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

Androgen receptors and sociosexual behaviors in mammals: The limits of generalization

Circulating testosterone is easily aromatized to estradiol and reduced to dihydrotestosterone in target tissues and elsewhere in the body. Thus, the actions of testosterone can be mediated either by the estrogen receptors, the androgen receptor or by simultaneous action at both receptors. To determine the role of androgens acting at the androgen receptor, we need to eliminate actions at the estrogen receptors. Alternatively, actions at the androgen receptor itself can be eliminated. In the present review, I will analyze the specific role of androgen receptors in male and female sexual behavior as well as in aggression. Some comments about androgen receptors and social recognition are also made. It will be shown that there are important differences between species, even between strains within a species, concerning the actions of the androgen receptor on the behaviors mentioned. This fact makes generalizations from one species to another or from one strain to another very risky. The existence of important species differences is often ignored, leading to many misunderstandings and much confusion.

Neural mechanisms of persistent aggression

While aggression is often conceptualized as a highly stereotyped, innate behavior, individuals within a species exhibit a surprising amount of variability in the frequency, intensity, and targets of their aggression. While differences in genetics are a source of some of this variation across individuals (estimates place the heritability of behavior at around 25-30%), a critical driver of variability is previous life experience. A wide variety of social experiences, including sexual, parental, and housing experiences can facilitate "persistent" aggressive states, suggesting that these experiences engage a common set of synaptic and molecular mechanisms that act on dedicated neural circuits for aggression. It has long been known that sex steroid hormones are powerful modulators of behavior, and also, that levels of these hormones are themselves modulated by experience. Several recent studies have started to unravel how experience-dependent hormonal changes during adulthood can create a cascade of molecular, synaptic, and circuit changes that enable behavioral persistence through circuit level remodeling. Here, we propose that sex steroid hormones facilitate persistent aggressive states by changing the relationship between neural activity and an aggression "threshold".

Testosterone dependent territorial aggression is modulated by cohabitation with a female in male Mongolian gerbils (Meriones unguiculatus)

Most mammal studies on the neuroendocrine mechanisms of territorial aggression have demonstrated that testosterone (T) is required for the display of territorial aggression. However, the relationship between T and aggression is more complex and may be modulated by social factor. The aim of this study was to determine the role of T in territorial aggression in the Mongolian gerbil (Meriones unguiculatus), and the effect of social factors on the modulation of this behavior. The relationship between T and territorial aggression was analyzed using castration and T replacement in two social contexts: male-male and male-female cohabitation. Plasma T concentrations in males of all groups were quantified by radioimmunoassay (RIA). T concentrations were compared using two-way ANOVA. Only sham-castrated and castrated males with T replacement in male-female cohabitation showed aggression, whereas castrated gerbils in the same condition were not aggressive. This indicates that T is the hormone that maintains territorial aggression, but mating is a modulator stimulus. The modulator effect of mating in territorial aggression was associated with an increase in T, but it seems that other mechanisms are involved in the regulation of this behavior, since castrated males with T replacement in the male-male cohabitation did not exhibit aggression, although they had T concentrations as high as these males that received the same treatment, but that cohabited with a female. These results suggest that T is involved in the mechanisms that regulate territorial aggression in the male Mongolian gerbil, and that the cohabitation with a female modulates this behavior.

Blood Testosterone Concentration and Testosterone-induced Aggressive Behavior in Male Layer Chicks: Comparison between Isolated- and Grouped-Raising

Testosterone (T) is known to induce aggressive behavior, mainly in male animals. Subcutaneous implantation of T-filled silastic tubes, rather than intramuscular injection of T, is generally recommended for long-term treatment using exogenous T. However, the effect of T implantation on chicken aggressive behavior has not been investigated. In addition, the concentration of T required to induce aggressive behavior or whether rearing conditions such as isolated- or grouped-raising affect T-induced aggressive behavior in chickens is not known. The present study aimed to examine the relationship between the lengths of T-filled tubes, blood T concentration, and aggressive behavior in group- and isolation-raised male layer chicks. The testes were bilaterally removed and silactic tubes of various lengths filled with crystalline T were subcutaneously implanted at 14 days of age. A social interaction test was performed to quantitatively assess chick aggressive behavior at 32 days of age. Comb weight and size were used to assess the activation of endogenous androgen receptors. Total aggression frequencies (TAF) and aggression establishment rate (AER) were used to evaluate aggressiveness. Significant positive correlations (P<0.001) were observed between the comb parameters and plasma T concentration. In the isolation-raised chicks, the TAF and AER were high irrespective of the lengths of the implanted T tubes or the corresponding plasma T concentrations. However, in the group-raised chicks, the AER tended to differ between the T-implanted aggressors (P=0.0902), and the AER significantly increased with implantation of 1.0-cm-long T-filled tubes (P<0.05), which corresponded to approximately 47 pg/mL plasma T concentration. These results suggest that both grouped raising and approximately 47 pg/mL plasma T concentration are required for the induction of T-dependent aggressive behavior, and that isolation-induced aggressive behavior is T-independent in male layer chicks.

Hormone-dependent medial preoptic/lumbar spinal cord/autonomic coordination supporting male sexual behaviors

Testosterone (T) can act directly through neural androgen receptors (AR) to facilitate male sexual behavior; however, T's metabolites also can play complicated and interesting roles in the control of mating. One metabolite, dihydrotestosterone (DHT) binds to AR with significantly greater affinity than that of T. Is that important behaviorally? Another metabolite, estradiol (E), offers a potential alternative route of facilitating male mating behavior by acting through estradiol receptors (ER). In this review we explore the roles and relative importance of T as well as E and DHT at various levels of the neuroaxis for the activation of male sex behavior in common laboratory animals and, when relevant research findings are available, in man.

