Marta Magdalena Suárez

Sexual dimorphism in rats: effects of early maternal separation and variable chronic stress on pituitary-adrenal axis and behavior

The pituitary‐adrenal axis response is gender‐dependent, showing lower activity in male rats. Furthermore, males showed low emotional behavior and females high emotionality when exposed to such chronic stress situations.

Neurobiological effects of neonatal maternal separation and post-weaning environmental enrichment

Throughout the lifespan, the brain has a considerable degree of plasticity and can be strongly influenced by sensory input from the outside environment. Given the importance of the environment in the regulation of the brain structure, behavior and physiology, the aim of the present work was to analyze the effects of different environmental qualities during two critical ontogenic periods (early life and peripuberty) on behavior and hippocampal physiology. Male Wistar rats were separated from their mothers for 4.5h daily during the first 3 weeks of life. They were weaned on day 21 and housed under either standard or enriched conditions. At 60 d of age, all animals were then housed in same-treatment groups, two per cage, until testing began on day 74. Emotional and cognitive responses were tested using the open field, novel object recognition test and step-down inhibitory avoidance learning. In the dorsal hippocampus, glucocorticoid receptor expression and neuronal activity were examined by immunoreactivity. Grooming behavior in the open field was found to be significantly lower in maternally separated animals, but post-weaning environmental enrichment completely reversed this tendency. Inhibitory avoidance but not object recognition memory was impaired in maternally separated animals, suggesting that early maternal separation alters learning and memory in a task-specific manner. Again, environmental enrichment reversed the effects of maternal separation on the inhibitory avoidance task. Even though maternal separation did not significantly affect Fos and glucocorticoid receptor (GR) expression, environmental enrichment increased both Fos expression in the total hippocampal area and also the overall number of GR positive cells per hippocampal area, mainly due to the changes in CA1. These findings suggest that differential rearing is a useful procedure to study behavioral and physiological plasticity in response to early experience and that, although the effects of adverse experience early in life such as maternal separation can persist until adulthood, some of them can be compensated by early favorable environments, possibly through nervous system plasticity.

Sex differences in rats: Effects of chronic stress on sympathetic system and anxiety

In this study we tested whether periodic maternal deprivation (MD) (4.5 h daily during the first 3 weeks of life) caused chronic changes in anxiety and medullo-adrenal responses to chronic stress in either male or female adult (2.5 months of age) rats, or both. Repeated maternal deprivation had a sex-specific effect on epinephrine (E) and norepinephrine (NE) levels: an increase in both measures was observed only in females. Unpredictable stress did not produce changes on plasma catecholamine levels either in males or females. However, when the females were maternally deprived as well as stressed they showed an increase in plasma NE p < 0.05. On the other hand, non-maternally deprived (NMD), maternally-deprived and stressed males showed high levels of catecholamines compared to females p < 0.001. In the elevated plus maze test, MD-treated males displayed a slight increase in anxiety-related behavior compared with NMD rats. This was indicated by a reduction in the time spent on the open arms, whereas females showed less anxiety, indicated by an increase in the number of entries, and in the time spent on the open arms. After exposure to chronic stress only the females displayed decreased anxiety-related behavior. These results suggest that there are sex-induced effects in emotional reactivity, perception of the stressor and in the evaluation of novel situations. Thus, maternal deprivation and chronic variable stress caused both long-term alterations in sympathetic response and gender-dependent changes in the anxiety index of adult rats.

Early maternal separation: neurobehavioral consequences in mother rats.

Repeated separation of dams from their pups during the postpartum period may evoke emotional stress in the dam. In the present study we investigated whether prolonged maternal separation is stressful for rat dams by studying different behavioral and central responses known to be affected by stress. After delivery, female Wistar rats were subjected to either animal facility rearing (AFR) conditions or daily 4.5 h of mother-litter separation from postpartum day (PPD) 1-21. Maternal care (pup retrieval) was evaluated at PPD 3. After weaning on PPD 21, anxiety (elevated plus maze) and depression-like behaviors (forced swimming test) were assessed in the dams. Memory abilities (one-trial step down inhibitory avoidance) were tested either 1 h (short-term memory) or 24 h (long-term memory) after training session. Finally, c-Fos expression was examined in the central nucleus of the amygdala. The results revealed that pup retrieval efficiency at PPD 3 was significantly impaired by maternal separation. AFR dams retrieved their pups sooner and engaged in more pup-directed activities (nest building and carrying pups). Separation from pups increased the number of entries in open arms of the plus maze and decreased latency times in the inhibitory avoidance test for both short and long-term memory in the dams. There were no differences in depression-related behavior as assessed using the forced swimming test. Furthermore, maternal separation yielded high c-Fos expression in the central nucleus of the amygdala. Together, these data indicate that repeated maternal separation in the early postpartum period reduces maternal care and impairs the retention memory, providing further evidence for the detrimental neurobehavioral effects of maternal separation in dams.

