Sexual differentiation of brain and behavior in quail and zebra finches: studies with a new aromatase inhibitor, R76713

How inversion variants can shape neural circuitry: Insights from the three-morph mating tactics of ruffs

Behavior polymorphisms underlying alternative mating tactics can evolve due to genetic inversions, especially when inversions capture sets of genes involved in hormonal regulation. In the three-morph system of the ruff (Calidris pugnax), two alternative morphs (Satellites and Faeders) with distinct behaviors and low circulating testosterone are genetically determined by an inverted region on an autosomal chromosome. Here, we discuss recent findings on the ruff and present novel insights into how an inversion that poses drastic constraints on testosterone production might lead to morph-specific differences in brain areas that regulate social behavior. A gene responsible for converting testosterone to androstenedione (HSD17B2) is located inside the inverted region and is a promising candidate. We identify a single missense mutation in the HSD17B2 gene of inverted alleles that is responsible for a 350–500% increase in testosterone to androstenedione conversion, when mutated in the human HSD17B2 protein. We discuss new evidence of morph differences in neural HSD17B2 expression in embryos and circulating androgens in sexually-immature juveniles. We suggest processes that shape morph differences in behavior likely begin early in ontogeny. We propose that the organization of behaviorally relevant neuron cell types that are canonically sexually dimorphic, such as subpopulations of aromatase and vasotocin neurons, should be particularly affected due to the life-long condition of low circulating testosterone in inversion morphs. We further emphasize how HSD17B2 catalytic activity extends beyond androgens, and includes estradiol oxidation into estrone and progesterone synthesis. Lastly, we underscore dimerization of HSD17B2 as an additional layer of complexity that merits consideration.

Non-reproductive Functions of Aromatase in the Central Nervous System Under Physiological and Pathological Conditions

The modulation of brain function and behavior by steroid hormones was classically associated with their secretion by peripheral endocrine glands. The discovery that the brain expresses the enzyme aromatase, which produces estradiol from testosterone, expanded this traditional concept. One of the best-studied roles of brain estradiol synthesis is the control of reproductive behavior. In addition, there is increasing evidence that estradiol from neural origin is also involved in a variety of non-reproductive functions. These include the regulation of neurogenesis, neuronal development, synaptic transmission, and plasticity in brain regions not directly related with the control of reproduction. Central aromatase is also involved in the modulation of cognition, mood, and non-reproductive behaviors. Furthermore, under pathological conditions aromatase is upregulated in the central nervous system. This upregulation represents a neuroprotective and likely also a reparative response by increasing local estradiol levels in order to maintain the homeostasis of the neural tissue. In this paper, we review the non-reproductive functions of neural aromatase and neural-derived estradiol under physiological and pathological conditions. We also consider the existence of sex differences in the role of the enzyme in both contexts.

Radial glia express aromatase in the injured zebra finch brain

Estrogens have neurotrophic and neuroprotective properties. The synthesis of estrogen occurs via the expression of aromatase. Previous studies have shown that injury to the vertebrate brain results in a rapid and dramatic up-regulation of aromatase expression in astrocytes around the lesion. As part of experiments examining injury-induced glial aromatization, we identified aromatase in radial glia of the zebra finch brain. Adult female zebra finches received a penetrating injury to the right hippocampus. Twenty-four hours after lesioning, birds were administered bromodeoxyuridine (BrdU) and sacrificed 2 hours, 1 day, or 7 days later. We determined the distribution of aromatase and BrdU labeling by using immunocytochemistry. Radial aromatase was localized to cells lining the lateral ventricle adjacent to the lesioned hippocampus. Injury also induced a dramatic accumulation of newly generated cells labeled with BrdU around the lesion. BrdU labeling was strongly associated with aromatase-positive radial fibers, suggesting the migration of newly generated cells along these fibers. In the songbird brain, estrogen supports neuronal recruitment and promotes the survival and addition of new neurons. The presence of aromatase in radial glia provides a mechanism of estrogen delivery to postmitotic cells. Radial aromatization may be a key feature in the repair of the vertebrate brain following neural injury.

Distribution of aromatase activity in brain and peripheral tissues of male sheep: effect of nutrition

Conversion of testosterone to oestradiol plays a major role in the feedback inhibition of gonadotrophin secretion in male sheep but little is known of the distribution or control of aromatase activity among central and peripheral tissues. Changes in activity at those sites may mediate alterations in the effectiveness of negative feedback following, for example, a change in nutrition. Using a tritiated-water assay, we quantified aromatase in several tissues in mature male sheep, assessed their contribution to oestradiol production, and tested whether activity at each site was affected by a nutritional treatment that stimulates gonadotrophin secretion. Among the brain tissues, the preoptic area had the highest concentration of activity, followed by the hypothalamus, amygdala and cortex. Among the peripheral tissues, liver and testis had the highest activity and, due to their mass, they are the major sources of circulating oestradiol. Pituitary, muscle, kidney and adipose tissues had very low aromatase levels. The nutritional stimulus increased activity in testis but not in liver or brain. We conclude that changes in aromatase activity do not mediate the effects of nutrition on steroid feedback, but aromatisation in testis, liver and brain is important in the endocrine regulation of reproduction in the mature ram.

