Adrenal MT1 melatonin receptor expression is linked with seasonal variation in social behavior in male Siberian hamsters.

Many animals exhibit pronounced changes in physiology and behavior on a seasonal basis, and these adaptations have evolved to promote survival and reproductive success. While the neuroendocrine pathways mediating seasonal reproduction are well-studied, far less is known about the mechanisms underlying seasonal changes in social behavior, particularly outside of the context of the breeding season. Our previous work suggests that seasonal changes in melatonin secretion are important in regulating aggression in Siberian hamsters (Phodopus sungorus); it is unclear, however, how melatonin acts via its receptors to modulate seasonal variation in social behavior. In this study, we infused a MT1 melatonin receptor-expressing (MT1) or control (CON) lentivirus into the adrenal glands of male Siberian hamsters. We then housed hamsters in long-day (LD) or short-day (SD) photoperiods, administered timed melatonin or control injections, and quantified aggressive and non-aggressive social behaviors (e.g., investigation, self-grooming) following 10 weeks of treatment. LD hamsters infused with the MT1 lentivirus had significantly higher adrenal mt1 expression than LD CON hamsters, as determined via quantitative PCR. While melatonin administration was necessary to induce SD-like reductions in body and relative reproductive mass, only LD hamsters infused with the MT1 lentivirus displayed SD-like changes in social behavior, including increased aggression and decreased investigation and grooming. In addition, SD CON and LD hamsters infused with the MT1 lentivirus exhibited similar relationships between adrenal mt1 expression and aggressive behavior. Together, our findings suggest a role for adrenal MT1 receptor signaling in regulating behavior, but not energetics or reproduction in seasonally breeding species.