Social interaction following prepubertal stress alters prefrontal gene expression associated with cell signalling and oligodendrocytes.

Early-life adversity is associated with an increased risk of psychopathology, including mood disorders, later in life. Early-life stress affects several physiological systems, however, the exact mechanisms underlying pathological risk are not fully understood. This knowledge is crucial in developing appropriate therapeutic interventions. The prepubertal period is documented as a key developmental period for the maturation of the prefrontal cortex (PFC), a brain region involved in higher cognitive functions, including social function. In this study, we performed RNA sequencing on the PFC of adult rats who had experienced prepubertal stress (PPS) and controls to investigate the genome-wide consequences of this stress. PPS alters social behaviour in adulthood, therefore we also performed RNA sequencing on PPS and control rats following a social interaction test to determine social activity-dependent gene changes. At a baseline state (1 week following a social interaction test), no genes were differentially expressed in the PPS group. However, 1603 genes were differentially expressed in PPS rats compared to controls following a social interaction. These genes were enriched in biological pathways associated with cell signalling and axon myelination dynamics. Cell enrichment analysis showed these genes were associated with oligodendrocytes, and a comparison with an existing early-life stress sequencing dataset showed that pathways linked to oligodendrocyte morphology are impacted in a range of models of early-life stress in rodents. In conclusion, we identify pathways, including those involved in axon myelination, that are differentially activated in the adult in response to social stimulation following PPS. These differential responses may contribute to vulnerability to psychiatric pathology.

Neuroimmunological effects of early life experiences.

Exposure to adverse experiences during development increases the risk of psychiatric illness later in life. Growing evidence suggests a role for the neuroimmune system in this relationship. There is now substantial evidence that the immune system is critical for normal brain development and behaviour, and responds to environmental perturbations experienced early in life. Severe or chronic stress results in dysregulated neuroimmune function, concomitant with abnormal brain morphology and function. Positive experiences including environmental enrichment and exercise exert the opposite effect, promoting normal brain and immune function even in the face of early life stress. The neuroimmune system may therefore provide a viable target for prevention and treatment of psychiatric illness. This review will briefly summarise the neuroimmune system in brain development and function, and review the effects of stress and positive environmental experiences during development on neuroimmune function. There are also significant sex differences in how the neuroimmune system responds to environmental experiences early in life, which we will briefly review.

Enduring neuroimmunological consequences of developmental experiences: From vulnerability to resilience.

The immune system is crucial for normal neuronal development and function (neuroimmune system). Both immune and neuronal systems undergo significant postnatal development and are sensitive to developmental programming by environmental experiences. Negative experiences from infection to psychological stress at a range of different time points (in utero to adolescence) can permanently alter the function of the neuroimmune system: given its prominent role in normal brain development and function this dysregulation may increase vulnerability to psychiatric illness. In contrast, positive experiences such as exercise and environmental enrichment are protective and can promote resilience, even restoring the detrimental effects of negative experiences on the neuroimmune system. This suggests the neuroimmune system is a viable therapeutic target for treatment and prevention of psychiatric illnesses, especially those related to stress. In this review we will summarise the main cells, molecules and functions of the immune system in general and with specific reference to central nervous system development and function. We will then discuss the effects of negative and positive environmental experiences, especially during development, in programming the long-term functioning of the neuroimmune system. Finally, we will review the sparse but growing literature on sex differences in neuroimmune development and response to environmental experiences.

Cacna1c Hemizygosity Results in Aberrant Fear Conditioning to Neutral Stimuli.

CACNA1C, a gene that encodes an alpha-1 subunit of L-type voltage-gated calcium channels, has been strongly associated with psychiatric disorders including schizophrenia and bipolar disorder. An important objective is to understand how variation in this gene can lead to an increased risk of psychopathology. Altered associative learning has also been implicated in the pathology of psychiatric disorders, particularly in the manifestation of psychotic symptoms. In this study, we utilize auditory-cued fear memory paradigms in order to investigate whether associative learning is altered in rats hemizygous for the Cacna1c gene. Cacna1c hemizygous (Cacna1c+/-) rats and their wild-type littermates were exposed to either delay, trace, or unpaired auditory fear conditioning. All rats received a Context Recall (24 h post-conditioning) and a Cue Recall (48 h post-conditioning) to test their fear responses. In the delay condition, which results in strong conditioning to the cue in wild-type animals, Cacna1c+/- rats showed increased fear responses to the context. In the trace condition, which results in strong conditioning to the context in wild-type animals, Cacna1c+/- rats showed increased fear responses to the cue. Finally, in the unpaired condition, Cacna1c+/- rats showed increased fear responses to both context and cue. These results indicate that Cacna1c heterozygous rats show aberrantly enhanced fear responses to inappropriate cues, consistent with key models of psychosis.

