Housing environment modulates physiological and behavioral responses to anxiogenic stimuli in trait anxiety male rats

The effect of acute crocin on behavioral changes and BDNF expression level in socially isolated rats

Social isolation is a reliable method used for the induction of depression and psychiatric disorders in rodents. It has been suggested that social isolation can lead to hyperlocomotion, as a schizophrenic-like symptom in rodents. On the other hand, crocin (the major constituent of Crocus sativus) induces a wide-range of neuroprotective and mood enhancer effects. In the present study, we aimed to investigate the effect of acute crocin on social isolation-induced behavioral changes and BDNF expression in the hippocampus. Novelty-suppressed feeding test, open field test, marble burying test, hot plate, forced swim test, and the shuttle box were used to assess anxiety-like behavior, locomotor activity, obsessive-compulsive-like (OCD-like) behavior, pain threshold, depressive-like behavior, and passive avoidance memory, respectively. Real-time PCR was used to assess BDNF hippocampal expression level. The results showed that social isolation decreased anxiety- and depressive-like behavior, pain threshold, and BDNF expression, and induced OCD-like behavior and hyperlocomotion. Crocin dose-dependently restored the effect of social isolation on pain threshold, locomotor activity, depressive-like behavior, OCD-like behavior, and BDNF expression. Passive avoidance memory performance was also unaffected. In conclusion, we showed a hyperlocomotion profile and OCD-like behaviors, and a robust decrease in pain threshold in socially isolated rats. It can be suggested that social isolation from adolescence induces a "hyperlocomotion state" that affects all the behavioral functions of rats. Also, the function of BDNF can be related to a hyperlocomotion state and OCD-like symptom. It seems that BDNF expression level can be related to the therapeutic effect of crocin.

Differences in Fluid, Electrolyte, and Energy Balance in C57BL/6J Mice (Mus musculus) in Metabolic Caging at Thermoneutral or Standard Room Temperatures

The Guide for the Care and Use of Laboratory Animals recommends mice be pair or group housed and provided with nesting materials. These provisions support social interactions and are also critical for thermoregulatory behaviors such as huddling and burrowing. However, studies of fluid and electrolyte balance and digestive function may involve use of metabolic caging (MC) systems in which mice are housed individually on wire-mesh floors that permit quantitative collection of urine and feces. MC housing prevents mice from performing their typical huddling and burrowing behaviors. Housing in MC can cause weight loss and behavioral changes in rodents. Here, we tested the hypothesis that MC housing of mice at standard room temperature (SRT, 22 to 23 °C) exposes them to cold stress, which causes metabolic changes in the mice as compared with standard housing. We hypothesized that performing MC studies at a thermoneutral temperature (TNT, 30 °C) would minimize these changes. Fluid, electrolyte, and energy balance and body composition were assessed in male and female C57BL/6J mice housed at SRT or TNT in MC, static microisolation cages, or a multiplexed metabolic phenotyping system designed to mimic static microisolation cages (Promethion, Sable Systems International). In brief, as compared with MC housing at SRT, MC housing at TNT was associated with lower food intake and energy expenditure, absence of weight loss, and lower urine and fecal corticosterone levels. These results indicate that housing in MC at SRT causes cold stress that can be mitigated if MC studies are performed at TNT.

Unscheduled changes in pre-clinical stroke model housing contributes to variance in physiological and behavioural data outcomes: A post hoc analysis

Ischaemic stroke presents a significant problem worldwide with no neuroprotective drugs available. Many of the failures in the search for neuroprotectants are attributed to failure to translate from pre-clinical models to humans, which has been combatted with rigorous pre-clinical stroke research guidelines. Here, we present post hoc analysis of a pre-clinical stroke trial, conducted using intraluminal filament transient middle cerebral artery occlusion in the stroke-prone spontaneously hypertensive rat, whereby unscheduled changes were implemented in the animal housing facility. These changes severely impacted body weight post-stroke resulting in a change from the typical body weight of 90.6% of pre-surgery weight post-stroke, to on average 80.5% of pre-surgery weight post-stroke. The changes also appeared to impact post-stroke blood pressure, with an increase from 215.4 to 240.3 mmHg between housing groups, and functional outcome post-stroke, with a 38% increased latency to contact in the sticky label test. These data highlight the importance of tightly controlled housing conditions when using physiological or behavioural measurements as a primary outcome.

Responses to Reduced Feeding Frequency in Captive-Born Cheetahs (Acinonyx jubatus): Implications for Behavioural and Physiological Stress and Gastrointestinal Health

Unnatural diet composition and frequent feeding regimes may play an aetiological role in the multiple diseases prevalent in captive cheetahs. This study investigated the responses of captive-born (hand-reared) cheetahs (n = 6) to a reduced feeding frequency schedule distinguished by offering larger quantities of food less frequently. The study cheetahs were fed four once-daily meals per week during the 3-week treatment period, followed by a 3-week control period in which they were fed two daily rations six days a week. Total weekly food intake was maintained throughout the study. Variations in behaviour, faecal consistency score (FCS), and faecal glucocorticoid metabolite concentration were measured. Less frequent feeding resulted in higher FCS (p < 0.01) and locomotory behaviour (p < 0.05) among the studied cheetahs. Faecal glucocorticoid metabolite concentration demonstrated an initial acute stress response to the change in feeding frequency (p < 0.05) and subsequent adaptation. The results of the FCS analysis suggest that the more natural feeding pattern could have benefited the studied cheetahs' gastrointestinal health without a significant behavioural or physiological stress response overall to the change in feeding frequency.

