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

Effects of pretest manipulation on elevated plus-maze behavior in adolescent and adult male and female Sprague-Dawley rats

The elevated plus-maze (EPM) is vulnerable to variations in pretest circumstances when testing adult rodents. Because of an increasing interest in adolescence, the present experiments examined the impact of pretest manipulations on anxiety levels in the EPM among adolescent and adult Sprague-Dawley rats of both sexes. In Exp. 1, animals removed from their home cage and immediately placed on the EPM were compared to rats tested following 30 min of social isolation, or following 30-min exposure to a novel context. These pretest manipulations only modestly decreased anxiety levels at both ages. In Exp. 2, more varied pretest conditions were examined: testing directly from the home cage; testing following 30 min of social isolation in a novel environment; or a large saline injection and rehousing 18 h prior to a 30-min period of social isolation in a novelty situation before testing. In adults, anxiety levels decreased linearly as pretest perturbation increased, whereas adolescents showed comparable levels of anxiety with both the moderate and large perturbations. As a result, observed age differences in anxiety differed as a function of pretest circumstances. Therefore, caution is urged when using the EPM for across-age comparisons of anxiolytic and anxiogenic effects of pharmacological or other manipulations.

Removing obstacles in neuroscience drug discovery: the future path for animal models

Despite great advances in basic neuroscience knowledge, the improved understanding of brain functioning has not yet led to the introduction of truly novel pharmacological approaches to the treatment of central nervous system (CNS) disorders. This situation has been partly attributed to the difficulty of predicting efficacy in patients based on results from preclinical studies. To address these issues, this review critically discusses the traditional role of animal models in drug discovery, the difficulties encountered, and the reasons why this approach has led to suboptimal utilization of the information animal models provide. The discussion focuses on how animal models can contribute most effectively to translational medicine and drug discovery and the changes needed to increase the probability of achieving clinical benefit. Emphasis is placed on the need to improve the flow of information from the clinical/human domain to the preclinical domain and the benefits of using truly translational measures in both preclinical and clinical testing. Few would dispute the need to move away from the concept of modeling CNS diseases in their entirety using animals. However, the current emphasis on specific dimensions of psychopathology that can be objectively assessed in both clinical populations and animal models has not yet provided concrete examples of successful preclinical-clinical translation in CNS drug discovery. The purpose of this review is to strongly encourage ever more intensive clinical and preclinical interactions to ensure that basic science knowledge gained from improved animal models with good predictive and construct validity readily becomes available to the pharmaceutical industry and clinical researchers to benefit patients as quickly as possible.

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.

Pitfalls in the interpretation of genetic and pharmacological effects on anxiety-like behaviour in rodents

Over the last 15 years, genetically modified mice have added important data to our knowledge on psychiatric diseases including anxiety. This has produced many behavioural publications, partially by non-behaviourists, in which differences between mutants and normal wild-type animals were described. The popularity of these novel tools allowing the study of new mechanisms also, however, led to observations that could not be confirmed. This review attempts to summarize various factors that can lead to difficult and partially incorrect interpretation of data collected in anxiety-related paradigms. These pitfalls are explained by using virtual data. Our analysis illustrates that determining anxiety in rodents is more complicated than measuring a single parameter in a particular paradigm. It is important to use proper controls such as additional measures in the same or other procedures, as well as a conservative estimation of the chance of finding an actual effect. In this way, it is possible to enhance confidence in the findings. Alternative explanations for findings, like side effects or main effects in a different domain, such as cognition, should always be taken into account. Finally, several examples from the literature are presented as illustrations of the theoretical issues discussed. We believe that considering the pitfalls presented here will help researchers to design optimized experiments that can be more readily interpreted and replicated across laboratories.

Thigmotactic responses in an open-field

The goal of the present study was to investigate the role of thigmotaxis (the tendency to remain close to vertical surfaces) in rat exploratory behavior in an open-field. Thigmotaxis was investigated in a parametric way, using 24 experimentally adult naive male Wistar rats (210-230 g). Exploratory behavior was studied in an open-field (N = 12) in 5-min sessions and behavior was analyzed in terms of where it occurred: in areas surrounded by two, one, or no walls. Another group of rats (N = 12) was studied in an open-field with blocks placed near two of the corners so as to make these corner areas surrounded by three walls. The floor of the open-fields was divided into 20-cm squares in order to locate the exact place of occurrence of each behavior. The following behaviors were recorded: entries into the squares, rearings, and groomings. In both types of open-field the rats chose to remain longer in the squares surrounded by the largest possible number of walls. In one of the open-fields, the mean time (seconds) spent in squares surrounded by two walls was longer than the time spent in squares surrounded by one or no walls (37.2, 7.7, and 1.8 s, respectively). In the other open-field, the mean time spent in squares surrounded by three walls was longer than the time spent in squares surrounded by two, one or no walls (41.7, 20.4, 7.0, and 2.6 s, respectively). Other measures presented a similar profile. These results indicate that rats are sensitive to the number of walls in an environment and prefer to remain close to them.

Nesfatin-1 increases anxiety- and fear-related behaviors in the rat

RATIONALE

Nesfatin-1, derived from the protein NEFA/nucleobindin2 (NUCB2), is a newly identified peptide that acts as a potent satiety agent. It has been reported that peptides involved in the regulation of ingestive behavior are also involved in the regulation of the stress response. However, the relation between nesfatin-1 and stressor-related behaviors like anxiety and/or fear has not yet been investigated.

OBJECTIVE

The effects of intracerebroventricular (ICV) injection of nesfatin-1 (0, 5, and 25 pmol/3 microl) were assessed in several paradigms that are thought to reflect anxiety and/or fear in rats.

RESULTS

Consistent with an anxiogenic effect, nesfatin-1 dose-dependently decreased the percentage of time spent on the open arms of the elevated plus maze, increased latency to approach, and decreased consumption of a palatable snack in an anxiogenic (unfamiliar) environment. Moreover, ICV nesfatin-1 increased the fear-potentiated startle response and the time spent freezing to both context and conditioned cues in a conditioned emotional response test.

CONCLUSIONS

These findings suggest that in addition to its role as a satiety peptide, nesfatin-1 may also be involved in the mediation of anxiety- and/or fear-related responses.

A single early-life seizure impairs short-term memory but does not alter spatial learning, recognition memory, or anxiety

The impact of a single seizure on cognition remains controversial. We hypothesized that a single early-life seizure (sELS) on rat Postnatal Day (P) 7 would alter only hippocampus-dependent learning and memory in mature (P60) rats. The Morris water maze, the novel object and novel place recognition tasks, and contextual fear conditioning were used to assess learning and memory associated with hippocampus/prefrontal cortex, perirhinal/hippocampal cortex, and amygdala function, respectively. The elevated plus maze and open-field test were used to assess anxiety associated with the septum. We report that sELS impaired hippocampus-dependent short-term memory, but not spatial learning or recall. sELS did not disrupt performance in the novel object and novel place recognition tasks. Contextual fear conditioning performance suggested intact amydgala function. sELS did not change anxiety levels as measured by the elevated plus maze or open-field test. Our data suggest that the long-term cognitive impact of sELS is limited largely to the hippocampus/prefrontal cortex.

