Affective responses to changes in day length in Siberian hamsters (Phodopus sungorus)

Seasonal Adaptation of Mammalian Development: Effect of Early-Life Photoperiod on Reproduction, Somatic Growth, and Neurobehavioral Systems

For the survival and efficient breeding of wild-living animals, it is crucial to predict seasonal changes and prepare appropriate physiological functions and neurobehavioral mechanisms. In mammals, photoperiod serves as a reliable cue for seasonal changes in the environment, primarily transmitted by melatonin. This review focuses on the seasonal adaptation of mammalian development, specifically the effect of early-life photoperiod on reproductive, somatic, and neurobehavioral development in small- and large-sized mammals. Prediction of seasons through early-life photoperiod is particularly important for small mammals, which have relatively short longevity, to adjust their maximum growth and breeding ability in appropriate seasons during the birth year or the following round. Brain plasticity, as well as cognitive and emotional behaviors, are also highly modulated by early-life photoperiods for successful mating and spatial memory for foraging. This review first summarizes the basic knowledge and recent progress in the programming and epigenetic regulatory mechanisms of reproductive and neurobehavioral development in small mammals, including C57BL/6J mice, which cannot produce detectable amounts of melatonin. The review then focuses on the influence of perinatal environmental conditions or birth season on adult phenotypes in large livestock and humans. Studies have advanced on the concept of the developmental origins of health and disease (DOHaD). Evidence from large mammals suggests that the prediction of seasons is crucial for high-fitness functions over several years. Finally, this review discusses the association of the season of birth with life course physiology and diseases in humans, and the possible mechanisms.

Mood variation under dual regulation of circadian clock and light

The intricate relationship between circadian rhythms and mood is well-established. Disturbances in circadian rhythms and sleep often precede the development of mood disorders, such as major depressive disorder (MDD), bipolar disorder (BD), and seasonal affective disorder (SAD). Two primary factors, intrinsic circadian clocks and light, drive the natural fluctuations in mood throughout the day, mirroring the patterns of sleepiness and wakefulness. Nearly all organisms possess intrinsic circadian clocks that coordinate daily rhythms, with light serving as the primary environmental cue to synchronize these internal timekeepers with the 24-hour cycle. Additionally, light directly influences mood states. Disruptions to circadian rhythms, such as those caused by jet lag, shift work, or reduced daylight hours, can trigger or exacerbate mood symptoms. The complex and often subtle connections between circadian disruptions and mood dysregulation suggest that focusing solely on individual clock genes is insufficient to fully understand their etiology and progression. Instead, mood instability may arise from systemic misalignments between external cycles and the internal synchronization of circadian clocks. Here, we synthesize past research on the independent contributions of circadian clocks and light to mood regulation, drawing particularly on insights from animal studies that illuminate fundamental mechanisms relevant to human health.

Orphan nuclear receptor nr4a1 regulates winter depression-like behavior in medaka

About 10% of the population suffers from depression in winter at high latitude. Although it has become a serious public health issue, its underlying mechanism remains unknown and new treatments and therapies are required. As an adaptive strategy, many animals also exhibit depression-like behavior in winter. Previously, it has been reported that celastrol, a traditional Chinese medicine, can rescue winter depression-like behavior in medaka, an excellent model of winter depression. Nuclear receptor subfamily 4 group A member 1 (nr4a1, also known as nur77) is a known target of celastrol, and the signaling pathway of nr4a1 was suggested to be inactive in medaka brain during winter, implying the association of nr4a1 and winter depression-like behavior. However, the direct evidence for its involvement in winter depression-like behavior remains unclear. The present study found that nr4a1 was suppressed in the medaka brain under winter conditions. Cytosporone B, nr4a1 chemical activator, reversed winter depression-like behavior under winter conditions. Additionally, nr4a1 mutant fish generated by CRISPR/Cas9 system showed decreased sociability under summer conditions. Therefore, our results demonstrate that the seasonal regulation of nr4a1 regulates winter depression-like behavior and offers potential therapeutic target.

Winter madness: Melatonin as a neuroendocrine regulator of seasonal aggression

Individuals of virtually all vertebrate species are exposed to annual fluctuations in the deterioration and renewal of their environments. As such, organisms have evolved to restrict energetically expensive processes and activities to a specific time of the year. Thus, the precise timing of physiology and behavior is critical for individual reproductive success and subsequent fitness. Although the majority of research on seasonality has focused on seasonal reproduction, pronounced fluctuations in other non-reproductive social behaviors, including agonistic behaviors (e.g., aggression), also occur. To date, most studies that have investigated the neuroendocrine mechanisms underlying seasonal aggression have focused on the role of photoperiod (i.e., day length); prior findings have demonstrated that some seasonally breeding species housed in short "winter-like" photoperiods display increased aggression compared with those housed in long "summer-like" photoperiods, despite inhibited reproduction and low gonadal steroid levels. While fewer studies have examined how the hormonal correlates of environmental cues regulate seasonal aggression, our previous work suggests that the pineal hormone melatonin acts to increase non-breeding aggression in Siberian hamsters (Phodopus sungorus) by altering steroid hormone secretion. This review addresses the physiological and cellular mechanisms underlying seasonal plasticity in aggressive and non-aggressive social behaviors, including a key role for melatonin in facilitating a "neuroendocrine switch" to alternative physiological mechanisms of aggression across the annual cycle. Collectively, these studies highlight novel and important mechanisms by which melatonin regulates aggressive behavior in vertebrates and provide a more comprehensive understanding of the neuroendocrine bases of seasonal social behaviors broadly.

Sex differences in the response to circadian disruption in diurnal sand rats

Most animal model studies on physiological functions and pathologies are conducted in males. However, diseases such as depression, type 2 diabetes (T2DM) and cardiovascular disease, all show different prevalence and characteristics in females and males. Moreover, most mammal studies are conducted in nocturnal mice and rats, while modelling diurnal humans. We therefore used male and female fat sand rats (Psammomys obesus), which are diurnal in the wild, as an animal model for T2DM, to explore the effects of mild circadian disruption on behavior, glucose tolerance, cholesterol and heart weight. We found significant differences between the sexes: on average, in response to short photoperiods (SP) acclimation, males showed higher levels of depression-like behavior, lower glucose tolerance, and increased plasma cholesterol levels compared with females, with no effect on heart/body weight ratio. Females, however did show an increase in heart/body weight ratio in response to SP acclimation. We also found that regardless of sex, arrhythmic animals showed higher blood glucose levels, cholesterol levels, heart/body weight ratio, and depressive-like behavior compared with rhythmic animals. Hence, we suggest that the expression of the Circadian Syndrome could be different between males and females. Additional work with females is required to clearly delineate the specific effects in both sexes, and promote sex-based health care, prevention measures and therapies.

