Light/dark cycle manipulation influences mice behaviour in the elevated plus maze

Mouse Testing Methods in Psychoneuroimmunology: Measuring Behavioral Responses

The field of psychoneuroimmunology (PNI) aims to uncover the processes and consequences of nervous, immune, and endocrine system relationships. Behavior is a consequence of such interactions and manifests from a complex interweave of factors including immune-to-neural and neural-to-immune communication. Often the signaling molecules involved during a particular episode of neuroimmune activation are not known, but behavioral response provides evidence that bioactives such as neurotransmitters and cytokines are perturbed. Immunobehavioral phenotyping is a first-line approach when examining the neuroimmune system and its reaction to immune stimulation or suppression. Behavioral response is significantly more sensitive than direct measurement of a single specific bioactive and can quickly and efficiently rule in or out relevance of a particular immune challenge or therapeutic to neuroimmunity. Classically, immunobehavioral research was focused on sickness symptoms related to bacterial infection, but neuroimmune activation is now a recognized complication of diseases and disorders ranging from cancer to diabesity to Alzheimer's. Immunobehaviors include lethargy, loss of appetite, and disinterest in social activity/surrounding environment. In addition, neuroimmune activation can diminish physical activity, precipitate feelings of depression and anxiety, and impair cognitive and executive function. Provided is a detailed overview of behavioral tests frequently used to examine neuroimmune activation in mice with a special emphasis on pre-experimental conditions that can confound or prevent successful immunobehavioral experimentation.

Locomotion changes in methamphetamine and amphetamine withdrawal: a systematic review

Despite extensive preclinical research over the years, a significant gap remains in our understanding of the specific effects of methamphetamine (METH) and amphetamine (AMPH) withdrawal. Understanding these differences could be pivotal to unveiling the unique pathophysiology underlying each stimulant. This may facilitate the development of targeted and effective treatment strategies tailored to the specific characteristics of each substance. Following PRISMA guidelines, this systematic review was conducted to examine alterations in spontaneous locomotor activity, specifically horizontal activity, in animals experiencing withdrawal from extended and repeated administration of AMPH or METH. Original articles were retrieved from four electronic databases, supplemented by a review of the references cited in the published papers. A total of thirty-one full-length articles (n = 31) were incorporated in the analysis. The results indicated that six studies documented a significant increase in horizontal activity among animals, seven studies reported decreased locomotion, and eighteen studies (8 AMPH; 10 METH) reported no significant alterations in the animals' locomotor activity. Studies reporting heightened locomotion mainly employed mice undergoing withdrawal from METH, studies reporting diminished locomotion predominantly involved rats undergoing withdrawal from AMPH, and studies reporting no significant changes in horizontal activity employed both rats and mice (12 rats; 6 mice). Drug characteristics, routes of administration, animal models, dosage regimens, duration, and assessment timing seem to influence the observed outcomes. Despite more than 50% of papers enlisted in this review indicate no significant changes in the locomotion during the stimulant withdrawal, the unique reactions of animals to withdrawal from METH and AMPH reported by some underscore the need for a more nuanced understanding of stimulant withdrawal.

Chronic clomipramine treatment reverses depressogenic-like effects of a chronic treatment with dexamethasone in rats

Corticosteroids are widely used in medicine, for their anti-inflammatory and immunosuppressive actions, but can lead to troubling psychiatric side-effects. In fact, corticosteroids can induce many symptoms and syndromes, for example, mood disorders, anxiety and panic disorder, suicidal thinking and behavior. Furthermore, chronic stress and the administration of exogenous glucocorticoids are reported to induce affective changes in humans and rodents that relate to depressive state. Animal models are highly useful tools for studying the depression etiology. Face validity, construct validity, and predictive validity are the main criteria to evaluate animal depression models. The present study aimed to investigate the behavioral, cognitive, and biochemical effects of a chronic administration of DEX on Wistar rats. Wistar rats were administered daily with DEX (1.5 mg/kg, i.p., 21 days) or saline, the clomipramine treatment (2 mg/kg, i.p.) was realized just after the DEX injections for 21 days. DEX induced changes were evaluated by: forced swimming, novelty suppressed feeding, saccharin preference, open field, Morris water maze, and oxidative stress state in the brain. Results showed that chronic DEX administration conduct to a range of depression-related behavioral traits, including anhedonia, despair, weight loss, anxiety-like behavior, and cognitive impairments, which fill the face validity criterion. The DEX induced behavioral changes may result from the massive production of oxidative stress agents. This sustains the etiological hypothesis claiming that hyper-circulating glucocorticoid resulting from HPA dysfunction induces damage in certain neural structures related to depressive disorder, essentially the hippocampus. The antidepressant treatment has restored the behavioral state of rats which fills the predictive validity criterion.

