Food deprivation modulates chronic stress effects on object recognition in male rats: role of monoamines and amino acids

Androgens Enhance Recognition Memory and Dendritic Spine Density in the Hippocampus and Prefrontal Cortex of Ovariectomized Female Rats

Estrogen replacement has been repeatedly shown to enhance memory and increase dendritic spine density in the hippocampus and prefrontal cortex of ovariectomized (OVX) female rats. Given the potential deleterious effects of chronic estrogen administration, the present study assessed cognitive function using recognition memory tasks and measured dendritic spine density in the CA1 region of the hippocampus and medial prefrontal cortex after subchronic androgen replacement to adult OVX female rats. All androgens enhanced recognition memory in OVX rats, but object placement (OP) and object recognition (OR) results differed. Only testosterone enhanced OR. Testosterone had no effect on OP while dehydroepiandrosterone (DHEA), dihydrotestosterone (DHT) and androstenedione (AD) enhanced OP. Dendritic spine density was increased by both TP and DHEA in both brain areas (DHT and AD were not tested). Lastly, we used the aromatase inhibitor, letrozole, to discriminate between potential androgenic and estrogenic effects of androgens on behavior. Letrozole alone did not alter recognition memory in OVX rats and did not block the effects of either TP or DHEA on recognition memory suggesting that effects were mediated via androgenic mechanisms. The present results expand previous information on gonadal hormone actions and show that, in addition to estrogens, androgens also improve memory and increase spine density in brains of OVX female rats. While requiring further investigation, these observations provide a basis for therapeutic interventions in the treatment of menopausal, age or disease related memory loss.

Transient impact of chronic social stress on effort-based reward motivation in non-food restricted mice: Involvement of corticosterone

Chronic stress has been connected to a reduced effort and motivational deficits. To study effort-based motivation in rodents, operant conditioning is often employed. However, caloric restriction is typically imposed simultaneously. Since caloric restriction is a stressor in its own right, this procedure interferes with data interpretation. Here, we investigate whether chronic social defeat stress (CSD), lasting 10 consecutive days, would alter effort-based reward motivation in mice trained under ad libitum food conditions. Utilizing operant FED3 boxes in home cages, mice were trained within eight days to nose poke for palatable food. After training completion, operant memory was retained for at least 16 days, and mice demonstrated sustained effort, as assessed with a progressive ratio schedule, to obtain reward pellets. Directly after CSD exposure (10th day), mice exhibited reduced effort for palatable food rewards, but also displayed reduced nose poking in general. The effects of CSD on effort were short-lived, with no lasting impact on effort-based reward motivation one week post-stress. As corticosterone (CORT) levels were increased at day 10 of CSD, but not at day 17, we hypothesized that CORT might mediate the acute effects of CSD on effort-based reward motivation. Indeed, CORT administration [100 μg/ml], supplied via the drinking water, mirrored the CSD-induced CORT spike and temporarily reduced reward motivation. Our findings emphasize that CSD does not result in long-term deficits in reward motivation, suggesting a resilient adaptive response in mice under unrestricted feeding conditions. This study underscores the necessity of considering temporal dynamics of stress impacts and highlights the modulating effects of CORT. These insights contribute to a deeper understanding of the resilience mechanisms in motivational impairments and pave the way for further research into factors facilitating this resilience.

Spared nerve injury decreases motivation in long-access homecage-based operant tasks in mice

ABSTRACT

Neuropathic pain causes both sensory and emotional maladaptation. Preclinical animal studies of neuropathic pain-induced negative affect could result in novel insights into the mechanisms of chronic pain. Modeling pain-induced negative affect, however, is variable across research groups and conditions. The same injury may or may not produce robust negative affective behavioral responses across different species, strains, and laboratories. Here, we sought to identify negative affective consequences of the spared nerve injury model on C57BL/6J male and female mice. We found no significant effect of spared nerve injury across a variety of approach-avoidance conflict, hedonic choice, and coping strategy assays. We hypothesized these inconsistencies may stem in part from the short test duration of these assays. To test this hypothesis, we used the homecage-based Feeding Experimentation Device version 3 to conduct 12-hour, overnight progressive ratio testing to determine whether mice with chronic spared nerve injury had decreased motivation to earn palatable food rewards. Our data demonstrate that despite equivalent task learning, spared nerve injury mice are less motivated to work for a sugar pellet than sham controls. Furthermore, when we normalized behavioral responses across all the behavioral assays we tested, we found that a combined normalized behavioral score is predictive of injury state and significantly correlates with mechanical thresholds. Together, these results suggest that homecage-based operant behaviors provide a useful platform for modeling nerve injury-induced negative affect and that valuable pain-related information can arise from agglomerative data analyses across behavioral assays-even when individual inferential statistics do not demonstrate significant mean differences.

Nrf2 activation rescues stress-induced depression-like behaviour and inflammatory responses in male but not female rats

BACKGROUND

Major depressive disorder (MDD) is a recurring affective disorder that is two times more prevalent in females than males. Evidence supports immune system dysfunction as a major contributing factor to MDD, notably in a sexually dimorphic manner. Nuclear factor erythroid 2-related factor 2 (Nrf2), a regulator of antioxidant signalling during inflammation, is dysregulated in many chronic inflammatory disorders; however, its role in depression and the associated sex differences have yet to be explored. Here, we investigated the sex-specific antidepressant and immunomodulatory effects of the potent Nrf2 activator dimethyl fumarate (DMF), as well as the associated gene expression profiles.

METHODS

Male and female rats were treated with vehicle or DMF (25 mg/kg) whilst subjected to 8 weeks of chronic unpredictable stress. The effect of DMF treatment on stress-induced depression- and anxiety-like behaviours, as well as deficits in recognition and spatial learning and memory were then assessed. Sex differences in hippocampal (HIP) microglial activation and gene expression response were also evaluated.

RESULTS

DMF treatment during stress exposure had antidepressant effects in male but not female rats, with no anxiolytic effects in either sex. Recognition learning and memory and spatial learning and memory were impaired in chronically stressed males and females, respectively, and DMF treatment rescued these deficits. DMF treatment also prevented stress-induced HIP microglial activation in males. Conversely, females displayed no HIP microglial activation associated with stress exposure. Last, chronic stress elicited sex-specific alterations in HIP gene expression, many of which were normalized in animals treated with DMF. Of note, most of the differentially expressed genes in males normalized by DMF were related to antioxidant, inflammatory or immune responses.

CONCLUSIONS

Collectively, these findings support a greater role of immune processes in males than females in a rodent model of depression. This suggests that pharmacotherapies that target Nrf2 have the potential to be an effective sex-specific treatment for depression.

Neurocognitive effects of stress: a metaparadigm perspective

Stressful experiences, both physical and psychological, that are overwhelming (i.e., inescapable and unpredictable), can measurably affect subsequent neuronal properties and cognitive functioning of the hippocampus. At the cellular level, stress has been shown to alter hippocampal synaptic plasticity, spike and local field potential activity, dendritic morphology, neurogenesis, and neurodegeneration. At the behavioral level, stress has been found to impair learning and memory for declarative (or explicit) tasks that are based on cognition, such as verbal recall memory in humans and spatial memory in rodents, while facilitating those that are based on emotion, such as differential fear conditioning in humans and contextual fear conditioning in rodents. These vertically related alterations in the hippocampus, procedurally observed after subjects have undergone stress, are generally believed to be mediated by recurrently elevated circulating hypothalamic-pituitary-adrenal (HPA) axis effector hormones, glucocorticoids, directly acting on hippocampal neurons densely populated with corticosteroid receptors. The main purposes of this review are to (i) provide a synopsis of the neurocognitive effects of stress in a historical context that led to the contemporary HPA axis dogma of basic and translational stress research, (ii) critically reappraise the necessity and sufficiency of the glucocorticoid hypothesis of stress, and (iii) suggest an alternative metaparadigm approach to monitor and manipulate the progression of stress effects at the neural coding level. Real-time analyses can reveal neural activity markers of stress in the hippocampus that can be used to extrapolate neurocognitive effects across a range of stress paradigms (i.e., resolve scaling and dichotomous memory effects issues) and understand individual differences, thereby providing a novel neurophysiological scaffold for advancing future stress research.

