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

Low-dose neonatal domoic acid causes persistent changes in behavioural and molecular indicators of stress response in rats

Appropriate stress responses rely on a finely-tuned neuronal balance that must continually adapt to a frequently changing external environment. Alterations in this balance can result in susceptibility to a variety of stress-related disorders, as well as exacerbate already existing conditions. We have previously reported that rat pups injected with a very low dose (20 μg/kg) of domoic acid during the second postnatal week of life display low-grade seizure behaviours when challenged with stressful tasks, and also exhibit a variety of structural and functional changes similar to those seen in temporal lobe epilepsy. The current study was designed to investigate markers of altered stress-response in this model. Following neonatal treatment, adult rats were tested in the elevated plus maze, as well as two water maze tasks, both of which involved a platform reversal challenge. Results indicated a modified behavioural stress/anxiety response, increased perseveration, and alterations in search strategy for all domoate-treated rats, as well as male-specific deficits in cognitive flexibility. In addition, 80% of treated males and 20% of treated females exhibited seizure behaviour. Western blot analysis revealed male-only increases in adrenergic receptor (α2a and α2c) and mineralocorticoid receptor expression, and subtle sex-specific changes in glucocorticoid receptor expression, but no differences in corticotropin-releasing factor receptors I/II, or dopamine D2 receptor expression. A significant decrease in glucocorticoid:mineralocorticoid ratio was also noted. We conclude that early exposure to DOM alters central mechanisms underlying stress response, and that this model may be valuable for investigating the connection between stress and neurological disorders.

Applications of positron emission tomography in animal models of neurological and neuropsychiatric disorders

Positron emission tomography (PET) provides dynamic images of the biodistribution of radioactive tracers in the brain. Through application of the principles of compartmental analysis, tracer uptake can be quantified in terms of specific physiological processes such as cerebral blood flow, cerebral metabolic rate, and the availability of receptors in brain. Whereas early PET studies in animal models of brain diseases were hampered by the limited spatial resolution of PET instruments, dedicated small-animal instruments now provide molecular images of rodent brain with resolution approaching 1mm, the theoretic limit of the method. Major applications of PET for brain research have consisted of studies of animal models of neurological disorders, notably Parkinson's disease (PD), Alzheimer's disease (AD), and Huntington's disease (HD), stroke, epilepsy and traumatic brain injury; these studies have particularly benefited from selective neurochemical lesion models (PD), and also transgenic rodent models (AD, HD). Due to their complex and uncertain pathophysiologies, corresponding models of neuropsychiatric disorders have proven more difficult to establish. Historically, there has been an emphasis on PET studies of dopamine transmission, as assessed with a range of tracers targeting dopamine synthesis, plasma membrane transporters, and receptor binding sites. However, notable recent breakthroughs in molecular imaging include the development of greatly improved tracers for subtypes of serotonin, cannabinoid, and metabotropic glutamate receptors, as well as noradrenaline transporters, amyloid-β and neuroinflammatory changes. This article reviews the considerable recent progress in preclinical PET and discusses applications relevant to a number of neurological and neuropsychiatric disorders in humans.

The Elevated Plus-Maze Test: Differential Psychopharmacology of Anxiety-Related Behavior

The role of individual factors in behavioral neuroscience is an important, but still neglected, area of research. For example, the Elevated Plus-Maze Test has been one of the most used paradigms to gauge unconditioned aversively motivated behavior in rodents. However, despite a great number of experiments with this test there have been only few efforts to assess systematic individual variations in the elevated plus-maze and related neurobiological functions. The present review aims to give, first, a general overview and introduction about the test, and second, an animal model of anxiety based on natural variance of plus-maze behavior within a given unselected population of rats. Finally, critical aspects of such approaches in animal research are discussed, and suggestions are given as to where to go from here.

Activation of amygdalar metabotropic glutamate receptors modulates anxiety, and risk assessment behaviors in ovariectomized estradiol-treated female rats

Anxiety disorders are more prevalent in females than males. The underlying reasons for this gender difference are unknown. Metabotropic glutamate receptors (mGluRs) have been linked to anxiety and it has been shown that interaction between estrogen receptors and mGluRs modulate sexual receptivity in rats. We investigated the role of mGluRs in anxiety-related behaviors in ovariectomized female rats with (OVX+EB) or without (OVX) estradiol implants. We centrally infused (s)-3,5-dihydroxyphenylglycine (DHPG), a group I mGluR agonist, into the basolateral amygdala (BLA) of OVX+EB and OVX rats at 0.1 and 1.0 μM. Male rats that normally have low estradiol levels were used to compare with OVX rats. Generalized anxiety, explorative activity and detection and analysis of threat were analyzed in the elevated plus maze (EPM) and risk assessment behaviors (RABs). DHPG (1.0 μM) increased the percentage of time spent in- and entries into- the open arms in OVX+EB, but not in OVX or male rats. Flat-back approaches and stretch-attend postures, two RABs, were significantly reduced by DHPG (0.1 and 1.0 μM) in OVX+EB rats only. DHPG did not modulate rearing and freezing, behaviors related to exploration and fear-like behavior, respectively. However, DHPG (1.0 μM) increased head dipping and decreased grooming behaviors in OVX rats, suggesting a weak explorative modulation. The effects of DHPG observed in OVX+EB, were blocked by 50 μM of (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA), a mGluR1 antagonist. AIDA and/or estradiol did not modulate anxiety and or RABs. Our results show that intra-BLA infusion of DHPG exerts an anxiolytic-like effect in OVX+EB, but not in OVX or male rats. This effect seems to depend upon mGluR1 subtype activation. Our findings led us to suggest that the effects observed in OVX+EB rats might be due to an interaction at the membrane level of estrogen receptors with mGlu1 within the BLA.

