Restraint stress-induced changes in saccharin preference: the effect of antidepressive treatment and diazepam

Behavioral and accumbens synaptic plasticity induced by cues associated with restraint stress

Exposure to acute stress can increase vulnerability to develop or express many psychiatric disorders, including post-traumatic stress disorder. We hypothesized that stress-induced psychiatric vulnerability is associated with enduring neuroplasticity in the nucleus accumbens core because stress exposure can alter drug addiction-related behaviors that are associated with accumbens synaptic plasticity. We used a single 2-h stress session and 3 weeks later exposed male and female rats to stress-conditioned odors in a modified defensive burying task, and quantified both active and avoidant coping strategies. We measured corticosterone, dendritic spine and astrocyte morphology in accumbens, and examined reward sensitivity using a sucrose two-bottle choice and operant sucrose self-administration. Exposure to stress odor increased burying (active coping) and immobility (avoidant coping) in the defensive burying task in female and male rats. Systemic corticosterone was transiently increased by both ongoing acute restraint stress and stress-conditioned odors. Three weeks after administering acute restraint stress, we observed increased dendritic spine density and head diameter, and decreased synaptic association with astroglia and the astroglial glutamate transporter, GLT-1. Exposure to conditioned stress further increased head diameter without affecting spine density or astroglial morphology, and this increase by conditioned stress was correlated with burying behavior. Finally, we found that stress-exposed females have a preference for sweet solutions and higher motivation to seek sucrose than stressed male rats. We conclude that acute stress produced enduring plasticity in accumbens postsynapses and associated astroglia. Moreover, conditioned stress odors induced active behavioral coping strategies that were correlated with dendritic spine morphology.

[18F]MPPF and [18F]FDG μPET imaging in rats: impact of transport and restraint stress

Background

Stress exposure can significantly affect serotonergic signaling with a particular impact on 5-HT_1A receptor expression. Positron emission tomography (PET) provides opportunities for molecular imaging of alterations in 5-HT_1A receptor binding following stress exposure. Considering the possible role of 5-HT_1A receptors in stress coping mechanisms, respective imaging approaches are of particular interest.

Material and methods

For twelve consecutive days, Sprague Dawley rats were exposed to daily transport with a 1 h stay in a laboratory or daily transport plus 1 h restraint in a narrow tube. Following, animals were subjected to μPET imaging with 2′-methoxyphenyl-(N-2′-pyridinyl)-p-[¹⁸F]fluoro-benzamidoethylpiperazine ([¹⁸F]MPPF) and 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG). Behavioral and biochemical parameters were analyzed to obtain additional information.

Results

In rats with repeated transport, hippocampal [¹⁸F]MPPF binding exceeded that in the naive group, while no difference in [¹⁸F]FDG uptake was detected between the groups. A transient decline in body weight was observed in rats with transport or combined transport and restraint. Thereby, body weight development correlated with [¹⁸F]MPPF binding.

Conclusions

Mild-to-moderate stress associated with daily transport and exposure to a laboratory environment can trigger significant alterations in hippocampal binding of the 5-HT_1A receptor ligand [¹⁸F]MPPF. This finding indicates that utmost care is necessary to control and report transport and associated handling procedures for animals used in μPET studies analyzing the serotonergic system in order to enhance the robustness of conclusions and allow replicability of findings. In view of earlier studies indicating that an increase in hippocampal 5-HT_1A receptor expression may be associated with a resilience to stress, it would be of interest to further evaluate 5-HT_1A receptor imaging approaches as a candidate biomarker for the vulnerability to stress.

Social support rescues acute stress-induced cognitive impairments by modulating ERK1/2 phosphorylation in adolescent mice

Social support can relieve stress-induced behavioural outcomes, although its underlying molecular mechanisms are not fully understood. Here, we evaluated whether social interactions can prevent the restraint stress (RS)-induced cognitive impairments in male adolescent mice by utilizing molecular, cellular, and behavioural approaches. Acute RS in adolescent ICR mice impaired the working memory in the Y-maze test and memory consolidation and retrieval in the novel-object-recognition test (NORT). In addition, RS increased the extracellular signal-regulated kinases 1/2 phosphorylation (p-ERK1/2) in the prefrontal cortex (PFC) and corticosterone levels in the plasma. Interestingly, these outcomes were normalized by the presence of a conspecific animal (social support) during RS. RS also significantly upregulated the expression levels of known stress-relevant genes such as Egr1, Crh, and Crhr1, which were normalized by social support. Systemic injection of SL327 (an inhibitor of MEK1/2 that also blocks its downstream signal ERK1/2) prior to RS rescued the working memory impairments and the increased p-ERK1/2 while normalizing the expression of Egr1. Our results suggest that social support can alleviate the RS-induced cognitive impairments partly by modulating ERK1/2 phosphorylation and gene transcription in the PFC, and provide novel insights into the molecular mechanisms of the stress-buffering effects of social support.

