Enhanced sensitivity of the MRL/MpJ mouse to the neuroplastic and behavioral effects of chronic antidepressant treatments

Volumetric alterations in subregions of the amygdala in adults with major depressive disorder

BACKGROUND

Although major depressive disorder (MDD) has been associated with volumetric abnormalities in the amygdala, studies investigating the association between structural alterations of the amygdala and depression have yielded varying results. Since the amygdala comprises several subregions, it is difficult to detect subtle regional changes by measuring the total amygdala volume. This study aimed to examine the volume in each amygdala subregion in adults with and without a diagnosis of MDD.

METHODS

A total of 147 participants with a current history of major depression and 144 healthy participants ranging in age from 19 to 64 years underwent 3T magnetic resonance imaging scanning. Automatic segmentation of the nine nuclei of the amygdala was performed using FreeSurfer. One-way analysis of covariance, with individual volumes as dependent variables, and age, sex, and total intracranial volume as covariates, was performed to analyze volume differences.

RESULTS

Patients with MDD had significantly lower volumes of the entire amygdala and subregions, including the lateral nucleus and anterior amygdaloid area, than healthy volunteers (HCs). There were no significant associations between subregion volumes and antidepressant use, illness duration, or depression severity.

LIMITATIONS

Our cross-sectional design cannot provide a causal relationship between the volume change in the amygdala subregion and the risk of MDD.

CONCLUSION

Our findings suggest that specific amygdala subregions are more susceptible to volumetric alterations in patients with MDD than in HCs. These findings may advance our understanding of the neuroanatomic basis on MDD.

The engine initiating tissue regeneration: does a common mechanism exist during evolution?

A successful tissue regeneration is a very complex process that requires a precise coordination of many molecular, cellular and physiological events. One of the critical steps is to convert the injury signals into regeneration signals to initiate tissue regeneration. Although many efforts have been made to investigate the mechanisms triggering tissue regeneration, the fundamental questions remain unresolved. One of the major obstacles is that the injury and the initiation of regeneration are two highly coupled processes and hard to separate from one another. In this article, we review the major events occurring at the early injury/regeneration stage in a range of species, and discuss the possible common mechanisms during initiation of tissue regeneration.

Targeting Serotonin Transporters in the Treatment of Juvenile and Adolescent Depression

Depression is a serious public health concern. Many patients are not effectively treated, but in children and adolescents this problem is compounded by limited pharmaceutical options. Currently, the Food and Drug Administration approves only two antidepressants for use in these young populations. Both are selective serotonin reuptake inhibitors (SSRIs). Compounding matters further, they are therapeutically less efficacious in children and adolescents than in adults. Here, we review clinical and preclinical literature describing the antidepressant efficacy of SSRIs in juveniles and adolescents. Since the high-affinity serotonin transporter (SERT) is the primary target of SSRIs, we then synthesize these reports with studies of SERT expression/function during juvenile and adolescent periods. Preclinical literature reveals some striking parallels with clinical studies, primary among them is that, like humans, juvenile and adolescent rodents show reduced antidepressant-like responses to SSRIs. These findings underscore the utility of preclinical assays designed to screen drugs for antidepressant efficacy across ages. There is general agreement that SERT expression/function is lower in juveniles and adolescents than in adults. It is well established that chronic SSRI treatment decreases SERT expression/function in adults, but strikingly, SERT expression/function in adolescents is increased following chronic treatment with SSRIs. Finally, we discuss a putative role for organic cation transporters and/or plasma membrane monoamine transporter in serotonergic homeostasis in juveniles and adolescents. Taken together, fundamental differences in SERT, and putatively in other transporters capable of serotonin clearance, may provide a mechanistic basis for the relative inefficiency of SSRIs to treat pediatric depression, relative to adults.

Deletion of the vesicular monoamine transporter 1 (vmat1/slc18a1) gene affects dopamine signaling

The vesicular monoamine transporter is involved in presynaptic catecholamine storage and neurotransmission. Two isoforms of the transporter exist, VMAT1 and VMAT2, and both are expressed in the brain, though VMAT2 expression is more robust and has been more widely studied. In this study we investigated the role of VMAT1 KO on markers of dopaminergic function and neurotransmission, and dopamine-related behaviors. Null-mutant VMAT1 mice were studied behaviorally using the tail suspension test, elevated zero maze and locomotor activity assessments. Tissue monoamines were measured both ex vivo and by using in vivo microdialysis. Protein expression of tyrosine hydroxylase and D2 dopamine receptors was measured using western blot analysis. Results show that VMAT1 KO mice have decreased dopamine levels in the frontal cortex, increased postsynaptic D2 expression, and lower frontal cortex tyrosine hydroxylase expression compared to WT mice. VMAT1 KO mice also show an exaggerated behavioral locomotor response to acute amphetamine treatment. We conclude that dopaminergic signaling is robustly altered in the frontal cortex of VMAT1 null-mutant mice and suggest that VMAT1 may be relevant to the pathogenesis and/or treatment of psychiatric illnesses including schizophrenia and bipolar disease.

Constitutive activity of 5-HT2C receptors is present after incomplete spinal cord injury but is not modified after chronic SSRI or baclofen treatment

After spinal cord injury (SCI), reflexes become hyperexcitable, leading to debilitating muscle spasms and compromised motor function. Previous work has described adaptations in spinal systems that might underlie this hyperexcitability, including an increase in constitutively active 5-HT_2C receptors in spinal motoneurons. That work, however, examined adaptations following complete transection SCI, whereas SCI in humans is usually anatomically and functionally incomplete. We therefore evaluated whether constitutive activity of 5-HT_2C receptors contributes to reflex hyperexcitability in an incomplete compression model of SCI and to spasms in vitro and in vivo. Our results confirm that 5-HT_2C receptor constitutive activity contributes to reflex excitability after incomplete SCI. We also evaluated whether constitutive activity could be altered by manipulation of neural activity levels after SCI, testing the hypothesis that it reflects homeostatic processes acting to maintain spinal excitability. We decreased neural activity after SCI by administering baclofen and increased activity by administering the selective serotonin reuptake inhibitor (SSRI) fluoxetine. We found that drug administration produced minimal alterations in in vivo locomotor function or reflex excitability. Similarly, we found that neither baclofen nor fluoxetine altered the contribution of constitutively active 5-HT_2C receptors to reflexes after SCI, although the contribution of 5-HT_2C receptors to reflex activity was altered after SSRIs. These results confirm the importance of constitutive activity in 5-HT_2C receptors to spinal hyperexcitability following SCI in the clinically relevant case of incomplete SCI but suggest that this activity is not driven by homeostatic processes that act to maintain overall levels of spinal excitability.NEW & NOTEWORTHY After spinal cord injury (SCI), most people will develop muscle spasms below their level of injury that can severely impact function. In this work, we examine the adaptations that occur within the spinal cord after SCI that contribute to these motor dysfunctions. We also evaluate one hypothesis about how these adaptations develop, which will potentially lead to intervention strategies to improve functional outcomes in persons with SCI.

Reversal of Stress-Induced Social Interaction Deficits by Buprenorphine

BACKGROUND

Patients with post-traumatic stress disorder frequently report persistent problems with social interactions, emerging after a traumatic experience. Chronic social defeat stress is a widely used rodent model of stress that produces robust and sustained social avoidance behavior. The avoidance of other rodents can be reversed by 28 days of treatment with selective serotonin reuptake inhibitors, the only pharmaceutical class approved by the U.S. Food and Drug Administration for treating post-traumatic stress disorder. In this study, the sensitivity of social interaction deficits evoked by 10 days of chronic social defeat stress to prospective treatments for post-traumatic stress disorder was examined.

METHODS

The effects of acute and repeated treatment with a low dose of buprenorphine (0.25 mg/kg/d) on social interaction deficits in male C57BL/6 mice by chronic social defeat stress were studied. Another cohort of mice was used to determine the effects of the selective serotonin reuptake inhibitor fluoxetine (10 mg/kg/d), the NMDA antagonist ketamine (10 mg/kg/d), and the selective kappa opioid receptor antagonist CERC-501 (1 mg/kg/d). Changes in mRNA expression of Oprm1 and Oprk1 were assessed in a separate cohort.