Do rats really express neophobia towards novel objects? Experimental evidence from exposure to novelty and to an object recognition task in an open space and an enclosed space

Three set of experiments were performed in an enclosed space (open-field) and in an open space (elevated platform). The surface of the open-field and the elevated platform were divided in nine equal squares. Rats were exposed (without previous habituation) in a unique session (experiment 1) or three consecutive sessions (experiment 2) either to an open-field (enclosed space) or to an elevated platform (open space) with and without an object on the centre of the field. In experiment 3, rats were exposed (without previous habituation) either to an enclosed or an open space on five consecutive sessions, one session a day. They were tested in an object recognition test in sessions 1, 3 and 5. In sessions 2 and 4, no objects were present. In experiment 1, we recorded the latency, frequency and duration of entries into different areas of the field. In experiment 3, we recorded the latency, frequency and duration of contacts with objects in addition to entries into different areas of the field. The first experiment demonstrates that rats exposed for the first time to an enclosed or an open space do not express neophobia toward novel objects in the absence of walls that surround an open-field. They crossed frequently into and spent more time in areas occupied with an object than in unoccupied areas. After two sessions of habituation to an empty open space or an empty enclosed space, the latency of first approach to a novel object is reduced while the frequency and duration of approaches are increased. The third experiment on object recognition confirmed that rats do not avoid novel objects; they made frequent visit and spent more time in the corner of the field occupied with an object than in empty corners. Recording of crossings provided detailed information about the patterns of exploratory behavior of rats but failed to reveal discrimination between novel and familiar objects which was evident in both open and enclosed space with recording of contacts with objects on the fifth exposure.

An endocannabinoid signaling system modulates anxiety-like behavior in male Syrian hamsters

RATIONALE

An endocannabinoid signaling system has not been identified in hamsters.

OBJECTIVE

We examined the existence of an endocannabinoid signaling system in Syrian hamsters using neuroanatomical, biochemical, and behavioral pharmacological approaches.

MATERIALS AND METHODS

The distribution of cannabinoid receptors was mapped, and membrane fatty-acid amide hydrolase (FAAH) activity and levels of fatty-acid amides were measured in hamster brain. The impact of cannabinoid CB1 receptor blockade and inhibition of FAAH was evaluated in the elevated plus maze, rota-rod test, and models of unconditioned and conditioned social defeat.

RESULTS

A characteristic heterogeneous distribution of cannabinoid receptors was detected in hamster brain using [3H]CP55,940 binding and autoradiography. The FAAH inhibitor URB597 inhibited FAAH activity (IC50 = 12.8 nM) and elevated levels of fatty-acid amides (N-palmitoyl ethanolamine and N-oleoyl ethanolamine) in hamster brain. Anandamide levels were not reliably altered. The cannabinoid agonist WIN55,212-2 (1- 10 mg/kg i.p.) induced CB1-mediated motor ataxia. Blockade of CB1 with rimonabant (5 mg/kg i.p.) induced anxiogenic-like behavior in the elevated plus maze. URB597 (0.1-0.3 mg/kg i.p.) induced CB1-mediated anxiolytic-like effects in the elevated plus maze, similar to the benzodiazepine diazepam (2 mg/kg i.p.). Diazepam (2-6 mg/kg i.p.) suppressed the expression, but not the acquisition, of conditioned defeat. By contrast, neither URB597 (0.3-3.0 mg/kg i.p.) nor rimonabant (5 mg/kg i.p.) altered unconditioned or conditioned social defeat or rota-rod performance.

CONCLUSIONS

Endocannabinoids engage functional CB1 receptors in hamster brain to suppress anxiety-like behavior and undergo enzymatic hydrolysis catalyzed by FAAH. Our results further suggest that neither unconditioned nor conditioned social defeat in the Syrian hamster is dependent upon cannabinoid CB1 receptor activation.

Attenuation of anxiety-related behaviour after the antagonism of transient receptor potential vanilloid type 1 channels in the rat ventral hippocampus

The neurochemical mechanisms supporting the role of the ventral hippocampus in anxiety are now under investigation. To address this issue, we examined whether the pharmacological blockade of transient receptor potential vanilloid type 1 (TRPV1) channels in the ventral hippocampus would attenuate anxiety-related behaviour. Rats infused with the TRPV1 channel antagonist capsazepine (2.0 nmol) showed less inhibitory avoidance than controls in the elevated plus-maze test. This result indicates an anxiolytic-like effect, and suggests a role for TRPV1 channels in regulating anxiety.

Animal models of anxiety: do I need multiple tests?

The combination of cutting-edge molecular technology and high-throughput phenotyping tools will not bring the expected contribution to the pre-clinical study of anxiety if not paralleled by an increase in our capacity to interpret behavioral data. Here, previous views about the multidimensional nature of emotional behaviors will be expanded and the psychological meaning and behavioral overlaps of widely used anxiety tests such as the open field, elevated plus maze and light-dark box will be discussed. It is proposed here that short-term, intra-individual variations in emotionality, although normally overlooked, constitute an important factor in the study of anxiety and can lead to unreliable estimates of the similarities between tests. The physical integration of different current tests in one single apparatus, in such a way that the emotional status of an animal becomes assessable through a series of distinct tasks, could contribute to increase reliability, rapidity and comprehensiveness in behavioral testing.

Involvement of 5HT1A receptors in the anxiolytic-like effects of cannabidiol injected into the dorsolateral periaqueductal gray of rats

RATIONALE

Cannabidiol (CBD) is a non-psychotomimetic constituent of Cannabis sativa plant that induces anxiolytic effects. However, the brain sites and mechanisms of these effects remain poorly understood. The dorsolateral periaqueductal gray (dlPAG) is a midbrain structure related to anxiety that contains receptors proposed to interact with CBD such as 5HT1A. In addition, since CBD has been shown to inhibit anandamide metabolism, CB1 receptors could also be involved in the effects of this cannabinoid.

OBJECTIVES

To investigate if the dlPAG could be a possible site of the anxiolytic effects induced by CBD and if these effects depend on CB1 or 5HT1A receptors.

MATERIALS AND METHODS

Male Wistar rats with cannulae aimed at the dlPAG were tested in the elevated plus maze (EPM) and the Vogel conflict test (VCT).

RESULTS

CBD injected into the dlPAG produced anxiolytic-like effects in the EPM with a bell-shaped dose-response curve. The anxiolytic effect of CBD was confirmed in the VCT. These effects were prevented by WAY100635, a 5HT1A receptor antagonist, but not by AM251, an antagonist of CB1 receptors.

CONCLUSION

These results suggest the CBD interacts with 5HT1A receptors to produce anxiolytic effects in the dlPAG.

