The effects of stress on avoidance in rodents: An unresolved matter

Effects of repeated social stress on risk assessment behaviors and response to diazepam in the elevated plus maze in adult male rats

Anxiety is highly common, and stress is a major trigger for anxiety. Anxiety includes heightened threat assessment and avoidance, but we do not fully understand which components are sensitive to stress. Rodents show a balance of exploration and avoidance that incorporates threat assessment prior to making the relatively risky decision to explore an open area. The purpose of this study was to determine if stress impacts risk assessment and if this is tied to the effects of stress on exploration. The present study used elevated plus maze (EPM) to test the effects of repeated social defeat stress (RSDS) on risk assessment behaviors in adult male rats. We then tested the effects of diazepam, an anxiolytic that reduces the impact of stress on EPM exploration, to further clarify the relationship between risk assessment and risky behavior in the EPM. We found that RSDS decreased time in the open arm, similar to prior studies. We also found that RSDS increased the likelihood of the primary risk assessment behavior, stretch and attend posture (SAP), increased SAP prior to entering an open arm, and decreased the likelihood that a rat would enter an open arm after SAP. Diazepam ameliorated the effects of RSDS on both SAP and exploratory behavior, further linking risk assessment and subsequent exploratory behaviors. These results suggest that increased risk assessment and reduced risky choices after risk assessment are tied to effects of stress on exploration and provide novel insight into how stress may increase avoidance by effects on risk assessment. (PsycInfo Database Record (c) 2025 APA, all rights reserved).

The effects of shared, depression-specific, and anxiety-specific internalizing symptoms on negative and neutral episodic memories following post-learning sleep

Emotional memory bias is a common characteristic of internalizing symptomatology and is enhanced during sleep. The current study employs bifactor S-1 modeling to disentangle depression-specific anhedonia, anxiety-specific anxious arousal, and the common internalizing factor, general distress, and test whether these internalizing symptoms interact with sleep to influence memory for emotional and neutral information. Healthy adults (N = 281) encoded scenes featuring either negative objects (e.g., a vicious looking snake) or neutral objects (e.g., a chipmunk) placed on neutral backgrounds (e.g., an outdoor scene). After a 12-hour period of daytime wakefulness (n = 140) or nocturnal sleep (n = 141), participants judged whether objects and backgrounds were the same, similar, or new compared with what they viewed during encoding. Participants also completed the mini version of the Mood and Anxiety Symptom Questionnaire. Higher anxious arousal predicted worse memory across all stimuli features, but only after a day spent being awake-not following a night of sleep. No significant effects were found for general distress and anhedonia in either the sleep or wake condition. In this study, internalizing symptoms were not associated with enhanced emotional memory. Instead, memory performance specifically in individuals with higher anxious arousal was impaired overall, regardless of emotional valence, but this was only the case when the retention interval spanned wakefulness (i.e., not when it spanned sleep). This suggests that sleep may confer a protective effect on general memory impairments associated with anxiety.

Neurocognitive impairments in rat offspring after maternal exposure to vortioxetine: Involvement of BDNF, apoptosis and cholinergic mediated signaling pathways

