Winning aggressive disputes can enhance future fighting ability and the desire to seek out additional contests. In some instances, these effects are long lasting and vary in response to the physical location of a fight. Thus, in principle, winning aggressive encounters may cause long-term and context-dependent changes to brain areas that control the output of antagonistic behavior or the motivation to fight (or both). We examined this issue in the territorial California mouse (Peromyscus californicus) because males of this species are more likely to win fights after accruing victories in their home territory but not after accruing victories in unfamiliar locations. Using immunocytochemistry and real-time quantitative PCR, we found that winning fights either at home or away increases the expression of androgen receptors (AR) in the medial anterior bed nucleus of the stria terminalis, a key brain area that controls social aggression. We also found that AR expression in brain regions that mediate motivation and reward, nucleus accumbens (NAcc) and ventral tegmental area (VTA), increases only in response to fights in the home territory. These effects of winning were likely exclusive to the neural androgenic system because they have no detectible impact on the expression of progestin receptors. Finally, we demonstrated that the observed changes in androgen sensitivity in the NAcc and VTA are positively associated with the ability to win aggressive contests. Thus, winning fights can change brain phenotype in a manner that likely promotes future victory and possibly primes neural circuits that motivate individuals to fight.aggression | androgen and progestin receptors | behavioral reinforcement | neurobiology | winner effect S ocial experiences that individuals accrue throughout life can modify the brain's morphology and hormonal milieu (1, 2). These changes are often thought to adjust future social behavior because they occur in brain areas that control affective state, arousal, and motivation (3). However, less is known about how the environment of a given social encounter modulates its subsequent effects on the brain. We examined this issue here by testing (i) whether winning fights modifies neural circuits that control social aggression and motivation and (ii) whether a fight's environment, in turn, modulates any of these effects.Winning aggressive disputes or competitions can affect future behavior (4, 5). For example, individuals that win fights are more likely to win later in life (6). In some species, this so-called winner effect is long lasting and forms only in response to victories in certain locations (6). Although mechanistic studies of the winner effect focus on how postencounter changes in steroid hormones adjust future winning behavior (7), this physiological model is less suitable for species in which the winner effect either persists well after excess hormones are cleared from circulation or depends on a fight's environment (8, 9). Thus, it is possible that the experience of victory induces long-term ...

Methyl-CpG-binding protein 2 (MeCP2) binds methylated DNA and recruits corepressor proteins to modify chromatin and alter gene transcription. Mutations of the MECP2 gene can cause Rett syndrome, whereas subtle reductions of MeCP2 expression may be associated with male-dominated social and neurodevelopmental disorders. We report that transiently decreased amygdala Mecp2 expression during a sensitive period of brain sexual differentiation disrupts the organization of sex differences in juvenile social play behavior. Interestingly, neonatal treatment with Mecp2 small interfering RNA within the developing amygdala reduced juvenile social play behavior in males but not females. Reduced Mecp2 expression did not change juvenile sociability or anxiety-like behavior, suggesting that this disruption is associated with subtle behavioral modification. This suggests that Mecp2 may have an overlooked role in the organization of sexually dimorphic behaviors and that male juvenile behavior is particularly sensitive to Mecp2 disruption during this period of development.

