Persistent ∆FosB expression limits recurrent seizure activity and provides neuroprotection in the dentate gyrus of APP mice.

Recurrent seizures lead to accumulation of the activity-dependent transcription factor ∆FosB in hippocampal dentate granule cells in both mouse models of epilepsy and mouse models of Alzheimer's disease (AD), which is also associated with increased incidence of seizures. In patients with AD and related mouse models, the degree of ∆FosB accumulation corresponds with increasing severity of cognitive deficits. We previously found that ∆FosB impairs spatial memory in mice by epigenetically regulating expression of target genes such as calbindin that are involved in synaptic plasticity. However, the suppression of calbindin in conditions of neuronal hyperexcitability has been demonstrated to provide neuroprotection to dentate granule cells, indicating that ∆FosB may act over long timescales to coordinate neuroprotective pathways. To test this hypothesis, we used viral-mediated expression of ∆JunD to interfere with ∆FosB signaling over the course of several months in transgenic mice expressing mutant human amyloid precursor protein (APP), which exhibit spontaneous seizures and develop AD-related neuropathology and cognitive deficits. Our results demonstrate that persistent ∆FosB activity acts through discrete modes of hippocampal target gene regulation to modulate neuronal excitability, limit recurrent seizure activity, and provide neuroprotection to hippocampal dentate granule cells in APP mice.

Hippocampal ΔFosB expression is associated with cognitive impairment in a subgroup of patients with childhood epilepsies.

Epilepsy is a chronic neurological disorder characterized by recurrent seizures, and is often comorbid with other neurological and neurodegenerative diseases, such as Alzheimer's disease (AD). Patients with recurrent seizures often present with cognitive impairment. However, it is unclear how seizures, even when infrequent, produce long-lasting deficits in cognition. One mechanism may be seizure-induced expression of ΔFosB, a long-lived transcription factor that persistently regulates expression of plasticity-related genes and drives cognitive dysfunction. We previously found that, compared with cognitively-intact subjects, the activity-dependent expression of ΔFosB in the hippocampal dentate gyrus (DG) was increased in individuals with mild cognitive impairment (MCI) and in individuals with AD. In MCI patients, higher ΔFosB expression corresponded to lower Mini-Mental State Examination scores. Surgically resected DG tissue from patients with temporal lobe epilepsy also showed robust ΔFosB expression; however, it is unclear whether ΔFosB expression also corresponds to cognitive dysfunction in non-AD-related epilepsy. To test whether DG ΔFosB expression is indicative of cognitive impairment in epilepsies with different etiologies, we assessed ΔFosB expression in surgically-resected hippocampal tissue from 33 patients with childhood epilepsies who had undergone Wechsler Intelligence Scale for Children (WISC) testing prior to surgery. We found that ΔFosB expression is inversely correlated with Full-Scale Intelligence Quotient (FSIQ) in patients with mild to severe intellectual disability (FSIQ < 85). Our data indicate that ΔFosB expression corresponds to cognitive impairment in epilepsies with different etiologies, supporting the hypothesis that ΔFosB may epigenetically regulate gene expression and impair cognition across a wide range of epilepsy syndromes.

ΔFosB is part of a homeostatic mechanism that protects the epileptic brain from further deterioration.

Activity induced transcription factor ΔFosB plays a key role in different CNS disorders including epilepsy, Alzheimer's disease, and addiction. Recent findings suggest that ΔFosB drives cognitive deficits in epilepsy and together with the emergence of small molecule inhibitors of ΔFosB activity makes it an interesting therapeutic target. However, whether ΔFosB contributes to pathophysiology or provides protection in drug-resistant epilepsy is still unclear. In this study, ΔFosB was specifically downregulated by delivering AAV-shRNA into the hippocampus of chronically epileptic mice using the drug-resistant pilocarpine model of mesial temporal epilepsy (mTLE). Immunohistochemistry analyses showed that prolonged downregulation of ΔFosB led to exacerbation of neuroinflammatory markers of astrogliosis and microgliosis, loss of mossy fibers, and hippocampal granule cell dispersion. Furthermore, prolonged inhibition of ΔFosB using a ΔJunD construct to block ΔFosB signaling in a mouse model of Alzheimer's disease, that exhibits spontaneous recurrent seizures, led to similar findings, with increased neuroinflammation and decreased NPY expression in mossy fibers. Together, these data suggest that seizure-induced ΔFosB, regardless of seizure-etiology, is part of a homeostatic mechanism that protects the epileptic brain from further deterioration.

