Influence of a natural stressor (predator odor) on locomotor activity in the meadow vole (Microtus pennsylvanicus): modulation by sex, reproductive condition and gonadal hormones

Short daylight photoperiod alleviated alarm substance-stimulated fear response of zebrafish

Photoperiod has been well-documented to be involved in regulating many activities of animals. However, whether photoperiod takes part in mood control, such as fear response in fish and the underlying mode(s) of action remain unclear. In this study, adult zebrafish males and females (Danio rerio) were exposed to different photoperiods, Blank (12 h light: 12 h dark), Control (12 h light: 12 h dark), Short daylight (SD, 6 h light: 18 h dark) and Long daylight (LD, 18 h light: 6 h dark) for 28 days. After exposure, fear response of the fish was investigated using a novel tank diving test. After alarm substance administration, the onset to higher half, total duration in lower half and duration of freezing in SD-fish were significantly decreased, suggesting that short daylight photoperiod is capable of alleviating fear response in zebrafish. In contrast, comparing with the Control, LD didn't show significant effect on fear response of the fish. Further investigation revealed that SD increased the levels of melatonin (MT), serotonin (5-HT) and dopamine (DA) in the brain while decreased the plasma level of cortisol comparing to the Control. Moreover, the expressions of genes in MT, 5-HT and DA pathways and HPI axis were also altered consistently. Our data indicated that short daylight photoperiod might alleviate fear response of zebrafish probably through interfering with MT/5-HT/DA pathways and HPI axis.

Effect of Chamomile Vaginal Gel on the Sexual Function in Postmenopausal Women: A Double-Blind Randomized Controlled Trial

BACKGROUND

Female sexual dysfunction (FSD) is a common complaint among postmenopausal women, which is largely because of the genitourinary syndrome in these women (GSM).

AIM

Considering the phytoestrogenic effects of chamomile, the present study was primarily aimed to investigate the effect of chamomile vaginal gel on the sexual function of postmenopausal women. The side effects of these drugs were evaluated as a secondary outcome of the study.

METHODS

This randomized double-blind clinical trial and placebo-controlled study was conducted on postmenopausal women with sexual dysfunction (FSFI ≤26.55). To this aim, 96 postmenopausal women were randomly assigned into three groups (n = 32 each) including women receiving (i) chamomile vaginal gel 5%, (ii) conjugated estrogen vaginal cream, and (iii) placebo vaginal gel, for 12 weeks (ie, every night in the first 2 weeks, and 2 nights per week in the next 10 weeks, each night 1 g was used). The sexual function was measured using female sexual function index (FSFI) before and after the intervention. Data analysis was performed by chi-square, one-way ANOVA, descriptive statistics, analysis of covariance (ANCOVA), and paired t test using SPSS software version 22. P < .05 was considered statistically significant.

OUTCOMES

The main study outcome measure was evaluate the effects of vaginal administration of chamomile gel in comparison with conjugated estrogen cream and placebo gel on postmenopausal FSD using the FSFI.

RESULTS

The findings showed that chamomile vaginal gel in compared to placebo vaginal gel caused a significant improvement in all six sexual function domains and the total FSFI score (effect size = +2.9 [95% CI, +2.1 to +3.6], P < .001). Also, there was no significant difference between the chamomile vaginal gel and conjugated estrogen vaginal cream groups in terms of the total score and all sub-domains of sexual function with the exception of orgasm (effect size = +0.13 [95% CI, -0.36 to +0.63], P = .02) and sexual satisfaction (effect size = 0 [95% CI, -0.49 to +0.49], P = .04). Two women in the chamomile group and one in the placebo group experienced a burning sensation (P = .345).

CLINICAL IMPLICATIONS

This treatment can be considered as a treatment option for postmenopausal women with sexual dysfunction who have contraindications to the use of hormone therapy.

STRENGTHS & LIMITATIONS

This study is the first study to investigate the effectiveness of chamomile vaginal gel on sexual function in postmenopausal women. However, in this study, treatment duration was 12 weeks and no follow up was performed beyond this time CONCLUSION: Based on the results of this study, the use of vaginal chamomile gel improved sexual function in postmenopausal women. Bosak Z, Iravani M, Moghimipour E, et al. Effect of Chamomile Vaginal Gel on the Sexual Function in Postmenopausal Women: A Double-Blind Randomized Controlled Trial. J Sex Med 2022;19:983-994.

Sexual differences in responses of meadow voles to environmental cues in the presence of mink odor

In rodents, defensive behaviors increase the chances of survival during a predator encounter. Observable rodent defensive behaviors have been shown to be influenced by the presence of predator odors and nearby environmental cues such as cover, odors from conspecifics and food availability. Our experiment tested whether a predator scent cue influenced refuge preference in meadow voles within a laboratory setting. We placed voles in an experimental apparatus with bedding soaked in mink scent versus olive oil as a control across from four tubes that either contained (a) a dark plastic covering, (b) opposite-sex conspecific odor, (c) a food pellet, or (d) an empty, unscented space. A three-way interaction of tube contents, subject sex, and the presence of mink or olive oil on the preference of meadow voles to spend time in each area of the experimental apparatus and their latency to enter each area of the apparatus revealed sex differences in the environmental preference of meadow voles facing the risk of predation. The environmental preference of female, but not male, meadow voles was altered by the presence of mink urine or olive oil. A similar trend was found in the latency of males and females to enter each area of the experimental apparatus. These differences suggest that each sex utilizes different methods to increase their fitness when experiencing a predation risk. The observed sex differences may be explained by the natural history of voles owing to the differences in territorial range and the dynamics of evasion of terrestrial predators.