The effects of intra-cerebral drug infusions on animals' unconditioned fear reactions: a systematic review

Intra-cerebral (i.c.) microinfusion of selective receptor agonists and antagonists into behaving animals can provide both neuroanatomical and neurochemical insights into the neural mechanisms of anxiety. However, there have been no systematic reviews of the results of this experimental approach that include both a range of unconditioned anxiety reactions and a sufficiently broad theoretical context. Here we focus on amino acid, monoamine, cholinergic and peptidergic receptor ligands microinfused into neural structures previously implicated in anxiety, and subsequent behavioral effects in animal models of unconditioned anxiety or fear. GABAA receptor agonists and glutamate receptor antagonists produced the most robust anxiolytic-like behavioral effects, in the majority of neural substrates and animal models. In contrast, ligands of the other receptor systems had more selective, site-specific anti-anxiety effects. For example, 5-HT1A receptor agonists produced anxiolytic-like effects in the raphe nuclei, but inconsistent effects in the amygdala, septum, and hippocampus. Conversely, 5-HT3 receptor antagonists produced anxiolytic-like effects in the amygdala but not in the raphe nuclei. Nicotinic receptor agonists produced anxiolytic-like effects in the raphe and anxiogenic effects in the septum and hippocampus. Unexpectedly, physostigmine, a general cholinergic agonist, produced anxiolytic-like effects in the hippocampus. Neuropeptide receptors, although they are popular targets for the development of selective anxiolytic agents, had the least reliable effects across different animal models and brain structures, perhaps due in part to the fact that selective receptor ligands are relatively scarce. While some inconsistencies in the microinfusion data can easily be attributed to pharmacological variables such as dose or ligand selectivity, in other instances pharmacological explanations are more difficult to invoke: e.g., even the same dose of a known anxiolytic compound (midazolam) with a known mechanism of action (the benzodiazepine-GABAA receptor complex), can selectively affect different fear reactions depending upon the different subregions of the nucleus into which it is infused (CeA versus BLA). These particular functional dissociations are important and may depend on the ability of a GABAA receptor agonist to interact with distinct isoforms and combinations of GABAA receptor subunits (e.g., alpha1-6, beta1-3, Upsilon1-2, delta), many of which are unevenly distributed throughout the brain. Although this molecular hypothesis awaits thorough evaluation, the microinfusion data overall give some support for a model of "anxiety" that is functionally segregated along different levels of a neural hierarchy, analogous in some ways to the organization of sensorimotor systems.

Nesfatin-1 increases anxiety- and fear-related behaviors in the rat

RATIONALE

Nesfatin-1, derived from the protein NEFA/nucleobindin2 (NUCB2), is a newly identified peptide that acts as a potent satiety agent. It has been reported that peptides involved in the regulation of ingestive behavior are also involved in the regulation of the stress response. However, the relation between nesfatin-1 and stressor-related behaviors like anxiety and/or fear has not yet been investigated.

OBJECTIVE

The effects of intracerebroventricular (ICV) injection of nesfatin-1 (0, 5, and 25 pmol/3 microl) were assessed in several paradigms that are thought to reflect anxiety and/or fear in rats.

RESULTS

Consistent with an anxiogenic effect, nesfatin-1 dose-dependently decreased the percentage of time spent on the open arms of the elevated plus maze, increased latency to approach, and decreased consumption of a palatable snack in an anxiogenic (unfamiliar) environment. Moreover, ICV nesfatin-1 increased the fear-potentiated startle response and the time spent freezing to both context and conditioned cues in a conditioned emotional response test.

CONCLUSIONS

These findings suggest that in addition to its role as a satiety peptide, nesfatin-1 may also be involved in the mediation of anxiety- and/or fear-related responses.

Age related differences in stress-induced neurobehavioral responses in rats: modulation by antioxidants and nitrergic agents

The effect of restraint stress (RS) on neurobehavioral and brain oxidative/nitrosative stress markers and their modulation by antioxidants and nitrergic agents were evaluated in young (2 months) and old (16 months) male Wistar rats. Exposure to RS, induced anxiogenesis when tested in the elevated plus maze (EPM) and open field (OF) tests and such changes were greater in the old as compared to the young rats. These behavioral alterations were associated with enhanced levels of malondialdehyde (MDA) and reductions in glutathione (GSH), catalase (CAT) and nitric oxide metabolites (NOx) levels in brain homogenates-the effects being greater in intensity in the old as compared to the young animals. Pretreatment with antioxidants, alpha-tocopherol (25 and 50mg/kg) and N-acetylcysteine (100 and 200mg/kg) consistently reversed the RS-induced behavioral and biochemical alterations in both young and old rats. Similar attenuations of RS-induced changes were seen after pretreatment with NO precursor L-arginine (500 and 1000mg/kg) while the NO synthase inhibitor N-nitro L-arginine methyl ester (L-NAME) (50 and 100mg/kg) tended to aggravate the effects of RS in both age groups of rats. The results suggest that susceptibility to stress-induced neurobehavioral alterations may increase with age and interactions of reactive oxygen species (ROS) and nitric oxide in the central nervous system may exert a regulatory influence in such age dependent responses to stress.

Anxiety-like behaviour in rats with mononeuropathy is reduced by the analgesic drugs morphine and gabapentin

Anxiety has been described as an important comorbidity in patients suffering from chronic pain. However, in animals the connection between persistent pain and anxiety has hardly been investigated. Therefore, in the current study it was assessed whether chronic pain also causes anxiety-like behaviour in animals and if it can be reversed by analgesic or anxiolytic drugs. Neuropathic pain was induced in rats by partial sciatic nerve ligation (PNL) and chronic constriction injury (CCI). Mechanical hypersensitivity was assessed by the "electronic algometer", while anxiety-like behaviour was measured by using the elevated plus maze. In both neuropathic pain models, rats exhibited mechanical hypersensitivity, whereas a significant increase in anxiety-like behaviour was observed only in CCI rats (time spent in open arms decreased significantly from 99+/-15.8s in sham animals to 33.4+/-7.5s in CCI animals). Furthermore, midazolam (0.5mg/kg; i.p.) significantly reduced anxiety-like behaviour in both sham- and CCI-operated animals without influencing mechanical hypersensitivity. Morphine (3mg/kg; i.p.) and gabapentin (30 mg/kg; i.p.) significantly attenuated anxiety-like behaviour in the CCI lesioned rats: morphine increased entries into open arms from 3.0+/-0.4 to 7.7+/-1.4 (P=0.01), gabapentin elevated this value from 4.7+/-1 to 7.5+/-0.9 (P=0.02). These data suggest that rats subjected to neuropathic pain models develop anxiety-like behaviour which can be reversed by appropriate analgesic treatment. Morphine and gabapentin had no anxiolytic-like effect in sham treated animals, thus their effect on anxiety-like behaviour in the neuropathic pain model is likely indirect via their anti-nociceptive properties.

