synthesis in the CA1 region of the dorsal hippocampus at the time of the first CS-no US presentation.

Hippocampal-dependent memory is impaired in heterozygous GAP-43 knockout mice

Cajal proposed that the rearrangement and growth of neurites and synaptic terminals is a substrate for the formation and storage of long-term memories. Proteins that regulate this learning-dependent growth are therefore likely to be "core determinants" (Sanes and Lichtman, Nat Neurosci 1999; 2:597-604) of such information storage processes. Although the growth-associated, protein kinase C (PKC) substrate GAP-43 has been oft-implicated in synaptic plasticity and memory, it has never been demonstrated that a reduction in the level of this protein has a deleterious effect on memory, because most homozygotes die perinatally. In this report, we observe significant memory impairments in heterozygous GAP-43 knockout mice with GAP-43 levels reduced by one-half. Impaired memory for a context was demonstrated in contextual fear conditioning. Importantly, no significant impairments in cued conditioning or on tests of nociceptive or auditory perception were observed in the heterozygous knockout, indicating that the observed impairments were unlikely related to performance or acquisition factors and are the result of reduced GAP-43 levels in the hippocampus. The present results, taken together with the prior demonstration of enhanced memory in transgenic mice overexpressing GAP-43, provide strong evidence for a pivotal role of hippocampal GAP-43 in the bidirectional regulation of mnemonic processing.

Angiotensin II blocks memory consolidation through an AT2 receptor-dependent mechanism

RATIONALE AND OBJECTIVES

Several studies suggest that the brain renin-angiotensin system is involved in memory consolidation. However, the participation of angiotensin II (AII) in this process is controversial. This is probably due to the fact that many of the studies carried out to elucidate this matter employed multitrial learning paradigms together with pretraining intracerebroventricular infusions, and therefore were unable to distinguish between consolidation and retrieval related events and lacked anatomical specificity. To circumvent this problem, we analyzed the role played in memory consolidation by AII using the hippocampal-dependent, one-trial, step-down inhibitory avoidance task (IA) in combination with stereotaxically localized intrahippocampal infusion of drugs.

METHODS AND RESULTS

Rats bilaterally implanted with infusion cannulae into the CA1 region of the dorsal hippocampus (CA1) were trained in IA and tested for memory retention 24 h later. We found that when infused into CA1 immediately or 30 min after training but not later, AII produced a dose-dependent amnesic effect without altering locomotor activity, exploratory behavior or anxiety state. The amnesic effect of AII was not mimicked by angiotensin IV (AIV) and was totally blocked by the AII-type 2 receptor (AT2) antagonist, PD123319, but not by the AII-type 1 receptor (AT1) antagonist, losartan. Importantly, when infused alone, neither PD123319 nor losartan produced any effect on memory retention.

CONCLUSIONS

Our data indicate that, when given into CA1, AII blocks memory formation through a mechanism involving activation of AT2 receptors; however, endogenous AII does not seem to participate in the consolidation of IA long-term memory.

Behavioral characterization in a comprehensive mouse test battery reveals motor and sensory impairments in growth-associated protein-43 null mutant mice

The growth-associated protein (GAP)-43 is a major neuronal protein associated with axonal growth, neuronal plasticity and learning. The observation that only 5-10% of mice with a full GAP-43 gene deletion survive weaning suggests that basic neural functions are disturbed. Here we used a comprehensive test battery to characterise and quantify the motor and sensory function of surviving adult homozygous GAP-43 (-/-) mice as compared with GAP-43 (+/-) and wild-type animals. The test battery was comprised of motor, sensory, and reflex tests producing 25 measures of locomotion, as well as epicritic, auditory, olfactory and visual function. The analysis revealed significant impairments in muscle strength, limb coordination and balance in GAP-43 (-/-) mice. Furthermore, GAP-43 (-/-) animals were hyperactive and showed reduced anxiety as measured by open field and light dark tests. In sensory tests, GAP-43 (-/-) mice were tested for impaired tactile and labyrinthine function. Abnormal reflexes were found in the contact and vibrissa placing responses, and in the crossed extensor reflex. GAP-43 (+/-) animals showed only moderate abnormalities as compared with wild-type animals. We conclude that GAP-43 is necessary for the development and function of a variety of neuronal systems. The results also show that the comprehensive test battery used in the present study represents a sensitive approach to assess the functional integrity of ascending and descending pathways in genetically manipulated mice.

