Pathogenic SYNGAP1 mutations impair cognitive development by disrupting maturation of dendritic spine synapses.

Mutations that cause intellectual disability (ID) and autism spectrum disorder (ASD) are commonly found in genes that encode for synaptic proteins. However, it remains unclear how mutations that disrupt synapse function impact intellectual ability. In the SYNGAP1 mouse model of ID/ASD, we found that dendritic spine synapses develop prematurely during the early postnatal period. Premature spine maturation dramatically enhanced excitability in the developing hippocampus, which corresponded with the emergence of behavioral abnormalities. Inducing SYNGAP1 mutations after critical developmental windows closed had minimal impact on spine synapse function, whereas repairing these pathogenic mutations in adulthood did not improve behavior and cognition. These data demonstrate that SynGAP protein acts as a critical developmental repressor of neural excitability that promotes the development of life-long cognitive abilities. We propose that the pace of dendritic spine synapse maturation in early life is a critical determinant of normal intellectual development.

A contextual binding theory of episodic memory: systems consolidation reconsidered.

Episodic memory reflects the ability to recollect the temporal and spatial context of past experiences. Episodic memories depend on the hippocampus but have been proposed to undergo rapid forgetting unless consolidated during offline periods such as sleep to neocortical areas for long-term storage. Here, we propose an alternative to this standard systems consolidation theory (SSCT) - a contextual binding account - in which the hippocampus binds item-related and context-related information. We compare these accounts in light of behavioural, lesion, neuroimaging and sleep studies of episodic memory and contend that forgetting is largely due to contextual interference, episodic memory remains dependent on the hippocampus across time, contextual drift produces post-encoding activity and sleep benefits memory by reducing contextual interference.

An expanded palette of dopamine sensors for multiplex imaging in vivo.

Genetically encoded dopamine sensors based on green fluorescent protein (GFP) enable high-resolution imaging of dopamine dynamics in behaving animals. However, these GFP-based variants cannot be readily combined with commonly used optical sensors and actuators, due to spectral overlap. We therefore engineered red-shifted variants of dopamine sensors called RdLight1, based on mApple. RdLight1 can be combined with GFP-based sensors with minimal interference and shows high photostability, permitting prolonged continuous imaging. We demonstrate the utility of RdLight1 for receptor-specific pharmacological analysis in cell culture, simultaneous assessment of dopamine release and cell-type-specific neuronal activity and simultaneous subsecond monitoring of multiple neurotransmitters in freely behaving rats. Dual-color photometry revealed that dopamine release in the nucleus accumbens evoked by reward-predictive cues is accompanied by a rapid suppression of glutamate release. By enabling multiplexed imaging of dopamine with other circuit components in vivo, RdLight1 opens avenues for understanding many aspects of dopamine biology.

Amnesia for context fear is caused by widespread disruption of hippocampal activity.

The hippocampus plays an essential role in the formation and retrieval of episodic memories in humans and contextual memories in animals. However, amnesia is not always observed when this structure is compromised. To determine why this is the case, we compared the effects of several different circuit manipulations on memory retrieval and hippocampal activity. Mice were first trained on context fear conditioning and then optogenetic and chemogenetic tools were used to alter activity during memory retrieval. We found that retrieval was only impaired when manipulations caused widespread changes (increases or decreases) in hippocampal activity. Widespread increases occurred when pyramidal cells were excited and widespread decreases were found when GABAergic neurons were stimulated. Direct hyperpolarization of excitatory neurons only moderately reduced activity and did not produce amnesia. Surprisingly, widespread decreases in hippocampal activity did not prevent retrieval if they occurred gradually prior to testing. This suggests that intact brain regions can express contextual memories if they are given adequate time to compensate for the loss of the hippocampus.

Prion-like behaviour and tau-dependent cytotoxicity of pyroglutamylated amyloid-ß.