Integrating the open field, elevated plus maze and light/dark box to assess different types of emotional behaviors in one single trial

Current anxiety tests do not provide, individually, a pure and complete picture of an animal's emotional profile. Therefore, many authors test their experimental hypotheses using a series of anxiety-related tests, which are thought to reflect different facets of emotionality. The objective of this study was to investigate the potential usefulness of integrating three widely used behavioral tests, the open field (OF), elevated plus maze (EPM), and light/dark box (LDB), to assess a wider range of emotional and non-emotional behaviors within one single trial. A protocol was developed where rats could freely explore an OF that was physically connected to an EPM and a LDB during 15min. Classical anxiety- and locomotion-related behaviors from each test were measured. Lewis and spontaneously hypertensive rats (SHR) inbred strains, known to present genetic differences in each of the individual tests, differed for all anxiety-related behaviors from the combined apparatus. Factor analyses revealed that similar anxiety- and locomotion-related factors were produced by the three tests applied either separately or in combination. Under both conditions, each test produced its own anxiety-related factor. Two benzodiazepines, chlordiazepoxide (at 5 and 10mg/kg) and midazolam (at 0.75mg/kg), facilitated the approach towards the EPM open arms, whereas pentylenetetrazole (10mg/kg) specifically inhibited exploration of the three aversive areas (OF center, EPM open arms, LDB light compartment). Together, these results suggest that the new integrated apparatus may contribute to the study of anxiety, by providing a rapid, comprehensive and reliable method of assessing emotionality-related behaviors and its underlying components.

Persistent alterations in heart rate variability, baroreflex sensitivity, and anxiety-like behaviors during development of heart failure in the rat

Depressed heart rate variability and mood are associated with increased mortality in patients with congestive heart failure (CHF). Here autonomic indexes were assessed 3 and 7 wk after left coronary artery ligation in telemetered rats, after which anxiety-like behaviors were assessed in an elevated plus maze. Low frequency (LF) and high frequency (HF) heart rate variability were reduced in CHF rats 3 wk after infarction (LF, 1.60 +/- 0.52 vs. 6.97 +/- 0.79 ms(2); and HF, 1.53 +/- 0.39 vs. 6.20 +/- 1.01 ms(2); P < 0.01). The number of sequences of interbeat intervals that correlated with arterial pressure was decreased in CHF rats at 3 and 7 wk (week 3, 26.60 +/- 10.85 vs. 59.75 +/- 11.4 sequences, P < 0.05; and week 7, 20.80 +/- 8.97 vs. 65.38 +/- 5.89 sequences, P < 0.01). Sequence gain was attenuated in CHF rats by 7 wk (1.34 +/- 0.06 vs. 2.70 +/- 0.29 ms/mmHg, P < 0.01). Coherence between interbeat interval and mean arterial blood pressure variability in the LF domain was reduced in CHF rats at 3 (0.12 +/- 0.03 vs. 0.26 +/- 0.05 k(2), P < 0.05) and 7 (0.16 +/- 0.02 vs. 0.31 +/- 0.05 k(2), P < 0.05) wk. CHF rats invariably entered the open arm of the elevated plus maze first and spent more time in the open arms (36.0 +/- 15% vs. 4.6 +/- 1.9%, P < 0.05). CHF rats also showed a tendency to jump head first off the apparatus, whereas controls did not. Together the data indicate that severe autonomic dysfunction is accompanied by escape-seeking behaviors in rats with verified CHF.

Analysis of the anxiolytic-like effect of TRH and the response of amygdalar TRHergic neurons in anxiety

Thyrotropin-releasing hormone (TRH) was first described for its neuroendocrine role in controlling the hypothalamus-pituitary-thyroid axis (HPT). Anatomical and pharmacological data evidence its participation as a neuromodulator in the central nervous system. Administration of TRH induces various behavioural effects including arousal, locomotion, analepsy, and in certain paradigms, it reduces fear behaviours. In this work we studied the possible involvement of TRHergic neurons in anxiety tests. We first tested whether an ICV injection of TRH had behavioural effects on anxiety in the defensive burying test (DBT). Corticosterone serum levels were quantified to evaluate the stress response and, the activity of the HPT axis to distinguish the endocrine response of TRH injection. Compared to a saline injection, TRH reduced cumulative burying, and decreased serum corticosterone levels, supporting anxiolytic-like effects of TRH administration. The response of TRH neurons was evaluated in brain regions involved in the stress circuitry of animals submitted to the DBT and to the elevated plus maze (EPM), tests that allow to correlate biochemical parameters with anxiety-like behaviour. In the DBT, the response of Wistar rats was compared with that of the stress-hypersensitive Wistar Kyoto (WKY) strain. Behavioural parameters were analysed in recorded videos. Animals were sacrificed 30 or 60min after test completion. In various limbic areas, the relative mRNA levels of TRH, its receptors TRH-R1 and -R2, and its inactivating ectoenzyme pyroglutamyl peptidase II (PPII), were determined by RT-PCR, TRH tissue content by radioimmunoassay (RIA). The extent of the stress response was evaluated by measuring the expression profile of CRH, CRH-R1 and GR mRNA in the paraventricular nucleus (PVN) of the hypothalamus and in amygdala, corticosterone levels in serum. As these tests demand increased physical activity, the response of the HPT axis was also evaluated. Both tasks increased the levels of serum corticosterone. WKY rats showed higher anxiety-like behaviour in the DBT than Wistar, as well as increased PVN mRNA levels of CRH and GR. TRH mRNA levels increased in the PVN and TSH values remained unchanged in both strains although TRH content decreased in the medial basal hypothalamus of Wistar rats only. TRH content was measured in several limbic regions but only amygdala showed specific task-related changes after DBT exposure of both strains: increased TRH content. Expression of TRH mRNA decreased in the amygdala of Wistar, suggesting inhibition of TRHergic neuronal activity in this region. The participation of amygdalar TRH neurons in anxiety was confirmed in the EPM where TRH expression and release correlated with the number of entries, and the % of time spent in open arms, supporting an anxiolytic role of these TRH-neurons. These results contribute to the understanding of the involvement of TRH during emotionally charged situations and shed light on the participation of particular circuits in related behaviours.

Combined exposure to nicotine and ethanol in adolescent mice differentially affects anxiety levels during exposure, short-term, and long-term withdrawal

Smoking and consumption of alcoholic beverages are frequently associated during adolescence. This association could be explained by the cumulative behavioral effects of nicotine and ethanol, particularly those related to anxiety levels. However, despite epidemiological findings, there have been few animal studies of the basic neurobiology of the combined exposure in the adolescent brain. In the present work we assessed, through the use of the elevated plus maze, the short- and long-term anxiety effects of nicotine (NIC) and/or ethanol (ETOH) exposure during adolescence (from the 30th to the 45th postnatal day) in four groups of male and female C57BL/6 mice: (1) Concomitant NIC (nicotine free-base solution (50 microg/ml) in 2% saccharin to drink) and ETOH (ethanol solution (25%, 2 g/kg) i.p. injected every other day) exposure; (2) NIC exposure; (3) ETOH exposure; (4) Vehicle. C57BL/6 mice were selected, in spite of the fact that they present slower ethanol metabolism, because they readily consume nicotine in the concentration used in the present study. During exposure (45th postnatal day: PN45), our results indicated that ethanol was anxiolytic in adolescent mice and that nicotine reverted this effect. Short-term drug withdrawal (PN50) elicited sex-dependent effects: exposure to nicotine and/or ethanol was anxiogenic only for females. Although neither nicotine nor ethanol effects persisted up to 1 month postexposure (PN75), the coadministration elicited an anxiogenic response. In spite of the fact that generalizations based on the results from a single strain of mice are prone to shortcomings, our results suggest that the deficient response to the anxiolytic effects of ethanol in adolescents co-exposed to nicotine may drive higher ethanol consumption. Additionally, increased anxiety during long-term smoking and drinking withdrawal may facilitate relapse to drug use.