Expression of androgen receptor mRNA in the late embryonic and early posthatch zebra finch brain

Zebra finch males sing and females do not, and the underlying neural circuitry in males is more developed than that in females. Sex steroid hormones influence the development of sex differences in this circuitry, including differences in androgen receptor (AR) expression, although the role of androgens has been controversial. We isolated a cDNA encoding a portion of the zebra finch AR and used in situ hybridization to examine the spatiotemporal pattern of AR mRNA expression in the brain during late embryonic development and at hatching. We detected AR mRNA in all the major subdivisions of the brain as early as embryonic day 10. No qualitative sex differences in AR mRNA expression patterns were observed. Cells lining the ventral arm of the lateral telencephalic ventricles expressed AR mRNA on embryonic day 11 and posthatching day 1, as did cells lining the third ventricle at all three developmental stages examined, suggesting that androgens may play a role in early stages of cellular proliferation, migration, or differentiation. AR mRNA was also detected in the hippocampus, neostriatum, septum, ventromedial archistriatum, hypothalamic regions, dorsal mesencephalon, and in and around the brainstem nucleus tracheosyringealis. Our results suggested that androgens act early in neural development and therefore may contribute to the process of sexual differentiation.

Effects of embryonic treatment with fadrozole on phenotype of gonads, syrinx, and neural song system in zebra finches

Previous studies have found that treatment of zebra finch embryos with an aromatase inhibitor on Day 5 or 8 of incubation caused partial sex reversal of gonadal phenotype in females. These females possessed both testicular and ovarian tissue, and the development of the neural circuit for song remained feminine. The present study attempted more complete gonadal reversal by treating zebra finch embryos earlier, on Day 3 of incubation, with Fadrozole (CGS 16949A), an aromatase inhibitor, or with saline. We examined the phenotype of the syrinx (androgen-dependent vocal organ), the gonads, and the telencephallic neural song system in 100-day-old birds. Treated females typically possessed a left ovotestis and a right testis, and significantly larger syringes than control females. The histology and steroid synthetic enzyme activity of the testicular tissue in treated females were quite masculine and similar to that of control males. At the time of sacrifice, the plasma concentrations of testosterone and estradiol for fadrozole-treated females did not differ from those of control females, but dihydrotestosterone was lower in treated females. Despite the large amount of functional testicular tissue and a masculine syrinx, the volumes and soma sizes of song system nuclei (HVC, RA) in treated females remained feminine. These results suggest that testicular secretions masculinize the syrinx, but are not sufficient to masculinize the song system in zebra finches.

Experimental analysis of sexual differentiation of the zebra finch brain

Classical theories of sexual differentiation of brain and behavior hold that sex differences in the brain arise because of the action of gonadal steroid hormones. In mammals, testosterone secretion by the testes stimulates a masculine pattern of neural differentiation, whereas feminine patterns of development occur in the absence of testicular secretions. In some bird species, estrogen secreted by the ovary is thought to trigger feminine patterns of neural development, whereas masculine development occurs in the absence of ovaries. Sexual differentiation of the neural circuit for song in zebra finches is not easily explained by these theories. Although female zebra finches can be masculinized by treatments with estrogen, it has proven difficult to prevent masculine neural development in genetic males by treating them with inhibitors of estrogen synthesis. Moreover, when genetic female embryos are treated with inhibitors of estrogen synthesis, they develop significant amounts of testicular tissue that causes little or no masculinization of the song system. Thus, testicular secretions alone appear to be insufficient to cause masculine neural differentiation, and other factors need to be invoked. These factors may include ovarian secretions that inhibit masculine development, or direct genetic (nonhormonal) effects on neural differentiation.

Effects of embryonic estrogen on differentiation of the gonads and secondary sexual characteristics of male zebra finches

Male zebra finches sing to court females, whereas females do not normally sing. In parallel, the telencephalic brain regions that control song are larger in volume and contain larger cells in males than in females. The vocal control organ (syrinx) is also larger in males. Some evidence suggests that the sexual differentiation of both anatomy and behavior is under the regulation of gonadal hormones during early development, yet recent data conflict with the idea that the sole source of masculinization of the neural song system is the testes. In the present experiment, we treated genetic males with estradiol benzoate on embryonic day 5 and measured the volume of and neuron soma size in robust nucleus of the archistriatum (RA) and the high vocal center (HVC), two telencephalic song control nuclei. We also weighed the syrinx, the muscles of which are the target of the motor pathway containing the two brain regions. The estrogen treatment disrupted testicular morphology, and induced an oviduct in six of seven animals, but it had no effect on any of four measures of masculinization of the neural song system. These results suggest that normal testicular tissue is not required for masculine development of the neural song system.