HIV-1 gp120 Impairs Spatial Memory Through Cyclic AMP Response Element-Binding Protein.

HIV-associated neurocognitive disorders (HAND) remain an unsolved problem that persists despite using antiretroviral therapy. We have obtained data showing that HIV-gp120 protein contributes to neurodegeneration through metabolic reprogramming. This led to decreased ATP levels, lower mitochondrial DNA copy numbers, and loss of mitochondria cristae, all-important for mitochondrial biogenesis. gp120 protein also disrupted mitochondrial movement and synaptic plasticity. Searching for the mechanisms involved, we found that gp120 alters the cyclic AMP response element-binding protein (CREB) phosphorylation on serine residue 133 necessary for its function as a transcription factor. Since CREB regulates the promoters of PGC1α and BDNF genes, we found that CREB dephosphorylation causes PGC1α and BDNF loss of functions. The data was validated in vitro and in vivo. The negative effect of gp120 was alleviated in cells and animals in the presence of rolipram, an inhibitor of phosphodiesterase protein 4 (PDE4), restoring CREB phosphorylation. We concluded that HIV-gp120 protein contributes to HAND via inhibition of CREB protein function.

Restoring activity in the thalamic reticular nucleus improves sleep architecture and reduces Aß accumulation in mice.

Sleep disruptions promote increases of amyloid ß (Aß) and tau in the brain and increase Alzheimer's disease (AD) risk, but the precise mechanisms that give rise to sleep disturbances have yet to be defined. The thalamic reticular nucleus (TRN) is essential for sleep maintenance and for the regulation of slow-wave sleep (SWS). We examined the TRN in transgenic mice that express mutant human amyloid precursor protein (APP) and found reduced neuronal activity, increased sleep fragmentation, and decreased SWS time as compared to nontransgenic littermates. Selective activation of the TRN using excitatory DREADDs restored sleep maintenance, increased time in SWS, and reduced amyloid plaque load in both hippocampus and cortex. Our findings suggest that the TRN may play a major role in symptoms associated with AD. Enhancing TRN activity might be a promising therapeutic strategy for AD.

Genes Bound by ΔFosB in Different Conditions With Recurrent Seizures Regulate Similar Neuronal Functions.

Seizure incidence is increased in Alzheimer's disease (AD) patients and mouse models, and treatment with the antiseizure drug levetiracetam improves cognition. We reported that one mechanism by which seizures can exert persistent effects on cognition is through accumulation of ΔFosB, a transcription factor with a long half-life. Even the infrequent seizures that spontaneously occur in transgenic mice expressing human amyloid precursor protein (APP) lead to persistent increases in ΔFosB in the hippocampus, similar to what we observed in patients with AD or temporal lobe epilepsy. ΔFosB epigenetically regulates expression of target genes, however, whether ΔFosB targets the same genes when induced by seizures in different neurological conditions is not clear. We performed ChIP-sequencing to assess the repertoire of ΔFosB target genes in APP mice and in pilocarpine-treated wildtype mice (Pilo mice), a pharmacological model of epilepsy. These mouse models allowed us to compare AD, in which seizures occur in the context of high levels of amyloid beta, and epilepsy, in which recurrent seizures occur without AD-specific pathophysiology. Network profiling of genes bound by ΔFosB in APP mice, Pilo mice, and respective control mice revealed that functional domains modulated by ΔFosB in the hippocampus are expanded and diversified in APP and Pilo mice (vs. respective controls). Domains of interest in both disease contexts involved neuronal excitability and neurotransmission, neurogenesis, chromatin remodeling, and cellular stress and neuroinflammation. To assess the gene targets bound by ΔFosB regardless of seizure etiology, we focused on 442 genes with significant ΔFosB binding in both APP and Pilo mice (vs. respective controls). Functional analyses identified pathways that regulate membrane potential, glutamatergic signaling, calcium homeostasis, complement activation, neuron-glia population maintenance, and chromatin dynamics. RNA-sequencing and qPCR measurements in independent mice detected altered expression of several ΔFosB targets shared in APP and Pilo mice. Our findings indicate that seizure-induced ΔFosB can bind genes in patterns that depend on seizure etiology, but can bind other genes regardless of seizure etiology. Understanding the factors that underlie these differences, such as chromatin accessibility and/or abundance of co-factors, could reveal novel insights into the control of gene expression in disorders with recurrent seizures.

