Gambel's White-crowned Sparrows Research Articles

Adult neurogenesis is necessary for functional regeneration of a forebrain neural circuit

In adult songbirds, new neurons are born in large numbers in the proliferative ventricular zone in the telencephalon and migrate to the adjacent song control region HVC (acronym used as proper name) [A. Reiner et al., J. Comp. Neurol. 473, 377-414 (2004)]. Many of these new neurons send long axonal projections to the robust nucleus of the arcopallium (RA). The HVC-RA circuit is essential for producing stereotyped learned song. The function of adult neurogenesis in this circuit has not been clear. A previous study suggested that it is important for the production of well-structured songs [R. E. Cohen, M. Macedo-Lima, K. E. Miller, E. A. Brenowitz, J. Neurosci. 36, 8947-8956 (2016)]. We tested this hypothesis by infusing the neuroblast migration inhibitor cyclopamine into HVC of male Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) to block seasonal regeneration of the HVC-RA circuit. Decreasing the number of new neurons in HVC prevented both the increase in spontaneous electrical activity of RA neurons and the improved structure of songs that would normally occur as sparrows enter breeding condition. These results show that the incorporation of new neurons into the adult HVC is necessary for the recovery of both electrical activity and song behavior in breeding birds and demonstrate the value of the bird song system as a model for investigating adult neurogenesis at the level of long projection neural circuits.

Brain-Derived Neurotrophic Factor Has a Transsynaptic Trophic Effect on Neural Activity in an Adult Forebrain Circuit.

While hormone-driven plasticity in the adult brain is well studied, the underlying cellular and molecular mechanisms are less well understood. One example of this is seasonal plasticity in the avian brain, where song nuclei exhibit hormonally driven changes in response to changing photoperiod and circulating sex steroid hormones. Hormone receptor activation in song nucleus HVC (proper name) elicits a robust change in activity in target nucleus RA (robust nucleus of the arcopallium), but the molecular signal responsible for this is unknown. This study addressed whether brain-derived neurotrophic factor (BDNF) mediates a transsynaptic effect from HVC to RA in male Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii). In situ hybridization confirmed an increase in BDNF expression in HVC neurons of birds switched to a long-day (LD) photoperiod plus systemically elevated testosterone (T) levels, compared with short-day (SD) conditions. BDNF expression was virtually absent in RA neurons of SD birds, increasing to barely detectable levels in a small subset of cells in LD+T birds. Infusion of BDNF protein adjacent to the RA of SD birds caused an increase in the spontaneous neuron firing rate. Conversely, the infusion of ANA12, a specific antagonist of the tyrosine-related kinase B (TrkB) for BDNF, prevented the increase in RA neuron firing rate in LD+T birds. These results indicate that BDNF is sufficient, and TrkB receptor activation is necessary, for the transsynaptic trophic effect exerted by HVC on RA. The dramatic change in the activity of RA neurons during the breeding season provides a clear example of transsynaptic BDNF effects in the adult brain in a functionally relevant circuit.SIGNIFICANCE STATEMENT Sex steroid hormones drive changes in brain circuits in all vertebrates, both within specific neurons and on their synaptic targets. Such changes can lead to profound changes in behavior, but little is known about the precise molecular mechanisms that underlie this process. We addressed this question in a seasonally breeding songbird and found that the trophic effects of one forebrain song nucleus on its target are mediated transsynaptically by the neurotrophin BDNF. This suggests that, in addition to their role in development, neurotrophins have critical roles in adult brain plasticity.

Phenotypic flexibility of glucocorticoid signaling in skeletal muscles of a songbird preparing to migrate