Prenatal ethanol exposure reduces mGluR5 receptor number and function in the dentate gyrus of adult offspring

BACKGROUND

Previous studies in our laboratory indicated that metabotropic glutamate receptor (mGluR)-stimulated phosphoinositide hydrolysis is markedly reduced in the hippocampal formation of adult rat offspring whose mothers drank moderate amounts of ethanol during pregnancy. In the present study, we extended these observations by measuring the impact of prenatal ethanol exposure on proteins associated with the mGluR5 receptor-effector system along with two mGluR5 agonist-mediated responses in dentate gyrus of adult offspring.

METHODS

Sprague-Dawley rat dams consumed one of three diets throughout gestation: (1) a BioServ liquid diet that contained 5% ethanol (v/v), (2) pair-fed an isocalorically equivalent amount of 0% ethanol liquid diet, or (3) lab chow ad libitum. Microdissected slices of dentate gyrus were prepared from adult female offspring from each diet group and used for (1) Western blot analyses of mGluR5, the G-proteins Galphaq and Galpha11, and phospholipase C-beta1; (2) 2-chloro-5-hydroxyphenylglycine (CHPG)-stimulated growth associated protein 43 (GAP-43) phosphorylation; or (3) CHPG potentiation of electrically evoked [H]-D-aspartate (D-ASP) release from dentate gyrus slices.

RESULTS

In tissue prepared from untreated control rats, CHPG produced a dose-dependent increase in phosphate incorporation into GAP-43, with maximal agonist stimulation occurring at 20 microM of CHPG. CHPG produced a quantitatively similar dose-dependent increase in the potentiation of electrically evoked D-ASP release from dentate gyrus slices from untreated controls. Fetal ethanol exposure reduced the amount of dentate gyrus mGluR5 receptor protein by 36% compared with the diet control groups. There were no significant differences between diet groups in the two G-proteins or phospholipase C-beta1 protein. Fetal ethanol exposure reduced CHPG-stimulated GAP-43 phosphorylation to approximately one half the amount of CHPG stimulation observed in the control diet groups. Prenatal ethanol exposure also reduced CHPG potentiation of D-ASP release to a similar degree compared with control.

CONCLUSIONS

These results indicate that prenatal exposure to moderate quantities of ethanol reduces mGluR5 expression in the dentate gyrus of adult offspring. Although the subcellular site(s) for reduced mGluR5 expression cannot be discerned from Western blot data, the quantitatively similar effects of prenatal ethanol exposure on mGluR5 agonist stimulation of presynaptically localized GAP-43 phosphorylation and CHPG potentiation of evoked D-ASP release suggest that the presynaptic nerve terminal is one site where prenatal ethanol exposure has reduced mGluR5 receptor number and function. Furthermore, these data implicate these neurochemical alterations as one factor contributing to the hippocampal synaptic plasticity and behavioral deficits that we have observed previously in prenatal ethanol-exposed offspring.

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.

Fetal Alcohol Exposure Alters GAP-43 Phosphorylation and Protein Kinase C Responses to Contextual Fear Conditioning in the Hippocampus of Adult Rat Offspring

BACKGROUND

The growth- and plasticity-associated protein GAP-43 plays a significant role in the establishment and remodeling of neuronal connections. We have previously shown that GAP-43 levels, protein kinase C (PKC) activity, and GAP-43 phosphorylation increase during contextual fear conditioning and that fetal alcohol exposure (FAE) decreases PKC activity and GAP-43 phosphorylation in the hippocampus of adult offspring. Drawing on these observations, we hypothesized that FAE manifests its cognitive impairment by disrupting PKC activation and membrane translocation, thereby decreasing GAP-43 phosphorylation and function.

METHODS

Three groups of pregnant rat dams (FAE and two control diet groups) were placed on different diet regimens. Offspring from each of these groups were placed into each of four test groups, a contextual fear conditioned (CFC) group, a naïve unhandled group, and two nonlearning stress control groups. Hippocampi were dissected, homogenized, and used to prepare a cytosolic and a membrane fraction. These fractions were probed for total GAP-43, PKC-phosphorylated GAP-43, and several PKC subtypes. PKC activity also was measured in total homogenates.