Although some DNA methylation patterns are altered by steroid hormone exposure in the developing brain, less is known about how changes in steroid hormone levels influence DNA methylation patterns in the adult brain. Steroid hormones act in the adult brain to regulate gene expression. Specifically, the expression of the socially relevant peptide vasopressin (AVP) within the bed nucleus of the stria terminalis (BST) of adult brain is dependent upon testosterone exposure. Castration dramatically reduces and testosterone replacement restores AVP expression within the BST. As decreases in mRNA expression are associated with increases in DNA promoter methylation, we explored the hypothesis that AVP expression in the adult brain is maintained through sustained epigenetic modifications of the AVP gene promoter. We find that castration of adult male rats resulted in decreased AVP mRNA expression and increased methylation of specific CpG sites within the AVP promoter in the BST. Similarly, castration significantly increased estrogen receptor α (ERα) mRNA expression and decreased ERα promoter methylation within the BST. These changes were prevented by testosterone replacement. This suggests that the DNA promoter methylation status of some steroid responsive genes in the adult brain is actively maintained by the presence of circulating steroid hormones. The maintenance of methylated or demethylated states of some genes in the adult brain by the presence of steroid hormones may play a role in the homeostatic regulation of behaviorally relevant systems.E pigenetic modification of chromatin (e.g., changes in DNA methylation status), by either steroid hormone exposure or changes in the social environment, can create changes in transcription rates of a number of genes within the developing brain, and in some cases these changes extend into adulthood (1-6). In adulthood, steroid hormones act to regulate the expression of steroid receptors as well as other factors in a fairly transient manner (7). Recent findings in vitro have shown that DNA methylation patterns on some genes cycle back and forth from methylated to unmethylated states within minutes (8,9). This supports the emerging concept that not all methylation patterns are stable. Whether a change in methylation status plays a role in the regulation of gene expression in the adult brain is not well understood. We begin to elucidate this possibility by examining the hormonal maintenance of expression and DNA promoter methylation status of the socially relevant neuropeptide vasopressin (AVP) in the adult brain.The extrahypothalamic AVP system in the rat brain is highly sexually dimorphic and steroid responsive (10). Adult male rats have two times more AVP-expressing cells in the bed nucleus of the stria terminals (BST) compared with females (11, 12). AVP expression within this area is dependent upon gonadal hormones, as castration results in a significant decrease in AVP mRNA and protein expression, and testosterone replacement restores AVP expression (13-15). The AVP cells wi...

Pervasive developmental disorder is a classification covering five related conditions including the neurodevelopmental disorder Rett syndrome (RTT) and autism. Of these five conditions, only RTT has a known genetic cause with mutations in Methyl-CpG-binding protein 2 (MeCP2), a global repressor of gene expression, responsible for the majority of RTT cases. However, recent evidence indicates that reduced MeCP2 expression or activity is also found in autism and other disorders with overlapping phenotypes. Considering the sex difference in autism diagnosis, with males diagnosed four times more often than females, we questioned if a sex difference existed in the expression of MeCP2, in particular within the amygdala, a region that develops atypically in autism. We found that male rats express significantly less mecp2 mRNA and protein than females within the amygdala, as well as the ventromedial hypothalamus (VMH), but not within the preoptic area (POA) on post-natal day 1 (PN1). At PN10 these differences were gone; however, on this day males had more mecp2 mRNA than females within the POA. The transient sex difference of mecp2 expression during the steroid-sensitive period of brain development suggests that mecp2 may participate in normal sexual differentiation of the rat brain. Considering the strong link between MeCP2 and neurodevelopmental disorders, the lower levels of mecp2 expression in males may also underlie a biological risk for mecp2-related neural disorders.

Social status and resource availability can strongly influence individual behavioral responses to conspecifics. In European starlings, males that acquire nest sites sing in response to females and dominate other males. Males without nest sites sing, but not to females, and they do not interact agonistically with other males. Little is known about the neural regulation of status- or resource-appropriate behavioral responses to conspecifics. Opioid neuropeptides are implicated in birdsong and agonistic behavior, suggesting that opioids may underlie differences in the production of these behaviors in males with and without nest sites. Here, we examined densities of immunolabeled mu-opioid receptors in groups of male starlings. Males that defended nest boxes dominated other males and sang at higher rates when presented with a female than males without nest boxes, independent of testosterone concentrations. Multiple regression analyses showed nest box ownership (not agonistic behavior or singing) predicted the optical density of receptor labeling in the medial bed nucleus of stria terminalis, paraventricular nucleus, ventral tegmental area and the medial preoptic nucleus. Compared to males without nest boxes, males with nest boxes had lower densities of immunolabeled mu-opioid receptors in these regions. Singing additionally predicted the area covered by labeling in the ventral tegmental area. The results suggest that elevated opioid activity in these regions suppresses courtship and agonistic behavioral responses to conspecifics in males without nest boxes. The findings are consistent with a dynamic role for opioid receptors in adjusting social behavior so that it is appropriate given the resources available to an individual.