Glucocorticoids are commonly associated with responses to stress, but other important functions include homeostatic regulation, energy metabolism and tissue remodeling. At low circulating levels, glucocorticoids bind to high-affinity mineralocorticoid receptors (MR) to activate tissue repair and homeostasis (anabolic pathways), whereas at elevated levels, glucocorticoids bind to glucocorticoid receptors (GR) to activate catabolic pathways. Long distance migrations, such as those performed by Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii), require modification of anatomy, physiology and behavior. Plasma corticosterone (CORT) increases in association with impending departure and flight and may promote muscle-specific anabolic states. To test this idea, we explored glucocorticoid signaling in the pectoralis (flight) and gastrocnemius (leg) muscles of male sparrows on the wintering grounds at three stages leading up to spring departure: winter (February), pre-nuptial molt (March), and pre-departure (April). CORT was detected in plasma and in both muscles, but measures of CORT signaling differed across muscles and stages. Expression of 11β-hydroxysteroid dehydrogenase (11β-HSD) Type 2 (inactivates CORT) increased in the pectoralis at pre-departure, whereas 11β-HSD Type 1 (regenerates CORT) did not change. Neither of the two 11β-HSD isoforms was detectable in the gastrocnemius. Expression of MR, but not GR, was elevated in the pectoralis at pre-departure, while only GR expression was elevated at pre-nuptial molt in gastrocnemius. These data suggest that anabolic functions predominate in the pectoralis only while catabolic activity is undetected in either muscle at pre-departure.

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.

Preparing to migrate: expression of androgen signaling molecules and insulin-like growth factor-1 in skeletal muscles of Gambel's white-crowned sparrows.

Migratory birds, including Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii), exhibit profound modifications of skeletal muscles prior to migration, notably hypertrophy of the pectoralis muscle required for powered flight. Muscle growth may be influenced by anabolic effects of androgens; however, prior to spring departure, circulating androgens are low in sparrows. A seasonal increase in local androgen signaling may occur within muscle to promote remodeling. We measured morphological parameters, plasma and tissue levels of testosterone, as well as mRNA expression levels of androgen receptor, 5α-reductase (converts testosterone to 5α-dihydrotestosterone), and the androgen-dependent myotrophic factor insulin-like growth factor-1. We studied the pectoralis muscle as well as the gastrocnemius (leg) muscle of male sparrows across three stages on the wintering grounds: winter (February), pre-nuptial molt (March), and pre-departure (April). Testosterone levels were low, but detectable, in plasma and muscles at all three stages. Androgen receptor mRNA and 5α-reductase Type 1 mRNA increased at pre-departure, but did so in both muscles. Notably, mRNA levels of insulin-like growth factor-1, an androgen-dependent gene critical for muscle remodeling, increased at pre-departure in the pectoralis but decreased in the gastrocnemius. Taken together, these data suggest a site-specific molecular basis for muscle remodeling that may serve to enable long-distance flight.

Seasonal changes in neuronal turnover in a forebrain nucleus in adult songbirds.

Neuronal death and replacement, or neuronal turnover, in the adult brain are one of many fundamental processes of neural plasticity. The adult avian song control circuit provides an excellent model for exploring mature neuronal death and replacement by new neurons. In the song control nucleus, HVC of adult male Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelli) nearly 68,000 neurons are added each breeding season and die during the subsequent nonbreeding season. To accommodate large seasonal differences in HVC neuron number, the balance between neuronal addition and death in HVC must differ between seasons. To determine whether maintenance of new HVC neurons changes within and between breeding and nonbreeding conditions, we pulse-labeled two different cohorts of new HVC neurons under both conditions and quantified their maintenance. We show that the maintenance of new HVC neurons, as well as new nonneuronal cells, was higher at the onset of breeding conditions than at the onset of nonbreeding conditions. Once a steady-state HVC volume and neuronal number were attained in either breeding or nonbreeding conditions, neuronal and nonneuronal maintenance were similarly low. We found that new neuronal number correlated with a new nonneuronal number within each cohort of new neurons. Together, these data suggest that sex steroids promote the survival of an initial population of new neurons and nonneuronal cells entering HVC. However, once HVC is fully grown or regressed, neuronal and nonneuronal cell turnover is regulated by a common mechanism likely independent of direct sex steroid signaling.

