Chronic adolescent stress alters GR-FKBP5 interactions in the hippocampus of adult female rats.

Chronic stress exposure during development can have lasting behavioral consequences that differ in males and females. More specifically, increased depressive behaviors in females, but not males, are observed in both humans and rodent models of chronic stress. Despite these known stress-induced outcomes, the molecular consequences of chronic adolescent stress in the adult brain are less clear. The stress hormone corticosterone activates the glucocorticoid receptor, and activity of the receptor is regulated through interactions with co-chaperones-such as the immunophilin FK506 binding proteins 5 (FKBP5). Previously, it has been reported that the adult stress response is modified by a history of chronic stress; therefore, the current study assessed the impact of chronic adolescent stress on the interactions of the glucocorticoid receptor (GR) with its regulatory co-chaperone FKBP5 in response to acute stress in adulthood. Although protein presence for FKBP5 did not differ by group, assessment of GR-FKBP5 interactions demonstrated that adult females with a history of chronic adolescent stress had elevated GR-FKBP5 interactions in the hippocampus following an acute stress challenge which could potentially contribute to a reduced translocation pattern given previous literature describing the impact of FKBP5 on GR activity. Interestingly, the altered co-chaperone interactions of the GR in the stressed female hippocampus were not coupled to an observable difference in transcription of GR-regulated genes. Together, these studies show that chronic adolescent stress causes lasting changes to co-chaperone interactions with the glucocorticoid receptor following stress exposure in adulthood and highlight the potential role that FKBP5 plays in these modifications. Understanding the long-term implications of adolescent stress exposure will provide a mechanistic framework to guide the development of interventions for adult disorders related to early life stress exposures.

Traumatic stress history interacts with sex and chronic peripheral inflammation to alter mitochondrial function of synaptosomes.

BACKGROUND: Repeated exposures to chronic stress can lead to long lasting negative behavioral and metabolic outcomes. Here, we aim to determine the impact of chronic stress and chronic low-level inflammation on behavior and synaptosomal metabolism. METHODS: Male (n = 31) and female (n = 32) C57Bl/6 mice underwent chronic repeated predation stress or daily handling for two rounds of 15 consecutive days of exposure during the adolescent and early adult timeframes. Subsequently, mice were exposed to repeated lipopolysaccharide (LPS; 7.5 × 105 EU/kg) or saline injections every third day for eight weeks. Exploratory and social behaviors were assessed in the open field and social interaction tests prior to examination of learning and memory with the Barnes Maze. Mitochondrial function and morphology were assessed in synaptosomes post-mortem using the Cell Mito Stress test and Seahorse XFe24 analyzer, TEM, and western analysis, respectively. In addition, expression of TNF-α, IL-1ß, and ROMO1 were examined in the hippocampus and prefrontal cortex with Taqman qPCR. Circulating pro- and anti-inflammatory cytokines in the periphery were assessed using the MSD V-plex Proinflammatory Panel 1 following the first and last LPS injection as well as at the time of tissue collection. Circulating ROMO1 was assessed in terminal samples via ELISA. RESULTS: Exposure to repeated predatory stress increased time spent in the corners of the open field, suggestive of anxiety-like behavior, in both males and females. There were no significant group differences in the social interaction test and minimal effects were evident in the Barnes maze. A history of chronic stress interacted with chronic LPS in male mice to lead to a deficit in synaptosomal respiration. Female mice were more sensitive to both chronic stress and chronic LPS such that either a history of chronic stress or chronic LPS exposure was sufficient to disrupt synaptosomal respiration in females. Both stress and chronic LPS were sufficient to increase inflammation and reactive oxygen in males centrally and peripherally. Females had increased markers of peripheral inflammation following acute LPS but no evidence of peripheral or central increases in inflammatory factors or reactive oxygen following chronic exposures. CONCLUSION: Collectively, these data suggest that while metrics of inflammation and reactive oxygen are disrupted in males following chronic stress and chronic LPS, only the combined condition is sufficient to alter synaptosomal respiration. Conversely, although evidence of chronic inflammation or chronic elevation in reactive oxygen is absent, females demonstrate profound shifts in synaptosomal mitochondrial function with either a history of chronic stress or a history of chronic inflammation. These data highlight that different mechanisms are likely in play between the sexes and that sex differences in neural outcomes may be precipitated by sex-specific effects of life experiences on mitochondrial function in the synapse.

