Widespread capacity for steroid synthesis in the avian brain and song system

Combined effects of DHEA and fadrozole on aggression and neural VIP immunoreactivity in the non-breeding male song sparrow

The male Song Sparrow, Melospiza melodia morphna, shows high levels of aggression in its non-breeding season, concomitant with basal levels of circulating testosterone (T) and estradiol (E(2)). However, administration of fadrozole, an aromatase inhibitor, decreases non-breeding aggression in the field. Circulating levels of dehydroepiandrosterone (DHEA), an androgen/estrogen precursor, correspond to the seasonal expression of aggression in this species, with high levels in the breeding and non-breeding seasons when aggression is also high, and lower levels during the molt when aggression is low. We test the hypothesis that circulating DHEA up-regulates non-breeding aggression via an aromatase-mediated mechanism. We also hypothesize that this up-regulation of aggression is partially mediated by changes in vasoactive intestinal polypeptide (VIP) in the lateral extent of the bed nucleus of the stria terminalis (BSTl) and lateral septum (LS). Birds were administered either DHEA, fadrozole, or both for 2 weeks and tested for aggression in a lab-based paradigm. As predicted, birds given DHEA were significantly more aggressive. However, fadrozole did not block this effect, and, when administered without DHEA, also led to increased aggression over controls. These results may be explained by the fact that the behaviors measured in field tests, which include more direct attack behaviors, may be under different hormonal regulation than the behaviors measured in the lab paradigm, which represent warning, or threat, behaviors. VIP immunoreactivity (VIP-ir) changed across multiple brain regions with this treatment regimen, most notably in the LSO/VFI subdivision of the lateral septum.

Rapid estrogen regulation of DHEA metabolism in the male and female songbird brain

In the songbird brain, dehydroepiandrosterone (DHEA) is metabolized to the active and aromatizable androgen androstenedione (AE) by 3beta-hydroxysteroid dehydrogenase/Delta5-Delta4 isomerase (3beta-HSD). Thus, brain 3beta-HSD plays a key role in regulating the steroidal milieu of the nervous system. Previous studies have shown that stress rapidly regulates brain 3beta-HSD activity in a sex-specific manner. To elucidate endocrine regulation of brain 3beta-HSD, we asked whether 17beta-estradiol (E(2)) regulates DHEA metabolism in adult zebra finch (Taeniopygia guttata) and whether there are sex-specific effects. Brain tissue was homogenized and centrifuged to obtain supernatant lacking whole cells and cell nuclei. Supernatant was incubated with [(3)H]DHEA and radioinert E(2)in vitro. Within only 10 min, E(2) significantly reduced 3beta-HSD activity in both male and female brain. Interestingly, the rapid effects of E(2) were more pronounced in females than males. These are the first data to show a rapid effect of estrogens on the songbird brain and suggest that rapid estrogen effects differ between male and female brains.

3beta-HSD activates DHEA in the songbird brain

Dehydroepiandrosterone (DHEA) is an abundant circulating prohormone in humans, with a variety of reported actions on central and peripheral tissues. Despite its abundance, the functions of DHEA are relatively unknown because common animal models (laboratory rats and mice) have very low DHEA levels in the blood. Over the past decade, we have obtained considerable evidence from avian studies demonstrating that (1) DHEA is an important circulating prohormone in songbirds and (2) the enzyme 3beta-hydroxysteroid dehydrogenase/isomerase (3beta-HSD), responsible for converting DHEA into a more active androgen, is expressed at high levels in the songbird brain. Here, we first review biochemical and molecular studies demonstrating the widespread activity and expression of 3beta-HSD in the adult and developing songbird brain. Studies examining neural 3beta-HSD activity show effects of sex, stress, and season that are region-specific. Second, we review studies showing seasonal and stress-related changes in circulating DHEA in captive and wild songbird species. Third, we describe evidence that DHEA treatment can stimulate song behavior and the growth of neural circuits controlling song behavior. Importantly, brain 3beta-HSD and aromatase can work in concert to locally metabolize DHEA into active androgens and estrogens, which are critical for controlling behavior and robust adult neuroplasticity in songbirds. DHEA is likely secreted by the avian gonads and/or adrenals, as is the case in humans, but DHEA may also be synthesized de novo in the songbird brain from cholesterol or other precursors. Irrespective of its source, DHEA seems to be an important prohormone in songbirds, and 3beta-HSD is a key enzyme in the songbird brain.