Licorice root components mimic estrogens in an object location task but not an object recognition task

This study investigated the efficacy of components of licorice root to alter performance on two different recognition tasks, a hippocampus-sensitive metric change in object location (MCOL) task and a striatum-sensitive double object recognition (DOR) task. Isoliquiritigenin (ISL), licorice root extract (LRE), and whole licorice root powder (LRP) were assessed. Young adult female rats were ovariectomized (OVX) and exposed to ISL, LRE or LRP at 0.075%, 0.5% or 5% respectively in the diet. An estradiol group was included as a positive control based on our prior findings. Rats were allowed to explore two objects for three 5-min study trials (separated by 3-min intervals) before a fourth 5-min test trial where the objects were moved closer together (MCOL task) or replaced with two new objects (DOR task). Rats typically habituate to the objects across the three study trials. An increase in object exploration time in the test trial suggests the rat detected the change. Estradiol improved MCOL performance and impaired DOR performance, similar to previously shown effects of estradiol and other estrogens, which tend to improve learning and memory on hippocampus-sensitive tasks and impair striatum-sensitive cognition. LRP had no effect on recognition while exposure to ISL and LRE improved MCOL performance. Exposure to ISL, LRE and LRP failed to attenuate DOR, contrary to effects of estradiol shown here and to previous reports in young-adult OVX rats. These findings suggest components of licorice root may prove to be effective therapies targeting memory enhancement without unintended deleterious cognitive effects.

Sex differences in risk-taking and associative learning in rats

In many species, males tend to have lower parental investment than females and greater variance in their reproductive success. Males might therefore be expected to adopt more high-risk, high-return behaviours than females. Next to risk-taking behaviour itself, sexes might also differ in how they respond to information and learn new associations owing to the fundamental link of these cognitive processes with the risk-reward axis. Here we investigated sex differences in both risk-taking and learned responses to risk by measuring male and female rats' (Rattus norvegicus) behaviour across three contexts in an open field test containing cover. We found that when the environment was novel, males spent more time out of cover than females. Males also hid less when exposed to the test arena containing predator odour. By contrast, females explored more than males when the predator odour was removed (associatively learned risk). These results suggest that males are more risk-prone but behave more in line with previous experiences, while females are more risk-averse and more responsive to changes in their current environment. Our results suggest that male and female rats differ in how they cope with risk and highlight that a general link may exist between risk-taking behaviour and learning style.

The response of rodents to scent marks: four broad hypotheses

This article is part of a Special Issue "Chemosignals and Reproduction". Many terrestrial mammals must be able to distinguish between the myriad of scent marks they encounter in order for them to facilitate or deter direct interactions with their scent donors. I review studies that examine how rodents, mainly meadow voles (Microtus pennsylvanicus), respond when they encounter the scent marks of conspecifics and heterospecifics, and how context, as well as the age and condition of senders and receivers, affect their responses. The review uses four broad hypotheses to discuss the response of rodents to scent marks. The four hypotheses are as follows: 1) Scent marks convey accurate information to the receiver about the sender's state and phenotype and genotype. 2) Scent marks are individually distinct. 3) The response of receivers to scent marks is flexible and would be modulated by the cognitive abilities of receivers. 4) Receivers respond to the information contained or conveyed by the scent mark in a manner that will increase their survival and fitness. The studies cited in this review show that scent marks signal accurate information about the sender's phenotype, genotype, and condition, which receivers use to distinguish among the scent marks of different conspecifics and heterospecifics, and by doing so, receivers tailor their response accordingly to increase their survival and fitness. Thus, the four broad hypotheses may serve as guide to increase our understanding of the response of receivers to scent marks and provide a conceptual framework for future research and the development of additional hypotheses.

Immediate early gene and neuropeptide expression following exposure to the predator odor 2,5-dihydro-2,4,5-trimethylthiazoline (TMT)

The immediate early gene c-fos and a number of neuropeptides have been widely used to help delineate the neural circuitry of innate fear to predator odors. The present study used in situ hybridization techniques to examine the expression of the immediate early gene transcription factors c-fos and egr-1, and the neuropeptides corticotropin-releasing hormone (crh) and enkephalin (enk) following exposure to the predator odor 2,5-dihydro-2,4,5-trimethylthiazoline (TMT). Rats were exposed to water (H2O), TMT, or the irritating odor butyric acid (BA) and freezing was used to measure fear behavior. Changes in gene expression were analyzed in the medial prefrontal cortex (mPFC), the bed nucleus of the stria terminalis (BNST), paraventricular nucleus of the hypothalamus (PVN), and central nucleus of the amygdala (CeA). Animals froze more to TMT than BA and H2O, and more to BA than H2O. Compared to H2O and BA, c-fos and egr-1 were elevated within the BNST, PVN, and CeA in rats exposed to TMT, but not the mPFC. Crh was also elevated in rats exposed to TMT within the CeA and PVN, but not the BNST or mPFC. Enk was elevated within the PVN in TMT and BA exposed rats compared to H2O exposure. These data indicate that exposure to the predator odor TMT induces similar expression patterns for c-fos and egr-1, but different patterns for crh and enk, with partial overlap of the immediate-early genes and neuropeptides within specific brain regions.

