Female rats in a competitive situation: medial hypothalamic lesions increase and ovariectomy decreases success and aggression

Evaluating social defeat as a model for psychopathology in adult female rodents

Social conflict is a predominant stressor in humans and is associated with increased risk for developing psychological illnesses including depression and anxiety. Overwhelmingly, more women suffer from these disorders, which may be due to increased stress sensitivity. Like humans, rodents experience a myriad of physiological and behavioral sequelae due to prolonged stress exposure. Although the motivation for social conflict may differ between humans and rodents, female rodents may provide an opportunity to explore the underlying mechanisms by which stress confers risk for psychopathology in women. Because most female rodents do not express spontaneous aggression, the majority of basic research examines the physiological and behavioral outcomes of social conflict in male rodents. However, there are instances where female rodents exhibit territorial (California mice and Syrian hamsters) and maternal aggression (rats, mice, and hamsters) creating a venue to examine sex differences in physiology and behavior in response to stress. While many studies rely upon nonsocial behavioral assays (e.g., elevated plus maze, forced swim test) to assess the impact of stress on emotionality, here we primarily focus on behavioral outcomes in social-based assays in rodents. This is critically important given that disruptions in social relationships can be a cause and consequence of neuropsychiatric diseases. Next, we briefly discuss how sex differences in the recruitment of neural circuitry and/or neurochemistry in response to stress may underlie sex differences in neuroendocrine and behavioral stress responses. Finally, the translational value of females in rodent stress models and considerations regarding behavioral interpretations of these models are discussed. © 2016 Wiley Periodicals, Inc.

Allopregnanolone promotes success in food competition in subordinate male rats

BACKGROUND/AIMS

Allopregnanolone or 3α-hydroxy-5α-pregnan-20-one (AlloP) is normally sedative and anxiolytic, but can under provoking circumstances paradoxically induce aggressive behavior. Therefore, it is of particular interest to determine if there is a relationship between an anxiolytic effect and aggressive behavior following AlloP administration.

METHOD

Male Wistar rats were housed in triads comprising of 1 young rat (35 days) and 2 older rats (55 days), with the intent of producing a social hierarchy. The triads were sampled for total serum testosterone and submitted to a social challenge in the form of a food competition test (FCT), where the rats competed for access to drinking sweetened milk. At baseline, the younger rats were identified as subordinates. To test for the behavioral effect of AlloP, the subordinate rats were given intravenous AlloP injections of 0.5 and 1 mg/kg. To assess the optimal AlloP effect, 6 intervals (5, 10, 15, 20, 30 and 40 min) between injection and the FCT were used. In separate studies, AlloP was also given by subcutaneous and intraperitoneal administration at 10 and 17 mg/kg.

RESULTS

AlloP (1 mg/kg, i.v.) increased drinking time and aggressive behavior in subordinate rats, with a positive correlation between these behaviors. The subcutaneous injection (17 mg/kg) also increased drinking time in subordinate animals. Serum testosterone concentration was higher in dominant compared to subordinate rats, and correlated with drinking time and weight.

CONCLUSIONS

AlloP increased drinking time and aggressive behavior, and the correlation indicates a relationship between an anxiolytic effect and aggressive behavior.

Sexual dimorphism of GABAA receptor levels in subcortical brain regions of a woodland rodent (Apodemus sylvaticus)

This is the first report of quantitative autoradiography results showing sex differences of GABAA receptor levels in brain regions of a wild rodent (wood mouse, Apodemus sylvaticus) living in its natural habitat. The labeling of this GABAergic site with its specific high affinity radioligand [3H] muscimol provided a heterogeneous and dimorphic binding pattern in some of the neural centers. In the female, higher (> or = 50 < or = 65%) to moderately higher (< 50%) binding levels than in the male, even after correction of the specific binding values using the calculated quenching coefficients, were observed in the substantia nigra pars reticulata and ventral lateral thalamic nucleus, brain centers that are relays of motor circuits. In the male, on the other hand, a higher level was only obtained in the caudateputamen. Relays of the stria terminalis-hypothalamic-central gray pathway such as the bed nucleus of the stria terminalis, the pontine central gray and the ventromedial hypothalamic nucleus, were among the other female brain areas with an extremely higher (> 65%) to higher and moderately higher binding activity than in the male. From the saturation analyses, it appeared that the binding differences were mainly due to Bmax variations, although closer examinations revealed that changes in the KD might have also accounted for [3H] muscimol binding differences, as shown by the high KD and Bmax values in the bed nucleus of the stria terminalis, the substantia nigra pars reticulata and the pontine central gray of the female wood mouse. These findings suggest that the dimorphic binding activity of GABAA receptors in the above brain regions might be involved in neuronal circuitry mechanisms related to sex-specific social behaviors in rodents living in their natural environmental conditions.

