Song Control Regions Research Articles

Neural sensitivity to sex steroids predicts individual differences in aggression: implications for behavioural evolution.

Testosterone (T) regulates many traits related to fitness, including aggression. However, individual variation in aggressiveness does not always relate to circulating T, suggesting that behavioural variation may be more closely related to neural sensitivity to steroids, though this issue remains unresolved. To assess the relative importance of circulating T and neural steroid sensitivity in predicting behaviour, we measured aggressiveness during staged intrusions in free-living male and female dark-eyed juncos (Junco hyemalis). We compared aggressiveness to plasma T levels and to the abundance of androgen receptor (AR), aromatase (AROM) and oestrogen receptor alpha (ORα) mRNA in behaviourally relevant brain areas (avian medial amygdala, hypothalamus and song control regions). We also asked whether patterns of covariation among behaviour and endocrine parameters differed in males and females, anticipating that circulating T may be a better predictor of behaviour in males than in females. We found that circulating T related to aggressiveness only in males, but that gene expression for ORα, AR and AROM covaried with individual differences in aggressiveness in both sexes. These findings are among the first to show that individual variation in neural gene expression for three major sex steroid-processing molecules predicts individual variation in aggressiveness in both sexes in nature. The results have broad implications for our understanding of the mechanisms by which aggressive behaviour may evolve.

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.

Individual differences in the motivation to communicate relate to levels of midbrain and striatal catecholamine markers in male European starlings

Individuals display dramatic differences in social communication even within similar social contexts. Across vertebrates dopaminergic projections from the ventral tegmental area (VTA) and midbrain central gray (GCt) strongly influence motivated, reward-directed behaviors. Norepinephrine is also rich in these areas and may alter dopamine neuronal activity. The present study was designed to provide insight into the roles of dopamine and norepinephrine in VTA and GCt and their efferent striatal target, song control region area X, in the regulation of individual differences in the motivation to sing. We used high pressure liquid chromatography with electrochemical detection to measure dopamine, norepinephrine and their metabolites in micropunched samples from VTA, GCt, and area X in male European starlings (Sturnus vulgaris). We categorized males as sexually motivated or non-sexually motivated based on individual differences in song produced in response to a female. Dopamine markers and norepinephrine in VTA and dopamine in area X correlated positively with sexually-motivated song. Norepinephrine in area X correlated negatively with non-sexually-motivated song. Dopamine in GCt correlated negatively with sexually-motivated song, and the metabolite DOPAC correlated positively with non-sexually-motivated song. Results highlight a role for evolutionarily conserved dopaminergic projections from VTA to striatum in the motivation to communicate and highlight novel patterns of catecholamine activity in area X, VTA, and GCt associated with individual differences in sexually-motivated and non-sexually-motivated communication. Correlations between dopamine and norepinephrine markers also suggest that norepinephrine may contribute to individual differences in communication by modifying dopamine neuronal activity in VTA and GCt.

Androgen receptor location in the dark-eyed junco using a probe for in situ hybridization histochemistry generated from zebra finch cDNA

Due to the role of sex steroids, namely testosterone (T), in the development and production of song in songbirds, androgen receptor (AR) densities in the brain regions controlling this behavior (i.e., the song control system) have long been studied. Many methods have been used to determine AR density and location to investigate the functional role of T in song development and production; however, a riboprobe developed from zebra finch (Taeniopygia guttata) cDNA was shown to be much more sensitive than previous methods. The zebra finch is a common model for song development and is sexually dimorphic, but does not breed seasonally or display seasonal changes in song control region volume. In this study, we used this riboprobe for in situ hybridization histochemistry (ISHH) to describe AR mRNA location in the brain of the dark-eyed junco (Junco hyemalis), a seasonally breeding model for which T has been shown to be important. Additionally, we provide a detailed comparison of AR mRNA location between these species. We found that this probe is indeed highly sensitive. We detected AR mRNA in four major regions of the song control system (HVC, MAN, RA and Area X). Additionally, we found that the location of AR mRNA in other regions varied only slightly between these two species. These findings suggest that this method is suitable for use across songbirds and it could be useful in the ongoing attempts to elucidate the roles of sex steroid hormones on the development of this and other sex steroid dependent behaviors in songbirds.

