Prior exposure to a brief restraint session facilitates the occurrence of fear in response to a conflict situation: behavioral and neurochemical correlates

Japanese quail classified by their permanence in proximity to a high or low density of conspecifics: a search for underpinning variables

Test of sociality in poultry is mainly based on the bird's individual ability to make quick social discriminations. In recent years, a density-related permanence (DRP) test has been developed that enables us to classify young birds (while in groups) according to their individual permanence in proximity to either a high or low density of conspecifics (HD or LD, respectively). Thus, the result of the classification depends not only on the bird's individual response but also on the outcome of the social interactions within the whole group. The birds' performance in DRP was associated with underlying differences in social responses of their individuals. Quails in homogeneous groups of LD residents responded with less compact groups and higher levels of agonistic interactions to the presence of an intruder and showed higher levels of agonistic interactions among cage-mates than the homogeneous groups of HD birds. An acute stressor also induced a higher corticosterone response in the LD birds than in their HD counterparts. The present study addressed the question of whether contrasting DRP performance by Japanese quail can also reflect underlying differences in fearfulness and social reinstatement responses. Thus, LD and HD categorized juvenile birds underwent one of the following tests: tonic immobility (TI), open-field (OF), or a one-way runway. Results showed that HD birds required more inductions and developed shorter responses (P ≤ 0.05) in the TI test and walked more, faster, and greater distances in the OF (P ≤ 0.05) than their LD counterparts. No differences between groups were found in short social reinstatement responses. The present findings suggest that underlying fearfulness is lower in the HD than in the LD birds. A reduced fearfulness could be regarded as an additional favorable trait of the HD-classified quail to cope with environmental challenging situations.

Restraint stress activates defensive behaviors in male rats depending on age and housing condition

Restraint is a widely used experimental stress manipulation in animal models. It is still unclear, however, whether restraint is associated with physical fatigue leading to overall behavioral inhibition, or if it induces activation of defensive behaviors and strategies to protect against subsequent challenges. The aim of this study was to systematically investigate restraint effects in rats based on housing condition (isolation- vs. pair-housed) and age at the time of testing, both of which are relevant to the expression of defensive strategies. Restraint induced behavioral inhibition in male rats younger than postnatal day 65 in an open-field paradigm, while it activated defensive behaviors in adult rats, depending on their housing condition; thereby pair-housed adult rats exhibited a heightened stretch-attend postures (SAPs) and it was suppressed by restraint, while isolation-housed adult rats displayed lower SAPs but it was enhanced by restraint. Restraint also enhanced pain tolerance, but not pain sensitivity, across all ages, regardless of housing conditions. These results suggest that restraint stress activates defensive systems of male rats, including sensory defenses and exploratory strategies in a novel environment, and these expression patterns vary with age from overall inhibition to changes in defensive behavior strategies. Understanding differential changes in these models could lead to greater consistency and better standardization of rodent models commonly used to assess the impact of stress on anxiety and defensive behaviors.

Prior stress promotes the generalization of contextual fear memories: Involvement of the gabaergic signaling within the basolateral amygdala complex

Fear generalization occurs when a response, previously acquired with a threatening stimulus, is transferred to a similar one. However, it could be maladaptive when stimuli that do not represent a real threat are appraised as dangerous, which is a hallmark of several anxiety disorders. Stress exposure is a major risk factor for the occurrence of anxiety disorders and it is well established that it influences different phases of fear memory; nevertheless, its impact on the generalization of contextual fear memories has been less studied. In the present work, we have characterized the impact of acute restraint stress prior to contextual fear conditioning on the generalization of this fear memory, and the role of the GABAergic signaling within the basolateral amygdala complex (BLA) on the stress modulatory effects. We have found that a single stress exposure promoted the generalization of this memory trace to a different context that was well discriminated in unstressed conditioned animals. Moreover, this effect was dependent on the formation of a contextual associative memory and on the testing order (i.e., conditioning context first vs generalization context first). Furthermore, we observed that increasing GABA-A signaling by intra-BLA midazolam administration prior to the stressful session exposure prevented the generalization of fear memory, whereas intra-BLA administration of the GABA-A antagonist (Bicuculline), prior to fear conditioning, induced the generalization of fear memory in unstressed rats. We concluded that stress exposure, prior to contextual fear conditioning, promotes the generalization of fear memory and that the GABAergic transmission within the BLA has a critical role in this phenomenon.

Chronic stress in Lizards: Studies on the Behavior and Benzodiazepine Receptors in Liolaemus koslowskyi and Cnemidophorus tergolaevigatus

Behavioral and physiological adaptive responses of animals facing chronic exposure to a single stressor may allow them to overcome its negative effects for future exposures to similar stressful situations. At chemical level, the GABA_A /benzodiazepine complex is considered one of the main receptor systems involved in the modulation of stress-induced responses. Here, we describe the behavioral responses of two different lizard species, Liolaemus koslowskyi and Cnemidophorus tergolaevigatus exposed to three potential chronic stressful treatments: (a) high temperature, (b) forced swimming, and (c) simulated predator. Additionally, we aimed to determine in those lizards whether the central-type benzodiazepine receptor (CBR; an allosteric modulator site of the GABA_A receptor) is related to adaptive responses to those stressful stimulations. Our results revealed that the simulated predator was the stress condition that showed the largest difference in behavioral responses between the two species, resembling previously described strategies in nature. The basal affinity of CBRs (obtained from undisturbed animals) showed differences between both species, and the simulated predator was the only stressor that altered the affinity of CBRs. L. koslowskyi CBRs showed a decreased receptor affinity, whereas C. tergolaevigatus showed an increased receptor affinity in comparison to their respective control groups. We show for the first time the effects of different types of stressors upon behavioral responses and CBR biochemical parameters in two lizard species. Our findings suggest a potential GABA/benzodiazepine role in the ability of lizards to cope with a repeated exposure to a stressful (e.g., predator) condition.

HPA Axis Interactions with Behavioral Systems

Perhaps the most salient behaviors that individuals engage in involve the avoidance of aversive experiences and the pursuit of pleasurable experiences. Engagement in these behaviors is regulated to a significant extent by an individual's hormonal milieu. For example, glucocorticoid hormones are produced by the hypothalamic-pituitary-adrenocortical (HPA) axis, and influence most aspects of behavior. In turn, many behaviors can influence HPA axis activity. These bidirectional interactions not only coordinate an individual's physiological and behavioral states to each other, but can also tune them to environmental conditions thereby optimizing survival. The present review details the influence of the HPA axis on many types of behavior, including appetitively-motivated behaviors (e.g., food intake and drug use), aversively-motivated behaviors (e.g., anxiety-related and depressive-like) and cognitive behaviors (e.g., learning and memory). Conversely, the manuscript also describes how engaging in various behaviors influences HPA axis activity. Our current understanding of the neuronal and/or hormonal mechanisms that underlie these interactions is also summarized. © 2016 American Physiological Society. Compr Physiol 6:1897-1934, 2016.

Activation of ERK2 in basolateral amygdala underlies the promoting influence of stress on fear memory and anxiety: influence of midazolam pretreatment

Exposure to emotionally arousing experiences elicits a robust and persistent memory and enhances anxiety. The amygdala complex plays a key role in stress-induced emotional processing and in the fear memory formation. It is well known that ERK activation in the amygdala is a prerequisite for fear memory consolidation. Moreover, stress elevates p-ERK2 levels in several areas of the brain stress circuitry. Therefore, given that the ERK1/2 cascade is activated following stress and that the role of this cascade is critical in the formation of fear memory, the present study investigated the potential involvement of p-ERK2 in amygdala subnuclei in the promoting influence of stress on fear memory formation and on anxiety-like behavior. A robust and persistent ERK2 activation was noted in the Basolateral amygdala (BLA), which was evident at 5min after restraint and lasted at least one day after the stressful experience. Midazolam, a short-acting benzodiazepine ligand, administered prior to stress prevented the increase in the p-ERK2 level in the BLA. Pretreatment with intra-BLA infusion of U0126 (MEK inhibitor), but not into the adjacent central nucleus of the amygdala, attenuated the stress-induced promoting influence on fear memory formation. Finally, U0126 intra-BLA infusion prevented the enhancement of anxiety-like behavior in stressed animals. These findings suggest that the selective ERK2 activation in BLA following stress exposure is an important mechanism for the occurrence of the promoting influence of stress on fear memory and on anxiety-like behavior.