Competitive and territorial fighting: two types of offense in the rat

Two types of fighting (offense) were compared and contrasted in three experiments on the laboratory rat. In Experiment 1, competitive fighting was obtained in pairs of hungry cagemates by placing one food pellet into their food hopper. In Experiment 2, territorial fighting was obtained by introducing an unfamiliar intruder into the home cage of a male and female pair. Both types of fighting had the same motor patterns. Whereas territorial fighting is strongest against intruders of the same sex, competitive fighting is stronger against the smaller opponent (in this case female) regardless of the sex of the test animal. Whereas territorial fighting is stronger in males, competitive fighting is stronger in females. Whereas gonadectomy reduces territorial fighting in males but not females, it reduces competitive fighting in both sexes. In experiment three, it was shown that food deprivation increases competitive fighting, while it reduces territorial fighting. On the basis of these findings a revised model of the organization of the offense motivational system is proposed.

Aggression in humans: what is its biological foundation?

Although human aggression is frequently inferred to parallel aggression based on testosterone in nonprimate mammals, there is little concrete support for this position. High- and low-aggression individuals do not consistently differ in serum testosterone. Aggression does not change at puberty when testosterone levels increase. Aggression does not increase in hypogonadal males (or females) when exogenous testosterone is administered to support sexual activity. Similarly, there are no reports that aggression increases in hirsute females even though testosterone levels may rise to 200% above normal. Conversely, castration or antiandrogen administration to human males is not associated with a consistent decrease in aggression. Finally, changes in human aggression associated with neuropathology are not consistent with current knowledge of the neural basis of testosterone-dependent aggression. In contrast, human aggression does have a substantial number of features in common with defensive aggression seen in nonprimate mammals. It is present at all age levels, is displayed by both males and females, is directed at both males and females, and is not dependent on seasonal changes in hormone levels or experiential events such as sexual activity. As would be expected from current knowledge of the neural system controlling defensive aggression, aggression in humans increases with tumors in the medial hypothalamus and septal region, and with seizure activity in the amygdala. It decreases with lesions in the amygdala. The inference that human aggression has its roots in the defensive aggression of nonprimate mammals is in general agreement with evidence on the consistency of human aggressiveness over age, with similarities in male and female aggressiveness in laboratory studies, and with observations that some neurological disturbances contribute to criminal violence. This evidence suggests that human aggression has its biological roots in the defensive aggression of nonprimate mammals and not in hormone-dependent aggression based on testosterone.

Influence of combined estradiol and testosterone implants on the aggressiveness of nonaggressive female rats

Female rats that had been cohabiting with a sterile male or with another female for 6 weeks were tested for aggression toward an unfamiliar female once each week for 3 weeks. Females that were not aggressive as a result of cohabitation with a sterile male were ovariectomized. Half were implanted with a Silastic tube containing estradiol (1 mm long hormone-filled space) and a tube containing testosterone (5 mm long hormone-filled space). The other half were implanted with empty tubes. All females that had been cohabiting with another female were ovariectomized and implanted with an estradiol- and a testosterone-filled tube. Three additional weekly tests of aggression were given beginning 1 week postoperatively. Females given hormone replacement displayed only a slight increase in aggression postoperatively. Females not given hormone replacement declined in aggressiveness. These results indicate that hormone replacement levels sufficient to maintain aggression in highly aggressive females following ovariectomy are not sufficient to produce a high level of aggression in females that have not become aggressive following cohabitation with a sterile male or that have been cohabiting with another female.

Hormone-dependent aggression in male and female rats: experiential, hormonal, and neural foundations

Hormone-dependent aggression in both male and female rats includes the distinctive behavioral characteristics of piloerection and lateral attack. In males the aggression is dependent on testicular testosterone and is commonly known as intermale aggression. In females, the aggression is most commonly observed as maternal aggression and is dependent on hormones whose identity is only beginning to emerge. The present review examines the experiential events which activate hormone-dependent aggression, the relation of the aggression to gonadal hormones, and the neural structures that participate in its modulation. In males and females, the aggression is activated by cohabitation with a conspecific of the opposite sex, by competitive experience, and by repeated exposure to unfamiliar conspecifics. In the female, the presence of pups also activates aggression. In both males and females, hormones are necessary for the full manifestation of the aggression. The essential hormone appears to be testosterone in males and a combination of testosterone and estradiol in females. The information available suggests the neural control systems for hormone-dependent aggression may be similar in males and females. It is argued that hormone-dependent aggression is behaviorally and biologically homologous in male and female rats.

