Dissociation between genes activated in long-term potentiation and in spatial learning in the rat

Hypertension-induced synapse loss and impairment in synaptic plasticity in the mouse hippocampus mimics the aging phenotype: implications for the pathogenesis of vascular cognitive impairment

Strong epidemiological and experimental evidence indicates that hypertension has detrimental effects on the cerebral microcirculation and thereby promotes accelerated brain aging. Hypertension is an independent risk factor for both vascular cognitive impairment (VCI) and Alzheimer's disease (AD). However, the pathophysiological link between hypertension-induced cerebromicrovascular injury (e.g., blood-brain barrier disruption, increased microvascular oxidative stress, and inflammation) and cognitive decline remains elusive. The present study was designed to characterize neuronal functional and morphological alterations induced by chronic hypertension and compare them to those induced by aging. To achieve that goal, we induced hypertension in young C57BL/6 mice by chronic (4 weeks) infusion of angiotensin II. We found that long-term potentiation (LTP) of performant path synapses following high-frequency stimulation of afferent fibers was decreased in hippocampal slices obtained from hypertensive mice, mimicking the aging phenotype. Hypertension and advanced age were associated with comparable decline in synaptic density in the stratum radiatum of the mouse hippocampus. Hypertension, similar to aging, was associated with changes in mRNA expression of several genes involved in regulation of neuronal function, including down-regulation of Bdnf, Homer1, and Dlg4, which may have a role in impaired synaptic plasticity. Collectively, hypertension impairs synaptic plasticity, reduces synaptic density, and promotes dysregulation of genes involved in synaptic function in the mouse hippocampus mimicking the aging phenotype. These hypertension-induced neuronal alterations may impair establishment of memories in the hippocampus and contribute to the pathogenesis and clinical manifestation of both vascular cognitive impairment (VCI) and Alzheimer's disease (AD).

Distinct and simultaneously active plasticity mechanisms in mouse hippocampus during different phases of Morris water maze training

Although the Morris water maze (MWM) is the most frequently used protocol to examine hippocampus-dependent learning in mice, not much is known about the spatio-temporal dynamics of underlying plasticity processes. Here, we studied molecular and cellular hippocampal plasticity mechanisms during early and late phases of spatial learning in the MWM. Quantitative in situ hybridization for the immediate early genes zif268 and Homer1a (H1a) revealed phase-dependent differences in their expression between areas CA1 and CA3. During the initial learning phase, CA1 expression levels of the molecular plasticity marker H1a, but not of the activity reporter gene zif268, were related to task proficiency; whereas no learning-specific changes could be detected in CA3. Simultaneously, the ratio of surface-expressed NMDAR subunits NR2A and NR2B was downregulated as measured by acute slice biotinylation assay, while the total number of surface NMDARs was unaltered. When intrinsic 'somatic' and synaptic plasticity in the CA1-region of hippocampal slices were examined, we found that early learning promotes intrinsic neuronal plasticity as manifested by a reduction of spike frequency adaptation and postburst afterhyperpolarization. At the synaptic level, however, maintenance of long-term potentiation (LTP) in all learning groups was impaired which is most likely due to 'intrinsic' learning-induced LTP which occluded any further electrically induced LTP. Late learning, in contrast, was characterized by re-normalized H1a, NR2A and NR2B expression and neuronal firing, yet a further strengthening of learning-induced LTP. Together, our data support a precisely timed cascade of complex molecular and subcellular transformations occurring from early to late MWM learning.

Transcription factors in long-term memory and synaptic plasticity

Transcription is a molecular requisite for long-term synaptic plasticity and long-term memory formation. Thus, in the last several years, one main interest of molecular neuroscience has been the identification of families of transcription factors that are involved in both of these processes. Transcription is a highly regulated process that involves the combined interaction and function of chromatin and many other proteins, some of which are essential for the basal process of transcription, while others control the selective activation or repression of specific genes. These regulated interactions ultimately allow a sophisticated response to multiple environmental conditions, as well as control of spatial and temporal differences in gene expression. Evidence based on correlative changes in expression, genetic mutations, and targeted molecular inhibition of gene expression have shed light on the function of transcription in both synaptic plasticity and memory formation. This review provides a brief overview of experimental work showing that several families of transcription factors, including CREB, C/EBP, Egr, AP-1, and Rel, have essential functions in both processes. The results of this work suggest that patterns of transcription regulation represent the molecular signatures of long-term synaptic changes and memory formation.

