Reward associated with singing behavior correlates with opioid-related gene expression in the medial preoptic nucleus in male European starlings

Experimentally elevated corticosterone increases song output and complexity in common mynas

Vocalization is an important communication tool that can reflect many aspects of an individual's internal and external condition. This is especially true for birds. Previous research has shown that bird calls and songs change in response to a variety of potential stressors, although the extent and direction of the changes depend on the nature of the stressor and the environment. Circulating glucocorticoids, such as corticosterone, often increase in response to stressors and mediate some of the observed changes via alterations of the individual's physiological state. Acute elevations of corticosterone often occur as a physiological response to short-term stressors; however, the effects of this elevation on adult vocalizations have not been well documented. Here, we experimentally elevated corticosterone at two different levels using a noninvasive method and examined the effects on the vocal communication of male and female adult common mynas (Acridotheres tristis). Corticosterone elevation temporarily increased song output and some measures of song complexity, while call output decreased. These effects were dosage dependent (higher corticosterone levels had a stronger effect), most evident 40 min after ingestion, and some vocal changes were sex-specific. Future studies should investigate whether the changes in vocal performance due to elevated glucocorticoids have consequences for the birds' behavior, reproductive success, and survival.

Conditioned preferences: Gated by experience, context, and endocrine systems

Central to the navigation of an ever-changing environment is the ability to form positive associations with places and conspecifics. The functions of location and social conditioned preferences are often studied independently, limiting our understanding of their interplay. Furthermore, a de-emphasis on natural functions of conditioned preferences has led to neurobiological interpretations separated from ecological context. By adopting a naturalistic and ethological perspective, we uncover complexities underlying the expression of conditioned preferences. Development of conditioned preferences is a combination of motivation, reward, associative learning, and context, including for social and spatial environments. Both social- and location-dependent reward-responsive behaviors and their conditioning rely on internal state-gating mechanisms that include neuroendocrine and hormone systems such as opioids, dopamine, testosterone, estradiol, and oxytocin. Such reinforced behavior emerges from mechanisms integrating past experience and current social and environmental conditions. Moreover, social context, environmental stimuli, and internal state gate and modulate motivation and learning via associative reward, shaping the conditioning process. We highlight research incorporating these concepts, focusing on the integration of social neuroendocrine mechanisms and behavioral conditioning. We explore three paradigms: 1) conditioned place preference, 2) conditioned social preference, and 3) social conditioned place preference. We highlight nonclassical species to emphasize the naturalistic applications of these conditioned preferences. To fully appreciate the complex integration of spatial and social information, future research must identify neural networks where endocrine systems exert influence on such behaviors. Such research promises to provide valuable insights into conditioned preferences within a broader naturalistic context.

Distinct patterns of activity within columns of the periaqueductal gray are associated with functionally distinct birdsongs

Male songbirds produce female-directed songs in spring that convey a state of sexual motivation. Many songbirds also sing in fall flocks in affiliative/gregarious contexts in which song is linked to an intrinsic positive affective state. The periaqueductal gray (PAG) in mammals, which is organized into functional columns, integrates information from multiple brain regions and relays this information to vocal motor areas so that an animal emits a vocal signal reflective of its affective state. Here, we test the hypothesis that distinct columns in the songbird PAG play roles in the distinct affective states communicated by sexually motivated and gregarious song. We quantified the numbers of immediate early gene ZENK-positive cells in 16 PAG subregions in male European starlings (Sturnus vulgaris) after singing gregarious or sexually motivated song. Results suggest that distinct PAG columns in songbirds context-specifically regulate song, agonistic, and courtship behaviors. A second exploratory, functional tract-tracing study also demonstrated that inputs to the PAG from specific subregions of the medial preoptic nucleus may contribute to gregarious song and behaviors indicative of social dominance. Together, findings suggest that conserved PAG columns and inputs from the preoptic nucleus may play a role in context-specific vocal and other social behaviors.

Distinct patterns of gene expression in the medial preoptic area are related to gregarious singing behavior in European starlings (Sturnus vulgaris)

BACKGROUND

Song performed in flocks by European starlings (Sturnus vulgaris), referred to here as gregarious song, is a non-sexual, social behavior performed by adult birds. Gregarious song is thought to be an intrinsically reinforced behavior facilitated by a low-stress, positive affective state that increases social cohesion within a flock. The medial preoptic area (mPOA) is a region known to have a role in the production of gregarious song. However, the neurochemical systems that potentially act within this region to regulate song remain largely unexplored. In this study, we used RNA sequencing to characterize patterns of gene expression in the mPOA of male and female starlings singing gregarious song to identify possibly novel neurotransmitter, neuromodulator, and hormonal pathways that may be involved in the production of gregarious song.

