Abnormal dendritic maturation of developing cortical neurons exposed to corticotropin releasing hormone (CRH): Insights into effects of prenatal adversity?

Abstract

Corticotropin releasing hormone (CRH) produced by the hypothalamus initiates the hypothalamic-pituitary-adrenal (HPA) axis, which regulates the body's stress response. CRH levels typically are undetectable in human plasma, but during pregnancy the primate placenta synthesizes and releases large amounts of CRH into both maternal and fetal circulations. Notably, placental CRH synthesis increases in response to maternal stress signals. There is evidence that human fetal exposure to high concentrations of placental CRH is associated with behavioral consequences during infancy and into childhood, however the direct effects on of the peptide on the human brain are unknown. In this study, we used a rodent model to test the plausibility that CRH has direct effects on the developing cortex. Because chronic exposure to CRH reduces dendritic branching in hippocampal neurons, we tested the hypothesis that exposure to CRH would provoke impoverishment of dendritic trees in cortical neurons. This might be reflected in humans as cortical thinning. We grew developing cortical neurons in primary cultures in the presence of graded concentrations of CRH. We then employed Sholl analyses to measure dendritic branching and total dendritic length of treated cells. A seven-day exposure to increasing levels of CRH led to a significant, dose-dependent impoverishment of the branching of pyramidal-like cortical neurons. These results are consistent with the hypothesis that, rather than merely being a marker of prenatal stress, CRH directly decreases dendritic branching. Because dendrites comprise a large portion of cortical volume these findings might underlie reduced cortical thickness and could contribute to the behavioral consequences observed in children exposed to high levels of CRH in utero.

Fig 1. The relative timing and cortical volume impacts of CRH on dendritic growth in the fetus.

A) The relative timing of placental CRH expression, blood brain barrier development, cortical CRH expression, and cortical neurogenesis, migration and dendritic outgrowth in human gestation [,–34]. B) Relative human and rat cortical neuronal development. The relative timing of the cell culture experiment and treatment with CRH is indicated by the grey and orange boxes, respectively. C) Contribution of different cell types and structures to cortical volume. Based on data from Braitenberg & Schüz (1998).

Fig 2. Images of cortical neurons treated with varying levels of CRH.

A-D: 40x images of representative neurons exposed to 0, 1, 10, 100 nM CRH respectively. Scale bar is 20 μm. A’-D’: Traces of the same cells in A-D and concentric Sholl circles spaced 20 μm apart.

Fig 3. CRH exposure of developing cortical neurons influences dendritic branching, but not total dendritic length.

A) Sholl analysis reveals the dose-dependent effects comparing varying levels of CRH. Significant decreases were found at 40 μm from the soma (10 nM, p = 0. 0006), 60 μm (10 nM, p = 0.009; 100 nM p = 0.014), and 80 μm (10 nM, p = 0.0017, 100 nM, p = 0025). Error bars represent SEM. B) Total combined dendritic length was not different between groups. (Error bars represent SEM. B) Total combined dendritic length was not different between groups Error bars represent 2.5–97.5 percentiles on box and whisker plots. For both figures, 0.01 nM and 0.1 nM had no significant effects and are not shown.