Olfactory learning-induced increase in spine density along the apical dendrites of CA1 hippocampal neurons

Impairment in novelty-promoted memory via behavioral tagging and capture before apparent memory loss in a knock-in model of Alzheimer's disease

Alzheimer's disease (AD) is associated with cognitive impairments and age-dependent memory deficits which have been studied using genetic models of AD. Whether the processes for modulating memory persistence are more vulnerable to the influence of amyloid pathology than the encoding and consolidation of the memory remains unclear. Here, we investigated whether early amyloid pathology would affect peri-learning novelty in promoting memory, through a process called behavioral tagging and capture (BTC). App^{NL-G-F/NL-G-F} mice and wild-type littermates were trained in an appetitive delayed matching-to-place (ADMP) task which allows for the assessment of peri-learning novelty in facilitating memory. The results show that novelty enabled intermediate-term memory in wild-type mice, but not in App^{NL-G-F/NL-G-F} mice in adulthood. This effect preceded spatial memory impairment in the ADMP task seen in middle age. Other memory tests in the Barnes maze, Y-maze, novel object or location recognition tasks remained intact. Together, memory modulation through BTC is impaired before apparent deficits in learning and memory. Relevant biological mechanisms underlying BTC and the implication in AD are discussed.

Prolonged and extended impacts of SARS-CoV-2 on the olfactory neurocircuit

The impact of SARS-CoV-2 on the olfactory pathway was studied over several time points using Syrian golden hamsters. We found an incomplete recovery of the olfactory sensory neurons, prolonged activation of glial cells in the olfactory bulb, and a decrease in the density of dendritic spines within the hippocampus. These data may be useful for elucidating the mechanism underlying long-lasting olfactory dysfunction and cognitive impairment as a post-acute COVID-19 syndrome.

A potential biomarker of preclinical Alzheimer's disease: The olfactory dysfunction and its pathogenesis-based neural circuitry impairments

The olfactory dysfunction can signal and act as a potential biomarker of preclinical AD. However, the precise regulatory mechanism of olfactory function on the neural pathogenesis of AD is still unclear. The impairment of neural networks in olfaction system has been shown to be tightly associated with AD. As key brain regions of the olfactory system, the olfactory bulb (OB) and the piriform cortex (PCx) have a profound influence on the olfactory function. Therefore, this review will explore the mechanism of olfactory dysfunction in preclinical AD in the perspective of abnormal neural networks in the OB and PCx and their associated brain regions, especially from two aspects of aberrant oscillations and synaptic plasticity damages, which help better understand the underlying mechanism of olfactory neural network damages related to AD.

Activation of 5-HT1A postsynaptic receptors by NLX-101 results in functional recovery and an increase in neuroplasticity in mice with brain ischemia

Pharmacological interventions that selectively activate serotonin 5-hydroxytryptramine-1A (5-HT_1A) heteroreceptors may prevent or attenuate the consequences of brain ischemic episodes. The present study investigated whether the preferential 5-HT_1A postsynaptic receptor agonist NLX-101 (a.k.a. F15599) mitigates cognitive and emotional impairments and affects neuroplasticity in mice that are subjected to the bilateral common carotid artery occlusion (BCCAO) model of brain ischemia. The selective serotonin reuptake inhibitor escitalopram (Esc) was used for comparative purposes because it is able to decrease morbidity and improve recovery in stroke patients and ischemic rodents. Sham and BCCAO mice received daily doses of NLX-101 (0.32 mg/kg, i.p) or Esc (20 mg/kg, i.p) for 28 days. During this period, they were evaluated for locomotor activity, anxiety- and despair-related behaviors and hippocampus-dependent cognitive function, using the open field, elevated zero maze, forced swim test and object location test, respectivelly. The mice's brains were processed for biochemical and histological analyses. BCCAO mice exhibited high anxiety and despair-like behaviors and performed worse than controls in the cognitive assessment. BCCAO induced neuronal and dendritic spine loss and decreases in the protein levels of neuronal plasticity markers, including brain-derived neurotrophic factor (BDNF), synaptophysin (SYN), and postsynaptic density protein-95 (PSD-95), in prefrontal cortex (PFC) and hippocampus. NLX-101 and Esc attenuated cognitive impairments and despair-like behaviors in BCCAO mice. Only Esc decreased anxiety-like behaviors due to brain ischemia. Both NLX-101 and Esc blocked the increase in plasma corticosterone levels and, restored BDNF, SYN and PSD-95 protein levels in the hippocampus. Moreover, both compounds impacted positively dentritic remodeling in the hippocampus and PFC of ischemic mice. In the PFC, NLX-101 increased the BDNF protein levels, while Esc in turn, attenuated the decrease in the PSD-95 protein levels induced by BCCAO. The present results suggest that activation of post-synaptic 5-HT_1A receptors is the molecular mechanism for serotonergic protective effects in BCCAO. Moreover, post-synaptic biased agonists such as NLX-101 might constitute promising therapeutics for treatment of functional and neurodegenerative outcomes of brain ischemia.

Genetic Perturbation of TIA1 Reveals a Physiological Role in Fear Memory

TIA1 is a prion-related RNA-binding protein whose capacity to form various types of intracellular aggregates has been implicated in neurodegenerative disease. However, its role in normal brain function is poorly understood. Here, we show that TIA1 bidirectionally modulates stress-dependent synaptic plasticity in the hippocampus, a brain region involved in fear memory and olfactory discrimination learning. At the behavioral level, conditioned odor avoidance is potentiated by TIA1 deletion, whereas overexpression of TIA1 in the ventral hippocampus inhibits both contextual fear memory and avoidance. However, the latter genetic manipulations have little impact on other hippocampus-dependent tasks. Transcriptional profiling indicates that TIA1 presides over a large network of immune system genes with modulatory roles in synaptic plasticity and long-term memory. Our results uncover a physiological and partly sex-dependent function for TIA1 in fear memory and may provide molecular insight into stress-related psychiatric conditions, such as post-traumatic stress disorder (PTSD) and anxiety.