RESULTS

Compared with both control diet groups, FAE animals showed a deficit in the activation of PKC in the hippocampus at 24 hr but not at 1.5 hr after CFC. Likewise, we found that the amount of GAP-43 and its phosphorylation were decreased 24 hr after CFC in FAE rats but not at early times after training. Analysis of the translocation of various PKC isoforms revealed that FAE animals had decreased levels of membrane-bound PKC beta2 and PKC epsilon 24 hr after CFC.

CONCLUSIONS

Considering the role of PKC activation and GAP-43 phosphorylation in synaptic plasticity, our results suggest that deficient translocation of PKC beta2 and PKC epsilon in the hippocampus may mediate the electrophysiological and behavioral deficits observed in fetal alcohol exposed animals.

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.

Changes in protein kinase C (PKC) activity, isozyme translocation, and GAP-43 phosphorylation in the rat hippocampal formation after a single-trial contextual fear conditioning paradigm

The hippocampus plays an important role in spatial learning and memory. However, the biochemical alterations that subserve this function remain to be fully elucidated. In this study, rats were subjected to a single-trial contextual fear conditioning (CFC) paradigm; the activation of different protein kinase C (PKC) subtypes and the levels and phosphorylation of the plasticity-associated protein GAP-43 were assayed in the hippocampus at varying times after training. We observed a rapid activation of hippocampal PKC (15 min through 24 h), with differential translocation of the PKC isotypes studied. At early times after CFC (15-90 min), PKCalpha and PKCgamma translocated to the membrane, while PKCbetaII and PKCepsilon moved more transiently (15 to 30 min) to the cytosol. These PKC isotypes returned to the membrane at later time points after CFC. Correlating with these changes in PKC translocation and activity, there was an early decrease in GAP-43 phosphorylation followed by a more sustained increase from 1.5-72 h. GAP-43 protein levels were also increased after 3 h, and these levels remained elevated for at least 72 h. These changes in PKC and GAP-43 were specific to the CFC trained animals and no changes were seen in animals exposed to the same stimuli in a non-associative fashion. Comparison of translocation of different PKC isotypes with the changes in GAP-43 phosphorylation suggested that PKCbetaII and PKCepsilon may mediate both the early changes in the phosphorylation of this protein and the increases in GAP-43 expression at later times after CFC.

Memory impairment induced by chronic intracerebroventricular infusion of beta-amyloid (1-40) involves downregulation of protein kinase C

Signaling pathways underlying the cognitive deficit of the Alzheimer's disease (AD) are not completely understood. Protein kinase C (PKC), a major neuronal protein plays a critical role in cellular signal transduction and it is known to be subjected to modulation in AD. We showed previously that, chronic infusion of beta-amyloid (1-40) into rat cerebroventricle leads to deficit in spatial and non-spatial memory formation. As an attempt to identify the cellular correlates of the memory deficit, in the present study we investigated the PKC activation in different brain areas. Chronic infusion of beta-amyloid (1-40) for 14 days into the rat cerebroventricle decreased the activity of soluble protein kinase C (PKC) in the hippocampus. Subcellular translocation of PKC to membrane fraction in hippocampal slices of rats treated with beta-amyloid (1-40) was completely abolished under acute stimulation with 0.5 microM phorbol-dibutyrate (PDBu). We also reported a decreased affinity (k(D)) for PDBu binding in the hippocampus, cerebral cortex and striatum. The total number of binding sites for PDBu (B(max)) was increased, in the three brain areas analyzed on the day 14, but the changes were not statistically significant. Our data indicate that chronic accumulation of beta-amyloid (1-40) into the rat brain reduced activation of PKC, effect that would substantially contribute to the memory deficit found in these animals.

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.

Individual differences in spatial memory among aged rats are related to hippocampal PKCgamma immunoreactivity

We reported previously that the extent of spatial memory impairment among aged rats was correlated positively with levels of protein kinase Cgamma in hippocampal homogenates measured by quantitative Western blotting (Colombo et al., 1997). In the current study, immunocytochemistry was used to test whether the relationship between elevated PKC-gamma and memory impairment among aged rats could be localized further within regions of the hippocampus. Six- and 24-month-old male Long-Evans rats were first trained in the water maze on a standard place-learning task and then trained 2 weeks later on a transfer task designed for rapid acquisition. In comparison with young rats, aged rats with impaired spatial memory had increased PKCgamma-immunoreactivity (PKCgamma-ir) in CA1 of the hippocampus, but not the dentate gyrus. In addition, PKCgamma-ir in CA1 was correlated positively with spatial memory impairment among aged rats on the standard place-learning and the transfer training tasks. The current results are consistent with our previous report of PKCgamma in hippocampal homogenates, and show further that the relationships between PKCgamma-ir and memory impairments among aged rats are most evident in area CA1. Thus age-related impairments of spatial memory, as well as deficits in the flexible use of previously acquired information, may result from dysregulation of PKCgamma.