Photoperiodic changes in hippocampal neurogenesis and plasma metabolomic profiles in relation to depression-like behavior in mice

Photoperiod alters affective behaviors and brain neuroplasticity in several mammalian species. We addressed whether neurogenesis and signaling pathways of insulin-like growth factor-I (IGF-I), a key modulator of neuroplasticity, are regulated by photoperiod in C57BL/6 J mice, a putative model of seasonal affective disorder. We also examined the effects of photoperiod on plasma metabolomic profiles in relation to depression-like behavior to understand a possible linkage between peripheral metabolism and behavior. Mice that were maintained under long-day conditions (LD) exhibited a higher number of 5-bromo-2'-deoxyuridine-positive cells and higher levels of astrocyte marker in the dentate gyrus of the hippocampus compared to that of mice under short-day conditions (SD). Plasma IGF-I levels and levels/expression of IGF-I signaling molecules in the hippocampus (Brn-4, NeuroD1, and phospho-Akt) involved in neuronal proliferation and differentiation were higher in the mice under LD. Metabolome analysis using plasma of the mice under LD and SD identified several metabolites that were highly correlated with immobility in the forced swim test, a depression-like behavior. Negative correlations with behavior occurred in the levels of 23 metabolites, including metabolites related to neurogenesis and antidepressant-like effects of exercise, metabolites in the biosynthesis of arginine, and the occurrence of branched chain amino acids. Three metabolites had positive correlations with the behavior, including guanidinosuccinic acid, a neurotoxin. Taken together, photoperiodic responses of neurogenesis and neuro-glial organization in the hippocampus may be involved in photoperiodic alteration of depression-like behavior, mediated through multiple pathways, including IGF-I and peripheral metabolites.

Rhythms, Reward, and Blues: Consequences of Circadian Photoperiod on Affective and Reward Circuit Function

Circadian disruptions, along with altered affective and reward states, are commonly associated with psychiatric disorders. In addition to genetics, the enduring influence of environmental factors in programming neural networks is of increased interest in assessing the underpinnings of mental health. The duration of daylight or photoperiod is known to impact both the serotonin and dopamine systems, which are implicated in mood and reward-based disorders. This review first examines the effects of circadian disruption and photoperiod in the serotonin system in both human and preclinical studies. We next highlight how brain regions crucial for the serotoninergic system (i.e., dorsal raphe nucleus; DRN), and dopaminergic (i.e., nucleus accumbens; NAc and ventral tegmental area; VTA) system are intertwined in overlapping circuitry, and play influential roles in the pathology of mood and reward-based disorders. We then focus on human and animal studies that demonstrate the impact of circadian factors on the dopaminergic system. Lastly, we discuss how environmental factors such as circadian photoperiod can impact the neural circuits that are responsible for regulating affective and reward states, offering novel insights into the biological mechanisms underlying the pathophysiology, systems, and therapeutic treatments necessary for mood and reward-based disorders.

Photoperiodic effects on monoamine signaling and gene expression throughout development in the serotonin and dopamine systems

Photoperiod or the duration of daylight has been implicated as a risk factor in the development of mood disorders. The dopamine and serotonin systems are impacted by photoperiod and are consistently associated with affective disorders. Hence, we evaluated, at multiple stages of postnatal development, the expression of key dopaminergic (TH) and serotonergic (Tph2, SERT, and Pet-1) genes, and midbrain monoamine content in mice raised under control Equinox (LD 12:12), Short winter-like (LD 8:16), or Long summer-like (LD 16:8) photoperiods. Focusing in early adulthood, we evaluated the midbrain levels of these serotonergic genes, and also assayed these gene levels in the dorsal raphe nucleus (DRN) with RNAScope. Mice that developed under Short photoperiods demonstrated elevated midbrain TH expression levels, specifically during perinatal development compared to mice raised under Long photoperiods, and significantly decreased serotonin and dopamine content throughout the course of development. In adulthood, Long photoperiod mice demonstrated decreased midbrain Tph2 and SERT expression levels and reduced Tph2 levels in the DRN compared Short photoperiod mice. Thus, evaluating gene × environment interactions in the dopaminergic and serotonergic systems during multiple stages of development may lead to novel insights into the underlying mechanisms in the development of affective disorders.

Blue Light Deprivation Produces Depression-Like Responses in Mongolian Gerbils

BACKGROUND

Depression is a leading cause of disability worldwide and is a major contributor to the overall global burden of disease, but its etiology is poorly understood. It has been reported that a disrupted biological rhythm, in terms of a shortened light duration and total darkness, can cause depression-like behaviors in animals. Blue light was reported to have an inhibitory effect on melatonin, which is considered an important clock rhythm biomarker. In the present study, we investigated the effects of blue light deprivation on depressive-like behaviors in gerbils and explored the underlying mechanisms.

METHODS

Gerbils were housed under white light with a filter to block the blue light or without a filter. The behaviors of the gerbils were observed. The biological rhythm, 5-HT, hypothalamic-pituitary-adrenal (HPA) axis and melanopsin pathway were analyzed.

RESULTS

We found that blue light deprivation (BLD) induced depression-like behavior in gerbils. Melatonin lost its rhythm, and corticosterone (CORT) levels decreased in the morning in the BLD group. Lower corticotropin-releasing hormone (CRH) in the hypothalamus and lower adrenocorticotropin hormone (ACTH)/CORT in serum were observed after BLD. Furthermore, 5-HT in the serum and brain were decreased after BLD. Additionally, BLD affected the blue light sensitivity protein melanopsin and its pathway, with downregulation of the proteins melanopsin, PKCα, and c-Fos and the mRNA levels of c-fos and trpc3 and upregulation of the protein p-PKCα.

CONCLUSIONS

Our findings indicated that BLD might produce depression-like behaviors in gerbils. Melatonin arrhythmicity, HPA axis abnormalities, 5-HT decreases and melanopsin pathway changes might be associated with the depression behavioral phenotype in gerbils.