Do testosterone declines during the transition to marriage and fatherhood relate to men's sexual behavior? Evidence from the Philippines

Testosterone (T) is thought to help facilitate trade-offs between mating and parenting in humans. Across diverse cultural settings married men and fathers have lower T than other men and couples' sexual activity often declines during the first years of marriage and after having children. It is unknown whether these behavioral and hormonal changes are related. Here we use longitudinal data from a large study in the Philippines (n=433) to test this model. We show that among unmarried non-fathers at baseline (n=153; age: 21.5 ± 0.3 years) who became newly married new fathers by follow-up (4.5 years later), those who experienced less pronounced longitudinal declines in T reported more frequent intercourse with their partners at follow-up (p<0.01) compared to men with larger declines in T. Controlling for duration of marriage, findings were similar for men transitioning from unmarried to married (without children) (p<0.05). Men who remained unmarried and childless throughout the study period did not show similar T-sexual activity outcomes. Among newly married new fathers, subjects who had frequent intercourse both before and after the transition to married fatherhood had more modest declines in T compared to peers who had less frequent sex (p<0.001). Our findings are generally consistent with theoretical expectations and cross-species empirical observations regarding the role of T in male life history trade-offs, particularly in species with bi-parental care, and add to evidence that T and sexual activity have bidirectional relationships in human males.

Chronic social stress induces cardiomyocyte contractile dysfunction and intracellular Ca2+ derangement in rats

Chronic psychosocial stress triggers cardiovascular diseases although underlying mechanisms are still elusive. This study examined the effect of social stress on cardiomyocyte contractile function and pathological changes in myocardium using the visible burrow system (VBS) model. Chronic social stress was induced using a mixed-sex VBS housing in adult Sprague-Dawley (SD) rats. Contractile and intracellular Ca(2+) properties were evaluated in isolated cardiomyocytes including peak shortening (PS), time-to-PS (TPS), time-to-90% relengthening (TR(90)), maximal velocity of shortening/relengthening (± dL/dt), Fura-2 fluorescence intensity, and intracellular Ca(2+) decay. Myocardial histology was evaluated using Masson trichrome staining. Social stress led to depressed PS, ± dL/dt, shortened TPS and prolonged TR(90) compared with the unstressed controls. Baseline and electrically-stimulated rise in Ca(2+) were reduced whereas intracellular Ca(2+) decay was delayed in stressed rats. Histological analyses exhibited overt interstitial fibrosis and cardiomyocyte hypertrophy in stressed rats. The GSH/GSSG ratio (indicative of oxidative stress status) was reduced whereas oxidative protein carbonyl formation was elevated in stressed rats. Western blot analysis showed unchanged expression of superoxide dismutase 1 (SOD1), β(1)-adrenoceptor (β(1)-AR) levels, reduced sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2a) levels, and elevated phosphorylation of the stress signaling protein kinase JNK but not ERK in myocardium from stressed rats. Short-term in vitro treatment of cardiomyocytes with the stress inducer phenylephrine mimicked cell damage and intracellular Ca(2+) mishandling, the effects of which were mitigated by antioxidant, JNK inhibition, carvedilol and SERCA2a adenovirus. These findings indicate that chronic social stress is detrimental to cardiac structure and function possibly via mechanisms associated with oxidative injury and intracellular Ca(2+) mishandling.

The effect of different levels of gonadotropin-releasing hormone antibody titres on plasma hormone concentrations, sexual and aggressive behaviour, testis size and performance of bulls

To determine the effect of different levels of gonadotropin-releasing hormone (GnRH) antibody titres (AT) on plasma hormone concentrations, behaviour, testes size and performance, Friesian bull calves (no. = 72) were immunized against either human serum albumin (HSA) conjugated to Cys-Gly-GnRH (no. = 48; immunized) or HSA (no. = 24; controls) at 8 to 10 weeks of age. One booster immunization was administered on either day 28 or 56 (no. = 24 GnRH-immunized and no. = 12 control per booster day). Based on AT 1 week post respective booster, 12 immunized and six control bulls were allocated to one of three AT groups, control (C), medium (M) and high (H) with mean AT of 0·3 (s.e. 0·1), 32 (s.e. 2) and 51 (s.e. 2)% binding at a plasma dilution of 1: 160; respectively. Bulls were blood sampled and weighed every 14 days, and testicular measurements taken every 28 days, from days 0 to 533. Behaviour was observed for 4 h once weekly while bulls were together at pasture and, within AT group, for 20 min 1 week before slaughter. At slaughter, testes and carcass measurements were made. Appropriate data were analysed by ANOVA and correlation coefficients (i) determined. Behavioural data were analysed using x2. Mean AT for M and H treatment groups during the experiment were 36·5 and 44·8% binding at a 1: 160 dilution (P > 0·05, pooled s.e.d. 5·9%), respectively. Level of AT decreased (P < 0·05) mean testosterone concentrations (1·74, 1·36 and 1·21 (s.e.d. 0·17) ng/ml for the C, M and H treatment groups, respectively) and testes size (e.g. length, 10·0, 8·9 and 8·1 (s.e.d. OS) cm) in the M and H groups compared with the C bulls. There was no effect of AT level on luteinizing hormone and insulin-like growth factor-1 concentrations or on average daily gain. Bulls in the M group had the lightest testes and lowest masculinity score compared with the C and H groups (both P < 0·05). Bulls in the H group had greater backfat thickness than the C bulls (P < 0·05). Bulls in the M and H groups were less aggressive and sexually active than the C bulls (P < 0·05). In conclusion, GnRH AT in both the M and H groups persisted for the duration of the experiment, and these AT were significantly different for a critical period pre-puberty; however they both had similar effects and caused extended periods of reduced testes growth, testosterone concentrations, and aggressive and sexual behaviour compared with C bulls but there was no detrimental effect of AT on growth or carcass characteristics.

A direct relationship between aggressive behavior in the resident/intruder test and cell oxidative status in adult male mice

Disturbances in oxidative metabolism are involved in many acute and chronic diseases, as well as in several other conditions. The objective of the present study was to examine the relationship between the level of intracellular reactive oxygen species in the peripheral blood granulocytes of mice, as evaluated by 2',7'-dichlorofluorescin diacetate (DCFH-DA), a sensor of reactive oxygen species, and the aggressive behavior of these mice, as estimated by the resident/intruder test. Our results showed a significant, linear and positive relationship (P<0.001) between the intracellular redox status of peripheral blood granulocytes and the aggressive behavior levels of adult male mice (correlation coefficients (R(2)) ranging from 0.75 to 0.77). This suggests that the granulocytes of aggressively behaving mice have high levels of oxidative stress.

