Does sex or photoperiodic condition influence ZENK induction in response to song in European starlings?

It takes a seasoned bird to be a good listener: communication between the sexes

Birds commonly use sound for communication between the sexes. In many songbird species, only males sing and there are pronounced sex differences in the neural song control circuits. By contrast, the auditory circuitry is largely similar in males and females. Both sexes learn to recognize vocalizations heard as juveniles and this shapes auditory response selectivity. Mating vocalizations are restricted to the breeding season, when sex steroid levels are elevated. Auditory cells, from the ear to the cortex, are hormone sensitive. Estrogens are synthesized in the brain and can modulate the activity of auditory neurons. In species that breed seasonally, elevated levels of estradiol in females transiently enhance their auditory responses to conspecific vocalizations, resulting in sex differences in audition.

Vocal production and playback of altered song do not affect ZENK expression in black-capped chickadees (Poecile atricapillus)

The two-note fee bee song of the black-capped chickadee (Poecile atricapillus) is sung at many different absolute frequencies, but the relative frequencies between the start and end of the fee note (the glissando) and between the fee and the bee notes (the inter-note ratio) are preserved regardless of absolute frequency. If these relative frequencies are experimentally manipulated, birds exhibit reduced behavioural responses to playback of altered songs both in field studies and laboratory studies. Interestingly, males appear to be sensitive to alterations in the glissando, while females appear to be sensitive to alterations in both the glissando and the inter-note ratio. In this study, we sought to determine whether the behaviour of male and female chickadees corresponds to differences in zenk protein immunoreactivity (ZENK-ir) in auditory perceptual regions following playback of fee bee songs with typical and altered pitch ratios. Overall, there was a small but significant sex difference in ZENK-ir (females>males), but altering relative frequencies did not reduce ZENK-ir compared to typical song. Birds did vocalize less in response to playback of songs that lacked an inter-note interval, but amount of singing fee bee song, chick-a-dee calls, or gargles was not correlated with ZENK-ir in perceptual regions (caudomedial nidopallium, NCM and caudomedial mesopallium, CMM) or in HVC, which is part of the song system. Our results confirm that ZENK-ir in NCM and CMM is not involved in fine-grain perceptual discrimination, however it did not support the idea that increased vocalizing increases ZENK-ir in HVC.

The importance of neural aromatization in the acquisition, recall, and integration of song and spatial memories in passerines

This article is part of a Special Issue "Estradiol and cognition". In addition to their well-studied and crucial effects on brain development and aging, an increasing number of investigations across vertebrate species indicate that estrogens like 17β-estradiol (E2) have pronounced and rapid effects on cognitive function. The incidence and regulation of the E2-synthesizing enzyme aromatase at the synapse in regions of the brain responsible for learning, memory, social communication and other complex cognitive processes suggest that local E2 production and action affect the acute and chronic activity of individual neurons and circuits. Songbirds in particular are excellent models for the study of this "synaptocrine" hormone provision given that aromatase is abundantly expressed in neuronal soma, dendrites, and at the synapse across many brain regions in both sexes. Additionally, songbirds readily acquire and recall memories in laboratory settings, and their stereotyped behaviors may be manipulated and measured with relative ease. This leads to a rather unparalleled advantage in the use of these animals in studies of the role of neural aromatization in cognition. In this review we describe the results of a number of experiments in songbird species with a focus on the influence of synaptic E2 provision on two cognitive processes: auditory discrimination reliant on the caudomedial nidopallium (NCM), a telencephalic region likely homologous to the auditory cortex in mammals, and spatial memory dependent on the hippocampus. Data from these studies are providing evidence that the local and acute provision of E2 modulates the hormonal, electrical, and cognitive outputs of the vertebrate brain and aids in memory acquisition, retention, and perhaps the confluence of memory systems.

Is it useful to view the brain as a secondary sexual characteristic?

Many sex differences in brain and behavior related to reproduction are thought to have evolved based on sexual selection involving direct competition for mates during male-male competition and female choice. Therefore, certain aspects of brain circuitry can be viewed as secondary sexual characteristics. The study of proximate causes reveals that sex differences in the brain of mammals and birds reflect organizational and activational effects of sex steroids as articulated by Young and collaborators. However, sex differences in brain and behavior have been identified in the cognitive domain with no obvious link to reproduction. Recent views of sexual selection advocate for a broader view of how intra-sexual selection might occur including such examples as competition within female populations for resources that facilitate access to mates rather than mating competition per se. Sex differences can also come about for other reasons than sexual selection and recent work on neuroendocrine mechanisms has identified a plethora of ways that the brain can develop in a sex specific manner. Identifying the brain as sexually selected requires careful hypothesis testing so that one can link a sex-biased aspect of a neural trait to a behavior that provides an advantage in a competitive mating situation.