Chronic Jet Lag Simulation Decreases Hippocampal Neurogenesis and Enhances Depressive Behaviors and Cognitive Deficits in Adult Male Rats

There is a long history that protracted periods of circadian disruption, such as through frequent transmeridian travel or rotating shift work, can have a significant impact on brain function and health. In addition, several studies have shown that chronic periods of circadian misalignment can be a significant risk factor for the development of depression and anxiety in some individuals with a history of psychiatric illness. In animal models, circadian disruption can be introduced through either phase advances or delays in the light-dark cycle. However, the impact of chronic phase shifts on affective behavior in rats has not been well-studied. In the present study, male rats were subjected to either weekly 6 h phase advances (e.g., traveling eastbound from New York to Paris) or 6 h phase delays (e.g., traveling westbound from New York to Hawaii) in their light/dark cycle for 8 weeks. The effect of chronic phase shifts was then examined on a range of emotional and cognitive behaviors. We found that rats exposed to frequent phase advances, which mirror conditions of chronic jet lag in humans, exhibited impairments in object recognition memory and showed signature symptoms of depression, including anhedonia, increased anxiety behavior, and higher levels of immobility in the forced swim test. In addition, rats housed on the phase advance schedule also had lower levels of hippocampal neurogenesis and immature neurons showed reduced dendritic complexity compared to controls. These behavioral and neurogenic changes were direction-specific and were not observed after frequent phase delays. Taken together, these findings support the view that circadian disruption through chronic jet lag exposure can suppress hippocampal neurogenesis, which can have a significant impact on memory and mood-related behaviors.

Prepubertal ovarian inhibition of Light/Dark Box exploration and novel object investigation in juvenile Siberian hamsters

The overwhelming majority of research on the role of gonadal hormones in behavioral development has focused on perinatal, pubertal, or adult life stages. The juvenile period has been overlooked because it is thought to be a time of gonadal quiescence. In the present study, we tested whether prepubertal gonadectomy impacts the behavior of male and female juvenile hamsters on the Light/Dark Box, Novel Object, and Social Approach tests (Experiment 1) and compared these findings to those obtained after adult gonadectomy (Experiment 2). Prepubertal ovariectomy increased exploration (i.e. time spent in the light zone of the Light/Dark Box) and novel object investigation of juveniles indicating an inhibitory role for the juvenile ovary; social approach was unaffected. In contrast, adult ovariectomy and castration (both prepubertal and adult) had no effect on any behavioral measure. Experiment 3 tested whether rearing hamsters in a short day length (SD), which delays puberty in this species, extends the interval of juvenile ovarian inhibition on exploration and novelty seeking. We also tested whether provision of estradiol reverses the effects of prepubertal ovariectomy. Hormonal manipulations and behavioral tests of Experiment 3 were conducted at ages when long day-reared hamsters are adult (as in Experiment 2), but SD-reared hamsters remain reproductively immature. Ovariectomy again increased exploration in the SD-reared juveniles despite the older age of surgery and testing. Estradiol treatment had no effect. These findings reveal a novel role for the juvenile ovary in exploration and novelty seeking that is unlikely to be mediated exclusively by estradiol.

Sequential Photoperiodic Programing of Serotonin Neurons, Signaling and Behaviors During Prenatal and Postnatal Development

Early life stimuli during critical developmental time frames have been linked to increased risk for neurodevelopmental disorders later in life. The serotonergic system of the brain is implicated in mood disorders and is impacted by the duration of daylight, or photoperiod. Here we sought to investigate sensitive periods of prenatal and postnatal development for photoperiodic programming of DRN serotonin neurons, midbrain serotonin and metabolite levels along with affective behaviors in adolescence (P30) or adulthood (P50). To address these questions we restricted the interval of exposure to prenatal development (E0-P0) for Long summer-like photoperiods (LD 16:8), or Short winter-like photoperiods (LD 8:16) with postnatal development and maturation then occurring under the opposing photoperiod. Prenatal exposure alone to Long photoperiods was sufficient to fully program increased excitability of DRN serotonin neurons into adolescence and adulthood, similar to maintained exposure to Long photoperiods throughout development. Interestingly, Long photoperiod exposure can elevate serotonin and its’ corresponding metabolite levels along with reducing affective behavior, which appear to have both pre and postnatal origins. Thus, exposure to Long photoperiods prenatally programs increased DRN serotonin neuronal excitability, but this step is insufficient to program serotonin signaling and affective behavior. Continuing influence of Long photoperiods during postnatal development then modulates serotonergic content and has protective effects for depressive-like behavior. Photoperiodic programing of serotonin function in mice appears to be a sequential process with programing of neuronal excitability as a first step occurring prenatally, while programing of circuit level serotonin signaling and behavior extends into the postnatal period.

Light as a modulator of emotion and cognition: Lessons learned from studying a diurnal rodent

Light profoundly affects the behavior and physiology of almost all animals, including humans. One such effect in humans is that the level of illumination during the day positively contributes to affective well-being and cognitive function. However, the neural mechanisms underlying the effects of daytime light intensity on affect and cognition are poorly understood. One barrier for progress in this area is that almost all laboratory animal models studied are nocturnal. There are substantial differences in how light affects nocturnal and diurnal species, e.g., light induces sleep in nocturnal mammals but wakefulness in diurnal ones, like humans. Therefore, the mechanisms through which light modulates affect and cognition must differ between the chronotypes. To further understand the neural pathways mediating how ambient light modulates affect and cognition, our recent work has developed a diurnal rodent model, the Nile grass rat (Arvicanthis niloticus), in which daytime light intensity is chronically manipulated in grass rats housed under the same 12:12 hour light/dark cycle. This simulates lighting conditions during summer-like bright sunny days vs. winter-like dim cloudy days. Our work has revealed that chronic dim daylight intensity results in higher depression- and anxiety-like behaviors, as well as impaired spatial learning and memory. Furthermore, we have found that hypothalamic orexin is a mediator of these effects. A better understanding of how changes in daytime light intensity impinge upon the neural substrates involved in affect and cognition will lead to novel preventive and therapeutic strategies for seasonal affective disorder, as well as for non-seasonal emotional or cognitive impairments associated with light deficiency.