Female HPA axis displays heightened sensitivity to pre-pubertal stress.

Early life stress (ELS) is a risk factor in the development of psychiatric disorders. The underlying biological mechanisms governing this phenomenon are not fully understood, but dysregulation of stress responses is likely to play a key role. Males and females differ in their propensity to develop psychiatric disorders, with far higher rates of anxiety, major depressive disorder, affective disorders and post-traumatic stress disorder found in women. We hypothesized that sex differences in response to ELS may play a crucial role in differential vulnerability between the sexes. To test this, we evaluated the consequences of pre-pubertal stress (PPS) on the HPA axis in adult female and male Lister Hooded rats. PPS animals were exposed to swim, restraint and elevated platform stress on postnatal days 25-27, controls remained in their home cage. Once adult, animals were either a) sacrificed directly and brains collected or b) sacrificed 20 minutes or 1 week after a social test and trunk blood collected. In the female hippocampal formation, PPS increased expression of FKBP5 and AVPR1a. In the female prefrontal cortex, PPS resulted in increased glucocorticoid receptor expression, increased glucocorticoid:mineralocorticoid (GR:MR) receptor expression ratio and decreased AVPR1a expression. Females exposed to PPS did not show the normal rise in blood corticosterone levels following a social interaction test. In contrast, PPS did not alter the expression of oxytocin or oxytocin receptors, and no effects of PPS were seen in males. However, striking sex differences were found. Females had higher oxytocin receptor expression in the prefrontal cortex and AVPR1a and oxytocin expression in the hypothalamus, whereas males demonstrated higher expression of GR, MR, GR:MR, FKBP5 and oxytocin receptor in the hypothalamus. These results demonstrate heightened reactivity of the female HPA axis to PPS and may help explain why in humans females display an increased susceptibility to certain stress-related psychopathologies.LAY SUMMARYWomen are at greater risk of developing several psychiatric illnesses. Using a rodent model, we show that the female stress system is more reactive to the lasting effects of early life stress. This heightened reactivity of the female stress response may help explain why women are at a greater risk of developing psychiatric disorders.

Childhood stress impairs social function through AVP-dependent mechanisms.

Impaired social function is a core feature of many psychiatric illnesses. Adverse experiences during childhood increase risk for mental illness, however it is currently unclear whether stress early in life plays a direct role in the development of social difficulties. Using a rat model of pre-pubertal stress (PPS), we investigated effects on social behaviour, oxytocin and arginine vasopressin (AVP) in the periphery (plasma) and centrally in the paraventricular and supraoptic hypothalamic nuclei. We also explored social performance and AVP expression (plasma) in participants with borderline personality disorder (BPD) who experienced a high incidence of childhood stress. Social behaviour was impaired and AVP expression increased in animals experiencing PPS and participants with BPD. Behavioural deficits in animals were rescued through administration of the AVPR1a antagonist Relcovaptan (SR49059). AVP levels and recognition of negative emotions were significantly correlated in BPD participants only. In conclusion, early life stress plays a role in the precipitation of social dysfunction, and AVP mediates at least part of this effect.

A shortened protocol for assessing cognitive bias in rats.

BACKGROUND: Reliable measurement of affective state in animals is a significant goal of animal welfare. Such measurements would also improve the validity of pre-clinical mental health research which relies on animal models. However, at present, affective states in animals are inaccessible to direct measurement. In humans, changes in cognitive processing can give reliable indications of emotional state. Therefore, similar techniques are increasingly being used to gain proxy measures of affective states in animals. In particular, the 'cognitive bias' assay has gained popularity in recent years. Major disadvantages of this technique include length of time taken for animals to acquire the task (typically several weeks), negative experiences associated with task training, and issues of motivation. NEW METHOD: Here we present a shortened cognitive bias protocol using only positive reinforcers which must actively be responded to. RESULTS: The protocol took an average of 4days to complete, and produced similar results to previous, longer methods (minimum 30days). Specifically, rats housed in standard laboratory conditions demonstrated negative cognitive biases when presented with ambiguous stimuli, and took longer to make a decision when faced with an ambiguous stimulus. COMPARISON WITH EXISTING METHODS: Compared to previous methods, this protocol is significantly shorter (average 4days vs. minimum 30days), utilises only positive reinforcers to avoid inducing negative affective states, and requires active responses to all cues, avoiding potential confounds of motivational state. CONCLUSIONS: We have successfully developed a shortened cognitive bias protocol, suitable for use with laboratory rats.

Prenatal glucocorticoid exposure in rats: programming effects on stress reactivity and cognition in adult offspring.

