Dysbiosis of gut microbiota has been shown to be related to depression (Zheng et al. 2016). MWD is a common neuropsychiatric symptom following prolonged morphine use. In the current study, we detected the gut microbial profiles in the morphine abstinence-induced depressive-like disorder. The difference in the diversity and abundance of the gut microbiota between groups were analyzed. Representative microbial genera for each group were identified. Finally, gene function prediction of the OTUs with differential abundance between the Mor-dep and Mor-nondep groups was performed.
Repeated morphine treatment was shown to decrease the motility (Girón et al. 2016) and increase the permeability (Thomaz et al. 2021) of the gut, thus may influencing the composition of the gut microbiota. Our data revealed that morphine abstinence for four weeks did not change the richness of the gut microbiota in mice. While, the composition of the microbiota altered significantly following morphine abstinence. This was consistent with previous studies indicating that intermittent or sustained morphine treatment influenced the β diversity of the gut microbiota, but not the α diversity (Lee et al. 2018; Zhang et al. 2020). In addition, according to our results, the abundance of Coprobacter increased after morphine abstinence both in the Mor-dep and Mor-nondep groups compared with the control group. Though with no significant difference in the abundance of gut microbial genera between the Mor-dep and Mor-nondep groups, the Enterorhabdus abundance was found higher in the Mor-dep group than that of the control group, but lower in the Mor-nondep group compared with the control group. This also supported a separation of the composition of the gut microbiota between the Mor-dep and Mor-nondep groups.
Representative microbial taxa can serve as potential markers for the diagnosis of diseases (Olovo et al. 2021; Tarracchini et al. 2021). We found several distinctive OTUs and gut microbial genera in the control, Mor-dep and Mor-nondep groups, respectively. These gut microbial markers might be used to distinguish different depressive states after morphine withdrawal. Meanwhile, the distinctive gut microbiota among groups demonstrates the differential alteration in the gut microbiota following morphine abstinence.
Metabolites provide a pathway for the interaction of gut microbiota and the function of CNS (Henriques et al. 2020), either directly across the blood-brain barrier (BBB) (Luo et al. 2021) or indirectly through the neuroendocrinology system (Cussotto et al. 2018). Supplementation of propionic acid, one of the short-chain fatty acids (SCFAs), could alleviate multiple sclerosis and the related immune dysfunction (Duscha et al. 2020). SCFAs intake was shown to help to improve the recovery of limb motor function after stroke in mice (Sadler et al. 2020). In our study, gene function prediction of the OTUs with differential abundance between the Mor-dep and Mor-nondep groups demonstrated that the pathway of fatty acid metabolism was enriched in the Mor-dep group compared with the Mor-nondep group. Hence, disturbance of several specific gut microbiota may be associated with morphine abstinence-induced depressive behavior through the pathway fatty acid metabolism.
To the best of our knowledge, this is the first study to demonstrate the characters of the gut microbial profiles after morphine abstinence-induced depressive behavior. However, there were several limitations in our study. First, the sample size is limited, which might restrict the identification of some gut microbiota with low abundances. Second, the resolution of the 16S rRNA gene sequencing is lower than that of the metagenome sequencing, which can determine the species of the organism. Third, only an association of dysbiosis of gut microbiota and morphine abstinence-induced depressive behavior was conducted in the present study, the functional roles of the gut microbiota in the MWD are needed to be clarified.
In summary, we observed characterized gut microbial profiles and distinctive microbial genera in mice following morphine abstinence-induced depressive-like behavior. Although no difference in the abundance of the gut microbiota was found between the Mor-dep and Mor-nondep groups, gene functions of the OTUs that with differential abundance between the above two groups included the pathways of fatty acid metabolism. Our findings suggest that there is an association between morphine abstinence-induced depressive-like behavior and dysbiosis of the gut microbiota. Further functional and mechanism research are needed to reveal the role of gut microbiota in the MWD.