Maturation of social reward in adult male Syrian hamsters does not depend on organizational effects of pubertal testosterone

Methods and Challenges in Investigating Sex-Specific Consequences of Social Stressors in Adolescence in Rats: Is It the Stress or the Social or the Stage of Development?

Adolescence is a time of social learning and social restructuring that is accompanied by changes in both the hypothalamic-pituitary-gonadal axis and the hypothalamic-pituitary-adrenal (HPA) axis. The activation of these axes by puberty and stressors, respectively, shapes adolescent development. Models of social stress in rats are used to understand the consequences of perturbations of the social environment for ongoing brain development. This paper reviews the challenges in investigating the sex-specific consequences of social stressors, sex differences in the models of social stress used in rats and the sex-specific effects on behaviour and provides an overview of sex differences in HPA responding to stressors, the variability in pubertal development and in strains of rats that require consideration in conducting such research, and directions for future research.

Preliminary evidence of sex differences in behavioral and neural responses to palatable food reward in rats

The female bias in eating disorder prevalence is the largest in all of psychiatry. Binge eating on palatable food (PF) is a core, maladaptive symptom that cuts across all major types of eating disorders and can be studied via animal models. Using an individual differences rat model of binge eating that identifies binge eating prone (BEP) and binge eating resistant (BER) phenotypes, we previously showed that, compared with males, females consume more PF and are more likely to be classified as BEP. One potential explanation for this sex difference is that PF is inherently more rewarding to females, leading to higher rates of binge eating. Here we tested the hypothesis that females have more robust behavioral and neural responses to PF reward than males. Adult male (N=18) and female (N=17) Sprague-Dawley rats were exposed to the Conditioned Place Preference paradigm using PF as the unconditioned stimulus. Select males (N=9) and females (N=9) were video-recorded during three of the PF-paired conditioning sessions to score feeding behavior. Following CPP, 13 male and 12 female rats were exposed to PF just prior to sacrifice to induce expression of the neural activation marker Fos, and Fos expression was quantified in mesocorticolimbic, hypothalamic, and amygdalar circuits. In the CPP paradigm, females displayed a more robust shift in preference for the chamber paired with PF compared with males, and behavioral analyses revealed that average duration of individual feeding bouts during pairing sessions was longer in females than in males. Fos expression was significantly higher in females vs. males in select regions of the mesocorticolimbic reward circuit, with no sex differences in hypothalamic or amygdalar regions. These results provide initial evidence that PF may be more rewarding to females than to males, possibly due to heightened responsiveness of neural substrates that mediate the hedonic and motivational responses to PF, which in part, may underlie sex differences in binge eating proneness.

Understanding the epigenetic basis of sex differences in depression

Epigenetics refers to potentially heritable processes that can mediate both lasting and transient changes in gene expression in the absence of genome sequence alterations. The field of epigenetics has introduced a novel understanding of the mechanisms through which the environment can shape an individual and potentially its offspring. This Mini-Review examines the current literature exploring the role of epigenetics in the development of mood disorders such as depression. Depression is twofold more common in females, yet the majority of preclinical research has been conducted exclusively in male subjects. Here we discuss what is known about sex differences in epigenetic regulation and function and how this may contribute to the etiology and onset of mood disorders. © 2016 Wiley Periodicals, Inc.

The organizational effects of pubertal testosterone on sexual proficiency in adult male Syrian hamsters

Social proficiency requires making appropriate behavioral adaptations as a result of social experience. For example, male rodents become sexually proficient with experience as demonstrated by a reduction in ectopic (misdirected) mounts, mount-to-intromission ratio, and latency to ejaculation. We previously found that over a series of timed tests with a receptive female, male hamsters deprived of testosterone specifically during puberty (NoT@P) have overall lower levels of sexual behavior and continue to display high levels of ectopic mounts, compared with males that experienced endogenous testosterone during puberty (T@P). These results suggested that pubertal testosterone programs sexual proficiency in adulthood, but because NoT@P males engaged in less sexual behavior than T@P males in these tests, the amount of sexual experience may have been insufficient to improve sexual proficiency. To more rigorously test the hypothesis that pubertal testosterone is necessary for social proficiency in adulthood, the present study compared the behavior of NoT@P and T@P males in a series of 4 trials with a 48-h interval between each trial. Sexual experience was equated by limiting each trial to 5 intromissions. Sexually-naïve males were either gonadectomized prepubertally (NoT@P) or in adulthood (T@P) and received subcutaneous testosterone capsules four weeks later. Two weeks after testosterone replacement, these groups and a group of adult gonad-intact controls began sexual behavior testing. We found that NoT@P males had more ectopic mounts/min across all four tests compared to gonad-intact and T@P males. Moreover, both gonad-intact and T@P males, but not NoT@P males, showed an increase in the number of mounts and intromissions/min between trials 1 and 3. Unexpectedly, both gonad-intact and T@P, but not NoT@P, males showed a decrease in sexual behaviors during trial 4. Thus, T@P males display multiple behavioral adaptations to sexual experience that are not observed in NoT@P males: a reduction in ectopic mounts after repeated encounters with a receptive female and an inverted U-shape pattern in mounts and intromissions when these encounters do not lead to ejaculations. These results support the hypothesis that pubertal testosterone organizes neural circuits underlying behavioral flexibility and adaptability to promote sexual proficiency in adulthood.