Circadian Forced Desynchrony of the Master Clock Leads to Phenotypic Manifestation of Depression in Rats

In mammals, a master circadian clock within the suprachiasmatic nucleus (SCN) of the hypothalamus maintains the phase coherence among a wide array of behavioral and physiological circadian rhythms. Affective disorders are typically associated with disruption of this fine-tuned “internal synchronization,” but whether this internal misalignment is part of the physiopathology of mood disorders is not clear. To date, depressive-like behavior in animal models has been induced by methods that fail to specifically target the SCN regulation of internal synchronization as the mode to generate depression. In the rat, exposure to a 22-h light-dark cycle (LD22) leads to the uncoupling of two distinct populations of neuronal oscillators within the SCN. This genetically, neurally, and pharmacologically intact animal model represents a unique opportunity to assess the effect of a systematic challenge to the central circadian pacemaker on phenotypic manifestations of mood disorders. We show that LD22 circadian forced desynchrony in rats induces depressive-like phenotypes including anhedonia, sexual dysfunction, and increased immobility in the forced swim test (FST), as well as changes in the levels and turnover rates of monoamines within the prefrontal cortex. Desynchronized rats show increased FST immobility during the dark (active) phase but decreased immobility during the light (rest) phase, suggesting a decrease in the amplitude of the normal daily oscillation in this behavioral manifestation of depression. Our results support the notion that the prolonged internal misalignment of circadian rhythms induced by environmental challenge to the central circadian pacemaker may constitute part of the etiology of depression.

Decreased daytime illumination leads to anxiety-like behaviors and HPA axis dysregulation in the diurnal grass rat (Arvicanthis niloticus)

The impact of ambient light on mood and anxiety is best exemplified in seasonal affective disorder, in which patients experience depression and anxiety in winter when there is less light in the environment. However, the brain mechanisms underlying light-dependent changes in affective state remain unclear. Our previous work revealed increased depression-like behaviors in the diurnal Nile grass rat (Arvicanthis niloticus) housed in a dim light-dark (dim-LD) cycle as compared to the controls housed in a bright light-dark (bright-LD) condition. As depression is often comorbid with anxiety and is associated with dysregulation of the body's stress response system, the present study examined the anxiety-like behaviors as well as indicators of the hypothalamic-pituitary-adrenal (HPA) axis functioning in the grass rats. Animals housed in dim-LD showed increased anxiety-like behaviors compared to bright-LD controls, as revealed by fewer entries and less time spent at the center in the open field test and more marbles buried during the marble-burying test. Following the marble-burying test, dim-LD animals showed higher plasma corticosterone (CORT) levels and hippocampal Fos expression. Although the daily CORT rhythm was comparable between bright-LD and dim-LD groups, the day/night variation of corticotropin-releasing hormone mRNA expression in the paraventricular nucleus was diminished in dim-LD animals. In addition, glucocorticoid receptor and mineralocorticoid receptor mRNA expression were higher in the hippocampus of dim-LD animals. The results suggest that in diurnal species, reduced daytime illumination can lead to increased anxiety-like behaviors and altered HPA axis functioning, providing insights into the link between decreased environmental illumination and negative emotion.

Short photoperiod condition increases susceptibility to stress in adolescent male rats

The seasonality of depressive symptoms is prevalent in children and adolescents. However, the mechanisms that underlie such susceptibility to seasonal influences on mood disorders are unclear. We examined the effects of a short photoperiod condition on the susceptibility to subchronic unpredictable mild stress (SCUS) and rhythmic alterations of plasma corticosterone (CORT), melatonin, and neuropeptide Y (NPY) in adolescent male rats. Compared with the 12h/12h light/dark photoperiod control (CON) rats, the 8h/16h photoperiod SCUS rats exhibited significant anhedonia, a core symptom of human depression, together with a blunted diurnal rhythm and elevation of 24h CORT, melatonin, and NPY levels. The 8h/16h photoperiod condition also blunted the rhythmicity of CORT, caused a phase inversion of melatonin, and caused a phase delay of NPY compared with 12h/12h CON rats. Such abnormalities of plasma CORT, NPY, and melatonin might cause adolescent individuals to present higher stress reactivity and greater vulnerability to stress over their lifetimes. The present study provides evidence of the susceptibility to the seasonality of stress-related disorders in adolescence.

Phase-delay in the light-dark cycle impairs clock gene expression and levels of serotonin, norepinephrine, and their metabolites in the mouse hippocampus and amygdala

OBJECTIVE

A number of animal studies have implicated circadian clock genes in the regulation of mood, anxiety, and reward. However, the effect of misalignment of the environmental light-dark and internal circadian clock on the monoamine system is not fully understood. In the present study, we examined whether an abnormal light-dark schedule would affect behavior-, circadian clock-, and monoamine-related gene expressions, along with monoamine contents in the amygdala and hippocampus of mice.

METHODS

Mice were subjected to an 8-hour phase delay in the light-dark cycle (Shift) every two days for four weeks, and locomotor activity was continuously measured. We examined the circadian expression of clock genes (Per1, Per2, and Bmal1) and genes of the NE/5HT uptake transporters (Net and Sert). In addition, the levels of NE/5HT and their metabolites MHPG/5HIAA were analyzed in the amygdala and hippocampus.

RESULTS

Locomotor activity showed a free-running phenotype with a longer period (>24 hours) and showed misalignment between the light-dark and inactive-active cycles. The amplitude of the day-night fluctuation of Bmal1 expression was reduced in the amygdala and hippocampus of light-dark-shifted mice. Net gene expression in the Shift group showed different profiles compared with the Control group. In addition, NE and 5HT levels in the amygdala of the Shift group increased during the active period.

CONCLUSIONS

The present results suggest that misalignment of the internal and external clocks by continuous shifting of the light-dark cycle affects the circadian clocks and monoamine metabolism in the amygdala and hippocampus of mice.

The first observation of seasonal affective disorder symptoms in Rhesus macaque

Diurnal animals are a better model for seasonal affective disorder (SAD) than nocturnal ones. Previous work with diurnal rodents demonstrated that short photoperiod conditions brought about depression-like behavior. However, rodents are at a large phylogenetic distance from humans. In contrast, nonhuman primates are closely similar to humans, making them an excellent candidate for SAD model. This study made the first attempt to develop SAD in rhesus macaque (Macaca mulatta) and it was found that short photoperiod conditions could lead monkeys to display depressive-like huddling behavior, less spontaneous locomotion, as well as less reactive locomotion. In addition to these depression-related behavioral changes, the physiological abnormalities that occur in patients with SAD, such as weight loss, anhedonia and hypercortisolism, were also observed in those SAD monkeys. Moreover, antidepressant treatment could reverse all of the depression-related symptoms, including depressive-like huddling behavior, less spontaneous locomotion, less reactive locomotion, weight loss, anhedonia and hypercortisolism. For the first time, this study observed the SAD symptoms in rhesus macaque, which would provide an important platform for the understanding of the etiology of SAD as well as developing novel therapeutic interventions in the future.

