Behavioral styles in European rabbits: social interactions and responses to experimental stressors

The existence and consistency of individual behavioral types in response to challenging situations is of increasing interest in behavioral biology. In our study on European rabbits (Oryctolagus cuniculus), we (1) investigated correlations between social behavior during early development and responses to experimental stressors during later life, and (2) tested for consistencies in these responses across different situations. For this, we observed juveniles living in field enclosures in early summer and recorded agonistic and positive social interactions. In autumn, the animals were (a) introduced singly into a novel environment and were (b) confronted with predator (red fox Vulpes vulpes) odor. We recorded behavioral and physiological stress responses. In addition, we evaluated the predator odor test with an independent sample of animals. These latter results showed a correlation between the animals' behavioral and physiological response: Individuals, which reacted to the presence of fox odor by low scanning rates showed a high increase in serum corticosterone challenge concentrations, whilst the levels in high scanners remained stable. Overall, we found correlations among social behavior displayed during early development and behavioral responses in the two experimental tests, however the correlations between the different traits of social behavior and the responses during the two different experimental tests were not consistent. Animals which were involved in more agonistic interactions during their early development started to explore faster when entered into the novel environment. During the second test we found that rabbits which previously showed a higher frequency of positive social behavior responded to the presence of predator odor by more scanning. Moreover, the behavioral responses during both experimental tests were not correlated: fast explorers in the novel environment test did not show a more active response during the predator odor test. Due to this lack of consistent behavioral styles across both tests we conclude that the study fails to support the existence of domain-general behavioral phenotypes in European rabbits.     

Photoperiod alters hypothalamic cytokine gene expression and sickness responses following immune challenge in female Siberian hamsters (Phodopus sungorus)

Rodents that live in changing environments display different immune responses mediated in part by photoperiod (day length) cues. Siberian hamsters maintained in winter-like (short) photoperiods display smaller physiological and behavioral responses to immune challenges as compared with hamsters housed in summer-like (long) photoperiods. We hypothesized that these different response patterns are attributable to altered cytokine production in the hypothalamus in response to photoperiod changes. Female hamsters were housed in long or short days for 10 weeks to induce photoperiodic alterations, then injected with either LPS (a bacterial endotoxin) or saline. Fever and food intake were assessed 3 h post-injection; hypothalami and blood were collected 3, 6, and 12 h post-injection. LPS induced lower fever and reduction in food intake responses in short-day hamsters as compared with long-day hamsters. Additionally, short-day hamsters reduced IL-1beta and Tnfalpha expression in the hypothalamus 6 h after LPS injection, as measured by quantitative RT-PCR. Plasma estradiol concentrations did not differ between long- and short-day hamsters. These data suggest that differences in cytokine production in the hypothalamus may underlie the photoperiod-induced differences in sickness responses, and that these changes are not mediated by estradiol.     

Detecting danger--or just another odorant? Olfactory sensitivity for the fox odor component 2,4,5-trimethylthiazoline in four species of mammals

2,4,5-trimethylthiazoline (TMT) is a volatile component of the anal gland secretion of the red fox and elicits behavioral and physiological fear responses in the rat. Using instrumental conditioning paradigms, we determined olfactory detection thresholds for TMT in three rats, a natural prey species of the red fox, and compared their performance to that of three squirrel monkeys, three spider monkeys and four pigtail macaques, all non-prey species of the red fox. We found that the rats were able to discriminate concentrations between 0.04 and 0.10 ppt (parts per trillion) of TMT from the odorless solvent which is by far the lowest olfactory detection threshold for an odorant reported in rats so far. In contrast, the spider monkeys needed 0.14-1.38 ppb (parts per billion), the pigtail macaques 0.41-4.07 ppb, and the squirrel monkeys 4.07-13.80 ppb to detect TMT which does not rank among the lowest olfactory thresholds reported for these three primate species. Thus, these results support the assumption that the behavioral relevance of an odorant may be an important determinant of a species' olfactory sensitivity.     

