Acute stress impairs spatial memory in male but not female rats: influence of estrous cycle

Septohippocampal cholinergic system at the intersection of stress and cognition: Current trends and translational implications

Deficits in hippocampus-dependent memory processes are common across psychiatric and neurodegenerative disorders such as depression, anxiety and Alzheimer's disease. Moreover, stress is a major environmental risk factor for these pathologies and it exerts detrimental effects on hippocampal functioning via the activation of hypothalamic-pituitary-adrenal (HPA) axis. The medial septum cholinergic neurons extensively innervate the hippocampus. Although, the cholinergic septohippocampal pathway (SHP) has long been implicated in learning and memory, its involvement in mediating the adaptive and maladaptive impact of stress on mnemonic processes remains less clear. Here, we discuss current research highlighting the contributions of cholinergic SHP in modulating memory encoding, consolidation and retrieval. Then, we present evidence supporting the view that neurobiological interactions between HPA axis stress response and cholinergic signalling impact hippocampal computations. Finally, we critically discuss potential challenges and opportunities to target cholinergic SHP as a therapeutic strategy to improve cognitive impairments in stress-related disorders. We argue that such efforts should consider recent conceptualisations on the dynamic nature of cholinergic signalling in modulating distinct subcomponents of memory and its interactions with cellular substrates that regulate the adaptive stress response.

Acute stress facilitates habitual behavior in female rats

Instrumental behavior can reflect the influence of goal-directed and habitual systems. Contemporary research suggests that stress may facilitate control by the habitual system under conditions where the behavior would otherwise reflect control by the goal-directed system. However, it is unclear how stress modulates the influence of these systems on instrumental responding to achieve this effect, particularly in females. Here, we examine whether a mild psychogenic stressor experienced before acquisition training (Experiment 1), or prior to the test of expression (Experiment 2) would influence goal-directed and habitual control of instrumental responding in female rats. In both experiments, rats acquired an instrumental nose-poke response for a sucrose reward. This was followed by a reinforcer devaluation phase in which half the rats in Stressed and Non-Stressed conditions received pairings of the sucrose pellet with illness induced by lithium chloride until they rejected the pellet when offered. The remaining rats received a control treatment consisting of pellets and illness on separate days (Unpaired). Control by goal-directed and habitual systems was evaluated in a subsequent nonreinforced test of nose poking. The results of Experiment 1 indicated that the Non-Stressed Paired group reduced nose-poking compared to the Unpaired controls, identifying the response as goal directed, whereas the Stressed Paired and Unpaired groups made a similar number of nose pokes identifying the response as habitual despite a similar amount of training. Results from Experiment 2 indicated habitual control of nose-poke responding was present when stress was experienced just prior to the test. Collectively, these data suggest that stress may facilitate habitual control by altering the relative influence of goal-directed and habitual processes underpinning instrumental behavior. These results may be clinically relevant for understanding the contributions of stress to dysregulated instrumental behavior in compulsive pathologies.

A terrified-sound stress causes cognitive impairment in female mice by impairing neuronal plasticity

Stress is an important environmental factor affecting mental health that cannot be ignored. Moreover, due to the great physiological differences between males and females, the effects of stress may vary by sex. Previous studies have shown that terrified-sound stress, meaning exposed mice to the recorded vocalizations in response to the electric shock by their kind to induce psychological stress, can cause cognitive impairment in male. In the study, we investigated the effects of the terrified-sound stress on adult female mice.

METHODS

32 adults female C57BL/6 mice were randomly divided into control (n = 16) and stress group (n = 16). Sucrose preference test (SPT)was carried out to evaluate the depressive-like behavior. Using Open field test (OFT) to evaluate locomotor and exploratory alterations in mice. Spatial learning and memory ability were measured in Morris Water maze test (MWM), Golgi staining and western blotting showed dendritic remodeling after stress. In addition, serum hormone quantifications were performed by ELISA.

RESULTS

we found the sucrose preference of stress group was significantly decreased (p < 0.05) compared with control group; the escape latency of the stress group was significantly prolonged (p < 0.05), the total swimming distance and the number of target crossings(p < 0.05) were significantly increased (p < 0.05) in MWM; Endocrine hormone, Testosterone (T) (p < 0.05), GnRH (p < 0.05), FSH and LH levels was decreased; Golgi staining and western blotting showed a significant decrease in dendritic arborization, spine density and synaptic plasticity related proteins PSD95 and BDNF in the stress group.

CONCLUSION

Terrified-sound stress induced depressive-like behaviors, locomotor and exploratory alterations. And impaired cognitive by altering dendritic remodeling and the expression of synaptic plasticity-related proteins. However, females are resilient to terrified-sound stress from a hormonal point of view.

Sex-specific alterations in corticotropin-releasing factor regulation of coerulear-cortical network activity

The locus coeruleus (LC)-norepinephrine system is a stress responsive system that regulates arousal and cognitive functions through extensive projections, including to the prefrontal cortex. LC-cortical circuits are activated by stressors, and this activation is thought to contribute to stress-induced impairments in executive function. Because corticotropin-releasing factor (CRF) is a mediator of stress-induced LC activation, we examined the effects of CRF administered into the LC of male and female rats on network activity of two functionally distinct regions of the PFC, the medial PFC (mPFC) and the orbitofrontal cortex (OFC). Network activity, measured as local field potentials, was recorded in awake animals before and after intra-LC infusion of aCSF or CRF (2 or 20 ng). CRF had qualitatively distinct effects on network activity in males and females with respect to dose, region and timecourse. CRF (20 ng) produced a prominent theta oscillation (7-9 Hz) selectively in female rats shortly after LC infusion and 20 min later. In contrast, in male rats, CRF (2 and 20 ng) decreased the amplitude of power in the 4-6 Hz range in the mPFC 10 min after injection. Lastly, CRF (20 ng) increased mPFC-OFC coherence in females and decreased mPFC-OFC coherence in males. In sum, these results show sex differences in CRF modulation of the LC-norepinephrine system that regulates prefrontal cortical networks, which may underlie sex differences in cognitive and behavioral responses to stress.

Sex differences in cognition following variations in endocrine status

Spatial memory, mediated primarily by the hippocampus, is responsible for orientation in space and retrieval of information regarding location of objects and places in an animal's environment. Since the hippocampus is dense with steroid hormone receptors and is capable of robust neuroplasticity, it is not surprising that changes in spatial memory performance occur following a variety of endocrine alterations. Here, we review cognitive changes in both spatial and nonspatial memory tasks following manipulations of the hypothalamic-pituitary-adrenal and gonadal axes and after exposure to endocrine disruptors in rodents. Chronic stress impairs male performance on numerous behavioral cognitive tasks and enhances or does not impact female cognitive function. Sex-dependent changes in cognition following stress are influenced by both organizational and activational effects of estrogen and vary depending on the developmental age of the stress exposure, but responses to gonadal hormones in adulthood are more similar than different in the sexes. Also discussed are possible underlying neural mechanisms for these steroid hormone-dependent, cognitive effects. Bisphenol A (BPA), an endocrine disruptor, given at low levels during adolescent development, impairs spatial memory in adolescent male and female rats and object recognition memory in adulthood. BPA's negative effects on memory may be mediated through alterations in dendritic spine density in areas that mediate these cognitive tasks. In summary, this review discusses the evidence that endocrine status of an animal (presence or absence of stress hormones, gonadal hormones, or endocrine disruptors) impacts cognitive function and, at times, in a sex-specific manner.