Hibernation Impairs Odor Discrimination - Implications for Alzheimer's Disease

Reversible formation of PHF-like phosphorylated tau, an early feature of Alzheimer's disease (AD) was previously shown to occur in torpor during hibernation in the Golden hamster (Syrian hamster, Mesocricetus auratus). Here, we tackled the question to what extent hibernating Golden hamsters can serve as a model for the early stage of AD. During early AD, anosmia, the loss of olfactory function, is a common and typical feature. We, thus, investigated tau phosphorylation, synaptic plasticity and behavioral physiology of the olfactory system during hibernation. Tau was phosphorylated on several AD-relevant epitopes, and distribution of PHF-like phosphorylated tau in the olfactory bulb was quite similar to what is seen in AD. Tau phosphorylation was not associated with a destabilization of microtubules and did not lead to fibril formation. Previously, we observed a transient spine reduction in pyramidal cells in the hippocampus, which is correlated with the distribution of phosphorylated tau. Here we show that granule cells in the olfactory bulb are devoid of phosphorylated tau and maintain their spines number during torpor. No reduction of synaptic proteins was observed. However, hibernation did impair the recall performance in a two-odor discrimination task. We conclude that hibernation is associated with a specific olfactory memory deficit, which might not be attributed to the formation of PHF-like phosphorylated tau within the olfactory bulb. We discuss a possible involvement of modulatory input provided by cholinergic neurons in the basal forebrain, which are affected by hibernation.

Dendritic spine density and EphrinB2 levels of hippocampal and anterior cingulate cortex neurons increase sequentially during formation of recent and remote fear memory in the mouse

Memory consolidation is a dynamic process that involves a sequential remodeling of hippocampal-cortical circuits. Although synaptic events underlying memory consolidation are well assessed, fine molecular events controlling this process deserve further characterization. To this aim, we challenged male C57BL/6N mice in a contextual fear conditioning (CFC) paradigm and tested their memory 24 h, 7 days or 36 days later. Mice displayed a strong fear response at all time points with an increase in dendritic spine density and protein levels of the cell adhesion factor EphrinB2 in CA1 hippocampal neurons 24 h and 7 days post conditioning (p.c.), and in anterior cingulate cortex (ACC) neurons 36 days p.c. We then investigated whether the formation of remote memory and neuronal modifications in the ACC would depend on p.c. protein synthesis in hippocampal neurons. Bilateral intrahippocampal infusions with the protein synthesis inhibitor anisomycin administered immediately p.c. decreased fear response, neuronal spine growth and EphrinB2 protein levels of hippocampal and ACC neurons 24 h and 36 days p.c., respectively. Anisomycin infusion 24 h p.c. had no effects on fear response, increase in spine density and in EphrinB2 protein levels in ACC neurons 36 days p.c. Our results thus confirm that early but not late p.c. hippocampal protein synthesis is necessary for the formation of remote memory and provide the first evidence of a possible involvement of EphrinB2 in neuronal plasticity in the ACC.

Electroacupuncture Regulates Hippocampal Synaptic Plasticity via miR-134-Mediated LIMK1 Function in Rats with Ischemic Stroke

MircoRNAs (miRs) have been implicated in learning and memory, by regulating LIM domain kinase (LIMK1) to induce synaptic-dendritic plasticity. The study aimed to investigate whether miRNAs/LIMK1 signaling was involved in electroacupuncture- (EA-) mediated synaptic-dendritic plasticity in a rat model of middle cerebral artery occlusion induced cognitive deficit (MICD). Compared to untreatment or non-acupoint-EA treatment, EA at DU20 and DU24 acupoints could shorten escape latency and increase the frequency of crossing platform in Morris water maze test. T2-weighted imaging showed that the MICD rat brain lesions were located in cortex, hippocampus, corpus striatum, and thalamus regions and injured volumes were reduced after EA. Furthermore, we found that the density of dendritic spine and the number of synapses in the hippocampal CA1 pyramidal cells were obviously reduced at Day 14 after MICD. However, synaptic-dendritic loss could be rescued after EA. Moreover, the synaptic-dendritic plasticity was associated with increases of the total LIMK1 and phospho-LIMK1 levels in hippocampal CA1 region, wherein EA decreased the expression of miR-134, negatively regulating LIMK1 to enhance synaptic-dendritic plasticity. Therefore, miR-134-mediated LIMK1 was involved in EA-induced hippocampal synaptic plasticity, which served as a contributor to improving learning and memory during the recovery stage of ischemic stroke.

High frequency stimulation of the infralimbic cortex induces morphological changes in rat hippocampal neurons

BACKGROUND

Although a significant subset of patients with major depressive disorder (MDD) fail to respond to medical or behavioural therapy, deep brain stimulation (DBS) applied to the subgenual cingulate cortex (SCC; sg25) has been shown to reduce depressive symptoms in a subset of patients. This area receives projections from neurons in the CA1 region and subiculum of the hippocampus (HC), a brain region implicated in the pathobiology and treatment of MDD.

OBJECTIVE

To assess whether high frequency stimulation (HFS) of the infralimbic cortex is associated with changes in cellular morphology in the HC.

METHODS

Rats were subjected to either infralimbic HFS or sham-stimulation. Measures of cellular morphology, including dendritic length and complexity, were assessed in pyramidal neurons in the CA1 region of the HC by means of the Golgi-Cox histological stain.

RESULTS

Dendritic length (p = 0.013) and number of branch points (p = 0.004) were significantly increased across the entire dendritic tree in animals subjected to HFS. Subsequent Scholl analysis revealed that for dendritic length these effects were localized to the region between 80 and 160 μm from the soma (p < 0.001 for either 40 μm interval) in the basal dendritic tree, while branch point number was predominantly increased between 120 and 160 μm from the soma (p < 0.001) in the apical dendritic tree.

CONCLUSIONS

High-frequency stimulation of the infralimbic cortex increases the complexity of apical dendrites and the length of basal dendritic trees of pyramidal neurons located in the CA1 hippocampal subfield relative to sham-stimulated animals.

Phthalates and neurotoxic effects on hippocampal network plasticity

Phthalates are synthetically derived chemicals used as plasticizers in a variety of common household products. They are not chemically bound to plastic polymers and over time, easily migrate out of these products and into the environment. Experimental investigations evaluating the biological impact of phthalate exposure on developing organisms are critical given that estimates of phthalate exposure are considerably higher in infants and children compared to adults. Extensive growth and re-organization of neurocircuitry occurs during development leaving the brain highly susceptible to environmental insults. This review summarizes the effects of phthalate exposure on brain structure and function with particular emphasis on developmental aspects of hippocampal structural and functional plasticity. In general, it appears that widespread disruptions in hippocampal functional and structural plasticity occur following developmental (pre-, peri- and post-natal) exposure to phthalates. Whether these changes occur as a direct neurotoxic effect of phthalates or an indirect effect through disruption of endogenous endocrine functions is not fully understood. Comprehensive investigations that simultaneously assess the neurodevelopmental, neurotoxic, neuroendocrine and behavioral correlates of phthalate exposure are needed to provide an opportunity to thoroughly evaluate the neurotoxic potential of phthalates throughout the lifespan.