The role of androgens in the mediation of seasonal territorial aggression in male Siberian hamsters (Phodopus sungorus)

Testosterone (T) mediates aggression in a wide range of species. In some species, however, aggressive behavior persists or increases during the non-breeding season when T levels are relatively low. Animals that do not display a positive correlation between aggression and gonadal steroids suggest the need for further investigation of alternative neuroendocrine mechanisms mediating seasonal aggression. Siberian hamsters (Phodopus sungorus) are an ideal study system because they display increased territorial aggression during the non-breeding season which may be independent of circulating T levels. The goals of the present study were to: 1) explore the role of T in the aggression of reproductive males, and 2) test the hypothesis that the adrenal steroid dehydroepiandrosterone (DHEA) acts as an endocrine regulator of seasonal aggression. In Experiment 1, individuals were housed in long day (breeding) photoperiod and received castrations, exogenous T capsules or both manipulations. In Experiment 2, animals were housed in either long or short days (non-breeding) photoperiod and received DHEA or empty capsules. In both experiments, serum hormone levels and aggressive behavior were assessed. In Experiment 1, castration did not reduce aggression whereas exogenous T actually inhibited aggressive behavior. In Experiment 2, short-day individuals were more aggressive than long-day animals but DHEA treatment did not affect aggressive behavior, regardless of photoperiod. The present study supports the hypothesis that circulating gonadal steroids are not necessary to activate aggressive behavior in adult male hamsters. Further, seasonal changes in territorial aggression appear independent of circulating levels of DHEA in Siberian hamsters.     

Changes in brain cholecystokinin and anxiety-like behavior following exposure of mice to predator odor

Exposure of CD-1 mice to a familiar environment lined with clean shavings (control odor) as well as a familiar environment lined with soiled rat shavings (predator odor) induced anxiety in the light/dark box. Mice exposed to the familiar environment or predator odor displayed decreased latency to enter the dark chamber of the light/dark box and spent less time in the light portion of the apparatus relative to home-caged mice. Mice exposed to the familiar environment lined with clean shavings or predator odor displayed elevated cholecystokinin mRNA levels from the ventral tegmental area, medial and basolateral nuclei of the amygdala relative to home-caged mice. Exposure of CD-1 mice to 2, 5 or 10 min of predator odor increased acoustic startle relative to mice merely exposed to the familiar environment lined with clean shavings at protracted intervals. Mice exposed to the familiar environment lined with clean shavings did not exhibit enhanced startle relative to home-caged mice. Exaggerated startle reactivity was in evidence immediately, 24, and 48 h following a 5-min exposure of mice to predator odor. In contrast, a 10-min exposure of mice to predator odor produced an oscillating pattern of enhanced startle evident during the immediate and 48-h post-stressor intervals only. However, when the startle stimulus was withheld 1 h following odor presentation, mice exhibited enhanced startle patterns reminiscent of the 5-min exposure. The 2-min exposure of mice to predator odor produced a delayed onset of enhanced startle observed at the 168-h test interval only. Potential anxiogenic influences of mesocorticolimbic cholecystokinin availability as well as the time course and underlying neuronal substrates of long-term behavioral disturbances as a result of psychogenic stressor manipulations are discussed.     

Seasonal changes in chronic social interactions and physiological states in female rat-like hamsters (Tscheskia triton)

We examined the behavioral characteristics and physiological states related to solitary lifestyle and dominant-subordinate relationships in female rat-like hamsters (Tscheskia triton), formerly known as Cricetulus triton. Wild adult hamsters were captured and caged singly in the laboratory during the non-breeding and breeding seasons. The experimental hamsters were subjected to 5 min staged dyadic encounters every day for 28 consecutive days by pairing two unfamiliar and weight matched females in a neutral arena. Aggressive behavior, defense and flank marking were quantified everyday within the first week and once each week during the last 3 weeks. Animals were then autopsied and their physiological and reproductive state assessed. Our results suggested that dominant-subordinate relationships could be established especially in non-breeding conditions, where the dominant displayed higher aggression and flank marking, and lower defense than its opponent. The breeding females followed this pattern except there was no difference in aggressive behavior, between the partners. The repeated encounters did not appear to reduce aggression or lead to amiable behavior or bonding. At the end of the experiment breeding females exhibited higher levels of serum estradiol, progesterone and corticosterone than non-breeding females. Both dominant and subordinate females in non-breeding condition had atrophied ovaries and uteri, whereas both dominant and subordinate females in breeding condition had hypertrophied ovaries and uteri. Non-breeding females possessed heavier and thicker flank glands than breeding females did. Dominant females displayed longer or thicker flank glands than subordinate did. Thus, our data suggest that the behavioral traits observed in our experiment support the solitary lifestyle of adult female rat-like hamsters and physiological state show some differences between social ranks or in both seasons.     

