Seasonal plasticity in the song control system: multiple brain sites of steroid hormone action and the importance of variation in song behavior

Photoperiodism, testosterone and adult neurogenesis in canaries (Serinus canaria)

Domestic strains of canaries (Serinus canaria) variably respond to photoperiod changes and apparently stay in breeding state for extended periods. Fife Fancy canaries are supposed to be similar to the native species living at 27-39° north where photoperiod significantly changes across the year. Our birds showed reproductive cycles when exposed to light regimes mimicking the annual cycle of photoperiod. However after 6 months in short days (SD: 8L:16D), males developed large testes, as observed by X-ray tomography, and intense singing. Switching to long days (LD: 16L:8D) did not further increase song rate nor testes size but increased song duration, number of syllables per song, and trill occurrence frequency. No sign of regression was observed after 12 weeks in LD but return to SD produced a rapid decrease in testes size and singing activity below values in birds maintained throughout in SD. Fife Fancy thus does not seem to develop absolute but only relative refractoriness. The relatively high singing activity expressed by SD-photosensitive males does not seem to depend on high testosterone (T) concentrations. Singing did not correlate with plasma testosterone (T). Treatment with ATD + Flutamide only marginally decreased song rate and did not affect song quality nor song control nuclei volume. These birds are either supersensitive to low T levels or their reproductive physiology is activated by other mechanisms. Neurogenesis is increased by T and by LD but the function of new neurons incorporated in HVC is poorly understood. We developed a procedure based on X-ray focal irradiation to deplete neural progenitors adjacent to HVC and study the functional consequences. The decrease in neurogenesis increased the variability of T-induced songs in females and decreased their bandwidth. Neurogenesis in HVC thus plays a role in song production and X-ray focal irradiation represents an excellent tool to analyze adult neurogenesis.

Vernal growth of vocal control nucleus Area X, but not HVC, precedes gonadal recrudescence in wild black-capped chickadees (Poecile atricapillus)

In temperate-zone songbirds, the neuroanatomical changes which occur in advance of breeding, including the growth of nuclei of the vocal control system, are believed to occur downstream of gonadal recrudescence. However, evidence from wild birds is mixed. Here, we captured black-capped chickadees from the wild in early spring (March-April), summer (August-September), and winter (December-January); in addition to measuring the volumes of two vocal control nuclei (Area X and HVC), we also quantified two indicators of reproductive state (gonads and circulating gonadal steroids). Most birds captured in early spring had regressed gonads and low levels of circulating gonadal steroids, indicating these birds were not yet in full breeding condition. However, these early spring birds still had a significantly larger Area X than winter birds, while HVC did not differ in size across groups. Using data from a previously published seasonal study of black-capped chickadees (Phillmore et al., Developmental Neurobiology, 2015;75:203-216), we then compared Area X and HVC volumes from our early spring group to a breeding group of chickadees captured 3-4 weeks later in the spring. While Area X volume did not differ between the studies, breeding males in Phillmore et al. (2015) had a significantly larger HVC. Taken together, this suggests that the vernal growth of Area X occurs ahead of HVC in black-capped chickadees, and that the overall vernal changes in the vocal control system occur at least partially in advance of the breeding-associated upregulation of the hypothalamic-pituitary-gonadal axis.

A dawn and dusk chorus will emerge if males sing in the absence of their mate

The spring dawn and dusk chorus of birds is a widespread phenomenon, yet its origin remains puzzling. We propose that a dawn and dusk chorus will inevitably arise if two criteria are met: (1) females leave their roost later in the morning and go to roost earlier in the evening than their mate, and (2) males sing more when separated from their mate. Previous studies on blue tits (Cyanistes caeruleus) support the first criterion. We here report that males sing at a higher rate whenever they are separated from their mate and that song rate increases with the duration of female absence. These findings can explain the existence of the dawn and dusk chorus in blue tits, and they can explain why the dawn chorus is more pronounced than the dusk chorus, as is typically observed. An exhaustive literature search provides support for both criteria of the 'absent mate' hypothesis in several passerine birds. We found no evidence contradicting the hypothesis. The new hypothesis is not inconsistent with many of the existing hypotheses about dawn singing, but may be a more general explanation for the occurrence of a dawn and dusk chorus. We describe how the 'absent mate' hypothesis leads to testable predictions about daily and seasonal variation in song output.

Using seasonality and birdsong to understand mechanisms underlying context-appropriate shifts in social motivation and reward

Social motivation and reward are dynamic and flexible, shifting adaptively across contexts to meet changing social demands. This is exceptionally apparent when seasonal contexts are considered in seasonally breeding songbirds as they cycle from periods of sexual motivation and reward during the breeding season to periods of extreme gregariousness outside the breeding season when non-sexual social interactions gain reward value, motivating birds to form flocks. Here we review evidence demonstrating a key integrative role for the medial preoptic area (mPOA) in the seasonally-appropriate adjustment of behaviors, with seasonal changes in dopamine activity in mPOA adjusting social motivation and changes in opioid activity modifying social reward. Experiments demonstrate that dramatic seasonal fluctuations in steroid hormone concentrations alter patterns of opioid- and dopamine-related protein and gene expression in mPOA to modify social motivation and reward to meet seasonal changes in social demands. These studies of birdsong and seasonality provide new insights into neural and endocrine mechanisms underlying adaptive changes in social motivation and reward and highlight an underappreciated, evolutionarily conserved role for the mPOA in important social behaviors in non-reproductive contexts.

Sex differences in seasonal brain plasticity and the neuroendocrine regulation of vocal behavior in songbirds

Birdsong is controlled in part by a discrete network of interconnected brain nuclei regulated in turn by steroid hormones and environmental stimuli. This complex interaction results in neural changes that occur seasonally as the environment varies (e.g., photoperiod, food/water availability, etc.). Variation in environment, vocal behavior, and neuroendocrine control has been primarily studied in male songbirds in both laboratory studies of captive birds and field studies of wild caught birds. The bias toward studying seasonality in the neuroendocrine regulation of song in male birds comes from a historic focus on sexually selected male behaviors. In fact, given that male song is often loud and accompanied by somewhat extravagant courtship behaviors, female song has long been overlooked. To compound this bias, the primary model songbird species for studies in the lab, zebra finches (Taeniopygia guttata) and canaries (Serinus canaria), exhibit little or no female song. Therefore, understanding the degree of variation and neuroendocrine control of seasonality in female songbirds is a major gap in our knowledge. In this review, we discuss the importance of studying sex differences in seasonal plasticity and the song control system. Specifically, we discuss sex differences in 1) the neuroanatomy of the song control system, 2) the distribution of receptors for androgens and estrogens and 3) the seasonal neuroplasticity of the hypothalamo-pituitary-gonadal axis as well as in the neural and cellular mechanisms mediating song system changes. We also discuss how these neuroendocrine mechanisms drive sex differences in seasonal behavior. Finally, we highlight specific gaps in our knowledge and suggest experiments critical for filling these gaps.