Sexual Differentiation of the Vocal Control System of Birds

Birds evolved neural circuits of various complexities in relation to their capacity to produce learned or unlearned vocalizations. These vocalizations, in particular those that function in the realm of reproduction, are frequently sexually dimorphic, both in vocal learners (songbirds, parrots, some hummingbirds) and vocal nonlearners (all other birds). In many cases, the development and/or the adult differentiation of vocalizations of sociosexual function is sensitive to sex hormones, androgens and estrogens. The underlying mechanisms have been studied in detail in songbirds, a bird group that comprises about half of all bird species. Next to unlearned calls, songbirds produce learned songs that require forebrain vocal control areas that express receptors for androgens and estrogens. These forebrain vocal areas are sexually dimorphic in many species, but a clear relation between the degree of "brain sex" and sex differences in vocal pattern is lacking, except that a minimum number of vocal neurons is necessary to sing learned songs. Genetic brain-intrinsic mechanisms are likely to determine the neuron pools that develop into forebrain song control areas. Subsequently, gonadal steroid hormones, androgens and estrogens, modulate the fate of these neurons and thus the functionality of the vocal control systems. Further action of gonadal hormones, and may be other factors signaling the sociosexual and physical environment, affect the phenotype of vocal control areas in adulthood. Despite the clear evidence of hormone dependency of both adult vocalizations and phenotypes of vocal neuron pools, their causal relation is little understood.

Aromatase expression and cell proliferation following injury of the adult zebra finch hippocampus

Estrogens can be neuroprotective following traumatic brain injury. Immediately after trauma to the zebra finch hippocampus, the estrogen-synthetic enzyme aromatase is rapidly upregulated in astrocytes and radial glia around the lesion site. Brain injury also induces high levels of cell proliferation. Estrogens promote neuronal differentiation, migration, and survival naturally in the avian brain. We suspect that glia are a source of estrogens promoting cell proliferation after neural injury. To explore this hypothesis, we examined the spatial and temporal relationship between glial aromatase expression and cell proliferation after neural injury in adult female zebra finches. Birds were ovariectomized and given a blank implant or one filled with estradiol; some birds were also administered an aromatase inhibitor or vehicle. All birds received penetrating injuries to the right hippocampus. Twenty-four hours after lesioning, birds were injected once with BrdU to label mitotically active cells and euthanized 2 h, 24 h, or 7 days later. The brains were processed for double-label BrdU and aromatase immunocytochemistry. Injury-induced glial aromatase expression was unaffected by survival time and aromatase inhibition. BrdU labeling was significantly reduced at 24 h by ovariectomy and by aromatase inhibition; effects were partially reversed by E2 replacement. Irrespective of ovariectomy, the densities of aromatase immunoreactive astrocytes and BrdU-labeled cells at known distances from the lesion site were highly correlated. These data suggest that injury-induced glial aromatization may influence the reorganization of injured tissue by providing a rich estrogenic environment available to influence cellular incorporation.

Steroidogenic enzymes along the ventricular proliferative zone in the developing songbird brain

Neural development requires regulation and coordination of the differentiation, migration, and survival of newly divided cells, most of which derive from the region surrounding the lateral ventricles. While many factors are involved in these maturational processes, studies of cell proliferation and neurogenesis in songbirds indicate that sex steroids may provide crucial cues to newly divided cells and may be fundamental to the organization of a specific neural circuit, the song system. In the case of the zebra finch, steroids that impact song system masculinization are most likely not synthesized from the gonads but from the brain, and evidence is mounting that both developing and adult zebra finches have the capacity for neurosteroidogenesis. Therefore, we hypothesized that during early development, all of the genes required for de novo sex steroid synthesis would be expressed in regions that would indicate a role for neurosteroids in neural organization. We found that the genes necessary for de novo neurosteroid synthesis at posthatch day 1 (P1) and P5 show a broad expression distribution. Most strikingly, the spatial distribution of expression for all of the genes necessary for androgen synthesis is similar to the previously described pattern of proliferating neuronal precursors along the lateral border of the lateral ventricle. Due to the increasing evidence for neurosteroid action on multiple cell traits, it may be that locally synthesized neurosteroids impact cells along the proliferative zone to influence early events in neural development generally and song system masculinization specifically.