Seasonal and individual variation in singing behavior correlates with alpha 2-noradrenergic receptor density in brain regions implicated in song, sexual, and social behavior

In seasonally breeding male songbirds, both the function of song and the stimuli that elicit singing behavior change seasonally. The catecholamine norepinephrine (NE) modulates attention and arousal across behavioral states, yet the role of NE in seasonally-appropriate vocal communication has not been well-studied. The present study explored the possibility that seasonal changes in alpha 2-noradrenergic receptors (α 2-R) within song control regions and brain regions implicated in sexual arousal and social behavior contribute to seasonal changes in song behavior in male European starlings ( Sturnus vulgaris). We quantified singing behavior in aviary housed males under spring breeding season conditions and fall conditions. α 2-R were identified with the selective ligand [ 3H]RX821002 using autoradiographic methods. The densities of α 2-R in song control regions (HVC and the robust nucleus of the arcopallium [RA]) and the lateral septum (LS) were lower in Spring Condition males. α 2-R densities in the caudal portion of the medial preoptic nucleus (POM) related negatively to singing behavior. Testosterone concentrations were highest in Spring Condition males and correlated with α 2-R in LS and POM. Results link persistent seasonal alterations in the structure or function of male song to seasonal changes in NE α 2-Rs in HVC, RA, and LS. Individual differences in α 2-R in the POM may in part explain individual differences in song production irrespective of the context in which a male is singing, perhaps through NE modification of male sexual arousal.

Opioid modulation of cell proliferation in the ventricular zone of adult zebra finches (Taenopygia guttata)

Besides modulating pain, stress, physiological functions, motivation, and reward, the opioid system has been implicated in developmental and adult mammalian neurogenesis and gliogenesis. In adult male songbirds including zebra finches, neurons generated from the ventricular zone (VZ) of the lateral ventricles are incorporated throughout the telencephalon, including the song control nuclei, HVC, and area X. Although the endogenous opioid met-enkephalin is present in neurons adjacent to the VZ and is upregulated in song control regions during singing, it is not known whether the opioid system can modulate adult neurogenesis/gliogenesis in zebra finches. We used quantitative RT-PCR and in situ hybridization to demonstrate that μ- and δ-opioid receptors are expressed by the VZ of adult male zebra finches. Treating cultured VZ cells from male birds with the opioid antagonist naloxone led to an increase in cell proliferation measured by 5-bromo-2-deoxyuridine incorporation, whereas administering met-enkephalin had the opposite effect, compared with saline-treated cultures. Systemically administering naloxone (2.5 mg/kg body wt) to adult male zebra finches for 4 d also led to a significant increase in cell proliferation in the ventral VZ of these birds, compared with saline-treated controls. Our results show that cell proliferation is augmented by naloxone in the VZ adjacent to the anterior commissure, suggesting that the endogenous opioids modulate adult neurogenesis/gliogenesis by inhibiting cell proliferation in songbirds.

A Basal Ganglia Pathway Drives Selective Auditory Responses in Songbird Dopaminergic Neurons via Disinhibition

Dopaminergic neurons in mammals respond to rewards and reward-predicting cues, and are thought to play an important role in learning actions or sensory cues that lead to reward. The anatomical sources of input that drive or modulate such responses are not well understood; these ultimately define the range of behavior to which dopaminergic neurons contribute. Primary rewards are not the immediate objective of all goal-directed behavior. For example, a goal of vocal learning is to imitate vocal-communication signals. Here, we demonstrate activation of dopaminergic neurons in songbirds driven by a basal ganglia region required for vocal learning, area X. Dopaminergic neurons in anesthetized zebra finches respond more strongly to the bird's own song (BOS) than to other sounds, and area X is critical for these responses. Direct pharmacological modulation of area X output, in the absence of auditory stimulation, is sufficient to bidirectionally modulate the firing rate of dopaminergic neurons. The only known pathway from song control regions to dopaminergic neurons involves a projection from area X to the ventral pallidum (VP), which in turn projects to dopaminergic regions. We show that VP neurons are spontaneously active and inhibited preferentially by BOS, suggesting that area X disinhibits dopaminergic neurons by inhibiting VP. Supporting this model, auditory-response latencies are shorter in area X than VP, and shorter in VP than dopaminergic neurons. Thus, dopaminergic neurons can be disinhibited selectively by complex sensory stimuli via input from the basal ganglia. The functional pathway we identify may allow dopaminergic neurons to contribute to vocal learning.

