Interest of using genetically manipulated mice as models of depression to evaluate antidepressant drugs activity: a review

Depression-/Anxiety-Like Behavior Alterations in Adult Slit2 Transgenic Mice

Background: Slit2 is a member of the Slit family of secreted glycoproteins that plays highly conserved roles in neuronal axon guidance and cellular migration. Our previous experimental results showed Alzheimer's disease-like alterations and increased permeability of the blood–brain barrier in Slit2-overexpressing transgenic (Slit2-Tg) mice aged 8–9 months. Nevertheless, relatively little is known about behavioral alterations in adult Slit2-Tg mice (2–6 months of age). To observe the age-related behavioral effects of Slit2 overexpression in adult mice, we performed a battery of behavioral tests with adult Slit2-Tg mice at 2–6 months of age.

Results: The body weight of Slit2-Tg mice was lower than that of the wild-type mice from 15 weeks of age. Compared with the control mice, depression-like behaviors were found in Slit2-Tg mice from 15 to 21 weeks of age in the sucrose preference test, although Slit2-Tg mice were hyperactive in the tail suspension test. The anxiety-like behaviors were found in Slit2-Tg mice in the open field test, as well as increased locomotor activity. The anxiety-like behaviors were also found in adult Slit2-Tg mice in the elevated plus maze. Compared to wild-type mice at 23 weeks old, impairment of the hippocampal neurons were found in Slit2-Tg mice at the same age in hematoxylin–eosin staining (H&E), including some eccentric dispersion and expansion of neuronal bodies. In addition, the messenger RNA (mRNA) expression of TNF-α was elevated in the hippocampus of adult Slit2-Tg mice.

Conclusions: Slit2 overexpression causes depression-/anxiety-like behaviors in adult mice that may be related to an increase in inflammatory factors and damage to hippocampal neurons.

CK2 regulates 5-HT4 receptor signaling and modulates depressive-like behavior

The serotonergic neurotransmitter system has been widely implicated in the pathophysiology of mood-related disorders such as anxiety and major depressive disorder (MDD). The onset of therapeutic efficacy of traditional antidepressants is delayed by several weeks. The 5-HT₄ receptor has emerged as a new therapeutic target since agonists of this receptor induce rapid antidepressant-like responses in rodents. Here we show that the 5-HT₄ receptor is regulated by CK2, at transcriptional and post-transcriptional levels. We present evidence, in two different CK2α knockout mouse lines, that this regulation is region-specific, with the 5-HT₄ receptor upregulated in prefrontal cortex (PFC) but not striatum or hippocampus where CK2α is also ablated. 5-HT₄ receptor signaling is enhanced in vitro, as evidenced by enhanced cAMP production or receptor plasma membrane localization in the presence of CK2 inhibitor or shRNA targeting CK2α. In vivo, 5-HT₄ receptor signaling is also upregulated since ERK activation is elevated and sensitive to the inverse agonist, GR113808 in the PFC of CK2α KO mice. Behaviorally, KO mice as well as mice with AAV-mediated deletion of CK2α in the PFC show a robust 'anti-depressed-like' phenotype and display an enhanced response to antidepressant treatment when tested in paradigms for mood and anxiety. Importantly, it is sufficient to overexpress the 5-HT₄ receptor in the mPFC to generate mice with a similar 'anti-depressed-like' phenotype. Our findings identify the mPFC as the region that mediates the effect of enhanced 5-HT₄ receptor activity and CK2 as modulator of 5-HT4 receptor levels in this brain region that regulates mood-related phenotypes.

