Influence of testosterone metabolites on song-control system neuroplasticity during photostimulation in adult European starlings (Sturnus vulgaris)

The song-control system is a network of discrete nuclei in the songbird brain that controls the production and learning of birdsong and exhibits some of the best-studied neuroplasticity found in the adult brain. Photoperiodic growth of the song-control system during the breeding season is driven, at least in part, by the gonadal steroid testosterone. When acting on neural tissue, however, testosterone can be metabolized into 5α-dihydrotestosterone (DHT) or 17β-estradiol (E2), which activate different hormonal signaling pathways. By treating adult starlings with both testosterone metabolites and metabolite antagonists, we attempted to isolate the effects of androgen and estrogen treatment on neuroplasticity during photostimulation in male and female European starlings (Sturnus vulgaris). Photostimulation resulted in a large HVC volume typical of the breeding season in all treatments independent of hormone treatment. E2 had additional effects on HVC growth by reducing neuron density and enhancing early survival of new neurons recruited to HVC in females but did not significantly affect HVC volume. Conversely, DHT reduced the migration of new neurons, assessed by the expression of doublecortin, to HVC. DHT also increased syrinx mass and maintained RA (robust nucleus of the arcopallium) cytoarchitecture in the presence of aromatase inhibitors. In addition, we document the first evidence of sex-specific neuroplastic responses of the song-control system to androgens and estrogens. These findings suggest that the contributions of DHT and E2 signaling in songbird neuroplasticity may be regulated by photoperiod and that future studies should account for species and sex differences in the brain.     

Birth,migration, incorporation,and death of vocal control neurons in adult songbirds

Neurogenesis continues in the brain of adult birds. These cells are born in the ventricular zone of the lateral ventricles. Young neurons then migrate long distances guided, in part, by radial cell processes and become incorporated throughout most of the telencephalon. In songbirds, the high vocal center (HVC), which is important for the production of learned song, receives many of its neurons after hatching. HVC neurons which project to the robust nucleus of the archistriatum to form part of the efferent pathway for song production, and HVC interneurons continue to be added throughout life. In contrast, Area X-projecting HVC cells, thought to be part of a circuit necessary for song learning but not essential for adult song production, are only born in the embryo. New neurons in HVC of juvenile and adult birds replace older cells that die. There is a correlation between seasonal cell turnover rates (addition and loss) and testosterone levels in adult male canaries. Available evidence suggests that steroid hormones control the recruitment and/or survival of new HVC neurons, but not their production. The functions of neuronal replacement in adult birds remain unclear. However, rates of HVC neuron turnover are highest at times of year when canaries modify their songs. Replaceable HVC neurons may participate in the modification of perceptual memories or motor programs for song production. In contrast, permanent HVC neurons could hold long-lasting song-related information. The unexpected large-scale production of neurons in the adult brain holds important clues about brain function and, in particular, about the neural control of a learned behavior—birdsong. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 585–601, 1997     

Effects of testosterone on Reelin expression in the brain of male European starlings

Reelin, a large glycoprotein defective in reeler mice, is assumed to determine the final location of migrating neurons in the developing brain. We studied the expression of Reelin in the brain of adult male European starlings that had been treated or not with exogenous testosterone. Reelin-immunoreactive cells and fibers were widely distributed in the forebrain including areas in and around the song control nucleus, HVC. No labeling was detected in other song control nuclei with the exception of nucleus uvaeformis, which was delineated by a dense cluster of Reelin-immunoreactive perikarya. Reelin is thus expressed in areas incorporating new neurons in adulthood, such as HVC. Reelin expression was sharply decreased by testosterone in HVC, nucleus uvaeformis and dorsal thalamus but not in other brain regions. These results are consistent with the idea that seasonal changes in Reelin expression modulate the incorporation of neurons within HVC. The presence of Reelin in other brain areas that do not incorporate new neurons in adulthood indicates, however, that this protein must play other unrelated roles in the adult brain. Additional studies should now be carried out to determine the specific role played by this protein in the seasonal plasticity of the songbird brain.     

