Adrenal mediation of intermale aggression maintained by aromatized and reduced metabolites of testosterone

Individually housed CD-1 mice were either sham castrated or castrated and treated with testosterone (T), estradiol benzoate (EB), dihydrotestosterone (DHT), a combination of EB and DHT (EB+DHT), or the injection vehicle. Following 16 days of isolation and subcutaneous injections, animals were tested repeatedly for fighting behavior in paired encounters with nonaggressive stimulus males. Results indicated that the T and EB+DHT groups fought to the same extent as the gonadally intact group. Both the EB and DHT groups fought more than the vehicle-treated group but less than the T, EB+DHT and sham castrated groups. A similar study was subsequently performed with adrenalectomized animsls. Adrenalectomy eliminated agonistic responses in animals receiving metabolites of testosterone (EB, DHT, EB+DHT) but had only slight effects in gonadally intact and T-treated, castrated mice. The results suggested that a) EB and DHT, either singly or in combination, maintain aggression through a synergism with adrenal steroids; b) the combined effects of EB and DHT reflect an additive action rather than synergistic interaction, notwithstanding the synergism with adrenal steroids; c) metabolism of testosterone to estrogen and dihydrotestosterone does not sufficiently account for the action of testosterone.     

The role of endocrines in isolation-induced intermale fighting in albino laboratory mice. I: PITUITARY-ADRENOCORTICAL INFLUENCES

Recent experiments concerning the possible role of components of the pituitary–adrenocortical axis (ACTH and glucocorticoids) in isolation-induced intermale fighting behavior in laboratory strains of mice are reviewed. A series of experiments which investigate factors that may influence the successful demonstration of this relationship are described. Differences in performances in standard-opponent tests of some outbred strains of albino mice are indicated, as well as an interesting positive correlation between the aggressiveness and the relative adrenal weights of the strains employed. Long-acting preparations of both ACTH and ACTH 4–10, injected throughout a period of isolation, suppressed fighting behavior in intact TO-strain mice in subsequent standard-opponent and trained-fighter tests. After it had been confirmed that vigorous isolation-induced fighting could be obtained in bilaterally castrated mice which had been subcutaneously implanted with testosterone pellets, the effects of long-acting preparations of ACTH, ACTH 4–10, ACTH 1–10, and ACTH 4–10 D-phe on the fighting behavior of such animals were studied. A significant suppression of fighting behavior was evident only with respect to ACTH, suggesting that ACTH 4–10 may have its action on this behavior in a manner rather different from that of the parent molecule. Some evidence was also obtained indicating that the zinc used in the preparation of long-acting injections of ACTH and its analogs may also cause a decline in the level of fighting behavior, in a standard-opponent test, in TO-strain mice which had been castrated and testosterone implanted. While such a finding does not affect the validity of the recorded behavioral influences of ACTH and ACTH 4–10 described earlier, it does seem likely that the presence of this substance in placebo injections would make the demonstration of the actions of the pep tides difficult, as there would be a low level of fighting in all categories. Rather less impressive results were obtained with respect to the influences of ACTH preparations and zinc on castrated—implanted CFW mice. The studies identify a number of the factors which may influence the successful demonstration of a relationship between the functioning of the pituitary—adrenocortical axis and isolation-induced agonistic behavior in the mouse. A number of general, if tentative, conclusions may also be listed. It seems likely, in spite of the interpretational difficulties caused by what seems to be a complex and rather unstable relationship, that ACTH and glucocorticoids may have profound influences on this type of fighting behavior in this species. One may also conclude that the evidence for an extraadrenal influence of ACTH on this behavior appears stronger as a result of these and related studies and that the actions of ACTH and its analogs on murine fighting behavior may be logically related to the actions of these compounds on the acquisition and extinction of conditioned avoidance reactions in hypophysectomized rats. The possible utility of such actions to the natural territorial habit of the adult male mouse is also indicated.     