Effects of the chinese traditional prescription xiaoyaosan decoction on chronic immobilization stress-induced changes in behavior and ultrastructure in rat hippocampus

Xiaoyaosan (XYS) decoction has been widely used as a traditional medicine for treating stress and depression-related disorders in China for thousands of years. Aim of the Study. To observe the potential mechanism of XYS decoction's antidepressant-like effect in α -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors related to synaptic plasticity in the hippocampus rats induced by chronic immobilization stress (CIS). Materials and Methods. Animals were randomly divided into five groups: (1) control group; (2) sham-operated group; (3) CIS group, in which rats were conducted CIS for 21 days; (4) XYS decoction treatment group; (5) 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) positive group, in which the amygdala of CIS rats was unilaterally microinjected with a competitive glutamate receptor antagonist, CNQX. After CIS for 21 days, the open field test (OPT) and elevated plus-maze test (EPM) were measured, the ultrastructure of hippocampus CA1 subregion was observed by the electron microscopy; both the GluR1 and GluR2 mRNA level of AMPA receptor subunits in hippocampus CA1 subregion were detected by real-time qPCR. Results. Rats subjected to CIS exhibited increases in time in central zone and decreases in total distance traveled in the OPT. In the EPM, they also showed decreases in center zone time and entries, open arm time and entries, and an increase in close arm time. Ultrastructural damage in the hippocampus CA1 was also observed. XYS decoction and CNQX showed significant improvement behavioral changes and the ultrastructural damage of the hippocampus CA1; XYS decoction also reversed CIS-induced decreases in GluR2 mRNA and increases in GluR1 mRNA in the hippocampus CA1 as well as CNQX. Conclusions. XYS decoction may effectively produce an antidepressant-like effect, which appears to be involved AMPA receptors related synaptic plasticity of hippocampus.

Repeated restraint stress increases basolateral amygdala neuronal activity in an age-dependent manner

Chronic stress is a precipitating factor for affective disorders such as depression and anxiety. This is associated with the effects of chronic stress on the amygdala. Adolescents may be more vulnerable to the effects of chronic stress, which may be related to its impact on amygdala function. However, the stress-induced changes in amygdala neuronal activity, and the age-dependent impact of chronic stress on amygdala neuronal activity have not been studied in depth. In this study, we investigated how repeated restraint impacts basolateral amygdala (BLA) projection neuron activity in both adolescent and adult rats. Using in vivo extracellular recordings from anesthetized rats, we found that repeated restraint increased the number of spontaneously firing neurons in the BLA of adolescent rats, but did not significantly increase the firing rate. In contrast, repeated restraint increased the firing rate of BLA neurons in adult rats, but did not change the number of spontaneously firing neurons. This is the first direct evidence of how stress differently impacts amygdala physiology in adolescent and adult rats. These findings may shed light on the mechanism by which chronic stress may age-dependently precipitate psychiatric disorders.

Risk assessment behaviors associated with corticosterone trigger the defense reaction to social isolation in rats: role of the anterior cingulate cortex

The extent to which the hypothalamic-pituitary-adrenal axis is activated by short-term and long-term consequences of stress is still open to investigation. This study aimed to determine (i) the correlation between plasma corticosterone and exploratory behavior exhibited by rats subjected to the elevated plus maze (EPM) following different periods of social isolation, (ii) the effects of the corticosterone synthesis blocker, metyrapone, on the behavioral consequences of isolation, and (iii) whether corticosterone produces its effects through an action on the anterior cingulate cortex, area 1 (Cg1). Rats were subjected to 30-min, 2-h, 24-h, or 7-day isolation periods before EPM exposure and plasma corticosterone assessments. Isolation for longer periods of time produced greater anxiogenic-like effects on the EPM. However, stretched attend posture (SAP) and plasma corticosterone concentrations were increased significantly after 30 min of isolation. Among all of the behavioral categories measured in the EPM, only SAP positively correlated with plasma corticosterone. Metyrapone injected prior to the 24 h isolation period reversed the anxiogenic effects of isolation. Moreover, corticosterone injected into the Cg1 produced a selective increase in SAP. These findings indicate that risk assessment behavior induced by the action of corticosterone on Cg1 neurons initiates a cascade of defensive responses during exposure to stressors.

Central, but not basolateral, amygdala involvement in the anxiolytic-like effects of midazolam in rats in the elevated plus maze

The role of the amygdala in the mediation of fear and anxiety has been extensively investigated. However, how the amygdala functions during the organization of the anxiety-like behaviors generated in the elevated plus maze (EPM) is still under investigation. The basolateral (BLA) and the central (CeA) nuclei are the main input and output stations of the amygdala. In the present study, we ethopharmacologically analyzed the behavior of rats subjected to the EPM and the tissue content of the monoamines dopamine (DA) and serotonin (5-HT) and their metabolites in the nucleus accumbens (NAc), dorsal hippocampus (DH), and dorsal striatum (DS) of animals injected with saline or midazolam (20 and 30 nmol/0.2 µL) into the BLA or CeA. Injections of midazolam into the CeA, but not BLA, caused clear anxiolytic-like effects in the EPM. These treatments did not cause significant changes in 5-HT or DA contents in the NAc, DH, or DS of animals tested in the EPM. The data suggest that the anxiolytic-like effects of midazolam in the EPM also appear to rely on GABA-benzodiazepine mechanisms in the CeA, but not BLA, and do not appear to depend on 5-HT and DA mechanisms prevalent in limbic structures.

The influence of stress on fear memory processes

It is well recognized that stressful experiences promote robust emotional memories, which are well remembered. The amygdaloid complex, principally the basolateral complex (BLA), plays a pivotal role in fear memory and in the modulation of stress-induced emotional responses. A large number of reports have revealed that GABAergic interneurons provide a powerful inhibitory control of the activity of projecting glutamatergic neurons in the BLA. Indeed, a reduced GABAergic control in the BLA is essential for the stress-induced influence on the emergence of associative fear memory and on the generation of long-term potentiation (LTP) in BLA neurons. The extracellular signal-regulated kinase (ERK) subfamily of the mitogen-activated protein kinase (MAPK) signaling pathway in the BLA plays a central role in the consolidation process and synaptic plasticity. In support of the view that stress facilitates long-term fear memory, stressed animals exhibited a phospho-ERK2 (pERK2) increase in the BLA, suggesting the involvement of this mechanism in the promoting influence of threatening stimuli on the consolidation fear memory. Moreover, the occurrence of reactivation-induced lability is prevented when fear memory is encoded under intense stressful conditions since the memory trace remains immune to disruption after recall in previously stressed animals. Thus, the underlying mechanism in retrieval-induced instability seems not to be functional in memories formed under stress. All these findings are indicative that stress influences both the consolidation and reconsolidation fear memory processes. Thus, it seems reasonable to propose that the emotional state generated by an environmental challenge critically modulates the formation and maintenance of long-term fear memory.

BDNF expression in perirhinal cortex is associated with exercise-induced improvement in object recognition memory

Physical exercise induces widespread neurobiological adaptations and improves learning and memory. Most research in this field has focused on hippocampus-based spatial tasks and changes in brain-derived neurotrophic factor (BDNF) as a putative substrate underlying exercise-induced cognitive improvements. Chronic exercise can also be anxiolytic and causes adaptive changes in stress-reactivity. The present study employed a perirhinal cortex-dependent object recognition task as well as the elevated plus maze to directly test for interactions between the cognitive and anxiolytic effects of exercise in male Long Evans rats. Hippocampal and perirhinal cortex tissue was collected to determine whether the relationship between BDNF and cognitive performance extends to this non-spatial and non-hippocampal-dependent task. We also examined whether the cognitive improvements persisted once the exercise regimen was terminated. Our data indicate that 4weeks of voluntary exercise every-other-day improved object recognition memory. Importantly, BDNF expression in the perirhinal cortex of exercising rats was strongly correlated with object recognition memory. Exercise also decreased anxiety-like behavior, however there was no evidence to support a relationship between anxiety-like behavior and performance on the novel object recognition task. There was a trend for a negative relationship between anxiety-like behavior and hippocampal BDNF. Neither the cognitive improvements nor the relationship between cognitive function and perirhinal BDNF levels persisted after 2weeks of inactivity. These are the first data demonstrating that region-specific changes in BDNF protein levels are correlated with exercise-induced improvements in non-spatial memory, mediated by structures outside the hippocampus and are consistent with the theory that, with regard to object recognition, the anxiolytic and cognitive effects of exercise may be mediated through separable mechanisms.

