Predation threat exerts specific effects on rat maternal behaviour and anxiety-related behaviour of male and female offspring.

Differences in the rate of maternal behaviours received by rodent offspring are associated with differential programming of molecular and behavioural components of anxiety and stress-related functions. To determine the degree to which maternal behaviours are sensitive to environmental conditions, Long-Evans rat dams were exposed to the odour of a predator (cat) at two different time points during the first week postpartum. Exposure on the day of birth (DOB), but not the third day following birth, increased levels of maternal care in predator-exposed dams relative to dams exposed to a control condition across the first 5 days post-partum. As adults, female offspring of dams exposed on DOB exhibited a less-anxious phenotype in a novel open-field, spending more time in the center and less time displaying thigmotaxis. In contrast, under the same conditions, male offspring showed the opposite behavioural response, consistent with an increasingly anxious phenotype. Results from a subsequent stressor test (response to a predator odour) were consistent with the notion that the rearing effects were specific to anxiety-related behaviours in offspring. Accordingly, we showed that rearing conditions did not affect GR mRNA or NGFI-A expression in the hippocampus of offspring or cross-fostered offspring. The dissociation between stress and anxiety, as well as the sex-specific alterations in behaviour, may reflect the specificity inherent to neural programming in the face of naturalistic early life conditions.

Investigation of sex differences in behavioural, endocrine, and neural measures following repeated psychological stressor exposure.

Animal models of repeated stressor exposure have generally been limited to physical stressors, despite the fact that the purpose of such models is to represent repeated stress in humans, which is usually psychological in nature. The present study was undertaken to investigate the behavioural, endocrine, and neural responses to a repeated psychological stressor exposure in male and female rats. Long-Evans rats were exposed to cat odour or a control condition for 1 h each day from Day 1 to Day 22. Every fourth day, defensive (e.g. hiding), and non-defensive (e.g. grooming) behaviour was quantified, during both the initial and the final 10 min of the hour. Defensive behaviours in cat odour-exposed animals remained vigorous during the initial 10 min of exposure across 22 exposure days. Non-defensive behaviours were suppressed during early exposures, but this suppression habituated across repeated exposures. Overall, the pattern of behavioural results indicated enhanced responses to novelty and to repeated cat odour exposure, in females, relative to males. Plasma corticosterone (CORT) levels were higher in females relative to males overall. However, males, but not females, exposed to cat odour had higher levels of CORT following exposure on Days 1 and 22, relative to controls. Finally, mRNA levels of glucocorticoid receptor, mineralocorticoid receptor, and brain-derived neurotrophic factor, all of which are modulated by CORT, were examined in hippocampus at the completion of stressor exposure, but none was affected by repeated stressor exposure. Results are discussed within the context of potential differences in effects of repeated psychological versus physical stressors.

Periadolescent stress exposure exerts long-term effects on adult stress responding and expression of prefrontal dopamine receptors in male and female rats.

Recent research has demonstrated that experiential/environmental factors in early life can program the adult stress response in rats, and this is manifest as altered hypothalamic-pituitary-adrenocortical activity and behavior in response to a stressor. Very little work has been devoted to investigating whether the environment during adolescence plays a similar role in modulating ongoing developmental processes and how this might affect adult stress responding. Periadolescent predator odor (PPO) exposure was used here as a naturalistic model of repeated psychological stress. Behavioral and endocrine responses to PPO changed across the exposure period, and behavioral alterations persisted into adulthood. While adolescent rats showed pronounced avoidance responses upon initial PPO exposure, hyperactivity increased across the exposure period, especially in females. Corticosterone (cort) responses to stressor exposure also changed in females, with higher physiological baseline levels observed at the end of the exposure period. In adulthood, relative to rats who had received a control manipulation during adolescence, PPO-exposed rats were more fearful in a novel open field and displayed altered responses to a predator odor stress test in adulthood. Moreover, lower levels of the D2 dopamine (DA) receptor were measured in prefrontal (infralimbic and dorsopeduncular) cortices of PPO-exposed rats. These findings suggest that the adolescent period may represent a sensitive period during which developmental programming of the stress response occurs.

Sex and ovarian steroids modulate brain-derived neurotrophic factor (BDNF) protein levels in rat hippocampus under stressful and non-stressful conditions.

Abnormal levels of brain-derived neurotrophic factor (BDNF) are associated with major depression, a disorder with a higher incidence in women than men. Stress affects BDNF levels in various brain regions and thus, a heightened stress response in females could contribute to the development of depression. As well, ovarian hormones directly affect brain levels of BDNF mRNA and protein. Two experiments were performed to investigate the effects of stress and sex and gonadal hormones on BDNF protein levels in CA1, CA3, and dentate gyrus (DG) subregions of the hippocampus. In the first experiment, male and female Sprague-Dawley rats were subjected to one hour of restraint stress or control handling prior to sacrifice. In the second experiment, fifty-one female rats were ovariectomized and separated into stress and control conditions, as described for the first experiment. Stressed and handled groups received a single injection of estrogen (E; 53h prior to stress), estrogen and progesterone (EP; E given at 53h and P given 5h prior to stress), or vehicle (OVX). In both experiments BDNF protein was quantified using an enzyme-linked immunosorbent enzyme assay (ELISA) in micropunches of hippocampus. Gonadally intact females had significantly higher levels of BDNF in CA3, but significantly lower levels in DG, relative to males. In CA3, stress significantly decreased BDNF in both males and females. In DG of ovariectomized female rats, the effects of stress were significantly different following EP vs. vehicle treatment. Thus, stress increased BDNF levels in EP-treated rats but decreased BDNF levels in vehicle-treated rats. Reduced trophic support in DG in the presence of estrogen and progesterone could jeopardize neurogenesis and under certain conditions could be a contributing factor to the hippocampal atrophy associated with stress-induced affective disorders. These results emphasize the need to consider sex, gonadal steroids, and hippocampal subregion when examining the effects of stress on the brain.

Corticosterone increases depression-like behavior, with some effects on predator odor-induced defensive behavior, in male and female rats.

This experiment examined the effect of repeated corticosterone injections on anxiety and depression-like behavior in male and female rats. Rats received either corticosterone or vehicle injections for 21 consecutive days prior to behavioral testing in the forced swim, open-field, and predator odor tests. The corticosterone injections significantly increased depression-like behavior in the forced swim test in both male and female rats but had no significant effect on anxiety in the open-field test. In the predator odor test, the corticosterone injections significantly increased a subset of defensive behaviors in the male rats. These results suggest that repeated exposure to corticosterone increases depression-like behavior, with some effects on anxiety, and that male rats may be more affected than female rats by this manipulation.

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

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

Getting excited about GABA and sex differences in the brain.

In a unique reversal of function, the ubiquitous and dominant inhibitory neurotransmitter, GABA, serves as the major source of neuronal excitation in the developing brain. Opening of voltage-gated Ca(2+) channels following GABA(A)-receptor-mediated depolarization translates the action of GABA into trophic responses, such as changes in cell death and synaptic patterning. During a perinatal sensitive period, steroid hormones, in particular estradiol, modulate these cellular responses to differentiate male and female brains. Profound sex differences are found in the volumes of specific nuclei, and the frequencies and types of synapses, in estradiol-concentrating brain regions. Recent evidence suggests that the mechanism of estradiol action involves enhancing, and extending the duration of, the developmental excitatory effects of GABA, resulting in divergence of the signal transduction pathways activated in males versus females. This is the first identification of a pivotal point in brain development that dictates the sex differences evident in adult physiology and behavior.