Seasonal female brain plasticity in processing social vs. sexual vocal signals

While cerebral plasticity has been extensively studied and demonstrated - during ontogenetic development, few studies have considered adult plasticity in different social contexts using relevant social communication signals. Communication requires adaptability throughout the life of an individual, especially in species for which breeding periods (when intersexual signaling prevails) are interspersed with more 'social' (non-sexual) periods when intrasexual bonding prevails. In songbirds, structure or frequency of songs or song elements may convey different information depending on the season. This is the case in the European starling, where some song structures characterize social bonds between females while other song structures are more characteristic of male courtship. We hypothesized that the female perceptual system may have adapted to these changes in song structure and function according to season, and we tested for potential seasonal brain plasticity. Electrophysiological recordings from adult female starlings during playback of song elements with different functions showed clear seasonal (breeding/non-breeding) changes in neuronal responses in the primary auditory area. The proportion of responsive sites was higher in response to social (non-sexual) songs during the non-reproductive season, and higher in response to sexual songs during the reproductive season.

Zenk expression in auditory regions changes with breeding condition in male Black-capped chickadees (Poecile atricapillus)

Black-capped chickadees (Poecile atricapillus) produce different vocalizations at different times of year: the fee-bee song is produced by males primarily in spring, whereas the chick-a-dee call is produced year-round but most frequently in the fall and winter. We wanted to determine if neural response to playback in auditory regions of the songbird brain varied with season in parallel to production. We captured adult male black-capped chickadees from the wild in either breeding condition or non-breeding condition and within 24-48 h of bringing them into the laboratory setting, played them recordings of either conspecific vocalizations (fee-bee songs or chick-a-dee calls), heterospecific vocalizations (Song Sparrow, Melospiza melodia, song), or silence. We then measured ZENK protein immunoreactivity (Zenk-ir) in caudomedial nidopallium (NCM) and caudomedial mesopallium (CMM), two regions important for perception of conspecific vocalizations. We found that, overall, non-breeding birds had greater Zenk-ir than breeding birds. In addition, we found that birds in non-breeding condition had significantly greater Zenk-ir to heterospecific song than birds in breeding condition, but this difference was not seen in birds that heard conspecific songs or calls. Finally, in NCMd chickadees had greater response to playback of conspecific vocalizations (when combining song and call groups) than playback of heterospecific vocalizations but only while in breeding condition. Our results qualify the claim that Zenk-ir is biased toward conspecific vocalizations, and indicate that specificity of neural response varies with season. Variation could be a result of increased production and perceptual demand in spring, or hormonal changes in breeding birds, possibly because chickadees display vocal plasticity in chick-a-dee calls in the fall, requiring a degree of neural plasticity across seasons.

Estradiol-dependent modulation of auditory processing and selectivity in songbirds

The steroid hormone estradiol plays an important role in reproductive development and behavior and modulates a wide array of physiological and cognitive processes. Recently, reports from several research groups have converged to show that estradiol also powerfully modulates sensory processing, specifically, the physiology of central auditory circuits in songbirds. These investigators have discovered that (1) behaviorally-relevant auditory experience rapidly increases estradiol levels in the auditory forebrain; (2) estradiol instantaneously enhances the responsiveness and coding efficiency of auditory neurons; (3) these changes are mediated by a non-genomic effect of brain-generated estradiol on the strength of inhibitory neurotransmission; and (4) estradiol regulates biochemical cascades that induce the expression of genes involved in synaptic plasticity. Together, these findings have established estradiol as a central regulator of auditory function and intensified the need to consider brain-based mechanisms, in addition to peripheral organ dysfunction, in hearing pathologies associated with estrogen deficiency.

From songs to synapses: molecular mechanisms of birdsong memory. Molecular mechanisms of auditory learning in songbirds involve immediate early genes, including zenk and arc, the ERK/MAPK pathway and synapsins

There are remarkable behavioral, neural, and genetic similarities between the way songbirds learn to sing and human infants learn to speak. Furthermore, the brain regions involved in birdsong learning, perception, and production have been identified and characterized in detail. In particular, the caudal medial nidopallium (the avian analog of the mammalian auditory-association cortex) has been found to contain the neural substrate of auditory memory, paving the way for analyses of the underlying molecular mechanisms. Recently, the zebra finch genome was sequenced, and annotated cDNA databases representing over 15,000 unique brain-expressed genes are available, enabling high-throughput gene expression analyses. Here we review the involvement of immediate early genes (e.g. zenk and arc), their downstream targets (e.g. synapsins), and their regulatory signaling pathways (e.g. MAPK/ERK) in songbird memory. We propose that in-depth investigations of zenk- and ERK-dependent cascades will help to further unravel the molecular basis of auditory memory.