Chronic mild stress induces differential depression-like symptoms and c-Fos and 5HT1A protein levels in high-anxiety female Long Evans rats

Depression and anxiety disorders overlap in clinical populations, suggesting common mechanisms that may be further investigated in reliable animal models. We used filial 8 female Long-Evans rats bred for high (HAn; n = 19) and low anxiety (LAn)-like behavior (n = 21) to assess forced swim test mobility strategies and chronic mild stress (CMS)-induced depression-like symptoms. We measured (1) weight, (2) fur piloerection, (3) sweet food consumption, (4) grooming behavior, and (5) circulating estradiol (E2). One month after CMS terminated and following a terminal forced swim test, brains were processed for immunohistochemistry targeting c-Fos and serotonin 1 A receptor (5-HT1AR) protein in the paraventricular nucleus (PVN) of the hypothalamus. HAn female rats showed increased anxiety-like behavior (i.e., lower open to closed arm ratios, increased closed arm entries), more swimming (i.e., mobility), and less floating (i.e., immobility) behavior in the forced swim test. Overall, HAn females weighed less than their LAn counterparts. After chronic mild stress, HAn lines displayed even greater mobility and consumed fewer Froot Loops™. Fur and grooming analyses indicated no significant differences in mean counts across experimental groups. One month after CMS, cycling E2 concentrations (pg/ml) did not differ between HAn and LAn animals. Elevated c-Fos and 5-HT1AR expression were observed in the PVN, where HAn CMS rats expressed the most c-Fos and 5-HT1AR immunoreactivity. In summary, outbred HAn rats show robust anxiety-like behavior, exhibit more mobility in the forced swim test, and are more sensitive to chronic mild stress-induced grooming and decline in palatable food ingestion.

Physiologic Effects of Housing Rats in Metabolic Cages

Currently, metabolic cages (MC) are the only way to achieve serial sampling of urine and feces in rodents. However, the use of this caging creates a dramatic change from an animal's usual microenvironment. Here we sought to examine the effect of MC on physiologic parameters that are stress-responsive in rats. We surgically implanted 8 male Wistar rats (weight, 150 to 175 g) with telemetric transmitters and allowed them to recover for at least 2 wk. At the beginning of the study, the rats were moved to conventional open-top cages, and telemetry recording was initiated. After 24 h, the rats were moved to MC or to another conventional cage and the recording continued for another 24 h. Finally, the rats were returned to their home cages, and telemetry recording was performed for a final 24 h. After 10 days, this process was then repeated, with MC and conventional assignments switched. During the 78-h monitoring period, we recorded heart rate, arterial blood pressure, locomotor activity, body weight, and food and water consumption. Heart rate and arterial blood pressure showed transient but significant changes. Locomotor activity during the dark phase was greatly decreased in MC compared with conventional cages, perhaps due to space constraints. In addition, when the rats were housed in MC, they showed a small but significant weight loss. Food consumption did not differ between housing environments, but water consumption was lower when rats were in MC. In conclusion, the housing of rats in MC for 24 h can elicit mild and reversible cardiovascular changes. This finding is consistent with European Directive 2010/63/EU, which considers short-term (less than 24 h) restraint in MC a procedure of mild severity.

Behavioral outcomes of complete Freund adjuvant-induced inflammatory pain in the rodent hind paw: a systematic review and meta-analysis

ABSTRACT

Many analgesics inadequately address the psychiatric comorbidities of chronic and persistent pain, but there is no standard preclinical model of pain-altered behavior to support the development of new therapies. To explore this conflicting and inconclusive literature, we conducted a focused systematic review and meta-analysis on the effect of complete Freund adjuvant-induced (CFA) rodent hind paw inflammation on multiple classical indicators of exploratory behavior, stress coping, and naturalistic behavior. Our primary objective was to define CFA's effect on assays including, but not limited to, the elevated plus maze and forced swim test. Our secondary objective was to discover how variables such as species and strain may influence outcomes in such assays. We searched Ovid MEDLINE, Embase, Scopus, and Web of Science in April and October 2020 for studies with adult rodents injected with CFA into the hind paw and subsequently tested for aspects of "anxiety-like" or "depressive-like" behaviors. Forty-four studies evaluated performance in the elevated plus or zero maze, open field test, light-dark box, place escape and avoidance paradigm, forced swim test, tail suspension test, sucrose preference test, wheel running, and burrowing assay. Complete Freund adjuvant modestly but significantly decreased exploratory behavior, significantly increased passive stress coping in the tail suspension test but not the forced swim test, and significantly decreased preference for sucrose and naturally rewarding activity. Subgroup analyses revealed significant differences between species and animal sourcing. Based on the evidence provided here, we conclude future studies should focus on CFA's effect on natural rewards and naturalistic behaviors.

Chronic hM4Di-DREADD-Mediated Chemogenetic Inhibition of Forebrain Excitatory Neurons in Postnatal or Juvenile Life Does Not Alter Adult Mood-Related Behavior

G-protein-coupled receptors (GPCRs) coupled to G_i signaling, in particular downstream of monoaminergic neurotransmission, are posited to play a key role during developmental epochs (postnatal and juvenile) in shaping the emergence of adult anxiodepressive behaviors and sensorimotor gating. To address the role of G_i signaling in these developmental windows, we used a CaMKIIα-tTA::TRE hM4Di bigenic mouse line to express the hM4Di-DREADD (designer receptor exclusively activated by designer drugs) in forebrain excitatory neurons and enhanced G_i signaling via chronic administration of the DREADD agonist, clozapine-N-oxide (CNO) in the postnatal window (postnatal days 2–14) or the juvenile window (postnatal days 28–40). We confirmed that the expression of the HA-tagged hM4Di-DREADD was restricted to CaMKIIα-positive neurons in the forebrain, and that the administration of CNO in postnatal or juvenile windows evoked inhibition in forebrain circuits of the hippocampus and cortex, as indicated by a decline in expression of the neuronal activity marker c-Fos. hM4Di-DREADD-mediated inhibition of CaMKIIα-positive forebrain excitatory neurons in postnatal or juvenile life did not impact the weight profile of mouse pups, and also did not influence the normal ontogeny of sensory reflexes. Further, postnatal or juvenile hM4Di-DREADD-mediated inhibition of CaMKIIα-positive forebrain excitatory neurons did not alter anxiety- or despair-like behaviors in adulthood and did not impact sensorimotor gating. Collectively, these results indicate that chemogenetic induction of G_i signaling in CaMKIIα-positive forebrain excitatory neurons in postnatal and juvenile temporal windows does not appear to impinge on the programming of anxiodepressive behaviors in adulthood.

