Subfield-specific immediate early gene expression associated with hippocampal long-term potentiation in vivo

Projection-TAGs enable multiplex projection tracing and multi-modal profiling of projection neurons

Single-cell multiomic techniques have sparked immense interest in developing a comprehensive multi-modal map of diverse neuronal cell types and their brain wide projections. However, investigating the spatial organization, transcriptional and epigenetic landscapes of brain wide projection neurons is hampered by the lack of efficient and easily adoptable tools. Here we introduce Projection-TAGs, a retrograde AAV platform that allows multiplex tagging of projection neurons using RNA barcodes. By using Projection-TAGs, we performed multiplex projection tracing of the mouse cortex and high-throughput single-cell profiling of the transcriptional and epigenetic landscapes of the cortical projection neurons. Projection-TAGs can be leveraged to obtain a snapshot of activity-dependent recruitment of distinct projection neurons and their molecular features in the context of a specific stimulus. Given its flexibility, usability, and compatibility, we envision that Projection-TAGs can be readily applied to build a comprehensive multi-modal map of brain neuronal cell types and their projections.

Insulin-induced long-term potentiation in the dentate gyrus of hippocampal formation

The discovery that brain areas involving in learning and memory express receptors for insulin hormone, led to the idea that insulin signaling may have a role in regulating cognitive function. Although previous studies have shown a role for insulin in regulation of the threshold of plasticity induction, no study has addressed whether insulin can induce a chemical plasticity per se. Young-adult male rats that are fed with standard diets with or without carbohydrate syrup (sucrose or high-fructose corn syrups) were enrolled in this study. Extracellular field potentials were recorded from the dentate gyrus in response to perforant pathway stimulation at 0.033 Hz in anesthetized rats. The slope of field excitatory postsynaptic potentials (fEPSPs) and the amplitude of population spike (PS) were measured 15 min after a 60-min infusion of insulin (500 nM), NT157 (an IRS inhibitor, 6 μM), alone or together, or physiological saline. mRNA expressions of insulin signaling proteins were measured by rt-PCR in the whole hippocampus. We did not observe any appreciable change in the fEPSP slope and the PS amplitude before and after saline infusion. However, intra-hippocampal insulin application results in the induction of LTP of fEPSP and of PS in the dentate gyrus. Insulin infusion together with NT157 inhibited fEPSP-LTP, but not PS-LTP, and rats that are fed with carbohydrate syrup did not express synaptic LTP. In rats that additional carbohydrate syrup is not given, insulin-induced LTP was accompanied with an increase in PI3K-mRNA, AKT-mRNA, and GSK-3β-mRNA which was not observed when co-administered with NT157. The GSK-3β-mRNA and IRS1-mRNA levels were found to be lower in rats that received supplemental carbohydrate and that not express insulin-induced synaptic LTP, compared to the rats expressing synaptic LTP and fed by standard diet. The results obtained provide a mechanistic link between insulin and synaptic plasticity. We concluded that insulin not only functions as a modulator of synaptic plasticity but also acts as a chemical inducer of LTP.

The Induction of Long-Term Potentiation by Medial Septum Activation under Urethane Anesthesia Can Alter Gene Expression in the Hippocampus

We studied changes in the expression of early genes in hippocampal cells in response to stimulation of the dorsal medial septal area (dMSA), leading to long-term potentiation in the hippocampus. Rats under urethane anesthesia were implanted with stimulating electrodes in the ventral hippocampal commissure and dMSA and a recording electrode in the CA1 area of the hippocampus. We found that high-frequency stimulation (HFS) of the dMSA led to the induction of long-term potentiation in the synapses formed by the ventral hippocampal commissure on the hippocampal CA1 neurons. One hour after dMSA HFS, we collected the dorsal and ventral hippocampi on both the ipsilateral (damaged by the implanted electrode) and contralateral (intact) sides and analyzed the expression of genes by qPCR. The dMSA HFS led to an increase in the expression of bdnf and cyr61 in the ipsilateral hippocampi and egr1 in the ventral contralateral hippocampus. Thus, dMSA HFS under the conditions of degeneration of the cholinergic neurons in the medial septal area prevented the described increase in gene expression. The changes in cyr61 expression appeared to be dependent on the muscarinic M1 receptors. Our data suggest that the induction of long-term potentiation by dMSA activation enhances the expression of select early genes in the hippocampus.

De novo variants in MAST4 related to neurodevelopmental disorders with developmental delay and infantile spasms: Genotype-phenotype association

Objective

This study aims to prove that the de novo variants in MAST4 gene are associated with neurodevelopmental disorders (NDD) with developmental delay (DD) and infantile spasm (IS) and to determine the genotype-phenotype correlations.

Methods

Trio-based exome sequencing (ES) was performed on the four families enrolled in this study. We collected and systematically reviewed the four probands' clinical data, magnetic resonance images (MRI), and electroencephalography (EEG). We also carried out bioinformatics analysis by integrating published exome/genome sequencing data and human brain transcriptomic data.

Results

We described four patients whose median age of seizure onset was 5 months. The primary manifestation was infantile spasms with typical hypsarrhythmia on EEG. Developmental delays or intellectual disabilities varied among the four individuals. Three de novo missense variants in MAST4 gene were identified from four families, including chr5:66438324 (c.2693T > C: p.Ile898Thr) z, chr5:66459419 (c.4412C > T: p.Thr1471Ile), and chr5:66462662 (c.7655C > G:p.Ser2552Trp). The missense variant p.Ile898Thr is mapped to the AGC-kinase C-terminal with phosphatase activity. The other variant p.Ser2552Trp is located in a phosphoserine-modified residue which may affect cell membrane stability and signal transduction. Besides, the variant p.Thr1471Ile is a recurrent site screened out in two unrelated patients. Compared to private mutations (found only in a single family or a small population) of MAST4 in the gnomAD non-neuro subset, all de novo variants were predicted to be damaging or probably damaging through different bioinformatic analyses. Significantly higher CADD scores of the variant p.Thr1471Ile indicate more deleteriousness of the recurrent site. And the affected amino acids are highly conserved across multiple species. According to the Brainspan Atlas database, MAST4 is expressed primarily in the mediodorsal nucleus of the thalamus and medial prefrontal cortex during the prenatal period, potentially contributing to embryonic brain development.

Conclusion

Our results revealed that the variants of MAST4 gene might lead to neurodevelopmental disorders with developmental delay and infantile spasm. Thus, MAST4 variants should be considered the potential candidate gene in patients with neurodevelopmental disorders clinically marked by infantile spasms.