Unilateral vocal nerve resection alters neurogenesis in the avian song system in a region-specific manner

New neurons born in the adult brain undergo a critical period soon after migration to their site of incorporation. During this time, the behavior of the animal may influence the survival or culling of these cells. In the songbird song system, earlier work suggested that adult-born neurons may be retained in the song motor pathway nucleus HVC with respect to motor progression toward a target song during juvenile song learning, seasonal song restructuring, and experimentally manipulated song variability. However, it is not known whether the quality of song per se, without progressive improvement, may also influence new neuron survival. To test this idea, we experimentally altered song acoustic structure by unilateral denervation of the syrinx, causing a poor quality song. We found no effect of aberrant song on numbers of new neurons in HVC, suggesting that song quality does not influence new neuron culling in this region. However, aberrant song resulted in the loss of left-side dominance in new neurons in the auditory region caudomedial nidopallium (NCM), and a bilateral decrease in new neurons in the basal ganglia nucleus Area X. Thus new neuron culling may be influenced by behavioral feedback in accordance with the function of new neurons within that region. We propose that studying the effects of singing behaviors on new neurons across multiple brain regions that differentially subserve singing may give rise to general rules underlying the regulation of new neuron survival across taxa and brain regions more broadly.

Song learning and plasticity in songbirds

Birdsong provides a fascinating system to study both behavioral and neural plasticity. Oscine songbirds learn to sing, exhibiting behavioral plasticity both during and after the song-learning process. As a bird learns, its song progresses from a plastic and highly variable vocalization into a more stereotyped, crystallized song. However, even after crystallization, song plasticity can occur: some species' songs become more stereotyped over time, whereas other species can incorporate new song elements. Alongside the changes in song, songbirds' brains are also plastic. Both song and neural connections change with the seasons in many species, and new neurons can be added to the song system throughout life. In this review, we highlight important research on behavioral and neural plasticity at multiple timescales, from song development in juveniles to lifelong modifications of learned song.

In vivo online monitoring of testosterone-induced neuroplasticity in a female songbird

Adult neuroplasticity in the song control system of seasonal songbirds is largely driven by photoperiod-induced increases in testosterone. Prior studies of the relationships between testosterone, song performance and neuroplasticity used invasive techniques, which prevent analyzing the dynamic changes over time and often focus on pre-defined regions-of-interest instead of examining the entire brain. Here, we combined (i) in vivo diffusion tensor imaging (DTI) to assess structural neuroplasticity with (ii) repeated monitoring of song and (iii) measures of plasma testosterone concentrations in thirteen female photosensitive starlings (Sturnus vulgaris) who received a testosterone implant for 3 weeks. We observed fast (days) and slower (weeks) effects of testosterone on song behavior and structural neuroplasticity and determined how these effects correlate on a within-subject level, which suggested separate contributions of the song motor and anterior forebrain pathways in the development of song performance. Specifically, the increase in testosterone correlated with a rapid increase of song rate and RA volume, and with changes in Area X microstructure. After implant removal, these variables rapidly reverted to baseline levels. In contrast, the more gradual improvement of song quality was positively correlated with the fractional anisotropy values (DTI metric sensitive to white matter changes) of the HVC-RA tract and of the lamina mesopallialis, which contains fibers connecting the song control nuclei. Thus, we confirmed many of the previously reported testosterone-induced effects, like the increase in song control nuclei volume, but identified for the first time a more global picture of the spatio-temporal changes in brain plasticity.

Rapid effects of 17β-estradiol on aggressive behavior in songbirds: Environmental and genetic influences

Contribution to Special Issue on Fast effects of steroids. 17β-estradiol (E2) has numerous rapid effects on the brain and behavior. This review focuses on the rapid effects of E2 on aggression, an important social behavior, in songbirds. First, we highlight the contributions of studies on song sparrows, which reveal that seasonal changes in the environment profoundly influence the capacity of E2 to rapidly alter aggressive behavior. E2 administration to male song sparrows increases aggression within 20 min in the non-breeding season, but not in the breeding season. Furthermore, E2 rapidly modulates several phosphoproteins in the song sparrow brain. In particular, E2 rapidly affects pCREB in the medial preoptic nucleus, in the non-breeding season only. Second, we describe studies of the white-throated sparrow, which reveal how a genetic polymorphism may influence the rapid effects of E2 on aggression. In this species, a chromosomal rearrangement that includes ESR1, which encodes estrogen receptor α (ERα), affects ERα expression in the brain and the ability of E2 to rapidly promote aggression. Third, we summarize studies showing that aggressive interactions rapidly affect levels of E2 and other steroids, both in the blood and in specific brain regions, and the emerging potential for steroid profiling by liquid chromatography tandem mass spectrometry (LC-MS/MS). Such studies of songbirds demonstrate the value of an ethologically informed approach, in order to reveal how steroids act rapidly on the brain to alter naturally-occurring behavior.

Questioning Seasonality of Neuronal Plasticity in the Adult Avian Brain

To date, studies that reported seasonal patterns of adult neurogenesis and neuronal recruitment have correlated them to seasonal behaviors as the cause or as a consequence of neuronal changes. The aim of our study was to test this correlation, and to investigate whether there is a seasonal pattern of new neuronal recruitment that is not correlated to behavior. To do this, we used adult female zebra finches (songbirds that are not seasonal breeders), kept them under constant social, behavioral, and spatial environments, and compared neuronal recruitment in their brains during two seasons, under natural and laboratory conditions. Under natural conditions, no significant differences were found in the pattern of new neuronal recruitment across seasons. However, under artificial indoor conditions that imitated the natural conditions, higher neuronal recruitment occurred in late summer (August) compared to early spring (February). Moreover, our data indicate that "mixing" temperature and day length significantly reduces new neuronal recruitment, demonstrating the importance of the natural combination of temperature and day length. Taken together, our findings show, for the first time, that neuroplasticity changes under natural vs. artificial conditions, and demonstrate the importance of both laboratory and field experiments when looking at complex biological systems.

Behavioral evidence for sex steroids hypersensitivity in castrated male canaries

In seasonally breeding songbirds such as canaries, singing behavior is predominantly under the control of testosterone and its metabolites. Short daylengths in the fall that break photorefractoriness are followed by increasing daylengths in spring that activate singing via both photoperiodic and hormonal mechanisms. However, we observed in a group of castrated male Fife fancy canaries maintained for a long duration under a short day photoperiod a large proportion of subjects that sang at high rates. This singing rate was not correlated with variation in the low circulating concentrations of testosterone. Treatment of these actively singing castrated male canaries with a combination of an aromatase inhibitor (ATD) and an androgen receptor blocker (flutamide) only marginally decreased this singing activity as compared to control untreated birds and did not affect various measures of song quality. The volumes of HVC and of the medial preoptic nucleus (POM) were also unaffected by these treatments but were relatively large and similar to volumes in testosterone-treated males. In contrast, peripheral androgen-sensitive structures such as the cloacal protuberance and syrinx mass were small, similar to what is observed in castrates. Together these data suggest that after a long-term steroid deprivation singing behavior can be activated by very low concentrations of testosterone. Singing normally depends on the activation by testosterone and its metabolites of multiple downstream neurochemical systems such as catecholamines, nonapeptides or opioids. These transmitter systems might become hypersensitive to steroid action after long term castration as they probably are at the end of winter during the annual cycle in seasonally breeding temperate zone species.