Corticosterone regulates both naturally occurring and cocaine-induced dopamine signaling by selectively decreasing dopamine uptake

Stressful and aversive events promote maladaptive reward-seeking behaviors such as drug addiction by acting, in part, on the mesolimbic dopamine system. Using animal models, data from our laboratory and others show that stress and cocaine can interact to produce a synergistic effect on reward circuitry. This effect is also observed when the stress hormone corticosterone is administered directly into the nucleus accumbens (NAc), indicating that glucocorticoids act locally in dopamine terminal regions to enhance cocaine's effects on dopamine signaling. However, prior studies in behaving animals have not provided mechanistic insight. Using fast-scan cyclic voltammetry, we examined the effect of systemic corticosterone on spontaneous dopamine release events (transients) in the NAc core and shell in behaving rats. A physiologically relevant systemic injection of corticosterone (2 mg/kg i.p.) induced an increase in dopamine transient amplitude and duration (both voltammetric measures sensitive to decreases in dopamine clearance), but had no effect on the frequency of transient release events. This effect was compounded by cocaine (2.5 mg/kg i.p.). However, a second experiment indicated that the same injection of corticosterone had no detectable effect on the dopaminergic encoding of a palatable natural reward (saccharin). Taken together, these results suggest that corticosterone interferes with naturally occurring dopamine uptake locally, and this effect is a critical determinant of dopamine concentration specifically in situations in which the dopamine transporter is pharmacologically blocked by cocaine.

Reducing the stress of drug administration: implications for the 3Rs

Restraint in animals is known to cause stress but is used during almost all scientific procedures in rodents, representing a major welfare and scientific issue. Administration of substances, a key part of most scientific procedures, almost always involves physical restraint of the animal. In this study, we developed a method to inject substances to rats using a non-restrained technique. We then compared the physiological, behavioral and emotional impacts of restrained versus non-restrained injection procedures. Our results highlight the negative welfare implications associated with physical restraint and demonstrate a method which can be used to avoid this. Our work shows how adopting strategies that avoid restraint can minimize a widespread source of stress in laboratory animals and improve welfare through refinement.

Role of the insular cortex in taste familiarity

Determining the role of the main gustatory cortical area within the insular cortex (IC), in conditioned taste aversion (CTA) has been elusive due to effective compensatory mechanisms that allow animals to learn in spite of lacking IC. IC lesions performed before CTA training induces mild if any memory impairments, while IC lesions done weeks after CTA produce amnesia. IC lesions before taste presentation have also been shown not to affect taste familiarity learning (attenuation of neophobia). This lack of effect could be either explained by compensation from other brain areas or by a lack of involvement of the IC in taste familiarity. To assess this issue, rats were bilaterally IC lesioned with ibotenic acid (200-300 nl.; 15 mg/ml) one week before or after taste familiarity, using either a preferred (0.1%) or a non-preferred (0.5%) saccharin solution. Rats lesioned before familiarity showed a decrease in neophobia to both solutions but no difference in their familiarity curve or their slope. When animals were familiarized and then IC lesioned, both IC lesioned groups treated the solutions as familiar, showing no differences from sham animals in their retention of familiarity. However, both lesioned groups showed increased latent inhibition (or impaired CTA) when CTA trained after repeated pre-exposures. The role of the IC in familiarity was also assessed using temporary inactivation of the IC, using bilateral micro-infusions of sodium channel blocker bupivacaine before each of 3 saccharin daily presentations. Intra-insular bupivacaine had no effects on familiarity acquisition, but did impair CTA learning in a different group of rats micro-infused before saccharin presentation in a CTA training protocol. Our data indicate that the IC is not essentially involved in acquisition or retention of taste familiarity, suggesting regional dissociation of areas involved in CTA and taste familiarity.