Human epidemiological studies have provided compelling evidence that prenatal exposure to stress is associated with significantly increased risks of developing psychiatric disorders in adulthood. Exposure to excessive maternal glucocorticoids may underlie this fetal programming effect. In the current study, we assessed how prenatal dexamethasone administration during the last week of gestation affects stress reactivity and cognition in adult offspring. Stress reactivity was assessed by evaluating anxiety-like behavior on an elevated plus maze and in an open field. In addition, to characterize the long-term cognitive outcomes of prenatal exposure to glucocorticoids, animals were assessed on two cognitive tasks, a spatial reference memory task with reversal learning and a delayed matching to position (DMTP) task. Our results suggest that prenatal exposure to dexamethasone had no observable effect on anxiety-like behavior, but affected cognition in the adult offspring. Prenatally dexamethasone-exposed animals showed a transient deficit in the spatial reference memory task and a trend to faster acquisition during the reversal-learning phase. Furthermore, prenatally dexamethasone-treated animals also showed faster learning of new platform positions in the DMTP task. These results suggest that fetal overexposure to glucocorticoids programs a phenotype characterized by cognitive flexibility and adaptability to frequent changes in environmental circumstances. This can be viewed as an attempt to increase the fitness of survival in a potentially hazardous postnatal environment, as predicted by intrauterine adversity. Collectively, our data suggest that prenatal exposure to dexamethasone in rats could be used as an animal model for studying some cognitive components of related psychiatric disorders.

Early life stress produces compulsive-like, but not impulsive, behavior in females.

Adverse experiences during childhood are associated with the development of psychiatric disorders later in life. In particular, childhood abuse and neglect are risk factors for addictive disorders, such as substance misuse and pathological gambling. Impulsivity and compulsivity are key features of these disorders. Therefore, we investigated whether childhood adversity might increase vulnerability for addictive disorders through promotion of compulsive and impulsive behaviors. Rats were exposed to a brief, variable childhood or prepubertal stress protocol (Postnatal Days 25-27), and their behavior in a delay discounting task was compared with that of control animals in adulthood. Prepubertal stress produced compulsive-type behavior in females. Specifically, stressed females displayed inappropriate responses during a choice phase of the task, perseverating with nosepoke responding instead of choosing between 2 levers. Stressed females also showed learning impairments during task training. However, prepubertal stress was not associated with the development of impulsive behavior, as rates of delay discounting were not affected in either sex. Childhood adversity may contribute to the establishment and maintenance of addictive disorders by increasing perseveration in females. Perseverative behavior may therefore provide a viable therapeutic target for preventing the development of addictive disorders in individuals exposed to childhood adversity. These effects were not seen in males, highlighting sex differences in response to early life stress.

Imaging learned fear circuitry in awake mice using fMRI.

Functional magnetic resonance imaging (fMRI) of learned behaviour in 'awake rodents' provides the opportunity for translational preclinical studies into the influence of pharmacological and genetic manipulations on brain function. fMRI has recently been employed to investigate learned behaviour in awake rats. Here, this methodology is translated to mice, so that future fMRI studies may exploit the vast number of genetically modified mouse lines that are available. One group of mice was conditioned to associate a flashing light (conditioned stimulus, CS) with foot shock (PG; paired group), and another group of mice received foot shock and flashing light explicitly unpaired (UG; unpaired group). The blood oxygen level-dependent signal (proxy for neuronal activation) in response to the CS was measured 24 h later in awake mice from the PG and UG using fMRI. The amygdala, implicated in fear processing, was activated to a greater degree in the PG than in the UG in response to the CS. Additionally, the nucleus accumbens was activated in the UG in response to the CS. Because the CS signalled an absence of foot shock in the UG, it is possible that this region is involved in processing the safety aspect of the CS. To conclude, the first use of fMRI to visualise brain activation in awake mice that are completing a learned emotional task is reported. This work paves the way for future preclinical fMRI studies to investigate genetic and environmental influences on brain function in transgenic mouse models of disease and aging.

Juvenile stress enhances anxiety and alters corticosteroid receptor expression in adulthood.

BACKGROUND: Exposure to stress in early life is correlated with the development of anxiety disorders in adulthood. The underlying mechanisms are not fully understood, but an imbalance in corticosteroid receptor (CR) expression in the limbic system, particularly the hippocampus, has been implicated in the etiology of anxiety disorders. However, little is known about how prepubertal stress in the so called "juvenile" period might alter the expression of these receptors. AIMS: Therefore, the aim of this study was to investigate how stress experienced in the juvenile phase of life altered hippocampal expression of CRs and anxiety behaviors in adulthood. MATERIALS AND METHODS: We used a rodent model to assess the effects of juvenile stress on hippocampal CR expression, and performance in three behavioral tests of anxiety in adulthood. RESULTS: Juvenile stress (JS) increased anxiety-like behavior on the elevated plus maze, increased mineralocorticoid receptor (MR) expression, and decreased the ratio of glucocorticoid receptor (GR) to MR expression in the hippocampus of adult animals. Females demonstrated lower levels of anxiety-type behavior and increased activity in three behavioral tests, and had greater expression of GR and GR:MR ratio than males, regardless of treatment. DISCUSSION AND CONCLUSION: These results demonstrate that JS can alter the expression and balance of CRs, providing a potential mechanism for the corresponding increase in anxiety behavior observed in adulthood. Further evidence for the role of CR expression in anxiety is provided by sex differences in anxiety behavior and corresponding alterations in CR expression.