Administration of growth hormone ameliorates adverse effects of total sleep deprivation

Total sleep deprivation (TSD) causes several harmful changes including anxiety, inflammation, and increased expression of extracellular signal-regulated kinase (ERK) and tropomyosin receptor kinase B (TrkB) genes in the hippocampus. The current study was conducted to explain the possible effects of exogenous GH against the above parameters caused by TSD and the possible mechanisms involved. Male Wistar rats were divided into 1) control, 2) TSD and 3) TSD + GH groups. To induce TSD, the rats received a mild repetitive electric shock (2 mA, 3 s) to their paws every 10 min for 21 days. Rats in the third group received GH (1 ml/kg, sc) for 21 days as treatment for TSD. The motor coordination, locomotion, the level of IL-6, and expression of ERK and TrkB genes in hippocampal tissue were measured after TSD. The motor coordination (p < 0.001) and locomotion indices (p < 0.001) were impaired significantly by TSD. The concentrations of serum corticotropin-releasing hormone (CRH) (p < 0.001) and hippocampal interleukin-6 (IL-6) (p < 0.001) increased. However, there was a significant decrease in the interleukin-4 (IL-4) concentration and expression of ERK (p < 0.001) and TrkB (p < 0.001) genes in the hippocampus of rats with TSD. Treatment of TSD rats with GH improved motor balance (p < 0.001) and locomotion (p < 0.001), decreased serum CRH (p < 0.001), IL-6 (p < 0.01) but increased the IL-4 and expression of ERK (p < 0.001) and TrkB (p < 0.001) genes in the hippocampus. Results show that GH plays a key role in modulating the stress hormone, inflammation, and the expression of ERK and TrkB genes in the hippocampus following stress during TSD.

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.

Sex-Specific Brain Transcriptional Signatures in Human MDD and Their Correlates in Mouse Models of Depression

Major depressive disorder (MDD) is amongst the most devastating psychiatric conditions affecting several millions of people worldwide every year. Despite the importance of this disease and its impact on modern societies, still very little is known about the etiological mechanisms. Treatment strategies have stagnated over the last decades and very little progress has been made to improve the efficiency of current therapeutic approaches. In order to better understand the disease, it is necessary for researchers to use appropriate animal models that reproduce specific aspects of the complex clinical manifestations at the behavioral and molecular levels. Here, we review the current literature describing the use of mouse models to reproduce specific aspects of MDD and anxiety in males and females. We first describe some of the most commonly used mouse models and their capacity to display unique but also shared features relevant to MDD. We then transition toward an integral description, combined with genome-wide transcriptional strategies. The use of these models reveals crucial insights into the molecular programs underlying the expression of stress susceptibility and resilience in a sex-specific fashion. These studies performed on human and mouse tissues establish correlates into the mechanisms mediating the impact of stress and the extent to which different mouse models of chronic stress recapitulate the molecular changes observed in depressed humans. The focus of this review is specifically to highlight the sex differences revealed from different stress paradigms and transcriptional analyses both in human and animal models.

Manifestations of domination: Assessments of social dominance in rodents

Social hierarchies are ubiquitous features of virtually all animal groups. The varying social ranks of members within these groups have profound effects on both physical and emotional health, with lower-ranked individuals typically being the most adversely affected by their respective ranks. Thus, reliable measures of social dominance in preclinical rodent models are necessary to better understand the effects of an individual's social rank on other behaviors and physiological processes. In this review, we outline the primary methodologies used to assess social dominance in various rodent species: those that are based on analyses of agonistic behaviors, and those that are based on resource competition. In synthesizing this review, we conclude that assays based on resource competition may be better suited to characterize social dominance in a wider variety of rodent species and strains, and in both males and females. Lastly, albeit expectedly, we demonstrate that similarly to many other areas of preclinical research, studies incorporating female subjects are lacking in comparison to those using males. These findings emphasize the need for an increased number of studies assessing social dominance in females to form a more comprehensive understanding of this behavioral phenomenon.

Effect of Cold Stress on Neurobehavioral and Physiological Parameters in Rats

Objective: Cold stress is an important current issue and implementing control strategies to limit its sometimes harmful effects is crucial. Cold is a common stressor that can occur in our work and our occupational or leisure time activities every day. There are substantial studies on the effects of chronic stress on memory and behavior, although, the cognitive changes and anxiety disorders that can occur after exposure to chronic intermittent cold stress are not completely characterized. Therefore, the present study was undertaken with an aim to investigate the effects of chronic intermittent cold stress on body weight, food intake and working memory, and to elucidate cold stress related anxiety disorders using cognitive and behavioral test batteries. Methods: We generated a cold stress model by exposing rats to chronic intermittent cold stress for 5 consecutive days and in order to test for the potential presence of sex differences, a comparable number of male and female rats were tested in the current study. Then, we measured the body weights, food intake and the adrenal glands weight. Working memory and recognition memory were assessed using the Y maze and the Novel Object Recognition (NOR) tasks. While, sex differences in the effects of chronic stress on behavior were evaluated by the elevated plus maze (EPM), open field maze (OF), and Marble burying (MB) tests. Results: We found that 2 h exposure to cold (4°C) resulted in an increase in the relative weight of the adrenal glands in male rats. Given the same chronic stress 5 days of cold exposure (2 h per day), increased weight gain in male rats, while females showed decreased food intake and no change in body weight. Both sexes successfully performed the Y maze and object recognition (OR) tasks, indicating intact spatial working memory performance and object recognition abilities in both male and female rats. In addition, we have shown that stress caused an increase in the level of anxiety in male rats. In contrast, the behavior of the female rats was not affected by cold exposure. Conclusion: Overall, the current results provide preliminary evidence that chronic intermittent cold stress model may not be an efficient stressor to female rats. Females exhibit resilience to cold exposure that causes an increase in the level of anxiety in male rats, which demonstrates that they are affected differently by stress and the gender is an important consideration in experimental design.

Effects of chemotherapy on operant responding for palatable food in male and female mice

Patients treated with cancer chemotherapeutics frequently report chemotherapy-induced peripheral neuropathy (CIPN), changes in mood (depression and anxiety) and functional impairments. Rodent models of CIPN elicit limited alterations in functional behaviors, which pose challenges in developing preclinical models of chemotherapy-induced behavioral depression. The study examined the consequences of chemotherapy-induced mechanical hypersensitivity (paclitaxel: 32 or 64 mg/kg, cumulative; oxaliplatin: 30 mg/kg, cumulative) on behavioral depression, as measured with operant responding for palatable food during periods of food restriction and ad libitum chow, consumption of noncontingently available palatable food in the presence of ad libitum chow, and voluntary wheel running. The study employed two inbred mouse strains (C57BL/6J and Balb/cJ) and examined potential sex differences. All chemotherapeutic regimens caused profound mechanical hypersensitivity for the duration of the observation periods (up to 7 months), but no treatments changed voluntary wheel running or consumption of noncontingent palatable food. The high dose of paclitaxel temporarily reduced operant responding for palatable food in male C57BL/6J mice undergoing food restriction or maintained on ad libitum chow. However, paclitaxel failed to decrease operant responding for palatable food in free-feeding female C57BL/6J mice or Balb/cJ mice of either sex. Moreover, oxaliplatin did not significantly alter operant responding for palatable food in male or female C57BL/6J mice maintained on ad libitum chow. These findings demonstrate a dissociation between chemotherapy-induced mechanical hypersensitivity and behavioral depression. The transient effects of paclitaxel on operant responding in male C57BL/6J mice may represent a fleeting behavioral correlate of chemotherapy-associated pain-like behaviors.