Paradoxical changes in mood-related behaviors on continuous social isolation after weaning

Continuous social isolation (SI) from an early developmental stage may have different effects in youth and adulthood. Moreover, SI is reported to impair neuronal plasticity. In this study, we used post-weaning rats to compare the impact of continuous SI on depressive-like, anxiety-related, and fear-related behaviors and neuronal plasticity in puberty and adulthood. Furthermore, we assessed the effect of lithium on behavioral changes and neuronal plasticity. Continuous SI after weaning induced depressive-like behaviors in puberty; however, in adulthood, depressive-like and anxiety-related behaviors did not increase, but—paradoxically—decreased in comparison with the controls. The decreased expression of neuronal plasticity-related proteins in the hippocampus in puberty was more prominent in the prefrontal cortex and hippocampus in adulthood. In contrast, SI after weaning tended to decrease fear-related behaviors in puberty, a decrease which was more prominent in adulthood with increased neuronal plasticity-related protein expression in the amygdala. Lithium administration over the last 14 days of the SI-induced period removed the behavioral and expression changes of neuronal plasticity-related proteins observed in puberty and adulthood. Our findings suggest that the extension of the duration of SI from an early developmental stage does not simply worsen depressive-like behaviors; rather, it induces a behavior linked to neuronal plasticity damage. Lithium may improve behavioral changes in puberty and adulthood by reversing damage to neuronal plasticity. The mechanisms underlying the depressive-like and anxiety-related behaviors may differ from those underlying fear-related behaviors.

Housing quality and behavior affect brain health and anxiety in healthy Japanese adults

Countless studies in animals have shown how housing environments and behaviors can significantly affect anxiety and brain health, giving valuable insight as to whether this is applicable in the human context. The relationship between housing, behavior, brain health, and mental wellbeing in humans remains poorly understood. We therefore explored the interaction of housing quality, weekend/holiday sedentary behavior, brain structure, and anxiety in healthy Japanese adults. Whole-brain structural magnetic resonance imaging (MRI) methods based on gray matter volume and fractional anisotropy were used as markers for brain health. Correlation tests were conducted, and then adjusted for multiple comparisons using the False Discovery Rate method. Housing quality and weekend/holiday sedentary behavior were associated with fractional anisotropy, but not with gray matter volume. Fractional anisotropy showed significant associations with anxiety. Lastly, both weekend/holiday sedentary behavior and housing quality were indirectly associated with anxiety through fractional anisotropy. These results add to the limited evidence surrounding the relationship among housing, behavior, and the brain. Furthermore, these results show that behavior and housing qualities can have an indirect impact on anxiety through neurobiological markers such as fractional anisotropy.

Social isolation at adolescence: a systematic review on behaviour related to cocaine, amphetamine and nicotine use in rats and mice

Adolescence is known for its high level of risk-taking, and neurobiological alterations during this period may predispose to psychoactive drug initiation and progression into more severe use patterns. Stress is a risk factor for drug consumption, and post-weaning social isolation increases drug self-administration in rodents. This review aimed to provide an overview of the effects of adolescent social isolation on cocaine, amphetamine and nicotine use-related behaviours, highlighting the specific period when animals were submitted to stress and these drugs. We wondered if there was a specific period during adolescence that isolation stress would increase drug use vulnerability. A total of 323 publications from the Scopus, Web of Science and PubMed (Medline) electronic databases were identified using the words "social isolation" and "adolescence" and "drug" or "cocaine" or "amphetamine" or "nicotine", resulting in 24 articles after analyses criteria following the PRISMA statement. The main points raised were social isolation during adolescence increased cocaine self-administration, amphetamine and nicotine locomotor activity. We did not observe a pattern of a specific moment during the adolescent period that could lead to an increased vulnerability to drug use. The precise conditions under which adolescent social stress alters drug use parameters are complex and likely depend on several factors.

CXCR4 inhibition with AMD3100 attenuates amphetamine induced locomotor activity in adolescent Long Evans male rats

Adolescent psychostimulant abuse has been on the rise over the past decade. This trend has demonstrable ramifications on adolescent behavior and brain morphology, increasing risk for development of addiction during adolescence and in later adulthood. Neuroimmune substrates are implicated in the etiology of substance use disorders. To add to this body of work, the current study was developed to explore the role of a chemokine receptor, CXC Chemokine Receptor 4 (CXCR4), in the development of amphetamine (AMPH) sensitization. We targeted CXCR4 as it is implicated in developmental processes, dopaminergic transmission, neuroimmune responses, and the potentiation of psychostimulant abuse pathology. To evaluate the role of CXCR4 activity on the development of AMPH sensitization, a CXCR4 antagonist (Plerixafor; AMD3100) was administered to rats as a pretreatment variable. Specifically, adolescent Long Evans male rats (N = 37) were divided into four groups: (1) AMD3100 (IP, 4.0 mg/kg) + AMPH (IP, 4.0 mg/kg), (2) saline (SAL; 0.9% NaCl) + AMPH, (3) AMD3100 + SAL, and (4) SAL + SAL. Animals were first habituated to locomotor activity (LMA) chambers, then injected with a pretreatment drug (AMD3100 or SAL) followed by AMPH or SAL every other for four days. After a one-week withdrawal period, all animals were administered a low challenge dose of AMPH (IP, 1.0 mg/kg). AMPH-injected rats displayed significantly more locomotor activity compared to controls across all testing days. CXCR4 antagonism significantly attenuated AMPH-induced locomotor activity. On challenge day, AMD3100 pre-treated animals exhibited diminutive AMPH-induced locomotor activity compared to SAL pre-treated animals. Postmortem analyses of brain tissue revealed elevated CXCR4 protein levels in the striatum of all experimental groups. Our results implicate CXCR4 signaling in the development of AMPH sensitization and may represent an important therapeutic target for future research in psychostimulant abuse.

Environment-dependent behavioral traits and experiential factors shape addiction vulnerability

The transition from controlled drug use to drug addiction depends on an interaction between a vulnerable individual, their environment and a drug. Here we tested the hypothesis that conditions under which individuals live influence behavioral vulnerability traits and experiential factors operating in the drug taking environment to determine the vulnerability to addiction. The role of behavioral vulnerability traits in mediating the influence of housing conditions on the tendency to acquire cocaine self-administration was characterized in 48 rats housed in either an enriched (EE) or a standard (SE) environment. Then, the influence of these housing conditions on the individual vulnerability to develop addiction-like behavior for cocaine or alcohol was measured in 72 EE or SE rats after several months of cocaine self-administration or intermittent alcohol drinking, respectively. The determining role of negative experiential factors in the drug taking context was further investigated in 48 SE rats that acquired alcohol drinking to self-medicate distress in a schedule-induced polydipsia procedure. The environment influenced the acquisition of drug intake through its effect on behavioral markers of resilience to addiction. In contrast, the initiation of drug taking as a coping strategy or in a negative state occasioned by the contrast between enriched housing conditions and a relatively impoverished drug taking setting, facilitated the development of compulsive cocaine and alcohol intake. These data indicate that addiction vulnerability depends on environmentally determined experiential factors, and suggest that initiating drug use through negative reinforcement-based self-medication facilitates the development of addiction in vulnerable individuals. SIGNIFICANCE STATEMENT: The factors that underlie an individual's vulnerability to switch from controlled, recreational drug use to addiction are not well understood. We showed that in individuals housed in enriched conditions, the experience of drugs in the relative social and sensory impoverishment of the drug taking context, and the associated change in behavioral traits of resilience to addiction, exacerbate the vulnerability to develop compulsive drug intake. We further demonstrated that the acquisition of alcohol drinking as a mechanism to cope with distress increases the vulnerability to develop compulsive alcohol intake. Together these results demonstrate that experiential factors in the drug taking context shape the vulnerability to addiction.

