Antidepressant drug treatment induces Arc gene expression in the rat brain

NeuropsychopharmARCology: Shaping Neuroplasticity through Arc/ Arg3.1 Modulation

Activity-regulated cytoskeleton-associated protein (aka activity-regulated gene Arg3.1) belongs to the effector gene family of the immediate early genes. This family encodes effector proteins, which act directly on cellular homeostasis and function. Arc/Arg3.1 is localized at dendritic processes, allowing the protein local synthesis on demand, and it is considered a reliable index of activity- dependent synaptic changes. Evidence also exists showing the critical role of Arc/Arg3.1 in memory processes. The high sensitivity to changes in neuronal activity, its specific localization as well as its involvement in long-term synaptic plasticity indeed make this effector gene a potential, critical target of the action of psychotropic drugs. In this review, we focus on antipsychotic and antidepressant drugs as well as on psychostimulants, which belong to the category of drugs of abuse but can also be used as drugs for specific disorders of the central nervous system (i.e., Attention Deficit Hyperactivity Disorder). It is demonstrated that psychotropic drugs with different mechanisms of action converge on Arc/Arg3.1, providing a means whereby Arc/Arg3.1 synaptic modulation may contribute to their therapeutic activity. The potential translational implications for different neuropsychiatric conditions are also discussed, recognizing that the treatment of these disorders is indeed complex and involves the simultaneous regulation of several dysfunctional mechanisms.

Astrocyte-derived lactate in stress disorders

Stress disorders are psychiatric disorders arising following stressful or traumatic events. They could deleteriously affect an individual's health because they often co-occur with mental illnesses. Considerable attention has been focused on neurons when considering the neurobiology of stress disorders. However, like other mental health conditions, recent studies have highlighted the importance of astrocytes in the pathophysiology of stress-related disorders. In addition to their structural and homeostatic support role, astrocytes actively serve several functions in regulating synaptic transmission and plasticity, protecting neurons from toxic compounds, and providing metabolic support for neurons. The astrocyte-neuron lactate shuttle model sets forth the importance of astrocytes in providing lactate for the metabolic supply of neurons under intense activity. Lactate also plays a role as a signaling molecule and has been recently studied regarding its antidepressant activity. This review discusses the involvement of astrocytes and brain energy metabolism in stress and further reflects on the importance of lactate as an energy supply in the brain and its emerging antidepressant role in stress-related disorders.

Arc Regulates Transcription of Genes for Plasticity, Excitability and Alzheimer’s Disease

The immediate early gene Arc is a master regulator of synaptic function and a critical determinant of memory consolidation. Here, we show that Arc interacts with dynamic chromatin and closely associates with histone markers for active enhancers and transcription in cultured rat hippocampal neurons. Both these histone modifications, H3K27Ac and H3K9Ac, have recently been shown to be upregulated in late-onset Alzheimer’s disease (AD). When Arc induction by pharmacological network activation was prevented using a short hairpin RNA, the expression profile was altered for over 1900 genes, which included genes associated with synaptic function, neuronal plasticity, intrinsic excitability, and signalling pathways. Interestingly, about 100 Arc-dependent genes are associated with the pathophysiology of AD. When endogenous Arc expression was induced in HEK293T cells, the transcription of many neuronal genes was increased, suggesting that Arc can control expression in the absence of activated signalling pathways. Taken together, these data establish Arc as a master regulator of neuronal activity-dependent gene expression and suggest that it plays a significant role in the pathophysiology of AD.

Attenuating effect of paroxetine on memory impairment following cerebral ischemia-reperfusion injury in rat: The involvement of BDNF and antioxidant capacity

Memory impairments are frequently reported in patients suffering from brain ischemic diseases. Oxidative/nitrosative stress, synaptic plasticity, and brain-derived neurotrophic factor (BDNF) are involved in the physiopathology of brain ischemia-induced memory disorders. In the present study, the effect of paroxetine as an efficacious antidepressant medication with antioxidant properties was evaluated on passive avoidance memory deficit following cerebral ischemia in rats. Transient occlusion of common carotid arteries was applied to induce ischemia-reperfusion injury in male Wistar rats. Paroxetine (5, 10, 20 mg/kg) was administered intraperitoneally once daily before (for 3 days) or after (for 7 days) the induction of ischemia. A week after ischemia-reperfusion injury, passive avoidance memory, long-term potentiation (LTP), BDNF levels, total antioxidant capacity, the activity of antioxidant enzymes (including catalase, glutathione peroxidase, and superoxide dismutase), the concentration of malondialdehyde (MDA), and nitric oxide (NO) were investigated in the hippocampus. In the passive avoidance test, paroxetine significantly increased the step-through latency and decreased the time spent in the dark compartment. This affirmative function of paroxetine on the passive avoidance memory was accompanied by the improvement of hippocampal LTP and an obvious augmentation in the BDNF contents. Besides, paroxetine caused a significant rise in the total antioxidant capacity and antioxidant enzyme activity; while decreased the hippocampal levels of NO and MDA. It was ultimately attained that paroxetine attenuates cerebral ischemia-induced passive avoidance memory dysfunction in rats by the enhancement of hippocampal synaptic plasticity and BDNF content together with the suppression of oxidative/nitrosative stress.

Effect of pharmacological manipulations on Arc function

Activity-regulated cytoskeleton-associated protein (Arc) is a brain-enriched immediate early gene that regulates important mechanisms implicated in learning and memory. Arc levels are controlled through a balance of induction and degradation in an activity-dependent manner. Arc further undergoes multiple post-translational modifications that regulate its stability, localization and function. Recent studies demonstrate that these features of Arc can be pharmacologically manipulated. In this review, we discuss some of these compounds, with an emphasis on drugs of abuse and psychotropic drugs. We also discuss inflammatory states that regulate Arc.

Dibutyl phthalate exposure during gestation and lactation in C57BL/6 mice: Maternal behavior and neurodevelopment in pups

OBJECTIVES

Neurotoxic effects of phthalate during pregnancy on immature brain of the offspring or mature brains of the mothers remain unclear. We examined the effect of dibutyl phthalate (DBP) exposure during gestation and lactation on the maternal behavior of mother mice and neurodevelopment in pups.

METHODS

Pregnant mice were treated orally with DBP (0, 50 and 100 mg/kg/day, N = 20 per group) from gestational day 13 to postnatal day (PND) 15. Maternal behavior was measured using pup retrieval and nest shape test at postpartum day 4. For the pups, the neurodevelopment was measured using negative geotaxis, cliff avoidance at PND 7, swimming test and olfactory orientation at PND 14. RNA and protein expressions in the brain cortex of 50 mg/kg/day and control group (0 mg/kg/day) were analyzed using microarray and Western blot analysis. Nissl-stained sections at the coronal level of interaural 2.56 mm, bregma -1,23 mm, were used for counting of dark cortical neurons in mother and pup mice.

RESULTS

DBP treated mother mice (50 and 100 mg/kg/day) showed poor maternal behavior, poor nesting and retrieval behavior compared to the control group (0 mg/kg/day). In brain cortex, DBP-treated mothers showed decrease in protein expression of Nr4a3, Egr1, Arc, BDNF and phosphorylation of AKT and CREB, were also decreased in cortex of DBP-treated mothers. Pups exposed to DBP showed significantly decreased scores in negative geotaxis at PND 7 and swimming scores and olfactory orientation tests at PND 14. The cortex of the DBP exposed pups showed increase in expression of dopamine receptor D2 gene. Nissl staining showed that the dark neurons were increased in cortex of DBP treated mothers and DBP exposed pups. Suggesting that phthalate may delay pup development indirectly through inadequate mothering as well as direct phthalate exposure on the brain.

