ΔJunD overexpression in the nucleus accumbens prevents sexual reward in female Syrian hamsters.

Motivated behaviors, including sexual experience, activate the mesolimbic dopamine system and produce long-lasting molecular and structural changes in the nucleus accumbens. The transcription factor ΔFosB is hypothesized to partly mediate this experience-dependent plasticity. Previous research in our laboratory has demonstrated that overexpressing ΔFosB in the nucleus accumbens of female Syrian hamsters augments the ability of sexual experience to cause the formation of a conditioned place preference. It is unknown, however, whether ΔFosB-mediated transcription in the nucleus accumbens is required for the behavioral consequences of sexual reward. We therefore used an adeno-associated virus to overexpress ΔJunD, a dominant negative binding partner of ΔFosB that decreases ΔFosB-mediated transcription by competitively heterodimerizing with ΔFosB before binding at promotor regions on target genes, in the nucleus accumbens. We found that overexpression of ΔJunD prevented the formation of a conditioned place preference following repeated sexual experiences. These data, when coupled with our previous findings, suggest that ΔFosB is both necessary and sufficient for behavioral plasticity following sexual experience. Furthermore, these results contribute to an important and growing body of literature demonstrating the necessity of endogenous ΔFosB expression in the nucleus accumbens for adaptive responding to naturally rewarding stimuli.

Aggressive experience increases dendritic spine density within the nucleus accumbens core in female Syrian hamsters.

Activity within the mesolimbic dopamine system is associated with the performance of naturally motivated behaviors, one of which is aggression. In male rats, aggressive behavior induces neurochemical changes within the nucleus accumbens, a key structure within the mesolimbic dopamine system. Corresponding studies have not been done in females. Female Syrian hamsters live as isolates and when not sexually responsive are aggressive toward either male or female intruders, making them an excellent model for studying aggression in females. We took advantage of this naturally expressed behavior to examine the effects of repeated aggressive experience on the morphology of medium spiny neurons in the nucleus accumbens and caudate nucleus, utilizing a DiOlistic labeling approach. We found that repeated aggressive experience significantly increased spine density within the nucleus accumbens core, with no significant changes in any other brain region examined. At the same time, significant changes in spine morphology were observed in all brain regions following repeated aggressive experience. These data are significant in that they demonstrate that repeated exposure to behaviors that form part of an animal's life history will alter neuronal structure in a way that may shift neurobiological responses to impact future social interactions.

Aggressiveness and brain amine concentration in dominant and subordinate finishing pigs fed the beta-adrenoreceptor agonist ractopamine.

Under farm conditions, aggression related to the formation of social hierarchy and competition for resources can be a major problem because of associated injuries, social stress, and carcass losses. Any factor that may affect the regulation and amount of aggression within a farmed system, for instance, feeding the beta-adrenoreceptor agonist ractopamine (RAC), is therefore worthy of investigation. The objectives of this study were to assess the effects of the widely used swine feed additive RAC, considering also the effects of sex and social rank on aggressiveness and concentrations of brain amines, neurotransmitters essential for controlling aggression, in finishing pigs. Thirty-two barrows and 32 gilts (4 pigs/pen by sex) were fed either a control diet or a diet with RAC (Paylean, Elanco Animal Health, Greenfield, IN) added (5 mg/kg for 2 wk, followed by 10 mg/kg for 2 wk). The top dominant and bottom subordinate pigs (16 pigs/sex) in each pen were determined after mixing by a 36-h period of continuous behavioral observation. These pigs were then subjected to resident-intruder tests (maximum 300 s) during the feeding trial to measure aggressiveness. At the end of wk 4, the amygdala, frontal cortex, hypothalamus, and raphe nuclei were dissected and analyzed for concentrations of dopamine (DA); serotonin (5-HT); their metabolites 3,4-dihydroxyphenyl acetic acid (DOPAC) and homovanillic acid, and 5-hydroxyindoleacetic acid (5-HIAA), respectively; norepinephrine; and epinephrine using HPLC. Ractopamine-fed gilts performed more attacks during the first 30 s of testing than pigs in all other subgroups (P < 0.05). By the end of the resident-intruder test (300 s), the dominant control gilts and barrows, and both dominant and subordinate RAC-fed gilts performed the greatest percentage of attacks (P < 0.05). Gilts had decreased norepinephrine and DOPAC concentrations in the amygdala and frontal cortex, and when fed RAC, gilts also had the least 5-HIAA concentration and greatest DA turnover rate in the amygdala (P < 0.05). The 5-HT concentration was less in the frontal cortex of gilts compared with barrows and in the raphe nuclei (single site for brain 5-HT synthesis) of dominant gilts (P < 0.05). Ractopamine may be affecting aggressive behavior through indirect action on central regulatory mechanisms such as the DA system. The aggressive pattern observed in the tested pigs, especially in gilts, is likely linked to brain monoamine profiling of a deficient serotonergic system in the raphe nuclei, amygdala, and frontal cortex, and enhanced DA metabolism in the amygdala, brain areas vital for aggression regulation.