No need to Talk, I Know You: Familiarity Influences Early Multisensory Integration in a Songbird's Brain

It is well known that visual information can affect auditory perception, as in the famous “McGurk effect,” but little is known concerning the processes involved. To address this issue, we used the best-developed animal model to study language-related processes in the brain: songbirds. European starlings were exposed to audiovisual compared to auditory-only playback of conspecific songs, while electrophysiological recordings were made in their primary auditory area (Field L). The results show that the audiovisual condition modulated the auditory responses. Enhancement and suppression were both observed, depending on the stimulus familiarity. Seeing a familiar bird led to suppressed auditory responses while seeing an unfamiliar bird led to response enhancement, suggesting that unisensory perception may be enough if the stimulus is familiar while redundancy may be required for unfamiliar items. This is to our knowledge the first evidence that multisensory integration may occur in a low-level, putatively unisensory area of a non-mammalian vertebrate brain, and also that familiarity of the stimuli may influence modulation of auditory responses by vision.

Topography of estradiol-modulated genomic responses in the songbird auditory forebrain

Sex steroids facilitate dramatic changes in behavioral responses to sociosexual signals and are increasingly implicated in the sensory processing of those signals. Our previous work demonstrated that in female white-throated sparrows, which are seasonal breeders, genomic responses in the auditory forebrain are selective for conspecific song over frequency-matched tones only when plasma estradiol (E2) reaches breeding levels. Here, we sought to map this E2-dependent selectivity in the best-studied area of the auditory forebrain, the caudomedial nidopallium (NCM). Nonbreeding females with low endogenous levels of E2 were treated with E2 or a placebo and exposed to conspecific song, tones, or no sound playback. Immunoreactive protein product of the immediate early gene zenk (egr-1) was then quantified within seven distinct subregions, or domains, of NCM. We report three main findings: (1) regardless of hormone treatment, the zenk response is significantly higher in dorsal than in ventral NCM, and higher in medial than in lateral NCM; (2) E2-dependent selectivity of the response is limited to the rostral and medial domains of NCM; in the more caudal domains, song induces more zenk expression than tones regardless of hormone treatment; (3) even when no sound stimuli were presented, E2 treatment significantly increased zenk expression in the rostral, but not the caudal, domains of NCM. Together, the latter two findings suggest that E2-dependent plasticity in NCM is concentrated in rostral NCM, which is hodologically and neurochemically distinct from caudal NCM. Activity in rostral NCM may therefore be seasonally regulated in this species.

Birdsong "transcriptomics": neurochemical specializations of the oscine song system

Background

Vocal learning is a rare and complex behavioral trait that serves as a basis for the acquisition of human spoken language. In songbirds, vocal learning and production depend on a set of specialized brain nuclei known as the song system.

Methodology/Principal Findings

Using high-throughput functional genomics we have identified ∼200 novel molecular markers of adult zebra finch HVC, a key node of the song system. These markers clearly differentiate HVC from the general pallial region to which HVC belongs, and thus represent molecular specializations of this song nucleus. Bioinformatics analysis reveals that several major neuronal cell functions and specific biochemical pathways are the targets of transcriptional regulation in HVC, including: 1) cell-cell and cell-substrate interactions (e.g., cadherin/catenin-mediated adherens junctions, collagen-mediated focal adhesions, and semaphorin-neuropilin/plexin axon guidance pathways); 2) cell excitability (e.g., potassium channel subfamilies, cholinergic and serotonergic receptors, neuropeptides and neuropeptide receptors); 3) signal transduction (e.g., calcium regulatory proteins, regulators of G-protein-related signaling); 4) cell proliferation/death, migration and differentiation (e.g., TGF-beta/BMP and p53 pathways); and 5) regulation of gene expression (candidate retinoid and steroid targets, modulators of chromatin/nucleolar organization). The overall direction of regulation suggest that processes related to cell stability are enhanced, whereas proliferation, growth and plasticity are largely suppressed in adult HVC, consistent with the observation that song in this songbird species is mostly stable in adulthood.

Conclusions/Significance

Our study represents one of the most comprehensive molecular genetic characterizations of a brain nucleus involved in a complex learned behavior in a vertebrate. The data indicate numerous targets for pharmacological and genetic manipulations of the song system, and provide novel insights into mechanisms that might play a role in the regulation of song behavior and/or vocal learning.

