Activity-dependent changes of the hippocampal CA3-CA1 synapse during the acquisition of associative learning in conscious mice

Dentate gyrus is needed for memory retrieval

The hippocampus is crucial for acquiring and retrieving episodic and contextual memories. In previous studies, the inactivation of dentate gyrus (DG) neurons by chemogenetic- and optogenetic-mediated hyperpolarization led to opposing conclusions about DG's role in memory retrieval. One study used Designer Receptors Exclusively Activated by Designer Drugs (DREADD)-mediated clozapine N-oxide (CNO)-induced hyperpolarization and reported that the previously formed memory was erased, thus concluding that denate gyrus is needed for memory maintenance. The other study used optogenetic with halorhodopsin induced hyperpolarization and reported and dentate gyrus is needed for memory retrieval. We hypothesized that this apparent discrepancy could be due to the length of hyperpolarization in previous studies; minutes by optogenetics and several hours by DREADD/CNO. Since hyperpolarization interferes with anterograde and retrograde neuronal signaling, it is possible that the memory engram in the dentate gyrus and the entorhinal to hippocampus trisynaptic circuit was erased by long-term, but not with short-term hyperpolarization. We developed and applied an advanced chemogenetic technology to selectively silence synaptic output by blocking neurotransmitter release without hyperpolarizing DG neurons to explore this apparent discrepancy. We performed in vivo electrophysiology during trace eyeblink in a rabbit model of associative learning. Our work shows that the DG output is required for memory retrieval. Based on previous and recent findings, we propose that the actively functional anterograde and retrograde neuronal signaling is necessary to preserve synaptic memory engrams along the entorhinal cortex to the hippocampal trisynaptic circuit.

Identification and characterization of long non-coding RNA Carip in modulating spatial learning and memory

CaMKII has long been known to be a key effector for synaptic plasticity. Recent studies have shown that a variety of modulators interact with the subunits of CaMKII to regulate the long-term potentiation (LTP) of hippocampal neurons. However, whether long non-coding RNAs modulate the activity of CaMKII and affect synaptic plasticity is still elusive. Here, we identify a previously uncharacterized long non-coding RNA Carip that functions as a scaffold, specifically interacts with CaMKIIβ, and regulates the phosphorylation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-d-aspartate (NMDA) receptor subunits in the hippocampus. The absence of Carip causes dysfunction of synaptic transmission and attenuates LTP in hippocampal CA3-CA1 synapses, which further leads to impairment of spatial learning and memory. In summary, our findings demonstrate that Carip modulates long-term synaptic plasticity by changing AMPA receptor and NMDA receptor activities, thereby affecting spatial learning and memory in mice.

l-Lactate: Food for Thoughts, Memory and Behavior

More and more evidence shows how brain energy metabolism is the linkage between physiological and morphological synaptic plasticity and memory consolidation. Different types of memory are associated with differential inputs, each with specific inputs that are upstream diverse molecular cascades depending on the receptor activity. No matter how heterogeneous the response is, energy availability represents the lowest common denominator since all these mechanisms are energy consuming and the brain networks adapt their performance accordingly. Astrocytes exert a primary role in this sense by acting as an energy buffer; glycogen granules, a mechanism to store glucose, are redistributed at glance and conveyed to neurons via the Astrocyte–Neuron Lactate Shuttle (ANLS). Here, we review how different types of memory relate to the mechanisms of energy delivery in the brain.

G-Protein-Gated Inwardly Rectifying Potassium (Kir3/GIRK) Channels Govern Synaptic Plasticity That Supports Hippocampal-Dependent Cognitive Functions in Male Mice

The G-protein-gated inwardly rectifying potassium (Kir3/GIRK) channel is the effector of many G-protein-coupled receptors (GPCRs). Its dysfunction has been linked to the pathophysiology of Down syndrome, Alzheimer's and Parkinson's diseases, psychiatric disorders, epilepsy, drug addiction, or alcoholism. In the hippocampus, GIRK channels decrease excitability of the cells and contribute to resting membrane potential and inhibitory neurotransmission. Here, to elucidate the role of GIRK channels activity in the maintenance of hippocampal-dependent cognitive functions, their involvement in controlling neuronal excitability at different levels of complexity was examined in C57BL/6 male mice. For that purpose, GIRK activity in the dorsal hippocampus CA3-CA1 synapse was pharmacologically modulated by two drugs: ML297, a GIRK channel opener, and Tertiapin-Q (TQ), a GIRK channel blocker. Ex vivo, using dorsal hippocampal slices, we studied the effect of pharmacological GIRK modulation on synaptic plasticity processes induced in CA1 by Schaffer collateral stimulation. In vivo, we performed acute intracerebroventricular (i.c.v.) injections of the two GIRK modulators to study their contribution to electrophysiological properties and synaptic plasticity of dorsal hippocampal CA3-CA1 synapse, and to learning and memory capabilities during hippocampal-dependent tasks. We found that pharmacological disruption of GIRK channel activity by i.c.v. injections, causing either function gain or function loss, induced learning and memory deficits by a mechanism involving neural excitability impairments and alterations in the induction and maintenance of long-term synaptic plasticity processes. These results support the contention that an accurate control of GIRK activity must take place in the hippocampus to sustain cognitive functions.SIGNIFICANCE STATEMENT Cognitive processes of learning and memory that rely on hippocampal synaptic plasticity processes are critically ruled by a finely tuned neural excitability. G-protein-gated inwardly rectifying K+ (GIRK) channels play a key role in maintaining resting membrane potential, cell excitability and inhibitory neurotransmission. Here, we demonstrate that modulation of GIRK channels activity, causing either function gain or function loss, transforms high-frequency stimulation (HFS)-induced long-term potentiation (LTP) into long-term depression (LTD), inducing deficits in hippocampal-dependent learning and memory. Together, our data show a crucial GIRK-activity-mediated mechanism that governs synaptic plasticity direction and modulates subsequent hippocampal-dependent cognitive functions.