Photoperiod mediated changes in olfactory bulb neurogenesis and olfactory behavior in male white-footed mice (Peromyscus leucopus)

Brain plasticity, in relation to new adult mammalian neurons generated in the subgranular zone of the hippocampus, has been well described. However, the functional outcome of new adult olfactory neurons born in the subventricular zone of the lateral ventricles is not clearly defined, as manipulating neurogenesis through various methods has given inconsistent and conflicting results in lab mice. Several small rodent species, including Peromyscus leucopus, display seasonal (photoperiodic) brain plasticity in brain volume, hippocampal function, and hippocampus-dependent behaviors; plasticity in the olfactory system of photoperiodic rodents remains largely uninvestigated. We exposed adult male P. leucopus to long day lengths (LD) and short day lengths (SD) for 10 to 15 weeks and then examined olfactory bulb cell proliferation and survival using the thymidine analog BrdU, olfactory bulb granule cell morphology using Golgi-Cox staining, and behavioral investigation of same-sex conspecific urine. SD mice did not differ from LD counterparts in granular cell morphology of the dendrites or in dendritic spine density. Although there were no differences due to photoperiod in habituation to water odor, SD mice rapidly habituated to male urine, whereas LD mice did not. In addition, short day induced changes in olfactory behavior were associated with increased neurogenesis in the caudal plexiform and granule cell layers of the olfactory bulb, an area known to preferentially respond to water-soluble odorants. Taken together, these data demonstrate that photoperiod, without altering olfactory bulb neuronal morphology, alters olfactory bulb neurogenesis and olfactory behavior in Peromyscus leucopus.

Male risk taking, female odors, and the role of estrogen receptors

Male risk-taking and decision making are affected by sex-related cues, with men making riskier choices and decisions after exposure to either women or stimuli associated with women. In non-human species females and, or their cues can also increase male risk taking. Under the ecologically relevant condition of predation threat, brief exposure of male mice to the odors of a sexually receptive novel female reduces the avoidance of, and aversive responses to, a predator. We briefly review evidence showing that estrogen receptors (ERs), ERα and ERβ, are associated with the mediation of these risk taking responses. We show that ERs influence the production of the female odors that affect male risk taking, with the odors of wild type (ERαWT, ERβWT), oxytocin (OT) wildtype (OTWT), gene-deleted 'knock-out' ERβ (ERβKO), but not ERαKO or oxytocin (OT) OTKO or ovariectomized (OVX) female mice reducing the avoidance responses of male mice to cat odor. We further show that administration of specific ERα and ERβ agonists to OVX females results in their odors increasing male risk taking and boldness towards a predator. We also review evidence that ERs are involved in the mediation of the responses of males to female cues, with ERα being associated with the sexual and both ERβ and ERα with the sexual and social mechanisms underlying the effects of female cues on male risk taking. The implications and relations of these findings with rodents to ERs and the regulation of human risk taking are briefly considered.

Effects of maternal L-tryptophan depletion and corticosterone administration on neurobehavioral adjustments in mouse dams and their adolescent and adult daughters

Major depressive disorder (MDD), a pathology characterized by mood and neurovegetative disturbances, depends on a multi-factorial contribution of individual predisposition (e.g., diminished serotonergic transmission) and environmental factors (e.g., neonatal abuse or neglect). Despite its female-biased prevalence, MDD basic research has mainly focused on male rodents. Most of present models of depression are also devalued due to the fact that they typically address only one of the aforementioned pathogenetic factors. In this paper we first describe the basic principles behind mouse model development and evaluation and then articulate that current models of depression are intrinsically devalued due to poor construct and/or external validity. We then report a first attempt to overcome this limitation through the design of a mouse model in which the genetic and the environmental components of early risk factors for depression are mimicked together. Environmental stress is mimicked through the supplementation of corticosterone in the maternal drinking water while biological predisposition is mimicked through maternal access to an L-tryptophan (the serotonin precursor) deficient diet during the first week of lactation. CD1 dams and their offspring exposed to the L-tryptophan deficient diet (T) and to corticosterone (80mg/l; C) were compared to animal facility reared (AFR) subjects. T and C mice served as intermediate reference groups. Adolescent TC offspring, compared to AFR mice, showed decreased time spent floating in the forced-swim test and increased time spent in the open sectors of an elevated 0-maze. Adult TC offspring showed reduced preference for novelty, decreased breakpoints in the progressive ratio operant procedure and major alterations in central BDNF levels and altered HPA regulation. The route of administration and the possibility to control the independent variables predisposing to depressive-like symptoms disclose novel avenues towards the development of animal models with increased external and construct validity. Furthermore, the observation that, compared to adult subjects, adolescent mice display an opposite profile suggests that peri-pubertal developmental processes may interact with neonatal predispositions to calibrate the adult abnormal phenotype.

Molecular and cellular sex differences at the intersection of stress and arousal

Elucidating the mechanisms underlying sex biases in the prevalence and severity of diseases can advance our understanding of their pathophysiological basis and serve as a guide for developing treatments. A well-established sex difference in psychiatry is the higher incidence of mood and anxiety disorders in females. These disorders share stress as a potential etiological contributor and hyperarousal as a core symptom, suggesting that the distinction between sexes lies at the intersection of stress and arousal systems. This review focuses on the link between the stress axis and the brain norepinephrine arousal system as a key point at which sex differences occur and are translated to differences in the expression of mood disorders. Evidence for a circuit designed to relay emotion-related information via the limbic corticotropin-releasing factor (CRF) system to the locus coeruleus (LC)-norepinephrine arousal system is reviewed. This is followed by recent novel findings of sex differences in CRF receptor signaling and trafficking that would result in an enhanced arousal response and a compromised ability to adapt to chronic stress in females. Finally, we discuss the evidence for sex differences in LC dendritic structure that allow for an increased receipt and processing of limbic information in females compared to males. Together these complementary sets of data suggest that in females, the LC arousal system is poised to process more limbic information and to respond to some of this information in an enhanced manner compared to males. The clinical and therapeutic considerations arising from this perspective are discussed. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Female fear: influence of estrus cycle on behavioral response and neuronal activation