Depression in pregnant women raises concerns about the safety of antidepressants use, particularly its impact on offspring's neurocognition. This study investigates the effects of maternal exposure to vortioxetine (VOX) on the neurocognitive development of rat offspring. Pregnant Wistar rats were administered clinically pertinent doses of VOX, 1 mg/kg/day or 2 mg/kg/day from gestational day 6-21. The dams delivered their offspring naturally and reared until postnatal day (PND) 70. Offspring of both sexes were assessed for postnatal growth by measuring body weight from PND 1-70 weekly and cognitive function using Morris water maze (MWM) test and passive avoidance learning test from PND 49-70. After behavioral assessments, adult rat offspring were sacrificed, and their brains were dissected out for assessment of brain morphology as well as biochemical analysis. The results demonstrated that VOX exposure potentially impaired cognitive performance, evidenced by increased latency in MWM and passive avoidance learning tests. Additionally, it led to decreased body weight, altered brain morphology, and disrupted neurobiochemical profiles. Specifically, VOX 2 mg/kg exposure significantly reduced brain-derived neurotrophic factor (BDNF) expression, increased pro-apoptotic BAX expression, decreased anti-apoptotic Bcl-2 expression, and elevated acetylcholinesterase (AChE) activity in the hippocampus. Lower dose of VOX (1 mg/kg) did not show significant adverse effects on neurocognition, suggesting a dose-dependent impact. No sex specific neurocognitive deficits were observed in current study. These findings indicate that while VOX may offer a safer profile compared to SSRIs, high doses during pregnancy can still result in neurocognitive impairments in offspring.

Acute stress yields a sex-dependent facilitation of signaled active avoidance in rats

Post-traumatic stress disorder (PTSD) is a debilitating disorder characterized by excessive fear, hypervigilance, and avoidance of thoughts, situations or reminders of the trauma. Among these symptoms, relatively little is known about the etiology of pathological avoidance. Here we sought to determine whether acute stress influences avoidant behavior in adult male and female rats. We used a stress procedure (unsignaled footshock) that is known to induce long-term sensitization of fear and potentiate aversive learning. Rats were submitted to the stress procedure and, one week later, underwent two-way signaled active avoidance conditioning (SAA). In this task, rats learn to prevent an aversive outcome (shock) by performing a shuttling response when exposed to a warning signal (tone). We found that acute stress significantly enhanced SAA acquisition rate in females, but not males. Female rats exhibited significantly greater avoidance responding on the first day of training relative to controls, reaching similar levels of performance by the second day. Males that underwent the stress procedure showed similar rates of acquisition to controls but exhibited resistance to extinction. This was manifest as both elevated avoidance and intertrial responding across extinction days relative to non-stressed controls, an effect that was not observed in females. In a second experiment, acute stress sensitized footshock unconditioned responses in males, not females. However, males and females exhibited similar levels of stress-enhanced fear learning (SEFL), which was expressed as sensitized freezing to a shock-paired context. Together, these results reveal that acute stress facilitates SAA performance in both male and female rats, though the nature of this effect is different in the two sexes. We did not observe sex differences in SEFL, suggesting that the stress-induced sex difference in performance was selective for instrumental avoidance. Future work will elucidate the neurobiological mechanisms underlying the differential effect of stress on instrumental avoidance in male and female rats.

Effects of chronic light cycle disruption during adolescence on circadian clock, neuronal activity rhythms, and behavior in mice

The advent of artificial lighting, particularly during the evening and night, has significantly altered the predictable daily light and dark cycles in recent times. Altered light environments disrupt the biological clock and negatively impact mood and cognition. Although adolescents commonly experience chronic changes in light/dark cycles, our understanding of how the adolescents' brain adapts to altered light environments remains limited. Here, we investigated the impact of chronic light cycle disruption (LCD) during adolescence, exposing adolescent mice to 19 h of light and 5 h of darkness for 5 days and 12 L:12D for 2 days per week (LCD group) for 4 weeks. We showed that LCD exposure did not affect circadian locomotor activity but impaired memory and increased avoidance response in adolescent mice. Clock gene expression and neuronal activity rhythms analysis revealed that LCD disrupted local molecular clock and neuronal activity in the dentate gyrus (DG) and in the medial amygdala (MeA) but not in the circadian pacemaker (SCN). In addition, we characterized the photoresponsiveness of the MeA and showed that somatostatin neurons are affected by acute and chronic aberrant light exposure during adolescence. Our research provides new evidence highlighting the potential consequences of altered light environments during pubertal development on neuronal physiology and behaviors.

