Gonadal Sources of Androgens

The role of androgens on aggressive behavior in males is well established (Soma, 2006). Early studies found that castration reduces aggression in males (Payne & Swanson, 1971; Vandenbergh, 1971), although now we know these effects may be mediated by estrogen in addition to testosterone (T) (discussed below). In many seasonally breeding species, increased T levels serve to coordinate increased territorial aggression with mating behavior (Wingfield, Hegner, Dufty, & Ball, 1990; Wingfield, Lynn, & Soma 2001). In females, there is increasing evidence that T is increased in the breeding season (Ketterson, Nolan, & Sandell, 2005), and high levels of T have been related to sexual behavior in women (Anders, Hamilton, Schmidt, & Watson, 2007). In addition, female T levels are higher in colonial bird species with elevated competition for nesting sites compared to solitary species that have less competition for nesting sites (Møller, Garamszegi, Gil, Hurtrez-Boussès, & Eens, 2005).

Dynamic changes in circulating T have been shown to have an important effect on aggression in males. For example, changes in T during intrasexual aggressive encounters can have important long-term effects on aggression in future competitions (Gleason, Fuxjager, Oyegbile, & Marler, 2009). This has been called the challenge effect, which predicts that concentrations of T should be elevated during social competition to promote aggressive behaviors (Wingfield et al., 1990). Interestingly, dynamic changes in T can also affect aggression in females. For example, in the absence of dominant males, females of the African cichlid fish (Astatotilapia burtoni) increase their T levels and show higher levels of territorial aggression (Renn, Fraser, Aubin-Horth, Trainor, & Hofmann, 2012). Similarly, females of the daffodil cichlid (Neolamprologus pulcher) and female marmosets (Callithrix kuhlii) show elevated levels of testosterone after aggressively defending their territories from intruders (Desjardins, Hazelden, Van der Kraak, & Balshine, 2006; Ross & French, 2011). Nonetheless, T elevation in competitive social environment is not always observed in females (Davis & Marler, 2003; Goymann, Wittenzellner, Schwabl, & Makomba, 2008; Rubenstein & Wikelski, 2005). In some cases, circulating T levels in females actually decreases following an aggressive encounter (Elekonich & Wingfield, 2000; Rubenstein & Wikelski, 2005). Currently, the functional basis for the variability in T responses to aggressive interactions in females is unclear. Although androgens are known to have important metabolic costs (Buchanan, Evans, Goldsmith, Bryant, & Rowe, 2001; Marler & Moore, 1989) and immunosuppressive effects (Hillgarth & Wingfield, 1997), they can have additional side effects in females. These can include breeding delay, altered mate choice behavior, and inhibition of maternal care (Gerlach & Ketterson, 2013; McGlothlin, Neudorf, Casto, Nolan, & Ketterson, 2004; Rosvall, 2013b). Thus, it has been hypothesized that the elevation of T in response to competition should be more common in species with low maternal care, or species in which offspring are relatively less susceptible to parental neglect (Rosvall, 2013a). In turn, species with higher maternal investment are expected to have evolved mechanisms other than elevation of T to mediate aggression. An alternative solution to limit the costs of T would be to employ local synthesis of T.