Neurocalcin protein labeling reveals a dimorphism within the developing zebra finch brain

The exact mechanism(s) responsible for sexual differentiation of the zebra finch song system remains unknown but likely involve a combination of hormonal and genetic factors. One product that may play a role is the calcium binding protein, neurocalcin. A previous study indicated that neurocalcin mRNA was widely distributed throughout the zebra finch telencephalon, overlapping with song control regions. Because it was not clear how much of that expression was functionally relevant, we labeled tissue with a specific antibody. At the three age ranges that were examined, there were region-specific fluctuations in the number of neurocalcin immunoreactive cells. During post-hatching developmental ages (P) 18–23, quantitative analyses indicated that females had significantly more neurons containing neurocalcin protein in HVC and RA than males. This difference was not detected at P10 or in adults. Labeling in LMAN did not reveal a sex difference at any of the ages. In other areas of the brain, semi-quantitative analyses of the relative number of cells immunopositive for neurocalcin varied across specific regions, but with the possible exception of the lateral striatum, no obvious sex differences were apparent. Taken together, these data are consistent with the idea that neurocalcin may be important for regulating sexual dimorphisms within the neural song system at a specific developmental period. In addition, because of variations in events related to post-transcriptional modification, a thorough study of a gene's role in development and/or behavior should include investigations of its transcription as well as translation.

Lesions to the medial preoptic nucleus affect immediate early gene immunolabeling in brain regions involved in song control and social behavior in male European starlings

The medial preoptic nucleus (POM) is a brain region outside of the song control system of songbirds. It has been implicated in song production, sexual motivation, and the integration of both sensory and hormonal information with appropriate behavioral responses. The POM is well positioned neuroanatomically to interact with multiple regions involved in song, social behavior, and motivation. However, little is known about the brain regions with which the POM directly or indirectly communicates to influence song. To gain insight into the neuronal circuits normally activated in association with POM activity during male song, we compared activity within multiple brain regions using immunolabeling for protein products of immediate early genes (IEGs) zenk (aka egr-1) and c-fos (indirect markers of neuronal activity) in sham and POM-lesioned male European starlings (Sturnus vulgaris). As compared to sham lesions, POM lesions disrupted song and interest in a nest box, and females responded less to POM-lesioned males. POM lesions reduced numbers of IEG-labeled cells and disrupted correlations between numbers of IEG-labeled cells and song within several song control, limbic, hypothalamic and midbrain regions. These results are consistent with the possibility that the POM integrates activity among nuclei involved in song control, social behavior and motivational state that work in concert to promote sexually motivated communication.

Expression of μ- and δ-opioid receptors in song control regions of adult male zebra finches ( Taenopygia guttata)

The opioid system is known to play a role in various aspects of learning and memory in diverse species of mammals and birds. Earlier studies have localized the endogenous opioids, met- and leu-enkephalin, in the song control regions of male zebra finches ( Taenopygia guttata), a sexually dimorphic species of songbirds wherein only males sing. Recent research has shown that levels of enkephalin increase in some of the song control regions during singing and that blocking opioid receptors with the antagonist naloxone decreases the frequency of singing in songbirds. However, the distribution of receptors specific to the opioid system has not been studied in zebra finches. In the present study, we used reverse transcriptase PCR (RT-PCR) and quantitative real-time PCR (qRT-PCR) to demonstrate that the song control regions lateral magnocellular nucleus of the anterior nidopallium (LMAN), Area X, MSt (medial striatum), HVC and RA (robust nucleus of the arcopallium) expressed higher levels of μ-OR mRNA compared to δ-OR mRNA. In situ hybridization was used to demonstrate that neither LMAN nor Area X could be delineated from the surrounding brain regions [anterior nidopallium (ANP) and MSt, respectively], based on OR mRNA expression. However, HVC and RA neurons expressed marginally higher levels of μ-OR mRNA compared to the posterior nidopallium, which was confirmed by immunohistochemical localization. We also found that the dorsolateral subdivision of DLM (dorsolateral nucleus of the medial thalamus) demonstrated high levels of μ-OR immunoreactivity. Our results suggest that the ORs may be involved in modulating different aspects of vocalization and/or gating auditory input, as well as motor control.