Trait pessimism predicts vulnerability to stress-induced anhedonia in rats

Depressive disorder is often associated with cognitive biases. In this study, we took a unique opportunity to investigate whether trait pessimism could predict vulnerability to stress-induced anhedonia in an animal model of depression. In a series of ambiguous-cue interpretation (ACI) tests, we identified animals displaying 'pessimistic' and 'optimistic' traits. Subsequently, the rats were subjected to chronic restraint, and the trait differences in response to stress were investigated using sucrose preference and ACI tests before, during and after the stress regime. Although stress resulted in anhedonia in both subgroups, it occurred faster and lasted longer in the 'pessimistic' compared with the 'optimistic' animals. Chronic stress exposure also increased the negative judgment bias in rats, although this effect was not dependent on the 'pessimistic' trait. For the first time, we demonstrated a link between cognitive judgment bias and vulnerability to stress-induced anhedonia in an animal model. We also introduced a cognitive biomarker, which may be of value for etiological depression studies.

Maternal neglect with reduced depressive-like behavior and blunted c-fos activation in Brattleboro mothers, the role of central vasopressin

Early mother-infant relationships exert important long-term effects in offspring and are disturbed by factors such as postpartum depression. We aimed to clarify if lack of vasopressin influences maternal behavior paralleled by the development of a depressive-like phenotype. We compared vasopressin-deficient Brattleboro mothers with heterozygous and homozygous normal ones. The following parameters were measured: maternal behavior (undisturbed and separation-induced); anxiety by the elevated plus maze; sucrose and saccharin preference and forced swim behavior. Underlying brain areas were examined by c-fos immunocytochemistry among rest and after swim-stress. In another group of rats, vasopressin 2 receptor agonist was used peripherally to exclude secondary changes due to diabetes insipidus. Results showed that vasopressin-deficient rats spend less time licking-grooming their pups through a centrally driven mechanism. There was no difference between genotypes during the pup retrieval test. Vasopressin-deficient mothers tended to explore more the open arms of the plus maze, showed more preference for sucrose and saccharin and struggled more in the forced swim test, suggesting that they act as less depressive. Under basal conditions, vasopressin-deficient mothers had more c-fos expression in the medial preoptic area, shell of nucleus accumbens, paraventricular nucleus of the hypothalamus and amygdala, but not in other structures. In these areas the swim-stress-induced activation was smaller. In conclusion, vasopressin-deficiency resulted in maternal neglect due to a central effect and was protective against depressive-like behavior probably as a consequence of reduced activation of some stress-related brain structures. The conflicting behavioral data underscores the need for more sex specific studies.

Effect of repeated restraint stress and clomipramine on Na+/K+-ATPase activity and behavior in rats

Activation of the limbic-hypothalamic-pituitary-adrenal axis (LHPA) and the release of glucocorticoids are fundamental for the adaptive response and immediate survival of an organism in reaction to acute stimuli. However, high levels of glucocorticoids in the brain may produce neuronal injury and a decrease of Na(+)/K(+)-ATPase activity, with effects on neurotransmitter signaling, neural activity, as well as the whole animal behavior. Clomipramine is a tricyclic antidepressant that inhibits the reuptake of serotonin and norepinephrine by indirect actions on the dopaminergic system and LHPA axis. Its chronic use increases the body's ability to cope with stress; however, high doses can potentiate its side effects on memory, learning, and sensory motor function. The purpose of the present study was to compare the effect of repeated restraint stress and clomipramine treatment on Na(+)/K(+)-ATPase activity and on the behavior of male rats. Changes in the behavioral response were evaluated by measuring the memory, learning, anxiety, and exploratory responses. Our results showed that exposure to repeated restraint stress reduced levels of Na(+)/K(+)-ATPase in brain structures and changed short and long-term memory, learning, and exploratory response when compared to the control group. Exposure to clomipramine treatment increased anxiety levels and reduced Na(+)/K(+)-ATPase activity in the cerebral cortex as well as short term memory, learning, and exploratory response. In conclusion, the present results provide additional evidence concerning how repeated restraint stress and clomipramine chronically administered at higher dose levels affect the neural activity and behavior of male rats.