Evidence for an allostatic epigenetic memory on chromatin footprints after double-hit acute stress

Stress induces allostatic responses, whose limits depend on genetic background and the nature of the challenges. Allostatic load reflects the cumulation of these reponses over the course of life. Acute stress is usually associated with adaptive responses, although, depending on the intensity of the stress and individual differences , some may experience maladaptive coping that persists through life and may influence subsequent responses to stressful events, as is the case of post-traumatic stress disorder. We investigated the behavioral traits and epigenetic signatures in a double-hit mouse model of acute stress in which heterotypic stressors (acute swim stress and acute restraint stress) were applied within a 7-day interval period. The ventral hippocampus was isolated to study the footprints of chromatin accessibility driven by exposure to double-hit stress. Using ATAC sequencing to determine regions of open chromatin, we showed that depending on the number of acute stressors, several gene sets related to development, immune function, cell starvation, translation, the cytoskeleton, and DNA modification were reprogrammed in both males and females. Chromatin accessibility for transcription factor binding sites showed that stress altered the accessibility for androgen, glucocorticoid, and mineralocorticoid receptor binding sites (AREs/GREs) at the genome-wide level, with double-hit stressed mice displaying a profile unique from either single hit of acute stress. The investigation of AREs/GREs adjacent to gene coding regions revealed several stress-related genes, including Fkbp5, Zbtb16, and Ddc, whose chromatin accessibility was affected by prior exposure to stress. These data demonstrate that acute stress is not truly acute because it induces allostatic signatures that persist in the epigenome and may manifest when a second challenge hits later in life.

Sex Differences in the Spatial Behavior Functions of Adult-Born Neurons in Rats

Adult neurogenesis modifies hippocampal circuits and behavior, but removing newborn neurons does not consistently alter spatial processing, a core function of the hippocampus. Additionally, little is known about sex differences in neurogenesis since few studies have compared males and females. Since adult-born neurons regulate the stress response, we hypothesized that spatial functions may be more prominent under aversive conditions and may differ between males and females given sex differences in stress responding. We therefore trained intact and neurogenesis-deficient rats in the spatial water maze at temperatures that vary in their degree of aversiveness. In the standard water maze, ablating neurogenesis did not alter spatial learning in either sex. However, in cold water, ablating neurogenesis had divergent sex-dependent effects: relative to intact rats, male neurogenesis-deficient rats were slower to escape the maze and female neurogenesis-deficient rats were faster. Neurogenesis promoted temperature-related changes in search strategy in females, but it promoted search strategy stability in males. Females displayed greater recruitment (Fos expression) of the dorsal hippocampus than males, particularly in cold water. However, blocking neurogenesis did not alter Fos expression in either sex. Finally, morphologic analyses revealed greater experience-dependent plasticity in males. Adult-born neurons in males and females had similar morphology at baseline but training increased spine density and reduced presynaptic terminal size, specifically in males. Collectively, these findings indicate that adult-born neurons contribute to spatial learning in stressful conditions and they provide new evidence for sex differences in their behavioral functions.

Two-hour acute restraint stress facilitates escape behavior and learning outcomes through the activation of the Cdk5/GR P S211 pathway in male mice

Acute stress exerts pleiotropic actions on learning behaviors. The induced negative effects are sometimes adopted to measure the efficacy of particular drugs. Until now, there are no detailed experimental data on the time-gradient effects of acute stress. Here, we developed the time gradient acute restraint stress (ARS) model to precisely assess the roles of different restrain times on inducing acute stress. Time gradient ARS facilitates escape behaviors and learning outcomes, peaking at 2 h-ARS and then declining to baseline at 3.5 h-ARS as confirmed by time gradient post-stress data. Furthermore, time gradient ARS activates glucocorticoid receptor (GR) phosphorylation site at Serine211 (P S221) as an inverted V-shaped pattern peaking at 2 h-ARS, whereas that of the GR phosphorylation site at Serine226 (P S226) from 2 h-ARS to 3.5 h-ARS. The 2 h-ARS but not 3.5 h-ARS enhances synaptic plasticity and genes transcription associated with learning and memory in the hippocampus of male mice. The Cdk5 inhibitor, roscovitine, blocks this facilitation effect by intervening in GR phosphorylation at Serine211 in the 2 h-ARS mice. Altogether, these findings show that the time gradient ARS selectively activates GR phospho-isoforms and differentially influences the behaviors along with maintaining a relationship between 2 h-ARS and Cdk5/GR P S211-mediated transcriptional activity.

Animal Models of Anxiety and Depression: Incorporating the Underlying Mechanisms of Sex Differences in Macroglia Biology

Animal models have been utilized to explore the mechanisms by which mood disorders develop. Ethologically based stress paradigms are used to induce behavioral responses consistent with those observed in humans suffering from anxiety and depression. While mood disorders are more often diagnosed in women, animal studies are more likely to be carried out in male rodents. However, understanding the mechanisms behind anxiety- and depressive-like behaviors in both sexes is necessary to increase the predictive and construct validity of the models and identify therapeutic targets. To understand sex differences following stress, we must consider how all cell types within the central nervous system are influenced by the neuroendocrine system. This review article discusses the effects of stress and sex steroids on the macroglia: astrocytes and oligodendrocytes. Glia are involved in shaping the synapse through the regulation of neurotransmitter levels and energy resources, making them essential contributors to neural dynamics following stress. As the role of glia in neuromodulation has become more apparent, studies exploring the mechanisms by which glia are altered by stress and steroids will provide insight into sex differences in animal models. These insights will facilitate the optimization of animal models of psychiatric disorders and development of future therapeutic targets.

The medial entorhinal cortex mediates basolateral amygdala effects on spatial memory and downstream activity-regulated cytoskeletal-associated protein expression

The basolateral amygdala (BLA) modulates the consolidation of dorsal hippocampus (DH)-dependent spatial and dorsolateral striatum (DLS)-dependent cued-response memories, often in competition with one another. Evidence suggests that a critical mechanism for BLA influences on memory consolidation is via effects on activity-regulated cytoskeletal-associated protein (ARC) in downstream brain regions. However, the circuitry by which the BLA modulates ARC in multiple competing memory systems remains unclear. Prior evidence indicates that optogenetic stimulation of BLA projections to the medial entorhinal cortex (mEC) enhances the consolidation of spatial learning and impairs the consolidation of cued-response learning, suggesting this pathway provides a circuit for favoring one system over another. Therefore, we hypothesized the BLA-mEC pathway mediates effects on downstream ARC-based synaptic plasticity related to these competing memory systems. To address this, male and female Sprague-Dawley rats underwent spatial or cued-response Barnes maze training and, 45 min later, were sacrificed for ARC analysis in synaptoneurosomes from the DH and DLS. Initial experiments found that spatial training alone increased ARC levels in the DH above those observed in control rats and rats that underwent a cued-response version of the task. Postspatial training optogenetic stimulation of the BLA-mEC pathway altered the balance of ARC expression in the DH vs. DLS, specifically shifting the balance in favor of the DH-based spatial memory system, although the precise region of ARC changes differed by sex. These findings suggest that BLA-mEC pathway influences on ARC in downstream regions are a mechanism by which the BLA can favor one memory system over another.

Protective effects of duloxetine against chronic immobilisation stress-induced anxiety, depression, cognitive impairment and neurodegeneration in mice

OBJECTIVES

This study aimed to evaluate the effect of duloxetine (10 and 20 mg/kg) against chronic immobilisation stress (CIS)-induced anxiety, depression, cognitive impairment and neurodegeneration in mice.