RESULTS

Differential gene expression analysis and rank rank hypergeometric analysis indicated that dopaminergic, cholinergic, and GABAergic systems were associated with the production of gregarious song, with multiple receptor genes (e.g., DRD2, DRD5, CHRM4, GABRD) upregulated in the mPOA of starlings who sang at high rates. Additionally, co-expression network analyses identified co-expressing gene clusters of glutamate signaling-related genes associated with song. One of these clusters contained five glutamate receptor genes and two glutamate scaffolding genes and was significantly enriched for genetic pathways involved in neurodevelopmental disorders associated with social deficits in humans. Two of these genes, GRIN1 and SHANK2, were positively correlated with performance of gregarious song.

CONCLUSIONS

This work provides new insights into the role of the mPOA in non-sexual, gregarious song in starlings and highlights candidate genes that may play a role in gregarious social interactions across vertebrates. The provided data will also allow other researchers to compare across species to identify conserved systems that regulate social behavior.

The medial preoptic area and its projections to the ventral tegmental area and the periaqueductal gray are activated in response to social play behavior in juvenile rats

The medial preoptic area (MPOA) is well known for its role in sexual and maternal behaviors. This region also plays an important role in affiliative social behaviors outside reproductive contexts. We recently demonstrated that the MPOA is a central nucleus in which opioids govern highly rewarding social play behavior in adolescent rats. However, the neural circuit mechanisms underlying MPOA-mediated social play remain largely unresolved. We hypothesized that the MPOA unites a complementary neural system through which social play induces reward via a projection to the ventral tegmental area (VTA) and reduces a negative affective state through a projection to the periaqueductal gray (PAG). To test whether the two projection pathways are activated in response to social play behavior, we combined retrograde tract tracing with immediate early gene (IEG) expression and immunofluorescent labeling to identify opioid-sensitive projection pathways from the MPOA to VTA and PAG that are activated after performance of social play. Retrograde tracer, fluoro-gold (FG), was microinjected into the VTA or PAG. IEG expression (i.e., Egr1) was assessed and triple immunofluorescent labeling for mu opioid receptor (MOR), Egr1, and FG in the MPOA was performed after social play. We revealed that play animals displayed an increase in neurons double labeled for Egr1 + FG and triple labeled for MOR + Egr1 + FG in the MPOA projecting to both the VTA and PAG when compared to no-play rats. The increased activation of projection neurons that express MORs from MPOA to VTA or PAG after social play suggests that opioids may act through these projection pathways to govern social play. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Dehydroepiandrosterone (DHEA) increases undirected singing behavior and alters dopaminergic regulation of undirected song in non-breeding male European starlings (Sturnus vulgaris)

Introduction

It has been proposed that in species that defend territories across multiple life history stages, brain metabolism of adrenal dehydroepiandrosterone (DHEA) regulates aggressive behavior at times when gonadal androgen synthesis is low (i.e. the non-breeding season). To date, a role for DHEA in the regulation of other forms of social behavior that are expressed outside of the context of breeding remains unknown.

Methods

In this experiment, we used the European starling (Sturnus vulgaris) model system to investigate a role for DHEA in the neuroendocrine regulation of singing behavior by males in non-breeding condition. Starling song in a non-breeding context is spontaneous, not directed towards conspecifics, and functions to maintain cohesion of overwintering flocks.

Results

Using within-subjects design, we found that DHEA implants significantly increase undirected singing behavior by non-breeding condition male starlings. Given that DHEA is known to modulate multiple neurotransmitter systems including dopamine (DA) and DA regulates undirected song, we subsequently used immunohistochemistry for phosphorylated tyrosine hydroxylase (pTH, the active form of the rate-limiting enzyme in DA synthesis) to investigate the effect of DHEA on dopaminergic regulation of singing behavior in a non-breeding context. Pearson correlation analysis revealed a positive linear association between undirected singing behavior and pTH immunoreactivity in the ventral tegmental area and midbrain central gray of DHEA-implanted, but not control-implanted, males.

Discussion

Taken together, these data suggest that undirected singing behavior by non-breeding starlings is modulated by effects of DHEA on dopaminergic neurotransmission. More broadly, these data expand the social behavior functions of DHEA beyond territorial aggression to include undirected, affiliative social communication.