Vortioxetine Improves Context Discrimination in Mice Through a Neurogenesis Independent Mechanism

Major Depressive Disorders (MDD) patients may exhibit cognitive deficits and it is currently unclear to which degree treatment with antidepressants may affect cognitive function. Preclinical and clinical observations showed that vortioxetine (VORT, an antidepressant with multimodal activity), presents beneficial effects on aspects of cognitive function. In addition, VORT treatment increases adult hippocampal neurogenesis (AHN) in rodents, a candidate mechanism for antidepressant activity. Pattern separation (PS) is the ability to discriminate between two similar contexts/events generating two distinct and non-overlapping representations. Impaired PS may lead to overgeneralization and anxiety disorders. If PS impairments were described in depressed patients, the consequences of antidepressant treatment on context discrimination (CD) are still in its infancy. We hypothesized that VORT-increased AHN may improve CD. Thus, in an attempt to elucidate the molecular mechanism underpinning VORT treatment effects on CD, a rodent model of PS, the role of AHN and stress-induced c-Fos activation was evaluated in the adult mouse hippocampus. Chronic treatment with VORT (1.8 g/kg of food weight; corresponding to a daily dose of 10 mg/kg, 3 weeks) improved CD in mice. Interestingly, chronic treatment with VORT reversed ablation of AHN-induced delay in CD and freezing behavior. VORT treatment decreased stress-induced c-Fos activation in the dorsal but not ventral dentate gyrus. VORT treatment did not affect c-Fos activity in the hippocampus of mice with ablated neurogenesis. This study highlights a role of VORT in CD, which may be independent from AHN and hippocampal c-Fos activation. Further studies elucidating the mechanisms underlying VORT’s effects in CD could contribute to future strategies for alleviating the disease burden for individuals suffering from depression and/or anxiety disorders.

BDNF isoforms: a round trip ticket between neurogenesis and serotonin?

The brain-derived neurotrophic factor, BDNF, was discovered more than 30 years ago and, like other members of the neurotrophin family, this neuropeptide is synthetized as a proneurotrophin, the pro-BDNF, which is further cleaved to yield mature BDNF. The myriad of actions of these two BDNF isoforms in the central nervous system is constantly increasing and requires the development of sophisticated tools and animal models to refine our understanding. This review is focused on BDNF isoforms, their participation in the process of neurogenesis taking place in the hippocampus of adult mammals, and the modulation of their expression by serotonergic agents. Interestingly, around this triumvirate of BDNF, serotonin, and neurogenesis, a series of recent research has emerged with apparently counterintuitive results. This calls for an exhaustive analysis of the data published so far and encourages thorough work in the quest for new hypotheses in the field. BDNF is synthetized as a pre-proneurotrophin. After removal of the pre-region, proBDNF can be cleaved by intracellular or extracellular proteases. Mature BDNF can bind TrkB receptors, promoting their homodimerization and intracellular phosphorylation. Phosphorylated-TrkB can activate three different signaling pathways. Whereas G-protein-coupled receptors can transactivate TrkB receptors, truncated forms can inhibit mBDNF signaling. Pro-BDNF binds p75(NTR) by its mature domain, whereas the pro-region binds co-receptors.

The role of serotonergic, adrenergic and dopaminergic receptors in antidepressant-like effect

Depression is a serious global illness, becoming more and more common in developed countries. Because of specific symptoms it is considered as a leading cause of disability all over the world with a high death factor due to suicides. There are many antidepressants used in the therapy, but still more than 30% of patients do not respond to the treatment. The heterogeneous nature of the illness and its complex, unclear aetiology may be responsible for these difficulties. Next to the main monoaminergic hypothesis of depression there are also many other approaches connected with the pathophysiology of the disease, including hypothalamic-pituitary-adrenal axis dysregulation, dopaminergic, cholinergic, glutamatergic or GABA-ergic neurotransmission. Nevertheless, it can be unambiguously stated that serotonergic, noradrenergic and dopaminergic systems are precisely connected with pathogenesis of depression, and should be therefore considered as valuable targets in patients' treatment. Bearing that in mind, this review presents the role of serotonergic, adrenergic and dopaminergic receptors in antidepressant-like effect.

