Effects of excess glucocorticoids on neural cell fate : long lasting consequences of adverse prenatal factors

Abstract

The developing nervous system is particularly vulnerable to high level of glucocorticoids (GCs). Excess GCs is often associated with decreased birth weight in relation to gestational age, and to increased risk of psychiatric disorders, like depression, later in life. Persistent deregulation of the hypothalamic-pituitary-adrenal axis drive on central and peripheral systems has been suggested to play a central role, however the link between developmental GCs exposure and late-onset depression is poorly understood. The aim of this thesis was to investigate and characterize behavioral alterations induced by developmental exposure to excess GCs in mice.

Twelve month old mice (C57Bl/6) exposed to dexamethasone (DEX) (0.05 mg/Kg/day s.c.) from gestational day 14 until delivery displayed depression-like behavior resistant to treatment with the SSRI antidepressant fluoxetine (FLX), and decreased neurogenesis. These alterations, not detected at younger age, were associated with deregulation of genes promoting progenitor stem cells quiescence and proliferation, such as Cdkn1c. Neuronal stem cells exposed to DEX in vitro exhibited changes in methylation of promoter regions of the same class of genes, suggesting the involvement of epigenetic mechanisms. Depression-like behavior is often associated with altered hippocampal connectivity of granule neurons in the dentate gyrus.

We investigated the morphology of adult-born hippocampal granule neurons and found remodeling in dendritic arborization accompanied by changes in the expression of TrkB, DISC1 and Reelin. In addition, DEX-exposed mice showed blunted circadian oscillations in corticosterone secretion, and down-regulation of glucocorticoid receptor expression in the hippocampus, which may explain the resistance to FLX treatment. We then tested the SNRI antidepressant desipramine (DMI), which reversed the depressed phenotype. Since modifications in corticosterone fluctuations are also associated with circadian rhythm alterations, we tested whether DEX might alter circadian rhythms. Twelve month old DEX-exposed mice showed abnormal circadian entrainment, which appeared to be more rigid and strongly dependent on photic drive. Interestingly, circadian alterations were displayed already at 6 months, long before the onset of depression-like behavior, and were reduced by chronic DMI administration.

We assessed the function of the central clock and its drive on the hippocampus. The expression of arginine-vasopressin, the main output of the suprachiasmatic nucleus (SCN), was downregulated in DEX-exposed mice, suggesting a decoupling of the SCN control on the hippocampus. The core clock gene Bmal1 showed robust circadian fluctuations in the DEX-exposed mice SCN, while the oscillations were abolished in the hippocampus. The marked de-synchronization of Bmal1 across the SCN and hippocampus was restored by chronic treatment with DMI. Neither depression nor morphological and circadian alterations were developed in mice that received chronic treatment with DMI at 6 months, suggesting that the restoration of norepinephrine transmission might prevent the appearance of the phenotype. All together our results indicate that prenatal exposure to DEX triggers early changes in circadian rhythmicity, alterations in adult neurogenesis and neuronal plasticity preceding the onset of depression-like behavior.