Testosterone-induced changes in adult canary brain are reversible

Brain nuclei that control song are larger in male canaries, which sing, than in females, which sing rarely or not at all. Treatment of adult female canaries with testosterone (T) induces song production and causes song-control nuclei to grow, approaching the volumes observed in males. For example, the higher vocal center (HVC) of adult females approximately doubles in size by 1 month following the onset of T treatment. Male HVC projects to a second telencephalic nucleus, RA (the robust nucleus of the archistriatum), which projects in turn to the vocal motor neurons. Whether HVC makes a similar connection in female canaries is not known, although HVC and RA are not functionally connected in female zebra finches, a species in which testosterone does not induce neural or behavioral changes in the adult song system. This experiment investigated whether HVC makes an efferent projection to RA in normal adult female canaries, or if T is necessary to induce the growth of this connection. In addition, we examined whether T-induced changes in adult female canary brain are reversible. Adult female canaries received systemic T implants that were removed after 4 weeks; these birds were killed 4 weeks after T removal (Testosterone-Removal, T-R). Separate groups of control birds received either (a) T implants for 4 weeks which were not removed (Testosterone-Control, T-C) or (b) empty implants (Untreated Control, øO-C). Crystals of the fluorescent tracer DiI were placed in the song-control nucleus HVC in order to anterogradely label both efferent targets of HVC, RA and Area X. Projections from HVC to RA and Area X were present in all treatment groups including untreated controls, and did not appear to differ either qualitatively or quantitatively. Thus, formation of efferent connections from HVC may be prerequisite to hormone-induced expression of song behavior in adult songbirds. The volumes of RA and Area X were measured using the distribution of anterograde label as well as their appearance in Nissl-stained tissue. RA was larger in T-treated control birds than in untreated controls. Experimental birds in which T was given and then removed (T-R) had RA volumes closer in size to untreated controls (ø-C). Because the volume of RA in T-treated controls (T-C) was larger than that of birds that did not receive T (ø-C), we conclude that the volume of RA increased in both T-C and T-R birds but regressed upon removal of T in T-R birds. Surprisingly, the volume of Area X did not increase in T-treated birds. Birds in this study were maintained on short days, suggesting that T-induced growth of Area X reported previously may have resulted from an interaction between T and another seasonal or photoperiodic factor induced by exposure to long daylengths. © 1993 John Wiley & Sons, Inc.     

Sexual Differences in Cell Proliferation in the Ventricular Zone,Cell Migration and Differentiation in the HVC of Juvenile Bengalese Finch

Song control nuclei have distinct sexual differences and thus are an ideal model to address how brain areas are sexually differentiated. Through a combination of histological analysis and electrical lesions, we first identified the ventricle site for HVC progenitor cells. We then found that there were significant sex differences in the cellular proliferation activity in the ventricular zone of the HVC, the number of migrating cells along the radial cells (positive immunoreactions to vimentin) and differentiation towards neurons. Through co-culturing of male and female slices containing the developing HVC in the same well, we found that the male slices could produce diffusible substances to masculinize the female HVC. By adding estrogen, an estrogen antagonist, brain-derived neurotrophic factor (BDNF) or its antibody into the culture medium, separately or in combination, we found that these diffusible substances may include estrogen and BDNF. Finally, we found that 1) estrogen-induced BDNF upregulation could be detected 48 hr after estrogen treatment and could not be blocked by a vascular endothelial growth factor (VEGF) receptor inhibitor and 2) the amount of VEGF mRNA expressed in the developing HVC and its adjacent area did not display any significant sex differences, as did the distribution of VEGF and laminin-expressing endothelial cells in the developing HVC. Because these findings are largely different from previous reports on the adult female HVC, it is suggested that our estrogen-induced BDNF up-regulation and the resultant sexual differentiation might not be mediated by VEGF and endothelial cells, but instead, may result from the direct effects of estrogen on BDNF.     