Hesperidin Ameliorates Anxiety-Depressive-Like Behavior in 6-OHDA Model of Parkinson's Disease by Regulating Striatal Cytokine and Neurotrophic Factors Levels and Dopaminergic Innervation Loss in the Striatum of Mice

The mechanisms underlying the neuroprotective effects of hesperidin in a murine model of PD are not fully elucidated. The current study was carried out to investigate the ability of hesperidin in modulating proinflammatory cytokines, neurotrophic factors, and neuronal recovery in 6-hydroxydopamine (6-OHDA)-induced nigral dopaminergic neuronal loss. Adult male C57BL/6 mice were randomly assigned into four groups: (I) sham/vehicle, (II) sham/hesperidin, (III) 6-OHDA/vehicle, and (IV) 6-OHDA/hesperidin. Mice received a unilateral intrastriatal injection of 6-OHDA and treated with hesperidin (50 mg/kg; per oral) for 28 days. After hesperidin treatment, mice were submitted to behavioral tests and had the striatum removed for neurochemical assays. Our results demonstrated that oral treatment with hesperidin ameliorated the anxiety-related and depressive-like behaviors in 6-OHDA-lesioned mice (p < 0.05). It also attenuated the striatal levels of proinflammatory cytokines tumor necrosis factor-α, interferon-gamma, interleukin-1β, interleukin-2, and interleukin-6 and increased the levels of neurotrophic factors, including neurotrophin-3, brain-derived neurotrophic factor, and nerve growth factor in the striatum of 6-OHDA mice (p < 0.05). Hesperidin treatment was also capable to increase striatal levels of dopamine and its metabolite 3,4-dihydroxyphenylacetic acid and protects against the impairment of dopaminergic neurons in the substantia nigra pars compacta (SNpc) (p < 0.05). In conclusion, this study indicated that hesperidin exerts anxiolytic-like and antidepressant-like effect against 6-OHDA-induced neurotoxicity through the modulation of cytokine production, neurotrophic factors levels, and dopaminergic innervation in the striatum.

Adverse Effects of Circadian Disorganization on Mood and Molecular Rhythms in the Prefrontal Cortex of Mice

Disturbance of the daily cycles in sleep and wakefulness induced by conditions such as shift work and jet lag can increase the risk of affective disorders including anxiety and depression. The way such circadian disorganization disrupts the regulation of mood, however, is not well understood. More specifically, the impact of circadian disorganization on the daily rhythms of the neuronal function that controls mood remains unclear. We therefore investigated the effects of circadian disorganization on expression rhythms of clock genes as well as immediate early genes (IEGs) in several mood-controlling regions of the brain. To introduce circadian disorganization of behaviors, we exposed male C57BL/6J mice to chronic reversal of the light-dark cycle and we found a marked negative mood phenotype in these mice. Importantly, the most adverse effect of circadian disorganization on expression rhythms of clock and IEGs was observed in the prefrontal cortex (PFC) when compared to that in other mood-related areas of the brain. Dysregulation of molecular rhythms in the PFC is therefore suggested to be associated with the development of mood disorders in conditions including shift work and jet lag.