Hypoxic-ischemic injury decreases anxiety-like behavior in rats when associated with loss of tyrosine-hydroxylase immunoreactive neurons of the substantia nigra

Neonatal Sprague-Dawley rats were randomly divided into normal control, mild hypoxia-ischemia (HI), and severe HI groups (N = 10 in each group at each time) on postnatal day 7 (P7) to study the effect of mild and severe HI on anxiety-like behavior and the expression of tyrosine hydroxylase (TH) in the substantia nigra (SN). The mild and severe HI groups were exposed to hypoxia (8% O2/92% N2) for 90 and 150 min, respectively. The elevated plus-maze (EPM) test was performed to assess anxiety-like behavior by measuring time spent in the open arms (OAT) and OAT%, and immunohistochemistry was used to determine the expression of TH in the SN at P14, P21, and P28. OAT and OAT% in the EPM were significantly increased in both the mild (1.88-, 1.99-, and 2.04-fold, and 1.94-, 1.51-, and 1.46-fold) and severe HI groups (1.69-, 1.68-, and 1.87-fold, and 1.83-, 1.43-, and 1.39-fold, respectively; P < 0.05). The percent of TH-positive cells occupying the SN area was significantly and similarly decreased in both the mild (17.7, 40.2, and 47.2%) and severe HI groups (16.3, 32.2, and 43.8%, respectively; P < 0.05). The decrease in the number of TH-positive cells in the SN and the level of protein expression were closely associated (Pearson correlation analysis: r = 0.991, P = 0.000 in the mild HI group and r = 0.974, P = 0.000 in the severe HI group) with the impaired anxiety-like behaviors. We conclude that neonatal HI results in decreased anxiety-like behavior during the juvenile period of Sprague-Dawley rats, which is associated with the decreased activity of TH in the SN. The impairment of anxiety and the expression of TH are not likely to be dependent on the severity of HI.

The standardized extract of Loeselia mexicana possesses anxiolytic activity through the γ-amino butyric acid mechanism

ETHNOPHARMACOLOGICAL RELEVANCE

Loeselia mexicana (Lam.) Brand has been used in Mexican Traditional Medicine to treat "espanto" or "susto" (fear), which is a culturally affiliated syndrome whose symptomatology comprises loss of appetite, difficulty in sleeping, and also nausea and fatigue, with a sensation of fear or risk - real or imagined - to external stimuli.

AIM OF THE STUDY

The anxiolytic effect of the standardized methanol extract of Loeselia mexicana, with regard to its content of coumarin daphnoretin, was researched utilizing the elevated plus maze (EPM) in order to demonstrate whether the biological effect produced by the plant is antagonized by drugs that block γ-amino butyric acid (GABA)ergic transmission.

MATERIALS AND METHODS

The methanolic extract of Loeselia mexicana (LmMeOH) was tested at different doses on the EPM and then the interaction of this extract was evaluated in the same model with different GABAergic drugs, such as flumazenil (FLU) 10mg/kg, bicuculline (BIC) 5mg/kg, pentylenetetrazole (PTZ) 10mg/kg, and picrotoxin (PTX) 2mg/kg. The effect of all of these treatments was evaluated by means of the open field test (OFT). Coumarin content was measured by the high performance liquid chromatography (HPLC) technique.

RESULTS

The 200- and 400-mg/kg doses of methanolic extract containing 3.14 and 6.28 mg of daphnoretin, respectively, induced an anxiolytic effect in the EPM without modification of the spontaneous motor activity. The anxiolytic activity of 200mg/kg of methanolic extract in EPM-exposed mice was antagonized by PTX, BIC, and FLU, but not by PTZ.

CONCLUSION

The data presented here indicate that the Loeselia mexicana Brand methanolic extract possesses a significant anxiolytic effect that appears to be mediated in part by activation of the GABAergic system.

Rats that binge eat fat-rich food do not show somatic signs or anxiety associated with opiate-like withdrawal: implications for nutrient-specific food addiction behaviors

Previous studies suggest that binge eating sugar leads to behavioral and neurochemical changes similar to those seen with drug addiction, including signs of opiate-like withdrawal. Studies are emerging that show multiple neurochemical and behavioral indices of addiction when animals overeat a fat-rich diet. The goal of the present study was to utilize liquid and solid diets high in sugar and fat content to determine whether opiate-like withdrawal is seen after binge consumption of these diets in Sprague-Dawley rats. Control groups were given ad libitum access to the sweet-fat food or standard chow. All rats were then given a battery of tests to measure signs of opiate-like withdrawal, which included somatic signs of distress, elevated plus-maze anxiety, and locomotor hypoactivity. Neither naloxone-precipitated (3 mg/kg) nor deprivation-induced withdrawal was observed in rats that were maintained on a nutritionally complete pelleted sweet-fat diet or a sweet, high-fat diet supplemented with standard rodent chow. Naloxone-precipitated withdrawal was also not seen in rats fed a liquid sweet-fat food. Further, body weight reduction to 85%, which is known to potentiate the reinforcing effects of substances of abuse, did not affect naloxone-precipitated signs of opiate-like withdrawal. Thus, unlike previous findings reported regarding rats with binge access to a sucrose solution, rats that binge eat sweet-fat combinations do not show signs of opiate-like withdrawal under the conditions tested. These data support the idea that excessive consumption of different nutrients can induce behaviors associated with addiction in different ways, and that the behaviors that could characterize "food addiction" may be subtyped based on the nutritional composition of the food consumed.

Behavioral traits predicting alcohol drinking in outbred rats: an investigation of anxiety, novelty seeking, and cognitive flexibility

BACKGROUND

Most adults in Western society consume alcohol regularly without negative consequences. For a small subpopulation, however, drinking can quickly progress to excessive and chronic intake. Given the dangers associated with alcohol abuse, it is critical to identify traits that may place an individual at risk for developing these behaviors. To that end, we used a rat model to determine whether anxiety-related behaviors, novelty seeking, or cognitive flexibility predict excessive alcohol drinking under both limited and continuous access conditions.

METHODS

Adult male rats were assessed in a series of behavioral tasks (elevated plus maze [EPM], locomotor activity, and discrimination/reversal learning in a Y-maze) followed by 6 weeks of daily, 1-hour access to alcohol in a free-choice, 2-bottle paradigm (10% alcohol vs. tap water). Next, subjects were given the opportunity to consume alcohol for 72 hours in drinking chambers that permit separate measures of each drinking bout. Half of the animals experienced a 2-week deprivation period between the limited and continuous access sessions.