CDKL5 Deficiency Augments Inhibitory Input into the Dentate Gyrus That Can Be Reversed by Deep Brain Stimulation

Cognitive impairment is a core feature of cyclin-dependent kinase-like 5 (CDKL5) deficiency, a neurodevelopmental disorder characterized by early epileptic seizures, intellectual disability, and autistic behaviors. Although loss of CDKL5 affects a number of molecular pathways, very little has been discovered about the physiological effects of these changes on the neural circuitry. We therefore studied synaptic plasticity and local circuit activity in the dentate gyrus of both Cdkl5-/y and Cdkl5+/- mutant mice. We found that CDKL5 haploinsufficiency in both male and female mice impairs hippocampus-dependent learning and memory in multiple tasks. In vivo, loss of CDKL5 reduced LTP of the perforant path to the dentate gyrus and augmented feedforward inhibition in this pathway; ex vivo experiments confirmed that excitatory/inhibitory input into the dentate gyrus is skewed toward inhibition. Injecting the GABAergic antagonist gabazine into the dentate improved contextual fear memory in Cdkl5-/y mice. Finally, chronic forniceal deep brain stimulation rescued hippocampal memory deficits, restored synaptic plasticity, and relieved feedforward inhibition in Cdkl5+/- mice. These results indicate that CDKL5 is important for maintaining proper dentate excitatory/inhibitory balance, with consequences for hippocampal memory.SIGNIFICANCE STATEMENT Cognitive impairment is a core feature of cyclin-dependent kinase-like 5 (CDKL5) deficiency disorder. Although CDKL5 deficiency has been found to affect a number of molecular pathways, little is known about its physiological effects on the neural circuitry. We find that CDKL5 loss reduces hippocampal synaptic plasticity and augments feedforward inhibition in the perforant path to the dentate gyrus in vivo in Cdkl5 mutant mice. Chronic forniceal deep brain stimulation rescued hippocampal memory deficits, restored synaptic plasticity, and relieved feedforward inhibition in Cdkl5+/- mice, as it had previously done with Rett syndrome mice, suggesting that such stimulation may be useful for other neurodevelopmental disorders.

A role for the endocannabinoid enzymes monoacylglycerol and diacylglycerol lipases in cue-induced cocaine craving following prolonged abstinence

Following exposure to drugs of abuse, long-term neuroadaptations underlie persistent risk to relapse. Endocannabinoid signaling has been associated with drug-induced neuroadaptations, but the role of lipases that mediate endocannabinoid biosynthesis and metabolism in regulating relapse behaviors following prolonged periods of drug abstinence has not been examined. Here, we investigated how pharmacological manipulation of lipases involved in regulating the expression of the endocannabinoid 2-AG in the nucleus accumbens (NAc) influence cocaine relapse via discrete neuroadaptations. At prolonged abstinence (30 days) from cocaine self-administration, there is an increase in the NAc levels of diacylglycerol lipase (DAGL), the enzyme responsible for the synthesis of the endocannabinoid 2-AG, along with decreased levels of monoacylglycerol lipase (MAGL), which hydrolyzes 2-AG. Since endocannabinoid-mediated behavioral plasticity involves phosphatase dysregulation, we examined the phosphatase calcineurin after 30 days of abstinence and found decreased expression in the NAc, which we demonstrate is regulated through the transcription factor EGR1. Intra-NAc pharmacological manipulation of DAGL and MAGL with inhibitors DO-34 and URB-602, respectively, bidirectionally regulated cue-induced cocaine seeking and altered the phosphostatus of translational initiation factor, eIF2α. Finally, we found that cocaine seeking 30 days after abstinence leads to decreased phosphorylation of eIF2α and reduced expression of its downstream target NPAS4, a protein involved in experience-dependent neuronal plasticity. Together, our findings demonstrate that lipases that regulate 2-AG expression influence transcriptional and translational changes in the NAc related to drug relapse vulnerability.

Contrasting Functions of Mitogen- and Stress-activated Protein Kinases 1 and 2 in Recognition Memory and In Vivo Hippocampal Synaptic Transmission

The mitogen-activated protein kinases (MAPK) are major signaling components of intracellular pathways required for memory consolidation. Mitogen- and stress-activated protein kinases 1 and 2 (MSK1 and MSK2) mediate signal transduction downstream of MAPK. MSKs are activated by Extracellular-signal Regulated Kinase 1/2 (ERK1/2) and p38 MAPK. In turn, they can activate cyclic AMP-response-element-binding protein (CREB), thereby modulating the expression of immediate early genes crucial for the formation of long-term memories. While MSK1 has been previously implicated in certain forms of learning and memory, little is known concerning MSK2. Our goal was to explore the respective contribution of MSK1 and MSK2 in hippocampal synaptic transmission and plasticity and hippocampal-dependent recognition memory. In Msk1- and Msk2-knockout mice, we evaluated object and object-place recognition memory, basal synaptic transmission, paired-pulse facilitation (PPF) and inhibition (PPI), and the capacity to induce and sustain long-term potentiation (LTP) in vivo. We also assessed the level of two proteins downstream in the MAPK/ERK1/2 pathway crucial for long-term memory, CREB and the immediate early gene (IEG) Early growth response 1 (EGR1). Loss of Msk1, but not of Msk2, affected excitatory synaptic transmission at perforant path-to-dentate granule cell synapses, altered short-term presynaptic plasticity, impaired selectively long-term spatial recognition memory, and decreased basal levels of CREB and its activated form. LTP in vivo and LTP-induced CREB phosphorylation and EGR1 expression were unchanged after Msk1 or Msk2 deletion. Our findings demonstrate a dissimilar contribution of MSKs proteins in cognitive processes and suggest that Msk1 loss-of-function only has a deleterious impact on neuronal activity and hippocampal-dependent memory consolidation.

Arc/Arg3.1 function in long-term synaptic plasticity: Emerging mechanisms and unresolved issues

Arc (activity-regulated cytoskeleton-associated protein) is posited as a critical regulator of long-term synaptic plasticity at excitatory synapses, including long-term potentiation, long-term depression, inverse synaptic tagging and homoeostatic scaling, with pivotal roles in memory and postnatal cortical development. However, the mechanisms underlying the bidirectional regulation of synaptic strength are poorly understood. Here we review evidence from different plasticity paradigms, highlight outstanding issues and discuss stimulus-specific mechanisms that dictate Arc function. We propose a model in which Arc bidirectionally controls synaptic strength by coordinate regulation of AMPA-type glutamate receptor (AMPAR) trafficking and actin cytoskeletal dynamics in dendritic spines. Key to this model, Arc is proposed to function as an activity-dependent regulator of AMPAR lateral membrane diffusion and trapping at synapses.

