The effects of estradiol on 17β-hydroxysteroid dehydrogenase type IV and androgen receptor expression in the developing zebra finch song system

Early life stress increases testosterone and corticosterone and alters stress physiology in zebra finches

Early life stress has enduring effects on behavior and physiology. However, the effects on hormones and stress physiology remain poorly understood. In the present study, parents of zebra finches of both sexes were exposed to an increased foraging paradigm from 3 to 33days post hatching. Plasma and brains were collected from chicks at 3 developmental time points: post hatching days 25, 60 and adulthood. Plasma was assayed for testosterone (T), estradiol (E2), and corticosterone (CORT). The paraventricular nucleus of the hypothalamus was assessed for corticotrophin releasing factor (CRH) and glucocorticoid receptor (GR) expression. As expected, body mass was lower in nutritionally stressed animals compared to controls at multiple ages. Nutritionally stressed animals overall had higher levels of CORT than did control and this was particularly apparent in females at post hatching day 25. Nutritionally stressed animals also had a higher number of cells expressing CRH and GR in the paraventricular nucleus of the hypothalamus than did controls. There was an interaction, such that both measures were higher in control animals at PHD 25, but higher in NS animals by adulthood. Females, regardless of treatment, had higher circulating CORT and a higher number of cells expressing CRH than did males. Nutritionally stressed animals also had higher levels of T than did control animals, and this difference was greatest for males at post hatching day 60. There were no effects of nutritional stress on E2. These findings suggest that nutritional stress during development has long-lasting effects on testosterone and stress physiology.

Genetic regulation of sex differences in songbirds and lizards

Sex differences in the morphology of neural and peripheral structures related to reproduction often parallel the frequency of particular behaviours displayed by males and females. In a variety of model organisms, these sex differences are organized in development by gonadal steroids, which also act in adulthood to modulate behavioural expression and in some cases to generate parallel anatomical changes on a seasonal basis. Data collected from diverse species, however, suggest that changes in hormone availability are not sufficient to explain sex and seasonal differences in structure and function. This paper pulls together some of this literature from songbirds and lizards and considers the information in the broader context of taking a comparative approach to investigating genetic mechanisms associated with behavioural neuroendocrinology.

Sex differences and rapid estrogen signaling: A look at songbird audition

The actions of estrogens have been associated with brain differentiation and sexual dimorphism in a wide range of vertebrates. Here we consider the actions of brain-derived 'neuroestrogens' in the forebrain and the accompanying differences and similarities observed between males and females in a variety of species. We summarize recent evidence showing that baseline and fluctuating levels of neuroestrogens within the auditory forebrain of male and female zebra finches are largely similar, and that neuroestrogens enhance auditory representations in both sexes. With a comparative perspective we review evidence that non-genomic mechanisms of neuroestrogen actions are sexually differentiated, and we propose a working model for nonclassical estrogen signaling via the MAPK intracellular signaling cascade in the songbird auditory forebrain that is informed by the way sex differences may be compensated. This view may lead to a more comprehensive understanding of how sex influences estradiol-dependent modulation of sensorimotor representations.

Dynamic variation in forebrain estradiol levels during song learning

Estrogens shape brain circuits during development, and the capacity to synthesize estrogens locally has consequences for both sexual differentiation and the acute modulation of circuits during early learning. A recently optimized method to detect and quantify fluctuations in brain estrogens in vivo provides a direct means to explore how brain estrogen production contributes to both differentiation and neuromodulation during development. Here, we use this method to test the hypothesis that neuroestrogens are sexually differentiated as well as dynamically responsive to song tutoring (via passive video/audio playback) during the period of song learning in juvenile zebra finches. Our results show that baseline neuroestradiol levels in the caudal forebrain do not differ between males and females during an early critical masculinization window. Instead, we observe a prominent difference between males and females in baseline neuroestradiol that emerges during the subadult stage as animals approach sexual maturity. Second, we observe that fluctuating neuroestradiol levels during periods of passive song tutoring exhibit a markedly different profile in juveniles as compared to adults. Specifically, neuroestrogens in the caudal forebrain are elevated following (rather than during) tutor song exposure in both juvenile males and females, suggesting an important role for the early consolidation of tutor song memories. These results further reveal a circadian influence on the fluctuations in local neuroestrogens during sensory/cognitive tasks. Taken together, these findings uncover several unexpected features of brain estrogen synthesis in juvenile animals that may have implications for secondary masculinization as well as the consolidation of recent sensory experiences.