RESULTS

Time spent on the open arms of the EPM, but not novelty seeking or discrimination/reversal learning, predicted alcohol consumption during limited, 1-h/d access sessions to alcohol. Anxiety-related behavior also predicted the escalation of intake when animals were given 72 hours of continuous access to alcohol. Bout size, but not frequency, was responsible for the increased consumption by high-anxiety subjects during this period. Finally, intake during limited access sessions predicted intake during continuous access, but only in subjects with low intake during limited access.

CONCLUSIONS

These findings confirm that preexisting anxiety-related behavior predicts alcohol intake under several schedules of alcohol access. Moreover, when access is unlimited, the high-anxiety-related group exhibited an increase in bout size, but not frequency, of drinking. In addition, we show that modest intake when alcohol is restricted may or may not progress to excessive intake when the drug is freely available.

Corticosterone does not change open elevated plus maze-induced antinociception in mice

It has been demonstrated that the exposure of rodents to the standard elevated plus-maze (sEPM: 2 open and 2 enclosed arms) elicits defensive behavioral reactions and antinociception and also activates the hypothalamo-pituitary-adrenal (HPA) axis. We have recently reported that EPM-induced antinociception is particularly observed when rats and mice are exposed to a totally open EPM (oEPM: 4 open arms). Given that the oEPM seems to be a more aversive situation than the sEPM, we hypothesized that oEPM exposure would induce higher plasma levels of corticosterone than sEPM exposure in mice. In this study, we investigated the influence of exposure to eEPM (enclosed EPM: 4 enclosed arms), sEPM or oEPM on plasma corticosterone levels in mice, with or without prior nociceptive stimulation (2.5% formalin injection into the right hind paw). We also tested whether the nociceptive response in the formalin test and oEPM-induced antinociception are altered by adrenalectomy. Results showed that oEPM-exposed mice spent less time licking the injected paw than sEPM- and eEPM-exposed animals. All three types of EPM exposure increased plasma corticosterone when compared to the basal group, but sEPM- and oEPM-exposed mice showed higher corticosterone levels than eEPM-exposed mice. Prior nociceptive stimulation (formalin injection) did not enhance the plasma corticosterone response induced by the three types of EPM exposure. Indeed, formalin injection appeared to provoke a ceiling effect on plasma corticosterone concentration. Furthermore, neither the nociceptive response in the formalin test nor oEPM-induced antinociception was changed by adrenalectomy. Present results suggest that oEPM antinociception does not depend on corticosterone release in mice.

Dietary n-3 polyunsaturated fatty acid deprivation together with early maternal separation increases anxiety and vulnerability to stress in adult rats

Low concentrations of n-3 polyunsaturated fatty acid (PUFA) and chronic stress are implicated in susceptibility to mood disorders. We have investigated the combined effects of chronic n-3 PUFA dietary deficiency and early maternal separation (MS) stress on the reactivity to stressful situations of rats as adults. Pups fed a control or an n-3 PUFA deficient diet were daily separated for two weeks before weaning They were all tested at 3 month-old to determine their anxiety, and their ability to learn two aversive tasks differing in the control they could exert on the situation: auditory fear conditioning and brightness avoidance discrimination. Neither the n-3 PUFA-deficient diet nor MS alone significantly affected behavior. But n-3 PUFA-deficient rats that had been separated were more anxious and fearful in inescapable situations, while their ability to cope with an aversive avoidance task remained unaffected. These results support the notion that PUFA-unbalanced diet, together with stress, may be a determinant risk factor in emotional disorders.

Behavioral and autonomic responses to acute restraint stress are segregated within the lateral septal area of rats

BACKGROUND

The Lateral Septal Area (LSA) is involved with autonomic and behavior responses associated to stress. In rats, acute restraint (RS) is an unavoidable stress situation that causes autonomic (body temperature, mean arterial pressure (MAP) and heart rate (HR) increases) and behavioral (increased anxiety-like behavior) changes in rats. The LSA is one of several brain regions that have been involved in stress responses. The aim of the present study was to investigate if the neurotransmission blockade in the LSA would interfere in the autonomic and behavioral changes induced by RS.

METHODOLOGY/PRINCIPAL FINDINGS

Male Wistar rats with bilateral cannulae aimed at the LSA, an intra-abdominal datalogger (for recording internal body temperature), and an implanted catheter into the femoral artery (for recording and cardiovascular parameters) were used. They received bilateral microinjections of the non-selective synapse blocker cobalt chloride (CoCl(2), 1 mM/ 100 nL) or vehicle 10 min before RS session. The tail temperature was measured by an infrared thermal imager during the session. Twenty-four h after the RS session the rats were tested in the elevated plus maze (EPM).

CONCLUSIONS/SIGNIFICANCE

Inhibition of LSA neurotransmission reduced the MAP and HR increases observed during RS. However, no changes were observed in the decrease in skin temperature and increase in internal body temperature observed during this period. Also, LSA inhibition did not change the anxiogenic effect induced by RS observed 24 h later in the EPM. The present results suggest that LSA neurotransmission is involved in the cardiovascular but not the temperature and behavioral changes induced by restraint stress.

Physical exercise during adolescence versus adulthood: differential effects on object recognition memory and brain-derived neurotrophic factor levels

It is well established that physical exercise can enhance hippocampal-dependent forms of learning and memory in laboratory animals, commensurate with increases in hippocampal neural plasticity (brain-derived neurotrophic factor [BDNF] mRNA/protein, neurogenesis, long-term potentiation [LTP]). However, very little is known about the effects of exercise on other, non-spatial forms of learning and memory. In addition, there has been little investigation of the duration of the effects of exercise on behavior or plasticity. Likewise, few studies have compared the effects of exercising during adulthood versus adolescence. This is particularly important since exercise may capitalize on the peak of neural plasticity observed during adolescence, resulting in a different pattern of behavioral and neurobiological effects. The present study addressed these gaps in the literature by comparing the effects of 4 weeks of voluntary exercise (wheel running) during adulthood or adolescence on novel object recognition and BDNF levels in the perirhinal cortex (PER) and hippocampus (HP). Exercising during adulthood improved object recognition memory when rats were tested immediately after 4 weeks of exercise, an effect that was accompanied by increased BDNF levels in PER and HP. When rats were tested again 2 weeks after exercise ended, the effects of exercise on recognition memory and BDNF levels were no longer present. Exercising during adolescence had a very different pattern of effects. First, both exercising and non-exercising rats could discriminate between novel and familiar objects immediately after the exercise regimen ended; furthermore there was no group difference in BDNF levels. Two or four weeks later, however, rats that had previously exercised as adolescents could still discriminate between novel and familiar objects, while non-exercising rats could not. Moreover, the formerly exercising rats exhibited higher levels of BDNF in PER compared to HP, while the reverse was true in the non-exercising rats. These findings reveal a novel interaction between exercise, development, and medial temporal lobe memory systems.