Aggression by a female rat cohabitating with a sterile male: termination of pseudopregnancy does not abolish aggression

At the end of that time, each female was assessed for aggressiveness toward an unfamiliar female intruder once each week for 3 weeks. Those females displaying a high level of aggression had their male cagemate changed. For half of the females, the new male cagemate was a castrated male with a testosterone implant. For the other half, the new cagemate was a castrated male without a testosterone implant. Replacement males had been subjected to surgery 9 weeks previously. There were no differences in the aggressiveness of females of the two groups on any of 3 subsequent weekly tests of aggression. In a 3-h evaluation of male sexual behavior, none of the 9 castrated males without testosterone replacement displayed sexual activity with an estrogen/progesterone primed ovariectomized female, but 6 of 9 males with testosterone replacement did. Reanalysis of the aggression data comparing the females whose males had no testosterone replacement and females housed with the 6 males that were sexually active also revealed no differences in aggression over the 21-day test period. Since pseudopregnancy is known to last 13 days, these results indicate that the continuous presence of pseudopregnancy is not required for maintenance of aggression by a female cohabiting with a sterile male.

Hormone-dependent aggression in the female rat: testosterone plus estradiol implants prevent the decline in aggression following ovariectomy

Female rats were individually housed with a sterile male for the duration of the experiment. Beginning 7 to 10 weeks after the start of cohabitation, each female was tested for aggression toward an unfamiliar female at weekly intervals for 3 weeks. Females that displayed consistent and substantial aggression were given one of the following treatments: ovariectomy followed by both testosterone and estradiol implants, ovariectomy followed by 2 empty implants, or sham ovariectomy followed by 2 empty implants. The implants were subcutaneously placed hormone-filled Silastic capsules. They were expected to produce a serum testosterone concentration of 0.5 ng/ml and an estradiol concentration of 15 pg/ml. Postoperatively, the aggression of each female continued to be assessed on a weekly basis for 3 weeks. Ovariectomized females with hormone implants displayed a level of aggression postoperatively similar to that of sham-ovariectomized females and significantly greater than that of ovariectomized females with empty implants. These results, together with others, suggest that estradiol and testosterone act together to form the hormonal foundation of hormone-dependent aggression by females cohabiting with a sterile male.

Enhanced defensiveness and increased food motivation each contribute to aggression and success in food competition by rats with medial hypothalamic lesions

Castrated male rats (N = 27) with medial hypothalamic lesions or sham lesions were placed on a 23-h food-deprivation schedule and adapted to a highly palatable liquid food. They were also given two tests of defensiveness toward an experimenter. All animals were then housed in medial hypothalamic lesion/sham lesion pairs and subjected to a series of 6 competition tests (1 per day). Following the competition tests, all animals were given individual food consumption tests and a third test of defensiveness toward an experimenter. Correlational analysis showed that postcompetition defensiveness scores but not precompetition defensiveness scores or individual food consumption were related to aggression during the food competition. Analysis by criterion groups indicated that animals high in precompetition defensiveness and with food consumption in the normal range were not more successful in the competition but were slightly more aggressive than their sham-lesioned competitors. Animals with high postcompetition defensiveness scores and with individual food consumption in the normal range were more successful than their sham-lesioned competitors and the most aggressive of the lesioned animals during the food competition. Animals that were high in food consumption and only moderately defensive were also more successful but only slightly more aggressive in the food competition than their sham-lesioned competitors. These results suggest that a high and stable level of defensiveness, and excessive food intake, each contribute to the success and aggressiveness of rats with medial hypothalamic lesions in a food competition situation.

Aggression by ovariectomized female rats: combined testosterone/estrogen implants support the development of hormone-dependent aggression

Female hooded rats were ovariectomized and implanted with a single estrogen-filled and a single testosterone-filled Silastic tube. Control animals were ovariectomized and implanted with empty tubes. The implants produced an estrogen concentration of 30 pg/ml and a testosterone concentration of 0.25 ng/ml, levels close to those found in intact females. Two weeks following surgery, all animals were housed in individual cages, placed on a 23-hr food-deprivation schedule, and adapted to a liquid food. They were then housed in hormone-implant/empty-implant pairs and given a series of 3 restricted-access competition tests and 3 free-access competition tests (1/day). The animals were then paired with new partners and given a second series of restricted-access and free-access competition tests. Ovariectomized females with hormone implants were more successful at maintaining access to the liquid food and more aggressive than their competitors without hormone replacement. The aggression was used to maintain access to food during free-access as well as restricted-access competition. Following the competition tests, animals with hormone implants were significantly more aggressive toward an unfamiliar conspecific than were their cagemates with empty implants. The level of success and aggression by females with testosterone + estrogen implants appears greater than that which occurs with either hormone alone and comparable to that observed in intact females.