Testosterone release and social context: when it occurs and why

The functions of rapid increases in testosterone seem paradoxical because they can occur in response to different social contexts, such as male-male aggressive encounters and male-female sexual encounters. This suggests that context may impact the functional consequences of changes in testosterone, whether transient or long term. Many studies, including those with California mice (Peromyscus californicus), have addressed these issues using manipulations and species comparisons, but many areas remain to be investigated. We report a study here that suggests transient increases in testosterone after social competition influence future competitive behavior, but social experience alone may also be critical in determining future behavior. In other rodents, a comparable testosterone surge occurs in response to sexual stimulation, but the function is not entirely understood. In addition to competitive and sexual behavior, testosterone impacts other systems instrumental to social behaviors, including paternal behavior and degree of monogamy. Thus, mechanisms regulated by testosterone, such as the vasopressin and aromatase systems, may also be influenced by rapid surges of testosterone in aggressive or sexual contexts. We discuss how the functions of testosterone may overlap in some contexts.

Subacute oral exposure to benzo[alpha]pyrene (B[alpha]P) increases aggressiveness and affects consummatory aspects of sexual behaviour in male mice

Benzo[alpha]pyrene (B[alpha]P) is a neurotoxic pollutant which is also able to affect some behaviour and cognitive function. Here we report that a subacute oral exposure to B[alpha]P increases aggressiveness and affects copulatory behaviour in male mice. Indeed, after 3 weeks of exposure to B[alpha]P at 0.02 and 0.2mg/kg, we have observed that B[alpha]P 0.02 mg/kg-treated male mice are more aggressive than control mice in resident-intruder test because a significant decrease in the latency time of the first attack and a significant increase in the number of attacks in B[alpha]P 0.02 mg/kg-treated mice were found. On the other hand, we have found that subacute exposure (4 weeks) to B[alpha]P, does not affect the appetitive aspects and sexual motivation in copulatory behaviour because the latency to the first mount between control and B[alpha]P-treated male mice was not significantly different. We have nevertheless, surprisingly found that B[alpha]P (0.02-0.2)mg/kg-treated mice have performed significantly more sexual behavioural acts including mounting, intromission latency and intromission frequency than control mice. Although these last results suggest that B[alpha]P improves the consummatory aspects of sexual behaviour, we cannot conclude that this neurotoxic pollutant has advantage of sexual function because B[alpha]P has been shown to alter the monoaminergic neurotransmitter system and causes endocrine dysregulation via toxic effect.

Androgen and estrogen (alpha) receptor localization on periaqueductal gray neurons projecting to the rostral ventromedial medulla in the male and female rat

The periaqueductal gray (PAG) is involved in many gonadal steroid-sensitive behaviors, including responsiveness to pain. The PAG projects to the rostral ventromedial medulla (RVM), comprising the primary circuit driving pain inhibition. Morphine administered systemically or directly into the PAG produces greater analgesia in male compared to female rats, while manipulation of gonadal hormones alters morphine potency in both sexes. It is unknown if these alterations are due to steroidal actions on PAG neurons projecting to the RVM. The expression of androgen (AR) and estrogen (ERalpha) receptors in the PAG of female rats and within this descending inhibitory pathway in both sexes is unknown. The present study used immunohistochemical techniques (1) to map the distribution of AR and ERalpha across the rostrocaudal axis of the PAG; and (2) to determine whether AR and/or ERalpha were colocalized on PAG neurons projecting to the RVM in male and female rats. AR and ERalpha immunoreactive neurons (AR-IR, ERalpha-IR) were densely distributed within the caudal PAG of male rats, with the majority localized in the lateral/ventrolateral PAG. Females had significantly fewer AR-IR neurons, while the quantity of ERalpha was comparable between the sexes. In both sexes, approximately 25-50% of AR-IR neurons and 20-50% of ERalpha-IR neurons were retrogradely labeled. This study provides direct evidence of the expression of steroid receptors in the PAG and the descending pathway driving pain inhibition in both male and female rats and may provide a mechanism whereby gonadal steroids modulate pain and morphine potency.

Anabolic androgenic steroids and aggression: studies using animal models

The use of anabolic androgenic steroids (AASs) has escalated in teenagers and is associated with increased violence. Adolescent exposure to chronic high levels of AASs is of particular concern because puberty is a hormonally sensitive period during which neural circuitry for adult male patterns of behavior develop. Thus, teenage AAS use may have long-term repercussions on the potential for displaying aggression and violence. Animal models have contributed valuable information on the effects of AAS use. For example, studies in rodents confirmed that exposure to the AASs testosterone and nandrolone, but not stanozolol, does indeed increase aggression. A side effect of AAS use reported in humans is "'roid rage," characterized by indiscriminate and unprovoked aggression. Results of animal studies demonstrated that pubertal rats receiving AASs respond appropriately to social cues as they are more aggressive toward intact males than are castrates. Also, testosterone-treated males recognize appropriate environmental cues as they are most aggressive in their home cage. Thus, adolescent AAS exposure increases aggressive behaviors, but does not induce indiscriminate aggression. To assess whether AAS exposure increases aggression after provocation, rats were tested following a mild tail-pinch. In adolescent males, provocation increased aggression after withdrawal from testosterone, nandrolone, and stanozolol, an effect which persisted for many weeks. The data suggest that AASs sensitize animals to their surroundings and lower the threshold to respond to provocation with aggression. Thus, in humans, pubertal AAS exposure may not cause violent behaviors, but may increase the likelihood that aggressive acts will result in violence. This may persist into adulthood.

Dynamic body weight and body composition changes in response to subordination stress

Social stress is prevalent in many facets of modern society. Epidemiological data suggest that stress is linked to the development of overweight, obesity and metabolic disease. Although there are strong associations between the incidence of obesity with stress and elevated levels of hormones such as cortisol, there are limited animal models to allow investigation of the etiology of increased adiposity resulting from exposure to stress. Perhaps more importantly, an animal model that mirrors the consequences of stress in humans will provide a vehicle to develop rational clinical therapy to treat or prevent adverse outcomes from exposure to chronic social stress. In the visible burrow system (VBS) model of chronic social stress mixed gender colonies are housed for 2 week periods during which male rats of the colony quickly develop a dominance hierarchy. We found that social stress has significant effects on body weight and body composition such that subordinate rats progressively develop characteristics of obesity that occurs, in part, through neuroendocrine alterations and changes in food intake amount. Although subordinate rats are hyperphagic following social stress they do not increase their intake of sucrose solution as control and dominants do suggesting that they are anhedonic. Consumption of a high fat diet does not appear to affect development of a social hierarchy and appears to enhance the effect that chronic stress has on body composition. The visible burrow system (VBS) model of social stress may be a potential laboratory model for studying stress-associated metabolic disease, including the metabolic syndrome.