Our observation that male rat's innate fear response differed with hormonal status, as well as the higher prevalence of fear and anxiety disorders in human females led to the current investigation of the impact of phases of the estrus cycle on innate fear responding. Female rats in different phases of the cycle were exposed to an innate fear-inducing stimulus (2,5-dihydro-2,4,5-trimethylthiazoline, TMT odor) and monitored for changes in behavior and brain activation. Behavioral data showed freezing responses to TMT were significantly enhanced during estrus as compared to other phases of the cycle. This data was supported by significant increases in pixel intensity in cortical and sub-cortical regions in estrus compared to proestrus and diestrus. Imaging results demonstrated significant increases in brain activation in the somatosensory and insular cortices when comparing estrus to diestrus. There were significant increases in neural activity in the bed nucleus of the stria terminals (BNST) and septum in estrus as compared to proestrus. Additionally, the hippocampus, hypothalamus, olfactory system, and cingulate cortex show significant increases in the estrus phase when compared to both diestrus and proestrus. Taken together, these results suggest that the female's hormonal status may be correlated with alterations in both neuronal and behavioral indices of fear.

Prior sexual experience increases hippocampal cell proliferation and decreases risk assessment behavior in response to acute predator odor stress in the male rat

Acute exposure to the predator odor trimethylthiazoline (TMT) induces defensive behavior in the male rat, and this response is associated with a decrease in cell proliferation within the dentate gyrus of the hippocampus. Sexual experience appears to be protective, as it exerts anxiolytic-like effects and sustains gonadal function in the face of stress. To examine the influence of sexual experience on subsequent stress-induced defensive behavior and cell proliferation in the hippocampus we exposed adult male rats to TMT odor with or without prior exposure to sexually receptive female rats. A subset of rats was injected with the DNA-synthesis marker bromodeoxyuridine (BrdU; 200 mg/kg) during TMT exposure and perfused 24 h later to provide an index of cell proliferation within the dentate gyrus. In response to TMT, sexual experience reduced the duration of stretched attend postures, but had no significant effect on defensive burying. Furthermore, TMT induced a significant increase in cell proliferation in the dentate gyrus, but only in males with sexual experience. The results demonstrate an influence of socio-sexual experience on the magnitude of the behavioral and neural responses to predator odor stress.

Effects of xenoestrogens on the differentiation of behaviorally-relevant neural circuits

It has become increasingly clear that environmental chemicals have the capability of impacting endocrine function. Moreover, these endocrine disrupting chemicals (EDCs) have long term consequences on adult reproductive function, especially if exposure occurs during embryonic development thereby affecting sexual differentiation. Of the EDCs, most of the research has been conducted on the effects of estrogen active compounds. Although androgen active compounds are also present in the environment, much less information is available about their action. However, in the case of xenoestrogens, there is mounting evidence for long-term consequences of early exposure at a range of doses. In this review, we present data relative to two widely used animal models: the mouse and the Japanese quail. These two species long have been used to understand neural, neuroendocrine, and behavioral components of reproduction and are therefore optimal models to understand how these components are altered by precocious exposure to EDCs. In particular we discuss effects of bisphenol A and methoxychlor on the dopaminergic and noradrenergic systems in rodents and the impact of these alterations. In addition, the effects of embryonic exposure to diethylstilbestrol, genistein or ethylene,1,1-dichloro-2,2-bis(p-chlorophenyl) is reviewed relative to behavioral impairment and associated alterations in the sexually dimorphic parvocellular vasotocin system in quail. We point out how sexually dimorphic behaviors are particularly useful to verify adverse developmental consequences produced by chemicals with endocrine disrupting properties, by examining either reproductive or non-reproductive behaviors.

Developmental exposure to low-dose estrogenic endocrine disruptors alters sex differences in exploration and emotional responses in mice

Estrogenic endocrine disruptors (EEDs) are naturally occurring or man-made compounds present in the environment that are able to bind to estrogen receptors and interfere with normal cellular development in target organs and tissues. There is mounting evidence that EEDs can interfere with the processes of sexual differentiation of brain and behavior in different animal models. We investigated the effects of maternal exposure to EEDs, at concentrations within the range of human exposure and not patently teratogenic, on behavioral responses of male and female house mice (Mus musculus domesticus) before and after puberty. Pregnant dams were trained to spontaneously drink daily doses of corn oil with or without the estrogenic plastic derivative, bisphenol A (BPA 10 microg/kg), or the estrogenic insecticide methoxychlor (MXC 20 microg/kg) from gestation day 11 to postpartum day 8. Their male and female offspring were examined at different ages to examine several components of explorative and emotional behaviors in 3 experimental paradigms: a novelty test before puberty and, as adults, a free-exploratory open-field test and the elevated plus maze test. The main results are sex differences in control mice on a number of behavioral responses at both ages and in all experimental paradigms, while perinatal exposure to BPA or MXC decreased or eliminated such sex differences. The present findings are evidence of long-term consequences of developmental exposure to BPA and MXC on neurobehavioral development and suggest a differential effect of low-dose exposure to these estrogenic chemicals in males and females.

