Hormones and Behavior

Estradiol (E2) masculinizes the developing song system of female zebra finches (Taeniopygia castanotis) if it is administered in early life, but its effect is blocked with the co-administration of an antiandrogen (Flutamide). The effects of E2 on the developing male song system are not uniform and reports of Flutamide administration in developing male zebra finches differ in their findings. Therefore. this study was conducted to further explore the effects of administering E2 alone, Flutamide (Flut) alone, or the two in combination during early post-hatch development. Brains and testes were examined after day 100. The results showed definite demasculinizing effects of early E2 on the song nucleus HVC (proper name)--its volume and neuron number were markedly reduced. Nonetheless, early E2 hypermasculinized HVC neuronal size. Flut slightly hypermasculinized RA volume (Robust nucleus of the Arcopallium), which replicates one prior study but the absence of additional effects is at odds with others. Early E2 resulted in markedly reducing testes size, which is likely to be a consequence of hijacking endogenous endocrine feedback mechanisms. Arguments are put forward suggesting 1) early E2 action on HVC could be an anachronistic consequence of actions on the genotype of developing male versus females or 2) a disruption of endocrine mechanisms inducing inappropriate hormonal states during development. These possibilities are not mutually exclusive.

The impact of variation in parental caregiving has lasting implications for the development of offspring. However, the ways in which parents impact each other in the context of caregiving is comparatively less understood, but can account for much of the variation observed in the postnatal environment. Prairie voles (Microtus ochrogaster) demonstrate a range of postnatal social groups, including biparental pairs and pups raised by their mothers alone. In addition to the challenges of providing parental care, prairie vole parents often experience acute natural stressors (e.g., predation, foraging demands, thermoregulation) that could alter the way co-parents interact. We investigated how variation in the experience of raising offspring impacts parental behavior and neurobiology by administering an acute handling stressor on prairie vole families of single mothers and biparental parents over the course of offspring postnatal development. Mothers and fathers exhibited robust behavioral plasticity in response to the age of their pups, but in sex-dependent ways. Pup-directed care from mothers did not vary as a function of their partners presence, but did covary with the number of hypothalamic vasopressin neurons in experience-dependent ways. The relationship between vasopressin neuron numbers and fathers behaviors was also contingent upon the stress handling manipulation, suggesting that brain-behavior associations exhibit stress-induced plasticity. These results demonstrate that the behavioral and neuroendocrine profiles of adults are sensitive to distinct and interacting experiences as a parent, and extend our knowledge of the neural mechanisms that may facilitate parental behavioral plasticity.

Estrogen is an essential hormone that critically impacts bodily and brain functions, supporting learning, memory, and motor activities. A decrease in estrogen levels is associated with cognitive decline and motor dysfunction, such as muscle weakness. While conventional hormone replacement treatments (HRT) exist, those have limitations and potentially severe side effects. NAP (davunetide) is the smallest neuroprotective peptide site of activity-dependent neuroprotective protein (ADNP), a master regulator of cognition, essential for brain formation. It is known that NAP restores ADNP activity in cases of deficiency and it has already shown potential in preventing cognitive impairment, protecting against tauopathy, and improving motor function in various animal models and in clinical trials. Based on the dynamic regulation of ADNP by the estrous cycle and its involvement in steroidogenic pathways, we hypothesize that NAP may restore ADNP activity and thus serve as an alternative to conventional hormonal treatments. To test this, 3-month-old female ICR mice underwent bilateral ovariectomy (OVX) or Sham surgery and received daily intranasal administration of NAP, estrogen, or vehicle. Results showed a significant reduction in weight-normalized forelimb grip strength in the OVX model. Daily administration of NAP or estrogen resulted in intermediate grip strength levels that did not statistically differ from either the Sham control or untreated OVX groups. Interestingly, grip strength was the only test that yielded significant results, and no significant differences were observed in the Novel Object Recognition (NOR) test or computed tomography (CT) scans. These findings suggest that NAP may effectively prevent the loss of physical force production typically seen following ovarian hormone depletion, presenting a viable, non-hormonal candidate strategy for managing musculoskeletal symptoms. We hypothesize that the lack of significance in other parameters was due to soy-derived phytoestrogens in the diet, which may have exerted a systemic estrogenic effect that masked the expected physiological phenotypes typically observed in OVX models. Future replication using phytoestrogen-deficient food is required to isolate the specific neuroprotective and musculoskeletal effects of NAP from dietary influence and clarify the broader therapeutic benefits of NAP.