Effect of intrauterine position on sex differences in the gabaergic system and behavior of rats

In multiparous species such as the rat (in this case the albino Wistar strain), steroid influence during fetal growth is affected by the relative intrauterine position of male and female fetuses and is stronger when the potential effects of contiguity and caudal position are combined. The effect of intrauterine position on gonadal steroid levels in neonatal and adult animals was examined using radioimmunoassay techniques. Since the organizing effect of prenatal steroids may influence the postnatal GABA content, HPLC was used to determine the gabaergic content in several hypothalamic and limbic areas in the adult rat. The effects of intrauterine position on adaptive behavior were examined by recording exploratory behavior (using the corridor and hole board tests) and intraspecific aggression (induced by isolation). Female pups influenced by males during development produced more testosterone. In adult males, those that developed closer to the cervix (and with no influence from other fetuses) produced more testosterone and less estradiol. The same animals also produced more hypothalamic GABA and showed greater exploratory capacity. No significant differences were seen between any experimental groups with respect to aggression. These results show increased variability between males with respect to adult exploratory behavior, and in the neurochemical and endocrine systems involved, due to intrauterine position during development. The effect of this physiological phenomenon on the structure of rodent populations is discussed.

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.

Positive effects of testosterone and immunochallenge on energy allocation to reproductive organs

A number of studies have suggested the incompatibility of simultaneous increases in immune and reproductive functions. Other research has indicated that immune responses may be modulated depending on the relative benefits of increased survival and prospects for current and future reproduction. We tested the hypothesis that energy allocation to reproductive and other organ systems is not affected by testosterone level and energy expenditure on immune functions. Adult male white-footed mice (Peromyscus leucopus) with or without elevated testosterone levels and with or without immunochallenges were tested. Testosterone treatment was associated with reduced humoral immune response indicating immunosuppressive effects, reduced masses of gastrointestinal organs, reduced corticosterone level, increased kidney and seminal vesicle masses, and increased hematocrit. Immunochallenge was associated with increased resting metabolic rate and testes and seminal vesicle masses. Reproductive organ masses were greatest in immunochallenged mice with exogenous testosterone. Simultaneous increases in energy allocation to immune and reproductive structures may be an adaptive response that would enhance survival and current prospects for reproduction.

Genotype/age interactions on aggressive behavior in gonadally intact estrogen receptor beta knockout (betaERKO) male mice

Estrogen, as an aromatized metabolite of testosterone, has a facilitatory effect on male aggressive behavior in mice. Two subtypes of estrogen receptors, alpha (ER-alpha) and beta (ER-beta), in the brain are known to bind estrogen. Previous studies revealed that the lack of ER-alpha gene severely reduced the induction of male aggressive behavior. In contrast, mice that lacked the ER-beta gene tended to be more aggressive than wild type (WT) control mice, although the behavioral effects of ER-beta gene disruption were dependent on their social experience. These findings lead us to hypothesize that estrogen may facilitate aggression via ER-alpha whereas it may inhibit aggression via ER-beta. In the present study, we further investigated the role of ER-beta in the regulation of aggressive behavior by examining developmental changes starting at the time of first onset, around the age of puberty. Aggressive behaviors of ER-beta gene knockout (betaERKO) mice were examined in three different age groups, puberty, young-adult, and adult. Each mouse was tested every other day for three times in a resident-intruder paradigm against olfactory bulbectomized intruder mice and their trunk blood was collected for measurements of serum testosterone after the completion of the study. Overall, betaERKO mice were significantly more aggressive than WT. These genotype differences were more pronounced in puberty and young adult age groups, but not apparent in the adult age group, in which betaERKO mice were less aggressive than those in two younger age groups. Serum testosterone levels of betaERKO mice were significantly higher than those of WT mice only in the pubertal age group, but not in young adult (when betaERKO mice were still significantly more aggressive than WT mice) and adult (when no genotype differences in aggression were found) age groups. These results suggest that ER-beta mediated actions of gonadal steroids may more profoundly be involved in the inhibitory regulation of aggressive behavior in pubertal and young adult mice.

Physical provocation potentiates aggression in male rats receiving anabolic androgenic steroids

Anabolic androgenic steroids (AAS) have been linked to indiscriminant and unprovoked aggression and violence. We employed a brief tail pinch to examine the effects of different AAS on intermale aggression in gonadally intact male rats in response to a mild physical provocation. Animals received 5 mg/kg testosterone propionate (TP), nandrolone (ND), or stanozolol (ST) 5 days/week. Controls received vehicle injections. After 12 weeks, rats were tested for aggression while treatments continued. Animals were paired with either gonadally intact or castrated opponents and were tested in the subject rat's home cage, the opponents's home cage, and a neutral cage. Aggression was tested during tail pinch of the subject rat and during tail pinch of the opponent rat. In TP-treated males, tail pinch significantly enhanced aggression in all social and environmental conditions compared to intact controls. TP treatment also significantly enhanced aggression when the opponents were tail pinched. Tail pinch did not increase aggression in ND-treated males, and aggression was significantly lower than controls in ST-treated males. As expected, cell nuclear androgen receptor binding was significantly elevated by the high dose of TP. Our results show that while AAS alone does not induce the indiscriminate and unprovoked aggression characteristic of 'roid rage, TP heightens the animals sensitivity to

Aggression in male rats receiving anabolic androgenic steroids: effects of social and environmental provocation

This study examined the effects of anabolic androgenic steroids (AAS) on aggression under different social and environmental conditions. Three AAS were tested in gonadally intact male rats: testosterone propionate (TP), nandrolone (ND), and stanozolol (ST). Doses of 5 mg/kg were given 5 times/week, with gonadally intact controls receiving vehicle only (propylene glycol). Animals received six weekly tests under each condition in a counterbalanced order. Results show that the three AAS differed in their ability to elicit aggression. Males receiving TP were more aggressive than controls, ND males were similar to controls, and ST males were less aggressive than controls. In the social and environmental provocation tests TP-treated males were more aggressive than other groups, but were able to discriminate between intact and castrated opponents and between their home cage and a neutral cage. In the environmental provocation test, TP males were also more aggressive against opponents when tested in the opponent's home cage. It is suggested that chronic exposure to high levels of TP does not eliminate the ability to discriminate between social or environmental cues, as might be expected if it induces a " 'roid rage." However, TP does increase the likelihood that the animal will respond with aggression/dominance in a provoking situation. All three AAS variably affected serum testosterone and LH levels, as well as testes, seminal vesicle, and prostate weights. No effect on body weight was observed.