Distinct patterns of dentate gyrus cell activation distinguish physiologic from aberrant stimuli.

Under physiologic conditions, the dentate gyrus (DG) exhibits exceptionally low levels of activity compared to other brain regions. A sparse activation pattern is observed even when the DG is engaged to process new information; for example, only ~1-3% of neurons in the DG granule cell layer (GCL) are activated after placing animals in a novel, enriched environment. Moreover, such physiologic stimulation of GCL neurons recruits young granule cells more readily than older cells. This sparse pattern of cell activation has largely been attributed to intrinsic circuit properties of the DG, such as reduced threshold for activation in younger cells, and increased inhibition onto older cells. Given these intrinsic properties, we asked whether such activation of young granule cells was unique to physiologic stimulation, or could be elicited by general pharmacological activation of the hippocampus. We found that administration of kainic acid (KA) at a low dose (5 mg/kg) to wildtype C57BL/6 mice activated a similarly sparse number of cells in the GCL as physiologic DG stimulation by exposure to a novel, enriched environment. However, unlike physiologic stimulation, 5 mg/kg KA activated primarily old granule cells as well as GABAergic interneurons. This finding indicates that intrinsic circuit properties of the DG alone may not be sufficient to support the engagement of young granule cells, and suggest that other factors such as the specificity of the pattern of inputs, may be involved.

TRPM4 Conductances in Thalamic Reticular Nucleus Neurons Generate Persistent Firing during Slow Oscillations.

During sleep, neurons in the thalamic reticular nucleus (TRN) participate in distinct types of oscillatory activity. While the reciprocal synaptic circuits between TRN and sensory relay nuclei are known to underlie the generation of sleep spindles, the mechanisms regulating slow (<1 Hz) forms of thalamic oscillations are not well understood. Under in vitro conditions, TRN neurons can generate slow oscillations in a cell-intrinsic manner, with postsynaptic Group 1 metabotropic glutamate receptor activation triggering long-lasting plateau potentials thought to be mediated by both T-type Ca2+ currents and Ca2+-activated nonselective cation currents (ICAN). However, the identity of ICAN and the possible contribution of thalamic circuits to slow rhythmic activity remain unclear. Using thalamic slices derived from adult mice of either sex, we recorded slow forms of rhythmic activity in TRN neurons, which were driven by fast glutamatergic thalamoreticular inputs but did not require postsynaptic Group 1 metabotropic glutamate receptor activation. For a significant fraction of TRN neurons, synaptic inputs or brief depolarizing current steps led to long-lasting plateau potentials and persistent firing (PF), and in turn, resulted in sustained synaptic inhibition in postsynaptic relay neurons of the ventrobasal thalamus (VB). Pharmacological approachesindicated that plateau potentials were triggered by Ca2+ influx through T-type Ca2+ channels and mediated by Ca2+- and voltage-dependent transient receptor potential melastatin 4 (TRPM4) channels. Together, our results suggest that thalamic circuits can generate slow oscillatory activity, mediated by an interplay of TRN-VB synaptic circuits that generate rhythmicity and TRN cell-intrinsic mechanisms that control PF and oscillation frequency.SIGNIFICANCE STATEMENT Slow forms of thalamocortical rhythmic activity are thought to be essential for memory consolidation during sleep and the efficient removal of potentially toxic metabolites. In vivo, thalamic slow oscillations are regulated by strong bidirectional synaptic pathways linking neocortex and thalamus. Therefore, in vitro studies in the isolated thalamus offer important insights about the ability of individual neurons and local circuits to generate different forms of rhythmic activity. We found that circuits formed by GABAergic neurons in the thalamic reticular nucleus and glutamatergic relay neurons in the ventrobasal thalamus generated slow oscillatory activity, which was accompanied by persistent firing in thalamic reticular nucleus neurons. Our results identify both cell-intrinsic and synaptic mechanisms that mediate slow forms of rhythmic activity in thalamic circuits.

