Local intracerebral implants of estrogen masculinize some aspects of the zebra finch song system

The song system of zebra finches is sexually dimorphic: the volumes of the song control nuclei and the neurons within these nuclei are larger in males. The song system of hatching female zebra finches is masculinized by systemic treatment with estrogen. We investigated the locus of this estrogen action by using microimplants of estradiol benzoate (EB). We implanted female zebra finch nestlings 10–13 days old with Silastic pellets containing approximately 2 μg EB at one of several sites: near the higher vocal center (HVC), in the brain distant from HVC, or in the periphery either under the skin of the breast or in the peritoneal cavity. Controls were either unimplanted or implanted near HVC with Silastic pellets without hormone. The brains were fixed by perfusion at 60 days, and the volumes of the song control regions as well as the sizes of individual neurons were measured. Neurons in HVC were lerger (more masculine) in the HVC-implanted group than in other groups, which did not differ among themselves. The size of neurons in the robust nucleus of the archistriatum (RA) and the lateral magnocellular nucleus ofthe neostriatum (lMAN) were inversely correlated with the distance of the EB pellet to HVC; neurons in RA and lMAN were larger when the EB pellets were closer to HVC. This result suggests that implants near HVC were at or near a site of estrogen action. To our knowledge, this is the first demonstration that localized brain implants of estrogen cause morphological masculinization in any species. 1994 John Wiley & Sons, Inc.     

Lack of a synergistic effect between estradiol and dihydrotestosterone in the masculinization of the zebra finch song system

Previous studies have suggested that both major active metabolites of testosterone, estradiol (E₂) and dihydrotestosterone (DHT), are needed for complete masculinization of the brain regions that control song in passerine birds. However, DHT treatment of hatchling female zebra finches has only small masculinizing effects on the song system. To assess whether E₂ and DHT have a synergistic effect on the masculinization of the zebra finch song system, female zebra finches were given Silastic implants of E₂ on the day of hatching (day 1) either without any additional hormone treatment or in combination with DHT on days 1, 14, or 70. At 105 to 110 days of age, we measured the volumes of Area X, higher vocal center (HVC), robust nucleus of the archistriatum (RA), soma sizes in HVC, RA, and the lateral magnocellular nucleus of the neostriatum (lMAN), and neuron density and number in RA. E₂ masculinized all of the measures in the song system with the exception of the number of neurons in RA. DHT did not synergize with E₂ to produce any additional masculinization of the attributes measured. These data demonstrate that the combination of E₂ and DHT did not result in the complete masculinization of the song control nuclei and argue against the importance of androgen in sexual differentiation of the song system. © 1995 John Wiley & Sons, Inc.     

A direct comparison of the masculinizing effects of testosterone,androstenedione, estrogen,and progesterone on the development of the zebra finch song system

To assess which hormones are capable of masculinizing the neural song system of zebra finch hatchlings, we implanted female hatchlings with estrogen (estradiol [E2], 75 μg, n = 9), testosterone (T, 75–88 μg, n = 13), androstenedione (AE, 75 μg, n = 7), progesterone (P, 117 μg, n = 10), or nothing (Blanks, n = 10) and compared these to unimplanted males (n = 7). Implants, consisting of a hormone and Silastic mixture encased in polyethylene tubing, were placed under the skin of the breast on the day of hatching. Birds were killed when they were subadult (58 to 68 days old). We measured volumes of area X, the higher vocal center (HVC), and the robust nucleus of the archistriatum (RA); measured soma sizes in the lateral magnocellular nucleus of the neostriatum (IMAN), HVC, and RA: and counted RA neurons. E2 masculinized all measures in the song system and nearly sex-reversed the size of RA neurons. T masculinized volumes of nuclei and soma sizes but not the number or spacing of RA neurons. E2 was always at least as effective as T in masculinizing measures of the song system and was usually more effective. AE and P did not significantly masculinize any measure. These data suggest that E2 is more potent than aromatizable androgens or P in masculinizing the female song system in development and that the action of E2 alone may be sufficient to masculinize the volume of song control nuclei and the size and number of neurons. © 1995 John Wiley & Sons, Inc.     