A potential neural substrate for processing functional classes of complex acoustic signals

Categorization is essential to all cognitive processes, but identifying the neural substrates underlying categorization processes is a real challenge. Among animals that have been shown to be able of categorization, songbirds are particularly interesting because they provide researchers with clear examples of categories of acoustic signals allowing different levels of recognition, and they possess a system of specialized brain structures found only in birds that learn to sing: the song system. Moreover, an avian brain nucleus that is analogous to the mammalian secondary auditory cortex (the caudo-medial nidopallium, or NCM) has recently emerged as a plausible site for sensory representation of birdsong, and appears as a well positioned brain region for categorization of songs. Hence, we tested responses in this non-primary, associative area to clear and distinct classes of songs with different functions and social values, and for a possible correspondence between these responses and the functional aspects of songs, in a highly social songbird species: the European starling. Our results clearly show differential neuronal responses to the ethologically defined classes of songs, both in the number of neurons responding, and in the response magnitude of these neurons. Most importantly, these differential responses corresponded to the functional classes of songs, with increasing activation from non-specific to species-specific and from species-specific to individual-specific sounds. These data therefore suggest a potential neural substrate for sorting natural communication signals into categories, and for individual vocal recognition of same-species members. Given the many parallels that exist between birdsong and speech, these results may contribute to a better understanding of the neural bases of speech.

Catecholaminergic cell groups and vocal communication in male songbirds

Birdsong is a species-typical vocal signal that facilitates reproduction and deters competitors. Song production is regulated by a clearly defined and specialized neural circuitry in which high concentrations of catecholamines are present. The nuclei within the song control circuit receive projections from catecholaminergic cell populations involved in attention, arousal and motivation, including periaqueductal gray (PAG), ventral tegmental area (VTA), locus coeruleus (LoC) and sub coeruleus (SC). Here, we examine whether catecholamine-containing neurons in these regions exhibit the immediate early gene, ZENK, during spontaneous, undirected song production in male zebra finches (Taeniopygia guttata). Males were assigned to "singing" or "silent" groups based on the total duration of spontaneous, undirected song produced within a 30 min period. We quantified the number of cells expressing both ZENK-ir and tyrosine hydroxylase (TH)-ir within the VTA, PAG, LoC and SC. The number of cells expressing co-localized ZENK and TH-ir was significantly elevated within the PAG in males that were singing compared to silent males. The number of cells expressing ZENK-ir alone was also elevated in the VTA and SC in singing males compared to silent males. Although ZENK expression is elevated in singing birds it does not positively correlate with the amount of singing produced. It is therefore likely that catecholaminergic PAG neurons are involved in motivational or attentional components of vocal expression rather than vocal motor output. Overall, our study is consistent with the hypothesis that PAG catecholamine-containing neurons as well as VTA and SC neurons play a role in vocal communication of male songbirds.

Neural mechanisms of birdsong memory

The process through which young male songbirds learn the characteristics of the songs of an adult male of their own species has strong similarities with speech acquisition in human infants. Both involve two phases: a period of auditory memorization followed by a period during which the individual develops its own vocalizations. The avian 'song system', a network of brain nuclei, is the probable neural substrate for the second phase of sensorimotor learning. By contrast, the neural representation of song memory acquired in the first phase is localized outside the song system, in different regions of the avian equivalent of the human auditory association cortex.

Estrogen-dependent selectivity of genomic responses to birdsong

Behavioral responses to sociosexual signals often depend on gonadal steroid hormones, which are thought to modulate behavior by acting on motivational systems in the brain. There is mounting evidence that sex steroids may also modulate perception of sociosexual signals by affecting sensory processing. In seasonally breeding songbirds such as the white-throated sparrow (Zonotrichia albicollis), the female's behavioral response to hearing male song depends on her plasma levels of estradiol (E2). Here, we examined whether plasma E2 also affects the selectivity of the song-induced zenk (egr-1) response in the auditory forebrain, which is known to vary according to the behavioral relevance of song stimuli. Non-breeding females were held on a winter-like photoperiod and implanted with silastic capsules containing either no hormone or E2. E2-treated birds hearing 42 min of conspecific song had more cells immunoreactive for the protein product of zenk in the auditory forebrain than did those hearing frequency-matched synthetic tones. In birds not treated with E2, however, the zenk response to song did not differ from that to tones. We found similar effects in the avian homolog of the inferior colliculus, indicating that E2 may affect the processing of auditory information upstream of the forebrain. Our data suggest that in females, zenk induction in the auditory system is selective for song only when plasma E2 exceeds non-breeding levels. E2-dependent plasticity of auditory pathways and processing centres may promote recognition of and attention to conspecific song during the breeding season.

Social context modulates behavioural and brain immediate early gene responses to sound in male songbird

Although it is well known that brain sensory information processing is a highly modulated phenomenon, how this brain function is shaped by experience and social context remains a question to explore. In this paper, we present the first attempt to investigate this problem using a songbird acoustic communication paradigm. Social context is well known to influence acoustic communicating behaviours in birds. The present paper investigates whether brain processing of auditory inputs can be modulated by this 'audience effect'. Given that call-based communication is known to be highly context-dependent, we focused on the response of male zebra finches (Taeniopygia guttata) to female calls. We tested to see if the current social context surrounding the hearing bird can modify a sound-induced immediate early gene (IEG) activation in the specific region of the caudomedial nidopallium (NCM), a songbird brain analogous to the superficial layers of the mammalian primary auditory cortex. Our results show that the expression of the sound-induced immediate early gene ZENK in the NCM is considerably enhanced when the hearing bird is in the presence of conspecifics, compared to when he is alone. This context-dependent increase of a sound-induced immediate early gene expression can be correlated with the differential behavioural response of males to the playback of the same acoustic stimulus as a function of social context.