A novel elevated plus-maze procedure to avoid the one-trial tolerance problem

The elevated plus-maze (EPM) test is one of the most commonly used behavioral assays to evaluate anxiety-related behavior in rodents. It is an economic test (5 min duration) without prior conditioning of the animals. The critical measure for anxiety is the time spent in the open arms of the maze. A confounding problem of the EPM is the so called one-trial tolerance (OTT), characterized by a marked decrease of open arm exploration in spite of treatment with anxiolytic acting benzodiazepines upon re-exposure to the EPM. This consistent finding is often raised as an evidence for the inappropriateness to re-test rodents in the EPM. However, a reliable re-test paradigm would broaden the usability and effectiveness of this test. Therefore, we tested how an extension of the inter-trial interval to 28 days (instead of the usual 24 h), and an additional change of the testing room would affect the open arm time and other behaviors on the EPM. In two experiments, drug-naive Wistar rats were exposed to the EPM on trial 1, and treated intraperitoneally with either vehicle or midazolam (0.25 mg/kg) 30 min before trial 2. Then, trial 2 (28 days after trial 1) was carried out in either the same testing room (Experiment 1) or in another unfamiliar room (Experiment 2). Twenty-eight days after trial 1 the open arm time of the rats in the vehicle treated control rats of both experimental groups was comparable to that of the first trial, independent of the testing room. Most importantly, we found that the treatment with the benzodiazepine midazolam had a significantly anxiolytic-like (i.e., increase of open arm time) effect in trial 2 only when conducted in the previously unfamiliar testing room (Experiment 2). We suggest that in order to reliably re-test the EPM and to prevent confounding effects due to the OTT, an inter-trial interval of 28 days and a change in testing rooms reinstates anxiolytic-like actions of benzodiazepines.

Infusions of neuropeptide Y into the lateral septum reduce anxiety-related behaviors in the rat

Neuropeptide Y (NPY) is one of the most abundant peptides in mammalian brain and NPY-like-immunoreactivity is highly expressed in the lateral septum, an area extensively involved in anxiety regulation. NPY counteracts the neurochemical and behavioral responses to acute threat in animal models, and intracerebroventricular (i.c.v.) administration of NPY at low doses is anxiolytic. Less is known about the specific contributions of the lateral septum to NPY-mediated anxiety regulation. In Experiment 1, the effects of infusions of NPY (1.5 μg) into the lateral septum were investigated in three animal models of anxiety: the elevated plus-maze, novelty-induced suppression of feeding, and shock-probe burying tests. Experiment 2 examined the role of the NPY Y1 receptor in these models by co-infusing the Y1 antagonist BIBO 3304 (0.15 μg, 0.30 μg) with NPY into the lateral septum. In the elevated plus-maze, there were no changes in rats' open arm exploration, the index of anxiety reduction in this test. In the novelty-induced suppression of feeding test, rats infused with NPY showed decreases in the latency to consume a palatable snack in a novel (but not familiar) environment, suggesting a reduction in anxiety independent of increases in appetite. This anxiolysis was attenuated by co-infusion with BIBO 3304 (0.30 μg) in Experiment 2. Lastly, rats infused with NPY showed decreases in the duration of burying behavior in the shock-probe burying test, also indicative of anxiety reduction. However, unlike in the feeding test, BIBO 3304 did not attenuate the NPY-induced anxiolysis in the shock-probe test. It is concluded that NPY produces anxiolytic-like actions in the lateral septum in two animal models of anxiety: the novelty-induced suppression of feeding, and shock-probe burying tests, and that this anxiolysis is dependent on Y1 receptor activation in the feeding test.

Therapeutic effects of propolis essential oil on anxiety of restraint-stressed mice

Propolis has a broad spectrum of biological activities; however, whether its essential oils have neuroprotective effects is unknown. In this study, we found that propolis essential oil (PEO) could significantly reverse the anxiety-like behavior of restraint-stressed mice, and has no effect on locomotor activity. Furthermore, PEO significantly decreased the plasma levels of cortisol (CORT), adrenocorticotropic hormone (ACTH) and malondialdehyde (MDA), whereas it increased the activity of superoxide dismutase (SOD) in restraint-stressed mice. These results strongly suggest that PEO has therapeutic effects on anxiety through antagonizing the hyperfunction of hypothalamic-pituitary-adrenal (HPA) axis and improving the ability of antioxidation in brain tissue.

Microinjection of histamine into the cerebellar vermis impairs emotional memory consolidation in mice

The biogenic amine histamine is an important neurotransmitter in the central nervous system that has been implicated in learning and memory processes. Experimental evidence indicates that the role of the cerebellum may be more complex than the simple regulation of motor responses, and recent studies have demonstrated significant involvement of the cerebellum in emotional memory consolidation. This study investigated the effect of histamine microinjected into the cerebellar vermis on emotional memory consolidation in mice in the elevated plus-maze (EPM). The cerebellar vermis of male mice (Swiss Albino) were implanted with guide cannulae. The mice weighed between 25 and 30 g. After three days of recovery, behavioral tests in the EPM were performed on two consecutive days; the testing periods were called, Trial 1 and Trial 2. Immediately after Trial 1, the animals received microinjections of histamine in the cerebellar vermis (0.54, 1.36, 2.72, and 4.07 nmol/0.1 μl). On both days, the test sessions were recorded to enable analysis of behavioral measures. The decrease in open arm exploration (% entries and % time spent in the open arms) in Trial 2 relative to Trial 1 was used as a measure of learning and memory. The data were analyzed using One-way Analysis of Variance (ANOVA) and Duncan's tests. The percentage of open arm entries (%OAE) and the percentage of time spent in the open arms (%OAT) were reduced in Trial 2 relative to Trial 1 for the control group; the same was true for the group that was microinjected with histamine at doses of 0.54 (%OAE and %OAT) and 1.36 nmol (%OAT). However, when the animals received histamine at doses of 2.72 and 4.07 nmol, their open arm exploration did not decrease. No significant changes were observed in the number of enclosed arm entries (EAE), an EPM index of general exploratory activity. These results suggest that there is a dose-dependent inhibitory effect of histamine microinjected into the cerebellar vermis on emotional memory consolidation.