Contribution of Ca2+ release channels to hippocampal synaptic plasticity and spatial memory: potential redox modulation

SIGNIFICANCE

Memory is an essential human cognitive function. Consequently, to unravel the cellular and molecular mechanisms responsible for the synaptic plasticity events underlying memory formation, storage and loss represents a major challenge of present-day neuroscience.

RECENT ADVANCES

This review article first describes the wide-ranging functions played by intracellular Ca2+ signals in the activity-dependent synaptic plasticity processes underlying hippocampal spatial memory, and next, it focuses on how the endoplasmic reticulum Ca2+ release channels, the ryanodine receptors, and the inositol 1,4,5-trisphosphate receptors contribute to these processes. We present a detailed examination of recent evidence supporting the key role played by Ca2+ release channels in synaptic plasticity, including structural plasticity, and the formation/consolidation of spatial memory in the hippocampus.

CRITICAL ISSUES

Changes in cellular oxidative state particularly affect the function of Ca2+ release channels and alter hippocampal synaptic plasticity and the associated memory processes. Emphasis is placed in this review on how defective Ca2+ release, presumably due to increased levels of reactive oxygen species, may cause the hippocampal functional defects that are associated to aging and Alzheimer's disease (AD).

FUTURE DIRECTIONS

Additional studies should examine the precise molecular mechanisms by which Ca2+ release channels contribute to hippocampal synaptic plasticity and spatial memory formation/consolidation. Future studies should test whether redox-modified Ca2+ release channels contribute toward generating the intracellular Ca2+ signals required for sustained synaptic plasticity and hippocampal spatial memory, and whether loss of redox balance and oxidative stress, by altering Ca2+ release channel function, presumably contribute to the abnormal memory processes that occur during aging and AD.

Anterior thalamic lesions reduce spine density in both hippocampal CA1 and retrosplenial cortex, but enrichment rescues CA1 spines only

Injury to the anterior thalamic nuclei (ATN) may affect both hippocampus and retrosplenial cortex thus explaining some parallels between diencephalic and medial temporal lobe amnesias. We found that standard-housed rats with ATN lesions, compared with standard-housed controls, showed reduced spine density in hippocampal CA1 neurons (basal dendrites, -11.2%; apical dendrites, -9.6%) and in retrospenial granular b cortex (Rgb) neurons (apical dendrites, -20.1%) together with spatial memory deficits on cross maze and radial-arm maze tasks. Additional rats with ATN lesions were also shown to display a severe deficit on spatial working memory in the cross-maze, but subsequent enriched housing ameliorated their performance on both this task and the radial-arm maze. These enriched rats with ATN lesions also showed recovery of both basal and apical CA1 spine density to levels comparable to that of the standard-housed controls, but no recovery of Rgb spine density. Inspection of spine types in the CA1 neurons showed that ATN lesions reduced the density of thin spines and mushroom spines, but not stubby spines; while enrichment promoted recovery of thin spines. Comparison with enriched rats that received pseudo-training, which provided comparable task-related experience, but no explicit spatial memory training, suggested that basal CA1 spine density in particular was associated with spatial learning and memory performance. Distal pathology in terms of reduced integrity of hippocampal and retrosplenial microstructure provides clear support for the influence of the ATN lesions on the extended hippocampal system. The reversal by postoperative enrichment of this deficit in the hippocampus but not the retrosplenial cortex may indicate region-specific mechanisms of recovery after ATN injury.

Sleep and olfactory cortical plasticity

In many systems, sleep plays a vital role in memory consolidation and synaptic homeostasis. These processes together help store information of biological significance and reset synaptic circuits to facilitate acquisition of information in the future. In this review, we describe recent evidence of sleep-dependent changes in olfactory system structure and function which contribute to odor memory and perception. During slow-wave sleep, the piriform cortex becomes hypo-responsive to odor stimulation and instead displays sharp-wave activity similar to that observed within the hippocampal formation. Furthermore, the functional connectivity between the piriform cortex and other cortical and limbic regions is enhanced during slow-wave sleep compared to waking. This combination of conditions may allow odor memory consolidation to occur during a state of reduced external interference and facilitate association of odor memories with stored hedonic and contextual cues. Evidence consistent with sleep-dependent odor replay within olfactory cortical circuits is presented. These data suggest that both the strength and precision of odor memories is sleep-dependent. The work further emphasizes the critical role of synaptic plasticity and memory in not only odor memory but also basic odor perception. The work also suggests a possible link between sleep disturbances that are frequently co-morbid with a wide range of pathologies including Alzheimer's disease, schizophrenia and depression and the known olfactory impairments associated with those disorders.

Adult neurogenesis and dendritic remodeling in hippocampal plasticity: which one is more important?

Accumulating knowledge has shown that a decrease in hippocampal neurogenesis is linked to the pathophysiology of mood disorders and some hippocampal-dependent learning and memory tasks. The role of adult neurogenesis has initially been proposed based on correlations between decreases or increases in neurogenesis and impairments or improvements, respectively, in animal behaviors following interventions. Its role has been further elucidated through the ablation of neurogenesis. However, the functional roles of neurogenesis in hippocampal-dependent behaviors have been challenged by inconsistent findings between different studies. Despite the fact that factors affecting neurogenesis also induce dendritic or synaptic changes in newborn or existing neurons, these two aspects of structural changes within the hippocampus have always been examined separately. Thus, it is difficult to interpret the functional role of adult neurogenesis or dendritic remodification in hippocampal-dependent behaviors. This review discusses the relative contribution of adult neurogenesis and dendritic/synaptic remodeling of existing neurons to hippocampal plasticity.