Qualitatively different hippocampal subfield engagement emerges with mastery of a spatial memory task by rats

The parallel, entorhinal cortex projections to different hippocampal regions potentially support separate mnemonic functions. To examine this possibility, rats were trained in a radial-arm maze task so that hippocampal activity could be compared after "early" (two sessions) or "late" (five sessions) learning. Induction of the immediate-early gene Zif268 was then measured, so revealing possible activity differences across hippocampal subfields and the parahippocampal cortices. Each rat in the two experimental groups (early, late) was also yoked to a control rat that obtained the same number of rewards, visited the same number of maze arms, and spent a comparable amount of time in the maze. Although overall Zif268 levels did not distinguish the four groups, significant correlations were found between spatial memory performance and levels of dentate gyrus Zif268 expression in the early but not the late training group. Conversely, hippocampal fields CA3 and CA1 Zif268 expression correlated with performance in the late but not the early training group. This reversal in the correlation pattern was echoed by structural equation modeling, which revealed dynamic changes in effective network connectivity. With early training, the dentate gyrus appeared to help determine CA1 activity, but by late training the dentate gyrus reduced its neural influence. Furthermore, CA1 was distinguished from CA3, each subfield developing opposite relations with task mastery. Thus, functional entorhinal cortex coupling with CA1 activity became more direct with additional training, so producing a trisynaptic circuit bypass. The present study reveals qualitatively different patterns of hippocampal subfield engagement dependent on task demands and mastery.

Cocaine self-administration improves performance in a highly demanding water maze task

RATIONALE

Long-term potentiation (LTP) is considered to be a cellular substrate of learning and memory. Indeed, the involvement of LTP-like mechanisms in spatial learning has consistently been demonstrated in the Morris water maze test. We have previously shown that hippocampal LTP in Lewis rats was modulated by cocaine self-administration, although the performance of cocaine-self-administered rats in the Morris water maze was not altered.

OBJECTIVE

Given that the ease of the task previously used could have masked any possible effects of the cocaine-induced LTP enhancement on spatial learning, a new and more difficult water maze task was devised to address this issue.

MATERIALS AND METHODS

Animals self-administered cocaine (1 mg/kg) or saline under a fixed ratio 1 schedule of reinforcement for 22 days. Spatial learning was assessed in a difficult water maze task (four sessions, two trials per session with a 90-min intertrial interval), and spatial memory was also evaluated 48 h after training (a 90-s test). Additionally, reversal learning and perseverance were also studied.

RESULTS

There was a reduced latency in finding the hidden platform during training, as well as improved memory of the platform location in cocaine-self-administered rats with respect to animals that self-administered saline. No differences were observed in reversal learning or perseverance between groups.

CONCLUSIONS

Our data suggest that cocaine self-administration facilitates learning and memory in the water maze test only when animals are submitted to highly demanding tasks, involving working memory or consolidation-like processes during the intertrial interval.

Hippocampal Synaptic Plasticity and Water Maze Learning in Cocaine Self-Administered Rats

Previously, we have shown that long-term potentiation (LTP) in hippocampus of Lewis rats was significantly modulated by cocaine self-administration. Using a single train of high-frequency stimulation of 100 Hz for 1s (HFS), we found an enhancement of LTP after cocaine self-administration that was maintained even during the extinction of this behavior. However, the effects of cocaine self-administration on a hippocampal-dependent spatial learning task were unknown. Therefore, in the present study our first objective was to analyze if cocaine self-administration might affect the performance in a hippocampus-dependent task, such as the Morris water maze test. Male adult Lewis (LEW) rats self-administered cocaine (1 mg/kg/injection) or saline (0.9% NaCl) for 3 weeks. Three hours after finishing the last self-administration session, animals were submitted to Morris water maze training for 3 consecutives days. A memory test was carried out 24 h after the last training session. No significant differences were found in escape latencies and time spent in the quadrant where the platform was located during training. Given that we did not find any cocaine effect on this spatial learning task, our second objective was to estimate indirectly if brain cocaine levels have failed to modulate LTP in animals that were performing the water maze trials. To this end, we tested if cocaine application to hippocampal slices of naïve subjects was able to evoke LTP. The results indicated that cocaine produced an enhanced LTP in these hippocampal slices. Taking together, the results of the present study suggest that hippocampal LTP-like processes generated after cocaine self-administration are not related to spatial learning hippocampal-dependent tasks, such as the water maze test.