Coordinated interaction of neurogenesis and angiogenesis in the adult songbird brain

Neurogenesis proceeds throughout life in the higher vocal center (HVC) of the adult songbird neostriatum. Testosterone induces neuronal addition and endothelial division in HVC. We asked if testosterone-induced angiogenesis might contribute importantly to HVC neuronal recruitment. Testosterone upregulated both VEGF and its endothelial receptor, VEGF-R2/Quek1/KDR, in HVC. This yielded a burst in local HVC angiogenesis. FACS-isolated HVC endothelial cells produced BDNF in a testosterone-dependent manner. In vivo, HVC BDNF rose by the third week after testosterone, lagging by over a week the rise in VEGF and VEGF-R2. In situ hybridization revealed that much of this induced BDNF mRNA was endothelial. In vivo, both angiogenesis and neuronal addition to HVC were substantially diminished by inhibition of VEGF-R2 tyrosine kinase. These findings suggest a causal interaction between testosterone-induced angiogenesis and neurogenesis in the adult forebrain.     

Estrogens and non-estrogenic ovarian influences combine to promote the recruitment and decrease the turnover of new neurons in the adult female canary brain

The higher vocal center (HVC) of the songbird forebrain exhibits persistent neurogenesis in adulthood, particularly in a region of the mediocaudal neostriatum that is associated with a subventricular layer of estrogen receptive cells. We asked whether estrogens might influence adult neurogenesis, by assessing the effect of ovariectomy on HVC neuronal production in the adult female canary. Fifteen 1-year-old females were separated into groups of ovariectomized, estradiol-replaced ovariectomized, and gonadally intact birds. To label dividing cells and their progency, the birds were given [³H]thymidine for 8 days, killed 32 days later, and their brains autoradiographed. A significant rise was noted in the number of HVC neurons per section in estradiol-treated birds relative to the untreated control birds. The number of [³H]-thymidine-labeled HVC neurons was also higher in the estrogen-treated birds; however, the neuronal labeling index (LI) did not vary as a function of estradiol replacement, as the total number of HVC neurons rose in parallel with the added new neurons. In contrast, the neuronal LI did rise as a result of ovariectomy, and this ovariectomy-associated increase in the LI was not reversed by estradiol. Among non-neuronal cell types, the endothelial LI was higher in estrogen-treated birds than in their untreated counterparts, suggesting estrogen-associated angiogenesis. Radioimmunoassay confirmed that serum estradiol was reduced in the castrated birds. Since estrogen appeared to promote the survival of [³H]thymidine⁺ neurons, we next sought to determine whether estrogen acted directly on the newly generated neurons, or rather indirectly through an intermediary cell population. To this end, we asked whether the new neurons or their precursors expressed estrogen receptor immunoreactivity (ER-IR). Five adult male canaries were given [³H]thymidine for periods ranging from 2 to 28 days, killed at varying times up to 3 weeks therafter, then probed for ER-IR and autoradiographed. [³H]thymidine⁺ cells displayed no detectable ER-IR within their first 4 weeks of postmitotic life. Rather, during migration from the ventricular zone (VZ), the new neurons traversed a layer of mitotically quiescent, ER⁺ subventricular cells. Double labeling for ER-IR and cell-type selective antigens confirmed that these ER⁺ cells were neurons. These results indicate that the early survival of new neurons in the adult songbird HVC is promoted by estrogen, and may be mediated by the estrogen-stimulated paracrine release of neurotrophic agents by ER-IR subventricular neurons. Our data suggest that estrogen's promotion of neuronal survival may operate concurrently with an estrogen-independent ovarian suppression of neuronal mitogenesis.     