Differential impact of stress and environmental enrichment on corticolimbic circuits

Stress exposure can produce profound changes in physiology and behavior that can impair health and well-being. Of note, stress exposure is linked to anxiety disorders and depression in humans. The widespread impact of these disorders warrants investigation into treatments to mitigate the harmful effects of stress. Pharmacological treatments fail to help many with these disorders, so recent work has focused on non-pharmacological alternatives. One of the most promising of these alternatives is environmental enrichment (EE). In rodents, EE includes social, physical, and cognitive stimulation for the animal, in the form of larger cages, running wheels, and toys. EE successfully reduces the maladaptive effects of various stressors, both as treatment and prophylaxis. While we know that EE can have beneficial effects under stress conditions, the morphological and molecular mechanisms underlying these behavioral effects are still not well understood. EE is known to alter neurogenesis, dendrite development, and expression of neurotrophic growth factors, effects that vary by type of enrichment, age, and sex. To add to this complexity, EE has differential effects in different brain regions. Understanding how EE exerts its protective effects on morphological and molecular levels could hold the key to developing more targeted pharmacological treatments. In this review, we summarize the literature on the morphological and molecular consequences of EE and stress in key emotional regulatory pathways in the brain, the hippocampus, prefrontal cortex, and amygdala. The similarities and differences among these regions provide some insight into stress-EE interaction that may be exploited in future efforts toward prevention of, and intervention in, stress-related diseases.

Impact of Exposure to Environmental Enrichment on the Anxiety-Like Behavior of Ovariectomized Mice

Objective: The aim of this study was to explore the influence of short-term (2-week) exposure to social (SE) and/or physical enrichment (PE) on the anxiety-like behavior of ovariectomized (OVX) NIH Swiss mice. Method : One week after surgery, each OVX mouse was housed under one of 4 social conditions: (1) isolated, (2) accompanied by an intact female, (3) accompanied by an intact male, or (4) in a community of 10 OVX individuals. The animals in each of these environments were divided into 2 subgroups, consisting of the presence and absence of PE. Following a 2-week exposure to the respective conditions, each OVX mouse was subjected to either the light/dark exploration test (LDT) or the elevated plus maze (EPM) to examine anxiety-like behavior. Results: The LDT and EPM showed very similar patterns. Compared to an impoverished environment, PE elicited a significant anxiolytic effect for OVX mice housed alone or in companion of an intact female (F [1, 54] = 16.11, P = 0.001). By contrast, mice living in community but without PE displayed anxiogenic-like behavior, perhaps due to crowding, compared to the animals living in isolation (F [1, 36] = 5.64, P = 0.023). Conclusion: This study emphasized the importance of taking housing conditions into account during the screening of new anxiolytic agents and the critical role of OVX in the regulation of anxiety.

A novel GPR55-mediated satiety signal in the oval Bed Nucleus of the Stria Terminalis

Nestled within feeding circuits, the oval (ov) region of the Bed Nucleus of the Stria Terminalis (BNST) may be critical for monitoring energy balance through changes in synaptic strength. Here we report that bidirectional plasticity at ovBNST GABA synapses was tightly linked to the caloric state of male rats, seesawing between long-term potentiation (iLTP, fed) and depression (iLTD, food restricted). L-α-lysophosphatidylinositol (LPI) acting on GPR55 receptors and 2-arachidonoylglycerol (2-AG) through CB1R were respectively responsible for fed (iLTP) and food restricted (iLTD) states. Thus, we have characterized a potential gating mechanism within the ovBNST that may signal metabolic state within the rat brain feeding circuitry.

Pair-housing rats does not protect from behavioral consequences of an acute traumatic experience

Posttraumatic stress disorder (PTSD) is an extremely debilitating disease with a broad array of associated symptoms, making the disorder difficult to diagnose and treat. In humans, patients seem to benefit from group therapy or other means of promoting social behavior. To test these effects on our rodent model of PTSD, adult, male rats were housed in either single or pair conditions prior to and during an acute stressor to induce PTSD-like behaviors in these rats. Subsequently, rats were assessed for PTSD-like symptoms to determine the effect of social housing on stress-induced phenotypes. Posttrauma phenotypes, including enhanced fear conditioning and anxiety-related behavior, persisted regardless of the animal's housing condition. It is possible that any housing driven improvements to stress-induced phenotypes would require longer periods of pair housing than were used in these experiments. Although PTSD patients show improved health outcomes following social interaction or group therapy, the fear and anxiety phenotypes seen following an acute stressor in an animal model of the disease endured despite an animal's housing condition. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Nesting Environment Provides Sex-Specific Neuroprotection in a Rat Model of Neonatal Hypoxic-Ischemic Injury

