Differences in forebrain androgen receptor expression in winners and losers of male anole aggressive interactions

Testing hormonal responses to real and simulated social challenges in a competitive female bird

Competitive interactions often occur in series; therefore animals may respond to social challenges in ways that prepare them for success in future conflict. Changes in the production of the steroid hormone testosterone (T) are thought to mediate phenotypic responses to competition, but research over the past few decades has yielded mixed results, leading to several potential explanations as to why T does not always elevate following a social challenge. Here, we measured T levels in tree swallows (Tachycineta bicolor), a system in which females compete for limited nesting cavities and female aggression is at least partially mediated by T. We experimentally induced social challenges in two ways: (1) using decoys to simulate territorial intrusions and (2) removing subsets of nesting cavities to increase competition among displaced and territory-holding females. Critically, these experiments occurred pre-laying, when females are physiologically capable of rapidly increasing circulating T levels. However, despite marked aggression in both experiments, T did not elevate following real or simulated social challenges, and in some cases, socially challenged females had lower T levels than controls. Likewise, the degree of aggression was negatively correlated with T levels following a simulated territorial intrusion. Though not in line with the idea that social challenges prompt T elevation in preparation for future challenges, these patterns nevertheless connect T to territorial aggression in females. Coupled with past work showing that T promotes aggression, these results suggest that T may act rapidly to allow animals to adaptively respond to the urgent demands of a competitive event.

Social boldness correlates with brain gene expression in male green anoles

Within populations, some individuals tend to exhibit a bold or shy social behavior phenotype relative to the mean. The neural underpinnings of these differing phenotypes - also described as syndromes, personalities, and coping styles - is an area of ongoing investigation. Although a social decision-making network has been described across vertebrate taxa, most studies examining activity within this network do so in relation to exhibited differences in behavioral expression. Our study instead focuses on constitutive gene expression in bold and shy individuals by isolating baseline gene expression profiles that influence social boldness predisposition, rather than those reflecting the results of social interaction and behavioral execution. We performed this study on male green anole lizards (Anolis carolinensis), an established model organism for behavioral research, which provides a crucial comparison group to investigations of birds and mammals. After identifying subjects as bold or shy through repeated reproductive and agonistic behavior testing, we used RNA sequencing to compare gene expression profiles between these groups within various forebrain, midbrain, and hindbrain regions. The ventromedial hypothalamus had the largest group differences in gene expression, with bold males having increased expression of neuroendocrine and neurotransmitter receptor and calcium channel genes compared to shy males. Conversely, shy males express more integrin alpha-10 in the majority of examined regions. There were no significant group differences in physiology or hormone levels. Our results highlight the ventromedial hypothalamus as an important center of behavioral differences across individuals and provide novel candidates for investigations into the regulation of individual variation in social behavior phenotype.

Rapid Effects of an Aggressive Interaction on Dehydroepiandrosterone, Testosterone and Oestradiol Levels in the Male Song Sparrow Brain: a Seasonal Comparison

Across vertebrates, aggression is robustly expressed during the breeding season when circulating testosterone is elevated, and testosterone activates aggression either directly or after aromatisation into 17β-oestradiol (E2 ) in the brain. In some species, such as the song sparrow, aggressive behaviour is also expressed at high levels during the nonbreeding season, when circulating testosterone is non-detectable. At this time, the androgen precursor dehydroepiandrosterone (DHEA) is metabolised within the brain into testosterone and/or E2 to promote aggression. In the present study, we used captive male song sparrows to test the hypothesis that an acute agonistic interaction during the nonbreeding season, but not during the breeding season, would alter steroid levels in the brain. Nonbreeding and breeding subjects were exposed to either a laboratory simulated territorial intrusion (L-STI) or an empty cage for only 5 min. Immediately afterwards, the brain was rapidly collected and flash frozen. The Palkovits punch technique was used to microdissect specific brain regions implicated in aggressive behaviour. Solid phase extraction followed by radioimmunoassay was used to quantify DHEA, testosterone and E2 in punches. Overall, levels of DHEA, testosterone and E2 were higher in brain tissue than in plasma. Local testosterone and E2 levels in the preoptic area, anterior hypothalamus and nucleus taeniae of the amygdala were significantly higher in the breeding season than the nonbreeding season and were not affected by the L-STI. Unexpectedly, subjects that were dominant in the L-STI had lower levels of DHEA in the anterior hypothalamus and medial striatum in both seasons and lower levels of DHEA in the nucleus taeniae of the amygdala in the breeding season only. Taken together, these data suggest that local levels of DHEA in the brain are very rapidly modulated by social interactions in a context and region-specific pattern.

Steroid Receptors and Aromatase Gene Expression in Different Brain Areas of Copulating and Sexually Sluggish Male Rats

INTRODUCTION

Sexually sluggish (SS) males have been identified in several species of mammals including rats. These animals take more than 30 minutes to ejaculate; they do not ejaculate or do so inconsistently despite being tested repeatedly with sexually receptive females. Different brain areas and hormones play an important role in the control of male sexual behavior.

AIMS

Determine gene expression for the androgen receptor (AR), the estrogen receptor alpha (ERα), the progesterone receptor (PR), and the aromatase enzyme (ARO), in brain regions important in the control of male sexual behavior including the medial preoptic area (MPOA), the amygdala (AMG), the olfactory bulb (OB), and, as a control, the cortex (CTX) of copulating (C) and SS male rats.

METHODS

Males that ejaculated within 30 minutes in three tests with receptive females were included in the C group, while those males that ejaculated in one or none of the four tests were included in the SS group. RNA was isolated 1 week after the last test of sexual behavior, and cDNA was synthesized from the brain areas listed above.

MAIN OUTCOMES MEASURES

Expression of the AR, ERα, PR, and ARO genes was determined by quantitative polymerase chain reaction (qPCR). Cyclophilin A (CycA) and tyrosine 3-monooxygenase-tryptophan activation protein zeta (Ywhaz) were housekeeping genes used to determine relative gene expression with the 2(-ΔΔCt) method.

RESULTS

The expression of mRNA for AR and ARO increased in the MPOA of SS males. ARO mRNA was increased in the AMG of SS males. In the OB, ERα mRNA was increased and AR mRNA reduced in SS males.

CONCLUSION

These results indicate SS and C males show differences in gene expression within brain regions controlling sexual behavior.