Fear learning increases the number of polyribosomes associated with excitatory and inhibitory synapses in the barrel cortex

Associative fear learning, resulting from whisker stimulation paired with application of a mild electric shock to the tail in a classical conditioning paradigm, changes the motor behavior of mice and modifies the cortical functional representation of sensory receptors involved in the conditioning. It also induces the formation of new inhibitory synapses on double-synapse spines of the cognate barrel hollows. We studied density and distribution of polyribosomes, the putative structural markers of enhanced synaptic activation, following conditioning. By analyzing serial sections of the barrel cortex by electron microscopy and stereology, we found that the density of polyribosomes was significantly increased in dendrites of the barrel activated during conditioning. The results revealed fear learning-induced increase in the density of polyribosomes associated with both excitatory and inhibitory synapses located on dendritic spines (in both single- and double-synapse spines) and only with the inhibitory synapses located on dendritic shafts. This effect was accompanied by a significant increase in the postsynaptic density area of the excitatory synapses on single-synapse spines and of the inhibitory synapses on double-synapse spines containing polyribosomes. The present results show that associative fear learning not only induces inhibitory synaptogenesis, as demonstrated in the previous studies, but also stimulates local protein synthesis and produces modifications of the synapses that indicate their potentiation.

Behavioral effects of Rho GTPase modulation in a model of Alzheimer's disease

Small GTPases of the Rho family, including Rho, Rac and CDC42 subfamilies, play key role in neural connectivity and cognition. The pharmacological modulation of these regulatory proteins is associated with enhancement of learning and memory. We sought to determine whether the modulation of cerebral Rho GTPases may correct behavioral disturbances in a mouse model of Alzheimer's disease (AD). TgCRND8 mice show early-onset Abeta amyloid deposits associated with deficits in several cognitive tasks. We report that four-month old TgCRND8 mice display (a) increased locomotor activity in an open field, (b) mild deficits in the learning of a fixed platform position in a water maze task. More markedly, after displacement of the escape platform, TgCRND8 mice exhibit impairment in the learning of the novel position (reversal learning), as they perseverate searching in the familiar position. The administration of the Rho GTPase activator Cytotoxic Necrotizing Factor 1 (CNF1, 1.0 fmol kg(-1) intracerebroventricularly) reduces locomotor hyperactivity and corrects the deficits in reversal learning, thus re-establishing normal behavioral plasticity. We conclude that the pharmacological modulation of Rho GTPase signaling might be beneficial for the treatment of AD. Reversal learning in TgCRND8 mice may represent a convenient pre-clinical assay for the efficacy of therapeutic interventions in AD.

CREB selectively controls learning-induced structural remodeling of neurons

The modulation of synaptic strength associated with learning is post-synaptically regulated by changes in density and shape of dendritic spines. The transcription factor CREB (cAMP response element binding protein) is required for memory formation and in vitro dendritic spine rearrangements, but its role in learning-induced remodeling of neurons remains elusive. Using transgenic mice conditionally expressing a dominant-negative CREB (CREBS133A: mCREB) mutant, we found that inhibiting CREB function does not alter spine density, spine morphology, and levels of polymerized actin in naive CA1 neurons. CREB inhibition, however, impaired contextual fear conditioning and produced a learning-induced collapse of spines associated with a blockade of learning-dependent increase in actin polymerization. Blocking mCREB expression with doxycycline rescued memory and restored a normal pattern of learning-induced spines, demonstrating that CREB controls structural adaptations of neurons selectively involved in memory formation.

The role of metaplasticity mechanisms in regulating memory destabilization and reconsolidation

Memory allows organisms to predict future events based on prior experiences. This requires encoded information to persist once important predictors are extracted, while also being modifiable in response to changes within the environment. Memory reconsolidation may allow stored information to be modified in response to related experience. However, there are many boundary conditions beyond which reconsolidation may not occur. One interpretation of these findings is that the event triggering memory retrieval must contain new information about a familiar stimulus in order to induce reconsolidation. Presently, the mechanisms that affect the likelihood of reconsolidation occurring under these conditions are not well understood. Here we speculate on a number of systems that may play a role in protecting memory from being destabilized during retrieval. We conclude that few memories may enter a state in which they cannot be modified. Rather, metaplasticity mechanisms may serve to alter the specific reactivation cues necessary to destabilize a memory. This might imply that destabilization mechanisms can differ depending on learning conditions.

Targeting of the Arpc3 actin nucleation factor by miR-29a/b regulates dendritic spine morphology

Previous studies have demonstrated that microribonucleic acids (miRs) are key regulators of protein expression in the brain and modulate dendritic spine morphology and synaptic activity. To identify novel miRs involved in neuronal plasticity, we exposed adult mice to chronic treatments with nicotine, cocaine, or amphetamine, which are psychoactive drugs that induce well-documented neuroadaptations. We observed brain region- and drug-specific changes in miR expression levels and identified miR-29a/b as regulators of synaptic morphology. In vitro imaging experiments indicated that miR-29a/b reduce mushroom-shaped dendritic spines on hippocampal neurons with a concomitant increase in filopodial-like outgrowths, suggesting an effect on synapse formation via actin cytoskeleton remodeling. We identified Arpc3, a component of the ARP2/3 actin nucleation complex, as a bona fide target for down-regulation by miR-29a/b. This work provides evidence that targeting of Arpc3 by miR-29a/b fine tunes structural plasticity by regulating actin network branching in mature and developing spines.

Contextual learning increases dendrite complexity and EphrinB2 levels in hippocampal mouse neurons

Although the role of hippocampus in memory processing is well assessed, an association of experience-dependent behavioural modifications with hippocampal neuron morphological and biochemical changes deserves further characterisation. Here, we present evidence of dendritic alterations together with rapid accumulation of EphrinB2, a factor known to influence cell plasticity, in pyramidal neurons of the CA1 area of mouse hippocampus, during the formation of recent contextual fear memory. Male C57BL/6N mice exhibited a robust fear response 24h after contextual and cued fear conditioning. At this time and in the absence of the memory test, conditioned mice showed morphological alterations in hippocampal and lateral amygdala neurons. Western blot analysis of extracts from conditioned but not pseudoconditioned or naive mice showed a specific increase in the amount of EphrinB2 in the hippocampus but not the cortex. However, levels of EphA4 receptor, known to interact trans-synaptically with EphrinB2, did not change upon conditioning in extracts from the same structures. Finally, immunohistochemical analysis of the hippocampus and amygdala of conditioned mice showed increased levels of EphrinB2 in pyramidal neurons of the CA1 area, when compared to pseudoconditioned and control mice. Such increase was not observed in other hippocampal areas or the amygdala. These results suggest that rapid accumulation of EphrinB2 in hippocampal CA1 neurons is involved in the behavioural and cellular modifications induced by contextual fear conditioning. A similar mechanism does not appear to occur in lateral amygdala neurons, in spite of the robust behavioural and cellular modifications induced in such structure by cued fear conditioning.