RESULTS

Buprenorphine significantly reversed social interaction deficits produced by chronic social defeat stress following 7 days of administration, but not after acute injection. Treatment with fluoxetine for 7 days, but not 24 hours, also reinstated social interaction behavior in mice that were susceptible to chronic social defeat. In contrast, CERC-501 and ketamine failed to reverse social avoidance. Gene expression analysis found: (1) Oprm1 mRNA expression was reduced in the hippocampus and increased in the frontal cortex of susceptible mice and (2) Oprk1 mRNA expression was reduced in the amygdala and increased in the frontal cortex of susceptible mice compared to non-stressed controls and stress-resilient mice.

CONCLUSIONS

Short-term treatment with buprenorphine and fluoxetine normalized social interaction after chronic social defeat stress. In concert with the changes in opioid receptor expression produced by chronic social defeat stress, we speculate that buprenorphine's efficacy in this model of post-traumatic stress disorder may be associated with the ability of this compound to engage multiple opioid receptors.

Genetic variation in the behavioral effects of buprenorphine in female mice derived from a murine model of the OPRM1 A118G polymorphism

Pharmacogenetic studies have identified the non-synonymous single nucleotide polymorphism (A118G) in the human mu opioid receptor (MOR) gene (OPRM1) as a critical genetic variant capable of altering the efficacy of opioid therapeutics. To date few studies have explored the potential impact of the OPRM1 A118G polymorphism on the pharmacological effects of buprenorphine (BPN), a potent MOR partial agonist and kappa opioid receptor antagonist, which is approved by the FDA for the treatment of opioid addiction and chronic pain. The goal of these studies was to determine whether the MOR-mediated behavioral effects of BPN were altered in the Oprm1 A112G mouse model of the human OPRM1 A118G SNP. All studies were conducted in female, AA, AG and GG mice. BPN's maximal analgesic effect in the hot plate test was significantly blunted in AG and GG mice compared to wild type AA mice. Similarly, the BPN-induced reduction of latency to consume food in the novelty induced hypophagia test was blocked entirely in AG and GG mice compared to their AA littermates. In addition, GG mice exhibited marked reductions in psychostimulant hyperlocomotor activity compared to the AA group. In contrast, reduced immobility in the forced swim test, an effect of BPN mediated by kappa opioid receptors, was not affected by genotype. These studies demonstrate the ability of the Oprm1 A112G SNP to attenuate the analgesic, anxiolytic and hyperlocomotor effects of BPN. Overall, these data suggest that the OPRM1 A118G SNP will significantly impact the clinical efficacy of BPN in its therapeutic applications.

Gamma-H2AX upregulation caused by Wip1 deficiency increases depression-related cellular senescence in hippocampus

The PP2C family member Wild-type p53-induced phosphatase 1 (Wip1) critically regulates DNA damage response (DDR) under stressful situations. In the present study, we investigated whether Wip1 expression was involved in the regulation of DDR-induced and depression-related cellular senescence in mouse hippocampus. We found that Wip1 gene knockout (KO) mice showed aberrant elevation of hippocampal cellular senescence and of γ-H2AX activity, which is known as a biomarker of DDR and cellular senescence, indicating that the lack of Wip1-mediated γ-H2AX dephosphorylation facilitates cellular senescence in hippocampus. Administration of the antidepressant fluoxetine had no significant effects on the increased depression-like behaviors, enriched cellular senescence, and aberrantly upregulated hippocampal γ-H2AX activity in Wip1 KO mice. After wildtype C57BL/6 mice were exposed to the procedure of chronic unpredictable mild stress (CUMS), cellular senescence and γ-H2AX activity in hippocampus were also elevated, accompanied by the suppression of Wip1 expression in hippocampus when compared to the control group without CUMS experience. These CUMS-induced symptoms were effectively prevented following fluoxetine administration in wildtype C57BL/6 mice, with the normalization of depression-like behaviors. Our data demonstrate that Wip1-mediated γ-H2AX dephosphorylation may play an important role in the occurrence of depression-related cellular senescence.

Antidepressant-like Effects of Buprenorphine are Mediated by Kappa Opioid Receptors

Previous studies have identified potential antidepressant effects of buprenorphine (BPN), a drug with high affinity for mu opioid receptor (MORs) and kappa opioid receptors (KORs) and some affinity at delta opioid receptor (DOR) and opioid receptor-like 1 (ORL-1) receptors. Therefore, these studies examined which opioid receptors were involved in BPN's effects on animal behavior tests sensitive to antidepressant drugs. The acute effects of BPN were tested in the forced swim test (FST) using mice with genetic deletion of individual opioid receptors or after pharmacological blockade of receptors. For evaluating the effects of BPN on chronic stress, separate groups of mice were exposed to unpredictable chronic mild stress (UCMS) for 3 weeks and treated with BPN for at least 7 days before behavioral assessment and subsequent measurement of Oprk1, Oprm1, and Pdyn mRNA expression in multiple brain regions. BPN did not reduce immobility in mice with KOR deletion or after pretreatment with norbinaltorphimine, even though desipramine remained effective. In contrast, BPN reduced immobility in MOR and DOR knockout mice and in mice pretreated with the ORL-1 antagonist JTC-801. UCMS reduced sucrose preference, decreased time in the light side of the light/dark box, increased immobility in the FST and induced region-specific alterations in Oprk1, Oprm1, and PDYN mRNA expression in the frontal cortex and striatum. All of these changes were normalized following BPN treatment. The KOR was identified as a key player mediating the effects of BPN in tests sensitive to antidepressant drugs in mice. These studies support further development of BPN as a novel antidepressant.

A simple assessment model to quantifying the dynamic hippocampal neurogenic process in the adult mammalian brain

Adult hippocampal neurogenesis is a highly dynamic process in which new cells are born, but only some of which survive. Of late it has become clear that these surviving newborn neurons have functional roles, most notably in certain forms of memory. Conventional methods to look at adult neurogenesis are based on the quantification of the number of newly born neurons using a simple cell counting methodology. However, this type of approach fails to capture the dynamic aspects of the neurogenic process, where neural proliferation, death and differentiation take place continuously and simultaneously. In this paper, we propose a simple mathematical approach to better understand the adult neurogenic process in the hippocampus which in turn will allow for a better analysis of this process in disease states and following drug therapies.

Two short-acting kappa opioid receptor antagonists (zyklophin and LY2444296) exhibited different behavioral effects from the long-acting antagonist norbinaltorphimine in mouse anxiety tests

Prototypical long-acting kappa opioid receptor (KOPR) antagonists [e.g., norbinaltorphimine (norBNI)] have been reported to exert anxiolytic-like effects in several commonly used anxiety tests in rodents including the novelty-induced hypophagia (NIH) and elevated plus maze (EPM) tests. It remains unknown if the short-acting KOPR antagonists (e.g., zyklophin and LY2444296) have similar effects. In this study effects of zyklophin and LY2444296 (s.c.) were investigated in the NIH and EPM tests in mice 1h post-injection and compared with norBNI (i.p.) 48h post-administration. In the NIH test, zyklophin at 3 and 1mg/kg, but not 0.3mg/kg, or LY2444296 at 30mg/kg decreased the latency of palatable food consumption in novel cages, but had no effect in training cages, similar to norBNI (10mg/kg). Zyklophin at 3 or 1mg/kg increased or had a trend of increasing the amount of palatable food consumption in novel cages, with no effects in training cages, further indicating its anxiolytic-like effect, but norBNI (10mg/kg) and LY2444296 (30mg/kg) did not. In the EPM test, norBNI (10mg/kg) increased open arm time and % open arm entries or time, but zyklophin at all three doses and LY2444296 (30mg/kg) had no effects. In addition, zyklophin at 3mg/kg increased numbers of close and total arm entries on EPM, suggesting increased activity; however, norBNI and LY2444296 had no effects on close and total arm entries. Thus, all three KOPR antagonists had anxiolytic-like effects in the NIH test. However, only the long-acting one (norBNI), but not the short-acting ones (zyklophin and LY2444296), demonstrated anti-anxiety like effects in the EPM test. It remains to be investigated if the differences are due to the differences in their durations of action and/or pharmacodynamic properties.