Distinct patterns of brain Fos expression in Carioca High- and Low-conditioned Freezing Rats

BACKGROUND

The bidirectional selection of high and low anxiety-like behavior is a valuable tool for understanding the neurocircuits that are responsible for anxiety disorders. Our group developed two breeding lines of rats, known as Carioca High- and Low-conditioned Freezing (CHF and CLF), based on defensive freezing in the contextual fear conditioning paradigm. A random selected line was employed as a control (CTL) comparison group for both CHF and CLF lines of animals. The present study performed Fos immunochemistry to investigate changes in neural activity in different brain structures among CHF and CLF rats when they were exposed to contextual cues that were previously associated with footshock.

RESULTS

The study indicated that CHF rats expressed high Fos expression in the locus coeruleus, periventricular nucleus of the hypothalamus (PVN), and lateral portion of the septal area and low Fos expression in the medial portion of the septal area, dentate gyrus, and prelimbic cortex (PL) compared to CTL animals. CLF rats exhibited a decrease in Fos expression in the PVN, PL, and basolateral nucleus of the amygdala and increase in the cingulate and perirhinal cortices compared to CTL animals.

CONCLUSIONS

Both CHF and CLF rats displayed Fos expression changes key regions of the anxiety brain circuitry. The two bidirectional lines exhibit different pattern of neural activation and inhibition with opposing influences on the PVN, the main structure involved in regulating the hypothalamic-pituitary-adrenal neuroendocrine responses observed in anxiety disorders.

Sox11 is an Activity-Regulated Gene with Dentate-Gyrus-Specific Expression Upon General Neural Activation

Neuronal activity initiates transcriptional programs that shape long-term changes in plasticity. Although neuron subtypes differ in their plasticity response, most activity-dependent transcription factors (TFs) are broadly expressed across neuron subtypes and brain regions. Thus, how region- and neuronal subtype-specific plasticity are established on the transcriptional level remains poorly understood. We report that in young adult (i.e., 6-8 weeks old) mice, the developmental TF SOX11 is induced in neurons within 6 h either by electroconvulsive stimulation or by exploration of a novel environment. Strikingly, SOX11 induction was restricted to the dentate gyrus (DG) of the hippocampus. In the novel environment paradigm, SOX11 was observed in a subset of c-FOS expressing neurons (ca. 15%); whereas around 75% of SOX11+ DG granule neurons were c-FOS+, indicating that SOX11 was induced in an activity-dependent fashion in a subset of neurons. Environmental enrichment or virus-mediated overexpression of SOX11 enhanced the excitability of DG granule cells and downregulated the expression of different potassium channel subunits, whereas conditional Sox11/4 knock-out mice presented the opposite phenotype. We propose that Sox11 is regulated in an activity-dependent fashion, which is specific to the DG, and speculate that activity-dependent Sox11 expression may participate in the modulation of DG neuron plasticity.

Long-lasting correction of in vivo LTP and cognitive deficits of mice modelling Down syndrome with an α5-selective GABAA inverse agonist

Background and Purpose

Excessive GABAergic inhibition contributes to cognitive dysfunctions in Down syndrome (DS). Selective negative allosteric modulators (NAMs) of α5‐containing GABA_A receptors such as the α5 inverse agonist (α5IA) restore learning and memory deficits in Ts65Dn mice, a model of DS. In this study we have assessed the long‐lasting effects of α5IA on in vivo LTP and behaviour in Ts65Dn mice.

Experimental Approach

We made in vivo LTP recordings for six consecutive days in freely moving Ts65Dn mice and their wild‐type littermates, treated with vehicle or α5IA. In parallel, Ts65Dn mice were assessed by various learning and memory tests (Y maze, Morris water maze, or the novel object recognition) for up to 7 days, following one single injection of α5IA or vehicle.

Key Results

LTP was not evoked in vivo in Ts65Dn mice at hippocampal CA3‐CA1 synapses. However, this deficit was sustainably reversed for at least six consecutive days following a single injection of α5IA. This long‐lasting effect of α5IA was also observed when assessing working and long‐term memory deficits in Ts65Dn mice.

Conclusion and Implications

We show for the first time in vivo LTP deficits in Ts65Dn mice. These deficits were restored for at least 6 days following acute treatment with α5IA and might be the substrate for the long‐lasting pharmacological effects of α5IA on spatial working and long‐term recognition and spatial memory tasks. Our results demonstrate the relevance of negative allosteric modulators of α5‐containing GABA_A receptors to the treatment of cognitive deficits associated with DS.

Sevoflurane Impairs Short-Term Memory by Affecting PSD-95 and AMPA Receptor in the Hippocampus of a Mouse Model

Objective

To explore the effects of sevoflurane on the latency and error times of the passive avoidance and levels of PSD-95 and AMPA receptors in the hippocampus. We evaluated the effects of sevoflurane on short-term memory in adult mice and explored the possible mechanism.

Methods

144 Kunming mice (2-3 months, 30-35 g) were randomly divided into two groups A (n = 64) and B (n = 80) and received the dark-avoidance (DA) and step-down avoidance (SA) tests, respectively. The groups DA and SA were further divided into control (inhaled 40% O₂ 2 h) and sevoflurane (3.3% sevoflurane and 40% O₂ 2 h) subgroups. Before inhalation intervention, all mice were trained to be familiar with the Morris water maze (MWM). According to the test points of behavioral indicators, 8 mice were randomly selected from each subgroup at point 12 h (T1), 24 h (T2), 48 h (T3), and 72 h (T4) after inhalation intervention. The step-through latency and error times were measured in 5 min. After the behavioral test, the mice were killed and the tissues of the hippocampus were taken for hematoxylin and eosin (H&E) staining. The expression level of PSD-95 and AMPA receptors in the hippocampus was detected by immunohistochemistry and Western Blot. The changes of synaptic transmission were measured via electrophysiology analysis of hippocampal slices.

Results

The mice in the control subgroups found the platform in a shorter pathway than those in the sevoflurane subgroups during an MWM test. The step-through latency of T1 and T2 in the sevoflurane subgroup was shorter than baseline time, and the error times were increased in 5 min and higher than baseline time when compared with the control subgroup (P < 0.05) in the A and B groups. Compared with the control subgroup, the expression level of PSD-95 and AMPA receptors in the hippocampus was decreased at T1 and T2 in the sevoflurane subgroup (P < 0.05). The nerve cells were partially swelling. Electrophysiology analysis showed that the levels of PSD-95 and AMPA receptor expression were associated with synaptic transmission.

Conclusion

Sevoflurane impaired short-term memory in adult mice by inhibiting the expression of PSD-95 and AMPA receptors in the hippocampus, which led to the decrease in synaptic transmission.