Deuterium content of water increases depression susceptibility: the potential role of a serotonin-related mechanism

Environmental factors can significantly affect disease prevalence, including neuropsychiatric disorders such as depression. The ratio of deuterium to protium in water shows substantial geographical variation, which could affect disease susceptibility. Thus the link between deuterium content of water and depression was investigated, both epidemiologically, and in a mouse model of chronic mild stress. We performed a correlation analysis between deuterium content of tap water and rates of depression in regions of the USA. Next, we used a 10-day chronic stress paradigm to test whether 2-week deuterium-depleted water treatment (91 ppm) affects depressive-like behavior and hippocampal SERT. The effect of deuterium-depletion on sleep electrophysiology was also evaluated in naïve mice. There was a geographic correlation between a content of deuterium and the prevalence of depression across the USA. In the chronic stress model, depressive-like features were reduced in mice fed with deuterium-depleted water, and SERT expression was decreased in mice treated with deuterium-treated water compared with regular water. Five days of predator stress also suppressed proliferation in the dentate gyrus; this effect was attenuated in mice fed with deuterium-depleted water. Finally, in naïve mice, deuterium-depleted water treatment increased EEG indices of wakefulness, and decreased duration of REM sleep, phenomena that have been shown to result from the administration of selective serotonin reuptake inhibitors (SSRI). Our data suggest that the deuterium content of water may influence the incidence of affective disorder-related pathophysiology and major depression, which might be mediated by the serotoninergic mechanisms.

Modulation of rat behaviour by using a rat-like robot

In this paper, we study the response of a rat to a rat-like robot capable of generating different types of behaviour (stressful, friendly, neutral). Experiments are conducted in an open-field where a rat-like robot called WR-4 is put together with live rats. The activity level of each rat subject is evaluated by scoring its locomotor activity and frequencies of performing rearing (rising up on its hind limbs) and body grooming (body cuddling and head curling) actions, whereas the degree of preference of that is indicated by the robot-rat distance and the frequency of contacting WR-4. The moving speed and behaviour of WR-4 are controlled in real-time based on the feedback from rat motion. The activity level and degree of preference of rats for each experimental condition are analysed and compared to understand the influence of robot behaviour. The results of this study show that the activity level and degree of preference of the rat decrease when exposed to a stressful robot, and increase when the robot exhibit friendly behaviour, suggesting that a rat-like robot can modulate rat behaviour in a controllable, predictable way.

Region-specific regulation of 5-HT1B receptors in the rat brain by chronic venlafaxine treatment

RATIONALE

Venlafaxine is a non-selective serotonin and noradrenaline reuptake inhibitor antidepressant drug for which clinical studies have suggested a high level efficacy and a possible early action onset compared to the classical antidepressants. Its therapeutic effects might be due, at least in part, to adaptive changes in serotonergic neurotransmission, through the activation of the different 5-HT receptor subtypes. 5-HT(1B) receptors are located in the axon terminals of both serotonergic and non-serotonergic neurons, where they act as inhibitory autoreceptors or heteroreceptors, respectively. However, the information about the involvement of this subtype in the mechanism of action of antidepressants is limited and quite controversial.

OBJECTIVES

The aim of this study was to evaluate the effect of venlafaxine (10 mg kg⁻¹ day⁻¹, p.o.) after 21 days of treatment on the density of 5-HT(1B) receptors and their functionality in rat brain.

METHODS

Effects of chronic venlafaxine were evaluated at different levels of 5-HT(1B) receptor by using receptor autoradiography, [³⁵S]GTPγS binding, and the regulation of body temperature induced by selective 5-HT(1B) agonist.

RESULTS

Our results show that venlafaxine induced an increase in sensitivity of 5-HT(1B) receptors in hypothalamus both at G-protein level and the control of core temperature without affecting the receptor density.

CONCLUSIONS

These results demonstrate that adaptive changes on 5-HT(1B) receptors induced by chronic administration of venlafaxine exhibit regional differences suggesting that the hypothalamus might be an important site of drug action.

A rat-like robot for interacting with real rats

We developed a novel small rat-like robot calledWasedaRat No.4(WR-4) to interact with real rats. WR-4 can perform both rearing (rising up on its hind limbs) and rotating (body bending during movement) actions faster than live mature rats. After robot–rat interaction involving rearing and body grooming (body cuddling and head curling) actions of WR-4, real rats showed more activity and greater interest in the robot. Similar results evident from rat–rat interaction suggest that a rat-like robot is able to interact with rats in the same way as real rats. Furthermore, lower variances between the rat subjects in robot–rat interaction reveals that a rat-like robot can more effectively impact rat's behavior in a controllable, predictable way.