Corticosterone exposure augments sensitivity to the behavioral and neuroplastic effects of fluoxetine in C57BL/6 mice

Both genetic background and pre-existing stress play critical roles in the effects of antidepressant drugs. The current studies showed this principal by demonstrating that exposure to the stress hormone corticosterone (CORT) allowed behavioral and neurogenic effects to emerge following chronic treatment with fluoxetine of C57BL/6 mice, a strain ordinarily resistant to these effects. Adult male mice were implanted subcutaneously with 21-day slow-release CORT pellets (10 mg) or placebo and then co-treated with 5 mg/kg fluoxetine (b.i.d., i.p.) or saline for 14 days. Animals were then assessed for approach behavior in the novelty-induced hypophagia (NIH) test, hippocampal cell proliferation, corticosteroid receptor expression, and CORT plasma levels. Co-treatment of CORT with fluoxetine significantly reduced approach behavior in the novel environment of the NIH test and increased hippocampal cell proliferation whereas fluoxetine given alone was ineffective. CORT given alone did not alter approach behavior in the novel environment and caused a smaller increase of cell proliferation. The CORT effect was blocked by adrenalectomy and was likely due to increased adrenal feedback. Cell proliferation in CORT-treated animals was associated with reduced mineralocorticoid, but not glucocorticoid, receptor mRNA expression. Although the pellets were advertised to release CORT for 21 days, plasma CORT levels were increased at 1 day after implantation but were not sustained when measured at 7 days or longer intervals. Nevertheless, the transient CORT increase was sufficient to induce long-lasting behavioral and molecular changes when followed by fluoxetine treatment. These studies warrant further investigation into the role of glucocorticoids and environmental stress as adjunctive facilitators of the response to antidepressants, especially for treatment-resistant patients.

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C57BL/6J mice are not responsive to behavioral and neurogenic effects of chronic fluoxetine.

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CORT reduced anxiogenic behavior, increased hippocampal neurogenesis in response to fluoxetine.

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CORT pellets are not effective in sustaining elevated plasma CORT levels.

Effects of buprenorphine on behavioral tests for antidepressant and anxiolytic drugs in mice

RATIONALE

Buprenorphine (BPN) has been shown to rapidly improve mood in treatment-resistant depressed patients in small clinical studies. However, BPN's effects in preclinical tests for mood and antidepressant efficacy are largely unexplored.

OBJECTIVE

The current study examined the effects of BPN in the forced swim test (FST) and novelty-induced hypophagia (NIH) test as measures of antidepressant and anxiolytic-like effects in C57BL/6 J mice. Microdialysis was used to measure whether BPN engaged kappa-opioid receptor (KORs) in the nucleus accumbens shell (NAcSh) at a behaviorally active dose (0.25 mg/kg).

METHODS

BPN was tested in the FST at both 30 min and 24 h post-administration. Also measured in the FST at 24 h post-administration were the KOR antagonist norbinaltorphimine (nor-BNI), the MOR agonist morphine and the reference antidepressant desipramine. The anxiolytic effects of BPN were examined in the NIH test 24 h after treatment. The effects of acute injection of BPN and the KOR agonist U50,488 were measured on extracellular dopamine (DA) levels in the NAcSh.

RESULTS

BPN produced significant reductions in FST immobility without changing locomotor activity and reduced approach latencies in the novel environment of the NIH test when tested 24 h after treatment. Repeated daily BPN injections for 6 days did not produce tolerance to these behavioral effects. nor-BNI produced a similar antidepressant-like response in the FST 24 h post-injection but morphine and desipramine were ineffective. BPN (0.25 mg/kg) did not alter DA levels when given alone but prevented the KOR agonist U50,488 from reducing DA levels.

CONCLUSIONS

Acute and subchronic treatment with BPN produced antidepressant and anxiolytic-like responses in mice at doses that engage KORs. These studies support the clinical evidence that BPN may be a novel rapid-acting antidepressant medication and provides rodent models for investigating associated neurochemical mechanisms.

Rodent models of treatment-resistant depression

Major depression is a prevalent and debilitating disorder and a substantial proportion of patients fail to reach remission following standard antidepressant pharmacological treatment. Limited efficacy with currently available antidepressant drugs highlights the need to develop more effective medications for treatment- resistant patients and emphasizes the importance of developing better preclinical models that focus on treatment- resistant populations. This review discusses methods to adapt and refine rodent behavioral models that are predictive of antidepressant efficacy to identify populations that show reduced responsiveness or are resistant to traditional antidepressants. Methods include separating antidepressant responders from non-responders, administering treatments that render animals resistant to traditional pharmacological treatments, and identifying genetic models that show antidepressant resistance. This review also examines pharmacological and non-pharmacological treatments regimes that have been effective in refractory patients and how some of these approaches have been used to validate animal models of treatment-resistant depression. The goals in developing rodent models of treatment-resistant depression are to understand the neurobiological mechanisms involved in antidepressant resistance and to develop valid models to test novel therapies that would be effective in patients that do not respond to traditional monoaminergic antidepressants.

Inflammation and Its Correlates in Regenerative Wound Healing: An Alternate Perspective

Objective: The wound healing response may be viewed as partially overlapping sets of two physiological processes, regeneration and wound repair with the former overrepresented in some lower species such as newts and the latter more typical of mammals. A robust and quantitative model of regenerative healing has been described in Murphy Roths Large (MRL) mice in which through-and-through ear hole wounds in the ear pinna leads to scarless healing and replacement of all tissue through blastema formation and including cartilage. Since these mice are naturally autoimmune and display many aspects of an enhanced inflammatory response, we chose to examine the inflammatory status during regenerative ear hole closure and observed that inflammation has a clear positive effect on regenerative healing. Approach: The inflammatory gene expression patterns (Illumina microarrays) of early healing ear tissue from regenerative MRL and nonregenerative C57BL/6 (B6) strains are presented along with a survey of innate inflammatory cells found in this tissue type pre and postinjury. The role of inflammation on healing is tested using a COX-2 inhibitor. Innovation and Conclusion: We conclude that (1) enhanced inflammation is consistent with, and probably necessary, for a full regenerative response and (2) the inflammatory gene expression and cell distribution patterns suggest a novel mast cell population with markers found in both immature and mature mast cells that may be a key component of regeneration.

Neurotrophins and psychiatric disorders

Increasing number of studies has during the last decade linked neurotrophic factors with the pathophysiology of neuropsychiatric disorders and with the mechanisms of action of drugs used for the treatment of these disorders. In particular, brain-derived neurotrophic factor BDNF and its receptor TrkB have been connected with the pathophysiology in mood disorders, and there is strong evidence that BDNF signaling is critically involved in the recovery from depression with both pharmacological and psychological means. Neurotrophins play a central role in neuronal plasticity and network connectivity in developing adult brain, and recent evidence links plasticity and network rewiring with mood disorders and their treatment. Therefore, neurotrophins should not be seen as happiness factors but as critical tools in the process where brain networks are optimally tuned to environment, and it is against this background that the effects of neurotrophins on neuropsychiatric disorders should be looked at.

Fine-mapping quantitative trait loci affecting murine external ear tissue regeneration in the LG/J by SM/J advanced intercross line

External ear hole closure in LG/J mice represents a model of regenerative response. It is accompanied by the formation of a blastema-like structure and the re-growth of multiple tissues, including cartilage. The ability to regenerate tissue is heritable. An F34 advanced intercross line of mice (Wustl:LG,SM-G34) was generated to identify genomic loci involved in ear hole closure over a 30-day healing period. We mapped 19 quantitative trait loci (QTL) for ear hole closure. Individual gene effects are relatively small (0.08 mm), and most loci have co-dominant effects with phenotypically intermediate heterozygotes. QTL support regions were limited to a median size of 2 Mb containing a median of 19 genes. Positional candidate genes were evaluated using differential transcript expression between LG/J and SM/J healing tissue, function analysis and bioinformatic analysis of single-nucleotide polymorphisms in and around positional candidate genes of interest. Analysis of the set of 34 positional candidate genes and those displaying expression differences revealed over-representation of genes involved in cell cycle regulation/DNA damage, cell migration and adhesion, developmentally related genes and metabolism. This indicates that the healing phenotype in LG/J mice involves multiple physiological mechanisms.