A species-specific view of song representation in a sensorimotor nucleus

Songbirds constitute a powerful model system for the investigation of how complex vocal communication sounds are represented and generated, offering a neural system in which the brain areas involved in auditory, motor and auditory–motor integration are well known. One brain area of considerable interest is the nucleus HVC. Neurons in the HVC respond vigorously to the presentation of the bird’s own song and display song-related motor activity. In the present paper, we present a synthesis of neurophysiological studies performed in the HVC of one songbird species, the canary (Serinus canaria). These studies, by taking advantage of the singing behavior and song characteristics of the canary, have examined the neuronal representation of the bird’s own song in the HVC. They suggest that breeding cues influence the degree of auditory selectivity of HVC neurons for the bird’s own song over its time-reversed version, without affecting the contribution of spike timing to the information carried by these two song stimuli. Also, while HVC neurons are collectively more responsive to forward playback of the bird’s own song than to its temporally or spectrally modified versions, some are more broadly tuned, with an auditory responsiveness that extends beyond the bird’s own song. Lastly, because the HVC is also involved in song production, we discuss the peripheral control of song production, and suggest that interspecific variations in song production mechanisms could be exploited to improve our understanding of the functional role of the HVC in respiratory–vocal coordination.     

Rapamycin blocks the neuroprotective effects of sex steroids in the adult birdsong system

In adult songbirds, the telencephalic song nucleus HVC and its efferent target RA undergo pronounced seasonal changes in morphology. In breeding birds, there are increases in HVC volume and total neuron number, and RA neuronal soma area compared to nonbreeding birds. At the end of breeding, HVC neurons die through caspase‐dependent apoptosis and thus, RA neuron size decreases. Changes in HVC and RA are driven by seasonal changes in circulating testosterone (T) levels. Infusing T, or its metabolites 5α‐dihydrotestosterone (DHT) and 17 β‐estradiol (E2), intracerebrally into HVC (but not RA) protects HVC neurons from death, and RA neuron size, in nonbreeding birds. The phosphoinositide 3‐kinase (PI3K)‐Akt (a serine/threonine kinase)‐mechanistic target of rapamycin (mTOR) signaling pathway is a point of convergence for neuroprotective effects of sex steroids and other trophic factors. We asked if mTOR activation is necessary for the protective effect of hormones in HVC and RA of adult male Gambel's white‐crowned sparrows (Zonotrichia leucophrys gambelii). We transferred sparrows from breeding to nonbreeding hormonal and photoperiod conditions to induce regression of HVC neurons by cell death and decrease of RA neuron size. We infused either DHT + E2, DHT + E2 plus the mTOR inhibitor rapamycin, or vehicle alone in HVC. Infusion of DHT + E2 protected both HVC and RA neurons. Coinfusion of rapamycin with DHT + E2, however, blocked the protective effect of hormones on HVC volume and neuron number, and RA neuron size. These results suggest that activation of mTOR is an essential downstream step in the neuroprotective cascade initiated by sex steroid hormones in the forebrain.     

Differential estrogen accumulation among populations of projection neurons in the higher vocal center of male canaries

The higher vocal center (HVC) of adult male canries undergoes a seasonal change in volume that corresponds to seasonal modifications of vocal behavior: HVC is large when birds produce stereotyped song (spring) and is small when birds produce plastic song and add new song syllables into their vocal repertoires (fall). We reported previously that systemic exposure to testosterone (T) produces an increase in the volume of HVC similar to that observed with long-day photoperiods. T-induced growth of HVC occured regardless of wheter the borders of HVC were defined by Nissl-staining, the distribution of androgen-concentrating cells, or the distribution of projection neurons [separate neuronal populations within HVC project to the robust nucleus of the archistriatum (RA) and to Area X of the avian striatum (X)]. In the present study we used steroid autoradiography to determine whether T can influence the distribution of HVC cells that bind estrogen, and we combined estrogen autoradiography with retrograde labeling to determine whether HVC neurons that project to RA versus X differ in their ability to accumulate estrogen. Results showed that T increased the volume of Nissl-defined HVC and although HVC contained a low density of estrogen-concentrating cells, T increased the spatial distribution of these cells to match the Nissl borders of HVC. We also identified a region containing a high density of estrogenconcentrating cells located medial to HVC [we call this region paraHVC (pHVC)], and T also increased the volume of pHVC. pHVC also contained numerous X-projecting neurons, but few if any RA-projecting neurons. Double-labeling analysis revealed the RA-projecting neurons did not accumulate estrogen, a small percentage of X-projecting neurons in HVC accumulated estrogen, and the majority of X-projecting neurons in pHVC showed heavy accumulation of estrogen. The data reported here and in our previous article suggest distinct roles for gonadal steroids within the HVC-pHVC complex: estrogens are concentrated by neurons that project to a striatal region that influences vocal production during song learning (X), whereas androgens are concentrated primarily by neurons that project to a motor region that is involved in vocal production during both song learning and the recitation of already-learned song (RA). © 1995 John Wiley & Sons, Inc.     