FoxP2 Expression in a Highly Vocal Teleost Fish with Comparisons to Tetrapods

Motivated by studies of speech deficits in humans, several studies over the past two decades have investigated the potential role of a forkhead domain transcription factor, FoxP2, in the central control of acoustic signaling/vocalization among vertebrates. Comparative neuroanatomical studies that mainly include mammalian and avian species have mapped the distribution of FoxP2 expression in multiple brain regions that imply a greater functional significance beyond vocalization that might be shared broadly across vertebrate lineages. To date, reports for teleost fish have been limited in number and scope to nonvocal species. Here, we map the neuroanatomical distribution of FoxP2 mRNA expression in a highly vocal teleost, the plainfin midshipman (Porichthys notatus). We report an extensive overlap between FoxP2 expression and vocal, auditory, and steroid-signaling systems with robust expression at multiple sites in the telencephalon, the preoptic area, the diencephalon, and the midbrain. Label was far more restricted in the hindbrain though robust in one region of the reticular formation. A comparison with other teleosts and tetrapods suggests an evolutionarily conserved FoxP2 phenotype important to vocal-acoustic and, more broadly, sensorimotor function among vertebrates.

Co-localization patterns of neurotensin receptor 1 and tyrosine hydroxylase in brain regions involved in motivation and social behavior in male European starlings

Animals communicate in distinct social contexts to convey information specific to those contexts, such as sexual or agonistic motivation. In seasonally-breeding male songbirds, seasonal changes in day length and increases in testosterone stimulate sexually-motivated song directed at females for courtship and reproduction. Dopamine and testosterone may act in the same brain regions to stimulate sexually-motivated singing. The neuropeptide neurotensin, acting at the neurotensin receptor 1 (NTR1), can strongly influence dopamine transmission. The goal of this study was to gain insight into the degree to which seasonal changes in physiology modify interactions between neurotensin and dopamine to adjust context-appropriate communication. Male European starlings were examined in physiological conditions that stimulate season-typical forms of communication: late summer/early fall non-breeding condition (low testosterone; birds sing infrequently), late fall non-breeding condition (low testosterone; birds produce non-sexually motivated song), and spring breeding condition (high testosterone; males produce sexually-motivated song). Double fluorescent immunolabeling was performed to detect co-localization patterns between tyrosine hydroxylase (TH; the rate-limiting enzyme in dopamine synthesis) and NTR1 in brain regions implicated in motivation and song production (the ventral tegmental area, medial preoptic nucleus, periaqueductal gray, and lateral septum). Co-localization between TH and NTR1 was present in the ventral tegmental area for all physiological conditions, and the number of co-localized cells did not differ across conditions. Immunolabeling for TH and NTR1 was also present in the other examined regions, although no co-localization was seen. These results support the hypothesis that interactions between NTR1 and dopamine in the ventral tegmental area may modulate vocalizations, but suggest that testosterone- or photoperiod-induced changes in NTR1/TH co-localization do not underlie seasonally-appropriate adjustment of communication.

The large and small SPEN family proteins stimulate axon outgrowth during neurosecretory cell remodeling in Drosophila

Split ends (SPEN) is the founding member of a well conserved family of nuclear proteins with critical functions in transcriptional regulation and the post-transcriptional processing and nuclear export of transcripts. In animals, the SPEN proteins fall into two size classes that perform either complementary or antagonistic functions in different cellular contexts. Here, we show that the two Drosophila representatives of this family, SPEN and Spenito (NITO), regulate metamorphic remodeling of the CCAP/bursicon neurosecretory cells. CCAP/bursicon cell-targeted overexpression of SPEN had no effect on the larval morphology or the pruning back of the CCAP/bursicon cell axons at the onset of metamorphosis. During the subsequent outgrowth phase of metamorphic remodeling, overexpression of either SPEN or NITO strongly inhibited axon extension, axon branching, peripheral neuropeptide accumulation, and soma growth. Cell-targeted loss-of-function alleles for both spen and nito caused similar reductions in axon outgrowth, indicating that the absolute levels of SPEN and NITO activity are critical to support the developmental plasticity of these neurons. Although nito RNAi did not affect SPEN protein levels, the phenotypes produced by SPEN overexpression were suppressed by nito RNAi. We propose that SPEN and NITO function additively or synergistically in the CCAP/bursicon neurons to regulate multiple aspects of neurite outgrowth during metamorphic remodeling.

Different Seasonal Patterns in Song System Volume in Willow Tits and Great Tits

In most species of seasonally breeding songbirds studied to date, the brain areas that control singing (i.e. the song control system, SCS) are larger during the breeding season than at other times of the year. In the family of titmice and chickadees (Paridae), one species, the blue tit (Cyanistes caeruleus), shows the typical pattern of seasonal changes, while another species, the black-capped chickadee (Poecile atricapillus), shows, at best, very reduced seasonal changes in the SCS. To test whether this pattern holds up in the two Parid lineages to which these two species belong, and to rule out that the differences in seasonal patterns observed were due to differences in geography or laboratory, we compared the seasonal patterns in two song system nuclei volumes (HVC and Area X) in willow tits (Poecile montanus), closely related to black-capped chickadees, and in great tits (Parus major), more closely related to blue tits, from the same area around Oulu, Finland. Both species had larger gonads in spring than during the rest of the year. Great tit males had a larger HVC in spring than at other times of the year, but their Area X did not change in size. Willow tits showed no seasonal change in HVC or Area X size, despite having much larger gonads in spring than the great tits. Our findings suggest that the song system of willow tits and their relatives may be involved in learning and producing nonsong social vocalizations. Since these vocalizations are used year-round, there may be a year-round demand on the song system. The great tit and blue tit HVC may change seasonally because the demand is only placed on the song system during the breeding season, since they only produce learned vocalizations during this time. We suggest that changes were not observed in Area X because its main role is in song learning, and there is evidence that great tits do not learn new songs after their first year of life. Further study is required to determine whether our hypothesis about the role of the song system in the learned, nonsong vocalizations of the willow tit and chickadee is correct, and to test our hypothesis about the role of Area X in the great tit song system.

Endocrine and social regulation of adult neurogenesis in songbirds

The identification of pronounced seasonal changes in the volume of avian song control nuclei stimulated the discovery of adult neurogenesis in songbirds as well as renewed studies in mammals including humans. Neurogenesis in songbirds is modulated by testosterone and other factors such as photoperiod, singing activity and social environment. Adult neurogenesis has been widely studied by labeling, with tritiated thymidine or its analog BrdU, cells duplicating their DNA in anticipation of their last mitotic division and following their fate as new neurons. New methods based on endogenous markers of cell cycling or of various stages of neuronal life have allowed for additional progress. In particular immunocytochemical visualization of the microtubule-associated protein doublecortin has provided an integrated view of neuronal replacement in the song control nucleus HVC. Multiple questions remain however concerning the specific steps in the neuronal life cycle that are modulated by various factors and the underlying cellular mechanisms.

Light pollution alters the phenology of dawn and dusk singing in common European songbirds

Artificial night lighting is expanding globally, but its ecological consequences remain little understood. Animals often use changes in day length as a cue to time seasonal behaviour. Artificial night lighting may influence the perception of day length, and may thus affect both circadian and circannual rhythms. Over a 3.5 month period, from winter to breeding, we recorded daily singing activity of six common songbird species in 12 woodland sites, half of which were affected by street lighting. We previously reported on analyses suggesting that artificial night lighting affects the daily timing of singing in five species. The main aim of this study was to investigate whether the presence of artificial night lighting is also associated with the seasonal occurrence of dawn and dusk singing. We found that in four species dawn and dusk singing developed earlier in the year at sites exposed to light pollution. We also examined the effects of weather conditions and found that rain and low temperatures negatively affected the occurrence of dawn and dusk singing. Our results support the hypothesis that artificial night lighting alters natural seasonal rhythms, independently of other effects of urbanization. The fitness consequences of the observed changes in seasonal timing of behaviour remain unknown.

