Environmental enrichment alters gene expression of steroidogenic enzymes in the rat hippocampus

Steroidogenic enzymes in the hippocampus: Transcriptional regulation aspects

Neurosteroids are steroids synthesized de novo from cholesterol in brain regions, and regulate processes associated with the development and functioning of the nervous system. Enzymes and proteins involved in the synthesis of these steroids have been detected in several brain regions, including hippocampus, hypothalamus, and cerebral cortex. Hippocampus has long been associated with learning and memory functions, while the loss of its functionality has been linked to neurodegenerative pathologies. In this sense, neurosteroids are critical for the maintenance of hippocampal functions and neuroprotective effects. Moreover, several factors have been shown to deregulate expression of steroidogenic enzymes in the rodent brain, including aging, enrichment experiences, diet habits, drug/alcohol consumption, hormone fluctuations, neurodegenerative processes and other diseases. These transcriptional deregulations are mediated mainly by transcription factors and epigenetic mechanisms. An epigenetic modification of chromatin involves changes in bases and associated proteins in the absence of changes in the DNA sequence. One of the most well-studied mechanisms related to gene silencing is DNA methylation, which involves a reversible addition of methyl groups in a cytosine base. Importantly, these epigenetic marks could be maintained over time and could be transmitted transgenerationally. The aim of this chapter is to present the most relevant steroidogenic enzymes described in rodent hippocampus; to discuss about their transcriptional regulation under different conditions; to show the main gene control regions and to propose DNA methylation as an epigenetic mechanism through which the expression of these enzymes could be controlled.

The role of allopregnanolone in depressive-like behaviors: Focus on neurotrophic proteins

Allopregnanolone (3α,5α-tetrahydroprogesterone; pharmaceutical formulation: brexanolone) is a neurosteroid that has recently been approved for the treatment of postpartum depression, promising to fill part of a long-lasting gap in the effectiveness of pharmacotherapies for depressive disorders. In this review, we explore the experimental research that characterized the antidepressant-like effects of allopregnanolone, with a particular focus on the neurotrophic adaptations induced by this neurosteroid in preclinical studies. We demonstrate that there is a consistent decrease in allopregnanolone levels in limbic brain areas in rodents submitted to stress-induced models of depression, such as social isolation and chronic unpredictable stress. Further, both the drug-induced upregulation of allopregnanolone or its direct administration reduce depressive-like behaviors in models such as the forced swim test. The main drugs of interest that upregulate allopregnanolone levels are selective serotonin reuptake inhibitors (SSRIs), which present the neurosteroidogenic property even in lower, non-SSRI doses. Finally, we explore how these antidepressant-like behaviors are related to neurogenesis, particularly in the hippocampus. The protagonist in this mechanism is likely the brain-derived neurotrophic factor (BFNF), which is decreased in animal models of depression and may be restored by the normalization of allopregnanolone levels. The role of an interaction between GABA and the neurotrophic mechanisms needs to be further investigated.

Early exposure to environmental enrichment modulates the effects of prenatal ethanol exposure upon opioid gene expression and adolescent ethanol intake

Prenatal ethanol exposure (PEE) promotes ethanol consumption in the adolescent offspring accompanied by the transcriptional regulation of kappa opioid receptor (KOR) system genes. This study analysed if environmental enrichment (EE, from gestational day 20 to postnatal day 26) exerts protective effects upon PEE-modulation of gene expression, ethanol intake and anxiety responses. Pregnant rats were exposed to PEE (0.0 or 2.0 g/kg ethanol, gestational days 17-20) and subsequently the dam and offspring were reared under EE or standard conditions. PEE upregulated KOR mRNA levels in amygdala (AMY) and prodynorphin (PDYN) mRNA levels in ventral tegmental area (VTA) with the latter effect associated with lower DNA methylation at the gene promoter. These effects were normalized by exposure to EE. PEE modulated BDNF mRNA levels in VTA and Nucleus accumbens (AcbN), and EE mitigated the changes in AcbN. EE induced a protective effect on ethanol intake and preference, an effect more noticeable in males than in females, and in prenatal vehicle-treated (PV) than in PEE rats. The male offspring drank significantly less ethanol than the female offspring. The latter result suggests that the protective effect of EE on ethanol drinking may only emerge at lower levels of drinking. In the dams, PEE induced an upregulation of PDYN and KOR in AcbN. PDYN gene expression was normalized by exposure to EE. These results suggest that EE is a promising treatment to inhibit the effects of PEE. The results confirm that PEE effects are mediated by alterations in the transcriptional regulation of KOR system genes.