Distribution of gonadal steroid receptor-containing neurons in the preoptic-periaqueductal gray-brainstem pathway: a potential circuit for the initiation of male sexual behavior

The present study used anterograde and retrograde tract tracing techniques to examine the organization of the medial preoptic-periaqueductal gray-nucleus paragigantocellularis pathway in the male rat. The location of neurons containing estrogen (alpha subtype; ER alpha) and androgen receptors (AR) were also examined. We report here that injection of the anterograde tracer biotinylated dextran amine (BDA) into the medial preoptic (MPO) produced dense labeling within the periaqueductal gray (PAG); anterogradely labeled fibers terminated in close juxtaposition to neurons retrogradely labeled from the nucleus paragigantocellularis (nPGi). Dual immunostaining for Fluoro-Gold (FG) and ER alpha or FG and AR showed that over one-third of MPO efferents to the PAG contain receptors for either estrogen or androgen. In addition, approximately 50% of PAG neurons retrogradely labeled from the nPGi were immunoreactive for either ER alpha or AR. These results are the first to establish an MPO-->PAG-->nPGi circuit and further indicate that gonadal steroids can influence neuronal synaptic activity within these sites. We reported previously that nPGi reticulospinal neurons terminate preferentially within the motoneuronal pools of the lumbosacral spinal cord that innervate the pelvic viscera. Together, we propose that the MPO-->PAG-->nPGi circuit forms the final common pathway whereby MPO neural output results in the initiation and maintenance of male copulatory reflexes.

Disinhibitory behavior and GABA(A) receptor function in serotonin-depleted adult male rats are reduced by gonadectomy

Impulsive and aggressive behaviors in, e.g., personality or substance abuse disorders in man and corresponding behaviors in rats may involve serotonin (5-HT), gamma-amino-butyric acid(A)/benzodiazepine receptor complexes (GABA(A)/BDZ-RC) and steroid hormones, e.g., testosterone. Here, we studied the effect of gonadectomy on disinhibitory behavior in individually housed 5-HT-depleted rats and on GABA(A)/BDZ-RC function in vitro, in corticohippocampal synaptoneurosomes prepared from the brain of these animals. 5-HT depletion by intracerebroventricular 5,7-dihydroxytryptamine (5,7-DHT)-induced disinhibitory behavior in a shock-induced behavioral inhibition model (punished conflict model) 14 days after operation. Gonadectomy in connection with the 5-HT depletion reduced the disinhibitory behavior and testosterone substitution prevented this effect. Shock threshold and drinking motivation were not affected by gonadectomy and/or 5-HT depletion. The relative epididymides weight was increased in 5-HT-depleted as compared to sham-operated rats. However, the serum concentrations of testosterone and the relative testes weights were not different in 5-HT-depleted rats as compared to controls. GABA-induced (30, 100, 300 microM) 36Cl(-)-uptake into synaptoneurosomes was lower in 5,7-DHT+gonadectomized rats compared to the control group. This effect was reversed by substitution with testosterone. These results demonstrate that gonadectomy reduces disinhibitory behavior in 5-HT-depleted rats and that GABA(A)/BDZ-RC may be involved in this effect.

Social stress effects on territorial marking and ultrasonic vocalizations in mice

Acute social defeat (SD) leads to transient and persistent physiological and behavioral changes. We examined the effects of acute SD on territorial urine marking and ultrasonic courtship vocalizations in DBA/2 male mice. Both behaviors are considered androgen dependent and are influenced by social status, with dominant mice displaying more of both behaviors. In Experiment 1, male mice that received SD displayed prolonged inhibition of territorial urine marking, relative to nondefeated control mice (NOSD). In addition, territorial marking increased with repeated tests. In Experiment 2, male mice that received 3 successive days of SD displayed fewer ultrasonic courtship vocalizations at 30 min. post-SD1 and 30 min. post-SD2, relative to NOSD mice. In Experiment 2, we also observed decreased territorial marking 4 weeks post-SD. In sum, SD induced prolonged inhibition of territorial marking, but had only transient effects on ultrasonic courtship vocalizations, suggesting that different mechanisms may mediate the maintenance of these behaviors.

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.

Effects of chronic treatment with testosterone propionate on aggression and hormonal levels in intact male mice

Effects of testosterone propionate, an anabolic-androgenic steroid (AAS), on aggression in gonadally intact male mice were examined. Animals were given weekly injections of 3.75, 7.5, 15, and 30 mg/kg of drug or sesame oil for 10 weeks. During the last 3 weeks, behavioral tests were conducted and at the end of the experiment, body, liver and testes weight and hormonal data were collected. The treatment had minimal behavioral and endocrine effects. It resulted in shorter latencies of 'threat' only in the last agonistic encounter, increases in testosterone levels and decreases in testes weight in a non-linear dose-dependant way. The action of treatment was different on threat and attack, the latter being unaffected. The behavioral effects in the total sample were only found in aggressive animals selected on the basis of their latency of attack in the first encounter.