The role of PKA, CaMKII, and PKC in avoidance conditioning: permissive or instructive?

This article explores the causal and correlative relationships between kinases and learning and memory. Specifically, the contributions of three kinases-protein kinase A (PKA), calcium calmodulin-dependent kinase II (CaMKII), and protein kinase C (PKC)-are assessed during the consolidation phase of avoidance conditioning. The following sources of evidence are considered: inhibitor data, activity monitoring, and transgenic studies. An exhaustive effort is made to address several issues regarding the participation of these kinases in (a) posttraining timing and magnitude, (b) location across many brain regions, and (c) the use of multiple pharmacological agents and assays. In addition, this article attempts to integrate the behavioral data with the purported role of kinases in long-term potentiation (LTP).

The effects of antiepileptic drugs on spatial learning and hippocampal protein kinase C gamma in immature rats

This study was conducted to determine if alterations in hippocampal protein kinase C (PKC) gamma is one of the cellular mechanisms by which conventional antiepileptic drugs affect learning and memory. Wistar Rats (21-day-old) were divided into five groups: (1) control (no training and drugs); (2) training group (no drugs); (3) phenobarbital (PB) group; (4) carbamazepine (CBZ) group; and (5) valproate (VPA) group. A hippocampus dependent learning task (spatial changing learning) was used in the latter four groups lasting a total of 10 days. Correct responding rate of training group was significantly higher (P < 0.05) than in the PB, CBZ and VPA group. The PKC gamma staining intensity in hippocampal CA1-2 region of training group was significant greater than that of the control and PB group. There was no difference in staining intensities between the CBZ, VPA group or training group. The amount of PKC gamma located in plasma membrane of hippocampal neurons was significantly higher in the training group (P < 0.05) than the control, PB and VPA groups. No differences were found between the training and CBZ group. Lastly, the amount of PKC gamma in cytosol of hippocampus did not significantly differ between any of the five groups. These results indicate that the three antiepileptic drugs used in this study all disturbed the spatial learning of immature rats. Spatial learning was concomitant with activation of PKC gamma in hippocampal neurons. PB and VPA likely adversely affect learning and memory by interfering with PKC gamma activation, whereas CBZ may act by a different mechanism, possibly in the post-translocation process or by a PKC gamma independent pathway.

Concentration-dependent effects of protein phosphatase (PP) inhibitors implicate PP1 and PP2A in different stages of memory formation

Numerous studies have demonstrated roles for protein phosphorylation and for specific kinases in memory formation; however, a role for specific protein phosphatases has not been established. Previous studies using pharmacobehavioral methods to implicate protein phosphatase activity in memory formation have been unable to discriminate between protein phosphatases 1 (PP1) and 2A (PP2A), as available cell-permeable agents generally inhibit both enzyme classes. To address this difficulty the present study exploited differences in the potency of the selective phosphatase inhibitor, okadaic acid, toward PP1 and PP2A. Within the context of a temporally precise animal model of memory, developed using the day-old chick (Gallus domesticus), acute administration of various concentrations of okadaic acid was found to disrupt two temporally distinct stages of memory formation. When administered bilaterally into an area of the chick brain implicated in memory formation, concentrations of okadaic acid known to selectively inhibit PP2A in vitro disrupted memory from 50 min posttraining. Higher concentrations, reported to inhibit both PP2A and PP1 in vitro, produced significant retention deficits from 20 min posttraining. Identical temporally specific effects were also obtained by varying the concentration and time of administration of calyculin A, a phosphatase inhibitor with equal potency toward both enzyme classes. Hence, different phosphatase enzymes may contribute to different stages of the enzymatic cascade believed to underlie memory formation.

Different hippocampal activity profiles for PKA and PKC in spatial discrimination learning

Protein kinases are considered essential for the processing and storage of information in the brain. However, the dynamics of protein kinase activation in the hippocampus during spatial learning are poorly understood. In this study, rats were trained to learn a holeboard spatial discrinmination task and the activity profiles for cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) and Ca2+/ phospholipid-dependent protein kinase C (PKC) in the hippocampus were examined. Hippocampal PKA activity increased rapidly on Day 1 of spatial learning and remained moderately high at later stages of acquisition. In contrast, PKC activity increased in particulate fractions compared with cytosolic fractions after habituation training and was maximal at Day 3 of spatial acquisition. The results establish a temporal dissociation between PKA and PKC during acquisition of spatial discrimination learning.

