Photoperiod affects estrogen receptor alpha, estrogen receptor beta and aggressive behavior

Day length and estradiol affect same-sex affiliative behavior in the female meadow vole

Non-sexual social bonding between adult mammals is poorly understood, despite its importance in many species. Female meadow voles are territorial and nest alone in long summer day lengths when circulating estradiol concentrations are high, but cohabit in groups in short winter photoperiods when estradiol secretion is low. The influence of day length and estradiol on same-sex huddling behavior was assessed in adult female pairs housed together in long day lengths (LDs) or short day lengths (SDs) from weaning. The behavior of intact, ovariectomized, and estradiol-treated ovariectomized females from each photoperiod was assessed during 3 h partner-preference tests. Intact SD voles, unlike intact LD voles, spent the majority of the test in proximity to their cage-mates. Estradiol treatment of SD voles significantly reduced time spent huddling with the partner. Neither ovariectomy nor estradiol treatment significantly affected the amount of time LD females spent in contact with their partners. Low estradiol availability is therefore a necessary but not sufficient condition for maintenance of high levels of huddling. These results establish that ovarian hormones interact with photoperiod to affect same-sex social behavior.

Paternal aggression in a biparental mouse: parallels with maternal aggression

Environmental and social factors have important effects on aggressive behaviors. We examined the effect of reproductive experience on aggression in a biparental species of mouse, Peromyscus californicus. Estrogens are important in mediating aggressive behavior so we also examined estrogen receptor expression and c-fos for insights into possible mechanisms of regulation. Parental males were significantly more aggressive than virgin males, but no significant differences in estrogen receptor alpha or beta expression were detected. Patterns of c-fos following aggression tests suggested possible parallels with maternal aggression. Parental males had more c-fos positive cells in the medial amygdala, and medial preoptic area relative to virgin males. The medial preoptic area is generally considered to be relatively less important for male-male aggression in rodents, but is known to have increased activity in the context of maternal aggression. We also demonstrated through habituation-dishabituation tests that parental males show exaggerated investigation responses to chemical cues from a male intruder, suggesting that heightened sensory responses may contribute to increased parental aggression. These data suggest that, in biparental species, reproductive experience leads to the onset of paternal aggression that may be analogous to maternal aggression.

Rapid effects of estradiol on male aggression depend on photoperiod in reproductively non-responsive mice

In three genuses and four species of rodents, housing in winter-like short days (8L:16D) increases male aggressive behavior. In all of these species, males undergo short-day induced regression of the reproductive system. Some studies, however, suggest that the effect of photoperiod on aggression may be independent of reproductive responses. We examined the effects of photoperiod on aggressive behavior in California mice (Peromyscus californicus), which do not display reproductive responsiveness to short days. As expected, short days had no effect on plasma testosterone. Estrogen receptor alpha and estrogen receptor beta immunostaining did not differ in the lateral septum, medial preoptic area, bed nucleus of the stria terminalis, or medial amygdala. However, males housed in short days were significantly more aggressive than males housed in long days. Similar to previous work in beach mice (Peromyscus polionotus), estradiol rapidly increased aggression when male California mice were housed in short days but not when housed in long days. These data suggest that the effects of photoperiod on aggression and estrogen signaling are independent of reproductive responses. The rapid action of estradiol on aggression in short-day mice also suggests that nongenomic mechanisms mediate the effects of estrogens in short days.