Androgens and aggressive behavior in primates: A review

This review deals with possible central and peripheral effects of androgens upon primate aggressive behavior. One problem that clouds interpretation of experimental work is that measurements of dominance have often been employed, such as competition tests for food and water. Such measures often do not correlate with those obtained by quantifying aggressive interactions. It should be remembered that very few of the 188 primate species have been studied experimentally and that great behavioral and physiological diversity occurs within the order. Therefore, generalizations about the effects of androgens upon aggressive behavior in primates (including man) should be made with caution. Testosterone has an organizing influence upon the foetal brain of rhesus monkeys and may affect the development of neural mechanisms which govern aggression in males. More data are required on primates, however, since rhesus monkeys show some important differences from rodents as regards the effects of androgen upon sexual differentiation of the hypothalamus. In future, marmosets may provide a suitable model for such studies, because there is evidence that sexual differentiation of brain by androgen occurs postnatally in these monkeys. At puberty, male primates show a variety of behavioral changes and, during adulthood, males of seasonally breeding species may be more aggressive during the mating season, when testosterone levels are maximal. This does not indicate a causative relationship between testosterone and aggressive responses, because castration and androgen treatments have little effect upon aggression in prepubertal or adult males of several primate species. Androgens have pronounced effects on sexual responses in adult male monkeys, but their central effects upon aggression are much less important than among rodents. Elec trical stimulation of hypothalamic pathways has been employed to evoke aggressive behavior in marmosets and rhesus monkeys. In the rhesus, preliminary evidence indicates that such pathways show some sensitivity to androgens. In rodents it is known that these areas are richly supplied with monoaminergic neurons, which play an important role in aggressive behavior. There is little evidence on primates, however, and this remains a crucial topic for future research. Peripheral effects of androgens should also be considered. Many prosimians and New World monkeys use scent-marking behaviors and, in males, androgen-dependent chemical cues may be involved in sexual recognition and territorial behavior. This possibility awaits investigation. Finally, plasma testosterone levels may alter as a function of aggression itself; thus levels decrease if male rhesus monkeys are defeated by conspecifics. This might occur because neural events associated with giving (or receiving) aggression also influence pituitary function and hence alter gonadal testosterone secretion. Theoretically, it is possible that such changes in circulating testosterone might affect aggressive behavior via a feedback action on the brain, but the experimental evidence does not support such a view.     

The effects of gormone manipulations on aggressive target-biting in mice

Aggressive biting of an inanimate target by mice was studied. Males attacked the bite-target more frequently than females, but this difference disappeared after castration when the response rate of the males approached that of the females. Ovariectomizing the females had little effect on their bite-attack frequencies. Subsequent androgen injections restored the biting-attack frequency of the castrated males to preoperative levels but had little effect on the intact males. Estrogen had little effect on the response frequency of the females, whereas androgen produced a slight increase in their bite-attack frequency. Results indicate that androgen is critical for the maintenance of this aggressive response and that the single subject paradigm utilized in this study was a sensitive measure of aggressive tendencies in mice.     

Relationship between sexual activity and intraspecific fighting in male mice

Significantly more male mice having cohabited and mated with intact females subsequently displayed intraspecific fighting behavior after castration than males having cohabited with noncycling (ovariectomized) females. Also, intact males that failed to achieve a criterion for aggression during three screening tests subsequently showed a marked increase in fighting after having had copulatory experience relative to males that lived with ovariectomized females. Lastly, spontaneously aggressive males copulated more frequently than nonfighters.     

Inter-male aggression in mice: Influence of gonadectomy and prior fighting experience

Groups of adult male mice of the Rockland-Swiss strain were paired 0, 2, 7, or 15 times with adult male stimulus animals rendered non-aggressive by extirpation of olfactory bulbs. All animals were castrated and tested until fighting ceased, following which each was given testosterone replacement and tested until fighting was reinstated. More than 50% of mice with no pre-castration fighting experience fought following gonadectomy. Castrated animals permitted 15 pre-castration pairings fought significantly longer than did animals of any of the other treatment groups. Finally, there was no difference among the groups with regard to the latency to resume fighting following the commencement of testosterone replacement. It is concluded that fighting experience enhances the persistence of the behavior following castration.     

Neonatal androgenic stimulation and adult sexual behavior in male and female golden hamsters.

Neonatally and adult castrated male hamsters as well as neonatally androgenized and nonandrogenized female hamsters were tested for both mounting and lordosis behaviors during treatment with either testosterone or ovarian hormones. Neonatal androgenization facilitated mounting behavior in adult animals administered either testosterone or ovarian hormones and suppressed lordosis behavior in adult ovarian-hormone-treated animals. Early androgen effects on the display of lordosis behavior during adult testosterone treatment were complex and varied with the exact timing of perinatal endogenous or exogenous androgenization. Species differences in hormone-behavior relationships and the possible role of perinatal androgenization in the development of rodents' ability to aromatize androgens were discussed.     