Progesterone inhibition of sexual behavior is accompanied by an activation of aggression in female Syrian hamsters

Previous studies have shown that progesterone can have biphasic activational and inhibitory roles in the regulation of sexual behavior in female Syrian hamsters. The purpose of this study was to determine whether these biphasic effects on sexual behavior are mirrored in a reciprocal regulation of aggression. Ovariectomized female Syrian hamsters were treated with estradiol followed 2 and 3 days later by either 1) two oil injections (O-O), 2) an oil and a progesterone injection (O-P), 3) a progesterone and oil injection (P-O), or 4) two progesterone injections (P-P). Females were tested for aggression and sexual behavior in conjunction with these hormone treatments. On the final test session, P-P females had significantly lower lordosis durations than did O-P females, confirming the progesterone inhibition of sexual behavior previously reported. Without hormone treatment all females showed high baseline levels of aggression. On the last behavioral test, aggression in O-O females and P-O females was comparable to their baseline levels of aggression. In contrast, aggression in P-P females was significantly higher than the baseline levels of aggression. These results suggest that like its effects on sexual behavior, progesterone can have both inhibitory and activational effects on aggression in female Syrian hamsters.     

Effects of thyroxine on the photoperiodic control of energy balance and reproductive status in Siberian hamsters.

Siberian hamsters (Phodopus sungorus sungorus) exhibit a variety of seasonal responses when exposed to short days (SD), including decreases in body weight and fat, gonadal regression, and changes in several nonsteroid serum hormone concentrations. One such SD-induced hormonal change is a modest decrease in serum thyroxine (T4) concentrations. In an attempt to determine any underlying hormonal influences for the SD-induced decreases in body weight and fat, we investigated the possible role of T4 and triiodothyronine (T3). This potential paradoxical effect of these hormones on body weight and fat, as compared with most other rodent species, is not without precedent in Siberian hamsters. Specifically, changes in the gonadal steroids have opposite effects on body weight and fat in Siberian hamsters compared with laboratory rats and mice, and Syrian hamsters. SD serum thyroid hormone concentrations were elevated to long-day (LD) levels via subcutaneous T4 injections. Vehicle- and noninjected controls were included, as well as three similar LD-housed groups. Although we found a trend towards decreased T4 serum concentrations in noninjected control hamsters following 9 weeks of SD exposure, this effect did not reach statistical significance. SD-housed, T4-injected hamsters had similar decreases in body, fat pad, and paired testes weights compared to the SD-housed, vehicle- and noninjected controls despite having LD-like serum T4 and T3 concentrations. Thus, no paradoxical effect of the thyroid hormones on body weight (fat) was found, nor do these hormones appear to play a role in the effects of SDs on reproductive status in this hamster species.     

The anxiety/defense test battery: influence of gender and ritanserin treatment on antipredator defensive behavior.

The anxiety/defense test battery has been developed to measure defensive reactions in laboratory rats to both direct exposure to, and stimuli associated with, a natural predator, the domestic cat. The present investigation confirmed earlier findings with each test providing a distinct behavioral profile following exposure to predator stimuli. In addition, the data showed a consistent gender difference in a number of these behavioral measures, indicating that females are more defensive than males. These effects included reliability higher levels of cat avoidance and crouching, with lower levels of transits, lying and drinking for cat-exposed females. Similarly, females exposed to a cat odor stimulus showed a reliably higher level of stretch attend and flat back approach behaviors (risk assessment) towards the stimulus block. The 5-HT2 antagonist, ritanserin, failed to provide significant indication of anxiolytic activity, and had minimal influence on antipredator defensive behavior. An important exception to this profile was a reliable decrease in stretch attend behavior to a cat odor stimulus in females but not males. Overall, these findings suggest a complex relationship between gender, antipredator defensive behavior, and anxiolytic drug treatment.     

