Programming for increased expression of hippocampal GAD67 mediated the hypersensitivity of the hypothalamic–pituitary–adrenal axis in male offspring rats with prenatal ethanol exposure

Abstract

An imbalance of excitatory and inhibitory signals in the brain has been proposed to be one of the main pathological features of various diseases related to hypothalamic–pituitary–adrenal axis (HPAA) dysfunction. Excessive glutamate release induces neuronal excitotoxicity, while glutamic acid decarboxylase (GAD) 67 promotes the transformation of excessive glutamate to γ-aminobutyric acid (GABA). Our previous studies demonstrated that prenatal ethanol exposure (PEE) causes foetal over-exposure to maternal corticosterone and hypersensitivity of the HPAA after birth, but its intrauterine programming mechanism is unknown. In this study, PEE was shown to lead to an enhanced potential excitatory ability of the hypothalamus and hypersensitivity of the HPAA, as well as mild abnormal hippocampal morphology, demethylation of the -1019 to -691-bp region in the hippocampal GAD67 promoter and upregulation of GAD67 expression accompanied by a reduction in glutamatergic neurons and increase in GABAergic neurons in PEE male offspring. Similar changes were also found in PEE male foetal rats. Furthermore, corticosterone increased the expression of the glucocorticoid receptor (GR) and GAD67 in foetal hippocampal H19-7 cells in a concentration-dependent manner, accompanied by demethylation of the GAD67 promoter, a decrease in glutamatergic neurons and increase in GABAergic neurons. The GR inhibitor, mifepristone, reversed the effects of corticosterone on H19-7 cells. These results suggested that PEE-induced excessive corticosterone can lead to upregulation of GAD67 through epigenetic modification mediated by the GR in the male foetal hippocampus, thereby weakening the negative regulation of the HPAA by the hippocampus and increasing the potential excitatory ability of the hypothalamus. These changes persisted until after birth, resulting in hypersensitivity of the HPAA. However, gender differences were observed in the hippocampal development, morphology and GAD67 expression associated with PEE. Programming for the increased expression of hippocampal GAD67 is a potential mechanism responsible for the hypersensitivity of the HPAA in PEE male rats.

Prenatal drug exposure and neurodevelopmental programming of glucocorticoid signalling

“…A similar pattern was observed for CRH (ie, significantly more induced by stress in alcohol‐exposed group). Baseline arginine vasopressin levels were lower in the alcohol‐exposed group, although, after stress, they were were significantly elevated compared to controls . Therefore, prenatal alcohol exposure may generate a hypersensitive programming of the HPA axis to stressful stimuli.…”

“…Animal, molecular and cellular studies have corroborated the clinical observation of abnormal stress response via characterisation of altered HPA function, GC signalling and stress‐related behavioural studies after prenatal alcohol exposure. That is, prenatal alcohol exposure can both variably affect basal corticosterone levels in rodents, and differentially regulate other HPA axis regulated hormones and associated stress responses.…”

“…A study by Lu et al evaluating adult male rats that were exposed chronically to alcohol in utero demonstrated differential regulation of HPA axis hormones compared to controls and after exposure to stressors. Alcohol‐exposed rats demonstrated decreased basal levels of ACTH compared to unexposed controls and, although basal corticosterone levels were not significantly different between the two groups, corticosterone levels in the alcohol‐exposed group were significantly increased relative to unexposed animals after exposure to a stressor.…”

“…Therefore, prenatal alcohol exposure may generate a hypersensitive programming of the HPA axis to stressful stimuli. In foetal rats exposed to prenatal alcohol, GR mRNA expression is increased in the hippocampus, along with a concurrent increase in GAD67 mRNA and protein expression in GABAergic neurones . The increase in GABAergic neurones may represent a compensatory response to the environment with excessive GC signalling, leading to alterations in hypothalamic function in adulthood via an imbalance of excitatory/inhibitory input .…”

“…In foetal rats exposed to prenatal alcohol, GR mRNA expression is increased in the hippocampus, along with a concurrent increase in GAD67 mRNA and protein expression in GABAergic neurones . The increase in GABAergic neurones may represent a compensatory response to the environment with excessive GC signalling, leading to alterations in hypothalamic function in adulthood via an imbalance of excitatory/inhibitory input . Of particular note, even periconceptional exposure to alcohol has been shown to trigger a sexually dimorphic alteration in HPA function in rats.…”