Hormone-dependent aggression in female rats: testosterone implants attenuate the decline in aggression following ovariectomy

Female rats were individually housed with a sterile male for a 4- to 5-week period. Each female was then tested for aggression toward an unfamiliar female intruder at weekly intervals. Those females that displayed a high level of aggression on each of three weekly tests were ovariectomized and given subcutaneous implants of testosterone-filled tubes, ovariectomized and given subcutaneous implants of empty tubes, or sham-ovariectomized and implanted with empty tubes. These implants should produce a serum testosterone concentration of about 0.6 ng/ml, compared to 0.17 ng/ml in intact females. Beginning 1 week postoperatively, the aggression of each female was tested weekly for 4 weeks. Ovariectomized females with testosterone implants displayed a level of aggression significantly higher than that of ovariectomized females with empty implants on 3 of 4 weekly tests. The level of aggression by females with testosterone implants was not significantly different from that of sham-ovariectomized females on the first postoperative test. Additional observations showed that testosterone implants did not produce an increase in aggression in females whose preoperative level of aggression was low. Further, Silastic implants containing estrogen (1 to 2 mm long) sufficient to maintain a serum estrogen level of 20 to 30 pg/ml also attenuated the decline of aggression following ovariectomy. These results suggest that testosterone and estrogen may both contribute to the biological substrate of hormone-dependent aggression in female rats.

Aggression by ovariectomized female rats with testosterone implants: competitive experience activates aggression toward unfamiliar females

Female hooded rats (250 to 325 g) were ovariectomized and bilaterally implanted with testosterone-filled or empty Silastic tubes. The testosterone-filled space in each tube was 10 mm long and this should produce a serum testosterone concentration 4 to 5 times that of an intact female, but well below that of a male. Three weeks following surgery, half of the animals with testosterone implants were housed with an animal with an empty implant and left for 6 weeks. The remaining animals were placed on a 23-hr food deprivation schedule, housed in testosterone implant/empty implant pairs, and then subjected to a series of food competition tests. Following the competition tests, all animals were individually tested in their living cage for aggression toward an unfamiliar female. In food competition, females with testosterone implants were more successful and more aggressive than their cagemates with empty implants. When tested for aggression toward an unfamiliar intruder, females with testosterone implants given competitive experience were more aggressive toward an intruder than were their cagemates with empty implants or females with testosterone implants not given the competitive experience. Females with testosterone implants but without competitive experience were not more aggressive toward an unfamiliar female than were their cagemates with empty implants. These results suggest that, in ovariectomized females with testosterone implants, hormone-dependent aggression fostered by a competitive situation is displayed toward unfamiliar females.

Testosterone supports hormone-dependent aggression in female rats

Female hooded rats were ovariectomized and implanted with a single testosterone-filled Silastic tube or an empty tube. The tube size was one which allowed a release of testosterone at the high end of the mean normal serum testosterone concentration for intact females. Following a 7-day recovery period, all rats were placed on a 23-hr food-deprivation schedule and adapted to a highly palatable liquid food over a 5-day period. Each animal with a testosterone implant was then housed with an animal of similar weight but an empty implant. The pairs were subjected to a series of 3 restricted-access competition tests (1/day) followed 4 days later by a series of 3 free-access competition tests. The animals were then separated, adapted to a bland liquid food, and paired with new partners. They were then subjected to the restricted- and free-access food-competition tests but with bland food as the incentive. During the first 6 competition tests there were no significant differences between groups in aggression or in time spent licking at the food spout. During the second series of tests, females with testosterone implants were more aggressive and more successful at maintaining access to the food than were their competitors with empty implants. The difference between groups occurred during the free- as well as the restricted-access tests. The effectiveness of physiological levels of testosterone in supporting aggression is attributed to the use of a test situation that activates as well as elicits hormone-dependent aggression. These results suggest that testosterone may be the hormonal substrate for hormone-dependent aggression in female rats.