Androgen action at the target musculature regulates brain‐derived neurotrophic factor protein in the spinal nucleus of the bulbocavernosus

We have previously demonstrated that brain‐derived neurotrophic factor (BDNF) interacts with testosterone to regulate dendritic morphology of motoneurons in the highly androgen‐sensitive spinal nucleus of the bulbocavernosus (SNB). Additionally, in adult male rats testosterone regulates BDNF in SNB motoneurons and its target muscle, the bulbocavernosus (BC). Because BDNF is retrogradely transported from skeletal muscles to spinal motoneurons, we hypothesized that testosterone could regulate BDNF in SNB motoneurons by acting locally at the BC muscle. To test this hypothesis, we restricted androgen manipulation to the SNB target musculature. After castration, BDNF immunolabeling in SNB motoneurons was maintained at levels similar to those of gonadally intact males by delivering testosterone treatment directly to the BC muscle. When the same implant was placed interscapularly in castrated males it was ineffective in supporting BDNF immunolabeling in SNB motoneurons. Furthermore, BDNF immunolabeling in gonadally intact adult males given the androgen receptor blocker hydroxyflutamide delivered directly to the BC muscle was decreased compared with that of gonadally intact animals that had the same hydroxyflutamide implant placed interscapularly, or when compared with castrated animals that had testosterone implants at the muscle. These results demonstrate that the BC musculature is a critical site of action for the androgenic regulation of BDNF in SNB motoneurons and that it is both necessary and sufficient for this action. Furthermore, the local action of androgens at the BC muscle in regulating BDNF provides a possible mechanism underlying the interactive effects of testosterone and BDNF on motoneuron morphology. © 2013 Wiley Periodicals, Inc. Develop Neurobiol 73: 587–598, 2013     

Aromatase inhibition affects testosterone‐induced masculinization of song and the neural song system in female canaries

Songbirds have a specialized steroid‐sensitive network of brain nuclei, the song system, for controlling song. Most nuclei of the song system express androgen receptors, and the sensory‐motor integration nucleus High Vocal Center (HVC) alone also expresses estrogen receptors. Apart from expressing estrogen receptors in the vocal control system, songbirds are unique among birds because they have high concentrations of the estrogen‐synthesizing enzyme aromatase in the neostriatum surrounding HVC. However, the role of estrogen in controlling the development of the song structure has been scarcely investigated. In this work, we show that blocking the production of estrogen during testosterone‐induced song motor development in adult female canaries alters the song pattern compared to control females treated with testosterone only. These effects were correlated with inhibition of the expression of estrogen‐sensitive genes, such as brain‐derived nerve growth factor, in HVC. The expression of the ATP‐synthase gene, an indicator of cell activity, in HVC, and the size of HVC, were not affected by the treatment. Our results provide the first example of estrogen‐sensitive mechanisms controlling the structural features of adult birdsong. © 2002 Wiley Periodicals, Inc. J Neurobiol 54: 370–379, 2003     

Hormone-induced changes in identified cell populations of the higher vocal center in male canaries