Distinct neuronal populations contribute to trace conditioning and extinction learning in the hippocampal CA1

Trace conditioning and extinction learning depend on the hippocampus, but it remains unclear how neural activity in the hippocampus is modulated during these two different behavioral processes. To explore this question, we performed calcium imaging from a large number of individual CA1 neurons during both trace eye-blink conditioning and subsequent extinction learning in mice. Our findings reveal that distinct populations of CA1 cells contribute to trace conditioned learning versus extinction learning, as learning emerges. Furthermore, we examined network connectivity by calculating co-activity between CA1 neuron pairs and found that CA1 network connectivity patterns also differ between conditioning and extinction, even though the overall connectivity density remains constant. Together, our results demonstrate that distinct populations of hippocampal CA1 neurons, forming different sub-networks with unique connectivity patterns, encode different aspects of learning.

Acute Effects of Two Different Species of Amyloid-β on Oscillatory Activity and Synaptic Plasticity in the Commissural CA3-CA1 Circuit of the Hippocampus

Recent evidence indicates that soluble amyloid-β (Aβ) species induce imbalances in excitatory and inhibitory transmission, resulting in neural network functional impairment and cognitive deficits during early stages of Alzheimer's disease (AD). To evaluate the in vivo effects of two soluble Aβ species (Aβ_25-35 and Aβ_1-40) on commissural CA3-to-CA1 (cCA3-to-CA1) synaptic transmission and plasticity, and CA1 oscillatory activity, we used acute intrahippocampal microinjections in adult anaesthetized male Wistar rats. Soluble Aβ microinjection increased cCA3-to-CA1 synaptic variability without significant changes in synaptic efficiency. High-frequency CA3 stimulation was rendered inefficient by soluble Aβ intrahippocampal injection to induce long-term potentiation and to enhance synaptic variability in CA1, contrasting with what was observed in vehicle-injected subjects. Although soluble Aβ microinjection significantly increased the relative power of γ-band and ripple oscillations and significantly shifted the average vector of θ-to-γ phase-amplitude coupling (PAC) in CA1, it prevented θ-to-γ PAC shift induced by high-frequency CA3 stimulation, opposite to what was observed in vehicle-injected animals. These results provide further evidence that soluble Aβ species induce synaptic dysfunction causing abnormal synaptic variability, impaired long-term plasticity, and deviant oscillatory activity, leading to network activity derailment in the hippocampus.

Long-Lasting Input-Specific Experience-Dependent Changes of Hippocampus Synaptic Function Measured in the Anesthetized Rat

How experience causes long-lasting changes in the brain is a central question in neuroscience. The common view is that synaptic function is altered by experience to change brain circuit functions that underlie conditioned behavior. We examined hippocampus synaptic circuit function in vivo, in three groups of animals, to assess the impact of experience on hippocampus function in rats. The “conditioned” group acquired a shock-conditioned place response during a cognitively-challenging, hippocampus synaptic plasticity-dependent task. The no-shock group had similar exposure to the environmental conditions but no conditioning. The home-cage group was experimentally naive. After the one-week retention test, under anesthesia, we stimulated the perforant path inputs to CA1, which terminate in stratum lacunosum moleculare (slm), and to the dentate gyrus (DG), which terminate in the molecular layer. We find synaptic compartment specific changes that differ amongst the groups. The evoked field EPSP (fEPSP) and pre-spike field response are enhanced only at the DG input layer and only in conditioned animals. The DG responses, measured by the population spiking activity and post-spike responses, are enhanced in both the conditioned and no-shock groups compared to home-cage animals. These changes are pathway specific because no differences are observed in slm of CA1. These findings demonstrate long-term, experience-dependent, pathway-specific alterations to synaptic circuit function of the hippocampus.

The analysis of hippocampus neuronal density (CA1 and CA3) after Ocimum sanctum ethanolic extract treatment on the young adulthood and middle age rat model

Aim:

This study aimed to assess the changes in neuronal density in CA1 and CA3 regions in the hippocampus of young adulthood and middle age rat model after feeding by Ocimum sanctum ethanolic extract.