Hypoxic-ischemic (HI) encephalopathy is a devastating injury that occurs when the fetal brain is deprived of oxygen and blood to a degree that may lead to neurological damage, seizing and cerebral palsy. In rodents, early environmental enrichment that promotes maternal care-taking behavior (mCTB) can improve neurobehavioral outcomes and protect against neurological decline. We hypothesized that an enhanced nesting environment would improve mCTB as measured by pup weight gain, and support greater HI recovery in developing rats. Pregnant dams (E15-16) were introduced to either control Standard Facility (SF) housing or closed nestbox (CN) conditions and maintained in larger cages through pup weaning. On postnatal day (PND) 7, male and female Long-Evans rat pups (N = 73) were randomly sorted into one of two surgical conditions: control and HI. HI pups received isoflurane anesthesia and right carotid artery ligation, a 2-h rest followed by 90 min exposure to a moist hypoxic (92% N, 8% O2) chamber. Pups (PND 8) were weighed daily, and tested on the Morris Water Maze (MWM) task (PND 35-50). Results demonstrate significant differences afforded to male and female pups based on weight measure, where CN-rearing modifies pre-weaning adolescent weights in females and increases post-weaning weights in males and females by an average of 10 g. Following successful MWM training and acquisition (PND 35-37), both male and female CN-raised animals demonstrated faster latency to find the hidden platform (HP) during HP trials (PND 38-42) and appeared to freely explore the MWM pool during an additional probe trial (PND 43). Moreover, after sacrifice (PND 60), CN rearing created sex-specific alterations in brain-derived neurotrophic factor (BDNF), glial-derived neurotrophic factor (GDNF) immunopositive cell staining of the dorsomedial striatum and CA1 of the hippocampus. CN-rearing afforded HI males higher BDNF levels in the striatum and produced greater GDNF levels in the hippocampus of HI-injured females. These results suggest that early life environmental enrichment positively modifies nesting environment, increases weight gain, as well as spatial learning and memory in a sex-specific directionality. Our findings also implicate correlative changes in corticolimbic neurotrophin protein levels in the CN-reared animals that may contribute to these benefits.

Sex and Trait Anxiety Differences in Psychological Stress are Modified by Environment

Evidence-based research has revealed how physiological and emotional responses to acute stress are adaptive. However, under conditions of unpredictable or protracted stress, health and drug vulnerability can be compromised. In this study, we examined anxiety-like behavioral responses of 4th generation adolescent male and female Long Evans rats selectively bred for high (HAn) and low (LAn) anxiety-like behavior when housed in an isolated environment (IE) versus a social environment (SE). After 35 days in IE or SE, animals were tested in the elevated plus maze (EPM), injected with amphetamine (AMPH: 0.5 mg/kg, IP) in the locomotor activity (LMA) chamber, measured for basal and post air puff-stressor core body temperature and blood pressure. Following select rearing, SE reduced the anxiogenic response in HAn rats with females displaying the lowest anxiety-like behavior in the EPM. During habituation in the LMA, IE rats remained active, while post-AMPH injection HAn females were hyperactive, followed closely by LAn females. Our findings from the post-stressor physiological measurements indicate that temperature differences due to environment are observed only in the SE females. We also observed group differences for diastolic (DBP) and systolic (SBP) blood pressure. HAn IE males experienced higher DBP and SBP but LAn IE females only experienced higher SBP. Not only do our findings corroborate earlier work on HAn/LAn lines but the findings obtained from this research offer new insights about the role of environment and the role of sex in (1) modulation of anxiety-like behavior, (2) AMPH sensitivity, and (3) basal and stress-induced physiological changes.

Neurobiology and consequences of social isolation stress in animal model-A comprehensive review

The brain is a vital organ, susceptible to alterations under genetic influences and environmental experiences. Social isolation (SI) acts as a stressor which results in alterations in reactivity to stress, social behavior, function of neurochemical and neuroendocrine system, physiological, anatomical and behavioral changes in both animal and humans. During early stages of life, acute or chronic SIS has been proposed to show signs and symptoms of psychiatric and neurological disorders such as anxiety, depression, schizophrenia, epilepsy and memory loss. Exposure to social isolation stress induces a variety of endocrinological changes including the activation of hypothalamic-pituitary-adrenal (HPA) axis, culminating in the release of glucocorticoids (GCs), release of catecholamines, activation of the sympatho-adrenomedullary system, release of Oxytocin and vasopressin. In several regions of the central nervous system (CNS), SIS alters the level of neurotransmitter such as dopamine, serotonin, gamma aminobutyric acid (GABA), glutamate, nitrergic system and adrenaline as well as leads to alteration in receptor sensitivity of N-methyl-D-aspartate (NMDA) and opioid system. A change in the function of oxidative and nitrosative stress (O&NS) mediated mitochondrial dysfunction, inflammatory factors, neurotrophins and neurotrophicfactors (NTFs), early growth response transcription factor genes (Egr) and C-Fos expression are also involved as a pathophysiological consequences of SIS which induce neurological and psychiatric disorders.

Amygdala - and serum - neurotrophic factor levels depend on rearing condition in male rats

Early life experiences could determine brain and behavioral development. Neurotrophic factors are likely to mediate the effects of the experience on brain structures and function. Brain-derived neurotrophic factor (BDNF) plays a central role in psychiatric disorders. To investigate the effects of early rearing condition on the amygdala - and serum - BDNF levels, we reared male Wistar rats from weaning (postnatal days 21) to adulthood (postnatal days 119) in three different rearing conditions: (1) enriched, (2) standard and (3) isolated. We found that long-term post-weaning environmental enrichment leads to lower amygdala - and serum - BDNF levels as well as lower brain weights. Grouped rearing in standard laboratory cages enhanced body weight. Thus, early rearing condition might play a crucial role in adult healthiness by predetermining individual BDNF profiles.

Novel approaches to alcohol rehabilitation: Modification of stress-responsive brain regions through environmental enrichment

Relapse remains the most prominent hurdle to successful rehabilitation from alcoholism. The neural mechanisms underlying relapse are complex, but our understanding of the brain regions involved, the anatomical circuitry and the modulation of specific nuclei in the context of stress and cue-induced relapse have improved significantly in recent years. In particular, stress is now recognised as a significant trigger for relapse, adding to the well-established impact of chronic stress to escalate alcohol consumption. It is therefore unsurprising that the stress-responsive regions of the brain have also been implicated in alcohol relapse, such as the nucleus accumbens, amygdala and the hypothalamus. Environmental enrichment is a robust experimental paradigm which provides a non-pharmacological tool to alter stress response and, separately, alcohol-seeking behaviour and symptoms of withdrawal. In this review, we examine and consolidate the preclinical evidence that alcohol seeking behaviour and stress-induced relapse are modulated by environmental enrichment, and these are primarily mediated by modification of neural activity within the key nodes of the addiction circuitry. Finally, we discuss the limited clinical evidence that stress-reducing approaches such as mindfulness could potentially serve as adjunctive therapy in the treatment of alcoholism. This article is part of the Special Issue entitled "Neurobiology of Environmental Enrichment".