Invivo antioxidant status: a putative target of antidepressant action

Oxidative stress is a critical route of damage in various psychological stress-induced disorders, such as depression. Antidepressants are widely prescribed to treat these conditions; however, few animal studies have investigated the effect of these drugs on endogenous antioxidant status in the brain. The present study employed a 21-day chronic regimen of random exposure to restraint stress to induce oxidative stress in brain, and behavioural aberrations, in rodents. The forced swimming (FST) and sucrose preference tests were used to identify depression-like phenotypes, and reversal in these indices indicated the effectiveness of treatment with fluoxetine (FLU; 20 mg/kg/day, p.o.; selective serotonin reuptake inhibitor), imipramine (IMI; 10 mg/kg/day, p.o.; tricyclic antidepressant) and venlafaxine (VEN; 10 mg/kg/day, p.o.; dual serotonin/norepinephrine reuptake inhibitor) following restraint stress. The antioxidant status was investigated in the brain of these animals. The results evidenced a significant recovery in the activities of superoxide dismutase (SOD), catalase (CAT), glutathione S-transferase (GST), glutathione reductase (GR) and glutathione (GSH) levels by antidepressant treatments following a restraint stress-induced decline of these parameters. The severely accumulated lipid peroxidation product malondialdehyde (MDA) and protein carbonyl contents in stressed animals were significantly normalized by antidepressant treatments. The altered oxidative status is implicated in various aspects of cellular function affecting the brain. Thus, it is possible that augmentation of in vivo antioxidant defenses could serve as a convergence point for multiple classes of antidepressants as an important mechanism underlying the neuroprotective pharmacological effects of these drugs observed clinically in the treatment of various stress disorders. Consequently, pharmacological modulation of stress-induced oxidative damage as a possible stress-management approach should be an important avenue of further research.

SSR181507, a putative atypical antipsychotic with dopamine D2 antagonist and 5-HT1A agonist activities: improvement of social interaction deficits induced by phencyclidine in rats

Social behaviour is frequently impaired in schizophrenic patients, and current antipsychotics appear poorly effective in alleviating this deficit. SSR181507 is a selective dopamine D2 receptor antagonist and 5-HT1A receptor agonist [Neuropsychopharmacology 28 (2003) 2064] with an atypical antipsychotic profile and additional antidepressant/anxiolytic activities [Neuropsychopharmacology 28 (2003) 1889]. Here, we sought to assess the efficacy of SSR181507, and of reference antipsychotics and antidepressant/anxiolytics, to counteract phencyclidine (PCP)-induced social interaction deficit in rats. Pairs of unfamiliar rats were placed for 10 min each day into a dimly lit arena, during four consecutive days. On the test day (5th day), each pair was placed into the arena 30 min after i.p. treatment with PCP (or vehicle) and a challenge compound or vehicle (same for both rats, i.p. or s.c.). The time spent in social interaction was scored during 10 min. PCP (1 mg/kg) decreased social interaction time by about 35%. This effect was fully antagonized by pre-treatment with SSR181507 (1 mg/kg). In contrast, neither haloperidol (0.05 and 0.1 mg/kg) nor clozapine (0.3 and 1 mg/kg) antagonized this PCP-induced deficit. The selective 5-HT1A receptor agonist 8-OH-DPAT (0.025 and 0.05 mg/kg s.c.), but not the anxiolytic diazepam (0.75 and 1.5 mg/kg), also improved social interaction impairment in PCP-treated rats: this would indicate that the 5-HT1A receptor agonist properties of SSR181507 are responsible for the reversal of PCP-induced social deficit. These data suggest that, in addition to its atypical antipsychotic profile and antidepressant/anxiolytic activities, SSR181507 has a potential therapeutic activity in another key feature of schizophrenia poorly controlled by current antipsychotics, namely deterioration in social functioning.

Relationship of anhedonia and anxiety to social rank, defeat and entrapment

There is good evidence to suggest that depression is highly correlated with perceptions of low rank and subordinate status (i.e. feeling inferior, low-self esteem, feeling that others look down on the self, and submissive behaviour). However, it is possible for people to feel inferior and anxious, and behave submissively but not necessarily be depressed. More recently two other processes, defeat and entrapment, have attracted attention as possible processes linked specifically to depression and anhedonia. This research explored the relationship of these variables (social rank variables and defeat and entrapment) to two measures of hedonic tone (low positive affectivity and anhedonia) and anxiety in both a clinical and student population. All variables were strongly associated with lowered hedonic tone and anxiety. However, partial correlations, and a structural equation model fitted to the data from combined groups, suggests that perceptions of defeat play a specifically important role in anhedonia as measured by low positive affect. Framed within an evolutionary model the data suggest that the mechanisms which evolved to help animals accommodate and respond to defeats may have important regulatory effects over positive affect, reducing exploration of and engagement with the environment.