CONCLUSION

DBP exposure during gestation and lactation cause impairment in maternal behaviors and downregulation of neuronal plasticity and survival signals. Pups of mothers with exposed to DBP, showed delayed neurodevelopment and dark neurons increase in brain cortex, suggesting that phthalate may delay pup development indirectly through inadequate mothering as well as direct phthalate exposure on the brain.

Selective Knockdown of TASK3 Potassium Channel in Monoamine Neurons: a New Therapeutic Approach for Depression

Current pharmacological treatments for major depressive disorder (MDD) are severely compromised by both slow action and limited efficacy. RNAi strategies have been used to evoke antidepressant-like effects faster than classical drugs. Using small interfering RNA (siRNA), we herein show that TASK3 potassium channel knockdown in monoamine neurons induces antidepressant-like responses in mice. TASK3-siRNAs were conjugated to cell-specific ligands, sertraline (Ser) or reboxetine (Reb), to promote their selective accumulation in serotonin (5-HT) and norepinephrine (NE) neurons, respectively, after intranasal delivery. Following neuronal internalization of conjugated TASK3-siRNAs, reduced TASK3 mRNA and protein levels were found in the brainstem 5-HT and NE cell groups. Moreover, Ser-TASK3-siRNA induced robust antidepressant-like behaviors, enhanced the hippocampal plasticity, and potentiated the fluoxetine-induced increase on extracellular 5-HT. Similar responses, yet of lower magnitude, were detected for Reb-TASK3-siRNA. These findings provide substantial support for TASK3 as a potential target, and RNAi-based strategies as a novel therapeutic approach to treat MDD.

Immediate Early Genes, Memory and Psychiatric Disorders: Focus on c-Fos, Egr1 and Arc

Many psychiatric disorders, despite their specific characteristics, share deficits in the cognitive domain including executive functions, emotional control and memory. However, memory deficits have been in many cases undervalued compared with other characteristics. The expression of Immediate Early Genes (IEGs) such as, c-fos, Egr1 and arc are selectively and promptly upregulated in learning and memory among neuronal subpopulations in regions associated with these processes. Changes in expression in these genes have been observed in recognition, working and fear related memories across the brain. Despite the enormous amount of data supporting changes in their expression during learning and memory and the importance of those cognitive processes in psychiatric conditions, there are very few studies analyzing the direct implication of the IEGs in mental illnesses. In this review, we discuss the role of some of the most relevant IEGs in relation with memory processes affected in psychiatric conditions.

Altered microglia morphology and higher resilience to stress-induced depression-like behavior in CX3CR1-deficient mice

Microglia are suggested to be involved in several neuropsychiatric diseases. Indeed changes in microglia morphology have been reported in different mouse models of depression. A crucial regulatory system for microglia function is the well-defined CX3C axis. Thus, we aimed to clarify the role of microglia and CX3CR1 in depressive behavior by subjecting CX3CR1-deficient mice to a particular chronic despair model (CDM) paradigm known to exhibit face validity to major depressive disorder. In wild-type mice we observed the development of chronic depressive-like behavior after 5days of repetitive swim stress. 3D-reconstructions of Iba-1-labeled microglia in the dentate molecular layer revealed that behavioral effects were associated with changes in microglia morphology towards a state of hyper-ramification. Chronic treatment with the anti-depressant venlafaxine ameliorated depression-like behavior and restored microglia morphology. In contrast, CX3CR1 deficient mice showed a clear resistance to either (i) stress-induced depressive-like behavior, (ii) changes in microglia morphology and (iii) antidepressant treatment. Our data point towards a role of hyper-ramified microglia in the etiology of chronic depression. The lack of effects in CX3CR1 deficient mice suggests that microglia hyper-ramification is controlled by neuron-microglia signaling via the CX3C axis. However, it remains to be elucidated how hyper-ramified microglia contribute to depressive-like behavior.

Social Isolation Stress Induces Anxious-Depressive-Like Behavior and Alterations of Neuroplasticity-Related Genes in Adult Male Mice

Stress is a major risk factor in the onset of several neuropsychiatric disorders including anxiety and depression. Although several studies have shown that social isolation stress during postweaning period induces behavioral and brain molecular changes, the effects of social isolation on behavior during adulthood have been less characterized. Aim of this work was to investigate the relationship between the behavioral alterations and brain molecular changes induced by chronic social isolation stress in adult male mice. Plasma corticosterone levels and adrenal glands weight were also analyzed. Socially isolated (SI) mice showed higher locomotor activity, spent less time in the open field center, and displayed higher immobility time in the tail suspension test compared to group-housed (GH) mice. SI mice exhibited reduced plasma corticosterone levels and reduced difference between right and left adrenal glands. SI showed lower mRNA levels of the BDNF-7 splice variant, c-Fos, Arc, and Egr-1 in both hippocampus and prefrontal cortex compared to GH mice. Finally, SI mice exhibited selectively reduced mGluR1 and mGluR2 levels in the prefrontal cortex. Altogether, these results suggest that anxious- and depressive-like behavior induced by social isolation stress correlates with reduction of several neuroplasticity-related genes in the hippocampus and prefrontal cortex of adult male mice.

A critical evaluation of the activity-regulated cytoskeleton-associated protein (Arc/Arg3.1)'s putative role in regulating dendritic plasticity, cognitive processes, and mood in animal models of depression

Major depressive disorder (MDD) is primarily conceptualized as a mood disorder but cognitive dysfunction is also prevalent, and may limit the daily function of MDD patients. Current theories on MDD highlight disturbances in dendritic plasticity in its pathophysiology, which could conceivably play a role in the production of both MDD-related mood and cognitive symptoms. This paper attempts to review the accumulated knowledge on the basic biology of the activity-regulated cytoskeleton-associated protein (Arc or Arg3.1), its effects on neural plasticity, and how these may be related to mood or cognitive dysfunction in animal models of MDD. On a cellular level, Arc plays an important role in modulating dendritic spine density and remodeling. Arc also has a close, bidirectional relationship with postsynaptic glutamate neurotransmission, since it is stimulated by multiple glutamatergic receptor mechanisms but also modulates α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor internalization. The effects on AMPA receptor trafficking are likely related to Arc's ability to modulate phenomena such as long-term potentiation, long-term depression, and synaptic scaling, each of which are important for maintaining proper cognitive function. Chronic stress models of MDD in animals show suppressed Arc expression in the frontal cortex but elevation in the amygdala. Interestingly, cognitive tasks depending on the frontal cortex are generally impaired by chronic stress, while those depending on the amygdala are enhanced, and antidepressant treatments stimulate cortical Arc expression with a timeline that is reminiscent of the treatment efficacy lag observed in the clinic or in preclinical models. However, pharmacological treatments that stimulate regional Arc expression do not universally improve relevant cognitive functions, and this highlights a need to further refine our understanding of Arc on a subcellular and network level.