Previous stress attenuates the susceptibility to Midazolam's disruptive effect on fear memory reconsolidation: influence of pre-reactivation D-cycloserine administration

It is well known that, under certain boundary conditions, the retrieval of a stable consolidated memory results into a labile one. During this unstable phase, memory can be vulnerable to interference by a number of pharmacological agents, including benzodiazepines. One of the goals of this study was to evaluate the vulnerability to midazolam (MDZ) after reactivation of recent and remote contextual fear memories in animals that experienced a stressful situation before learning. Animals were subjected to a restraint session and trained in a contextual fear paradigm the following day; consolidated memories were reactivated at different times after learning and different MDZ doses (1.5, 3.0 mg/kg) were administered to rats after reactivation. Our results show that MDZ did not affect memory reconsolidation in older-than-one-day memories of stressed animals, even after the administration of a higher MDZ dose and a longer reactivation session (5 min). In contrast, MDZ was effective in blocking reconsolidation at all memory ages in unstressed animals. In addition, the current research investigated whether activating NMDA sites before reactivation promotes the destabilization of resistant memories such as those of stressed animals. We tested the influence of pre-reactivation D-cycloserine (DCS), a partial NMDA agonist, on MDZ's effect on fear memory reconsolidation in stressed animals. Our findings indicate that DCS before reactivation promotes retrieval-induced lability in resistant memory traces, as MDZ-induced memory impairment in stressed rats became evident with pre-reactivation DCS but not after pre-reactivation sterile isotonic saline.

Effect of environmental enrichment on fearful behavior and gastrin-releasing peptide receptor expression in the amygdala of prenatal stressed rats

Prenatal stressed offspring exhibit more fearful behavior in behavioral tests, which can be reversed by environmental enrichment (EE). However, the physiological basis of these phenomena remains unclear. Previous studies revealed that abnormal fearful behavior of prenatally stressed offspring may be a consequence of increased activities of CRFergic systems (corticotropin-releasing factor and its receptors) in the amygdala. Gastrin-releasing peptide receptors (GRPR) also have an important role in regulating amygdala-dependent, fear-related learning. The aim of this study was to examine weather prenatal stress and EE can affect the expression of GRPR in the amygdala. We reported here that prenatal chronic stress (subjected to immobilization and bright light stress for 45 min three times per day) caused increased fearfulness in defensive withdrawal test but had no effect on the expression of GRPR in the amygdala. However, enriched environment housing treatment on postnatal days 21-60 can dramatically increase the expression of GRPR in amygdala and reduce fearfulness in the defensive withdrawal test. Our results demonstrate for the first time that EE can modify the expression of GRPR in the amygdala, which might contribute to our understanding of the physiological effects of environmental enrichment.

Priming of rotational behavior by a dopamine receptor agonist in Hemiparkinsonian rats: movement-dependent induction

Repetitive stimulation of dopamine receptors located in the basal ganglia may lead to the manifestation of sensitized, abnormal, motor responses in dopamine-denervated rats. In order to study the role of motor behavior execution on the expression of these altered motor responses, we evaluated how "priming", a phenomenon displaying neurochemical and behavioral features peculiar to a sensitized abnormal motor response in dopamine-denervated rats, depends on actual movement performance. To this end, unilaterally 6-hydroxydopamine-lesioned rats received apomorphine (0.2 mg/kg s.c.), being either allowed to move or immobilized (1 h) before, concomitantly to, or after its administration, respectively. Three days after apomorphine, the dopamine D(1) receptor agonist 1-Phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol (SKF 38393, 3 mg/kg s.c.) was administered to all animals. Rats that had performed rotational behavior following apomorphine administration displayed robust contraversive rotational behavior in response to SKF 38393, whereas rats that had been immobilized concomitantly to, but neither before nor after apomorphine, did not. To clarify whether stress, which may be increased by immobilization, mediated the results observed, additional rats received apomorphine paired with immobilization plus the corticosterone-synthesis inhibitor metyrapone (100 mg/kg i.p.), or apomorphine paired with a tail stressor, being not immobilized. Metyrapone did not affect the capacity of immobilization to prevent priming and tail stressor imposition did not affect priming magnitude, suggesting that stress has minimal or no effect on the results observed. This study demonstrates how movement performance following initial dopaminergic stimulation governs the occurrence of a sensitized, abnormal, motor response to a subsequent dopaminergic challenge in dopamine-denervated rats.

Increased fear- and stress-related anxiety-like behavior in mice lacking tuberoinfundibular peptide of 39 residues

Tuberoinfundibular peptide of 39 residues (TIP39) is synthesized by two groups of neurons, one in the subparafascicular area at the caudal end of the thalamus and the other in the medial paralemniscal nucleus within the lateral brainstem. The subparafascicular TIP39 neurons project to a number of brain regions involved in emotional responses, and these regions contain a matching distribution of a receptor for TIP39, the parathyroid hormone 2 receptor (PTH2-R). We have now evaluated the involvement of TIP39 in anxiety-related behaviors using mice with targeted null mutation of the TIP39 gene (Tifp39). Tifp39(-/-) mice (TIP39-KO) did not significantly differ from wild-type (WT) littermates in the open field, light/dark exploration and elevated plus-maze assays under standard test conditions. However, the TIP39-KO engaged in more active defensive burying in the shock-probe test. In addition, when tested under high illumination or after restraint, TIP39-KO displayed significantly greater anxiety-like behavior in the elevated plus-maze than WT. In a Pavlovian fear-conditioning paradigm, TIP39-KO froze more than WT during training and during tone and context recall but showed normal fear extinction. Disruption of TIP39 projections to the medial prefrontal cortex, lateral septum, bed nucleus of the stria terminalis, hypothalamus and amygdala likely account for the fear- and anxiety-related phenotype of TIP39-KO. Current data support the hypothesis that TIP39 modulates anxiety-related behaviors following environmental provocation.

Involvement of septal Cdk5 in the emergence of excessive anxiety induced by stress

The aim of the present study was to evaluate whether the activation of Cdk5, a protein that has been suggested to participate in higher cognitive functions, is required for the onset of a sensitized anxiety-related behavior induced by stress. The exposure to restraint enhanced both Cdk5 expression in certain subareas of the septohippocampal system, principally in the lateral septum (LS) and septal Cdk5 kinase activity in rats. Behaviorally, restrained wild type mice showed a behavior indicative of enhanced anxiety in the elevated plus maze (EPM). In contrast, unstressed mice and stressed knockout mice, which lacked the p35 protein, the natural activator of Cdk5, displayed similar anxiety-like behavior in the EPM. Finally, the intra-LS infusion of olomoucine - a Cdk5 inhibitor - blocked the enhanced anxiety in the EPM induced by prior stress in rats. All these data provide evidence that septal Cdk5 is required in the emergence of a sensitized emotional process induced by stress.

Distinct behavioral and neurochemical alterations induced by intermittent hypoxia or paradoxical sleep deprivation in rats

The current study investigated the effects of paradoxical sleep deprivation and intermittent hypoxia by examining whether a combination of the two would induce anxiety-like alterations in behavior. The neurochemical effects of these manipulations were investigated by measuring cortical, striatal and hippocampal monoamine concentrations. Wistar Hannover rats were submitted to subchronic (3 days) intermittent hypoxia exposure (alternating cycles of 2 min room air-2 min 10% O2 from 0700-1900 h) and paradoxical sleep deprivation using the single platform method. Rats were randomly assigned to four different protocols: 1) control, 2) intermittent hypoxia during the light period (12 h/day), 3) paradoxical sleep deprivation (24 h/day), and 4) intermittent hypoxia combined with paradoxical sleep deprivation. Rats subjected to intermittent hypoxia showed no modification in the behavioral or neurochemical parameters assessed. Although paradoxical sleep deprivation did not produce alterations in anxiety-like behavior, the rats did increase exploratory activity in the elevated plus-maze. Moreover, a significant increase in striatal epinephrine and hippocampal homovanilic acid (HVA) concentrations was found in the paradoxical sleep deprivation groups, but not in the intermittent hypoxia/paradoxical sleep deprivation group. Of note, both paradoxical sleep deprivation and intermittent hypoxia/paradoxical sleep deprivation groups showed an increase in plasma corticosterone concentration. These results suggest that paradoxical sleep deprivation induces behavioral alterations, and these abnormalities may reflect altered neurotransmission in the brain. When paradoxical sleep deprivation was combined with intermittent oxygen depletion, the behavioral and biochemical parameters were comparable to those of control rats.

