High-fat diet triggers anxiety in male rats through gut-brain axis disruption

A recent study published in Biological Research investigates the impact of a high-fat diet (HFD) on gut microbiome diversity and its subsequent effects on the expression of brainstem serotonergic genes associated with anxiety-related symptoms.

Study: High-fat diet, microbiome-gut-brain axis signaling, and anxiety-like behavior in male rats. Image Credit: beauty-box / Shutterstock.com

The health effects of HFD

Previous animal studies have shown that HFD-induced obesity increases neuroinflammation and anxiety-related behaviors. Although the underlying mechanisms responsible for the effects of a HFD on anxiety-related defensive behavioral responses are not completely understood, a strong link with modified microbiome-gut-brain (MGB) axis signaling has been established.

HFD modifies the microbial diversity, composition, and abundance of the gut microbiome. These alterations invariably influence an individual’s innate immunity, cognitive function, and inflammation state, thereby increasing their risk of disease.

An imbalanced gut microbiome, which is also referred to as gut dysbiosis, alters brain serotonergic signaling. The serotonergic system regulates an individual’s emotional state and promotes anxiety-like defensive behavioral responses. In the brain, the dorsal raphe nucleus (DR) is the key source of serotonin.

Previous studies have shown that serotonergic neuron activation in the caudal region of the dorsal raphe nucleus (DRC) and dorsal part of the dorsal raphe nucleus (DRD) induces anxiety-like responses. The caudal part of the DRD nucleus (cDRD) is strongly associated with the regulation of anxiety-related defensive behavioral responses in rodents.

Increased expression of tryptophan hydroxylase 2 (TPH2) messenger ribonucleic acid (mRNA), which encodes for the neuronal isoform of TPH, has been detected in depressed individuals who committed suicide. Increased mRNA expression of htr1a and slc6a4 genes has also been associated with anxiety-related defensive behavioral responses in rodents.

About the study

The current study hypothesized that HFD modifies gut microbiome diversity and composition, thereby increasing serotonergic gene expression in the cDRD, which triggers anxiety-related defensive behavioral responses in male rodents.

To this end, rodents were subjected to HFD or control diet (CD) for nine weeks, following which their gut microbial composition and diversity were assessed. Additionally, tph2, htr1a, and slc6a4 gene expression in the brainstem raphe nuclei were evaluated, along with the rats’ defensive behavioral responses.

Rats were kept under controlled noise (60-80 dB) and temperature (23 ± 1 ºC) conditions with free access to food and water for nine weeks. Fecal pellets were collected for microbial analysis.

At the end of the study, all rats were euthanized using rapid decapitation and brains were analyzed using in situ hybridization histochemistry to measure tph2, htr1a, and slc6a4 mRNA expression, pontomesencephalic reticular formation, median raphe nucleus, and B9 serotonergic cell levels. Adipose tissues were also collected to determine the effects of HFD on adiposity.

Study findings

As compared to CD, HFD consumption impacted serotonergic gene expression in the pontine raphe nuclei and midbrain, as demonstrated by increased tph2, htr1a, and slc6a4 mRNA expression in subregions of the DR. In previous studies, the increased expression of these genes has been associated with anxiety-like responses in the cDRD or dorsomedial DR. Moreover, increased serotonergic gene expression can lead to serotonergic projections to forebrain anxiety circuits.

These findings strongly suggest that HFD triggers anxiety-like behavioral symptoms in rats. Furthermore, as compared to CD, HFD caused obesity, which was associated with reduced alpha diversity in the gut microbiome.

Although dissimilar gut microbiome composition was observed in HFD-rats relative to CD-rats, significant changes in gut microbial diversity were also observed in CD-rats across life stages. This observation implies the importance of life stages in microbiome community structure.

A relatively lower abundance of Prevotella and Lactobacillus, as well as higher levels of Blautia, were observed in the gut microbiome profile of HFD rats as compared to CD rats. The HFD diet also increased the Firmicutes/Bacteroidetes (F/B) ratio relative to CD, which has been associated with obesity. Furthermore, a positive correlation was observed between the F/B ratio, adipose index, and final weight in HFD rats.

Prevotella is a beneficial bacterium that is present in high concentrations following the consumption of a fiber-rich diet. Previously, a high abundance of Prevotella was negatively correlated with slc6a4 mRNA expression in the cDRD. Therefore, a lower abundance of Prevotella may promote stress-like conditions due to increased slc6a4 mRNA expression.

Conclusions

The current study highlights that nine weeks of exposure to HFD impacted gut microbiome diversity and composition, which influenced serotonergic gene expression in the brain and caused anxiety-like behavioral responses in rats. Taken together, these findings provide new insights into the possibility of developing microbiome-based interventions to prevent stress-related psychiatric disorders like anxiety.