Role of GABA in anxiety and depression

This review assesses the parallel data on the role of gamma-aminobutyric acid (GABA) in depression and anxiety. We review historical and new data from both animal and human experimentation which have helped define the key role for this transmitter in both these mental pathologies. By exploring the overlap in these conditions in terms of GABAergic neurochemistry, neurogenetics, brain circuitry, and pharmacology, we develop a theory that the two conditions are intrinsically interrelated. The role of GABAergic agents in demonstrating this interrelationship and in pointing the way to future research is discussed.

Modulation of defensive responses and anxiety-like behaviors by group I metabotropic glutamate receptors located in the dorsolateral periaqueductal gray

Glutamatergic neurotransmission in the dorsolateral periaqueductal gray (dlPAG) is related to defensive responses. However, the role of group I glutamate metabotropic receptors (mGluR) in these responses has been poorly investigated. The objective of the present study, therefore, was to test the hypothesis that interference with group I mGluR-mediated neurotransmission in dlPAG could modulate defensive responses. Male Wistar rats with cannulae aimed at the dlPAG were submitted to the following experiments: 1. intra dlPAG injections of vehicle (veh, 0.2 microL) or (RS)1-aminoindan-1,5-dicarboxylic acid (AIDA, 30-100 nmol, an mGluR1 receptor competitive antagonist) followed, 5 min later, by veh or trans-(+)-1-amino-1,3-ciclopentanedicarboxylic acid (tACPD, a group I and II mGluR agonist, 30 nmol); 2. intra-dlPAG injections of veh, AIDA (30 nmol) or 2-methyl-6-(phenylethynyl)-pyridine (MPEP, an mGluR5 receptor non-competitive antagonist, 50 nmol) followed by trans-azetidine-2,4-dicarboxylic acid (tADA, a group I mGluR agonist, 10 nmol); 3. and 4. intra-dlPAG injections of vehicle, AIDA (10-30 nmol) or MPEP (10-50 nmol) before the elevated plus maze (EPM) test; 5. intra-dlPAG injections of vehicle, AIDA (30 nmol) or MPEP (50 nmol) before the Vogel punished licking test. tACPD induced defensive responses characterized by jumps and an increased number of crossings in the observation box. These responses were attenuated by AIDA (30 nmol). tADA produced similar responses, although of lower intensity. tADA effects were prevented by AIDA and MPEP. Both drugs also produced anxiolytic-like effects in the EPM and Vogel tests when injected alone. The results suggest that group I metabotropic glutamate receptors in the dlPAG facilitate defensive responses and may also be involved in regulating anxiety-like behavior.

Reduced anxiety-like behaviour induced by genetic and pharmacological inhibition of the endocannabinoid-degrading enzyme fatty acid amide hydrolase (FAAH) is mediated by CB1 receptors

Anandamide and 2-arachidonoyl glycerol, referred to as endocannabinoids (eCBs), are the endogenous agonists for the cannabinoid receptor type 1 (CB1). Several pieces of evidence support a role for eCBs in the attenuation of anxiety-related behaviours, although the precise mechanism has remained uncertain. The fatty acid amid hydrolase (FAAH), an enzyme responsible for the degradation of eCBs, has emerged as a promising target for anxiety-related disorders, since FAAH inhibitors are able to increase the levels of anandamide and thereby induce anxiolytic-like effects in rodents. The present study adopted both genetic and pharmacological approaches and tested the hypothesis that FAAH-deficient (FAAH(-/-)) mice as well as C57BL/6N mice treated with an FAAH inhibitor (URB597) would express reduced anxiety-like responses. Furthermore, as it is known that anandamide can bind several other targets than CB1 receptors, we investigated whether FAAH inhibition reduces anxiety via CB1 receptors. FAAH(-/-) mice showed reduced anxiety both in the elevated plus maze and in the light-dark test. These genotype-related differences were prevented by the CB1 receptor antagonist rimonabant (3mg/kg). Moreover, URB597 (1mg/kg) induced an anxiolytic-like effect in C57BL/6N mice exposed to the elevated plus maze, which was prevented by rimonabant (3mg/kg). The present work provides genetic and pharmacological evidence supporting the inhibition of FAAH as an important mechanism for the alleviation of anxiety. In addition, it indicates an increased activation of CB1 receptors as a mechanism underlying the effects of FAAH inhibition in two models of anxiety.

Social isolation in prairie voles induces behaviors relevant to negative affect: toward the development of a rodent model focused on co-occurring depression and anxiety

Recent evidence suggests substantial overlap between mood and anxiety disorders, both in clinical presentation and associated features. A theoretical framework to account for this overlap focuses on negative affectivity, defined as the disposition to experience negative emotional states, including fear, sadness, and guilt. This model has been successful in explaining the co-occurrence of depressive and anxiety disorders in humans. As a next step, development of an animal model focused on both depression- and anxiety-relevant behaviors may advance understanding of depression-anxiety symptom overlap, relations of these disorders with associated medical conditions and responses to treatment. This study was designed to investigate inducible and quantifiable depression- and anxiety-like behaviors in prairie voles (Microtus ochrogaster). Adult, female prairie voles were exposed to 4 weeks of social pairing (control) or isolation, an established stressor for socially monogamous mammals (including humans). Operational measures of depression (sucrose intake and behaviors in the forced swim test), anxiety (behaviors in the elevated plus maze), and aggression (responses to an unrelated prairie vole pup) were investigated. Social isolation induced a progressive decline in sucrose intake and increased immobility time during the forced swim test. Social isolation also decreased the amount of time spent in the open arms of the elevated plus maze, and increased pup-directed attack behavior. The current findings suggest that isolation induces behaviors reflecting elevated negative affect. These results may provide a foundation for creating a rodent model to examine the mechanisms underlying comorbid mood and anxiety disorders.

Behavioral and neurochemical consequences of multiple MDMA administrations in the rat: role of individual differences in anxiety-related behavior

Using the elevated plus-maze (EPM), Wistar rats can be distinguished into high (HA) or low anxiety (LA) subjects. These differences seem to reflect traits, since HA and LA rats vary also in other anxiety-dependent tasks, neurochemical mechanisms, and psychopharmacological reactivity, including lasting consequences after single treatment with 3,4-methylenedioxymethamphetamine (MDMA). Here, we tested whether multiple MDMA treatments also have subject-dependent effects. Based on routine EPM screening, male Wistar rats were divided into HA and LA sub-groups, which received five (i.e. multiple) daily injections of MDMA (5 mg/kg) or saline, followed by a test battery, including a challenge test with MDMA, a retest in the EPM, a novel-object test, and a final neurochemical analysis. Acutely, MDMA led to comparable hyperactivity in HA and LA rats. After multiple MDMA, behavioral sensitization was observed, especially in LA rats. Open arm time during the EPM retest (min 0-5) correlated with that of the initial one only in those rats, which had received a single injection of MDMA. Rats with multiple MDMA, especially LA-rats, showed more open-arm time and locomotion during the subsequent 5-10 min of the retest. In a novel-object test, rats with multiple MDMA, again especially LA subjects, showed more exploratory bouts towards the novel object. Neurochemically, multiple MDMA led to moderately lower serotonin in the ventral striatum, and higher dopamine levels in the frontal cortex as compared to single MDMA; these effects were also moderated by subject-dependent factors. Our data show that low-dosed multiple MDMA can lead to behavioral sensitization and outlasting consequences, which affect behavior in the EPM and a novel object task. Detecting such sequels partly requires consideration of individual differences.