Effects of caloric restriction on monoaminergic neurotransmission, peripheral hormones, and olfactory memory in aged rats

Aging is associated with a reduced ability to identify and discriminate scents, and olfactory dysfunction has been linked to preclinical stages of neurodegenerative diseases in humans. Moreover, emerging evidence suggests that smell-driven behaviors are regulated by hormones like insulin or leptin, and by metabolic parameters like glucose, which in turn may influence monoaminergic neurotransmission in brain areas related to cognition. Several studies have suggested that dietary interventions like caloric restriction (CR) can mitigate the age-induced decline in memory by modifying metabolic parameters and brain monoaminergic levels. The present study explored the effects of CR on age-dependent olfactory memory deficits, as well as their relationship with peripheral leptin, insulin and glucose levels, and brain monoamines. To this end, aged rats (24-months-old) fed on a CR diet or with ad libitum access to food, and adult rats (3-4 months), were trained in an odor discrimination task (ODT). The peripheral plasma levels of insulin, leptin, and glucose, and of monoamines and metabolites/precursors in brain areas related to olfactory learning and memory processes, such as the striatum and frontal cortex (FC), were determined. The data obtained indicated that CR attenuated the age-dependent decline in olfactory sensitivity in old animals fed ad libitum, which was correlated with the performance in ODT retention trial, as well as with leptin plasma levels. CR enhanced dopamine levels in the striatum, while it attenuated the age-related decline in serotonin levels in the striatum and FC. Such findings support a positive effect of CR on age-dependent olfactory sensitivity decline and dysfunctions in brain monoamine levels.

Recognition of novelty in leopard geckos (Eublepharis macularius) and tiger salamanders (Ambystoma tigrinum)

Novelty recognition helps organisms identify changes over time. Studies to date have usually involved mammals, particularly rodents. We explored leopard geckos' (Eublepharis macularius; Experiment 1) and tiger salamanders' (Ambystoma tigrinum, Experiment 2) sensitivity to spatial and object novelty. We used an exploratory paradigm adapted from rodents where time spent near objects in an open-field box was compared. Subjects first habituated to three objects. To evaluate spatial novelty recognition, one object was moved to a new location. Subjects again habituated to the objects' locations. To evaluate object novelty recognition, one object that had not been moved earlier was replaced with an unfamiliar object. Results indicated when one object was moved to a new location, geckos and salamanders spent more time near that spatially-displaced object. Additionally, when a familiar object was replaced with a new object, geckos and salamanders spent more time near the substituted object. These results suggest geckos and salamanders recognized changes in objects' identities and locations. Geckos and salamanders acted differentially depending on familiarity in both spatial and object domains. These results support attempts to include lesser-studied species in our efforts to characterize cognition.

Remediation of chronic immobilization stress-induced negative affective behaviors and altered metabolism of monoamines in the prefrontal cortex by inactivation of basolateral amygdala

Exposure to chronic stress precipitates depression and anxiety. Stress-induced responses are differentially regulated by the prefrontal cortex (PFC) and basolateral amygdala (BLA). For instance, repeated stress leads to hypertrophy of BLA, resulting in the emergence of affective symptoms. Chronic stress-induced changes in the metabolism of monoamines are central in the manifestation of affective symptoms. Interestingly, BLA via its reciprocal connections modulates prefrontal cortical monoaminergic responses to acute stress. However, the effects of BLA inactivation on chronic stress-induced affective behaviors and monoaminergic changes in the PFC are relatively unknown. Thus, we hypothesized that inactivation of BLA might prevent chronic immobilization stress (CIS)-induced depressive-, anxiety-like behaviors, and associated monoaminergic alterations in the prelimbic (PrL) and anterior cingulate cortex (ACC) subregions of PFC. We used two different BLA silencing strategies, namely ibotenic acid lesion and reversible temporary inactivation using lidocaine. We found that CIS precipitates depressive- and anxiety-like behaviors. Further, CIS-induced negative affective behaviors were associated with decreased levels of 5-HT, DA, and NE, and increased 5-HIAA/5-HT, DOPAC + HVA/DA, and MHPG/NE ratio in the PrL and ACC, suggesting enhanced metabolism. Interestingly, BLA lesion prior to CIS blocked the emergence of depressive- and anxiety-like behaviors. Moreover, the lesion of BLA prior to CIS was sufficient to prevent alterations in levels of monoamines and their metabolites in the PrL and ACC. Thereafter, we evaluated whether the effects of BLA lesion could be mirrored by temporary inactivation of BLA, specifically during stress. Remarkably, temporary inactivation of BLA during stress recapitulated the effects of lesion. Our results have implications for understanding the role of BLA in chronic stress-induced metabolic alterations in prefrontal cortical monoaminergic systems, and associated mood and anxiety disorders. The current study supports the hypothesis that combating amygdalar hyperactivity might be a viable strategy for the management of stress and associated affective disorders.

Rigor and reproducibility in rodent behavioral research

Behavioral neuroscience research incorporates the identical high level of meticulous methodologies and exacting attention to detail as all other scientific disciplines. To achieve maximal rigor and reproducibility of findings, well-trained investigators employ a variety of established best practices. Here we explicate some of the requirements for rigorous experimental design and accurate data analysis in conducting mouse and rat behavioral tests. Novel object recognition is used as an example of a cognitive assay which has been conducted successfully with a range of methods, all based on common principles of appropriate procedures, controls, and statistics. Directors of Rodent Core facilities within Intellectual and Developmental Disabilities Research Centers contribute key aspects of their own novel object recognition protocols, offering insights into essential similarities and less-critical differences. Literature cited in this review article will lead the interested reader to source papers that provide step-by-step protocols which illustrate optimized methods for many standard rodent behavioral assays. Adhering to best practices in behavioral neuroscience will enhance the value of animal models for the multiple goals of understanding biological mechanisms, evaluating consequences of genetic mutations, and discovering efficacious therapeutics.

Chronic unpredictable mild stress-induced behavioral changes are coupled with dopaminergic hyperfunction and serotonergic hypofunction in mouse models of depression

Accumulating evidence shows that stressful events evoke molecular alterations in the brain, considered a pathology in major depressive disorder (MDD). However, the abnormalities of neurotransmissions as well as intracellular signaling pathways affected by chronic stress in brain have not been fully explored. We investigated the effect of chronic unpredictable mild stress (CUMS) on the emotional behaviors, dopaminergic and serotoninergic function, and intracellular signaling in the nucleus accumbens, hippocampus and prefrontal cortex. Male C57BL/6J mice were exposed to CUMS for 4 weeks. CUMS was shown to induce hyperactivity in a novel environment, decrease interaction time in the social interaction test, prolong feeding latency in the novelty suppressed feeding test and enhance immobility in the forced swimming test. The levels of dopamine, its metabolites and turnover, and protein level of tyrosine hydroxylase (TH) were increased by CUMS in the nucleus accumbens (NAc). The level of serotonin and protein levels of tryptophan hydroxylase (TPH) and TH were decreased by CUMS in the hippocampus (HPC) and prefrontal cortex (PFC). Accompanying the increase in dopaminergic function, phosphorylation levels of extracellular signal-regulated kinases (ERK), protein kinase B (Akt) and cAMP response element-binding protein (CREB) were increased by CUMS in the NAc. Administration of fluoxetine (selective serotonin re-uptake inhibitor: 20 mg/kg i.p.) and aripiprazole (dopamine D₂ receptor partial agonist: 0.1 mg/kg i.p.) during CUMS, prevented behavioral changes and increase of dopamine level in the NAc. These data suggest that CUMS-induced depression-like behaviors are coupled with dopaminergic hyperfunction in the NAc and serotonergic hypofunction in the HPC and PFC.

