Testosterone supports hormone-dependent aggression in female rats

No correlation between serum testosterone levels and state-level anger intensity in transgender people: Results from the European Network for the Investigation of Gender Incongruence

INTRODUCTION

Anger is a state of emotions ranging from irritation to intense rage. Aggression implies externalizing anger through destructive/punitive behaviour. The World Professional Association for Transgender Health (WPATH) Standards of Care, Edition 7 (SOC7) guidelines warn about aggression in transgender men (TM) on testosterone treatment. We aimed to assess whether anger intensity increases in TM and decreases in transgender women (TW) after initiation of gender affirming hormone therapy and to identify predictors for anger intensity in transgender people.

METHODS

This prospective cohort study was part of the European Network for the Investigation of Gender Incongruence (ENIGI). Anger intensity was prospectively assessed in 898 participants (440 TM, 468 TW) by STAXI-2 (State-Trait Anger Expression Inventory-2) State Anger (S-Anger) during a three-year follow-up period, starting at the initiation of hormone treatment. Data were analysed cross-sectionally and prospectively.

RESULTS

There was no change in STAXI-2 S-Anger scores. At three, twelve and thirty-six months of gender affirming hormone therapy, STAXI-2 S-Anger scores were not correlated to serum testosterone levels, although there was a correlation with various psychological measures after three and twelve months. TM experiencing menstrual spotting after three months had higher STAXI-2 S-Anger scores compared to those without (median 26.5 [18.0-29.8] versus 15.0 [15.0-17.0], P = 0.020). Changes in STAXI-2 S-Anger scores were not correlated to changes in serum testosterone levels after three, twelve and thirty-six months in TM or TW.

CONCLUSIONS

State-level anger intensity is associated with psychological and/or psychiatric vulnerability, but not exogenous testosterone therapy or serum testosterone levels in transgender people.

Brood reduction via intra-clutch variation in testosterone--an experimental test in the great tit

In birds, yolk androgen concentrations in eggs can increase or decrease over the laying sequence and common hypotheses hold that this serves to favour the competitive ability of either first- or last-hatched chicks depending on the prevailing conditions, and thus promote brood reduction or maintenance of original brood size respectively. Intra-clutch variation of testosterone can shift relative competitive ability of siblings and hence competitive dynamics. In a natural population of great tits, we experimentally investigated the effects and function of maternal testosterone on offspring phenotype in relation to the laying position of the egg in a context of hatching asynchrony. To this end, we created three types of clutches where either the first three or the last three eggs of a clutch were injected with testosterone (T) dissolved in sesame oil, and the remaining eggs with sesame oil only, or where all eggs of a clutch were injected with sesame oil. Increased levels of yolk T in the last-laid eggs resulted in the last-hatched chicks being significantly lighter and smaller than their siblings, while increased levels of T in the first-laid eggs had no direct effect on the first-hatched chicks, but an indirect negative effect on their siblings. Our results suggest that females can potentially adjust offspring phenotype by modulating, over the laying sequence, the amounts of T deposited in the eggs. These results are in contradiction, however, with current hypotheses and previous findings, which suggest that under good conditions higher levels of maternally derived T in the last-laid eggs should mitigate the negative effects of hatching asynchrony.

The role of estrogen and testosterone in female rats in behavioral models of relevance to schizophrenia

RATIONALE

The sex steroid hormone, estrogen, may play a protective role in schizophrenia. We previously found that estrogen treatment inhibited serotonin-1A (5-HT(1A)) and dopamine D(2) receptor-mediated disruptions of prepulse inhibition (PPI), a measure of sensorimotor gating which is deficient in schizophrenia.

OBJECTIVES

The present study aimed to further explore the role of sex steroid hormones in schizophrenia. Part 1 of this study examined whether estrogen could inhibit PPI disruption induced by the N-methyl-D: -aspartate (NMDA) receptor antagonist, MK-801. Part 2 investigated whether the functionally protective effect of estrogen occurs in another animal model of schizophrenia, amphetamine-induced locomotor hyperactivity. Part 3 compared our previous PPI findings in estrogen-treated rats, to treatment with testosterone.

METHODS

Female Sprague-Dawley rats were ovariectomized (OVX) or sham-operated. Some OVX rats received silastic implants filled with either a low (E20) or high dose (E100) of estradiol, or a low (T5) or high dose (T20) of testosterone, for at least 2 weeks before behavioral testing.