Effects of environmental enrichment and voluntary exercise on neurogenesis, learning and memory, and pattern separation: BDNF as a critical variable?

Adult-generated neurons in the dentate gyrus of the hippocampus have been the focus of many studies concerned with learning and memory (L&M). It has been shown that procedures like environmental enrichment (EE) or voluntary physical exercise (Vex) can increase neurogenesis (NG) and also enhance L&M. It is tempting to conclude that improvements in L&M are due to the increased NG; that is, a causal relationship exists between enhancement of NG and enhancement of L&M. However, it remains unclear whether the L&M enhancement observed after these treatments is causally dependent on the increase in newborn neurons in the dentate gyrus. It remains a possibility that some unspecified change--a "third variable"--brought about by EE and/or Vex could be a causal determinant of both NG and L&M. We suggest that this third variable could be neurotrophic and/or plasticity-related factors such as BDNF. Indeed, both EE and Vex can induce expression of such proteins, and BDNF in particular has long been linked with L&M. In addition, we argue that a very likely source of variation in previous experiments was the load on "pattern separation", a process that keeps similar memories distinct, and in which NG has been shown to be critically involved. To attempt to bring these ideas together, we present preliminary evidence that BDNF is also required for pattern separation, which strengthens the case for BDNF as a candidate third variable. Other ways in which BDNF might be involved are also discussed.

Influence of aging and neurodegeneration on dendritic spine morphology

In neuronal circuits, excitatory synaptic transmission predominantly occurs at postsynaptic protrusions called dendritic spines. Spines are highly plastic structures capable of formation, enlargement, shrinkage, and elimination over time. Individual spine morphology is widely variable, and evidence suggests these differences in morphology are relevant to spine function. Recent reports provide evidence that spine structural plasticity underlies functional synaptic changes, including those seen in animal models of learning and memory plasticity. Conversely, impairments in cognitive functions, such as those commonly seen in aging, have recently been linked to and correlated with alterations in spine density and morphology. In addition, dendritic spine density and morphology also appear to be altered in various transgenic animal models of neurodegenerative diseases. Ultimately, an understanding of the synaptic basis of age- and disease-related cognitive impairments may lead to the development of drug treatments that can restore or protect synaptic profiles in neural circuits that mediate cognition.

Role of the hippocampus in goal-oriented tasks requiring retrieval of spatial versus non-spatial information

The role of the hippocampus in non-spatial memory has been issue of some controversy. To investigate the nature of dorsal hippocampus engagement in spatial and non-spatial memory we performed discrete excitotoxic lesions of this region before mice (C57/BL6) were trained in one of two tasks that required the animals to retrieve a hidden food reward. In the visuospatial task animals had to remember a particular spatial location, independent of odor cues. In contrast, in a non-spatial olfactory task animals had to remember a particular odor, independent of spatial location. The mice were trained in one of these tasks over a period of three days. We found that lesions restricted to the dorsal hippocampus affected performance only in the spatial task. In contrast, lesions that also encompassed a larger portion of the ventral hippocampus caused a moderate deficit in the olfactory task. These results are consistent with the role of the dorsal hippocampus in long-term spatial episodic memory, and support the involvement of larger portions of the hippocampus on the encoding of non-spatial olfactory representations.

Structural plasticity and hippocampal function

The hippocampus is a region of the mammalian brain that shows an impressive capacity for structural reorganization. Preexisting neural circuits undergo modifications in dendritic complexity and synapse number, and entirely novel neural connections are formed through the process of neurogenesis. These types of structural change were once thought to be restricted to development. However, it is now generally accepted that the hippocampus remains structurally plastic throughout life. This article reviews structural plasticity in the hippocampus over the lifespan, including how it is investigated experimentally. The modulation of structural plasticity by various experiential factors as well as the possible role it may have in hippocampal functions such as learning and memory, anxiety, and stress regulation are also considered. Although significant progress has been made in many of these areas, we highlight some of the outstanding issues that remain.

A simple role for BDNF in learning and memory?

Since its discovery almost three decades ago, the secreted neurotrophin brain-derived neurotrophic factor (BDNF) has been firmly implicated in the differentiation and survival of neurons of the CNS. More recently, BDNF has also emerged as an important regulator of synaptogenesis and synaptic plasticity mechanisms underlying learning and memory in the adult CNS. In this review we will discuss our knowledge about the multiple intracellular signalling pathways activated by BDNF, and the role of this neurotrophin in long-term synaptic plasticity and memory formation as well as in synaptogenesis. We will show that maturation of BDNF, its cellular localization and its ability to regulate both excitatory and inhibitory synapses in the CNS may result in conflicting alterations in synaptic plasticity and memory formation. Lack of a precise knowledge about the mechanisms by which BDNF influences higher cognitive functions and complex behaviours may constitute a severe limitation in the possibility to devise BDNF-based therapeutics for human disorders of the CNS.

The formation of recent and remote memory is associated with time-dependent formation of dendritic spines in the hippocampus and anterior cingulate cortex

Although hippocampal-cortical interactions are crucial for the formation of enduring declarative memories, synaptic events that govern long-term memory storage remain mostly unclear. We present evidence that neuronal structural changes, i.e., dendritic spine growth, develop sequentially in the hippocampus and anterior cingulate cortex (aCC) during the formation of recent and remote contextual fear memory. We found that mice placed in a conditioning chamber for one 7 min conditioning session and exposed to five footshocks (duration, 2 s; intensity, 0.7 mA; interstimulus interval, 60 s) delivered through the grid floor exhibited robust fear response when returned to the experimental context 24 h or 36 d after the conditioning. We then observed that their fear response at the recent, but not the remote, time point was associated with an increase in spine density on hippocampal neurons, whereas an inverse temporal pattern of spine density changes occurred on aCC neurons. At each time point, hippocampal or aCC structural alterations were achieved even in the absence of recent or remote memory tests, thus suggesting that they were not driven by retrieval processes. Furthermore, ibotenic lesions of the hippocampus impaired remote memory and prevented dendritic spine growth on aCC neurons when they were performed immediately after the conditioning, whereas they were ineffective when performed 24 d later. These findings reveal that gradual structural changes modifying connectivity in hippocampal-cortical networks underlie the formation and expression of remote memory, and that the hippocampus plays a crucial but time-limited role in driving structural plasticity in the cortex.