METHODS

CIS, 2 h/10 days (11:00 AM-1:00 PM) was applied after 30 min of pretreatment with saline, duloxetine 10 mg/kg and 20 mg/kg to the respective groups of animals, except the control group. Animals were examined for physiological (body weight, locomotion and grip strength), psychological (memory impairment, anxiety and depression), neurochemical (GABA and glutamate), biochemical (MDA, catalase, glutathione, superoxide dismutase) and histopathological changes.

KEY FINDINGS

CIS exposure revealed anxiety-like behaviour, depression-like behaviour, motor in-coordination and learning and memory impairment in mice. Besides, CIS induction decreased the antioxidant enzymes (GSH, SOD and catalase), GABA and the viable neuronal cell count, whereas CIS exposure significantly elevated the MDA, AChE activity and glutamate content in the cortex and hippocampus. Pretreatment with duloxetine10 and 20 mg/kg showed dose-dependent ameliorated effect against the CIS-induced alterations in mice.

CONCLUSION

In conclusion, the results of this study demonstrated the protective effect of duloxetine against neuropsychiatric symptoms, memory impairment caused by CIS-induction through inhibition of oxidative stress, AChE activity and glutamate release.

Sleep loss mediates the effect of stress on nitrergic signaling in female mice

Nitric oxide (NO) has been implicated as an important neurotransmitter in stress responses and sleep regulatory processes. However, the role of NO in the relationship between stress and sleep remains unclear. The medial septum (MS) and vertical diagonal band (VDB), regions of the basal forebrain involved in sleep regulation, contain nitric oxide synthase (NOS) producing neurons. Additionally, NOS neurons in the dorsal raphe nucleus (DRN) encode information about stress duration. The role of nitrergic neurons in these regions in subserving sex-specific responses to stress and sleep loss has yet to be elucidated. In this study, NADPH-d, an index of NOS activity, was used to examine the effects of acute restraint stress and sleep loss on NOS activity in the MS, VDB, and DRN. We show that NOS activity in response to restraint stress, total sleep deprivation (TSD), and partial sleep restriction (PSR) differs based on sex and region. Initial analysis showed no effect of restraint stress or TSD on NOS activity in the basal forebrain. However, investigation of each sex separately revealed that restraint stress and TSD significantly decrease NOS activity in the MS of females, but not males. Interestingly, the difference in NOS activity between restraint stress and TSD in females was not significant. Furthermore, PSR was not sufficient to affect NOS activity in males or females. These data suggest that restraint stress and sleep loss regulate NOS activation in a sex-dependent manner, and that the NOS stress response in females may be mediated by sleep loss.

Central CRF and acute stress differentially modulate probabilistic reversal learning in male and female rats

Acute stress can have variable and sometimes sex-dependent effects on different executive functions, including cognitive flexibility, some of which may be mediated by increased corticotropin releasing factor (CRF). Previous studies on the effects of stress and CRF on cognitive flexibility have used procedures entailing deterministic rewards, yet how they may alter behavior when outcomes are probabilistic is unclear. The present study examined how acute stress and increased CRF activity alters probabilistic reversal learning (PRL) in male and female rats. Rats learned to discriminate between a 'correct' lever rewarded on 80 % of trials, and an "incorrect" lever delivering reward on 20 % of trials, with reward contingencies reversed after 8 consecutive correct choices. Separate groups received either intracerebroventricular infusions of CRF (3 μg) or restraint stress prior to a PRL session. Experiments examined how these manipulations affected learning when given prior to a one-day acquisition test or during performance in well-trained rats. Exogenous CRF, and to a lesser extent acute stress, impaired motivation across sexes, slowing deliberation times and increasing the number of trials omitted, particularly following a switch in reward contingencies. Neither manipulation significantly altered errors or reversal performance. However, increased CRF activity reduced negative feedback sensitivity. Across manipulations, females showed increased omissions and choice latencies, and were less sensitive to feedback than males. These results reveal the complexity with which stress, CRF, sex, and experience interact to alter aspects of motivation and probabilistic reinforcement learning and provide insight into how CRF activity may contribute to symptoms of stress-related disorders.

Estradiol promotes and progesterone reduces anxiety-like behavior produced by nicotine withdrawal in female rats

This study assessed sex differences and the role of ovarian hormones in nicotine withdrawal. Study 1 compared physical signs, anxiety-like behavior, and corticosterone levels in male, intact female, and ovariectomized (OVX) female rats during nicotine withdrawal. Estradiol (E2) and progesterone levels were also assessed in intact females that were tested during different phases of the 4-day estrous cycle. Study 2 assessed the role of ovarian hormones in withdrawal by comparing the same measures in OVX rats that received vehicle, E2, or E2+progesterone prior to testing. Briefly, rats received a sham surgery or an ovariectomy procedure. Fifteen days later, rats were prepared with a pump that delivered nicotine for 14 days. On the test day, rats received saline or the nicotinic receptor antagonist, mecamylamine to precipitate withdrawal. Physical signs and anxiety-like behavior were assessed on the elevated plus maze (EPM) and light-dark transfer (LDT) tests. During withdrawal, intact females displayed greater anxiety-like behavior and increases in corticosterone levels as compared to male and OVX rats. Females tested in the estrus phase (when E2 is relatively low) displayed less anxiety-like behavior and had lower corticosterone levels versus all other phases. Anxiety-like behavior and corticosterone levels were positively correlated with E2 and negatively correlated with progesterone levels. Intact females displaying high E2/low progesterone showed greater anxiety-like behavior and corticosterone levels as compared to females displaying low E2/high progesterone. Lastly, OVX-E2 rats displayed greater anxiety-like behavior than OVX-E2+progesterone rats. These data suggest that E2 promotes and progesterone reduces anxiety-like behavior produced by nicotine withdrawal.

Protective effects of Hydrocotyle umbellata var. bonariensis Lam. (Araliaceae) on memory in sleep-impaired female mice

ETHNOPHARMACOLOGICAL RELEVANCE

Hydrocotyle umbellata var.bonariensis Lam. (Hb), popularly known in Brazil as acariçoba and outside Brazil Hb by a number of names including marsh-pennywort, and many-flower, has traditionally been used in Ayurvedic medicine in the retardation of aging (Rasayana effect).

AIM OF THE STUDY

The present study evaluated the effect of Hb treatment before and during paradoxical sleep deprivation (PSD) and sleep restriction (SR) on learning, memory, and acetylcholinesterase (AChE) brain activity.

MATERIAL AND METHODS

Adult Swiss nulliparous female mice were randomly distributed among the experimental groups. The treated groups received the aqueous solution of Hb leaves orally at concentrations of 500 and 1.000 mg/kg. PSD and SR were induced by the multiple platform method, in which the animals remained for 3-days in PSD or 15-days in SR for 22 h per day. The collection of the vaginal epithelium occurred daily to determine the estrous cycle. Body mass gain was determined. The animals were submitted to the passive avoidance test and were then euthanized for the collection of brain tissue and the determination of cerebral cholinesterase activity.

RESULTS

The aqueous solution of Hb was associated with a significant reduction in cholinesterase activity at both doses in the SR model, and at the dose of 1.000 mg/kg in the PSD model. Regarding the learning and memory test, the PSD group treated with 1.000 mg/kg presented significant improvement, whereas in the SR experiment none of the treated-groups showed any improvement in learning and memory. In the analysis of SR/PSD interference and/or Hb treatment on the estrous cycle, it was possible to observe that the treatment acted as a protector in the SR group, maintaining a normal cycle.

CONCLUSIONS

The analyses showed that Hb was safe to use during periods of SR or PSD, acting as an adaptogen for these situations, in addition to being able to reduce cholinesterase activity, which suggests its neuroprotective action. In relation to the estrous cycle, Hb can act as a protector in SR situations.