FOS and ZENK responses in 45-day-old zebra finches vary with auditory stimulus and brain region, but not sex

Male zebra finches (Taeniopygia guttata) begin to sing around 45 days posthatch (d45) and tune their songs to match learned templates. Females never develop song, but they use male conspecific vocalizations for mate choice. While auditory perception is critical for both sexes, the responses of the immediate early genes (IEGs) ZENK and FOS differ in auditory brain areas of d30 males and females. The present study examined expression of these IEGs in the caudomedial nidopallium (NCM), caudomedial mesopallium (CMM; formerly cHV), and the hippocampus (HP) in both sexes at d45 in response to conspecific, heterospecific, or no songs. Overall, zebra finch song presentations resulted in the highest density of ZENK and FOS cells in each region analyzed, but expression varied across brain areas. Contrary to d30 birds, the IEG response patterns did not differ between the sexes. ZENK-immunoreactivity was significantly increased following exposure to conspecific songs compared to no songs, and zebra finch song presentations produced more FOS-immunoreactive nuclei than both heterospecific and no songs. While the pattern was consistent, significant effects of stimulus type were seen only in the NCM when the brain regions were analyzed separately. Furthermore, levels of FOS- and ZENK-immunoreactive neurons were higher in the lateral than medial NCM in both sexes. Along with previous work from our lab and others, these data suggest that at d45 neuronal responses within perceptual regions are still maturing on some levels, but IEG expression has acquired a number of adult characteristics.

Neural constraints on the complexity of avian song

Why do birds sing? In many species, because the song attracts or retains a mate. Why do females pay attention? This paper reviews evidence that females may do so because male song can be an honest indicator of attributes of a male's brain that could contribute to his fitness or that of his young. Male songbirds learn and produce their songs using a set of brain regions collectively known as the song system. The learning has distinct auditory and motor components, and current data suggest that the neural changes that encode these forms of learning primarily occur in different subdivisions of the song system. There are positive correlations between song complexity and the volume of motor song system nucleus HVC, both between and within species. The correlations appear to arise because individual differences in volume lead to differences in capacity for learning. The differences in HVC volume are correlated with differences in the volumes of other song system components and with the volume of the forebrain. They are heritable. Thus, a complex song can be a signal to a female of immediate fitness (the male has a larger brain) and ultimate fitness (he has attractive characteristics that will be passed on to progeny).

A role for norepinephrine in the regulation of context-dependent ZENK expression in male zebra finches (Taeniopygia guttata)

Singing drives expression of the immediate-early gene ZENK in a context-dependent manner in certain nuclei within the avian song circuit of male zebra finches (Taeniopygia guttata). ZENK mRNA expression is low when males are engaged in female- or male-directed song, but high during solo song. Neurotransmitter systems like catecholamines with diffuse projections to forebrain regions are good candidates for regulation of such context-dependent brain activity. We investigated whether the noradrenergic system regulates the dramatic switch in ZENK expression across contexts in male zebra finches. We systemically injected a noradrenergic neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP-4) and found a marked increase in the resultant ZENK expression in area X of the medial striatum in male zebra finches singing directed song. ZENK protein expression in saline-treated males across different contexts mirrored the pattern of previously reported ZENK mRNA expression. We corroborated DSP-4 specificity via immunohistochemical procedures for tyrosine hydroxylase and dopamine-beta hydroxylase, which revealed decreases in norepinephrine synthesizing nuclei and certain song control nuclei. Based on these results we propose a mechanism by which the noradrenergic system usually downregulates ZENK expression in area X during directed song. By depleting this system we induced a disruption of this regulation and reversion back to the default situation characterized by an increase in motor-driven ZENK expression in the song circuit. These data demonstrate that the noradrenergic system (probably in concert with other modulatory neurotransmitters) plays an important role in the response of the brain to salient events that occur in the context of a natural behavior--singing.