The impact of sensory and motor enrichment on the epigenetic control of steroidogenic-related genes in rat hippocampus

In the present study, we analyzed the effects of a short-term environmental enrichment on the mRNA expression and DNA methylation of steroidogenic enzymes in the hippocampus. Thus, young adult (80-day-old) and middle-aged (350-day-old) Wistar female rats were exposed to sensory (SE) or motor (ME) enrichment during 10 days and compared to animals housed under standard conditions. SE was provided by an assortment of objects that included plastic tubes and toys; for ME, rodent wheels were provided. In young adult animals, SE and ME increased the mRNA expression of cytochrome P450 17α-hydroxylase/c17,20-lyase, steroid 5α-reductase type 1 (5αR-1) and 3α-hydroxysteroid dehydrogenase and decreased the methylation levels of 5αR-1 gene. In middle-aged rats, ME and SE upregulated the gene expression of aldosterone synthase and decreased the methylation state of its promoter. These results propose that SE and ME differentially regulate the transcription of neurosteroidogenic enzymes through epigenetic mechanisms in young and aged rats.

Gestational chronodisruption leads to persistent changes in the rat fetal and adult adrenal clock and function

KEY POINTS

Light at night is essential to a 24/7 society, but it has negative consequences on health. Basically, light at night induces an alteration of our biological clocks, known as chronodisruption, with effects even when this occurs during pregnancy. Here we explored the developmental impact of gestational chronodisruption (chronic photoperiod shift, CPS) on adult and fetal adrenal biorhythms and function. We found that gestational chronodisruption altered fetal and adult adrenal function, at the molecular, morphological and physiological levels. The differences between control and CPS offspring suggest desynchronization of the adrenal circadian clock and steroidogenic pathway, leading to abnormal stress responses and metabolic adaptation, potentially increasing the risk of developing chronic diseases.

ABSTRACT

Light at night is essential to a 24/7 society, but it has negative consequences on health. Basically, light at night induces an alteration of our biological clocks, known as chronodisruption, with effects even when this occurs during pregnancy. Indeed, an abnormal photoperiod during gestation alters fetal development, inducing long-term effects on the offspring. Accordingly, we carried out a longitudinal study in rats, exploring the impact of gestational chronodisruption on the adrenal biorhythms and function of the offspring. Adult rats (90 days old) gestated under chronic photoperiod shift (CPS) decrease the time spent in the open arm zone of an elevated plus maze to 62% and increase the rearing time to 170%. CPS adults maintained individual daily changes in corticosterone, but their acrophases were distributed from 12.00 h to 06.00 h. CPS offspring maintained clock gene expression and oscillation, nevertheless no daily rhythm was observed in genes involved in the regulation and synthesis of steroids. Consistent with adult adrenal gland being programmed during fetal life, blunted daily rhythms of corticosterone, core clock gene machinery, and steroidogenic genes were observed in CPS fetal adrenal glands. Comparisons of the global transcriptome of CPS versus control fetal adrenal gland revealed that 1078 genes were differentially expressed (641 down-regulated and 437 up-regulated). In silico analysis revealed significant changes in Lipid Metabolism, Small Molecule Biochemistry, Cellular Development and the Inflammatory Response pathway (z score: 48-20). Altogether, the present results demonstrate that gestational chronodisruption changed fetal and adult adrenal function. This could translate to long-term abnormal stress responses and metabolic adaptation, increasing the risk of developing chronic diseases.