Pleiotropic Control by Testosterone of a Learned Vocal Behavior and Its Underlying Neuroplasticity(1,2,3)

Steroid hormones coordinate multiple aspects of behavior and physiology. The same hormone often regulates different aspects of a single behavior and its underlying neuroplasticity. This pleiotropic regulation of behavior and physiology is not well understood. Here, we investigated the orchestration by testosterone (T) of birdsong and its neural substrate, the song control system. Male canaries were castrated and received stereotaxic implants filled with T in select brain areas. Implanting T solely in the medial preoptic nucleus (POM) increased the motivation to sing, but did not enhance aspects of song quality such as acoustic structure and stereotypy. In birds implanted with T solely in HVC (proper name), a key sensorimotor region of the song control system, little or no song was observed, similar to castrates that received no T implants of any sort. However, implanting T in HVC and POM simultaneously rescued all measures of song quality. Song amplitude, though, was still lower than what was observed in birds receiving peripheral T treatment. T in POM enhanced HVC volume bilaterally, likely due to activity-dependent changes resulting from an enhanced song rate. T directly in HVC, without increasing song rate, enhanced HVC volume on the ipsilateral side only. T in HVC enhanced the incorporation and recruitment of new neurons into this nucleus, while singing activity can independently influence the incorporation of new neurons into HVC. These results have broad implications for how steroid hormones integrate across different brain regions to coordinate complex social behaviors.

Effectiveness of the GnRH agonist Deslorelin as a tool to decrease levels of circulating testosterone in zebra finches

Songbirds are widely used in studies of the neurobiology underlying learning, memory and performance of the sounds used in vocal communication. Development and activity of neurons in many brain sites implicated in those behaviors are closely related to levels of circulating testosterone. Approaches to understand the effects of testosterone in songbirds are presently limited to testosterone implants, which elevate testosterone levels to supraphysiological values, or castration, which eliminates gonadal production of testosterone. Previous studies in mammals indicate that GnRH agonists may be an effective tool to reduce testosterone within that range of extremes and without invasive surgery. To evaluate the effectiveness of the GnRH agonist Deslorelin as a tool to modulate levels of testosterone in songbirds, we recorded the effects of Deslorelin in adult male zebra finches. We recorded songs, body mass and blood testosterone levels pre-treatment, then we gave each bird a small subcutaneous implant of Deslorelin. We measured blood plasma testosterone levels weekly and recorded song behavior and gross morphology of brain, testes and heart at the end of each experiment. Testosterone levels were reduced at the 5mg/kg dose, and the very slight song changes we observed at that dose were like those reported for castrated zebra finches. As expected, there were no changes in the number of cells in androgen-sensitive brain structures. Suppression of testosterone at the 5mg/kg dose was reversible through implant removal. Thus, Deslorelin is a new tool to transiently suppress testosterone levels without the invasiveness and undesirable aftereffects of surgical castration.

Testosterone Mediates Seasonal Growth of the Song Control Nuclei in a Tropical Bird

In mid- to high-latitude songbirds, seasonal reproduction is stimulated by increasing day length accompanied by elevated plasma sex steroid levels, increased singing, and growth of the song control nuclei (SCN). Plasticity of the SCN and song behavior are primarily mediated by testosterone (T) and its metabolites in most species studied thus far. However, the majority of bird species are tropical and have less pronounced seasonal reproductive cycles. We have previously documented that equatorial rufous-collared sparrows (Zonotrichia capensis) exhibit seasonal neuroplasticity in the SCN. Manipulating T in these birds, however, did not alter singing behavior. In the current study, we investigated whether T mediates plasticity of the SCN in a similar manner to temperate songbirds. In the first experiment, we treated captive male birds with T or blank implants during the nonbreeding season. In a second experiment, we treated captive male birds with either blank implants, T-filled implants, T with flutamide (FLU; an androgen receptor antagonist) or T with FLU and 1,4,6-androstatriene-3,17-dione (ATD; an estrogen synthesis inhibitor) during the breeding season. In both experiments, the volumes of the brain areas high vocal center (HVC), Area X, and robust nucleus of the arcopallium (RA) were measured along with singing behavior. In summary, T stimulated growth of HVC and RA, and the combined effect of FLU and ATD reversed this effect in HVC. Area X was not affected by T treatment in either experiment. Neither T-treated birds nor controls sang in captivity during either experiment. Together, these data indicate that T mediates seasonal changes in the HVC and RA of both tropical and higher- latitude bird species even if the environmental signals differ. However, unlike most higher-latitude songbirds, we found no evidence that motivation to sing or growth of Area X are stimulated by T under captive conditions.

Duration of melatonin regulates seasonal plasticity in subtropical Indian weaver bird, Ploceus philippinus

Day length regulates seasonal plasticity connected with reproduction in birds. Rhythmic pineal melatonin secretion is a reliable indicator of the night length, hence day length. Removal of rhythmic melatonin secretion by exposure to constant bright light (LLbright) or by pinealectomy renders several species of songbirds including Indian weaver bird (Ploceus philippinus) arrhythmic. Present study investigated whether rhythmic melatonin is involved in the regulation of key reproductive neuropeptides (GnRH I and GnIH) and reproduction linked neural changes, viz. song control nuclei, in Indian weaver birds. Two experiments were performed using birds in an arrhythmic condition with low (under LLbright) or no (in the absence of pineal gland) endogenous melatonin. In experiment I, three groups of birds (n=5 each) entrained to 12L:12D were exposed to LLbright (25lux) for two weeks. Beginning on day 15 of LLbright, a control group received vehicle for 16h and two treatment groups were given melatonin in drinking water for 8h or 16h. In experiment II, one group of sham-operated and three groups of pinealectomized birds (n=5 each) entrained to 12L:12D were exposed to constant dim light (LLdim, 0.5lux). Beginning on day 15 of LLdim, three groups received similar treatment as in experiment I. Birds were perfused after thirty cycles of the melatonin treatment, and brain sections were immunohistochemically double-labeled for GnRH I and GnIH or Nissl stained. Activity was recorded throughout the experiments, while body mass and testes were measured at the beginning and end of the experiment. Birds were synchronized with melatonin cycles and measured the duration of melatonin as "night". Pinealectomized birds that received 16h of melatonin had significantly higher GnIH-ir cells than those received 8h melatonin; there was no difference in the GnRH I immunoreactivity between two treatment groups however. Intact birds that received long duration melatonin cycles exhibited small song control nuclei, specifically the high vocal center (HVC) and the robust nucleus of the arcopallium (RA), while birds that received short duration melatonin or no melatonin exhibited large HVC and RA. Thus, melatonin possibly regulates seasonal reproduction via GnIH secretion, and also controls seasonal neuroplasticity in the song control system in songbirds.