T Synthetized in the Brain

Although there is little direct evidence for T synthesis within the brain, recent data suggest that the enzymes necessary to produce T de novo from cholesterol are present in many brain areas (Do Rego et al., 2009). In addition, it is possible that T could be synthesized from dehydroepiandrosterone (DHEA), a steroid precursor that can be synthesized in nervous tissue (Corpéchot, Robel, Axelson, Sjövall, & Baulieu, 1981; Do Rego et al., 2007; Hojo et al., 2004) and adrenal glands (Labrie et al., 2005). DHEA can be metabolized into androgens and/or estrogens in peripheral tissues (Labrie et al., 2005). Data from rodents and songbirds suggest that this conversion of DHEA in to active steroids can occur in the brain where it could affect behavior (Dupont, Simard, Luu-The, Labrie, & Pelletier, 1994; Soma, Alday, Hau, & Schlinger, 2004). For example, plasma levels of DHEA are elevated in male sparrows during the non-breeding season (Maddison, Anderson, Prior, Taves, & Soma, 2012; Newman & Soma, 2009), when territorial aggression levels are still elevated but plasma T levels are very low (Soma, Schlinger, Wingfield, & Saldanha, 2003). Interestingly, levels of the enzyme necessary for conversion of DHEA to androgens, 3β‎-hydroxysteroid dehydrogenase/δ‎5-δ‎4 isomerase (3β‎-HSD), are highest during this time in centromedial and caudal telencephalon (Pradhan, Newman, Wacker, Wingfield, Schlinger, & Soma, 2010). In addition, the activity of these enzymes is further increased during territorial challenges and engaging in aggression. These data suggest that male sparrows have the potential for T synthesis within the brain, and that it can be rapidly modulated. However the hypothesis that the conversion of DHEA to T increases aggression still needs to be tested directly. In females, DHEA has been linked to territorial aggression in Siberian hamsters. During winter-like days, female Siberian hamsters show increased levels of territorial aggression and have elevated levels of serum DHEA and adrenal DHEA responsiveness (see Figue 2) (Rendon, Rudolph, Sengelaub, & Demas, 2015). Further, aggression reduces serum DHEA levels during short days (Rendon & Demas, 2016). One possible explanation for this result is that DHEA metabolism to androgens during aggressive encounters might reduce circulating DHEA levels, but again, this is not direct evidence that DHEA is being converted to androgens to modulate aggression, as alternative mechanisms of action are possible. For example, DHEA can bind to estrogen and androgen receptors in the brain (Webb, Geoghegan, Prough, & Miller, 2006) and has been shown to modulate action of various receptor systems known to modulate aggression including GABA_A and NMDA (Bergeron, Montigny, & Debonnel, 1996; Compagnone & Mellon, 2000; Majewska, 1992). Further studies will be needed to directly test which are the underlying mechanisms mediating the effects of DHEA on aggressive behavior.

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Figure 2. ACTH challenge time course for cortisol and DHEA across photoperiods. Cortisol levels for (a) long-day females, (b) short-day females; DHEA levels for (c) long-day females and (d) short-day females receiving either an ACTH or control treatment. White bars, long days; grey bars, short days. Bar heights represent means ± SEM *p < 0.05.

Reproduced with permission from “The Agonistic Adrenal: Melatonin Elicits Female Aggression Via Regulation Of Adrenal Androgens,” by N. M. Rendon, L. M. Rudolph, D. R. Sengelaub, and G. E. Demas, 2015, Proceedings of the Royal Society of Biological Sciences, 282(91819).

Androgens: Mechanisms of Action

The effects of androgens on behavior can be mediated through direct binding to intracellular androgen receptors (AR), which then migrate to the nucleus to regulate gene expression (Heemers & Tindall, 2007). This so-called genomic effect is relatively slow, with the effects on cell function observed over a period of hours to days. Alternatively, T can rapidly influence aggressive behavior by non-genomic mechanisms in which AR are deposited in the cell membrane (Lösel & Wehling, 2003) or through intracellular AR that remain outside the nucleus. These non-genomic actions are mediated by second messenger pathways that can lead to rapid changes in cell function within seconds or minutes (Foradori, Weiser, & Handa, 2008). Membrane associated AR have been described in dendrites (Tabori et al., 2005) and axons (DonCarlos et al., 2006).