Mu opioid receptor stimulation in the medial preoptic area or nucleus accumbens facilitates song and reward in flocking European starlings

It has been proposed that social cohesion in gregarious animals is reinforced both by a positive affective state induced by social interactions and by the prevention of a negative state that would be caused by social separation. Opioids that bind to mu opioid receptors (MORs) act in numerous brain regions to induce positive and to reduce negative affective states. Here we explored a potential role for MORs in affective states that may impact flocking behavior in mixed-sex flocks of nonbreeding European starlings, Sturnus vulgaris. Singing behavior, which is considered central to flock cohesion, and other social behaviors were quantified after infusions of the MOR agonist D-Ala2, N-Me-Phe4, glycinol5-ENK (DAMGO) into either the medial preoptic area (POM) or the nucleus accumbens (NAC), regions previously implicated in affective state and flock cohesion. We focused on beak wiping, a potential sign of stress or redirected aggression in this species, to provide insight into a presumed negative state. We also used conditioned place preference (CPP) tests to provide insight into the extent to which infusions of DAMGO into POM or NAC that stimulated song might be rewarding. We found that MOR stimulation in either POM or NAC dose-dependently promoted singing behavior, reduced beak wiping, and induced a CPP. Subtle differences in responses to MOR stimulation between NAC and POM also suggest potential functional differences in the roles of these two regions. Finally, because the location of NAC has only recently been identified in songbirds, we additionally performed a tract tracing study that confirmed the presence of dopaminergic projections from the ventral tegmental area to NAC, suggesting homology with mammalian NAC. These findings support the possibility that MORs in POM and NAC play a dual role in reinforcing social cohesion in flocks by facilitating positive and reducing negative affective states.

Birdsong and the Neural Regulation of Positive Emotion

Birds are not commonly admired for emotional expression, and when they are, the focus is typically on negative states; yet vocal behavior is considered a direct reflection of an individual's emotional state. Given that over 4000 species of songbird produce learned, complex, context-specific vocalizations, we make the case that songbirds are conspicuously broadcasting distinct positive emotional states and that hearing songs can also induce positive states in other birds. Studies are reviewed that demonstrate that that the production of sexually motivated song reflects an emotional state of anticipatory reward-seeking (i.e., mate-seeking), while outside the mating context song in gregarious flocks reflects a state of intrinsic reward. Studies are also reviewed that demonstrate that hearing song induces states of positive anticipation and reward. This review brings together numerous studies that highlight a potentially important role for the songbird nucleus accumbens, a region nearly synonymous with reward in mammals, in positive emotional states that underlie singing behavior and responses to song. It is proposed that the nucleus accumbens is part of an evolutionarily conserved circuitry that contributes context-dependently to positive emotional states that motivate and reward singing behavior and responses to song. Neural mechanisms that underlie basic emotions appear to be conserved and similar across vertebrates. Thus, these findings in songbirds have the potential to provide insights into interventions that can restore positive social interactions disrupted by mental health disorders in humans.

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

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

Immunolabeling Provides Evidence for Subregions in the Songbird Nucleus Accumbens and Suggests a Context-Dependent Role in Song in Male European Starlings (Sturnus vulgaris)

Birdsong is well known for its role in mate attraction during the breeding season. However, many birds, including European starlings (Sturnus vulgaris), also sing outside the breeding season as part of large flocks. Song in a breeding context can be extrinsically rewarded by mate attraction; however, song in nonbreeding flocks, referred to here as gregarious song, results in no obvious extrinsic reward and is proposed to be intrinsically rewarded. The nucleus accumbens (NAC) is a brain region well known to mediate reward and motivation, which suggests it is an ideal candidate to regulate reward associated with gregarious song. The goal of this study was to provide new histochemical information on the songbird NAC and its subregions (rostral pole, core, and shell) and to begin to determine subregion-specific contributions to gregarious song in male starlings. We examined immunolabeling for tyrosine hydroxylase (TH), neurotensin, and enkephalin (ENK) in the NAC. We then examined the extent to which gregarious and sexually motivated song differentially correlated with immunolabeling for the immediate early genes FOS and ZENK in each subdivision of the NAC. We found that TH and ENK labeling within subregions of the starling NAC was generally similar to patterns seen in the core and shell of NACs in mammals and birds. Additionally, we found that gregarious song, but not sexually motivated song, positively correlated with FOS in all NAC subregions. Our observations provide further evidence for distinct subregions within the songbird NAC and suggest the NAC may play an important role in regulating gregarious song in songbirds.