An analysis of the neural representation of birdsong memory

Songbirds, such as zebra finches, learn their song from a tutor early in life. Forebrain nuclei in the "song system" are important for the acquisition and production of song. Brain regions [including the caudomedial part of the neostriatum (NCM) and of the hyperstriatum ventrale (CMHV)] outside the song system show increased neuronal activation, measured as expression of immediate early genes (IEGs), when zebra finch males are exposed to song. IEG expression in the NCM in response to tutor song is significantly positively correlated with the strength of song learning (i.e., the number of elements copied). Here, we exposed three groups of adult zebra finch males to tutor song, to their own song, or to novel conspecific song. The two control groups were included to examine an alternative explanation of our previous results in terms of variation in predisposed levels of attentiveness. Expression of Zenk, the protein product of the IEG ZENK, was measured in the NCM, CMHV, and hippocampus. There were no significant differences in overall Zenk expression between the three experimental groups. However, there was a significant positive correlation between Zenk expression in the NCM (but not in the other two regions) and strength of song learning in the males that were exposed to the tutor song. There was no such correlation in the other two groups. These results suggest that experience-related neuronal activation is specific to the tutor song and thus unlikely to be a result of differences in attention.

Localized brain activation specific to auditory memory in a female songbird

Song acquisition in songbird males is a prominent model system for the study of the brain mechanisms of memory. Male zebra finches (Taeniopygia guttata) learn their songs from an adult conspecific tutor early in life. Previous work has shown that exposure of males to their tutor song leads to increased expression of immediate early genes (IEGs) in the caudomedial nidopallium (NCM) and in the caudomedial mesopallium (CMM). In addition, IEG expression in the NCM correlates significantly with the strength of song learning. Interpretation of these findings is complicated, as males both learn the characteristics of tutor song and learn to produce a similar own song. Female zebra finches do not sing, but nevertheless they learn the characteristics of a song to which they were exposed when young, and form a preference for it. Here, adult zebra finch females reared with their fathers showed a significant preference for their father's song. Females that were later reexposed to their father's song showed significantly greater expression of Zenk, the protein product of the IEG ZENK, than controls that were exposed to a novel song, in the CMM, but not in the NCM or hippocampus. These results suggest that in female zebra finches the CMM may be (part of) the neural substrate for the representation of the memory of their father's song.

Neuronal activation related to auditory perception in the brain of a non-songbird, the ring dove

In songbirds, parrots, and hummingbirds, which need to learn their songs, exposure to conspecific song leads to increased expression of the immediate early gene (IEG) ZENK in a number of forebrain regions, including the caudomedial nidopallium (NCM) and the caudomedial mesopallium (CMM). Here we investigated the pattern of IEG expression in response to auditory stimulation in the brain of the ring dove (Streptopelia risoria), a non-songbird that does not need to learn its vocalizations. Ring dove males were exposed to conspecific vocalizations (coos), to heterospecific vocalizations (zebra finch song) or they were kept in silence. IEG expression was investigated by means of immunocytochemical analysis of the distribution of the ZENK protein product Zenk. In all three groups there was Zenk expression in several forebrain regions including the NCM and the CMM, similar to earlier findings in song-learning species. Quantitative analysis of the NCM, the CMM, and the hippocampus revealed significantly greater Zenk expression in birds exposed to conspecific vocalizations than in birds kept in silence, in the CMM only. These results show that there is substantial Zenk expression in the forebrain of a non-song-learning bird, comparable to that in avian song-learning taxa. These findings suggest that there is IEG expression specific to conspecific auditory stimulation in the CMM in both songbirds and non-songbirds.

Background noise does not modify song-induced genic activation in the bird brain

Specialised brain structures allow songbirds to process acoustic signals. One of these brain areas, the NCM (caudomedial neostriatum), shows an immediate-early gene ZENK response when a bird hears a conspecific song. Using a neuro-ethological approach, we investigate if high level of background noise added to conspecific song can modify this song-induced genic activation. We test the ZENK activation in the NCM of adult male Zebra finches Taeniopygya guttata (n = 17) by playing back conspecific signals mixed with different levels of noise, the successful discrimination being reflected by the birds' (n = 6) behavioural responses to these stimuli. From our results, it appears that a high genic activation of the NCM does not necessarily require the audition of an undegraded species-specific signal. Nevertheless, it requires that the signal still contains sufficient information to elicit a behavioural response. The genic activation of the NCM remains thus stable against very high levels of a wide-band background noise, as far as the signal recognition remains possible for the bird.

Effects of songs and calls on ZENK expression in the auditory telencephalon of field- and isolate-reared black capped chickadees

We examined the effects of hearing two different conspecific vocalizations on expression of the immediate-early gene ZENK in the caudomedial neostriatum (NCM) and the caudomedial portion of the ventral hyperstriatum (cmHV) in male and female black-capped chickadees (Poecile atricapilla). Both the fee-bee song and the chick-a-dee call induced Zenk protein expression in NCM and in cmHV, however, patterns of expression to songs and calls varied across brain region. In the dorsal region of NCM, fee-bee songs induced more Zenk expression than chick-a-dee calls. In ventral NCM and cmHV, Zenk expression did not differ between songs and calls. We found that sex of the listener also affected Zenk expression: there was more robust ZENK response in males than in females. Finally, we compared field- and isolate-reared chickadees and found similar Zenk expression to fee-bee song in each group. These findings indicated that the type of conspecific vocalization, as well as the sex of the listener, appear to modulate IEG expression in the songbird ascending auditory pathway.