RESULTS

The disruption of PPI caused by MK-801 (0.1 mg/kg) was significantly reduced by treatment with estradiol (E20 and E100). However, estradiol treatment did not alter amphetamine-induced (0.25 and 0.5 mg/kg) locomotor hyperactivity, in terms of distance traveled, ambulation, or vertical counts. In contrast to estrogen, testosterone treatment did not affect disruption of PPI after administration of 8-OH-DPAT (0.5 mg/kg) or apomorphine (0.3 mg/kg). Testosterone treatment significantly enhanced the MK-801-induced (0.1 mg/kg) PPI disruption.

CONCLUSIONS

Estrogen is functionally protective against 5-HT(1A)-, dopamine D(2)-, and NMDA receptor-induced PPI disruptions, while testosterone treatment enhances NMDA receptor-mediated PPI disruptions.

Testosterone and human aggression: an evaluation of the challenge hypothesis

Research on testosterone-behavior relationships in humans is assessed in relation to a version of the challenge hypothesis, originally proposed to account for testosterone-aggression associations in monogamous birds. Predictions were that that testosterone would rise at puberty to moderate levels, which supported reproductive physiology and behavior. Sexual arousal and challenges involving young males would raise testosterone levels further. In turn, this would facilitate direct competitive behavior, including aggression. When males are required to care for offspring, testosterone levels will decrease. Testosterone levels will also be associated with different behavioral profiles among men, associated with life history strategies involving emphasis on either mating or parental effort. Most of these predictions were supported by the review of current research, although most studies were not designed to specifically test the challenge hypothesis.

Food availability affects behavior but not circulating gonadal hormones in maternal Belding's ground squirrels

We tested predictions of hypotheses suggesting that the steroid hormones, testosterone (T), progesterone (P), and estradiol (E2), contribute to the energetic regulation of behaviors associated with rearing young in free-living female Belding's ground squirrels (Spermophilus beldingi). We provisioned some female S. beldingi with food rich in fat and calories, and used unprovisioned females as controls. We observed the behavior of females throughout the reproductive cycle, and regularly collected blood samples to measure plasma hormone concentrations. Circulating concentrations of T, P, and E2 were similar in provisioned and unprovisioned females, as were temporal patterns of variation in these hormones. Peaks in rates of nest maintenance and aggressive behavior occurred during gestation and were associated with elevated concentrations of circulating T, P, and E2, raising the possibility that one or more of these hormones mediates behaviors that help females establish maternal nest sites and territories after mating. Temporal patterns of variation in behavior were similar among provisioned and unprovisioned females; however, rates of resting, vigilance, and aggression were higher among provisioned females, whereas unprovisioned females devoted significantly more time to feeding and locomotion. Thus, our data suggest that in maternal S. beldingi, gonadal steroids play a role in mediating behavior associated with raising offspring, but do not facilitate changes in rates of behavior associated with increased energy availability.

Analysis of steroid hormone levels in female mice at high population density

Populations of predominantly female house mice (Mus musculus) were created by placing virgin female mice in cages (0.045 m2 to 0.48 m2) with a single stud male, and removing all ensuing male offspring at weaning. At maximum population size, the females in these all-female/one-male populations exhibited male-like aggressive behavior. Termination of the populations and subsequent measurement of steroid hormone levels indicated that the aggressive females had high circulating level of testosterone and corticosterone, and elevated baseline levels of progesterone. The high levels of corticosterone could be lowered by dexamethasone, but not the high levels of testosterone.

The role of androgens in female vertebrates

The role of androgens in vertebrate females has been overlooked until recently. We examine the functional significance of androgens in females by reviewing studies that document relatively high levels of circulating plasma androgens, androgen receptors, or androgen-metabolizing enzymes in females. Among the mechanisms of androgenic action identified are enhanced neuron survival, stimulation of muscle satellite cell proliferation, alteration of ion current kinetics, and release of somatostatin. These mechanisms are not sex specific and thus we hypothesize that androgens play a significant role in normal female development. We encourage study in this nontraditional research area.