Stabilizing dendritic structure as a novel therapeutic approach for epilepsy

People with epilepsy often experience long-term cognitive dysfunction and other neurological deficits, including memory loss, learning disabilities and neurobehavioral disorders, which may exhibit a progressive course correlating with worsening seizure control. Furthermore, a third of epilepsy patients have seizures that are intractable to all available treatments. Thus, novel therapies for seizures and the neurological comorbidities of epilepsy are desperately needed. As most current treatments are merely symptomatic therapies that suppress seizures, epilepsy researchers have recently realized the critical need for novel therapeutic strategies targeting the underlying mechanisms of epileptogenesis and seizure-related brain injury. Yet, to date, few such antiepileptogenic therapies have emerged or are even in developmental stages. Although many seizure medications modulate the functional or physiological activity of neurons, the methods for stabilizing the structure of neurons are relatively unexplored therapeutic strategies for epilepsy. Human pathological studies and animal models of epilepsy demonstrate obvious structural abnormalities in dendrites of neurons, which could contribute to neuronal dysfunction, epileptogenesis and cognitive/neurological deficits in epilepsy patients. This dendritic injury may be caused by activity-dependent breakdown of cytoskeletal elements, such as actin. Mechanistically targeted approaches to limit seizure-related structural changes in dendrites may represent a novel therapeutic strategy for treating epilepsy and its complications.

Spine-type-specific recruitment of newly synthesized AMPA receptors with learning

The stabilization of long-term memories requires de novo protein synthesis. How can proteins, synthesized in the soma, act on specific synapses that participate in a given memory? We studied the dynamics of newly synthesized AMPA-type glutamate receptors (AMPARs) induced with learning using transgenic mice expressing the GluR1 subunit fused to green fluorescent protein (GFP-GluR1) under control of the c-fos promoter. We found learning-associated recruitment of newly synthesized GFP-GluR1 selectively to mushroom-type spines in adult hippocampal CA1 neurons 24 hours after fear conditioning. Our results are consistent with a "synaptic tagging" model to allow activated synapses to subsequently capture newly synthesized receptor and also demonstrate a critical functional distinction in the mushroom spines with learning.

Running induces widespread structural alterations in the hippocampus and entorhinal cortex

Physical activity enhances hippocampal function but its effects on neuronal structure remain relatively unexplored outside of the dentate gyrus. Using Golgi impregnation and the lipophilic tracer DiI, we show that long-term voluntary running increases the density of dendritic spines in the entorhinal cortex and hippocampus of adult rats. Exercise was associated with increased dendritic spine density not only in granule neurons of the dentate gyrus, but also in CA1 pyramidal neurons, and in layer III pyramidal neurons of the entorhinal cortex. In the CA1 region, changes in dendritic spine density are accompanied by changes in dendritic arborization and alterations in the morphology of individual spines. These findings suggest that physical activity exerts pervasive effects on neuronal morphology in the hippocampus and one of its afferent populations. These structural changes may contribute to running-induced changes in cognitive function.

Upregulation of neurotrophic factors selectively in frontal cortex in response to olfactory discrimination learning

We have previously shown that olfactory discrimination learning is accompanied by several forms of long-term enhancement in synaptic connections between layer II pyramidal neurons selectively in the piriform cortex. This study sought to examine whether the previously demonstrated olfactory-learning-task-induced modifications are preceded by suitable changes in the expression of mRNA for neurotrophic factors and in which brain areas this occurs. Rats were trained to discriminate positive cues in pair of odors for a water reward. The relationship between the learning task and local levels of mRNA for brain-derived neurotrophic factor, tyrosine kinase B, nerve growth factor, and neurotrophin-3 in the frontal cortex, hippocampal subregions, and other regions were assessed 24 hours post olfactory learning. The olfactory discrimination learning activated production of endogenous neurotrophic factors and induced their signal transduction in the frontal cortex, but not in other brain areas. These findings suggest that different brain areas may be preferentially involved in different learning/memory tasks.

Ultrastructural plasticity associated with hippocampal-dependent learning: A meta-analysis

In order to develop a profile of how individual synapses in the hippocampal formation alter their structure following learning experience, a meta-analysis synthesized the available literature on morphological change following hippocampal-dependent learning. Analysis of the 132 calculated effect sizes suggest a consistent profile of morphological change in the hippocampus following learning experience. Across the hippocampal formation, dendritic complexity, spine density, and the size of perforated postsynaptic densities showed consistent increases following training. Both the density of synapses in general and perforated synapses in particular showed unique responses to training, depending on the duration of training and/or different cell layers of the hippocampal formation. Most importantly, it seems that this profile, while consistent, is small and specific--only a select few of the morphological parameters typically measured in anatomical studies of plasticity showed significant change following training. Collectively, these data suggest that the distinct electrophysiological properties of neocortical versus hippocampal synapses may be at least partially mediated by distinct morphological cascades. That is, on the basis of theory, and with the support of the current data, it seems that synaptogenesis correlates with enduring neocortical plasticity, while structural changes correlate with more transient hippocampal plasticity. To be able to state these conclusions with conviction, however, more data are needed in several key areas for continued pursuit of the morphological correlates of hippocampal-dependent learning.

Dopamine D1/D5 receptors gate the acquisition of novel information through hippocampal long-term potentiation and long-term depression

Hebbian learning models require that neurons are able to both strengthen and weaken their synaptic connections. Hippocampal synaptic plasticity, in the form of long-term potentiation (LTP) and long-term depression (LTD), has been implicated in both spatial memory formation as well as novelty acquisition. In addition, the ventral tegmental area-hippocampal loop has been proposed to control the entry of information into long-term memory, whereas the dopaminergic system is believed to play an important role in information acquisition and synaptic plasticity. D1/D5 dopamine receptors are positively coupled to adenylyl cyclase and have been to modulate certain forms of synaptic plasticity, particularly in vitro. We investigated how D1/D5 dopamine receptors modify long-lasting synaptic plasticity at CA1 synapses of adult freely moving rats and found that receptor activation lowered the threshold for the induction of both LTP and LTD. Specific types of learning are associated with specific types of hippocampal synaptic plasticity. We found that object-configuration learning, facilitation of late-phase LTD by object exploration, and late-phase LTP by exploration of empty space were all prevented by D1/D5 receptor antagonism. Furthermore, receptor antagonism prevented electrically induced late-LTP, whereas receptor activation facilitated induction of both LTP and LTD by patterned electrical stimulation. These findings suggest that the dopaminergic system, acting via D1/D5 receptors, gates long-term changes in synaptic strength and that these changes are a critical factor in the acquisition of novel information.