The vomeronasal organ is involved in discrimination of individual odors by males but not by females in golden hamsters

The vomeronasal organ (VNO) has a wide variety of functions in terrestrial vertebrates, some of which involve responses to classical pheromones whereas others do not. We examined the role of the VNO in discrimination of individual differences in odors of male and female golden hamsters using a habituation paradigm. Removal of the VNO resulted in elimination of the ability of male hamsters to discriminate between some individually distinctive odors (e.g., flank gland secretion), but not others (e.g., urine). In females, such lesions had no effect. The type of test trial also influenced the results; in test trials employing a single, novel odor, removal of the VNO in males did have an effect but in test trials in which both the novel and the familiar odor were presented, VNO removal had no effect. It is concluded that (a) there is a sex difference in the role of the VNO in the discrimination of individual odors, (b) the role of the VNO in discrimination of individual odors varies from odor to odor, and (c) deficits due to VNO removal are more readily observed in more difficult tasks.     

Genetic influences on behavioral and neuroendocrine responses to predator-odor stress in rats

The exposure of animals to a variety of stressful events can induce behavioral and physiological responses, which can be modulated by anxiety levels. It is well recognized that genetic factors play a substantial role in both anxiety and stress reactivity. The present study examined the effect of exposure to 2,4,5-trimethylthiazoline (TMT), a component of fox feces, on nociception and corticosterone levels in Lewis (LEW) and Spontaneously Hypertensive (SHR) inbred rat strains (which display genetic differences in anxiety models such as the elevated plus-maze and open-field). The influence of two quantitative trait loci (QTL), named Ofil1 and Ofil2, which are known to affect emotionality in LEW versus SHR intercrosses on the responses to TMT was also investigated. LEW and SHR rats of both sexes displayed similar levels of behavioral and neuroendocrine responses after TMT exposure. As expected, TMT odor stress produced analgesia and enhanced corticosterone levels. Ofil1 on chromosome 4 affected stress-induced analgesia in males only. Ofil2 on chromosome 7 had no effect. The results suggest that behaviors measured in classical models of generalized anxiety and reactivity to stress produced by predator odors can be genetically dissociated. Finding a locus with an effect on the behavioral responses to stress represents the starting point in the search for genes responsible for stress-related traits.     

Comparative studies on the effects of aminoglutethimide on hamster and rat adrenal cortex.

Adult female, intact or steroid suppressed hamsters were treated twice daily with 7 mg aminoglutethimide phosphate (AG) for 5 days while intact female rats received 14 mg AG per injection. AG resulted in an increase in adrenal gland weight of both hamster and rat. In the hamster AG had no effect on the amount of lipid droplets while in the rat a slightly higher number of fine lipid vacuoles was seen. In the hamster, enlargement of the gland was due to hyperplasia of the zona reticularis cells while in the rat the number of glomerulosa and fasciculata cells increased. AG had no effect on the adrenal cortex of steroid suppressed hamsters. The serum cortisol level was markedly higher in AG-treated hamsters while the corticosterone level was notably lower in AG-administered rats. In both hamsters and rats, AG-treatment did not change the serum ACTH level. Thus the study demonstrated different responses of the hamster and rat adrenal cortex to AG.     

Social defeat differentially affects immune responses in Siberian hamsters (Phodopus sungorus)

Social defeat is a complex, multi-faceted behavioral interaction capable of eliciting a wide range of physiological and behavioral responses. The behavioral components responsible for eliciting these changes, however, remain unspecified. The goal of the present study was to examine the effects of chronic social defeat on serum cortisol concentration as well as innate and acquired immune responses in adult male Siberian hamsters (Phodopus sungorus). Specifically, we experimentally manipulated the nature of the social interaction among conspecific animals (i.e., no social interaction, exposure to the sight or smell of a conspecific, or full social defeat) in order to determine the important components contributing to potential stress-induced changes in immunity. We found that immunoglobulin G (IgG) levels were decreased in defeated animals relative to both control animals and those exposed only to the olfactory cues of conspecifics. In contrast, serum bactericidal activity was increased in the defeated animals relative to controls. Prolonged social defeat did not elevate serum cortisol levels as compared with control animals. The results of this study suggest that social defeat alters immune responses and that specific behavioral components (i.e., defeat) contribute to this response. Importantly, these findings also demonstrate that social defeat exerts opposite effects on innate and acquired measures of immunity. Collectively, these results contribute to our understanding of complex social behaviors and their differential effects on endocrine and immune responses in vertebrates.     