Forebrain-specific knockout of B-raf kinase leads to deficits in hippocampal long-term potentiation, learning, and memory

Raf kinases are downstream effectors of Ras and upstream activators of the MEK-ERK cascade. Ras and MEK-ERK signaling play roles in learning and memory (L&M) and neural plasticity, but the roles of Raf kinases in L&M and plasticity are unclear. Among Raf isoforms, B-raf is preferentially expressed in the brain. To determine whether B-raf has a role in synaptic plasticity and L&M, we used the Cre-LoxP gene targeting system to derive forebrain excitatory neuron B-raf knockout mice. This conditional knockout resulted in deficits in ERK activation and hippocampal long-term potentiation (LTP) and impairments in hippocampus-dependent L&M, including spatial learning and contextual discrimination. Despite the widespread expression of B-raf, this mutation did not disrupt other forms of L&M, such as cued fear conditioning and conditioned taste aversion. Our findings demonstrate that B-raf plays a role in hippocampal ERK activation, synaptic plasticity, and L&M.

A gene for neuronal plasticity in the mammalian brain: Zif268/Egr-1/NGFI-A/Krox-24/TIS8/ZENK?

Zif268 is a transcription regulatory protein, the product of an immediate early gene. Zif268 was originally described as inducible in cell cultures; however, it was later shown to be activated by a variety of stimuli, including ongoing synaptic activity in the adult brain. Recently, mice with experimentally mutated zif268 gene have been obtained and employed in neurobiological research. In this review we present a critical overview of Zif268 expression patterns in the naive brain and following neuronal stimulation as well as functional data with Zif268 mutants. In conclusion, we suggest that Zif268 expression and function should be considered in a context of neuronal activity that is tightly linked to neuronal plasticity.

Acute amygdalar activation induces an upregulation of multiple monoamine G protein coupled pathways in rat hippocampus

A "partial" rodent model for schizophrenia has been used to characterize the regulation of hippocampal genes in response to amygdalar activation. At 96 h after the administration of picrotoxin into the basolateral nucleus, we have observed an increase in the expression of genes associated with 18 different monoamine (ie adrenergic alpha 1, alpha 2 and beta 2, serotonergic 5HT5b and 5HT6, dopamine D4 and muscarinic m1, m2 and m3) and peptide (CCK A and B, angiotensin 1A, mu and kappa opiate, FSH, TSH, LH, GNRH, and neuropeptide Y) G-protein coupled receptors (GPCRs). These latter receptors are associated with three different G protein signaling pathways (Gq, Gs, and Gi) in which significant changes in gene expression were also noted for adenylate cyclase (AC4), phosphodiesterase (PDE4D), protein kinase A (PKA), and protein kinase C (PKC). Quantitative RT-PCR was used to validate the results and demonstrated that there were predictable increases of three GPCRs selected for this analysis, including the dopamine D4, alpha 1b, and CCK-B receptors. Eight out of the nine monoamine receptors showing these changes have moderate to high affinity for the atypical antipsychotic, clozapine. Taken together, these results suggest that amygdalar activation may play a role in the pathophysiology and treatment of psychosis by regulating the activity of multiple GPCR and metabolic pathways in hippocampal cells.

Gene expression during memory formation

For several decades, neuroscientists have provided many clues that point out the involvement of de novo gene expression during the formation of long-lasting forms of memory. However, information regarding the transcriptional response networks involved in memory formation has been scarce and fragmented. With the advent of genome-based technologies, combined with more classical approaches (i.e., pharmacology and biochemistry), it is now feasible to address those relevant questions--which gene products are modulated, and when that processes are necessary for the proper storage of memories--with unprecedented resolution and scale. Using one-trial inhibitory (passive) avoidance training of rats, one of the most studied tasks so far, we found two time windows of sensitivity to transcriptional and translational inhibitors infused into the hippocampus: around the time of training and 3-6 h after training. Remarkably, these periods perfectly overlap with the involvement of hippocampal cAMP/PKA (protein kinase A) signaling pathways in memory consolidation. Given the complexity of transcriptional responses in the brain, particularly those related to processing of behavioral information, it was clearly necessary to address this issue with a multi-variable, parallel-oriented approach. We used cDNA arrays to screen for candidate inhibitory avoidance learning-related genes and analyze the dynamic pattern of gene expression that emerges during memory consolidation. These include genes involved in intracellular kinase networks, synaptic function, DNA-binding and chromatin modification, transcriptional activation and repression, translation, membrane receptors, and oncogenes, among others. Our findings suggest that differential and orchestrated hippocampal gene expression is necessary in both early and late periods of long-term memory consolidation. Additionally, this kind of studies may lead to the identification and characterization of genes that are relevant for the pathogenesis of complex psychiatric disorders involving learning and memory impairments, and may allow the development of new methods for the diagnosis and treatment of these diseases.