Dendritic plasticity of CA1 pyramidal neurons after transient global ischemia

Dendrites and spines undergo dynamic changes in physiological conditions, such as learning and memory, and in pathological conditions, such as Alzheimer's disease and epilepsy. Long-term dendritic plasticity has also been reported after ischemia/hypoxia, which might be compensatory effects of surviving neurons for the functional recovery after the insults. However, the dendritic changes shortly after ischemia, which might be associated with the pathogenesis of ischemic cell death, remain largely unknown. To reveal the morphological changes of ischemia-vulnerable neurons after ischemia, the present study investigated the alteration of dendritic arborization of CA1 pyramidal neurons in rats after transient cerebral ischemia using intracellular staining technique in vivo. The general appearance of dendritic arborization of CA1 neurons within 48 h after ischemia was similar to that of control neurons. However, a dramatic increase of dendritic disorientation was observed after ischemia with many basal dendrites coursed into the territory of apical dendrites and apical dendrites branched into the region of basal dendrites. In addition, a significant increase of apical dendritic length was found 24 h after ischemia. The increase of dendritic length after ischemia was mainly due to the dendritic sprouting rather than the extension of individual dendrites, which mainly occurred in the middle segment of the apical dendrites. These results reveal a plasticity change in dendritic arborization of CA1 neurons shortly after cerebral ischemia.

Passive avoidance training decreases synapse density in the hippocampus of the domestic chick

The bird hippocampus (Hp), although lacking the cellular lamination of the mammalian Hp, possesses comparable roles in spatial orientation and is implicated in passive avoidance learning. As in rodents it can be divided into dorsal and ventral regions based on immunocytochemical, tracing and electrophysiological studies. To study the effects of passive avoidance learning on synapse morphometry in the Hp, spine and shaft synapse densities of 1-day-old domestic chicks were determined in dorsal and ventral Hp of each hemisphere by electron microscopy, 6 and 24 h following training to avoid pecking at a bead coated with a bitter-tasting substance, methyl anthranilate (MeA). The density of asymmetric spine and shaft synapses in MeA-trained birds at 6 h post-training was significantly lower in the dorsal and ventral Hp of the right hemisphere relative to control (untrained) chicks, but by 24 h this difference was absent. A hemispheric asymmetry was apparent in the ventral Hp where the water-trained group showed enhanced shaft and spine synapse density in the left hemisphere, whilst in the MeA-trained group only asymmetric shaft synapses follow the same pattern in relation to the right hemisphere. There were no differences in asymmetric shaft synapses in the dorsal Hp at 6 h post-training, but at 24 h post-training there was a reduction in the density of shaft synapses in the right hemisphere in MeA compared with control birds. These data are discussed in relation to the pruning effects of stress and learning on synapse density in chick Hp.

Simultaneous olfactory discrimination elicits a strain-specific increase in dendritic spines in the hippocampus of inbred mice

This study examines the extent to which simultaneous olfactory discrimination learning increases spine density on hippocampal CA1 pyramidal neurons in C57BL/6J (C57) and DBA/2J (DBA) inbred mice, characterized by spontaneous differences in hippocampal plasticity and hippocampus-related learning. The behavioral data first showed a clear-cut difference in performance between the two strains. C57 mice learned to identify the positively reinforced olfactory cue whereas DBA did not. Both strains, however, similarly acquired the procedural aspects of the task. The morphological analysis performed 24 h post-training revealed that spine density was significantly increased along apical, oblique, and basal dendrites in trained C57 mice compared to trained DBA mice, and to pseudotrained as well as to control cage mice of both strains. These findings confirm the ability of C57 mice to solve hippocampal-dependent tasks and provide the first evidence that simultaneous olfactory discrimination learning elicits spine growth in the mouse hippocampus. In addition, the finding that DBA mice failed to discriminate between the two olfactory cues but were as efficient as C57 mice in learning the procedural aspects of the task outlines that the structural changes observed in the latter strain were independent from any procedural learning component.

Modulation of dendritic spines in epilepsy: cellular mechanisms and functional implications

Epilepsy patients often suffer from significant neurological deficits, including memory impairment, behavioral problems, and psychiatric disorders. While the causes of neuropsychological dysfunction in epilepsy are multifactorial, accumulating evidence indicates that seizures themselves may directly cause brain injury. Although seizures sometimes result in neuronal death, they may also cause more subtle pathological changes in neuronal structure and function, including abnormalities in synaptic transmission. Dendritic spines receive a majority of the excitatory synaptic inputs to cortical neurons and are critically involved in synaptic plasticity and learning. Studies of human epilepsy and experimental animal models demonstrate that seizures may directly affect the morphological and functional properties of dendritic spines, suggesting that seizure-related changes in spines may represent a mechanistic basis for cognitive deficits in epilepsy. Novel therapeutic strategies directed at modulation of spine motility may prevent the detrimental effects of seizures on cognitive function in epilepsy.

Insulin receptor signaling in long-term memory consolidation following spatial learning

Evidence has shown that the insulin and insulin receptor (IR) play a role in cognitive function. However, the detailed mechanisms underlying insulin's action on learning and memory are not yet understood. Here we investigated changes in long-term memory-associated expression of the IR and downstream molecules in the rat hippocampus. After long-term memory consolidation following a water maze learning experience, gene expression of IR showed an up-regulation in the CA1, but a down-regulation in the CA3 region. These were correlated with a significant reduction in hippocampal IR protein levels. Learning-specific increases in levels of downstream molecules such as IRS-1 and Akt were detected in the synaptic membrane accompanied by decreases in Akt phosphorylation. Translocation of Shc protein to the synaptic membrane and activation of Erk1/2 were also observed after long-term memory formation. Despite the clear memory-correlated alterations in IR signaling pathways, insulin deficits in experimental diabetes mellitus (DM) rats induced by intraperitoneal injections of streptozotocin resulted in only minor memory impairments. This may be due to higher glucose levels in the DM brain, and to compensatory mechanisms from other signaling pathways such as the insulin-like growth factor-1 receptor (IGF-1R) system. Our results suggest that insulin/IR signaling plays a modulatory role in learning and memory processing, which may be compensated for by alternative pathways in the brain when an insulin deficit occurs.

