Lauren V. Riters

Neuroendocrinology of song behavior and avian brain plasticity: multiple sites of action of sex steroid hormones.

Seasonal changes in the brain of songbirds are one of the most dramatic examples of naturally occurring neuroplasticity that have been described in any vertebrate species. In males of temperate-zone songbird species, the volumes of several telencephalic nuclei that control song behavior are significantly larger in the spring than in the fall. These increases in volume are correlated with high rates of singing and high concentrations of testosterone in the plasma. Several song nuclei express either androgen receptors or estrogen receptors, therefore it is possible that testosterone acting via estrogenic or androgenic metabolites regulates song behavior by seasonally modulating the morphology of these song control nuclei. However, the causal links among these variables have not been established. Dissociations among high concentrations of testosterone, enlarged song nuclei, and high rates of singing behavior have been observed. Singing behavior itself can promote cellular changes associated with increases in the volume of the song control nuclei. Also, testosterone may stimulate song behavior by acting in brain regions outside of the song control system such as in the preoptic area or in catecholamine cell groups in the brainstem. Thus testosterone effects on neuroplasticity in the song system may be indirect in that behavioral activity stimulated by testosterone acting in sites that promote male sexual behavior could in turn promote morphological changes in the song system.

Seasonal Plasticity in the Song Control System: Multiple Brain Sites of Steroid Hormone Action and the Importance of Variation in Song Behavior

Abstract: Birdsong, in non‐tropical species, is generally more common in spring and summer when males sing to attract mates and/or defend territories. Changes in the volumes of song control nuclei, such as HVC and the robust nucleus of the arcopallium (RA), are observed seasonally. Long photoperiods in spring stimulate the recrudescence of the testes and the release of testosterone. Androgen receptors, and at times estrogen receptors, are present in HVC and RA as are co‐factors that facilitate the transcriptional activity of these receptors. Thus testosterone can act directly to induce changes in nucleus volume. However, dissociations have been identified at times among long photoperiods, maximal concentrations of testosterone, large song control nuclei, and high rates of song. One explanation of these dissociations is that song behavior itself can influence neural plasticity in the song system. Testosterone can act via brain‐derived neurotrophic factor (BDNF) that is also released in HVC as a result of song activity. Testosterone could enhance song nucleus volume indirectly by acting in the preoptic area, a region regulating sexual behaviors, including song, that connects to the song system through catecholaminergic cells. Seasonal neuroplasticity in the song system involves an interplay among seasonal state, testosterone action, and behavioral activity.

Seasonal changes in the densities of α2‐noradrenergic receptors are inversely related to changes in testosterone and the volumes of song control nuclei in male European starlings

The functions of song and the contextual cues that elicit song change seasonally in parallel with testosterone (T) concentrations in male European starlings. T is high in spring when at least one function of male song is that of immediate mate attraction, and low outside the context of breeding, when starlings primarily use song for dominance or flock maintenance. Several brain nuclei that control song contain high densities of α2‐adrenergic receptors. T can regulate the density of α2‐adrenergic receptors in the avian brain, indicating that the density of α2‐adrenergic receptors within the song system might change seasonally. Although the function of seasonal brain variation is not entirely clear, in many songbirds the volumes of song nuclei are largest when T is high and males sing most. Male starlings, however, sing both when T is high and when T is low. Therefore, exploring seasonal changes in T and the volumes of song nuclei could provide insight into the function of these changes. The present study was performed to explore the relationships among T, the volumes of song nuclei, and the densities of α2‐adrenergic receptors within the song system of male starlings. Song nuclei (the high vocal center [HVc], robust nucleus of the archistriatum [RA], and Area X) were largest, T was highest, and the density of α2‐adrenergic receptors (within HVc and RA) was lowest during the breeding season. The reverse pattern was observed outside of the breeding season. These results suggest that changes in T, volumes of song nuclei, and α2‐receptor densities might regulate seasonal changes in song behavior or the context that will elicit song in male starlings. J. Comp. Neurol. 444:63–74, 2002.

Neuroanatomical evidence for indirect connections between the medial preoptic nucleus and the song control system: possible neural substrates for sexually motivated song

In European starlings (Sturnus vulgaris) as in other seasonally breeding songbirds, a major function of song during the breeding season is mate attraction, and song in this context is highly sexually motivated. Song learning, perception, and production are regulated by nuclei of the song control system, but there is no evidence that these nuclei participate in the motivation to sing. Evidence suggests that the medial preoptic nucleus (POM), a diencephalic nucleus outside of the song control system, might regulate the motivation to sing, at least in a sexual context. If the POM is involved in the regulation of sexually motivated song, then this structure must interact with the song control system. To examine possible neuroanatomical connections between the POM and song control nuclei a tract-tracing study was performed in male starlings using the antero- and retrograde tract tracer, biotinylated dextran amine (BDA). No direct connections were identified between the POM and song control nuclei; however, labeled fibers were found to terminate in a region bordering dorsal-medial portions of the robust nucleus of the archistriatum (RA). Additionally, several indirect routes via which the POM might communicate with the song control system were identified. Specifically, POM projected to dorsomedial nucleus intercollicularis (DM), mesencephalic central gray (GCt), area ventralis of Tsai (AVT), and locus ceruleus (LoC), structures projecting directly to nuclei involved in song production (DM → vocal-patterning and respiratory nuclei; GCt, AVT, LoC → RA and HVC, and the context in which song is sung (AVT → area X). These results are consistent with the possibility that the POM regulates sexually motivated song through interactions with brain regions involved in vocal production.

Vocal production in different social contexts relates to variation in immediate early gene immunoreactivity within and outside of the song control system.