Hypothalamic dopaminergic neurons in an animal model of seasonal affective disorder

Light has profound effects on mood regulation as exemplified in seasonal affective disorder (SAD) and the therapeutic benefits of light therapy. However, the underlying neural pathways through which light regulates mood are not well understood. Our previous work has developed the diurnal grass rat, Arvicanthis niloticus, as an animal model of SAD. Following housing conditions of either 12:12 h dim light:dark (DLD) or 8:16 h short photoperiod (SP), which mimic the lower light intensity or short day-length of winter, respectively, grass rats exhibit an increase in depression-like behavior compared to those housed in a 12:12 h bright light:dark (BLD) condition. Furthermore, we have shown that the orexinergic system is involved in mediating the effects of light on mood and anxiety. To explore other potential neural substrates involved in the depressive phenotype, the present study examined hypothalamic dopaminergic (DA) and somatostatin (SST) neurons in the brains of grass rats housed in DLD, SP and BLD. Using immunostaining for tyrosine hydroxylase (TH) and SST, we found that the number of TH- and SST-ir cells in the hypothalamus was significantly lower in the DLD and SP groups compared to the BLD group. We also found that treating BLD animals with a selective orexin receptor 1 (OX1R) antagonist SB-334867 significantly reduced the number of hypothalamic TH-ir cells. The present study suggests that the hypothalamic DA neurons are sensitive to daytime light deficiency and are regulated by an orexinergic pathway. The results support the hypothesis that the orexinergic pathways mediate the effects of light on other neuronal systems that collectively contribute to light-dependent changes in the affective state.

Photoperiod programs dorsal raphe serotonergic neurons and affective behaviors

The serotonergic raphe nuclei of the midbrain are principal centers from which serotonin neurons project to innervate cortical and sub-cortical structures. The dorsal raphe nuclei receive light input from the circadian visual system and indirect input from the biological clock nuclei. Dysregulation of serotonin neurotransmission is implicated in neurobehavioral disorders, such as depression and anxiety, and alterations in the serotonergic phenotype of raphe neurons have dramatic effects on affective behaviors in rodents. Here, we demonstrate that day length (photoperiod) during development induces enduring changes in mouse dorsal raphe serotonin neurons—programming their firing rate, responsiveness to noradrenergic stimulation, intrinsic electrical properties, serotonin and norepinephrine content in the midbrain, and depression/anxiety-related behavior in a melatonin receptor 1 (MT1)-dependent manner. Our results establish mechanisms by which seasonal photoperiods may dramatically and persistently alter the function of serotonin neurons.

Anabolic/androgenic steroid administration during adolescence and adulthood differentially modulates aggression and anxiety

Anabolic/androgenic steroid (AAS) use remains high in both teens and adults in the U.S. and worldwide despite studies showing that AAS use is associated with a higher incidence of aggression and anxiety. Recently we showed that chronic exposure to AAS through adolescence increases aggression and decreases anxious behaviors, while during AAS-withdrawal aggression is lowered to species-normative levels and anxiety increases. AAS exposure is known to differentially alter behaviors and their underlying neural substrates between adults and adolescents and thus the current study investigated whether exposure to AAS during adulthood affects the relationship between aggression and anxiety in a manner similar to that previously observed in adolescents. Male hamsters were administered a moderate dose of AAS (5.0mg/kg/day×30days) during adolescence (P27-56) or young adulthood (P65-P94) and then tested for aggression and anxiety during AAS exposure (i.e., on P57 or P95) and during AAS withdrawal (i.e., 30days later on P77 or P115). Adolescent exposure to AAS increased aggressive responding during the AAS exposure period and anxiety-like responding during AAS withdrawal. Neither behavior was similarly influenced by adult exposure to AAS. Adult AAS exposure produced no difference in aggressive responding during AAS exposure (P95) or AAS withdrawal (P115); however, while AAS exposure during adulthood produced no difference in anxiety-like responding during AAS exposure, adult hamsters administered AAS were less anxious than vehicle control animals following AAS withdrawal. Together these data suggest that the aggression and anxiety provoking influence of AAS are likely a developmental phenomenon and that adult exposure to AAS may be anxiolytic over the long term.

Tryptophan hydroxylase 2 in seasonal affective disorder: underestimated perspectives?

Seasonal affective disorder (SAD) is characterized by recurrent depression occurring generally in fall/winter. Numerous pieces of evidence indicate the association of SAD with decreased brain neurotransmitter serotonin (5-HT) system functioning. Tryptophan hydroxylase 2 (TPH2) is the key and rate-limiting enzyme in 5-HT synthesis in the brain. This paper concentrates on the relationship between TPH2 activity and mood disturbances, the association between human

Social interaction with rat exposed to constant light during lactation prevents depressive-like behavior induced by constant light in adulthood

Circadian rhythm disruptions are often observed in depressed patients, and changes in the light/dark cycle promote depressive-like behavior in animal models. Prolonged exposure to constant light (LL) is known to lead to arrhythmicity of circadian locomotor activity and depressive-like behavior in rats. Interestingly, neonatal exposure to LL prevents both arrhythmicity and depressive behavior in adulthood. Arrhythmic rats under LL conditions that cohabitate with a rhythmic rat exhibit improvement in circadian rhythms. We tested whether such cohabitation also protects against LL-induced depressive-like behavior. Wistar rats were assigned to conditions of either neonatal constant light (neonatal-LL) on postnatal days 10-22 or a regular light/dark cycle (neonatal-LD). On day 45, the animals were assigned to three possible pair combinations. After a baseline sucrose preference test, half of the pairs were placed under LL conditions. Weekly sucrose preference tests were conducted to evaluate depressive-like behavior. The animals were isolated by an aluminum wall on the test day. At week 2 of LL, sucrose preference was reduced in neonatal-LD/neonatal-LD pairs of animals. At week 5, neonatal-LD/neonatal-LD pairs exhibited anhedonic-like behavior, but the pairs with at least one neonatal-LL rat did not. The LL cycle was returned to an LD cycle, and the neonatal-LD/neonatal-LD pairs exhibited a restoration of sucrose preference 2 weeks later. We conclude that social interaction can prevent depressive-like behavior induced by circadian rhythm disruption as long as one of the animals is more prone to present a strong rhythm.

