Jérôme Mairesse

Prenatal restraint stress generates two distinct behavioral and neurochemical profiles in male and female rats

Prenatal Restraint Stress (PRS) in rats is a validated model of early stress resulting in permanent behavioral and neurobiological outcomes. Although sexual dimorphism in the effects of PRS has been hypothesized for more than 30 years, few studies in this long period have directly addressed the issue. Our group has uncovered a pronounced gender difference in the effects of PRS (stress delivered to the mothers 3 times per day during the last 10 days of pregnancy) on anxiety, spatial learning, and a series of neurobiological parameters classically associated with hippocampus-dependent behaviors. Adult male rats subjected to PRS (“PRS rats”) showed increased anxiety-like behavior in the elevated plus maze (EPM), a reduction in the survival of newborn cells in the dentate gyrus, a reduction in the activity of mGlu1/5 metabotropic glutamate receptors in the ventral hippocampus, and an increase in the levels of brain-derived neurotrophic factor (BDNF) and pro-BDNF in the hippocampus. In contrast, female PRS rats displayed reduced anxiety in the EPM, improved learning in the Morris water maze, an increase in the activity of mGlu1/5 receptors in the ventral and dorsal hippocampus, and no changes in hippocampal neurogenesis or BDNF levels. The direction of the changes in neurogenesis, BDNF levels and mGlu receptor function in PRS animals was not consistent with the behavioral changes, suggesting that PRS perturbs the interdependency of these particular parameters and their relation to hippocampus-dependent behavior. Our data suggest that the epigenetic changes in hippocampal neuroplasticity induced by early environmental challenges are critically sex-dependent and that the behavioral outcome may diverge in males and females.

The Fat-Induced Satiety Factor Oleoylethanolamide Suppresses Feeding through Central Release of Oxytocin

Oleoylethanolamide (OEA) is a biologically active lipid amide that is released by small-intestinal enterocytes during the absorption of dietary fat and inhibits feeding by engaging the nuclear receptor, peroxisome proliferator-activated receptor-α (PPAR-α). Previous studies have shown that the anorexic effects of systemically administered OEA require the activation of sensory afferents of the vagus nerve. The central circuits involved in mediating OEA-induced hypophagia remain unknown. In the present study, we report the results of in situ hybridization and immunohistochemistry experiments in rats and mice, which show that systemic injections of OEA (5–10 mg kg−1, intraperitoneal) enhance expression of the neuropeptide oxytocin in magnocellular neurons of the paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus. No such effect is observed with other hypothalamic neuropeptides, including vasopressin, thyrotropin-releasing hormone and pro-opiomelanocortin. The increase in oxytocin expression elicited by OEA was absent in mutant PPAR-α-null mice. Pharmacological blockade of oxytocin receptors in the brain by intracerebroventricular infusion of the selective oxytocin antagonist, L-368,899, prevented the anorexic effects of OEA. The results suggest that OEA suppresses feeding by activating central oxytocin transmission.

Prenatal stress alters Fos protein expression in hippocampus and locus coeruleus stress-related brain structures

Prenatal stress (PS) durably influences responses of rats from birth throughout life by inducing deficits of the hypothalamo-pituitary-adrenal (HPA) axis feedback. The neuronal mechanisms sustaining such alterations are still unknown. The purpose of the present study was to determine whether in PS and control rats, the exposure to a mild stressor differentially induces Fos protein in hippocampus and locus coeruleus, brain areas involved in the feedback control of the HPA axis. Moreover, Fos protein expression was also evaluated in the hypothalamic paraventricular nucleus (PVN) that reflect the magnitude of the hormonal response to stress. Basal plasma corticosterone levels were not different between the groups, while, PS rats exhibited higher number of Fos-immunoreactive neurons than controls, in the hippocampus and locus coeruleus in basal condition. A higher basal expression of a marker of GABAergic synapses, the vGAT, was also observed in the hypothalamus of PS rats. Fifteen minutes after the end of the exposure to the open arm of the elevated plus-maze (mild stress) a similar increased plasma corticosterone levels was observed in both groups in parallel with an increased number of Fos-immunoreactive neurons in the PVN. Return to basal plasma corticosterone values was delayed only in the PS rats. On the contrary, after stress, no changes in Fos-immunoreactivity were observed in the hippocampus and locus coeruleus of PS rats compared to basal condition. After stress, only PS rats presented an elevation of the number of activated catecholaminergic neurons in the locus coeruleus. In conclusion, these results suggest for the first time that PS alters the neuronal activation of hippocampus and locus coeruleus implicated in the feedback mechanism of the HPA axis. These data give anatomical substrates to sustain the HPA axis hyperactivity classically described in PS rats after stress exposure.