Life-long norepinephrine transporter (NET) knock-out leads to the increase in the NET mRNA in brain regions rich in norepinephrine terminals

These studies aimed to identify the genes differentially expressed in the frontal cortex of mice bearing a life-long norepinephrine transporter knock-out (NET-KO) and wild-type animals (WT). Differences in gene expression in the mouse frontal cortex were studied using a whole-genome microarray approach. Using an alternative approach, i.e. RT-PCR (reverse transcription polymerase chain reaction) with primers complementary to various exons of the NET gene, as well as TaqMan arrays, the level of mRNA encoding the NET in other brain regions of the NET-KO mice was also examined. The analyses revealed a group of 92 transcripts (27 genes) that differentiated the NET-KO mice from the WT mice. Surprisingly, the studies have shown that the mRNA encoding NET accumulated in the brain regions rich in norepinephrine nerve endings in the NET-KO mice. Because there is no other source of NET mRNA besides the noradrenergic terminals in the brain regions studied, these results might speak in favor of the presence of mRNA in axon terminals. RNA-Binding Protein Immunoprecipitation approach indicated that mRNA encoding NET was detected in the Ago2 protein/mRNA complex. In addition, the amount of Ago2 protein in the frontal cortex was significantly higher in NET-KO mice as compared with that of the WT animals. These results are important for further characterization of the NET-KO mice, which - besides other merits - might serve as a good model to study the fate of truncated mRNA in neurons.

Chronic administration of haloperidol and clozapine induces differential effects on the expression of Arc and c-Fos in rat brain

The modulation of genes implicated in synaptic plasticity following administration of antipsychotic drugs has been instrumental in understanding their possible mode of action. Arc (Arg 3.1) is one such gene closely associated with changes in synaptic plasticity. In this study we have investigated the changes in expression of Arc protein following acute and chronic administration of a typical antipsychotic (haloperidol) and an atypical antipsychotic (clozapine) by means of immunohistochemistry compared to the prototypic gene marker c-Fos. In dorsal striatum haloperidol (1 mg/kg) significantly increased Arc expression following both acute and chronic (21 day) administration with evidence of modulation in induction after repeated dosing. No significant changes were observed following either acute or chronic administration of clozapine (20 mg/kg). In the nucleus accumbens shell both clozapine and haloperidol induced Arc expression following acute administration, again with evidence of modulation after chronic dosing. The pattern of induction of Arc expression following haloperidol and clozapine in both dorsal and ventral striatum was similar to that for c-Fos. In medial prefrontal and cingulate cortex, Arc expression was significantly decreased by clozapine but not haloperidol without any indication of modulation following chronic dosing, whereas no significant changes in c-Fos expression were observed with either drug. Since synaptic modulation mediated by Arc is associated with down-regulation of the AMPA glutamate receptor, this study suggests a mechanism whereby enhanced glutamate receptor efficacy in medial cortical areas may be a component of antipsychotic drug action.

Agomelatine: mechanism of action and pharmacological profile in relation to antidepressant properties

Agomelatine behaves both as a potent agonist at melatonin MT1 and MT2 receptors and as a neutral antagonist at 5-HT2C receptors. Accumulating evidence in a broad range of experimental procedures supports the notion that the psychotropic effects of agomelatine are due to the synergy between its melatonergic and 5-hydroxytryptaminergic effects. The recent demonstration of the existence of heteromeric complexes of MT1 and MT2 with 5-HT2C receptors at the cellular level may explain how these two properties of agomelatine translate into a synergistic action that, for example, leads to increases in hippocampal proliferation, maturation and survival through modulation of multiple cellular pathways (increase in trophic factors, synaptic remodelling, glutamate signalling) and key targets (early genes, kinases). The present review focuses on the pharmacological properties of this novel antidepressant. Its mechanism of action, strikingly different from that of conventional classes of antidepressants, opens perspectives towards a better understanding of the physiopathological bases underlying depression.

Anatomical specificity in the modulation of activity-regulated genes after acute or chronic lurasidone treatment

Lurasidone is a novel second generation antipsychotic drug characterized by a multi-receptor profile. Besides the high affinity for 5-HT2A and D2 receptors, it is also characterized by potent 5-HT7 receptor antagonism, which may be beneficial for mood and cognition. Considering that dose-dependent changes in receptor occupancy may differentially impact gene transcription, we aimed at investigating the effects of acute and chronic treatments with different doses of lurasidone (1, 3 and 10mg/kg) in rats on the expression of the activity-regulated genes Arc, Zif268 and Npas4, which are markers of neuronal activation and are also associated with neuroadaptive mechanisms. Our results show dose-dependent and anatomically-selective differences after acute and chronic lurasidone treatment. Indeed, the effects produced by acute treatment seem to reflect the modulatory activity of lurasidone at selected neurotransmitter receptors. In fact, low doses of the drug acted in the hippocampus, while high doses acted in the striatum, reflecting the high predominance of D2 receptor expression in this brain region. On the contrary, chronic treatment with lurasidone revealed a different profile of IEGs modulation, possibly reflecting neuroadaptive changes set in motion in response to repetitive drug exposure. In summary, the multi-receptor profile of lurasidone leads to the recruitment of different brain structures in a dose-related manner and this may be important for its therapeutic properties, particularly with respect to antidepressant activity and cognition.

5-HTT deficiency affects neuroplasticity and increases stress sensitivity resulting in altered spatial learning performance in the Morris water maze but not in the Barnes maze

The purpose of this study was to evaluate whether spatial hippocampus-dependent learning is affected by the serotonergic system and stress. Therefore, 5-HTT knockout (-/-), heterozygous (+/-) and wildtype (+/+) mice were subjected to the Barnes maze (BM) and the Morris water maze (WM), the latter being discussed as more aversive. Additionally, immediate early gene (IEG) expression, hippocampal adult neurogenesis (aN), and blood plasma corticosterone were analyzed.

While the performance of 5-HTT-/- mice in the BM was undistinguishable from both other genotypes, they performed worse in the WM. However, in the course of the repeated WM trials 5-HTT-/- mice advanced to wildtype level. The experience of a single trial of either the WM or the BM resulted in increased plasma corticosterone levels in all genotypes. After several trials 5-HTT-/- mice exhibited higher corticosterone concentrations compared with both other genotypes in both tests. Corticosterone levels were highest in 5-HTT-/- mice tested in the WM indicating greater aversiveness of the WM and a greater stress sensitivity of 5-HTT deficient mice.

Quantitative immunohistochemistry in the hippocampus revealed increased cell counts positive for the IEG products cFos and Arc as well as for proliferation marker Ki67 and immature neuron marker NeuroD in 5-HTT-/- mice compared to 5-HTT+/+ mice, irrespective of the test. Most differences were found in the suprapyramidal blade of the dentate gyrus of the septal hippocampus. Ki67-immunohistochemistry revealed a genotype x environment interaction with 5-HTT genotype differences in naïve controls and WM experience exclusively yielding more Ki67-positive cells in 5-HTT+/+ mice. Moreover, in 5-HTT-/- mice we demonstrate that learning performance correlates with the extent of aN.

Overall, higher baseline IEG expression and increased an in the hippocampus of 5-HTT-/- mice together with increased stress sensitivity may constitute the neurobiological correlate of raised alertness, possibly impeding optimal learning performance in the more stressful WM.