Tamoxifen's effects on the zebra finch song system are estrogenic,not antiestrogenic

Antiestrogens fail to block the masculine ontogeny of the zebra finch song system that is hypothesized to occur as a result of early estrogen action. Moreover, they hypermasculinize the male, and masculinize the female song systems. In experiment 1, we assessed whether these antiestrogenic effects might mimic estrogenic actions. Zebra finch chicks received one of two treatments. They were given estradiol benzoate (EB) or vehicle daily for the first 20 days after hatching and sacrificed at 60 days of age, or they received EB or vehicle for the first 25 days after hatching, at which time they were sacrificed. In the day 60 group, certain attributes of the song system were hypermasculinized in males and masculinized in females by EB, when compared with controls. In the day 25 group, males treated with EB were partially demasculinized, while the females were partially masculinized. In experiment 2, we assessed whether simultaneous treatment with tamoxifen was capable of antagonizing the effects of EB obtained in experiment 1 (day 60 group). Sixty-day-old females, previously treated with both EB and tamoxifen for the first 20 days after hatching, had more masculine song regions than females treated with either EB alone or tamoxifen alone. In males, the effects of the combined treatment of EB and tamoxifen over those produced by tamoxifen alone were not as dramatic as in the female. These results are similar to those obtained in systems where tamoxifen is purely estrogenic and suggest that in the song system, tamoxifen acts as an estrogen, not an antiestrogen.     

Muscle‐dependent and hormone‐dependent differentiation of the vocal control premotor nucleus robustus archistriatalis and the motornucleus hypoglossus pars tracheosyringealis of the zebra finch

Sex differences in the vertebrate brain (brain sex) are thought to develop owing to the tissue specific action of gonadal hormones similar to the development of secundary sex characteristics of the body. Small sex differences in body anatomy could, however, retrogradely control the sexual differentiation of the central nervous system. This possibility has so far been verified only for motorneuron pools, since the connectivity of sex‐specific higher brain areas to the sexual dimorphic periphery is frequently not well known. Here, we tested whether somatic sex differences feed back on higher brain areas by bilateral denervation of the syringeal musculature of zebra finches before, during, and after onset of estrogen‐sensitive sexual differentiation of forebrain vocal nuclei such as RA (nucleus robustus archistriatalis). In the zebra finch, the sound‐producing musculature (the syrinx), the syrinx motornucleus hypolossus pars tracheosyringealis (nXIIts), and the RA are much larger in males compared to females. Tract tracing studies revealed that the volume and neuron size distribution of the nXIIts was sexually dimorphic in intact but not in animals denervated as juveniles. In contrast, the volume of RA and size of RA neurons of denervated animals were highly sexually dimorphic. Furthermore, estrogen masculinized the RA of denervated females. Thus, sexual differentiation of the RA but not of the nXIIts appears independent of somatic sex differences. The syrinx muscles are, however, important for the soma size of those RA neurons that project to the nXIIts. © 2000 John Wiley & Sons, Inc. J Neurobiol 42: 220–231, 2000     

Social context affects testosterone-induced singing and the volume of song control nuclei in male canaries (Serinus canaria)

The contribution of social factors to seasonal plasticity in singing behavior and forebrain nuclei controlling song, and their interplay with gonadal steroid hormones are still poorly understood. In many songbird species, testosterone (T) enhances singing behavior but elevated plasma T concentrations are not absolutely required for singing to occur. Singing is generally produced either to defend a territory or to attract a mate and it is therefore not surprising that singing rate can be influenced by the sex and behavior of the social partner. We investigated, based on two independent experiments, the effect of the presence of a male or female partner on the rate of song produced by male canaries. In the first experiment, song rate was measured in dyads composed of one male and one female (M-F) or two males (M-M). Birds were implanted with T-filled Silastic capsules or with empty capsules as control. The number of complete song bouts produced by all males was recorded during 240 min on week 1, 2, 4, and 8 after implantation. On the day following each recording session, brains from approximately one-fourth of the birds were collected and the volumes of the song control nuclei HVC and RA were measured. T increased the singing rate and volume of HVC and RA but these effects were affected by the social context. Singing rates were higher in the M-M than in the M-F dyads. Also, in the M-M dyads a dominance-subordination relationship soon became established and dominant males sang at higher rates than subordinates in T-treated but not in control pairs. The differences in song production were not reflected in the size of the song control nuclei: HVC was larger in M-F than in M-M males and within the M-M dyads, no difference in HVC or RA size could be detected between dominant and subordinate males. At the individual level, the song rate with was positively correlated with RA and to a lower degree HVC volume, but this relationship was observed only in M-M dyads, specifically in dominant males. A second experiment, carried out with castrated males that were all treated with T and exposed either to another T-treated castrate or to an estradiol-implanted female, confirmed that song rate was higher in the M-M than in the M-F condition and that HVC volume was larger in heterosexual than in same-sex dyads. The effects of T on singing rate and on the volume of the song control nuclei are thus modulated by the social environment, including the presence/absence of a potential mate and dominance status among males.     