Behavior and peripheral amine concentrations in relation to ractopamine feeding, sex, and social rank of finishing pigs.

Aggression can impair productivity and well-being. The association between aggression in finishing pigs and the feed additive ractopamine (RAC), a beta-adrenoreceptor agonist, is unknown and warrants further investigation. Our goal was to examine behavioral activity, including aggression, in the home pen and concentrations of peripheral amines in barrows and gilts, taking into account diet (RAC) and social rank. Sixty-four finishing pigs, housed in pens of 4 by sex, were fed either a control (CTL) or RAC-added (5 mg/kg for 2 wk plus 10 mg/kg for another 2 wk) diet. The top dominant and bottom subordinate pigs in each pen were determined at mixing (2 wk pretrial). The behavior of all pigs was recorded continuously during the pretrial week (baseline) and for the following 4 wk. These behavioral data were used to evaluate home pen aggression, including the number of agonistic interactions (AINX) and constituent aggressive actions, during a 3-h period (0800 to 1100 h) once per week and their change in relation to the baseline. Time-budget behaviors and postures were analyzed over eight 24-h periods (2 d/wk) using 10-min instantaneous scan sampling that focused on only the dominant and subordinate pigs in each pen. These 2 pigs were also subjected to blood collection once per week during the trial to determine concentrations of dopamine, norepinephrine, epinephrine, and serotonin (5-HT) using HPLC. Gilts performed more bites and total actions per AINX than barrows, and RAC-fed gilts increased bites and pursuits, whereas these behaviors decreased compared with baseline values in all other subgroups (P < 0.05). Gilts fed RAC increased the total number actions per AINX, whereas the occurrence of AINX decreased for all subgroups (P < 0.01). Overall, RAC-fed pigs were more behaviorally active, spending more time alert, bar biting, and sham chewing compared with CTL pigs (P < 0.05). The dominant RAC-fed pigs tended to have the greatest norepinephrine concentrations among the tested subgroups (P = 0.08). Dominant barrows had greater epinephrine concentrations than subordinate barrows (P < 0.05). The RAC-fed gilts tended to have lesser 5-HT concentrations than CTL gilts (P = 0.08), whereas concentrations were similar in barrows (P > 0.10). Greater activity and the increase in oral-related behaviors observed in RAC-fed pigs may be mediated by the increase in arousal caused by RAC. Intensified aggression in gilts, especially when fed RAC, may be linked to reduced central 5-HT and greater noradrenergic activity, and further research on brain neurotransmitters in gilts is needed.

Delta FosB overexpression in the nucleus accumbens enhances sexual reward in female Syrian hamsters.

Repeated activation of the mesolimbic dopamine system results in persistent behavioral alterations accompanied by a pattern of neural plasticity in the nucleus accumbens (NAc). As the accumulation of the transcription factor Delta FosB may be an important component of this plasticity, the question addressed in our research is whether Delta FosB is regulated by sexual experience in females. We have shown that female Syrian hamsters, given sexual experience, exhibit several behavioral alterations including increased sexual efficiency with naïve male hamsters, sexual reward and enhanced responsiveness to psychomotor stimulants (e.g. amphetamine). We recently demonstrated that sexual experience increased the levels of Delta FosB in the NAc of female Syrian hamsters. The focus of this study was to explore the functional consequences of this induction by determining if the constitutive overexpression of Delta FosB by adeno-associated virus (AAV) vectors in the NAc could mimic the behavioral effects of sexual experience. Animals with AAV-mediated overexpression of Delta FosB in the NAc showed evidence of sexual reward in a conditioned place preference paradigm under conditions in which control animals receiving an injection of AAV-green fluorescent protein (GFP) into the NAc did not. Sexual behavior tests further showed that males paired with the AAV-Delta FosB females had increased copulatory efficiency as measured by the proportion of mounts that included intromission compared to males mated with the AAV-GFP females. These results support a role for Delta FosB in mediating natural motivated behaviors, in this case female sexual behavior, and provide new insight into the possible endogenous actions of Delta FosB.

