Bacteria and Bellicosity: Photoperiodic Shifts in Gut Microbiota Drive Seasonal Aggressive Behavior in Male Siberian Hamsters.

The existence of a microbiome-gut-brain axis has been established wherein gut microbiota significantly impacts host behavior and physiology, with increasing evidence suggesting a role for the gut microbiota in maintaining host homeostasis. Communication between the gut microbiota and the host is bidirectional, and shifts in the composition of the gut microbiota are dependent on both internal and external cues (host-derived signals, such as stress and immunity, and endocrine and environmental signals, such as photoperiod). Although there is host-driven seasonal variation in the composition of the microbiota, the mechanisms linking photoperiod, gut microbiota, and host behavior have not been characterized.

The paraventricular thalamus serves as a nexus in the regulation of stress and immunity.

Many temperate zone animals exhibit seasonal rhythms in physiology and behavior, including seasonal cycles of reproduction, energetics, stress responsiveness, and immune function, among many others. These rhythms are driven by seasonal changes in the duration of pineal melatonin secretion. The neural melatonin target tissues that mediate several of these rhythms have been identified, though the target(s) mediating melatonin's regulation of glucocorticoid secretion, immune cell numbers, and bacterial killing capacity remain unspecified.

A novel role for the pineal gland: Regulating seasonal shifts in the gut microbiota of Siberian hamsters.

The gut microbiota plays a significant role in a variety of host behavioral and physiological processes. The mechanisms by which the gut microbiota and the host communicate are not fully resolved but include both humoral and direct neural signals. The composition of the microbiota is affected by internal (host) factors and external (environmental) factors.