Corticotropin-releasing hormone receptors in the medial prefrontal cortex regulate hypothalamic-pituitary-adrenal activity and anxiety-related behavior regardless of prior stress experience

The hypothalamic-pituitary-adrenal (HPA) axis habituates, or gradually decreases its activity, with repeated exposure to the same stressor. During habituation, the HPA axis likely requires input from cortical and limbic regions involved in the processing of cognitive information that is important in coping to stress. Brain regions such as the medial prefrontal cortex (mPFC) are recognized as important in mediating these processes. The mPFC modulates stress-related behavior and some evidence suggests that the mPFC regulates acute and repeated stress-induced HPA responses. Interestingly, corticotropin-releasing hormone (CRH)-1 receptors, which integrate neuroendocrine, behavioral and autonomic responses to stress, are localized in the mPFC but have not been specifically examined with respect to HPA regulation. We hypothesized that CRH receptor activity in the mPFC contributes to stress-induced regulation of HPA activity and anxiety-related behavior and that CRH release in the mPFC may differentially regulate HPA responses in acutely compared to repeatedly stressed animals. In the present experiments, we found that blockade of CRH receptors in the mPFC with the non-selective receptor antagonist d-Phe-CRH (50 ng or 100 ng) significantly inhibited HPA responses compared to vehicle regardless of whether animals were exposed to a single, acute 30 min restraint or to the eighth 30 min restraint. We also found that intra-mPFC injections of CRH (20 ng) significantly increased anxiety-related behavior in the elevated plus maze in both acutely and repeatedly restrained groups compared to vehicle. Together, these results suggest an excitatory influence of CRH in the mPFC on stress-induced HPA activity and anxiety-related behavior regardless of prior stress experience.

Lack of feedback inhibition on rat basolateral amygdala following stress or withdrawal from sedative-hypnotic drugs

Previous research has demonstrated that suppression of inhibition in projection neurons of the basolateral complex of the amygdala (BLA) represents an essential mechanism underlying the emergence of negative emotional responses, including exaggerated fear and anxiety. The present work evaluates inhibitory postsynaptic potentials (IPSPs) in pyramidal projection neurons of the BLA in rats subjected to either diazepam or ethanol withdrawal or uncontrollable stress. These are experimental paradigms conducive to a negative emotional state. In slices containing the BLA, IPSPs were studied using whole-cell patch clamp. In control animals, a small IPSP was evoked by sub-threshold stimulation of the external capsule. When an action potential (AP) was evoked by supra-threshold stimuli, IPSPs were considerably larger; these IPSPs were sensitive to blockade of GABA(A) receptors by picrotoxin. However, IPSPs were clearly reduced in diazepam- or ethanol-withdrawn and in stressed rats. Firing of an AP by a depolarizing pulse applied through the patch pipette consistently evoked an inhibitory postsynaptic current (IPSC) in the pyramidal neurons of control animals from all three experimental models; these IPSCs were mediated by GABA(A) receptor activation and were blocked after suppression of glutamatergic transmission. In contrast, no IPSCs were observed in slices from diazepam- or ethanol-withdrawn or stressed animals, although the depolarizing pulse regularly evoked an AP in pyramidal neurons. It is concluded that, in withdrawn or stressed rats, GABAergic disinhibition occurs due to attenuation or suppression of feedback inhibition.

Social defeat-induced contextual conditioning differentially imprints behavioral and adrenal reactivity: a time-course study in the rat

The present experiments were based on the rat resident-intruder paradigm and aimed at better understanding the long-term conditioning properties of this social stress model. Intruders were exposed to aggressive conspecifics residents. During 3 daily encounters, intruders were either defeated or threatened by residents, providing the defeated-threatened (DT) and threatened-threatened (TT) groups respectively, or exposed to a novel empty cage (EC). The effect of such exposures was assessed in 3 separate experiments 8, 14, or 21 days following the last session on both behavior and hypothalamus-pituitary-adrenal (HPA) axis parameters. A specific and persistent behavioral conditioning due to social defeat but also to the sole social threat experience was observed as defensive behaviors and anxiety-like behaviors were observed respectively in DT and TT rats, highlighting a lack of habituation for the conditioning properties of this social stressor. On the other hand, at the earlier time points examined a less specific activation of the HPA axis parameters was found, starting to show habituation at day 21 in EC but not in DT or TT rats. These data give further support to the lasting effects of this social stress model, bestowing a special emphasis upon the impact of its psychological component and upon the relevance of its development and maintenance over time.

5-HT2 receptor mechanisms of the dorsal periaqueductal gray in the conditioned and unconditioned fear in rats

RATIONALE

It is well known that 5-HT(2) mechanisms modulate the defensive behavior produced by the stimulation of the dorsal periaqueductal gray (dPAG). However, in spite of the notion that past stressful experiences play a role in certain types of anxiety, only studies with the stimulation of the dPAG of rats without previous aversive experience have been conducted so far.

OBJECTIVES

We investigated the mediation of 5-HT(2) receptors of the dPAG in rats previously submitted to contextual fear conditioning (CFC). Defensive behaviors induced by the activation of the dPAG were assessed by measuring the lowest intensity of electric current applied to this structure (threshold) able to produce freezing and escape responses during the testing sessions of CFC in which animals were placed in a context previously paired to footshocks. The 5-HT(2) function of the dPAG in this condition was evaluated by local injections of alpha-methyl-5-HT (20 nmol/0.2 mul) and ketanserin (5 and 10 nmol/0.2 mul), selective agonist and antagonist of 5-HT(2) receptors, respectively.

RESULTS

In accordance with previous studies, alpha-methyl-5-HT increased the aversive thresholds (antiaversive effects) in naive rats, and injection of ketanserin into the dPAG did not produce significant effects. On the other hand, ketanserin decreased in a dose-dependent manner the freezing threshold (proaversive effect) determined by the dPAG electrical stimulation, whereas alpha-methyl-5-HT continued to show antiaversive effects in animals under CFC.

CONCLUSIONS

The present results suggest that past stressful experience can produce changes in the synaptic function of 5-HT(2) receptors within the dPAG with important impact on the expression of defensive behaviors.

Amphetamine and pentylenetetrazole given post-trial 1 enhance one-trial tolerance to the anxiolytic effect of diazepam in the elevated plus-maze in mice

There are several hypotheses to explain the lack of an anxiolytic effect on animals with previous maze experience (one-trial tolerance). Some of these hypotheses are related to learning and memory, so the reduction of trial 1 duration to 1 min or amnesic drug administration before trial 1 prevents the lack of an anxiolytic effect in trial 2. Amphetamine and pentylenetetrazole have been shown to enhance memory consolidation when administered immediately after training. Thus, the aim of the present study was to evaluate the effect of amphetamine (1.0-3.0 mg/kg) or pentylenetetrazole (30.0 mg/kg), at putative memory-enhancing doses, on the effect of diazepam (2.5 mg/kg) in the elevated plus-maze trial 2 on mice exposed to a 1-min long trial 1. Mice were submitted to 1-min trial 1 in the elevated plus-maze immediately followed by drug treatment (saline, amphetamine, or pentylenetetrazole) and to elevated plus-maze trial 2 after 48 h. Animals were treated with vehicle or diazepam 30 min before trial 2. The results showed that post-trial 1 saline and 1.0 mg/kg amphetamine did not induce one-trial tolerance. On the other hand, 2.0 and 3.0 mg/kg amphetamine and 30 mg/kg pentylenetetrazole induced a lack of anxiolytic effect of diazepam on trial 2 even with 1-min trial 1 length. Furthermore, these data were not due to novelty exposure in trial 1 or to amphetamine treatment so that mice exposed to an activity chamber instead of the plus-maze (trial 1) and then immediately submitted to amphetamine treatment (2.0 mg/kg) did not show one-trial tolerance 48 h after trial 1. Taken as a whole, these data support the hypothesis that memory is involved in the lack of an anxiolytic effect in the elevated plus-maze trial 2.