Changes in gene expression within the nucleus accumbens and striatum following sexual experience.

Sexual experience, like repeated drug use, produces long-term changes including sensitization in the nucleus accumbens and dorsal striatum. To better understand the molecular mechanisms underlying the neuroadaptations following sexual experience, we employed a DNA microarray approach to identify genes differentially expressed between sexually experienced and sexually naive female hamsters within the nucleus accumbens and dorsal striatum. For 6 weeks, a stimulus male was placed in the home cage of one-half of the hormonally primed, ovariectomized female hamsters. On the seventh week, the two experimental groups were subdivided, with one half paired with a stimulus male. In comparison with sexually naive animals, sexually experienced hamsters receiving a stimulus male on week 7 exhibited an increase in a large number of genes. Conversely, sexually experienced female hamsters not receiving a stimulus male on week 7 exhibited a reduction in the expression of many genes. For directional changes and the categories of genes regulated by the experimental conditions, data were consistent across the nucleus accumbens and dorsal striatum. However, the specific genes exhibiting changes in expression were disparate. These experiments, among the first to profile genes regulated by female sexual behavior, will provide insight into the mechanisms by which both motivated behaviors and drugs of abuse induce long-term changes in the mesolimbic and nigrostriatal dopamine pathways.

Localization of protein Ser/Thr phosphatase 5 in rat brain.

Protein phosphatase 5 is a recently discovered Ser/Thr phosphatase that is structurally related to calcineurin and protein phosphatases 1 and 2. Northern blot and in situ hybridization studies have shown that protein phosphatase 5 mRNA is present at high levels in brain and is localized to discrete regions. In the present study, we used immunocytochemistry and immunoblot analyses to examine the regional and subcellular distribution of this enzyme in brain. Our work demonstrates that protein phosphatase 5 is widely expressed throughout brain, but is not uniformly distributed. The most intense staining occurred in neurons of the cerebellum, cerebral cortex, and the supraoptic nucleus of the hypothalamus. Other areas also contained immunoreactive cell bodies, including the globus pallidus, hippocampus, thalamus, lateral preoptic area of the hypothalamus, substantia nigra and other brainstem nuclei. Staining in these cells was observed primarily in perikarya and proximal processes.

Sexual behavior induction of c-Fos in the nucleus accumbens and amphetamine-stimulated locomotor activity are sensitized by previous sexual experience in female Syrian hamsters.

Dopamine transmission in the nucleus accumbens can be activated by drugs, stress, or motivated behaviors, and repeated exposure to these stimuli can sensitize this dopamine response. The objectives of this study were to determine whether female sexual behavior activates nucleus accumbens neurons and whether past sexual experience cross-sensitizes neuronal responses in the nucleus accumbens to amphetamine. Using immunocytochemical labeling, c-Fos expression in different subregions (shell vs core at the rostral, middle, and caudal levels) of the nucleus accumbens was examined in female hamsters that had varying amounts of sexual experience. Female hamsters, given either 6 weeks of sexual experience or remaining sexually naive, were tested for sexual behavior by exposure to adult male hamsters. Previous sexual experience increased c-Fos labeling in the rostral and caudal levels but not in the middle levels of the nucleus accumbens. Testing for sexual behavior increased labeling in the core, but not the shell, of the nucleus accumbens. To validate that female sexual behavior can sensitize neurons in the mesolimbic dopamine pathway, the locomotor responses of sexually experienced and sexually naive females to an amphetamine injection were then compared. Amphetamine increased general locomotor activity in all females. However, sexually experienced animals responded sooner to amphetamine than did sexually naive animals. These data indicate that female sexual behavior can activate neurons in the nucleus accumbens and that sexual experience can cross-sensitize neuronal responses to amphetamine. In addition, these results provide additional evidence for functional differences between the shell and core of the nucleus accumbens and across its anteroposterior axis.

Sexual experience sensitizes mating-related nucleus accumbens dopamine responses of female Syrian hamsters.