Extracellular plaques of amyloid-ß and intraneuronal neurofibrillary tangles made from tau are the histopathological signatures of Alzheimer's disease. Plaques comprise amyloid-ß fibrils that assemble from monomeric and oligomeric intermediates, and are prognostic indicators of Alzheimer's disease. Despite the importance of plaques to Alzheimer's disease, oligomers are considered to be the principal toxic forms of amyloid-ß. Interestingly, many adverse responses to amyloid-ß, such as cytotoxicity, microtubule loss, impaired memory and learning, and neuritic degeneration, are greatly amplified by tau expression. Amino-terminally truncated, pyroglutamylated (pE) forms of amyloid-ß are strongly associated with Alzheimer's disease, are more toxic than amyloid-ß, residues 1-42 (Aß(1-42)) and Aß(1-40), and have been proposed as initiators of Alzheimer's disease pathogenesis. Here we report a mechanism by which pE-Aß may trigger Alzheimer's disease. Aß(3(pE)-42) co-oligomerizes with excess Aß(1-42) to form metastable low-n oligomers (LNOs) that are structurally distinct and far more cytotoxic to cultured neurons than comparable LNOs made from Aß(1-42) alone. Tau is required for cytotoxicity, and LNOs comprising 5% Aß(3(pE)-42) plus 95% Aß(1-42) (5% pE-Aß) seed new cytotoxic LNOs through multiple serial dilutions into Aß(1-42) monomers in the absence of additional Aß(3(pE)-42). LNOs isolated from human Alzheimer's disease brain contained Aß(3(pE)-42), and enhanced Aß(3(pE)-42) formation in mice triggered neuron loss and gliosis at 3 months, but not in a tau-null background. We conclude that Aß(3(pE)-42) confers tau-dependent neuronal death and causes template-induced misfolding of Aß(1-42) into structurally distinct LNOs that propagate by a prion-like mechanism. Our results raise the possibility that Aß(3(pE)-42) acts similarly at a primary step in Alzheimer's disease pathogenesis.

Encoding and storage of spatial information in the retrosplenial cortex.

The retrosplenial cortex (RSC) is part of a network of interconnected cortical, hippocampal, and thalamic structures harboring spatially modulated neurons. The RSC contains head direction cells and connects to the parahippocampal region and anterior thalamus. Manipulations of the RSC can affect spatial and contextual tasks. A considerable amount of evidence implicates the role of the RSC in spatial navigation, but it is unclear whether this structure actually encodes or stores spatial information. We used a transgenic mouse in which the expression of green fluorescent protein was under the control of the immediate early gene c-fos promoter as well as time-lapse two-photon in vivo imaging to monitor neuronal activation triggered by spatial learning in the Morris water maze. We uncovered a repetitive pattern of cell activation in the RSC consistent with the hypothesis that during spatial learning an experience-dependent memory trace is formed in this structure. In support of this hypothesis, we also report three other observations. First, temporary RSC inactivation disrupts performance in a spatial learning task. Second, we show that overexpressing the transcription factor CREB in the RSC with a viral vector, a manipulation known to enhance memory consolidation in other circuits, results in spatial memory enhancements. Third, silencing the viral CREB-expressing neurons with the allatostatin system occludes the spatial memory enhancement. Taken together, these results indicate that the retrosplenial cortex engages in the formation and storage of memory traces for spatial information.

Memory retrieval along the proximodistal axis of CA1.

The proximal and distal segments of CA1 are thought to perform distinct computations. Neurons in proximal CA1 are reciprocally connected with the medial entorhinal cortex (MEC) and exhibit precise spatial firing. In contrast, cells in distal CA1 communicate with the lateral entorhinal cortex (LEC), exhibit more diffuse spatial firing and are affected by the presence of objects in the environment. To determine if these segments make unique contributions to memory retrieval, we examined cellular activity along the proximodistal axis of CA1 using transgenic reporter mice. Neurons tagged during context learning in proximal CA1 were more likely to be reactivated during testing than those in distal CA1. This was true following context fear conditioning and after exposure to a novel environment. Reactivation was also higher in brain regions connected to proximal CA1 (MEC, distal CA3) than those connected to the distal segment (LEC, proximal CA3). To examine contributions to memory retrieval, we performed neurotoxic lesions of proximal or distal CA1 after training. Lesions of the proximal segment significantly impaired memory retrieval while damage to distal CA1 had no effect. These data suggest that context memories are retrieved by a hippocampal microcircuit that involves the proximal but not distal segment of CA1. © 2016 Wiley Periodicals, Inc.

Light enhances learned fear.