Differential expression of the immediate early genes FOS and ZENK following auditory stimulation in the juvenile male and female zebra finch

The brains of adult zebra finches (Taeniopygia guttata) are tuned to the songs of conspecifics. In adult males, the caudomedial neostriatum (NCM) responds to zebra finch song, and in adult females the NCM and hippocampus (HP) are active following exposure to zebra finch song more than other auditory stimuli. The caudal hyperstriatum ventrale (cHV) in both sexes also responds to song, but in females not as selectively as the NCM and HP. While much is known about the adult perceptual circuit, less is known about its development. The present study exposed d30 male and female zebra finches to conspecific or heterospecific song, tones or silence, and examined the densities of FOS- and ZENK-immunoreactive nuclei in the NCM, cHV and HP. Significant interactions existed between sex and auditory stimulus condition for both immediate early genes, but they were in opposite directions. That is, across the three regions, FOS-immunoreactive neurons were increased in females following presentation of conspecific songs; males did not show an effect of stimulus exposure. In contrast, the density of ZENK-positive neurons was increased in males, but not females, following zebra finch song exposure. The FOS results demonstrate that some neural responses required for song perception may develop earlier in females than males; data on ZENK induction suggest the opposite. Overall, differences in juvenile immediate early gene activation suggest either that males and females employ divergent neural mechanisms for song perception or that the developmental trajectories leading to common neural responses differ.

Experimental analysis of the processes of systems genesis: expression of the c-fos gene in the chick brain during treatments inducing the development of the species-specific results-of-action acceptor

The aim of the present work was to identify which parts of the chick brain are activated during treatments inducing the development of the preference to follow a species-specific object (a stuffed chick). Expression of the gene for the transcription factor c-fos was used as the molecular marker for neuron activation. Day-old chicks lacking visual experience were placed for 90 min in a freely rotating squirrel wheel or were subjected to stimulation with a loud noise for 180 min. The animals' preference was tested 24 h after stimulation ended. Both types of stimulation induced the formation of a marked preference to follow the "natural" object in the chicks. c-fos expression was analyzed in sections from the brains of chicks stimulated for 45 min. Increases in the quantity of c-fos mRNA were seen after each type of stimulation in the medial part of the caudal neostriatum. In addition, stimulation in the squirrel wheel was accompanied by high levels of c-fos expression in the paraolfactory lobes, while sound stimulation gave high levels of c-fos expression in the ventral and caudal parts of the archistriatum. These structures of the chick brain are of great interest for studies of the cellular and molecular mechanisms of the formation of the species-specific results-of-action acceptor in the mother-following functional system.

Neuronal activation in female budgerigars is localized and related to male song complexity

Females of several songbird species have been shown to respond preferentially to a more complex song. The male budgerigar (Melopsittacus undulatus) sings complex songs consisting of discrete components, known as syllables. We exposed female budgerigars to either standard male song, complex song, or simple song, the iteration of only one syllable (either frequency-modulated or unmodulated). Using immunocytochemistry, we analysed the expression of the protein product of the immediate early gene ZENK in a number of forebrain regions. The level of Zenk protein expression caused by song stimuli varied among each of the brain regions. Expression was highest in the caudomedial neostriatum (NCM), lower in the caudomedial hyperstriatum ventrale (CMHV), and lowest in the hippocampus. There was a significant effect of song complexity on the number of Zenk-immunoreactive cells in the NCM, but not in the hippocampus. Zenk protein expression correlated significantly and positively with the number of different syllables to which the females were exposed in the NCM and to a lesser extent in the CMHV, but not in the hippocampus. For the NCM this correlation was also significant within the group exposed to natural song. These results suggest that the NCM is involved in the perception of song complexity in female budgerigars.

In vivo manganese-enhanced magnetic resonance imaging reveals connections and functional properties of the songbird vocal control system

Injection of manganese (Mn(2+)), a paramagnetic tract tracing agent and calcium analogue, into the high vocal center of starlings labeled within a few hours the nucleus robustus archistriatalis and area X as observed by in vivo magnetic resonance imaging. Structures highlighted by Mn(2+) accumulation assumed the expected tri-dimensional shape of the nucleus robustus archistriatalis and area X as identified by classical histological or neurochemical methods. The volume of these nuclei could be accurately calculated by segmentation of the areas highlighted by Mn(2+). Besides confirming previously established volumetric sex differences, Mn(2+) uptake into these nuclei revealed new functional sex differences affecting Mn(2+) transport. A faster transport was observed in males than in females and different relative amounts of Mn(2+) were transported to nucleus robustus archistriatalis and area X in males as compared to females. This new in vivo approach, allowing repeated measures, opens new vistas to study the remarkable seasonal plasticity in size and activity of song-control nuclei and correlate neuronal activity with behavior. It also provides new insights on in vivo axonal transport and neuronal activity in song-control nuclei of oscines.