Environmental Enrichment as a Positive Behavioral Intervention Across the Lifespan

BACKGROUND

In recent decades, the interest in behavioral interventions has been growing due to the higher prevalence of age-related cognitive impairments. Hence, behavioral interventions, such as cognitive stimulation and physical activity, and along with these, our lifestyle (education level, work position, frequency of cognitive and social activities) have shown important benefits during the cognitive impairment, dementia and even recovery after brain injury. This is due to the fact that this type of intervention and activities promote the formation of a cognitive and brain reserve that allows tolerating brain damage during a long period of time without the appearance of cognitive symptoms. With regard to this, animal models have proved very useful in providing information about the brain mechanisms involved in the development of these cognitive and brain reserves and how they interact with each other.

METHODS

We summarize several studies showing the positive effects of Environmental Enrichment (EE), understood as a housing condition in which animals benefit from the sensory, physical, cognitive and social stimulation provided, on brain and cognitive functions usually impaired during aging.

RESULTS

Most of studies have shown that EE is a successful protocol to improve cognitive functions and reduce anxiety-related behaviors across the lifespan, as well as in animal models of neurodegenerative diseases.

CONCLUSION

Therefore, EE is a laboratory condition in which some aspects of an active lifestyle are reproduced.

Effects of environmental enrichment during abstinence in morphine dependent parents on anxiety, depressive-like behaviors and voluntary morphine consumption in rat offspring

Chronic morphine exposure during puberty increased morphine-induced rewarding effects and sensitization in the next generation. Given the well-known beneficial effects of environmental enrichment on the severity of physical and psychological dependence on morphine, we examined effects of enriched environment during morphine abstinence in morphine dependent parental rats before mating on the anxiety and depressive-like behaviors, and voluntary morphine consumption in their offspring. Paternal and/or maternal rats were injected with bi-daily doses (10mg/kg, 12h intervals) of morphine for 14days followed by rearing in a standard environment (SE) or enriched environment (EE) during 30days of morphine abstinence before mating. The pubertal male and female rat offspring were tested for anxiety (the elevated plus maze- EPM) and depression (sucrose preference test-SPT), and voluntary morphine consumption using a two-bottle choice (TBC) paradigm. The results showed that EE experience in morphine-dependent both parents result in an increase in the percentage of time spent into open arms/time spent on both arms using EPM in male offspring, higher levels of sucrose preference in female offspring and lower levels of voluntary morphine consumption in male and female offspring. Thus, EE experience in morphine-dependent both parents reduced anxiety, depressive-like behavior and also the voluntary morphine consumption in their offspring during puberty which may prevent the vulnerability of the next generation to drug abuse.

Environmental enrichment ameliorates chronic immobilisation stress-induced spatial learning deficits and restores the expression of BDNF, VEGF, GFAP and glucocorticoid receptors

Severe and prolonged stress is the main environmental factor that precipitates depression, anxiety and cognitive dysfunctions. On the other hand, exposure to environmental enrichment (EE) has been shown to induce progressive plasticity in the brain and improve learning and memory in various neurological and psychiatric disorders. It is not known whether exposure to enriched environment could ameliorate chronic immobilisation stress-induced cognitive deficits and altered molecular markers. Hence, in the present study we aimed to evaluate the effect of enriched environment on chronic immobilisation stress (CIS) associated changes in spatial learning and memory, behavioural measures of anxiety, depression and molecular markers as well as structural alterations. Male Wistar rats were subjected to chronic immobilisation stress for 2h/day/10days followed by 2weeks of exposure to EE. CIS resulted in weight loss, anhedonia, increased immobility, spatial learning and memory impairment, enhanced anxiety, and reduced expression of BDNF, VEGF, GFAP and glucocorticoid receptors (GR) in discrete brain regions. Interestingly, stressed rats exposed to enrichment ameliorated behavioural depression, spatial learning and memory impairment and reduced anxiety behaviour. In addition, EE restored BDNF, VEGF, GFAP and GR expression and normalized hypotrophy of dentate gyrus and hippocampus in CIS rats. In contrast, EE did not restore hypertrophy of the amygdalar complex. Thus, EE ameliorates stress-induced cognitive deficits by modulating the neurotrophic factors, astrocytes and glucocorticoid receptors in the hippocampus, frontal cortex and amygdala.

The Evidence for Altered BDNF Expression in the Brain of Rats Reared or Housed in Social Isolation: A Systematic Review

There is evidence that development and maintenance of neural connections are disrupted in major mental disorders, which indicates that neurotrophic factors could play a critical role in their pathogenesis. Stress is a well-established risk factor for psychopathology and recent research suggests that disrupted signaling via brain-derived neurotrophic factor (BDNF) may be involved in mediating the negative effects of stress on the brain. Social isolation of rats elicits chronic stress and is widely used as an animal model of mental disorders such as schizophrenia and depression. We carried out a systematic search of published studies to review current evidence for an altered expression of BDNF in the brain of rats reared or housed in social isolation. Across all age groups (post-weaning, adolescent, adult), majority of the identified studies (16/21) reported a decreased expression of BDNF in the hippocampus. There are far less published data on BDNF expression in other brain regions. Data are also scarce to assess the behavioral changes as a function of BDNF expression, but the downregulation of BDNF seems to be associated with increased anxiety-like symptoms. The reviewed data generally support the putative involvement of BDNF in the pathogenesis of stress-related mental illness. However, the mechanisms linking chronic social isolation, BDNF expression and the elicited behavioral alterations are currently unknown.

Short-term environmental enrichment is sufficient to counter stress-induced anxiety and associated structural and molecular plasticity in basolateral amygdala

Moderate levels of anxiety enable individual animals to cope with stressors through avoidance, and could be an adaptive trait. However, repeated stress exacerbates anxiety to pathologically high levels. Dendritic remodeling in the basolateral amygdala is proposed to mediate potentiation of anxiety after stress. Similarly, modulation of brain-derived neurotrophic factor is thought to be important for the behavioral effects of stress. In the present study, we investigate if relatively short periods of environmental enrichment in adulthood can confer resilience against stress-induced anxiety and concomitant changes in neuronal arborisation and brain derived neurotrophic factor within basolateral amygdala. Two weeks of environmental enrichment countermanded the propensity of increased anxiety following chronic immobilization stress. Environmental enrichment concurrently reduced dendritic branching and spine density of projection neurons of the basolateral amygdala. Moreover, stress increased abundance of BDNF mRNA in the basolateral amygdala in agreement with the dendritic hypertrophy post-stress and role of BDNF in promoting dendritic arborisation. In contrast, environmental enrichment prevented stress-induced rise in the BDNF mRNA abundance. Gain in body weights and adrenal weights remained unaffected by exposure to environmental enrichment. These observations suggest that a short period of environmental enrichment can provide resilience against maladaptive effects of stress on hormonal, neuronal and molecular mediators of anxiogenesis.