The ability to learn, remember, and respond to emotional events is a powerful survival strategy. However, dysregulated behavioral and physiological responses to these memories are maladaptive. To fully understand learned fear and the pathologies that arise during response malfunction we must reveal the environmental variables that influence learned fear responses. Light, a ubiquitous environmental feature, modulates cognition and anxiety. We hypothesized that light modulates responses to learned fear. Using tone-cued fear conditioning, we found that light enhances behavioral responses to learned fear in C57BL/6J mice. Mice in light freeze more in response to a conditioned cue than mice in darkness. The absence of significant freezing during a 2-wk habituation period and during intertrial intervals indicated that light specifically modulates freezing to the learned acoustic cue rather than the context of the experimental chamber. Repeating our assay in two photoreceptor mutant models, Pde6b(rd1/rd1) and Opn4(-/-) mice, revealed that light-dependent enhancement of conditioned fear is driven primarily by the rods and/or cones. By repeating our protocol with an altered lighting regimen, we found that lighting conditions acutely modulate responses when altered between conditioning and testing. This is manifested either as an enhancement of freezing when light is added during testing or as a depression of freezing when light is removed during testing. Acute enhancement, but not depression, requires both rod/cone- and melanopsin-dependent photoreception. Our results demonstrate a modulation by light of behavioral responses to learned fear.

New methods for understanding systems consolidation.

According to the standard model of systems consolidation (SMC), neocortical circuits are reactivated during the retrieval of declarative memories. This process initially requires the hippocampus. However, with the passage of time, neocortical circuits become strengthened and can eventually retrieve memory without input from the hippocampus. Although consistent with lesion data, these assumptions have been difficult to confirm experimentally. In the current review, we discuss recent methodological advances in behavioral neuroscience that are making it possible to test the basic assumptions of SMC for the first time. For example, new transgenic mice can be used to monitor the activity of individual neurons across the entire brain while optogenetic approaches provide precise control over the activity of these cells using light stimulation. These tools can be used to examine the reactivation of neocortical neurons during recent and remote memory retrieval and determine if this process requires the hippocampus.

The hippocampus contributes to retroactive stimulus associations during trace fear conditioning.

Binding events that occur at different times are essential for memory formation. In trace fear conditioning, animals associate a tone and footshock despite no temporal overlap. The hippocampus is thought to mediate this learning by maintaining a memory of the tone until shock occurrence, however, evidence for sustained hippocampal tone representations is lacking. Here, we demonstrate a retrospective role for the hippocampus in trace fear conditioning. Bulk calcium imaging revealed sustained increases in CA1 activity after footshock that were not observed after tone termination. Optogenetic silencing of CA1 immediately after footshock impaired subsequent memory. Additionally, footshock increased the number of sharp-wave ripples compared to baseline during conditioning. Therefore, post-shock hippocampal activity likely supports learning by reactivating and linking latent tone and shock representations. These findings highlight an underappreciated function of post-trial hippocampal activity in enabling retroactive temporal associations during new learning, as opposed to persistent maintenance of stimulus representations.

Phasic locus coeruleus activity enhances trace fear conditioning by increasing dopamine release in the hippocampus.

Locus coeruleus (LC) projections to the hippocampus play a critical role in learning and memory. However, the precise timing of LC-hippocampus communication during learning and which LC-derived neurotransmitters are important for memory formation in the hippocampus are currently unknown. Although the LC is typically thought to modulate neural activity via the release of norepinephrine, several recent studies have suggested that it may also release dopamine into the hippocampus and other cortical regions. In some cases, it appears that dopamine release from LC into the hippocampus may be more important for memory than norepinephrine. Here, we extend these data by characterizing the phasic responses of the LC and its projections to the dorsal hippocampus during trace fear conditioning in mice. We find that the LC and its projections to the hippocampus respond to task-relevant stimuli and that amplifying these responses with optogenetic stimulation can enhance long-term memory formation. We also demonstrate that LC activity increases both norepinephrine and dopamine content in the dorsal hippocampus and that the timing of hippocampal dopamine release during trace fear conditioning is similar to the timing of LC activity. Finally, we show that hippocampal dopamine is important for trace fear memory formation, while norepinephrine is not.