In the context of aggression, both slow and rapid effects of T have been reported. For example, a study in the coral reef fish Stegastes nigricans used implants of flutamide, an AR antagonist, to show that AR are necessary for the expression of territorial aggression in males but not females (Vullioud, Bshary, & Ros, 2013). Here flutamide was administered over the course of 5-6 days, which is consistent with slow acting genomic effects of steroid hormones. However, T can also have more rapid effects. In white-footed mice (Peromyscus leucopus), one pulse of T can modify territorial marking within 20 minutes, suggesting that non-genomic regulation can modify this agonistic behavior (Fuxjager, Knaebe, & Marler, 2015). Although these studies did not directly test whether these effects were mediated by AR, other evidence suggests that estrogens are not necessary. For example, male California mice (Peromyscus californicus) injected with T immediately following aggressive encounters are more aggressive in future encounters than males receiving saline, and this effect is not blocked by aromatase (the enzyme responsible for converting T to estrogens) inhibitor treatment (Trainor, Bird, & Marler, 2004). Rapid effects of T on aggressive behavior have also been observed in females. Lythrypnus dalli is a fish species that can show bidirectional sex changes. Dominant females show increased territorial aggression within 2 hours of exogenous administration of 11-ketotestosterone, a teleost analog to dihydrotestosterone, compared to controls (Pradhan, Connor, Pritchett, & Grober, 2014) (Figure 3).

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Figure 3. Rates of agonistic interactions between alpha and beta Lythrypnus dalli relative to the time before or after alphas were treated with either 11-ketotestosterone or cholesterol. (A) Alpha approaches beta; (B) alpha displaces beta; (C) beta approaches alpha; and (D) beta displaces alpha. *p < 0.05 (n = 8 cholesterol and n = 10 ketotestosterone). Differences in aggressive behavior of both alphas and betas are seen within 2 h of implanting the alpha. Dotted lines represent the transient window of social instability that follows male removal.

Reprinted from “Contextual Modulation of Androgen Effects on Agonistic Interactions” by D. S. Pradhan, K. R. Connor, E. M. Pritchett, and M. S. Grober, 2014, Hormones and Behavior, 65(1), 47–56. Copyright with permission from Elsevier.

Interestingly, in several cases AR expression in the brain is promoted by T, which in turn can promote sex differences in brain AR expression. In Mus musculus, AR expression is higher in intact males than intact females in BNST, LS, and MPOA; a difference that is abolished by gonadectomy (Lu, McKenna, Cologer-Clifford, Nau, & Simon, 1998). In addition, females treated with T implants show AR expression levels equivalent to males in these nuclei. However, this is not true for every species. For example, although T increases AR gene expression in male rats, no effects of T implants were observed on AR expression in females (Roselli, 1991). Further, in females central AR expression can be higher than in males even with lower circulating T. An intriguing example is the African black coucal (Centropus grillii), a cuckoo bird in which females are more aggressive than males. Females have lower levels of circulating T, but show higher expression of AR mRNA in the taeniae of the amygdala (a brain region analogous to the mammalian MeA) than males (Voigt & Goymann, 2007) (Figure 4). In addition to sex specific expression of steroid receptors, activation of the same receptors in males and females may result in sex-specific gene expression profiles. For example, in the sexually dimorphic dark-eyed junco (Junco hyemalis), treatment with T has different effects on gene expression in the brain for males versus females. Testosterone increased expression of aromatase in hypothalamus in both sexes, but decreased expression of monoamine oxidase A in MeA in females but not males (Peterson et al., 2013). Together, this suggests that different region specific expression pattern of AR, as well as downstream effects of AR in males vs. females, may allow females to harness the beneficial effects of T on certain behaviors while avoiding some of the adverse effects that T may cause on other behaviors, metabolism and reproduction (Wingfield et al., 2001).

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Figure 4. Mean (± SEM) expression level of AR, ERα‎ (ER) and aromatase (ARO) mRNA in nucleus taeniae (TnA) of male and female black coucals. The expression of AR mRNA in TnA was significantly higher in females than males (c), particularly in central and caudal part of the nucleus (d).

Reprinted with permission from “Sex-Role Reversal Is Reflected in the Brain of African Black Coucals (Centropus grillii),” by C. Voigt and W. Goymann, 2007, Developmental Neurobiology, 67, 1560–1573.

Peripheral Sources of Estrogens

Estrogens have very important effects on aggression in both males and females. Early studies in males showed that E administration (Payne & Swanson, 1972a; Vandenbergh, 1971), as well as ovarian implantation (Payne & Swanson, 1971), could increase inter-male aggression. During development, T secreted by the male gonads is converted to estradiol (E2) in the brain, which has long lasting effects on male typical behaviors, including aggression, and inhibition of female like behaviors (Lenz, Nugent, & McCarthy, 2012; Wu et al., 2009).