Aggressive behavior in male mice lacking the gene for neuronal nitric oxide synthase requires testosterone

Nitric oxide acts as a neural messenger in both the central and peripheral nervous systems. Mice with targeted disruption of the neuronal isoform of nitric oxide synthase (nNOS - / -) are extremely aggressive relative to wild-type (WT) mice. Male nNOS - / - mice exhibit an increase in the number and duration of aggressive encounters compared to WT animals when tested in a variety of paradigms used to test rodent aggression. This inappropriate aggressive behavior has only been observed in male nNOS - /- mice; nNOS - /- females, like female WT mice, exhibit little or no aggression. The present study sought to test the dependence of increased aggressive behavior in nNOS - / - males on testosterone. Intact nNOS - / - males exhibited elevated levels of aggression relative to intact WT males. Castration reduced aggression in both WT and nNOS - /- males to equivalent low levels. Testosterone replacement restored aggression to precastration levels in both genotypes. These data provide evidence that increased aggressive behavior of nNOS - /- mice, like aggression in WT mice, is testosterone-dependent.

Actions of testosterone in prepubertal and postpubertal male hamsters: dissociation of effects on reproductive behavior and brain androgen receptor immunoreactivity

This study was conducted to determine whether there is a increase in responsiveness to the activating effects of testosterone on male reproductive behavior during puberty in male golden hamsters and whether responsiveness to behavioral actions of testosterone is correlated with the ability of testosterone to upregulate brain androgen receptor immunoreactivity (AR-ir). Sexually naive male hamsters were castrated at 21 or 42 days of age and implanted subcutaneously with a pellet containing 0, 2.5, or 5 mg of testosterone. One week later, males were given a 10-min mating test with a receptive female. Animals were euthanized 1 hr after the behavioral test, and blood samples and brains were collected. Plasma testosterone levels were equivalent in prepubertal and adult males that had been administered the same dose of testosterone. However, adult males exhibited more mounts, intromissions, and ejaculations than prepubertal males, demonstrating that postpubertal males are more responsive than prepubertal males to the effects of testosterone on sexual behavior. In both age groups, testosterone increased the number of AR-ir cells per unit area in several brain regions involved in male sexual behavior, including the medial preoptic nucleus (MPN), medial amygdala, posteromedial bed nucleus of the stria terminalis, and magnocellular preoptic nucleus (MPNmag). Surprisingly, testosterone increased AR-ir in the latter three regions to a greater extent in prepubertal males than in adults. Thus, prepubertal males are more responsive to the effects of testosterone on AR-ir in these regions. In a separate experiment, a pubertal increase in the number of AR-ir cells per unit area was found in both the MPN and MPNmag of intact male hamsters. These results indicate that a testosterone-dependent increase in brain AR during puberty may be necessary, but is not sufficient, to induce an increase in behavioral responsiveness to testosterone.

Systemically administered gonadotrophin-releasing hormone enhances copulatory behaviour in castrated, testosterone-treated hyperprolactinaemic male rats

Hyperprolactinaemic male rats exhibit deficits in copulatory behaviour which can be reversed by a single injection of GnRH. We tested whether systemically administered GnRH can stimulate copulatory behaviour independently of LH-mediated increases in plasma testosterone levels. Gonadectomized, pituitary-grafted adult male Fischer 344 rats bearing implants of 5, 10 or 20 mm capsules of testosterone were administered a single injection of 500 ng GnRH or saline s.c., 30 min prior to copulation tests. Pituitary-grafted castrates displayed copulatory deficits, relative to sham-operated castrates with identical levels of testosterone replacement. Administration of 500 ng GnRH to pituitary-grafted castrates bearing 10 mm testosterone implants significantly increased the proportion of rats that mounted, intromitted and ejaculated during a 30 min test. This treatment also reduced significantly the latency of intromission and ejaculation, and increased significantly the frequency of intromission. The copulatory behaviour of the sexually unresponsive, pituitary-grafted castrates bearing 5 mm testosterone implants, or of the more sexually responsive castrates bearing 20 mm testosterone implants, was not altered significantly by GnRH injections. These results support the hypothesis that copulatory deficits in moderately hyperprolactinaemic rats are due in part to reduced hypothalamic GnRH release, and suggest that GnRH can stimulate sexual behaviour in these animals via mechanisms that are independent of luteinizing hormone-induced testosterone release. However, a threshold level of testosterone (achieved with 10 mm implants) appears to be required for GnRH to elicit this effect.

Hyperactivity in hyposexual male rats

It is widely accepted that some male rats fail to copulate because of a decrease in arousability, measured as decreased general locomotor activity (hypoactivity). This relationship, however, failed to explain an observation made in our laboratory that rats that failed to copulate exhibited increased general locomotor behavior. To directly address this issue, we quantified open-field and male sexual behaviors in 360 rats from two different strains. Twenty-two out of 49 hyposexual males were also hyperactive; this was a significantly greater number than would be expected by chance (p < 0.002, binomial test). Interestingly, only 6 of the 49 hyposexual males were hypoactive; this number was actually significantly smaller than would be expected by chance (p < 0.02). There was no correlation between behavioral measures and plasma levels of testosterone or progesterone. A decrease in selective attention and a failure to be stimulated by amphetamine was apparent in all hyperactive rats--those normal-sexual as well as hyposexual. The hyperactive rats were not hypertensive. We conclude that a significant percentage of hyposexual rats are hyperactive, and that hypoactive rats generally exhibit normal levels of sexual behavior. Decreased selective attention and decreased responsiveness to amphetamine do not explain this result, which is also not related to blood pressure or androgen levels.

Lesions of the medial prefrontal cortex and sexual behavior in the male rat

Lesions of the cerebral cortex near the midline in the frontal region appearing to destroy most of the cingulate cortex and adjacent prefrontal areas had profound effects on male rat sexual behavior. At the first postoperative tests, one week after the lesion, the mount and intromission latencies were extremely long (> 60 min). They continued elevated at every fortnightly test until postlesion week 13, when they were no longer different from controls. However, the proportion of animals that intromitted or ejaculated was reduced at this time. The lesion had a slight effect on the number of intromissions and on the intromission ratio, but did not reliably modify other parameters of sexual behavior in those males that copulated after operation. These data suggest that the medial prefrontal cortex is important for the initiation of sexual behaviour but less so for its execution. It is proposed that the elaboration and/or interpretation of environmental stimuli are rendered deficient by the lesion. Consequently, sexual behavior is activated only with difficulty. This coincides with the arousal hypothesis proposed by Beach [8]. There appears to exist a spontaneous recovery of the mechanisms responsible for the activation of sex behavior because the lesioned group was not different from the sham or intact groups 13 weeks postlesion. Remaining cortical tissue or other brain structures may compensate for the initial deficiencies. However, the lesion's effect on intromission behavior did not diminish with time. This could suggest that possible motor deficiencies produced by the lesion are irreversible.