白腰文鸟发声行为的神经发育

本文研究了 5～ 15 0日龄雄性白腰文鸟 (Lonchurastriataswinhoei)不同年龄段的声谱变化以及这种变化的神经调制机制。结果如下 :(1)HVC、RA和AreaX三个发声核团的神经联系基本接近成年鸟的水平后 ,幼鸟才开始学习鸣叫 (约 45日龄 ) ;(2 )HVC、RA和AreaX达到成年核团体积时 (约 80日龄 ) ,幼鸟才具有成年雄鸟的鸣叫模式 ;(3)发声控制核团的发育与核团间的神经支配有关 ,而基本不受鸣唱行为的影响 ,HVC、RA和AreaX的最快增长时间段各不相同 ,三个核团随年龄增长而呈现体积增长的显著变化 (one wayANOVA ,P <0 0 5 ) ,但各核团在任意两个时间段的体积差异并不都显著。结果提示 :发声行为产生的时间和发展与发声控制核团的发育、核团间的神经联系有关 ,最终的体积发育程度受内在遗传力的作用 ,同时可能还受神经核团建立正常神经联系时间的影响     

Social context affects testosterone‐induced singing and the volume of song control nuclei in male canaries (Serinus canaria)

The contribution of social factors to seasonal plasticity in singing behavior and forebrain nuclei controlling song, and their interplay with gonadal steroid hormones are still poorly understood. In many songbird species, testosterone (T) enhances singing behavior but elevated plasma T concentrations are not absolutely required for singing to occur. Singing is generally produced either to defend a territory or to attract a mate and it is therefore not surprising that singing rate can be influenced by the sex and behavior of the social partner. We investigated, based on two independent experiments, the effect of the presence of a male or female partner on the rate of song produced by male canaries. In the first experiment, song rate was measured in dyads composed of one male and one female (M‐F) or two males (M‐M). Birds were implanted with T‐filled Silastic capsules or with empty capsules as control. The number of complete song bouts produced by all males was recorded during 240 min on week 1, 2, 4, and 8 after implantation. On the day following each recording session, brains from approximately one‐fourth of the birds were collected and the volumes of the song control nuclei HVC and RA were measured. T increased the singing rate and volume of HVC and RA but these effects were affected by the social context. Singing rates were higher in the M‐M than in the M‐F dyads. Also, in the M‐M dyads a dominance‐subordination relationship soon became established and dominant males sang at higher rates than subordinates in T‐treated but not in control pairs. The differences in song production were not reflected in the size of the song control nuclei: HVC was larger in M‐F than in M‐M males and within the M‐M dyads, no difference in HVC or RA size could be detected between dominant and subordinate males. At the individual level, the song rate with was positively correlated with RA and to a lower degree HVC volume, but this relationship was observed only in M‐M dyads, specifically in dominant males. A second experiment, carried out with castrated males that were all treated with T and exposed either to another T‐treated castrate or to an estradiol‐implanted female, confirmed that song rate was higher in the M‐M than in the M‐F condition and that HVC volume was larger in heterosexual than in same‐sex dyads. The effects of T on singing rate and on the volume of the song control nuclei are thus modulated by the social environment, including the presence/absence of a potential mate and dominance status among males. 2006 Wiley Periodicals, Inc. J Neurobiol, 2006     

Sex difference among nonneuronal cells precedes sexually dimorphic neuron growth and survival in an avian song control nucleus

