Blocking beta-adrenergic receptors during stress mitigates gut microbiome dysbiosis

Technical Summary: Blocking Beta-Adrenergic Receptors Mitigates Stress-Induced Gut Dysbiosis

Problem Statement
Stress-related psychiatric disorders (e.g., Major Depressive Disorder, PTSD) and inflammatory bowel diseases (e.g., IBD, IBS) exhibit high comorbidity and reciprocal symptom severity. A shared mechanism may involve stress-induced shifts in the gut microbiome, specifically the reduction of short-chain fatty acid (SCFA)-producing bacteria like Lachnospiraceae and the phylum Firmicutes. While psychological stress activates the sympathetic nervous system (SNS) and releases noradrenaline, the specific mechanisms by which noradrenergic signaling drives gut dysbiosis remain largely unknown. This study investigates whether pharmacological inhibition of $\alpha$- or $\beta$-adrenergic receptors (ARs) during chronic stress can prevent these microbiome alterations.

Methodology
The study utilized a Chronic Social Defeat Stress (CSDS) paradigm in 10–14-week-old C57BL/6 mice (intruders) over 10 consecutive days.

• Experimental Groups: Mice were treated 15 minutes prior to daily defeat sessions with either:
  • Vehicle (sterile saline)
  • Prazosin (1 mg/kg), an $\alpha_1$-AR antagonist
  • Propranolol (10 mg/kg), a $\beta$-AR antagonist
• Behavioral Assessment: 24 hours after the final stress session (Day 11), mice underwent a social interaction test to measure sociability and social anxiety-like behavior.
• Microbiome Analysis: Fecal samples were collected on Day 1 (pre-stress) and Day 10 (post-stress). DNA was extracted and subjected to 16S rRNA sequencing (V3/V4 region) to determine relative abundance of taxa from kingdom to species level.
• Statistical Analysis: Non-parametric tests (Aligned Rank Transform ANOVA) were used for behavioral and abundance data. Alpha diversity (Shannon-Weiner Index) and beta diversity (Bray-Curtis distance) were calculated. Correlations between specific taxa abundance and social interaction ratios were assessed using Spearman's Rank Correlation.

Key Results

1. Sociability: Both prazosin ($\alpha_1$-blockade) and propranolol ($\beta$-blockade) increased sociability (time spent in the social interaction zone with a target mouse) compared to vehicle-treated mice following CSDS. Prazosin showed a stronger effect on increasing time in the social zone compared to vehicle, while propranolol showed a modest increase.
2. Alpha Diversity: CSDS significantly reduced alpha diversity (Shannon Index) in vehicle-treated mice. This reduction was also observed in prazosin-treated mice but was prevented in propranolol-treated mice, which maintained higher diversity scores comparable to baseline.
3. Taxonomic Shifts:
   • Firmicutes: CSDS reduced the relative abundance of Firmicutes in vehicle and prazosin groups. Propranolol treatment prevented this reduction, maintaining higher Firmicutes abundance compared to vehicle.
   • Clostridia: The reduction in the Clostridia class (and orders Eubacteriales, Lachnospirales, Peptococcales) caused by CSDS in vehicle mice was blocked by propranolol.
   • SCFA Producers: Propranolol specifically preserved anaerobic, SCFA-producing taxa, including the Lachnospiraceae and Peptococceae families, the Anaerovax and Lachnospiraceae genera, and an uncultured species in the Butyricoccus genus. These taxa were significantly reduced in vehicle and prazosin groups.
   • Beta Diversity: While CSDS altered beta diversity (community composition shifts), neither prazosin nor propranolol significantly altered the overall beta diversity distance compared to vehicle, suggesting the drugs preserved specific taxa rather than reshaping the entire community structure.
4. Correlations: The abundance of anaerobic, SCFA-producing Clostridia (specifically Lachnospiraceae and related genera) on Day 10 positively correlated with the social interaction ratio across all treatment groups.

Significance and Claims
The paper claims that pharmacological inhibition of $\beta$-adrenergic receptors, but not $\alpha_1$-adrenergic receptors, is sufficient to mitigate stress-induced gut dysbiosis. Specifically, the authors demonstrate that:

• $\beta$-AR activation during stress drives the depletion of Firmicutes and anaerobic SCFA-producing Clostridia.
• Blocking $\beta$-ARs with propranolol preserves these beneficial bacteria, which correlates with improved sociability.
• While both $\alpha_1$ and $\beta$ blockade improve social behavior, only $\beta$-blockade preserves the gut microbiome composition. This suggests that the mechanism by which propranolol improves sociability may involve the preservation of the gut microbiome, whereas prazosin likely acts through central nervous system mechanisms independent of the gut flora.
• The authors propose a mechanistic hypothesis where stress-induced $\beta$-AR activation redirects blood flow away from the gut, impairing oxidative phosphorylation in intestinal epithelial cells. This metabolic shift increases luminal oxygen levels, creating a hostile environment for obligate anaerobes (SCFA producers), leading to their depletion. Propranolol may prevent this by maintaining normal gut blood flow and hypoxic conditions.

The study concludes that $\beta$-AR activation is a critical link between psychological stress and gut dysbiosis, offering a potential pathway for mitigating the comorbidity between stress-related psychiatric disorders and inflammatory gut diseases.