Anti-aversive effects of the atypical antipsychotic, aripiprazole, in animal models of anxiety

Aripiprazole is a unique antipsychotic that seems to act as a partial agonist at dopamine D2-receptors, contrasting with other drugs in this class, which are silent antagonists. Aripiprazole may also bind to serotonin receptors. Both neurotransmitters may play major roles in aversion-, anxiety- and panic-related behaviours. Thus, the present work tested the hypothesis that this antipsychotic could also have anti-aversive properties. Male Wistar rats received injections of aripiprazole (0.1-10 mg/kg) and were tested in the open field, in the elevated plus and T mazes (EPM and ETM, respectively) and in a contextual fear conditioning paradigm. Aripiprazole (1 mg/kg) increased the percentage of entries onto the open arms of the EPM and attenuated escape responses in the ETM. In the latter model, the dose of 0.1 mg/kg also decreased the latency to leave the enclosed arm, suggesting anxiolytic- and panicolytic-like properties. This dose also decreased the time spent in freezing in a contextual fear conditioning. No significant motor effects were observed at these doses. The present data support the hypothesis that aripiprazole could inhibit anxiety-related responses. Acting as a partial agonist at dopamine receptors, this drug could effectively treat schizophrenia and, in contrast with most antipsychotic drugs, alleviate aversive states.

Central effects of a local inflammation in three commonly used mouse strains with a different anxious phenotype

As in humans, genetic background in rodents may influence a peculiar set of behavioural traits such as sensitivity to pain and stressors or anxiety-related behaviours. Therefore, we tested the hypothesis that mice with different genetic backgrounds [outbred (CD1), inbred (C57BL/6J) and hybrid (B6C3F1) adult male mice] display altered reactivity to pain, stress and anxiety related behaviours. We demonstrated that B6C3F1 mice displayed the more anxious phenotype with respect to C57BL/6J or CD1 animals, with the latter being the less anxious strain when tested in an open field and on an elevated plus maze. No difference was observed across strains in thermal sensitivity to a radiant heat source. Mice were then treated with a sub-plantar injection of the inflammatory agent Complete Freund's Adjuvant (CFA), 24h later they were hyperalgesic with respect to saline exposed animals, irrespective of strain. We then measured intra-strain differences and CFA-induced inter-strain effects on the expression of various genes with a recognized role in pain and anxiety: BDNF, IL-6, IL-1β, IL-18 and NMDA receptor subunits in the mouse thalamus, hippocampus and hypothalamus. The more anxious phenotype observed in B6C3F1 hybrid mice displayed lower levels of BDNF mRNA in the hippocampus and hypothalamus when compared to outbred CD1 and C57BL/6J inbred mice. CFA led to a general decrease in central gene expression of the evaluated targets especially in CD1 mice, while BDNF hypothalamic downregulation stands out as a common effect of CFA in all three strains evaluated.

Neurons in the amygdala with response-selectivity for anxiety in two ethologically based tests

The amygdala is a key area in the brain for detecting potential threats or dangers, and further mediating anxiety. However, the neuronal mechanisms of anxiety in the amygdala have not been well characterized. Here we report that in freely-behaving mice, a group of neurons in the basolateral amygdala (BLA) fires tonically under anxiety conditions in both open-field and elevated plus-maze tests. The firing patterns of these neurons displayed a characteristic slow onset and progressively increased firing rates. Specifically, these firing patterns were correlated to a gradual development of anxiety-like behaviors in the open-field test. Moreover, these neurons could be activated by any impoverished environment similar to an open-field; and introduction of both comfortable and uncomfortable stimuli temporarily suppressed the activity of these BLA neurons. Importantly, the excitability of these BLA neurons correlated well with levels of anxiety. These results demonstrate that this type of BLA neuron is likely to represent anxiety and/or emotional values of anxiety elicited by anxiogenic environmental stressors.

The effect of enriched environment on the outcome of traumatic brain injury; a behavioral, proteomics, and histological study

De novo hippocampal neurogenesis contributes to functional recovery following traumatic brain injury (TBI). Enriched environment (EEN) can improve the outcome of TBI by positively affecting neurogenesis. Blast induced traumatic brain injury (bTBI) characterized by memory impairment and increased anxiety levels, is a leading cause of chronic disability among soldiers. Using a rodent model of bTBI we asked: (a) whether long-term exposure to EEN after injury can ameliorate behavioral abnormalities and (b) what the effects of EEN are at the molecular and cellular levels and on de novo neurogenesis. We found that housing injured animals in EEN resulted in significantly improved spatial memory while animals in normal housing (NH) showed persistent memory impairment. VEGF and Tau protein but not Interleukin-6 (IL-6) levels were normalized in the dorsal hippocampus (DHC) of EEN rats while all three markers remained elevated in NH rats. Interestingly, after peaking at 6 weeks post-injury, anxiety returned to normal levels at 2 months independent of housing conditions. Housing animals in EEN had no significant effect on VEGF and Tau protein levels in the ventral hippocampus (VHC) and the amygdala (AD). We also found that EEN reduced IL-6 and IFNγ levels in the VHC; these markers remained elevated following NH. We observed an increase in GFAP and DCX immunoreactivities in the VHC of NH animals at 2 months post-injury. Conversely, injured animals housed in EEN showed no increase in GFAP or DCX immunoreactivity in their VHC. In summary, long-term exposure of injured animals to EEN appears to play a positive role in the restoration of memory functions but not on anxiety, which returned to normal levels after a significant period of time. Cellular and molecular changes in response to EEN appear to be a part of neurogenesis-independent as well as dependent recovery processes triggered by bTBI.

