The phenomenon of one-trial tolerance to the anxiolytic effect of chlordiazepoxide in the elevated plus-maze test is abolished by previous administration of chlordiazepoxide or buspirone

Behavioural characteristics and sex differences of a treatment-resistant depression model: Chronic mild stress in the Wistar-Kyoto rat

Depression affects 20% of the general population and is a leading cause of disability worldwide, with a strong female prevalence. Current pharmacotherapies have significant limitations, and one third of patients are unresponsive. Male Wistar-Kyoto rats exposed to chronic mild stress (CMS) were recently proposed as a model to study antidepressant resistance. However, sex differences and interindividual vulnerability to stress are yet unexplored in this model. We aimed to investigate these in the context of the behavioural impact of CMS in the sucrose preference test, elevated plus maze (EPM), forced swim test (FST), open field test and daily locomotor activity rhythms, in male and female WKY rats exposed or not to a 4-week CMS protocol. CMS-exposed animals were clustered through K-means into subgroups based on the EPM and FST results. In both sexes, one subgroup behaved similarly to non-stressed animals and was labelled stress-non vulnerable; the second exhibited less open arms exploration in the EPM and higher immobility in the FST and was named stress-vulnerable. Vulnerable males presented phase delay in daily locomotor activity following CMS, but no significant rhythm could be determined in females. CMS-exposed males of both groups showed hyperlocomotion in reaction to novelty and slower weight gain through the course of CMS, while CMS-exposed females showed smaller sucrose intake. Unexpectedly, CMS did not affect sucrose preference. Our findings strengthen the view that in models of psychiatric pathologies based on stress exposure it is important to consider the effect of sex and to differentiate the non vulnerable and vulnerable subpopulations.

Model of theta frequency perturbations and contextual fear memory

Theta oscillations in the hippocampal local field potential (LFP) appear during translational movement and arousal, modulate the activity of principal cells, and are associated with spatial cognition and episodic memory function. All known anxiolytics slightly but consistently reduce hippocampal theta frequency. However, whether this electrophysiological effect is mechanistically related to the decreased behavioral expression of anxiety is currently unclear. Here, we propose that a reduction in theta frequency affects synaptic plasticity and mnemonic function and that this can explain the reduction in anxiety behavior. We test this hypothesis in a biophysical model of contextual fear conditioning. First, we confirm that our model reproduces previous empirical results regarding the dependence of synaptic plasticity on presynaptic firing rate. Next, we investigate how theta frequency during contextual conditioning impacts learning. These simulations demonstrate that learned associations between threat and context are attenuated when learning takes place under reduced theta frequency. Additionally, our simulations demonstrate that learned associations result in increased theta activity in the amygdala, consistent with empirical data. In summary, we propose a mechanism that can account for the behavioral effect of anxiolytics by impairing the integration of threat attributes of an environment into the cognitive map due to reduced synaptic potentiation.

Preclinical animal anxiety research - flaws and prejudices

The current tests of anxiety in mice and rats used in preclinical research include the elevated plus-maze (EPM) or zero-maze (EZM), the light/dark box (LDB), and the open-field (OF). They are currently very popular, and despite their poor achievements, they continue to exert considerable constraints on the development of novel approaches. Hence, a novel anxiety test needs to be compared with these traditional tests, and assessed against various factors that were identified as a source of their inconsistent and contradictory results. These constraints are very costly, and they are in most cases useless as they originate from flawed methodologies. In the present report, we argue that the EPM or EZM, LDB, and OF do not provide unequivocal measures of anxiety; that there is no evidence of motivation conflict involved in these tests. They can be considered at best, tests of natural preference for unlit and/or enclosed spaces. We also argued that pharmacological validation of a behavioral test is an inappropriate approach; it stems from the confusion of animal models of human behavior with animal models of pathophysiology. A behavioral test is developed to detect not to produce symptoms, and a drug is used to validate an identified physiological target. In order to overcome the major methodological flaws in animal anxiety studies, we proposed an open space anxiety test, a 3D maze, which is described here with highlights of its various advantages over to the traditional tests.