In many male temperate-breeding songbirds, increased plasma testosterone in the early breeding season regenerates song control nuclei that regulate song. Females of some temperate species also sing but have lower circulating testosterone concentrations. We hypothesized that upregulation of steroid receptors in females could compensate for low circulating testosterone, focusing on the northern house wren (Troglodytes aedon), a temperate-breeding songbird in which both sexes sing. We collected brain tissue from both sexes during the early breeding, late breeding, and nonbreeding season. Using quantitative PCR, we quantified mRNA expression of four genes--androgen receptor (AR), estrogen receptors (ER and ER{beta}), and aromatase (AROM)--in three song nuclei--HVC, Area X, and RA--and compared sexes. We found females had lower expression than males of AR, AROM and ER in most song nuclei, especially of AR in HVC in the breeding season. Both sexes, however, had low ER expression in Area X and RA. In males, expression differed seasonally: breeding males had higher expression of AR in RA and AROM in HVC than nonbreeding males. In both sexes, expression differed among song nuclei: in most genes, HVC had the highest expression, followed by RA, then Area X. These findings suggest singing female house wrens do not compensate for low plasma testosterone by upregulating steroid receptors beyond male levels or during early breeding, when they sing most. Conversely, increased AR expression in breeding males indicates differences in the mechanisms regulating male and female song, with testosterone playing a greater role in male birdsong.

Sex hormones are essential modulators of birdsong. Testosterone, and its active androgenic and estrogenic metabolites, 5-dihydrotestosterone (DHT) and estradiol, can re-shape the brain circuits responsible for song learning and production. The differential mechanisms of action of these different hormones during song development and song maintenance are, nonetheless, not fully understood. Here we demonstrate that unlike testosterone, DHT treatment does not induce singing behavior in naive adult female canaries that have never previously produced song. However, in birds with previous testosterone-induced singing experience, DHT alone is enough to promote the re-acquisition of high quality songs, even after months of silence. In addition, we show that the synaptic reorganization that accompanies vocal motor skill development requires more than DHT-induced androgen receptor activation. These results indicate that vocal motor practice will persistently modify the hormone-sensitive brain circuit responsible for song production, suggesting a mechanistic differentiation in the hormone-dependent regulation of the initial vocal motor skill acquisition and later re-acquisition.

17--estradiol (17aE2) treatment extends lifespan in male mice and can reduce neuroinflammatory responses in the hypothalamus of 12-month-old males. Although 17aE2 improves longevity in males, female mice are unaffected, suggesting a sexually dimorphic pattern of lifespan regulation. We tested whether the sex-specific effects of 17aE2 on neuroinflammatory responses are mediated by sex hormones and whether hypothalamic changes extend to other brain regions in old age. Manipulating sex hormone levels through gonadectomy, we show that sex-specific effects of 17aE2 on age-associated gliosis are brain region-specific and are partially dependent on gonadal hormone production. 17aE2 treatment started at 4 months of age protected 25-month-old males from hypothalamic inflammation. Castration prior to 17aE2 exposure reduced the effect of 17aE2 on hypothalamic astrogliosis. By contrast, sex-specific changes in microgliosis with 17aE2 were not significantly affected by castration in males. While 17aE2 treatment had no effect of hypothalamic astrocytes or microglia in intact females, ovariectomy significantly increased the occurrence of hypothalamic gliosis evaluated in 25-month-old females, which was partially reduced by 17aE2. In the hippocampus, both male and female gonadally-derived hormones influenced the severity of gliosis and the responsiveness to 17aE2 in a regiondependent manner. The male-specific effects of 17aE2 correlate with changes in hypothalamic ER expression, highlighting a receptor through which 17aE2 could act. The results of this study demonstrate that neuroinflammatory responses to 17aE2 are partially controlled by the presence of sex-specific gonads. Interactions between sex-steroids and neuroinflammation could, therefore, influence late-life health and disease onset, leading to sexual dimorphism in aging.