Embracing Complexity in Defensive Networks

The neural basis of defensive behaviors continues to attract much interest, not only because they are important for survival but also because their dysregulation may be at the origin of anxiety disorders. Recently, a dominant approach in the field has been the optogenetic manipulation of specific circuits or cell types within these circuits to dissect their role in different defensive behaviors. While the usefulness of optogenetics is unquestionable, we argue that this method, as currently applied, fosters an atomistic conceptualization of defensive behaviors, which hinders progress in understanding the integrated responses of nervous systems to threats. Instead, we advocate for a holistic approach to the problem, including observational study of natural behaviors and their neuronal correlates at multiple sites, coupled to the use of optogenetics, not to globally turn on or off neurons of interest, but to manipulate specific activity patterns hypothesized to regulate defensive behaviors.

Modulation of Apoptosis Controls Inhibitory Interneuron Number in the Cortex

Cortical networks are composed of excitatory projection neurons and inhibitory interneurons. Finding the right balance between the two is important for controlling overall cortical excitation and network dynamics. However, it is unclear how the correct number of cortical interneurons (CIs) is established in the mammalian forebrain. CIs are generated in excess from basal forebrain progenitors, and their final numbers are adjusted via an intrinsically determined program of apoptosis that takes place during an early postnatal window. Here, we provide evidence that the extent of CI apoptosis during this critical period is plastic and cell-type specific and can be reduced in a cell-autonomous manner by acute increases in neuronal activity. We propose that the physiological state of the emerging neural network controls the activity levels of local CIs to modulate their numbers in a homeostatic manner.

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Lhx6 is required for survival of CIs generated in the MGE

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MGE-derived CI loss is compensated for by a decrease in CGE-derived interneuron apoptosis

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Increases in cortical network activity are correlated with improved CI survival

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Transient, cell-autonomous depolarization improves the survival of grafted CIs

Frontal Cortex Epigenetic Dysregulation During the Progression of Alzheimer's Disease

Although the frontal cortex plays an important role in cognitive function and undergoes neuronal dysfunction in Alzheimer's disease (AD), the factors driving these cellular alterations remain unknown. Recent studies suggest that alterations in epigenetic regulation play a pivotal role in this process in AD. We evaluated frontal cortex histone deacetylase (HDAC) and sirtuin (SIRT) levels in tissue obtained from subjects with a premortem diagnosis of no-cognitive impairment (NCI), mild cognitive impairment (MCI), mild to moderate AD (mAD), and severe AD (sAD) using quantitative western blotting. Immunoblots revealed significant increases in HDAC1 and HDAC3 in MCI and mAD, followed by a decrease in sAD compared to NCI. HDAC2 levels remained stable across clinical groups. HDAC4 was significantly increased in MCI and mAD, but not in sAD compared to NCI. HDAC6 significantly increased during disease progression, while SIRT1 decreased in MCI, mAD, and sAD compared to NCI. HDAC1 levels negatively correlated with perceptual speed, while SIRT1 positively correlated with perceptual speed, episodic memory, global cognitive score, and Mini-Mental State Examination. HDAC1 positively, while SIRT1 negatively correlated with cortical neurofibrillary tangle counts. These findings suggest that dysregulation of epigenetic proteins contribute to neuronal dysfunction and cognitive decline in the early stage of AD.

Transcriptional Regulation Involved in Fear Memory Reconsolidation

Memory reconsolidation has been demonstrated to offer a potential target period during which the fear memories underlying fear disorders can be disrupted. Reconsolidation is a labile stage that consolidated memories re-enter after memories are reactivated. Reactivated memories, induced by cues related to traumatic events, are susceptible to strengthening and weakening. Gene transcription regulation and protein synthesis have been suggested to be required for fear memory reconsolidation. Investigating the transcriptional regulation mechanisms underlying reconsolidation may provide a therapeutic method for the treatment of fear disorders such as post-traumatic stress disorder (PTSD). However, the therapeutic effect of treating a fear disorder through interfering with reconsolidation is still contradictory. In this review, we summarize several transcription factors that have been linked to fear memory reconsolidation and propose that transcription factors, as well as related signaling pathways can serve as targets for fear memory interventions. Then, we discuss the application of pharmacological and behavioral interventions during reconsolidation that may or not efficiently treat fear disorders.

Somatic Arc protein expression in hippocampal granule cells is increased in response to environmental change but independent of task-specific learning

Activated neurons express immediate-early genes, such as Arc. Expression of Arc in the hippocampal granule cell layer, an area crucial for spatial learning and memory, is increased during acquisition of spatial learning; however, it is unclear whether this effect is related to the task-specific learning process or to nonspecific aspects of the testing procedure (e.g. exposure to the testing apparatus and exploration of the environment). Herein, we show that Arc-positive cells numbers are increased to the same extent in the granule cell layer after both acquisition of a single spatial learning event in the active place avoidance task and exploration of the testing environment, as compared to naïve (i.e. caged) mice. Repeated exposure the testing apparatus and environment did not reduce Arc expression. Furthermore, Arc expression did not correlate with performance in both adult and aged animals, suggesting that exploration of the testing environment, rather than the specific acquisition of the active place avoidance task, induces Arc expression in the dentate granule cell layer. These findings thus suggest that Arc is an experience-induced immediate-early gene.

Age-related changes in Egr1 transcription and DNA methylation within the hippocampus

Aged animals show functional alterations in hippocampal neurons that lead to deficits in synaptic plasticity and changes in cognitive function. Transcription of immediate-early genes (IEGs), including Egr1, is necessary for processes such as long-term potentiation and memory consolidation. Here, we show an age-related reduction in the transcription of Egr1 in the dentate gyrus following spatial behavior, whereas in the area CA1, Egr1 is reduced at rest, but its transcription can be effectively driven by spatial behavior to levels equivalent to those observed in adult animals. One mechanism possibly contributing to these aging-related changes is an age-associated, CpG site-specific change in methylation in DNA associated with the promoter region of the Egr1 gene. Our results add to a growing body of work demonstrating that complex transcriptional and epigenetic changes in the hippocampus significantly contribute to brain and cognitive aging. © 2016 Wiley Periodicals, Inc.

Variants of contextual fear conditioning induce differential patterns of Egr-1 activity within the young adult prefrontal cortex

Contextual fear conditioning is a form of associative learning where animals must experience a context before they can associate it with an aversive stimulus. Single-trial contextual fear conditioning (sCFC) and the context preexposure facilitation effect (CPFE) are two variants of CFC where learning about the context is temporally contiguous (sCFC) with or separated (CPFE) from receiving a footshock in that context. Neural activity within CA1 of the dorsal hippocampus (CA1), amygdala (LA), and prefrontal cortex (PFC) may play a critical role when animals learn to associate a context with a footshock (i.e., training). Previous studies from our lab have found that early-growth-response gene 1 (Egr-1), an immediate early gene, exhibits unique patterns of activity within regions of the PFC following training in sCFC and the CPFE of juvenile rats. In the present study, we extended our studies by examining Egr-1 expression in young adult rats to determine (1) if our previous work reflected changes unique to development or extend into adulthood and (2) to contrast expression profiles between sCFC and the CPFE. Rats that learned context fear with sCFC showed increased Egr-1 in the anterior cingulate, orbitofrontal and infralimbic cortices relative to non-associative controls following training, but expression in prelimbic cortex did not differ between fear conditioned and non-associative controls. In contrast, rats trained in the CPFE also showed increased Egr-1 in all the prefrontal cortex regions, including prelimbic cortex. These findings replicate our previous findings in juveniles and suggest that Egr-1 in specific PFC subregions may be uniquely involved in learning context-fear in the CPFE compared to sCFC.