Early exposure to chronic variable stress facilitates the occurrence of anhedonia and enhanced emotional reactions to novel stressors: reversal by naltrexone pretreatment

The present research studied the influence of an early chronic variable stress (CVS) paradigm - an animal model of depression - on behavioral responses to subsequent environmental challenges suggested to model anhedonia and emotional reactions such as anxiety and fear. In order to explore a potential involvement of an endogenous opiate mechanism - presumably activated during CVS exposure - in the development of such behavioral reactions, in all experiments rats were administered naltrexone (NAL, 2 mg/kg, i.p.) or vehicle (VH) prior to each daily stressor of the CVS procedure. Animals were exposed to CVS and 1 week later tested for sucrose preference (1%) in a free choice paradigm after the presentation or not of a 90-min restraint period. Only CVS treated animals that were later exposed to restraint showed a reduction of sucrose preference, this reduction was absent when CVS rats were pretreated previously with NAL. Moreover, CVS rats were one week later tested on the elevated plus maze (EPM) and in their conditioned and unconditioned freezing response to a single shock session. Early chronic stress resulted in an anxiogenic behavior in the EPM and in an enhanced conditioned and unconditioned freezing which were all abolished by NAL pretreatment. These behavioral findings suggest that the potential activation of an endogenous opiate mechanism during CVS participates in the development of anhedonia and exaggerated emotional reactions in response to subsequent stressful experiences.

Prevention of stress-induced weight loss by third ventricle CRF receptor antagonist

We previously reported that rats exposed to repeated restraint (3 h/day for 3 days) experience temporary hypophagia and a sustained reduction in body weight compared with nonrestrained controls. Studies described here determined the involvement of central corticotropin-releasing factor (CRF) receptors in the initiation of this chronic response to acute stress. In experiment 1, Sprague-Dawley rats were fitted with cannulas in the lateral ventricle and infused with 50 micrograms of alphahCRF-(9-41) or saline immediately before restraint on each of the 3 days of restraint. The receptor antagonist inhibited hypophagia and weight loss on day 1 of restraint but not on days 2 and 3. In experiment 2, 10 micrograms of alphahCRF-(9-41) or saline were infused into the third ventricle immediately before each restraint. The receptor antagonist totally blocked stress-induced hypophagia and weight loss. These results demonstrate that CRF receptors located in or near the hypothalamus mediate the acute responses to stress that lead to a permanent change in the hormonal or metabolic processes that determine body weight and body composition.

Chronic stress sensitizes frontal cortex dopamine release in response to a subsequent novel stressor: reversal by naloxone

The present study examined the influence of an early chronic variable stress procedure with or without concurrent naloxone administration at different doses (1, 2 or 3 mg/kg, i.p.) on stress (restraint)-induced dopamine release in the frontal cortex in vivo. A higher increase in cortical dopamine release in response to a subsequent restraint event was observed in chronically stressed rats as compared with those without chronic stress exposure. Naloxone pretreatment normalized this sensitized response only at the higher dose (3 mg/kg, i.p.). The present results indicate that cortical dopamine response to a novel and uncontrollable stressor sensitizes after exposure to a chronic variable stress procedure and that an endogenous opiate mechanism, presumably activated during chronic stress, may be involved in the development of such a sensitization process.

Prior morphine facilitates the occurrence of immobility and anhedonia following stress

The role of the activation of the opiate system either induced by a 120-min restraint session or by a single morphine administration (10 mg/kg, i.p.) on the behaviors performed in a subsequent forced-swim test has been evaluated. In addition, animals were pretreated with naloxone (2 mg/kg, i.p.) prior to restraint or to morphine. Furthermore, in order to evaluate if this opioid mechanism could participate in the effect of stress on the response to a rewarding stimulus, rats were administered with morphine (10 mg/kg, i.p.)--whether associated or not with prior naloxone (2 mg/kg, i.p.) administration--and subsequently exposed to a 90-min restraint period. Following stress, all rats were submitted to a sucrose (1%) preference test. Both morphine and restraint enhanced the time spent in immobility in the forced-swim test. Both behavioral effects were attenuated by naloxone pretreatment thus suggesting that the increased immobility is probably modulated by the previous activation of an opiate mechanism. Furthermore, only animals with the associated treatment with morphine and restraint showed a clear reduction in sucrose preference. The fact that this effect was blocked by naloxone suggests the involvement of an opiate process in this decreased response to reward. These behavioral data suggest that the activation of an endogenous opiate mechanism facilitates the occurrence of enhanced immobility and anhedonia in response to a subsequent stress experience.

The anhedonias: Clinical and neurobiological aspects

During the last 30 years there has been renewed clinical interest in the state of 'lack of pleasure' (anhedonia) seen in conditions such as schizophrenia and depression. In spite of some important work, confusion still remains about the term, behaviours and explanatory concepts pertaining to anhedonia. This paper reviews the clinical and basic scientific studies that throw light on this interesting clinical phenomenon and then presents a new model of anhedonia which can be tested empirically and should facilitate research in this field.