We examined the effects of prior sexual experience on extracellular concentrations of dopamine in the nucleus accumbens of female hamsters. Nucleus accumbens dopamine was measured by in vivo microdialysis during mating in female Syrian hamsters that had previously been given six prior sexual encounters with a male, three prior encounters, or were sexually naive. High levels of sexual behavior were observed in all three groups, which were accompanied by increases in dialysate dopamine during periods when the male was present. However, females that received six prior sexual encounters had significantly elevated and prolonged increases in dialysate dopamine compared with those of the sexually naive females or females with only three prior sexual encounters with a male. The data indicate that the mesolimbic system can be sensitized by repeated experiences associated with a motivated behavior.

Effects of serotonin 1A or 1B receptor agonists on social aggression in male and female Syrian hamsters.

Numerous studies have demonstrated that activation of serotonin 5-HT1A or 5-HT1B receptor decreases aggression in male mammals. To determine whether female mammals also show decreased aggression in response to 5-HT1A or 5-HT1B activation, we assessed the effects of the serotonin receptor agonists 8-OH-DPAT (5-HT1A) and CGS-12066A (5-HT1B) on aggression in female Syrian hamsters. Female Syrian hamsters were tested for interfemale aggression 2 days before and 15 min after receiving intracerebroventricular infusions of 8-OH-DPAT (5, 10, 20 microg) or CGS-12066A (5, 10, 20 microg). Neither drug affected aggression as measured by the latency and frequency of attacks or uprights, although the highest dose of 8-OH-DPAT increased general activity. For male hamsters, intraventricular infusions of 10 microg of 8-OH-DPAT essentially eliminated aggression, whereas 5 microg of 8-OH-DPAT or 20 microg of CGS-12066A were without effect. Systemic treatment with 8-OH-DPAT (1 mg/kg body weight) did reduce aggression in females, although there was an attendant increase in symptoms of nonspecific serotonergic activity. There were no behavioral effects of systemic CGS-12066A (4 mg/kg body weight) on female hamsters. These results indicate that there may be sex differences in the neurochemical regulation of aggression and point to a need for more studies directed at this issue.

Intromissive stimulation from the male increases extracellular dopamine release from fluoro-gold-identified neurons within the midbrain of female hamsters.

Extracellular concentrations of dopamine in the nucleus accumbens were monitored using microdialysis in ovariectomized female Syrian hamsters hormonally primed with estradiol and progesterone or with a similar regimen of oil injections. Some females in each of these groups had their vaginas occluded with tape, whereas the remaining females' vaginas stayed unoccluded. When exposed to a male, both groups of hormonally primed females showed high levels of lordosis. However, only in the hormone-primed, unoccluded females were there significant elevations of dialysate dopamine during the sexual interactions with the male. There were no significant elevations in dopamine levels in the oil-treated females during interactions with the male. These data suggest that nucleus accumbens dopamine is responsive to stimuli associated with the vaginocervical stimulation received by the female during intromissions by the male. Histological analyses were based on Fluoro-Gold efflux through the probes combined with immunocytochemistry for tyrosine hydroxylase. Probe placements in the rostral accumbens, caudal accumbens, or rostral bed nucleus of the stria terminalis were not distinguishable based on analyses of basal dopamine levels, volume of Fluoro-Gold injection sites, or Fluoro-Gold labeling of midbrain, tyrosine hydroxylase-stained neurons. The number of midbrain neurons containing Fluoro-Gold was positively related to basal dopamine levels, indicating that the amount of dopamine recovered from the nucleus accumbens in microdialysis studies is a function of the number of neurons contributing to the terminal field in the region of the probe.

Dopamine receptor antagonists attenuate conditioned place preference following sexual behavior in female Syrian hamsters.

We assessed the effects of the dopamine D2 receptor antagonists, sulpiride and raclopride, on conditioned place preference produced by sexual behavior in female Syrian hamsters. Female hamsters treated with sulpiride or raclopride showed high levels of sexual behavior (lordosis) that were equivalent to control females receiving vehicle injections. The degree of place preference conditioning for sulpiride-treated females was marginally reduced, whereas females treated with raclopride showed no evidence of conditioning. These results indicate that conditioned place preference is a useful means for probing the appetitive components of female sexual behavior, and that dopamine D2 receptors are involved in this appetitive process.

-Fos expression in female hamster brain following sexual and aggressive behaviors.