Regulation of complex behavioural transitions: migration to breeding

The evolution of migration combines the selective advantages of breeding with ephemeral resources at high latitude or altitude with the benefits of overwintering at distant sites with adequate resources to promote survival. Migration between sites involves changes in behaviour, especially the transition from migration to nesting, or in autumn, the transition to wintering stage. We focus here on the vernal transition. Because migration and onset of nesting are mutually exclusive events, transition from a social, hyperphagic and highly mobile state to territoriality and pair bonding involves complex behavioural changes. In Arctic regions, weather in spring is unpredictable adding another dimension of complexity to the migration/breeding transition. Nevertheless, the brief Arctic summer requires that songbirds arrive on the breeding grounds while conditions may still be unfavourable, thus delaying nesting until conditions ameliorate. This requires flexibility and coordination of migration behaviour to be able to leave if weather deteriorates, or to become territorial, attract a mate and begin nesting within hours when conditions permit. In Gambel's white-crowned sparrows, Zonotrichia leucophrys gambelii, neuroendocrine regulation of reproductive development progresses throughout vernal migration so that males and females are mature when they arrive on the nesting grounds, but final ovarian maturation leading to ovulation and reproductive behaviours is suppressed. What provides the ‘brake’ to the onset of nesting? We focus on the adrenocortical response to acute stress that is greatly increased at arrival, especially in males, probably to enable flexibility in behavioural responses to severe weather. More recent evidence suggests that a hypothalamic peptide, gonadotropin-inhibiting hormone (GnIH) may provide the brake for the onset of nesting without suppressing development of a functional reproductive system. Experimental investigations show that gene silencing of GnIH in the brain increases singing and reproductive behaviours accompanying the onset of nesting. Thus we suggest that corticosteroids and GnIH play important regulatory roles specifically at the transition from vernal migration to the onset of nesting.

Airborne laser scanning and spectral remote sensing give a bird's eye perspective on arctic tundra breeding habitat at multiple spatial scales

Abstract The effects of climate change are more acute in the Arctic than any other region, and as such, arctic tundra wildlife habitats are changing in ways that are not yet well understood. Remote sensing tools are capable of assessing dynamics in wildlife-habitat associations over a wide range of spatial scales and in a variety of ecosystems. As already well-established in other ecosystems, Light Detection and Ranging (LiDaR) technology has the potential to greatly expand our understanding of tundra wildlife-habitat associations because unlike the most commonly used spectral vegetation index - the Normalized Difference Vegetation Index (NDVI) - LiDaR directly quantifies three-dimensional (3-D) vegetation structure. Only recently has airborne laser scanning (ALS) technology evolved to having vertical resolution and point densities high enough to quantify small differences in vegetation structure, such as characterize arctic tundra ecosystems. Our objective was to employ ALS metrics, airborne spectral data (for NDVI), and Landsat 7 ETM + data (for snow cover) to determine the relative importance of a suite of ecological characteristics - considered at multiple spatial scales - on habitat use and reproductive success of two of the most common migratory songbird species breeding in northern Alaska. Our most important findings are that (1) combining ALS metrics, but not NDVI, with Landsat derived snow cover data provided useful information, revealing that while Gambel's white-crowned sparrows use breeding territories in areas with high canopy volume and large snow-free areas, Lapland longspurs will establish territories in areas with low canopy volume, small patches of snow-free tundra, and minimal surface wetness and; (2) while habitat characteristics were important determinants of habitat use for both species at the territory scale, those in the immediate vicinity of nest-sites were not important. Contrary to expectation, we also found that the reproductive success of both species was unaffected by variation in our hypothesized metrics of shelter and food availability (canopy volume, standard deviation of micro-terrain height) at both the nest-site or territory scales. Our study is the first arctic demonstration of how ALS yields novel insights into wildlife-habitat associations, suggesting it has great potential in other ecosystems with similarly small - yet often ecologically important - ranges in 3-D vegetation structure.

Regulation of vernal migration in Gambel's white-crowned sparrows: Role of thyroxine and triiodothyronine

Appropriate timing of migratory behavior is critical for migrant species. For many temperate zone birds in the spring, lengthening photoperiod is the initial cue leading to morphological, physiological and behavior changes that are necessary for vernal migration and breeding. Strong evidence has emerged in recent years linking thyroid hormone signaling to the photoinduction of breeding in birds while more limited information suggest a potential role in the regulation of vernal migration in photoperiodic songbirds. Here we investigate the development and expression of the vernal migratory life history stage in captive Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) in a hypothyroidic state, induced by chemical inhibition of thyroid hormone production. To explore possible variations in the effects of the two thyroid hormones, triiodothyronine and thyroxine, we subsequently performed a thyroid inhibition coupled with replacement therapy. We found that chemical inhibition of thyroid hormones resulted in complete abolishment of mass gain, fattening, and muscle hypertrophy associated with migratory preparation as well as resulting in failure to display nocturnal restlessness behavior. Replacement of thyroxine rescued all of these elements to near control levels while triiodothyronine replacement displayed partial or delayed rescue. Our findings support thyroid hormones as being necessary for the expression of changes in morphology and physiology associated with migration as well as migratory behavior itself.