RNAi-mediated serotonin transporter suppression rapidly increases serotonergic neurotransmission and hippocampal neurogenesis

Current antidepressants, which inhibit the serotonin transporter (SERT), display limited efficacy and slow onset of action. Here, we show that partial reduction of SERT expression by small interference RNA (SERT-siRNA) decreased immobility in the tail suspension test, displaying an antidepressant potential. Moreover, short-term SERT-siRNA treatment modified mouse brain variables considered to be key markers of antidepressant action: reduced expression and function of 5-HT(1A)-autoreceptors, elevated extracellular serotonin in forebrain and increased neurogenesis and expression of plasticity-related genes (BDNF, VEGF, Arc) in hippocampus. Remarkably, these effects occurred much earlier and were of greater magnitude than those evoked by long-term fluoxetine treatment. These findings highlight the critical role of SERT in serotonergic function and show that the reduction of SERT expression regulates serotonergic neurotransmission more potently than pharmacological blockade of SERT. The use of siRNA-targeting genes in serotonin neurons (SERT, 5-HT(1A)-autoreceptor) may be a novel therapeutic strategy to develop fast-acting antidepressants.

Molecular and cellular mechanisms of antidepressant action

A long-standing theory is that brain monoamine signalling is critically involved in the mechanisms of antidepressant drug treatment. Theories on the nature of these mechanisms commenced with ideas developed in the 1960s that the drugs act simply by increasing monoamine availability in the synapse. However, this thinking has advanced remarkably in the last decade to concepts which position that antidepressant drug action on monoamine signalling is just the starting point for a complex sequence of neuroadaptive molecular and cellular changes that bring about the therapeutic effect. These changes include activation of one or more programmes of gene expression that leads to the strengthening of synaptic efficacy and connectivity, and even switching neural networks into a more immature developmental state. It is thought that through this increase in plasticity, key neural circuits within the limbic system are more easily remodelled by incoming emotionally relevant stimuli. This article attempts to bring together previous and current knowledge of antidepressant drug action on monoamine signalling at molecular and cellular levels, and introduces current thinking that these changes interact with neuropsychological processes ultimately to elevate mood.

The Role of Neurotrophins in Major Depressive Disorder

Neurotrophins and other growth factors have been advanced as critical modulators of depressive behavior. Support for this model is based on analyses of knockout and transgenic mouse models, human genetic studies, and screens for gene products that are regulated by depressive behavior and/or antidepressants. Even subtle alteration in the regulated secretion of brain-derived neurotrophic factor (BDNF), for example, due to a single nucleotide polymorphism (SNP)-encoded Val-Met substitution in proBDNF that affects processing and sorting, impacts behavior and cognition. Alterations in growth factor expression result in changes in neurogenesis as well as structural changes in neuronal cytoarchitecture, including effects on dendritic length and spine density, in the hippocampus, nucleus accumbens, and prefrontal cortex. These changes have the potential to impact the plasticity and stability of synapses in the CNS, and the complex brain circuitry that regulates behavior. Here we review the role that neurotrophins play in the modulation of depressive behavior, and the downstream signaling targets they regulate that potentially mediate these behavioral pro-depressant and antidepressant effects.

Molecular adaptation to chronic antidepressant treatment: evidence for a more rapid response to the novel α₂-adrenoceptor antagonist/5-HT-noradrenaline reuptake inhibitor (SNRI), S35966, compared to the SNRI, venlafaxine

Evidence of early changes in neural plasticity may aid the prediction of rapid-onset antidepressant drugs. Here we compared the dual α₂-adrenoceptor antagonist/5-HT-noradrenaline reuptake inhibitor (SNRI), S35966, to the SNRI, venlafaxine, with regards to their effect on rat brain expression of a panel of neural plasticity-related genes: Arc, BDNF, and VGLUT1, as well as Homer1a and Shank1B (not studied previously). Abundance of mRNA was determined by in-situ hybridization in cortical and hippocampal regions 2 h and 16 h following drug administration for 14, 7 and 1 d. After 14 d, both S35966 and venlafaxine increased mRNA of all genes, including Homer1a and Shank1B, and effects were similarly time- and region-dependent. After 7 d, S35966 elevated Arc, Shank1B and BDNF mRNA, whereas venlafaxine increased Shank1B mRNA only. Finally, after 1 d (acute administration), S35966 increased Arc and Homer1a mRNA whereas venlafaxine had no effect on any gene examined. In summary, a 14-d course of treatment with S35966 or venlafaxine induced similar region- and time-dependent increases in expression of neural plasticity-related genes including Shank1B and Homer1a. Some genes responded earlier to S35966, suggesting that drugs with combined α₂-adrenoceptor antagonist/SNRI properties may elicit more rapid changes in markers of neural plasticity than a SNRI alone.

Differential immediate and sustained memory enhancing effects of alpha7 nicotinic receptor agonists and allosteric modulators in rats

The α7 nicotinic acetylcholine receptor (nAChR) is a potential target for the treatment of cognitive deficits in patients with schizophrenia, ADHD and Alzheimer's disease. Here we test the hypothesis that upregulation of α7 nAChR levels underlies the enhanced and sustained procognitive effect of repeated administration of α7 nAChR agonists. We further compare the effect of agonists to that of α7 nAChR positive allosteric modulators (PAMs), which do not induce upregulation of the α7 nAChR. Using the social discrimination test as a measure of short-term memory, we show that the α7 nAChR agonist A-582941 improves short-term memory immediately after repeated (7× daily), but not a single administration. The α7 nAChR PAMs PNU-120596 and AVL-3288 do not affect short-term memory immediately after a single or repeated administration. This demonstrates a fundamental difference in the behavioral effects of agonists and PAMs that may be relevant for clinical development. Importantly, A-582941 and AVL-3288 increase short-term memory 24 hrs after repeated, but not a single, administration, suggesting that repeated administration of both agonists and PAMs may produce sustained effects on cognitive performance. Subsequent [¹²⁵I]-bungarotoxin autoradiography revealed no direct correlation between α7 nAChR levels in frontal cortical or hippocampal brain regions and short-term memory with either compound. Additionally, repeated treatment with A-582941 did not affect mRNA expression of RIC-3 or the lynx-like gene products lynx1, lynx2, PSCA, or Ly6H, which are known to affect nAChR function. In conclusion, both α7 nAChR agonists and PAMs exhibit sustained pro-cognitive effects after repeated administration, and altered levels of the α7 nAChR per se, or that of endogenous regulators of nAChR function, are likely not the major cause of this effect.

Effects of voluntary ethanol consumption on emotional state and stress responsiveness in socially isolated rats

Social isolation of rats immediately after weaning is thought to represent an animal model of anxiety-like disorders. This mildly stressful condition reduces the cerebrocortical and plasma concentrations of 3α-hydroxy-5α-pregnan-20-one (3α,5α-TH PROG) as well as increases the sensitivity of rats to the effects of acute ethanol administration on the concentrations of this neuroactive steroid. We further investigated the effects of voluntary consumption of ethanol at concentrations increasing from 2.5 to 10% over 4 weeks of isolation. Isolated rats showed a reduced ethanol preference compared with group-housed animals. Ethanol consumption did not affect the isolation-induced down-regulation of BDNF or Arc, but it attenuated the increase in the cerebrocortical concentration of 3α,5α-TH PROG induced by foot-shock stress in both isolated and group-housed animals as well as increased the percentage of number of entries made by socially isolated rats into the open arms in the elevated plus-maze test. Ethanol consumption did not affect expression of the α₄ subunit of the GABA(A) receptor in the hippocampus of group-housed or isolated rats, whereas it up-regulated the δ subunit throughout the hippocampus under both conditions. The results suggest that low consumption of ethanol may ameliorate some negative effects of social isolation on stress sensitivity and behavior.