Our brains are more likely to remember activities or incidents that stand out from typical day-to-day experiences. For instance, if your phone is stolen on the way to work, you will have a stronger memory of this experience compared to other uneventful commutes. These are known as salient events and can be emotional, surprising, or even just out of the ordinary. During salient events, an area of the brain known as the hippocampus receives chemicals called neuromodulators from other parts of the brain. These neuromodulators enhance the formation of the memory by modifying how neurons connect together in the hippocampus. One of the regions that signals to the hippocampus ­ called the locus coeruleus ­ was thought to enhance memory by releasing the neuromodulator norepinephrine. Recent studies indicate that the locus coeruleus also releases a second neuromodulator called dopamine. However, it remained unclear what causes the locus coeruleus to release dopamine, and what effect this neuromodulator has on the hippocampus. To investigate these questions, Wilmot et al. recorded and manipulated the activity of the locus coeruleus in the brains of mice experiencing salient, fearful events. The mice were exposed to a sound and, a few seconds later, a shock to the foot to illicit the formation of an aversive salient memory. If the next day, the mice responded to just the sound as if they were expecting a shock, this indicated they had remembered the aversive experience. Wilmot et al. observed that neurons in the locus coeruleus were active during the salient event, resulting in increased dopamine in the hippocampus. When the activity of these neurons was forcefully increased during relatively non-salient events, such as a quiet tone and a very mild shock, the animals still showed strong memory formation. Finally, blocking the action of dopamine in the hippocampus substantially affected memory formation, whereas blocking the action of norepinephrine did not have the same effect. These findings suggest that the locus coeruleus enhances the memory of salient events by increasing the levels of dopamine in the hippocampus not norepinephrine, as was previously thought. Developing a better understanding of how the locus coeruleus regulates memory may lead to improved treatments for various neurological disorders, like Alzheimer's disease, which are associated with neuromodulators taking on different roles in the hippocampus.

Systems consolidation and the content of memory.

Systems consolidation is the process by which memories become independent of the hippocampus and stored in regions of the neocortex. This process is commonly studied in rodents using context fear conditioning. It is becoming increasingly clear, however, that context memories do not always undergo systems consolidation. To explain this fact, the current review describes a number of factors that determine whether or not context fear can be retrieved without the hippocampus during remote memory tests. These include neurogenesis, the presentation of reminder cues after learning, the quality of the memory that is retrieved during testing and the method that is used to inactivate the hippocampus. Based on these data, we propose that remote context fear memories can be retrieved by either the hippocampus or the neocortex. Tests of memory quality (e.g. context discrimination) can typically be used to determine which system is engaged during retrieval. The same is not true of recently formed context fear memories, which appear to always require the hippocampus during retrieval.

The cellular mechanisms of memory are modified by experience.

The N-methyl-D-aspartate receptor (NMDAR) is thought to be essential for synaptic plasticity and learning. However, recent work indicates that the role of this receptor depends on the prior history of the research subject. For example, animals trained on a hippocampus-dependent learning task are subsequently able to acquire new information in the absence of NMDAR activation. The current experiments were designed to identify the types of experiences that lead to NMDAR-independent learning. Using contextual fear conditioning in mice, we find that NMDAR-independent learning is only observed when (1) animals are trained on the same behavioral task and (2) initial learning is successfully encoded into long-term memory.

High-Frequency Stimulation of Ventral CA1 Neurons Reduces Amygdala Activity and Inhibits Fear.

The hippocampus can be divided into distinct segments that make unique contributions to learning and memory. The dorsal segment supports cognitive processes like spatial learning and navigation while the ventral hippocampus regulates emotional behaviors related to fear, anxiety and reward. In the current study, we determined how pyramidal cells in ventral CA1 respond to spatial cues and aversive stimulation during a context fear conditioning task. We also examined the effects of high and low frequency stimulation of these neurons on defensive behavior. Similar to previous work in the dorsal hippocampus, we found that cells in ventral CA1 expressed high-levels of c-Fos in response to a novel spatial environment. Surprisingly, however, the number of activated neurons did not increase when the environment was paired with footshock. This was true even in the subpopulation of ventral CA1 pyramidal cells that send direct projections to the amygdala. When these cells were stimulated at high-frequencies (20 Hz) we observed feedforward inhibition of basal amygdala neurons and impaired expression of context fear. In contrast, low-frequency stimulation (4 Hz) did not inhibit principal cells in the basal amygdala and produced an increase in fear generalization. Similar results have been reported in dorsal CA1. Therefore, despite clear differences between the dorsal and ventral hippocampus, CA1 neurons in each segment appear to make similar contributions to context fear conditioning.

Acute Disruption of the Dorsal Hippocampus Impairs the Encoding and Retrieval of Trace Fear Memories.

A major function of the hippocampus is to link discontiguous events in memory. This process can be studied in animals using Pavlovian trace conditioning, a procedure where the conditional stimulus (CS) and unconditional stimulus (US) are separated in time. While the majority of studies have found that trace conditioning requires the dorsal segment of the hippocampus, others have not. This variability could be due to the use of lesion and pharmacological techniques, which lack cell specificity and temporal precision. More recent studies using optogenetic tools find that trace fear acquisition is disrupted by decreases in dorsal CA1 (dCA1) activity while increases lead to learning enhancements. However, comparing these results is difficult given that some studies manipulated the activity of CA1 pyramidal neurons directly and others did so indirectly (e.g., via stimulation of entorhinal cortex inputs). The goal of the current experiments, therefore, was to compare the effects of direct CA1 excitation and inhibition on the encoding and expression of trace fear memories. Our data indicates that stimulation of ArchT in dCA1 pyramidal neurons reduces activity and impairs both the acquisition and retrieval of trace fear. Unlike previous work, direct stimulation of CA1 with ChR2 increases activity and produces deficits in trace fear learning and expression. We hypothesize that this is due to the artificial nature of optogenetic stimulation, which could disrupt processing throughout the hippocampus and in downstream structures.