Acute stress yields a sex-dependent facilitation of signaled active avoidance in rats

Post-traumatic stress disorder (PTSD) is a debilitating disorder characterized by excessive fear, hypervigilance, and avoidance of thoughts, situations or reminders of the trauma. Among these symptoms, relatively little is known about the etiology of pathological avoidance. Here we sought to determine whether acute stress influences avoidant behavior in adult male and female rats. We used a stress procedure (unsignaled footshock) that is known to induce long-term sensitization of fear and potentiate aversive learning. Rats were submitted to the stress procedure and, one week later, underwent two-way signaled active avoidance conditioning (SAA). In this task, rats learn to prevent an aversive outcome (shock) by performing a shuttling response when exposed to a warning signal (tone). We found that acute stress significantly enhanced SAA acquisition rate in females, but not males. Female rats exhibited significantly greater avoidance responding on the first day of training relative to controls, reaching similar levels of performance by the second day. Males that underwent the stress procedure showed similar rates of acquisition to controls but exhibited resistance to extinction. This was manifest as both elevated avoidance and intertrial responding across extinction days relative to non-stressed controls, an effect that was not observed in females. In a second experiment, acute stress sensitized footshock unconditioned responses in males, not females. However, males and females exhibited similar levels of stress-enhanced fear learning (SEFL), which was expressed as sensitized freezing to a shock-paired context. Together, these results reveal that acute stress facilitates SAA performance in both male and female rats, though the nature of this effect is different in the two sexes. We did not observe sex differences in SEFL, suggesting that the stress-induced sex difference in performance was selective for instrumental avoidance. Future work will elucidate the neurobiological mechanisms underlying the differential effect of stress on instrumental avoidance in male and female rats.

Effects of chronic social equality and inequality conditions on passive avoidance memory and PTSD-like behaviors in rats under chronic empathic stress

INTRODUCTION

Social inequality conditions induce aversion and affect brain functions and mood. This study investigated the effects of chronic social equality and inequality (CSE and CSI, respectively) conditions on passive avoidance memory and post-traumatic stress disorder (PTSD)-like behaviors in rats under chronic empathic stress.

METHODS

Rats were divided into different groups, including control, sham-observer, sham-demonstrator, observer, demonstrator, and co-demonstrator groups. Chronic stress (2 h/day) was administered to all stressed groups for 21 days. Fear learning, fear memory, memory consolidation, locomotor activity, and PTSD-like behaviors were evaluated using the passive avoidance test. Apart from the hippocampal weight, the correlations of memory and right hippocampal weight with serum corticosterone (CORT) levels were separately assessed for all experimental groups.

RESULTS

Latency was significantly higher in the demonstrator and sham-demonstrator groups compared to the control group. It was decreased significantly in other groups compared to the control group. Latency was also decreased in the observer and co-demonstrator groups compared to the demonstrator group. Moreover, the right hippocampal weight was significantly decreased in the demonstrator and sham-demonstrator groups compared to the control group. Pearson's correlation of memory and hippocampal weight with serum CORT levels supported the present findings.

CONCLUSION

Maladaptive fear responses occurred in demonstrators and sham-demonstrators. Also, extremely high levels of psychological stress, especially under CSI conditions (causing abnormal fear learning) led to heightened fear memory and PTSD-like behaviors. Right hippocampal atrophy confirmed the potential role of CSI conditions in promoting PTSD-like behaviors. Compared to inequality conditions, the abnormal fear memory was reduced under equality conditions.