An updated overview of animal models in neuropsychiatry

Animal models are vital tools to study the genetic, molecular, cellular, and environmental parameters involved in several neuropsychiatric disorders. Over the years, these models have expanded our understanding of the pathogenesis of many neuropsychiatric disorders and neurodegenerative diseases. Although animal models have been widely used in psychiatry, and despite several years of extensive research with these models, their validity is still being investigated and presents a challenge to both investigators and clinicians as well. In this concise review, we will describe the most common animal models utilized in neuropsychiatry, including animal models of depression, anxiety, and psychosis. In addition, we will also discuss the validity and reliability of these models and current challenges in this domain. Furthermore, this work will discuss the role of gene-environment interaction as an additional contributing factor that modulates neuropsychological outcome and its implication on animal models. This overview will give a succinct summary of animal models in psychiatry which will be useful both to the seasoned researcher, as well as novices in the field.

Chronic 5-HT4 receptor activation decreases Aβ production and deposition in hAPP/PS1 mice

Lowering the production and accumulation of Aβ has been explored as treatment for Alzheimer's disease (AD), because Aβ is postulated to play an important role in the pathogenesis of AD. 5-HT4 receptors are an interesting drug target in this regard, as their activation might stimulate α-secretase processing, which increases sAPPα and reduces Aβ, at least according to the central dogma in APP processing. Here we describe a novel high-affinity 5-HT4 receptor agonist SSP-002392 that, in cultured human neuroblastoma cells, potently increases the levels of cAMP and sAPPα at 100-fold lower concentrations than the effective concentrations of prucalopride, a known selective 5-HT4 receptor agonist. Chronic administration of this compound in a hAPP/PS1 mouse model of Alzheimer's disease decreased soluble and insoluble Aβ in hippocampus, but the potential mechanisms underlying these observations seem to be complex. We found no evidence for direct α-secretase stimulation in the brain in vivo, but observed decreased APP and BACE-1 expression and elevated astroglia and microglia responses. Taken together these results provide support for a potential disease-modifying aspect when stimulating central 5-HT4 receptors; however, the complexity of the phenomena warrants further research.

Genetic mouse models of depression

This chapter focuses on the use of genetically modified mice in investigating the neurobiology of depressive behaviour. First, the behavioural tests commonly used as a model of depressive-like behaviour in rodents are described. These tests include those sensitive to antidepressant treatment such as the forced swim test and the tail suspension test, as well as other tests that encompass the wider symptomatology of a depressive episode. A selection of example mutant mouse lines is then presented to illustrate the use of these tests. As our understanding of depression increases, an expanding list of candidate genes is being investigated using mutant mice. Here, mice relevant to the monoamine and corticotrophin-releasing factor hypotheses of depression are covered as well as those relating to the more recent candidate, brain-derived neurotrophic factor. This selection provides interesting examples of the use of complimentary lines, such as those that have genetic removal or overexpression, and also opposing behavioural changes seen following manipulation of closely related genes. Finally, factors such as the issue of background strain and influence of environmental factors are reflected upon, before considering what can realistically be expected of a mouse model of this complex psychiatric disorder.

Effects of maternal L-tryptophan depletion and corticosterone administration on neurobehavioral adjustments in mouse dams and their adolescent and adult daughters