Motivation to eat and not to eat - The psycho-biological conflict in anorexia nervosa

Anorexia nervosa is a severe psychiatric illness with high mortality. Brain imaging research has indicated altered reward circuits in the disorder. Here we propose a disease model for anorexia nervosa, supported by recent studies, that integrates psychological and biological factors. In that model, we propose that there is a conflict between the conscious motivation to restrict food, and a body-homeostasis driven motivation to approach food in response to weight loss. These opposing motivations trigger anxiety, which maintains the vicious cycle of ongoing energy restriction and weight loss.

Association of Brain Reward Learning Response With Harm Avoidance, Weight Gain, and Hypothalamic Effective Connectivity in Adolescent Anorexia Nervosa

IMPORTANCE

Anorexia nervosa (AN) is associated with adolescent onset, severe low body weight, and high mortality as well as high harm avoidance. The brain reward system could have an important role in the perplexing drive for thinness and food avoidance in AN.

OBJECTIVE

To test whether brain reward learning response to taste in adolescent AN is altered and associated with treatment response, striatal-hypothalamic connectivity, and elevated harm avoidance.

DESIGN, SETTING, AND PARTICIPANTS

In this cross-sectional multimodal brain imaging study, adolescents and young adults with AN were matched with healthy controls at a university brain imaging facility and eating disorder treatment program. During a sucrose taste classical conditioning paradigm, violations of learned associations between conditioned visual and unconditioned taste stimuli evoked the dopamine-related prediction error (PE). Dynamic effective connectivity during sweet taste receipt was studied to investigate hierarchical brain activation across the brain network that regulates eating. The study was conducted from July 2012 to May 2017, and data were analyzed from June 2017 to December 2017.

MAIN OUTCOMES AND MEASURES

Prediction error brain reward response across the insula, caudate, and orbitofrontal cortex; dynamic effective connectivity between hypothalamus and ventral striatum; and treatment weight gain, harm avoidance scores, and salivary cortisol levels and their correlations with PE brain response.

RESULTS

Of 56 female participants with AN included in the study, the mean (SD) age was 16.6 (2.5) years, and the mean (SD) body mass index (BMI; calculated as weight in kilograms divided by height in meters squared) was 15.9 (0.9); of 52 matched female controls, the mean (SD) age was 16.0 (2.8) years, and the mean (SD) BMI was 20.9 (2.1). Prediction error response was elevated in participants with AN in the caudate head, nucleus accumbens, and insula (multivariate analysis of covariance: Wilks λ, 0.707; P = .02; partial η2 = 0.296), which correlated negatively with sucrose taste pleasantness. Bilateral AN orbitofrontal gyrus rectus PE response was positively correlated with harm avoidance (right ρ, 0.317; 95% CI, 0.091 to 0.539; P < .02; left ρ, 0.336; 95% CI, 0.112 to 0.550; P < .01) but negatively correlated with treatment BMI change (right ρ, -0.282; 95% CI, -0.534 to -0.014; P < .04; left ρ, -0.268; 95% CI, -0.509 to -0.018; P < .045). Participants with AN showed effective connectivity from ventral striatum to hypothalamus, and connectivity strength was positively correlated with insula and orbitofrontal PE response. Right frontal cortex PE response was associated with cortisol, which correlated with body dissatisfaction.

CONCLUSIONS AND RELEVANCE

These results further support elevated PE signal in AN and suggest a link between PE and elevated harm avoidance, brain connectivity, and weight gain in AN. Prediction error may have a central role in adolescent AN in driving anxiety and ventral striatal-hypothalamus circuit-controlled food avoidance.

Association of 24 h maternal deprivation with a saline injection in the neonatal period alters adult stress response and brain monoamines in a sex-dependent fashion

Maternal deprivation (MD) disinhibits the adrenal glands, rendering them responsive to various stressors, including saline injection, and this increased corticosterone (CORT) response can last for as long as 2 h. In the present study, we tested the hypothesis that association of MD on day 11 with a saline injection would alter emotional behavior, CORT response, and brain monoamine levels, in male and female adult rats. Rats were submitted to the novelty suppressed feeding (NSF), the sucrose negative contrast test (SNCT), social investigation test (SIT), and the elevated plus maze (EPM). One quarter of each group was not tested (providing basal values of CORT and brain monoamines) and the remainder was decapitated 15, 45, or 75 min after the EPM, to assess CORT reactivity. Monoamine levels were determined in the hypothalamus (HPT), frontal cortex (FC), amygdala (AMY), ventral, and dorsal hippocampus (vHPC, dHPC, respectively). MD reduced food intake, in the home-cage, and latency to eat in the NSF in both sexes; females explored less the target animal in the SIT and explored more the open arms of the EPM than males; the CORT response to the EPM was greater in maternally-deprived males and females than in their control counterparts, and this response was further elevated in maternally-deprived females injected with saline. Regarding monoamine levels, females were less affected, showing isolated effects of the stressors, while in males, MD increased 5-HT levels in the HPT and decreased this monoamine in the FC, MD associated with saline reduced dopamine levels in all brain regions, except the HPT. MD at 11 days did not alter emotional behaviors in adult rats, but had an impact in neurobiological parameters associated with this class of behaviors. The impact of MD associated with saline on dopamine levels suggests that males may be vulnerable to motivation-related disorders.

Differential dendritic remodeling in prelimbic cortex of male and female rats during recovery from chronic stress

Chronic stress produces differential dendritic remodeling of pyramidal neurons in medial prefrontal cortex of male and female rats. In males, this dendritic remodeling is reversible. However, the timeline of recovery, as well as the potential for reversibility in females, is unknown. Here, we examined dendritic recovery of pyramidal neurons in layer II-II of prelimbic cortex in male and female rats following chronic restraint stress (3h/day for 10days). Dendritic morphology and spine density were analyzed immediately following the cessation of stress, or following a 7- or 10-day recovery period. Chronic stress produced apical dendritic retraction in males, which was coupled with a decrease in the density of stubby spine on apical dendrites. Further, following a 10-day recovery period, the morphology of neurons from stressed rats resembled that of unstressed rats. Male rats given a 7-day recovery period had apical dendritic outgrowth compared to unstressed rats. Immediately after cessation of stress, females showed only minimal dendritic remodeling. The morphology of neurons in stressed females resembled those of unstressed rats following only 7days of recovery, at which time there was also a significant increase in stubby spine density. Males and females also showed different changes in baseline corticosterone concentrations during recovery. These findings not only indicate that dendritic remodeling in prelimbic cortex following chronic stress is different between males and females, but also suggest chronic stress induces differential hypothalamic-pituitary-adrenal axis dysregulation in males and females. These differences may have important implications for responses to subsequent stressors.

Sex differences in chronic stress effects on cognition in rodents

Chronic stress causes deleterious changes in physiological function in systems ranging from neural cells in culture to laboratory rodents, sub-human primates and humans. It is notable, however, that the vast majority of research in this area has been conducted in males. In this review, we provide information about chronic stress effects on cognition in female rodents and contrast it with responses in male rodents. In general, females show cognitive resilience to chronic stressors which impair male cognitive function using spatial tasks including the radial arm maze, radial arm water maze, Morris water maze, Y-maze and object placement. Moreover, stress often enhances female performance in some of these cognitive tasks. Memory in females is not affected by stress in non-spatial memory tasks like recognition memory and temporal order recognition memory while males show impaired memory following stress. We discuss possible bases for these sex-dependent differences including the use of different strategies by the sexes to solve cognitive tasks. Whether the sex differences result from changes in non-mnemonic factors is also considered. Sex-dependent differences in alcohol and drug influences on stress responses are also described. Finally, the role of neurally derived estradiol in driving sex differences and providing resilience to stress in females is shown. The importance of determining the nature and extent of sex differences in stress responses is that such differences may provide vital information for understanding why some stress related diseases have different incidence rates between the sexes and for developing novel therapeutic treatments.