Materials and Methods:

In this research, 30 male Wistar rats consist of young to middle-aged rats were divided into three groups (3, 6, and 9 months old) and treated with a different dosage of O. sanctum ethanolic extract (0, 50, and 100 mg/kg b.w.) for 45 days. Furthermore, cresyl violet staining was performed to analyze hippocampus formation mainly in CA1 and CA3 area. The concentrations of acetylcholine (Ach) in brain tissues were analyzed by enzyme-linked immunosorbent assay.

Results:

In our in vivo models using rat model, we found that the administration of O. sanctum ethanolic extract with a dosage of 100 mg/kg b.w. for 45 days induced the density of pyramidal cells significantly in CA1 and CA3 of the hippocampus. These results were supported by an increase of Ach concentrations on the brain tissue.

Conclusions:

The administration of O. sanctum ethanolic extract may promote the density of the pyramidal cells in the CA1 and CA3 mediated by the up-regulated concentration of Ach.

Cognitive dysfunction in Huntington's disease: mechanisms and therapeutic strategies beyond BDNF

One of the main focuses in Huntington's disease (HD) research, as well as in most neurodegenerative diseases, is the development of new therapeutic strategies, as currently there is no treatment to delay or prevent the progression of the disease. Neuronal dysfunction and neuronal death in HD are caused by a combination of interrelated pathogenic processes that lead to motor, cognitive and psychiatric symptoms. Understanding how mutant huntingtin impacts on a plethora of cellular functions could help to identify new molecular targets. Although HD has been classically classified as a neurodegenerative disease affecting voluntary movement, lately cognitive dysfunction is receiving increased attention as it is very invalidating for patients. Thus, an ambitious goal in HD research is to find altered molecular mechanisms that contribute to cognitive decline. In this review, we have focused on those findings related to corticostriatal and hippocampal cognitive dysfunction in HD, as well as on the underlying molecular mechanisms, which constitute potential therapeutic targets. These include alterations in synaptic plasticity, transcriptional machinery and neurotrophic and neurotransmitter signaling.

CA1 pyramidal cells have diverse biophysical properties, affected by development, experience, and aging

Neuron types (e.g., pyramidal cells) within one area of the brain are often considered homogeneous, despite variability in their biophysical properties. Here we review literature demonstrating variability in the electrical activity of CA1 hippocampal pyramidal cells (PCs), including responses to somatic current injection, synaptic stimulation, and spontaneous network-related activity. In addition, we describe how responses of CA1 PCs vary with development, experience, and aging, and some of the underlying ionic currents responsible. Finally, we suggest directions that may be the most impactful in expanding this knowledge, including the use of text and data mining to systematically study cellular heterogeneity in more depth; dynamical systems theory to understand and potentially classify neuron firing patterns; and mathematical modeling to study the interaction between cellular properties and network output. Our goals are to provide a synthesis of the literature for experimentalists studying CA1 PCs, to give theorists an idea of the rich diversity of behaviors models may need to reproduce to accurately represent these cells, and to provide suggestions for future research.

Learning as a Functional State of the Brain: Studies in Wild-Type and Transgenic Animals

Contemporary neuroscientists are paying increasing attention to subcellular, molecular, and electrophysiological mechanisms underlying learning and memory processes. Recent studies have examined the development of transgenic mice affected at different stages of the learning process, or have emulated in animals various human pathological conditions involving cognition and motor learning. However, a parallel effort is needed to develop stimulating and recording techniques suitable for use in behaving mice in order to understand activity-dependent synaptic changes taking place during the very moment of the learning process. The in vivo models should incorporate information collected from different molecular and in vitro approaches. Long-term potentiation (LTP) has been proposed as the neural mechanism underlying synaptic plasticity, and NMDA receptors have been proposed as the molecular substrate of LTP. It now seems necessary to study the relationship of both LTP and NMDA receptors to functional changes in synaptic efficiency taking place during actual learning in selected cerebral cortical structures. Here, we review data collected in our laboratory during the past 10 years on the involvement of different hippocampal synapses in the acquisition of the classically conditioned eyelid responses in behaving mice. Overall the results indicate a specific contribution of each cortical synapse to the acquisition and storage of new motor and cognitive abilities. Available data show that LTP, evoked by high-frequency stimulation of Schaffer collaterals, disturbs both the acquisition of conditioned eyelid responses and the physiological changes that occur at hippocampal synapses during learning. Moreover, the administration of NMDA-receptor antagonists is able not only to prevent LTP induction in vivo, but also to hinder both the formation of conditioned eyelid responses and functional changes in the strength of the CA3-CA1 synapse. Nevertheless, many other neurotransmitter receptors, intracellular mediators, and transcription factors are also involved in learning and memory processes. In summary, it can be proposed that learning and memory in behaving mammals are the result of the activation of complex and distributed functional states involving many different cerebral cortical synapses, with the participation also of various neurotransmitter systems.