Sex Differences in Risk and Resilience: Stress Effects on the Neural Substrates of Emotion and Motivation

Risk for stress-sensitive psychopathologies differs in men and women, yet little is known about sex-dependent effects of stress on cellular structure and function in corticolimbic regions implicated in these disorders. Determining how stress influences these regions in males and females will deepen our understanding of the mechanisms underlying sex-biased psychopathology. Here, we discuss sex differences in CRF regulation of arousal and cognition, glucocorticoid modulation of amygdalar physiology and alcohol consumption, the age-dependent impact of social stress on prefrontal pyramidal cell excitability, stress effects on the prefrontal parvalbumin system in relation to emotional behaviors, contributions of stress and gonadal hormones to stress effects on prefrontal glia, and alterations in corticolimbic structure and function after cessation of chronic stress. These studies demonstrate that, while sex differences in stress effects may be nuanced, nonuniform, and nonlinear, investigations of these differences are nonetheless critical for developing effective, sex-specific treatments for psychological disorders.

Neurobiological mechanisms underlying sex-related differences in stress-related disorders: Effects of neuroactive steroids on the hippocampus

Men and women differ in their vulnerability to a variety of stress-related illnesses, but the underlying neurobiological mechanisms are not well understood. This is likely due to a comparative dearth of neurobiological studies that assess male and female rodents at the same time, while human neuroimaging studies often don't model sex as a variable of interest. These sex differences are often attributed to the actions of sex hormones, i.e. estrogens, progestogens and androgens. In this review, we summarize the results on sex hormone actions in the hippocampus and seek to bridge the gap between animal models and findings in humans. However, while effects of sex hormones on the hippocampus are largely consistent in animals and humans, methodological differences challenge the comparability of animal and human studies on stress effects. We summarise our current understanding of the neurobiological mechanisms that underlie sex-related differences in behavior and discuss implications for stress-related illnesses.

Effect of supplementation with methyl-donor nutrients on neurodevelopment and cognition: considerations for future research

Pregnancy represents a critical period in fetal development, such that the prenatal environment can, in part, establish a lifelong trajectory of health or disease for the offspring. Poor nutrition (macro- or micronutrient deficiencies) can adversely affect brain development and significantly increase offspring risk for metabolic and neurological disease development. The concentration of dietary methyl-donor nutrients is known to alter DNA methylation in the brain, and alterations in DNA methylation can have long-lasting effects on gene expression and neuronal function. The decreased availability of methyl-donor nutrients to the developing fetus in models of poor maternal nutrition is one mechanism hypothesized to link maternal malnutrition and disease risk in offspring. Animal studies indicate that supplementation of both maternal and postnatal (early- and later-life) diets with methyl-donor nutrients can attenuate disease risk in offspring; however, clinical research is more equivocal. The objective of this review is to summarize how specific methyl-donor nutrient deficiencies and excesses during pre- and postnatal life alter neurodevelopment and cognition. Emphasis is placed on reviewing the current literature, highlighting challenges within nutrient supplementation research, and considering potential strategies to ensure robust findings in future studies.

Memory impairment induced by different types of prolonged stress is dependent on the phase of the estrous cycle in female rats

A growing body of evidence demonstrates that estrogen and corticosterone (CORT) impact on cognition and emotion. On the one hand, ovarian hormones may have beneficial effects on several neurophysiological processes, including memory. On the other hand, chronic exposure to stressful conditions has negative effects on brain structures related to learning and memory. In the present study, we used the plus-maze discriminative avoidance task (PMDAT) to evaluate the influence of endogenous variations of sex hormones and exposure to different types of prolonged stressors on learning, memory, anxiety-like behavior and locomotion. Female Wistar rats were submitted to seven consecutive days of restraint stress (4 h/day), overcrowding (18 h/day) or social isolation (18 h/day) and tested in different phases of the estrous cycle. The main results showed that: (1) neither stress conditions nor estrous cycle modified PMDAT acquisition; (2) restraint stress and social isolation induced memory impairments; (3) this impairment was observed particularly in females in metestrus/diestrus; (4) stressed females in estrus displayed less risk assessment behavior, suggesting reduced anxiety-like behavior; (5) restraint stress and social isolation, but not overcrowding, elevated corticosterone levels. Taken together, our findings suggest that the phase of the estrous cycle is an important modulatory factor of the cognitive processing disrupted by stress in female rats. Negative effects were observed in metestrus/diestrus, indicating that the peak of sex hormones may protect females against stress-induced memory impairment.

Sex differences in the effects of acute stress on cerebral glucose metabolism: A microPET study

Stress has been considered as a risk factor for the development and aggravation of several diseases. The hypothalamic-pituitary-adrenal axis (HPA) is one of the main actors for the stress response and homeostasis maintenance. Positron emission tomography (PET) has been used to evaluate neuronal activity and to study brain regions that may be related to the HPA axis response. Since neuroimaging is an important tool in detecting neuroendocrine-related changes, we used fluorodeoxyglucose-18 (¹⁸F-FDG) and positron emission microtomography (microPET) to evaluate sexual differences in the glucose brain metabolism after 10, 30 and 40 min of acute stress in Balb/c mice. We also investigated the effects of restraint stress in blood, liver and adrenal gland ¹⁸F-FDG biodistribution using a gamma counter. A decreased glucose uptake in the whole brain in both females and males was found. Additionally, there were time and sex-dependent alterations in the ¹⁸F-FDG uptake after restraint stress in specific brain regions, indicating that males could be more vulnerable to the short-term effects of acute stress. According to the gamma counter biodistribution, only females showed a significant decreased glucose uptake in the blood, liver and right adrenal after restraint stress. In addition, in comparisons between the sexes, males showed a decreased glucose uptake in the whole brain and in several brain regions compared to females. In conclusion, exposure to acute restraint stress resulted in significant decreased glucose metabolism in the brain, with particular effects in different regions and organs in a sex-specific manner.

Cotinine Plus Krill Oil Decreased Depressive Behavior, and Increased Astrocytes Survival in the Hippocampus of Mice Subjected to Restraint Stress

Restraint stress (RS) is a condition affecting millions of people worldwide. The investigation of new therapies to alleviate the consequences of prolonged RS is much needed. Cotinine, a nicotine-derivative, has shown to prevent the decrease in cerebral synaptic density, working memory deficits, anxiety, and depressive-like behavior after prolonged restraint stress (RS) in mice. Furthermore, post-treatment with cotinine reduced the adverse effects of chronic RS on astrocyte survival and architecture. On the other hand, the nutritional supplement krill oil (KO), has shown to be beneficial in decreasing depressive-like behavior and oxidative stress. In this study, in the search for effective preventative treatments to be used in people subjected to reduced mobility, the effect of co-treatment with cotinine plus KO in mice subjected to prolonged RS was investigated. The results show that cotinine plus KO prevented the loss of astrocytes, the appearance of depressive-like behavior and cognitive impairment induced by RS. The use of the combination of cotinine plus KO was more effective than cotinine alone in preventing the depressive-like behavior in the restrained mice. The potential use of this combination to alleviate the psychological effects of reduced mobility is discussed.