Metaplasticity contributes to memory formation in the hippocampus.

Prior learning can modify the plasticity mechanisms that are used to encode new information. For example, NMDA receptor (NMDAR) activation is typically required for new spatial and contextual learning in the hippocampus. However, once animals have acquired this information, they can learn new tasks even if NMDARs are blocked. This finding suggests that behavioral training alters cellular plasticity mechanisms such that NMDARs are not required for subsequent learning. The mechanisms that mediate this change are currently unknown. To address this issue, we tested the idea that changes in intrinsic excitability (induced by learning) facilitate the encoding of new memories via metabotropic glutamate receptor (mGluR) activation. Consistent with this hypothesis, hippocampal neurons exhibited increases in intrinsic excitability after learning that lasted for several days. This increase was selective and only observed in neurons that were activated by the learning event. When animals were trained on a new task during this period, excitable neurons were reactivated and memory formation required the activation of mGluRs instead of NMDARs. These data suggest that increases in intrinsic excitability may serve as a metaplastic mechanism for memory formation.

New circuits for old memories: the role of the neocortex in consolidation.

Studies of learning and memory have provided a great deal of evidence implicating hippocampal mechanisms in the initial storage of facts and events. However, until recently, there were few hints as to how and where this information was permanently stored. A recent series of rodent molecular and cellular cognition studies provide compelling evidence for the involvement of specific neocortical regions in the storage of information initially processed in the hippocampus. Areas of the prefrontal cortex, including the anterior cingulate and prelimbic cortices, and the temporal cortex show robust increases in activity specifically following remote memory retrieval. Importantly, damage to or inactivation of these areas produces selective remote memory deficits. Additionally, transgenic studies provide glimpses into the molecular and cellular mechanisms underlying cortical memory consolidation. The studies reviewed here represent the first exciting steps toward the understanding of the molecular, cellular, and systems mechanisms of how the brain stores our oldest and perhaps most defining memories.

Interactions between the NR2B receptor and CaMKII modulate synaptic plasticity and spatial learning.

The NR2B subunit of the NMDA receptor interacts with several prominent proteins in the postsynaptic density, including calcium/calmodulin-dependent protein kinase II (CaMKII). To determine the function of these interactions, we derived transgenic mice expressing a ligand-activated carboxy-terminal NR2B fragment (cNR2B) by fusing this fragment to a tamoxifen (TAM)-dependent mutant of the estrogen receptor ligand-binding domain LBD(G521R). Here, we show that induction by TAM allows the transgenic cNR2B fragment to bind to endogenous CaMKII in neurons. Activation of the LBD(G521R)-cNR2B transgenic protein in mice leads to the disruption of CaMKII/NR2B interactions at synapses. The disruption decreases Thr286 phosphorylation of alphaCaMKII, lowers phosphorylation of a key CaMKII substrate in the postsynaptic membrane (AMPA receptor subunit glutamate receptor 1), and produces deficits in hippocampal long-term potentiation and spatial learning. Together our results demonstrate the importance of interactions between CaMKII and NR2B for CaMKII activity, synaptic plasticity, and learning.

NF-kappa B functions in synaptic signaling and behavior.

Ca(2+)-regulated gene transcription is essential to diverse physiological processes, including the adaptive plasticity associated with learning. We found that basal synaptic input activates the NF-kappa B transcription factor by a pathway requiring the Ca(2+)/calmodulin-dependent kinase CaMKII and local submembranous Ca(2+) elevation. The p65:p50 NF-kappa B form is selectively localized at synapses; p65-deficient mice have no detectable synaptic NF-kappa B. Activated NF-kappa B moves to the nucleus and could directly transmute synaptic signals into altered gene expression. Mice lacking p65 show a selective learning deficit in the spatial version of the radial arm maze. These observations suggest that long-term changes to adult neuronal function caused by synaptic stimulation can be regulated by NF-kappa B nuclear translocation and gene activation.