Glucocorticoid receptors in lateral septum are involved in the modulation of the emotional sequelae induced by social defeat

The current research studied the behavior adopted in the elevated plus maze (EPM) of rats previously subjected to a social defeat using the resident-intruder paradigm. One day after defeat, intruder animals exhibited an anxiogenic-like behavior in the EPM. In addition, we also evaluated the role of the corticosteroid receptor system (minerlocorticoid - MR - and glucocorticoid - GR - receptors) from the lateral septum (LS) on the anxiety generated by social defeat. The LS is an area of the aversive circuitry that is preferentially activated in passive defensive postures, and participates - together with other brain areas - in the modulation of aversive states. Intruder animals were infused into the LS with the MR or GR antagonist (ZK 91587 and RU 38486, respectively) and then submitted to social stress. All rats were tested in the EPM 1 day later. Only the administration of the GR antagonist, but not the MR antagonist, into the LS normalized the anxiogenic response induced by defeat. Furthermore, we examined whether a single injection of corticosterone (CS) could induce the same influence on the behavior in the EPM as that observed after social defeat. Moreover, we explored the effect of local infusions of MR or GR antagonists into the LS on the behavior exhibited by CS-treated rats in a subsequent EPM exposure. CS administration also exerted an increased anxiogenic-like behavior, which was normalized only by the local infusion of the GR antagonist. Based on these findings, we suggest that CS secreted by emotionally relevant stimuli acting via GR in LS plays an important role in the modulation of the emotional sequelae induced by social defeat.

Models of anxiety: responses of rats to novelty in an open space and an enclosed space

Exposure to novelty has been shown to induce anxiety responses in a variety of behavioural paradigms. The purpose of the present study was to investigate whether exposition of naïve rats to novelty would result in a comparable or a different pattern of responses in an open space versus enclosed space with or without the presence of an object in the centre of the field. Lewis and Wistar rats of both genders were used to illustrate and discuss the value and validity of these anxiety paradigms. We examined a wide range of measures, which cover several aspects of animals' responses. The results of this study revealed significant differences between the behaviour of animals in an open space and in the enclosed space. It also revealed significant differences in animal's responses to the presence and absence of an object in the open space and in the enclosed space. In the enclosed space, rats spent most of their time in the outer area with lower number of exits and avoided the object area except when there was an object, while in the open space rats displayed frequent short duration re-entries in the outer area and spent longer time in the object area in presence of an object. The time spent in the inner area (away from the outer area and the object area) was significantly longer and the number of faecal boli was significantly higher in the open space than in the enclosed space. In the present report, we will discuss the fundamental differences between enclosed space and open space models, and we will examine some methodological issues related to the current animal models of human behaviour in anxiety. In the enclosed space, animals can avoid the potential threat associated with the centre area of a box and chose the safety of walls and corners, whereas, in the open space animals have to avoid every parts of the field from which there was no safe escape. The response of animals to novelty in an open space model appears more relevant to anxiety than in an enclosed space. The present studies revealed no correlations between the measures of behaviour in enclosed space and the measures of behaviour in open space, which suggest that these two models do not involve the same construct. Our results suggest that the enclosed space model involves avoidance responses while the open space model involves anxiety responses. The open space model can be very useful in understanding the underlying neural mechanisms of anxiety responses, and in assessing the effects of potential anxiolytic drugs.

Fear state induced by ethanol withdrawal may be due to the sensitization of the neural substrates of aversion in the dPAG

The neural substrate underlying the aversive effects induced by ethanol abstinence is still unclear. One candidate for such effects is the dorsal periaqueductal gray (dPAG), a core structure of the brain aversion system. The main aim of this study is to examine the role of the dPAG as a possible locus of the aversive effects following abrupt alcohol withdrawal. To this end, rats were subjected to an oral ethanol self-administration procedure, in which animals were offered 6-8% (v/v) ethanol solution for a period of 21 days followed by an abrupt discontinuation of the treatment on the two subsequent days. Control animals received control dietary fluid for similar periods of time. The effects of ethanol withdrawal were examined in the elevated plus-maze (EPM) (Exp. I), on the prepulse inhibition of startle to loud sounds (Exp. II) and on the freezing and escape responses induced by electrical stimulation of the dPAG (Exp. III). In Experiment III, rats were implanted with an electrode aimed at the dPAG and the number and duration of ultrasonic vocalizations (USVs) were also recorded in the rats that received dPAG stimulation at freezing and escape thresholds. Data obtained showed that ethanol withdrawal elicited significant "anxiety-like" behaviors, as revealed by the decrease in the number of entries into and time spent onto the open arms of the EPM. Startle reflex and prepulse inhibition remained unchanged in withdrawn animals. In addition, discontinuation from the chronic ethanol regimen caused a reduction in the stimulation thresholds for freezing and escape and in the number and duration of USVs. Together, these effects have been interpreted in the frame of a high fear state elicited by activation of the dPAG. These findings are indicative that ethanol withdrawal sensitizes the substrates of fear at the level of this midbrain structure.

Prior electrical stimulation of dorsal periaqueductal grey matter or deep layers of the superior colliculus sensitizes rats to anxiety-like behaviors in the elevated T-maze test

Electrical stimulation of the dorsal periaqueductal grey matter (DPAG) and deep layers of the superior colliculus (DLSC) of the rat elicits anxiety-like reactions such as freezing and flight. The temporal course of the effects of the aversive electrical stimulation of the DPAG (5, 15 and 30 min afterward) and DLSC (5, 10 and 15 min afterward) on the defensive response of rats exposed to elevated T-maze were determined. The elevated T-maze generates two defensive behaviors, inhibitory avoidance and one-way escape, which have been related, respectively, to generalized anxiety and panic disorders. Prior electrical stimulation of the DPAG (15 min) and DLSC (5 min) enhanced inhibitory avoidance when compared to no-operated and sham animals, although not affecting escape. Therefore, stimulation of the DPAG and DLSC causes a heightened responsivity to anxiogenic stimulus, but not to panicogenic stimulus, inherent to elevated T-maze. These findings support the participation of the DPAG and DLSC in the elaboration of adaptive responses to stressful situations. Besides, the data supports the view that prior electrical stimulation of DPAG and DLSC is selective in sensitizing rats to anxiety-like behaviors, but not to panic-like behaviors in the elevated T-maze test.

Previous stress facilitates fear memory, attenuates GABAergic inhibition, and increases synaptic plasticity in the rat basolateral amygdala

In experiments designed to investigate the relationship between stress and the acquisition of new fear memories, it was found that previous exposure to a restraint session increased fear conditioning in a contextual fear paradigm. Moreover, the infusion of bicuculline, a competitive antagonist of GABAA receptors, into the basolateral amygdala complex (BLA), but not into the central amygdaloid nucleus, induced the same behavioral effect. Pretreatment with midazolam (MDZ), a positive modulator of GABAA sites, prevented the facilitating influence on fear memory of both stress and GABAA receptor blockade in the BLA. These data suggest that facilitation of fear conditioning could be causally related to increased neuronal excitability attributable to depressed GABAergic inhibition in the BLA. To test this hypothesis, evoked potentials were studied in brain slices from stressed animals. Potentials evoked in the BLA by single stimuli applied to the external capsule showed multispike responses, suggestive of GABAergic disinhibition. These multiple responses were no longer evident after the slices were perfused with diazepam or if the stressed animals were pretreated with MDZ. In slices from stressed rats, paired-pulse inhibition (GABA dependent) was suppressed. Also, in stressed animals, long-term potentiation (LTP) was induced with a single train of high-frequency stimulation, which did not induce LTP in control rats. Moreover, MDZ pretreatment prevented the facilitating influence of stress on LTP induction. All of these findings support the hypothesis that previous stress attenuates inhibitory GABAergic control in the BLA, leading to neuronal hyperexcitability and increased plasticity that facilitates fear learning.

Changes in the biogenic amine content of the prefrontal cortex, amygdala, dorsal hippocampus, and nucleus accumbens of rats submitted to single and repeated sessions of the elevated plus-maze test

It has been demonstrated that exposure to a variety of stressful experiences enhances fearful reactions when behavior is tested in current animal models of anxiety. Until now, no study has examined the neurochemical changes during the test and retest sessions of rats submitted to the elevated plus maze (EPM). The present study uses a new approach (HPLC) by looking at the changes in dopamine and serotonin levels in the prefrontal cortex, amygdala, dorsal hippocampus, and nucleus accumbens in animals upon single or double exposure to the EPM (one-trial tolerance). The study involved two experiments: i) saline or midazolam (0.5 mg/kg) before the first trial, and ii) saline or midazolam before the second trial. For the biochemical analysis a control group injected with saline and not tested in the EPM was included. Stressful stimuli in the EPM were able to elicit one-trial tolerance to midazolam on re-exposure (61.01%). Significant decreases in serotonin contents occurred in the prefrontal cortex (38.74%), amygdala (78.96%), dorsal hippocampus (70.33%), and nucleus accumbens (73.58%) of the animals tested in the EPM (P < 0.05 in all cases in relation to controls not exposed to the EPM). A significant decrease in dopamine content was also observed in the amygdala (54.74%, P < 0.05). These changes were maintained across trials. There was no change in the turnover rates of these monoamines. We suggest that exposure to the EPM causes reduced monoaminergic neurotransmission activity in limbic structures, which appears to underlie the "one-trial tolerance" phenomenon.