Down-regulation of hippocampal BDNF and Arc associated with improvement in aversive spatial memory performance in socially isolated rats

Rats deprived of social contact with other rats at a young age experience a form of prolonged stress that leads to long-lasting changes in behavioral profile. Such isolation is thought to be anxiogenic for these normally gregarious animals, and the abnormal reactivity of isolated rats to environmental stimuli is thought to be a product of prolonged stress. We now show that isolation of rats at weaning reduced immobility time in the forced swim test, decreased sucrose intake and preference, and down-regulated both brain-derived neurotrophic factor (BDNF) and activity-regulated cytoskeletal associated protein (Arc) in the hippocampus. In the Morris water maze, isolated rats showed a reduced latency to reach the hidden platform during training, indicative of an improved learning performance, compared with group-housed rats. The cumulative search error during place training trials indicated a reliable difference between isolated and group-housed rats on days 4 and 5. The probe trial revealed a significant decrease of the average proximity to the target location in the isolated rats suggesting an improvement in spatial memory. Isolated rats also showed an increase in the plasma level of corticosterone on the 5th day of training and increased expression of BDNF and Arc in the hippocampus on both days 1 and 5. These results show that social isolation from weaning in rats results in development of depressive-like behavior but has a positive effect on spatial learning, supporting the existence of a facilitating effect of stress on cognitive function.

Shared changes in gene expression in frontal cortex of four genetically modified mouse models of depression

This study aimed to identify whether genetic manipulation of four systems implicated in the pathogenesis of depression converge on shared molecular processes underpinning depression-like behaviour in mice. Altered 5HT function was modelled using the 5-HT transporter knock out mouse, impaired glucocorticoid receptor (GR) function using an antisense-induced knock down mouse, disrupted glutamate function using a heterozygous KO of the vesicular glutamate transporter 1 gene, and impaired cannabinoid signalling using the cannabinoid 1 receptor KO mouse. All 4 four genetically modified mice were previously shown to show exaggerated helpless behaviour compared to wild-type controls and variable degrees of anxiety and anhedonic behaviour. mRNA was extracted from frontal cortex and hybridised to Illumina microarrays. Combined contrast analysis was used to identify genes showing different patterns of up- and down-regulation across the 4 models. 1823 genes were differentially regulated. They were over-represented in gene ontology categories of metabolism, protein handling and synapse. In each model compared to wild-type mice of the same genetic background, a number of genes showed increased expression changes of >10%, other genes showed decreases in each model. Most of the genes showed mixed effects. Several previous array findings were replicated. The results point to cellular stress and changes in post-synaptic remodelling as final common mechanisms of depression and resilience.

α7 nicotinic receptor agonism mitigates phencyclidine-induced changes in synaptophysin and Arc gene expression in the mouse prefrontal cortex

Repeated phencyclidine (PCP) administration in mice reproduces several histopathological features of schizophrenia, such as reduced synaptophysin and parvalbumin mRNA expression in the frontal cortex. These changes can be prevented by co-administering the α7 nicotinic acetylcholine receptor (nAChR) agonist SSR180711 with PCP, but it is not known to what extent PCP-induced changes can be normalized once they have already occurred. Here we use semi-quantitative in situ hybridization to show that repeated administration of SSR180711 (3 mg/kg b.i.d. for 5 days) subsequent to repeated PCP administration (10 mg/kg/day for 10 days) is able to mitigate the reduction of synaptophysin mRNA expression induced by PCP in two prefrontal cortical regions, the medial prefrontal cortex (mPFC) and the ventrolateral orbitofrontal cortex (VLO). This effect is accompanied by a normalization of the PCP-induced increase in Arc mRNA expression in the same regions. In contrast, subsequent administration of SSR180711 does not affect PCP-induced decreases in parvalbumin mRNA in the mPFC, and glutamate decarboxylase 67 mRNA in the mPFC or VLO. These data demonstrate that it is possible to restore some, but not all, of the molecular dysregulations induced by repeated PCP administration with an α7 nAChR agonist. They also suggest that the previously demonstrated cognitive improvement with SSR180711 subsequent to PCP treatment does not require normalization of parvalbumin expression, but may instead be related to a restoration of synaptophysin and/or Arc levels in the frontal cortex. These data lend support to the potential for development of α7 nAChR agonists for the treatment of cognitive deficits in schizophrenia.

Repeated administration of alpha7 nicotinic acetylcholine receptor (nAChR) agonists, but not positive allosteric modulators, increases alpha7 nAChR levels in the brain

The alpha7 nicotinic acetylcholine receptor (nAChR) is an important target for treatment of cognitive deficits in schizophrenia and Alzheimer's disease. However, the receptor desensitizes rapidly in vitro, which has led to concern regarding its applicability as a clinically relevant drug target. Here we investigate the effects of repeated agonism on alpha7 nAChR receptor levels and responsiveness in vivo in rats. Using [(125)I]-alpha-bungarotoxin (BTX) autoradiography we show that acute or repeated administration with the selective alpha7 nAChR agonist A-582941 increases the number of alpha7 nAChR binding sites in several brain regions, particularly in the prefrontal cortex. The alpha7 nAChR agonists SSR180711 and PNU-282987 also increase [(125)I]-BTX binding, suggesting that this is a general consequence of alpha7 nAChR agonism. Interestingly, the alpha7 nAChR positive allosteric modulators PNU-120596 and NS1738 do not increase [(125)I]-BTX binding. Furthermore, A-582941-induced increase in Arc and c-fos mRNA expression in the prefrontal cortex is enhanced and unaltered, respectively, after repeated administration, demonstrating that the alpha7 nAChRs remain responsive. Contrarily, A-582941-induced phosphorylation of Erk2 in the prefrontal cortex occurs following acute, but not repeated administration. Our results demonstrate that repeated agonist administration increases the number of alpha7 nAChRs in the brain, and leads to coupling versus uncoupling of specific intracellular signaling pathways. Additionally, our data suggest a fundamental difference between the sequelae of repeated administration with agonists and allosteric modulators of the alpha7 nAChR.

Combined Pharmacotherapy and Cognitive-Behavioral Therapy for Anxiety Disorders: Medication Effects, Glucocorticoids, and Attenuated Treatment Outcomes

Despite the success of both pharmacologic and cognitive-behavioral interventions for the treatment of anxiety disorders, the combination of these modalities in adults has not resulted in substantial improvements in outcome relative to either strategy alone, raising questions about whether there are interfering effects that attenuate the magnitude of combination treatment benefits. In this article, we introduce an accounting of potential interference effects that expands upon arguments asserting the necessity of arousal for successful fear exposure. Specifically, recent advances in the study of the effects of cortisol on memory--suggesting that glucocorticoids are crucial to the learning of emotional material--have led us to posit that the attenuation of glucocorticoid activity by anxiolytic medications may interfere with extinction learning in exposure-based therapies. Implications for the efficacy of combination treatments for the anxiety disorders are discussed.