Maternal fructose intake disturbs ovarian estradiol synthesis in rats

AIMS

Recent increases in fructose consumption have raised concerns regarding the potential adverse intergenerational effects, as maternal fructose intake may induce physiological dysfunction in offspring. However, no reports are available regarding the effect of excess maternal fructose on reproductive tissues such as the ovary. Notably, the maternal intrauterine environment has been demonstrated to affect ovarian development in the subsequent generation. Given the fructose is transferred to the fetus, excess fructose consumption may affect offspring ovarian development. As ovarian development and its function is maintained by 17β-estradiol, we therefore investigated whether excess maternal fructose intake influences offspring ovarian estradiol synthesis. Rats received a 20% fructose solution during gestation and lactation. After weaning, offspring ovaries were isolated.

KEY FINDINGS

Offspring from fructose-fed dams showed reduced StAR and P450(17α) mRNA levels, along with decreased protein expression levels. Conversely, attenuated P450arom protein level was found in the absence of mRNA expression alteration. Consistent with these phenomena, decreased circulating levels of estradiol were observed. Furthermore, estrogen receptor α (ERα) protein levels were also down-regulated. In accordance, the mRNA for progesterone receptor, a transcriptional target of ERα, was decreased. These results suggest that maternal fructose might alter ovarian physiology in the subsequent generation.

Neurosteroids in Adult Hippocampus of Male and Female Rodents: Biosynthesis and Actions of Sex Steroids

The brain is not only the target of steroid hormones but also is able to locally synthesize steroids de novo. Evidence of the local production of steroids in the brain has been accumulating in various vertebrates, including teleost fish, amphibia, birds, rodents, non-human primates, and humans. In this review, we mainly focus on the local production of sex steroids in the hippocampal neurons of adult rodents (rats and mice), a center for learning and memory. From the data of the hippocampus of adult male rats, hippocampal principal neurons [pyramidal cells in CA1-CA3 and granule cells in dentate gyrus (DG)] have a complete system for biosynthesis of sex steroids. Liquid chromatography with tandem-mass-spectrometry (LC-MS/MS) enabled us to accurately determine the levels of hippocampal sex steroids including 17β-estradiol (17β-E2), testosterone (T), and dihydrotestosterone (DHT), which are much higher than those in blood. Next, we review the steroid synthesis in the hippocampus of female rats, since previous knowledge had been biased toward the data from males. Recently, we clarified that the levels of hippocampal steroids fluctuate in adult female rats across the estrous cycle. Accurate determination of hippocampal steroids at each stage of the estrous cycle is of importance for providing the account for the fluctuation of female hippocampal functions, including spine density, long-term potentiation (LTP) and long-term depression (LTD), and learning and memory. These functional fluctuations in female had been attributed to the level of circulation-derived steroids. LC-MS/MS analysis revealed that the dendritic spine density in CA1 of adult female hippocampus correlates with the levels of hippocampal progesterone and 17β-E2. Finally, we introduce the direct evidence of the role of hippocampus-synthesized steroids in hippocampal function including neurogenesis, LTP, and memory consolidation. Mild exercise (2 week of treadmill running) elevated synthesis of DHT in the hippocampus, but not in the testis, of male rats, resulting in enhancement of neurogenesis in DG. Concerning synaptic plasticity, hippocampus-synthesized E2 is required for LTP induction, whereas hippocampus-synthesized DHT is required for LTD induction. Furthermore, hippocampus-synthesized E2 is involved in memory consolidation tested by object recognition and object placement tasks, both of which are hippocampus-dependent.

The role of cognitive activity in cognition protection: from Bedside to Bench

BACKGROUND

Cognitive decline poses a great concern to elderly people and their families. In addition to pharmacological therapies, several varieties of nonpharmacological intervention have been developed. Most training trials proved that a well-organized task is clinically effective in cognition improvement.

MAIN BODY

We will first review clinical trials of cognitive training for healthy elders, MCI and AD patients, respectively. Besides, potential neuroprotective and compensatory mechanisms in animal models of AD are discussed. Despite controversy, cognitive training has promising effect on cognitive ability. In animal model of AD, environmental enrichment showed beneficial effect for cognitive ability, as well as neuronal plasticity. Neurotrophin, neurotransmitter and neuromodulator signaling pathway were also involved in the process. Well-designed cognitive activity could benefit cognitive function, and thus life quality of patients and their families.