A comparative study with two types of elevated plus-maze (transparent vs. opaque walls) on the anxiolytic effects of midazolam, one-trial tolerance and fear-induced analgesia

The phenomenon known as one-trial tolerance (OTT) to the anxiolytic effects of benzodiazepines observed in rats submitted to the elevated plus-maze test (EPM) is considered to be due to the emergence of phobic states across the test/retest sessions. Antinociception is a usual component of the defense reaction. Until now, no study has examined antinociception and OTT together in freely behaving rats in the EPM. This work is a new approach looking at the sensorimotor gatings underlying OTT through the examination of the changes in reactivity to noxious stimuli during OTT development. We used the tail-flick test to assess the reactivity of rats to noxious stimulus during the effects of midazolam in test/retest sessions using two types of EPM, one with opaque (standard EPM) and another one with transparent walls (modified EPM). The authors had previously shown that this modified test caused an overall stressful situation more related to anxiety while the standard test coursed with a mixture of anxiety and high fear levels. In both plus mazes, the study was conducted in two experiments: (i) midazolam before the first trial, and (ii) midazolam before the second trial. In each experimental condition the effects of midazolam were tested under two doses (0.5 and 1.0 mg/kg) against a control group that received injections of saline. The anxiolytic effects of midazolam were more pronounced in animals tested in the modified EPM than in the standard EPM. Stressful stimuli present in both types of maze were able to elicit one-trial tolerance to midazolam on re-exposure. However, anxiolytic-insensitive behaviors in the first and the reduction in exploratory activity in the second trial are more pronounced in the standard EPM indicating that this test is more prone to transfer fear-related states across trials than the modified maze test. Antinociception is not present upon the re-exposure of rats to the EPM. These findings show that animals tested in the modified EPM showed higher sensitivity to the anxiolytic effects of midazolam than the standard EPM. Antinociception was not a concomitant of the shift in the emotional state present in the retest sessions of the EPM. These results are in agreement with the premises that repeated stressful experience leads to anxiolytic-insensitive fear state different from anxiety.

Pharmacological dissociation of moderate and high contextual fear as assessed by freezing behavior and fear-potentiated startle

The amplitude of the whole-body acoustic startle response is reliably enhanced when elicited in the presence of foreground signals, such as light, previously paired with footshocks. It has been shown that this enhancement is evident by moderate fear levels, but is less affected by high fear levels. Potentiation of the acoustic startle reflex has also been reported in the presence of background cues previously associated with footshocks. However, the effects of anxiolytic drugs on different levels of fear elicited by moderate and intense contextual fear conditioning associated with startle reflex have not been examined yet. To approach this issue, we examined the effects of the anxiolytic, midazolam, on two intensities of contextual fear; freezing behavior and the startle response to loud noise. First, we compared the magnitude of the freezing behavior and the startle amplitude during the testing sessions in groups of rats submitted to fear conditioning using 0.3 and 0.6 mA as unconditioned stimuli (10 stimuli of 1 s each, intertrial interval from 60 to 180 s). Afterwards, the effects of midazolam (0.5 and 1.0 mg/kg) were assessed in these two conditions. Rats showed a potentiated startle reflex and a significant freezing behavior to moderate fear conditioning, which were both attenuated by midazolam. Higher levels of fear conditioning caused more intense freezing behavior without enhancing the startle reflex. Whereas midazolam reduced this freezing response, the startle response was unaffected. These results are indicative that anxiolytic-sensitive freezing and fear-potentiated startle are triggered by moderate contextual fear conditioning, while contextual conditioning with the use of high footshocks causes a distinct pattern of behavioral responses, which is only partially affected by midazolam. Due to the differential sensitivity to midazolam of these two patterns of startle responses generated as a function of the intensity of contextual fear conditioning, it is proposed that they represent moderate and intense aversive states that may be related to anxiety or panic/phobic conditions, respectively.

Long-term effects of social stress on brain and behavior: a focus on hippocampal functioning

In order to study mechanisms involved in the etiology of human affective disorders, there is an abundant use of various animal models. Next to genetic factors that predispose for psychopathologies, environmental stress is playing an important role in the etiology of these mental diseases. Since the majority of stress stimuli in humans that lead to psychopathology are of social nature, the study of consequences of social stress in experimental animal models is very valuable. The present review focuses on one of these models that uses the resident-intruder paradigm. In particular the long-lasting effects of social defeat in rats will be evaluated. Data from our laboratory on the consequences of social defeat on emotional behavior, stress responsivity and serotonergic functionality are presented. Furthermore, we will go into detail on hippocampal functioning in socially stressed rats. Very recent results show that there is a differential effect of a brief double social defeat and repetitive social defeat stress on dendritic remodeling in hippocampal CA3 neurons and that this has repercussions on hippocampal LTP and LTD. Both the structural and electrophysiological changes of principal neurons in the hippocampal formation after defeat are discussed as to their relationship with the maintenance in cognitive performance that was observed in socially stressed rats. The results are indicative of a large dynamic range in the adaptive plasticity of the brain, allowing the animals to adapt behaviorally to the previously occurred stressful situation with the progression of time.

The interaction of housing condition and acute immobilization stress on the elevated plus-maze behaviors of protein-malnourished rats

Protein malnutrition induces structural, neurochemical and functional alterations in the central nervous system, leading to behavioral alterations. In the present study, we used the elevated plus-maze (EPM) as a measure of anxiety to evaluate the interaction between acute immobilization and housing conditions on the behavior of malnourished rats. Pups (6 males and 2 females) were fed by Wistar lactating dams receiving a 6% (undernourished) or 16% (well-nourished) protein diet. After weaning, the animals continued to receive the same diets ad libitum until 49 days of age when they started to receive a regular lab chow diet. From weaning to the end of the tests on day 70, the animals were housed under two different conditions, i.e., individual or in groups of three. On the 69th day, half of the animals were submitted to immobilization for 2 h, while the other half were undisturbed, and both groups were tested 24 h later for 5 min in the EPM. Independent of other factors, protein malnutrition increased, while immobilization and social isolation per se decreased, EPM exploration. Analysis of the interaction of diet vs immobilization vs housing conditions showed that the increased EPM exploration presented by the malnourished group was reversed by acute immobilization in animals reared in groups but not in animals reared individually. The interaction between immobilization and housing conditions suggests that living for a long time in social isolation is sufficiently stressful to reduce the responses to another anxiogenic procedure (immobilization), while living in groups prompts the animals to react to acute stress. Thus, it is suggested that housing condition can modulate the effects of an anxiogenic procedure on behavioral responses of malnourished rats in the EPM.

Antidepressant-like effects of NMDA-receptor antagonist injected into the dorsal hippocampus of rats

Exposure to uncontrollable stressors causes behavioral changes that have been related to depressive states in humans. Poststress intrahippocampal administration of amino-7-phosphonoheptanoic acid (AP-7), a glutamate NMDA-receptor antagonist, attenuated the restraint-induced decreased exploration of an elevated plus maze 24 h later. The objective of the study was to test if this treatment would also attenuate the increased immobility seem in the forced swim test (FST) due to preexposition to this stressful situation. Male Wistar rats with cannulae aimed at the dorsal hippocampus were submitted to 15 min of forced swimming and tested 24 h later. They received bilateral intrahippocampal injections of AP-7 (10 nmol) either before or after the pretest swimming session or before the test. Poststress treatment increased latency to display the first episode of immobility and tended to reduce total immobility time. The drug was ineffective when given before stress or before test and in nonstressed animals. This suggests that glutamate NMDA receptors located in the dorsal hippocampus are involved in the behavioral changes observed in the FST.