Sex modifies the consequences of extended fructose consumption on liver health, motor function, and physiological damage in rats.

Sex differences are evident in the presentation of metabolic symptoms. A shift of sex hormones that signal the onset of puberty combined with a poor diet consumed in adolescence is likely to have sex-specific, long-term impacts on adult physiology. Here, we expanded on existing literature to elucidate the sex-specific mechanisms driving physiological deficits following high fructose consumption. Male and female Wistar rats were fed a high-fructose (55%) diet beginning immediately postweaning for 10 wk. Female rats fed the high-fructose diet displayed elevated weight gain and extensive liver pathology consistent with markers of nonalcoholic fatty liver disease (NAFLD). Male rats fed the high-fructose diet exhibited increased circulating glucose along with moderate hepatic steatosis. Levels of cytokines and gene expression of inflammatory targets were not altered by fructose consumption in either sex. However, circulating levels of markers for liver health, including alanine transaminase and uric acid, and markers for epithelial cell death were altered by fructose consumption. From the alterations in these markers for liver health, along with elevated circulating triglycerides, it was evident that liver health had deteriorated significantly and that a number of factors were at play. Both adult fructose-fed male and female rats displayed motor deficits that correlated with aberrant structural changes at the neuromuscular junction; however, these deficits were exacerbated in males. These data indicate that consumption of a high-fructose diet beginning in adolescence leads to adult pathology that is modified by sex. Identification of these sex-specific changes has implications for treatment of clinical presentation of metabolic syndrome and related disorders.

Estrogen-dependent modifications to hippocampal plasticity in paternal California mice (Peromyscus californicus).

In many biparental species, mothers and fathers experience similar modifications to circulating hormones. With these modifications come alterations in neural structure and function suggesting that neuroendocrine mechanisms may underlie postpartum plasticity in both males and females. In the biparental California mouse (Peromyscus californicus), adult neurogenesis is maintained and anxiety-like behavior is attenuated in fathers during the mid-postpartum period. Given a causal relationship between estrogen and regulation of both adult neurogenesis and anxiety, we aimed to elucidate the role of estrogen-dependent mechanisms in paternal experience-related modifications to hippocampal neuroplasticity in California mice. In Experiment 1, hippocampal estrogen receptor beta (ERß) mRNA expression, along with circulating estradiol concentrations, were determined throughout the postpartum period. An upregulation in ERß expression was observed in postnatal day 16 males compared to virgins. Additionally, a rise in circulating estradiol concentrations was detected on postnatal day 2 compared to virgins; levels began to decline toward virgin levels on postnatal day 16 and postnatal day 30. In Experiment 2, we determined the role of estrogen-dependent mechanisms in adult neurogenesis and anxiety-like behavior by treating virgin and paternal males with saline or the selective estrogen receptor modulator, tamoxifen (TMX), during the time of axon extension (i.e., one week after bromodeoxyuridine injection). While TMX failed to alter elevated plus maze performance, TMX treatment inhibited survival of adult born neurons but only in paternal mice. These findings highlight the potential for estrogen-dependent pathways to mediate hippocampal adult neurogenesis in paternal mice.

Behavioral training and predisposed coping strategies interact to influence resilience in male Long-Evans rats: implications for depression.

Effective coping strategies and adaptive behavioral training build resilience against stress-induced pathology. Both predisposed and acquired coping strategies were investigated in rats to determine their impact on stress responsiveness and emotional resilience. Male Long-Evans rats were assigned to one of the three coping groups: passive, active, or variable copers. Rats were then randomly assigned to either an effort-based reward (EBR) contingent training group or a non-contingent training group. Following EBR training, rats were tested in appetitive and stressful challenge tasks. Physiological responses included changes in fecal corticosterone and dehydroepiandrosterone (DHEA) metabolites as well as neuropeptide Y (NPY)-immunoreactivity in the hippocampus and amygdala. Regardless of a rat's predisposed coping strategy, EBR rats persisted longer than non-contingent rats in the appetitive problem-solving task. Furthermore, training and coping styles interacted to yield the seemingly most adaptive DHEA/corticosterone ratios in the EBR-trained variable copers. Regardless of training group, variable copers exhibited increased NPY-immunoreactivity in the CA1 region.

Characteristic neurobiological patterns differentiate paternal responsiveness in two Peromyscus species.

