Long-Term Effects of Prenatal Severe Hypoxia on Central and Peripheral Components of the Glucocorticoid System in Rats

Prenatal Hypoxia Triggers a Glucocorticoid-Associated Depressive-like Phenotype in Adult Rats, Accompanied by Reduced Anxiety in Response to Stress

Fetal hypoxia and maternal stress frequently culminate in neuropsychiatric afflictions in life. To replicate this condition, we employed a model of prenatal severe hypoxia (PSH) during days 14-16 of rat gestation. Subsequently, both control and PSH rats at 3 months old were subjected to episodes of inescapable stress to induce learned helplessness (LH). The results of the open field test revealed an inclination towards depressive-like behavior in PSH rats. Following LH episodes, control (but not PSH) rats displayed significant anxiety. LH induced an increase in glucocorticoid receptor (GR) levels in extrahypothalamic brain structures, with enhanced nuclear translocation in the hippocampus (HPC) observed both in control and PSH rats. However, only control rats showed an increase in GR nuclear translocation in the amygdala (AMG). The decreased GR levels in the HPC of PSH rats correlated with elevated levels of hypothalamic corticotropin-releasing hormone (CRH) compared with the controls. However, LH resulted in a reduction of the CRH levels in PSH rats, aligning them with those of control rats, without affecting the latter. This study presents evidence that PSH leads to depressive-like behavior in rats, associated with alterations in the glucocorticoid system. Notably, these impairments also contribute to increased resistance to severe stressors.

The Prenatal Hypoxic Pathology Associated with Maternal Stress Predisposes to Dysregulated Expression of the chrna7 Gene and the Subsequent Development of Nicotine Addiction in Adult Offspring

INTRODUCTION

Previous studies have shown that fetal hypoxia predisposes individuals to develop addictive disorders in adulthood. However, the specific impact of maternal stress, mediated through glucocorticoids and often coexisting with fetal hypoxia, is not yet fully comprehended.

METHODS

To delineate the potential effects of these pathological factors, we designed models of prenatal severe hypoxia (PSH) in conjunction with maternal stress and prenatal intrauterine ischemia (PII). We assessed the suitability of these models for our research objectives by measuring HIF1α levels and evaluating the glucocorticoid neuroendocrine system. To ascertain nicotine dependence, we employed the conditioned place aversion test and the startle response test. To identify the key factor implicated in nicotine addiction associated with PSH, we employed techniques such as Western blot, immunohistochemistry, and correlational analysis between chrna7 and nr3c1 genes across different brain structures.

RESULTS

In adult rats exposed to PSH and PII, we observed increased levels of HIF1α in the hippocampus (HPC). However, the PSH group alone exhibited reduced glucocorticoid receptor levels and disturbed circadian glucocorticoid rhythms. Additionally, they displayed signs of nicotine addiction in the conditioned place aversion and startle response tests. We also observed elevated levels of phosphorylated DARPP-32 protein in the nucleus accumbens (NAc) indicated compromised glutamatergic efferent signaling. Furthermore, there was reduced expression of α7 nAChR, which modulates glutamate release, in the medial prefrontal cortex (PFC) and HPC. Correlation analysis revealed strong associations between chrna7 and nr3c1 expression in both brain structures.

CONCLUSION

Perturbations in the glucocorticoid neuroendocrine system and glucocorticoid-dependent gene expression of chrna7 associated with maternal stress response to hypoxia in prenatal period favor the development of nicotine addiction in adulthood.

Prenatal Hypoxia-Induced Adverse Reaction to Mild Stress is Associated with Depressive-Like Changes in the Glucocorticoid System of Rats

The effects of prenatal hypoxia on neurodevelopment are predominantly associated with impaired maternal glucocorticoid stimulation of the fetus, which is "imprinted" in altered sensitivity of glucocorticoid reception in brain structures of offspring and can affect brain plasticity during lifespan. This study aimed to investigate response of the brain glucocorticoid system to mild stress (MS) in adult rats that survived prenatal severe hypoxia (PSH) on embryonic days 14-16. In response to MS the control (but not PSH) rats demonstrate increased corticosterone levels, a decrease in exploratory activity and increased anxiety. In the raphe nuclei of adult PSH rats the expression of glucocorticoid receptors (GR) is increased without changes in serotonin levels in comparison with the control. MS induces a decrease in GR expression accompanied by up-regulation of tryptophan hydroxylase 2 (tph2) and down-regulation of monoamine oxidase A (maoa) transcription in the raphe nuclei of both control and PSH groups. PSH also causes significant deviations in GR expression and GR-dependent transcription in the hippocampus, the medial prefrontal cortex, but not in the amygdala of rats. However, in response to MS, PSH rats demonstrate mild changes in their activity, while in control animals the MS-induced activity of the glucocorticoid system in these brain structures is similar to intact PSH animals. Impaired activity of the glucocorticoid system in the extrahypothalamic brain structures of PSH rats is accompanied by increase in the hypothalamic corticotropin-releasing hormone (CRH) levels in comparison with the control regardless of MS. Synthesis of proopiomelanocortin (POMC) and release of adrenocorticotropic hormone (ACTH) into the blood are decreased in response to MS in the pituitary of control rats, which demonstrates a negative glucocorticoid feedback mechanism. Meanwhile, in the pituitary of PSH rats reduced POMC levels were found regardless of MS. Thus, prenatal hypoxia causes depression-like patterns in the brain glucocorticoid system with adverse reaction to mild stressors.