Therapeutic inhibition of mTORC2 rescues the behavioral and neurophysiological abnormalities associated with Pten-deficiency.

Dysregulation of the mammalian target of rapamycin (mTOR) signaling, which is mediated by two structurally and functionally distinct complexes, mTORC1 and mTORC2, has been implicated in several neurological disorders1-3. Individuals carrying loss-of-function mutations in the phosphatase and tensin homolog (PTEN) gene, a negative regulator of mTOR signaling, are prone to developing macrocephaly, autism spectrum disorder (ASD), seizures and intellectual disability2,4,5. It is generally believed that the neurological symptoms associated with loss of PTEN and other mTORopathies (for example, mutations in the tuberous sclerosis genes TSC1 or TSC2) are due to hyperactivation of mTORC1-mediated protein synthesis1,2,4,6,7. Using molecular genetics, we unexpectedly found that genetic deletion of mTORC2 (but not mTORC1) activity prolonged lifespan, suppressed seizures, rescued ASD-like behaviors and long-term memory, and normalized metabolic changes in the brain of mice lacking Pten. In a more therapeutically oriented approach, we found that administration of an antisense oligonucleotide (ASO) targeting mTORC2's defining component Rictor specifically inhibits mTORC2 activity and reverses the behavioral and neurophysiological abnormalities in adolescent Pten-deficient mice. Collectively, our findings indicate that mTORC2 is the major driver underlying the neuropathophysiology associated with Pten-deficiency, and its therapeutic reduction could represent a promising and broadly effective translational therapy for neurological disorders where mTOR signaling is dysregulated.

Early Seizure Activity Accelerates Depletion of Hippocampal Neural Stem Cells and Impairs Spatial Discrimination in an Alzheimer's Disease Model.

Adult hippocampal neurogenesis has been reported to be decreased, increased, or not changed in Alzheimer's disease (AD) patients and related transgenic mouse models. These disparate findings may relate to differences in disease stage, or the presence of seizures, which are associated with AD and can stimulate neurogenesis. In this study, we investigate a transgenic mouse model of AD that exhibits seizures similarly to AD patients and find that neurogenesis is increased in early stages of disease, as spontaneous seizures became evident, but is decreased below control levels as seizures recur. Treatment with the antiseizure drug levetiracetam restores neurogenesis and improves performance in a neurogenesis-associated spatial discrimination task. Our results suggest that seizures stimulate, and later accelerate the depletion of, the hippocampal neural stem cell pool. These results have implications for AD as well as any disorder accompanied by recurrent seizures, such as epilepsy.

Corticothalamic network dysfunction and Alzheimer's disease.

Alzheimer's disease (AD) is a devastating neurodegenerative disease that is characterized by progressive cognitive decline and a prominent loss of hippocampal-dependent memory. Therefore, much focus has been placed on understanding the function and dysfunction of the hippocampus in AD. However, AD is also accompanied by a number of other debilitating cognitive and behavioral alterations including deficits in attention, cognitive processing, and sleep maintenance. The underlying mechanisms that give rise to impairments in such diverse behavioral domains are unknown, and identifying them would shed insight into the multifactorial nature of AD as well as reveal potential new therapeutic targets to improve overall function in AD. We present here several lines of evidence that suggest that dysregulation of the corticothalamic network may be a common denominator that contributes to the diverse cognitive and behavioral alterations in AD. First, we will review the mechanisms by which this network regulates processes that include attention, cognitive processing, learning and memory, and sleep maintenance. Then we will review how these behavioral and cognitive domains are altered in AD. We will also discuss how dysregulation of tightly regulated activity in the corticothalamic network can give rise to non-convulsive seizures and other forms of epileptiform activity that have also been documented in both AD patients and transgenic mouse models of AD. In summary, the corticothalamic network has the potential to be a master regulator of diverse cognitive and behavioral domains that are affected in AD.

Genome-wide profiling reveals functional diversification of ∆FosB gene targets in the hippocampus of an Alzheimer's disease mouse model.