Selective and compartmentalized myelin expression of HspB5

In the present study, we reveal myelin-specific expression and targeting of mRNA and biochemical pools of HspB5 in the mouse CNS. Our observations are based on in situ hybridization, electron microscopy and co-localization with 2',3'-Cyclic-Nucleotide 3'-Phosphodiesterase (CNPase), reinforcing this myelin-selective expression. HspB5 mRNA might be targeted to these structures based on its presence in discrete clusters resembling RNA granules and the presence of a putative RNA transport signal. Further, sub-cellular fractionation of myelin membranes reveals a distinct sub-compartment-specific association and detergent solubility of HspB5. This is akin to other abundant myelin proteins and is consistent with HspB5's association with cytoskeletal/membrane assemblies. Oligodendrocytes have a pivotal role in supporting axonal function via generating and segregating the ensheathing myelin. This specialization places extreme structural and metabolic demands on this glial cell type. Our observations place HspB5 in oligodendrocytes which may require selective and specific chaperone capabilities to maintain normal function and neuronal support.

Exploring Memory Representations with Activity-Based Genetics

The brain is thought to represent specific memories through the activity of sparse and distributed neural ensembles. In this review, we examine the use of immediate early genes (IEGs), genes that are induced by neural activity, to specifically identify and genetically modify neurons activated naturally by environmental experience. Recent studies using this approach have identified cellular and molecular changes specific to neurons activated during learning relative to their inactive neighbors. By using opto- and chemogenetic regulators of neural activity, the neurons naturally recruited during learning can be artificially reactivated to directly test their role in coding external information. In contextual fear conditioning, artificial reactivation of learning-induced neural ensembles in the hippocampus or neocortex can substitute for the context itself. That is, artificial stimulation of these neurons can apparently cause the animals to "think" they are in the context. This represents a powerful approach to testing the principles by which the brain codes for the external world and how these circuits are modified with learning.

Defective synaptic transmission and structure in the dentate gyrus and selective fear memory impairment in the Rsk2 mutant mouse model of Coffin-Lowry syndrome

The Coffin-Lowry syndrome (CLS) is a syndromic form of intellectual disability caused by loss-of-function of the RSK2 serine/threonine kinase encoded by the rsk2 gene. Rsk2 knockout mice, a murine model of CLS, exhibit spatial learning and memory impairments, yet the underlying neural mechanisms are unknown. In the current study, we examined the performance of Rsk2 knockout mice in cued, trace and contextual fear memory paradigms and identified selective deficits in the consolidation and reconsolidation of hippocampal-dependent fear memories as task difficulty and hippocampal demand increase. Electrophysiological, biochemical and electron microscopy analyses were carried out in the dentate gyrus of the hippocampus to explore potential alterations in neuronal functions and structure. In vivo and in vitro electrophysiology revealed impaired synaptic transmission, decreased network excitability and reduced AMPA and NMDA conductance in Rsk2 knockout mice. In the absence of RSK2, standard measures of short-term and long-term potentiation (LTP) were normal, however LTP-induced CREB phosphorylation and expression of the transcription factors EGR1/ZIF268 were reduced and that of the scaffolding protein SHANK3 was blocked, indicating impaired activity-dependent gene regulation. At the structural level, the density of perforated and non-perforated synapses and of multiple spine boutons was not altered, however, a clear enlargement of spine neck width and post-synaptic densities indicates altered synapse ultrastructure. These findings show that RSK2 loss-of-function is associated in the dentate gyrus with multi-level alterations that encompass modifications of glutamate receptor channel properties, synaptic transmission, plasticity-associated gene expression and spine morphology, providing novel insights into the mechanisms contributing to cognitive impairments in CLS.

Maternal marginal iodine deficiency affects the expression of relative proteins during brain development in rat offspring

Marginal iodine deficiency is a major health problem in pregnant women, but its impact on nerve and intelligence development in offspring has been rarely reported. Our study aimed to investigate the effects of maternal marginal iodine deficiency on nerve and cognitive development in offspring and the related mechanisms. Marginal iodine-deficient rats were given 3  μg iodine per day, while normal control rats were given 4  μg iodine daily. Western blot was used to detect the amounts of brain-derived neurotropic factor (BDNF) and early growth response protein 1 (EGR1) in the hippocampus of each group. Immunohistochemistry was used to measure c-jun and c-fos expression in the hippocampal CA1 region. Finally, the water maze method was used to measure spatial performance. Free thyroxine (FT₄) levels in marginal iodine-deficient rats decreased by about 30%. Seven days after birth, EGR1 and BDNF protein levels significantly decreased in the hippocampus of marginal iodine deficiency rats compared with the normal control group. In addition, c-jun and c-fos expression in the hippocampus of 40-day-old rats was decreased in marginal iodine-deficient rats, compared with control. The spatial learning and memory ability of 40-day-old marginal iodine-deficient rats had a downward trend compared with the normal control group. FT₄ significantly decreased after pregnancy in rats with marginal iodine deficiency, affecting the expression of related proteins in the brain of offspring.

Differentiation in the protein synthesis-dependency of persistent synaptic plasticity in mossy fiber and associational/commissural CA3 synapses in vivo

Long-term potentiation (LTP) and long-term depression (LTD) are two mechanisms involved in the long-term storage of information in hippocampal synapses. In the hippocampal CA1 region, the late phases of LTP and LTD are protein-synthesis dependent. In the dentate gyrus, late-LTP but not LTD requires protein synthesis. The protein synthesis-dependency of persistent plasticity at CA3 synapses has not yet been characterized. Here, the roles of protein transcription and translation at mossy fiber (mf) and associational/commissural (AC)- synapses were studied in freely behaving rats. In control animals, low-frequency stimulation (LFS) evoked robust LTD (>24 h), whereas high-frequency stimulation (HFS) elicited robust LTP (>24 h) at both mf-CA3 and AC-CA3 synapses. Translation inhibitors prevented early and late phases of LTP and LTD at mf-CA3 synapses. In contrast, at AC–CA3 synapses, translation inhibitors prevented intermediate/late-LTP and late-LTD only. Transcription effects were also synapse-specific: whereas transcription inhibitors inhibited late-LTP and late-LTD (>3 h) at mf-CA3 synapses, at AC–CA3 synapses, protein transcription affected early-LTP and late-LTD. These results show that the AC-CA3 and mf-CA3 synapses display different properties in terms of their protein synthesis dependency, suggesting different roles in the processing of short- and long term synaptic plasticity.