Animal Models of Early-Life Adversity

From the prenatal period throughout the first years of life, the brain undergoes its most rapid development, a period during which it is highly sensitive to external experiences. The timing of brain development differs from one region to another, as it also differs between substrates, neurotransmitter systems, and central endocrine circuitries. These discontinuities are part of the "critical periods of brain development." Early-life adversity (ELA), such as exposure to infection, maternal deprivation, and substance use, disrupts the programmed brain development, yielding a myriad of deviations in brain circuitry, stress responsivity, cognitive function, and general health. This is applicable to both humans and animal models.In our laboratory, several experimental animal designs have been developed that allow investigating the long-lasting consequences of ELA on brain function, cognitive and emotional development, and the risk to develop stress-related psychopathology later in adulthood. This book chapter will provide a review of such animal models, in particular, designs related to infections (LPS-induced), the quality of mother-infant relationship (maternal deprivation and separation), and substance use (ethanol intoxication). The behavior tests, biochemical, and immunohistochemistry assays applied after ELA will be explained. The behavioral tests encompass the open-field, elevated plus maze, forced swim, sucrose preference, Y-maze, object recognition, and Morris water maze tests. These experiments allow the assessment of several outcomes of interest, pertaining to locomotor activity, anxiety-like symptoms, depressive-like symptoms, working memory, recognition memory, spatial memory, and learning performance. The biochemical assays are employed to measure the level of oxidative stress and inflammation in brain areas after application of adversity. Immunohistochemistry puts into perspective the degree of immunoreactivity in the brain subjected to adversity. The findings from our laboratory indicate that the nature and timing of exposure play a critical role in sensitivity to develop neurodevelopmental disorders.

Aging exacerbates cognitive and anxiety alterations induced by an intracerebroventricular injection of amyloid-β1-42 peptide in mice

An increasing body of evidence indicates that the activation of indoleamine-2,3-dyoxigenase (IDO), a first and rate-limiting enzyme in the kynurenine (KYN) pathway, is involved in Aβ_1-42-neurotoxicity and AD pathogenesis. We have reported for the first time that brain IDO activation is related to Aβ_1-42 exposure in young mice. Because aging is characterized by a brain dyshomeostasis and because it remains the most dominant risk factor for AD, the purpose of this study was to determine whether aging is associated with a higher sensitivity to behavioural and neurochemical alterations elicited by an intracerebroventricular (i.c.v.) injection of Aβ_1-42 (400 pmol/mice), and whether KYN pathway is involved in these effects. We confirmed that aged mice displayed higher cognitive deficit in the object recognition test and higher anxiety-like behaviour in the elevated plus-maze and open field tests after the Aβ_1-42 administration. Aged mice also responded to Aβ_1-42 with a higher deficiency of brain-derived neurotrophic factor, glutathione levels and total radical-trapping antioxidant capacity, a higher IDO activity, and a higher KYN and KYN/tryptophan ratio in the prefrontal cortex and hippocampus. These effects of Aβ_1-42 were associated with a higher proinflammatory status, as measured by higher levels of interleukin-6, lower levels of interleukin-10 and higher expression of glial fibrillary acidic protein (GFAP) and allograft inflammatory factor 1 (Iba1) in the brain of aged mice. These results represent primary evidence suggesting that age-associated inflammatory signature and down-regulation of neuroprotectants in the brain render aged mice more vulnerable to Aβ_1-42-induced memory loss, anxiety symptoms and KYN pathway dysregulation.

Mouse Testing Methods in Psychoneuroimmunology 2.0: Measuring Behavioral Responses

The field of psychoneuroimmunology (PNI) aims to uncover the processes and consequences of nervous, immune, and endocrine system relationships. Behavior is a consequence of such interactions and manifests from a complex interweave of factors including immune-to-neural and neural-to-immune communication. Often the signaling molecules involved during a particular episode of neuroimmune activation are not known but behavioral response provides evidence that bioactives such as neurotransmitters and cytokines are perturbed. Immunobehavioral phenotyping is a first-line approach when examining the neuroimmune system and its reaction to immune stimulation or suppression. Behavioral response is significantly more sensitive than direct measurement of a single specific bioactive and can quickly and efficiently rule in or out relevance of a particular immune challenge or therapeutic to neuroimmunity. Classically, immunobehavioral research was focused on sickness symptoms related to bacterial infection but neuroimmune activation is now a recognized complication of diseases and disorders ranging from cancer to diabesity to Alzheimer's. Immunobehaviors include lethargy, loss of appetite, and disinterest in social activity/surrounding environment. In addition, neuroimmune activation can diminish physical activity, precipitate feelings of depression and anxiety, and impair cognitive and executive function. Provided is a detailed overview of behavioral tests frequently used to examine neuroimmune activation in mice with a special emphasis on pre-experimental conditions that can confound or prevent successful immunobehavioral experimentation.