Effects of Metabolic Cage Housing on Rat Behavior and Performance in the Social Interaction Test

Although the metabolic cage is commonly used for housing nonhuman animals in the laboratory, it has been recognized as constituting a unique stressor. Such an environment would be expected to affect behavioral change in animals housed therein. However, few studies have specifically addressed the nature or magnitude of this change. The current study sought to characterize the behavioral time budget of rats in metabolic cage housing in comparison to that of individually housed animals in standard open-top cages. Rats in metabolic cages spent less time moving, manipulating enrichment, and carrying out rearing behaviors, and there was a corresponding shift toward inactivity. In an applied Social Interaction Test, behavioral scoring implied that metabolic cage housing had an anxiogenic effect. In conclusion, metabolic cage housing produces measurable effects on spontaneous and evoked behavior in rats in the laboratory. These behavioral changes may lead to a negative emotional state in these animals, which could have negative welfare consequences. Further research is needed to quantify the existence and magnitude of such an effect on rat well being.

Preclinical animal anxiety research - flaws and prejudices

The current tests of anxiety in mice and rats used in preclinical research include the elevated plus-maze (EPM) or zero-maze (EZM), the light/dark box (LDB), and the open-field (OF). They are currently very popular, and despite their poor achievements, they continue to exert considerable constraints on the development of novel approaches. Hence, a novel anxiety test needs to be compared with these traditional tests, and assessed against various factors that were identified as a source of their inconsistent and contradictory results. These constraints are very costly, and they are in most cases useless as they originate from flawed methodologies. In the present report, we argue that the EPM or EZM, LDB, and OF do not provide unequivocal measures of anxiety; that there is no evidence of motivation conflict involved in these tests. They can be considered at best, tests of natural preference for unlit and/or enclosed spaces. We also argued that pharmacological validation of a behavioral test is an inappropriate approach; it stems from the confusion of animal models of human behavior with animal models of pathophysiology. A behavioral test is developed to detect not to produce symptoms, and a drug is used to validate an identified physiological target. In order to overcome the major methodological flaws in animal anxiety studies, we proposed an open space anxiety test, a 3D maze, which is described here with highlights of its various advantages over to the traditional tests.

A Possible Role of Anhedonia as Common Substrate for Depression and Anxiety

Depression and anxiety are often comorbid, in up to 70% of cases, and the level of one or the other may fluctuate, leading now to a diagnosis of depression, now to a diagnosis of anxiety. For these reasons, and for the presence of many other common factors, it has been suggested that both are part of the same continuum of problems and that they have a common substrate. This paper proposes the possibility that anhedonia may be an important component of this possible common substrate, and it tries to identify the mechanism with which anhedonia could contribute to causing both depression and anxiety. It also proposes an explanation why an intense pleasure could improve both depression and anxiety.

Neuroplastin 65 mediates cognitive functions via excitatory/inhibitory synapse imbalance and ERK signal pathway

Neuroplastin 65 (NP65) is a brain-specific glycoprotein component of synaptic membrane, which is predominantly located in the forebrain such as the cortex, amygdala and striatum and hippocampus. Previous studies have shown that NP65 is implicated in synaptic plasticity, so it was hypothesized to play roles in cognitive functions. To test this hypothesis, we generated NP65 knock-out (KO) mice and found that the null mice exhibited enhanced hippocampus-dependent learning and memory as manifested by Morris water maze test and step-through passive avoidance test, but showed anxiety-like behaviors as manifested by open field test and light/dark exploration test. In addition, molecular and cellular studies revealed several alterations including: (1) the enhanced ratio of excitatory to inhibitory synapses; (2) increased expression of NMDA receptors NR2A; (3) enhanced activation of ERK signaling; (4) lowered number of the mushroom- and bifurcate-shaped dendritic spines in NP65 KO mice. Together, our findings suggest that NP65 may mediate cognitive functions.

The impact of early postnatal environmental enrichment on maternal care and offspring behaviour following weaning

The early postnatal period is a sensitive period in rodents as behavioural systems are developing and maturing during this time. However, relatively little information is available about the impact of environmental enrichment on offspring behaviour if enrichment is implemented only during this period. Here, environmental enrichment was provided from postnatal day 1 until weaning. On post-natal day 9, maternal behaviour and nonmaternal behaviour of the dam was observed. Nursing time in the enriched group was reduced but dams showed more non-maternal appetitive behaviours. Offspring were exposed to either the open field or the elevated plus maze (EPM) after weaning. In the open field, rats from the enriched group approached the more aversive inner zone of the open field later than control rats. Offspring from the enriched group made fewer entries into the inner zone and spent less time in this part of the arena. Enrichment had no impact on behaviour in the EPM. The present study provides evidence that postnatal enrichment can interfere with maternal behaviour in rats and can possibly lead to increased anxiety in the offspring. The findings suggest that enrichment procedures can have potentially unintended effects, interfering with the development of emotional behaviours in rats.

Post-weaning environmental enrichment improves BDNF response of adult male rats

The environment could have long lasting effects on the individual phenotype through developmental plasticity. Early environmental enrichment exerts profound biological effects, most of which are quite beneficial ones. To explore the enduring effects of rearing condition quality on BDNF(1) responses, we reared male Wistar rats from weaning to young-adulthood in three different environmental conditions: 1. Enriched 2. Standard, and 3. Isolated. Then, at the age of 16 weeks, 10 rats from each group were randomly chosen and allocated to six common mix cages. They were kept together for 14 weeks. At the end of the experiment, each rat received ten inescapable foot-shocks. Twelve hours later, the BDNF contents of the amygdala and CA1 sub-region of the dorsal hippocampus were measured. The serum BDNF levels, hematocrit values as well as brain and testis weights were also measured. Results showed that the environmental enrichment led to stronger dorsal hippocampal BDNF response and higher serum BDNF levels, while rats from standard laboratory condition showed higher amygdala BDNF response. Also, enriched animals showed higher brain weight compared to isolation reared rats as well as higher testis weight and hematocrit value compared to animals reared in standard laboratory condition. Rats showed less body weights in isolated condition. In conclusion, the BDNF profile of enriched animals might represent the neurobiological correlate of resilience phenotype under a stressful situation.