Effects of chronic fluoxetine treatment on anxious behaviour of BALB/c mice in a 3-dimensional maze

Here we used a 3-dimensional (3D) maze, a modification of the radial maze, to assess the effects of treatment for two weeks with a single daily dose of fluoxetine (20 mg/kg, i.p.) on anxiety in male BALB/c mice. We examined whether anxiolytic effects of fluoxetine can be detected over three daily test sessions. We examined also whether repeated handling associated with chronic treatment interferes with effects of fluoxetine on anxiety responses. The 3D maze comprises nine arms, each connected to an upward inclined bridge radiating from a central platform. In this maze, BALB/c mice cross frequently into the bridges but avoid the arms. This avoidance is used as an index of anxiety. Two separate groups received once a day either saline (SALCH, n = 8) or fluoxetine (FLUCH, n = 8) for 14 days, and up to 30 min before the test during the subsequent 3 days. A third group received saline (SALAC, n = 8) 30 min before the test, once a day for 3 days. SALAC mice did not cross into the arms, and continued this avoidance over 3 sessions. SALCH mice avoided the arms in session 1 whereas FLUCH mice did cross into the arms, and like SALCH mice, increased number of crossings into and time on the arms in subsequent sessions. Fluoxetine evidently had an anxiolytic effect but only in the first session. These results indicate that handling experience decreased fear and anxiety in the mice, which may have masked the anxiolytic effect of fluoxetine in the second and third test sessions.

Implication of the 5-HT2A and 5-HT2C (but not 5HT1A) receptors located within the periaqueductal gray in the elevated plus-maze test-retest paradigm in mice

A single exposure to the elevated plus-maze test (EPM) increases open arms avoidance and reduces or abolishes the anxiolytic-like effect of benzodiazepines assessed during a second trial, a phenomenon defined as "one-trial tolerance" (OTT). It has been emphasized that the dorsal portion of the midbrain periaqueductal gray (dPAG) plays a role on this enhanced aversion phenomenon in maze-experienced rodents. Given that intra-dPAG injections of a wide range of serotonergic 5-HT(1A), 5-HT(2A) and 5-HT(2C) receptor agonists produce anxiolytic-like effects in maze-naïve rodents, the present study examined the effects of the 5-HT(1A) receptor agonist 8-OH-DPAT (5.6 and 10.0nmol in 0.15microl) the preferential 5-HT(2A) receptor agonist DOI (2.0 and 8.0nmol in 0.1microl) and the preferential 5-HT(2C) receptor agonist MK-212 (21.2 and 63.6nmol in 0.1microl) microinjected into the dPAG prior to Trial 1 and Trial 2 on the behaviour of mice in the EPM. Test sessions were recorded and subsequently scored for anxiety-like behaviour (percentage of open arms entries and time) as well as general locomotor activity (closed arm entries). The results showed a lack of 8-OH-DPAT (5.6 and 10.0nmol) effect on the behaviour of maze-naïve and maze-experienced mice, while intra-dPAG microinfusions of DOI (8nmol) reduced anxiety-like behaviour only in maze-experienced mice that had received a similar treatment prior to Trial 1. Furthermore, intra-dPAG MK-212 (63.6nmol) showed an anxiolytic-like effect on both Trial 1 and Trial 2. Importantly, these effects were observed in the absence of any significant change in closed arm entries, the parameter considered to be a valid index of locomotor activity in the plus-maze. These results support the dPAG as a crucial structure involved in the neurobiology of the OTT phenomenon as well as accounting the role of the 5-HT(2A) and 5-HT(2C) receptors located within this midbrain structure on the emotional state induced by EPM test and retest paradigm mice.

The partial 5-HT1A receptor agonist buspirone enhances neurogenesis in the opossum (Monodelphis domestica)

We demonstrate for the first time that neurogenesis in the adult Monodelphis opossum has a typical mammalian pattern and occurs only in the dentate gyrus (DG) and subventricular zone (SVZ) of the lateral ventricles. In these two brain regions neurogenesis is present throughout the lifespan, although its rate is reduced by half in the old age. Treatment with buspirone, a partial 5-HT1A receptor agonist which is used in human clinic as an anxiolytic agent, boosts proliferation in the SVZ and DG in both adult and aged opossums. The neuronal phenotype dominates among newly generated cells in both non-treated and buspirone-treated opossums. We suggest that if functional importance of adult neurogenesis is in improving olfactory discrimination and generation of hippocampus-dependent memory, both spatial and emotional, then administration of drugs increasing the rate of neurogenesis via activation of 5-HT1A receptors may be a valuable aid in combating problems of the advanced age.

