Effects of photoperiod and experience on aggressive behavior in female California mice

Abstract

Aggressive behavior among females is observed in many species, but the mechanisms of this behavior have historically been understudied. In many species of rodents, winter-like short day photoperiods induce increased aggression levels compared to summer-like long day photoperiods. Recent reports in hamsters show that short days also increase aggression in females. We examined the effects of photoperiod on aggression in female California mice, and for the first time compare brain activity of aggression-tested female rodents under different photoperiods. We observed that female California mice were more aggressive when housed in short days versus long days. Intriguingly, we also observed that under long days female attack latency decreases with repeated testing in resident-intruder tests. These data suggest that winner effects that have been described in males may also occur in females. We also used the expression of phosphorylated extracellular signal-regulated kinases (pERK) in the brain to estimate brain activity during aggression tests. pERK can alter neuronal activity in the short term and in the long term can act as a transcription factor. Using immunoblot analyses we observed that aggression-induced pERK expression in the female bed nucleus of the stria terminalis and medial amygdala occurs under both long and short days. Thus, the mechanisms controlling increased aggression under short days are still unclear and additional study is needed.

Figure 1

Photomicrographs of phosphorylated extracellular signal regulated kinase (pERK) immunoreactivity in the BNST (A), MEA (B), VMH (C), and BLA (D). Abbreviations: a.c., anterior commissure; f, fornix; o.t., optic tract; e.c. external capsule. Scale bars = 1 mm.

Figure 2

Short days increased the average number of bites across three resident-intruder tests (A). Repeated measures analyses of variance showed that attack latency significantly decreased across the three tests in long day mice but not short day mice (B). Note that individual dots represent individual data points and lines indicate medians. * p < 0.05.

Figure 3

Estradiol levels in females from experiment 2 (n=4–6 per group). Estradiol levels were significantly higher in proestrus females than diestrus females. Among diestrus females, estradiol levels were higher in short days. † p < 0.05.

Figure 4

Effects of photoperiod on aggression in a single aggression test. Females bit intruders more frequently and had shorter attack latencies when housed in short days. * P < 0.05.

Figure 5

Effects of aggression testing on pERK activation (relative to total ERK activation) in the BNST (A, B) and MEA (C, D). Aggression testing significantly increased expression of the p42 isoform in the BNST (A) and MEA (C). Aggression testing significantly increased expression of the p44 isoform in the MEA (D) whereas there was a nonsignificant trend in the BNST (B). * P < 0.05 effect of aggression testing, + p = 0.055 effect of aggression testing.

Figure 6

Representative images from western blot analysis of protein extracted from BNST (A) and MEA (B) punch samples. A = aggression, C = control.