Depression-prone mice with reduced glucocorticoid receptor expression display an altered stress-dependent regulation of brain-derived neurotrophic factor and activity-regulated cytoskeleton-associated protein

Increasing evidence suggests that depression is characterised by impaired brain plasticity that might originate from the interaction between genetic and environmental risk factors. Hence, the aim of this study was to investigate changes in neuroplasticity following exposure to stress, an environmental condition highly relevant to psychiatric disorders, in glucocorticoid receptor-deficient mice (GR(+/-)), a genetic model of predisposition to depression. Specifically, we have analysed the neurotrophin brain-derived neurotrophic factor (BDNF) and the immediate-early gene activity-regulated cytoskeletal-associated protein (Arc), two closely related molecules that can contribute to neuroplastic and morphological changes observed in depression. We found a region-specific influence of the GR-genotype on BDNF levels both under basal and stress conditions. Steady-state levels of BDNF mRNA were unchanged in hippocampus while up-regulated in frontal lobe of GR(+/-) mice. Following exposure to an acute stress, increased processing from pro- to mature BDNF was observed in hippocampal synaptosomes of wild-type mice, but not in GR mutants. Furthermore, the stress-dependent modulation of Arc was impaired in the hippocampus of GR(+/-) mice. These results indicate that GR(+/-) mice show overt differences in the stress-induced modulation of neuroplastic proteins, which may contribute to pathologic conditions that may originate following gene x environment interaction.

Alpha2-adrenoceptor blockade accelerates the neurogenic, neurotrophic, and behavioral effects of chronic antidepressant treatment

Slow-onset adaptive changes that arise from sustained antidepressant treatment, such as enhanced adult hippocampal neurogenesis and increased trophic factor expression, play a key role in the behavioral effects of antidepressants. alpha(2)-Adrenoceptors contribute to the modulation of mood and are potential targets for the development of faster acting antidepressants. We investigated the influence of alpha(2)-adrenoceptors on adult hippocampal neurogenesis. Our results indicate that alpha(2)-adrenoceptor agonists, clonidine and guanabenz, decrease adult hippocampal neurogenesis through a selective effect on the proliferation, but not the survival or differentiation, of progenitors. These effects persist in dopamine beta-hydroxylase knock-out (Dbh(-/-)) mice lacking norepinephrine, supporting a role for alpha(2)-heteroceptors on progenitor cells, rather than alpha(2)-autoreceptors on noradrenergic neurons that inhibit norepinephrine release. Adult hippocampal progenitors in vitro express all the alpha(2)-adrenoceptor subtypes, and decreased neurosphere frequency and BrdU incorporation indicate direct effects of alpha(2)-adrenoceptor stimulation on progenitors. Furthermore, coadministration of the alpha(2)-adrenoceptor antagonist yohimbine with the antidepressant imipramine significantly accelerates effects on hippocampal progenitor proliferation, the morphological maturation of newborn neurons, and the increase in expression of brain derived neurotrophic factor and vascular endothelial growth factor implicated in the neurogenic and behavioral effects of antidepressants. Finally, short-duration (7 d) yohimbine and imipramine treatment results in robust behavioral responses in the novelty suppressed feeding test, which normally requires 3 weeks of treatment with classical antidepressants. Our results demonstrate that alpha(2)-adrenoceptors, expressed by progenitor cells, decrease adult hippocampal neurogenesis, while their blockade speeds up antidepressant action, highlighting their importance as targets for faster acting antidepressants.

Altered expression and modulation of activity-regulated cytoskeletal associated protein (Arc) in serotonin transporter knockout rats

A gene variant in the human serotonin transporter (SERT) can increase the vulnerability to mood disorders. SERT knockout animals show similarities to the human condition and represent an important tool to investigate the mechanisms underlying the pathologic condition in humans. Along this line of thinking, we used SERT KO rats (SERT(+/-) and SERT(-/-)) to investigate abnormalities in the expression and function of the activity-regulated gene Arc (Activity-regulated cytoskeletal associated protein) and the early inducible gene Zif-268, (zinc finger binding protein clone 268), which are important players in neuronal plasticity. We found lower basal Arc mRNA levels in hippocampus and prefrontal cortex of mutant rats in comparison with wild-type animals. Moreover SERT mutant rats show altered stress responsiveness. Indeed an acute swim stress significantly up-regulated the levels of Arc mRNA in hippocampus and prefrontal cortex, as well as of Zif-268 in frontal cortex, only in SERT(+/-) and SERT(-/-) rats. These alterations may be associated to behavioral traits linked to SERT and may contribute to the neuroplastic and morphological changes observed in depression.

Opposite effect of phencyclidine on activity-regulated cytoskeleton-associated protein (Arc) in juvenile and adult limbic rat brain regions

The psychotomimetic effect of NMDA antagonists such as phencyclidine (PCP) in humans spurred the hypoglutamatergic theory of schizophrenia. This theory is supported by animal studies demonstrating schizophrenia-like behavioral and molecular changes following PCP administration to adult or neonatal animals. However, schizophrenia is believed to develop in part due to neurodevelopmental dysfunction during adolescence. Therefore, the effects of PCP in juvenile animals may better reflect the pathophysiology of schizophrenia. Here, we compare the effect of PCP (5mg/kg/day for 5 days) on activity-regulated cytoskeleton-associated protein (Arc) and parvalbumin mRNA expression in juvenile and adult rats. Arc is a marker for excitatory neurotransmission. Parvalbumin is a marker for GABAergic neurotransmission, known to be reduced in postmortem brains of schizophrenics. PCP reduced parvalbumin mRNA expression in the medial prefrontal cortex (mPFC), ventrolateral orbitofrontal cortex (VLO) and shell of the nucleus accumbens (ACCshell) in both juvenile and adult rats. Contrarily, PCP produced opposite effects on Arc mRNA expression in the mPFC, VLO and ACCshell, leading to decreased expression in juvenile and increased expression in adult rats. The differential effect of PCP in juvenile and adult rats may be caused by the immature functional state of the prefrontal cortex in juvenile rats. These results demonstrate differences between the effects of PCP in juvenile and adult rats. The decrease in Arc mRNA in juvenile rats corresponds best with the proposed "hypofrontality" in schizophrenia, suggesting the merits of using PCP in juvenile animals as a model for schizophrenia, as this would relate better to the typical onset and clinical features of schizophrenia.

Decrease in the kainate-induced wet dog shake behavior in genetically epilepsy-prone rats: possible involvement of an impaired synaptic transmission to the 5-HT(2A) receptor

Genetically epilepsy-prone rats (GEPR-9s) were derived from Sprague-Dawley rats (SD). The number of kainate-induced wet dog shake behavior (WDS) responses was found to decrease significantly in GEPR-9s compared to SD. WDS responses were potentiated by 5-hydroxytryptophan or 2,5-dimethoxy-4-iodoamphetamine and antagonized by ritanserin. The antagonizing effect of ritanserin on WDS latency was more evident in GEPR-9s than in SD, and hippocampal expression of activity-regulated cytoskeleton-associated protein paralleled the severity of WDS. The results suggest that downstream serotonergic synaptic activation is less pronounced in GEPR-9s than in SD and that the serotonergic agent may directly activate postsynaptic 5-HT2A receptors in both strains.

The Arc of synaptic memory

The immediate early gene Arc is emerging as a versatile, finely tuned system capable of coupling changes in neuronal activity patterns to synaptic plasticity, thereby optimizing information storage in the nervous system. Here, we attempt to overview the Arc system spanning from transcriptional regulation of the Arc gene, to dendritic transport, metabolism, and translation of Arc mRNA, to post-translational modification, localization, and degradation of Arc protein. Within this framework we discuss the function of Arc in regulation of actin cytoskeletal dynamics underlying consolidation of long-term potentiation (LTP) and regulation of AMPA-type glutamate receptor endocytosis underlying long-term depression (LTD) and homeostatic plasticity. Behaviorally, Arc has a key role in consolidation of explicit and implicit forms of memory, with recent work implicating Arc in adaptation to stress as well as maladaptive plasticity connected to drug addiction. Arc holds considerable promise as a “master regulator” of protein synthesis-dependent forms of synaptic plasticity, but the mechanisms that modulate and switch Arc function are only beginning to be elucidated.