Rodent paternal models provide unique opportunities to investigate the emergence of affiliative social behavior in mammals. Using biparental and uniparental Peromyscus species (californicus and maniculatus, respectively) we assessed paternal responsiveness by exposing males to biological offspring, unrelated conspecific pups, or familiar brothers following a 24-hour separation. The putative paternal circuit we investigated included brain areas involved in fear/anxiety [cingulate cortex (Cg), medial amygdala (MeA), paraventricular nucleus of the hypothalamus (PVN), and lateral septum (LS)], parental motivation [medial preoptic area (MPOA)], learning/behavioral plasticity (hippocampus), olfaction [pyriform cortex (PC)], and social rewards (nucleus accumbens). Paternal experience in californicus males reduced fos immunoreactivity (ir) in several fear/anxiety areas; additionally, all californicus groups exhibited decreased fos-ir in the PC. Enhanced arginine vasopressin (AVP) and oxytocin (OT)-ir cell bodies and fibers, as well as increased neuronal restructuring in the hippocampus, were also observed in californicus mice. Multidimensional scaling analyses revealed distinct brain activation profiles differentiating californicus biological fathers, pup-exposed virgins, and pup-naïve virgins. Specifically, associations among MPOA fos, CA1 fos, dentate gyrus GFAP, CA2 nestin-, and PVN OT-ir characterized biological fathers; LS fos-, Cg fos-, and AVP-ir characterized pup-exposed virgins, and PC-, PVN-, and MeA fos-ir characterized pup-naïve virgins. Thus, whereas fear/anxiety areas characterized pup-naïve males, neurobiological factors involved in more diverse functions such as learning, motivation, and nurturing responses characterized fatherhood in biparental californicus mice. Less distinct paternal-dependent activation patterns were observed in uniparental maniculatus mice. These data suggest that dual neurobiological circuits, leading to the inhibition of social-dependent anxiety as well as the activation of affiliative responses, characterize the transition from nonpaternal to paternal status in californicus mice.

High Fructose Diet Induces Sex-specific Modifications in Synaptic Respiration and Affective-like Behaviors in Rats.

The consequences of excessive fructose intake extend beyond those of metabolic disorder to changes in emotional regulation and cognitive function. Long-term consumption of fructose, particularly common when begun in adolescence, is more likely to lead to deleterious consequences than acute consumption. These long-term consequences manifest differently in males and females, suggesting a sex-divergent mechanism by which fructose can impair physiology and neural function. The purpose of the current project was to investigate a possible sex-specific mechanism by which elevated fructose consumption drives behavioral deficits and accompanying metabolic symptoms - specifically, synaptic mitochondrial function. Male and female rats were fed a high fructose diet beginning at weaning and maintained into adulthood. Measures of physiological health across the diet consumption period indicated that females were more likely to gain weight than males while both displayed increased circulating blood glucose. As adults, females fed the high fructose diet displayed increased floating behavior in the forced swim task while males exhibited increased exploratory behavior in the open field. Synaptic respiration was altered by diet in both females and males but the effect was sex-divergent - fructose-fed females had increased synaptic respiration while males showed a decrease. When exposed to an acute energetic challenge, the pattern was reversed. Taken together, these data indicate that diet-induced alterations to neural function and physiology are sex-specific and highlight the need to consider sex as a biological variable when treating metabolic disease. Furthermore, these data suggest that synaptic mitochondrial function may contribute directly to the behavioral consequences of elevated fructose consumption.

Acute LPS exposure increases synaptosomal metabolism during estrus but not diestrus.

The hormones estrogen and progesterone alter physiological functions, including the estrus cycle and relevant neurological and synaptic activity. Here, we determined the extent to which estrus cycle stage interacts with an inflammatory stimulus, lipopolysaccharide (LPS), to alter synaptic mitochondrial respiration in female rats. LPS elevated synaptic mitochondrial respiration of rats in estrus, but not diestrus. Likewise, estrogen concentration correlated with multiple respiratory metrics in LPS treated females in estrus. These data suggest estrogen likely modulates synaptic mitochondrial respiration in a high progesterone environment.

Adolescent stress sensitizes the adult neuroimmune transcriptome and leads to sex-specific microglial and behavioral phenotypes.

