Stressful life experiences are known to be a precipitating factor for many mental disorders. The social defeat model induces behavioral responses in rodents (e.g. reduced social interaction) that are similar to behavioral patterns associated with mood disorders. The model has contributed to the discovery of novel mechanisms regulating behavioral responses to stress, but its utility has been largely limited to males. This is disadvantageous because most mood disorders have a higher incidence in women versus men. Male and female California mice (Peromyscus californicus) aggressively defend territories, which allowed us to observe the effects of social defeat in both sexes. In two experiments, mice were exposed to three social defeat or control episodes. Mice were then behaviorally phenotyped, and indirect markers of brain activity and corticosterone responses to a novel social stimulus were assessed. Sex differences in behavioral responses to social stress were long lasting (4 wks). Social defeat reduced social interaction responses in females but not males. In females, social defeat induced an increase in the number of phosphorylated CREB positive cells in the nucleus accumbens shell after exposure to a novel social stimulus. This effect of defeat was not observed in males. The effects of defeat in females were limited to social contexts, as there were no differences in exploratory behavior in the open field or light-dark box test. These data suggest that California mice could be a useful model for studying sex differences in behavioral responses to stress, particularly in neurobiological mechanisms that are involved with the regulation of social behavior.

Monogamous species are usually considered to be less likely to exhibit sex differences in behavior or brain structure. Most previous studies examining sex differences in stress hormone responses have used relatively sexually dimorphic species such as rats. We examined the stress hormone responses of monogamous California mice (Peromyscus californicus) to resident-intruder tests. We also tested males and females under different photoperiods, because photoperiod has been shown to affect both aggression and stress hormone responses. Females, but not males showed a significant increase in corticosterone levels immediately following a resident-intruder test. Males but not females showed elevated corticosterone levels under short days. Females tested in aggression tests also showed a significant increase in plasma oxytocin levels, but only when housed in long days. This was consistent with our observation that females but not males had more oxytocin positive cells in the paraventricular nucleus (PVN) when housed under long days. Our data show that sex differences in glucocorticoid responses identified in other rodents are present in a monogamous species.

Using a variety of experimental methods, a network of brain areas regulating aggressive behaviors has been outlined in several groups of vertebrates. However, aggressive behavior expressed in different contexts is associated with different patterns of activity across hypothalamic and limbic brain regions. Previous studies in rodents demonstrated that short day photoperiods reliably increase both male and female aggression versus long day photoperiods. Here we used immunohistochemistry and western blots to examine the effect of photoperiod on phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK) in male California mice (Peromyscus californicus) during resident-intruder tests. Phosphorylated ERK (pERK) can alter neuronal activity in the short term and in the long term acts as a transcription factor. In the posterior bed nucleus of the stria terminalis (BNST) males tested in aggression tests had more phosphorylated ERK (pERK) positive cells when housed in short days but not long days. This result was replicated in western blot analyses from microdissected BNST samples. In the medial amygdala, immunostaining and western analyses showed that pERK expression also was generally increased in short days. Immunostaining was also used to examine phosphorylation of cyclic AMP response element binding protein (CREB). CREB can be phosphorylated by pERK as well as other kinases and functions primarily as a transcription factor. Intriguingly, aggressive interactions reduced the number of cells stained positive for phosphorylated CREB in the infralimbic cortex, ventral lateral septum and MEA. This effect was observed in mice housed in long days but not short days. Overall, these data suggest that different (but overlapping) networks of aggressive behavior operate under different environmental conditions.

In several vertebrate species, the effects of estrogens on male aggressive behavior can be modulated by environmental cues. In song sparrows and rodents, estrogens modulate aggression in the nonbreeding season or winter-like short days, respectively. The behavioral effects of estrogens are rapid, which generally is considered indicative of nongenomic processes. The current study further examined the hypothesis that estradiol acts nongenomically under short days by utilizing a protein synthesis inhibitor, cycloheximide (CX). Mice were housed in either short or long day photoperiods, and treated with an aromatase inhibitor. One hour before resident-intruder testing mice were injected with either CX or saline vehicle, and 30 minutes later were treated orally with either cyclodextrin conjugated estradiol or vehicle. Under short days, mice treated with estradiol showed a rapid decrease in aggressive behavior, independent of CX administration. CX alone had no effect on aggression. These results show that protein synthesis is not required for the rapid effects of estradiol on aggression, strongly suggesting that these effects are mediated by nongenomic processes. We also showed that estradiol suppressed c-fosimmunoreactivity in the caudal bed nucleus of the stria terminalis under short days. No effects of estradiol on behavior or c-fos expression were observed in mice housed under long days. Previously we had also demonstrated that cage beddinginfluenced the directional effects of estrogens on aggression. Here, we show that the phenomenon of rapid action of estradiol on aggression under short days is a robust result that generalizes to different bedding conditions.

Food restriction has been reported to have positive effects on cognition. This study examines how another environmental factor, daylength, can alter the impact of food restriction on the brain and behavior. Female California mice (Peromyscus californicus), housed on either long days (16L:8D) or short days (8L:16D) were restricted to 80% of their normal baseline food intake or provided food ad libitum. Testing in a Barnes maze revealed that the effects of food restriction depend on photoperiod, and that these effects differed for acquisition vs. reversal learning. During acquisition testing, food restriction increased latency to find the target hole in short-day mice but not long-day mice. In reversal testing, food restriction decreased latency to find the target hole in long-day mice but not short-day mice. Latency to find the hole was positively and independently correlated with both errors and time spent freezing, suggesting that changes in both spatial learning and anxiety-like behavior contributed to performance. Short days increased hippocampal expression of the synaptic protein, synapsin I, which was reversed by food restriction. Short days also reduced plasma corticosterone levels, but diet had no effect. There was no effect of diet or photoperiod on hippocampal expression of the glial marker, glial acidic fibrillary protein. The present findings suggest that in female California mice the differential effects of food restriction on acquisition and reversal learning are photoperiod-dependent. These results justify further testing of the relationship between food restriction and hippocampal synapsin I in the context of spatial learning.