Genome-brain-behavior interdependencies as a framework to understand hormone effects on learned behavior

Hormones have profound effects on the maturation and function of the zebra finch song system. Hormones often signal through receptors that directly or indirectly regulate transcription. In this way, hormones and the genome are functionally connected. Genome-brain-behavior interdependencies are often studied on evolutionary timescales but we can now apply and test these relationships on short timescales, relevant to an individual. Here, we begin to place patterns of hormone-related gene expression into the timeframe of an individual's lifespan to consider how hormones contribute to organization of neural systems necessary for learned behavior, and how they might signal during experience in ways that affect future behavior. This framework illustrates both how much investigations into genome and hormone function are intertwined, and how much we still need to learn.

Sex-stratified genome-wide association studies including 270,000 individuals show sexual dimorphism in genetic loci for anthropometric traits

Given the anthropometric differences between men and women and previous evidence of sex-difference in genetic effects, we conducted a genome-wide search for sexually dimorphic associations with height, weight, body mass index, waist circumference, hip circumference, and waist-to-hip-ratio (133,723 individuals) and took forward 348 SNPs into follow-up (additional 137,052 individuals) in a total of 94 studies. Seven loci displayed significant sex-difference (FDR<5%), including four previously established (near GRB14/COBLL1, LYPLAL1/SLC30A10, VEGFA, ADAMTS9) and three novel anthropometric trait loci (near MAP3K1, HSD17B4, PPARG), all of which were genome-wide significant in women (P<5×10(-8)), but not in men. Sex-differences were apparent only for waist phenotypes, not for height, weight, BMI, or hip circumference. Moreover, we found no evidence for genetic effects with opposite directions in men versus women. The PPARG locus is of specific interest due to its role in diabetes genetics and therapy. Our results demonstrate the value of sex-specific GWAS to unravel the sexually dimorphic genetic underpinning of complex traits.

17β-Hydroxysteroid dehydrogenase type IV, a Z-linked gene, is higher in females than in males in visual and auditory regions of developing zebra finches

One of the most important decisions in a monogamous animal's life is the choice of a partner (partner preference), but the process by which this occurs remains poorly understood. The present study tests the hypothesis that hormones and genes play a role in sexual differentiation of partner preferences, as in the song system. We focused on a Z-linked gene, 17β-hydroxysteroid dehydrogenase type IV (HSD17B4), coding for a steroidogenic enzyme that converts estradiol (E2) into an inactive metabolite. HSD17B4 mRNA is expressed more in the song regions of males compared to females throughout development, suggesting that regulation of E2 is important for male-typical song development. Here, we focused on four regions associated with sexual partner preferences. Females had significantly higher levels of HSD17B4 mRNA in auditory (caudomedial nidopallium) and visual (hyperpallium apicale) regions than did males at day 25. HSD17B4 was expressed in the hippocampus and caudolateral nidopallium, but there were no sex differences. In a second experiment, animals of both sexes were treated with E2 and HSD17B4 and androgen receptor (AR) mRNA were measured, since masculinization of the song system is, in part, accomplished by AR. AR was low across the four regions and was not sexually differentiated. E2 treatments increased HSD17B4 mRNA in the auditory region of males, which is contrary to findings in the song system. Our research suggests that different behaviors may be guided by the same genes and hormones, but that the exact nature of the gene-hormone relationships may differ according to brain region and behavior.