Long-lasting LTP requires neither repeated trains for its induction nor protein synthesis for its development

Current thinking about LTP triggered in the area CA1 of hippocampal slices is ruled by two “dogmas”: (1) A single train of high-frequency stimulation is sufficient to trigger short-lasting LTP (1 – 3 h), whereas multiple trains are required to induce long-lasting LTP (L-LTP, more than 4 h). (2) The development of the late phase of L-LTP requires the synthesis of new proteins. In this study, we found that a single high-frequency train could trigger an LTP lasting more than 8 h that was not affected by either anisomycin or cycloheximide (two inhibitors of protein synthesis). We ascertained that the induction of this L-LTP made use of the same mechanisms as those usually reported to be involved in LTP induction: it was dependent on NMDA receptors and on the activation of two “core” kinases, CaMKII and PI3K. These findings call into question the two “dogmas” about LTP.

Experience-dependent recruitment of Arc expression in multiple systems during rest

The patterns of ensemble activity in the hippocampal formation during wakeful, attentive behavior are recapitulated during subsequent resting states. This replay of activity has also been found in several brain regions across many species, indicating a very general biological phenomenon. Concomitantly, transcription of immediate-early genes (IEGs) such as Arc also reoccurs in the same hippocampal neurons, suggesting that IEGs contribute to "off-line" consolidation. If continued IEG expression during rest reflects a correlate of ensemble replay, then the same generality should be observed in IEG transcription patterns. This hypothesis was tested by examining Arc in F344 rats engaging in spatial exploration alongside a rest episode. The probability that an individual neuron participates in "constitutive" Arc expression during rest is increased by recent experience in multiple cortical regions as well as across the septal and temporal poles of the hippocampus, consistent with memory trace reactivation. That is, neurons that were recently active during spatial exploration are preferentially recruited into further Arc expression during subsequent rest. The continued Arc expression, however, occurs in only a small fraction of the cells that were engaged in transcription during previous behavior. This fraction is greatest in CA3 and progressively decreases in CA1, superficial, and deep cortical layers and is consistent with the idea that consolidation occurs rapidly in the hippocampus (centering on the CA3 recurrent network) while changes are much more gradual in neocortical synaptic networks.

Mechanisms of age-related cognitive change and targets for intervention: epigenetics

Epigenetic regulation of gene expression plays an important role in learning and memory, mediating the influence of experience on critical mechanisms of plasticity. Speakers in the opening session of the Summit reviewed research on epigenetic contributions to age-related cognitive decline and discussed strategies for the development of interventions targeting epigenetic mechanisms. The presentations focused on experience-dependent DNA methylation, the regulatory role of microRNAs, histone deacetylases as potential therapeutic targets, and strategies for exploring epigenetic contributions to the aging of brain and cognition. This session established useful mileposts for gaging progress in this rapidly advancing area of research.

Time-dependent involvement of adult-born dentate granule cells in behavior

Adult-born neurons are continuously generated and incorporated into the circuitry of the hippocampus throughout life in mammals. Cumulative evidence supports a physiological role for adult-born neurons, yet it not clear whether this subset of dentate granule cells makes a unique contribution to hippocampal function. Perturbation or ablation of adult hippocampal neurogenesis leads to deficits in the acquisition of learned associations or memory recall, whereas an increase in adult hippocampal neurogenesis enhances some forms of learning and memory. The observed effects thus far appear to be task-dependent, species-specific, and sensitive to the timing of manipulations. Here, we review the recent evidence correlating adult-born dentate granule cells (DGCs) with hippocampal-dependent behavior and focus on the dynamic properties of this neuronal population that may underlie its function. We further discuss a framework for future investigations of how newly integrated neurons may contribute to hippocampal processing using advanced genetic techniques with enhanced temporal resolution.

Contribution of Egr1/zif268 to Activity-Dependent Arc/Arg3.1 Transcription in the Dentate Gyrus and Area CA1 of the Hippocampus

Egr1, a member of the Egr family of transcription factors, and Arc are immediate early genes known to play major roles in synaptic plasticity and memory. Despite evidence that Egr family members can control Arc transcriptional regulation, demonstration of a selective role of Egr1 alone is lacking. We investigated the extent to which activity-dependent Arc expression is dependent on Egr1 by analyzing Arc mRNA expression using fluorescence insitu hybridization in the dorsal dentate gyrus and CA1 of wild-type (WT) and Egr1 knockout mice. Following electroconvulsive shock, we found biphasic expression of Arc in area CA1 in mice, consisting in a rapid (30 min) and transient wave followed by a second late-phase of expression (8 h), and a single but prolonged wave of expression in the dentate gyrus. Egr1 deficiency abolished the latest, but not the early wave of Arc expression in CA1, and curtailed that of the dentate gyrus. Since the early wave of Arc expression was not affected in Egr1 mutant mice, we next analyzed behaviorally induced Arc expression patterns as an index of neural ensemble activation in the dentate gyrus and area CA1 of WT and Egr1 mutant mice. Spatial exploration of novel or familiar environments induced in mice a single early and transient wave of Arc expression in the dentate gyrus and area CA1, which were not affected in Egr1 mutant mice. Analyses of Arc-expressing cells revealed that exploration recruits similar size dentate gyrus and CA1 neural ensembles in WT and Egr1 knockout mice. These findings suggest that hippocampal neural ensembles are normally activated immediately following spatial exploration in Egr1 knockout mice, indicating normal hippocampal encoding of information. They also provide evidence that in condition of strong activation Egr1 alone can control late-phases of activity-dependent Arc transcription in the dentate gyrus and area CA1 of the hippocampus.