The Preventive Effect of Urinary Trypsin Inhibitor on Postoperative Cognitive Dysfunction, on the Aspect of Behavior, Evaluated by Y-Maze Test, via Modulation of Microglial Activity

This experimental study was designed to evaluate the effect of ulinastatin, a urinary trypsin inhibitor, on postoperative cognitive dysfunction (POCD) in rats under general anesthesia with isoflurane, on the aspect of behavior, as evaluated using a Y-maze test and focusing on microglial activity. Ulinastatin (50,000 U/mL) and normal saline (1 mL) were randomly (1:1) administered intraperitoneally to the ulinastatin and control groups, respectively, before general anesthesia. Anesthesia with isoflurane 1.5 volume% was maintained for 2 h. The Y-maze test was used to evaluate cognitive function. Neuronal damage using caspase-1 expression, the degree of inflammation through cytokine detection, and microglial activation with differentiation of the phenotypic expression were evaluated. Twelve rats were enrolled in the study and evenly allocated into the two groups, with no dropouts from the study. The Y-maze test showed similar results in the two groups before general anesthesia (63 ± 12% in the control group vs. 64 ± 12% in the ulinastatin group, p = 0.81). However, a significant difference was observed between the two groups after general anesthesia (17 ± 24% in the control group vs. 60 ± 12% in the ulinastatin group, p = 0.006). The ulinastatin group showed significantly lower expression of caspase-1. Pro-inflammatory cytokine levels were significantly lower in the ulinastatin group than in the control group. The ulinastatin group had a significantly lower microglial activation (41.74 ± 10.56% in the control group vs. 4.77 ± 0.56% in the ulinastatin, p < 0.001), with a significantly lower activation of M1 phenotypes (52.19 ± 7.83% in the control group vs. 5.58 ± 0.76% in the ulinastatin group, p < 0.001). Administering ulinastatin before general anesthesia prevented neuronal damage and cognitive decline after general anesthesia, in terms of the aspect of behavior, as evaluated by the Y-maze test. The protective effect of ulinastatin was associated with the inhibition of microglial activation, especially the M1 phenotype.

Stress, associative learning, and decision-making

Exposure to acute and chronic stress has significant effects on the basic mechanisms of associative learning and memory. Stress can both impair and enhance associative learning depending on type, intensity, and persistence of the stressor, the subject's sex, the context that the stress and behavior is experienced in, and the type of associative learning taking place. In some cases, stress can cause or exacerbate the maladaptive behavior that underlies numerous psychiatric conditions including anxiety disorders, obsessive-compulsive disorder, post-traumatic stress disorder, substance use disorder, and others. Therefore, it is critical to understand how the varied effects of stress, which may normally facilitate adaptive behavior, can also become maladaptive and even harmful. In this review, we highlight several findings of associative learning and decision-making processes that are affected by stress in both human and non-human subjects and how they are related to one another. An emerging theme from this work is that stress biases behavior towards less flexible strategies that may reflect a cautious insensitivity to changing contingencies. We consider how this inflexibility has been observed in different associative learning procedures and suggest that a goal for the field should be to clarify how factors such as sex and previous experience influence this inflexibility.

Thermal Response of Laboratory Rats (Rattus norvegicus) during the Application of Six Methods of Euthanasia Assessed by Infrared Thermography

Refinement is one of the principles aiming to promote welfare in research animals. The techniques used during an experimental protocol, including euthanasia selection, must prevent and minimize suffering. Although the current euthanasia methods applied to laboratory rodents are accepted, the controversial findings regarding the potential stress/distress they can cause is a field of research. The objective was to assess the thermal response of Wistar rats during various euthanasia methods using infrared thermography (IRT) to determine the method that prevents or diminishes the stress response and prolonged suffering. Pentobarbital (G1), CO2 (G2), decapitation (G3), isoflurane (G4), ketamine + xylazine (G5), and ketamine + CO2 (G6) were evaluated at five evaluation times with IRT to identify changes in the surface temperature of four anatomical regions: ocular (T°ocu), auricular (T°ear), interscapular (T°dor), and caudal (T°tai). Significant differences (p < 0.05) were found in G2 and G4, registering temperature increases from the administration of the drug to the cessation of respiratory rate and heart rate. Particularly, isoflurane showed a marked thermal response in T°ocu, T°ear, T°dor, and T°tai, suggesting that, in general, inhalant euthanasia methods induce stress in rats and that isoflurane might potentially cause distress, an effect that must be considered when deciding humane euthanasia methods in laboratory rodents.