Indoleamine-2,3-dioxygenase mediates neurobehavioral alterations induced by an intracerebroventricular injection of amyloid-β1-42 peptide in mice

Alzheimer's disease (AD) is a neurodegenerative disorder that is characterized by a progressive cognitive decline along with various neuropsychiatric symptoms, including depression and anxiety. Increasing evidence has been proposed the activation of the tryptophan-degrading indoleamine-2,3-dyoxigenase (IDO), the rate-limiting enzyme of kynurerine pathway (KP), as a pathogenic factor of amyloid-beta (Aβ)-related inflammation in AD. In the current study, the effects of an intracerebroventricular (i.c.v.) injection of Aβ1-42 peptide (400pmol/mice; 3μl/site) on the regulation of KP biomarkers (IDO activity, tryptophan and kynurerine levels) and the impact of Aβ1-42 on neurotrophic factors levels were investigated as potential mechanisms linking neuroinflammation to cognitive/emotional disturbances in mice. Our results demonstrated that Aβ1-42 induced memory impairment in the object recognition test. Aβ1-42 also induced emotional alterations, such as depressive and anxiety-like behaviors, as evaluated in the tail suspension and elevated-plus maze tests, respectively. We observed an increase in levels of proinflammatory cytokines in the Aβ1-42-treated mice, which led to an increase in IDO activity in the prefrontal cortex (PFC) and the hippocampus (HC). The IDO activation subsequently increased kynurerine production and the kynurenine/tryptophan ratio and decreased the levels of neurotrophic factors in the PFC and HC, which contributed to Aβ-associated behavioral disturbances. The inhibition of IDO activation by IDO inhibitor 1-methyltryptophan (1-MT), prevented the development of behavioral and neurochemical alterations. These data demonstrate that brain IDO activation plays a key role in mediating the memory and emotional disturbances in an experimental model based on Aβ-induced neuroinflammation.

Diphenyl diselenide supplementation in infected mice by Toxoplasma gondii: Protective effect on behavior, neuromodulation and oxidative stress caused by disease

The aim of this study was to evaluate the effect of subcutaneous administration of diphenyl diselenide (PhSe)2 on animal behavior and activities of acetylcholinesterase (AChE), adenylate kinase (AK), and creatine kinase (CK) in the brain of mice infected by Toxoplasma gondii. In addition, thiobarbituric acid reactive species (TBARS) levels and glutathione (GR, GPx and GST) activity were also evaluated. For the study, 40 female mice were divided into four groups of 10 animals each: group A (uninfected and untreated), group B (uninfected and treated with (PhSe)2), group C (infected and untreated) and group D (infected and treated with (PhSe)2). The mice were inoculated with 50 cysts of the ME49 strain of T. gondii. After infection the animals of the groups B and D were treated on days 1 and 20 post-infection (PI) with 5.0 μmol/kg of (PhSe)2 subcutaneously. Behavioral tests were conducted on days 29 PI to assess memory loss (object recognition), anxiety (elevated plus maze), locomotor and exploratory activity (Open Field) and it was found out that infected and untreated animals (group C) had developed anxiety and memory impairment, and the (PhSe)2 treatment did not reverse these behavioral changes on infected animals treated with (PhSe)2 (group D). The results showed an increase on AChE activity (P < 0.01) in the brain of infected and untreated animals (group C) compared to the uninfected and untreated animals (group A). The AK and CK activities decreased in infected and untreated animals (group C) compared to the uninfected and untreated animals (group A) (P < 0.01), however the (PhSe)2 treatment did not reverse these alterations. Infected and untreated animals (group C) showed increased TBARS levels and GR activity, and decreased GPx and GST activities when compared to uninfected and untreated animals (group A). Infected animals treated with (PhSe)2 (group D) decreased TBARS levels and GR activity, while increased GST activity when compared to infected and untreated animals (group C). It was concluded that (PhSe)2 showed antioxidant activity, but the dose used had no anti-inflammatory effect and failed to reverse the behavioral changes caused by the parasite.

Synergistic effects of resveratrol (free and inclusion complex) and sulfamethoxazole-trimetropim treatment on pathology, oxidant/antioxidant status and behavior of mice infected with Toxoplasma gondii