The hippocampus and caudomedial neostriatum show selective responsiveness to conspecific song in the female zebra finch

The perception of song is vital to the reproductive success of both male and female songbirds. Several neural structures underlying this perception have been identified by examining expression of immediate early genes (IEGs) following the presentation of conspecific or heterospecific song. In the few avian species investigated, areas outside of the circuit for song production contain neurons that are active following song presentation, specifically the caudal hyperstriatum ventrale (cHV) and caudomedial neostriatum (NCM). While studied in detail in the male zebra finch, IEG responses in these neural substrates involved in song perception have not been quantified in females. Therefore, adult female zebra finches were presented with zebra finch song, nonzebra finch song, randomly generated tones, or silence for 30 min. One hour later they were sacrificed, and their brains removed, sectioned, and immunocytochemically processed for FOS expression. Animals exposed to zebra finch song had a significantly higher density of FOS-immunoreactive cells in the NCM than those presented with other songs, tones, or silence. Neuronal activation in the cHV was equivalent in birds that heard zebra finch and non-zebra finch song, expression that was higher than that observed in the groups that heard no song. Interestingly, the hippocampus (HP) and adjacent parahippocampal area (AHP) were activated in a manner comparable to the NCM. These results suggest a general role for the cHV in song perception and a more specific role for the NCM and HP/AHP in facilitating recognition of and responsiveness to species-specific song in female zebra finches.

Localized immediate early gene expression related to the strength of song learning in socially reared zebra finches

Recent evidence showed that exposure of tape-tutored zebra finch (Taeniopygia guttata castanotis) males to the tutor song involves neuronal activation in brain regions outside the conventional 'song control pathways', particularly the caudal part of the neostriatum (NCM) and of the hyperstriatum ventrale (CMHV). Zebra finch males were reared with a live tutor during the sensitive period for song learning. When, as adults, they were re-exposed to the tutor song, the males showed increased expression of Fos, the protein product of the immediate early gene c-fos, in the NCM and CMHV, compared with expression in two conventional 'song control nuclei', high vocal centre (HVC) and Area X. The strength of the Fos response (which is a reflection of neuronal activation) in the NCM (but not in the other three regions) correlated significantly and positively with the number of song elements that the birds had copied from the tutor song. Thus, socially tutored zebra finch males show localized neural activation in response to tutor song exposure, which correlates with the strength of song learning.

Response biases in auditory forebrain regions of female songbirds following exposure to sexually relevant variation in male song

In many species of songbirds, individual variation between the songs of competing males is correlated with female behavioral preferences. The neural mechanisms of song based female preference in songbirds are not known. Working with female European starlings (Sturnus vulgaris), we used immunocytochemistry for ZENK protein to localize forebrain regions that respond to sexually relevant variation in conspecific male song. The number of ZENK-ir cells in ventral caudo-medial neostriatum [NCMv] was significantly higher in females exposed to longer songs than in those exposed to shorter songs, whereas variation in the total duration of song exposure yielded no significant differences in ZENK expression. ZENK expression in caudo-medial ventral hyperstriatum [cmHV] was uniformly high in all subjects, and did not vary significantly among the three groups. These results suggest that subregions of NCM in female starlings are tuned to variation in male song length, or to song features correlated therewith. Female starlings exhibit robust behavioral preferences for longer over shorter male songs (Gentner and Hulse; Anim Behav 59:443-458, 2000). Therefore, the results of this study strongly implicate NCM in at least a portion of the perceptual processes underlying the complex natural behavior of female choice.

Sexually stimulating signals of canary (Serinus canaria) songs: evidence for a female-specific auditory representation in the HVc nucleus during the breeding season

During the breeding season under long-day conditions, male canaries sing sexually attractive songs and females respond behaviorally to such songs. This study assessed whether auditory response properties of neurons in nucleus HVc of female and male canaries are tuned to sexually salient song features: special song phrases and canary song segmentation. In sexually receptive female canaries, neurons responded to special song phrases with a decreased spike rate and were sensitive to canary song segmentation. The nonreceptive females showed no clear response to special song phrases. In females on short days, neurons responded to song phrases with an increase in activity. In males on long days, they exhibited phasic responses after the phrase onset, whatever the song phrase and song segmentation. This study demonstrates both a plasticity in relation to females' sexual responsiveness and a sexual dimorphism in the auditory processing performed in the HVc.