Lesions targeted to the anterior forebrain disrupt vocal variability associated with testosterone-induced sensorimotor song development in adult female canaries, Serinus canaria

Learned communication was a trait observed in a limited number of vertebrates such as humans but also songbirds (i.e., species in the suborder passeri sometimes called oscines). Robust male-biased sex-differences in song development and production have been observed in several songbird species. However, in some of these species treating adult females with testosterone (T) induced neuro-behavioral changes such that females become more male-like in brain and behavior. T-treatment in these adult females seemed to stimulate sensorimotor song development to facilitate song masculinization. In male songbirds it was known that the lateral magnocellular nucleus of the anterior nidopallium (LMAN) played a modulatory role during song development. LMAN was androgen sensitive and may be a key target of a T-induced recapitulation of a developmental process in adult females. This hypothesis was tested. Adult female canaries were given either a chemical lesion of LMAN or a control sham-surgery. Prior to surgery birds were individually housed for 2-weeks in sound-attenuated chambers to record baseline vocal behavior. Post-surgery birds were given 1-week to recover before subcutaneous implantation with silastic capsules filled with crystalline-T. Birds remained on treatment for 3-weeks (behavioral recordings continued throughout). Birds with a lesion to LMAN had less variability in their song compared with controls. The diversity of syllable and phrase type(s) was greater in sham controls as compared with birds with LMAN lesions. Birds did not differ in song rate. These data suggested that the sustention and conclusion of T-induced sensorimotor song development in adult female canaries required an intact LMAN.

Seasonal Variation in Forebrain Region Sizes in Male Ruffed Grouse (Bonasa umbellus)

The song system of songbirds has provided significant insight into the underlying mechanisms and behavioural consequences of seasonal neuroplasticity. The extent to which seasonal changes in brain region volumes occur in non-songbird species has, however, remained largely untested. Here, we tested whether brain region volumes varied with season in the ruffed grouse (Bonasa umbellus), a gallinaceous bird that produces a unique wing-beating display known as 'drumming' as its primary form of courtship behaviour. Using unbiased stereology, we measured the sizes of the cerebellum, nucleus rotundus, telencephalon, mesopallium, hippocampal formation, striatopallidal complex and arcopallium across spring males, fall males and fall females. The majority of these brain regions did not vary significantly across these three groups. The two exceptions were the striatopallidal complex and arcopallium, both of which were significantly larger in spring males that are actively drumming. These seasonal changes in volume strongly implicate the striatopallidal complex and arcopallium as key structures in the production and/or modulation of the ruffed grouse drumming display and represent the first evidence of seasonal plasticity in the telencephalon underlying a non-vocal courtship behaviour. Our findings also suggest that seasonal plasticity in the striatopallidal complex and arcopallium might be a trait that is shared across many bird species and that both structures are related to the production of multiple forms of courtship and not just learned song.

Real-time monitoring of electrically evoked catecholamine signals in the songbird striatum using in vivo fast-scan cyclic voltammetry

Fast-scan cyclic voltammetry is a powerful technique for monitoring rapid changes in extracellular neurotransmitter levels in the brain. In vivo fast-scan cyclic voltammetry has been used extensively in mammalian models to characterize dopamine signals in both anesthetized and awake preparations, but has yet to be applied to a non-mammalian vertebrate. The goal of this study was to establish in vivo fast-scan cyclic voltammetry in a songbird, the European starling, to facilitate real-time measurements of extracellular catecholamine levels in the avian striatum. In urethane-anesthetized starlings, changes in catecholamine levels were evoked by electrical stimulation of the ventral tegmental area and measured at carbon-fiber microelectrodes positioned in the medial and lateral striata. Catecholamines were elicited by different stimulations, including trains related to phasic dopamine signaling in the rat, and were analyzed to quantify presynaptic mechanisms governing exocytotic release and neuronal uptake. Evoked extracellular catecholamine dynamics, maximal amplitude of the evoked catecholamine signal, and parameters for catecholamine release and uptake did not differ between striatal regions and were similar to those determined for dopamine in the rat dorsomedial striatum under similar conditions. Chemical identification of measured catecholamine by its voltammogram was consistent with the presence of both dopamine and norepinephrine in striatal tissue content. However, the high ratio of dopamine to norepinephrine in tissue content and the greater sensitivity of the carbon-fiber microelectrode to dopamine compared to norepinephrine favored the measurement of dopamine. Thus, converging evidence suggests that dopamine was the predominate analyte of the electrically evoked catecholamine signal measured in the striatum by fast-scan cyclic voltammetry. Overall, comparisons between the characteristics of these evoked signals suggested a similar presynaptic regulation of dopamine in the starling and rat striatum. Fast-scan cyclic voltammetry thus has the potential to be an invaluable tool for investigating the neural underpinnings of behavior in birds.

Seasonal change in the avian hippocampus

The hippocampus plays an important role in cognitive processes, including memory and spatial orientation, in birds. The hippocampus undergoes seasonal change in food-storing birds and brood parasites, there are changes in the hippocampus during breeding, and further changes occur in some species in association with migration. In food-storing birds, seasonal change in the hippocampus occurs in fall and winter when the cognitively demanding behaviour of caching and retrieving food occurs. The timing of annual change in the hippocampus of food-storing birds is quite variable, however, and appears not to be under photoperiod control. A variety of factors, including cognitive performance, exercise, and stress may all influence seasonal change in the avian hippocampus. The causal processes underlying seasonal change in the avian hippocampus have not been extensively examined and the more fully described hormonal influences on the mammalian hippocampus may provide hypotheses for investigating the control of hippocampal seasonality in birds.

Transsynaptic trophic effects of steroid hormones in an avian model of adult brain plasticity

The avian song control system provides an excellent model for studying transsynaptic trophic effects of steroid sex hormones. Seasonal changes in systemic testosterone (T) and its metabolites regulate plasticity of this system. Steroids interact with the neurotrophin brain-derived neurotrophic factor (BDNF) to influence cellular processes of plasticity in nucleus HVC of adult birds, including the addition of newborn neurons. This interaction may also occur transsynpatically; T increases the synthesis of BDNF in HVC, and BDNF protein is then released by HVC neurons on to postsynaptic cells in nucleus RA where it has trophic effects on activity and morphology. Androgen action on RA neurons increases their activity and this has a retrograde trophic effect on the addition of new neurons to HVC. The functional linkage of sex steroids to BDNF may be of adaptive value in regulating the trophic effects of the neurotrophin and coordinating circuit function in reproductively relevant contexts.

Contextual modulation of social and endocrine correlates of fitness: insights from the life history of a sex changing fish

Steroid hormones are critical regulators of reproductive life history, and the steroid sensitive traits (morphology, behavior, physiology) associated with particular life history stages can have substantial fitness consequences for an organism. Hormones, behavior and fitness are reciprocally associated and can be used in an integrative fashion to understand how the environment impacts organismal function. To address the fitness component, we highlight the importance of using reliable proxies of reproductive success when studying proximate regulation of reproductive phenotypes. To understand the mechanisms by which the endocrine system regulates phenotype, we discuss the use of particular endocrine proxies and the need for appropriate functional interpretation of each. Lastly, in any experimental paradigm, the responses of animals vary based on the subtle differences in environmental and social context and this must also be considered. We explore these different levels of analyses by focusing on the fascinating life history transitions exhibited by the bi-directionally hermaphroditic fish, Lythrypnus dalli. Sex changing fish are excellent models for providing a deeper understanding of the fitness consequences associated with behavioral and endocrine variation. We close by proposing that local regulation of steroids is one potential mechanism that allows for the expression of novel phenotypes that can be characteristic of specific life history stages. A comparative species approach will facilitate progress in understanding the diversity of mechanisms underlying the contextual regulation of phenotypes and their associated fitness correlates.