A robust animal model of state anxiety: fear-potentiated behaviour in the elevated plus-maze

Fear (i.e., decreased percentage time spent on open-arm exploration) in the elevated plus-maze can be potentiated by prior inescapable stressor exposure, but not by escapable stress. The use of fear-potentiated plus-maze behaviour has several advantages as compared to more traditional animal models of anxiety. (a) In contrast to the traditional (spontaneous) elevated plus-maze, which measures innate fear of open spaces, fear-potentiated plus-maze behaviour reflects an enhanced anxiety state (allostatic state). This "state anxiety" can be defined as an unpleasant emotional arousal in face of threatening demands or dangers. A cognitive appraisal of threat is a prerequisite for the experience of this type of emotion. (b) Depending on the stressor used (e.g., fear of shock, predator odour, swim stress, restraint, social defeat, predator stress (cat)), this enhanced anxiety state can last from 90 min to 3 weeks. Stress effects are more severe when rats are isolated in comparison to group housing. (c) Drugs can be administered in the absence of the original stressor and after stressor exposure. As a consequence, retrieval mechanisms are not affected by drug treatment. (d) Fear-potentiated plus-maze behaviour is sensitive to proven/putative anxiolytics and anxiogenics which act via mechanisms related to the benzodiazepine-gamma-aminobutyric acid receptor, but it is also sensitive to corticotropin-releasing receptor antagonists and glucocorticoid receptor antagonists and serotonin receptor agonists/antagonists complex (high predictive validity). (e) Fear-potentiated plus-maze behaviour is very robust, and experiments can easily be replicated in other labs. (f) Fear-potentiated plus-maze behaviour can be measured both in males and females. (g) Neural mechanisms involved in contextual fear conditioning, fear potentiation and state anxiety can be studied.Thus, fear-potentiated plus-maze behaviour may be a valuable measure in the understanding of neural mechanisms involved in the development of anxiety disorders and in the search for novel anxiolytics. Finally, the involvement of corticotropin-releasing factor and corticosteroid-corticotropin-releasing factor interactions in the production of fear-potentiated plus-maze behaviour are discussed.

Interaction of "readthrough" acetylcholinesterase with RACK1 and PKCbeta II correlates with intensified fear-induced conflict behavior

Behavioral reactions to stress are altered in numerous psychiatric and neurodegenerative syndromes, but the corresponding molecular processes and signal transduction pathways are yet unknown. Here, we report that, in mice, the stress-induced splice variant of acetylcholinesterase, AChE-R, interacts intraneuronally with the scaffold protein RACK1 and through it, with its target, protein kinase CbetaII (PKCbetaII), which is known to be involved in fear conditioning. In stress-responsive brain regions of normal FVBN mice, the mild stress of i.p. injection increased AChE and PKCbetaII levels in a manner suppressible by antisense prevention of AChE-R accumulation. Injection stress also prolonged conflict between escape and hiding in the emergence into an open field test. Moreover, transgenic FVBN mice overexpressing AChE-R displayed prolonged delay to emerge into another field (fear-induced behavioral inhibition), associated with chronically intensified neuronal colabeling of RACK1 and PKCbetaII in stress-responsive brain regions. These findings are consistent with the hypothesis that stress-associated changes in cholinergic gene expression regulate neuronal PKCbetaII functioning, promoting fear-induced conflict behavior after stress.

Gabaergic modulation of the stress response in frontal cortex and amygdala

GABAergic neurotransmission is thought to play an important role in the modulation of the central response to stress. In the present study we evaluate the influence of a brief restraint exposure on GABA-stimulated chloride influx in diverse brain areas presumed to have a major role in the mediation of emotional behaviors following aversive stimulation. A reduced chloride uptake after stress exposure was only observed in frontal cortex and amygdala. Moreover, rats subjected to such stressor performed an anxiogenic behavior when exposed later to the elevated plus-maze. A comparable behavior in the elevated plus-maze was observed between animals that were allowed to chew during the restraint experience and those without any stressful manipulation, suggesting that chewing served as an efficient coping behavioral strategy during such threatening situations. In order to explore if chewing during the restraint experience could suppress the reduction in GABA-stimulated chloride uptake induced by this stressor, rats were allowed or not to chew during restraint and in both cases GABA-stimulated chloride influx was assayed in frontal cortex and amygdala. The finding of this experiment showed that restrained rats that have the possibility to chew exhibited a similar GABA-stimulated chloride uptake in cortical tissue to that shown by control, unstressed rats. Moreover, chewing in response to restraint attenuated the reduction of GABA-stimulated chloride uptake in amygdala, supporting the notion that chewing is an effective coping response to restraint. These experiments suggest that a reduced GABAergic inhibitory control in these areas could be implicated in the emotional sequelae generated by this uncontrollable stressor and that the suppression of this reduction seems to be associated with the occurrence of coping behavioral response to such fear-inducing stimulus.

A comparison of female and male rats' ETOH-induced ataxia and exploration following restraint or swim stress

Animal models of stress reactivity are often employed in developing treatments for humans. Many studies use shock stress, and most use male rats. These experiments compare female and male rats exposed to either restraint stress (RS) or ambient-temperature swim stress (SS), using two durations of each stressor and naive controls. The ataxic effects of a 0.6 g/kg i.p. dose of ethanol (ETOH) were measured. Females exhibited less ataxia than males following ETOH administration. There were no significant effects of stress on ETOH-induced ataxia. Exploration was also measured in an open-field test (OFT) both pre- and poststress. In the prestress OFT, females were more active than males. For the no-stress groups and the shorter-duration stress groups, exploration decreased between the first and second OFTs. However, the groups exposed to the longer-duration stress did not show this expected decrease in exploration. A key finding of this research is that while sex differences may be present at baseline, the sexes may react similarly to stress. These data extend knowledge on sex differences in stress, alcohol reactivity and exploratory behavior.

Sex differences in central benzodiazepine receptor densities and circulating corticosterone release after acute stress in broiler chicks

The purpose of the study was to determine the effect of sex on central benzodiazepine receptor (CBR) and serum corticosterone (CS) responses to an acute stressor in broiler chicks. Birds were housed in ten mixed-sex groups of eight chicks per cage. At 15 d of age, chicks were taken from a randomly selected cage and blood was immediately sampled (undisturbed controls), or they were taken from the same cage and immersed up to their necks in warm water (partial water immersion, PWI) for 15 min before blood was sampled. After blood sampling, forebrains were dissected for preparation of membranes, and bird sex was determined by gonadal inspection. Serum CS levels were determined by a competitive protein-binding assay. CBR densities were determined by radiolabeled receptor binding assay. There were no sex differences in serum CS levels or benzodiazepine receptor densities in controls. Exposure to PWI significantly increased (P < 0.01) circulating CS levels in both sexes, and this elevation was more pronounced (P < 0.01) in males than in females. Male, but not female, chicks also showed a significant stressor-induced increase (P < 0.01) in CBR densities. These findings showed sexual differences in acute, stressor-induced benzodiazepine and adrenocortical responses that suggest broiler males are more stress-susceptible than females.

A dopaminergic mechanism is involved in the 'anxiogenic-like' response induced by chronic amphetamine treatment: a behavioral and neurochemical study

The purpose of this study was to examine the influence of chronic d-amphetamine (AMPH) treatment (2 mg/kg i.p., for 9 consecutive days) on behavioral and neurochemical responses to a subsequent exposure - 4 days after the last AMPH injection--to the elevated plus-maze (EPM), as well as to determine the involvement of a dopaminergic mechanism in that influence. Results showed that chronic AMPH treatment induced an 'anxiogenic-like' response when animals were evaluated in the EPM test. Pretreatment with either haloperidol (HAL, 1 mg/kg i.p., 20 min prior to each injection) or SCH-23390 (0.1 mg/kg i.p., 10 min prior to each injection) completely abolished the chronic AMPH-induced 'anxiogenic-like' effect displayed in the EPM test. However, sulpiride pretreatment (60 mg/kg i.p., 10 min prior to each AMPH injection) did not modify such effect. In addition, rats treated with AMPH and subsequently exposed to the EPM, showed a decrease in the maximal GABA-stimulated chloride uptake in cortical microsacs. HAL pretreatment restored the maximal chloride uptake induced by chronic AMPH. Altogether, these results suggest that: (1) previous exposure to chronic AMPH treatment induces an increased emotional response following a conflict situation, (2) dopamine D(1) receptors are mainly involved in chronic AMPH-induced changes in the behavior displayed in EPM test, and (3) an interaction between GABAergic and dopaminergic mechanisms may be implicated in neurochemical and behavioral changes induced by chronic AMPH treatment.