Changes in Behaviour and Body Weight Following a Single or Double Social Defeat in Rats

In a series of experiments, the consequences of a single and double social conflict on various behaviours and body weight in rats were studied. Animals were subjected to social defeat by placing them into the territory of an aggressive male conspecific for one hour, either once, or twice at the same time on two consecutive days. To assess the consequences of social defeat, three experiments were performed with independent groups of rats. In the first experiment, an open field test was performed two days after the last conflict. Locomotor activity was strongly reduced after social defeat. There were no differences between the single and double defeat group. To assess the effects of social defeat on subsequent social behaviour, a second experiment was performed in which experimental animals were confronted with an unfamiliar non-aggressive rat two days after a single or double conflict. Social defeat resulted in a reduction of social contact with the unfamiliar conspecific. There was no difference between the single and double conflict group. In the third experiment, the effects of social conflict on food intake, body weight and saccharine preference were measured. Food intake was not affected after a single conflict, but in the double conflict group food intake was decreased for several days. Body weight gain was decreased after both single and double social defeat. The decrease was stronger in the double conflict group. Water intake and saccharine preference were not significantly affected. This study revealed that social defeat in rats causes pronounced changes in various behaviours and body weight. Different aspects of behaviour are differentially affected by defeat with respect to the magnitude and time course of the changes induced. Moreover, different behavioural parameters are differentially sensitive to repetition of the stressor.

An endogenous opiate mechanism seems to be involved in stress-induced anhedonia

This study assessed the effect of an uncontrollable stressor on the preference for a palatable solution (sucrose 1%), and on the preference for a context associated with a single administration of D-amphetamine (3 mg/kg i.p.) by means of the conditioning place preference test. We also evaluated the effect of prior naloxone (2 mg/kg, i.p.) administration on the influence of this stressful stimulus in both tests. Animals previously submitted to a 120-min--but not 60-min--restraint period showed a selective reduction in the preference for sucrose intake as compared to unstressed animals. Similarly, an identical restraint exposure elicited a diminished preference for the place previously paired with amphetamine. Both stress-induced effects were blocked by prior naloxone administration. These data demonstrate that a highly aversive experience decreased the reinforcing efficacy of sucrose and amphetamine, suggesting that uncontrollable stress may lead to an impaired capacity to experience pleasure, which could resemble the anhedonia observed in clinical depression. Furthermore, an endogenous opiate mechanism activated by stress seems to be involved in stress-induced anhedonia since naloxone normalized the reduction of the rewarding induced by both reinforcers.

Effects of single restraint on the defensive behavior of male and female rats

The effects of single aversive stimulation due to restraint (RT) on behavioral responses to unfamiliar conspecifics were studied in male and female rats. The Resident-intruder paradigm was adopted, RT animals and their controls playing the role of intruders. Introductory and agonistic behaviors of both intruders and residents were recorded. In males, RT increased both the number of subjects which showed freezing and freezing duration, and this was independent of the amount of aggression received by the residents. By contrast, no change was found in active defense. Increased passive defense was not paralleled by a complete inhibition of aggression. The latter was rare, but not absent, and occurred in RT males as often as in their controls. Females never showed freezing and, unlike males, resorted to a fully active defensive strategy. RT females were the preferential targets of residents' high-intensity aggression, but showed the same rate of defensive responding as control females. The crucial role played in studies of social behavior by testing conditions and mutual influences between the behavior of experimental subjects and residents are discussed.

Social stress by repeated defeat: effects on social behaviour and emotionality

The consequences of repeated defeat stress on social and non-social behaviours were assessed in male rats 24 h after the last defeat. Aggressive, defensive, introductory and affiliative items of both experimental animals and their opponents were recorded in a social behaviour test, while emotionality, exploration and general motor activity were scored in the Emergency, Hole-Board, and Elevated Plus-Maze tests. In addition to a dramatic loss of body weight, a selective inhibition of aggression was observed in the stressed experimental subjects, paralleled by decreased defence in their opponents. In the stressed animals, no change was found in other social and non-social behaviours; in particular, defence and emotionality were unaffected. This shows that, under our experimental conditions, the inhibition of aggression, which has often been reported to parallel an increase in defence after social and non-social aversive stimulation, was not dependent on a concomitant activation of a prevailing defensive motivational system, sustained by increased emotionality and fear. As the same result, namely a selective inhibition of aggression with no effect on defence, was obtained after exposure to a non-social stressor (restraint), the hypothesis is advanced that the threshold for stress-induced behavioural changes is lower for aggression than for any other behavioural and motivational system, including that leading to defence. The inhibition of aggression would therefore be a direct response to stress and not a by-product of the activation of a fear-based defensive system.

