Acute administration of fluoxetine increases social avoidance and risk assessment behaviors in a sex- and social stress-dependent manner in Syrian hamsters (Mesocricetus auratus)

Effects of social dominance and acute social stress on morphology of microglia and structural integrity of the medial prefrontal cortex

Chronic stress increases activity of the brain's innate immune system and impairs function of the medial prefrontal cortex (mPFC). However, whether acute stress triggers similar neuroimmune mechanisms is poorly understood. Across four studies, we used a Syrian hamster model to investigate whether acute stress drives changes in mPFC microglia in a time-, subregion-, and social status-dependent manner. We found that acute social defeat increased expression of ionized calcium binding adapter molecule 1 (Iba1) in the infralimbic (IL) and prelimbic (PL) and altered the morphology Iba1+ cells 1, 2, and 7 days after social defeat. We also investigated whether acute defeat induced tissue degeneration and reductions of synaptic plasticity 2 days post-defeat. We found that while social defeat increased deposition of cellular debris and reduced synaptophysin immunoreactivity in the PL and IL, treatment with minocycline protected against these cellular changes. Finally, we tested whether a reduced conditioned defeat response in dominant compared to subordinate hamsters was associated with changes in microglia reactivity in the IL and PL. We found that while subordinate hamsters and those without an established dominance relationships showed defeat-induced changes in morphology of Iba1+ cells and cellular degeneration, dominant hamsters showed resistance to these effects of social defeat. Taken together, these findings indicate that acute social defeat alters microglial morphology, increases markers of tissue degradation, and impairs structural integrity in the IL and PL, and that experience winning competitive interactions can specifically protect the IL and reduce stress vulnerability.

Sex-dependent regulation of social avoidance by oxytocin signaling in the ventral tegmental area

It has been hypothesized that oxytocin increases the salience of social stimuli, whether the valence is positive or negative, through its interactions with the ventral tegmental area (VTA). Indeed, oxytocin neurons project to the VTA and activate dopamine neurons that are necessary for social experiences with positive valence. Surprisingly, though, there has not been an investigation of the role of oxytocin in the VTA in mediating social experiences with negative valence (e.g., social stress). Given that there are sex differences in how oxytocin regulates the salience of positively-valenced social interactions, we hypothesized that oxytocin acting in the VTA also alters the salience of social stress in a sex-dependent manner. To test this, female and male Syrian hamsters were site-specifically infused with either saline, oxytocin (9 μM), or oxytocin receptor antagonist (90 μM) into the VTA. Subjects were then exposed to either no defeat or a single, 15 min defeat by one RA. The day following social defeat, subjects underwent a 5 min social avoidance test. There was an interaction between sex and drug treatment, such that the oxytocin antagonist increased social avoidance compared to saline treatment in socially stressed females, while oxytocin decreased social avoidance compared to saline treatment in socially stressed males. Contrary to expectations, these results suggest that oxytocin signaling generally acts to decrease social avoidance, regardless of sex. These sex differences in the efficacy of oxytocin and oxytocin receptor antagonists to alter negatively-valenced social stimuli, however, should be considered when guiding pharmacotherapies for disorders involving social deficits.

Alpha-melanocyte-stimulating hormone (αMSH) modulates the rewarding properties of social interactions in an oxytocin receptor-dependent manner in Syrian hamsters (Mesocricetus Auratus)

A reduction in the rewarding properties of social interactions is frequently a key contributor to neuropsychiatric disorders. Although much remains to be learned about the neural mechanisms governing social reward, numerous studies have found that oxytocin can enhance the salience of rewarding social interactions. As a result, oxytocin has been suggested as a pharmacotherapy for disorders characterized by a dampening of social motivation. However, exogenous oxytocin does not cross the blood-brain barrier effectively, which has led to the investigation of alternative approaches to induce central oxytocin release, such as pharmaceuticals targeting melanocortins. Although oxytocin treatment is widely viewed to increase social reward, there is also recent evidence that high concentrations of oxytocin can decrease social reward. In the present study we tested the hypothesis that alpha-melanocyte-stimulating hormone (αMSH) influences the rewarding properties of social interactions by acting on oxytocin receptors. Male and female Syrian hamsters were given intracerebroventricular infusions of saline, αMSH, or a cocktail containing αMSH and an oxytocin receptor antagonist during social conditioning with a same-sex hamster and then tested for a conditioned place preference. αMSH decreased preference for the socially-paired chamber compared to saline treatment, and administration of the oxytocin antagonist concurrent with αMSH administration returned subjects' preference to control levels. Importantly, αMSH treatments did not affect any measures of body composition or the specific social behaviors displayed during conditioning. These data suggest that melanocortin-targeting drugs should be administered carefully to avoid the possibility of decreasing the rewarding properties of social interactions.