Similar anxiolytic-like effects following intra-amygdala infusions of benzodiazepine receptor agonist and antagonist: evidence for the release of an endogenous benzodiazepine inverse agonist in mice exposed to elevated plus-maze test

Previous studies have demonstrated that microinjections of midazolam, a benzodiazepine receptor agonist, into the amygdala produce anxiolytic-like effects in elevated plus-maze (EPM)-naïve rodents. However, systemic or intracerebral administration of benzodiazepines (BDZ) fails to alter anxiety in maze-experienced animals, a phenomenon defined as "one trial tolerance" (OTT). This study focused on the effects of intra-amygdala infusion of midazolam in maze-experienced mice. In addition, the effects of flumazenil in the amygdala of maze-naïve and experienced mice were also investigated. To investigate intrinsic effects of intra-amygdala flumazenil on anxiety, animals were systemically treated with the BDZ receptor inverse agonist, DMCM (4-ethyl-6,7-dimethoxy-9H-pyrido[3,4-b]indole-3-carboxylic acid methyl ester hydrochloride). Conventional measures of anxiety (% open arm entries and % open arm time), locomotor activity (frequency of closed arm entries) and a range of ethological measures related to risk assessment were recorded. Intra-amygdala midazolam (3.0 and 30 nmol) attenuated anxiety in maze-experienced mice. A similar behavioral profile was produced by intra-amygdala flumazenil in maze-naïve (4.0 and 16 nmol) and maze-experienced (16 nmol) mice. Intra-amygdala flumazenil (at 2.0 nmol, a dose devoid of any intrinsic effect on anxiety measures in the EPM) selectively and completely blocked the anxiogenic-like effects of systemic administration of DMCM (1.0 mg/kg, i.p.) in maze-naïve mice. Together, these results demonstrate that the GABA(A)-benzodiazepine receptor complex located within the amygdala does not play a role in the OTT phenomenon. Present results also suggest that the release of an endogenous BDZ receptor inverse agonist within the amygdala seems to be an important correlate of the emotional state induced by the plus-maze test.

Animal models of anxiety and anxiolytic drug action

Animal models of anxiety attempt to represent some aspect of the etiology, symptomatology, or treatment of human anxiety disorders, in order to facilitate their scientific study. Within this context, animal models of anxiolytic drug action can be viewed as treatment models relevant to the pharmacological control of human anxiety. A major purpose of these models is to identify novel anxiolytic compounds and to study the mechanisms whereby these compounds produce their anxiolytic effects. After a critical analysis of "face," "construct," and "predictive" validity, the biological context in which animal models of anxiety are to be evaluated is specified. We then review the models in terms of their general pharmacological profiles, with particular attention to their sensitivity to 5-HTIA agonists and antidepressant compounds. Although there are important exceptions, most of these models are sensitive to one or perhaps two classes of anxiolytic compounds, limiting their pharmacological generality somewhat, but allowing in depth analysis of individual mechanisms of anxiolytic drug action (e.g., GABAA agonism). We end with a discussion of possible sources of variability between models in response to 5-HTIA agonists and antidepressant drugs.

Ethological and temporal analyses of anxiety-like behavior: the elevated plus-maze model 20 years on

As well as being considered a reliable measurement instrument of animal anxiety-like behavior, the elevated plus-maze (EPM) is also used as a post-hoc test to evaluate emotionality in genetically modified rodents. The present review considers factors which may further improve the validity (predictive/face/construct) of the EPM model: (1) the importance of measuring defensive patterns of response such as risk assessment in addition to traditional measures such as open arm time; (2) other methodological refinements such as min-by-min scoring and use of a test/retest protocol; and (3) the identification and control of major sources of variability in this test. To estimate whether current use of the EPM by researchers takes the above factor into account, a survey of the recent literature was conducted. Results showed that the majority of studies have not yet assimilated these important considerations into their use of the EPM. For example, although risk assessment measures may be more sensitive to anxiety modulating drugs than traditional measures, only a quarter of studies have adopted them. It is hoped that this review can provide insights into the optimal use of the EPM, a simple task that can be very complex in terms of behavioral analysis.