Effect of fluoxetine on disease progression in a mouse model of ALS

Selective serotonin reuptake inhibitors (SSRIs) and other antidepressants are often prescribed to amyotrophic lateral sclerosis (ALS) patients; however, the impact of these prescriptions on ALS disease progression has not been systematically tested. To determine whether SSRIs impact disease progression, fluoxetine (Prozac, 5 or 10 mg/kg) was administered to mutant superoxide dismutase 1 (SOD1) mice during one of three age ranges: neonatal [postnatal day (P)5-11], adult presymptomatic (P30 to end stage), and adult symptomatic (P70 to end stage). Long-term adult fluoxetine treatment (started at either P30 or P70 and continuing until end stage) had no significant effect on disease progression. In contrast, neonatal fluoxetine treatment (P5-11) had two effects. First, all animals (mutant SOD1(G93A) and control: nontransgenic and SOD1(WT)) receiving the highest dose (10 mg/kg) had a sustained decrease in weight from P30 onward. Second, the high-dose SOD1(G93A) mice reached end stage ∼8 days (∼6% decrease in life span) sooner than vehicle and low-dose animals because of an increased rate of motor impairment. Fluoxetine increases synaptic serotonin (5-HT) levels, which is known to increase spinal motoneuron excitability. We confirmed that 5-HT increases spinal motoneuron excitability during this neonatal time period and therefore hypothesized that antagonizing 5-HT receptors during the same time period would improve disease outcome. However, cyproheptadine (1 or 5 mg/kg), a 5-HT receptor antagonist, had no effect on disease progression. These results show that a brief period of antidepressant treatment during a critical time window (the transition from neonatal to juvenile states) can be detrimental in ALS mouse models.

Cell cycle regulation and regeneration

Regeneration of ear punch holes in the MRL mouse and amputated limbs of the axolotl show a number of similarities. A large proportion of the fibroblasts of the uninjured MRL mouse ear are arrested in G2 of the cell cycle, and enter nerve-dependent mitosis after injury to form a ring-shaped blastema that regenerates the ear tissue. Multiple cell types contribute to the establishment of the regeneration blastema of the urodele limb by dedifferentiation, and there is substantial reason to believe that the cells of this early blastema are also arrested in G2, and enter mitosis under the influence of nerve-dependent factors supplied by the apical epidermal cap. Molecular analysis reveals other parallels, such as; (1) the upregulation of Evi5, a centrosomal protein that prevents mitosis by stabilizing Emi1, a protein that inhibits the degradation of cyclins by the anaphase promoting complex and (2) the expression of sodium channels by the epidermis. A central feature in the entry into the cell cycle by MRL ear fibroblasts is a natural downregulation of p21, and knockout of p21 in wild-type mice confers regenerative capacity on non-regenerating ear tissue. Whether the same is true for entry into the cell cycle in regenerating urodele limbs is presently unknown.

Metabolic changes and DNA hypomethylation in cerebellum are associated with behavioral alterations in mice exposed to trichloroethylene postnatally

Previous studies demonstrated that low-level postnatal and early life exposure to the environmental contaminant, trichloroethylene (TCE), in the drinking water of MRL+/+ mice altered glutathione redox homeostasis and increased biomarkers of oxidative stress indicating a more oxidized state. Plasma metabolites along the interrelated transmethylation pathway were also altered indicating impaired methylation capacity. Here we extend these findings to further characterize the impact of TCE exposure in mice exposed to water only or two doses of TCE in the drinking water (0, 2, and 28mg/kg/day) postnatally from birth until 6weeks of age on redox homeostasis and biomarkers of oxidative stress in the cerebellum. In addition, pathway intermediates involved in methyl metabolism and global DNA methylation patterns were examined in cerebellar tissue. Because the cerebellum is functionally important for coordinating motor activity, including exploratory and social approach behaviors, these parameters were evaluated in the present study. Mice exposed to 28mg/kg/day TCE exhibited increased locomotor activity over time as compared with control mice. In the novel object exploration test, these mice were more likely to enter the zone with the novel object as compared to control mice. Similar results were obtained in a second test when an unfamiliar mouse was introduced into the testing arena. The results show for the first time that postnatal exposure to TCE causes key metabolic changes in the cerebellum that may contribute to global DNA methylation deficits and behavioral alterations in TCE-exposed mice.

Adult depression-like behavior, amygdala and olfactory cortex functions are restored by odor previously paired with shock during infant's sensitive period attachment learning

Maltreatment from the caregiver induces vulnerability to later life psychopathologies, yet attraction and comfort is sometimes provided by cues associated with early life maltreatment. We used a rat model of early life maltreatment with odor-0.5 mA shock conditioning to produce depressive-like behaviors and questioned whether stimuli associated with maltreatment would restore emotional neurobehavioral function to control levels. Pups received daily novel odor-0.5 mA shock conditioning from postnatal day 8 to 12. This procedure produces a new maternal odor that controls pups' attachment behaviors. In adulthood, either with or without the infant odor, animals received a Forced Swim Test, Sucrose Preference Test or assessment of amygdala and olfactory system functioning using field potential signal evoked by olfactory bulb paired-pulse electrical stimulation. Following neonatal odor-shock pairings, but not unpaired controls, adults without the odor present showed increased depression-like behavior in the Forced Swim Test and Sucrose Preference Test and a deficit in paired-pulse inhibition in amygdala and piriform (olfactory) cortex. All effects were brought to control levels when the infant conditioned odor was presented during behavioral and neural tests. The ability of cues associated with early life maltreatment to normalize behavior and amygdala activity suggests these cues provide adaptive value in adulthood.

Epigenetics of the depressed brain: role of histone acetylation and methylation

Major depressive disorder is a chronic, remitting syndrome involving widely distributed circuits in the brain. Stable alterations in gene expression that contribute to structural and functional changes in multiple brain regions are implicated in the heterogeneity and pathogenesis of the illness. Epigenetic events that alter chromatin structure to regulate programs of gene expression have been associated with depression-related behavior, antidepressant action, and resistance to depression or 'resilience' in animal models, with increasing evidence for similar mechanisms occurring in postmortem brains of depressed humans. In this review, we discuss recent advances in our understanding of epigenetic contributions to depression, in particular the role of histone acetylation and methylation, which are revealing novel mechanistic insight into the syndrome that may aid in the development of novel targets for depression treatment.

Brain monoamines and antidepressant-like responses in MRL/MpJ versus C57BL/6J mice

The MRL/MpJ mouse demonstrates enhanced wound healing and tissue regeneration and increased neurotrophic mobilization to chronic antidepressant drug treatments. This study compared brain monoamine systems between MRL/MpJ and C57BL/6J mice as a potential basis for strain differences after chronic antidepressant treatment. MRL/MpJ mice had significantly higher tissue levels of serotonin and dopamine in multiple brain regions. Microdialysis studies demonstrated that baseline levels of extracellular serotonin did not differ between strains. However, acute administration of the selective serotonin reuptake inhibitor citalopram produced an increase in extracellular serotonin in the ventral hippocampus of MRL/MpJ mice that was twice as large as achieved in C57BL/6J mice. The greater effects in MRL/MpJ mice on 5-HT levels were not maintained after local perfusion of citalopram, suggesting that mechanisms outside of the hippocampus were responsible for the greater effect of citalopram after systemic injection. The density of serotonin and norepinephrine transporters in the hippocampus was significantly higher in MRL/MpJ mice. In addition, the expression of 5-HT(1A) mRNA was lower in the hippocampus, 5-HT(1B) mRNA was higher in the hippocampus and brainstem and SERT mRNA was higher in the brain stem of MRL/MpJ mice. The exaggerated neurotransmitter release in MRL/MpJ mice was accompanied by reduced baseline immobility in the tail suspension test and a greater reduction of immobility produced by citalopram or the tricyclic antidepressant desipramine. These data suggest that differences in the response to acute and chronic antidepressant treatments between the two strains could be attributed to differences in serotonin or catecholamine transmission.