The role of the ubiquitin proteasome system in the memory process

Quite intuitive is the notion that memory formation and consolidation is orchestrated by protein synthesis because of the synaptic plasticity necessary for those processes. Nevertheless, recent advances have begun accumulating evidences of a high requirement for protein degradation on the molecular mechanisms of the memory process in the mammalian brain. Because degradation determines protein half-life, degradation has been increasingly recognized as an important intracellular regulatory mechanism. The proteasome is the main player in the degradation of intracellular proteins. Proteasomal substrates are mainly degraded after a post-translational modification by a poly-ubiquitin chain. Latter process, namely poly-ubiquitination, is highly regulated at the step of the ubiquitin molecule transferring to the protein substrate mediated by a set of proteins whose genes represent almost 2% of the human genome. Understanding the role of polyubiquitin-mediated protein degradation has challenging researchers in many fields of investigation as a new source of targets for therapeutic intervention, e.g. E3 ligases that transfer ubiquitin moieties to the substrate. The goal of present work was to uncover mechanisms underlying memory processes regarding the role of the ubiquitin-proteasome system (UPS). For that purpose, preceded of a short review on UPS and memory processes a top-down systems biology approach was applied to establish central proteins involved in memory formation and consolidation highlighting their cross-talking with the UPS. According to that approach, the pattern of expression of several elements of the UPS were found overexpressed in regions of the brain involved in processing cortical inputs.

Impact of Increased Astrocyte Expression of IL-6, CCL2 or CXCL10 in Transgenic Mice on Hippocampal Synaptic Function

An important aspect of CNS disease and injury is the elevated expression of neuroimmune factors. These factors are thought to contribute to processes ranging from recovery and repair to pathology. The complexity of the CNS and the multitude of neuroimmune factors that are expressed in the CNS during disease and injury is a challenge to an understanding of the consequences of the elevated expression relative to CNS function. One approach to address this issue is the use of transgenic mice that express elevated levels of a specific neuroimmune factor in the CNS by a cell type that normally produces it. This approach can provide basic information about the actions of specific neuroimmune factors and can contribute to an understanding of more complex conditions when multiple neuroimmune factors are expressed. This review summarizes studies using transgenic mice that express elevated levels of IL-6, CCL2 or CXCL10 through increased astrocyte expression. The studies focus on the effects of these neuroimmune factors on synaptic function at the Schaffer collateral to CA1 pyramidal neuron synapse of the hippocampus, a brain region that plays a key role in cognitive function.

Prolonged maternal separation induces undernutrition and systemic inflammation with disrupted hippocampal development in mice

OBJECTIVE

Prolonged maternal separation (PMS) in the first 2 wk of life has been associated with poor growth with lasting effects in brain structure and function. This study aimed to investigate whether PMS-induced undernutrition could cause systemic inflammation and changes in nutrition-related hormonal levels, affecting hippocampal structure and neurotransmission in C57BL/6J suckling mice.

METHODS

This study assessed mouse growth parameters coupled with insulin-like growth factor-1 (IGF-1) serum levels. In addition, leptin, adiponectin, and corticosterone serum levels were measured following PMS. Hippocampal stereology and the amino acid levels were also assessed. Furthermore, we measured myelin basic protein and synapthophysin (SYN) expression in the overall brain tissue and hippocampal SYN immunolabeling. For behavioral tests, we analyzed the ontogeny of selected neonatal reflexes. PMS was induced by separating half the pups in each litter from their lactating dams for defined periods each day (4 h on day 1, 8 h on day 2, and 12 h thereafter). A total of 67 suckling pups were used in this study.

RESULTS

PMS induced significant slowdown in weight gain and growth impairment. Significant reductions in serum leptin and IGF-1 levels were found following PMS. Total CA3 area and volume were reduced, specifically affecting the pyramidal layer in PMS mice. CA1 pyramidal layer area was also reduced. Overall hippocampal SYN immunolabeling was lower, especially in CA3 field and dentate gyrus. Furthermore, PMS reduced hippocampal aspartate, glutamate, and gamma-aminobutyric acid levels, as compared with unseparated controls.

CONCLUSION

These findings suggest that PMS causes significant growth deficits and alterations in hippocampal morphology and neurotransmission.

Modulating Hippocampal Plasticity with In Vivo Brain Stimulation

Investigations into the use of transcranial direct current stimulation (tDCS) in relieving symptoms of neurological disorders and enhancing cognitive or motor performance have exhibited promising results. However, the mechanisms by which tDCS effects brain function remain under scrutiny. We have demonstrated that in vivo tDCS in rats produced a lasting effect on hippocampal synaptic plasticity, as measured using extracellular recordings. Ex vivo preparations of hippocampal slices from rats that have been subjected to tDCS of 0.10 or 0.25 mA for 30 min followed by 30 min of recovery time displayed a robust twofold enhancement in long-term potentiation (LTP) induction accompanied by a 30% increase in paired-pulse facilitation (PPF). The magnitude of the LTP effect was greater with 0.25 mA compared with 0.10 mA stimulations, suggesting a dose-dependent relationship between tDCS intensity and its effect on synaptic plasticity. To test the persistence of these observed effects, animals were stimulated in vivo for 30 min at 0.25 mA and then allowed to return to their home cage for 24 h. Observation of the enhanced LTP induction, but not the enhanced PPF, continued 24 h after completion of 0.25 mA of tDCS. Addition of the NMDA blocker AP-5 abolished LTP in both control and stimulated rats but maintained the PPF enhancement in stimulated rats. The observation of enhanced LTP and PPF after tDCS demonstrates that non-invasive electrical stimulation is capable of modifying synaptic plasticity.