Vibrissal sense is not the main sensory modality in rat exploratory behavior in the elevated plus-maze

Four groups of male Wistar rats were submitted to acute bilateral removal of mystacial vibrissae at different lengths from the follicle. Each group was divided into two subgroups, tested under high (150 Lux) and low environmental illumination (2 Lux). All the subjects were allowed to freely explore an elevated plus-maze for 5 min. Results indicated that rats tested under low illumination tended to explore the open arms more frequently and longer then rats tested under high illumination. When tested under low illumination, rats in the group that suffered whole vibrissa removal stayed longer in the open arms than those in the other groups but did not differ in the number of entries. The average increase in the length of open arm entries, rather than a decrease in aversion to the open arms, may be due to the need of more time to obtain information about the environment since there is no light and the vibrissae were removed. This effect was not seen with rats tested under high illumination, possibly because vision could be used to obtain relevant information.

Behavioral effects in the elevated plus maze of an NMDA antagonist injected into the dorsal hippocampus: influence of restraint stress

The objective of the study was to investigate the influence of restraint stress on the effects of 2-amino-7-phosphonoheptanoic acid (AP7), an NMDA receptor antagonist, injected into the hippocampus of rats submitted to the elevated plus maze (EPM). Male Wistar rats with cannulas aimed to the dorsal hippocampus were forced immobilized for 2 h. Twenty four hours later they received bilateral injections of saline or AP7 (10 nmol/0.5 microl), and were tested in the EPM. In another experiment the animals received the treatment immediately before or after the restraint period, and were tested in the EPM 24 h later. AP7 had no effect in any anxiety measure in non-stressed rats. In stressed animals the drug increased the percentage of open arm entries when injected before the test in the EPM. When administered immediately after the restraint period, AP7 increased the percentage of time spent in the open arms and tended to do the same with the percentage of entries in these same arms. The results suggest that interference with hippocampal NMDA receptors modify the anxiogenic effect of restraint stress in an EPM.

Anticonvulsive effect of swim stress in mice

To explore the possible involvement of glucocorticoids in the previously observed anticonvulsive effect of swim stress, mice were, prior to administration of convulsants, subjected to treatments that diminish or enhance plasma corticosterone levels. Aminoglutethimide, the inhibitor of steroid synthesis, failed to modify convulsant doses of picrotoxin, but enhanced threshold doses of pentylenetetrazole producing myoclonus and death, both in unstressed and stressed animals. The same drug prevented the effect of stress on pentylenetetrazole-induced running bouncing clonus (RB clonus) and abolished the appearance of tonic hindlimb extension (THE). Doses of kainic acid producing convulsions and death were not affected by stress, but they were enhanced by aminoglutethimide. Corticosterone administration could not imitate the effect of swim stress. Finasteride, a 5 alpha-reductase inhibitor, did not interfere with the effect of stress on picrotoxin-induced convulsions. Swim stress failed to modify the binding of the convulsant t[3H]-butylbicycloorthobenzoate [3H]TBOB, to washed mouse forebrain membranes. The results confirmed an anticonvulsant effect of swim stress against convulsions produced by GABA-related convulsants, but they do not support the hypothesis suggesting the involvement of glucocorticoids or neurosteroids in this effect.

Prior electrical stimulation of the inferior colliculus sensitizes rats to the stress of the elevated plus-maze test

Besides its primary function in the transmission of acoustic signals, the inferior colliculus (IC) is involved in conveying auditory information of aversive nature to higher cortical structures. In the field of anxiety research, one widely used animal model is the electrical stimulation of a given structure supposed to be involved in the neural circuitry underlying emotional behavior. Indeed, electrical stimulation of the inferior colliculus produces fear-like responses. Moreover, prior exposure to stressful events sensitizes rodents' responsivity to fearful stimuli when they are subsequently tested in the elevated plus-maze. Based on these evidence it seems to be important to know whether animals stimulated in the inferior colliculus would show a heightened behavioral responsivity to subsequent stressful events. To this end, we examined the temporal course of the effects of the electrical stimulation of this midbrain region (5, 10 and 15 min afterward) on the conventional and ethological measures of the behavior of rats exposed to the elevated plus-maze test. Prior electrical stimulation of the inferior colliculus produced an 'anxiogenic' profile in the elevated plus-maze, i.e. a significant reduction in the number of entries and time spent into the open arms. Still, previous electrical stimulation of the inferior colliculus caused a significant decrease in rearing, scanning, peeping out, head dipping and end-arm activity while increased immobility. All these changes occurred after 5 min of inferior colliculus electrical stimulation. Therefore, stimulation of the inferior colliculus causes a heightened responsivity to anxiogenic stimuli inherent to the elevated plus-maze test. These findings bring additional support to the proposed role of this midbrain structure in the elaboration of adaptive responses to stressful situations.

Prolactin levels in juvenile and adult rats following acute restraint and the open field

Acute restraint and exposure to a novel environment alter behavior and increase prolactin levels in rats quickly and reliably. However, little research is available that examines behavior and levels of prolactin as a result of acute exposure to one stressor immediately followed by a second stressor. Similarly, a relationship between prolactin and behavior has not been established. In the present study, juvenile (35-day-old) and adult (5-month-old) rats were either placed in a novel open field for 10 min or restrained for 10 min prior to exposure to the open field. Restrained juveniles groomed more than control juveniles and restrained adults. Conversely, restraint + open field reduced ambulation and rearing among juvenile males and females, and adult females ambulated less than control females and restrained males across both behaviors. In addition, results from the present study demonstrated the first reported relationship between prolactin and open-field behaviors. Prolactin was positively correlated with rearing and number of fecal boli, and levels were negatively correlated with freezing. Among adult females, prolactin levels were lower following restraint + open field than after exposure only to the open field. This may be explained by the psychological response to the open field when it directly followed the physical stressor of acute restraint.

Restraint-induced hypoactivity in an elevated plus-maze

Rodents submitted to restraint stress show decreased activity in an elevated plus-maze (EPM) 24 h later. The objective of the present study was to determine if a certain amount of time is needed after stress for the development of these changes. We also wanted to verify if behavioral tolerance of repeated daily restraint would be detectable in this model. Male Wistar rats were restrained for 2 h and tested in the EPM 1, 2, 24 or 48 h later. Another group of animals was immobilized daily for 2 h for 7 days, being tested in the EPM 24 h after the last restraint period. Restraint induced a significant decrease in the percent of entries and time spent in the open arms, as well as a decrease in the number of enclosed arm entries. The significant effect in the number of entries and the percentage of time spent in the open arms disappeared when the data were submitted to analysis of covariance using the number of enclosed arm entries as a covariate. This suggests that the restraint-induced hypoactivity influences the measures of open arm exploration. The modifications of restraint-induced hypoactivity are evident 24 or 48 h, but not 1 or 2 h, after stress. In addition, rats stressed daily for seven days became tolerant to this effect.

The relationship between anxiety and depression in animal models: a study using the forced swimming test and elevated plus-maze

The present study evaluated the correlation between the behavior of mice in the forced swimming test (FST) and in the elevated plus-maze (PM). The effect of the order of the experiments, i.e., the influence of the first test (FST or PM) on mouse behavior in the second test (PM or FST, respectively) was compared to handled animals (HAND). The execution of FST one week before the plus-maze (FST-PM, N = 10), in comparison to mice that were only handled (HAND-PM, N = 10) in week 1, decreased % open entries (HAND-PM: 33.6 +/- 2.9; FST-PM: 20.0 +/- 3.9; mean +/- SEM; P<0.02) and % open time (HAND-PM: 18.9 +/- 3.3; FST-PM: 9.0 +/- 1.9; P<0.03), suggesting an anxiogenic effect. No significant effect was seen in the number of closed arm entries (FST-PM: 9.5 (7.0-11.0); HAND-PM: 10.0 (4.0-14.5), median (interquartile range); U = 46.5; P>0.10). A prior test in the plus-maze (PM-FST) did not change % immobility time in the FST when compared to the HAND-FST group (HAND-FST: 57.7 +/- 3.9; PM-FST: 65.7 +/- 3.2; mean +/- SEM; P>0.10). Since these data suggest that there is an order effect, the correlation was evaluated separately with each test sequence: FST-PM (N = 20) and PM-FST (N = 18). There was no significant correlation between % immobility time in the FST and plus-maze indexes (% time and entries in open arms) in any test sequence (r: -0.07 to 0.18). These data suggest that mouse behavior in the elevated plus-maze is not related to behavior in the forced swimming test and that a forced swimming test before the plus-maze has an anxiogenic effect even after a one-week interval.