Male canaries revise their vocal repertoire every year. Early work indicated that the volume and neuron number of the song-control nucleus HVC (Higher Vocal Center) declined in late-summer/fall as birds added and deleted syllables from their repertoire, and increased in spring as the set of song syllables stabilized to a fixed number. Seasonal variation in serum testosterone levels suggested that these changes in brain and behavior were regulated by testosterone (T). However, although initial studies describing growth and regression of HVC used Nissl-staining to define its borders, recent experiments that have measured the distribution of identified populations of HVC cells (projection neurons, hormone target cells) suggest that there are no seasonal changes in HVC volume or neuron number. In order to clarify the role of T in the regulation of HVC morphology, we castrated male canaries, maintained them on short (fall-like) days, and treated them with either T, antisteroid drugs, or nothing. After 1 month of treatment, we used a double-labeling technique to characterize HVC projection neurons and androgen target cells. The results showed that hormonal manipulation influenced HVC volume, the density and size of HVC cells, and the absolute number and percentage of androgen target cells in HVC. Hormonal manipulation did not influence the absolute number of cells in HVC. Moreover, the distribution of projection neurons, androgen target cells, and the Nissl-defined borders of HVC were closely aligned in all experimental groups, indicating that exposure to T and/or its metabolites (estradiol and dihydrotestosterone) regulates the overall size of HVC by affecting the distributions of both projection neurons and androgen target cells. Analysis of double-labeling results suggests that T specifically influences both cell size and the ability to accumulate androgen among HVC neurons that project to the robust nucleus of the archistriatum (RA). The results of this study show that steroid hormones exert potent effects on HVC morphology in male canaries, but differences between our results and studies of seasonal males suggest there may be additional factors that can regulate HVC morphology. © 1993 John Wiley & Sons, Inc.     

Hormonal and environmental control of song control region growth and new neuron addition in adult male house finches, Carpodacus mexicanus

In songbirds, testosterone (T) mediates seasonal changes in the sizes and neuroanatomical characteristics of brain regions that control singing (song control regions; SCRs). One model explaining the mechanisms of the growth of one SCR, the HVC, postulates that in the spring increasing photoperiod and circulating T concentrations enhance new neuron survival, thus increasing total neuron number. However, most research investigating the effects of T on new neuron survival has been done in autumn. The present study investigated the effects of photoperiod and T treatment on SCR growth and new neuron survival in the HVC in photosensitive adult male House Finches, Carpodacus mexicanus, under simulated spring-like conditions. Birds were castrated, given T-filled or empty Silastic capsules and maintained on short days (SD; 8L:16D) or long days (LD; 16L:8D). To mark new cells, birds received bromodeoxyuridine injections 11 days after experimental manipulations began and were sacrificed 28 days later. Testosterone treatment increased the sizes of two SCRs, the HVC and Robust nucleus of the arcopallium (RA). Exposure to LD did not affect HVC volume, but did increase RA volume. Testosterone treatment increased the total number of HVC neurons, but did not affect the number of new HVC neurons. Thus, T initiates SCR growth and increases neuron survival, but effects of T on new neuron incorporation may be limited in photosensitive birds under spring-like conditions. These results provide new insight into the effects of photoperiod and T treatment on vernal SCR growth and new neuron incorporation and support current models explaining this growth.     

Aromatase inhibition affects testosterone-induced masculinization of song and the neural song system in female canaries

Songbirds have a specialized steroid-sensitive network of brain nuclei, the song system, for controlling song. Most nuclei of the song system express androgen receptors, and the sensory-motor integration nucleus High Vocal Center (HVC) alone also expresses estrogen receptors. Apart from expressing estrogen receptors in the vocal control system, songbirds are unique among birds because they have high concentrations of the estrogen-synthesizing enzyme aromatase in the neostriatum surrounding HVC. However, the role of estrogen in controlling the development of the song structure has been scarcely investigated. In this work, we show that blocking the production of estrogen during testosterone-induced song motor development in adult female canaries alters the song pattern compared to control females treated with testosterone only. These effects were correlated with inhibition of the expression of estrogen-sensitive genes, such as brain-derived nerve growth factor, in HVC. The expression of the ATP-synthase gene, an indicator of cell activity, in HVC, and the size of HVC, were not affected by the treatment. Our results provide the first example of estrogen-sensitive mechanisms controlling the structural features of adult birdsong.     