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.

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.

Enhanced learning after genetic overexpression of a brain growth protein

Ramón y Cajal proposed 100 years ago that memory formation requires the growth of nerve cell processes. One-half century later, Hebb suggested that growth of presynaptic axons and postsynaptic dendrites consequent to coactivity in these synaptic elements was essential for such information storage. In the past 25 years, candidate growth genes have been implicated in learning processes, but it has not been demonstrated that they in fact enhance them. Here, we show that genetic overexpression of the growth-associated protein GAP-43, the axonal protein kinase C substrate, dramatically enhanced learning and long-term potentiation in transgenic mice. If the overexpressed GAP-43 was mutated by a Ser --> Ala substitution to preclude its phosphorylation by protein kinase C, then no learning enhancement was found. These findings provide evidence that a growth-related gene regulates learning and memory and suggest an unheralded target, the GAP-43 phosphorylation site, for enhancing cognitive ability.

Rapid and transient learning-associated increase in NMDA NR1 subunit in the rat hippocampus

Several lines of evidence indicate that glutamate NMDA receptors are critically involved in long-term potentiation (LTP) and in certain forms of learning. It was previously demonstrated that memory formation of an inhibitory avoidance task in chick is specifically associated with an increase in the density of NMDA receptor in selected brain regions. Here we report on the effect of a one trial inhibitory avoidance training in rats, a hippocampal-dependent learning task, on the levels of different subunits of the glutamate NMDA receptor in synaptic plasma membranes (SPM) isolated from the hippocampus. Training rats on a one trial inhibitory avoidance task results in a rapid, transient and selective increase (+33%, p < 0.05) in NMDA NRI subunit expression in hippocampal SPM of rats sacrificed 30 min posttraining. No changes were observed at 0 or 120 min after training or in shocked animals in comparison to naive control rats. In addition, no training-associated increase in the levels of NMDA NR2A and NR2B or AMPA GluR 2/3 subunits was observed at any timepoint tested. In conclusion, the present findings support the hypothesis that alterations in expression of synaptic NMDA NR1 subunits in the hippocampus are specifically associated with memory formation of an inhibitory avoidance task and strongly suggest that hippocampal NMDA receptors are crucially involved in the neural mechanisms underlying certain forms of learning.

Alterations in hippocampal GAP-43 phosphorylation and protein level following contextual fear conditioning

C57BL/6 (B6) mice display better contextual learning than the DBA/2 (D2) mice. The possibility that GAP-43, is differentially affected as a function of strain and learning was investigated in the present study. No basal difference between C57BL/6J (B6) and DBA/2J (D2) mice in the amount of hippocampal GAP-43 was observed, but naive D2 mice have slightly lower basal levels of GAP-43 phosphorylation than do B6 mice. Interestingly, alterations in hippocampal GAP-43 protein levels and phosphorylation state in response to training for contextual learning were observed only in B6 mice. Immediate-shocked mice, serving as nonlearning controls, showed no GAP-43 alterations, nor did D2 mice subjected to either training condition. These results suggest that modulation of hippocampal GAP-43 may be important for contextual learning and that strain-specific alterations in GAP-43 may be part of a disrupted pathway in D2 mice that is essential for learning.

Learning-associated activation of nuclear MAPK, CREB and Elk-1, along with Fos production, in the rat hippocampus after a one-trial avoidance learning: abolition by NMDA receptor blockade

It is widely accepted that the formation of long-term memory (LTM) requires neuronal gene expression, protein synthesis and the remodeling of synaptic contacts. From mollusk to mammals, the cAMP/PKA/CREB signaling pathway has been shown to play a pivotal role in the establishment of LTM. More recently, the MAPK cascade has been also involved in memory processing. Here, we provide evidence for the participation of hippocampal PKA/CREB and MAPK/Elk-1 pathways, via activation of NMDA receptors, in memory formation of a one-trial avoidance learning in rats. Learning of this task is associated with an activation of p44 and p42 MAPKs, CREB and Elk-1, along with an increase in the levels of the catalytic subunit of PKA and Fos protein in nuclear-enriched hippocampal fractions. These changes were blocked by the immediate posttraining intra-hippocampal infusion of APV, a selective blocker of glutamate NMDA receptors, which renders the animals amnesic for this task. Moreover, no changes were found in control-shocked animals. Thus, inhibitory avoidance training in the rat is associated with an increase in the protein product of an IEG, c-fos, which occurs concomitantly with the activation of nuclear MAPK, CREB and Elk-1. NMDA receptors appear to be a necessary upstream step for the activation of these intracellular cascades during learning.