Effects of corticosterone on mild auditory fear conditioning and extinction; role of sex and training paradigm

Multiple lines of evidence suggest that glucocorticoid hormones enhance memory consolidation of fearful events. However, most of these studies involve male individuals. Since anxiety, fear, and fear-associated disorders present differently in male and female subjects we investigated in mice whether male and female mice perform differently in a mild, auditory fear conditioning task and tested the modulatory role of glucocorticoid hormones. Using an auditory fear conditioning paradigm with different footshock intensities (0.1, 0.2, and 0.4 mA) and frequencies (1× or 3×), we find that intraperitoneal injections with corticosterone (2 mg/kg) immediately after training, altered freezing behavior when repeated footshocks were applied, and that the direction of the effects were opposite in male and female mice. Effects were independent of footshock intensity. In male mice, corticosterone consistently increased freezing behavior in response to the tone, whereas in female mice, corticosterone reduced freezing behavior 24 h after training. These effects were not related to the phase of the oestrous cycle. In addition, corticosterone enhanced extinction learning for all tones, in both male and female mice. These results emphasize that glucocorticoid hormones influence memory consolidation and retrieval, and underscore sex-specific effects of glucocorticoid hormones in modulating conditioned fear responses.

Factors Influencing Individual Variation in Farm Animal Cognition and How to Account for These Statistically

For farmed species, good health and welfare is a win-win situation: both the animals and producers can benefit. In recent years, animal welfare scientists have embraced cognitive sciences to rise to the challenge of determining an animal's internal state in order to better understand its welfare needs and by extension, the needs of larger groups of animals. A wide range of cognitive tests have been developed that can be applied in farmed species to assess a range of cognitive traits. However, this has also presented challenges. Whilst it may be expected to see cognitive variation at the species level, differences in cognitive ability between and within individuals of the same species have frequently been noted but left largely unexplained. Not accounting for individual variation may result in misleading conclusions when the results are applied both at an individual level and at higher levels of scale. This has implications both for our fundamental understanding of an individual's welfare needs, but also more broadly for experimental design and the justification for sample sizes in studies using animals. We urgently need to address this issue. In this review, we will consider the latest developments on the causes of individual variation in cognitive outcomes, such as the choice of cognitive test, sex, breed, age, early life environment, rearing conditions, personality, diet, and the animal's microbiome. We discuss the impact of each of these factors specifically in relation to recent work in farmed species, and explore the future directions for cognitive research in this field, particularly in relation to experimental design and analytical techniques that allow individual variation to be accounted for appropriately.

The gist and details of sex differences in cognition and the brain: How parallels in sex differences across domains are shaped by the locus coeruleus and catecholamine systems

Across three different domains, there are similar sex differences in how men and women process information. There tends to be a male advantage in attending to and remembering the gist (essential central information of a scene or situation), but a female advantage in attending to and remembering the details (non-essential peripheral information of a scene or situation). This is seen in emotional memory, where emotion enhances gist memory more for males than for females, but enhances detail memory more for females than for males. It also occurs in spatial memory, where men tend to notice and remember the gist of where they or objects are in space, allowing them to more flexibly manipulate themselves or objects within that space, whereas women tend to recall the details of the space around them, allowing them to accurately remember the locations of objects. Finally, such sex differences have also been noted in perception of stimuli such that men attend to global aspects of stimuli (such as a large letter E) more than women, whereas women attend more to the local aspects (such as the many smaller letter Ts making up the E). We review the parallel sex differences seen across these domains in this paper and how they relate to the different brain systems involved in each of these task domains. In addition, we discuss how sex differences in evolutionary pressures and in the locus coeruleus and norepinephrine system may account for why parallel sex differences occur across these different task domains.

Using a memory systems lens to view the effects of estrogens on cognition: Implications for human health

Understanding the organizing and activating effects of gonadal steroids on adult physiology can guide insight into sex differences in and hormonal influences on health and disease, ranging from diabetes and other metabolic disorders, emotion and stress regulation, substance abuse, pain perception, immune function and inflammation, to cognitive function and dysfunction accompanying neurological disorders. Because the brain is highly sensitive to many forms of estrogens, it is not surprising that many adult behaviors, including cognitive function, are modulated by estrogens. Estrogens are known for their facilitating effects on learning and memory, but it is becoming increasingly clear that they also can impair learning and memory of some classes of tasks and may do so through direct actions on specific neural systems. This review takes a multiple memory systems approach to understanding how estrogens can at the same time enhance hippocampus-sensitive place learning and impair striatum-sensitive response learning by exploring the role estrogen receptor signaling may play in the opposing cognitive effects of estrogens. Accumulating evidence suggests that neither receptor subtype nor the timing of treatment, i.e. rapid vs slow, explain the bidirectional effects of estrogens on different types of learning. New findings pointing to neural metabolism and the provision of energy substrates by astrocytes as a candidate mechanism for cognitive enhancement and impairment are discussed.

Effects of testosterone dose on spatial memory among castrated adult male rats

Previous research on the activational effects of testosterone on spatial memory has produced mixed results, possibly because such effects are dose-dependent. We tested a wide range of testosterone doses using two spatial memory tasks: a working-reference memory version of the radial-arm maze (RAM) and an object location memory task (OLMT). Adult male Sprague-Dawley rats were castrated or sham-castrated and given daily injections of drug vehicle (Oil Sham and Oil GDX) or one of four doses of testosterone propionate (0.125, 0.250, 0.500, and 1.000 mg T) beginning seven days before the first day of behavioral tests and continuing throughout testing. For the RAM, four arms of the maze were consistently baited on each day of testing. Testosterone had a significant effect on working memory on the RAM, with the Oil Sham, 0.125 mg T, and 0.500 mg T groups performing better than the Oil GDX group. In contrast, there was no significant effect of testosterone on spatial reference memory on the RAM. For the OLMT, we tested long-term memory using a 2 h inter-trial interval between first exposure to two identical objects and re-exposure after one object had been moved. Only the 0.125 and 0.500 mg T groups showed a significant increase in exploration of the moved object during the testing trials, indicating better memory than all other groups. Testosterone replacement restored spatial memory among castrated male rats on both behavioral tasks, but there was a complex dose-response relationship; therefore, the therapeutic value of testosterone is likely sensitive to dose.

Acute stress evokes sexually dimorphic, stressor-specific patterns of neural activation across multiple limbic brain regions in adult rats

Stress enhances the risk for psychiatric disorders such as anxiety and depression. Stress responses vary across sex and may underlie the heightened vulnerability to psychopathology in females. Here, we examined the influence of acute immobilization stress (AIS) and a two-day short-term forced swim stress (FS) on neural activation in multiple cortical and subcortical brain regions, implicated as targets of stress and in the regulation of neuroendocrine stress responses, in male and female rats using Fos as a neural activity marker. AIS evoked a sex-dependent pattern of neural activation within the cingulate and infralimbic subdivisions of the medial prefrontal cortex (mPFC), lateral septum (LS), habenula, and hippocampal subfields. The degree of neural activation in the mPFC, LS, and habenula was higher in males. Female rats exhibited reduced Fos positive cell numbers in the dentate gyrus hippocampal subfield, an effect not observed in males. We addressed whether the sexually dimorphic neural activation pattern noted following AIS was also observed with the short-term stress of FS. In the paraventricular nucleus of the hypothalamus and the amygdala, FS similar to AIS resulted in robust increases in neural activation in both sexes. The pattern of neural activation evoked by FS was distinct across sexes, with a heightened neural activation noted in the prelimbic mPFC subdivision and hippocampal subfields in females and differed from the pattern noted with AIS. This indicates that the sex differences in neural activation patterns observed within stress-responsive brain regions are dependent on the nature of stressor experience.