Prenatal Hypoxia Affects Nicotine Consumption and Withdrawal in Adult Rats via Impairment of the Glutamate System in the Brain

The role of damaging factors in the prenatal period as a basis for drug addiction in offspring is of great interest. In this study, we aim at deciphering the effects and possible mechanisms of prenatal severe hypoxia (PSH) on predisposition to nicotine addiction in adult rats. In PSH rats, we found an increasing tendency to nicotine consumption in the two-bottle choice test. After 2 weeks of chronic treatment with nicotine via osmotic minipump (9 mg/kg per day), we assessed the symptoms of withdrawal in the conditioned place aversion test after mecamylamine (an antagonist of nicotinic acetylcholine receptors, nAChR) treatment. We showed that the mecamylamine-precipitated withdrawal aversion was stronger in the PSH group than in the control group. This suggests that PSH acts as a predisposing factor for developing nicotine addiction in adulthood. PSH rats also demonstrated an increased level of phosphorylated DARPP-32 protein (known as the relay for dopamine and glutamate signaling) at 34 threonine (pThr34DARPP-32) in relation to its total amount in the nucleus accumbens of the striatum (NAc). Meanwhile, no changes in both the content of dopamine in the mesolimbic pathway and the first type of dopamine receptors (DAR1) in NAc were found. The increased rate of DARPP-32 phosphorylation in adult PSH rats might result from excessive glutamatergic stimulation of the dopaminergic (DA) neurons of the ventral tegmental area (VTA) caused by activation of presynaptic nAChR by nicotine. This hypothesis is supported by the observed increase in VGluT2-positive terminals to Nurr1-positive neuronal bodies in VTA in PSH animals. Thus, the altered glutamate signaling phenotype might play a significant role in the development of PSH-related nicotine addiction.

Effects of Prenatal Hypoxia on Nervous System Development and Related Diseases

The fetal origins of adult disease (FOAD) hypothesis, which was proposed by David Barker in the United Kingdom in the late 1980s, posited that adult chronic diseases originated from various adverse stimuli in early fetal development. FOAD is associated with a wide range of adult chronic diseases, including cardiovascular disease, cancer, type 2 diabetes and neurological disorders such as schizophrenia, depression, anxiety, and autism. Intrauterine hypoxia/prenatal hypoxia is one of the most common complications of obstetrics and could lead to alterations in brain structure and function; therefore, it is strongly associated with neurological disorders such as cognitive impairment and anxiety. However, how fetal hypoxia results in neurological disorders remains unclear. According to the existing literature, we have summarized the causes of prenatal hypoxia, the effects of prenatal hypoxia on brain development and behavioral phenotypes, and the possible molecular mechanisms.

Glucocorticoid-Dependent Mechanisms of Brain Tolerance to Hypoxia

Adaptation of organisms to stressors is coordinated by the hypothalamic-pituitary-adrenal axis (HPA), which involves glucocorticoids (GCs) and glucocorticoid receptors (GRs). Although the effects of GCs are well characterized, their impact on brain adaptation to hypoxia/ischemia is still understudied. The brain is not only the most susceptible to hypoxic injury, but also vulnerable to GC-induced damage, which makes studying the mechanisms of brain hypoxic tolerance and resistance to stress-related elevation of GCs of great importance. Cross-talk between the molecular mechanisms activated in neuronal cells by hypoxia and GCs provides a platform for developing the most effective and safe means for prevention and treatment of hypoxia-induced brain damage, including hypoxic pre- and post-conditioning. Taking into account that hypoxia- and GC-induced reprogramming significantly affects the development of organisms during embryogenesis, studies of the effects of prenatal and neonatal hypoxia on health in later life are of particular interest. This mini review discusses the accumulated data on the dynamics of the HPA activation in injurious and non-injurious hypoxia, the role of the brain GRs in these processes, interaction of GCs and hypoxia-inducible factor HIF-1, as well as cross-talk between GC and hypoxic signaling. It also identifies underdeveloped areas and suggests directions for further prospective studies.

Bidirectional Crosstalk Between Hypoxia Inducible Factors and Glucocorticoid Signalling in Health and Disease

Glucocorticoid-induced (GC) and hypoxia-induced transcriptional responses play an important role in tissue homeostasis and in the regulation of cellular responses to stress and inflammation. Evidence exists that there is an important crosstalk between both GC and hypoxia effects. Hypoxia is a pathophysiological condition to which cells respond quickly in order to prevent metabolic shutdown and death. The hypoxia inducible factors (HIFs) are the master regulators of oxygen homeostasis and are responsible for the ability of cells to cope with low oxygen levels. Maladaptive responses of HIFs contribute to a variety of pathological conditions including acute mountain sickness (AMS), inflammation and neonatal hypoxia-induced brain injury. Synthetic GCs which are analogous to the naturally occurring steroid hormones (cortisol in humans, corticosterone in rodents), have been used for decades as anti-inflammatory drugs for treating pathological conditions which are linked to hypoxia (i.e. asthma, ischemic injury). In this review, we investigate the crosstalk between the glucocorticoid receptor (GR), and HIFs. We discuss possible mechanisms by which GR and HIF influence one another, in vitro and in vivo, and the therapeutic effects of GCs on HIF-mediated diseases.