Acting in the adult brain, estrogens are mainly associated with promoting aggressive behaviors. In females, estrogens promote aggression in reptiles (Rubenstein & Wikelski, 2005), birds (Rosvall et al., 2012), and mammals (Albert, Petrovic, & Walsh, 1989). The role of estrogens on aggression has been particularly well studied in the context of maternal behavior. For example, ovariectomy on gestation day 16 in rats significantly reduces maternal aggression in comparison to controls (sham operated gestating rats), and this effect can be reversed by injections of E2 (Mayer & Rosenblatt, 1987). In males, evidence from several bird and rodent species shows that E2 can increase territorial aggressive behavior (Laredo, Villalon Landeros, & Trainor, 2014). In California mice, estrogens increase territorial aggression if males are housed in cages containing corncob bedding (Trainor et al., 2008a), which increases blood levels of E2-like tetrahydrofuran-diols (Villalon Landeros, Morisseau, Yoo, Fu, Hammock, & Trainor, 2012). However, if California mice are housed on cardboard-based bedding then estrogens decrease aggression (Laredo et al., 2013). These results show that factors such as social experience and diet can have important effects how estrogens modulate aggression. These factors may alter how estrogens interact with estrogen receptors (Byrnes, Babb, & Bridges, 2009; Byrnes, Casey, & Bridges, 2012). Finally, estrogens are involved at multiple levels of processing of social information (Ervin, Lymer, Matta, Clipperton-Allen, Kavaliersv, & Choleris, 2015), and can interact with other steroid hormones and neurotransmitter systems to influence aggression (Murakami, 2016; Soma, Rendon, Boonstra, Albers, & Demas, 2015)

Brain Sources of Estrogens

Aromatase, is expressed in brain areas that regulate aggressive behaviors across species (Balthazart, Baillien, Charlier, Cornil, & Ball, 2003; Cohen & Wade, 2011; Naftolin, Horvath, & Balthazart, 2001; Shen, Schlinger, Campagnoni, & Arnold, 1995), and its activity has been associated with aggression in males and females. For example, studies in male Japanese quail (Coturnix japonica) showed that individual differences in territorial aggressive behavior during the breeding season were positively correlated with aromatase in hypothalamus, but no correlation was seen between circulating T and this behavior (Schlinger & Callard, 1989). Further, administration of an aromatase inhibitor, but not an AR antagonist, significantly reduces aggression in this species (Schlinger & Callard, 1990). Similarly, genetic deletion of aromatase completely eliminates territorial aggression in mice (Toda, Saibara, Okada, Onishi, & Shizuta, 2001). In general, aromatase activity in the brain is higher in males than females (Roselli, Horton, & Resko, 1985), a difference mediated by at least two mechanisms. First, aromatase enzyme velocity is higher in males than females in the BNST and VMH (Roselli, Klosterman, & Fasasi, 1996b). Second, androgen receptor is a more effective enhancer of aromatase mRNA expression in male hypothalamus than females (Roselli, Abdelgadir, Jorgensen, & Resko, 1996a). Interestingly, recent data showed that aromatase producing neurons in the MeA had important effects on aggression in both males and females (Unger, Burke, Yang, Bender, Fuller, & Shah, 2015). Aromatase expressing neurons in the MeApd were selectively ablated in adult mice, such that the MeApd was allowed to develop normally in the presence of aromatase. When these aromatase expressing neurons were lesioned, both intermale aggression and maternal aggression were reduced but reproductive behaviors were unaffected (Figure 5, a & b).

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Figure 5.(a.) Ablation of aromatase + meA neurons reduces specific components of male and maternal aggression. No difference between aro^Cre/IPIN and aro^+/IPIN males in mating with an estrous female. Comparable percent of aro^Cre/IPIN and aro^+/IPIN males sniff and attack an intruder male. Aro Cre/IPIN males take significantly longer to attack and tail rattle, and they attack intruder less. (Unger et al., 2015).