Hormone-dependent adolescent organization of socio-sexual behaviors in mammals

The adolescent transition from childhood to adulthood requires both reproductive and behavioral maturation as individuals acquire the ability to procreate. Gonadal steroid hormones are key players in the maturation of behaviors required for reproductive success. Beyond activating behavior in adulthood, testicular and ovarian hormones organize the adolescent brain and program adult-typical and sex-typical expression of sociosexual behaviors. Testicular hormones organize sexual and agonistic behaviors, including social proficiency-the ability to adapt behavior as a function of social experience. Ovarian hormones organize behaviors related to energy balance and maternal care. These sex differences in the behaviors that are programmed by gonadal hormones during adolescence suggest that evolution has selected for hormone-dependent sex-specific organization of behaviors that optimize reproductive fitness.

Peri-pubertal exposure to testicular hormones organizes response to novel environments and social behaviour in adult male rats

Previous research has shown that exposure to testicular hormones during the peri-pubertal period of life has long-term, organizational effects on adult sexual behaviour and underlying neural mechanisms in laboratory rodents. However, the organizational effects of peri-pubertal testicular hormones on other aspects of behaviour and brain function are less well understood. Here, we investigated the effects of manipulating peri-pubertal testicular hormone exposure on later behavioural responses to novel environments and on hormone receptors in various brain regions that are involved in response to novelty. Male rodents generally spend less time in the exposed areas of novel environments than females, and this sex difference emerges during the peri-pubertal period. Male Lister-hooded rats (Rattus norvegicus) were castrated either before puberty or after puberty, then tested in three novel environments (elevated plus-maze, light-dark box, open field) and in an object/social novelty task in adulthood. Androgen receptor (AR), oestrogen receptor (ER1) and corticotropin-releasing factor receptor (CRF-R2) mRNA expression were quantified in the hypothalamus, hippocampus and medial amygdala. The results showed that pre-pubertally castrated males spent more time in the exposed areas of the elevated-plus maze and light-dark box than post-pubertally castrated males, and also confirmed that peri-pubertal hormone exposure influences later response to an opposite-sex conspecific. Hormone receptor gene expression levels did not differ between pre-pubertally and post-pubertally castrated males in any of the brain regions examined. This study therefore demonstrates that testicular hormone exposure during the peri-pubertal period masculinizes later response to novel environments, although the neural mechanisms remain to be fully elucidated.

A decrease in the addition of new cells in the nucleus accumbens and prefrontal cortex between puberty and adulthood in male rats

Adolescence involves shifts in social behaviors, behavioral flexibility, and adaptive risk-taking that coincide with structural remodeling of the brain. We previously showed that new cells are added to brain regions associated with sexual behaviors, suggesting that cytogenesis may be a mechanism for acquiring adult-typical behaviors during adolescence. Whether pubertal cell addition occurs in brain regions associated with behavioral flexibility or motivation and whether these patterns differ between pubertal and adult animals had not been determined. Therefore, we assessed patterns of cell proliferation or survival in the prefrontal cortex and nucleus accumbens. Pubertal and adult male rats were given injections of bromo-deoxyuridine (BrdU). To assess cell proliferation, half of the animals from each group were sacrificed 24 h following the last injection. The remaining animals were sacrificed at Day 30 following the last injection to evaluate cell survival. Adult animals had significantly lower densities of BrdU-immunoreactive (ir) cells in the prefrontal cortex, irrespective of post-BrdU survival time, whereas in the nucleus accumbens, adult animals had a lower density of BrdU-ir cells at the short survival time; however, the density of BrdU-ir cells was equivalent in pubertal and adult animals at the longer survival time. These data provide evidence that cell addition during puberty may contribute to the remodeling of brain regions associated with behavioral flexibility and motivation, and this cell addition continues into adulthood, albeit at lower levels. Higher levels of cell proliferation or survival in younger animals may reflect a higher level of plasticity, possibly contributing to the dynamic remodeling of the pubertal brain.