The goal of these experiments was to use c-Fos immunocytochemistry to determine areas of the female hamster brain that are active during lordosis and aggression. Ovariectomized hamsters were given (i) estradiol and progesterone, plus a lordosis test, (ii) estradiol and progesterone, but no lordosis test, (iii) oil, plus an aggressive behavior test, or (iv) oil, but no behavior test. Results showed that following lordosis, there was increased c-Fos expression in the medial bed nucleus of the stria terminalis, medial accumbens, medial preoptic nucleus, paraventricular nucleus and medial amygdala. Following a single aggression test, c-Fos was significantly increased only within the medial amygdala. There was no effect of lordosis or aggression on c-Fos expression within the lateral or central ventromedial hypothalamus, suprachiasmatic nucleus or dorsal midbrain central gray. In a second experiment, ovariectomized female hamsters were given (i) repeated aggressive experience, (ii) a single aggression test or (iii) no aggression test. Because some females were not aggressive towards males, they became a separate group post hoc. The number of cells expressing c-Fos was higher in the medial preoptic nucleus and medial amygdala of females given a single aggressive test and in non-aggressive females vs control females. Females given prior aggressive experience showed higher c-Fos expression only in the medial preoptic nucleus. These results demonstrate that increased neural activation in several forebrain nuclei is seen after sexual or aggressive behaviors in female hamsters. However, because the pattern of c-Fos staining in the non-aggressive females was similar to the pattern in aggressive females, this questions previous conclusions regarding the behavioral specificity of these effects and suggests instead that such activation is common to social interactions in general.

Conditioned place preference in female hamsters following aggressive or sexual encounters.

Prior studies have demonstrated the utility of conditioned place preference procedures for examining the motivational or rewarding properties of behavior. The purpose of this experiment was to assess whether female Syrian hamsters would show evidence of conditioned place preference for aggression or sexual behaviors. Weekly conditioning sessions were conducted for three groups of female hamsters for 5 weeks. One group of female hamsters engaged in sexual activity with a male hamster in the gray compartment of a place preference apparatus. A second group of females experienced aggressive interactions with a male when placed together also in the gray compartment. Females in each of these conditioning groups were placed alone in the white compartment within 1 h of the behavioral interactions. A control group of hormone-treated females was placed alone in both compartments of the apparatus. Following the conditioning sessions, all females were given free access to the conditioning apparatus. Females with prior sexual or aggressive experience spent significantly more time in the gray compartment than they did before conditioning. Control females did not show any significant change in their preference for either compartment of the apparatus. The results suggest that female hamsters prefer an environment associated with the prior rewarding properties of sexual or aggressive interactions.

A microdialysis study of ventral striatal dopamine during sexual behavior in female Syrian hamsters.

Microdialysis was used to study the effects of exposure to a male hamster on extracellular concentrations of dopamine, dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindole acetic acid (5-HIAA) in the ventral striatum of ovariectomized female Syrian hamsters pretreated with either estradiol and progesterone, or a similar regimen of oil injections. The hormone-treated females showed high levels of lordosis throughout the hour of exposure to the male. In hormone-treated females, there was a rapid elevation of dialysate dopamine within the first 15 min of exposure to the male. Dialysate dopamine gradually declined over the next 45 min, though remaining significantly above baseline during the entire period of exposure to the male. None of the oil-treated females showed any indication of lordosis, and the addition of the male produced only a small increase in dopamine at 30 min, after which dopamine returned to pre-male basal levels. DOPAC, HVA, and 5-HIAA were all elevated following introduction of the male for both groups of females. These results suggest that ovarian hormones modulate the responsivity of ventral striatal dopamine to incentive stimuli associated with mating behavior in females, although extracellular levels of dopamine in the ventral striatum do not seem to be directly coupled to the display of lordosis.

Forebrain sites of estradiol-17 beta action on sexual behavior and aggression in female Syrian hamsters.

The effects of intracranial implants of estradiol in the ventromedial hypothalamus (VMH), the anterior hypothalamus (AH), or the medial amygdala (AMG) on aggression, sexual behavior, and serum estradiol were examined in female Syrian hamsters. Estradiol implants in the VMH, followed by systemic progesterone, stimulated sexual behavior and inhibited aggression. Estradiol implants in other intracranial sites activated sexual behavior but did not reliably inhibit aggression. Intracranially implanted and systemically treated animals had equivalent peripheral estradiol concentrations at sacrifice. These results suggest that: (a) the VMH is an important neural site for estradiol actions on sexual and aggressive behavior, (b) the caudal AH and AMG also may be sites of estradiol action on sexual behavior, and (c) these intracranial implants may only be effective given systemic estradiol exposure or the concurrent stimulation of multiple brain areas.