Prolonged alterations in the serotonin neural system following the cessation of adolescent anabolic-androgenic steroid exposure in hamsters (Mesocricetus auratus)

In hamsters (Mesocricetus auratus), anabolic-androgenic steroid (AAS) exposure during adolescence facilitates offensive aggression that is modulated, in part, by serotonin (5-HT) signaling and development and by signaling and expression of 5-HT1B receptors. To examine whether these effects are persistent or reversible, the authors administered AAS to hamsters, then examined them for aggression at 1, 4, 11, 18, or 25 days following cessation of AAS treatment. Then, 1 day later, hamsters were killed by transcardial perfusion and examined for 5-HT afferents to and 5-HT1B receptor-containing neuronal puncta and somata in areas of the brain altered by AAS, namely, the anterior hypothalamus, ventrolateral hypothalamus, and medial amygdala. Although aggression resulting from AAS exposure returned to control, nonaggressive levels by 18 days following cessation of AAS treatment, alterations in 5-HT afferent innervation and 5-HT1B receptor localization were observed throughout the extended time period examined. These data suggest that adolescent AAS exposure may have long-term, irreversible effects on 5-HT neural systems and that return to nonaggressive behavioral phenotypes following adolescent AAS exposure may not be a function of plasticity in central 5-HT systems.     

Repeated social stress and the development of agonistic behavior: individual differences in coping responses in male golden hamsters

In male golden hamsters, repeated social subjugation during puberty accelerates the development of adult aggressive behavior and enhances its intensity in the presence of smaller individuals. The current study is focused on the characterization of the hormonal and behavioral responses to social subjugation during puberty. Subjugation consisted of daily exposure to an aggressive adult for 20-min periods from postnatal day 28 (P-28) to P-42, while controls were placed into an empty clean cage. Plasma cortisol levels were measured prior to or immediately after treatment on P-28 and P-42. On P-28, exposure to an aggressive adult or a clean and empty cage caused an increase in plasma cortisol levels. However, only social subjugation resulted in elevated cortisol levels on P-42, showing that juvenile hamsters habituate to an unfamiliar environment but not to social subjugation. In addition, we found a relationship between the frequency of submissive responses during social subjugation and the development of aggressive behavior. The transition from play fighting to adult aggression was most accelerated in the least submissive animals. These data show that behavioral response to social subjugation determines the development of aggressive behavior in golden hamsters. Our data also suggest that submissive behavior is a form of coping that attenuates the behavioral consequences of social subjugation in male golden hamsters.     

Plasticity in anterior hypothalamic vasopressin correlates with aggression during anabolic-androgenic steroid withdrawal in hamsters

In hamsters, adolescent anabolic-androgenic steroid (AAS) exposure facilitates offensive aggression, in part by altering the development and activity of anterior hypothalamic arginine vasopressin (AH-AVP). This study assessed whether these effects were lasting by examining aggression and AH-AVP during AAS withdrawal. Adolescent hamsters administered AAS were tested as adults for aggression at 1, 4, 11, 18, or 25 days of withdrawal, sacrificed the following day, and examined for AH-AVP afferent innervation using immunohistochemistry. Through Day 12 of withdrawal, aggression and AVP were significantly higher in AAS-treated hamsters than in controls. These differences were no longer observable by Day 19 of withdrawal, at which point the behavior and neurobiology of AAS-treated hamsters reverted to that observed in controls. These data indicate that adolescent AAS exposure has short-term, reversible effects on both aggression and AH-AVP, correlating AH-AVP with the aggressive/nonaggressive behavioral phenotype during AAS withdrawal.     