Paradoxical sleep deprivation in female rats alters drug-induced behaviors

Paradoxical sleep deprivation (PSD) induces changes in behaviors induced by dopaminergic and cholinergic agonists, including increased aggressive behavior and stereotypy, decreased number of yawns, and shedding of bloody tears in male rats. In female rats, however, very little is known about the relationship between PSD and the effect of these drugs. The present study sought to examine this issue. As in males, PSD in females resulted in increased apomorphine-induced stereotypy, decreased pilocarpine-induced chromodacryorrhea, and hyperthermia. Unlike males, however, no apomorphine-induced aggressiveness or apomorphine- and pilocarpine-induced yawning were observed in PSD females. These findings suggest that female sexual hormones may affect the expression of some behaviors and not the neurotransmission as a whole, because drug-induced behaviors in PSD females were partly similar to those observed in PSD males.

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.

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.

Serum estradiol concentration required to maintain body weight, attractivity, proceptivity, and receptivity in the ovariectomized female rat

Female hooded rats (230 to 260 g) were ovariectomized and given a subcutaneous implant of an estradiol-filled Silastic tube. The length of the tube was varied in order to produce a variety of serum estradiol levels. In the first experiment, animals were weighed over a 6-week period following surgery and then tested for sexual responsiveness to a male. The results demonstrated that ovariectomized females with an implant maintaining a serum estradiol concentration at about 15 pg/ml maintained body weight at the same level as that of intact females. A smaller implant gave rise to a higher weight gain and a larger implant to a lower weight gain. All implants resulted in a continuous state of receptivity. In a second experiment, ovariectomized females were implanted with smaller estradiol-filled implants in order to determine the threshold for maintaining proceptivity and receptivity. The results indicated that with a serum estradiol concentration below 15 pg/ml, the frequency of lordosis and of ear wiggling and darting decreased. Progesterone injections facilitated both proceptive and receptive behavior. In addition, following progesterone injections, the time required for a male to mount a female 10 times was decreased in females with low or no estradiol replacement. These results indicate that a constant concentration of estradiol at about the mean level present throughout the estrous cycle will result in normal body weight regulation and will maintain sexual behaviors that normally occur only during estrus. These results emphasize that Silastic implants of estradiol do not mimic normal endocrine function since, even at low levels, estradiol implants produce continuous receptivity.

Hormone-dependent aggression in male rats is proportional to serum testosterone concentration but sexual behavior is not

Male hooded rats were castrated and implanted with Silastic capsules (1.57 mm i.d.; 3.18 mm o.d.) having a testosterone-filled space 0, 7, 22, 60, or 90 mm long. All animals were returned to their original group cages for a three-week period to allow hormone concentrations and behavioral tendencies to stabilize. Each male was then housed with an intact female in a large cage. Aggression by the male toward an unfamiliar male was tested at weekly intervals for three weeks. Sexual behavior with an estrogen/progesterone-primed ovariectomized female was tested on each of the subsequent two weeks. Serum testosterone was measured during the following week. The frequency of aggression was correlated with serum testosterone concentration up to the normal level and did not increase with higher serum testosterone concentrations. In contrast, sexual behavior was virtually absent in animals with no testosterone replacement and normal in all other groups. These results demonstrate a clear dissociation in the dependence of hormone-dependent aggression and sexual behavior on serum testosterone concentration. In a male cohabiting with a female, sexual experience activates hormone-dependent aggression toward an unfamiliar male but the level of aggression that develops depends on the serum testosterone concentration in the resident male.

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.

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

Competition for food was observed between pairs of ovariectomized female rats with medial hypothalamic lesions or sham lesions. A second series of tests observed food-competition between ovariectomized and sham-ovariectomized females. Competing pairs were continuously housed together and maintained on a 23-hr food-deprivation schedule. Competition occurred as each rat attempted to maintain access to a spout containing liquid food that only one animal could lick at a time. Female rats made hyperdefensive by medial hypothalamic lesions maintained access to a food spout significantly longer than their sham-lesioned cagemates. The lesioned animals were also significantly more aggressive than their sham-lesioned cagemates. Sham-ovariectomized rats maintained access to the food spout significantly longer than their ovariectomized cagemates. The intact cagemates were also more aggressive. These results suggest that defensive aggression heightened by medial hypothalamic lesions is displayed in a competitive situation by females as has been demonstrated previously with males. Further, ovariectomy in females appears to decrease aggression and success in a competitive situation as does gonadectomy in males. These results suggest that homologous biological mechanisms modulate aggressive behavior in male and female rats.