Activational levels of androgens influence risk assessment behaviour but do not influence stress-induced suppression in hippocampal cell proliferation in adult male rats

Adult male, but not female, rats demonstrate a suppression in hippocampal cell proliferation in response to an acute predator odour stress. The present study examined the effect of activational levels of androgens on stress-induced changes in hippocampal cell proliferation and defensive and non-defensive behaviours in adult male rats. Adult male Sprague-Dawley rats were castrated and exposed to trimethylthiazoline (TMT, the main component of fox feces). Androgen status did not significantly affect TMT-induced suppression in hippocampal cell proliferation or expression of defensive burying. However, castrated males did not show an increase in duration of stretch attends (a risk assessment behaviour) in response to TMT. The results of this study suggest that activational levels of androgens in the male rat do not directly regulate the sex difference in stress-induced suppression of hippocampal cell proliferation but do regulate risk assessment behaviour.

Influence of the estrous cycle on tolerance development to LPS-induced sickness behaviors in rats

The relations between the estrous cycle, inflammatory responses and the development of tolerance to endotoxin were examined. Female Long-Evans rats were injected intraperitoneally with lipopolysaccharide (LPS; 200 microg/kg) or saline vehicle at 08:00h on either diestrus (D) or proestrus (P). Ninety-five minutes after injection locomotor activity was assessed in an automated non-novel open-field for 20 min. To assess tolerance development to LPS, rats were re-injected at the next identical stage (i.e. 4 days later; groups: DD, PP) or at the alternate stage (i.e. 6 days later; groups: DP, PD) of the estrous cycle and locomotor activity was again assessed. On Test Day 1 all groups injected with LPS exhibited similar significant activity decrements, regardless of the stage of the estrous cycle. However, on Test Day 2 rats which received both injections of LPS during proestrus (PP) showed no signs of tolerance development, whereas rats in all other groups were tolerant to LPS. In a follow up study, the time between injections was extended to 8 days. Still the animals injected both times at proestrus showed no signs of tolerance to LPS after the second injection. Thus, the stages of the estrous cycle both at the time of initial exposure and of re-exposure appear critical in the formation of behavioral tolerance to LPS in rats.

Do captive male meadow voles experience acute stress in response to weasel odour?

The hormonal stress response is adapted to deal with acute (short-term) stressors; however, chronic (long-term) stressors have negative effects on survival and fitness. Field and laboratory evidence suggest that voles respond behaviourally to predator odours. However, it is unknown whether voles mount an acute hormonal stress response to predator odour. We determined whether reproductively active, captive male meadow voles ( Microtus pennsylvanicus (Ord, 1815)) mounted a more pronounced hormonal stress response to weasel odour (ermine, Mustela erminea L., 1758), one of their principal mammalian predators, than to nonpredator and control odours. We compared the corticosterone response of captive voles to weasel, jumping mouse ( Zapus hudsonius (Zimmermann, 1780)), and control odours following acute (20 min) exposure. The hormonal stress response to the treatment odours did not differ, indicating that captive male voles in the reproductive season do not mount an acute stress response to predator odour. We hypothesize that voles do not respond to weasel odour because, independent of other stimuli, olfactory signals are not reliable enough to outweigh the costs, such as suppression of reproduction and reproductive behaviour, associated with a response.

Influence of photoperiod and sex on locomotor behavior of meadow voles (Microtus pennsylvanicus) in an automated light-dark 'anxiety' test

This study examined the influence of photoperiod on affective behavior (anxiety) of adult male and female meadow voles (Microtus pennsylvanicus), maintained in either a long or short day photoperiod, when tested in an automated (VersaMax) light-dark test. The light-dark test is based on an innate aversion of rodents to novel, brightly illuminated spaces and has been used with laboratory raised species, such as mice, to assess anxiety and/or fear related behaviors. Male and female meadow voles, housed either in a long day (LD: 16 h light) or short day (SD: 8 h light) photoperiod, were tested in the light-dark apparatus for 30 min on 3 consecutive days. All animals spent significantly (p < 0.001) less time in the brightly lit chamber (900 lux) than in the dark chamber. LD voles, especially females, spent significantly less time in the brightly lit area than did SD voles. Both horizontal and vertical movements occurred less frequently per unit time in the dark area relative to the light, but only in the LD voles. LD female voles were the least active group in the dark area on the first test day but the most active group in the light area, despite spending the least amount of time in this area on the second and third test days. The present results show that LD voles exhibit more anxiety related behaviors in this test situation than do SD voles. LD females avoided the brightly lit area the most, particularly when the apparatus was novel. Thus, both photoperiod and sex influence situation-based anxiety in this species. These findings suggest that meadow voles are an excellent animal model in which to examine the role of gonadal hormones, and their modulation of defence related neural systems, in the induction of anxiety.

Sex and repeated restraint stress interact to affect cat odor-induced defensive behavior in adult rats

The overall objective of the present experiment was to assess sex differences in the effects of repeated restraint stress on fear-induced defensive behavior and general emotional behavior. Groups of male and female Long-Evans rats received either daily restraint stress (stressed) or daily brief handling (nonstressed) for 21 consecutive days. On days 22-25, a number of behavioral tests were administered concluding with a test of defensive behavior in response to a predatory odor. Stressed and nonstressed males and females were exposed to a piece of cat collar previously worn by a female domestic cat (cat odor) or a piece of collar never worn by a cat (control odor) in a familiar open field containing a hide barrier. Rats displayed pronounced defensive behavior (increased hiding and risk assessment) and decreased nondefensive behavior (grooming, rearing) in response to the cat odor. Nonstressed females exposed to cat odor displayed less risk assessment behavior relative to nonstressed males exposed to cat odor. Restraint stress had little effect on defensive behavior in male rats but significantly increased risk assessment behaviors in females. Behavior on the Porsolt forced swim test (a measure of depression-like behavior) and the open field test (a measure of anxiety-like behavior) was not affected by stress or sex. These findings indicate the utility of the predator odor paradigm in detecting subtle shifts in naturally occurring anxiety-like behaviors that may occur differentially in males and females.