Predator odor-induced conditioned fear involves the basolateral and medial amygdala

The basolateral (BLA) and medial nucleus (MeA) of the amygdala participate in the modulation of unconditioned fear induced by predator odor. However, the specific role of these amygdalar nuclei in predator odor-induced fear memory is not known. Therefore, fiber-sparing lesions or temporary inactivation of the BLA or MeA were made either prior to or after exposure to cat odor, and conditioned contextual fear behavior was examined the next day. BLA and MeA lesions produced significant reductions in cat odor-induced unconditioned and conditioned fear-related behavior. In addition, temporary pharmacological neural inactivation methods occurring after exposure to cat odor revealed subtle behavioral alterations indicative of a role of the BLA in fear memory consolidation but not memory retrieval. In contrast, the MeA appears to play a specific role in retrieval but not consolidation. Results show that the BLA participates in the conditioned and unconditioned cat odor stimulus association that underlies fear memory, underscore a novel role of the MeA in predator odor contextual conditioning, and demonstrate different roles of the BLA and MeA in modulating consolidation and retrieval of predator odor fear memory.     

Ferret odor as a processive stress model in rats: neurochemical, behavioral, and endocrine evidence

Predator odors have been shown to elicit stress responses in rats. The present studies assessed the use of domestic ferret odor as a processive stress model. Plasma corticosterone and adrenocorticotropin hormone levels were higher after 30 min of exposure to ferret odor (fur/skin) but not control odors, ferret feces, urine, or anal gland secretions. Behavioral differences were also found between ferret and the control odors as tested in a defensive withdrawal paradigm. In addition, c-fos messenger RNA expression in several brain areas previously associated with processive stress was significantly higher in ferret odor-exposed rat brains than in control odor-exposed brains. These results suggest that ferret odor produces a reliable unconditioned stress response and may be useful as a processive stress model.     

Alterations in anterior hypothalamic vasopressin, but not serotonin, correlate with the temporal onset of aggressive behavior during adolescent anabolic-androgenic steroid exposure in hamsters (Mesocricetus auratus)

In hamsters (Mesocricetus auratus), anabolic-androgenic steroid (AAS) exposure during adolescence facilitates offensive aggression that is correlated with the enhanced development of the arginine vasopressin (AVP) neural system and reduced development of the serotonin (5-HT) neural system in the anterior hypothalamus (AH). This study examined the temporal onset of these effects by measuring aggression and AH AVP and 5-HT during progressively shorter periods of AAS exposure during adolescent development. The authors tested adolescent hamsters that received AAS for 3, 7, 14, or 28 days for offensive aggression and then examined the hamsters for AVP/5-HT afferent innervation to the AH using immunohistochemistry. While reductions in AH 5-HT afferent innervation were detectable by 7 days of AAS exposure, no concomitant increases in offensive aggression were observed compared to oil-treated littermates. In contrast, by Day 14 of AAS treatment, AH AVP and offensive aggression were significantly higher than oil-treated controls. These data indicate that relatively short-term adolescent AAS exposure alters aggression and AH 5-HT and AVP development, yet only alterations in AH AVP development correlate with temporal onset of the aggressive behavioral phenotype during adolescent AAS exposure.     

Differential effects of paternal presence on pup survival in two species of dwarf hamster (Phodopus sungorus and Phodopus campbelli)

The effect of the presence or absence of the male, and of decreased ambient temperature (21 degrees C vs. 4 degrees C) on litter survival, pup survival and pup growth was measured from birth through day 18 after birth in Siberian (P. sungorus) and Djungarian (P. campbelli) hamsters. Siberian hamsters were not significantly affected by the experimental manipulations. In contrast, whereas 100% of litters and 95% of pups were successfully raised to weaning at 21 degrees C by paired Djungarian hamsters, survival fell to 47% when the mate was absent and even further, to 32%, when the ambient temperature was lowered. No significant differences in litter size or pup weight at birth were detected between species at the warmer temperature. However, P. sungorus pups gained weight significantly faster through day 12 after birth (while dependence upon the mother for food was absolute) than P. campbelli pups under all experimental conditions. Although the species are closely related, these data show that male Djungarian hamsters are essential to offspring survival under conditions where Siberian hamsters do not require conspecific help. Species differences in metabolism and thermoregulation may account for the differential pup survival.