Prepubertal stress and hippocampal function: sex-specific effects.

The chances of developing psychiatric disorders in adulthood are increased when stress is experienced early in life. In particular, stress experienced in the childhood or 'prepubertal' phase is associated with the later development of disorders such as depression, anxiety, post-traumatic stress disorder, and psychosis. Relatively little is known about the biological basis of this effect, but one hypothesis is that prepubertal stress produces long-lasting changes in brain development, particularly in stress sensitive regions such as the hippocampus, leaving an individual vulnerable to disorders in adulthood. In this study, we used an animal model of prepubertal stress to investigate the hypothesis that prepubertal stress induces alterations in hippocampal function in adulthood. Male and female rats were exposed to a brief, variable prepubertal stress protocol (postnatal days 25-27), and their performance in two distinct hippocampal-dependent tasks (contextual fear and spatial navigation) was compared with controls in adulthood. Prepubertal stress significantly impaired contextual fear responses in males and enhanced performance in spatial navigation in females. These results demonstrate that exposure to a brief period of stress in the prepubertal phase alters hippocampal-dependent behaviors in adulthood in a sex-specific manner.

Imaging conditioned fear circuitry using awake rodent fMRI.

Functional magnetic resonance imaging (fMRI) is a powerful method for exploring emotional and cognitive brain responses in humans. However rodent fMRI has not previously been applied to the analysis of learned behaviour in awake animals, limiting its use as a translational tool. Here we have developed a novel paradigm for studying brain activation in awake rats responding to conditioned stimuli using fMRI. Using this method we show activation of the amygdala and related fear circuitry in response to a fear-conditioned stimulus and demonstrate that the magnitude of fear circuitry activation is increased following early life stress, a rodent model of affective disorders. This technique provides a new translatable method for testing environmental, genetic and pharmacological manipulations on emotional and cognitive processes in awake rodent models.

The effects of juvenile stress on anxiety, cognitive bias and decision making in adulthood: a rat model.

Stress experienced in childhood is associated with an increased risk of developing psychiatric disorders in adulthood. These disorders are particularly characterized by disturbances to emotional and cognitive processes, which are not currently fully modeled in animals. Assays of cognitive bias have recently been used with animals to give an indication of their emotional/cognitive state. We used a cognitive bias test, alongside a traditional measure of anxiety (elevated plus maze), to investigate the effects of juvenile stress (JS) on adulthood behaviour using a rodent model. During the cognitive bias test, animals were trained to discriminate between two reward bowls based on a stimulus (rough/smooth sandpaper) encountered before they reached the bowls. One stimulus (e.g. rough) was associated with a lower value reward than the other (e.g. smooth). Once rats were trained, their cognitive bias was explored through the presentation of an ambiguous stimulus (intermediate grade sandpaper): a rat was classed as optimistic if it chose the bowl ordinarily associated with the high value reward. JS animals were lighter than controls, exhibited increased anxiety-like behaviour in the elevated plus maze and were more optimistic in the cognitive bias test. This increased optimism may represent an optimal foraging strategy for these underweight animals. JS animals were also faster than controls to make a decision when presented with an ambiguous stimulus, suggesting altered decision making. These results demonstrate that stress in the juvenile phase can increase anxiety-like behaviour and alter cognitive bias and decision making in adulthood in a rat model.

Habitat stability and predation pressure affect temperament behaviours in populations of three-spined sticklebacks.

1. There is growing interest in the causes and consequences of animal temperaments. Temperament behaviours often have heritable components, but ecological variables can also affect them. Numerous variables are likely to differ between habitats, and these may interact to influence temperament behaviours. 2. Temperament behaviours may be correlated within populations (behavioural syndromes), although the underlying causes of such correlations are currently unclear. 3. We analysed three different temperament behaviours and learning ability in three-spined sticklebacks, Gasterosteus aculeatus, to determine how different ecological variables influence them both within and between populations. We selected populations from four ponds and four rivers that varied naturally in their exposure to predators. 4. High-predation river populations were significantly less bold than a high-predation pond and low-predation river populations, and low-predation pond populations were significantly less bold than a high-predation pond population. Within populations, temperament behaviours were correlated in one high-predation river population only. 5. These results suggest that multiple ecological factors can interact to affect temperament behaviours between populations, and also correlations in those behaviours within populations.