F15063, a potential antipsychotic with dopamine D(2)/D(3) receptor antagonist and 5-HT(1A) receptor agonist properties: influence on immediate-early gene expression in rat prefrontal cortex and striatum

Brain region-specific modulation of immediate-early gene (IEG) may constitute a marker of antipsychotic drug-like activity. We investigated the effects of the putative antipsychotic drug N-[(2,2-dimethyl-2,3-dihydro-benzofuran-7-yloxy)ethyl]-3-(cyclopent-1-enyl)-benzylamine (F15063), a compound that targets both dopamine D(2) and serotonin 5-HT(1A) receptors, in comparison with haloperidol and clozapine on rat mRNA expression of IEG i.e. the zinc-fingered transcription factors c-fos, fosB, zif268, c-jun and junB, two transcription factors of the nuclear receptor family nur77 and nor1, and the effector IEG arc. F15063 (10 mg/kg) and clozapine (10 mg/kg), but not haloperidol (0.63 mg/kg), induced c-fos and fosB mRNA expression in prefrontal cortex, a region associated with control of cognition and negative symptoms of schizophrenia. In striatum, only c-fos, fosB, junB and nur77 were induced by clozapine whereas all IEG mRNAs were increased by haloperidol and F15063 (from 2.5 mg/kg) with similar high efficacy despite a total absence of F15063-induced catalepsy. However, at 0.63 mg/kg, F15063 induced a lower degree of striatal IEG mRNA expression than haloperidol and pretreatment with the serotonin 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl-N-(2-pyridinyl)cyclohexane carboxamide trihydrochloride (WAY100635) (0.63 mg/kg) increased the level of IEG mRNA induction by F15063. Furthermore, (+)-8-hydroxy-2-(di-n-propylamino)tetralin [(+)-8-OH-DPAT] at 0.16 mg/kg decreased haloperidol-induced striatal IEG mRNA expression although it exerted no effects on its own. These results are consistent with an activation of serotonin 5-HT(1A) receptors by F15063, thus reducing D(2) blockade-induced striatal IEG mRNA. Furthermore, the substantial F15063-induced expression of IEGs such as c-fos in striatum is not related to cataleptogenic activity and may act more as a marker of efficacious dopamine D(2) receptor blockade.

Fluorescent Arc/Arg3.1 indicator mice: a versatile tool to study brain activity changes in vitro and in vivo

The brain-specific immediate early gene Arc/Arg3.1 is induced in response to a variety of stimuli, including sensory and behavior-linked neural activity. Here we report the generation of transgenic mice, termed TgArc/Arg3.1-d4EGFP, expressing a 4-h half-life form of enhanced green fluorescent protein (d4EGFP) under the control of the Arc/Arg3.1 promoter. We show that d4EGFP-mediated fluorescence faithfully reports Arc/Arg3.1 induction in response to physiological, pathological and pharmacological stimuli, and that this fluorescence permits electrical recording from activated neurons in the live mouse. Moreover, the fluorescent Arc/Arg3.1 indicator revealed activity changes in circumscribed brain areas in distinct modes of stress and in a mouse model of Alzheimer's disease. These findings identify the TgArc/Arg3.1-d4EGFP mouse as a versatile tool to monitor Arc/Arg3.1 induction in neural circuits, both in vitro and in vivo.

Antipsychotic drugs modulate Arc expression in the rat brain

We found that, in the striatum, acute injections of the first generation antipsychotic (FGA) haloperidol or the second generation antipsychotic (SGA) olanzapine enhanced Arc mRNA levels, however such induction persisted for at least 2 h in haloperidol-treated rats whereas it waned as early as 1 h after olanzapine injection. Conversely, repeated injections led to a persistent decrease of striatal Arc gene expression, regardless of the agent examined. In the frontal cortex, acute injection of both antipsychotics caused a reduction of Arc mRNA levels which persisted for at least 2 h. Following repeated treatment, olanzapine reduced Arc mRNA levels 2 h, but not 24 h, post-treatment whereas haloperidol was ineffective. Of note, the SGA quetiapine regulated the Arc gene expression similarly to olanzapine. Given the particular nature of Arc, our findings show its fine tuning following antipsychotic administration to be highly dependent on the length of the treatment and on the brain region investigated and suggest that antipsychotic drugs affect this marker of neuronal activity differently.

Basal and stress-induced modulation of activity-regulated cytoskeletal associated protein (Arc) in the rat brain following duloxetine treatment

RATIONALE

Therapeutic efficacy of antidepressant drugs appears to be related to their ability in producing neuroadaptive changes that restore normal brain function. Activity-regulated cytoskeletal associated protein (Arc) is an effector immediate early gene that plays a fundamental role in activity-dependent neural plasticity in corticolimbic brain regions and has been implicated in the modulation of several functions known to be profoundly perturbed in depressive states.

OBJECTIVE

In the present study, we investigated transcriptional and translational changes of Arc in response to acute or chronic treatment with the novel antidepressant duloxetine.

RESULTS

Although a limited increase of Arc messenger RNA (mRNA) levels was found in some structures after acute antidepressant administration, a marked up-regulation of its gene expression was found after chronic treatment, primarily at the level of frontal cortex. The changes observed after prolonged duloxetine administration strongly correlates with those previously reported on brain-derived neurotrophic factor mRNA levels Calabrese et al. (Neuropsychopharmacol 32:2351-2359, 2007). In addition, we found an anatomical-specific influence of chronic duloxetine on stress-dependent Arc modulation, which was limited to the frontal cortex.

CONCLUSIONS

We suggest that these neuroadaptive changes, among others, might contribute to the normalization of neuroplastic defects associated with mood disorders.

Acute nicotine activates c-fos and activity-regulated cytoskeletal associated protein mRNA expression in limbic brain areas involved in the central stress-response in rat pups during a period of hypo-responsiveness to stress

In adult rats, acute nicotine, the major psychoactive ingredient in tobacco smoke, stimulates the hypothalamic-pituitary-adrenal axis (HPA), resulting in activation of brain areas involved in stress and anxiety-linked behavior. However, in rat pups the first two postnatal weeks are characterized by hypo-responsiveness to stress, also called the 'stress non-responsive period' (SNRP). Therefore, we wanted to address the question if acute nicotine stimulates areas involved in the stress response during SNRP. To determine neuronal activation, the expression of the immediate-early genes c-fos and activity-regulated cytoskeletal associated protein (Arc) was studied in the central nucleus of the amygdala (CeA), bed nucleus stria terminalis (BST) and paraventricular hypothalamic nucleus (PVN), which are areas involved in the neuroendocrine and central stress response. Rat pups received nicotine tartrate (2 mg/kg) or saline by i.p. injection at postnatal days (P) 5, 7 and 10 and their brains were removed after 30 min. We used semi-quantitative radioactive in situ hybridization with gene specific antisense cRNA probes in coronal sections. In control pups, c-fos expression was low in most brain regions, but robust Arc hybridization was found in several areas including cingulate cortex, hippocampus and caudate. Acute nicotine resulted in significant induction of c-fos expression in the PVN and CeA at P5, P7 and P10, and in the BST at P7 and P10. Acute nicotine significantly induced expression of Arc in CeA at P5, P7 and P10, and in the BST at P10. In conclusion, acute nicotine age dependently activated different brain areas of the HPA axis during the SNRP. After P7, the response was more pronounced and included the BST, suggesting differential maturation of the HPA axis in response to nicotine.

