Interactions Between Gut Microbiota and Acute Restraint Stress in Peripheral Structures of the Hypothalamic-Pituitary-Adrenal Axis and the Intestine of Male Mice.

Karla Vagnerová,Petra Hermanová,Kateřina Balounová,Jiří Pácha,Dagmar Šrůtková,Tomáš Hudcovic,Petra Klusoňová,Peter Ergang,Martin Vodička

doi:10.3389/fimmu.2019.02655

Abstract

The gut microbiota play an important role in shaping brain functions and behavior, including the activity of the hypothalamus-pituitary-adrenocortical (HPA) axis. However, little is known about the effect of the microbiota on the distinct structures (hypothalamus, pituitary, and adrenals) of the HPA axis. In the present study, we analyzed the influence of the microbiota on acute restraint stress (ARS) response in the pituitary, adrenal gland, and intestine, an organ of extra-adrenal glucocorticoid synthesis. Using specific pathogen-free (SPF) and germ-free (GF) male BALB/c mice, we showed that the plasma corticosterone response to ARS was higher in GF than in SPF mice. In the pituitary, stress downregulated the expression of the gene encoding CRH receptor type 1 (Crhr1), upregulated the expression of the Fkbp5 gene regulating glucocorticoid receptor sensitivity and did not affect the expression of the proopiomelanocortin (Pomc) and glucocorticoid receptor (Gr) genes. In contrast, the microbiota downregulated the expression of pituitary Pomc and Crhr1 but had no effect on Fkbp5 and Gr. In the adrenals, the steroidogenic pathway was strongly stimulated by ARS at the level of the steroidogenic transcriptional regulator Sf-1, cholesterol transporter Star and Cyp11a1, the first enzyme of steroidogenic pathway. In contrast, the effect of the microbiota was significantly detected at the level of genes encoding steroidogenic enzymes but not at the level of Sf-1 and Star. Unlike adrenal Sf-1, the expression of the gene Lrh-1, which encodes the crucial transcriptional regulator of intestinal steroidogenesis, was modulated by the microbiota and ARS and this effect differed between the ileum and colon. The findings demonstrate that gut microbiota have an impact on the response of the pituitary, adrenals and intestine to ARS and that the interaction between stress and the microbiota during activation of glucocorticoid steroidogenesis differs between organs. The results suggest that downregulated expression of pituitary Pomc and Crhr1 in SPF animals might be an important factor in the exaggerated HPA response of GF mice to stress.

Highlights

Stressful stimuli induce a cascade of events in the hypothalamic– pituitary–adrenal (HPA) axis, which culminate in the secretion of glucocorticoids from the adrenal gland
To establish the impact of microbiota on glucocorticoid and neuropeptide signaling pathways in the pituitary, we examined the expression of the corticotropin-releasing hormone (CRH) receptor type 1 (Crhr1) gene encoding the CRHR1 receptor, whose activation enhances the transcription of Pomc, a gene encoding the adrenocorticotropic hormone (ACTH) precursor (Figure 1)
Similar to Sudo et al [6] and Clarke et al [7], we showed an exaggerated response of the HPA axis to acute restraint stress in GF mice, but our study extends this finding by demonstrating that the microbiota have a profound modulatory effect on brain neurochemistry [6, 8, 31] and on the pituitary and adrenal glands and extra-adrenal tissues

Summary

Introduction

Stressful stimuli induce a cascade of events in the hypothalamic– pituitary–adrenal (HPA) axis, which culminate in the secretion of glucocorticoids from the adrenal gland. GF mice exposed to acute restraint stress exhibited an exaggerated response of the HPA axis with elevated plasma adrenocorticotropic hormone (ACTH) and corticosterone levels, and this discrepancy was normalized after colonization of GF mice with commensal bacteria [6]. A similar exaggerated response of the HPA axis was observed in response to acute novel-environment stress in GF mice and rats [7, 8]. Treatment with prebiotics [9] or probiotics [10, 11] attenuated the HPA response to acute restraint or forced swim stress, even if this was not confirmed in all studies [12], probably due to strain-specific effects of the probiotic bacteria

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