How necessary is the activation of the immediate early gene zif268 in synaptic plasticity and learning?

The immediate early genes (IEGs) are activated rapidly and transiently in response to a multitude of stimuli. The zif268 belongs to a category of regulatory IEGs that activate downstream target genes and is considered to be a triggering mechanism to activate the genomic response in neurons. Several studies have shown that zif268 mRNA is upregulated during different forms of associative learning, and following tetanic stimulation that induces long-lasting LTP. To date, there is a general consensus that zif268 activation may constitute a critical mechanism for the encoding of long-lasting memories, however this is based on relatively few studies. Given the fact that zif268 can be activated by a number of different types of stimuli, it becomes important to determine exactly how it may be implicated in memory. Examination of the current literature suggests that zif268 is necessary in the processing of several types of memory, however, it is not entirely clear what aspects of memory zif268 may be implicated in. Here, we review the existing literature and emphasise that understanding the signalling pathways that lead to activation of the IEGs and the downstream targets of these genes will advance our understanding of how functional activation of zif268 may be implicated in processing long-term memories.

Expression of the specific type IV phosphodiesterase gene PDE4B3 during different phases of long-term potentiation in single hippocampal slices of rats in vitro

Hippocampal long-term potentiation (LTP), the most prominent cellular model for learning and memory formation, consists of phases: early-LTP (<4 h) and late-LTP (>4 h), with the latter dependent upon protein translation and transcription. To explore the molecular processes that might be specifically regulated during late-LTP, we have modified standard electrophysiological and molecular biological methods, which allowed the cloning of activated genes and their products from single hippocampal slices in vitro 8 h after LTP induction. From one such screen we identified a specific type IV phosphodiesterase gene, PDE4B3, the first cAMP-specific phosphodiesterase to be associated with LTP. Previous studies documented an integral role for the cAMP-PKA system in late-LTP and recently, inhibition of cAMP degradation facilitates LTP and ameliorates mnemonic deficits. We now report that PDE4B3 is modulated during LTP phases. Its activation is NMDA-receptor dependent and its transcription is transiently up-regulated 2 h after tetanization. Protein expression peaks 6 h after LTP induction and is rapidly down-regulated at 8 h, whereas cAMP levels decrease during LTP phases. Immunohistochemical studies identified that the majority of type IV phosphodiesterase protein staining is localized to the cell bodies and dendrites of neurones in hippocampal CA1.

Unpacking the cognitive map: the parallel map theory of hippocampal function

In the parallel map theory, the hippocampus encodes space with 2 mapping systems. The bearing map is constructed primarily in the dentate gyrus from directional cues such as stimulus gradients. The sketch map is constructed within the hippocampus proper from positional cues. The integrated map emerges when data from the bearing and sketch maps are combined. Because the component maps work in parallel, the impairment of one can reveal residual learning by the other. Such parallel function may explain paradoxes of spatial learning, such as learning after partial hippocampal lesions, taxonomic and sex differences in spatial learning, and the function of hippocampal neurogenesis. By integrating evidence from physiology to phylogeny, the parallel map theory offers a unified explanation for hippocampal function.

Staring, a novel E3 ubiquitin-protein ligase that targets syntaxin 1 for degradation

Syntaxin 1 is an essential component of the neurotransmitter release machinery, and regulation of syntaxin 1 expression levels is thought to contribute to the mechanism underlying learning and memory. However, the molecular events that control the degradation of syntaxin 1 remain undefined. Here we report the identification and characterization of a novel RING finger protein, Staring, that interacts with syntaxin 1. Staring is expressed throughout the brain, where it exists in both cytosolic and membrane-associated pools. Staring binds and recruits the brain-enriched E2 ubiquitin-conjugating enzyme UbcH8 to syntaxin 1 and facilitates the ubiquitination and proteasome-dependent degradation of syntaxin 1. These findings suggest that Staring is a novel E3 ubiquitin-protein ligase that targets syntaxin 1 for degradation by the ubiquitin-proteasome pathway.