Anxiety-Like Behavior of Prenatally Stressed Rats Is Associated with a Selective Reduction of Glutamate Release in the Ventral Hippocampus

Abnormalities of synaptic transmission and plasticity in the hippocampus represent an integral part of the altered programming triggered by early life stress. Prenatally restraint stressed (PRS) rats develop long-lasting biochemical and behavioral changes, which are the expression of an anxious/depressive-like phenotype. We report here that PRS rats showed a selective impairment of depolarization- or kainate-stimulated glutamate and [3H]d-aspartate release in the ventral hippocampus, a region encoding memories related to stress and emotions. GABA release was unaffected in PRS rats. As a consequence of reduced glutamate release, PRS rats were also highly resistant to kainate-induced seizures. Abnormalities of glutamate release were associated with large reductions in the levels of synaptic vesicle-related proteins, such as VAMP (synaptobrevin), syntaxin-1, synaptophysin, synapsin Ia/b and IIa, munc-18, and Rab3A in the ventral hippocampus of PRS rats. Anxiety-like behavior in male PRS (and control) rats was inversely related to the extent of depolarization-evoked glutamate release in the ventral hippocampus. A causal relationship between anxiety-like behavior and reduction in glutamate release was demonstrated using a mixture of the mGlu2/3 receptor antagonist, LY341495, and the GABAB receptor antagonist, CGP52432, which was shown to amplify depolarization-evoked [3H]d-aspartate release in the ventral hippocampus. Bilateral microinfusion of CGP52432 plus LY341495 in the ventral hippocampus abolished anxiety-like behavior in PRS rats. These findings indicate that an impairment of glutamate release in the ventral hippocampus is a key component of the neuroplastic program induced by PRS, and that strategies aimed at enhancing glutamate release in the ventral hippocampus correct the “anxious phenotype” caused by early life stress.

Prenatal stress has pro-inflammatory consequences on the immune system in adult rats

The in utero environment is critical for initiating the ontogeny of several physiological systems, including the immune surveillance. Yet, little is known about adverse early experiences on the offsprings immunity and vulnerability to disease. The present work aimed at investigating the impact of restraint prenatal stress (PS) on the development and responsiveness of in vitro and in vivo cellular and humoral immunity of male progeny aged 7 weeks and 6 months. In adult 6-month-old rats, we detected increased circulating CD8(+)-expressing and NK cells in PS rats as compared to controls, associated with higher mRNA expression of IFN-gamma. In addition, in vitro stimulation with phytohemagglutinin-A induced an increase in both the proliferation of T lymphocytes and the secretion of IFN-gamma in PS rats. Interestingly, these alterations were undetectable in younger PS rats (7-week old), except for a slight increase in the mRNA expression of several pro-inflammatory cytokines in peripheral blood mononuclear cells. Moreover, in vivo neutralization of IFN-gamma in young rats had no effects in PS group. In conclusion, we report for the first time long-lasting pro-inflammatory consequences of PS in rats.

The Effects of Antidepressant Treatment in Prenatally Stressed Rats Support the Glutamatergic Hypothesis of Stress-Related Disorders

Abnormalities of synaptic transmission in the hippocampus represent an integral part of the altered programming triggered by early life stress, which enhances the vulnerability to stress-related disorders in the adult life. Rats exposed to prenatal restraint stress (PRS) develop enduring biochemical and behavioral changes characteristic of an anxious/depressive-like phenotype. Most neurochemical abnormalities in PRS rats are found in the ventral hippocampus, a region that encodes memories related to stress and emotions. We have recently demonstrated a causal link between the reduction of glutamate release in the ventral hippocampus and anxiety-like behavior in PRS rats. To confer pharmacological validity to the glutamatergic hypothesis of stress-related disorders, we examined whether chronic treatment with two antidepressants with different mechanisms of action could correct the defect in glutamate release and associated behavioral abnormalities in PRS rats. Adult unstressed or PRS rats were treated daily with either agomelatine (40 mg/kg, i.p.) or fluoxetine (5 mg/kg, i.p.) for 21 d. Both treatments reversed the reduction in depolarization-evoked glutamate release and in the expression of synaptic vesicle-associated proteins in the ventral hippocampus of PRS rats. Antidepressant treatment also corrected abnormalities in anxiety-/depression-like behavior and social memory performance in PRS rats. The effect on glutamate release was strongly correlated with the improvement of anxiety-like behavior and social memory. These data offer the pharmacological demonstration that glutamatergic hypofunction in the ventral hippocampus lies at the core of the pathological phenotype caused by early life stress and represents an attractive pharmacological target for novel therapeutic strategies.

