Samuel S. Newton

Glial pathology in an animal model of depression: reversal of stress-induced cellular, metabolic and behavioral deficits by the glutamate-modulating drug riluzole.

Growing evidence indicates that glia pathology and amino-acid neurotransmitter system abnormalities contribute to the pathophysiology and possibly the pathogenesis of major depressive disorder. This study investigates changes in glial function occurring in the rat prefrontal cortex (PFC) after chronic unpredictable stress (CUS), a rodent model of depression. Furthermore, we analyzed the effects of riluzole, a Food and Drug Administration-approved drug for the treatment of amyotrophic laterosclerosis, known to modulate glutamate release and facilate glutamate uptake, on CUS-induced glial dysfunction and depressive-like behaviors. We provide the first experimental evidence that chronic stress impairs cortical glial function. Animals exposed to CUS and showing behavioral deficits in sucrose preference and active avoidance exhibited significant decreases in 13C-acetate metabolism reflecting glial cell metabolism, and glial fibrillary associated protein (GFAP) mRNA expression in the PFC. The cellular, metabolic and behavioral alterations induced by CUS were reversed and/or blocked by chronic treatment with the glutamate-modulating drug riluzole. The beneficial effects of riluzole on CUS-induced anhedonia and helplessness demonstrate the antidepressant action of riluzole in rodents. Riluzole treatment also reversed CUS-induced reductions in glial metabolism and GFAP mRNA expression. Our results are consistent with recent open-label clinical trials showing the drugs effect in mood and anxiety disorders. This study provides further validation of hypothesis that glial dysfunction and disrupted amino-acid neurotransmission contribute to the pathophysiology of depression and that modulation of glutamate metabolism, uptake and/or release represent viable targets for antidepressant drug development.

A negative regulator of MAP kinase causes depressive behavior

The lifetime prevalence (∼16%) and the economic burden (

Antidepressant actions of the exercise-regulated gene VGF.

100 billion annually) associated with major depressive disorder (MDD) make it one of the most common and debilitating neurobiological illnesses. To date, the exact cellular and molecular mechanisms underlying the pathophysiology of MDD have not been identified. Here we use whole-genome expression profiling of postmortem tissue and show significantly increased expression of mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1, encoded by DUSP1, but hereafter called MKP-1) in the hippocampal subfields of subjects with MDD compared to matched controls. MKP-1, also known as dual-specificity phosphatase-1 (DUSP1), is a member of a family of proteins that dephosphorylate both threonine and tyrosine residues and thereby serves as a key negative regulator of the MAPK cascade, a major signaling pathway involved in neuronal plasticity, function and survival. We tested the role of altered MKP-1 expression in rat and mouse models of depression and found that increased hippocampal MKP-1 expression, as a result of stress or viral-mediated gene transfer, causes depressive behaviors. Conversely, chronic antidepressant treatment normalizes stress-induced MKP-1 expression and behavior, and mice lacking MKP-1 are resilient to stress. These postmortem and preclinical studies identify MKP-1 as a key factor in MDD pathophysiology and as a new target for therapeutic interventions.

Dysregulation of Protein Kinase A Signaling in the Aged Prefrontal Cortex: New Strategy for Treating Age-Related Cognitive Decline

Exercise has many health benefits, including antidepressant actions in depressed human subjects, but the mechanisms underlying these effects have not been elucidated. We used a custom microarray to identify a previously undescribed profile of exercise-regulated genes in the mouse hippocampus, a brain region implicated in mood and antidepressant response. Pathway analysis of the regulated genes shows that exercise upregulates a neurotrophic factor signaling cascade that has been implicated in the actions of antidepressants. One of the most highly regulated target genes of exercise and of the growth factor pathway is the gene encoding the VGF nerve growth factor, a peptide precursor previously shown to influence synaptic plasticity and metabolism. We show that administration of a synthetic VGF-derived peptide produces a robust antidepressant response in mice and, conversely, that mutation of VGF in mice produces the opposite effects. The results suggest a new role for VGF and identify VGF signaling as a potential therapeutic target for antidepressant drug development.

Sweet taste signaling functions as a hypothalamic glucose sensor.

Activation of the cAMP/protein kinase A (PKA) pathway has been proposed as a mechanism for improving age-related cognitive deficits based on studies of hippocampal function. However, normal aging also afflicts prefrontal cortical cognitive functioning. Here, we report that agents that increase PKA activity impair rather than improve prefrontal cortical function in aged rats and monkeys with prefrontal cortical deficits. Conversely, PKA inhibition ameliorates prefrontal cortical cognitive deficits. Western blot and immunohistochemical analyses of rat brain further indicate that the cAMP/PKA pathway becomes disinhibited in the prefrontal cortex with advancing age. These data demonstrate that PKA inhibition, rather than activation, is the appropriate strategy for restoring prefrontal cortical cognitive abilities in the elderly.

Altered expression of synapse and glutamate related genes in post-mortem hippocampus of depressed subjects.