Olfactory learning-induced morphological modifications in single dendritic spines of young rats

Learning-related morphological modifications in single dendritic spines were studied quantitatively in the brains of young Sprague-Dawley rats. We have previously shown that olfactory discrimination rule-learning results in transient physiological and morphological modifications in piriform cortex pyramidal neurons. In particular, spine density along the apical dendrites of neurons from trained rats is increased after learning. The aim of the present study was to identify and describe olfactory learning-induced modifications in the morphology of single spines along apical dendrites of the same type of neurons. By using laser-scanning confocal microscopy, we show that 3 days after training completion spines on neurons from olfactory discrimination trained rats are shorter as compared to spines on neurons from control rats. Further analysis revealed that spine shortening attributed to olfactory discrimination learning derives from shortening of spine head and not from shortening of spine neck. In addition, detailed analysis of spine head volume suggests that spines with large heads are absent after learning. As spine head size may be related to the efficacy of the synapse it bears, we suggest that modifications in spine head dimensions following olfactory rule-learning enhance the cortical network ability to enter into a 'learning mode', in which memories of new odours can be acquired rapidly and efficiently.

A Cellular Correlate of Learning-induced Metaplasticity in the Hippocampus

Metaplasticity, the plasticity of synaptic plasticity, is thought to have a pivotal role in activity-dependent modulation of synaptic connectivity, which underlies learning and memory. Metaplasticity is usually attributed to modifications in glutamate receptor-mediated synaptic transmission. However, experimental evidence and theoretical considerations suggest that learning reduces the predisposition for further synaptic strengthening, while behavioral studies show that learning capability is enhanced by prior learning. Here we show that enhanced neuronal excitability in CA1 pyramidal neurons, but not enhanced synaptic transmission, occurs prior to rule learning of an olfactory discrimination task. This transient enhancement lasts for 1 day after rule learning, is apparent throughout the cell population and results from reduction in the medium and slow after-hyperpolarizations that control spike frequency adaptation. Such olfactory learning-induced increased excitability in hippocampal neurons enhances the rats' learning capability in another hippocampus-dependent task, the Morris water maze. Once olfactory discrimination rule learning is acquired, its maintenance is not dependent on the reduced post-burst AHP in hippocampal neurons. However, the enhanced spatial learning capability of olfactory-trained rats in the water maze is diminished once the post burst AHP in CA1 pyramidal cells resumes its initial value. We suggest that enhanced excitability of CA1 neurons may serve as a mechanism for generalized enhancement of hippocampus-dependent learning capability. In the presence of such enhanced neuronal excitability, the hippocampal network enters into a 'learning mode' in which a variety of hippocampus-dependent skills are acquired rapidly and efficiently.

Dendritic spines and long-term plasticity

A recent flurry of time-lapse imaging studies of live neurons have tried to address the century-old question: what morphological changes in dendritic spines can be related to long-term memory? Changes that have been proposed to relate to memory include the formation of new spines, the enlargement of spine heads and the pruning of spines. These observations also relate to a more general question of how stable dendritic spines are. The objective of this review is to critically assess the new data and to propose much needed criteria that relate spines to memory, thereby allowing progress in understanding the morphological basis of memory.

Dynamics of learning-induced spine redistribution along dendrites of pyramidal neurons in rats

We have previously shown that olfactory-discrimination (OD) learning is accompanied by enhanced spine density along proximal apical dendrites of layer II pyramidal neurons in the piriform (olfactory) cortex. Here we studied the temporal dynamics of learning-induced modifications in dendritic spine density throughout the dendritic trees of these neurons. We observed a transient increase in proximal apical spine density after OD learning, suggesting a strengthening of intrinsic excitatory inputs interconnecting neurons within the olfactory cortex. By contrast, the afferent pathway receiving direct input from the olfactory bulb shows spine pruning, suggesting that the connectivity is weakened. The changes in spine density can be attributed to a net change in number of spines, as the morphometric parameters of the dendrites are unaffected by learning. We suggest that spine density changes may represent a mechanism of selective synaptic reorganization required for olfactory learning consolidation.

Olfactory learning-related NCAM expression is state, time, and location specific and is correlated with individual learning capabilities

The notion that long-term synaptic plasticity is generated by activity-induced molecular modifications is widely accepted. It is well established that neural cell adhesion molecule (NCAM) is one of the prominent modulators of synaptic plasticity. NCAM can be polysialylated (PSA-NCAM), a reaction that provides it with anti-adhesion properties. In this study we have focused on NCAM and on its polysialylated state, and their relation to learning of an olfactory discrimination task, which depends on both the piriform (olfactory) cortex and hippocampus. We trained rats to distinguish between pairs of odors until rule learning was achieved, a process that normally lasts 6-8 days. At four time points, during training and after training completion, synaptic NCAM and PSA-NCAM expression were assessed in the piriform cortex and hippocampus. We report that NCAM modulation is specific to PSA-NCAM, which is upregulated in the hippocampus one day after training completion. We also report a correlation between the performance of individual rats in an early training stage and their NCAM expression, both in the piriform cortex and hippocampus. Since individual early performance in our odor discrimination task is correlated with the performance throughout the training period, we conclude that early NCAM expression is associated with odor learning capability. We therefore suggest that early synaptic NCAM expression may be one of the factors determining the capability of rats to learn.

Dynamics of olfactory learning-induced up-regulation of L1 in the piriform cortex and hippocampus

L1 is a cell adhesion molecule implicated in the formation of neural circuits and synaptic plasticity. We have examined the sequence and time-frame in which modifications in the synaptic expression of L1 occur in the piriform cortex and hippocampus in the course of rule learning of an olfactory discrimination task. Rats were trained to choose the correct odour in a pair to be rewarded with drinking water. Such training requires 6-8 days on average before rats reach maximal performance. We observed a learning-induced L1 up-regulation that occurred at an early training stage in the piriform cortex but only after rule-learning establishment in the hippocampus. We suggest that the dynamics of L1 up-regulation may reflect the functional role of these brain regions in olfactory rule learning.