Date palm spathe extract reverses chronic stress-induced changes in dendritic arborization in the amygdala and impairment of hippocampal long-term potentiation

Chronic restraint stress induces anxiety-like behaviors and emotional abnormalities via an alteration of synaptic remodeling in the amygdala and the hippocampus. Given that the date palm spathe has been shown to have neuroprotective effects on different experimental models, this study aimed to address whether the date palm spathe extract (hydroalcoholic extract of date palm spathe [HEDPP]) can reduce chronic restraint stress-induced behavioral, electrophysiological, and morphological changes in the rat model. Thirty-two male Wistar rats (weight 200-220 g) were randomly divided into control, stress, HEDPP, and stress + HEDPP for 14 days. Animals were submitted to restraint stress for 2 h per day for 14 consecutive days. The animals of the HEDPP and stress + HEDPP groups were supplemented with HEDPP (125 mg/kg) during these 14 days, 30 min before being placed in the restraint stress tube. We used passive avoidance, open-field test, and field potential recording to assess emotional memory, anxiety-like behavioral and long-term potentiation in the CA1 region of the hippocampus, respectively. Moreover, Golgi-Cox staining was used to investigate the amygdala neuron dendritic arborization. Results showed that stress induction was associated with behavioral changes (anxiety-like behavioral and emotional memory impairment), and the administration of HEDPP effectively normalized these deficits. HEDPP remarkably amplified the slope and amplitude of mean-field excitatory postsynaptic potentials (fEPSPs) in the CA1 area of the hippocampus in stressed rats. Chronic restraint stress significantly decreased the dendritic arborization in the central and basolateral nucleus of the amygdala neuron. HEDPP suppressed this stress effect in the central nucleus of the amygdala. Our findings indicated that HEDPP administration improves stress-induced learning impairment and memory and anxiety-like behaviors by preventing adverse effects on synaptic plasticity in the hippocampus and amygdala.

Latent toxoplasmosis impairs learning and memory yet strengthens short-term and long-term hippocampal synaptic plasticity at perforant pathway-dentate gyrus, and Schaffer collatterals-CA1 synapses

Investigating long-term potentiation (LTP) in disease models provides essential mechanistic insight into synaptic dysfunction and relevant behavioral changes in many neuropsychiatric and neurological diseases. Toxoplasma (T) gondii is an intracellular parasite causing bizarre changes in host's mind including losing inherent fear of life-threatening situations. We examined hippocampal-dependent behavior as well as in vivo short- and long-term synaptic plasticity (STP and LTP) in rats with latent toxoplasmosis. Rats were infected by T. gondii cysts. Existence of REP-529 genomic sequence of the parasite in the brain was detected by RT-qPCR. Four and eight weeks after infection, spatial, and inhibitory memories of rats were assessed by Morris water maze and shuttle box tests, respectively. Eight weeks after infection, STP was assessed in dentate gyrus (DG) and CA1 by double pulse stimulation of perforant pathway and Shaffer collaterals, respectively. High frequency stimulation (HFS) was applied to induce LTP in entorhinal cortex-DG (400 Hz), and CA3-CA1 (200 Hz) synapses. T. gondii infection retarded spatial learning and memory performance at eight weeks post-infection period, whereas inhibitory memory was not changed. Unlike uninfected rats that normally showed paired-pulse depression, the infected rats developed paired-pulse facilitation, indicating an inhibitory synaptic network disruption. T. gondii-infected rats displayed strengthened LTP of both CA1-pyramidal and DG-granule cell population spikes. These data indicate that T. gondii disrupts inhibition/excitation balance and causes bizarre changes to the post-synaptic neuronal excitability, which may ultimately contribute to the abnormal behavior of the infected host.