Reprinted from “Medial Amygdalar Aromatase Neurons Regulate Aggression in Both Sexes,” by E. K. Unger, K. J. Burke, C. F. Yang, K. J. Bender, P. M. Fuller, and N. M. Shah, 2015, Cell Reports, 10(4), 453–462. Copyright (2015), with permission from Elsevier.

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Figure 5.(b.) Ablation of aromatase + meA neurons reduces specific components of male and maternal aggression. The vast majority of aro^cre/IPIN and aro^+/IPIN females retrieved all pups to the nest, and they did so with similar latencies. Comparable percentages of aro^Cre/IPIN and aro^+/IPIN females sniffed and attacked an intruder male. There was a significant decrease in number of attacks directed to the intruder male by aro^Cre/IPIN females.

Reprinted from “Medial Amygdalar Aromatase Neurons Regulate Aggression in Both Sexes,” by E. K. Unger, K. J. Burke, C. F. Yang, K. J. Bender, P. M. Fuller, and N. M. Shah, 2015, Cell Reports, 10(4), 453–462. Copyright (2015), with permission from Elsevier.

Estrogen: Mechanisms of Action

Estrogens exert their action by binding to estrogen receptors (ER) ERα‎ and ERβ‎, which are expressed throughout the neural circuit of aggression (Cushing, 2016; Kaiser, Kruijver, Swaab, & Sachser, 2003; Kramer, Simmons, & Freeman 2008). Similar to androgen receptors, these receptors can be expressed in the nucleus (Heldring et al., 2007; Micevych & Dominguez, 2009), as well as outside of the nucleus including dendrites, axons, and neuronal terminals (Blaustein, Lehman, Turcotte, & Greene, 1992; Milner, McEwen, Hayashi, Li, Reagan, & Alves, 2001; Milner et al., 2005; Towart, Alves, Znamensky, Hayashi, McEwen, & Milner, 2003). Activation of these receptors results in activation second messenger systems that can rapidly modulate cell function (Heimovics, Trainor, & Soma, 2015b)

Initial knock out studies assessing global effects of ER in males found that functional ERα‎ facilitate normal expression of inter-male aggression in mice (Ogawa, Lubahn, Korach, & Pfaff, 1997; Scordalakes & Rissman, 2003). Furthermore male aggression in CD-1 mice is positively correlated with the expression of ERα‎ in areas important for aggression including BNST, LS, and LH (Trainor, Greiwe, & Nelson, 2006). ERβ‎, on the other side, has been associated with the inhibition of aggression, although this effect seems to depend on age and sexual experience (Nomura et al., 2002; Ogawa et al., 1999). However, studies comparing the effects of selective ERα‎ and ERβ‎ agonists on male aggression have found to both increase or decrease territorial aggression depending on the light cycles used for testing (Trainor et al., 2007). Similar results have been reported in females. Gonadectomized female mice treated with selective ERα‎ agonists have increased aggressive attacks towards intruders (Clipperton-Allen, Almey, Melichercik, Allen, & Choleris, 2011) while females treated with selective ERβ‎ agonists show increased non-attack agonistic behaviors such as social investigation and dominance (Clipperton-Allen, Cragg, Wood, Pfaff, & Choleris, 2010). Interestingly knockout of ERα‎ increases levels of offensive aggression in females (Ogawa, Eng, Taylor, Lubahn, Korach, & Pfaff, 1998). Together these results indicate that in males and females ERa and ERb have distinct organizational and activational effects on aggressive behaviors.