The ontogeny of anxiety-like behavior in rats from adolescence to adulthood

In human beings, susceptibility to anxiety disorders can be relatively high during adolescence. Understanding the ontogeny of anxiety-like behavior in laboratory rodents has implications for developing anxiolytic drugs that are suitable for this age group. Given the dearth of information about adolescent rodents, this study examined the response of both male and female adolescent, late adolescent, young adult, and older adult rats to three tests of anxiety-like behavior: the emergence test (ET), open field (OF), and elevated plus-maze (EPM). The results showed that adolescent rats exhibited a higher anxiety-like response than adults on each test; the amount of locomotion in the OF and percentage of time spent on the open arms of the EPM increased across the age groups, while older adult rats made the fewest start box re-entries in the ET. These results support the hypothesis that adolescent rats have a more pronounced response to stressors than do adults.

Decision-making performance is related to levels of anxiety and differential recruitment of frontostriatal areas in male rats

In humans, high levels of anxiety are associated with poor performance in the Iowa Gambling Task (IGT). The IGT measures decision-making under conditions of uncertainty. In this study, we investigated the association between anxiety and decision-making in rats. Rats were screened for anxiety on the elevated plus maze (EPM) and subsequently tested in a rat analogue of the IGT (r-IGT). We explored the role of frontostriatal areas related to r-IGT performance using c-fos immunohistochemistry following the last training-session. High levels of anxiety were associated with poor r-IGT performance: high anxious rats made fewer choices for the advantageous option and collected fewer sucrose pellets in the r-IGT than low anxious rats. Analysis of win-stay/lose-shift behaviour of choices for the advantageous option revealed that good performing-low anxious subjects showed an increase in win-stays and a decrease in lose-shifts across trial blocks while poor performing-high anxious subjects did not. Furthermore, decision-making performance and, indirectly, anxiety levels were related to neural activity in parts of the medial prefrontal cortex, that is prelimbic and infralimbic cortex, and in parts of the striatum, that is nucleus accumbens shell and core. These data suggest a similar frontostriatal circuitry underlying affective decision-making in humans and rats.

Interpreting the effects of exercise on fear conditioning: the influence of time of day

Previous studies indicate that physical exercise improves contextual fear memory, as evidenced by increased freezing behavior when rats are returned to a training environment that was initially paired with footshock. However, freezing behavior could also be affected by fatigue, especially because rats were tested shortly after the end of the dark cycle, which is when most wheel running was likely to occur. In addition, exercise has been shown to have anxiolytic effects, further confounding interpretation of the effects of exercise on cognition when using aversive conditioning tasks. These factors were examined in the present study by comparing freezing behavior in exercising and nonexercising rats that were tested at different times in the light cycle. In addition, all rats were tested on an elevated plus maze to assess anxiety-like behavior and in an open-field apparatus to measure locomotor activity in order to directly examine interactions between freezing, anxiety-like behavior, and locomotion. Consistent with prior studies, exercising rats exhibited more context freezing than did sedentary rats when tested early in the light cycle. However, the opposite pattern of results was obtained when testing occurred late in the light cycle, an effect driven by a difference in the amount of freezing exhibited by the sedentary control groups. Indeed, the levels of context freezing exhibited by exercising rats were comparable regardless of when the rats were tested during the light cycle. These data have implications for interpreting the effects of exercise on aversive conditioning.

Stress and traumatic brain injury: a behavioral, proteomics, and histological study

Psychological stress and traumatic brain injury (TBI) can both result in lasting neurobehavioral abnormalities. Post-traumatic stress disorder and blast induced TBI (bTBI) have become the most significant health issues in current military conflicts. Importantly, military bTBI virtually never occurs without stress. In this experiment, we assessed anxiety and spatial memory of rats at different time points after repeated exposure to stress alone or in combination with a single mild blast. At 2 months after injury or sham we analyzed the serum, prefrontal cortex (PFC), and hippocampus (HC) of all animals by proteomics and immunohistochemistry. Stressed sham animals showed an early increase in anxiety but no memory impairment at any measured time point. They had elevated levels of serum corticosterone (CORT) and hippocampal IL-6 but no other cellular or protein changes. Stressed injured animals had increased anxiety that returned to normal at 2 months and significant spatial memory impairment that lasted up to 2 months. They had elevated serum levels of CORT, CK-BB, NF-H, NSE, GFAP, and VEGF. Moreover, all of the measured protein markers were elevated in the HC and the PFC; rats had an increased number of TUNEL-positive cells in the HC and elevated GFAP and Iba1 immunoreactivity in the HC and the PFC. Our findings suggest that exposure to repeated stress alone causes a transient increase in anxiety and no significant memory impairment or cellular and molecular changes. In contrast, repeated stress and blast results in lasting behavioral, molecular, and cellular abnormalities characterized by memory impairment, neuronal and glial cell loss, inflammation, and gliosis. These findings may have implications in the development of diagnostic and therapeutic measures for conditions caused by stress or a combination of stress and bTBI.

Reduced anxiety-like behavior and central neurochemical change in germ-free mice

BACKGROUND

There is increasing interest in the gut-brain axis and the role intestinal microbiota may play in communication between these two systems. Acquisition of intestinal microbiota in the immediate postnatal period has a defining impact on the development and function of the gastrointestinal, immune, neuroendocrine and metabolic systems. For example, the presence of gut microbiota regulates the set point for hypothalamic-pituitary-adrenal (HPA) axis activity.

METHODS

We investigated basal behavior of adult germ-free (GF), Swiss Webster female mice in the elevated plus maze (EPM) and compared this to conventionally reared specific pathogen free (SPF) mice. Additionally, we measured brain mRNA expression of genes implicated in anxiety and stress-reactivity.