Nestling growth rates in relation to food abundance and weather in the Arctic

Raising nestlings to fledging is energetically demanding for songbirds, requiring parents to balance several major tradeoffs. Nestling growth rates are highly susceptible to variation in environmental conditions and parental investment, and highly variable environments with short breeding seasons such as the Arctic magnify these tradeoffs. Arctic-nesting passerines provide a good model system in which to explore variation within and between species in growth rates with regard to environmental conditions and the timing of clutch initiation. Here we investigated interannual and interspecies variation in nestling mass gain for 2 species of Arctic-breeding passerine, Gambel's White-crowned Sparrow (Zonotrichia leucophrys gambelii) and Lapland Longspur (Calcarius lapponicus), across 2 years. The nestling period of 2014 was both colder (with lower minimum and maximum temperatures) and wetter (with 73% more rainfall) than 2013. Arthropod biomass was also reduced in shrub tundra in 2014 compared to 2013. Both species showed reductions in rate of daily mass gain of nestlings in 2014 compared to 2013, but we observed no significant difference between species. Furthermore, we found that in 2014 early nesting birds had higher rates of nestling growth than those initiating clutches later in the season. These findings suggest that overall environmental conditions were more challenging for raising nestlings in 2014 compared to 2013 and that these differences were manifested in a reduced rate of nestling mass gain in both species. Furthermore, both species showed a negative correlation between precipitation and growth rates, whereas only Lapland Longspur showed a positive correlation between growth rates and temperature.

Network analysis of microRNA and mRNA seasonal dynamics in a highly plastic sensorimotor neural circuit.

BackgroundAdult neurogenesis and the incorporation of adult-born neurons into functional circuits requires precise spatiotemporal coordination across molecular networks regulating a wide array of processes, including cell proliferation, apoptosis, neurotrophin signaling, and electrical activity. MicroRNAs (miRs) - short, non-coding RNA sequences that alter gene expression by post-transcriptional inhibition or degradation of mRNA sequences - may be involved in the global coordination of such diverse biological processes. To test the hypothesis that miRs related to adult neurogenesis and related cellular processes are functionally regulated in the nuclei of the avian song control circuit, we used microarray analyses to quantify changes in expression of miRs and predicted target mRNAs in the telencephalic nuclei HVC, the robust nucleus of arcopallium (RA), and the basal ganglia homologue Area X in breeding and nonbreeding Gambel’s white-crowned sparrows (Zonotrichia leucophrys gambelli).ResultsWe identified 46 different miRs that were differentially expressed across seasons in the song nuclei. miR-132 and miR-210 showed the highest differential expression in HVC and Area X, respectively. Analyzing predicted mRNA targets of miR-132 identified 33 candidate target genes that regulate processes including cell cycle control, calcium signaling, and neuregulin signaling in HVC. Likewise, miR-210 was predicted to target 14 mRNAs differentially expressed across seasons that regulate serotonin, GABA, and dopamine receptor signaling and inflammation.ConclusionsOur results identify potential miR–mRNA regulatory networks related to adult neurogenesis and provide opportunities to discover novel genetic control of the diverse biological processes and factors related to the functional incorporation of new neurons to the adult brain.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-015-2175-z) contains supplementary material, which is available to authorized users.

Breeding on the leading edge of a northward range expansion: differences in morphology and the stress response in the arctic Gambel's white-crowned sparrow.