SIGNIFICANCE STATEMENT

Researchers have used brain stimulation such as transcranial direct current stimulation on human subjects to alleviate symptoms of neurological disorders and enhance their performance. Here, using rats, we have investigated the potential mechanisms of how in vivo brain stimulation can produce such effect. We recorded directly on viable brain slices from rats after brain stimulation to detect lasting changes in pattern of neuronal activity. Our results showed that 30 min of brain stimulation in rats induced a robust enhancement in synaptic plasticity, a neuronal process critical for learning and memory. Understanding such molecular effects will lead to a better understanding of the mechanisms by which brain stimulation produces its effects on cognition and performance.

Sirtuin-2 mediates male specific neuronal injury following experimental cardiac arrest through activation of TRPM2 ion channels

OBJECTIVE

Sirtuins (Sirt) are a class of deacetylase enzymes that play an important role in cell proliferation. Sirt2 activation produces O-acetylated-ADPribose (OAADPr) which can act as a ligand for transient receptor potential cation channel, M2 (TRPM2). We tested the hypothesis that Sirt2 is activated following global cerebral ischemia and contributes to neuronal injury through activation of TRPM2.

METHODS

Adult male and female mice (8-12 weeks old) C57Bl/6 and TRPM2 knock-out mice were subjected to 8 min of cardiac arrest followed by cardiopulmonary resuscitation (CA/CPR). The Sirt2 inhibitor AGK-2 was administered intravenously 30 min after resuscitation. Hippocampal CA1 injury was analyzed at 3 days after CA/CPR. Acute Sirt2 activity was analyzed at 3 and 24 h after CA/CPR. Long-term hippocampal function was assessed using slice electrophysiology 7 days after CA/CPR.

RESULTS

AGK-2 significantly reduced CA1 injury in WT but not TRPM2 knock-out males and had no effect on CA1 injury in females. Elevated Sirt2 activity was observed in hippocampal tissue from males at 24 h after cardiac arrest and was reduced by AGK-2. In contrast, Sirt2 activity in females was increased at 3 but not 24 h. Finally, we observed long-term benefit of AGK-2 on hippocampal function, with a protection of long-term potentiation at CA1 synapses at 7 and 30 days after ischemia.

CONCLUSIONS

In summary, we observed a male specific activation of Sirt2 that contributes to neuronal injury and functional deficits after ischemia specifically in males. These results are consistent with a role of Sirt2 in activating TRPM2 following global ischemia in a sex specific manner. These results support the growing body of literature showing that oxidative stress mechanisms predominate in males and converge on TRPM2 activation as a mediator of cell death.

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.

Learning to learn - intrinsic plasticity as a metaplasticity mechanism for memory formation

"Use it or lose it" is a popular adage often associated with use-dependent enhancement of cognitive abilities. Much research has focused on understanding exactly how the brain changes as a function of experience. Such experience-dependent plasticity involves both structural and functional alterations that contribute to adaptive behaviors, such as learning and memory, as well as maladaptive behaviors, including anxiety disorders, phobias, and posttraumatic stress disorder. With the advancing age of our population, understanding how use-dependent plasticity changes across the lifespan may also help to promote healthy brain aging. A common misconception is that such experience-dependent plasticity (e.g., associative learning) is synonymous with synaptic plasticity. Other forms of plasticity also play a critical role in shaping adaptive changes within the nervous system, including intrinsic plasticity - a change in the intrinsic excitability of a neuron. Intrinsic plasticity can result from a change in the number, distribution or activity of various ion channels located throughout the neuron. Here, we review evidence that intrinsic plasticity is an important and evolutionarily conserved neural correlate of learning. Intrinsic plasticity acts as a metaplasticity mechanism by lowering the threshold for synaptic changes. Thus, learning-related intrinsic changes can facilitate future synaptic plasticity and learning. Such intrinsic changes can impact the allocation of a memory trace within a brain structure, and when compromised, can contribute to cognitive decline during the aging process. This unique role of intrinsic excitability can provide insight into how memories are formed and, more interestingly, how neurons that participate in a memory trace are selected. Most importantly, modulation of intrinsic excitability can allow for regulation of learning ability - this can prevent or provide treatment for cognitive decline not only in patients with clinical disorders but also in the aging population.

Synaptic plasticity studies and their applicability in mouse models of neurodegenerative diseases

During the past few years, there have been many important contributions to the better understanding of the different types of memory and the putative neural structures that generate them. Moreover, various studies on neurodegenerative diseases in human beings have added useful information about learning and memory formation, and about their loss in patients with these disabling diseases. The development of sophisticated pharmacogenetic tools applied to mouse models, reproducing different types of neurodegenerative disease or, at least, some of their main symptoms, has turned out to be extremely useful for the further development of contemporary neuroscience. In addition, ingenious behavioral and electrophysiological approaches have been developed to study the activity-dependent changes in synaptic strength during learning and memory processes in the best possible way — that is, in the alert behaving animal. Collected data from these numerous studies have enabled us to know more about the role of many different molecular components integrating the synaptic cleft, and to draw some conclusions about the concordance between in vitro and in vivo recorded data, and the generalization of the results to other types of learning and/or brain-related structures. As described here, changes in synaptic strength studied in key neural synapses during learning and memory processes in genetically manipulated mice can represent an interesting and powerful approach to the better understanding of neural processes underlying the acquisition of new motor and cognitive abilities and how they are affected by different human diseases involving the neural tissue.