The activity-induced transcription factor ∆FosB has been implicated in Alzheimer's disease (AD) as a critical regulator of hippocampal function and cognition downstream of seizures and network hyperexcitability. With its long half-life (> 1 week), ∆FosB is well-poised to modulate hippocampal gene expression over extended periods of time, enabling effects to persist even during seizure-free periods. However, the transcriptional mechanisms by which ∆FosB regulates hippocampal function are poorly understood due to lack of identified hippocampal gene targets. To identify putative ∆FosB gene targets, we employed high-throughput sequencing of genomic DNA bound to ∆FosB after chromatin immunoprecipitation (ChIP-sequencing). We compared ChIP-sequencing results from hippocampi of transgenic mice expressing mutant human amyloid precursor protein (APP) and nontransgenic (NTG) wild-type littermates. Surprisingly, only 52 ∆FosB gene targets were shared between NTG and APP mice; the vast majority of targets were unique to one genotype or the other. We also found a functional shift in the repertoire of ∆FosB gene targets between NTG and APP mice. A large number of targets in NTG mice are involved in neurodevelopment and/or cell morphogenesis, whereas in APP mice there is an enrichment of targets involved in regulation of membrane potential and neuronal excitability. RNA-sequencing and quantitative PCR experiments confirmed that expression of putative ∆FosB gene targets were altered in the hippocampus of APP mice. This study provides key insights into functional domains regulated by ∆FosB in the hippocampus, emphasizing remarkably different programs of gene regulation under physiological and pathological conditions.

Epigenetic suppression of hippocampal calbindin-D28k by ΔFosB drives seizure-related cognitive deficits.

The calcium-binding protein calbindin-D28k is critical for hippocampal function and cognition, but its expression is markedly decreased in various neurological disorders associated with epileptiform activity and seizures. In Alzheimer's disease (AD) and epilepsy, both of which are accompanied by recurrent seizures, the severity of cognitive deficits reflects the degree of calbindin reduction in the hippocampal dentate gyrus (DG). However, despite the importance of calbindin in both neuronal physiology and pathology, the regulatory mechanisms that control its expression in the hippocampus are poorly understood. Here we report an epigenetic mechanism through which seizures chronically suppress hippocampal calbindin expression and impair cognition. We demonstrate that ΔFosB, a highly stable transcription factor, is induced in the hippocampus in mouse models of AD and seizures, in which it binds and triggers histone deacetylation at the promoter of the calbindin gene (Calb1) and downregulates Calb1 transcription. Notably, increasing DG calbindin levels, either by direct virus-mediated expression or inhibition of ΔFosB signaling, improves spatial memory in a mouse model of AD. Moreover, levels of ΔFosB and calbindin expression are inversely related in the DG of individuals with temporal lobe epilepsy (TLE) or AD and correlate with performance on the Mini-Mental State Examination (MMSE). We propose that chronic suppression of calbindin by ΔFosB is one mechanism through which intermittent seizures drive persistent cognitive deficits in conditions accompanied by recurrent seizures.

ΔFosB Regulates Gene Expression and Cognitive Dysfunction in a Mouse Model of Alzheimer's Disease.

Alzheimer's disease (AD) is characterized by cognitive decline and 5- to 10-fold increased seizure incidence. How seizures contribute to cognitive decline in AD or other disorders is unclear. We show that spontaneous seizures increase expression of ΔFosB, a highly stable Fos-family transcription factor, in the hippocampus of an AD mouse model. ΔFosB suppressed expression of the immediate early gene c-Fos, which is critical for plasticity and cognition, by binding its promoter and triggering histone deacetylation. Acute histone deacetylase (HDAC) inhibition or inhibition of ΔFosB activity restored c-Fos induction and improved cognition in AD mice. Administration of seizure-inducing agents to nontransgenic mice also resulted in ΔFosB-mediated suppression of c-Fos, suggesting that this mechanism is not confined to AD mice. These results explain observations that c-Fos expression increases after acute neuronal activity but decreases with chronic activity. Moreover, these results indicate a general mechanism by which seizures contribute to persistent cognitive deficits, even during seizure-free periods.

LINGO-1-Fc-Transduced Neural Stem Cells Are Effective Therapy for Chronic Stage Experimental Autoimmune Encephalomyelitis.