The Role of the Endogenous Opioid System in the Vocal Behavior of Songbirds and Its Possible Role in Vocal Learning

The opioid system in the brain is responsible for processing affective states such as pain, pleasure, and reward. It consists of three main receptors, mu- (μ-ORs), delta- (δ-ORs), and kappa- (κ-ORs), and their ligands – the endogenous opioid peptides. Despite their involvement in the reward pathway, and a signaling mechanism operating in synergy with the dopaminergic system, fewer reports focus on the role of these receptors in higher cognitive processes. Whereas research on opioids is predominated by studies on their addictive properties and role in pain pathways, recent studies suggest that these receptors may be involved in learning. Rodents deficient in δ-ORs were poor at recognizing the location of novel objects in their surroundings. Furthermore, in chicken, learning to avoid beads coated with a bitter chemical from those without the coating was modulated by δ-ORs. Similarly, μ-ORs facilitate long term potentiation in hippocampal CA3 neurons in mammals, thereby having a positive impact on spatial learning. Whereas these studies have explored the role of opioid receptors on learning using reward/punishment-based paradigms, the role of these receptors in natural learning processes, such as vocal learning, are yet unexplored. In this review, we explore studies that have established the expression pattern of these receptors in different brain regions of birds, with an emphasis on songbirds which are model systems for vocal learning. We also review the role of opioid receptors in modulating the cognitive processes associated with vocalizations in birds. Finally, we discuss the role of these receptors in regulating the motivation to vocalize, and a possible role in modulating vocal learning.

Intrinsic motivation for singing in songbirds is enhanced by temporary singing suppression and regulated by dopamine

Behaviors driven by intrinsic motivation are critical for development and optimization of physical and brain functions, but their underlying mechanisms are not well studied due to the complexity and autonomy of the behavior. Songbirds, such as zebra finches, offer a unique opportunity to study neural substrates of intrinsic motivation because they spontaneously produce many renditions of songs with highly-quantifiable structure for vocal practice, even in the absence of apparent recipients (“undirected singing”). Neural substrates underlying intrinsic motivation for undirected singing are still poorly understood partly because singing motivation cannot be easily manipulated due to its autonomy. Also, undirected singing itself acts as an internal reward, which could increase singing motivation, leading to difficulty in measuring singing motivation independent of singing-associated reward. Here, we report a simple procedure to easily manipulate and quantify intrinsic motivation for undirected singing independent of singing-associated reward. We demonstrate that intrinsic motivation for undirected singing is dramatically enhanced by temporary suppression of singing behavior and the degree of enhancement depends on the duration of suppression. Moreover, by examining latencies to the first song following singing suppression as a measure of singing motivation independent of singing-associated reward, we demonstrate that intrinsic singing motivation is critically regulated by dopamine through D2 receptors. These results provide a simple experimental tool to manipulate and measure the intrinsic motivation for undirected singing and illustrate the importance of zebra finches as a model system to study the neural basis of intrinsically-motivated behaviors.

μ-Opioid Receptor Stimulation in the Nucleus Accumbens Increases Vocal-Social Interactions in Flocking European Starlings, Sturnus Vulgaris

Social connections in gregarious species are vital for safety and survival. For these reasons, many bird species form large flocks outside the breeding season. It has been proposed that such large social groups may be maintained via reward induced by positive interactions with conspecifics and via the reduction of a negative affective state caused by social separation. Moreover, within a flock optimal social spacing between conspecifics is important, indicating that individuals may optimize spacing to be close but not too close to conspecifics. The μ-opioid receptors (MORs) in the nucleus accumbens (NAc) are well known for their role in both reward and the reduction of negative affective states, suggesting that MOR stimulation in NAc may play a critical role in flock cohesion. To begin to test this hypothesis, social and nonsocial behaviors were examined in male and female European starlings (Sturnus vulgaris) in nonbreeding flocks after intra-NAc infusion of saline and three doses of the selective MOR agonist d-Ala²-N-Me-Phe⁴-Glycol⁵-enkephalin (DAMGO). DAMGO in NAc dose-dependently increased singing behavior and facilitated social approaches while at the same time promoting displacements potentially used to maintain social spacing. These findings support the hypothesis that MORs in NAc promote social interactions important for group cohesion in nonsexual contexts and suggest the possibility that MORs in the NAc play a role in optimizing the pull of joining a flock with the push of potential agonistic encounters.