Long-term enhancement of synaptic transmission between antennal lobe and mushroom body in cultured Drosophila brain

In Drosophila, the mushroom body (MB) is a critical brain structure for olfactory associative learning. During aversive conditioning, the MBs are thought to associate odour signals, conveyed by projection neurons (PNs) from the antennal lobe (AL), with shock signals conveyed through ascending fibres of the ventral nerve cord (AFV). Although synaptic transmission between AL and MB might play a crucial role for olfactory associative learning, its physiological properties have not been examined directly. Using a cultured Drosophila brain expressing a Ca(2+) indicator in the MBs, we investigated synaptic transmission and plasticity at the AL-MB synapse. Following stimulation with a glass micro-electrode, AL-induced Ca(2+) responses in the MBs were mediated through Drosophila nicotinic acetylcholine receptors (dnAChRs), while AFV-induced Ca(2+) responses were mediated through Drosophila NMDA receptors (dNRs). AL-MB synaptic transmission was enhanced more than 2 h after the simultaneous 'associative-stimulation' of AL and AFV, and such long-term enhancement (LTE) was specifically formed at the AL-MB synapses but not at the AFV-MB synapses. AL-MB LTE was not induced by intense stimulation of the AL alone, and the LTE decays within 60 min after subsequent repetitive AL stimulation. These phenotypes of associativity, input specificity and persistence of AL-MB LTE are highly reminiscent of olfactory memory. Furthermore, similar to olfactory aversive memory, AL-MB LTE formation required activation of the Drosophila D1 dopamine receptor, DopR, along with dnAChR and dNR during associative stimulations. These physiological and genetic analogies indicate that AL-MB LTE might be a relevant cellular model for olfactory memory.

Effect of 17ß-estradiol on zinc content of hippocampal mossy fibers in ovariectomized adult rats

Sex hormones such as estrogen (17ß-estradiol) may modulate the zinc content of the hippocampus during the female estrous cycle. The mossy fiber system is highly plastic in the adult brain and is influenced by multiple factors including learning, memory, and stress. However, whether 17ß-estradiol is able to modulate the morphological plasticity of the mossy fibers throughout the estrous cycle remains unknown. Ovariectomized (Ovx) female 70- to 90-day-old Sprague-Dawley rats without or with estrogen supplement (OvxE) were compared with control rats in three stages of the estrous cycle: diestrus, proestrus, and estrus. The brain tissue from each of the five groups was processed with Timm's silver sulfide technique using the Image J program to measure the mossy fiber area in the stratum lucidum of CA3. Total zinc in the hippocampus was measured using Graphite Furnace Atomic Absorption Spectrophotometry. Two additional (Ovx and OvxE) groups were examined in spatial learning and memory tasks using the Morris water maze. Similar increases in total zinc content and mossy fiber area were observed. The mossy fiber area decreased by 26 ± 2 % (difference ± SEM percentages) in Ovx and 23 ± 4 % in estrus as compared to the proestrus group and by 18 ± 2 % in Ovx compared to OvxE. Additionally, only the OvxE group learned and remembered the task. These results suggest that estradiol has a significant effect on zinc content in hippocampal CA3 during the proestrus stage of the estrous cycle and is associated with correct performance in learning and memory.

Frequency dependency of NMDA receptor-dependent synaptic plasticity in the hippocampal CA1 region of freely behaving mice

Hippocampal synaptic plasticity in the form of long-term potentiation (LTP) and long-term depression (LTD) is likely to enable synaptic information storage in support of memory formation. The mouse brain has been subjected to intensive scrutiny in this regard; however, a multitude of studies has examined synaptic plasticity in the hippocampal slice preparation, whereas very few have addressed synaptic plasticity in the freely behaving mouse. Almost nothing is known about the frequency or N-methyl-D-aspartate receptor (NMDAR) dependency of hippocampal synaptic plasticity in the intact mouse brain. Therefore, in this study, we investigated the forms of synaptic plasticity that are elicited at different afferent stimulation frequencies. We also addressed the NMDAR dependency of this phenomenon. Adult male C57BL/6 mice were chronically implanted with a stimulating electrode into the Schaffer collaterals and a recording electrode into the Stratum radiatum of the CA1 region. To examine synaptic plasticity, we chose protocols that were previously shown to produce either LTP or LTD in the hippocampal slice preparation. Low-frequency stimulation (LFS) at 1 Hz (900 pulses) had no effect on evoked responses. LFS at 3 Hz (ranging from 200 up to 2 × 900 pulses) elicited short-term depression (STD, <45 min). LFS at 3 Hz (1,200 pulses) elicited slow-onset potentiation, high-frequency stimulation (HFS) at 100 Hz (100 or 200 pulses) or at 50 Hz was ineffective, whereas 100 Hz (50 pulses) elicited short-term potentiation (STP). HFS at 100 Hz given as 2 × 30, 2 × 50, or 4 × 50 pulses elicited LTP (>24 h). Theta-burst stimulation was ineffective. Antagonism of the NMDAR prevented STD, STP, and LTP. This study shows for the first time that protocols that effectively elicit persistent synaptic plasticity in the slice preparation elicit distinctly different effects in the intact mouse brain. Persistent LTD could not be elicited with any of the protocols tested. Plasticity responses are NMDAR dependent, suggesting that these phenomena are relevant for hippocampus-dependent learning.

