Meeting Abstract

P1.216  Friday, Jan. 4  Timing Matters: Exogenous Melatonin Mimics Short-Day Increases in Aggression in Female Siberian Hamsters (Phodopus sungorus) RENDON, N.M.*; DEMAS, G.E.; Indiana University, Bloomington nrendon@indiana.edu

Among the suite of seasonal adaptations displayed by temperate rodents, males and females of some species demonstrate increased territorial aggression in short compared with long day lengths despite its inverse relationship with gonadal steroids. The precise physiological mechanisms mediating such seasonal fluctuations in female aggression, however, remain mostly unknown. The goal of the present study was to determine if melatonin, the major biochemical cue signaling day length, regulates seasonal changes in aggression. To test this, female Siberian hamsters were housed in either long or short days and given daily injections of melatonin (15 μg/day in 0.1 ml saline) or vehicle (0.1ml saline) for 10 weeks. Injections were precisely administered 2 or 8 hours before their entrained night period for 10 weeks to mimic short-day-like and non-short-day-like patterns of melatonin, respectively. Levels of aggression were then determined using the resident-intruder paradigm, and sex steroid (i.e., testosterone and estradiol) levels were quantified following the aggressive interactions. Short-day responsive animals displayed gonadal regression and were significantly more aggressive compared to long-day, and short-day nonresponsive animals that remained reproductively active. Long-day hamsters receiving timed, but not mis-timed melatonin, displayed elevated aggression comparable to short-day animals. These results confirm previous findings of short-day aggression in female hamsters and suggest that environmentally relevant patterns of melatonin regulate these behavioral responses.