Adolescent exposure to chronic stress, a risk factor for mood disorders in adulthood, sensitizes the neuroinflammatory response to a subsequent immune challenge. We previously showed that chronic adolescent stress (CAS) in rats led to distinct patterns of neuroimmune priming in adult male and female rats. However, sex differences in the neuroimmune consequences of CAS and their underlying mechanisms are not fully understood. Here we hypothesized that biological sex would dictate differential induction of inflammation-related transcriptomic pathways and immune cell involvement (microglia activation and leukocyte presence) in the hippocampus of male and female rats with a history of CAS. Adolescent rats underwent CAS (six restraint and six social defeat episodes during postnatal days 38-49), and behavioral assessments were conducted in adolescence and adulthood. Neuroimmune measures were obtained following vehicle or a systemic lipopolysaccharide (LPS) challenge in adulthood. CAS led to increased time in the corners of the open field in adolescence. In males, CAS also increased social avoidance. As adults, CAS rats displayed an exaggerated enrichment of the nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB) pathway and chemokine induction following LPS challenge, and increased number of perivascular CD45+ cells in the hippocampus. However, CAS females, but not males, showed exaggerated glucocorticoid receptor (GR) pathway enrichment and increased microglial complexity. These results provide further insight to the mechanisms by which peripheral immune events may influence neuroimmune responses differentially among males and females and further demonstrate the importance of adolescent stress in shaping adult responses.

Acute forced exercise increases Bdnf IV mRNA and reduces exploratory behavior in C57BL/6J mice.

Acute exercise has been shown to improve memory in humans. Potential mechanisms include increased Bdnf expression, noradrenergic activity and modification of glutamate receptors. Because mice are commonly used to study exercise and brain plasticity, it is important to explore how acute exercise impacts behavior in this model. C57BL/6J mice were assigned to three groups: control, moderate-intensity running, and high-intensity running. Control mice were placed on a stationary treadmill for 30 minutes and moderate- and high-intensity mice ran for 30 minutes at 12 and 15-17 m/min, respectively. Mice were sacrificed immediately after running and the hippocampus removed. Total Bdnf, Bdnf exon IV, and glutamate receptor subunits were quantified with quantitative polymerase chain reaction. Total and phosphorylated GluR1 (Ser845 and Ser831) protein was quantified following immunoblotting. Utilizing the same protocol for control and high-intensity running, object location memory was examined in a separate cohort of mice. Anxiety-like behavior was assessed in the open field task (OFT) in a third cohort of mice that were separated into four groups: control-saline, control-DSP-4, acute exercise-saline, and acute exercise-DSP-4. DSP-4 was used to lesion the central noradrenergic system. We observed higher Bdnf IV mRNA in high-intensity runners compared to controls, but no effects of acute exercise on memory. In the OFT, runners traveled less distance and spent more time grooming than controls. DSP-4 did not attenuate the effects of exercise. A single bout of exercise increases Bdnf IV mRNA in an intensity-dependent manner; however, high-intensity running reduces exploratory behavior in C57BL/6J mice.

Chronic adolescent stress sex-specifically alters the hippocampal transcriptome in adulthood.

Chronic adolescent stress alters behavior in a sex-specific manner at the end of adolescence and in adulthood. Although prolonged behavioral repercussions of chronic adolescent stress have been documented, the potential underlying mechanisms are incompletely understood. In this study we demonstrate that a history of chronic adolescent stress modified the adult stress response, as measured by corticosterone concentration, such that a history of chronic adolescent stress resulted in a blunted response to a novel acute stressor. In order to begin to address potential mechanistic underpinnings, we assessed the extent to which chronic adolescent stress impacted global DNA methylation. Reduced global hippocampal methylation was evident in females with a history of chronic adolescent stress; thus, it was possible that chronic adolescent stress altered global transcription in the whole hippocampi of adult male and female rats. In addition, because acute stress can stimulate a genomic response, we assessed the transcriptome following exposure to an acute novel stressor to determine the extent to which a history of chronic adolescent stress modifies the adult transcriptional response to an acute stressor in males and females. In addition to the reduction in global methylation, chronic adolescent stress resulted in distinct patterns of gene expression in the adult hippocampus that differentiated by sex. Furthermore, both sex and a history of chronic adolescent stress influenced the transcriptional response to an acute novel stressor in adulthood, suggesting both latent and functional effects of chronic adolescent stress at the level of gene transcription. Pathway analysis indicated that ESR1 and IFN-α may be particularly influential transcription factors mediating these transcriptional differences and suggest candidate mechanisms for future studies. Collectively, these studies demonstrate sex-specific and enduring effects of adolescent stress exposure that are more pronounced in females than in males.