In zebra finches only males sing, and several song control nuclei contain more neurons in adult males than in females. In the robust nucleus of the archistriatum (RA), this sex difference in neuron number arises because neuron survival is greater in young males than in females. The events initiating this sex difference in neuron survival are not known, but in earlier studies we observed that during sexual differentiation the proliferation and/or survival of RA cells exhibiting glial morphology is greater in males than in females. Because glia and glia-derived molecules are known to exert trophic effects on developing neurons, we wanted to determine when the sex difference in RA glia develops relative to the sexually dimorphic growth and survival of RA neurons. Male and female zebra finches were injected twice daily with ³[H]thymidine for 2 days beginning either on day 15 or 27. Two days later (day 18 or 30) sections through the RA were processed for autoradiography. Virtually all of the ³[H]thymidine labeled cells within the RA exhibited morphological features characteristic of glia and were not immunoreactive for the neuron-specific antigen, Hu. The number of these ³[H]thymidine labeled cells was measured, as were the number and soma size of RA neurons. Sex differences in RA neuron number and soma size were not evident at day 18, but emerged by day 30. However, at both ages the density of ³[H]thymidine labeled RA cells and their total number/RA neuron were significantly greater in males than in females. No such sexual dimorphism in the density of ³[H]thymidine labeled cells was evident in the archistriatum lateral to the RA, or within the RA of adult birds. These data indicate that sexually dimorphic gliogenesis is an early event in the sexual differentiation of the RA, preceding sex differences in RA neuron growth and survival. The possibility that glia (or glia-derived substances) may contribute to the neurotrophic effects of masculinization within the RA is discussed. © 1996 John Wiley & Sons, Inc.     

Tamoxifen's effects on the zebra finch song system are estrogenic, not antiestrogenic.

Antiestrogens fail to block the masculine ontogeny of the zebra finch song system that is hypothesized to occur as a result of early estrogen action. Moreover, they hypermasculinize the male, and masculinize the female song systems. In experiment 1, we assessed whether these antiestrogenic effects might mimic estrogenic actions. Zebra finch chicks received one of two treatments. They were given estradiol benzoate (EB) or vehicle daily for the first 20 days after hatching and sacrificed at 60 days of age, or they received EB or vehicle for the first 25 days after hatching, at which time they were sacrificed. In the day 60 group, certain attributes of the song system were hypermasculinized in males and masculinized in females by EB, when compared with controls. In the day 25 group, males treated with EB were partially demasculinized, while the females were partially masculinized. In experiment 2, we assessed whether simultaneous treatment with tamoxifen was capable of antagonizing the effects of EB obtained in experiment 1 (day 60 group). Sixty-day-old females, previously treated with both EB and tamoxifen for the first 20 days after hatching, had more masculine song regions than females treated with either EB alone or tamoxifen alone. In males, the effects of the combined treatment of EB and tamoxifen over those produced by tamoxifen alone were not as dramatic as in the female. These results are similar to those obtained in systems where tamoxifen is purely estrogenic and suggest that in the song system, tamoxifen acts as an estrogen, not an antiestrogen.     

Androgens and estrogens induce seasonal‐like growth of song nuclei in the adult songbird brain

In seasonally breeding songbirds, the brain regions that control song behavior undergo dramatic structural changes at the onset of each annual breeding season. As spring approaches and days get longer, gonadal testosterone (T) secretion increases and triggers the growth of several song control nuclei. T can be converted to androgenic and estrogenic metabolites by enzymes expressed in the brain. This opens the possibility that the effects of T may be mediated via the androgen receptor, the estrogen receptor, or both. To test this hypothesis, we examined the effects of two bioactive T metabolites on song nucleus growth and song behavior in adult male white‐crowned sparrows. Castrated sparrows with regressed song control nuclei were implanted with silastic capsules containing either crystalline T, 5α‐dihydrotestosterone (DHT), estradiol (E₂), or a combination of DHT+E₂. Control animals received empty implants. Song production was highly variable within treatment groups. Only one of seven birds treated with E₂ alone was observed singing, whereas a majority of birds with T or DHT sang. After 37 days of exposure to sex steroids, we measured the volumes of the forebrain song nucleus HVc, the robust nucleus of the archistriatum (RA), and a basal ganglia homolog (area X). All three steroid treatments increased the volumes of these three song nuclei when compared to blank‐implanted controls. These data demonstrate that androgen and estrogen receptor binding are sufficient to trigger seasonal song nucleus growth. These data also suggest that T's effects on seasonal song nucleus growth may depend, in part, upon enzymatic conversion of T to bioactive metabolites. © 2003 Wiley Periodicals, Inc. J Neurobiol 57:130–140, 2003     