Aggression is attenuated by ovariectomy in pregnant female rats given progesterone and estradiol replacement to maintain pregnancy

Female rats were tested for aggression toward an unfamiliar female conspecific 13 days following the beginning of cohabitation with a male. Forty-eight hours later, half of the females that displayed aggression were ovariectomized. To maintain pregnancy, they were also given Silastic implants of estradiol (1-mm tube) and progesterone (6 tubes, 30-mm long; Dow-Corning tubing #602-305) sufficient to maintain pregnancy but at lower levels than those normally present. Control females were given a sham ovariectomy and implanted with empty tubes. Each animal received a second aggression test at 48 h following surgery and a third test, 3 days following the second. Hormone implants successfully maintained pregnancy and supported the normal development of the pups. However, Ovariectomized females with hormone implants did not spontaneously give birth. Ovariectomized females displayed less aggression than their sham-ovariectomized counterparts at the second and third aggression tests. The difference was due to an increasing level of aggression by the control but not the experimental animals. These results are consistent with other evidence that circulating gonadal hormones influence the onset of maternal aggression in the female rat.

Anabolic-androgenic steroids and aggression in castrated male rats

The resident-intruder paradigm of aggression was utilized to evaluate the aggression-inducing properties of two anabolic-androgenic steroid (AAS) compounds, methyltestosterone and stanozolol, in castrated male rats. Three weekly tests were conducted. On test week three, castrated males treated with methyltestosterone displayed levels of aggression equivalent to the levels displayed by castrated males treated with testosterone propionate on most of the behavioral indices assessed. In contrast, treatment with stanozolol at the dose used in this study was completely ineffective in eliciting aggressive behavior. AAS effects on aggression were mirrored by their ability to stimulate seminal vesicle growth. There were no effects of AAS treatments on the levels of locomotor activity. These findings highlight the heterogeneity of AAS effects on the nervous system and behavior and indicate that the psychological effects reported by human AAS abusers may depend upon the distinct chemical structures of the abused steroids.

Interactions between 5-hydroxytryptamine (5-HT) and testosterone in the control of sexual and nonsexual behaviour in male and female rats

Two 5-hydroxytryptamine2 (5-HT2) agents, ritanserin and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI) (both at 0.25 mg/kg IP), were administered to castrated males bearing graded testosterone implants (empty, 2.5-, 5-, and 10-mm length) and to normal and neonatally androgenized ovariectomized females bearing 10-mm testosterone implants. The results indicate that testosterone stimulates male sexual behaviour and appears to have a dose-related anxiolytic effect, but no effect on other nonsexual activities. 5-HT and testosterone had opposite effects on male sexual behavior, with ritanserin (5-HT antagonist) enhancing activity in both sexes and DOI (5-HT agonist) inhibiting behaviour in males, the latter being testosterone-dependent. Independent of testosterone, ritanserin reduced locomotion and exploration and increased anxiety in males, while DOI increased locomotion and exploration in both sexes. Ritanserin had a gender-specific effect on anxiety which was independent of testosterone, since in castrated males it was anxiogenic whether they bore testosterone implants or not, while in females it was anxiolytic whether the female were neonatally androgenized (250 micrograms/pup testosterone proprionate [TP] on day 1) or not. These results show that 5-HT and testosterone have opposite effects on male sexual behaviour and these may be interrelated. In adulthood, their effects on nonsexual activities are not inversely related and are independent of each other in contrast to the relationship seen in the neonatal period.

Effects of chronically high doses of the anabolic androgenic steroid, testosterone, on intermale aggression and sexual behavior in male rats

To determine if chronic exposure to high doses of anabolic androgenic steroids (AAS) increase aggression and sexual activity in gonadally intact rats, we administered SC injections of testosterone propionate (TP: 1 mg) or propylene glycol (PG: as a vehicle control) three times per week for 10 consecutive weeks. Weekly tests for male copulation were conducted by pairing each male with a sexually receptive female in a glass arena and recording various parameters of copulation. Chronic treatment with TP did not alter any parameter of male copulation including ejaculation and post ejaculatory interval. Sexually experienced males were tested once per week for intermale aggression in a neutral environment. For aggression tests, each experimental animal was placed into an arena with an opponent male for 15 min. The opponent, a gonadally intact male, was used only for the purpose of the aggression test. Frequencies of dominance, submissive, threat, and fight postures, as well as approaches were scored for each animal. TP-treated males did not differ in body weight from PG-treated males, suggesting that the increased aggression was not due to increased body mass. The results of this study show that chronic exposure to the anabolic androgenic steroid, testosterone, potentiates male aggressive response patterns but does not alter male sexual behavior or body weight.

Attractiveness of male odors to females varies directly with plasma testosterone concentration in meadow voles

We examined the hypothesis that differences in androgen concentrations may cause differences in the attractiveness of male odors to female meadow voles, Microtus pennsylvanicus. Males that were to be used as odor donors were castrated and implanted with Silastic capsules filled with no hormone or one of four different amounts of testosterone. Four different odors from males were assessed for each comparison; the odors were from the anogenital area, feces, urine, and posterolateral region. Treatment of scent donor males with the lowest dosage of testosterone was not sufficient to increase the attractiveness of their odors above that of males receiving no hormone. The three higher dosages increased attractiveness in a dose-dependent manner. The shape of the dose-response curve differed across scents: two scents reached maximum attractiveness in the middle of the dose range and remained there at higher doses, whereas the other two scents continued to increase in attractiveness as testosterone titers increased. The results suggest that scent glands produce graded signals that reflect circulating testosterone titers.

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.