Short- and long-term memory are differentially affected by metabolic inhibitors given into hippocampus and entorhinal cortex

Rats were implanted with cannulae in the CA1 area of the dorsal hippocampus or in the entorhinal cortex and trained in one-trial step-down inhibitory avoidance. Two retention tests were carried out in each animal, one at 1.5 h to measure short-term memory (STM) and another at 24 h to measure long-term memory (LTM). The purpose of the present study was to screen the effect on STM of various drugs previously shown to affect LTM of this task when given posttraining at the same doses that were used here. The drugs and doses were the guanylyl cyclase inhibitor LY83583 (LY, 2.5 microMg), the inhibitor of Tyr-protein kinase at low concentrations and of protein kinase G (PKG) at higher concentrations lavendustin A (LAV, 0.1 and 0.5 microMg), the PKG inhibitor KT5823 (2.0 microMg), the protein kinase C (PKC) inhibitor staurosporin (STAU, 2.5 microMg), the inhibitor of calcium/ calmodulin protein kinase II (CaMKII) KN62 (3.6 microMg), the protein kinase A (PKA) inhibitor KT5720 (0.5 microMg), and the mitogen-activated protein kinase kinase (MAPKK) inhibitor PD098059 (PD, 0.05 microMg). PD was dissolved in saline; all the other drugs were dissolved in 20% dimethyl sulfoxide. In all cases the drugs affected LTM as had been described in previous papers. The drugs affected STM and LTM differentially depending on the brain structure into which they were infused. STM was inhibited by KT5720, LY, and PD given into CA1 and by STAU and KT5720 given into the entorhinal cortex. PD given into the entorhinal cortex enhanced STM. LTM was inhibited by STAU, KN62, KT5720, KT5823, and LAV (0.5 microMg) given into CA1 and by STAU, KT5720, and PD given into the entorhinal cortex. The results suggest that STM and LTM involve different physiological mechanisms but are to an extent linked. STM appears to require PKA, guanylyl cyclase, and MAPKK activity in CA1 and PKA and PKC activity in the entorhinal cortex; MAPKK seems to play an inhibitory role in STM in the entorhinal cortex. In contrast, LTM appears to require PKA and PKC activity in both structures, guanylyl cyclase, PKG, and CaMKII activity in CA1, and MAPKK activity in the entorhinal cortex.

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

Several evidences demonstrate that protein kinase C (PKC) is involved in hippocampal long-term potentiation (LTP) and in different forms of learning, including inhibitory avoidance training in rats. Here, we evaluated the levels of conventional PKC isozymes (alpha, betaI, betaII, gamma) in synaptic plasma membrane (SPM) fractions isolated from hippocampus of rats subjected to a one-trial inhibitory avoidance paradigm. At 0, 30 and 120 min after training, there was a significant increase in the total amount of PKCbetaI. Densitometric analysis of the immunoblots showed an increase of 142+/-11% at 0 min, 193+/-16% at 30 min and 156+/-6% at 120 min after training relative to shocked control values. No changes were found in PKCbetaI levels in SPM fractions of the shocked animals relative to naive control values. No training-specific increments in the levels of PKCalpha, betaII and gamma were observed at any time point tested. However, an increase in PKCgamma levels was found in trained and shocked animals sacrificed 120 min after each experimental procedure. In addition, bilateral microinjections of a fairly selective inhibitor of PKCbetaI isozyme into the CA1 of the dorsal hippocampus produced amnesia when given 10 min before training, or 50, 110, but not 170 min, after training. Thus, the present findings demonstrate the participation of PKCbetaI in the early synaptic events responsible for the acquisition and consolidation of an inhibitory avoidance learning, and suggest a putative role of this presynaptic isozyme on the enhanced PKC-dependent B-50/GAP-43 phosphorylation previously detected by us during this associative learning.