Androgens and estrogens induce seasonal-like growth of song nuclei in the adult songbird brain

In seasonally breeding songbirds, the brain regions that control song behavior undergo dramatic structural changes at the onset of each annual breeding season. As spring approaches and days get longer, gonadal testosterone (T) secretion increases and triggers the growth of several song control nuclei. T can be converted to androgenic and estrogenic metabolites by enzymes expressed in the brain. This opens the possibility that the effects of T may be mediated via the androgen receptor, the estrogen receptor, or both. To test this hypothesis, we examined the effects of two bioactive T metabolites on song nucleus growth and song behavior in adult male white-crowned sparrows. Castrated sparrows with regressed song control nuclei were implanted with silastic capsules containing either crystalline T, 5alpha-dihydrotestosterone (DHT), estradiol (E(2)), or a combination of DHT+E(2). Control animals received empty implants. Song production was highly variable within treatment groups. Only one of seven birds treated with E(2) alone was observed singing, whereas a majority of birds with T or DHT sang. After 37 days of exposure to sex steroids, we measured the volumes of the forebrain song nucleus HVc, the robust nucleus of the archistriatum (RA), and a basal ganglia homolog (area X). All three steroid treatments increased the volumes of these three song nuclei when compared to blank-implanted controls. These data demonstrate that androgen and estrogen receptor binding are sufficient to trigger seasonal song nucleus growth. These data also suggest that T's effects on seasonal song nucleus growth may depend, in part, upon enzymatic conversion of T to bioactive metabolites.     

Lesions of HVc block the developmental masculinizing effects of estradiol in the female zebra finch song system

The neural song control system of female zebra finches is permanently masculinized if the females are given estradiol within 1 month after hatching. One hypothesis is that estradiol acts on neurons in the caudal nucleus of the ventral hyperstriatum (HVc) to cause developmental changes that lead to masculinizing influences in other song control regions. To test whether lesions of HVc block the masculinizing effects of estradiol elsewhere in the song system, we gave 20-day-old females either a Silastic pellet containing estradiol or no implant, and they received either a unilateral lesion of HVc or no lesion. At 60 days of age, they were sacrificed. The volumes of brain regions and sizes of neurons were measured in four song nuclei: HVc, robust nucleus of the archistriatum (RA), lateral magnocellular nucleus of the neostriatum (lMAN), and Area X. Lesions of HVc blocked the masculinizing effects of estradiol on RA and Area X on the side of the lesion. Thus, HVc must be intact in order for estradiol to masculinize these two nuclei. This observation is compatible with the hypothesis that estradiol acts on or near HVc to masculinize several song nuclei, although other interpretations are also possible.     

Sex-dependent loss of projection neurons involved in avian song learning

In zebra finches, only males sing, and the neural regions controlling song exhibit prominent, hormone-induced sex diffences in neuron number. In order to understand how sexual differentiation regulates neuron number within one song nucleus, the lateral magnocellular nucleus of the anterior neostriatum (IMAN), we studied the development of sex differences among IMAN neurons that project to the robust nucleus of the archistriatum (RA). The IMAN is implicated in song learning, and previous ontogenetic studies have indicated that males lose over 50% of their IMAN neurons during the juvenile song learning period. Based on developmental changes in both the extent of androgen accumulation within the IMAN and its appearance in Nissl-stained tissue, it had been hypothesized that IMAN neuron loss was even greater in young females, resulting in sex differences in neuron number. However, this hypothesis has not been tested directly because the Nissl-stained boundaries of the IMAN sometimes are ambiguous in young animals, and are not evident at all in adult females. To circumvent these problems, we employed the retrograde tracer fast blue to study the development of IMAN neurons defined on the basis of their projections to the RA. We find that the number of these IMAN-RA projection neurons is much greater in adult males than in females, and that this sex difference develops during the juvenile period of sexual differentiation and song learning because a significant number of these neurons are lost in females but not in males. With respect to sexual differentiation, we conclude that masculinization (which is stimulated by the hormone estradiol) promotes the retention of IMAN-RA projection neurons. In addition, our results indicate that any loss of IMAN neurons that may occur in young males does not include cells projecting to the RA. © 1992 John Wiley & Sons, Inc.     