Aggression by a female rat cohabiting with a sterile male declines within 27 h following ovariectomy

Females that had become aggressive as a result of cohabiting with a sterile male were ovariectomized and implanted with empty Silastic tubes. Control groups were either sham ovariectomized or ovariectomized and implanted with Silastic tubes providing replacement levels of estradiol and testosterone. Twenty-seven hours following surgery, all animals were tested for aggressiveness toward an unfamiliar female rat. The aggression of ovariectomized females without hormone replacement declined to a minimal level postoperatively and was significantly lower than that of sham-ovariectomized females or ovariectomized females with estradiol and testosterone replacement. Both sham-ovariectomized females and ovariectomized females given hormone replacement displayed a level of aggression close to that observed preoperatively. The aggression of a female rat cohabiting with a sterile male appears to be highly sensitive to the removal of gonadal hormones. This corresponds with observations made on pregnant females and contrasts with those made on lactating females.

A review of hormonal factors influencing the sexual and aggressive behavior of macaques

The effects of gonadal hormones on the sexual and aggressive behavior of adult macaques are reviewed. Similarities among findings from field, colony, and laboratory studies strengthen the view that testosterone facilitates the sexual and aggressive behavior of males, while sexual and perhaps aggressive behavior by the female is mainly dependent on estradiol, which increases both the sexual motivation of the female and her attractiveness to males. Differences between results from different settings help to emphasize the role of environmental and social factors in modulating the effects of hormones. © 1993 Wiley-Liss, Inc.

Cohabitation with a sterile male facilitates the development of retrieval behavior in nulliparous female rats exposed to pups

Female rats were housed with a sterile male or another female. After 3 weeks, half of the females that had been housed with a female were rehoused with an intact male. At the end of 6 weeks female or sterile male cagemates were removed. Intact male cagemates and pups were removed 3 to 12 h following parturition. All females were tested for retrieval of three unfamiliar pups placed in their cage on the day following removal of their cagemate. Three unfamiliar pups were placed with each female and the female's behavior observed for 10 min. Observations were made in this way for 13 days or until the female retrieved all three pups within the 10-min interval. Pups were left with the female on days they were not retrieved. Females housed with a sterile male reached criterion for pup retrieval in 2.9 days, significantly fewer days than were required for females housed with another female (6.6 days) but significantly more than were required for a postpartum female (0.8 days). By demonstrating that cohabitation with a male fosters the development of retrieval, these results support evidence from the study of aggressive behavior that pseudopregnancy facilitates the development of behaviors associated with pregnancy and lactation.

Interaction of estradiol, testosterone, and progesterone in the modulation of hormone-dependent aggression in the female rat

Female rats that had become aggressive as a result of cohabiting with a sterile male were ovariectomized and implanted with Silastic tubes of estradiol, testosterone, and progesterone, estradiol and testosterone alone, or with empty tubes. The implants were designed to model serum concentrations present during the last week of pregnancy (estradiol, 0.06 ng/ml; testosterone, 2.6 ng/ml; progesterone, 70 ng/ml). Following a test of aggression 1 week postoperatively, estradiol and testosterone implants were replaced with ones designed to maintain the lower hormone levels present following parturition (0.02 ng/ml; 0.6 ng/ml, respectively). Progesterone was not replaced. At the first aggression test, females with estradiol and testosterone alone displayed significantly more aggression than females with these hormones plus progesterone. Both groups were more aggressive than females without hormone replacement. Following the exchange of large implants for small ones, females that previously had progesterone increased in aggression while females that previously had only estradiol and testosterone decreased in aggression. Both groups continued to be more aggressive than the group without hormone replacement. High serum progesterone present near the end of pregnancy appears to moderate the expression of aggression supported by estradiol and testosterone. Conversely, progesterone's decline at parturition appears to produce a rebound facilitation of aggression even though serum estradiol and testosterone simultaneously decline.

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.

Hormonal reactions to fighting in rat colonies: prolactin rises during defence, not during offence

Male Wistar rats living in colonies of 4 males plus 4 females were compared to noncolony males, cohabitating with a female. Irreversible dominance relationships developed between one dominant male (D) and three subordinates (S). Dominants developed high basal testosterone levels and large preputial glands. Subordinates had reduced preputial glands despite normal testicle weights and normal basal testosterone levels. Basal corticosterone was elevated in both ranks, in S more so than in D, while in acute encounters both ranks showed a similar increase in the corticosterone-to-ACTH ratio. They also underwent a similar reduction in thymus weight, while an increase in adrenal weight was more pronounced in D. In D-S-I encounters, during which D simultaneously attacked S and an intruder (I) for 20 minutes, both defenders showed a 3-4 fold increase in plasma prolactin, while in the offensive dominant the level remained low. Similar, but weaker hormonal contrasts between offence and defence were found for beta-endorphin, ACTH, and corticosterone, while alpha-MSH and testosterone did not discriminate. In our view, the marked hyporesponsiveness of prolactin to acute offence may be associated with a specific offensive setting of dopaminergic inhibitory and beta-adrenergic stimulatory influences.

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.

Reciprocal relationships between pulsatile androgen secretion and the expression of mating behavior in adult male ferrets

The pulsatile secretion of androgen was similar over a 12-hr period in breeding male ferrets implanted with jugular catheters which either achieved an intromission with an estrous female or received no socio-sexual contact. This negative result contrasts with the previous demonstration (Carroll, Erskine, and Baum, 1987, Endocrinology 121, 1349-1359) of a significant, delayed rise in mean plasma androgen concentrations in breeding male ferrets 5-12 hr after mating. Males used in that previous study had lower initial mean plasma levels of androgen and smaller testis diameters than the present males. We therefore asked whether differences in circulating androgen levels, characteristic of males in different phases of the seasonal breeding cycle, might affect the expression of mating behavior. Castrated males given 0, 0.2, 2.0, or 5.0 mg/kg of testosterone propionate (TP) showed dose-related increases in the expression of different components of sexual behavior, including neck gripping, mounting, and intromitting. Surprisingly, intromissive performance was significantly better in intact breeding males than in castrates given even the highest dosage of TP. These results suggest that ferrets' mating performance may vary with seasonal variations in androgen availability, and that the ability of males to exhibit a postcoital increase in the testicular secretion of androgen may be limited to the beginning or end of the breeding season, when circulating levels of androgen are relatively low. Mating-induced increments in androgen secretion at these times may enhance subsequent reproductive success by facilitating males' intromissive capacity, which is required for the induction of ovulation and optimal sperm transport in female partners.