Evidence that dopamine within motivation and song control brain regions regulates birdsong context-dependently

Vocal communication is critical for successful social interactions among conspecifics, but little is known about how the brain regulates context-appropriate communication. The neurotransmitter dopamine (DA) is involved in modulating highly motivated, goal-directed behaviors (including sexually motivated singing behavior), and emerging data suggest that the role of DA in vocal communication may differ depending on the context in which it occurs. To address this possibility, relationships between immunolabeled tyrosine hydroxylase (TH, the rate-limiting enzyme in catecholamine synthesis) and song produced within versus outside of a breeding context were explored in male European starlings ( Sturnus vulgaris). Immunocytochemistry for dopamine beta-hydroxylase (DBH; the enzyme that converts DA to norepinephrine) was also performed to provide insight into whether relationships between song and TH immunoreactivity reflected dopaminergic or noradrenergic neurotransmission. Measures of TH and DBH were quantified in song control regions (HVC, Area X, robust nucleus of the acropallium) and regions implicated in motivation (medial preoptic nucleus (POM), ventral tegmental area (VTA), and midbrain central gray). In Area X, POM, and VTA measures of TH correlated with song produced within, but not outside of a breeding context. DBH in these regions did not correlate with song in either context. Together, these data suggest DA in both song control and motivation brain regions may be more tightly linked to the regulation of highly goal-directed, sexually motivated vocal behavior.

Photoperiodic induced changes in reproductive state of border canaries ( Serinus canaria) are associated with marked variation in hypothalamic gonadotropin-releasing hormone immunoreactivity and the volume of song control regions

In temperate zone songbirds, such as canaries ( Serinus canaria), seasonal variation in gonadal activity and behavior are associated with marked brain changes. These include gonadotropin-releasing hormone (GnRH) expression and the volume of brain areas controlling song production. Questions have been raised about the consistency of seasonal brain changes in canaries. Laboratory studies of the American singer strain raised doubts as to whether this strain exhibits a robust photoperiodic response along with changes in brain GnRH content, and studies of free-living canaries have failed to identify seasonal changes in volume of song control nuclei. We assessed differences in brain GnRH and the song control system associated with photoperiod-induced variation in reproductive state in Border canaries. We found that males and females maintained for 10 weeks on long days (14L:10D) regress their gonads, exhibit a decline in testosterone and initiate molt; a response consistent with the onset of absolute photorefractoriness (i.e., failed to respond to previously stimulating daylengths). All birds regained photosensitivity (i.e., exhibited gonadal response to stimulating daylengths) after experiencing short days (8L:16D) for 6 weeks. Furthermore, comparisons of birds in either a photosensitive, photostimulated, or photorefractory state revealed a marked increase in GnRH protein expression in the photosensitive and photostimulated birds over photorefractory birds. A similar variation was observed in the volume of key forebrain song nuclei. Thus, Border canaries demonstrate measurable neuroplasticity in response to photoperiodic manipulations. These data, along with previous work on other strains of canaries, indicate the presence of intra-specific variation in photoperiodically regulated neuroplasticity.

CB1 cannabinoid receptor activation dose dependently modulates neuronal activity within caudal but not rostral song control regions of adult zebra finch telencephalon