In placental mammals, estradiol levels are chronically elevated during pregnancy, but quickly drop to prepartum levels following birth. This may produce an "estrogen withdrawal" state that has been linked to changes in affective states in humans and rodents during the postpartum period. The neural mechanisms underlying these affective changes, however, are understudied. We used a hormone-simulated pseudopregnancy (HSP), a model of postpartum estrogen withdrawal, in adult female C57BL/6 mice to test the impact of postpartum estrogen withdrawal on several behavioral measures of anxiety and motivation. We found that estrogen withdrawal following HSP increased anxiety-like behavior in the elevated plus maze, but not in the open field or marble burying tests. Although hormone treatment during HSP consistently increased sucrose consumption, sucrose preference was generally not impacted by hormone treatment or subsequent estrogen withdrawal. In the social motivation test, estrogen withdrawal decreased the amount of time spent in proximity to a social stimulus animal. These behavioral changes were accompanied by changes in the expression of {Delta}FosB, a transcription factor correlated with stable long-term plasticity, in the nucleus accumbens (NAc). Specifically, estrogen-withdrawn females had higher {Delta}FosB expression in the nucleus accumbens core. Using transgenic reporter mice, we found that this increase in {Delta}FosB occurred in both D1- and D2-expressing cells in the NAc core. Together, these results suggest that postpartum estrogen withdrawal impacts anxiety and motivation and increases {Delta}FosB in the NAc core.

Anxiety disorders are more prevalent in females than males, and frequently emerge during adolescence. Despite this, preclinical research commonly focuses on adult males. Here we use Pavlovian fear conditioning and extinction in adolescent male and female rats to understand sex- and age-dependent processes relevant to anxiety disorders. In experiment 1, 35-day-old male and female rats were exposed to 6 pairings of a conditioned stimulus (CS, a tone) with an aversive unconditioned stimulus (US, a footshock). The next day they were extinguished in a contextually distinct chamber, via 60 presentations of the CS without the US. Extinction recall was tested 24 hours later in the extinction context. Estrous phase was monitored by cytology on vaginal smears taken 1 hour after each behavioral session. In experiment 2, male and female rats were given sham surgery or gonadectomy at 21 days of age. They were then trained and tested as for experiment 1. We observed that females in proestrus or met/diestrus during extinction showed delayed extinction and impaired extinction recall the next day compared to males. Ovariectomy enhanced extinction for female rats, but orchidectomy delayed extinction for males. Plasma analyses showed that met/di/proestrus phases were associated with high estradiol levels. These findings suggest that high plasma estradiol levels impair extinction for adolescent females. While these results contradict what is observed in adult animals, they are consistent with the prevalence of anxiety disorders observed in females. Our findings have important implications for understanding and treating anxiety in adolescents, particularly where treatment involves extinction-based therapies.

Female-female mounting is widespread in mammals. Although the behavioral functions of female-female mounting likely vary according to species and to behavioral context, a large body of work has investigated the relationship of female-female mounting to female sexual receptivity and hormonal status. While this relationship has been extensively explored in female rats, fewer instances of female-female mounting have been described in mice, and correspondingly, less is known about the potential relationship in mice between female-female mounting and estrous state. Recently, we found that short-term social isolation robustly promotes same-sex mounting in C57BL/6J female mice. In the current study, we tested whether displays of female-female mounting by naturally cycling 3-days-single-housed females during interactions with naturally cycling, group-housed stimulus females are related to estrous state. We found no evidence that same-sex mounting by single-housed females is related to the estrous state of either female in the pair. These findings suggest that same-sex mounting displayed by single-housed female mice is not related to female sexual behavior, and future work remains to determine the functions of female-female mounting in this species and behavioral context.