KEY RESULTS

Germ-free mice, compared to SPF mice, exhibited basal behavior in the EPM that can be interpreted as anxiolytic. Altered GF behavior was accompanied by a decrease in the N-methyl-D-aspartate receptor subunit NR2B mRNA expression in the central amygdala, increased brain-derived neurotrophic factor expression and decreased serotonin receptor 1A (5HT1A) expression in the dentate granule layer of the hippocampus.

CONCLUSIONS & INFERENCES

We conclude that the presence or absence of conventional intestinal microbiota influences the development of behavior, and is accompanied by neurochemical changes in the brain.

The anxiolytic-like effects of cannabidiol injected into the bed nucleus of the stria terminalis are mediated by 5-HT1A receptors

RATIONALE

Cannabidiol (CBD) is a non-psychotomimetic compound from Cannabis sativa that induces anxiolytic-like effects in rodents and humans after systemic administration. Previous results from our group showed that CBD injection into the bed nucleus of the stria terminalis (BNST) attenuates conditioned aversive responses. The aim of this study was to further investigate the role of this region on the anxiolytic effects of the CBD. Moreover, considering that CBD can activate 5-HT1A receptors, we also verified a possible involvement of these receptors in those effects.

METHODS

Male Wistar rats received injections of CBD (15, 30, or 60 nmol) into the BNST and were exposed to the elevated plus-maze (EPM) or to the Vogel conflict test (VCT), two widely used animal models of anxiety.

RESULTS

CBD increased open arms exploration in the EPM as well as the number of punished licks in the VCT, suggesting an anxiolytic-like effect. The drug did not change the number of entries into the enclosed arms of the EPM nor interfered with water consumption or nociceptive threshold, discarding potential confounding factors in the two tests. Moreover, pretreatment with the 5-HT1A receptor antagonist WAY100635 (0.37 nmol) blocked the effects of CBD in both models.

CONCLUSIONS

These results give further support to the proposal that BNST is involved in the anxiolytic-like effects of CBD observed after systemic administration, probably by facilitating local 5-HT1A receptor-mediated neurotransmission.

l-histidine induces state-dependent memory deficit in mice mediated by H(1) receptor

This study investigated the role of H(1) receptor in the state-dependent memory deficit induced by l-histidine (LH) in mice using Trial 1/2 protocol in the elevated plus-maze (EPM). The test was performed for two consecutive days: Trial 1 (T1) and Trial 2 (T2). Before both trials, mice received a combined injection i.p. of saline+saline (SAL/SAL), 500 mg/kg L-histidine+saline (LH/SAL), 500 mg/kg L-histidine+16 mg/kg chlorpheniramine (LH/CPA) or saline+16 mg/kg chlorpheniramine (SAL/CPA). The trials were performed in the EPM 10 min after the last injection. Each animal was placed in the center of the maze facing the open arm and had five minutes to explore it. On both days, test sessions were videotaped. The behavioral measures were scored from videotape. Data were analyzed based on Analysis of Variance (ANOVA) and the Fisher's LSD test. The data showed no effects on anxiety since there was no difference between the SAL/SAL and the other groups in Trial 1, respectively, open arm entries (OAE), open arm time (OAT) and their percentages (%OAE and %OAT). During Trial 2, OAE, OAT, %OAE and %OAT were reduced in mice treated with SAL/SAL, LH/CPA and SAL/CPA, while the group LH/SAL did not show any difference in these measures. No significant changes were observed in enclosed arm entries (EAE), an EPM index of general exploratory activity. Thus, it can be suggested that LH induces emotional memory deficit and the treatment with chlorpheniramine was able to revert this effect, suggesting this action of LH was mediated by the H(1) receptor.

Ventrolateral periaqueductal gray lesion attenuates nociception but does not change anxiety-like indices or fear-induced antinociception in mice

The exposure of rodents to an open elevated plus-maze (oEPM: four open arms raised from the floor) elicits naloxone-insensitive antinociception. Midazolam infusion into the dorsal portion of the periaqueductal gray (dPAG), a structure of the descending inhibitory system of pain, failed to alter oEPM-induced antinociception. Chemical lesion of dorsomedial and dorsolateral PAG attenuated defensive behavior in the standard EPM (sEPM), an animal model of anxiety, but failed to change oEPM-induced antinociception. The present study investigated the effects of bilateral lesion, with the injection of NMDA (N-methyl-D-aspartic acid), of the ventrolateral column of PAG (vlPAG) (i) on nociceptive response induced by 2.5% formalin injected into the right hind paw (nociception test) in mice exposed to the enclosed EPM (eEPM: four enclosed arms - a non-aversive situation) or to the oEPM and (ii) on anxiety indices in mice exposed to the sEPM without prior formalin injection. Results showed that oEPM-induced antinociception was not altered by lesion of vlPAG. Nevertheless, the lesion reduced the nociceptive response in mice exposed to the eEPM and increased general locomotor activity during the eEPM and oEPM exposure. Furthermore, vlPAG lesion did not alter anxiety-like indices in mice exposed to the sEPM. The results suggest that vlPAG does not play a role in oEPM-induced antinociception or in defensive reactions assessed in the sEPM. Moreover, vlPAG inactivation induces pain inhibition in mice not exposed to an aversive situation and seems to increase general activity.

Chemical composition of the essential oil from kelussia odoratissima Mozaff. and the evaluation of its sedative and anxiolytic effects in mice

OBJECTIVE

The present study aimed to evaluate the sedative and anxiolytic effects of the essential oils and hydroalcoholic extract of Kelussia odoratissima Mozaff. (K. odoratissima) in mice by utilizing an elevated plus maze. The chemical composition of its essential oil was also determined.

METHODS

The hydroalcoholic extract or essential oil fraction from this plant were administered intraperitoneally to male mice at various doses 30 min before testing. The anxiolytic and sedative effects were determined by an elevated plus maze and locomotor activity tests, respectively.

RESULTS

According to the results, none of the administered doses of hydroalcoholic extract or essential oil fraction of K. odoratissima changed the percentage of the time spent or number of entries into the open arms of the elevated plus maze. In contrast, the cumulative spontaneous locomotor activity of mice treated with the essential oil or hydroalcoholic extract was significantly decreased. Chemical analysis of the essential oil by Gas chromatography-mass spectromentry (GC-MS) showed that 3-butylidene-4,5-dihydrophthalide (85.9%) was the major component.