Brain glucosensing is essential for normal body glucose homeostasis and neuronal function. However, the exact signaling mechanisms involved in the neuronal sensing of extracellular glucose levels remain poorly understood. Of particular interest is the identification of candidate membrane molecular sensors that would allow neurons to change firing rates independently of intracellular glucose metabolism. Here we describe for the first time the expression of the taste receptor genes Tas1r1, Tas1r2 and Tas1r3, and their associated G-protein genes, in the mammalian brain. Neuronal expression of taste genes was detected in different nutrient-sensing forebrain regions, including the paraventricular and arcuate nuclei of the hypothalamus, the CA fields and dentate gyrus of the hippocampus, the habenula, and cortex. Expression was also observed in the intra-ventricular epithelial cells of the choroid plexus. These same regions were found to express the corresponding gene products that form the heterodimeric T1R2/T1R3 and T1R1/T1R3 sweet and l-amino acid taste G-protein coupled receptors, respectively, along with the taste G-protein α-gustducin. Moreover, in vivo studies in mice demonstrated that the hypothalamic expression of taste-related genes is regulated by the nutritional state of the animal, with food deprivation significantly increasing expression levels of Tas1r1 and Tas1r2 in hypothalamus, but not in cortex. Furthermore, exposing mouse hypothalamic cells to a low-glucose medium, while maintaining normal l-amino acid concentrations, specifically resulted in higher expression levels of the sweet-associated gene Tas1r2. This latter effect was reversed by adding the non-metabolizable artificial sweetener sucralose to the low-glucose medium, indicating that taste-like signaling in hypothalamic neurons does not require intracellular glucose oxidation. Taken together, our findings suggest that the heterodimeric G-protein coupled sweet receptor T1R2/T1R3 is a candidate membrane-bound brain glucosensor.

Chronic electroconvulsive seizure up-regulates β-catenin expression in rat hippocampus: Role in adult neurogenesis

Major depressive disorder (MDD) has been linked to changes in function and activity of the hippocampus, one of the central limbic regions involved in regulation of emotions and mood. The exact cellular and molecular mechanisms underlying hippocampal plasticity in response to stress are yet to be fully characterized. In this study, we examined the genetic profile of micro-dissected subfields of post-mortem hippocampus from subjects diagnosed with MDD and comparison subjects matched for sex, race and age. Gene expression profiles of the dentate gyrus and CA1 were assessed by 48K human HEEBO whole genome microarrays and a subgroup of identified genes was confirmed by real-time polymerase chain reaction (qPCR). Pathway analysis revealed altered expression of several gene families, including cytoskeletal proteins involved in rearrangement of neuronal processes. Based on this and evidence of hippocampal neuronal atrophy in MDD, we focused on the expression of cytoskeletal, synaptic and glutamate receptor genes. Our findings demonstrate significant dysregulation of synaptic function/structure related genes SNAP25, DLG2 (SAP93), and MAP1A, and 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid receptor subunit genes GLUR1 and GLUR3. Several of these human target genes were similarly dysregulated in a rat model of chronic unpredictable stress and the effects reversed by antidepressant treatment. Together, these studies provide new evidence that disruption of synaptic and glutamatergic signalling pathways contribute to the pathophysiology underlying MDD and provide interesting targets for novel therapeutic interventions.

Peripheral insulin-like growth factor-I produces antidepressant-like behavior and contributes to the effect of exercise

BACKGROUND Beta-catenin was discovered as a cytoskeletal protein, constituting a link between the cadherins to the actin cytoskeleton. Aside from this function, beta-catenin is a key effector molecule in the Wnt signaling pathway, serving as a downstream transcription factor. METHODS In this study, we examined the influence of electroconvulsive seizures (ECS) on the expression of beta-catenin, as well as expression of Wnt-2, in rat hippocampus. Repeated administration of generalized seizures increased levels of beta-catenin immunoreactivity in the subgranular zone of the hippocampus. To assess the relationship of beta-catenin to cell division in the dentate gyrus of the adult rat hippocampus, colocalization of beta-catenin with a marker of cell division was examined. RESULTS Beta-catenin immunoreactivity was consistently localized in newborn cells in this region, indicating a possible role in cell division and differentiation in adult hippocampus. We also found that ECS treatment significantly increased levels of Wnt-2, one of the ligands that activates beta-catenin signaling. CONCLUSIONS These results demonstrate that ECS increases Wnt-beta-catenin signaling and suggest that this pathway could mediate in part the neuronal adaptations underlying the therapeutic action of this treatment paradigm.

Gene expression profiling in postmortem prefrontal cortex of major depressive disorder.

Growth factors in the brain are important to depression and its treatment and we assessed the ability of peripherally administered insulin-like growth factor-I (IGF-I) to influence behavior related to depression. We found that mice that received chronic IGF-I treatment showed antidepressant-like behavior in forced-swim and novelty-induced hypophagia (NIH) tests and increased sucrose consumption after chronic mild unpredictable stress exposure. Additionally, peripheral anti-IGF-I administration blocked exercise-induced antidepressant effects in the forced-swim test (FST). These results support the functional relevance of neurotrophic mechanisms to depression and extend this idea to include neurotrophic factors in the periphery.

A Postpartum Model in Rat : Behavioral and Gene Expression Changes Induced by Ovarian Steroid Deprivation

Investigations of the molecular mechanisms underlying major depressive disorder (MDD) have been hampered by the complexity of brain tissue and sensitivity of gene expression profiling approaches. To address these issues, we used discrete microdissections of postmortem dorsolateral prefrontal cortex (DLPFC) (area 9) and an oligonucleotide (60mer) microarray hybridization procedure that increases sensitivity without RNA amplification. Mixed-effects statistical methods were used to rigorously control for medication usage in the subset of medicated depressed subjects. These analyses yielded a rich profile of dysregulated genes. Two of the most highly dysregulated genes of interest were stresscopin, a neuropeptide involved in stress responses, and Forkhead box D3 (FOXD3), a transcription factor. Secondary cell-based analysis demonstrated that stresscopin and FoxD3 are increased in neurons of DLPFC gray matter of MDD subjects. These findings identify abnormal gene expression in a discrete region of MDD subjects and contribute to further elucidation of the molecular alterations of this complex mood disorder.