Anxiolytic effects induced by inhibition of the nitric oxide-cGMP pathway in the rat dorsal hippocampus

RATIONALE

Conflicting results have been reported regarding the role of the nitric oxide (NO)-cyclic guanosine monophosphate (cGMP) pathway in the hippocampus on anxiety modulation.

OBJECTIVES

To investigate the effects of intrahippocampal injections of drugs that modify the NO-cGMP pathway in rats submitted to two animal models that are sensitive to anxiolytic drugs, the elevated plus-maze and the Vogel punished licking test.

MATERIALS AND METHODS

Male Wistar rats with cannulae aimed at the dentate gyrus of the dorsal hippocampus received microinjections of the NO synthase (NOS) inhibitors N (G)-nitro-L: -arginine methyl ester (LNAME, 15-300 nmol/0.2 microl), N (G)-nitro-L: -arginine (LNOARG, 50-300 nmol/0.2 microl), 7-nitroindazole (7NI, 10-100 nmol/0.2 microl), or the soluble guanylate cyclase inhibitor 1H-oxadiazolo-quinoxalin-1 one (ODQ, 10-100 nmol/0.2 microl), and were submitted to the elevated plus-maze. In a second group, the animals received 7NI, LNAME, or ODQ and were submitted to the Vogel punished licking test. To control for drug-induced changes in locomotor behavior, the animals were submitted to an open arena or to the Rota-rod test.

RESULTS

All drugs increased the exploration of the open arms of the elevated plus-maze. They also increased the number of punished licks in the Vogel test, indicating an anxiolytic effect. The anxiolytic effect of LNAME was prevented by previous treatment with L: -arginine (300 nmol/0.2 microl). Except for the lower dose of LNAME (15 nmol), administration of the NOS inhibitors or ODQ did not change exploratory activity in the open field nor cause any gross locomotor impairment in the Rota-rod test.

CONCLUSION

The results suggest that NO plays an anxiogenic role in the dentate gyrus of the dorsal hippocampus.

Targeting ASIC1a reduces innate fear and alters neuronal activity in the fear circuit

BACKGROUND

The molecular mechanisms underlying innate fear are poorly understood. Previous studies indicated that the acid sensing ion channel ASIC1a influences fear behavior in conditioning paradigms. However, these differences may have resulted from an ASIC1a effect on learning, memory, or the expression of fear.

METHODS

To test the hypothesis that ASIC1a influences the expression of fear or anxiety independent of classical conditioning, we examined the effects of disrupting the mouse ASIC1a gene on unconditioned fear in the open field test, unconditioned acoustic startle, and fear evoked by the predator odor trimethylthiazoline (TMT). In addition, we tested the effects of acutely inhibiting ASIC1a with PcTx, an ASIC1a antagonist in tarantula venom. Our immunohistochemistry suggested ASIC1a is expressed in the bed nucleus of the stria terminalis, medial amygdala, and periaqueductal gray, which are thought to play important roles in the generation and expression of innate fear. Therefore, we also tested whether ASIC1a disruption altered c-fos expression in these structures following TMT exposure.

RESULTS

We found that the loss of ASIC1a reduced fear in the open field test, reduced acoustic startle, and inhibited the fear response to TMT. Similarly, intracerebroventricular administration of PcTx reduced TMT-evoked freezing in ASIC1a(+/+) mice but not ASIC1a(-/-) mice. In addition, loss of ASIC1a altered TMT-evoked c-fos expression in the medial amydala and dorsal periaqueductal gray.

CONCLUSIONS

These findings suggest that ASIC1a modulates activity in the circuits underlying innate fear. Furthermore, the data indicate that targeting the ASIC1a gene or acutely inhibiting ASIC1a suppresses fear and anxiety independent of conditioning.

Social isolation disrupts autonomic regulation of the heart and influences negative affective behaviors

BACKGROUND

There is a documented association between affective disorders (e.g., depression and anxiety) and cardiovascular disease in humans. Chronic social stressors may play a mechanistic role in the development of behavioral and cardiac dysregulation. The current study investigated behavioral, cardiac, and autonomic responses to a chronic social stressor in prairie voles, a rodent species that displays social behaviors similar to humans.

METHODS

Female prairie voles were exposed to 4 weeks of social isolation (n = 8) or pairing (control conditions; n = 7). Electrocardiographic parameters were recorded continuously during isolation, and behavioral tests were conducted during and following this period.

RESULTS

Isolation induced a significant increase in resting heart rate, reduction in heart rate variability (standard deviation of normal-to-normal intervals and amplitude of respiratory sinus arrhythmia), and exaggerated cardiac responses during an acute resident-intruder paradigm. Isolation led also to both depression-like and anxiety-like behaviors in validated operational tests. These changes in response to social isolation showed predictable interrelations and were mediated by a disruption of autonomic balance including both sympathetic and parasympathetic (vagal) mechanisms.

CONCLUSIONS

These findings indicate that social isolation induces behavioral, cardiac, and autonomic alterations related to those seen after other stressors and which are relevant to cardiovascular disease and affective disorders. This model may provide insight into the mechanisms that underlie these co-occurring conditions.

Valeriana officinalis does not alter the orofacial dyskinesia induced by haloperidol in rats: role of dopamine transporter

Chronic treatment with classical neuroleptics in humans can produce a serious side effect, known as tardive dyskinesia (TD). Here, we examined the effects of V. officinalis, a medicinal herb widely used as calming and sleep-promoting, in an animal model of orofacial dyskinesia (OD) induced by long-term treatment with haloperidol. Adult male rats were treated during 12 weeks with haloperidol decanoate (38 mg/kg, i.m., each 28 days) and with V. officinalis (in the drinking water). Vacuous chewing movements (VCMs), locomotor activity and plus maze performance were evaluated. Haloperidol treatment produced VCM in 40% of the treated rats and the concomitant treatment with V. officinalis did not alter either prevalence or intensity of VCMs. The treatment with V. officinalis increased the percentage of the time spent on open arm and the number of entries into open arm in the plus maze test. Furthermore, the treatment with haloperidol and/or V. officinalis decreased the locomotor activity in the open field test. We did not find any difference among the groups when oxidative stress parameters were evaluated. Haloperidol treatment significantly decreased [(3)H]-dopamine uptake in striatal slices and V. officinalis was not able to prevent this effect. Taken together, our data suggest a mechanism involving the reduction of dopamine transport in the maintenance of chronic VCMs in rats. Furthermore, chronic treatment with V. officinalis seems not produce any oxidative damage to central nervous system (CNS), but it also seems to be devoid of action to prevent VCM, at least in the dose used in this study.