Separate mechanisms for short- and long-term memory

It has been assumed for over a century that short-term memory (STM) processes are in charge of cognition while long-term memory (LTM) is being formed, a process that takes hours. A major question is whether STM is merely a step towards LTM, or a separate entity. Recent experiments have shown that many treatments with specific molecular actions given into the hippocampus, entorhinal or parietal cortex immediately after one-trial avoidance training can effectively block STM without affecting LTM formation. This shows that STM and LTM involve separate mechanisms. Some treatments even affect STM and LTM in opposite directions. Others, however, influence both memory types similarly, suggesting links between the two both at the receptor and at the post-receptor level. Drug effects on working memory (WM) were also studied. In some brain regions WM is affected by receptor blockers that alter either STM or LTM; in others it is not. This suggests links between the three memory types at the receptor level. The anterolateral prefrontal cortex is crucial for WM and LTM but is not involved in STM. The hippocampus, entorhinal and parietal cortex are crucial for the three types of memory, in some cases using different receptors for each. The amygdala is not involved in WM or STM, but it plays a key role in the modulation of the early phase of LTM.

Decreased GAP-43/B-50 phosphorylation in striatal synaptic plasma membranes after circling motor behavior during development

We evaluated the in vitro phosphorylation of the presynaptic substrate of protein kinase C (PKC), GAP-43/B-50 and the PKC activity in the striatum of rats submitted to a circling training (CT) test during postnatal development. Motor activity at 30 days of age, but not at other ages, produced a unilateral reduction (-29.5%; p<0.001) in the level of GAP-43/B-50 endogenous phosphorylation in the contralateral striatum with respect to the ipsilateral side, while non-trained control animals did not show asymmetric differences. Compared to controls, the contralateral striatum of trained animals also showed a significant reduction (-29.3%; p<0. 001) in the incorporation of 32P-phosphate into GAP-43. This decreased in vitro GAP-43 phosphorylation was seen at 30 min, but not immediately after circling motor behavior. This contralateral change in GAP-43 phosphorylation correlated with the running speed developed by the animals [(r=0.9443, p=0.0046, n=6, relative to control group) and (r=0.8813, p=0.0203, n=6, with respect to the ipsilateral side of the exercised animals)]. On the contrary, GAP-43/B-50 immunoblots did not show changes in the amount of this phosphoprotein among the different experimental groups. Back phosphorylation assays, performed in the presence of bovine purified PKC, increased the level of GAP-43/B-50 phosphorylation in the striatum contralateral to the sense of turning [(+22%; p<0.05, with respect to ipsilateral side of the same trained group) and (+21%; p<0.05, relative to control group)]. Taken together, these results demonstrate that the activity developed in the CT test induces a reduction in the phosphorylation state of GAP-43/B-50 in the specific site for PKC. We conclude that general markers of activity-dependent neuronal plasticity are also altered in the same period that long-lasting changes in striatal neuroreceptors are triggered by circling motor behavior.

Decreased phosphorylation of GAP-43/B-50 in striatal synaptic plasma membranes after circling motor activity

The effects of spontaneous circling motor activity on the in vitro phosphorylation of the protein kinase C substrate GAP-43/B-50 was studied on striatal membranes of developing rats (30 days of age). At this time of postnatal development, permanent plastic changes in cholinergic and dopaminergic systems are produced by physiological motor activity. Exercised animals showed a significant reduction of 31% in the level of GAP-43/B-50 endogenous phosphorylation in the contralateral striatum respect to the ipsilateral side (P < 0.01), while control animals did not show asymmetric differences. Compared to controls, the contralateral striatum of exercised animals showed a 33% reduction in the incorporation of 32P-phosphate into GAP-43/B-50 30 minutes post-exercise (P < 0.01). This change in GAP-43/B-50 phosphorylation was correlated with the running speed developed by the animals (r:0.8986, P = 0.015). GAP-43/B-50 immunoblots revealed no changes in the amount of this protein in any group. Moreover, a significant variation of 25% (P < 0.05) in the PKC activity was seen between both exercised striata. Interhemispheric differences were not found in control animals. We conclude that endogenous phosphorylation of this protein is also altered by motor activity in the same period that permanent changes in striatal neuroreceptors are triggered after motor training.