Adrenal function response to chronic stress in rats with anterodorsal thalami nuclei lesions

The effects of chronic stress (forced immobilization, 15 min/day during 12 days), on the plasma corticosterone and the adrenal catecholamines response in rats with anterodorsal thalami nuclei (ADTN) lesions were studied. In sham lesioned rats, chronic stress produced a significant increase in plasma corticosterone (C), as compared to unstressed animals (p < 0.05). The adrenal C was, however, similar in both groups. There were no differences in plasma C values between unstressed and stressed lesioned rats. The adrenal C content, was significantly lower (P < 0.005) in stressed lesioned rats when compared with unstressed lesioned animals. Adrenal norepinephrine (NE) values, in sham lesioned rats after forced immobilization, were significantly below those found in unstressed sham lesioned ones (P < 0.05). There were no changes in adrenal epinephrine (E) response after forced immobilization. In lesioned rats, NE response to chronic stress showed the opposite pattern to that in sham lesioned ones; the adrenal glands of these animals showed a significant increase in NE content as compared to unstressed lesioned rats (P < 0.01). Similar alterations in the adrenal E concentration were found, post stress values were significantly higher (P < 0.01).

Periodic maternal deprivation and lesion of anterodorsal thalami nuclei induce alteration on hypophyso adrenal system activity in adult rats.

The hypothalamic-pituitary-adrenal (HPA) axis is normally regulated by extrahypothalamic limbic structures, among these, the anterodorsal thalami nuclei (ADTN), which exert an inhibitory influence on HPA, in basal and acute stress conditions in rats. In the present work we have investigated whether neonatal maternal deprivation (MD) produces long-term changes in the ADTN regulation of HPA activity. Maternal deprivation, in female rats, for 4.5 hs daily, during the first 3 weeks of life, produced at 3 months old, a significant decrease in plasma ACTH concentration (p<0.001) and an increase in plasma corticosterone (C) (p<0.001), compared to control non-deprived rats (NMD). Also MD showed higher plasma epinephrine (E) and norepinephrine (NE) levels than NMD rats. The increase of NE (66.6% p<0.001) was higher than that observed in E (19%). After 30 days of ADTN lesion, plasma ACTH values were higher than in sham lesioned rats, in both NMD and MD animals. ACTH response was greater in MD rats. Plasma C, in NMD, was higher, whereas in MD lesioned animals, it was significantly lower than in sham lesioned. In MD rats, lesion produced a significant increase in plasma E and NE (p<0.001), and again, NE increase was higher than E increase. The more accentuated increase of NE than E, suggests sympathetic nervous system hyperactivity. In summary, neonatal maternal deprivation induces long-term alterations on HPA axis sensitivity and medullo adrenal secretion; enhanced sympathetic nervous system activity and, therefore affected the ADTN inhibitory influence on ACTH and adrenal glands secretion.

Gender-Dependent Effects of Early Maternal Separation and Variable Chronic Stress on Vasopressinergic Activity and Glucocorticoid Receptor Expression in Adult Rats

The aim of the present study was to investigate the influence of early maternal separation on Fos, arginine vasopressin (AVP) and glucocorticoid receptor (GR) expression in the medial parvocellular portion of the paraventricular hypothalamic nucleus (PaMP), and GR expression in the hippocampus of adult male and female rats subjected to variable chronic stress (VCS). Male and female Wistar rats were isolated 4.5 h daily, during the first 3 weeks of life. At 48 days of age, the rats were exposed to VCS. Nonmaternally separated (NMS) females had a higher number of activated AVP neurons than NMS male rats. Maternally separated (MS) females subjected to VCS also showed a higher number of Fos/AVP double-labeled neurons than males with the same treatment. Males and females subjected to early maternal separation and VCS, compared with the MS animals, showed a decrease in the expression of GR in the PaMP. As regards GR expression in the hippocampus, MS animals subjected to VCS as adults, both males and females, showed an increase in GR expression in the subfields CA1, CA2 and CA3. The increase in AVP-immunoreactive neurons coexpressing Fos in response to stress in females exposed to early maternal separation suggests that perhaps early life stress results in a more reactive neuroendocrine stress response in females. Furthermore, our results demonstrate that the different anatomical levels of the hypothalamic-pituitary-adrenal axis have different roles related to its stress response and support the evidence of regional specificity in GR regulation.