CONCLUSION

The positive effects of cognitive activity is closely related with neural plasticity, neurotrophin, neurotransmitter and neuromodulator signaling pathway changes.

Fructose intake during gestation and lactation differentially affects the expression of hippocampal neurosteroidogenic enzymes in rat offspring

Neurosteroids, steroidal hormones synthesized de novo from cholesterol within the brain, stimulate hippocampal functions such as neuron protection and synapse formation. Previously, we examined the effect of maternal fructose on the transcriptional regulation of neurosteroidogenic enzymes. We found that the mRNA expression level of the steroidogenic acute regulatory protein (StAR), peripheral benzodiazepine receptor (PBR), cytochrome P450(11β), 11β-hydroxysteroid dehydrogenase (HSD), and 17β-HSD was altered. However, we could not determine whether maternal fructose intake played a role in the gestation or lactation period because the dam rats were fed fructose solution during both periods. Thus, in this study, we analyzed the hippocampi of the offspring of dams fed fructose during the gestation or lactation period. Maternal fructose consumption during either the gestation or lactation period did not affect the mRNA levels of StAR, P450(17α), 11β-HSD-2, and 17β-HSD-1. PBR expression was down-regulated, even when rats consumed fructose during the lactation period only, while fructose consumption during gestation tended to activate the expression of P450(11β)-2. We found that maternal fructose intake during gestation and lactation differentially affected the expression of hippocampal neurosteroidogenic enzymes in the offspring.

Environmental enrichment attenuates the age-related decline in the mRNA expression of steroidogenic enzymes and reduces the methylation state of the steroid 5α-reductase type 1 gene in the rat hippocampus

We analyzed the effects of aging and environmental enrichment on the mRNA expression and DNA methylation state of steroidogenic enzymes in the hippocampus. The effects of aging were evaluated by comparing young adult (90-day-old) and middle-aged (450-day-old) female Wistar rats. To elucidate the effects of environmental enrichment, a subgroup of middle-aged rats exposed to sensory and social stimulation for 105 days was compared to rats housed under standard laboratory conditions. Aging decreased the transcription of neurosteroidogenic-related genes and increased the promoter methylation state of cytochrome P450 side chain cleavage, 3α-hydroxysteroid dehydrogenase (3α-HSD) and 5α-reductase-1. Exposure of middle-aged rats to environmental enrichment increased mRNA levels of 5α-reductase-1, 3α-HSD and cytochrome P450 17α-hydroxylase/c17,20-lyase and decreased the methylation state of the 5α-reductase-1 gene. Thus, sensory and social stimulation attenuate the age-related decline in the mRNA expression of hippocampal steroidogenic enzymes. Epigenetic mechanisms associated with differential promoter methylation could be involved.

Stimulation of estradiol biosynthesis by tributyltin in rat hippocampal slices

Hippocampal functions are influenced by steroid hormones, such as testosterone and estradiol. It has been demonstrated that hippocampus-derived steroid hormones play important roles in neuronal protection and synapse formation. Our research groups have demonstrated that estradiol is de novo synthesized in the rat hippocampus. However, the mechanism(s) regulating this synthesis remains unclear. It has been reported that tributyltin, an environmental pollutant, binds to the retinoid X receptor (RXR) and modifies estrogen synthesis in human granulosa-like tumor cells. This compound can penetrate the blood brain barrier, and tends to accumulate in the brain. Based on these facts, we hypothesized that tributyltin could influence the hippocampal estradiol synthesis. A concentration of 0.1 μM tributyltin induced an increase in the mRNA content of P450(17α) and P450arom in hippocampal slices, as determined using real-time PCR. The transcript levels of other steroidogenic enzymes and a steroidogenic acute regulatory protein were not affected. The estradiol level in rat hippocampal slices was subsequently determined using a radioimmunoassay. We found that the estradiol synthesis was stimulated by ∼2-fold following a 48-h treatment with 0.1 μM tributyltin, and this was accompanied by transcriptional activation of P450(17α) and P450arom. Tributyltin stimulated de novo hippocampal estradiol synthesis by modifying the transcription of specific steroidogenic enzymes.