Peri-adolescence is a critical developmental stage marked by profound changes in the valence of social interactions with parents and peers. We hypothesized that the oxytocin (OXT) and vasopressin (AVP) systems, known for influencing social behavior, would be involved in the maintenance and breaking of bonding behavior expressed by peri-adolescent males and females. In rodents, OXT is associated with mother-pup bonding and may promote social attachment to members of the natal territory. AVP, on the other hand, can act in contrasting ways to OXT and has been associated with aggression and territoriality. Specifically, we predicted that in peri-adolescent male and female juveniles of the biparental and territorial California mouse (Peromyscus californicus), a) OXT would increase the social preferences for the parents over unfamiliar age-matched peers (one male and one female), and b) AVP would break the parent-offspring bond and either increase time in the neutral chamber and/or approach to their unfamiliar and novel peers. We examined anxiety and exploratory behavior using an elevated plus maze and a novel object task as a control. Peri-adolescent mice were administered an acute intranasal (IN) treatment of 0.5 IU/kg IN AVP, 0.5 IU/kg IN OXT, or saline control; five minutes later, the behavioral tests were conducted. As predicted, we found that IN OXT enhanced social preference for parents; however, this was only in male and not female peri-adolescent mice. IN AVP did not influence social preference in either sex. These effects appear specific to social behavior and not anxiety, as neither IN OXT nor AVP influenced behavior during the elevated plus maze or novel object tasks. To our knowledge, this is the first evidence indicating that OXT may play a role in promoting peri-adolescent social preferences for parents and delaying weaning in males. HIGHLIGHTSO_LIIn a 3-chambered choice test, peri-adolescent female and male California mice prefer their parents over peers or an empty chamber C_LIO_LIIntranasal oxytocin (IN OXT) enhances male but not female peri-adolescent social preference for their parents C_LIO_LIIntranasal arginine vasopressin (IN AVP) did not influence social preference in either sex C_LIO_LINeither IN OXT nor AVP alter peri-adolescent behavior in an elevated plus maze or novel object task C_LIO_LIOXT may play a role in delaying weaning in males C_LI

Social bonds are fundamental to health and well-being, and their disruption through partner separation is associated with significant physiological and psychological consequences. The kappa opioid receptor (KOR) system has been proposed as a key modulator of oxytocin (OT) release during partner separation, with prolonged separation hypothesized to trigger KOR downregulation in the paraventricular nucleus of the hypothalamus that disinhibits OT release over time (Bales & Rogers, 2022). However, this model has been difficult to assess non-invasively in primates. The present study used positron emission tomography (PET) with the KOR-selective radiotracer [11C]GR103545 to examine KOR availability in vivo across brain regions implicated in social bonding and separation distress in 16 pair-bonded titi monkeys using a within-subject two-week separation paradigm. Plasma OT, cerebrospinal fluid OT, and plasma cortisol were collected at each scan as complementary indices of peripheral and central OT signaling and physiological stress. We hypothesized that long-term separation would downregulate KORs in the hypothalamus and pituitary, indexed by reduced non-displaceable binding potential (BPND), consistent with the Bales and Rogers model, and increase plasma OT consistent with KOR downregulation disinhibiting OT release. Partner separation significantly elevated plasma cortisol in both sexes, confirming the physiological stress of the manipulation. A significant Condition x Sex interaction was observed for plasma OT, reflecting a crossover pattern in which males, who had significantly higher plasma OT than females at baseline, showed a significant decrease during separation while females showed a non-significant increase, though the sex difference during separation did not reach significance. KOR availability was significantly reduced in the nucleus accumbens during separation, with a non-significant trend toward reduction in the anterior cingulate cortex, while no significant condition effects were observed in the remaining a priori chosen regions. No significant sex effects or Condition x Sex interactions were observed in any a priori chosen PET region. Together, these findings provide the first in vivo neuroimaging evidence of KOR system engagement during partner separation in a pair-bonded primate species, and reveal a sex-dependent OT response to separation that extends and adds nuance to the Bales and Rogers (2022) framework.