In songbirds, a major function of song during the breeding season is mate attraction, and song in this context can be highly sexually motivated. Vocal learning, perception, and production are regulated by the song control system, but there is no evidence that this system participates in the motivation to sing. Instead, brain regions involved in sexual motivation and arousal, including the medial preoptic nucleus (POM), bed nucleus of the stria terminalis (BST), nucleus taeniae (Tn), and area ventralis of Tsai (AVT) might regulate the motivation to sing, at least in a sexual context. The role of these nuclei and song control nuclei (area X and HVC) in vocal production within a breeding context, and other courtship behaviors, was investigated using immunocytochemistry for protein products of immediate early genes (IEGs), ZENK and c-fos (Fos), in flocks of male house sparrows (Passer domesticus) presented with females. Compared to vocalizations from other perches, vocal behavior from a nest box is more likely directed toward females, and sexually motivated. The numbers of ZENK and Fos labeled cells within rostral, but not caudal POM related positively only to vocalizations produced from a nest box. In contrast, the number of ZENK-labeled cells within area X related negatively to vocalizations from a nest box. Additionally, numbers of IEG-labeled cells within rPOM, Tn and AVT related positively to mount attempts. The results support the hypothesis that the POM interacts with the song control system to regulate sexually motivated vocal expression, and are consistent with work indicating that (a) rostral and caudal POM play distinct roles in sexual behavior, and (b) involvement of area X in song is context specific.

D1-like dopamine receptor density in nuclei involved in social behavior correlates with song in a context-dependent fashion in male European starlings

Research in songbirds shows that singing behavior is regulated by both brain areas involved in vocal behavior as well as those involved in social behavior. Interestingly, the precise role of these regions in song can vary as a function of the social, environmental and breeding context. To date, little is known about the neurotransmitters underlying such context-dependent regulation of song. Dopamine (DA) modulates highly motivated, goal-directed behaviors (including sexually motivated song) and emerging data implicate DA in the context-dependent regulation of singing behavior. This study was performed to begin to examine whether differences in DA receptors may underlie, in part, context-dependent differences in song production. We used autoradiographic procedures to label D1-like and D2-like DA receptors to examine the relationship between DA receptor density and singing behavior in multiple contexts in male European starlings (Sturnus vulgaris). Within a breeding context (when testosterone (T) was high), D1-like receptor density in the medial preoptic nucleus (POM) and midbrain central gray (GCt) negatively correlated with song used to attract a female. Additionally in this context, D1-like receptor density in POM, GCt, medial bed nucleus of the stria terminalis (BSTm), and lateral septum (LS) negatively correlated with song likely used to defend a nest box. In contrast, in a non-breeding context (when T was low), D1-like receptor density in POM and LS positively correlated with song used to maintain social flocks. No relationships were identified between song in any context and D2-like receptor densities. Differences in the brain regions and directional relationships between D1-like receptor binding and song suggest that dopaminergic systems play a region and context-specific role in song. These data also suggest that individual variation in singing behavior may, in part, be explained by individual differences in D1-like receptor density in brain regions implicated in social behavior.

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.

The role of motivation and reward neural systems in vocal communication in songbirds.

Many vertebrates are highly motivated to communicate, suggesting that the consequences of communication may be rewarding. Past studies show that dopamine and opioids in the medial preoptic nucleus (mPOA) and ventral tegmental area (VTA) play distinct roles in motivation and reward. In songbirds, multiple lines of recent evidence indicate that the roles of dopamine and opioid activity in mPOA and VTA in male birdsong differ depending upon whether song is used to attract females (sexually-motivated) or is produced spontaneously (undirected). Evidence is reviewed supporting the hypotheses that (1) mPOA and VTA interact to influence the context in which a male sings, (2) distinct patterns of dopamine activity underlie the motivation to produce sexually-motivated and undirected song, (3) sexually-motivated communication is externally reinforced by opioids released as part of social interactions, and (4) undirected communication is facilitated and rewarded by immediate opioid release linked to the act of singing.

Pharmacological manipulations of dopamine and opioids have differential effects on sexually motivated song in male European starlings

Vocal communication is common among social vertebrates, though little is known about the neural mechanisms regulating the motivation to communicate. This study examined a possible role for dopamine and opioids in sexually motivated song in male European starlings. The dopamine reuptake inhibitor GBR-12909 increased singing behavior, whereas the D1 dopamine receptor antagonist SCH-23390 decreased song, suggesting a role for dopamine in the motivation to sing. In contrast, the opioid agonist fentanyl decreased song, and the antagonist naloxone has previously been shown to increase song, findings consistent with a role for opioids in reward associated with song production. These results suggest that dopamine and opioids play opposing roles in the regulation of the motivation to communicate.

Lesions to the medial preoptic nucleus differentially affect singing and nest box-directed behaviors within and outside of the breeding season in European starlings ( Sturnus vulgaris ).

Little is known about how the brain regulates context-appropriate communication. European starlings produce song in various social contexts. During the breeding season, males with nest sites sing high levels of sexually motivated song in response to a female. Outside of this context, song rates are not affected by female presence. The medial preoptic nucleus (POM) regulates male sexual behavior, and studies in songbirds implicate the POM in sexually motivated song. Recent data suggest that the role of the POM might extend to song produced in other contexts as well. To examine this possibility, effects of bilateral electrolytic lesions of the POM on singing and other behaviors in adult male starlings within sexually relevant and nonsexual contexts were studied. Lesions to the POM exclusively reduced song and nest box-directed behaviors within highly sexually relevant contexts. Unexpectedly, POM lesions increased song in a nonsexual context, suggesting an inhibitory role for the POM in this context. These data suggest that the POM interacts with the song control system so that song occurs in an appropriate social context in response to appropriate stimuli.