CB(1) cannabinoid receptors are distinctly expressed at high density within several regions of zebra finch telencephalon, including those known to be involved in song learning (lMAN and Area X) and production (HVC and RA) because (1) exposure to cannabinoid agonists during developmental periods of auditory and sensory-motor song learning alters song patterns produced later in adulthood and (2) densities of song region expression of CB(1) waxes and wanes during song learning. It is becoming clear that CB(1)-receptor-mediated signaling is important to normal processes of vocal development. To better understand the mechanisms involved in cannabinoid modulation of vocal behavior, we have investigated the dose-response relationship between systemic cannabinoid exposure and changes in neuronal activity (as indicated by expression of the transcription factor, c-Fos) within telencephalic brain regions, with established involvement in song learning and/or control. In adults, we have found that low doses (0.1 mg/kg) of the cannabinoid agonist WIN-55212-2 decrease neuronal activity (as indicated by densities of c-fos-expressing nuclei) within vocal motor regions of caudal telencephalon (HVC and RA) while higher doses (3 mg/kg) stimulate activity. Both effects were reversed by pretreatment with the CB(1)-selective antagonist rimonabant. Interestingly, no effects of cannabinoid treatment were observed within the rostral song regions lMAN and Area X, despite distinct and dense CB(1) receptor expression within these areas. Overall, our results demonstrate that, depending on dosage, CB(1) agonism can both inhibit and stimulate neuronal activity within brain regions controlling adult vocal motor output, implicating involvement of multiple CB(1)-sensitive neuronal circuits.

Testosterone and social context affect singing behavior but not song control region volumes in adult male songbirds in the fall

Testosterone (T) induces singing behavior and mediates changes in the sizes and neuroanatomical characteristics of brain regions controlling singing behavior (song control regions, SCRs) in songbirds. These effects may require the enzymatic conversion of T into androgenic and estrogenic metabolites by brain tissues and can be modulated by factors such as season and social context. Testosterone administration to adult male House Finches, Carpodacus mexicanus, in the spring increases the size of their SCRs. Here, we used males of this species to investigate effects of T and T metabolism on brain morphology and singing behavior in the fall. Birds received Silastic capsules containing androgens, estrogens, and/or inhibitors of androgenic action or estrogen synthesis to determine effects of these hormones on song rates and SCR volumes. We also manipulated the social environment by changing the number of birds in visual contact with each other. Testosterone treatment stimulated singing behavior in finches held in small, visually isolated groups and exposed to song playbacks. However, administration of T or T metabolites did not increase SCR sizes. The data suggest that photoperiodic condition and social context may modulate the effects of steroids on SCRs and singing behavior.

Plasticity of the Rufous-winged Sparrow, Aimophila carpalis, song control regions during the monsoon-associated summer breeding period

In most temperate zone songbirds, exposure to increasing photoperiod in the spring stimulates the reproductive system and induces reproductive behaviors. Additionally, the brain regions that control singing (song control regions; SCRs) are larger during the breeding season, thus paralleling changes in the activity of the reproductive system. However, in some birds, environmental factors other than photoperiod initiate breeding. For example, free-living male Rufous-winged Sparrows develop their testes in March due to increasing photoperiod, but have relatively low plasma T until after they begin to breed, usually in July, during the monsoon period when day length is declining. We tested the hypothesis that SCRs grow and singing behavior increases after the monsoon rains begin. We captured adult male Rufous-winged Sparrows in July 2002, 7 days before and 20 days after the monsoon rains began, euthanized birds in the field, collected their brains, and measured SCR volumes from sections immunostained for the neuronal marker NeuN. In June and July 2006, we measured song rates in the field before and after the monsoon rains. SCR volumes were larger and singing behavior increased after the onset of the monsoon rains, coinciding with the initiation of breeding. Unlike in other species studied so far, SCR volumes grew as day length was decreasing. Comparative studies utilizing species that do not breed when day length is increasing may provide information on the relative contributions of various environmental factors to SCR neuroplasticity.

Evidence that female endocrine state influences catecholamine responses to male courtship song in European starlings