Effects of olfactory bulb ablation and androgen on marking and agonistic behavior in male Mongolian gerbils (Meriones unguiculatus).

Male Mongolian gerbils, selected for high marking frequency, were paired with male opponents, and marking and fighting behavior were recorded. Animals then underwent castration, bilateral bulbectomy, unilateral bulbectomy, the combined operations, or a sham operation; and their behavior was again observed. All operated animals showed drastic reduction in both marking and aggressive encounters. Injections of testosterone propionate (TP) produced complete restoration of marking in castrates, but not in bilaterally or unilaterally bulbectomized animals or combined operates. The exhibition of aggression after injections of TP, however, was enhanced to supernormal levels in bulbectomized or bulbectomized-castrated animals. The results suggest the following: that removal of the olfactory bulbs may eliminate a critical neural input necessary for the normal expression of marking and agonistic; that although marking and fighting are influences by olfactory input and gonadal steroids, their regulatory mechanisms may not be identical; and that removal of the bulbs may sensitize a neural mechanism controlling aggression, thus potentiating its elicitation following administration of exogenous androgen.     

Male-female differences and the influence of neonatal and adult testosterone on intraspecies aggression in rats.

Male and female albino rats were tested for intraspecies aggression without the use of shock. In the first experiment, male pairs showed more biting attacks, offensive sideways movements, and self-grooming than did female pairs; male pairs also showed more stereotyped defensive/submissive behaviors and were wounded more frequently. The second experiment examined the effects of neonatal castration and testosterone propionate (TP) administration on fighting. Males castrated at birth attacked other males less frequently than did controls when tested with TP treatment as adults. The TP given at birth to neonatally castrated males restored attacks to control levels. Females given TP as neonates did not differ from either male or female controls. Other aggressive/defensive behaviors, however, did not show this pattern. The results suggest that while the presence of testosterone during a brief postnatal period and during adulthood is necessary for attack behavior to occur, other related behaviors may not be affected in a similar manner.     

Genotype modulates prenatal stress effects on aggression in male and female mice

Prenatal stress (heat and restraint) significantly increased postpartum aggression (proportion of animals fighting and/or the intensity of the behavior) in C57BL/6J female mice and reduced the behavior in DBA/2J females. For intermale aggression, prenatal stress increased the behavior (intensity of aggression) in C57BL/6J males but did not affect aggressive behavior in DBA/2J animals. Infanticidal behavior (the killing of young) exhibited by male mice was not influenced by prenatal stress in either strain. Relative anogenital distance measurements in neonates at birth did not serve as a reliable predictor of strain variation in prenatal stress effects. Prenatal stress did not influence this measure of prenatal androgen exposure in DBA/2J or C57BL/6J females. For males, prenatal stress elevated relative anogenital distance in C57BL/6J mice and decreased this measure in DBA/2J animals. Prenatal stress effects on aggressive behavior in male and female mice therefore depend upon genotype. Strain-dependent differences may be modulated by differences in endocrine reactivity to prenatal stress/and or differential central neural tissue sensitivity to hormones.     

Hormonal regulation of aggression: Evidence for a relationship among genotype,receptor binding,and behavioral sensitivity to androgen and estrogen

Gonadectomized male mice from the CF-1, CFW, and CD-1 strains were exposed to different androgens (testosterone, dihydrotestosterone, methyltrienolone) or estrogens (estradiol, diethylstilbestrol) and tested for aggressive behavior. Genetic differences in sensitivity to the aggression-promoting property of the various treatments were found. CF-1 mice responded to either androgenic or estrogenic stimulation, CFW males were most sensitive to the estrogen treatments, while CD-1 males responded to androgen but were relatively insensitive to the aggression-promoting property of estrogens. These data suggest that there may be multiple aggression-activating systems and that the functional pathway varies with genotype. Additional experiments examined whether receptor binding in the hypothalamic-preoptic-septal region was related to the genetic differences in behavioral responsiveness. The binding data suggested that sensitivity to the aggression-promoting property of estrogens was related to either a higher concentration of binding sites or to higher affinity binding between estrogen and its receptor. A systematic relationship between dihydrotestosterone binding and behavioral responsiveness to androgen was not found.     