The chronic stage multiple sclerosis (MS), an inflammatory demyelinating disease of the central nervous system (CNS), remains refractory to current treatments. This refractory nature may be due to the fact that current treatments are primarily immunomodulatory, which prevent further demyelination but lack the capacity to promote remyelination. Several approaches, including transplantation of neural stem cells (NSCs) or antagonists to LINGO-1, a key part of the receptor complex for neuroregeneration inhibitors, have been effective in suppressing the acute stage of experimental autoimmune encephalomyelitis (EAE), an animal model of MS. However, their effect on the chronic stage EAE is not known. Here, we show that transplantation of NSCs had only a slight therapeutic effect when treatment started at the chronic stage of EAE (e.g., injected at day 40 postimmunization). However, NSCs engineered to produce LINGO-1-Fc, a soluble LINGO-1 antagonist, significantly promoted neurological recovery as demonstrated by amelioration of clinical signs, improvement in axonal integrity, and enhancement of oligodendrocyte maturation and neuron repopulation. Significantly enhanced NAD production and Sirt2 expression were also found in the CNS of mice treated with LINGO-1-Fc-producing NSC. Moreover, differentiation of LINGO-1-Fc-producing NSCs into oligodendrocytes in vitro was largely diminished by an NAMPT inhibitor, indicating that LINGO-1-Fc enhances the NAMPT/NAD/Sirt2 pathway. Together, our study establishes a CNS-targeted, novel LINGO-1-Fc delivery system using NSCs, which represents a novel and effective NSC-based gene therapy approach for the chronic stage of MS.

Corticothalamic network dysfunction and behavioral deficits in a mouse model of Alzheimer's disease.

Alzheimer's disease is associated with cognitive decline and seizures. Growing evidence indicates that seizures contribute to cognitive deficits early in disease, but how they develop and impact cognition are unclear. To investigate potential mechanisms, we studied a mouse model that overexpresses mutant human amyloid precursor protein with high levels of amyloid beta (Aß). These mice develop generalized epileptiform activity, including nonconvulsive seizures, consistent with alterations in corticothalamic network activity. Amyloid precursor protein mice exhibited reduced activity marker expression in the reticular thalamic nucleus, a key inhibitory regulatory nucleus, and increased activity marker expression in downstream thalamic relay targets that project to cortex and limbic structures. Slice recordings revealed impaired cortical inputs to the reticular thalamic nucleus that may contribute to corticothalamic dysfunction. These results are consistent with our findings of impaired sleep maintenance in amyloid precursor protein mice. Finally, the severity of sleep impairments predicted the severity of deficits in Morris water maze, suggesting corticothalamic dysfunction may relate to hippocampal dysfunction, and may be a pathophysiological mechanism underlying multiple behavioral and cognitive alterations in Alzheimer's disease.

Delayed thalamic astrocytosis and disrupted sleep-wake patterns in a preclinical model of traumatic brain injury.

Traumatic brain injury (TBI) involves diffuse axonal injury and induces subtle but persistent changes in brain tissue and function and poses challenges for early detection of neurological injury. The present study uses an automated behavioral analysis system to assess alterations in rodent behavior in the subacute phase in a preclinical mouse model of TBI, controlled cortical impact (CCI) injury. In the first few weeks following CCI, mice demonstrated normal exploratory behaviors and other typical home-cage behaviors. However, beginning 4 weeks post-injury, CCI mice developed disruptions in sleep-wake patterns, including an increased number of awakenings from sleep. Such impaired sleep maintenance was accompanied by an increased latency to reach peak sleep in CCI mice. These sleep disruptions implicate involvement of the thalamocortical network, the activity of which must be tightly regulated to control sleep maintenance. After injury, there was an increase in reactive microglia in thalamic regions as well as delayed reactive astrocytosis that was evident in the thalamic reticular nucleus, which preceded the development of sleep disruptions. These data suggest that cortical injury may trigger inflammatory responses in deeper neuroanatomical structures, including the thalamic reticular nucleus. Such engagement of the thalamus may perturb the thalamocortical network that regulates sleep/awake patterns and contribute to sleep disruptions observed in this model as well as those documented in patients with TBI.

Chronic at-level thermal hyperalgesia following rat cervical contusion spinal cord injury is accompanied by neuronal and astrocyte activation and loss of the astrocyte glutamate transporter, GLT1, in superficial dorsal horn.