E can act rapidly to modulate aggression. These effects may originate from the ability of aromatase activity to be rapidly modulated by environmental factors such as stress or social conflict (Balthazart et al., 2003; Dickens, Balthazart, & Cornil, 2012; Dickens, Bournonville, Balthazar, & Cornil 2014). Although the specific directional effects of E on aggression (either increasing or decreasing) depend on factors such as diet (Villalon Landeros et al., 2012), a robust observation is that rapid E action is more likely to be observed in the non-breeding season. In both Peromyscus polionotus (Trainor et al., 2007) and Peromyscus californicus (Trainor et al., 2008a), E rapidly modulates aggression in winter-like short day photoperiods but not in summer-like long day photoperiods. In P. californicus, the specificity of rapid E action under short days has been replicated (Laredo et al., 2014). Similarly, a study in male song sparrows showed that administration of E2 rapidly (within 20 min) increases territorial aggression in non-breeding males only (Heimovics, Ferris, & Soma, 2015a). Currently it is unclear whether rapid activation of ER modulates aggression in females, but rapid effects of E on brain and other behaviors, including memory formation (Gabor, Lymer, Phan, & Choleris, 2015), modulation of pain (An, Li, Yan, & Li, 2014), and anxiety-like behaviors (Holm, Liang, Thorsell, & Hilke, 2014), have been reported in females. This suggests that it is likely that this type of E signaling could be involved in modulation of female aggression.

Gonadal and Adrenal P4

The effects of circulating progesterone (P4) on aggression have been mainly studied in the context of parental behaviors. In mammals, plasma levels of P4 decrease towards the end of gestation and then rise progressively from post-partum day 3 to 10 (Finley, Zhang, & Fewell, 2015; Taya & Greenwald, 1982). The administration of P4 receptor (PR) antagonist in day 6 post-partum increases maternal aggression (de Sousa et al., 2010) (Figure 6). Since maternal aggression sharply increases during peripartum and remains high for around 2 weeks post-partum, the authors proposed that increasing levels of P4 would have a role in the progressive reduction of maternal aggression after that period. The sharp decline in P4 around parturition has also been shown to be a key component of the onset of pup-directed behavior in females (Bridges, Rosenblatt, & Feder, 1978; Sheehan & Numan, 2002). Interestingly, studies using the biparental species Peromyscus californicus have found that in males, paternal behavior is also associated with a decrease in plasma P4 (Trainor et al., 2003) and PR expression in the BNST (Perea-Rodriguez, Takahashi, Amador, Hao, Saltzman, & Trainor, 2015; Trainor, Bird, Alday, Schlinger, & Marler, 2003). Transgenic mice lacking PR receptors show reduced aggression towards pups (Schneider et al., 2003; Schneider, Burgess, Horton, & Levine, 2009), suggesting that P4 and PR activation may facilitate infanticide.

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Figure 6. (A) Maternal aggressive behavior of lactating females [6th day postpartum] who underwent SHAM surgery or ovariectomy [OVX] on the first day postpartum. A Student t test was used between the two experimental groups [with a p < 0.005 level of significance]. Indicates a significant difference between the groups [SHAM and OVX]. (B) Maternal aggressive behavior of lactating females [sixth day postpartum] that received OIL, RU 486 (0.2, 1.0, 2.0, and 4.0 mg/kg); and FIN injected SC, 2 h before test one-way ANOVA was used between the three experimental groups [with a p < 0.05 level of significance]. Indicates a significant difference between the groups [OIL, RU 0.2 mg and FIN]. The data are expressed as mean [± SEM] of the frequencies of behaviors studied. The number of animals [n] is given between parentheses.

Reprinted from “Progesterone and Maternal Aggressive Behavior in Rats,” by F. L. de Sousa, V. Lazzari, M. S. de Azevedo, S. de Almeida, G. L. Sanvitto, A. B. Lucion, et al., 2010, Behavioural Brain Research, 212(1), 84–89. Copyright (2010), with permission from Elsevier.