This study aimed to investigate the synergistic effects of resveratrol and sulfamethoxazole-trimethoprim (ST) on the treatment of mice experimentally infected by Toxoplasma gondii during the chronic phase of the disease considering infection, behavior, and oxidative/antioxidants profile aspects. For the study, 60 mice were initially divided into two groups: uninfected (n = 24) and infected by T. gondii (n = 36). These two groups were later subdivided into other groups and treated with resveratrol (free and inclusion complex containing resveratrol) alone and co-administered with ST: groups A to D were composed by healthy mice and groups E to J were consisted of animals infected by T. gondii (VEG strain). Treatments began 20 days post-infection for 10 consecutive days with oral doses of 0.5 mg kg(-1) of ST (groups B and F), 100 mg kg(-1) of free resveratrol (groups C and G) and inclusion complex of resveratrol (nanoparticles containing resveratrol) (groups D and H), and lastly an co-administration of both drugs (groups I and J). Behavioral tests (memory, anxiety and locomotion) were performed after treatment. Liver and brain fragments were collected to evaluate pathological changes, brain cysts counts, as well as oxidant and antioxidant levels. A reduction on the number of cysts in the brain of animals treated with both drugs combined was observed; there was also reduced number of lesions on both organs. This drug combined effect was also able to reduce oxidative and increase antioxidant levels in infected mice, which might be interpreted as a resveratrol protective effect. In addition, the combination of ST and resveratrol was able to prevent behavioral changes in infected mice. Therefore, the use of co-administration drugs enhances the therapeutic effect acting on a synergic way, reducing the oxidizing effects of the chemical treatment for toxoplasmosis. In addition, resveratrol in inclusion complex when co-administered with ST showed an improved therapeutic effect of ST reducing oxidative damage, liver damage and the number of cysts in the brain of T. gondii infected mice.

Physical exercise prevents stress-induced activation of granule neurons and enhances local inhibitory mechanisms in the dentate gyrus

Physical exercise is known to reduce anxiety. The ventral hippocampus has been linked to anxiety regulation but the effects of running on this subregion of the hippocampus have been incompletely explored. Here, we investigated the effects of cold water stress on the hippocampus of sedentary and runner mice and found that while stress increases expression of the protein products of the immediate early genes c-fos and arc in new and mature granule neurons in sedentary mice, it has no such effect in runners. We further showed that running enhances local inhibitory mechanisms in the hippocampus, including increases in stress-induced activation of hippocampal interneurons, expression of vesicular GABA transporter (vGAT), and extracellular GABA release during cold water swim stress. Finally, blocking GABAA receptors in the ventral hippocampus, but not the dorsal hippocampus, with the antagonist bicuculline, reverses the anxiolytic effect of running. Together, these results suggest that running improves anxiety regulation by engaging local inhibitory mechanisms in the ventral hippocampus.

What are we measuring when we test strain differences in anxiety in mice?

We examined measures of locomotor and anxiety-like behavior in male and female mice of 15 inbred strains on the elevated-plus maze, light/dark transition box and open field. Strain differences were found on all measures of locomotor activity and anxiety. Strain means for measures of locomotor activity on the three apparatus were significantly correlated, but strain means for commonly used measures of anxiety were not correlated. Principal component analysis revealed a common locomotor activity factor, which accounted for 28.6 % of the variance, but no common anxiety factor. Species-typical behaviors (defecations, stretch-attend postures, grooming) accounted for smaller proportions (<11 %) of the variance. These results plus comparisons with previously published data suggest that the elevated-plus maze, light/dark box and open field measure different facets of anxiety, and that the reliability of genetic differences on anxiety is highly dependent on apparatus, procedural variables and laboratory factors. Locomotor activity, however, is a stable trait that differs across strains and is reliably measured in different apparatus and laboratories. We conclude that anxiety traits of inbred mouse strains are best reflected by species-typical behaviors in each apparatus. These results suggest that new ways of measuring trait anxiety are required in order to determine the neural and genetic correlates of anxiety-like behaviour in mice.

Mouse testing methods in psychoneuroimmunology: an overview of how to measure sickness, depressive/anxietal, cognitive, and physical activity behaviors

The field of psychoneuroimmunology (PNI) aims to uncover the processes and consequences of nervous, immune, and endocrine system relationships. Behavior is a consequence of such interactions and manifests from a complex interweave of factors including immune-to-neural and neural-to-immune communication. Often the signaling molecules involved during a particular episode of neuroimmune activation are not known but behavioral response provides evidence that bioactives such as neurotransmitters and cytokines are perturbed. Immunobehavioral phenotyping is a first-line approach when examining the neuroimmune system and its reaction to immune stimulation or suppression. Behavioral response is significantly more sensitive than direct measurement of a single specific bioactive and can quickly and efficiently rule in or out relevance of a particular immune challenge or therapeutic to neuroimmunity. Classically, immunobehavioral research was focused on sickness symptoms related to bacterial infection but neuroimmune activation is now a recognized complication of diseases and disorders ranging from cancer to diabesity. Immunobehaviors include lethargy, loss of appetite, and disinterest in social activity and the surrounding environment. In addition, neuroimmune activation can precipitate feelings of depression and anxiety while negatively impacting cognitive function and physical activity. Provided is a detailed overview of behavioral tests frequently used to examine neuroimmune activation in mice with a special emphasis on preexperimental conditions that can confound or prevent successful immunobehavioral experimentation.