CONCLUSION

These data confirm the sedative properties of K. odoratissima, yet there were no profound anxiolytic effects observed.

Progressive metabolic and structural cerebral perturbations after traumatic brain injury: an in vivo imaging study in the rat

Traumatic brain injury (TBI) has a high incidence of long-term neurologic and neuropsychiatric morbidity. Metabolic and structural changes in rat brains were assessed after TBI using serial (18)F-FDG PET and 3-dimensional MRI in vivo.

METHODS

Rats underwent lateral fluid percussion injury (FPI; n = 16) or a sham procedure (n = 11). PET and MR images were acquired at 1 wk and at 1, 3, and 6 mo after injury. Morphologic changes were assessed using MRI-based regions of interest, and hippocampal shape changes were assessed with large-deformation high-dimensional mapping. Metabolic changes were assessed using region-of-interest analysis and statistical parametric mapping with the flexible factorial analysis. Anxiety-like behavior and learning were assessed at 1, 3, and 6 mo after injury.

RESULTS

PET analyses showed widespread hypometabolism in injured rats, in particular involving the ipsilateral cortex, hippocampus, and amygdalae, present at 1 wk after FPI, most prominent at 1 mo, and then decreasing. Compared with the sham group, rats in the FPI group had decreased structural volume which progressively increased over 3-6 mo, occurring in the ipsilateral cortex, hippocampus, and ventricles after FPI (P < 0.05). Large-deformation high-dimensional mapping showed evolving hippocampal shape changes across the 6 mo after FPI. Injured rats displayed increased anxiety-like behavior (P < 0.05), but there were no direct correlations between the severity of the behavior abnormalities and functional or structural imaging changes.

CONCLUSION

In selected brain structures, FPI induces early hypometabolism and delayed progressive atrophic changes that are dynamic and continue to evolve for months. These findings have implications for the understanding of the pathophysiology and evolution of long-term neurologic morbidity following TBI, and indicate an extended window for targeted neuroprotective interventions.

Chronic all-trans retinoic acid administration induced hyperactivity of HPA axis and behavioral changes in young rats

Although clinical reports suggest a possible relationship between excess retinoids and the development of depression, the effect of retinoids on mood-related behavior remains controversial. Hyperactivity of the hypothalamus-pituitary-adrenal (HPA) axis plays a key role in the development of affective disorders. The present study aimed to elucidate the effect of retinoid on the activity of HPA axis in rat and whether this goes together with behavioral changes. All-trans retinoic acid (ATRA) was administered to juvenile male rats by daily intraperitoneal injection for 6 weeks. ATRA treatment increased basal serum corticosterone concentration as well as the thickness of adrenal cortex in young rat. Furthermore, the mRNA expression of corticotropin release factor (CRF) and retinoic acid receptor-α (RAR-α) in the hypothalamus was both markedly increased in ATRA-treated rats compared with vehicle. Some behavioral alterations were also observed. ATRA-treated rats showed anxiety-like behavior in elevated-plus maze and decreased spontaneous exploratory activities in novel open field. However, in the sucrose preference test chronic ATRA treatment did not modify behavior in the juvenile animals. Chronic administration of ATRA did not impair physical motor ability in either the prehensile traction or the beam balance/walk test. In conclusion, long-term ATRA administration resulted in hyperactivated HPA axis which was accompanied by several behavioral changes in young rat.

Orexins in the paraventricular nucleus of the thalamus mediate anxiety-like responses in rats

RATIONALE

Anatomical studies have shown that the paraventricular nucleus of the thalamus (PVT) innervates areas of the forebrain involved in the expression and regulation of emotional behaviors including fear and anxiety. In addition, the PVT is densely innervated by fibers containing orexin-A (OXA) and orexin-B (OXB), peptides that are well-known for their arousal effects on behavior.

OBJECTIVES

In this study, we investigate whether microinjections of orexin receptor agonists and antagonists in the PVT region alter expression of anxiety-like behaviors in the rat as measured in the elevated plus maze.

RESULTS

We report that microinjections of OXA and OXB in the PVT region elicited anxiety-like response as indicated by a reduction in open arm time and entries. In addition, OXA and OXB produced changes in ethological measures indicative of an anxiety state. Central administrations of antagonists for corticotropin releasing factor (CRF) or the opioid kappa receptors attenuated the anxiogenic effects produced by microinjections of OXA in the PVT region. We also provide evidence that endogenously released orexins act at the PVT to produce anxiety by showing that microinjections of TCSOX229, an orexin-2 receptor antagonist, in the PVT region attenuated the anxiogenic effects produced by a previous exposure to footshock stress.

CONCLUSIONS

This study indicates that endogenously released orexins act on the PVT to regulate anxiety levels through mechanisms involving the brain kappa and CRF receptors.

Characterization of the rat exploratory behavior in the elevated plus-maze with Markov chains

The elevated plus-maze is an animal model of anxiety used to study the effect of different drugs on the behavior of the animal. It consists of a plus-shaped maze with two open and two closed arms elevated 50cm from the floor. The standard measures used to characterize exploratory behavior in the elevated plus-maze are the time spent and the number of entries in the open arms. In this work, we use Markov chains to characterize the exploratory behavior of the rat in the elevated plus-maze under three different conditions: normal and under the effects of anxiogenic and anxiolytic drugs. The spatial structure of the elevated plus-maze is divided into squares, which are associated with states of a Markov chain. By counting the frequencies of transitions between states during 5-min sessions in the elevated plus-maze, we constructed stochastic matrices for the three conditions studied. The stochastic matrices show specific patterns, which correspond to the observed behaviors of the rat under the three different conditions. For the control group, the stochastic matrix shows a clear preference for places in the closed arms. This preference is enhanced for the anxiogenic group. For the anxiolytic group, the stochastic matrix shows a pattern similar to a random walk. Our results suggest that Markov chains can be used together with the standard measures to characterize the rat behavior in the elevated plus-maze.