Testosterone treatment increases the metabolic capacity of adult avian song control nuclei

In songbirds, the size of brain nuclei that control song learning and production change seasonally. These changes are mainly controlled by seasonal changes in plasma testosterone (T) concentration. One hypothesis to explain why it may be adaptive for these areas to regress in the fall is that this would decrease the metabolic demand of maintaining a large song system when singing is reduced or absent. We used a marker for cellular metabolism to examine birds with regressed song nuclei and compared them to birds whose song nuclei were induced to grow by administration of exogenous T. Photorefractory male Gambel's white-crowned sparrows were captured during their autumnal migration and kept in outdoor aviaries on a natural photoperiod. We implanted birds with Silastic capsules containing T or with empty implants. Three weeks later the birds were sacrificed. We assayed the brains for cytochrome oxidase (CO) activity and measured the volume of four song nuclei: HVc, RA, 1MAN, and area X. All four nuclei increased in volume in response to T treatment. T treatment increased the metabolic capacity of area X, HVc, and RA relative to surrounding tissue but had no effect on the metabolic capacity of 1MAN. These results support the hypothesis that song nuclei are more metabolically active under the influence of T than they are when plasma T levels are low.     

Estrogen contributes to seasonal plasticity of the adult avian song control system

Songbirds show dramatic neural plasticity as adults, including large-scale anatomical changes in discrete brain regions ("song control nuclei") controlling the production of singing behavior. The volumes of several song control nuclei are much larger in the breeding season than in the nonbreeding season, and these seasonal neural changes are regulated by plasma testosterone (T) levels. In many cases, the effects of T on the central nervous system are mediated by neural conversion to estradiol (E(2)) by the enzyme aromatase. The forebrain of male songbirds expresses very high levels of aromatase, in some cases adjacent to song control nuclei. We examined the effects of aromatase inhibition and estrogen treatment on song nuclei size using wild male songbirds in both the breeding and nonbreeding seasons. In breeding males, aromatase inhibition caused the volume of a telencephalic song control nucleus (HVC) to decrease, and this effect was partially rescued by concurrent estrogen replacement. In nonbreeding males, estradiol treatment caused HVC to grow to maximal spring size within 2 weeks. Overall, these data suggest that aromatization of T is an important mediator of song control system plasticity, and that estradiol has neurotrophic effects in adult male songbirds. This study demonstrates that estrogen can affect adult neural plasticity on a gross anatomical scale and is the first examination of estrogen effects on the brain of a wild animal.     

Paradoxical hypermasculinization of the zebra finch song system by an antiestrogen

Exogenous estrogens, when administered to hatchling female zebra finches, masculinize the morphology and function of their neural vocal control system. The first of two experiments evaluated whether tamoxifen citrate is an antiestrogen in zebra finches, and the second determined whether it would block the masculinization hypothesized to be caused in hatchling males by the males' endogenous estradiol. In the first experiment adult female zebra finches were ovariectomized and injected for 10 days with estradiol benzoate (EB), tamoxifen, EB and tamoxifen combined, or vehicle (control). The dependent variable was oviduct weight. The EB-stimulated growth of the oviduct was blocked by tamoxifen, which had no effects when administered alone. Thus, tamoxifen acts as an antiestrogen in the zebra finch oviduct. In Experiment 2, male and female zebra finches were treated with tamoxifen or vehicle for the first 20 days after hatching. The males were castrated at 20 days. At 60 days we compared the song control regions of experimental and control males and females. In both sexes tamoxifen increased the somatic areas of neurons in RA (robust nucleus of the archistriatum), HVc (caudal nucleus of the ventral hyperstriatum), and MAN (magnocellular nucleus of the anterior neostriatum). Tamoxifen also increased the volumes of HVc, RA, MAN, and Area X in males. Thus, tamoxifen failed to block masculinization of males, but masculinized females and hypermasculinized males. Tamoxifen's hypermasculinization of the male and masculinization of the female song system is paradoxical given that (1) estradiol does not have similar effects on the male song system, and (2) tamoxifen antagonizes the effects of EB in the oviduct.     