Individuals at the forefront of a range shift are likely to exhibit phenotypic traits that distinguish them from the population breeding within the historic range. Recent studies have examined morphological, physiological and behavioral phenotypes of individuals at the edge of their range. Several studies have found differences in the hypothalamic–pituitary–adrenal (HPA) axis activity in response to acute restraint stress in individuals at the range limits. HPA axis activation leads to elevations in glucocorticoids that regulate physiology and behavior. Here we compare the hormonal profiles and morphometrics from Gambel’s white-crowned sparrows (Zonotrichia leucophrys gambelii) breeding at the northern limit of the population’s range to those birds breeding within the historic population range. Birds breeding at the northern limit experienced a harsher environment with colder temperatures; however, we found no differences in arthropod prey biomass between the northern limit and more southern (historic) sites. Males at the northern limit had higher body condition scores (mass corrected for body size) compared to individuals within the historic range, but no differences were found in beak and tarsus lengths, wing chord, muscle profile or fat stores. In males during the pre-parental stage, before breeding commenced, HPA axis activity was elevated in birds at the northern limit of the range, but no differences were found during the parental or molt stages. Females showed no differences in HPA axis activity during the parental stage. This study suggests that “pioneering” individuals at the limits of their breeding range exhibit physiology and morphology that are distinct from individuals within the historic range.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-015-3447-7) contains supplementary material, which is available to authorized users.

Seasonal plasticity of precise spike timing in the avian auditory system.

Vertebrate audition is a dynamic process, capable of exhibiting both short- and long-term adaptations to varying listening conditions. Precise spike timing has long been known to play an important role in auditory encoding, but its role in sensory plasticity remains largely unexplored. We addressed this issue in Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii), a songbird that shows pronounced seasonal fluctuations in circulating levels of sex-steroid hormones, which are known to be potent neuromodulators of auditory function. We recorded extracellular single-unit activity in the auditory forebrain of males and females under different breeding conditions and used a computational approach to explore two potential strategies for the neural discrimination of sound level: one based on spike counts and one based on spike timing reliability. We report that breeding condition has robust sex-specific effects on spike timing. Specifically, in females, breeding condition increases the proportion of cells that rely solely on spike timing information and increases the temporal resolution required for optimal intensity encoding. Furthermore, in a functionally distinct subset of cells that are particularly well suited for amplitude encoding, female breeding condition enhances spike timing-based discrimination accuracy. No effects of breeding condition were observed in males. Our results suggest that high-resolution temporal discharge patterns may provide a plastic neural substrate for sensory coding.

Decreases in mineralocorticoid but not glucocorticoid receptor mRNA expression during the short Arctic breeding season in free-living Gambel's white-crowned sparrow (Zonotrichia leucophrys gambelii).

The acute stress response in vertebrates is a highly adaptive suite of physiological and behavioural mechanisms that promote survival in the face of deleterious stimuli from the environment. Facultative changes of physiology and behaviour are mediated through changes in circulating levels of glucocorticoids (corticosterone, cortisol) and their subsequent binding to the high-affinity mineralocorticoid receptor (MR) or the low-affinity glucocorticoid receptor (GR). Free-living male wild Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) display annual fluctuations in the stress response with marked attenuation during the transition from the pre-parental to the parental stage. We investigated whether this rapid reduction in the stress response is mediated through changes in MR and GR mRNA expression in the brain using in situ hybridisation. MR mRNA expression was found to be significantly lower in the hippocampus as the male birds became parental. No changes were observed in GR mRNA expression in the paraventricular nucleus (PVN) or preoptic area (POA) at this time. No significant correlations were found between initial capture levels of corticosterone and GR or MR mRNA expression. No differences were found in basal levels of corticosterone between pre-parental and parental in birds collected for in situ hybridisation. Stress response data revealed no difference at baseline but reductions in peak levels of corticosterone as birds became parental. These data suggest that changes in MR expression may be important for the regulation of the stress response or behavioural stress sensitivity with respect to promoting parental care and investment.

Greater shrub dominance alters breeding habitat and food resources for migratory songbirds in Alaskan arctic tundra.