Some studies have also assessed the role of P4 in the context of territorial aggression, although data here is less clear. Pharmacological manipulations of P4 in rodents have shown that P4 can both reduce and increase territorial aggression. For example, one study showed that administration of P4 reduces aggression in both male and female gonadectomized Syrian hamsters (Fraile, McEwen, & Pfaff, 1987), but another study in the same species showed that in ovariectomized females, daily injections of P4 for ten days increase intrasexual aggression (Payne & Swanson, 1972b). In female bank voles (Clethrionomys glareolus), administration of P4 also increases aggression (Kapusta, 1998). Similarly, studies assessing physiological levels of P4 have yielded conflicting results. One study in California mice (Peromyscus californicus) observed decreases in plasma P4 in females after engaging in aggression (Davis & Marler, 2003b). A similar result was observed in free-living black coucals (Centropus grillii), in which females also have decreased serum P4 after engaging in aggression (Goymann et al., 2008). Furthermore, females treated with P4 implants show reduced territorial aggression compared to females given empty implants. In contrast, P4 is higher in aggressive compared to non-aggressive female Iguanas (Amblyrhynchus cristatus) (Rubenstein & Wikelski, 2005), and studies in sparrows (Melospiza melodia) have found no association between P4 and female aggression (Elekonich & Wingfield, 2000b). Together, the data available suggests that while P4 seems to be necessary for the expression of maternal aggression, P4 in other contexts can have different effects, and this may be related to the relationship between levels of P and other steroid hormones. During lactation, T and E2 levels are low compared to P4 (Taya & Greenwald, 1982). Interestingly, female California mice showing territorial aggression show the opposite; P4/T ratio is decreased compared to controls (Davis & Marler, 2003a). Further studies are needed to test this hypothesis.

Progesterone: Mechanisms of Action

Similar to estrogens, P4 can exert its effect through genomic and non-genomic mechanisms (Taraborrelli, 2015; Wendler, Albrecht, & Wehling, 2012). Nuclear and membrane PR are widely expressed throughout the brain (Schumacher et al., 2014), and have been related to the regulation of a variety of behaviors, including sexual behavior, social recognition and motivation (Bychowski & Auger, 2012; Frye, Koonce, & Walf, 2013; Frye, Walf, Kohtz, & Zhu, 2014; Yang et al., 2013a). To our knowledge, little work has assessed the relationship between female aggression and PR. Deletion of the PR gene has no effect on intermale aggression, but female aggression was not examined (Schneider et al., 2003). On the other hand, ablation of PR-expressing neurons in VMH reduces territorial aggressive and sexual behavior in male mice but has no effect on territorial aggressive behavior in females (Yang et al., 2013a). This line of mice may not be optimal for assessing the role of PR on aggression in females, as C57Bl6 normally do not engage in significant levels of aggression outside of maternal defense.

Intriguingly, it is possible for P4 to affect behavior independently of PR. P4 can be metabolized in the brain to allopregnanolone (Dong et al., 2001a; Pinna et al., 2008), which is an efficient positive allosteric modulator of GABA_A receptors (Belelli & Lambert, 2005; Herd, Belelli, & Lambert, 2007; Pinna et al., 2000; Puia et al., 2003). The activation of GABA_A by allopregananolone has been mainly associated with an inhibition of aggression. For example, aggression in socially isolation male mice is associated with reduced levels of brain allopregnanolone (Dong et al., 2001a; Pinna, Agis-Balboa, Pibiri, Nelson, Guidotti, & Costa, 2008). Further, administration of fluoxetine, which prevents declines in allopregnanolone, blocks the effects of social isolation on aggression (Pinna et al., 2003). In females, social isolation does not reduce levels of allopregnanolone, and it also does not increase aggression (Pinna, Dong, Matsumoto, Costa, & Guidotti, 2003). Interestingly, long-term administration of T in female mice results in both increased levels of territorial aggression and reduction of allopregnanolone levels (Pinna, Costa, & Guidotti, 2005), which suggests that T actions on aggression could be mediated in part by down regulation of brain allopreganolone biosynthesis.