Sex differences and the role of acute stress in the open-field tower maze

Many studies provide evidence that differences in spatial learning exist between males and females. However, it is necessary to consider non-mnemonic factors that may influence these findings. The present experiment investigated acquisition, retention, and the effects of stress on response- and place-learning in male and female rats. Rats were trained in an open-field tower maze. Procedures were used to minimize stress in the rats, and their ability to solve place- or response-learning in the maze was determined by analyzing a response variable (i.e., first choice correct response) that was not influenced by general locomotor activity. The results revealed that male and female rats acquire place- and response-learning at the same rate even though females moved significantly faster in the maze. However, females showed better retrieval of place-, but not response-learning compared to male rats. This effect appeared to be enhanced when the rats were tested immediately following an acute restraint stress. Furthermore, both female and male rats that were exposed to acute restraint stress showed less impairment than controls when subsequently tested in a novel situation. These findings have clinical implications that a mild physiological stress response can make one more cognitively resistant to adversities later in life.

Beta estradiol and norepinephrine treatment of differentiated SH-SY5Y cells enhances tau phosphorylation at (Ser396) and (Ser262) via AMPK but not mTOR signaling pathway

Hyperphosphorylation of tau is one of the main hallmarks for Alzheimer's disease (AD) and many other tauopathies. Norepinephrine (NE), a stress-related hormone and 17-β-estradiol (E2) thought to influence tau phosphorylation (p-tau) and AD pathology. The controversy around the impact of NE and E2 requires further clarification. Moreover, the combination effect of physiological and psychological stress and estrogen alteration during menopause, which affect p-tau, has not been addressed. Exposure to E2 is believed to reduce NE release, however, the link between these two hormones and AD at cellular level was also remained unknown. Here, we examined whether NE and E2 treatment of differentiated SH-SY5Y cells affected tau phosphorylation. The involvement of adenosine monophosphate kinase protein kinase (AMPK) and target of Rapamycin (mTOR) as the possible mechanisms, underlying this effect was also investigated. Subsequent to SH-SY5Y differentiation to mature neurons, we treated the cells with NE, E2 and NE plus E2 in presence and absence of Compound C and Rapamycin. Cell viability was not affected by our treatment while our Western blot and immunofluorescent findings showed that exposure to NE and E2 separately, and in combination enhanced p-tau (Ser³⁹⁶) and (Ser²⁶²)/tau but not (Ser²⁰²/Thr²⁰⁵)/tau. Blocking AMPK by Compound C reduced p-tau (Ser³⁹⁶) and (Ser²⁶²), while GSK-3β and PP2A activities were remained unchanged. We also found that blocking mTOR by Rapamycin did not change increased p-tau (Ser³⁹⁶) and (Ser²⁶²) due to NE + E2 treatment. Collectively, our results suggested that tau hyperphosphorylation due to exposure to NE/E2 was mediated by AMPK, the main energy regulator of cells during stress with no significant involvement of mTOR, GSK-3β and PP2A.

Elevated paternal glucocorticoid exposure modifies memory retention in female offspring

Recent studies have demonstrated that behavioral traits are subject to transgenerational modification by paternal environmental factors. We previously reported on the transgenerational influences of increased paternal stress hormone levels on offspring anxiety and depression-related behaviors. Here, we investigated whether offspring sociability and cognition are also influenced by paternal stress. Adult C57BL/6J male mice were treated with corticosterone (CORT; 25mg/L) for four weeks prior to paired-matings to generate F1 offspring. Paternal CORT treatment was associated with decreased body weights of female offspring and a marked reduction of the male offspring. There were no differences in social behavior of adult F1 offspring in the three-chamber social interaction test. Despite male offspring of CORT-treated fathers displaying hyperactivity in the Y-maze, there was no observable difference in short-term spatial working memory. Spatial learning and memory testing in the Morris water maze revealed that female, but not male, F1 offspring of CORT-treated fathers had impaired memory retention. We used our recently developed methodology to analyze the spatial search strategy of the mice during the learning trials and determined that the impairment could not be attributed to underlying differences in search strategy. These results provide evidence for the impact of paternal corticosterone administration on offspring cognition and complement the cumulative knowledge of transgenerational epigenetic inheritance of acquired traits in rodents and humans.

Melatonin inhibits neuronal dysfunction-associated with neuroinflammation by atopic psychological stress in NC/Nga atopic-like mouse models

Atopic dermatitis (AD), also known as atopic eczema, is chronic pruritic skin disease. AD can increase psychological stress as well, increasing glucocorticoid release and exacerbating the associated symptoms. Chronic glucocorticoid elevation disturbs neuroendocrine signaling and can induce neuroinflammation, neurotoxicity, and cognitive impairment; however, it is unclear whether AD-related psychological stress elevates glucocorticoids enough to cause neuronal damage. Therefore, we assessed the effects of AD-induced stress in a mouse AD model. AD-related psychological stress increased astroglial and microglial activation, neuroinflammatory cytokine expression, and markers of neuronal loss. Notably, melatonin administration inhibited the development of skin lesions, scratching behavior, and serum IgE levels in the model mice, and additionally caused a significant reduction in corticotropin-releasing hormone responsiveness, and a significant reduction in neuronal damage. Finally, we produced similar results in a corticosterone-induced AD-like skin model. This is the first study to demonstrate that AD-related psychological stress increases neuroendocrine dysfunction, exacerbates neuroinflammation, and potentially accelerates other neurodegenerative disease states.

Sex Differences in Context Fear Generalization and Recruitment of Hippocampus and Amygdala during Retrieval

Anxiety disorders are commonly associated with increased generalization of fear from a stress- or trauma-associated environment to a neutral context or environment. Differences in context-associated memory in males and females may contribute to increased susceptibility to anxiety disorders in women. Here we examined sex differences in context fear generalization and its neural correlates. We observed stronger context fear conditioning and more generalization of fear to a similar context in females than males. In addition, context preexposure increased fear conditioning in males and decreased generalization in females. Accordingly, males showed stronger cFos activity in dorsal hippocampus during memory retrieval and context generalization, whereas females showed preferential recruitment of basal amygdala. Together, these findings are consistent with previous research showing that hippocampal activity correlates with reduced context fear generalization. Differential competition between hippocampus and amygdala-dependent processes may thus contribute to sex differences in retrieval of context fear and greater generalization of fear-associated memory.

Outbred CD1 mice are as suitable as inbred C57BL/6J mice in performing social tasks

Inbred mouse strains have been used preferentially for behavioral testing over outbred counterparts, even though outbred mice reflect the genetic diversity in the human population better. Here, we compare the sociability of widely available outbred CD1 mice with the commonly used inbred C57BL/6J (C57) mice in the one-chamber social interaction test and the three-chamber sociability test. In the one-chamber task, intra-strain pairs of juvenile, non-littermate, male CD1 or C57 mice display a series of social and aggressive behaviors. While CD1 and C57 pairs spend equal amount of time socializing, CD1 pairs spend significantly more time engaged in aggressive behaviors than C57 mice. In the three-chamber task, sociability of C57 mice was less dependent on acclimation paradigms than CD1 mice. Following acclimation to all three chambers, both groups of age-matched male mice spent more time in the chamber containing a stranger mouse than in the empty chamber, suggesting that CD1 mice are sociable like C57 mice. However, the observed power suggests that it is easier to achieve statistical significance with C57 than CD1 mice. Because the stranger mouse could be considered as a novel object, we assessed for a novelty effect by adding an object. CD1 mice spend more time in the chamber with a stranger mouse than that a novel object, suggesting that their preference is social in nature. Thus, outbred CD1 mice are as appropriate as inbred C57 mice for studying social behavior using either the single or the three-chamber test using a specific acclimation paradigm.