Sexually-motivated song is predicted by androgen-and opioid-related gene expression in the medial preoptic nucleus of male European starlings (Sturnus vulgaris)

Across vertebrates, communication conveys information about an individual's motivational state, yet little is known about the neuroendocrine regulation of motivational aspects of communication. For seasonally breeding songbirds, increases in testosterone in spring stimulate high rates of sexually-motivated courtship song, though not all birds sing at high rates. It is generally assumed that testosterone or its metabolites act within the medial preoptic nucleus (POM) to stimulate the motivation to sing. In addition to androgen receptors (ARs) and testosterone, opioid neuropeptides in the POM influence sexually-motivated song production, and it has been proposed that testosterone may in part regulate song by modifying opioid systems. To gain insight into a possible role for androgen-opioid interactions in the regulation of communication we examined associations between sexually-motivated song and relative expression of ARs, mu opioid receptors (muORs), and preproenkephalin (PENK) in the POM (and other regions) of male European starlings using qPCR. Both AR and PENK expression in POM correlated positively with singing behavior, whereas muOR in POM correlated negatively with song. Furthermore, the ratio of PENK/muOR expression correlated negatively with AR expression in POM. Finally, in the ventral tegmental area (VTA), PENK expression correlated negatively with singing behavior. Results support the hypothesis that ARs may alter opioid gene expression in POM to fine-tune singing to reflect a male's motivational state. Data also suggest that bidirectional relationships may exist between opioids and ARs in POM and song, and additionally support a role for opioids in the VTA, independent of AR activity in this region.

Endogenous versus exogenous markers of adult neurogenesis in canaries and other birds: advantages and disadvantages

Although the existence of newborn neurons had originally been suggested, but not broadly accepted, based on studies in adult rodent brains, the presence of an active neurogenesis process in adult homoeothermic vertebrates was first firmly established in songbirds. Adult neurogenesis was initially studied with the tritiated thymidine technique, later replaced by the injection and detection of the marker of DNA replication 5-bromo-2'-deoxyuridine (BrdU). More recently, various endogenous markers were used to identify young neurons or cycling neuronal progenitors. We review here the respective advantages and pitfalls of these different approaches in birds, with specific reference to the microtubule-associated protein, doublecortin (DCX), that has been extensively used to identify young newly born neurons in adult brains. All these techniques of course have limitations. Exogenous markers label cells replicating their DNA only during a brief period and it is difficult to select injection doses that would exhaustively label all these cells without inducing DNA damage that will also result in some form of labeling during repair. On the other hand, specificity of endogenous markers is difficult to establish due to problems related to the specificity of antibodies (these problems can be, but are not always, addressed) and more importantly because it is difficult, if not impossible, to prove that a given marker exhaustively and specifically labels a given cell population. Despite these potential limitations, these endogenous markers and DCX staining in particular clearly represent a useful approach to the detailed study of neurogenesis especially when combined with other techniques such as BrdU.

Dissociable effects of social context on song and doublecortin immunoreactivity in male canaries

Variation in environmental factors such as day length and social context greatly affects reproductive behavior and the brain areas that regulate these behaviors. One such behavior is song in songbirds, which males use to attract a mate during the breeding season. In these species the absence of a potential mate leads to an increase in the number of songs produced, while the presence of a mate greatly diminishes singing. Interestingly, although long days promote song behavior, producing song itself can promote the incorporation of new neurons in brain regions controlling song output. Social context can also affect such neuroplasticity in these song control nuclei. The goal of the present study was to investigate in canaries (Serinus canaria), a songbird species, how photoperiod and social context affect song and the incorporation of new neurons, as measured by the microtubule-associated protein doublecortin (DCX) in HVC, a key vocal production brain region of the song control system. We show that long days increased HVC size and singing activity. In addition, male canaries paired with a female for 2 weeks showed enhanced DCX-immunoreactivity in HVC relative to birds housed alone. Strikingly, however, paired males sang fewer songs that exhibited a reduction in acoustic features such as song complexity and energy, compared with birds housed alone, which sang prolifically. These results show that social presence plays a significant role in the regulation of neural and behavioral plasticity in songbirds and can exert these effects in opposition to what might be expected based on activity-induced neurogenesis.

Models of vocal learning in the songbird: Historical frameworks and the stabilizing critic

Birdsong is a form of sensorimotor learning that involves a mirror-like system that activates with both song hearing and production. Early models of song learning, based on behavioral measures, identified key features of vocal plasticity, such as the requirements for memorization of a tutor song and auditory feedback during song practice. The concept of a comparator, which compares the memory of the tutor song to auditory feedback, featured prominently. Later models focused on linking anatomically-defined neural modules to behavioral concepts, such as the comparator. Exploiting the anatomical modularity of the songbird brain, localized lesions illuminated mechanisms of the neural song system. More recent models have integrated neuronal mechanisms identified in other systems with observations in songbirds. While these models explain multiple aspects of song learning, they must incorporate computational elements based on unknown biological mechanisms to bridge the motor-to-sensory delay and/or transform motor signals into the sensory domain. Here, I introduce the stabilizing critic hypothesis, which enables sensorimotor learning by (1) placing a purely sensory comparator afferent of the song system and (2) endowing song system disinhibitory interneuron networks with the capacity both to bridge the motor-sensory delay through prolonged bursting and to stabilize song segments selectively based on the comparator signal. These proposed networks stabilize an otherwise variable signal generated by both putative mirror neurons and a cortical-basal ganglia-thalamic loop. This stabilized signal then temporally converges with a matched premotor signal in the efferent song motor cortex, promoting spike-timing-dependent plasticity in the premotor circuitry and behavioral song learning.

The role of the pineal gland in the photoperiodic control of bird song frequency and repertoire in the house sparrow, Passer domesticus

Temperate zone birds are highly seasonal in many aspects of their physiology. In mammals, but not in birds, the pineal gland is an important component regulating seasonal patterns of primary gonadal functions. Pineal melatonin in birds instead affects seasonal changes in brain song control structures, suggesting the pineal gland regulates seasonal song behavior. The present study tests the hypothesis that the pineal gland transduces photoperiodic information to the control of seasonal song behavior to synchronize this important behavior to the appropriate phenology. House sparrows, Passer domesticus, expressed a rich array of vocalizations ranging from calls to multisyllabic songs and motifs of songs that varied under a regimen of different photoperiodic conditions that were simulated at different times of year. Control (SHAM) birds exhibited increases in song behavior when they were experimentally transferred from short days, simulating winter, to equinoctial and long days, simulating summer, and decreased vocalization when they were transferred back to short days. When maintained in long days for longer periods, the birds became reproductively photorefractory as measured by the yellowing of the birds' bills; however, song behavior persisted in the SHAM birds, suggesting a dissociation of reproduction from the song functions. Pinealectomized (PINX) birds expressed larger, more rapid increases in daily vocal rate and song repertoire size than did the SHAM birds during the long summer days. These increases gradually declined upon the extension of the long days and did not respond to the transfer to short days as was observed in the SHAM birds, suggesting that the pineal gland conveys photoperiodic information to the vocal control system, which in turn regulates song behavior.