The Teenage Brain: Social Reorientation and the Adolescent Brain-The Role of Gonadal Hormones in the Male Syrian Hamster

Maturation of social cognition and a gain in social proficiency are universal aspects of adolescent development that prepare individuals for adulthood. Social cognition involves the perception and interpretation of social cues, followed by the generation of a behavioral response. Social proficiency is acquired through the ability to make behavioral adaptations as one learns from social experience; increased social proficiency facilitates successful social interactions. In males, the neuroendocrine bases of these developmental changes involve both activational and organizational influences of testicular hormones. Using the male Syrian hamster as a model, this review provides evidence that social stimuli acquire rewarding properties during adolescence via activational effects of pubertal testosterone, whereas the adolescent gain in social proficiency depends on organizational actions of pubertal testosterone.

Dopamine mediates testosterone-induced social reward in male Syrian hamsters

Adolescent maturation of responses to social stimuli is essential for adult-typical sociosexual behavior. Naturally occurring developmental changes in male Syrian hamster responses to a salient social cue, female hamster vaginal secretions (VS), provide a good model system for investigating neuroendocrine mechanisms of adolescent change in social reward. Sexually naïve adult, but not juvenile, males show a conditioned place preference (CPP) to VS, indicating that VS is not rewarding before puberty. In this series of experiments, the authors examined the roles of testosterone and dopamine receptor activation in mediating the adolescent gain in positive valence of VS. Experiment 1 showed that testosterone replacement is necessary for gonadectomized adult hamsters to form a CPP to VS. Experiment 2 showed that testosterone treatment is sufficient for juvenile hamsters to form a CPP to VS, and that the dopamine receptor antagonist haloperidol blocks formation of a CPP to VS in these animals. Experiments 3 and 4 demonstrated that the disruption of VS CPP with low doses of haloperidol is the result of a reduction in the attractive properties of VS and not attributable to aversive properties of haloperidol. Together, these studies demonstrate that the unconditioned rewarding properties of a social cue necessary for successful adult sociosexual interactions come about as the result of the pubertal increase in circulating testosterone in male hamsters. Furthermore, this social reward can be prevented by dopamine receptor antagonism, indicating that hypothalamic and/or mesocorticolimbic dopaminergic circuits are targets for hormonal activation of social reward.

Pubertal testosterone programs context-appropriate agonistic behavior and associated neural activation patterns in male Syrian hamsters

Pubertal testosterone programs the level of aggressive behavior displayed by male Syrian hamsters during resident-intruder interactions. To further explore the pubertal programming of adult male agonistic behaviors, the current study investigated the formation, stability, and maintenance of dominant-subordinate relationships in males that either did (T@P) or did not (NoT@P) experience testicular hormones during adolescent development. NoT@P males were gonadectomized prepubertally and T@P males were gonadectomized in adulthood. Four weeks after gonadectomy, all males received testosterone-replacement. Two weeks later, two males of the same hormonal history were given a 60 min introductory trial in a neutral arena, followed immediately and again 24h later by three 5-min trials. During the introductory trial, each male was deemed dominant, subordinate, or no-status. Brains were collected 1h after the last trial and sections were stained for Fos-immunoreactivity. Dominant T@P males flank marked more frequently than subordinate and no-status T@P males; this difference was not found in NoT@P males. NoT@P males showed an increase in the number of offensive postures the day after the first series of tests, whereas T@P males did not. Dominant T@P males had significantly more Fos expression than no-status T@P males in anterior cingulate cortex; this relationship was not observed in NoT@P males. Additionally, dominant T@P males had higher Fos expression than dominant NoT@P males in lateral septum. Thus, pubertal testosterone does not organize the formation or stability of male-male relationships, but does program the behavioral strategies used to maintain these relationships over time and the neural correlates of status.

Adolescent gain in positive valence of a socially relevant stimulus: engagement of the mesocorticolimbic reward circuitry

A successful transition from childhood to adulthood requires adolescent maturation of social information processing. The neurobiological underpinnings of this maturational process remain elusive. This research employed the male Syrian hamster as a tractable animal model for investigating the neural circuitry involved in this critical transition. In this species, adult and juvenile males display different behavioral and neural responses to vaginal secretions, which contain pheromones essential for expression of sexual behavior in adulthood. These studies tested the hypothesis that vaginal secretions acquire positive valence over adolescent development via remodeling of neural circuits underlying sexual reward. Sexually naïve adult, but not juvenile, hamsters showed a conditioned place preference for vaginal secretions. Differences in behavioral response to vaginal secretions between juveniles and adults correlated with a difference in the vaginal secretion-induced neural activation pattern in mesocorticolimbic reward circuitry. Fos immunoreactivity increased in response to vaginal secretions in the medial amygdala and ventral tegmental dopaminergic cells of both juvenile and adult males. However, only in adults was there a Fos response to vaginal secretions in non-dopaminergic cells in interfascicular ventral tegmental area, nucleus accumbens core and infralimbic medial prefrontal cortex. These results demonstrate that a socially relevant chemosensory stimulus acquires the status of an unconditioned reward during adolescence, and that this adolescent gain in social reward is correlated with experience-independent engagement of specific cell groups in reward circuitry.