Postnatal proteasome inhibition induces neurodegeneration and cognitive deficiencies in adult mice: a new model of neurodevelopment syndrome

Defects in the ubiquitin-proteasome system have been related to aging and the development of neurodegenerative disease, although the effects of deficient proteasome activity during early postnatal development are poorly understood. Accordingly, we have assessed how proteasome dysfunction during early postnatal development, induced by administering proteasome inhibitors daily during the first 10 days of life, affects the behaviour of adult mice. We found that this regime of exposure to the proteasome inhibitors MG132 or lactacystin did not produce significant behavioural or morphological changes in the first 15 days of life. However, towards the end of the treatment with proteasome inhibitors, there was a loss of mitochondrial markers and activity, and an increase in DNA oxidation. On reaching adulthood, the memory of mice that were injected with proteasome inhibitors postnatally was impaired in hippocampal and amygdala-dependent tasks, and they suffered motor dysfunction and imbalance. These behavioural deficiencies were correlated with neuronal loss in the hippocampus, amygdala and brainstem, and with diminished adult neurogenesis. Accordingly, impairing proteasome activity at early postnatal ages appears to cause morphological and behavioural alterations in adult mice that resemble those associated with certain neurodegenerative diseases and/or syndromes of mental retardation.

Modulation of ischemia-induced NMDAR1 activation by environmental enrichment decreases oxidative damage

In this study, we examined whether enriched environment (EE) housing has direct neuroprotective effects on oxidative damage following transient global cerebral ischemia. Fifty-two adult male Wistar rats were included in the study and received either ischemia or sham surgery. Once fully awake, rats in each group were randomly assigned to either: EE housing or socially paired housing (CON). Animals remained in their assigned environment for 7 days, and then were killed. Our data showed that glutamate receptor expression was significantly higher in the hippocampus of the ischemia CON group than in the ischemia EE group. Furthermore, the oxidative DNA damage, protein oxidation, and neurodegeneration in the hippocampus of the ischemia CON group were significantly increased compared to the ischemia EE group. These results suggest that EE housing possibly modulated the ischemia-induced glutamate excitotoxicity, which then attenuated the oxidative damage and neurodegeneration in the ischemia EE rats.

Postnatal alterations in induction threshold and expression magnitude of long-term potentiation and long-term depression at hippocampal synapses

Activity-dependent synaptic plasticity refines neural networks during development and subserves information processing in adulthood. Previous research has revealed postnatal alterations in synaptic plasticity at nearly all forebrain synapses, suggesting different forms of synaptic plasticity may contribute to network development and information processing. To assess possible relationships between modifications in synaptic plasticity and maturation of cognitive ability, we examined excitatory synaptic function in area CA1 of the mouse hippocampus ∼3 weeks of age, when hippocampal-dependent learning and memory abilities first emerge. Long-term potentiation (LTP) and depression (LTD) of synaptic efficacy were observed in slices from juvenile animals younger than 3 weeks of age. Both pre- and postsynaptic mechanisms supported LTP and LTD in juveniles. After the third postnatal week, the magnitude of LTP was reduced and the threshold for postsynaptic induction was reduced, but the threshold for presynaptic induction was increased. The reduced threshold for postsynaptic LTP appeared to be due, partly, to an increase in baseline excitatory synaptic strength, which likely permitted greater postsynaptic depolarization during induction. Low frequency stimulation did not induce LTD at this more mature stage, but it blocked subsequent induction of LTP, suggesting metaplastic differences across age groups. Late postnatal modifications in activity-dependent synaptic plasticity might reflect attenuation of mechanisms more closely tied to network formation (presynaptic potentiation and pre- and postsynaptic depression) and unmasking of mechanisms underlying information processing and storage (associative postsynaptic potentiation), which likely impact the integrative capacity of the network and regulate the emergence of adult-like cognitive abilities.