Globally, stress levels among women of reproductive age are rising, while fertility rates continue to decline. Despite this correlation, a causal link between stress and reduced fertility remains unclear. Experimental studies have shown that severe and chronic stress can disrupt reproductive function, but the effects of mild stress, more representative of the daily stress experienced by most women, are still poorly understood. This study aims to identify how mild stress affects the mouse estrous cycle. Nineteen mice were vaginally lavaged daily one week before stress, during 3-day stress, and one week after stress. The mild stress paradigm consisted of two hours of repeated restraint stress each day for three days. Restraint stress disrupted the estrous cycle causing a longer cycle length in stressed mice, characterized by an extended duration in the diestrus phase. These findings suggest that even moderate stress perturbs normal reproductive cycling, potentially contributing to reduced fertility. This work highlights the need to further explore how everyday stressors may subtly impair reproductive health.

Biological rhythms in core body temperature (CBT) provide informative markers of adolescent development under controlled laboratory conditions. However, it is unknown if these markers are preserved under more variable naturalistic conditions, and if CBT may therefore prove useful in a real-world setting. To evaluate this possibility, we examined fecal steroid concentrations and CBT rhythms from pre-adolescence (p26) through early adulthood (p76) in intact male and female rats under natural light and climate at the University of California, Berkeley Field Station. Despite greater environmental variability, CBT markers of pubertal onset and its rhythmic progression were comparable to those previously reported in laboratory conditions in female rats and extend actigraphy-based findings in males. Specifically, sex differences emerged in circadian rhythm (CR) power and temperature amplitude prior to pubertal onset and persisted into early adulthood, with females exhibiting elevated CBT and decreased CR power compared to males. Within-day (ultradian rhythm; UR) patterns also exhibited a pronounced sex difference associated with estrous cyclicity. Pubertal onset, defined by vaginal opening, preputial separation, and sex steroid concentrations, occurred later than previously reported under lab conditions for both sexes. Vaginal opening and increased fecal estradiol concentrations were closely tied to the commencement of 4-day oscillations in CBT and UR power in female rats. By contrast, preputial separation and the first rise in testosterone concentration were not associated with adolescent changes to CBT rhythms in male rats. Together, males and females exhibited unique temporal patterning of CBT and sex steroids across pubertal development, with tractable associations between hormonal concentrations, external development, and temporal structure in females. The preservation of these features outside the laboratory supports CBT as a strong candidate for translational pubertal monitoring under naturalistic conditions in females.

Prolactin, a hormone involved in vertebrate parental care, is hypothesized to inhibit reproductive hypothalamic-pituitary-gonadal (HPG) axis activity during parenting, thus maintaining investment in the current brood as opposed to new reproductive efforts. While prolactin underlies many parental behaviors in birds, its effects on other reproductive behaviors, such as courtship, remain unstudied. How prolactin affects neuropeptide and hormone receptor expression across the avian HPG axis also remains unknown. To address these questions, we administered ovine prolactin (oPRL) or a vehicle control to both sexes in experienced pairs of the biparental rock dove (Columba livia), after nest removal at the end of incubation. We found that oPRL promoted parental responses to novel chicks and stimulated crop growth compared to controls, consistent with other studies. However, we found that neither courtship behaviors, copulation rates nor pair maintenance differed with oPRL treatment. Across the HPG, we found oPRL had little effect on gene expression in hypothalamic nuclei, but increased expression of FSHB and hypothalamic hormone receptor genes in the pituitary. In the gonads, oPRL increased testes size and gonadotropin receptor expression, but did not affect ovarian state or small white follicle gene expression. However, the oviducts of oPRL-treated females were smaller and had lower estrogen receptor expression compared with controls. Our results highlight that some species, especially those that show multiple brooding, may be able to maintain mating behavior despite elevated prolactin. Thus, mechanisms may exist for prolactin to promote investment in parental care without concurrent inhibition of reproductive function or HPG axis activity.