Enduring Effects of Paternal Deprivation in California Mice (Peromyscus californicus): Behavioral Dysfunction and Sex-Dependent Alterations in Hippocampal New Cell Survival.

Early-life experiences with caregivers can significantly affect offspring development in human and non-human animals. While much of our knowledge of parent-offspring relationships stem from mother-offspring interactions, increasing evidence suggests interactions with the father are equally as important and can prevent social, behavioral, and neurological impairments that may appear early in life and have enduring consequences in adulthood. In the present study, we utilized the monogamous and biparental California mouse (Peromyscus californicus). California mouse fathers provide extensive offspring care and are essential for offspring survival. Non-sibling virgin male and female mice were randomly assigned to one of two experimental groups following the birth of their first litter: (1) biparental care: mate pairs remained with their offspring until weaning; or (2) paternal deprivation (PD): paternal males were permanently removed from their home cage on postnatal day (PND) 1. We assessed neonatal mortality rates, body weight, survival of adult born cells in the dentate gyrus of the hippocampus, and anxiety-like and passive stress-coping behaviors in male and female young adult offspring. While all biparentally-reared mice survived to weaning, PD resulted in a ~35% reduction in survival of offspring. Despite this reduction in survival to weaning, biparentally-reared and PD mice did not differ in body weight at weaning or into young adulthood. A sex-dependent effect of PD was observed on new cell survival in the dentate gyrus of the hippocampus, such that PD reduced cell survival in female, but not male, mice. While PD did not alter classic measures of anxiety-like behavior during the elevated plus maze task, exploratory behavior was reduced in PD mice. This observation was irrespective of sex. Additionally, PD increased some passive stress-coping behaviors (i.e., percent time spent immobile) during the forced swim task-an effect that was also not sex-dependent. Together, these findings demonstrate that, in a species where paternal care is not only important for offspring survival, PD can also contribute to altered structural and functional neuroplasticity of the hippocampus. The mechanisms contributing to the observed sex-dependent alterations in new cell survival in the dentate gyrus should be further investigated.

Sex Differences in Synaptic Plasticity: Hormones and Beyond.

Notable sex-differences exist between neural structures that regulate sexually dimorphic behaviors such as reproduction and parenting. While anatomical differences have been well-characterized, advancements in neuroimaging and pharmacology techniques have allowed researchers to identify differences between males and females down to the level of the synapse. Disparate mechanisms at the synaptic level contribute to sex-specific neuroplasticity that is reflected in sex-dependent behaviors. Many of these synaptic differences are driven by the endocrine system and its impact on molecular signaling and physiology. While sex-dependent modifications exist at baseline, further differences emerge in response to stimuli such as stressors. While some of these mechanisms are unifying between sexes, they often have directly opposing consequences in males and females. This variability is tied to gonadal steroids and their interactions with intra- and extra-cellular signaling mechanisms. This review article focuses on the various mechanisms by which sex can alter synaptic plasticity, both directly and indirectly, through steroid hormones such as estrogen and testosterone. That sex can drive neuroplasticity throughout the brain, highlights the importance of understanding sex-dependent neural mechanisms of the changing brain to enhance interpretation of results regarding males and females. As mood and stress responsivity are characterized by significant sex-differences, understanding the molecular mechanisms that may be altering structure and function can improve our understanding of these behavioral and mental characteristics.

Separation increases passive stress-coping behaviors during forced swim and alters hippocampal dendritic morphology in California mice.

Individuals within monogamous species form bonds that may buffer against the negative effects of stress on physiology and behavior. In some species, involuntary termination of the mother-offspring bond results in increased symptoms of negative affect in the mother, suggesting that the parent-offspring bond may be equally as important as the pair bond. To our knowledge, the extent to which affect in paternal rodents is altered by involuntary termination of the father-offspring bond is currently unknown. Here, we investigated to what extent separation and paternal experience alters passive stress-coping behaviors and dendritic morphology in hippocampal subfields of California mice (Peromyscus californicus). Irrespective of paternal experience, separated mice displayed shorter latencies to the first bout of immobility, longer durations of immobility, and more bouts of immobility than control (non-separated) mice. This effect of separation was exacerbated by paternal experience in some measures of behavioral despair-separation from offspring further decreased the latency to immobility and increased bouts of immobility. In the dentate gyrus, separation reduced dendritic spine density regardless of paternal experience. Increased spine density was observed on CA1 basal, but not apical, dendrites following paternal experience. Regardless of offspring presence, fatherhood was associated with reduced apical dendritic spine density in area CA3 of the hippocampus. Separation enhanced complexity of both basal and apical dendrites in CA1, while fatherhood reduced dendritic complexity in this region. Our data suggest that forced dissolution of the pair bond induces passive stress-coping behaviors and contributes to region-specific alterations in hippocampal structure in California mouse males.