The Neurobiology of Depression and Anxiety: How Do We Change from Models of Drug Efficacy to Understanding Mood and Anxiety Disorders?

Current treatments for depression and anxiety disorders are only effective in approximately half of the patient population. Effective treatments have negative side-effects including sexual dysfunction, weight gain and gastrointestinal problems. Furthermore, even when patients achieve remission, they often need to increase dosage or change treatment across their lifetime as efficacy weakens. The majority of treatments being used today are based on the monoamine hypothesis of depression, a theory of depression that was based on the effectiveness of drugs discovered by chance to alleviate the symptoms of depression. This chapter provides an overview of the neurobiology of depression and anxiety disorders within the context of drug discovery. The chapter starts with what we currently know about these disorders through the lens of the monoamine hypothesis of depression. We then provide a background into the animal models of depression and anxiety that are being used to understand the underlying biology of these disorders and test new treatments. Work conducted using these animal models has directed human imagining and has provided us with new information about both the molecular and cellular mechanism of depression and anxiety as well as the neural circuitry controlling these disorders. Finally, we will conclude with a discussion of new treatments being developed based on empirical evidence about the neurobiology of depression and anxiety and the need to develop more personalized treatments in the future. It is hoped that these new drugs will be able to provide effective treatment for more people, with fewer negative side-effects.

5-HT2A receptors in the orbitofrontal cortex facilitate reversal learning and contribute to the beneficial cognitive effects of chronic citalopram treatment in rats

Chronic stress is a risk factor for depression, and chronic stress can induce cognitive impairments associated with prefrontal cortical dysfunction, which are also major components of depression. We have previously shown that 5 wk chronic intermittent cold (CIC) stress induced a reversal-learning deficit in rats, associated with reduced serotonergic transmission in the orbitofrontal cortex (OFC) that was restored by chronic treatment with a selective serotonin reuptake inhibitor (SSRI). However, the mechanisms underlying the beneficial cognitive effects of chronic SSRI treatment are currently unknown. Thus, the purpose of this study was to investigate the potential modulatory influence specifically of 5-HT2A receptors (5-HT2ARs) in the OFC on reversal learning, and their potential contribution to the beneficial cognitive effects of chronic SSRI treatment. Bilateral microinjections of the selective 5-HT2AR antagonist, MDL 100,907 into OFC (0.02-2.0 nmol) had a dose-dependent detrimental effect on a reversal-learning task, suggesting a facilitatory influence of 5-HT2ARs in the OFC. In the next experiment, rats were exposed to 5 wk CIC stress, which compromised reversal learning, and treated chronically with the SSRI, citalopram (20 mg/kg.d) during the final 3 wk of chronic stress. Chronic citalopram treatment improved reversal learning in the CIC-stressed rats, and bilateral microinjection of MDL 100,907 (0.20 nmol, the optimal dose from the preceding experiment) into OFC once again had a detrimental effect on reversal learning, opposing the beneficial effect of citalopram. We conclude that 5-HT2ARs in the OFC facilitate reversal learning, and potentially contribute to the beneficial cognitive effects of chronic SSRI treatment.

Optimal dosages of fluoxetine in the treatment of hypoxic brain injury induced by 3-nitropropionic acid: implications for the adjunctive treatment of patients after acute ischemic stroke

AIM

The serotonin selective reuptake inhibitor fluoxetine (Flx) has tried to treat patients suffered acute ischemic stroke because of its possible neuroprotective actions. However, besides the neuroprotective effect, Flx at high concentration also induces some actions in contradiction to neuroprotection in the brain. The purpose of this study was to investigate whether Flx presents neuroprotective effect against 3-nitropropionic acid (3-NP)-induced hypoxic brain injury, and what is the most suitable dosage of Flx.

METHODS

Mouse model was established by subacute systemic administration of 3-NP. Rotarod and pole tests were used to evaluate motor deficit. The oxidative stress and oxidative DNA damage were assessed respectively by measuring malondialdehyde and 8-hydroxydeoxyguanosine content in brain homogenates.

RESULTS

According to measurements in the rotarod test, 7 days pretreatment plus 5 days treatment of Flx at low (2.5 mg/kg/day) and, to a lesser degree, medium (5 mg/kg/day) doses exerted a rapid and strong protection against the neurotoxicity induced by 3-NP, whereas Flx at high dose (10mg/kg/day) showed a much late and light effect. Similarly, in the pole test, Flx at 2.5 mg/kg/day had the strongest protective effects. Again, only Flx administration at 2.5 mg/kg/day canceled out the enhancement of malondialdehyde and 8-hydroxydeoxyguanosine in striatum following 3-NP neurotoxication.

CONCLUSIONS

Flx attenuated the motor deficits induced by 3-NP in a dose-dependent manner. In contrary to the high dose, Flx at the lower doses had a more remarkable effect against 3-NP insult, similar to acute ischemic stroke.

Strain differences in the effects of chronic corticosterone exposure in the hippocampus

Stress hormones are thought to be involved in the etiology of depression, in part, because animal models show they cause morphological damage to the brain, an effect that can be reversed by chronic antidepressant treatment. The current study examined two mouse strains selected for naturalistic variation of tissue regeneration after injury for resistance to the effects of chronic corticosterone (CORT) exposure on cell proliferation and neurotrophin mobilization. The wound healer MRL/MpJ and control C57BL/6J mice were implanted subcutaneously with pellets that released CORT for 7 days. MRL/MpJ mice were resistant to reductions of hippocampal cell proliferation by chronic exposure to CORT when compared to vulnerable C57BL/6J mice. Chronic CORT exposure also reduced protein levels of brain-derived neurotrophic factor (BDNF) in the hippocampus of C57BL/6J but not MRL/MpJ mice. CORT pellet exposure increased circulating levels of CORT in the plasma of both strains in a dose-dependent manner although MRL/MpJ mice may have larger changes from baseline. The strains did not differ in circulating levels of corticosterone binding globulin (CBG). There were also no strain differences in CORT levels in the hippocampus, nor did CORT exposure alter glucocorticoid receptor or mineralocorticoid receptor expression in a strain-dependent manner. Strain differences were found in the N-methyl-D-aspartate (NMDA) receptor, and BDNF I and IV promoters. Strain and CORT exposure interacted to alter tropomyosine-receptor-kinase B (TrkB) expression and this may be a potential mechanism protecting MRL/MpJ mice. In addition, differences in the inflammatory response of matrix metalloproteinases (MMPs) may also contribute to these strain differences in resistance to the deleterious effects of CORT to the brain.

The super super-healing MRL mouse strain

The Murphy Roths Large (MRL/MpJ) mice provide unique insights into wound repair and regeneration. These mice and the closely related MRL/MpJ-Fas^lpr /J and Large strains heal wounds made in multiple tissues without production of a fibrotic scar. The precise mechanism of this remarkable ability still eludes researchers, but some data has been generated and insights are being revealed. For example, MRL cells reepithelialize over dermal wound sites faster than cells of other mouse strains. This allows a blastema to develop beneath the protective layer. The MRL mice also have an altered basal immune system and an altered immune response to injury. In addition, MRL mice have differences in their tissue resident progenitor cells and certain cell cycle regulatory proteins. The difficulty often lies in separating the causative differences from the corollary differences. Remarkably, not every tissue in these mice heals scarlessly, and the specific type of wound and priming affect regeneration ability as well. The MRL/MpJ, MRL/MpJ-Fas^lpr /J, and Large mouse strains are also being investigated for their autoimmune characteristic. Whether the two phenotypes of regeneration and autoimmunity are related remains an enigma.