Menopause is characterized by the cessation of ovarian hormone production. During postmenopause, cisgender women face increased risks of obesity, cognitive decline, and mood disorder. Mood disorders are associated with exposure to chronic stress. We investigated the combined effects of a high-fat diet (HFD) and chronic stress exposure in a mouse model of menopause using 4-vinylcyclohexene diepoxide (VCD), a selective ovotoxicant that gradually depletes ovarian follicles and hormones. Starting at 6 months, 82 female WT C57BL/6J mice received saline or VCD (130 mg/kg i.p.) 5 days per week for 3 weeks. One month after injection, mice were fed either low-fat diet (LFD) or HFD for 8 weeks followed by 6 weeks of chronic variable mild stress (CVMS). Post-CVMS, mice were either processed for gene expression of the anterodorsal BNST or behavior tests to assess cognitive and anxiety-related behaviors. Plasma samples were collected to analyze metabolic hormones and corticosterone levels. VCD-treated HFD-fed mice had higher fat and body mass, and elevated fasting glucose levels compared to controls and more pronounced avoidance behaviors and cognitive impairments. LFD-fed, VCD-treated mice exhibited less exploration of novel objects and open spaces compared to OIL and HFD counterparts. VCD elevated corticosterone levels on LFD and increased BNST Pacap gene expression on HFD. These findings highlight cognitive repercussions of estrogen deficiency and suggest a potential protective effect of a HFD against some of the adverse outcomes associated with menopause. Our study emphasizes the importance of considering dietary and hormonal interactions in the development of therapeutic strategies.

RationaleThe -opioid receptor (MOR) is widely expressed across tissues and plays crucial roles in pain and stress responses, social behaviour, and immune regulation. Recent evidence indicates seasonal variation in in vivo MOR signalling; for example, short photoperiods are associated with reduced central MOR availability, increased MOR expression in brown adipose tissue (BAT), and strengthened brain-BAT interactions. However, despite this coupling with photoperiod, it remains unclear whether static daylength and dynamically changing photoperiods exert distinct effects, as adaptation to photoperiod transitions may itself induce stress-related modulation of the MOR system. Elucidating how seasonal, stress-related adaptations influence MOR signalling is essential for advancing our understanding of seasonal fluctuations in mood and stress regulation. MethodsWe compared rats housed under seasonal photoperiod cycling with those maintained under constant photoperiod conditions, using ex vivo radioligand binding to directly assess MOR density in central and peripheral tissues. ResultsRats exposed to seasonal photoperiod cycling showed markedly reduced inter-individual variability in MOR density in both the brain (including the cerebellum and striatum) and peripheral tissues (adrenal glands), whereas no tissues exhibited substantially increased variability. ConclusionsThese findings demonstrate that seasonal photoperiod cycling stabilizes MOR dynamics at the population level, suggesting stress-related synchronization of MOR signalling. The findings deepen our understanding of seasonal effects on endogenous MOR signalling, and further underscore the role of seasonal light variation in modulating mood-related processes.

Voluntary physical activity is a highly motivated behavior with important implications for physical and mental health, yet the neural and endocrine mechanisms underlying motivation to exercise remain poorly understood. In contrast, motivation for sugar/palatable foods, drugs, and sex has been extensively characterized using operant paradigms. Here, we describe a novel progressive ratio operant task to measure motivation to run, independent of running ability. Using female Long Evans rats, which exhibit robust voluntary running behavior, we validated this paradigm by applying a manipulation well known to enhance the motivation to run: calorie restriction. Calorie-restricted animals exhibited increased operant responding to gain access to a running wheel, thus demonstrating heightened motivation for exercise. More specifically, calorie-restricted rats completed more ratios, reached a higher breakpoint in the progressive ratio task, ran more, and spent more time in the operant chamber. We did not observe any effects of calorie restriction on the estrous cycle or steroids (e.g. corticosterone, testosterone) in the blood or brain. Importantly, our task dissociates the motivational drive for physical activity from the ability to perform the physical activity itself, providing a new paradigm for studying the neural and endocrine mechanisms that regulate exercise motivation.