Behavioral profile of amisulpride in agonistic encounters between male mice

Amisulpride is a substituted benzamide derivative that acts as a selective dopamine D2/D3 receptor antagonist. Although the anti‐aggressive properties of neuroleptic drugs are well known, the effects of amisulpride on agonistic interactions have not been explored, and there are no studies comparing acute and subchronic effects of this compound on aggression in rodents. In this study, we examined the action of amisulpride (5–25 mg/kg, i.p), administered acutely or subchronically for 10 days, on agonistic behavior elicited by isolation in male mice. Individually housed mice were exposed to anosmic "standard opponents" 30 min after drug administration, and the encounters were videotaped and evaluated using an ethologically based analysis. After acute treatment, amisulpride (5–20 mg/kg) exhibited an ethopharmacological profile characterized by a marked decrease of offensive behaviors (threat and attack) without an impairment of motor activity. By contrast, the anti‐aggressive action of the highest dose used (25 mg/kg) was accompanied by a weak increase of immobility. Body care was also significantly enhanced after treatment with the drug (20 and 25 mg/kg), emphasizing the involvement of dopaminergic receptors in this behavior. After subchronic treatment, no tolerance to amisulpride anti‐aggressive activity was observed. Overall, this behavioral profile is similar to that observed by other atypical neuroleptics. Aggr. Behav. 25:225–232, 1999. © 1999 Wiley‐Liss, Inc.     

Effects of clean and soiled sawdust substrates and of different urine types upon aggressive behavior in male mice

The effects of sawdust taken from the cages of grouped or isolated male mice upon the aggressiveness of trained fighter mice toward castrated opponents were examined. The results indicate that (1) substrate soiled by stable groups of male mice does not provide aggression-promoting odors, (2) application of the urine of isolate male mice to castrate opponents increases the aggressiveness of fighters toward the castrates, and (3) application of isolate male urine to the test substrate inhibits the aggressive behavior of a fighter toward its castrated opponent. The concepts that male mouse urine possesses both aggression-promoting and aggression-inhibiting cues are not mutually exclusive, but the action of the urine depends upon whether it is placed onto a mouse or onto the substrate. The findings are discussed in relation to those of previous studies on soiled sawdust and aggression, and an attempt is made to relate these results to a territorial situation.     

Rat strain differences in copulatory behavior after para-chlorophenylalanine and hormone treatment.

The potentiation of masculine copulatory behavior in castrated male rats following systemic para-chlorophenylalanine (PCPA) treatment was found to be dependent upon the strain of rat and the recency of castration. Wistar (W) and Sprague-Dawley (SD) males displayed a decline in behavioral responsiveness to PCPA treatment following castration, the W males retaining their behavioral responsiveness longer than SD males. Castrated W males were also behaviorally more responsive to androgen replacement than were SD males. Ovariectomized W and SD females receiving estrogen replacement displayed a strain difference in hormonal sensitivity. In tests for feminine copulatory behavior, ovariectomized W females were behaviorally more sensitive to estrogen treatment than SD females. Ovariectomized and estrogen-primed females of both strains displayed potentiated lordotic behavior following chronic PCPA treatment. Concomitant treatment with dexamethasone to reduce adrenal steroid output abolished the potentiation of lordotic behavior found with PCPA treatment.     

Role of estrogens in androgen-induced spontaneous activity in male rats.

Three experiments tested the hypothesis that testosterone may be aromatized to an estrogen to stimulate running-wheel activity in rats. Aromatizable (testosterone propionate: TP) and nonaromatizable (dihydrotestosterone propionate; DHTP) androgens were compared with estradiol benzoate (EB) for the ability to induce running in castrated male rats. The DHTP had no effect on running. The TP increased running, but EB was more than 100 times as effective. A relatively small dose of a specific estrogen antagonist, MER-25, was shown to attenuate the effects of both EB and TP on male running. The MER-25 did not affect the running of castrated, oil-treated male rats and did not inhibit the running induced by food deprivation.     

Differential sexual activity of isolated and grouped male mice despite testosterone administration

In Experiment 1, adult male C57 mice were castrated, housed individually or in groups of four, and repeatedly injected with either of two doses of testosterone. Control mice were sham-castrated, individually or group housed, and injected with oil vehicle. In repeated tests of sexual behavior with receptive females, isolated males in all surgery-dose combinations showed significantly more mounts and intromissions than did their group-housed counterparts. Ejaculations were fully restored by testosterone in castrated grouped males but not in castrated isolated males. In Experiment 2, administration of either of two doses of testosterone failed to elevate the sexual behavior of intact group-housed males. These experiments show that housing with other males depresses all major measures of sexual behavior, and suggest that this is probably independent of testicular hormones.     