Chronic fluoxetine treatment induces brain region-specific upregulation of genes associated with BDNF-induced long-term potentiation

Several lines of evidence implicate BDNF in the pathogenesis of stress-induced depression and the delayed efficacy of antidepressant drugs. Antidepressant-induced upregulation of BDNF signaling is thought to promote adaptive neuronal plasticity through effects on gene expression, but the effector genes downstream of BDNF has not been identified. Local infusion of BDNF into the dentate gyrus induces a long-term potentiation (BDNF-LTP) of synaptic transmission that requires upregulation of the immediate early gene Arc. Recently, we identified five genes (neuritin, Narp, TIEG1, Carp, and Arl4d) that are coupregulated with Arc during BDNF-LTP. Here, we examined the expression of these genes in the dentate gyrus, hippocampus proper, and prefrontal cortex after antidepressant treatment. We show that chronic, but not acute, fluoxetine administration leads to upregulation of these BDNF-LTP-associated genes in a brain region-specific pattern. These findings link chronic effects of antidepressant treatment to molecular mechanisms underlying BDNF-induced synaptic plasticity.

The immediate early gene Arc is associated with behavioral resilience to stress exposure in an animal model of posttraumatic stress disorder

Mechanisms involved in adaptative and maladaptive changes in neural plasticity and synaptic efficacy in various brain areas are pivotal to understanding the physiology of the response to stress and the pathophysiology of posttraumatic stress disorder (PTSD). Activity-regulated cytoskeletal-associated protein (Arc) is an effector immediate early gene (IEG) which has direct effects on intracellular homeostatic functions. Increased expression of Arc has been associated with increased neuronal activity and with consolidation of long-term memory. It may thus play an important role in mediating experience-induced reorganization and/or development of synaptic connections. This study sought to characterize the pattern of expression of mRNA for the Arc gene in selected brain areas of test subjects classified according to their individual pattern of behavioral response to a stressor, correlated with circulating levels of corticosterone (as a physiological marker of stress response). The hippocampal CA1 and CA3 subregions of individuals whose behavior was minimally or partially disrupted in response to predator scent stress demonstrated significantly increased levels of mRNA for Arc, compared to unexposed controls. The group whose behavior was severely disrupted demonstrated no such upregulation. Consistent with the hypothesis that the Arc gene has a promoting effect on neuronal function and/or structural changes, the lack of Arc expression in the behaviorally and physiologically more severely affected individuals raises the possibility that Arc may be associated with resilience and/or recovery after stress exposure.

Circadian gene expression is suppressed during sevoflurane anesthesia and the suppression persists after awakening

General anesthesia is routinely used as a surgical procedure and its safety has been endorsed by clinical outcomes; however, its effects at the molecular level have not been elucidated. We previously showed that inhalation anesthesia affects the expression of 1.5% of 10,000 genes, which included so-called circadian genes. In the current study, we confirmed that inhalation of sevoflurane alters circadian gene expression, and investigated whether this alteration persists after awakening from anesthesia. Rats were anesthetized with 4.0% sevoflurane for 0 h, 2 h and 6 h (n=9 each group), before being sacrificed. Rats were also anesthetized for 6 h and allowed to recover after anesthesia, then sacrificed 2 h, 6 h and 24 h after awakening (n=9 each group). Anesthesia was started for each group so that all rats would be sacrificed at 13:00, and gene expression in the whole brain was examined using real-time RT-PCR. Expression of the genes encoding Per2, Dbp, Arc, Egr1, Krox20 and NGFI-B was suppressed during inhalation of sevoflurane for 2 h and 6 h. Although the suppression tended to be alleviated after awakening from anesthesia, the expression levels after a recovery period of 24 h remained significantly lower than the control levels of these genes, except for Krox20. We demonstrated that circadian gene expression is suppressed in whole brain during sevoflurane anesthesia, and the suppression continues for at least 24 h after termination of sevoflurane treatment. This suggests that sevoflurane anesthesia may have effects at the molecular level and that these effects are long lasting.

Modulation of stress consequences by hippocampal monoaminergic, glutamatergic and nitrergic neurotransmitter systems

Several findings relate the hippocampal formation to the behavioural consequences of stress. It contains a high concentration of corticoid receptors and undergoes plastic modifications, including decreased neurogenesis and cellular remodelling, following stress exposure. Various major neurotransmitter systems in the hippocampus are involved in these effects. Serotonin (5-HT) seems to exert a protective role in the hippocampus and attenuates the behavioural consequences of stress by activating 5-HT1A receptors in this structure. These effects may mediate the therapeutic actions of several antidepressants. The role of noradrenaline is less clear and possibly depends on the specific hippocampal region (dorsal vs. ventral). The deleterious modifications induced in the hippocampus by stress might involve a decrease in neurotrophic factors such as brain derived neurotrophic factor (BDNF) following glutamate N-methyl-D-aspartate (NMDA) receptor activation. In addition to glutamate, nitric oxide (NO) could also be related to these effects. Systemic and intra-hippocampal administration of nitric oxide synthase (NOS) inhibitors attenuates stress-induced behavioural consequences. The challenge for the future will be to integrate results related to these different neurotransmitter systems in a unifying theory about the role of the hippocampus in mood regulation, depressive disorder and antidepressant effects.

Acute onset by 5-HT6-receptor activation on rat brain brain-derived neurotrophic factor and activity-regulated cytoskeletal-associated protein mRNA expression

A number of previous studies have shown that chronic but not acute treatment with antidepressant drugs targeting the central 5-HT system, enhances mRNA expression for a number of genes including, brain-derived neurotrophic factor (BDNF) and the effector immediate early gene (IEG), activity-regulated, cytoskeletal-associated protein (Arc). The present study investigated the effects of 5-HT(6)-receptor activation on hippocampal and cortical levels of mRNA expression of BDNF and Arc in the rat. The selective 5-HT(6)-receptor agonist LY-586713 was administered acutely (0.1-10 mg/kg, s.c.) and mRNA levels of BDNF and Arc were measured 18 h later. Administration of LY-586713 caused a bell-shaped dose response on hippocampal BDNF mRNA expression, having no effect at 0.1 mg/kg, a significant up-regulation at 1 mg/kg and no effect at 10 mg/kg. The up-regulation in BDNF expression observed at 1 mg/kg was completely blocked by pre-treatment with the selective 5-HT(6)-receptor antagonist SB-271046 (10 mg/kg, s.c.). The effective dose (1 mg/kg) of LY-586713 on the induction of BDNF expression was also tested on Arc expression. Acute administration of LY-586713 at this dose caused marked increases of the Arc mRNA levels in cortical and hippocampal regions. These increases were also attenuated by SB-271046 (10 mg/kg) in all regions of the hippocampus, as well as the parietal cortex. However, in frontal cortical regions there was no attenuation by the antagonist. Moreover, SB-271046 alone increased Arc expression in these regions. The results presented here provide the first evidence for the involvement of the 5-HT(6) receptor in regulating BDNF and Arc mRNA expression, suggesting that LY-586713 has potential effects on neuronal plasticity. Overall, these findings suggest that, as opposed to more general 5-HT receptor activation by, for example, antidepressants, direct 5-HT(6)-receptor activation results in a more rapid rise in BDNF and Arc mRNA expression which does not require repeated administration.