Little is known about the neural control of female responses to male courtship. Female European starlings in breeding condition with high concentrations of estrogen select mates based on variation in song and approach nest boxes broadcasting male song. In contrast, outside of the breeding season (when estrogen is low) females do not display the same response to male song. The catecholamines dopamine and norepinephrine regulate behaviors important for mate choice such as arousal, attention, sexual motivation, and goal-directed approach responses, suggesting a role for catecholamines in female responses to male song. In the present study, treating females with a dopamine agonist inhibited, whereas an antagonist stimulated female interest in nest boxes broadcasting male song. In a second study immunocytochemistry was used to examine the distribution of the phosphorylated (i.e., active) form of tyrosine hydroxylase (pTH), the rate-limiting enzyme for catecholamine synthesis. Exposure to male song in breeding condition females reduced pTH density in brain regions involved in social behavior (lateral septum, ventromedial nucleus of the hypothalamus) and a region involved in visual processing (nucleus of Edinger-Westphal) but not song control regions. Opposite patterns of pTH labeling densities were observed in the same regions in response to song in non-breeding condition females. pTH in the ventral tegmental area was also affected by song and female endocrine condition. Overall, the present data support an inhibitory role for dopamine in female responses to courtship and suggest that endocrine state and catecholamines interact to regulate this behavior.

Developmental origin and identity of song system neurons born during vocal learning in songbirds

New neurons are added to the forebrain song control regions high vocal center (HVC) and Area X of juvenile songbirds but the identity and site of origin of these cells have not been fully characterized. We used oncoretroviral vectors to genetically label neuronal progenitors in different regions of the zebra finch lateral ventricle. A region corresponding to the mammalian medial and lateral ganglionic eminences generated medium spiny neurons found in Area X and in the striatum surrounding Area X, and at least two classes of interneurons found in HVC. In addition, our experiments indicate that the HVC projection neurons that project into nucleus robust nucleus of the arcopallium (RA) are born locally from the ventricular region immediately dorsal to HVC. The ability to genetically target neuron subpopulations that give rise to different song system cell types provides a tool for specific genetic manipulations of these cell types. In addition, our results suggest striking similarities between neurogenesis in the embryonic mammalian brain and in the brain of the juvenile songbird and provide further evidence for the existence of conserved cell types in the forebrain for birds and mammals.

Evidence that norepinephrine influences responses to male courtship song and activity within song control regions and the ventromedial nucleus of the hypothalamus in female European starlings

Little is known about the neural regulation of female responses to male courtship displays. Female European starlings use attributes of male song to assess potential mates and can be highly motivated to approach nest boxes broadcasting male song. The catecholamine norepinephrine (NE) regulates attention and arousal and has recently been implicated in motivated/goal-directed behaviors, suggesting a role for NE in female responses to male song. In the present study, treating females with the noradrenergic neurotoxin N-(2-chloroethyl)- N-ethyl-2-bromobenzylamine hydrochloride (DSP-4) increased the number of times females were observed on or near a nest box broadcasting sexually motivated male song, suggesting an inhibitory role for norepinephrine in the regulation of female responses to male song. DSP-4 treatment reduced immunolabeling for the immediate early gene ZENK within area X, a region involved in song learning and possibly discrimination, and the ventromedial nucleus of the hypothalamus, a region involved in female sexual behavior. Furthermore, female responses to male song and numbers of ZENK-labeled cells within area X related positively in DSP-4-treated birds but not controls. Female responses to male song and the density of dopamine-β-hydroxylase in area X and another song nucleus LMAN related positively exclusively in DSP-4-treated birds. Similar trends were consistently observed for the ventromedial nucleus of the hypothalamus. These data suggest roles for area X, LMAN, and the ventromedial nucleus of the hypothalamus in female responses to male song and highlight norepinephrine as an important neuromodulator of this behavior.

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

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

Sexually dimorphic SCAMP1 expression in the forebrain motor pathway for song production of juvenile zebra finches

Mechanisms regulating sexual differentiation of the zebra finch song system are not well understood. The present study was designed to more fully characterize secretory carrier membrane protein 1 (SCAMP1), which was identified in a cDNA microarray screen as showing increased expression in the forebrains of developing male compared with female zebra finches. We completed the sequence of the open reading frame and used in situ hybridization to compare mRNA in song control regions of juvenile (25-day-old) individuals. Expression was significantly greater in the HVC (used as a proper name) and robust nucleus of the arcopallium (RA) in males than in females. Immunohistochemistry revealed that SCAMP1 protein is also expressed in these two brain regions, and qualitatively appears greater in males. Western analysis confirmed that the protein is increased in the telencephalon of males when compared with females at 25 days of age. These results are consistent with the idea that SCAMP1 is involved in masculinization of these brain areas, perhaps facilitating the survival of cells within them.