Sickness-induced changes in physiology do not affect fecundity or same-sex behavior

Early-life sickness may predispose Siberian hamsters to behavioral changes following alterations of the gut microbiome in adulthood

Although it is well-established that the immune system plays an important role in the development of physiology and behavior, the gut microbiome has recently become of interest in the study of developmental origins of behavior. Studies suggest that the effects of early-life immune activation may not occur until a secondary stressor is introduced, though the precise nature and timing of the stressor may be critical in the response. Further, recent work suggests that the microbiome and the immune system develop in parallel, and therefore any perturbations to one of these systems early in life will likely affect the other. Here, we sought to determine whether early-life activation of the immune system had long-term consequences on how the gut microbiome responds to antibiotic treatment in adulthood and whether those changes influence adult same-sex social behavior. In order to test the hypothesis that an early-life immune challenge makes individuals more vulnerable to the effects of antibiotics, we mimicked an early-life infection by injecting pups at postnatal day 3 and 5 with lipopolysaccharide (LPS; cell wall component of gram-negative bacteria) or saline, and subsequently exposed the same animals to antibiotic treatment (known to influence microbial community composition and behavior) or water in adulthood. We tracked physiology across development, and paired males and females with a novel individual of the same age and sex in adulthood to score same-sex behavior (e.g., aggression, investigation, grooming) before antibiotic treatment, immediately following treatment, and after recovery from antibiotics. LPS-treated females exhibited impaired reproductive physiology and function in adulthood (e.g., smaller ovaries and abnormal estrous cycles), and female and male gut microbial communities were strongly affected by antibiotic treatment in adulthood, but only slightly affected by postnatal LPS alone. Interestingly, LPS-treated males exhibited more robust changes in their behavioral response following adult antibiotic treatment, including decreased investigation and increased grooming, suggestive of changes in anxiety-like behaviors. These data suggest that males may be more vulnerable than females to behavioral abnormalities after being predisposed to an immune challenge early in life. Collectively, these results provide novel evidence that some of the sex-specific behavioral consequences of an early-life immune challenge may not transpire until an individual is faced with a secondary challenge, and the context in which an individual is exposed can greatly influence the response.

A gut feeling: Microbiome-brain-immune interactions modulate social and affective behaviors

The expression of a wide range of social and affective behaviors, including aggression and investigation, as well as anxiety- and depressive-like behaviors, involves interactions among many different physiological systems, including the neuroendocrine and immune systems. Recent work suggests that the gut microbiome may also play a critical role in modulating behavior and likely functions as an important integrator across physiological systems. Microbes within the gut may communicate with the brain via both neural and humoral pathways, providing numerous avenues of research in the area of the gut-brain axis. We are now just beginning to understand the intricate relationships among the brain, microbiome, and immune system and how they work in concert to influence behavior. The effects of different forms of experience (e.g., changes in diet, immune challenge, and psychological stress) on the brain, gut microbiome, and the immune system have often been studied independently. Though because these systems do not work in isolation, it is essential to shift our focus to the connections among them as we move forward in our investigations of the gut-brain axis, the shaping of behavioral phenotypes, and the possible clinical implications of these interactions. This review summarizes the recent progress the field has made in understanding the important role the gut microbiome plays in the modulation of social and affective behaviors, as well as some of the intricate mechanisms by which the microbiome may be communicating with the brain and immune system.