Induction of c-Fos, Zif268, and Arc from acute bouts of voluntary wheel running in new and pre-existing adult mouse hippocampal granule neurons

The functional significance of newly formed granule neurons in the adult mammalian hippocampus remains a mystery. Recently, it was demonstrated that wheel running increases new neuron survival and c-Fos expression in new and pre-existing granule cells in an activity-dependent manner. It is currently unknown whether other immediate early genes (IEGs) become expressed in granule neurons from running. Further, it is unknown whether locomotor activity in home cages without wheels can influence neurogenesis and IEG expression similar to running. The purpose of this study was three-fold: (1) to determine if Arc and Zif268 expression are also induced from wheel running in both pre-existing and newly formed neurons (2) to determine if neurogenesis and IEG induction is related to horizontal distance traveled in home cages without wheels, and (3) to determine whether IEG induction is related to acute bouts of running or chronic effects. Adult C57BL/6J female mice were placed in cages with or without running wheels for 31 days. The first 10 days, mice received daily injections of 5-Bromo-2'-deoxyuridine (BrdU) to label dividing cells. On day 1, running and non-running animals were euthanized either 2 h after peak activity, or during a period of relative inactivity. Immunohistochemistry was performed on hippocampal sections with antibodies against BrdU, mature neuron marker NeuN, c-Fos, Arc, and Zif268. Results demonstrate that Arc, Zif268, and c-Fos are induced from wheel running but not movement in cages without wheels. All IEGs were expressed in new neurons from running. Further, IEGs were induced acutely by running, as increased expression did not continue into the light cycle, a period of relative inactivity. The results suggest that robust movements, like running, are necessary to stimulate IEG expression and neurogenesis. Moreover, results suggest new neurons from running may be processing information about running behavior itself.

Distinctive features of Egr transcription factor regulation and DNA binding activity in CA1 of the hippocampus in synaptic plasticity and consolidation and reconsolidation of fear memory

Activity-dependent regulation of Egr1/Zif268, a transcription factor (TF) of the Egr family, is essential for stabilization of dentate gyrus synaptic plasticity and consolidation and reconsolidation of several forms of memory. The gene can be rapidly induced in selective brain circuits after certain types of learning or after recall. Here, we focused on area CA1 and examined regulation of Egr1, Egr2, and Egr3 mRNA and protein, and their DNA binding activity to the Egr response element (ERE) at different times after LTP in vivo and after learning and recall of a fear memory. We found LTP in CA1 leads to rapid induction of the three Egrs, however only Egr1 protein was overexpressed without a co-ordinated change in binding activity, indicating a fundamental difference between CA1 and dentate gyrus LTP. Our investigations in fear memory reveal that both learning and retrieval lead to an increase in binding of constitutively expressed Egr1 and Egr3 to the ERE, but not Egr2. Memory recall was also associated with increased Egr1 protein translation. The nature and temporal dynamics of these changes and tests for interactions between TFs suggest that in addition to ERE-mediated transcription, Egr1 in CA1 may interact with the TF c-Fos to regulate genes via other DNA response elements.

Attenuated long-term Arc expression in the aged fascia dentata

One prominent component of aging is a defect in memory stabilization. To understand how the formation of enduring memories is altered in the aged brain, long-term markers of the biological events that may mediate memory consolidation were used to examine the activity dynamics of hippocampal circuits over extended intervals. The immediate early gene Arc, which is implicated in both durable memory and synaptic plasticity, is expressed in the fascia dentata (FD) for long periods following behavioral experience. To test the hypothesis that aging alters long-term Arc transcription in the FD, a region critical for spatial memory and impaired with progressive age, young and aged rats explored a novel environment twice, separated by an 8-hour interval, and FD Arc transcription was assessed. Relative to young rats, (a) fewer granule cells in the aged FD transcribe arc 8 hours after spatial exploration, and (b) this decrease is correlated with impaired spatial memory. These findings are consistent with behavioral evidence of age-related decline in hippocampal-dependent memory processing long after an event is to be remembered, and reaffirm the integral role of the FD in the neural circuits supporting durable memory.

LTP- and LTD-inducing stimulations cause opposite changes in arc/arg3.1 mRNA level in hippocampal area CA1 in vivo

Immediate early genes (IEGs) typically are the first genetic responders to a variety of cellular activations. The IEG that encodes activity-regulated cytoskeleton-associated protein (arc/arg3.1) has attracted much interest because its mRNA is transported to and translated near activated synapses. Moreover, arc has been implicated in both long-term potentiation (LTP) and long-term depression (LTD). However, little is known about the time course of altered arc expression during LTP and LTD. Here we characterized arc mRNA levels in area CA1 of the adult rat hippocampus in vivo after LTP- and LTD-inducing stimulations that were identical, except for the temporal patterning of the stimulation pulses. We observed a persistent increase in arc mRNA level during LTP. In contrast, during LTD, arc mRNA level first was decreased and then transiently increased relative to control level. These findings demonstrate that arc mRNA is regulated differently during LTP and LTD, and they provide evidence for stimulation-induced downregulation of mRNA availability during LTD. Findings of abbreviated LTD when transcription was inhibited indicate that the prolonged maintenance of the type of N-methyl-D-aspartate receptor-dependent LTD studied here requires de novo transcription. Furthermore, lack of evidence for a LTD-associated change in the mRNA level of the IEG zif268 demonstrates that the decrease in arc mRNA during LTD is not a general genetic response. Thus, the regulation of arc expression not only differs between LTP and LTD, but also diverges from that of other IEGs implicated in activity-dependent synaptic plasticity.

An epigenetic hypothesis of aging-related cognitive dysfunction

This brief review will focus on a new hypothesis for the role of epigenetic mechanisms in aging-related disruptions of synaptic plasticity and memory. Epigenetics refers to a set of potentially self-perpetuating, covalent modifications of DNA and post-translational modifications of nuclear proteins that produce lasting alterations in chromatin structure. These mechanisms, in turn, result in alterations in specific patterns of gene expression. Aging-related memory decline is manifest prominently in declarative/episodic memory and working memory, memory modalities anatomically based largely in the hippocampus and prefrontal cortex, respectively. The neurobiological underpinnings of age-related memory deficits include aberrant changes in gene transcription that ultimately affect the ability of the aged brain to be "plastic". The molecular mechanisms underlying these changes in gene transcription are not currently known, but recent work points toward a potential novel mechanism, dysregulation of epigenetic mechanisms. This has led us to hypothesize that dysregulation of epigenetic control mechanisms and aberrant epigenetic "marks" drive aging-related cognitive dysfunction. Here we focus on this theme, reviewing current knowledge concerning epigenetic molecular mechanisms, as well as recent results suggesting disruption of plasticity and memory formation during aging. Finally, several open questions will be discussed that we believe will fuel experimental discovery.

The function of activity-regulated genes in the nervous system

The mammalian brain is plastic in the sense that it shows a remarkable capacity for change throughout life. The contribution of neuronal activity to brain plasticity was first recognized in relation to critical periods of development, when manipulating the sensory environment was found to profoundly affect neuronal morphology and receptive field properties. Since then, a growing body of evidence has established that brain plasticity extends beyond development and is an inherent feature of adult brain function, spanning multiple domains, from learning and memory to adaptability of primary sensory maps. Here we discuss evolution of the current view that plasticity of the adult brain derives from dynamic tuning of transcriptional control mechanisms at the neuronal level, in response to external and internal stimuli. We then review the identification of "plasticity genes" regulated by changes in the levels of electrical activity, and how elucidating their cellular functions has revealed the intimate role transcriptional regulation plays in fundamental aspects of synaptic transmission and circuit plasticity that occur in the brain on an every day basis.