Fatherhood contributes to increased hippocampal spine density and anxiety regulation in California mice.

INTRODUCTION: Parenting alters the hippocampus, an area of the brain that undergoes significant experience-induced plasticity and contributes to emotional regulation. While the relationship between maternal care and hippocampal neuroplasticity has been characterized, the extent to which fatherhood alters the structure and function of the hippocampus is far less understood. METHODS: Here, we investigated to what extent fatherhood altered anxiety regulation and dendritic morphology of the hippocampus using the highly paternal California mouse (Peromyscus californicus). RESULTS: Fathers spent significantly more time on the open arms of the elevated plus maze, compared to non-fathers. Total distance traveled in the EPM was not changed by paternal experience, which suggests that the increased time spent on the open arms of the maze indicates decreased anxiety-like behavior. Fatherhood also increased dendritic spine density of granule cells in the dentate gyrus and basal dendrites of pyramidal cells in area CA1 of the hippocampus. CONCLUSIONS: These findings parallel those observed in maternal rodents, suggesting that the hippocampus of fathers and mothers respond similarly to offspring.

Contingency-based emotional resilience: effort-based reward training and flexible coping lead to adaptive responses to uncertainty in male rats.

Emotional resilience enhances an animal's ability to maintain physiological allostasis and adaptive responses in the midst of challenges ranging from cognitive uncertainty to chronic stress. In the current study, neurobiological factors related to strategic responses to uncertainty produced by prediction errors were investigated by initially profiling male rats as passive, active or flexible copers (n = 12 each group) and assigning to either a contingency-trained or non-contingency trained group. Animals were subsequently trained in a spatial learning task so that problem solving strategies in the final probe task, as well-various biomarkers of brain activation and plasticity in brain areas associated with cognition and emotional regulation, could be assessed. Additionally, fecal samples were collected to further determine markers of stress responsivity and emotional resilience. Results indicated that contingency-trained rats exhibited more adaptive responses in the probe trial (e.g., fewer interrupted grooming sequences and more targeted search strategies) than the noncontingent-trained rats; additionally, increased DHEA/CORT ratios were observed in the contingent-trained animals. Diminished activation of the habenula (i.e., fos-immunoreactivity) was correlated with resilience factors such as increased levels of DHEA metabolites during cognitive training. Of the three coping profiles, flexible copers exhibited enhanced neuroplasticity (i.e., increased dentate gyrus doublecortin-immunoreactivity) compared to the more consistently responding active and passive copers. Thus, in the current study, contingency training via effort-based reward (EBR) training, enhanced by a flexible coping style, provided neurobiological resilience and adaptive responses to prediction errors in the final probe trial. These findings have implications for psychiatric illnesses that are influenced by altered stress responses and decision-making abilities (e.g., depression).

Effects of apomorphine on mating behavior, flank marking and aggression in male hamsters.

In male rats, the dopamine agonist apomorphine (APO) generally facilitates copulatory behavior. However, disruptive effects of high APO doses have been reported. These have been interpreted in diverse ways, as products of a dopaminergic system that inhibits sexual behavior or as consequences of APO's stimulation of competing responses. To test the generality of these effects, we observed APO's impact on copulatory behavior in male hamsters. Several effects were observed, all attributable to a relatively high dose and involving the disruption of male behavior. More unexpectedly, APO treatment caused males to attack estrous stimulus females in the course of these tests. To clarify these effects, we observed the effects of APO on flank marking, a type of scent marking closely allied to aggression and dominance in hamsters. Treatment reliably decreased the latency of marking. It also increased the rate of marking when appropriate measures were taken to prevent this effect from being obscured by drug-induced cheek pouching. Together, these results confirm and extend APO's well-known ability to increase aggression. Further, they suggest that APO-induced aggression can intrude into other contexts so as to disrupt, or possibly facilitate, other forms of social behavior.