Estradiol: Mediator of memories, spine density and cognitive resilience to stress in female rodents

Estradiol rapidly activates, within minutes, various physiological functions and behaviors including cognition in rodents. This review describes rapid effects of estradiol on hippocampal dependent learning and memory tasks in rodents. Mechanisms underlying the memory enhancements including the activation of signaling molecules and the enhancement of dendritic spinogenesis are briefly reviewed. In addition, the role of estradiol in the cognitive resilience to chronic stress exhibited only in females is discussed including contributions of ovarian as well as intra-hippocampally derived estrogens to this sex difference. Finally, speculations on possible physiologic functions for rapid mnemonic changes mediated by estrogens are made. Overall, the emergence of a novel and powerful mechanism for regulation of cognition by estradiol is described.

Estrogen in prefrontal cortex blocks stress-induced cognitive impairments in female rats

Animal and human studies have found that males and females show distinct stress responses. Recent studies suggest the contribution of estrogen in the brain to this sexual dimorphism. Repeated stress has been found to impair cognitive behaviors via suppressing glutamatergic transmission and glutamate receptor surface expression in pyramidal neurons of prefrontal cortex (PFC) in male rats. On the contrary, female rats exposed to the same stress paradigms show normal synaptic function and PFC-mediated cognition. The level of aromatase, the enzyme for the biosynthesis of estrogen, is significantly higher in the PFC of females than males. The stress-induced glutamatergic deficits and memory impairment are unmasked by blocking estrogen receptors or aromatase in females, suggesting a protective role of estrogen against the detrimental effects of repeated stress.

Maternal separation prior to neonatal hypoxia-ischemia: Impact on emotional aspects of behavior and markers of synaptic plasticity in hippocampus

Exposure to early-life stress is associated with long-term alterations in brain and behavior, and may aggravate the outcome of neurological insults. This study aimed at investigating the possible interaction between maternal separation, a model of early stress, and subsequent neonatal hypoxia-ischemia on emotional behavior and markers of synaptic plasticity in hippocampus. Therefore, rat pups (N=60) were maternally separated for a prolonged (MS 180min) or a brief (MS 15min) period during the first six postnatal days, while a control group was left undisturbed. Hypoxia-ischemia was applied to a subgroup of each rearing condition on postnatal day 7. Emotional behavior was examined at three months of age and included assessments of anxiety (elevated plus maze), depression-like behavior (forced swimming) and spontaneous exploration (open field). Synaptic plasticity was evaluated based on BDNF and synaptophysin expression in CA3 and dentate gyrus hippocampal regions. We found that neonatal hypoxia-ischemia caused increased levels of anxiety, depression-like behavior and locomotor activity (ambulation). Higher anxiety levels were also seen in maternally separated rats (MS180min) compared to non-maternally separated rats, but prolonged maternal separation prior to HI did not potentiate the HI-associated effect. No differences among the three rearing conditions were found regarding depression-like behavior or ambulation. Immunohistochemical evaluation of synaptophysin revealed that both prolonged maternal separation (MS180min) and neonatal hypoxia-ischemia significantly reduced its expression in the CA3 and dentate gyrus. Decreases in synaptophysin expression in these areas were not exacerbated in rats that were maternally separated for a prolonged period prior to HI. Regarding BDNF expression, we found a significant decrease in immunoreactivity only in the hypoxic-ischemic rats that were subjected to the prolonged maternal separation paradigm. The above findings suggest that early-life stress prior to neonatal hypoxia-ischemia leads to significant alterations in synaptic plasticity of the dorsal hippocampus during adulthood, but does not exacerbate HI-related changes in emotional behavior.

Role of hippocampus mitogen-activated protein kinase phosphatase-1 mRNA expression and DNA methylation in the depression of the rats with chronic unpredicted stress

Stressful life events especially the chronic unpredictable stress are the obvious precipitating factors of depression. The biological information transduction in cells plays an important role in the molecular biology mechanism of depression. Mitogen-activated protein kinase phosphatase-1 (MKP-1) regulates the cell physiological activity and involves in the adjustment of neural plasticity, function, and survival. This experiment tried to explore the possible effects of MKP-1 in hippocampus on depression of rats by determining the expression of MKP-1 mRNA and DNA methylation in MKP-1 gene promoter. The animal model was established by chronic unpredictable stress, and evaluated by open-field test and weight changes. All the rats were divided into the sham stimulation, the physiological saline, and the fluoxetine (1.25, 2.50, and 5.00 mg/kg) groups randomly. The expression of MKP-1 mRNA in the hippocampus was measured by RT-PCR and the methylation of MKP-1 promoter DNA was detected by COBRA. The chronic unpredicted stress (1) increased the animal movement scores in open-field test, and fluoxetine could prevent this increasement; (2) increased the body weight, and fluoxetine could not prevent this increasement; and (3) increased MKP-1 mRNA expression in the hippocampus, and fluoxetine could prevent it. However, fluoxetine did not influence the DNA methylation of MKP-1 gene promoter in the hippocampus during the chronic unpredicted stress. MKP-1 in the hippocampus might be involved in the etiology of depression, and DNA methylation of MKP-1 gene promoter in the hippocampus did not related with the depression.

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.

Ventromedial prefrontal cortex stimulation enhances memory and hippocampal neurogenesis in the middle-aged rats

Memory dysfunction is a key symptom of age-related dementia. Although recent studies have suggested positive effects of electrical stimulation for memory enhancement, its potential targets remain largely unknown. In this study, we hypothesized that spatially targeted deep brain stimulation of ventromedial prefrontal cortex enhanced memory functions in a middle-aged rat model. Our results show that acute stimulation enhanced the short-, but not the long-term memory in the novel-object recognition task. Interestingly, after chronic high-frequency stimulation, both the short- and long-term memories were robustly improved in the novel-object recognition test and Morris water-maze spatial task compared to sham. Our results also demonstrated that chronic ventromedial prefrontal cortex high-frequency stimulation upregulated neurogenesis-associated genes along with enhanced hippocampal cell proliferation. Importantly, these memory behaviors were strongly correlated with the hippocampal neurogenesis. Overall, these findings suggest that chronic ventromedial prefrontal cortex high-frequency stimulation may serve as a novel effective therapeutic target for dementia-related disorders.

DOI:http://dx.doi.org/10.7554/eLife.04803.001

Memory loss in older people is a serious and widespread problem that affects up to 50% of those over the age of 85. It is a key symptom of dementia, but despite the growing impact of this disease on society, there are no treatments currently available that can effectively stop or delay the progression of the symptoms.

One therapy that may reduce memory loss is called deep brain stimulation. Electrodes are implanted into the brain and used to stimulate brain cells in particular areas of the brain to alter mental and emotional processes. Deep brain stimulation is already used to treat Parkinson's disease, depression and other conditions that affect how the brain works.

Liu et al. studied the effect of deep brain stimulation on memory in rats. The experiments show that middle-aged rats performed less well in short- and long-term memory tests than young rats. Next, Liu et al. investigated whether deep brain stimulation could improve memory in the middle-aged rats. The electrodes were positioned to stimulate a region near the front of the brain called the ‘ventromedial prefrontal cortex’; this region is important for the formation and recall of memories. Liu et al. then gave the rats a series of memory tasks that tested their recall after 90 minutes (to test their short-term memory), and after 24 hours (to test long-term memory).