In search of a depressed mouse: utility of models for studying depression-related behavior in genetically modified mice

The ability to modify mice genetically has been one of the major breakthroughs in modern medical science affecting every discipline including psychiatry. It is hoped that the application of such technologies will result in the identification of novel targets for the treatment of diseases such as depression and to gain a better understanding of the molecular pathophysiological mechanisms that are regulated by current clinically effective antidepressant medications. The advent of these tools has resulted in the need to adopt, refine and develop mouse-specific models for analyses of depression-like behavior or behavioral patterns modulated by antidepressants. In this review, we will focus on the utility of current models (eg forced swim test, tail suspension test, olfactory bulbectomy, learned helplessness, chronic mild stress, drug-withdrawal-induced anhedonia) and research strategies aimed at investigating novel targets relevant to depression in the mouse. We will focus on key questions that are considered relevant for examining the utility of such models. Further, we describe other avenues of research that may give clues as to whether indeed a genetically modified animal has alterations relevant to clinical depression. We suggest that it is prudent and most appropriate to use convergent tests that draw on different antidepressant-related endophenotypes, and complimentary physiological analyses in order to provide a program of information concerning whether a given phenotype is functionally relevant to depression-related pathology.

Optimal body size and energy expenditure during winter: why are voles smaller in declining populations?

Winter is energetically challenging for small herbivores because of greater energy requirements for thermogenesis at a time when little energy is available. We formulated a model predicting optimal wintering body size, accounting for the scaling of both energy expenditure and assimilation to body size, and the trade-off between survival benefits of a large size and avoiding survival costs of foraging. The model predicts that if the energy cost of maintaining a given body mass differs between environments, animals should be smaller in the more demanding environments, and there should be a negative correlation between body mass and daily energy expenditure (DEE) across environments. In contrast, if animals adjust their energy intake according to variation in survival costs of foraging, there should be a positive correlation between body mass and DEE. Decreasing temperature always increases equilibrium DEE, but optimal body mass may either increase or decrease in colder climates depending on the exact effects of temperature on mass-specific survival and energy demands. Measuring DEE with doubly labeled water on wintering Microtus agrestis at four field sites, we found that DEE was highest at the sites where voles were smallest despite a positive correlation between DEE and body mass within sites. This suggests that variation in wintering body mass between sites was due to variation in food quality/availability and not adjustments in foraging activity to varying risks of predation.

Estrogen and progesterone dose-dependently reduce disruptive effects of restraint on lordosis behavior

Ovariectomized rats were hormonally primed with various doses of estradiol benzoate (EB; 0.5-10 microg) in combination with various doses of progesterone (2.5-500 microg) to induce sexual receptivity. Females were then subjected to 5 min restraint and the effect on lordosis behavior was monitored for the next 30 min. Such mild stress has been previously shown to transiently reduce lordosis behavior of ovariectomized females hormonally primed only with 10 microg EB. In the current study, doses of progesterone of 25 microg or more in combination with 10 microg EB reduced the effects of restraint. Also priming doses of EB from 4.0 to 10 microg in combination with 250 microg progesterone prevented the lordosis-inhibiting effects of restraint. These findings reinforce prior observations of the dose-dependency of both estrogen and progesterone in the facilitation of lordosis behavior and introduce the female's lordosis response to mild restraint as a potentially useful index of the female's response to stress.

Conditioning and residual emotionality effects of predator stimuli: some reflections on stress and emotion

The advantages of using predator-related odor stimuli to study emotional responses in laboratory tests depend on whether such stimuli do elicit a relatively complete pattern of emotionality. This has been confirmed for cat fur/skin odor stimuli, which elicit a range of defensive behaviors in rats that may be reduced by anxiolytic drugs, produce residual anxiety-like behavior in the elevated plus maze and support rapid aversive conditioning to the context in which they were encountered. Although the synthetic fox fecal odor, trimethylthiazoline (TMT), elicits avoidance similar to that seen in response to cat fur/skin odor, this avoidance does not respond to anxiolytic drugs. In addition, TMT does not produce residual anxiety-like behaviors in the elevated plus maze, nor does it support conditioning. As natural cat feces also elicit avoidance but fail to support conditioning, it is possible that the ability of a predator-related odor to serve as an effective unconditioned stimulus (US) relates to its predictive status with reference to the actual presence of the predator. Avoidance per se may reflect that a stimulus is aversive but not necessarily capable of eliciting an emotional response. This view is consonant with findings in a Mouse Defense Test Battery (MDTB) measuring a wide range of defensive responses to predator exposure. A contextual defense measure that may reflect either conditioned or residual but unconditioned emotional responses was almost never reduced by drug effects unless these also reduced risk assessment or defensive threat/attack measures. However, reductions in contextual defense without changes in flight/avoidance measures were much more common. These findings suggest that flight/avoidance, although it obviously may occur as one component of a full pattern of defensive and emotional behaviors, is also somewhat separable from the others. When-as appears to be the case with TMT-it is the major or perhaps only consistent defensive behavior elicited, this may reflect a stimulus that is aversive or noxious but with little ability to predict the presence of threat or danger. That such stimuli fail to support rapid aversive conditioning suggests the need for a reanalysis of the characteristics required for an effective aversive US.