Postnatal exposure to trichloroethylene alters glutathione redox homeostasis, methylation potential, and neurotrophin expression in the mouse hippocampus

Previous studies have shown that continuous exposure throughout gestation until the juvenile period to environmentally relevant doses of trichloroethylene (TCE) in the drinking water of MRL+/+ mice promoted adverse behavior associated with glutathione depletion in the cerebellum indicating increased sensitivity to oxidative stress. The purpose of this study was to extend our findings and further characterize the impact of TCE exposure on redox homeostasis and biomarkers of oxidative stress in the hippocampus, a brain region prone to oxidative stress. Instead of a continuous exposure, the mice were exposed to water only or two environmentally relevant doses of TCE in the drinking water postnatally from birth until 6 weeks of age. Biomarkers of plasma metabolites in the transsulfuration pathway and the transmethylation pathway of the methionine cycle were also examined. Gene expression of neurotrophins was examined to investigate a possible relationship between oxidative stress, redox imbalance and neurotrophic factor expression with TCE exposure. Our results show that hippocampi isolated from male mice exposed to TCE showed altered glutathione redox homeostasis indicating a more oxidized state. Also observed was a significant, dose dependent increase in glutathione precursors. Plasma from the TCE treated mice showed alterations in metabolites in the transsulfuration and transmethylation pathways indicating redox imbalance and altered methylation capacity. 3-Nitrotyrosine, a biomarker of protein oxidative stress, was also significantly higher in plasma and hippocampus of TCE-exposed mice compared to controls. In contrast, expression of key neurotrophic factors in the hippocampus (BDNF, NGF, and NT-3) was significantly reduced compared to controls. Our results demonstrate that low-level postnatal and early life TCE exposure modulates neurotrophin gene expression in the mouse hippocampus and may provide a mechanism for TCE-mediated neurotoxicity.

Pharmacological modulation of stress-induced behavioral changes in the light/dark exploration test in male C57BL/6J mice

Psychological stress is a major risk factor for mood and anxiety disorders. However, the phenotypic manifestation of stress effects varies across individuals, likely due, in part, to genetic variation. Modeling the behavioral and neural consequences of stress across genetically diverse inbred mouse strains is a valuable approach to studying gene × stress interactions. Recent work has shown that C57BL/6J mice exposed to ten daily sessions of restraint stress exhibited increased exploration of the aversive light compartment in the light/dark exploration (LDE) test. Here we sought to clarify the nature of this stress-induced phenotype by testing the ability of treatment with various clinically efficacious drugs of different therapeutic classes to rescue it. Ten days of restraint increased light compartment exploration, reduced body weight and sensitized the corticosterone response to swim stress. Subchronic administration (during stress and LDE testing) of fluoxetine, and to a lesser extent, lithium chloride, rescued stress-induced LDE behavior. Chronic fluoxetine treatment prior to (plus during stress and testing) failed to block the LDE stress effect. Acute administration of antipsychotic haloperidol, anti-ADHD medication methylphenidate or anxiolytic drug chlordiazepoxide, prior to LDE testing, was also unable to normalize the LDE stress effect. Collectively, these data demonstrate a treatment-selective prophylactic rescue of a restraint stress-induced behavioral abnormality in the C57BL/6J inbred strain. Further work with this novel model could help elucidate genetic and neural mechanisms mediating stress-induced changes in mouse 'emotion-relevant' behaviors and, ultimately, further understanding of the pathophysiology of stress-related neuropsychiatric disorders. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Lithium-induced effects on adult hippocampal neurogenesis are topographically segregated along the dorso-ventral axis of stressed mice

Adult hippocampal neurogenesis is an important process in the regulation of cognition, stress responsivity, and sensitivity to antidepressant and mood stabiliser drugs. Increasing evidence suggests that the hippocampus is functionally divided along its axis with the ventral hippocampus (vHi) playing a preferential role in stress- and anxiety-related processes, while the dorsal hippocampus (dHi) is crucial for spatial learning and memory. However, it is currently unclear whether stress or the medications used to treat stress-related disorders, preferentially affect neurogenesis in the vHi rather than dHi. The aim of this study was to determine whether the mood stabiliser, lithium, preferentially affects cell proliferation and survival in the vHi rather than dHi under stress conditions. To this end, mice of the stress-sensitive strain, BALB/c, underwent chronic exposure to immobilisation stress plus lithium treatment (0.2% lithium-supplemented diet), and the rates of cell proliferation and survival were compared in the dHi and vHi. Lithium preferentially increased cell proliferation in the vHi under stress conditions only. This increase in cell proliferation was secondary to reductions in the survival of newly-born cells. Moreover, lithium-induced decreases in cell survival in the vHi were only observed under stress conditions. Taken together, the data suggest that the turnover of newly-born cells in response to chronic stress and lithium treatment occurs predominantly in the vHi rather than the dHi. This article is part of a Special Issue entitled 'Anxiety and Depression'.

The role of p21 in regulating mammalian regeneration

The MRL (Murphy Roths Large) mouse has provided a unique model of adult mammalian regeneration as multiple tissues show this important phenotype. Furthermore, the healing employs a blastema-like structure similar to that seen in amphibian regenerating tissue. Cells from the MRL mouse display DNA damage, cell cycle G2/M arrest, and a reduced level of p21^CIP1/WAF. A functional role for p21 was confirmed when tissue injury in an adult p21^-/- mouse showed a healing phenotype that matched the MRL mouse, with the replacement of tissues, including cartilage, and with hair follicle formation and a lack of scarring. Since the major canonical function of p21 is part of the p53/p21 axis, we explored the consequences of p53 deletion. A regenerative response was not seen in a p53^-/- mouse and the elimination of p53 from the MRL background had no negative effect on the regeneration of the MRL.p53^-/- mouse. An exploration of other knockout mice to identify p21-dependent, p53-independent regulatory pathways involved in the regenerative response revealed another significant finding showing that elimination of transforming growth factor-β1 displayed a healing response as well. These results are discussed in terms of their effect on senescence and differentiation.

Effects of repeated treatment with phosphodiesterase-4 inhibitors on cAMP signaling, hippocampal cell proliferation, and behavior in the forced-swim test

The effects of repeated treatment with the phosphodiesterase-4 (PDE4) inhibitors rolipram, piclamilast, and 4-(2-(3-(cyclopentyloxy)-4-methoxyphenyl)-2-phenylethyl)pyridine (CDP840), which differ in their interactions with high- and low-affinity binding conformers of the enzyme, were contrasted to those of acute treatment on cAMP signaling, hippocampal cell proliferation, and immobility in the forced-swim test in rats. Repeated treatment with rolipram (1 and 3 mg/kg), piclamilast (0.3 and 1 mg/kg), or CDP840 (10 and 30 mg/kg) for 16 days increased cAMP and phosphorylation of cAMP response element binding protein (pCREB) in hippocampus and prefrontal cortex. In addition, repeated treatment with the PDE4 inhibitors increased proliferation and survival of newborn cells in the hippocampus and produced antidepressant-like effects on behavior, as evidenced by decreased immobility in the forced-swim test. Acute treatment with rolipram (3 mg/kg), piclamilast (1 mg/kg), or CDP840 (30 mg/kg) induced transient increases in cAMP and pCREB in hippocampus and prefrontal cortex, but the dose and time dependence of these effects did not parallel the behavioral effects. Compared with rolipram and piclamilast, repeated treatment with CDP840 exerted lesser effects on neural and behavioral measures, probably because of its weak interaction with the high-affinity binding conformer of PDE4. This suggests the relative importance of the high-affinity binding conformer in the mediation of the long-term effects of PDE4 inhibition on cAMP/pCREB signaling, hippocampal cell proliferation, and antidepressant-like effects on behavior.