Rules of engagement: factors that regulate activity-dependent synaptic plasticity during neural network development

Overproduction and pruning during development is a phenomenon that can be observed in the number of organisms in a population, the number of cells in many tissue types, and even the number of synapses on individual neurons. The sculpting of synaptic connections in the brain of a developing organism is guided by its personal experience, which on a neural level translates to specific patterns of activity. Activity-dependent plasticity at glutamatergic synapses is an integral part of neuronal network formation and maturation in developing vertebrate and invertebrate brains. As development of the rodent forebrain transitions away from an over-proliferative state, synaptic plasticity undergoes modification. Late developmental changes in synaptic plasticity signal the establishment of a more stable network and relate to pronounced perceptual and cognitive abilities. In large part, activation of glutamate-sensitive N-methyl-d-aspartate (NMDA) receptors regulates synaptic stabilization during development and is a necessary step in memory formation processes that occur in the forebrain. A developmental change in the subunits that compose NMDA receptors coincides with developmental modifications in synaptic plasticity and cognition, and thus much research in this area focuses on NMDA receptor composition. We propose that there are additional, equally important developmental processes that influence synaptic plasticity, including mechanisms that are upstream (factors that influence NMDA receptors) and downstream (intracellular processes regulated by NMDA receptors) from NMDA receptor activation. The goal of this review is to summarize what is known and what is not well understood about developmental changes in functional plasticity at glutamatergic synapses, and in the end, attempt to relate these changes to maturation of neural networks.

Random distance dependent attachment as a model for neural network generation in the Caenorhabditis elegans

MOTIVATION

The topology of the network induced by the neurons connectivity's in the Caenorhabditis elegans differs from most common random networks. The neurons positions of the C.elegans have been previously explained as being optimal to induce the required network wiring. We here propose a complementary explanation that the network wiring is the direct result of a local stochastic synapse formation process.

RESULTS

We show that a model based on the physical distance between neurons can explain the C.elegans neural network structure, specifically, we demonstrate that a simple model based on a geometrical synapse formation probability and the inhibition of short coherent cycles can explain the properties of the C.elegans' neural network. We suggest this model as an initial framework to discuss neural network generation and as a first step toward the development of models for more advanced creatures. In order to measure the circle frequency in the network, a novel graph-theory circle length measurement algorithm is proposed.

Altered mnemonic functions and resistance to N-METHYL-d-Aspartate receptor antagonism by forebrain conditional knockout of glycine transporter 1

Converging evidence from pharmacological and molecular studies has led to the suggestion that inhibition of glycine transporter 1 (GlyT1) constitutes an effective means to boost N-methyl-d-aspartate receptor (NMDAR) activity by increasing the extra-cellular concentration of glycine in the vicinity of glutamatergic synapses. However, the precise extent and limitation of this approach to alter cognitive function, and therefore its potential as a treatment strategy against psychiatric conditions marked by cognitive impairments, remain to be fully examined. Here, we generated mutant mice lacking GlyT1 in the entire forebrain including neurons and glia. This conditional knockout system allows a more precise examination of GlyT1 downregulation in the brain on behavior and cognition. The mutation was highly effective in attenuating the motor-stimulating effect of acute NMDAR blockade by phencyclidine, although no appreciable elevation in NMDAR-mediated excitatory postsynaptic currents (EPSC) was observed in the hippocampus. Enhanced cognitive performance was observed in spatial working memory and object recognition memory while spatial reference memory and associative learning remained unaltered. These findings provide further credence for the potential cognitive enhancing effects of brain GlyT1 inhibition. At the same time, they indicated potential phenotypic differences when compared with other constitutive and conditional GlyT1 knockout lines, and highlighted the possibility of a functional divergence between the neuronal and glia subpopulations of GlyT1 in the regulation of learning and memory processes. The relevance of this distinction to the design of future GlyT1 blockers as therapeutic tools in the treatment of cognitive disorders remains to be further investigated.

Developmental changes in short-term facilitation are opposite at temporoammonic synapses compared to Schaffer collateral synapses onto CA1 pyramidal cells

CA1 pyramidal neurons receive two distinct excitatory inputs that are each capable of influencing hippocampal output and learning and memory. The Schaffer collateral (SC) input from CA3 axons onto the more proximal dendrites of CA1 is part of the trisynaptic circuit, which originates in Layer II of the entorhinal cortex (EC). The temporoammonic (TA) pathway to CA1 provides input directly from Layer III of the EC onto the most distal dendrites of CA1 pyramidal cells, and is involved in spatial memory and memory consolidation. We have previously described a developmental decrease in short-term facilitation from juvenile (P13-18) to young adult (P28-42) rats at SC synapses that is due to feedback inhibition via synaptically activated mGluR1 on CA1 interneurons. It is not known how short-term changes in synaptic strength are regulated at TA synapses, nor is it known how short-term plasticity is balanced at SC and TA inputs during development. Here we describe a novel developmental increase in short-term facilitation at TA synapses, which is the opposite of the decrease in facilitation occurring at SC synapses. Although short-term facilitation is much lower at TA synapses when compared with SC synapses in juveniles, short-term plasticity at SC and TA synapses converges at similar levels of paired-pulse facilitation in the young adult rat. However, in young adults CA3-CA1 synapses still exhibit more facilitation than TA-CA1 synapses during physiologically-relevant activity, suggesting that the two pathways are each poised to uniquely modulate CA1 output in an activity-dependent manner. Finally, we show that there is a developmental decrease in the initial release probability at TA synapses that underlies their developmental decrease in facilitation, but no developmental change in release probability at SC synapses. This represents a fundamental difference in the presynaptic function of the two major inputs to CA1, which could alter the flow of information in hippocampus during development.