Seasonal patterns of melatonin alter aggressive phenotypes of female Siberian hamsters

Many animal species exhibit year-round aggression, a behaviour that allows
individuals to compete for limited resources in their environment
(e.g., food and mates). Interestingly, this high degree of territoriality
persists during the non-breeding season, despite low levels of
circulating gonadal steroids (i.e., testosterone [T] and oestradiol [E2]).
Our previous work suggests that the pineal hormone melatonin mediates a
‘seasonal switch’ from gonadal to adrenal regulation of
aggression in Siberian hamsters (Phodopus sungorus); solitary, seasonally
breeding mammals that display increased aggression during the
short, ‘winter-like’ days (SDs) of the non-breeding season. To test the
hypothesis that melatonin elevates non-breeding aggression by increasing
circulating and neural steroid metabolism, we housed female
hamsters in long days (LDs) or SDs, administered them timed or mis-timed
melatonin injections (mimic or do not mimic a SD-like signal,
respectively), and measured aggression, circulating hormone profiles, and
aromatase (ARO) immunoreactivity in brain regions associated with
aggressive or reproductive behaviours (paraventricular
hypothalamic nucleus [PVN], periaqueductal gray [PAG], and ventral
tegmental area [VTA]). Females that were responsive to SD
photoperiods (SD-R) and LD females given timed melatonin injections
(Mel-T) exhibited gonadal regression and reduced circulating E2, but
increased aggression and circulating dehydroepiandrosterone
(DHEA). Furthermore, aggressive challenges differentially altered
circulating hormone profiles across seasonal
phenotypes; reproductively inactive females (ie, SD-R and Mel-T
females) reduced circulating DHEA and T, but increased E2 after an
aggressive interaction, whereas reproductively active females
(i.e., LD females, SD non-responder females, and LD females given
mis-timed melatonin injections) solely increased circulating E2.
Although no differences in neural ARO abundance were observed, LD
and SD-R females showed distinct associations between ARO cell
density and aggressive behaviour in the PVN, PAG, and VTA. Taken
together, these results suggest that melatonin increases non-breeding
aggression by elevating circulating steroid metabolism after an aggressive
encounter and by regulating behaviourally relevant neural
circuits in a region-specific manner.