Major depressive disorder (MDD), a pathology characterized by mood and neurovegetative disturbances, depends on a multi-factorial contribution of individual predisposition (e.g., diminished serotonergic transmission) and environmental factors (e.g., neonatal abuse or neglect). Despite its female-biased prevalence, MDD basic research has mainly focused on male rodents. Most of present models of depression are also devalued due to the fact that they typically address only one of the aforementioned pathogenetic factors. In this paper we first describe the basic principles behind mouse model development and evaluation and then articulate that current models of depression are intrinsically devalued due to poor construct and/or external validity. We then report a first attempt to overcome this limitation through the design of a mouse model in which the genetic and the environmental components of early risk factors for depression are mimicked together. Environmental stress is mimicked through the supplementation of corticosterone in the maternal drinking water while biological predisposition is mimicked through maternal access to an L-tryptophan (the serotonin precursor) deficient diet during the first week of lactation. CD1 dams and their offspring exposed to the L-tryptophan deficient diet (T) and to corticosterone (80mg/l; C) were compared to animal facility reared (AFR) subjects. T and C mice served as intermediate reference groups. Adolescent TC offspring, compared to AFR mice, showed decreased time spent floating in the forced-swim test and increased time spent in the open sectors of an elevated 0-maze. Adult TC offspring showed reduced preference for novelty, decreased breakpoints in the progressive ratio operant procedure and major alterations in central BDNF levels and altered HPA regulation. The route of administration and the possibility to control the independent variables predisposing to depressive-like symptoms disclose novel avenues towards the development of animal models with increased external and construct validity. Furthermore, the observation that, compared to adult subjects, adolescent mice display an opposite profile suggests that peri-pubertal developmental processes may interact with neonatal predispositions to calibrate the adult abnormal phenotype.

Regulation of dorsal raphe nucleus function by serotonin autoreceptors: a behavioral perspective

Neurotransmission by serotonin (5-HT) is tightly regulated by several autoreceptors that fine-tune serotonergic neurotransmission through negative feedback inhibition at the cell bodies (predominantly 5-HT(1A)) or at the axon terminals (predominantly 5-HT(1B)); however, more subtle roles for 5-HT(1D) and 5-HT(2B) autoreceptors have also been detected. This review provides an overview of 5-HT autoreceptors, focusing on their contribution in animal behavioral models of stress and emotion. Experiments targeting 5-HT autoreceptors in awake, behaving animals have generally shown that increasing autoreceptor feedback is anxiolytic and rewarding, while enhanced 5-HT function is aversive and anxiogenic; however, the role of serotonergic activity in behavioral models of helplessness is more complex. The prevailing model suggests that 5-HT autoreceptors become desensitized in response to stress exposure and antidepressant administration, two seemingly opposite manipulations. Thus there are still unresolved questions regarding the role of these receptors-and serotonin in general-in normal and pathological states.

Rodent models in depression research: classical strategies and new directions

Depression, among other mood disorders, represents one of the most common health problems worldwide, with steadily increasing incidence and major socio-economic consequences. However, since the knowledge about the underlying pathophysiological principles is still very scanty, depression and other mood disorders are currently diagnosed solely on clinical grounds. Currently used treatment modalities would therefore benefit enormously from the development of alternative therapeutic interventions. The implementation of proper animal models is a prerequisite for increasing the understanding of the neurobiological basis of mood disorders and is paving the way for the discovery of novel therapeutic targets. In the past thirty years, since the seminal description of the Forced Swim Test as a system to probe antidepressant activity in rodents, the use of animals to model depression and antidepressant activity has come a long way. In this review we describe some of the most commonly used strategies, ranging from screening procedures, such as the Forced Swim Test and the Tail Suspension Test and animal models, such as those based upon chronic stress procedures, to genetic approaches. Finally we also discuss some of the inherent limitations and caveats that need to be considered when using animals as models for mental disorders in basic research.

Genetically modified animals and pharmacological research

This chapter reviews the use of genetically modified animals and the increasingly detailed knowledge of the genomes of the domestic species. The different approaches to genetic modification are outlined as are the advantages and disadvantages of the techniques in different species. Genetically modified mice have been fundamental in understanding gene function and in generating affordable models of human disease although these are not without their drawbacks. Transgenic farm animals have been developed for nutritionally enhanced food, disease resistance and xenografting. Transgenic rabbits, goats, sheep and cows have been developed as living bioreactors producing potentially high value biopharmaceuticals, commonly referred to as "pharming". Domestic animals are also important as a target as well as for testing genetic-based therapies for both inherited and acquired disease. This latter field may be the most important of all, in the future development of novel therapies.