Investigating anxiety and depressive-like phenotypes in genetic mouse models of serotonin depletion

Emotional disorders such as depression, panic attacks, generalized anxiety, phobias and post-traumatic stress have been associated to decreased serotonin (5-HT) function, based on the positive effects of treatments that enhance 5-HT neurotransmission. However, it has been difficult to establish a primary role for 5-HT deficiency in these diseases, making preclinical models particularly useful. Over the last ten years a variety of genetic mouse models of 5-HT depletion have been produced, complementing previous pharmacologically-based models. Initial models hindered the differentiation of the raphe 5-HT neurons, while more recently produced models suppressed 5-HT production or incapacitated 5-HT vesicular packaging and release in normally developed raphe neurons. Here, we provide an overview of 11 genetic mouse models with lowered 5-HT transmission and summarize the available behavioural investigations concerning their anxiety and depression phenotypes. Although these studies are still ongoing, some common anxiety-related traits and behavioural phenotypes have emerged. Most studies have reported decreased innate anxiety to novelty but heightened fear responses to conditioned aversive cues. This complex phenotype is in general agreement with the proposed dual function of 5-HT in modulating different defensive behaviours. Surprisingly, the depressive-like behaviours have been less studied and, so far, did not yield a consistent phenotype in standard tests. Future studies should be conducted using more ethological relevant models to conclude on the causal role of 5-HT depletion in depression. This review also describes the differences in level and regional distribution of 5-HT depletion among the available mouse models, which could contribute to the diverse phenotypes observed. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Reversed light-dark cycle and cage enrichment effects on ethanol-induced deficits in motor coordination assessed in inbred mouse strains with a compact battery of refined tests

The laboratory environment existing outside the test situation itself can have a substantial influence on results of some behavioral tests with mice, and the extent of these influences sometimes depends on genotype. For alcohol research, the principal issue is whether genotype-related ethanol effects will themselves be altered by common variations in the lab environment or instead will be essentially the same across a wide range of lab environments. Data from 20 inbred strains were used to reduce an original battery of seven tests of alcohol intoxication to a compact battery of four tests: the balance beam and grip strength with a 1.25 g/kg ethanol dose and the accelerating rotarod and open-field activation tests with 1.75 g/kg. The abbreviated battery was then used to study eight inbred strains housed under a normal or reversed light-dark cycle, or a standard or enriched home cage environment. The light-dark cycle had no discernable effects on any measure of behavior or response to alcohol. Cage enrichment markedly improved motor coordination in most strains. Ethanol-induced motor coordination deficits were robust; the well-documented strain-dependent effects of ethanol were not altered by cage enrichment.

Effects of photoperiod regimen on emotional behaviour in two tests for anxiolytic activity in Wistar rat

Seasonal changes are often gone with mood and behaviour changes which are probably linked to change in day length or photoperiod. The experiments developed in this work are based on the hypothesis that changes in photoperiod affect emotionality in rats. To check this hypothesis, female rats were exposed to four different photoperiods (LP: 16L/8D; MP: 12L/12D; SP: 8L/16D; SP-F: 8L/16D with a light pulse in midpoint of the dark phase). Eight or 14 weeks later, rats were subjected to two behavioural tests to quantify anxiety level. Independently of duration, rats exposed to SP exhibited higher levels of anxious-like behaviour than rats raised in LP and SP-F, in an open field test (OFT) and in elevated plus maze (EPM). Significant differences in EPM are obtained only after 14 weeks of treatment. Moreover rats treated more long time showed greater suprarenal gland mass. Compared to all other groups, females exposed to SP had greater suprarenal gland. Our results indicate that changes in day length are associated with different levels of anxious-like behaviours consistent with the conjecture that short days may have an anxiogenic effect in female rats.