Evidence for aggression-modulating pheromones in prepuberal pigs

A series of behavioral bioassays were conducted to determine the aggression-influencing properties of urine and other fluids. Subjects were prepuberal castrated male and female domestic pigs from commercial stocks. In the behavior assay, pigs were painted with a test fluid and grouped for a videotaped 90 min observation period. Experiment 1 validated use of videotape recording by showing that duration of aggressive behavior registered live was correlated with that obtained from video records (R = .98). In experiment 2, urine and plasma collected from actively aggressive pigs reduced the durations of aggressive behavior of test pigs compared with the effects of urine and plasma collected from socially stable, handled pigs. In Experiment 3, a new set of test pigs confirmed that urine from fighting pigs reduced the duration of attack by test pigs compared with urine from nonfighting, handled pigs. In addition, the suggested reproductive pheromone, 5 alpha-androst-16-en-3-one, substantially reduced the duration of attack. The effects of gender and aggressive state of urine-donor pigs on test pigs was determined in Experiment 4. Again, urine from castrated male and female aggressive pigs reduced attack by test pigs compared with the level of attack shown by test pigs coated with urine from handled castrated males and females. Urine from fighting and nonfighting intact males had similar effects on test pig aggression. In Experiment 5, urine was obtained from nonhandled, socially stable pigs in their home pens and again from the same pigs after they had been regrouped (aggressive). These urine types had no significant influence on test pigs' aggression over the entire 90-min observation. However, during the first 30 min nonhandled, nonfighting pigs' urine induced less aggression in test pigs than did urine from fighting pigs. Results indicate that urine and blood plasma from aggressive pigs reduces aggression by test pigs compared with the effects of urine from handled pigs. Handling may increase the aggression-promoting properties of urine, and aggression may inhibit this aggression-promoting property.     

Differential sensitivity to the effects of nicotine and bupropion in adolescent and adult male OF1 mice during social interaction tests

Few studies have compared the action of both nicotine (NIC) and bupropion (BUP), an antidepressant used to treat NIC dependence, on social and aggressive behavior at different ages. This study aims to determine whether these drugs produce differential effects in adolescent (postnatal day: 36-37) and adult (postnatal day: 65-66) mice that have been housed individually for 2 weeks in order to induce aggressive behavior. Mice received BUP (40, 20, or 10 mg/kg), NIC (1, 0.5, and 0.25 mg/kg as base), or vehicle earlier to a social interaction test. BUP (40 mg/kg) decreased social investigation and increased nonsocial exploration in both adolescent and adult mice. The same effects were also observed in adult mice administered with a lower dose of the same drug (20 mg/kg). In adolescents, NIC (1 mg/kg) decreased social investigation, but this effect did not reach statistical significance in adults. In conclusion, a differential sensitivity to the effects of NIC or BUP emerged in some of the behavioral categories when the two age groups were compared.     

Female behavior in populations of mice in the presence and absence of male hierarchy

Female aggression may be the regulator of population size in small mammals. Freely growing populations of house mice showed several differences in aggressive female behavior in the presence and the absence of a male hierarchy. Territoriality in females and not in males appeared to maintain social order and regulate population density. Certain females were seen patrolling and guarding the territory and chasing and fighting with both male and female intruders. These females did not fight amongst themselves, suggesting that they were not fighting for rank (as do the males) but for territory. Although these aggressive females produced young, the pups were neglected, and few were weaned. The non-aggressive females were the successful breeders. Aggression by the females only occurred when there was reproduction and increased densities. Assembled females with no males present never show this aggression. The occurrence of “male-type” behavior became most apparent when the males were removed at peak population densities. The removed males were then castrated and injected with testosterone cyprionate. Doses were increased by population cage, and therefore all males returned to each freely growing population were given the same dose. The males given oil placebo injections showed no return of a male hierarchy and the females showed high levels of aggression toward them. Males injected with testosterone cyprionate showed return of male aggression and fighting and mounting of females. But the new “dominant” females continued their patrols and chased males away from their territories and did not permit these males to mount. Male-male fighting consisted primarily of frontal attacks to the face and roll and tumble fights. Female-male aggression consisted primarily of attacks to the posterior region targeted at the base of the tail and the genitals of the male. The males were rarely seen attacking females and then only during mating. Females only attacked each other in defense of their territories.