Mouse handling limits the impact of stress on metabolic endpoints

Studies focused on end-points that are confounded by stress are best performed under minimally stressful conditions. The objective of this study was to demonstrate the impact of handling designed to reduce animal stress on measurements of glucose tolerance. A cohort of mice (CD1.C57BL/6) naïve to any specific handling was subjected to either a previously described "cup" handling method, or a "tail-picked" method in which the animals were picked up by the tail (as is common for metabolic studies). Following training, an elevated plus maze (EPM) test was performed followed by measurement of blood glucose and plasma corticosterone. A second cohort (CD1.C57BL/6) was rendered obese by exposure to a high fat diet, handled with either the tail-picked or cup method and subjected to an intraperitoneal glucose tolerance test. A third cohort of C57BL/6 mice was exposed to a cup regimen that included a component of massage and was subjected to tests of anxiety-like behavior, glucose homeostasis, and corticosterone secretion. We found that the cup mice showed reduced anxiety-like behaviors in the EPM coupled with a reduction in blood glucose levels compared to mice handled by the tail-picked method. Additionally, cup mice on the high fat diet exhibited improved glucose tolerance compared to tail-picked controls. Finally, we found that the cup/massage group showed lower glucose levels following an overnight fast, and decreased anxiety-like behaviors associated with lower stress-induced plasma corticosterone concentration compared to tail-picked controls. These data demonstrate that application of handling methods that reduce anxiety-like behaviors in mice mitigates the confounding contribution of stress to interpretation of metabolic endpoints (such as glucose tolerance).

Life-long environmental enrichment counteracts spatial learning, reference and working memory deficits in middle-aged rats subjected to perinatal asphyxia

Continuous environmental stimulation induced by exposure to enriched environment (EE) has yielded cognitive benefits in different models of brain injury. Perinatal asphyxia results from a lack of oxygen supply to the fetus and is associated with long-lasting neurological deficits. However, the effects of EE in middle-aged rats suffering perinatal asphyxia are unknown. Therefore, the aim of the present study was to assess whether life-long exposure to EE could counteract the cognitive and behavioral alterations in middle-aged asphyctic rats. Experimental groups consisted of rats born vaginally (CTL), by cesarean section (C+), or by C+ following 19 min of asphyxia at birth (PA). At weaning, rats were assigned to standard (SE) or enriched environment (EE) for 18 months. During the last month of housing, animals were submitted to a behavioral test battery including Elevated Plus Maze, Open Field, Novel Object Recognition and Morris water maze (MWM). Results showed that middle-aged asphyctic rats, reared in SE, exhibited an impaired performance in the spatial reference and working memory versions of the MWM. EE was able to counteract these cognitive impairments. Moreover, EE improved the spatial learning performance of middle-aged CTL and C+ rats. On the other hand, all groups reared in SE did not differ in locomotor activity and anxiety levels, while EE reduced locomotion and anxiety, regardless of birth condition. Recognition memory was altered neither by birth condition nor by housing environment. These results support the importance of environmental stimulation across the lifespan to prevent cognitive deficits induced by perinatal asphyxia.

Effects of environmental enrichment on ERK1/2 phosphorylation in the rat prefrontal cortex following nicotine-induced sensitization or nicotine self-administration

Rats raised in an enriched condition (EC) exhibit alterations in the neurobiological and behavioral response to nicotine compared with rats reared in an impoverished condition (IC) or a standard condition (SC). The current study determined whether environmental enrichment differentially regulates extracellular signal-regulated kinase1/2 (ERK1/2) activity in the prefrontal cortex in rats following nicotine sensitization or nicotine self-administration. Under the saline control condition, EC rats displayed diminished baseline activity and greater sensitization to repeated administration of nicotine compared with IC and SC rats. After repeated saline injections, the basal levels of phosphorylated ERK1/2 (pERK1/2) were higher in EC compared with IC and SC rats, which was negatively correlated with their respective baseline activities. Repeated nicotine (0.35 mg/kg) injections induced pERK1/2 to similar levels in SC and IC rats; however, the induction of pERK1/2 in EC rats by nicotine was not significantly different from saline controls, owing to their high baseline. In the self-administration paradigm, EC rats self-administered less nicotine (0.03 mg/kg/infusion) relative to IC or SC rats on a fixed ratio-1 schedule of reinforcement. Accordingly, no differences in pERK1/2 were found between EC and IC rats self-administering saline, whereas nicotine self-administration resulted in an increase in pERK1/2 in IC rats but not in EC rats. Furthermore, the levels of pERK1/2 in EC and IC rats were positively correlated with their respective total number of nicotine infusions. Thus, these findings suggest that environmental enrichment alters the basal and nicotine-mediated pERK1/2, which may contribute to enrichment-induced behavioral alterations in response to nicotine.

The assessment of general well-being using spontaneous burrowing behaviour in a short-term model of chemotherapy-induced mucositis in the rat

Mucositis is a common and serious side-effect experienced by cancer patients during treatment with chemotherapeutic agents. Consequently, programmes of research focus on the elucidation of novel therapeutics for alleviation of mucositis symptoms, and these frequently use animal models. However, although these models are assumed to be painful and distressing to the animal, endpoints are difficult to determine. The aim of this study was to evaluate whether a change in burrowing behaviour could provide an indication of disease onset and potentially be applied as a humane endpoint. Baseline burrowing behaviour was measured in healthy animals on three occasions by determining the weight of gravel displaced from a hollow tube. Mucositis was then induced in the same animals by intraperitoneal injection of 5-fluorouracil (150 mg/kg) and burrowing behaviour recorded over three consecutive days. Standard measures of disease progression, including body weight loss and clinical score, were also made. The presence of mucositis was confirmed at necropsy by findings of decreased duodenal and colon lengths, and reduced liver, spleen and thymus weights in comparison with non-treated control animals. Histological score of the jejunum and ileum was also significantly increased. Mucositis onset coincided with a decrease in mean burrowing behaviour which was progressive, however this result did not achieve statistical significance (P = 0.66).We conclude that burrowing may be a useful indicator of inflammation in the mucositis model, although this requires further characterization. Pre-selection of animals into treatment groups based on their prior burrowing performance should be pursued in further studies.