Effects of the Female Estrous Cycle on the Sexual Behaviors and Ultrasonic Vocalizations of Male C57BL/6 and Autistic BTBR T+ tf/J Mice

A primary characteristic of autism, which is a neurodevelopmental disorder, is impaired social interaction and communication. Furthermore, patients with autism frequently show abnormal social recognition. In mouse models of autism, social recognition is usually assessed by examining same-sex social behavior using various tests, such as the three-chamber test. However, no studies have examined the ability of male mice with autism to recognize the estrous cycle of female partners. In this study, we investigated the sexual behaviors, especially mounting and ultrasonic vocal communication (USV), of BTBR T+ tf/J (BTBR) mice, which are used as a well-known mouse model of autism, when they encountered estrus or diestrus female mice. As expected, C57BL/6 mice mounted more female mice in the estrus stage compared with the diestrus stage. We found that BTBR mice also mounted more female mice in the estrus stage than female mice in the diestrus stage. Although the USV emission of male mice was not different between estrus and diestrus female mice in both strains, the mounting result implies that BTBR mice distinguish sexual receptivity of females.

TNF-α from hippocampal microglia induces working memory deficits by acute stress in mice

The role of microglia in stress responses has recently been highlighted, yet the underlying mechanisms of action remain unresolved. The present study examined disruption in working memory due to acute stress using the water-immersion resistant stress (WIRS) test in mice. Mice were subjected to acute WIRS, and biochemical, immunohistochemical, and behavioral assessments were conducted. Spontaneous alternations (working memory) significantly decreased after exposure to acute WIRS for 2h. We employed a 3D morphological analysis and site- and microglia-specific gene analysis techniques to detect microglial activity. Morphological changes in hippocampal microglia were not observed after acute stress, even when assessing ramification ratios and cell somata volumes. Interestingly, hippocampal tumor necrosis factor (TNF)-α levels were significantly elevated after acute stress, and acute stress-induced TNF-α was produced by hippocampal-ramified microglia. Conversely, plasma concentrations of TNF-α were not elevated after acute stress. Etanercept (TNF-α inhibitor) recovered working memory deficits in accordance with hippocampal TNF-α reductions. Overall, results suggest that TNF-α from hippocampal microglia is a key contributor to early-stage stress-to-mental responses.

Enhancement of striatum-dependent memory by conditioned fear is mediated by beta-adrenergic receptors in the basolateral amygdala

Emotional arousal can have a profound impact on various learning and memory processes. For example, unconditioned emotional stimuli (e.g., predator odor or anxiogenic drugs) enhance dorsolateral striatum (DLS)-dependent habit memory. These effects critically depend on a modulatory role of the basolateral complex of the amygdala (BLA). Recent work indicates that, like unconditioned emotional stimuli, exposure to an aversive conditioned stimulus (CS) (i.e., a tone previously paired with shock) can also enhance consolidation of DLS-dependent habit memory. The present experiments examined whether noradrenergic activity, particularly within the BLA, is required for a fear CS to enhance habit memory consolidation. First, rats underwent a fear conditioning procedure in which a tone CS was paired with an aversive unconditioned stimulus. Over the course of the next five days, rats received training in a DLS-dependent water plus-maze task, in which rats were reinforced to make a consistent body-turn response to reach a hidden escape platform. Immediately after training on days 1–3, rats received post-training systemic (Experiment 1) or intra-BLA (Experiment 2) administration of the β-adrenoreceptor antagonist, propranolol. Immediately after drug administration, half of the rats were re-exposed to the tone CS in the conditioning context (without shock). Post-training CS exposure enhanced consolidation of habit memory in vehicle-treated rats, and this effect was blocked by peripheral (Experiment 1) or intra-BLA (Experiment 2) propranolol administration. The present findings reveal that noradrenergic activity within the BLA is critical for the enhancement of DLS-dependent habit memory as a result of exposure to conditioned emotional stimuli.

Delayed restraint procedure enhances cognitive recovery of spatial function after fimbria-fornix transection

PURPOSE

To i) evaluate the effect of a restraint procedure (7 days, 2 h/day) on place learning after fimbria-fornix transection (FF), ii) investigate effects of early vs. late administration of restraint, and iii) establish effects of the restraint procedure on expression of brain derived neurotrophic factor (BDNF) in prefrontal cortex and hippocampus.

METHODS

Fifty rats subjected to FF or sham surgery and divided into groups exposed to restraint immediately (early restraint) or 21 days (late restraint) after surgery were trained to acquire an allocentric place learning task. In parallel, 29 animals were subjected to FF or sham surgery and an identical restraint procedure in order to measure concentrations of BDNF and corticosterone.

RESULTS

The performance of the sham operated rats was positively affected by the late restraint. In FF-lesioned animals, the late restraint significantly improved task performance compared to the lesioned group with no restraint, while the early restraint was associated with a negative impact on task acquisition. Biochemical analysis after restraint procedure revealed a lesion-induced upregulation of BDNF in FF animals.

CONCLUSIONS

The improved task performance of lesioned animals suggests a therapeutic effect of this manipulation, independent of BDNF. This effect is sensitive to the temporal administration of treatment.

Glucocorticoid enhancement of dorsolateral striatum-dependent habit memory requires concurrent noradrenergic activity

Previous findings indicate that post-training administration of glucocorticoid stress hormones can interact with the noradrenergic system to enhance consolidation of hippocampus- or amygdala-dependent cognitive/emotional memory. The present experiments were designed to extend these findings by examining the potential interaction of glucocorticoid and noradrenergic mechanisms in enhancement of dorsolateral striatum (DLS)-dependent habit memory. In experiment 1, different groups of adult male Long-Evans rats received training in two DLS-dependent memory tasks. In a cued water maze task, rats were released from various start points and were reinforced to approach a visibly cued escape platform. In a response-learning version of the water plus-maze task, animals were released from opposite starting positions and were reinforced to make a consistent egocentric body-turn to reach a hidden escape platform. Immediately post-training, rats received peripheral injections of the glucocorticoid corticosterone (1 or 3 mg/kg) or vehicle solution. In both tasks, corticosterone (3 mg/kg) enhanced DLS-dependent habit memory. In experiment 2, a separate group of animals received training in the response learning version of the water plus-maze task and were given peripheral post-training injections of corticosterone (3 mg/kg), the β-adrenoreceptor antagonist propranolol (3 mg/kg), corticosterone and propranolol concurrently, or control vehicle solution. Corticosterone injections again enhanced DLS-dependent memory, and this effect was blocked by concurrent administration of propranolol. Propranolol administration by itself (3 mg/kg) did not influence DLS-dependent memory. Taken together, the findings indicate an interaction between glucocorticoid and noradrenergic mechanisms in DLS-dependent habit memory. Propranolol administration may be useful in treating stress-related human psychopathologies associated with a dysfunctional DLS-dependent habit memory system.

Sex steroid hormones matter for learning and memory: estrogenic regulation of hippocampal function in male and female rodents

Ample evidence has demonstrated that sex steroid hormones, such as the potent estrogen 17β-estradiol (E2), affect hippocampal morphology, plasticity, and memory in male and female rodents. Yet relatively few investigators who work with male subjects consider the effects of these hormones on learning and memory. This review describes the effects of E2 on hippocampal spinogenesis, neurogenesis, physiology, and memory, with particular attention paid to the effects of E2 in male rodents. The estrogen receptors, cell-signaling pathways, and epigenetic processes necessary for E2 to enhance memory in female rodents are also discussed in detail. Finally, practical considerations for working with female rodents are described for those investigators thinking of adding females to their experimental designs.