Maternal Exposure to Low Levels of Corticosterone during Lactation Protects the Adult Offspring against Ischemic Brain Damage

A growing body of evidence underscores the importance of early life events as predictors of health in adulthood. Abnormalities in maternal care or other forms of early postnatal stress induce long-term changes in behavior and influence the vulnerability to illnesses throughout life. Some of these changes may be produced by the activation of the hypothalamic-pituitary-adrenal (HPA) axis, which is invariably associated with stress. We used a model in which neonate rats are fed by mothers drinking water supplemented with 0.2 mg/ml corticosterone, the main glucocorticoid hormone in rodents. Plasma corticosterone levels increased in the dams to an extent similar to that induced by a mild stress. Corticosterone-treated dams also showed an increase in maternal care. Remarkably, adult rats that had been nursed by corticosterone-treated mothers were protected against neuronal damage and cognitive impairment produced by transient global brain ischemia. Neuroprotection was associated with a reduced HPA response to ischemia and was primarily decreased when corticosterone was injected at a dose that eliminated any difference in endogenous corticosterone levels between rats raised by mothers supplemented with corticosterone and their matched controls. These data suggest that an increased maternal care protects the offspring against ischemic neuronal damage and that at least a component of neuroprotection is mediated by a reduced response of the HPA axis to ischemia.

Proteomic characterization in the hippocampus of prenatally stressed rats.

Rats exposed to early life stress are considered as a valuable model for the study of epigenetic programming leading to mood disorders and anxiety in the adult life. Rats submitted to prenatal restraint stress (PRS) are characterized by an anxious/depressive phenotype associated with neuroadaptive changes in the hippocampus. We used the model of PRS to identify proteins that are specifically affected by early life stress. We therefore performed a proteomic analysis in the hippocampus of adult male PRS rats. We found that PRS induced changes in the expression profile of a number of proteins, involved in the regulation of signal transduction, synaptic vesicles, protein synthesis, cytoskeleton dynamics, and energetic metabolism. Immunoblot analysis showed significant changes in the expression of proteins, such as LASP-1, fascin, and prohibitin, which may lie at the core of the developmental programming triggered by early life stress.

Prenatal stress alters the negative correlation between neuronal activation in limbic regions and behavioral responses in rats exposed to high and low anxiogenic environments

Behavioral adaptation to an anxiogenic environment involves the activity of various interconnected limbic regions, such as the amygdala, hippocampus and prefrontal cortex. Prenatal stress (PS) in rats affects the ability to cope with environmental challenges and alters brain plasticity, leading to long-lasting behavioral and neurobiological alterations. We examined in PS and control animals whether behavioral reactivity was correlated to neuronal activation by assessing Fos protein expression in limbic regions of rats exposed to a low or high anxiogenic environment (the closed and open arms of an elevated plus maze, respectively). A negative correlation was found between behavioral and neuronal activation, with a lower behavioral reactivity and a higher neuronal response observed in rats exposed to the more anxiogenic environment (the open arm) with respect to the less anxiogenic environment (the closed arm). Interestingly, the variation in the neurobehavioral response between the two arms of the maze was less pronounced in rats that had been subjected to PS. This study provides a remarkable example of how long-lasting changes in brain plasticity induced by PS affect the ability of limbic neurons to cope with anxiogenic stimuli of different strength.

Impact of early life stress on alcohol consumption and on the short- and long-term responses to alcohol in adolescent female rats

We examined the interaction between early life stress and vulnerability to alcohol in female rats exposed to prenatal restraint stress (PRS rats). First we studied the impact of PRS on ethanol preference during adolescence. PRS slightly increased ethanol preference per se, but abolished the effect of social isolation on ethanol preference. We then studied the impact of PRS on short- and long-term responses to ethanol focusing on behavioral and neurochemical parameters related to depression/anxiety. PRS or unstressed adolescent female rats received 10% ethanol in the drinking water for 4 weeks from PND30 to PND60. At PND60, the immobility time in the forced-swim test did not differ between PRS and unstressed rats receiving water alone. Ethanol consumption had no effect in unstressed rats, but significantly reduced the immobility time in PRS rats. In contrast, a marked increase in the immobility time was seen after 5 weeks of ethanol withdrawal only in unstressed rats. Hippocampal levels of neuropeptide Y (NPY) and mGlu1a metabotropic glutamate receptors were increased at the end of ethanol treatment only in unstressed rats. Ethanol treatment had no effect on levels of corticotropin-releasing hormone (CRH) in the hippocampus, striatum, and prefrontal cortex of both groups of rats. After ethanol withdrawal, hippocampal levels of mGlu1 receptors were higher in unstressed rats, but lower in PRS rats, whereas NPY and CRH levels were similar in the two groups of rats. These data indicate that early life stress has a strong impact on the vulnerability and responsiveness to ethanol consumption during adolescence.