High vocal center growth and its relation to neurogenesis, neuronal replacement and song acquisition in juvenile canaries.

It is generally thought that most circuits of the adult central nervous system (CNS) are sculpted, in part at least, by selective elimination of some of the neurons present in an initial overabundant set. In this scenario, the birth of neurons precedes the period when brain functions, such as learning, first occur. In contrast to this form of brain assembly, we describe here the delayed development of the high vocal center (HVC) and one of its efferent pathways in canaries. The retrograde tracer Fluoro-Gold (FG) was injected into one of HVC's two efferent targets, the nucleus robustus archistriatalis (RA), to define the boundaries of HVC. The HVC grows markedly between 1 and 4 months, invading neighboring territories of the caudal telencephalon. During this same period, 0.43%-0.64% of the HVC neurons present at 1 year of age are labeled per day of [3H]-thymidine injection. [3H]-Thymidine labeling is a marker of cell birth, and during the first 4 months HVC neuron number increases, probably accounting for part of the HVC growth observed. Thereafter, the number of HVC neurons remains constant, but neuronal birth persists. We infer from this that neuronal replacement starts as early as 4 months after hatching and perhaps before then. About half of the neurons born after posthatching day 10 grow an axon to RA to form the main efferent pathway exiting from HVC. HVC growth, neurogenesis, axogenesis, and the observed replacement of neurons happen during the period of juvenile vocal learning. However, the recruitment of neurons that are still present at 1 year shows no particular inflections corresponding to the various stages in song learning, and continues at essentially the same rate after the more stereotyped adult song has been acquired. We suggest that a combination of neurogenesis and neuronal replacement provides unique advantages for learning.     

Bi-directional sexual dimorphisms of the song control nucleus HVC in a songbird with unison song

Sexually dimorphic anatomy of brain areas is thought to be causally linked to sex differences in behaviour and cognitive functions. The sex with the regional size advantage (male or female) differs between brain areas and species. Among adult songbirds, males have larger brain areas such as the HVC (proper name) and RA (robust nucleus of the arcopallium) that control the production of learned songs. Forest weavers (Ploceus bicolor) mated pairs sing a unison duet in which male and female mates learn to produce identical songs. We show with histological techniques that the volume and neuron numbers of HVC and RA were > or =1.5 times larger in males than in females despite their identical songs. In contrast, using in-situ hybridizations, females have much higher (30-70%) expression levels of mRNA of a number of synapse-related proteins in HVC and/or RA than their male counterparts. Male-typical and female-typical sexual differentiation appears to act on different aspects of the phenotypes within the same brain areas, leading females and males to produce the same behaviour using different cellular mechanisms.     

Regional distribution of brain-derived neurotrophic factor mRNA in the adult mouse brain.

Brain-derived neurotrophic factor (BDNF) is a protein that allows the survival of specific neuronal populations. This study reports on the distribution of the BDNF mRNA in the adult mouse brain, where the BDNF gene is strongly expressed, using quantitative Northern blot analysis and in situ hybridization. All brain regions examined were found to contain substantial amounts of BDNF mRNA, the highest levels being found in the hippocampus followed by the cerebral cortex. In the hippocampus, which is also the site of highest nerve growth factor (NGF) gene expression in the central nervous system (CNS), there is approximately 50-fold more BDNF mRNA than NGF mRNA. In other brain regions, such as the granule cell layer of the cerebellum, the differences between the levels of BDNF and NGF mRNAs are even more pronounced. The BDNF mRNA was localized by in situ hybridization in hippocampal neurons (pyramidal and granule cells). These data suggest that BDNF may play an important role in the CNS for a wide variety of adult neurons.