What's wrong with my mouse cage? Methodological considerations for modeling lifestyle factors and gene-environment interactions in mice

The mechanistic understanding of lifestyle contributions to disease has been largely driven by work in laboratory rodent models using environmental interventions. These interventions show an array of methodologies and sometimes unclear collective conclusions, hampering clinical interpretations. Here we discuss environmental enrichment, exercise and stress interventions to illustrate how different protocols can affect the interpretations of environmental factors in disease. We use Huntington's disease (HD) as an example because its mouse models exhibit excellent validity and HD was the first genetic animal model in which environmental stimulation was found to be beneficial. We make a number of observations and recommendations. Firstly, environmental enrichment and voluntary exercise generally show benefits across laboratories and mouse models. However, the extent to which these environmental interventions have beneficial effects depends on parameters such as the structural complexity of the cage in the case of enrichment, the timing of the intervention and the nature of the control conditions. In particular, clinical interpretations should consider deprived control living conditions and the ethological relevance of the enrichment. Secondly, stress can have negative effects on the phenotype in mouse models of HD and other brain disorders. When modeling stress, the effects of more than one type of experimental stressor should be investigated due to the heterogeneity and complexity of stress responses. With stress in particular, but ideally in all studies, both sexes should be used and the randomized group sizes need to be sufficiently powered to detect any sex effects. Opportunities for clinical translation will be guided by the 'environmental construct validity' of the preclinical data, including the culmination of complementary protocols across multiple animal models. Environmental interventions in mouse models of HD provide illustrative examples of how valid preclinical studies can lead to conclusions relevant to clinical populations.

Hippocampal learning, memory, and neurogenesis: Effects of sex and estrogens across the lifespan in adults

This article is part of a Special Issue "Estradiol and Cognition". There are sex differences in hippocampus-dependent cognition and neurogenesis suggesting that sex hormones are involved. Estrogens modulate certain forms of spatial and contextual memory and neurogenesis in the adult female rodent, and to a lesser extent male, hippocampus. This review focuses on the effects of sex and estrogens on hippocampal learning, memory, and neurogenesis in the young and aged adult rodent. We discuss how factors such as the type of estrogen, duration and dose of treatment, timing of treatment, and type of memory influence the effects of estrogens on cognition and neurogenesis. We also address how reproductive experience (pregnancy and mothering) and aging interact with estrogens to modulate hippocampal cognition and neurogenesis in females. Given the evidence that adult hippocampal neurogenesis plays a role in long-term spatial memory and pattern separation, we also discuss the functional implications of regulating neurogenesis in the hippocampus.

Strain and sex differences in puberty onset and the effects of THC administration on weight gain and brain volumes

The use of recreational marijuana is widespread and frequently begins and persists through adolescence. Some research has shown negative consequences of adolescent marijuana use, but this is not seen across studies, and certain factors, like genetic background and sex, may influence the results. It is critical to identify which characteristics predispose an individual to be susceptible to the negative consequences of chronic exposure to marijuana in adolescence on brain health and behavior. To this end, using males and females of two strains of rats, Long-Evans hooded (LER) and Wistar (WR) rats, we explored whether these anatomically and behaviorally dimorphic strains demonstrated differences in puberty onset and strain-specific effects of adolescent exposure to Δ9-tetrahydrocannabinol (THC), the main psychoactive component of marijuana. Daily 5 mg/kg treatment began on the day of puberty onset and continued for 14 days. Of particular interest were metrics of growth and volumetric estimates of brain areas involved in cognition that contain high densities of cannabinoid receptors, including the hippocampus and its subregions, the amygdala, and the frontal cortex. Brain volumetrics were analyzed immediately following the treatment period. LER and WR females started puberty at different ages, but no strain differences were observed in brain volumes. THC decreased weight gain throughout the treatment period for all groups. Only the hippocampus and some of its subregions were affected by THC, and increased volumes with THC administration was observed exclusively in females, regardless of strain. Long-term treatment of THC did not affect all individuals equally, and females displayed evidence of increased sensitivity to the effects of THC, and by extension, marijuana. Identifying differences in adolescent physiology of WR and LER rats could help determine the cause for strain and sex differences in brain and behavior of adults and help to refine the use of animal models in marijuana research.

Stress time-dependently influences the acquisition and retrieval of unrelated information by producing a memory of its own

Stress induces several temporally guided “waves” of psychobiological responses that differentially influence learning and memory. One way to understand how the temporal dynamics of stress influence these cognitive processes is to consider stress, itself, as a learning experience that influences additional learning and memory. Indeed, research has shown that stress results in electrophysiological and biochemical activity that is remarkably similar to the activity observed as a result of learning. In this review, we will present the idea that when a stressful episode immediately precedes or follows learning, such learning is enhanced because the learned information becomes a part of the stress context and is tagged by the emotional memory being formed. In contrast, when a stressful episode is temporally separated from learning or is experienced prior to retrieval, such learning or memory is impaired because the learning or memory is experienced outside the context of the stress episode or subsequent to a saturation of synaptic plasticity, which renders the retrieval of information improbable. The temporal dynamics of emotional memory formation, along with the neurobiological correlates of the stress response, are discussed to support these hypotheses.

Differential effects of stress-induced cortisol responses on recollection and familiarity-based recognition memory

Stress-induced changes in cortisol can impact memory in various ways. However, the precise relationship between cortisol and recognition memory is still poorly understood. For instance, there is reason to believe that stress could differentially affect recollection-based memory, which depends on the hippocampus, and familiarity-based recognition, which can be supported by neocortical areas alone. Accordingly, in the current study we examined the effects of stress-related changes in cortisol on the processes underlying recognition memory. Stress was induced with a cold-pressor test after incidental encoding of emotional and neutral pictures, and recollection and familiarity-based recognition memory were measured one day later. The relationship between stress-induced cortisol responses and recollection was non-monotonic, such that subjects with moderate stress-related increases in cortisol had the highest levels of recollection. In contrast, stress-related cortisol responses were linearly related to increases in familiarity. In addition, measures of cortisol taken at the onset of the experiment showed that individuals with higher levels of pre-learning cortisol had lower levels of both recollection and familiarity. The results are consistent with the proposition that hippocampal-dependent memory processes such as recollection function optimally under moderate levels of stress, whereas more cortically-based processes such as familiarity are enhanced even with higher levels of stress. These results indicate that whether post-encoding stress improves or disrupts recognition memory depends on the specific memory process examined as well as the magnitude of the stress-induced cortisol response.

Neurochemical and behavioral effects of chronic unpredictable stress

Chronic stress can influence behaviors associated with medial prefrontal cortex (mPFC) function, such as cognition and emotion regulation. Dopamine in the mPFC is responsive to stress and modulates its behavioral effects. The current study tested whether exposure to 10 days of chronic unpredictable stress (CUS) altered the effects of acute elevation stress on dopamine release in the mPFC and on spatial recognition memory. Male rats previously exposed to CUS or nonstressed controls were tested behaviorally, underwent microdialysis to assess mPFC dopamine levels or underwent blood sampling for corticosterone analysis. Dopamine in the mPFC significantly increased in both groups during acute elevation stress compared with baseline levels, but the level was attenuated in CUS rats compared with controls. Control rats exposed to elevation stress immediately before the T-maze test showed impaired performance, whereas CUS rats did not. No group differences were observed in general motor activity or plasma corticosterone levels following elevation stress. The present results indicate that prior exposure to this CUS procedure reduced dopamine release in the mPFC during acute elevation stress and prevented the impairment of performance on a spatial recognition test following an acute stressor. These findings may contribute to an understanding of the complex behavioral consequences of stress.