The experiments reveal that a brief stimulation of brain cells in this region of the brain improved the rats' short-term memory, but not their long-term memory. However, more sustained stimulation of this region of the brain improved both the short-term and long-term memory of the rats. Furthermore, deep brain stimulation led to the formation of new brain cells in another region of the brain called the hippocampus, which is also involved in memory. The hippocampus had not been in direct contact with the electrodes so the increase in brain cells was due to its connections with the stimulated ventromedial prefrontal cortex.

Liu et al.'s findings suggest that deep brain stimulation of the ventromedial prefrontal cortex has the potential to be developed into a therapy to treat dementia and other conditions that lead to memory loss in humans.

DOI:http://dx.doi.org/10.7554/eLife.04803.002

A 1-night operant learning task without food-restriction differentiates among mouse strains in an automated home-cage environment

Individuals are able to change their behavior based on its consequences, a process involving instrumental learning. Studying instrumental learning in mice can provide new insights in this elementary aspect of cognition. Conventional appetitive operant learning tasks that facilitate the study of this form of learning in mice, as well as more complex operant paradigms, require labor-intensive handling and food deprivation to motivate the animals. Here, we describe a 1-night operant learning protocol that exploits the advantages of automated home-cage testing and circumvents the interfering effects of food restriction. The task builds on behavior that is part of the spontaneous exploratory repertoire during the days before the task. We compared the behavior of C57BL/6J, BALB/cJ and DBA/2J mice and found various differences in behavior during this task, but no differences in learning curves. BALB/cJ mice showed the largest instrumental learning response, providing a superior dynamic range and statistical power to study instrumental learning by using this protocol. Insights gained with this home-cage-based learning protocol without food restriction will be valuable for the development of other, more complex, cognitive tasks in automated home-cages.

Estradiol and cognitive function: past, present and future

A historical perspective on estradiol's enhancement of cognitive function is presented, and research, primarily in animals, but also in humans, is reviewed. Data regarding the mechanisms underlying the enhancements are discussed. Newer studies showing rapid effects of estradiol on consolidation of memory through membrane interactions and activation of inter-cellular signaling pathways are reviewed as well as studies focused on traditional genomic mechanisms. Recent demonstrations of intra-neuronal estradiol synthesis and possible actions as a neurosteroid to promote memory are discussed. This information is applied to the critical issue of the current lack of effective hormonal (or other) treatments for cognitive decline associated with menopause and aging. Finally, the critical period hypothesis for estradiol effects is discussed along with novel strategies for hormone/drug development. Overall, the historical record documents that estradiol positively impacts some aspects of cognitive function, but effective therapeutic interventions using this hormone have yet to be realized.

Chronic scream sound exposure alters memory and monoamine levels in female rat brain

Chronic scream sound alters the cognitive performance of male rats and their brain monoamine levels, these stress-induced alterations are sexually dimorphic. To determine the effects of sound stress on female rats, we examined their serum corticosterone levels and their adrenal, splenic, and thymic weights, their cognitive performance and the levels of monoamine neurotransmitters and their metabolites in the brain. Adult female Sprague-Dawley rats, with and without exposure to scream sound (4h/day for 21 day) were tested for spatial learning and memory using a Morris water maze. Stress decreased serum corticosterone levels, as well as splenic and adrenal weight. It also impaired spatial memory but did not affect the learning ability. Monoamines and metabolites were measured in the prefrontal cortex (PFC), striatum, hypothalamus, and hippocampus. The dopamine (DA) levels in the PFC decreased but the homovanillic acid/DA ratio increased. The decreased DA and the increased 5-hydroxyindoleacetic acid (5-HIAA) levels were observed in the striatum. Only the 5-HIAA level increased in the hypothalamus. In the hippocampus, stress did not affect the levels of monoamines and metabolites. The results suggest that scream sound stress influences most physiologic parameters, memory, and the levels of monoamine neurotransmitter and their metabolites in female rats.

Recognition memory tasks in neuroendocrine research

The recognition memory tasks, novel object and novel object location, have been beneficial to neuroendocrine research concerning the effects of gonadal and adrenal hormones on cognitive function. This review discusses the advantages of these tasks in comparison with other learning and memory tasks. Experiments conducted across a number of laboratories show that gonadal hormones, both estradiol and testosterone, promote memory while the adrenal hormone, corticosterone, impairs memory. The effects of these steroid hormones on spine density in the prefrontal cortex and hippocampus are also briefly presented. Overall, results show that these steroid hormones are potent modulators of memory consolidation in rodent models.

Estrogen protects against the detrimental effects of repeated stress on glutamatergic transmission and cognition

Converging evidence suggests that females and males show different responses to stress; however, little is known about the mechanism underlying the sexually dimorphic effects of stress. In this study, we found that young female rats exposed to 1 week of repeated restraint stress show no negative effects on temporal order recognition memory (TORM), a cognitive process controlled by the prefrontal cortex (PFC), which was contrary to the impairment in TORM observed in stressed males. Concomitantly, normal glutamatergic transmission and glutamate receptor surface expression in PFC pyramidal neurons were found in repeatedly stressed females, in contrast to the significant reduction seen in stressed males. The detrimental effects of repeated stress on TORM and glutamate receptors were unmasked in stressed females when estrogen receptors were inhibited or knocked down in PFC, and were prevented in stressed males with the administration of estradiol. Blocking aromatase, the enzyme for the biosynthesis of estrogen, revealed the stress-induced glutamatergic deficits and memory impairment in females, and the level of aromatase was significantly higher in the PFC of females than in males. These results suggest that estrogen protects against the detrimental effects of repeated stress on glutamatergic transmission and PFC-dependent cognition, which may underlie the stress resilience of females.

Effect of xiaoyaosan decoction on learning and memory deficit in rats induced by chronic immobilization stress

Xiaoyaosan (XYS) decoction is a famous prescription which can protect nervous system from stress and treat liver stagnation and spleen deficiency syndrome (LSSDS). In this experiment, we observed the effect of XYS decoction on chronic immobilization stress (CIS) induced learning and memory deficit in rats from behaviors and changes of proteins in hippocampus. We used XYS decoction to treat CIS induced learning and memory deficit in rats with rolipram as positive control, used change of body weight and behavioral tests to determine whether the rats have LSSDS and have learning and memory deficit or not. We used Western blotting to determine the content of postsynaptic density protein 95 (PSD-95) and synaptophysin (SYP) in hippocampus. Results showed that XYS could improve the situation of slow weight gain induced by CIS, improve the ability of learning and memory, reverse the symptom of liver stagnation and spleen deficiency syndrome (LSSDS) in rats, and increase the levels of PSD-95 and SYP on the hippocampal nerve synapses. These findings suggested that XYS decoction may be helpful in reversing CIS induced learning and memory deficit by increasing the levels of PSD-95 and SYP on the hippocampal nerve synapses and improving synaptic plasticity.

Stressors impair odor recognition memory via an olfactory bulb-dependent noradrenergic mechanism

Non-associative habituation and odor recognition tasks have been widely used to probe questions of social recognition, odor memory duration, and odor memory specificity. Among others, these paradigms have provided valuable insight into how neuromodulation, and specifically norepinephrine/noradrenaline (NE) influences odor memory. In general, NE levels are modulated by arousal, stress, and behavioral state, but there is sparse evidence of a direct relationship between NE and odor memory in adult rodents. The present study uses simple mild psychological stressors (bright light and sound) to modulate NE levels physiologically in order to probe stressors NE-dependent effect on odor recognition memory. In rats with bilateral bulbar cannulations, we show that these stressors modulate olfactory memory and that this effect is at least partially mediated by the olfactory bulb. Specifically, we show that the presence of stressors during the acquisition of odor memory suppresses memory for an odor when tested 30 min after familiarization to that odor. This suppression is blocked by infusing NE antagonists into the olfactory bulb prior to odor acquisition. Additionally, we find that infusion of bulbar NE is sufficient to suppress odor memory in a manner mimicking that of our stressors. These effects are unlikely to be solely mediated by locomotor/exploratory changes produced by stressors, although these stressors influence certain behaviors not directly related to odor investigation. This study provides important information about how behaviorally relevant changes in NE can influence top-down sensory processing and odor memory.