Sex differences in cell proliferation, cell death and defensive behavior following acute predator odor stress in adult rats

Males show suppressed cell proliferation in the hippocampus in response to acute stress but no studies to date have examined cell proliferation in response to acute stress in females. In the current study, we examined the effects of acute exposure to a predator odor stressor [trimethyl thiazoline (TMT); the main component of fox feces] or a control odor on cell proliferation and cell death in the dentate gyrus and on behavior in adult male and female [intact, ovariectomized (OVX) or OVX+estradiol benzoate (EB)] rats. Further, we examined whether TMT-induced changes in behavior were related to cellular changes. During TMT exposure, rats were injected with the cell synthesis marker bromodeoxyuridine and perfused 24 h later. Acute TMT exposure suppressed both cell proliferation and death in males but not in any group of females. Interestingly, in the OVX females we observed an increase in cell death that was eliminated by EB treatment. Consistent with prior studies, estradiol treatment increased cell proliferation regardless of odor condition. Regardless of sex or hormone treatment, TMT increased defensive behavior, suggesting that the behavioral response to TMT is dissociated from this cellular response. This is the first demonstration of a sex difference in cell proliferation and death in the adult dentate gyrus in response to stress.

Failure to produce conditioning with low-dose trimethylthiazoline or cat feces as unconditioned stimuli

Trimethylthiazoline (TMT), a derivative of fox feces, has been reported to fail to produce aversive conditioning as an unconditioned stimulus (UCS) when presented in large amounts (I. S. McGregor, L. Schrama, P. Ambermoon, & R. A. Dielenberg, 2002). Experiment I evaluated very low TMT levels that nonetheless produced defensive behaviors in rats during exposure. Although each level (0.01, 0.05, and 0.10 microl TMT) produced significant change in defensiveness, none resulted in significant changes the following day in the absence of TMT. Experiment 2 evaluated cat urine, cat feces, and cat fur/skin odor against a no-odor control. Urine produced no significant changes, but feces and fur/skin odors elicited virtually identical changes in defensive behaviors during exposure. When tested the next day in the absence of odor, the fur/skin odor-exposed group showed significant differences on the same behaviors as during exposure, but the feces-exposed group showed no differences on any measure. Results suggest that lack of conditioning to TMT may relate to the type of predator odor rather than the amount, predator species, or possible lack of odor components in TMT that are present in natural feces. Predator feces may also be less effective as a UCS because they are poorly predictive of the actual presence of the predator, suggesting the need for a reevaluation of UCS functions in aversive conditioning.

Temporary inactivation of the bed nucleus of the stria terminalis but not of the amygdala blocks freezing induced by trimethylthiazoline, a component of fox feces

Presentation of trimethylthiazoline (TMT, a component of fox feces) to laboratory rats elicits freezing, a prominent behavioral sign of anxiety or fear. The present study investigated the neural basis of this unlearned response. Muscimol, a GABA(A) receptor agonist, was injected (4.4 nmol/0.5 microl) into the bed nucleus of the stria terminalis (BNST) as well as into the amygdala, two brain areas known to be involved in anxiety and fear. Temporary inactivation of the BNST but not of the amygdala significantly blocked TMT-induced freezing. This effect was not caused by an enhancement of motor activity after BNST inactivation. In addition, these results confirm previous studies showing that freezing is possible despite amygdala inactivation. These results, and other findings in the literature, suggest that the BNST is critically involved in unlearned fear, whereas the amygdala is more involved in the acquisition and expression of learned fear.

Sex differences in the effects of muramyl dipeptide and lipopolysaccharide on locomotor activity and the development of behavioral tolerance in rats

Administration of bacterial agents, such as muramyl dipeptide (MDP) or lipopolysaccharide (LPS), induces a number of illness symptoms including decreased locomotor activity and weight loss. This study provides a detailed multivariate assessment of the effects of repeated exposures of various doses of MDP and LPS, alone and in combination, on various aspects of locomotion in male and female rats. Animals were given a single intraperitoneal injection of either MDP (0.8 or 1.6 mg/kg), LPS (100 or 200 microg/kg), a combination of MDP and LPS (0.8 mg/kg and 100 microg/kg, respectively), or vehicle on Days 1, 4, and 7. Two hours after each injection, locomotor activity was recorded for 30 min in an automated open-field. Both doses of LPS and the high dose of MDP produced significant decrements in locomotor activity in male and female rats, with tolerance becoming evident over repeated administrations, although LPS decreased activity more robustly than MDP. Sex differences were evident in the combined effects of MDP and LPS. Together, MDP and LPS reduced male activity levels in an additive manner but significantly potentiated both horizontal and vertical activity decrements in females. In addition, the rate of behavioral tolerance development to repeated bacterial injections was significantly higher in females than in males. These findings provide evidence for sex differences in the actions of MDP and LPS on various aspects of locomotor activity and in the development of behavioral tolerance to infection.

Cue and context conditioning of defensive behaviors to cat odor stimuli

Exposure of rats to a cat odor block in a previously familiarized situation was followed by three extinction days to the same or a different situation, and with or without an identical but odor-free block, and, testing in the original apparatus with an odor-free block (cue). Initial exposure produced risk assessment (stretch attend), avoidance of the block, and crouch/freeze with sniffing/head movements. Avoidance continued during extinction, but context-only exposed rats showed predominantly crouch/freeze with sniff/head movements, while rats exposed to the context+cue showed higher levels of stretch attend. During the test day, rats exposed to the cue during extinction showed reduced defensive responding compared to those not extinguished with the cue, but context extinction had less effect, possibly due in part to initial familiarization with the situation. These data indicate that both cue and context conditioning to cat odor did occur, and that the type of conditioned stimulus (context-only vs. context+cue) influenced the type of defensive behaviors elicited by this stimulus, although the all animals received the same conditioning protocol. Particular behaviors disappeared at different rates during extinction, with avoidance the most persistent. However, in this context there was no incentive for approach behaviors inconsistent with avoidance, and stretch attend behaviors could and did occur while subjects were located far from the block or the area in which it had been encountered. In addition, immobile crouch/freeze did not occur at higher than control levels, while the crouch/freeze activities that did increase incorporated sensory sampling in a relevant modality (sniffing/head movements). Thus, the behaviors seen to the conditioned stimulus appeared to reflect combinations of different defense strategies, appropriate to the type of conditioned stimulus and responsive to its extinction. Differences between these data and those from studies using fecal predator odorants suggest that the latter may not elicit a complete range of conditioned defenses.