Melatonin adjusts the expression pattern of clock genes in the suprachiasmatic nucleus and induces antidepressant-like effect in a mouse model of seasonal affective disorder

Recently, we have shown that C57BL/6J mice exhibit depression-like behavior under short photoperiod and suggested them as an animal model for investigating seasonal affective disorder (SAD). In this study, we tested if manipulations of the circadian clock with melatonin treatment could effectively modify depression-like and anxiety-like behaviors and brain serotonergic system in C57BL/6J mice. Under short photoperiods (8-h light/16-h dark), daily melatonin treatments 2 h before light offset have significantly altered the 24-h patterns of mRNA expression of circadian clock genes (per1, per2, bmal1 and clock) within the suprachiasmatic nuclei (SCN) mostly by increasing amplitude in their expressional rhythms without inducing robust phase shifts in them. Melatonin treatments altered the expression of genes of serotonergic neurotransmission in the dorsal raphe (tph2, sert, vmat2 and 5ht1a) and serotonin contents in the amygdala. Importantly, melatonin treatment reduced the immobility in forced swim test, a depression-like behavior. As a key mechanism of melatonin-induced antidepressant-like effect, the previously proposed phase-advance hypothesis of the circadian clock could not be confirmed under conditions of our experiment. However, our findings of modest adjustments in both the amplitude and phase of the transcriptional oscillators in the SCN as a result of melatonin treatments may be sufficient to associate with the effects seen in the brain serotonergic system and with the improvement in depression-like behavior. Our study confirmed a predictive validity of C57BL/6J mice as a useful model for the molecular analysis of links between the clock and brain serotonergic system, which could greatly accelerate our understanding of the pathogenesis of SAD, as well as the search for new treatments.

Photoperiodic responses of depression-like behavior, the brain serotonergic system, and peripheral metabolism in laboratory mice

Seasonal affective disorder (SAD) is characterized by depression during specific seasons, generally winter. The pathophysiological mechanisms underlying SAD remain elusive due to a limited number of animal models with high availability and validity. Here we show that laboratory C57BL/6J mice display photoperiodic changes in depression-like behavior and brain serotonin content. C57BL/6J mice maintained under short-day conditions, as compared to those under long-day conditions, demonstrated prolonged immobility times in the forced swimming test with lower brain levels of serotonin and its precursor l-tryptophan. Furthermore, photoperiod altered multiple parameters reflective of peripheral metabolism, including the ratio of plasma l-tryptophan to the sum of other large neutral amino acids that compete for transport across the blood-brain barrier, responses of circulating glucose and insulin to glucose load, sucrose intake under restricted feeding condition, and sensitivity of the brain serotonergic system to peripherally administered glucose. These data suggest that the mechanisms underlying SAD involve the brain-peripheral tissue network, and C57BL/6J mice can serve as a powerful tool for investigating the link between seasons and mood.

Diurnal rodents as an advantageous model for affective disorders: novel data from diurnal degu (Octodon degus)

Circadian rhythms are strongly associated with affective disorders and recent studies have suggested utilization of diurnal rodents as model animal for circadian rhythms-related domains of these disorders. Previous work with the diurnal fat sand rat and Nile grass rat demonstrated that short photoperiod conditions result in behavioral changes including anxiety- and depression-like behavior. The present study examined the effect of manipulating day length on activity rhythms and behavior of the diurnal degu. Animals were housed for 3 weeks under either a short photoperiod (5-h:19-h LD) or a neutral photoperiod (12-h:12-h LD) and then evaluated by sweet solution test and the forced swim test for depression-like behavior, and in the light/dark box and open field for anxiety-like behavior. Results indicate that short photoperiod induced depression-like behavior in the forced swim test and the sweet solution preference test and anxiety-like behavior in the open field compared with animals maintained in a neutral photoperiod. No effects were shown in the light/dark box. Short photoperiod-acclimated degu showed reduced total activity duration and activity was not restricted to the light phase. The present study further supports the utilization of diurnal rodents to model circadian rhythms-related affective change. Beyond the possible diversity in the mechanisms underlying diurnality in different animals, there are now evidences that in three different diurnal species, the fat sand rat, the grass Nile rat and the degu, shortening of photoperiod results in the appearance of anxiety- and depression-like behaviors.

Circadian arrhythmia dysregulates emotional behaviors in aged Siberian hamsters

Emotional behaviors are influenced by the circadian timing system. Circadian disruptions are associated with depressive-like symptoms in clinical and preclinical populations. Circadian rhythm robustness declines markedly with aging and may contribute to susceptibility to emotional dysregulation in aged individuals. The present experiments used a model of chronic circadian arrhythmia generated noninvasively, via a series of circadian-disruptive light treatments, to investigate interactions between circadian desynchrony and aging on depressive- and anxiety-like behaviors, and on limbic neuroinflammatory gene expression that has been linked with emotionality. We also examined whether a social manipulation (group housing) would attenuate effects of arrhythmia on emotionality. In aged (14-18 months of age) male Siberian hamsters, circadian arrhythmia increased behavioral despair and decreased social motivation, but decreased exploratory anxiety. These effects were not evident in younger (5-9 months of age) hamsters. Social housing (3-5 hamsters/cage) abolished the effects of circadian arrhythmia on emotionality. Circadian arrhythmia alone was without effect on hippocampal or cortical interleukin-1β (IL-1β) and indoleamine 2,3-dioxygenase (Ido) mRNA expression in aged hamsters, but social housing decreased hippocampal IL-1β and Ido mRNAs. The data demonstrate that circadian disruption can negatively impact affective state, and that this effect is pronounced in older individuals. Although clear associations between circadian arrhythmia and constitutive limbic proinflammatory activity were not evident, the present data suggest that social housing markedly inhibits constitutive hippocampal IL-1β and Ido activity, which may contribute to the ameliorating effects of social housing on a number of emotional behaviors.