Environmental manipulations alter age differences in attribution of incentive salience to reward-paired cues

Cues repeatedly paired with rewards often themselves become imbued with enhanced motivational value, or incentive salience. During Pavlovian conditioned approach procedures, a cue repeatedly preceding reward delivery often elicits conditioned responses at either the reward delivery location ("goal-tracking") or the cue itself ("sign-tracking"). Sign-tracking behavior is thought to reflect the individual differences in attribution of incentive salience to reward-paired cues that may contribute to addiction vulnerability. Adolescent rats typically demonstrate less sign-tracking behavior than adult rats, a surprising finding given that adolescence is hypothesized to be a time of heightened addiction vulnerability. Given evidence that adult sign-tracking behavior can be influenced by environmental conditions, the present study compared the effects of isolate housing and food deprivation on expression of sign-tacking and goal-tracking behavior in adolescent and adult male rats across eight days of a Pavlovian conditioned approach procedure. Pair-housed adults exhibited more sign-tracking behavior than pair-housed adolescents; however, this age difference was not apparent in isolate-housed subjects. Adolescents often appeared more sensitive than adults to both food restriction- and isolate housing-induced changes in behavior, with food restriction promoting an increase in sign-tracking among isolate-housed adolescents and an increase in goal-tracking among pair-housed adolescents. For adults, food restriction resulted in a modest increase in overall expression of both sign- and goal-tracking behavior. To the extent that sign-tracking behavior reflects attribution of incentive salience to reward-paired cues, results from the present study provide evidence that reactivity to rewards during adolescence is strongly related to the nature of the surrounding environment.

Estrogens facilitate memory processing through membrane mediated mechanisms and alterations in spine density

Estrogens exert sustained, genomically mediated effects on memory throughout the female life cycle, but here we review new studies documenting rapid effects of estradiol on memory, which are exerted through membrane-mediated mechanisms. Use of recognition memory tasks in rats shows that estrogens enhance memory consolidation within 1h. 17α-Estradiol is more potent than 17β-estradiol, and the dose response relationship between estrogens and memory is an inverted U shape. Use of specific estrogen receptor (ER) agonists suggests mediation by an ERβ-like membrane receptor. Enhanced memory is associated with increased spine density and altered noradrenergic activity in the medial prefrontal cortex and hippocampus within 30 min of administration. The environmental chemical, bisphenol-A, rapidly antagonizes enhancements in memory in both sexes possibly through actions on spines. Thus, estradiol and related compounds exert rapid alterations in cognition through non-genomic mechanisms, a finding which may provide a basis for better understanding and treating memory impairments.

Novel object exploration in mice: not all objects are created equal

Object exploration is an increasingly popular experimental paradigm in behavioral sciences. We have begun a series of studies with mice specifically looking at the parameters that influence behaviors in this test. The aim of the present study was to examine the effect of object type on performance in the object exploration test. More specifically, adult male C57BL/6J mice were trained and tested using objects that could be climbed (CLIMB) or with those that could only be touched (TOUCH). The results show that activity is affected by the presentation of objects, with object type interacting with some of these changes. C57 mice explored objects that can be climbed over significantly longer than objects that can only be touched and a more rapid habituation was observed using objects that could only be touched. Robust recognition memory was observed in both groups of mice, however mice in the CLIMB group exhibited a significantly greater discrimination index compared to mice in the TOUCH group. Taken together, these findings demonstrate that the selection of objects is of critical importance and it is recommended that special attention be given to the functional (affordant) properties of the objects to-be-used in future studies.

Reversal of stress-induced dendritic atrophy in the prefrontal cortex by intracranial self-stimulation

The mammalian prefrontal cortex (PFC) has been implicated in a variety of motivational and emotional processes underlying working memory, attention and decision making. The PFC receives dopaminergic projections from the ventral tegmental area (VTA) and contains high density of D1 and D2 receptors and these projections are important in higher integrative cortical functions. The neurons of the PFC have been shown to undergo atrophy in response to stress. In an earlier study, we demonstrated that the chronic stress-induced atrophy of hippocampal neurons and behavioral impairment in the T-maze task were reversed by the activation of dopaminergic pathway by intracranial self-stimulation (ICSS) of the VTA. The stress-induced decrease in hippocampal dopamine (DA) levels was also restored by ICSS. Whether the reversal of stress-induced behavioral deficits by ICSS involves changes in the morphology of PFC neurons is unknown and the current study addresses this issue. Male Wistar rats underwent 21 days of restraint stress followed by ICSS for 10 days. The dendritic morphology of the PFC neurons was studied in Golgi-impregnated sections. Stress produced atrophy of the layer II/III and V PFC pyramidal neurons and ICSS to naïve rats significantly increased the dendritic arborization of these neurons compared to control. Interestingly, ICSS of stressed rats resulted in the reversal of the dendritic atrophy. Further, these structural changes were associated with a restored tissue levels of DA, norepinephrine and serotonin in the PFC. These results indicate that the behavioral restoration in stressed rats could involve changes in the plasticity of the PFC neurons and these results further our understanding of the role of dopaminergic neurotransmitter system in the amelioration of stress-induced deficits.

Chronic stress and a cyclic regimen of estradiol administration separately facilitate spatial memory: relationship with hippocampal CA1 spine density and dendritic complexity

This study investigated the effects of chronic restraint stress and repeated cyclic estradiol pulses on hippocampal CA3 and CA1 dendritic and/or spine morphology and spatial memory in female rats. Sprague-Dawley adult female rats were ovariectomized and then injected over 2 days with 17β-estradiol (10 μg, s.c.), which was repeated every 4-5 days. While all rats received similar estradiol injection histories, half of the rats were chronically restrained and/or given a final cyclic pulse of estradiol prior to testing on a hippocampal-dependent object placement (OP) task to assess spatial memory. OP testing was performed 2 days after the last restraint session, as well as when the last 2 estradiol pulses best captured the maximal effect on hippocampal CA1 spine density. The data revealed several novel findings: (a) chronic stress or estradiol separately facilitated spatial memory, but did not have the same effects when coadministered, (b) CA1 spine densities negatively correlated with spatial memory, and (c) repeated estradiol pulses failed to prevent stress-induced CA3 dendritic retraction. We also corroborated previous studies showing increased CA1 spine density following estradiol, chronic stress, and behavioral manipulations. The present study uniquely combined chronic stress, repeated estradiol pulses, hippocampal morphology, and behavior within the same animals, allowing for correlational analyses to be performed between CA1 spine morphology and spatial memory. We demonstrate novel findings that chronic stress or estradiol pulses independently facilitate spatial memory, but not when coadministered, and that these effects may involve a balance of CA1 apical spine expression that is independent of CA3 dendritic complexity.

Food hoarding and associated neuronal activation in brain reward circuitry in Mongolian gerbils

Mongolian gerbils (Meriones unguiculatus) display food hoarding and thus provide an opportunity to study the neuromechanisms underlying this behavior. In the present study, male gerbils exhibited a bimodal expression of food hoarding behavior-some displayed high levels of food hoarding whereas others virtually lacked this behavior under normal laboratory conditions with free access to food. Food hoarding was found to be associated with an increase in neuronal activation, indicated by Fos immunoreactive (ir) staining, in several brain areas including the nucleus accumbens, ventral tegmental area (VTA), and lateral hypothalamus. Food hoarding was also associated with increases in the number of cells labeled for tyrosine hydroxylase (TH-ir), the rate limiting enzyme for dopamine conversion, and the number of cells co-labeled for TH-ir/Fos-ir in the VTA, suggesting that dopamine in the brain reward circuitry may be involved in food hoarding. Further, we found that 22 h of food deprivation induced food hoarding in some, but not all, males that naturally did not display food hoarding. In these males, however, food hoarding did not increase TH-ir or TH-ir/Fos-ir expression in the VTA. Together, these data indicate that male Mongolian gerbils display diverse phenotypes of food hoarding behavior and that dopamine in the brain reward circuitry may be involved in the control of naturally occurring, but not food deprivation-induced, food hoarding.