Defensive behavior in rats towards predatory odors: a review

Studies of the response of rodents to predatory odors (mainly cat) have provided useful insights into the nature of defensive behavior. This article reviews work in this area with a focus on a behavioral paradigm recently developed in our laboratory in which we present rats with a piece of fabric collar that has been previously worn by a cat. Rats presented with this stimulus spent most of their time engaged in a behavior we call 'head out' in which the rat pokes its head out from a hide box and scans the environment. Periodic 'flat back approaches' and 'vigilant rearing' towards the cat odor source are seen as well as inhibition of non-defensive behaviors such as locomotor activity and grooming. Cat odor causes a sustained increase in blood pressure (> 15mm Hg) without greatly affecting heart beat rate. Rats will develop conditioned fear to both contexts and cues that have been paired with cat odor. C-fos immunohistochemistry indicates that cat odor selectively activates a defensive behavior circuit involving the medial amygdala, ventromedial and dorsomedial hypothalamus, dorsal premammillary nucleus and the periaqueductal gray. The defensive response to cat odor is attenuated by acute administration of the benzodiazepine midazolam (0.375 mg/kg), with chronically administered SSRI antidepressants and acute alcohol exerting more modest anxiolytic effects. The behavioral response to cat odor is very different to that seen to trimethylthiazoline (TMT: fox odor) which has effects more like those seen to an aversive putrid odor. It is concluded that cat odor is a useful tool for elucidating behavioral, neural, pharmacological and autonomic aspects of defensive behavior and anxiety.

Antipredator responses and defensive behavior: ecological and ethological approaches for the neurosciences

In nature, animals are exposed to a wide range of threats and dangers with predators being amongst the more prominent and intensely studied of these. The responses of prey to predators and various predator avoidance and antipredator behaviors have been extensively evaluated from ecological and ethological perspectives and more recent ethopharmacological and neuroscience approaches. Unfortunately, there has been relatively little interchange between the ecological-ethological and neuroscience areas with the latter often using responses to predators just simply as another 'model' system. There is, however, now a growing realization that integrative approaches incorporating ecological, evolutionary and neurobiological explanations are required for the understanding of behavior and its functions. This necessitates an incorporation of ecological and ethological concepts and validity with neuroscience approaches to the analysis of antipredator responses and defensive behavior. A number of selected ecological approaches that are used for the investigation of predator avoidance mechanisms and antipredator defensive behavior patterns are briefly reviewed here. These include examinations of how predation risk and its variation affect decision making in animals and how learning affects these responses. The trade-offs that are involved, how the risk of predation affects decisions concerning foraging behavior, mating and reproduction, as well as how varying levels of risk affect decisions relative to the type of defensive mechanisms utilized are briefly outlined. The utility of these approaches and their relevance to the design and interpretation of various neuroscience studies is addressed here.

Social stress in mice: gender differences and effects of estrous cycle and social dominance

A large discrepancy in the possibility of inducing social stress in the two genders exists. Since generalizations of findings from one sex to the other appear not to be valid, reliable models of social stress in females are needed. We examined the effects of social context in the housing environment, as a possible source of stress, on exploration and anxiety in male and female mice, taking into account the estrous phase for females and the social status for males as additional variables. Mice housed individually or with siblings were tested in a free-exploratory paradigm of anxiety (where test animals have a choice to stay in their home cage or to explore an open field, OF). Individually housed females did not leave their home cage for long periods, explored less the unfamiliar area and displayed higher risk assessment, a behavioral profile suggestive of lower propensity for exploration and higher level of anxiety compared with group-housed females. Individually housed males tended to show an opposite profile. Proestrus mice were less sensitive to the decrease of exploratory propensity induced by individually housing compared to estrus and diestrus mice. Social dominants and social subordinates in sibling groups did not differ in their exploratory responses to the OF. Different housing procedures, as means to provide different social environment, may differentially induce mild social stress in male and female mice.

Locomotor activity changes following lipopolysaccharide treatment in mice: a multivariate assessment of behavioral tolerance

To determine the effects of repeated, acute endotoxin exposure on locomotor behavior, male laboratory mice were injected intraperitoneally with lipopolysaccharide (LPS: 50, 100 or 200 microg/kg) or saline vehicle on experimental Days 1, 4 and 7. At 2 h after each treatment, locomotor activity was assessed in a nonnovel, automated open-field apparatus (Digiscan) for 30 min. On Day 1, all horizontal and vertical activity measures were significantly reduced to near zero values by each dose of LPS. Behavioral tolerance to LPS formed rapidly, as locomotor activity of the treated groups did not differ from the control group on Days 4 or 7. In a second study, mice were given LPS (50, 100 or 150 microg/kg ip) or saline vehicle on two test days, 28 days apart. Activity was assessed, 1 h after injection, in a novel open field on the first test day and in a nonnovel open field on the second test day. Significant locomotor activity decrements were readily apparent in LPS-treated mice only in the nonnovel open field. This latter finding indicates that environmental novelty mediates, at least partially, the locomotor-reducing effects of LPS in mice.