Climate warming is affecting the Arctic in multiple ways, including via increased dominance of deciduous shrubs. Although many studies have focused on how this vegetation shift is altering nutrient cycling and energy balance, few have explicitly considered effects on tundra fauna, such as the millions of migratory songbirds that breed in northern regions every year. To understand how increasing deciduous shrub dominance may alter breeding songbird habitat, we quantified vegetation and arthropod community characteristics in both graminoid and shrub dominated tundra. We combined measurements of preferred nest site characteristics for Lapland longspurs (Calcarius lapponicus) and Gambel's White-crowned sparrows (Zonotrichia leucophrys gambelii) with modeled predictions for the distribution of plant community types in the Alaskan arctic foothills region for the year 2050. Lapland longspur nests were found in sedge-dominated tussock tundra where shrub height does not exceed 20cm, whereas White-crowned sparrows nested only under shrubs between 20cm and 1m in height, with no preference for shrub species. Shrub canopies had higher canopy-dwelling arthropod availability (i.e. small flies and spiders) but lower ground-dwelling arthropod availability (i.e. large spiders and beetles). Since flies are the birds' preferred prey, increasing shrubs may result in a net enhancement in preferred prey availability. Acknowledging the coarse resolution of existing tundra vegetation models, we predict that by 2050 there will be a northward shift in current White-crowned sparrow habitat range and a 20-60% increase in their preferred habitat extent, while Lapland longspur habitat extent will be equivalently reduced. Our findings can be used to make first approximations of future habitat change for species with similar nesting requirements. However, we contend that as exemplified by this study's findings, existing tundra modeling tools cannot yet simulate the fine-scale habitat characteristics that are critical to accurately predicting future habitat extent for many wildlife species.

Reactive neurogenesis in response to naturally occurring apoptosis in an adult brain.

Neuronal birth and death are tightly coordinated to establish and maintain properly functioning neural circuits. Disruption of the equilibrium between neuronal birth and death following brain injury or pharmacological insult often induces reactive, and in some cases regenerative, neurogenesis. Many neurodegenerative disorders are not injury-induced, however, so it is critical to determine if and how reactive neurogenesis occurs under noninjury-induced neurodegenerative conditions. Here, we used a model of naturally occurring neural degradation in a neural circuit that controls song behavior in Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) and examined the temporal dynamics between neuronal birth and death. We found that during seasonal-like regression of the song, control nucleus HVC (proper name), caspase-mediated apoptosis increased within 2 d following transition from breeding to nonbreeding conditions and neural stem-cell proliferation in the nearby ventricular zone (VZ) increased shortly thereafter. We show that inhibiting caspase-mediated apoptosis in HVC decreased neural stem-cell proliferation in the VZ. In baseline conditions the extent of neural stem-cell proliferation correlated positively with the number of dying cells in HVC. We demonstrate that as apoptosis increased and the number of both recently born and pre-existing neurons in HVC decreased, the structure of song, a learned sensorimotor behavior, degraded. Our data illustrate that reactive neurogenesis is not limited to injury-induced neuronal death, but also can result from normally occurring degradation of a telencephalic neural circuit.

Changes in plasma concentrations of progesterone, dehydroepiandrosterone and corticosterone in response to acute stress of capture, handling and restraint in two subspecies of white-crowned sparrows

The aim of this study was to determine circulating patterns of the three major adrenal steroids in blood in response to stress during acute restraint handling in two subspecies of white-crowned sparrow. Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii) are long distance migrants that breed at high latitudes and Nuttall's white-crowned sparrows (Zonotrichia leucophrys nuttalli) are residents of coastal California. Column partition chromatography was developed to separate progesterone, dehydroepiandrosterone (DHEA), and corticosterone from a small plasma sample. Each of these steroids has the capability to modulate the stress response through various mechanisms. For example, progesterone is bound to corticosterone binding globulin (CBG) with a higher affinity than corticosterone. If plasma levels of progesterone rise during acute stress, then this could displace corticosterone from CBG and increase the amount of biologically active, free, corticosterone in blood. Dehydroepiandrosterone (DHEA) has been implicated to have many anti-stress properties with the potential to mitigate some of the actions of corticosterone. Results indicate that progesterone levels in both subspecies are elevated in response to acute stress handling. DHEA levels declined in Gambel's but did not change in Nuttall's. Thus DHEA does not follow the same secretory pattern as in mammals. Corticosterone levels were elevated in response to acute stress handling in both subspecies. This study provides new insight into an integrated stress response among three steroids.