The CCK-system mediates adaptation to novelty-induced stress in the rat: a pharmacological evidence

Brain cholecystokinin (CCK) and its receptor CCK(2) have been implicated in the etiology of anxiety. CCK(2) antagonists, however, fail to ameliorate anxiety in humans. In this study, a role for CCK in adaptation to stress is investigated by testing carry-over effects of Ly225.910, a potent CCK(2) antagonist, in a rat model of individual differences in novelty-induced emotionality. Novelty-seeking behavior in the rat is thought to model some aspects of sensation-seeking, a personality trait closely associated with risk activities including substance abuse. Animals were categorized as high-responders (HR) and low-responders (LR) based on the activity response to an inescapable novel environment. High-responders exhibit increased exploration and proactive behavior while low-responders are less exploratory and deemed to behave more anxiously. We analyzed the effects of the CCK(2) antagonist Ly225.910 (0.1 mg/kg or 0.5 mg/kg, i.p.) on the anxiety displayed by HR and LR rats in the light-dark (LD) box test (Day 1). Treatment and phenotype effects were not acutely evident. LD-experienced rats were then re-exposed to drug-free LD-box (Days 4 and 11) and elevated plus-maze (EPM) test (Day 14). Drug-naïve HR rats behaved less anxiously than drug-naïve LR rats while exploring the open arms. Previous exposure to the antagonist curtailed these differences. The emotional responses in drug-naïve HR and LR rats to the EPM test could reflect different degrees of adaptation to anxiety-like training. Long-term effects of Ly225.910 on EPM-induced risk assessment in HR and LR rats suggest that CCK-system may be involved in modulating preparedness to arousing environmental changes.

Stimulation of lateral septum CRF2 receptors promotes anorexia and stress-like behaviors: functional homology to CRF1 receptors in basolateral amygdala

The corticotropin-releasing factor (CRF) system is the primary central mediator of stress-like states, coordinating behavioral, endocrine, and autonomic responses to stress. Although induction of anorexia is a well documented effect of CRF receptor agonist administration, the central sites and behavioral processes underlying this phenomenon are poorly understood. The present studies addressed this question by examining the neuroanatomical, behavioral, and pharmacological mechanisms mediating decreases in feeding produced by the CRF1/CRF2 receptor agonist urocortin. Separate groups of food-restricted male Sprague Dawley rats were given infusions of urocortin (0, 50, 125, 250 ng/0.5 microl) into the lateral septum (LS) and immediately afterward were rated on a wide array of behaviors (locomotion, rearing, grooming, stereotypies) including a microstructural analysis of ingestive behavior. Intra-LS urocortin infusion dose-dependently reduced feeding and drinking while concomitantly increasing grooming, stereotypies, and ethological plus traditional measures of anxiety-like responses in the elevated plus-maze. Urocortin infusion into neighboring sites (lateral ventricle, medial caudate) had no effects. Coinfusion into the LS of the mixed CRF1/CRF2 receptor antagonist D-Phe-CRF(12-41) (0, 100, 1000 ng/0.5 microl) or the novel selective CRF2 receptor antagonist Astressin2B (0, 500, 1000 ng/0.5 microl) blocked urocortin-induced effects, but the CRF1-selective antagonist NBI27914 (0, 500, 1000 ng/0.5 microl) had no effect, although it completely reversed the behavioral sequelae of CRF when infused into the basolateral amygdala. These results indicate that one of the modes through which the CRF system promotes anorexia is the recruitment of stress-like states after stimulation of CRF2 receptors within the LS.

Developmental differences in acute ethanol withdrawal in adolescent and adult rats

BACKGROUND

Withdrawal from an acute high ethanol dose induces behaviors reminiscent of withdrawal from chronic ethanol exposure. While such "hangover"-related anxiety has previously been shown to be considerably less pronounced in adolescent compared to adult male rats, ontogenetic studies are limited and few experiments have directly compared sex- and age-related differences in sensitivity to ethanol hangover.

METHODS

The current experiments examined consequences of a previous ethanol challenge (4.0 g/kg i.p. injection, 20% v/v) on anxiety and exploratory behavior in the elevated plus-maze (EPM) and holeboard (HB) tests, respectively, in adolescent and adult male and female Sprague-Dawley rats.

RESULTS

In Exp. 1, evidence of hangover-related anxiety and withdrawal-induced hypoactivity emerged at both ages and in both sexes. As several procedural variables were changed in Exp.1 relative to previous studies from our laboratory showing age-related differences in these hangover measures, Exp. 2 explored the possible contribution of 2 variables to ontogenetic expression of withdrawal-induced anxiogenesis: (1) isolation vs. social context during the postchallenge recovery period and (2) EPM testing alone or immediately following a 5 minute HB test. Results of Exp. 2 revealed few significant interactions of these variables with age- and ethanol exposure-related anxiety measures, although sequential testing (HB before EPM) notably suppressed activity in the EPM and altered the major underlying component of EPM behavior from anxiety to activity as revealed in factor analyses of these data. Additional analyses conducted on animals tested only in the EPM revealed attenuations in withdrawal anxiogenesis among adolescents, along with withdrawal-related decreases in activity at both ages.

CONCLUSIONS

Taken together, these results suggest that adolescents do show an attenuated sensitivity to hangover-induced anxiogenesis in the EPM, an age difference not evident under other pretest conditions. Therefore, caution should be exerted when using the EPM to index anxiety across age. The robustness of withdrawal-related hypoactivity at both ages suggests that adolescents may not be globally insensitive to the consequences of previous binge-like exposure to ethanol, but rather less likely to express certain hangover-related consequences.

The role of hippocampus in anxiety: intracerebral infusion studies

Although current models of hippocampal function stress its well-known role in cognitive functions, historically it has also been viewed as a neural mediator of emotion. Here, we review recent evidence from intrahippocampal infusion studies in animals that support a distinctive role of the hippocampus in anxiety, independent of its roles in learning and memory. Specifically, gamma-aminobutyric acid type A receptor agonists, both direct and indirect, reliably inhibit a number of animals' untrained anxiety reactions when microinfused into the hippocampus, whereas gamma-aminobutyric acid type A receptor antagonists do not. Intrahippocampal infusions of glutamatergic, serotonergic and cholinergic compounds also produce statistically reliable antianxiety effects, but the results vary as a function of specific anxiety reactions, and to some extent specific intrahippocampal targets. One hypothesis that may accommodate some of this variability is that anxiety is functionally segregated within the hippocampus, with ventral subregions more involved in anxiety-related processes, and dorsal subregions more involved with cognitive processes. Another possibility is that different hippocampal functions (e.g. memory and anxiety) are mediated by different neurotransmitter systems and/or different receptor subtypes within the hippocampus. Although there is some evidence that supports the latter hypothesis, the evidence for the former is not conclusive. Overall, however, the data clearly suggest that the hippocampus is importantly and directly involved in the mediation of untrained anxiety reactions in animals.