Applications of the Morris water maze in the study of learning and memory

The Morris water maze (MWM) was described 20 years ago as a device to investigate spatial learning and memory in laboratory rats. In the meanwhile, it has become one of the most frequently used laboratory tools in behavioral neuroscience. Many methodological variations of the MWM task have been and are being used by research groups in many different applications. However, researchers have become increasingly aware that MWM performance is influenced by factors such as apparatus or training procedure as well as by the characteristics of the experimental animals (sex, species/strain, age, nutritional state, exposure to stress or infection). Lesions in distinct brain regions like hippocampus, striatum, basal forebrain, cerebellum and cerebral cortex were shown to impair MWM performance, but disconnecting rather than destroying brain regions relevant for spatial learning may impair MWM performance as well. Spatial learning in general and MWM performance in particular appear to depend upon the coordinated action of different brain regions and neurotransmitter systems constituting a functionally integrated neural network. Finally, the MWM task has often been used in the validation of rodent models for neurocognitive disorders and the evaluation of possible neurocognitive treatments. Through its many applications, MWM testing gained a position at the very core of contemporary neuroscience research.

Cellular imaging of zif268 expression in the hippocampus and amygdala during contextual and cued fear memory retrieval: selective activation of hippocampal CA1 neurons during the recall of contextual memories

The neuroanatomical and molecular basis of fear memory retrieval was studied by analyzing the expression of the plasticity-associated immediate early gene zif268. Cellular quantitative in situ hybridization revealed that zif268 is expressed within specific regions of the hippocampus and amygdala during fear memory retrieval. Within the hippocampus, increased expression of zif268 was observed within CA1 neurons, but not dentate gyrus neurons, during the retrieval of contextual, but not cued, fear associations. In contrast, zif268 expression was increased within neurons of the amygdala (lateral, basal, and central nuclei) during the retrieval of both contextual and cued fear memories. These results demonstrate activation of hippocampal CA1 neurons in contextual fear memory retrieval that was not merely a correlate of the behavioral expression of fear itself, because it was limited to the retrieval of contextual, and not cued, fear memories. Further studies revealed that the selective increase in hippocampal CA1 zif268 expression seen after contextual fear memory retrieval was limited to the retrieval of recent (24 hr) but not older (28 d) memories. These experiments represent the first demonstration that zif268 expression in specific neuronal populations is associated with memory retrieval and suggest that this gene may contribute to plasticity and reconsolidation accompanying the retrieval process.

Modulation of hippocampal long-term potentiation by the amygdala: a synaptic mechanism linking emotion and memory

Why are emotionally arousing experiences well-remembered? Since the amygdala and hippocampus play pivotal roles in emotion and memory, respectively, the interaction between these brain regions may underlie the formation of enhanced memory for emotionally arousing events. Behavioral experiments using animals have demonstrated that lesions of the amygdaloid nuclei or infusions of drugs into the amygdaloid nuclei impair or enhance hippocampal-dependent learning. In addition, we have obtained direct evidence that neural inputs from the amygdala modulate synaptic plasticity in the hippocampus, through electrophysiological experiments using anesthetized rats. Electrical stimulation of the basolateral amygdala evoked synaptic potentials in the dentate gyrus of the hippocampus, indicating that there is a neural connection from the amygdala to the hippocampus. Lesion of the basolateral or basomedial, but not central, amygdala resulted in attenuation of long-term potentiation (LTP) at the perforant path-dentate gyrus granule cell synapses. High-frequency stimulation of the basolateral or basomedial amygdala alone did not induce LTP in the dentate gyrus, but facilitated the induction of LTP when applied at the same time as tetanic stimulation of the perforant path. The activity-dependent facilitation of hippocampal LTP by the basomedial and basolateral amygdala may be a synaptic mechanism underlying memory enhancement associated with emotions.

Effects of saffron extract and its constituent crocin on learning behaviour and long-term potentiation

Crocus sativus L., commonly known as saffron, is used in folk medicine for various purposes. Modern pharmacological studies have demonstrated that saffron extracts have antitumour effects, radical scavenger properties or hypolipaemic effects. Among the constituents of saffron extract, crocetin is mainly responsible for these pharmacological activities. In addition, recent behavioural and electrophysiological studies have demonstrated that saffron extract affects learning and memory in experimental animals. Saffron extract improved ethanol-induced impairments of learning behaviours in mice, and prevented ethanol-induced inhibition of hippocampal long-term potentiation, a form of activity-dependent synaptic plasticity that may underly learning and memory. This effect of saffron extract is attributed to crocin (crocetin di-gentiobiose ester), but not crocetin. Saffron extract or its active constituents, crocetin and crocin, could be useful as a treatment for neurodegenerative disorders accompanying memory impairment.