Status-appropriate singing behavior, testosterone and androgen receptor immunolabeling in male European starlings (Sturnus vulgaris)

Vocalizations convey information about an individual's motivational, internal, and social status. As circumstances change, individuals respond by adjusting vocal behavior accordingly. In European starlings, a male that acquires a nest site socially dominates other males and dramatically increases courtship song. Although circulating testosterone is associated with social status and vocal production it is possible that steroid receptors fine-tune status-appropriate changes in behavior. Here we explored a possible role for androgen receptors. Male starlings that acquired nest sites produced high rates of courtship song. For a subset of males this occurred even in the absence of elevated circulating testosterone. Immunolabeling for androgen receptors (ARir) was highest in the medial preoptic nucleus (POM) in males with both a nest site and elevated testosterone. For HVC, ARir was higher in dominant males with high testosterone (males that sang longer songs) than dominant males with low testosterone (males that sang shorter songs). ARir in the dorsal medial portion of the nucleus intercollicularis (DM) was elevated in males with high testosterone irrespective of dominance status. Song bout length related positively to ARir in POM, HVC and DM, and testosterone concentrations related positively to ARir in POM and DM. Results suggest that the role of testosterone in vocal behavior differs across brain regions and support the hypothesis that testosterone in POM underlies motivation, testosterone in HVC relates to song quality, and testosterone in DM stimulates vocalizations. Our data also suggest that singing may influence AR independent of testosterone and that alternative androgen-independent pathways regulate status-appropriate singing behavior.

Avian circadian organization: a chorus of clocks

In birds, biological clock function pervades all aspects of biology, controlling daily changes in sleep: wake, visual function, song, migratory patterns and orientation, as well as seasonal patterns of reproduction, song and migration. The molecular bases for circadian clocks are highly conserved, and it is likely the avian molecular mechanisms are similar to those expressed in mammals, including humans. The central pacemakers in the avian pineal gland, retinae and SCN dynamically interact to maintain stable phase relationships and then influence downstream rhythms through entrainment of peripheral oscillators in the brain controlling behavior and peripheral tissues. Birds represent an excellent model for the role played by biological clocks in human neurobiology; unlike most rodent models, they are diurnal, they exhibit cognitively complex social interactions, and their circadian clocks are more sensitive to the hormone melatonin than are those of nocturnal rodents.

Functional changes between seasons in the male songbird auditory forebrain

Songbirds are an excellent model for investigating the perception of learned complex acoustic communication signals. Male European starlings (Sturnus vulgaris) sing throughout the year distinct types of song that bear either social or individual information. Although the relative importance of social and individual information changes seasonally, evidence of functional seasonal changes in neural response to these songs remains elusive. We thus decided to use in vivo functional magnetic resonance imaging (fMRI) to examine auditory responses of male starlings that were exposed to songs that convey different levels of information (species-specific and group identity or individual identity), both during (when mate recognition is particularly important) and outside the breeding season (when group recognition is particularly important). We report three main findings: (1) the auditory area caudomedial nidopallium (NCM), an auditory region that is analogous to the mammalian auditory cortex, is clearly involved in the processing/categorization of conspecific songs; (2) season-related change in differential song processing is limited to a caudal part of NCM; in the more rostral parts, songs bearing individual information induce higher BOLD responses than songs bearing species and group information, regardless of the season; (3) the differentiation between songs bearing species and group information and songs bearing individual information seems to be biased toward the right hemisphere. This study provides evidence that auditory processing of behaviorally-relevant (conspecific) communication signals changes seasonally, even when the spectro-temporal properties of these signals do not change.

Differential effects of global versus local testosterone on singing behavior and its underlying neural substrate

Steroid hormones regulate multiple but distinct aspects of social behaviors. Testosterone (T) has multiple effects on learned courtship song in that it regulates both the motivation to sing in a particular social context as well as the quality of song produced. The neural substrate(s) where T acts to regulate the motivation to sing as opposed to other aspects of song has not been definitively characterized. We show here that T implants in the medial preoptic nucleus (POM) of castrated male canaries (Serinus canaria) increase song rate but do not enhance acoustic features such as song stereotypy compared with birds receiving peripheral T that can act globally throughout the brain. Strikingly, T action in the POM increased song control nuclei volume, consistent with the hypothesis that singing activity induces neuroplasticity in the song control system independent of T acting in these nuclei. When presented with a female canary, POM-T birds copulated at a rate comparable to birds receiving systemic T but produced fewer calls and songs in her presence. Thus, POM is a key site where T acts to activate copulation and increase song rate, an appetitive sexual behavior in songbirds, but T action in other areas of the brain or periphery (e.g., HVC, dopaminergic cell groups, or the syrinx) is required to enhance the quality of song (i.e., stereotypy) as well as regulate context-specific vocalizations. These results have broad implications for research concerning how steroids act at multiple brain loci to regulate distinct sociosexual behaviors and the associated neuroplasticity.

Recent evidence for rapid synthesis and action of oestrogens during auditory processing in a songbird

It is now clear that oestrogens are not only circulating reproductive hormones, but that they also have neurotransmitter-like properties in a wide range of brain circuits. The view of oestrogens as intrinsic neuromodulators that shape behaviour has been bolstered by a series of recent developments from multiple vertebrate model systems. Here, we review several recent findings from studies of songbirds showing how the identified neural circuits that govern auditory processing and sensorimotor integration are modulated by the local and acute production of oestrogens. First, studies using in vivo microdialysis demonstrate that oestrogens fluctuate in the auditory cortex (30-min time bin resolution) when songbirds are hearing song and interacting with conspecifics. Second, oestrogens rapidly boost the auditory-evoked activity of neurones in the same auditory cortical region, enhancing auditory processing. Third, local pharmacological blockade of oestrogen signalling in this region impairs auditory neuronal responsiveness, as well as behavioural song preferences. Fourth, the rapid actions of oestrogens that occur within the auditory cortex can propagate downstream (trans-synaptically) to sensorimotor circuits to enhance the neural representation of song. Lastly, we present new evidence showing that the receptor for the rapid actions of oestradiol is likely in neuronal membranes, and that traditional nuclear oestrogen receptor agonists do not mimic these rapid actions. Broadly speaking, many of these findings are observed in both males and females, emphasising the fundamental importance of oestrogens in neural circuit function. Together, these and other emergent studies provide support for rapid, brain-derived oestrogen signalling in regulating sensorimotor integration, learning and perception.

Testosterone depletion in adult male rats increases mossy fiber transmission, LTP, and sprouting in area CA3 of hippocampus

Androgens have dramatic effects on neuronal structure and function in hippocampus. However, androgen depletion does not always lead to hippocampal impairment. To address this apparent paradox, we evaluated the hippocampus of adult male rats after gonadectomy (Gdx) or sham surgery. Surprisingly, Gdx rats showed increased synaptic transmission and long-term potentiation of the mossy fiber (MF) pathway. Gdx rats also exhibited increased excitability and MF sprouting. We then addressed the possible underlying mechanisms and found that Gdx induced a long-lasting upregulation of MF BDNF immunoreactivity. Antagonism of Trk receptors, which bind neurotrophins, such as BDNF, reversed the increase in MF transmission, excitability, and long-term potentiation in Gdx rats, but there were no effects of Trk antagonism in sham controls. To determine which androgens were responsible, the effects of testosterone metabolites DHT and 5α-androstane-3α,17β-diol were examined. Exposure of slices to 50 nm DHT decreased the effects of Gdx on MF transmission, but 50 nm 5α-androstane-3α,17β-diol had no effect. Remarkably, there was no effect of DHT in control males. The data suggest that a Trk- and androgen receptor-sensitive form of MF transmission and synaptic plasticity emerges after Gdx. We suggest that androgens may normally be important in area CA3 to prevent hyperexcitability and aberrant axon outgrowth but limit MF synaptic transmission and some forms of plasticity. The results also suggest a potential explanation for the maintenance of hippocampal-dependent cognitive function after androgen depletion: a reduction in androgens may lead to compensatory upregulation of MF transmission and plasticity.