Situational factors, conditions and individual variables which can determine ultrasonic vocalizations in male adult Wistar rats

The fact that rats emit different types of ultrasonic vocalizations in a variety of contexts has received increasing experimental attention, since such calls might serve as indices of the animal's subjective state, and/or as social signals in various types of interactions with other rats. Here, we present two experiments in adult male Wistar rats where we tested several different situations and conditions with respect to the occurrence of high-frequency (50-kHz) and low-frequency (22-kHz) calls. These experiments showed that rats emitted high-frequency calls when tested singly in a housing cage, which was situated in a room with no other rats present. Calling did not habituate with repeated testing, and occurred in the animal's own home cage, or a fresh housing cage, and irrespective of whether the animal's motivational status was high or low, that is, irrespective of whether they were food-deprived or fed ad libitum. Furthermore, high- and low-frequency calls were observed when applying a standardized new tickling procedure, which provided evidence for effective types of tickling. Most, but not all, young adult rats still accepted this stimulation as play. Therefore, this procedure might be a useful method to elicit high-frequency calls in adult rats. Overall, substantial evidence for inter-individual variability and intra-individual stability in vocalization was provided both, within and between housing cage and tickle tests. This variability seems to depend at least partly on dispositions or traits, which can be gauged by specific screening tests, like measuring risk-assessment in the elevated plus-maze, since animals with more risk-assessment were more likely to emit high-frequency calls during cage and tickle tests. These findings are discussed with respect to the major hypotheses concerning the functional significance of ultrasonic vocalizations, namely the social/communicatory and the motivational/emotional hypothesis.

Distinct ventral and dorsal hippocampus AP5 anxiolytic effects revealed in the elevated plus-maze task in rats

The hippocampal formation (HPC) mediates processes associated with learning, memory, anxiety and fear. The glutamate N-methyl-d-aspartate (NMDA)-receptor subtype is involved in many HPC functional processes related to learning and memory. Although not tested for the HPC, NMDA-receptor antagonists reduced fear and anxiety related responses when applied to other brain regions mediating defensive behaviour. Consequently, this study evaluated the effects of ventral or dorsal HPC application of the NMDA-receptor antagonist, AP5, in rats submitted to the Trial 1/Trial 2 elevated plus-maze (EPM) task. Ventral, but not dorsal, infusions of AP5 (6 and 24 nmol) before EPM Trial 1 increased open arms exploration and reduced risk assessment behavior, suggesting an anxiolytic-like effect. Furthermore, no interference in the avoidance responses was detected during EPM Trial 2 after AP5 infusion into the ventral or dorsal HPC before Trial 1, post-trial 1, or before Trial 2. These data support the notion of differential involvement of ventral HPC, but not dorsal, in mechanisms associated with anxiety and suggest the participation of the glutamatergic transmission, through NMDA receptor, into the ventral HPC in the mediation of defensive behavior.

The relationship between anxiety and sleep-wake behavior after stressor exposure in the rat

Disturbed sleep is a common subjective complaint among individuals diagnosed with anxiety disorders. In rodents, sleep is often recorded after exposure to various foot-shock paradigms designed to induce an anxiety state. Although differences in sleep-wake architecture are noted, the relationship to specific level of anxiety is often assumed or absent. Utilizing the elevated plus-maze (EPM) after exposure to escapable shock (ES), inescapable shock (IS) or fear conditioning (FC), resulting differences in sleep architecture were compared to an objective measure of anxiety. Male Wistar rats were implanted with EEG, EMG and hippocampal theta electrodes to record sleep-wake behavior. After recovery and recording of baseline sleep, rats were exposed to one of five manipulations: ES, IS, FC or control (CES or CIS; utilizing either chamber with no shock exposure). Shortly after experimental manipulation, the EPM was employed to quantify traditional and ethological measures of anxiety and polygraphic signs of sleep-wake behavior were recorded continuously for 6 h. Although no significance was observed in EPM measurements across groups, profound differences in sleep architecture were present. Individual correlation analysis revealed no differences in anxiety level and total percentage of time spent in sleep-wake states. These results indicate that differences in sleep architecture after foot-shock exposure may not be simply due to increased anxiety. Rather, individual anxiety may be exacerbated by disrupted sleep. To fully understand the relationship between anxiety and sleep-wake behavior, a more objective analysis of anxiety after stressor exposure is mandated.

Prenatal stress alters the negative correlation between neuronal activation in limbic regions and behavioral responses in rats exposed to high and low anxiogenic environments

Behavioral adaptation to an anxiogenic environment involves the activity of various interconnected limbic regions, such as the amygdala, hippocampus and prefrontal cortex. Prenatal stress (PS) in rats affects the ability to cope with environmental challenges and alters brain plasticity, leading to long-lasting behavioral and neurobiological alterations. We examined in PS and control animals whether behavioral reactivity was correlated to neuronal activation by assessing Fos protein expression in limbic regions of rats exposed to a low or high anxiogenic environment (the closed and open arms of an elevated plus maze, respectively). A negative correlation was found between behavioral and neuronal activation, with a lower behavioral reactivity and a higher neuronal response observed in rats exposed to the more anxiogenic environment (the open arm) with respect to the less anxiogenic environment (the closed arm). Interestingly, the variation in the neurobehavioral response between the two arms of the maze was less pronounced in rats that had been subjected to PS. This study provides a remarkable example of how long-lasting changes in brain plasticity induced by PS affect the ability of limbic neurons to cope with anxiogenic stimuli of different strength.

Locomotory patterns, spatiotemporal organization of exploration and spatial memory in serotonin transporter knockout mice

Serotonin transporter knockout (SERT-/-) mice are extensively used as a genetic model of several neuropsychiatric disorders, and consistently display anxiety-like behaviors and inactivity in different tests. To better understand how these mice organize their behavior, we assessed the open field and elevated plus maze spatiotemporal patterning of activity in adult male SERT wild type (+/+), heterozygous (+/-) and -/- mice on C57BL/6J genetic background using new videotracking and analytic procedures. In addition, we analyzed their spatial memory, assessing within- and between-trial habituation, and examined specific motor characteristics of their movement in these two tests. In the open field test, SERT-/- mice showed reduced vertical exploration throughout the arena, reduced central (but not peripheral) horizontal exploration, unaltered within-trial habituation, and slightly poorer between-trial habituation for horizontal activity. In the elevated plus maze, SERT-/- mice demonstrated anxiety-like avoidance of open arms, hypoactivity, as well as unaltered within-trial and between-trial habituation (except for poorer between-trial habituation of total horizontal activity). In both tests, SERT-/- mice showed greater prevalence of horizontal over vertical dimension of their exploration in the areas protected by the walls (open field periphery, plus maze closed arms), but not in open aversive areas, such as the center of the open field or center or open arms of the maze. In both arenas, SERT-/- mice consistently displayed increased turning behavior, potentially representing a perseverance-like phenotype or aberrant spatial strategies in novel environments. Overall, using a fine-graded behavioral analysis in two different novelty tests, this study revealed alterations in motor and spatiotemporal patterning of activity in SERT-/- mice. Given the relevance of exploratory strategies to human personality traits and brain disorders, our data may be useful for developing further neurobehavioral models using these mice.