Neuropathic pain is a form of pathological nociception that occurs in a significant portion of traumatic spinal cord injury (SCI) patients, resulting in debilitating and often long-term physical and psychological burdens. While many peripheral and central mechanisms have been implicated in neuropathic pain, central sensitization of dorsal horn spinothalamic tract (STT) neurons is a major underlying substrate. Furthermore, dysregulation of extracellular glutamate homeostasis and chronic astrocyte activation play important underlying roles in persistent hyperexcitability of these superficial dorsal horn neurons. To date, central sensitization and astrocyte changes have not been characterized in cervical SCI-induced neuropathic pain models, despite the fact that a major portion of SCI patients suffer contusion trauma to cervical spinal cord. In this study, we have characterized 2 rat models of unilateral cervical contusion SCI that behaviorally result in chronic persistence of thermal hyperalgesia in the ipsilateral forepaw. In addition, we find that STT neurons are chronically activated in both models when compared to laminectomy-only uninjured rats. Finally, persistent astrocyte activation and significantly reduced expression of the major CNS glutamate transporter, GLT1, in superficial dorsal horn astrocytes are associated with both excitability changes in STT neurons and the neuropathic pain behavioral phenotype. In conclusion, we have characterized clinically-relevant rodent models of cervical contusion-induced neuropathic pain that result in chronic activation of both STT neurons and astrocytes, as well as compromise in astrocyte glutamate transporter expression. These models can be used as important tools to further study mechanisms underlying neuropathic pain post-SCI and to test potential therapeutic interventions.

Impairments in neurogenesis are not tightly linked to depressive behavior in a transgenic mouse model of Alzheimer's disease.

Alzheimer's disease (AD), the most common cause of dementia, is also associated with depression. Although the precise mechanisms that lead to depression in AD are unknown, the impairments in adult hippocampal neurogenesis observed in AD may play a role. Adult-born neurons play a critical role in regulating both cognition and mood, and reduced hippocampal neurogenesis is associated with depression in other neurological disorders. To assess the relationship between Alzheimer's disease, neurogenesis, and depression, we studied human amyloid precursor protein (hAPP) transgenic mice, a well-characterized model of AD. We report that reductions in hippocampal neurogenesis are evident early in disease progression in hAPP mice, but a mild depressive phenotype manifests only in later stages of disease. We found that hAPP mice exhibited a reduction in BrdU-positive cells in the subgranular zone of the dentate gyrus in the hippocampus, as well as a reduction in doublecortin-expressing cells, relative to nontransgenic controls at 5-7 months of age. These alterations in neurogenesis appeared to worsen with age, as the magnitude of reduction in doublecortin-expressing cells was greater in hAPP mice at 13-15 months of age. Only 13-15 month old hAPP mice exhibited depressive behavior in the tail suspension test. However, mice at both age groups exhibited deficits in spatial memory, which was observed in the Morris water maze test for hippocampus-dependent memory. These findings indicate that neurogenesis impairments are accompanied by cognitive deficits, but are not tightly linked to depressive behavior in hAPP mice.

Expression of c-fos in hilar mossy cells of the dentate gyrus in vivo.

Granule cells (GCs) of the dentate gyrus (DG) are considered to be quiescent--they rarely fire action potentials. In contrast, the other glutamatergic cell type in the DG, hilar mossy cells (MCs) often have a high level of spontaneous activity based on recordings in hippocampal slices. MCs project to GCs, so activity in MCs could play an important role in activating GCs. Therefore, we investigated whether MCs were active under basal conditions in vivo, using the immediate early gene c-fos as a tool. We hypothesized that MCs would exhibit c-fos expression even if rats were examined randomly, under normal housing conditions. Therefore, adult male rats were perfused shortly after removal from their home cage and transfer to the laboratory. Remarkably, most c-fos immunoreactivity (ir) was in the hilus, especially temporal hippocampus. C-fos-ir hilar cells co-expressed GluR2/3, suggesting that they were MCs. C-fos-ir MCs were robust even when the animal was habituated to the investigator and laboratory where they were euthanized. However, c-fos-ir in dorsal MCs was reduced under these circumstances, suggesting that ventral and dorsal MCs are functionally distinct. Interestingly, there was an inverse relationship between MC and GC layer c-fos expression, with little c-fos expression in the GC layer in ventral sections where MC expression was strong, and the opposite in dorsal hippocampus. The results support the hypothesis that a subset of hilar MCs are spontaneously active in vivo and provide other DG neurons with tonic depolarizing input.