Age matters

The age of an experimental animal can be a critical variable, yet age matters are often overlooked within neuroscience. Many studies make use of young animals, without considering possible differences between immature and mature subjects. This is especially problematic when attempting to model traits or diseases that do not emerge until adulthood. In this commentary we discuss the reasons for this apparent bias in age of experimental animals, and illustrate the problem with a systematic review of published articles on long-term potentiation. Additionally, we review the developmental stages of a rat and discuss the difficulty of using the weight of an animal as a predictor of its age. Finally, we provide original data from our laboratory and review published data to emphasize that development is an ongoing process that does not end with puberty. Developmental changes can be quantitative in nature, involving gradual changes, rapid switches, or inverted U-shaped curves. Changes can also be qualitative. Thus, phenomena that appear to be unitary may be governed by different mechanisms at different ages. We conclude that selection of the age of the animals may be critically important in the design and interpretation of neurobiological studies.

Rapid activation of plasticity-associated gene transcription in hippocampal neurons provides a mechanism for encoding of one-trial experience

The hippocampus is hypothesized to support rapid encoding of ongoing experience. A critical prerequisite for such function is the ability to readily recruit enduring synaptic plasticity in hippocampal neurons. Hippocampal long-term potentiation (LTP) and memory consolidation require expression of the immediate-early gene (IEG) Arc. To determine whether Arc transcription could be driven by limited and controlled behavioral experience, we used a rectangular track paradigm. In past electrophysiological studies, pyramidal neurons recorded from rats running in one direction on similar tracks typically exhibited a single firing field. Using fluorescence in situ hybridization, we show that the behavioral activity associated with a single lap around the track was sufficient to trigger Arc transcription in complete CA3 neuronal ensembles, as predicted given the role of CA3 in one-trial learning. In contrast, Arc transcription in CA1 ensembles was recruited incrementally, with maximal activation achieved after four laps a day for 4 consecutive days. To test whether Arc transcription is linked to learning and plasticity, or merely elicited by location-specific firing, we inactivated the medial septum, a treatment that compromises hippocampus-dependent learning and LTP but spares location-specific firing in CA1 neurons. Septal inactivation abolished track training-induced Arc transcription in CA1 and CA3 neurons, showing that Arc transcription requires plasticity-inducing stimuli. Accordingly, LTP induction activated Arc transcription in CA1 neurons in vivo. These findings demonstrate for the first time that a single brief experience, equivalent to a single crossing of a firing field, can trigger IEG expression required for long-term plasticity in the hippocampus.

Transcription factors in long-term memory and synaptic plasticity

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

Expression of the MAST family of serine/threonine kinases

The Microtubule-Associated Serine/Threonine Kinase family (MAST1-4, and MAST-like) is characterised by the presence of a serine/threonine kinase domain and a postsynaptic density protein-95/discs large/zona occludens-1 domain (PDZ). This latter domain gives the MAST family the capacity to scaffold its own kinase activity. In the present study we have profiled the mRNA for each member of the MAST family transcripts across various tissues, with particular focus on rodent brain. Reverse-transcriptase polymerase chain reaction (RT-PCR) has shown equivalent patterns of expression for MAST1 and 2 in multiple tissues. Both MAST3 and 4 show more distinct expression in several tissues, and MAST-like appears to be predominantly expressed in heart and testis. In situ hybridisation reveals overlapping expression of MAST1 and 2 in specific brain regions. In contrast, MAST3 shows selective expression in the striatum and cerebral cortex. MAST4 also exhibits distinct expression in oligodendrocytes of white matter containing brain regions. In keeping with previous results, this family member also shows increased expression in the hippocampus following seizure-like activity. Our analysis of MAST family expression provides support for the role of these kinases in a broad range of neural functions.

A form of perforant path LTP can occur without ERK1/2 phosphorylation or immediate early gene induction

Stimulation paradigms that induce perforant path long-term potentiation (LTP) initiate phosphorylation of ERK1/2 and induce expression of a variety of immediate early genes (IEGs). These events are thought to be critical components of the mechanism for establishing the changes in synaptic efficacy that endure for hours or longer. Here we show that in mice, perforant path LTP can be induced using a standard protocol (repeated trains at 250 Hz), without accompanying increases in immunostaining for p-ERK1/2 or increased in expression of representative IEGs (Arc and c-fos). Signaling pathways capable of inducing ERK phosphorylation and IEG transcription are intact in mice because ERK phosphorylation differs strikingly in awake versus anesthetized mice, and IEG expression is strongly induced by electroconvulsive seizures. In pursuing the reasons for the lack of induction with LTP, we found that in rats, one of the stimulation paradigms used to induce perforant path LTP (trains at 250 Hz) also does not activate MAP kinase or induce IEG expression, despite the fact that the LTP induced by 250 Hz stimulation requires NMDA receptor activation and persists for hours. These findings indicate that there are different forms of perforant path LTP, one of which does not require MAP kinase activation or IEG induction. Moreover, these data demonstrate that different LTP induction paradigms do not have identical molecular consequences, which may account for certain discrepancies between previous studies.

Expression of FOS, EGR-1, and ARC in the amygdala and hippocampus of female rats during formation of the intromission mnemonic of pseudopregnancy

Pseudopregnancy (PSP) in the female rat is a neuroendocrine condition that is induced by repeated and intermittent vaginocervical stimulation received during mating and involves the expression of bicircadian prolactin surges and cessation of normal estrous cyclicity for 10-12 days postmating. The temporal patterning and number of intromissions received by the female are critical for PSP initiation, and thus, short-term encoding of VCS occurs during transduction of intromissions into PSP. In this experiment, we characterized and compared the mating-induced neural activation patterns within amygdalar and hippocampal regions using expression of the immediate early genes FOS, EGR-1, and ARC. Cycling female rats mated on proestrus received 15 or 5 intromissions under paced or nonpaced mating conditions. High numbers of intromissions during nonpaced mating or low numbers received during paced mating are sufficient to induce PSP, while five nonpaced intromissions and mounts without intromission are insufficient. Here we demonstrate that the CA1 region of the hippocampus was selectively sensitive to PSP-sufficient but not PSP-insufficient mating stimulation by showing significant effects of paced mating for all three IEGs. Paced mating also stimulated the expression of ARC within the basolateral, cortical, and central nuclei of the amygdala. The posterodorsal medial amygdala also showed selective EGR-1 responses to PSP-sufficient mating stimulation. There was no effect of hemisphere on IEG expression. The postmating expression profiles of these IEGs provide evidence that limbic areas involved in encoding and consolidation of memory are involved in initiating the neuroendocrine memory of PSP.