Selective deletion of a cell cycle checkpoint kinase (ATR) reduces neurogenesis and alters responses in rodent models of behavioral affect

Hippocampal function has been implicated in mood and anxiety disorders, as well as in the response to antidepressant (AD) treatment. However, the significance of new neurons in the therapeutic mechanism of ADs remains unclear. In this study, the proliferation of new neurons was inhibited through conditional deletion of ataxia telangeictasia-mutated and rad-3 related (ATR), a cell cycle checkpoint kinase, and cellular and behavioral outcomes following AD exposure were evaluated. ATR was conditionally deleted by microinjecting a Cre recombinase-expressing virus into the hippocampus of floxed-ATR mice. Behavioral assessment in multiple rodent models of affective state revealed anxiolytic-like behavior in the elevated zero maze, marble burying test, and novelty-induced hypophagia (NIH) test. The efficacy of chronic desipramine (DMI) treatment was evaluated in the NIH test, as this paradigm is thought to be sensitive to increases in neurogenesis by chronic AD exposure. Chronic exposure to DMI reduced hyponeophagia in the NIH test in control mice, whereas DMI had no behavioral effect in ATR-deleted mice. Although DMI did not alter cell proliferation in either group, it did produce a robust increase in dendritic spine density in control mice, indicative of enhanced neuronal plasticity. This effect of DMI on spine density was severely attenuated following ATR deletion. These findings demonstrate that reductions in basal neurogenesis produce an anxiolytic phenotype and reduce AD efficacy in behaviors requiring chronic exposure. Furthermore, attenuated capacity for synaptic remodeling may underlie these behaviors. ATR deletion may serve as a valuable model to study the various proposed roles of newborn neurons in the hippocampus.

The role of serotonin receptor subtypes in treating depression: a review of animal studies

RATIONALE

Serotonin reuptake inhibitors (SSRIs) are effective in treating depression. Given the existence of different families and subtypes of 5-HT receptors, multiple 5-HT receptors may be involved in the antidepressant-like behavioral effects of SSRIs.

OBJECTIVE

Behavioral pharmacology studies investigating the role of 5-HT receptor subtypes in producing or blocking the effects of SSRIs were reviewed.

RESULTS

Few animal behavior tests were available to support the original development of SSRIs. Since their development, a number of behavioral tests and models of depression have been developed that are sensitive to the effects of SSRIs, as well as to other types of antidepressant treatments. The rationale for the development and use of these tests is reviewed. Behavioral effects similar to those of SSRIs (antidepressant-like) have been produced by agonists at 5-HT(1A), 5-HT(1B), 5-HT(2C), 5-HT(4), and 5-HT(6) receptors. Also, antagonists at 5-HT(2A), 5-HT(2C), 5-HT(3), 5-HT(6), and 5-HT(7) receptors have been reported to produce antidepressant-like responses. Although it seems paradoxical that both agonists and antagonists at particular 5-HT receptors can produce antidepressant-like effects, they probably involve diverse neurochemical mechanisms. The behavioral effects of SSRIs and other antidepressants may also be augmented when 5-HT receptor agonists or antagonists are given in combination.

CONCLUSIONS

The involvement of 5-HT receptors in the antidepressant-like effects of SSRIs is complex and involves the orchestration of stimulation and blockade at different 5-HT receptor subtypes. Individual 5-HT receptors provide opportunities for the development of a newer generation of antidepressants that may be more beneficial and effective than SSRIs.

The roles of BDNF, pCREB and Wnt3a in the latent period preceding activation of progenitor cell mitosis in the adult dentate gyrus by fluoxetine

The formation of new neurons continues into adult life in the dentate gyrus of the rat hippocampus, as in many other species. Neurogenesis itself turns out to be highly labile, and is regulated by a number of factors. One of these is the serotoninergic system: treatment with drugs (such as the SSRI fluoxetine) markedly stimulates mitosis in the progenitor cells of the dentate gyrus. But this process has one remarkable feature: it takes at least 14 days of continuous treatment to be effective. This is despite the fact that the pharmacological action of fluoxetine occurs within an hour or so of first administration. This paper explores the role of BDNF in this process, using the effect of a Trk antagonist (K252a) on the labelling of progenitor cells with the mitosis marker Ki67 and the associated expression of pCREB and Wnt3a. These experiments show that (i) Fluoxetine increased Ki67 counts, as well as pCREB and Wnt3a expression in the dentate gyrus. The action of fluoxetine on the progenitor cells and on pCREB (but not Wnt3a) depends upon Trk receptor activation, since it was prevented by icv infusion of K252a. (ii) These receptors are required for both the first 7 days of fluoxetine action, during which no apparent change in progenitor mitosis occurs, as well as the second 7 days. Increased pCREB was always associated with progenitor cell mitosis, but Wnt3a expression may be necessary but not sufficient for increased progenitor cell proliferation. These results shed new light on the action of fluoxetine on neurogenesis in the adult dentate gyrus, and have both clinical and experimental interest.

Strain-specific outcomes of repeated social defeat and chronic fluoxetine treatment in the mouse

Social stress is a risk factor for affective disorders in vulnerable individuals. Although the biological nature of stress susceptibility/resilience remains to be elucidated, genetic variation is considered amongst the principal contributors to brain disorders. Furthermore, genetic predisposition may be determinant for the therapeutic outcome, as proposed for antidepressant treatments. In the present studies we compared the inherently diverse genetic backgrounds of 2 mouse strains by assessing the efficacy of a chronic antidepressant treatment in a repeated social stress procedure. C57BL/6J and BalbC mice underwent 10-day social defeats followed by 28-day fluoxetine treatment (10 mg/kg/mL, p.o.). In C57BL/6J, most of the social defeat-induced changes were of metabolic nature including persistently altered feed efficiency and decreased abdominal fat stores that were ameliorated by fluoxetine. BalbC mouse behavior was persistently affected by social defeat both in the social avoidance and the forced swim tests, and in either procedure it was restored by chronic fluoxetine, whereas their endocrine parameters were mostly unaffected. The highlighted strain-specific responsivity to the metabolic and behavioral consequences of social defeat and to the chronic antidepressant treatment offers a promising research tool to further explore the underlying neural mechanisms and genetic basis of stress susceptibility and treatment response.

Fluoxetine treatment induces dose dependent alterations in depression associated behavior and neural plasticity in female mice

Antidepressant-induced increases in neurogenesis and neurotrophin mobilization in rodents and primates are proposed to be necessary for behavioral efficacy. The current study examines the relationship between the effects of fluoxetine treatment on behavior, cell proliferation and the neurotrophin BDNF in females. Female MRL/MpJ mice were treated acutely (5 and 10mg/kg) or chronically (2.5, 5 and 10mg/kg b.i.d.) with fluoxetine and tested in the tail suspension test (TST) and or novelty-induced hypophagia test (NIH), respectively. Mice treated chronically with fluoxetine received 4 (100mg/kg) injections of 5-bromo-2'-deoxyuridine (BrdU) on the last 4 days of treatment to measure DNA synthesis. The other half of the hippocampus and the frontal cortex was removed and examined for BDNF levels. Fluoxetine treatment decreased immobility in the TST and latency to eat in the NIH test, but only the highest dose of fluoxetine significantly altered behavior in both tests. Chronic treatment with 5 and 10mg/kg of fluoxetine significantly increased cell proliferation and BDNF levels in the hippocampus. Only chronic treatment with the highest of fluoxetine increased BDNF levels in the frontal cortex. Behavioral measures in the NIH test correlated with BDNF levels in the frontal cortex but not in the hippocampus or with cell proliferation in the hippocampus. These data suggest that females require high doses of fluoxetine for behavioral efficacy regardless of elevations of neurogenesis and BDNF mobilization in the hippocampus. Elevations in BDNF levels in the frontal cortex are related to the behavioral efficacy of fluoxetine.

The neuropeptide VGF is reduced in human bipolar postmortem brain and contributes to some of the behavioral and molecular effects of lithium

Recent studies demonstrate that the neuropeptide VGF (nonacronymic) is regulated in the hippocampus by antidepressant therapies and animal models of depression and that acute VGF treatment has antidepressant-like activity in animal paradigms. However, the role of VGF in human psychiatric disorders is unknown. We now demonstrate using in situ hybridization that VGF is downregulated in bipolar disorder in the CA region of the hippocampus and Brodmann's area 9 of the prefrontal cortex. The mechanism of VGF in relation to LiCl was explored. Both LiCl intraperitoneally and VGF intracerebroventricularly reduced latency to drink in novelty-induced hypophagia, and LiCl was not effective in VGF(+/-) mice, suggesting that VGF may contribute to the effects of LiCl in this behavioral procedure that responds to chronic antidepressant treatment. VGF by intrahippocampal injection also had novel activity in an amphetamine-induced hyperlocomotion assay, thus mimicking the actions of LiCl injected intraperitoneally in a system that phenocopies manic-like behavior. Moreover, VGF(+/-) mice exhibited increased locomotion after amphetamine treatment and did not respond to LiCl, suggesting that VGF is required for the effects of LiCl in curbing the response to amphetamine. Finally, VGF delivered intracerebroventricularly in vivo activated the same signaling pathways as LiCl and is necessary for the induction of mitogen-activated protein kinase and Akt by LiCl, thus lending insight into the molecular mechanisms underlying the actions of VGF. The dysregulation of VGF in bipolar disorder as well as the behavioral effects of the neuropeptide similar to LiCl suggests that VGF may underlie the pathophysiology of bipolar disorder.