The expression of delta opioid receptor mRNA in adult male zebra finches (Taenopygia guttata)

The endogenous opioid system is evolutionarily conserved across reptiles, birds and mammals and is known to modulate varied brain functions such as learning, memory, cognition and reward. To date, most of the behavioral and anatomical studies in songbirds have mainly focused on μ-opioid receptors (ORs). Expression patterns of δ-ORs in zebra finches, a well-studied species of songbird have not yet been reported, possibly due to the high sequence similarity amongst different opioid receptors. In the present study, a specific riboprobe against the δ-OR mRNA was used to perform fluorescence in situ hybridization (FISH) on sections from the male zebra finch brain. We found that δ-OR mRNA was expressed in different parts of the pallium, basal ganglia, cerebellum and the hippocampus. Amongst the song control and auditory nuclei, HVC (abbreviation used as a formal name) and NIf (nucleus interfacialis nidopallii) strongly express δ-OR mRNA and stand out from the surrounding nidopallium. Whereas the expression of δ-OR mRNA is moderate in LMAN (lateral magnocellular nucleus of the anterior nidopallium), it is low in the MSt (medial striatum), Area X, DLM (dorsolateral nucleus of the medial thalamus), RA (robust nucleus of the arcopallium) of the song control circuit and Field L, Ov (nucleus ovoidalis) and MLd (nucleus mesencephalicus lateralis, pars dorsalis) of the auditory pathway. Our results suggest that δ-ORs may be involved in modulating singing, song learning as well as spatial learning in zebra finches.

Undirected singing rate as a non-invasive tool for welfare monitoring in isolated male zebra finches

Research on the songbird zebra finch (Taeniopygia guttata) has advanced our behavioral, hormonal, neuronal, and genetic understanding of vocal learning. However, little is known about the impact of typical experimental manipulations on the welfare of these birds. Here we explore whether the undirected singing rate can be used as an indicator of welfare. We tested this idea by performing a post hoc analysis of singing behavior in isolated male zebra finches subjected to interactive white noise, to surgery, or to tethering. We find that the latter two experimental manipulations transiently but reliably decreased singing rates. By contraposition, we infer that a high-sustained singing rate is suggestive of successful coping or improved welfare in these experiments. Our analysis across more than 300 days of song data suggests that a singing rate above a threshold of several hundred song motifs per day implies an absence of an acute stressor or a successful coping with stress. Because singing rate can be measured in a completely automatic fashion, its observation can help to reduce experimenter bias in welfare monitoring. Because singing rate measurements are non-invasive, we expect this study to contribute to the refinement of the current welfare monitoring tools in zebra finches.

Endogenous opioids facilitate intrinsically-rewarded birdsong

Many songbirds sing in non-reproductive contexts while in flocks. Singing in such gregarious contexts is critical for maintaining and learning songs; however, song is not directed towards other individuals and has no obvious, immediate social consequences. Studies using conditioned place preference (CPP) tests of reward indicate that song production in gregarious contexts correlates positively with a bird’s intrinsic reward state and with opioid markers in the medial preoptic nucleus (mPOA). However, the causal involvement of opioids in gregarious song is unknown. Here we report that the selective mu opioid receptor (MOR) agonist fentanyl dose-dependently facilitates gregarious song and reduces stress/anxiety-related behavior in male and female European starlings. Furthermore, infusion of siRNA targeting MORs specifically in mPOA both suppresses gregarious song and disrupts the positive association between affective state and singing behavior, as revealed using CPP tests of song-associated reward. Results strongly implicate opioids in gregarious song and suggest that endogenous opioids in the mPOA may facilitate song by influencing an individual’s intrinsic reward state.

Mu opioid receptors in the medial preoptic area govern social play behavior in adolescent male rats