Glucocorticoids and Aggression

GC production has been involved in both the promotion and inhibition of aggression in humans and animals, and many times this effect is context specific. For example, while acute systemic (Haller, Albert, & Makara, 1997; Mikics, Kruk, & Haller, 2004) and hypothalamic (Hayden-Hixson & Ferris, 1991) injections of GC promote territorial aggression in male rodents, chronic administration of high levels of GC suppresses aggression in a variety of species (Leshner, Korn, Mixon, Rosenthal, & Besser, 1980; Wingfield & Silverin, 1986). Interestingly, GC seems to have a particular important effect on abnormal levels of aggression. Animals selected for high levels of aggression show lower GC responses (Carere, Groothuis, Möstl, Daan, & Koolhaas, 2003; Veenema, Meijer, de Kloet, & Koolhaas, 2003a; Veenema, Meijer, de Kloet, Koolhaas, & Bohus, 2003b), and chronic glucocorticoid deficit induced by adrenalectomy results in escalated aggression in male rats (Haller, Schraaf, & Kruk, 2001). This has also been described in humans. Aggression in habitually violent offenders (Virkkunen, 1985) and patients with antisocial personality disorder (Dolan, Anderson, & Deakin, 2001) is associated with low GC levels. In females, HPA activity has been mainly associated with the reduction of aggression. In female rodents increased corticotrophin releasing hormone (CRH) signaling, as well as administration of dexamethasone, reduces maternal aggression (Gammie, Negron, Newman, & Rhodes, 2004; Gammie, Hasen, Stevenson, Bale, & D’Anna 2005; Vilela & Giusti-Paiva, 2011). Similarly, female crows (Corvus macrorhynchos) show a negative correlation between cortisol metabolites and dominance rank, which is the opposite of what happens in males (Ode, Asaba, Miyazawa, Mogi, Kikusui, & Izawa, 2015). A study in young women found a negative correlation between circulating cortisol and reactive aggression (Stoppelbein, Greening, Luebbe, Fite, & Becker, 2014). The apparent promotion or inhibition of GC on aggressive behavior may be explained by the fact that GC do not directly activate or inhibit aggression circuits, rather, they modulate neurons properties (Reul & de Kloet, 1985). Thus, the activation of specific neural circuits may be the primary variable modulating aggressive response, which in turn can be secondarily modulated by GC signaling.

Glucocprticoids: Mechanisms of Action

The modulating actions of GC are regulated through two types of receptors, the mineralocorticoid receptors (MR) and the GC receptor (GR) (Herman et al., 2016). MR primarily mediates the action of baseline levels of GC, while lower affinity GR are more important for mediating the effects of stress-induced increases of GC (Myers, McKlveen, & Herman, 2012). Both MR and GR can act as transcription factors, or as mediators of rapid non-genomic signaling (Groeneweg, Karst, de Kloet, & Joëls, 2012). MR are expressed in limbic areas including PFC, LS, and hippocampus, where they co-localize with GR. GR are more widely distributed throughout the brain, but also show lower affinity for GC than MR (Reul & de Kloet, 1985). It has been a challenge to directly assess the specific role of these receptors in regulating aggression, especially GR. This is because the field does not have access to a specific GR antagonist. For example, two groups have reported that RU486 treatment reduces territorial aggression in male rainbow trout (Schjolden, Basic, & Winberg, 2009) and electric fish (Dunlap, Jashari, & Pappas, 2011). The problem for interpreting these results is that RU486 is very strong antagonist of PR receptors also, so it is impossible to rule out a role for PR. Transgenic mice overexpressing or underexpressing GR have been produced (Ridder et al., 2005), but aggressive behavior has not be examined. An ideal strategy would be to examine the effect of inducible knockout of GR in brain regions mediating aggression such as the lateral septum. For MR, the specific antagonist spironolactone has been a useful tool. Intriguingly, a single injection of spironolactone reduced offensive behavior in male rats against a naïve intruder, whereas repeated spironolactone treatment enhanced aggression (Ruiz-Aizpurua, Buwalda, & de Boer, 2013). Thus, while it is clear that GCs can modulate aggression, the specific receptors and site of action mediating these effects have not been identified. Moreover, virtually no studies examining GC regulation of aggression have been conducted in females, despite that fact that, in many rodent species, GC levels are significantly higher in females than in males (Critchlow, Liebelt, Bar-Sela, Mountcastle, & Lipscomb, 1963; Trainor, Takahashi, Silva, Crean, & Hostetler, 2010; Weiser & Handa, 2009).