Estradiol Selectively Enhances Auditory Function in Avian Forebrain Neurons

Sex steroids modulate vertebrate sensory processing, but the impact of circulating hormone levels on forebrain function remains unclear. We tested the hypothesis that circulating sex steroids modulate single-unit responses in the avian telencephalic auditory nucleus, field L. We mimicked breeding or nonbreeding conditions by manipulating plasma 17β-estradiol levels in wild-caught female Gambel's white-crowned sparrows (Zonotrichia leucophrys gambelii). Extracellular responses of single neurons to tones and conspecific songs presented over a range of intensities revealed that estradiol selectively enhanced auditory function in cells that exhibited monotonic rate level functions to pure tones. In these cells, estradiol treatment increased spontaneous and maximum evoked firing rates, increased pure tone response strengths and sensitivity, and expanded the range of intensities over which conspecific song stimuli elicited significant responses. Estradiol did not significantly alter the sensitivity or dynamic ranges of cells that exhibited non-monotonic rate level functions. Notably, there was a robust correlation between plasma estradiol concentrations in individual birds and physiological response properties in monotonic, but not non-monotonic neurons. These findings demonstrate that functionally distinct classes of anatomically overlapping forebrain neurons are differentially regulated by sex steroid hormones in a dose-dependent manner.

Seasonal Changes in Patterns of Gene Expression in Avian Song Control Brain Regions

Photoperiod and hormonal cues drive dramatic seasonal changes in structure and function of the avian song control system. Little is known, however, about the patterns of gene expression associated with seasonal changes. Here we address this issue by altering the hormonal and photoperiodic conditions in seasonally-breeding Gambel's white-crowned sparrows and extracting RNA from the telencephalic song control nuclei HVC and RA across multiple time points that capture different stages of growth and regression. We chose HVC and RA because while both nuclei change in volume across seasons, the cellular mechanisms underlying these changes differ. We thus hypothesized that different genes would be expressed between HVC and RA. We tested this by using the extracted RNA to perform a cDNA microarray hybridization developed by the SoNG initiative. We then validated these results using qRT-PCR. We found that 363 genes varied by more than 1.5 fold (>log2 0.585) in expression in HVC and/or RA. Supporting our hypothesis, only 59 of these 363 genes were found to vary in both nuclei, while 132 gene expression changes were HVC specific and 172 were RA specific. We then assigned many of these genes to functional categories relevant to the different mechanisms underlying seasonal change in HVC and RA, including neurogenesis, apoptosis, cell growth, dendrite arborization and axonal growth, angiogenesis, endocrinology, growth factors, and electrophysiology. This revealed categorical differences in the kinds of genes regulated in HVC and RA. These results show that different molecular programs underlie seasonal changes in HVC and RA, and that gene expression is time specific across different reproductive conditions. Our results provide insights into the complex molecular pathways that underlie adult neural plasticity.

Plastic and stable electrophysiological properties of adult avian forebrain song-control neurons across changing breeding conditions.

Steroid sex hormones drive changes in the nervous system and behavior in many animal taxa, but integrating the former with the latter remains challenging. One useful model system for meeting this challenge is seasonally breeding songbirds. In these species, plasma testosterone levels rise and fall across the seasons, altering song behavior and causing dramatic growth and regression of the song-control system, a discrete set of nuclei that control song behavior. Whereas the cellular mechanisms underlying changes in nucleus volume have been studied as a model for neural growth and degeneration, it is unknown whether these changes in neural structure are accompanied by changes in electrophysiological properties other than spontaneous firing rate. Here we test the hypothesis that passive and active neuronal properties in the forebrain song-control nuclei HVC and RA change across breeding conditions. We exposed adult male Gambel's white-crowned sparrows to either short-day photoperiod or long-day photoperiod and systemic testosterone to simulate nonbreeding and breeding conditions, respectively. We made whole-cell recordings from RA and HVC neurons in acute brain slices. We found that RA projection neuron membrane time constant, capacitance, and evoked and spontaneous firing rates were all increased in the breeding condition; the measured electrophysiological properties of HVC interneurons and projection neurons were stable across breeding conditions. This combination of plastic and stable intrinsic properties could directly impact the song-control system's motor control across seasons, underlying changes in song stereotypy. These results provide a valuable framework for integrating how steroid hormones modulate cellular physiology to change behavior.