Restraint stress in biomedical research: an update

Since the publication of our initial review of restraint stress in 1986, much work has continued with this technique, either as a tool for the investigation of other pharmacological, physiological, or pathologic phenomena or with restraint stress itself serving as the object of the study. As we noted in 1986, the major use of restraint has been for the induction of stress responses in animals and, more specifically, for the investigation of drug effects, particularly as they affect typical stress-related pathology--gastrointestinal, neuroendocrine, and immunological agents have been extensively studied. In compiling this update on restraint stress and its effects, we noted an increasing emphasis on central nervous system mechanisms in peripheral disease, especially gastrointestinal disease. In particular, many CNS-active agents have been tested for their effects on gastric and duodenal lesion formation and gastric secretion, including antidepressants, antipsychotics, anxiolytics, noradrenergic, serotonergic, dopaminergic, and peptidergic compounds. Some of these agents are especially active in the gastrointestinal tract even when administered centrally, further solidifying the concept of a brain-gut axis. The present update includes studies of: methods and procedures, pre-restraint manipulations, post-restraint/healing effects, and drug effects. In addition, a current bibliography of reports that have employed restraint is included.

Chronic mild stress-induced anhedonia: greater effect in a genetic rat model of depression

The effects of acute and chronic stressors on saccharin intake and preference in the hypercholinergic Flinders Sensitive Line (FSL) rat, a putative genetic animal model of depression, were studied and compared to the control Flinders Resistant Line (FRL) rats. Overall, the FRL rats drank significantly less saccharin and water than the FSL rats when compared over a wide range of saccharin concentrations (0.01-5%) under baseline conditions. A 0.02% saccharin concentration was used in subsequent experiments. We observed a significant suppression of saccharin intake/preference at 1 h following a single 5-min exposure to cold swim stress only in FSL rats. There was a tendency to increase saccharin intake in both lines at 1 h following a scrambled foot shock stress. These effects of acute stressors disappeared upon retesting for saccharin consumption/preference 23 h after the stress. Chronic 4-week exposure to unpredictable mild stressors significantly (p < 0.01) decreased saccharin consumption in the FSL rats, but not in the FRL rats. The FSL rats also exhibited a significantly greater decrease in saccharin preference (-24% vs. prestress baseline, as compared to -7% in FRL controls, p < 0.05). In conclusion, FSL rats appear more prone than the FRL rats to chronic, as well as immediate acute, stress-induced anhedonic effects. This outcome further supports the notion that the FSL rat is a useful model of a genetic predisposition to depressive-like reactions.

Effects of repeated mild stress and two antidepressant treatments on the behavioral response to 5HT1C receptor activation in rats

This study investigated the possible involvement of 5HT1C receptors in the development of depressive states and in the mode of action of antidepressants. The effects of repeated unpredictable mild stress (a regimen known to induce an anhedonic state in the rat) and of chronic administration of either of two recognized antidepressant treatments (sleep deprivation or inhibition of monoamine oxidase type A) in rats were studied on a 5HT1C receptor initiated response, i.e. mCPP-induced penile erection. A 3-week period of repeated, but unpredictable exposure to mild stressors induced a shift to the left of the dose-response curve for mCPP-induced penile erection. In contrast, 72-h REM sleep deprivation resulted in a shift to the right of the mCPP dose-response curve and 10-day administration of the monoamine oxidase type A inhibitor moclobemide (20 mg/kg IP bid) also resulted in a decreased number of mCPP-induced penile erections. These findings support the hypothesis that neuronal activities initiated via 5HT1C receptor stimulation may play a role in the pathophysiology and treatment of depression.

Intake of sweet water attenuates restraint-stress-induced potentiation of morphine analgesia in rats

The analgesia induced in rats by morphine is potentiated by restraint-stress exposure and is reduced in rats that have been consuming a sweet solution. The purpose of the present study was to determine whether the potentiation of morphine-induced analgesia following restraint immobilization would be attenuated in rats consuming a sweet solution. Groups of rats were maintained on unsweetened water or allowed 2 h of daily access to a solution of saccharin and glucose (SG). Half of the rats in each of these groups were subjected to 1 h of restraint stress (groups RS and RS+SG) and the other half in each group were not stressed (groups NS and NS+SG). Rats then underwent 1 h of RS treatment or were nonstressed (NS). The next day all rats were injected subcutaneously with morphine (0.0, 4.0, 8.0, or 16 mg/kg) and analgesia was assessed using the tail flick assay. ED50S (mg/kg) were calculated for each treatment group; NS = 5.8, RS = 1.6, NS+SG = 6.4, and RS+SG = 4.4. Our results demonstrate that RS potentiated morphine-induced analgesia in rats given access to SG as well as non-SG exposed rats that displayed ED50S 1.5 and 3.9 times lower than their respective controls. RS-treated rats that consumed SG solution had significantly lower tail flick latencies than did non-SG exposed rats. Additionally, tail flick latencies of rats in the nonstressed and NS+SG groups did not significantly differ. We conclude that the brain mechanism(s) responsible for RS-induced potentiation of morphine antinociception are attenuated by intake of a sweet solution.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of stress-induced long-term behavioural changes in rats: evidence in favor of anxiety