Characterization of the carboxyl-terminal sequences responsible for protein retention in the endoplasmic reticulum.

The COOH-terminal sequence KDEL has been shown to be essential for the retention of several proteins in the lumen of the endoplasmic reticulum (Munro S., and Pelham, H. R. B. (1987) Cell 48, 899-907; Pelham, H. R. B. (1988) EMBO J. 7, 913-918; Mazzarella; R. A., Srinivasan, M., Haugejorden, S. M., and Green, M. (1990) J. Biol. Chem. 265, 1092-1101). We have previously demonstrated that variants to the KDEL retention signal, particularly at the initial two positions of the tetrapeptide, can be made without affecting its ability to direct intracellular retention when appended to the neuropeptide Y precursor (pro-NPY) (Andres, D. A., Dickerson, I. M., and Dixon, J. E. (1990) J. Biol. Chem. 265, 5952-5955). To further investigate the nature of the KDEL retention signal, oligonucleotide-directed mutagenesis and transfection was used to generate stable mouse anterior pituitary AtT-20 cell lines expressing pro-NPY mutants with variants of the KDEL sequence added to their direct carboxyl terminus. Analyses of dibasic processing and indirect immunofluorescent microscopy of AtT-20 subclones were consistent with the retention of the pro-NPY mutants bearing the COOH-terminal extensions QDEL, KEDL, or KDEI within the endoplasmic reticulum. A change in the final amino acid of the tetrapeptide from Leu to Val abolished retention completely, and the peptide hormone was processed and secreted. These results indicate that only a limited number of conservative changes can be made to the final two positions of the tetrapeptide without abolishing activity and suggest a highly specific interaction of the retention signal and the KDEL receptor.

High-dose intravenous gamma globulin: does it have a role in the treatment of severe erythroblastosis fetalis?

The role of high-dose intravenous (IV) gamma globulin in the treatment of erythroblastosis fetalis was assessed in five pregnancies with severe Rh (four) or Kell (one) isoimmunization. These women were treated with IV gamma globulin (1.0 g/kg body weight) once a week. In addition, fetal blood transfusions were performed when indicated. In four patients with Rh sensitization, high-dose IV gamma globulin treatment had no apparent effect on the total number of intrauterine transfusions required, the interval between transfusions, or the volume of blood required at each transfusion. The treatment did not prevent fetal hydrops and had no effect on maternal antibody titers. In one patient with Kell sensitization, however, the course of the disease was less severe than anticipated, suggesting that IV gamma globulin treatment may have modified the severity of the disease. We conclude that high-dose IV gamma globulin does not appear to be useful in the treatment of severe Rh disease. Its role in Kell and other types of red-cell isoimmunization deserves further evaluation.

Estradiol increases the dendritic length of ventromedial hypothalamic neurons in female Syrian hamsters.

We examined the effects of ovarian hormones on dendritic architecture of neurons in the ventromedial nucleus of the hypothalamus in female Syrian hamsters. Treatment with 10 micrograms of estradiol benzoate for two days, or estradiol benzoate for two days followed by an injection of 500 micrograms of progesterone, increased the total dendritic length of ventromedial nucleus neurons by almost 50% compared with neurons from the ventromedial nucleus of ovariectomized, oil-treated females. Neurons in a control region, the dorsomedial nucleus of the hypothalamus, were unaffected by these hormone treatments. These results demonstrate that steroids can induce changes in dendritic structure within 48 hr, suggesting that such morphological reconfiguration of hypothalamic neurons may underlie variations in behavior associated with the female's 4-day estrous cycle.

Induction of obesity by group housing in female Syrian hamsters.

We examined the effects of group housing on body weight in adult female Syrian hamsters. Over a 10-week period, female hamsters housed in groups of 5 per cage increased their body weight by 61% compared with an 18% increase in body weight for female hamsters housed individually. The divergence in body weight between females housed in groups and females housed individually was evident as early as 2 weeks after the start of the experiment. At the end of the 10 weeks, group-housed females were significantly longer, had a higher percentage of body fat, and larger adrenal glands compared with these measures from individually housed hamsters. These results demonstrate that housing conditions can have a powerful effect on body weight and body composition in female Syrian hamsters. These effects are discussed in the context of social stress mediating obesity in Syrian hamsters, and offer the possibility for a socially based animal model of obesity.