Increased plasma d-lactic acid associated with impaired memory in rats

AIM

d-Lactic acidosis is associated with memory impairment in humans. Recent research indicates that d-lactic acid may inhibit the supply of energy from astrocytes to neurons involved with memory formation. However, little is known about the effects of increased hind-gut fermentation due to changes in diet on circulating lactic acid concentrations and memory.

METHOD

Thirty-six male Wistar rats were fed three dietary treatments: a commercial rat and mouse chow, a soluble carbohydrate based diet or a fermentable carbohydrate based diet. The parameters estimating memory were examined by employing the object recognition test. Physical parameters of fermentation including hind-gut and plasma lactic acid concentrations were examined after sacrifice, either 3 or 21h after feeding.

RESULTS

Increased fermentation in the hind-gut of rats, indicated by lower caecum pH, was associated with increased plasma l-lactic acid (r=-0.41, p=0.020) and d-lactic acid (r=-0.33, p=0.087). Memory, being able to discriminate between a familiar and a novel object during the object recognition test, was reduced with increasing plasma d-lactic acid (r=-0.51, p=0.021).

CONCLUSIONS

Memory impairment was associated with alterations in plasma d-lactic acid following the fermentation of carbohydrate in the hind-gut. Further work is still required to determine whether these effects are mediated centrally or via direct connections through the enteric nervous system.

Acute stress and chronic stress change brain-derived neurotrophic factor (BDNF) and tyrosine kinase-coupled receptor (TrkB) expression in both young and aged rat hippocampus

PURPOSE

The purpose of this study is to explore the dynamic change of brainderived neurotrophic factor (BDNF) mRNA, protein, and tyrosine kinase-coupled receptor (TrkB) mRNA of the rat hippocampus under different stress conditions and to explore the influence of senescence on the productions expression.

MATERIALS AND METHODS

By using forced-swimming in 4 degrees C cold ice water and 25 degrees C warm water, young and aged male rats were randomly divided into acute stress (AS) and chronic mild repeated stress (CMRS) subgroups, respectively. BDNF productions and TrkB mRNA in the hippocampus were detected by using Western-blotting and reverse transcription-polymerase chain reaction (RT-PCR), separately, at 15, 30, 60, 180, and 720 min after the last stress session.

RESULTS

The short AS induced a significant increase in BDNF mRNA and protein in both age groups, but the changes in the young group were substantially greater than those of the aged group (p < 0.005). The CMRS resulted in a decrease in BDNF mRNA and protein, but a significant increase in TrkB mRNA in both young and age groups. The expression of BDNF mRNA and protein in the AS groups were higher than in the CMRS groups at 15, 30, and 60 min after stress.

CONCLUSION

The results indicated that the up/down-regulation of BDNF and TrkB were affected by aging and the stimulus paradigm, which might reflect important mechanisms by which the hippocampus copes with stressful stimuli.

Acute estrogen treatment facilitates recognition memory consolidation and alters monoamine levels in memory-related brain areas

Acute effects of estrogens on mnemonic processes were examined at the behavioral and neurochemical levels. 17beta-estradiol and 17alpha-estradiol influences on memory consolidation were assessed using object placement (OP) and object recognition (OR) tasks. Subjects received treatment immediately after a sample trial (exploring two novel objects), and memory of objects (OR memory) or location of objects (OP memory) was tested 4h later. Both isomers of estradiol enhanced memory. For spatial memory, 15 and 20 microg/kg of 17beta-estradiol facilitated OP, while lower and higher doses were ineffective. 17alpha-estradiol had a similar pattern, but a lower dose was effective. When treatment was delayed until 45 min after a sample trial, memory was not enhanced. For non-spatial memory, OR was facilitated at 5 microg/kg of 17beta-estradiol and at 1 and 2 microg/kg of 17alpha-estradiol and, similar to OP, lower and higher doses were ineffective. These data demonstrate that beneficial effects of estrogens are dose, time and task dependent, and the dose-response pattern is an inverted U. Because monoamines are known to have contributions to memory, brains were removed 30 min after treatment for measurements of dopamine (DA), norepinephrine (NE), serotonin (5-HT), and metabolites. Estrogen elevated 5HT, NE metabolite MHPG, turnover ratio of NE to MHPG, and DA metabolite DOPAC levels in the prefrontal cortex, while NE and MHPG were decreased in the hippocampus. Thus, acute estrogens exert rapid effects on memory consolidation and neural function, which suggests that its mnemonic effects may involve activation of membrane associated estrogen receptors and subsequent signaling cascades, and that monoamines may contribute to this process.

A critical review of chronic stress effects on spatial learning and memory

The purpose of this review is to evaluate the effects of chronic stress on hippocampal-dependent function, based primarily upon studies using young, adult male rodents and spatial navigation tasks. Despite this restriction, variability amongst the findings was evident and how or even whether chronic stress influenced spatial ability depended upon the type of task, the dependent variable measured and how the task was implemented, the type and duration of the stressors, housing conditions of the animals that include accessibility to food and cage mates, and duration from the end of the stress to the start of behavioral assessment. Nonetheless, patterns emerged as follows: For spatial memory, chronic stress impairs spatial reference memory and has transient effects on spatial working memory. For spatial learning, however, chronic stress effects appear to be task-specific: chronic stress impairs spatial learning on appetitively motivated tasks, such as the radial arm maze or holeboard, tasks that evoke relatively mild to low arousal components from fear. But under testing conditions that evoke moderate to strong arousal components from fear, such as during radial arm water maze testing, chronic stress appears to have minimal impairing effects or may even facilitate spatial learning. Chronic stress clearly impacts nearly every brain region and thus, how chronic stress alters hippocampal spatial ability likely depends upon the engagement of other brain structures during behavioral training and testing.

Food seeking in spite of harmful consequences is under prefrontal cortical noradrenergic control

Background

Eating disorders are multifactorial psychiatric disorders. Chronic stressful experiences and caloric restriction are the most powerful triggers of eating disorders in human and animals. Although compulsive behavior is considered to characterize pathological excessive food intake, to our knowledge, no evidence has been reported of continued food seeking/intake despite its possible harmful consequences, an index of compulsive behavior. Brain monoamine transmission is considered to have a key role in vulnerability to eating disorders, and norepinephrine in medial prefrontal cortex has been shown to be critical for food-related motivated behavior.

Here, using a new paradigm of conditioned suppression, we investigated whether the ability of a foot-shock-paired conditioned stimulus to suppress chocolate-seeking behavior was reversed by previous exposure to a food restriction experience, thus modeling food seeking in spite of harmful consequences in mice. Moreover, we assessed the effects of selective norepinephrine inactivation in medial prefrontal cortex on conditioned suppression test in stressed and caloric restricted mice.

Results

While Control (non food deprived) animals showed a profound conditioned suppression of chocolate seeking during presentation of conditioned stimulus, previously food restricted animals showed food seeking/intake despite its possible harmful consequences. Moreover, food seeking in spite of harmful consequences was prevented by selective norepinephrine inactivation, thus showing that prefrontal cortical norepinephrine is critical also for maladaptive food-related behavior.

Conclusions

These findings indicate that adaptive food seeking/intake can be transformed into maladaptive behaviors and point to "top-down" influence on eating disturbances and to new targets for therapy of aberrant eating behaviors.