Recently, we reported that rats exposed to one brief session of inescapable footshocks showed a gradually developing and long-lasting decrease in behavioural activity and an increase in defecation in an open field. The aim of the present study was to further characterize the long-lasting changes in behavioural responsiveness to environmental stimuli. For this purpose, behavioural paradigms validated as tools in the preclinical study of the psychobiology of depression were used. Footshocked rats (S) showed a decreased response latency in an one-way avoidance-escape task and decreased immobility in a forced swim test as compared to nonshocked control rats (C) 14 days after shock exposure. These S rats showed decreased behavioural activity and increased defecation as compared to the C rats in an open field test carried out 28 days after footshock exposure. In addition, footshock exposure did not affect the preference for or consumption of a 0.05% saccharin solution on a long-term basis, although a decreased consumption of this solution was evident in S rats on day 1 postshock. These S rats showed an exaggerated immobility response to a sudden reduction in background noise level compared to C rats while placed in a novel environment on day 11 postshock. We conclude that the long-term effects of one short session of inescapable footshocks are not compatible with what is supposed to represent behavioural manifestations of depression in animals. It is argued that the common denominator of shock-induced long-lasting changes is increased behavioural defensiveness, which is more likely related to increased fear and/or anxiety.

Divergent responses to saccharin vs. sucrose availability after stress in rats

Rats were exposed to 100 5-s inescapable, unpredictable shocks then had access to water and saccharin or sucrose solution for 5-6 days. Shock reduced daily drinking during saccharin tests (Experiments 1, 2B, 4) and increased daily drinking during sucrose tests (Experiment 2B). In addition, shock reduced body weight when saccharin, but not when sucrose, was available (Experiments 1, 2B). The specificity of the reductions to saccharin tests (Experiments 2B, 4), equal intake of flavors shortly after stress (Experiments 2B to 4), and failure of saccharin-naive and saccharin-familiar groups to differ (Experiment 3) argue against a neophobia interpretation. Normal or above-normal drinking during sucrose (Experiment 2B) or water-only (Experiment 4) tests indicate an absence of general hedonic, motoric, or hydrational deficits. Qualitative differences in the tastes of saccharin and sucrose may underlie their disparate effects on drinking after shock. Whatever the mechanisms, the present results show that even a severe stressor can decrease or increase ingestion, depending on the flavors available.

Antidepressant treatment prevents chronic unpredictable mild stress-induced anhedonia as assessed by ventral tegmentum self-stimulation behavior in rats

The effect of chronic unpredictable mild stress on sensitivity to reward was evaluated using the brain self-stimulation procedure. Rats were allowed to electrically self-stimulate the ventral tegmental area, one of the main cerebral structures subserving positive reinforcement. Stimulation thresholds (frequency of stimuli) for self-stimulation responses were determined prior to, during, and following a 19-day period of exposure to a variety of mild unpredictable stressors. Stimulation threshold was increased in stressed rats, suggesting a decrease in the rewarding properties of brain stimulation. This deficit became evident after about 1 week of mild stress, lasted throughout the stress period, and progressively diminished following termination of the stress regime. In stressed rats concomitantly treated with the tricyclic antidepressant desipramine (5 mg/kg b.i.d.), no stress-induced increase in self-stimulation threshold was observed. However, desipramine did not modify self-stimulation threshold in non-stressed animals. Thus, the increased threshold for brain self-stimulation produced by a period of chronic unpredictable mild stress can be completely prevented by concomitant antidepressant treatment and may provide an heuristic animal model of depression.

Chronic mild stress-induced anhedonia: a realistic animal model of depression

Chronic sequential administration of a variety of mild stressors causes a decrease in responsiveness to rewards in rats, which is reversed by chronic administration of antidepressant drugs. This paper reviews the validity of chronic mild stress-induced anhedonia as an animal model of depression, and the evidence that changes in hedonic responsiveness in this model are mediated by changes in the sensitivity of dopamine D2 receptors in the nucleus accumbens. The review opens with an analysis of the design features of animal models of depression, and ends with a brief account of other animal models of anhedonia.