Sex differences in the telencephalic song control circuitry in Bengalese finches (Lonchura striata var. domestica)

Bengalese finches, Lonchura striata, are extremely sexually dimorphic in their singing behavior; males sing complex songs, whereas females do not sing at all. This study describes the developmental differentiation of the brain song system in Bengalese finches. Nissl staining was used to measure the volumes of four telencephalic song nuclei: Area X, HVC, the robust nucleus of the arcopallium (RA), and the lateral portion of the magnocellular nucleus of the anterior nidopallium (LMAN). In juveniles (circa 35 days old), Area X and the HVC were well developed in males, while they were absent or not discernable in females. The RA was much larger in males but barely discernable in females. In males, the volumes of Area X and the RA increased further into adulthood, but that of the HVC remained unchanged. The LMAN volume was greater in juveniles than in adults, and there was no difference in the LMAN volume between the sexes. The overall tendency was similar to that described in zebra finches, except for the volume of the RA, where the degree of sexual dimorphism is larger and the timing of differentiation occurs earlier in Bengalese finches. Motor learning of the song continues until day 90 in zebra finches, but up to day 120 in Bengalese finches. Earlier neural differentiation and a longer learning period in Bengalese finches compared with zebra finches may be related to the more elaborate song structures of Bengalese finches.     

Muscle-dependent and hormone-dependent differentiation of the vocal control premotor nucleus robustus archistriatalis and the motornucleus hypoglossus pars tracheosyringealis of the zebra finch

Sex differences in the vertebrate brain (brain sex) are thought to develop owing to the tissue specific action of gonadal hormones similar to the development of secondary sex characteristics of the body. Small sex differences in body anatomy could, however, retrogradely control the sexual differentiation of the central nervous system. This possibility has so far been verified only for motorneuron pools, since the connectivity of sex-specific higher brain areas to the sexual dimorphic periphery is frequently not well known. Here, we tested whether somatic sex differences feed back on higher brain areas by bilateral denervation of the syringeal musculature of zebra finches before, during, and after onset of estrogen-sensitive sexual differentiation of forebrain vocal nuclei such as RA (nucleus robustus archistriatalis). In the zebra finch, the sound-producing musculature (the syrinx), the syrinx motornucleus hypoglossus pars tracheosyringealis (nXIIts), and the RA are much larger in males compared to females. Tract tracing studies revealed that the volume and neuron size distribution of the nXIIts was sexually dimorphic in intact but not in animals denervated as juveniles. In contrast, the volume of RA and size of RA neurons of denervated animals were highly sexually dimorphic. Furthermore, estrogen masculinized the RA of denervated females. Thus, sexual differentiation of the RA but not of the nXIIts appears independent of somatic sex differences. The syrinx muscles are, however, important for the soma size of those RA neurons that project to the nXIIts.     

Testosterone increases singing and aggression but not male-typical sexual partner preference in early estrogen treated female zebra finches

Female zebra finches given estradiol benzoate (EB) as nestlings and testosterone propionate (TP) as adults show masculinized sexual partner preference, preferring females instead of males. This suggests an organizational effect of EB on sexual partner preference in a socially monogamous species that pairs for life. It is not known whether there is an activational hormone effect on sexual partner preference in this species, or whether adult testosterone treatment is necessary for masculinized preference to be expressed. In this experiment females were injected with EB daily for the first 2 weeks posthatching. As adults they were given TP filled or empty implants. Subjects were then given two-choice preference tests with male vs female stimuli, in which singing as well as proximity to the stimuli was recorded, followed by tests in a group aviary for social behavior and pairing preference. Females with TP implants sang more than females with empty implants and were more aggressive toward other females. They did not, however, differ from females with empty implants in any measure of sexual partner preference. Neither group showed a marked preference for males; instead both groups were equally interested in males and females. Thus adult testosterone treatment is not necessary for early estrogen treated females to show a shift in sexual partner preference in the male-typical direction.