Sensorimotor modulation of mood and depression: in search of an optimal mode of stimulation

Depression involves a dysfunction in an affective fronto-limbic circuitry including the prefrontal cortices, several limbic structures including the cingulate cortex, the amygdala, and the hippocampus as well as the basal ganglia. A major emphasis of research on the etiology and treatment of mood disorders has been to assess the impact of centrally generated (top-down) processes impacting the affective fronto-limbic circuitry. The present review shows that peripheral (bottom-up) unipolar stimulation via the visual and the auditory modalities as well as by physical exercise modulates mood and depressive symptoms in humans and animals and activates the same central affective neurocircuitry involved in depression. It is proposed that the amygdala serves as a gateway by articulating the mood regulatory sensorimotor stimulation with the central affective circuitry by emotionally labeling and mediating the storage of such emotional events in long-term memory. Since both amelioration and aggravation of mood is shown to be possible by unipolar stimulation, the review suggests that a psychophysical assessment of mood modulation by multimodal stimulation may uncover mood ameliorative synergisms and serve as adjunctive treatment for depression. Thus, the integrative review not only emphasizes the relevance of investigating the optimal levels of mood regulatory sensorimotor stimulation, but also provides a conceptual springboard for related future research.

Effects of circadian phase and melatonin injection on anxiety-like behavior in nocturnal and diurnal rodents

Animals show daily rhythms in most bodily functions, resulting from the integration of information from an endogenous circadian clock and external stimuli. These rhythms are adaptive and are expected to be related to activity patterns, i.e., to be opposite in diurnal and nocturnal species. Melatonin is secreted during the night in all mammalian species, regardless of their activity patterns. Consequently, in diurnal species the nocturnal secretion of melatonin is concurrent with the resting phase, whereas in nocturnal species it is related to an increase in activity. In this research, we examined in three diurnal and three nocturnal rodent species whether a daily rhythm in anxiety-like behavior exists; whether it differs between nocturnal and diurnal species; and how melatonin affects anxiety-like behavior in species with different activity patterns. Anxiety-like behavior levels were analyzed using the elevated plus-maze. We found a daily rhythm in anxiety-like behavior and a significant response to daytime melatonin administration in all three nocturnal species, which showed significantly lower levels of anxiety during the dark phase, and after melatonin administration. The diurnal species showed either an inverse pattern to that of the nocturnal species in anxiety-like behavior rhythm and in response to daytime melatonin injection, or no rhythm and, accordingly, no response to melatonin.

Depression-like responses induced by daytime light deficiency in the diurnal grass rat (Arvicanthis niloticus)

Seasonal Affective Disorder (SAD) is one of the most common mood disorders with depressive symptoms recurring in winter when there is less sunlight. The fact that light is the most salient factor entraining circadian rhythms leads to the phase-shifting hypothesis, which suggests that the depressive episodes of SAD are caused by misalignments between the circadian rhythms and the habitual sleep times. However, how changes in environmental lighting conditions lead to the fluctuations in mood is largely unknown. The objective of this study is to develop an animal model for some of the features/symptoms of SAD using the diurnal grass rats Arvichantis niloticus and to explore the neural mechanisms underlying the light associated mood changes. Animals were housed in either a 12∶12 hr bright light∶dark (1000lux, BLD) or dim light∶dark (50lux, DLD) condition. The depression-like behaviors were assessed by sweet-taste Saccharin solution preference (SSP) and forced swimming test (FST). Animals in the DLD group showed higher levels of depression-like behaviors compared to those in BLD. The anxiety-like behaviors were assessed in open field and light/dark box test, however no significant differences were observed between the two groups. The involvement of the circadian system on depression-like behaviors was investigated as well. Analysis of locomotor activity revealed no major differences in daily rhythms that could possibly contribute to the depression-like behaviors. To explore the neural substrates associated with the depression-like behaviors, the brain tissues from these animals were analyzed using immunocytochemistry. Attenuated indices of 5-HT signaling were observed in DLD compared to the BLD group. The results lay the groundwork for establishing a novel animal model and a novel experimental paradigm for SAD. The results also provide insights into the neural mechanisms underlying light-dependent mood changes.

Health consequences of circadian disruption in humans and animal models

Daily rhythms in behavior and physiology are programmed by a hierarchical collection of biological clocks located throughout the brain and body, known as the circadian system. Mounting evidence indicates that disruption of circadian regulation is associated with a wide variety of adverse health consequences, including increased risk for premature death, cancer, metabolic syndrome, cardiovascular dysfunction, immune dysregulation, reproductive problems, mood disorders, and learning deficits. Here we review the evidence for the pervasive effects of circadian disruption in humans and animal models, drawing from both environmental and genetic studies, and identify questions for future research.

Cellular Circadian Clocks in Mood Disorders

Bipolar disorder (BD) and major depressive disorder (MDD) are heritable neuropsychiatric disorders associated with disrupted circadian rhythms. The hypothesis that circadian clock dysfunction plays a causal role in these disorders has endured for decades but has been difficult to test and remains controversial. In the meantime, the discovery of clock genes and cellular clocks has revolutionized our understanding of circadian timing. Cellular circadian clocks are located in the suprachiasmatic nucleus (SCN), the brain’s primary circadian pacemaker, but also throughout the brain and peripheral tissues. In BD and MDD patients, defects have been found in SCN-dependent rhythms of body temperature and melatonin release. However, these are imperfect and indirect indicators of SCN function. Moreover, the SCN may not be particularly relevant to mood regulation, whereas the lateral habenula, ventral tegmentum, and hippocampus, which also contain cellular clocks, have established roles in this regard. Dysfunction in these non-SCN clocks could contribute directly to the pathophysiology of BD/MDD. We hypothesize that circadian clock dysfunction in non-SCN clocks is a trait marker of mood disorders, encoded by pathological genetic variants. Because network features of the SCN render it uniquely resistant to perturbation, previous studies of SCN outputs in mood disorders patients may have failed to detect genetic defects affecting non-SCN clocks, which include not only mood-regulating neurons in the brain but also peripheral cells accessible in human subjects. Therefore, reporters of rhythmic clock gene expression in cells from patients or mouse models could provide a direct assay of the molecular gears of the clock, in cellular clocks that are likely to be more representative than the SCN of mood-regulating neurons in patients. This approach, informed by the new insights and tools of modern chronobiology, will allow a more definitive test of the role of cellular circadian clocks in mood disorders.

