RasGRF1 regulates the hypothalamic–pituitary–adrenal axis specifically in early-adolescent female mice

Abstract

Dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis has been implicated in the induction and prolongation of a variety of psychiatric disorders. As such, much effort has been made to understand the molecular mechanisms involved in its control. However, the vast majority of the studies on the HPA axis have used adult animals, and among these the majority has used males. Here we show that in knockout mice lacking the guanine nucleotide exchange factor, RasGRF1, habituation to 30 minutes a day of restraint stress is markedly accelerated, such that these mice do not display elevated corticosterone levels or enhanced locomotion after 7 days of stress exposure, like WT mice do. Strikingly, this phenotype is present in early-adolescent female RasGRF1 knockout mice, but not in their early-adolescent male, mid-adolescent female, adult female or adult male counterparts. Moreover, not only is there a clear response to restraint stress in early-adolescent female RasGRF1 knockout mice, their response after 1, 3, and 5 exposures is magnified ~3-fold compared to WT mice. These findings imply that distinct mechanisms exist to regulate the HPA axis in early-adolescent females that involves RasGRF1. A full understanding of how RasGRF1 controls the HPA axis response to stress may be required to design effective strategies to combat stress-associated psychiatric disorders initiated in young females.

Ras-GRF1 in CRF Cells Controls the Early Adolescent Female Response to Repeated Stress

“…After injection, baseline circulating CORT levels in unstressed animals were similar between Adeno-GFP and Adeno-CRE injected mice, consistent with our global GRF1 knockout mice (Uzturk et al 2015) (Figs. 2B-D).…”

“…We demonstrated previously that GRF1 negatively regulates the HPA axis response to short-term restraint stress (1-5 days; 30 min/day) in early-adolescent (EA) (prepubescent (peri-adolescent)) females (pn days 28-35)(Uzturk et al 2015). In particular, GRF1 knockout mice display an exaggerated enhancement of serum corticosterone (CORT) levels after these exposures.…”

“…After waiting 7 days to allow virus expression, mice were exposed to 7DRS, or control handling, and then CORT levels in serum were measured. This timeline was employed so that at the end of the experiment mice were still in early-adolescence, because we showed that after this developmental stage, GRF1 no longer plays a role in HPA axis regulation (Uzturk et al 2015). As expected, qPCR analysis of whole PVN RNA shows that injection of Adeno-CRE severely inhibited GRF1 mRNA levels compared to GFP controls in both EA females and males (Figure 2A).…”

“…However, we also showed, using these knockout mice, that GRF1 plays the opposite role after 7 exposures to restraint stress, where it contributes positively to the stress response specifically in EA females. It also contributes to the anxiolytic behavior we observed in EA female mice after restraint stress (Uzturk et al 2015). Finally, puberty-associated hormonal changes do not contribute to this phenotype because the effects are observed before puberty and ovariectomy had no effect on GRF1 knockout phenotypes after puberty.…”

“…GRF1 can also mediate dopamine signaling through Ras/Erk signaling in the striatum (Cerovic, et al 2014). Recently, we showed using knockout mice that GRF1 also contributes to regulation of the HPA axis in mice, but in a sex, age, and stimulus dependent manner (Uzturk, et al 2015). In particular, GRF1 contributes to the negative feedback the hippocampus plays in regulating the HPA axis response up to 5 exposures to 30min/day restraint stress, in prepuberal early-adolescent (EA) (pn-d28-35) females, but not their EA male, or late adolescent or adult female counterparts.…”