Uncovering the genetic, physiological, and developmental mechanisms underlying phenotypic variation is necessary for understanding how genetic and genomic variation shape phenotypic variation and for discovering possible targets of selection. Although the neural and endocrine mechanisms underlying social behavior are evolutionarily ancient, we lack an understanding of the proximate causes and evolutionary consequences of variation in these mechanisms. Here, we examine in the natural environment the behavioral, neuromolecular, and fitness consequences of a morpholino-mediated knockdown of the mineralocorticoid receptor (MR) in the brain of nesting males of the ocellated wrasse, Symphodus ocellatus, a species with male alternative reproductive tactics. Even though MR knockdown did not significantly change male behavior directly, this experimental manipulation strongly altered glucocorticoid signaling and neuroplasticity in the preoptic area, the putative hippocampus homolog, and the putative basolateral amygdala homolog. We also found that individual variation in stress axis gene expression and neuroplasticity is strongly associated with variation in male behavior and fitness-related traits. The brain region-specific effects of MR knockdown on phenotypic integration in the wild reported here suggest specific neuroendocrine and neuroplasticity pathways that may be targets of selection.

Social connectedness strongly influences health and longevity, and adult pair bonds provide psychological benefits distinct from other social relationships. Oxytocin (OT), corticotropin-releasing hormone (CRH), and opioids, play an important role in the formation and maintenance of pair bonds. Evidence suggests that OT modulates the stress response via the hypothalamic-pituitary-adrenal (HPA) axis, while the kappa ({kappa}) opioid system interacts with and may modulate OT signaling in contexts of stress and separation. In this study 20 coppery titi monkeys were exposed to a physical stressor under three social conditions: baseline (no stressor, partner present), stress (stressor, no partner present) and buffering (stressor, partner present). We predicted stress-induced dynorphin release would reduce {kappa}-opioid receptor availability measured via [{superscript 1}{superscript 1}C]GR103545 Positron Emission Tomography (PET) and lower cerebrospinal fluid (CSF) OT, whereas partner presence would mitigate dynorphin release and increase CSF OT, with reduced dynorphin inferred from higher {kappa}-opioid receptor radioligand binding. Our results show condition-dependent differences in [{superscript 1}{superscript 1}C]GR103545 binding in several brain regions, including the amygdala and hippocampus, with altered binding in both the stress and social buffering conditions. Cortisol levels were elevated in the stress condition compared to baseline. Females exhibited lower CSF OT levels during stress than at baseline, whereas plasma OT levels did not differ across conditions or between sexes. Spearman correlations revealed no significant associations between plasma and CSF OT. Together, these findings highlight the complex interaction between {kappa}-opioid signaling, OT, and HPA axis activity in the context of social relationships and highlight neuroendocrine mechanisms underlying stress regulation in pair-bonded species.

Chronic social subordination stress in macaques, particularly beginning early in life, is associated with negative health and cognitive aging outcomes. Utilizing a longitudinal, translational monkey model of early life stress in Rhesus macaques (Macaca mulatta), we assessed adult female cognitive performance that had received social subordination stress associated with Low Social Status (LSS) since birth. This chronic social stress in lower ranked monkeys produces physiological stress responses that over time can accelerate biological and cognitive aging. We compared 14 adult females of Low Birth Rank that had received higher levels of social subordination stress since infancy with 11 adult females of High Birth Rank that had received lower levels of chronic social stress. We followed these two groups longitudinally from birth to adolescence to assess long-term behavioral, physiological, and neural consequences of social stress. As they reached adulthood (ages 7-8 years), we measured executive function/cognitive flexibility using the Intra-/Extra-dimensional shift task (ID/ED), relevant for age-related cognitive decline. Only mild differences between High and Low ranking subjects were observed on the simple discrimination, and the reversal learning stages of the task. We did find an interaction between High Birth Rank and performance across the three dimensional-shift stages that was not present in the low-ranking subjects. Thus, although at this first age, we did not yet detect the presence of accelerated cognitive decline following chronic social stress, the low-ranking females showed mild deficits in cognitive flexibility compared to high-ranking subjects. We discuss additional factors impacting performance, and comparisons with neuroimaging data.