Complex interaction of circadian and non-circadian effects of light on mood: shedding new light on an old story

In addition to its role in vision, light exerts strong effects on behavior. Its powerful role in the modulation of mood is well established, yet remains poorly understood. Much research has focused on the effects of light on circadian rhythms and subsequent interaction with alertness and depression. The recent discovery of a third photoreceptor, melanopsin, expressed in a subset of retinal ganglion cells, allows major improvement of our understanding of how photic information is processed. Light affects behavior in two ways, either indirectly through the circadian timing system, or directly through mechanisms that are independent of the circadian system. These latter effects have barely been studied in regard to mood, but recent investigations on the direct effects of light on sleep and alertness suggest additional pathways through which light could influence mood. Based on our recent findings, we suggest that light, via melanopsin, may exert its antidepressant effect through a modulation of the homeostatic process of sleep. Further research is needed to understand how these mechanisms interplay and how they contribute to the photic regulation of mood. Such research could improve therapeutic management of affective disorders and influence the management of societal lighting conditions.

Short day lengths alter stress and depressive-like responses, and hippocampal morphology in Siberian hamsters

Many psychological disorders comprise a seasonal component. For instance, seasonal affective disorder (SAD) is characterized by depression during autumn and winter. Because hippocampal atrophy may underlie the symptoms of depression and depressive-like behaviors, one goal of this study was to determine whether short days also induce structural changes in the hippocampus using photoperiod responsive rodents--Siberian hamsters. Exposure to short days increases depressive-like responses (increased immobility in the forced swim test) in hamsters. Male hamsters were housed in either short (LD 8:16) or long days (LD 16:8) for 10 weeks and tested in the forced swim test. Brains were removed and processed for Golgi impregnation. HPA axis function may account for photoperiod-related changes in depressive-like responses. Thus, stress reactivity was assessed in another cohort of photoperiod-manipulated animals. Short days reduced soma size and dendritic complexity in the CA1 region. Photoperiod did not induce gross changes in stress reactivity, but an acute stressor disrupted the typical nocturnal peak in cortisol concentrations. These data reveal that immobility induced by exposure to short days is correlated with reduced CA1 cell complexity (and perhaps connectivity). This study is the first to investigate hippocampal changes in the context of short-day induced immobility and may be relevant for understanding psychological disorders with a seasonal component.

Interactions of the serotonin and circadian systems: nature and nurture in rhythms and blues

The serotonin and circadian systems are principal regulatory networks of the brain. Each consists of a unique set of neurons that make widespread neural connections and a defined gene network of transcriptional regulators and signaling genes that subserve serotonergic and circadian function at the genetic level. These master regulatory networks of the brain are extensively intertwined, with reciprocal circuit connections, expression of key genetic elements for serotonin signaling in clock neurons and expression of key clock genes in serotonergic neurons. The reciprocal connections of the serotonin and circadian systems likely have importance for neurobehavioral disorders, as suggested by their convergent contribution to a similar range of mood disorders including seasonal affective disorder (SAD), bipolar disorder, and major depression, and as suggested by their overlapping relationship with the developmental disorder, autism spectrum disorder. Here we review the neuroanatomical and genetic basis for serotonin-circadian interactions in the brain, their potential relationship with neurobehavioral disorders, and recent work examining the effects on the circadian system of genetic perturbation of the serotonergic system as well as the molecular and behavioral effects of developmental imprinting of the circadian system with perinatal seasonal light cycles.

Circadian rhythms and mood regulation: insights from pre-clinical models

Affective disorders such as major depression, bipolar disorder, and seasonal affective disorder are associated with major disruptions in circadian rhythms. Indeed, altered sleep/wake cycles are a critical feature for diagnosis in the DSM IV and several of the therapies used to treat these disorders have profound effects on rhythm length and stabilization in human populations. Furthermore, multiple human genetic studies have identified polymorphisms in specific circadian genes associated with these disorders. Thus, there appears to be a strong association between the circadian system and mood regulation, although the mechanisms that underlie this association are unclear. Recently, a number of studies in animal models have begun to shed light on the complex interactions between circadian genes and mood-related neurotransmitter systems, the effects of light manipulation on brain circuitry, the impact of chronic stress on rhythms, and the ways in which antidepressant and mood-stabilizing drugs alter the clock. This review will focus on the recent advances that have been gleaned from the use of pre-clinical models to further our understanding of how the circadian system regulates mood.

Influence of photoperiod on hormones, behavior, and immune function

Photoperiodism is the ability of plants and animals to measure environmental day length to ascertain time of year. Central to the evolution of photoperiodism in animals is the adaptive distribution of energetically challenging activities across the year to optimize reproductive fitness while balancing the energetic tradeoffs necessary for seasonally-appropriate survival strategies. The ability to accurately predict future events requires endogenous mechanisms to permit physiological anticipation of annual conditions. Day length provides a virtually noise free environmental signal to monitor and accurately predict time of the year. In mammals, melatonin provides the hormonal signal transducing day length. Duration of pineal melatonin is inversely related to day length and its secretion drives enduring changes in many physiological systems, including the HPA, HPG, and brain-gut axes, the autonomic nervous system, and the immune system. Thus, melatonin is the fulcrum mediating redistribution of energetic investment among physiological processes to maximize fitness and survival.

Antidepressants reverse short-photoperiod-induced, forced swim test depression-like behavior in the diurnal fat sand rat: further support for the utilization of diurnal rodents for modeling affective disorders

Recent findings demonstrate strong links between abnormalities in circadian rhythms and sleep and the etiology, pathophysiology and treatment of major affective disorders. Further exploration of these interactions requires the development, identification and utilization of good and predictive animal models. The biology and behavior related to circadian rhythms are significantly different in diurnal and nocturnal rodents. Accordingly, it is possible that exploring the interactions between these mechanisms and affective change in diurnal animals may be advantageous. Recent studies demonstrate that diurnal fat sand rats and Nile grass rats show depression-like behavior when maintained under short-photoperiod (SP) conditions compared with animals maintained under neutral photoperiod (NP) conditions. Moreover, these behaviors were ameliorated after treatment with bright light. The present study further explores the possible utility of sand rats as animal models by testing the effects of antidepressants on the SP-induced depression-like behaviors of sand rats. Sand rats maintained in SP or NP conditions for 3 weeks were treated subchronically (5 injections) with the clinically effective antidepressant bupropion, and their behavior was tested in a number of depression-related tests. Results show that antidepressant treatment reverses the effects of SP conditions in the forced swim test, but that neither SP conditions nor antidepressants influenced sweet solution preference. These results partly support the validity of the sand rat model, but suggest that not all tests that were validated in nocturnal laboratory rodents can be applied to other rodent species and that additional tests should be applied to further explore the validity of the model.