On brain lesions, the milkman and Sigmunda

Lesion studies have been of historical importance in establishing the brain systems involved in memory processes. Many of those studies, however, have been overinterpreted in terms of the actual role of each system and of connections between systems. The more recent molecular pharmacological approach has produced major advances in these two areas. The main biochemical steps of memory formation in the CAI region of the hippocampus have been established by localized microinfusions of drugs acting on specific enzymes of receptors, by subcellular measurements of the activity or function of those enzymes and receptors at definite times, and by transgenic deletions or changes of those proteins. The biochemical steps of long-term memory formation in CAI have been found to be quite similar to those of long-term potentiation in the same region, and of other forms of plasticity. Connections between the hippocampus and the entorhinal and parietal cortices in the formation and modulation of short- and long-term memory have also been elucidated using these techniques. Lesion studies, coupled with imaging studies, still have a role to play; with regard to human memory, this role is in many ways unique. But these methods by themselves are not informative as to the mechanisms of memory processing, storage or modulation.

Selective changes in protein kinase C isoforms and phosphorylation of endogenous substrate proteins in rat cerebral cortex during pre- and postnatal ethanol exposure

The effect of pre- and postnatal ethanol exposure on protein kinase C (PKC) activity, immunochemical analysis of PKC alpha, betaI, betaII, gamma, delta, epsilon, eta, and zeta by isoform-specific antibodies, and in vitro phosphorylation of endogenous substrate proteins was investigated in rat cerebral cortex. The PKC activity was increased throughout the development. However, the activity at the age of 8 days was significantly high in cytosolic and membrane fractions from ethanol-treated rats. Immunochemical analysis showed increased levels of PKC betaI and betaII at the age of 8 days, and a decrease in delta isoform at 8, 30, and 90 days of age. PKC isoforms alpha, gamma, epsilon, and eta showed no appreciable change in ethanol-treated rats. PKC zeta levels were high in the cytosolic fraction from ethanol-treated samples of 90 days age. In vitro phosphorylation of endogenous substrate proteins in the presence of Ca2+/phospholipid showed increased phosphorylation of selective membrane and cytosolic proteins with 87, 65, 50, 43, 36, and 29 kDa in ethanol-treated rats. The phosphorylation of these proteins decreased in the presence of staurosporine, which also supported PKC-mediated phosphorylation. Increased PKC activity, activation of betaI and betaII isoforms, decreased levels of delta isoform, and phosphorylation of selective substrate proteins in cerebral cortex due to alcohol exposure might be relevant in ethanol-induced central nervous system dysfunction and fetal alcohol syndrome.

Learning-specific, time-dependent increases in hippocampal Ca2+/calmodulin-dependent protein kinase II activity and AMPA GluR1 subunit immunoreactivity

Ca2+/calmodulin-dependent protein kinase II (CAMK II) and one of its target, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), glutamate receptors have been shown to participate in both long-term potentiation (LTP) in the hippocampus, and in spatial, as well as in a variety, of learning paradigms. Recently, we were able to demonstrate that the intrahippocampal infusion of a specific inhibitor of CAMK II (KN62) provoked full retrograde amnesia of an inhibitory avoidance learning in rats when given immediately, but not 120 or 240 min, after training. Furthermore, this task is accompanied by a rapid, selective and reversible increase in hippocampal [3H] AMPA receptor binding. Here we report the effect of this aversively motivated learning task on CAMK II activity, and AMPA GluR1 subunit phosphorylation and immunoreactivity in the hippocampus. One trial inhibitory avoidance training is associated with a learning-specific, time-dependent increase (25-78%) in both total and Ca2+-independent activities of CAMK II in the hippocampus of rats killed immediately (0 min), but not 120 min, after training. In addition, immunoblotting experiments showed an increment in the amount of the alpha-subunit of CAMK II at 0, 30 and 120 min after training. An increase in the in vitro phosphorylation of alpha- and beta-subunits of CAMK II was also observed in hippocampal synaptosomal membranes (SPM) of trained rats killed immediately and 30 min post-training. In addition, inhibitory avoidance is accompanied by a 20% increase in GluR1 phosphorylation and a 33% increase in GluR1 immunoreactivity 120 min after training. No significant changes were observed in shocked animals. Phosphorylation of hippocampal SPM from naive control animals in conditions suitable for CAMK II activation resulted in a large increase in the density of [3H] AMPA binding (+ 100%). Taken together, these findings confirm and extend previous data suggesting that CAMK II and AMPA glutamate receptors in the hippocampus participate in the early phase of memory formation of an inhibitory avoidance learning.