Effect of maternal separation and chronic stress on hippocampal-dependent memory in young adult rats: evidence for the match-mismatch hypothesis.

Abstract Adverse experiences early in life may sensitize the hippocampus to subsequent stressors throughout the individuals life. We analyzed in male rats, whether, the interaction between early maternal separation and chronic stress affects: (1) the volume of the dorsal hippocampus, (2) CA1, CA2/3 and dentate gyrus (DG) and (Figure 3) hippocampal-dependent memory in adulthood. Male Wistar rats were subjected to daily maternal separation for 4.5 h between postnatal days 1–21. From postnatal day 50, animals were exposed to a chronic unpredictable stress paradigm during 24 days. The volumes of the dorsal hippocampus, their areas or strata did not reveal significant differences between treatments. Non-maternally separated and stressed animals showed poor hippocampal performance in a contextual fear conditioning test, with a significant reduction in freezing behavior during post-conditioning compared with control and maternally separated and stressed animals. Also, memory retrieval 24 h after conditioning was significantly weaker in this group than in control animals. Memory performance in maternally separated and stressed rats was similar to control animals. Our results show an interaction between early environment experiences and chronic variable stress in young adulthood as evidence that early stressful experiences do not necessarily lead to a negative outcome but can help in maintaining brain plasticity and increase fitness when animals reach adulthood.

Early maternal separation and chronic variable stress in adulthood changes the neural activity and the expression of glucocorticoid receptor in limbic structures.

There is increasing evidence that early adverse experience contributes to the development of stress susceptibility, and increases the onset of stress‐related psychiatric disorders in stressful environments in adulthood. This study addressed whether or not prolonged maternal separation, a well‐established model of early stress, affects adult limbic areas related to the regulation of the hypothalamic‐pituitary‐adrenal axis in exposure to chronic variable stress in adulthood. Rats were subjected to daily maternal separation for 4.5 h during postnatal days 1–21. As adults, the animals were exposed to a variable chronic stress paradigm of 24 days. Persistent changes were assessed in glucocorticoid receptor density and Fos activity in the anterodorsal thalamic nuclei, mammillary nuclei and retrosplenial cortex. Immunohistochemical analysis revealed that adult maternally separated animals had increased levels of c‐Fos immunoreactivity in the anterodorsal thalamic nuclei as well as in the mammillary nuclei compared to normal non‐maternally separated animals. Chronic variable stress in maternally separated and non‐maternally separated animals diminished glucocorticoid receptor density in the anterodorsal thalamic nuclei but not in the rest of the nuclei analyzed. These results indicate that c‐Fos immunoreactivity as well as glucocorticoid receptor expression in the anterodorsal thalamic nuclei and mammillary nuclei exhibit long‐term alterations in adult rats following repeated maternal separation and subsequent stress exposure. Recognition of these adaptations helps to define the brain regions and neural circuitry associated with persistent alterations induced by early life environment and the development of stress‐associated disorders.

Long-term effects of maternal separation on chronic stress response suppressed by amitriptyline treatment

Abstract The early-life environment has many long-term effects on mammals. Maternal interaction and early stressful events may affect regulation of the HPA axis during adulthood, leading to differential glucocorticoid secretion in response to stressful situations. These adverse experiences during postnatal development may even sensitize specific neurocircuits to subsequent stressors. Later in life, the overreaction of the HPA axis to stress can constitute a risk factor for metabolic and mental diseases. As tricyclic antidepressants are known to correct glucocorticoid hypersecretion during depression, we treated maternally separated animals with amitriptyline, at a lower dose than habitually used in depression models, to prevent the response to chronic stress during adulthood. Male Wistar rats were separated from the mother for 4.5 h every day for the first 3 weeks of life. From postnatal day 50, animals were subjected to chronic variable stress during 24 d (five types of stressors at different times of day). During the stress, protocol rats were orally administered amitriptyline (5 mg/kg) daily. We observed that maternal separation caused a reduction in plasma ACTH levels (p < 0.05), but evoked hypersecretion of corticosterone (p < 0.05) when it was combined with stress in adulthood. This rise was completely prevented by antidepressant treatment with amitriptyline.