With a little help from my friends: androgens tap BDNF signaling pathways to alter neural circuits

Gonadal androgens are critical for the development and maintenance of sexually dimorphic regions of the male nervous system, which is critical for male-specific behavior and physiological functioning. In rodents, the motoneurons of the spinal nucleus of the bulbocavernosus (SNB) provide a useful example of a neural system dependent on androgen. Unless rescued by perinatal androgens, the SNB motoneurons will undergo apoptotic cell death. In adulthood, SNB motoneurons remain dependent on androgen, as castration leads to somal atrophy and dendritic retraction. In a second vertebrate model, the zebra finch, androgens are critical for the development of several brain nuclei involved in song production in males. Androgen deprivation during a critical period during postnatal development disrupts song acquisition and dimorphic size-associated nuclei. Mechanisms by which androgens exert masculinizing effects in each model system remain elusive. Recent studies suggest that brain-derived neurotrophic factor (BDNF) may play a role in androgen-dependent masculinization and maintenance of both SNB motoneurons and song nuclei of birds. This review aims to summarize studies demonstrating that BDNF signaling via its tyrosine receptor kinase (TrkB) receptor may work cooperatively with androgens to maintain somal and dendritic morphology of SNB motoneurons. We further describe studies that suggest the cellular origin of BDNF is of particular importance in androgen-dependent regulation of SNB motoneurons. We review evidence that androgens and BDNF may synergistically influence song development and plasticity in bird species. Finally, we provide hypothetical models of mechanisms that may underlie androgen- and BDNF-dependent signaling pathways.

Estrogen promotes learning-related plasticity by modifying the synaptic cytoskeleton

Estrogen's acute, facilitatory effects on glutamatergic transmission and long-term potentiation (LTP) provide a potential explanation for the steroid's considerable influence on behavior. Recent work has identified mechanisms underlying these synaptic actions. Brief infusion of 17ß-estradiol (E2) into adult male rat hippocampal slices triggers actin polymerization within dendritic spines via a signaling cascade beginning with the GTPase RhoA and ending with inactivation of the filament-severing protein cofilin. Blocking this sequence, or actin polymerization itself, eliminates E2's effects on synaptic physiology. Notably, the theta burst stimulation used to induce LTP activates the same signaling pathway as E2 plus events that stabilize the reorganization of the sub-synaptic cytoskeleton. These observations suggest that E2 elicits a partial form of LTP, resulting in an increase of fast excitatory postsynaptic potentials (EPSPs) and a reduction in the threshold for lasting synaptic changes. While E2's effects on the cytoskeleton could be direct, results described here indicate that the hormone activates synaptic tropomyosin-related kinase B (TrkB) receptors for brain-derived neurotrophic factor (BDNF), a releasable neurotrophin that stimulates the RhoA to cofilin pathway. It is therefore possible that E2 acts via transactivation of neighboring receptors to modify the composition and structure of excitatory contacts. Finally, there is the question of whether a loss of acute synaptic actions contributes to the memory problems associated with estrogen depletion. Initial tests found that ovariectomy in middle-aged rats disrupts RhoA signaling, actin polymerization, and LTP consolidation. Acute applications of E2 reversed these defects, a result consistent with the idea that disturbances to actin management are one cause of behavioral effects that emerge with reductions in steroid levels.

High throughput analysis reveals dissociable gene expression profiles in two independent neural systems involved in the regulation of social behavior

Background

Production of contextually appropriate social behaviors involves integrated activity across many brain regions. Many songbird species produce complex vocalizations called ‘songs’ that serve to attract potential mates, defend territories, and/or maintain flock cohesion. There are a series of discrete interconnect brain regions that are essential for the successful production of song. The probability and intensity of singing behavior is influenced by the reproductive state. The objectives of this study were to examine the broad changes in gene expression in brain regions that control song production with a brain region that governs the reproductive state.

Results

We show using microarray cDNA analysis that two discrete brain systems that are both involved in governing singing behavior show markedly different gene expression profiles. We found that cortical and basal ganglia-like brain regions that control the socio-motor production of song in birds exhibit a categorical switch in gene expression that was dependent on their reproductive state. This pattern is in stark contrast to the pattern of expression observed in a hypothalamic brain region that governs the neuroendocrine control of reproduction. Subsequent gene ontology analysis revealed marked variation in the functional categories of active genes dependent on reproductive state and anatomical localization. HVC, one cortical-like structure, displayed significant gene expression changes associated with microtubule and neurofilament cytoskeleton organization, MAP kinase activity, and steroid hormone receptor complex activity. The transitions observed in the preoptic area, a nucleus that governs the motivation to engage in singing, exhibited variation in functional categories that included thyroid hormone receptor activity, epigenetic and angiogenetic processes.

Conclusions

These findings highlight the importance of considering the temporal patterns of gene expression across several brain regions when engaging in social behaviors.

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.

Variation in the gonadotrophin-releasing hormone-1 and the song control system in the tropical breeding rufous-collared sparrow (Zonotrichia capensis) is dependent on sex and reproductive state

Seasonal breeding in temperate zone vertebrates is characterised by pronounced variation in both central and peripheral reproductive physiology as well as behaviour. In contrast, many tropical species have a comparatively longer and less of a seasonal pattern of breeding than their temperate zone counterparts. These extended, more "flexible" reproductive periods may be associate with a lesser degree of annual variation in reproductive physiology. Here we investigated variation in the neuroendocrine control of reproduction in relation to the changes in the neural song control system in a tropical breeding songbird the rufous-collared sparrows (Zonotrichia capensis). Using in situ hybridization, we show that the optical density of GnRH1 mRNA expression is relatively constant across pre-breeding and breeding states. However, males were found to have significantly greater expression compared to females regardless of breeding state. Both males and females showed marked variation in measures of peripheral reproductive physiology with greater gonadal volumes and concentrations of sex steroids in the blood (i.e. testosterone in males; estrogen in females) during the breeding season as compared to the pre-breeding season. These findings suggest that the environmental cues regulating breeding in a tropical breeding bird ultimately exert their effects on physiology at the level of the median eminence and regulate the release of GnRH1. In addition, histological analysis of the song control system HVC, RA and Area X revealed that breeding males had significantly larger volumes of these brain nuclei as compared to non-breeding males, breeding females, and non-breeding females. Females did not exhibit a significant difference in the size of song control regions across breeding states. Together, these data show a marked sex difference in the extent to which there is breeding-associated variation in reproductive physiology and brain plasticity that is dependent on the reproductive state in a tropical breeding songbird.

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.