Differential effects of acute and repeated citalopram in mouse models of anxiety and depression

Clinically, SSRIs are widely prescribed in the treatment of several anxiety disorders, although very few pre-clinical studies have observed a beneficial effect of this class of drugs in animal models of anxiety. Furthermore, the biphasic pattern observed clinically, an exacerbation of anxiety followed by beneficial effects, is rarely observed in animal studies. In the present study we document this clinical phenomenon in several behavioural paradigms. While a single injection of citalopram induced anxiogenic effects, three administrations of citalopram were sufficient to elicit anxiolytic effects. Congruent with these data, we observed that short-term repeated administration of citalopram was accompanied by increased activation of cAMP response element-binding protein (CREB) in the hippocampus and desensitization of 5-HT1A receptors, two phenomena well associated with chronic rather than acute actions of antidepressants. Moreover, effects of citalopram were abolished in CREBalphaDelta mutant animals in the elevated zero maze (EZM) and tail suspension test (TST), but not in novelty-induced hypophagia (NIH). Further, the desensitization of 5-HT1A receptors elicited by citalopram was not affected by CREB deficiency. The significance of the EZM and TST paradigms in predicting therapeutic efficacy is well known while effects in NIH and 5-HT1A sensitization are less well-established. These data demonstrate that behavioural responses to citalopram are dependent on the frequency of its administration, and that these responses are differentially dependent on CREB function.

Enhanced sensitivity of the MRL/MpJ mouse to the neuroplastic and behavioral effects of acute and chronic antidepressant treatments

Adult hippocampal neurogenesis has been implicated in the pathophysiology of depression and in the therapeutic effects of antidepressant drugs. Current immunohistochemical methods that study neurogenesis are time consuming and labor intensive. Therefore, a significantly more rapid flow cytometric method was characterized to measure neurogenesis in the adult mouse brain. The sensitivity of mice to the effects of antidepressant treatments is dependent on genetic background. Thus, studies were conducted comparing the responsiveness of 2 inbred mouse strains, MRL/MpJ and C57BL/6J, to the acute and chronic effects of antidepressants on neurochemistry and behavior. Acutely, MRL/MpJ mice displayed more robust behavioral and neurochemical responses to pharmacologically distinct antidepressants than C57BL/6J mice. Chronic administration of the antidepressant drugs fluoxetine and desipramine produced robust elevations in hippocampal cell proliferation and brain-derived neurotrophic factor (BDNF) protein levels in MRL/MpJ mice. C57BL/6J mice treated similarly with antidepressant drugs were mainly unresponsive on these measures. Mice were tested in the novelty-induced hypophagia (NIH) paradigm to examine a behavioral response associated with chronic, but not acute, antidepressant treatment. Only MRL/MpJ mice were behaviorally responsive to chronic antidepressant administration in the NIH paradigm. The positive effects of chronic antidepressants on hippocampal cell proliferation and BDNF paralleled the ability of these drugs to produce changes in NIH behavior. These studies highlight the advantages of using flow cytometry to study hippocampal neurogenesis and identify the MRL/MpJ mouse as a strain with superior response to antidepressant drug treatments that may lead to a better understanding of the genetics behind antidepressant efficacy and sensitivity.

Role of the amygdala in antidepressant effects on hippocampal cell proliferation and survival and on depression-like behavior in the rat

The stimulation of adult hippocampal neurogenesis by antidepressants has been associated with multiple molecular pathways, but the potential influence exerted by other brain areas has received much less attention. The basolateral complex of the amygdala (BLA), a region involved in anxiety and a site of action of antidepressants, has been implicated in both basal and stress-induced changes in neural plasticity in the dentate gyrus. We investigated here whether the BLA modulates the effects of the SSRI antidepressant fluoxetine on hippocampal cell proliferation and survival in relation to a behavioral index of depression-like behavior (forced swim test). We used a lesion approach targeting the BLA along with a chronic treatment with fluoxetine, and monitored basal anxiety levels given the important role of this behavioral trait in the progress of depression. Chronic fluoxetine treatment had a positive effect on hippocampal cell survival only when the BLA was lesioned. Anxiety was related to hippocampal cell survival in opposite ways in sham- and BLA-lesioned animals (i.e., negatively in sham- and positively in BLA-lesioned animals). Both BLA lesions and low anxiety were critical factors to enable a negative relationship between cell proliferation and depression-like behavior. Therefore, our study highlights a role for the amygdala on fluoxetine-stimulated cell survival and on the establishment of a link between cell proliferation and depression-like behavior. It also reveals an important modulatory role for anxiety on cell proliferation involving both BLA-dependent and -independent mechanisms. Our findings underscore the amygdala as a potential target to modulate antidepressants' action in hippocampal neurogenesis and in their link to depression-like behaviors.

Adult-born hippocampal neurons are more numerous, faster maturing, and more involved in behavior in rats than in mice

Neurons are born throughout adulthood in the hippocampus and show enhanced plasticity compared with mature neurons. However, there are conflicting reports on whether or not young neurons contribute to performance in behavioral tasks, and there is no clear relationship between the timing of maturation of young neurons and the duration of neurogenesis reduction in studies showing behavioral deficits. We asked whether these discrepancies could reflect differences in the properties of young neurons in mice and rats. We report that young neurons in adult rats show a mature neuronal marker profile and activity-induced immediate early gene expression 1-2 weeks earlier than those in mice. They are also twice as likely to escape cell death, and are 10 times more likely to be recruited into learning circuits. This comparison holds true in two different strains of mice, both of which show high rates of neurogenesis relative to other background strains. Differences in adult neurogenesis are not limited to the hippocampus, as the density of new neocortical neurons was 5 times greater in rats than in mice. Finally, in a test of function, we find that the contribution of young neurons to fear memory is much greater in rats than in mice. These results reveal substantial differences in new neuron plasticity and function between these two commonly studied rodent species.

Sex-specific effects of chronic fluoxetine treatment on neuroplasticity and pharmacokinetics in mice

Neurogenesis is a mechanism through which antidepressants may produce therapeutic effects. There is a dearth of information regarding the effects of antidepressants on neurogenesis and neurotrophin mobilization in females. This study examined sex differences in the alteration of cell proliferation and survival in multiple regions of the brain. Additional experiments examined brain-derived neurotrophic factor (BDNF) levels and pharmacokinetics of fluoxetine to determine whether they mediate sex differences. MRL/MpJ mice were treated with fluoxetine (5 and 10 mg/kg b.i.d.) for 21 days and received injections of 5-bromo-2'-deoxyuridine (200 mg/kg) to measure DNA synthesis. In the hippocampus, fluoxetine increased cell proliferation at both doses; females treated with 10 mg/kg produced more new cells than males. Fluoxetine did not alter survival in males, but 10 mg/kg reduced survival in females. In the frontal cortex, fluoxetine increased cell proliferation and survival in males treated with 10 mg/kg. In the cerebellum and amygdala, 10 mg/kg fluoxetine increased cell proliferation in both sexes but did not alter the incorporation of the new cells. Fluoxetine increased BDNF levels in the hippocampus of both sexes. BDNF levels correlated with cell proliferation in males but not females. Brain and plasma levels indicated that females metabolized fluoxetine faster than males and produced more of the metabolite norfluoxetine. These data suggest that fluoxetine acts on multiple areas of the brain to increase cell proliferation, and the pattern of activation differs between males and females. Sex-specific effects of fluoxetine on neurotrophin mobilization and pharmacokinetics may contribute to these differences in neural plasticity.