Neural systems underlying important behaviors are usually highly conserved across species. The medial preoptic area (MPOA) has been demonstrated to play a crucial role in reward associated with affiliative, nonsexual, social communication in songbirds. However, the role of MPOA in affiliative, rewarding social behaviors (eg, social play behavior) in mammals remains largely unknown. Here we applied our insights from songbirds to rats to determine whether opioids in the MPOA govern social play behavior in rats. Using an immediate early gene (ie, Egr1, early growth response 1) expression approach, we identified increased numbers of Egr1-labeled cells in the MPOA after social play in adolescent male rats. We also demonstrated that cells expressing mu opioid receptors (MORs, gene name Oprm1) in the MPOA displayed increased Egr1 expression when adolescent rats were engaged in social play using double immunofluorescence labeling of MOR and Egr1. Furthermore, using short hairpin RNA-mediated gene silencing we revealed that knockdown of Oprm1 in the MPOA reduced the number of total play bouts and the frequency of pouncing. Last, RNA sequencing differential gene expression analysis identified genes involved in neuronal signaling with altered expression after Oprm1 knockdown, and identified Egr1 as potentially a key modulator for Oprm1 in the regulation of social play behavior. Altogether, these results show that the MPOA is involved in social play behavior in adolescent male rats and support the hypothesis that the MPOA is part of a conserved neural circuit across vertebrates in which opioids act to govern affiliative, intrinsically rewarded social behaviors.

Conditioned place preferences induced by hearing song outside the breeding season relate to neural dopamine D1 and cannabinoid CB1 receptor gene expression in female European starlings (Sturnus vulgaris)

The affective state induced by sensory stimuli changes to adaptively modify behaviors that are critical for survival and reproduction. In European starlings, during the spring breeding season, male courtship song is rewarding to females, but only to those that possess resources that are necessary for reproduction (i.e., nesting sites). In fall, starling song is non-sexual and proposed to maintain flocks. This suggests that in fall it may be adaptive for females to be rewarded by fall rather than spring, courtship song. We used a conditioned place preference (CPP) test to evaluate song-induced affective state in fall condition females and quantitative real-time PCR to measure expression of genes that modulate affective state (CB₁ endocannabinoid and D₁ dopamine receptors) in brain regions that were previously implicated in song-induced reward (i.e., the medial preoptic nucleus (mPOA) and ventromedial hypothalamus (VMH)). Fall condition females developed an aversion to a place that had been paired with playback of both male fall and courtship song, indicating that in general male song induces a negative affective state outside the breeding season. Song-induced aversion was stronger in birds conditioned towards an initial place preference. For mPOA, CB₁ receptor expression correlated positively with fall and spring song-induced CPP. D₁ receptor expression correlated negatively with fall (but not spring) song-induced CPP, and the ratio of CB₁ to D₁ receptor expression correlated positively with fall (but not spring) song-induced CPP. These correlations suggest that interactions between D₁ and CB₁ receptors in mPOA may play a role in modifying affective responses to song.

Why Do Birds Flock? A Role for Opioids in the Reinforcement of Gregarious Social Interactions

The formation of social groups provides safety and opportunities for individuals to develop and practice important social skills. However, joining a social group does not result in any form of obvious, immediate reinforcement (e.g., it does not result in immediate copulation or a food reward), and individuals often remain in social groups despite agonistic responses from conspecifics. Much is known about neural and endocrine mechanisms underlying the motivation to perform mate- or offspring-directed behaviors. In contrast, relatively little is known about mechanisms underlying affiliative behaviors outside of these primary reproductive contexts. Studies on flocking behavior in songbirds are beginning to fill this knowledge gap. Here we review behavioral evidence that supports the hypothesis that non-sexual affiliative, flocking behaviors are both (1) rewarded by positive social interactions with conspecifics, and (2) reinforced because affiliative contact reduces a negative affective state caused by social isolation. We provide evidence from studies in European starlings, Sturnus vulgaris, that mu opioid receptors in the medial preoptic nucleus (mPOA) play a central role in both reward and the reduction of a negative affective state induced by social interactions in flocks, and discuss potential roles for nonapeptide/opioid interactions and steroid hormones. Finally, we develop the case that non-sexual affiliative social behaviors may be modified by two complementary output pathways from mPOA, with a projection from mPOA to the periaqueductal gray integrating information during social interactions that reduces negative affect and a projection from mPOA to the ventral tegmental area integrating information leading to social approach and reward.

Co-localization of mu-opioid and dopamine D1 receptors in the medial preoptic area and bed nucleus of the stria terminalis across seasonal states in male European starlings

In seasonally breeding animals, changes in photoperiod and sex-steroid hormones may modify sexual behavior in part by altering the activity of neuromodulators, including opioids and dopamine. In rats and birds, activation of mu-opioid receptors (MOR) and dopamine D1 receptors in the medial preoptic area (mPOA) often have opposing effects on sexual behavior, yet mechanisms by which the mPOA integrates these opposing effects to modulate behavior remain unknown. Here, we used male European starlings (Sturnus vulgaris) to provide insight into the hypothesis that MOR and D1 receptors modify sexual behavior seasonally by altering activity in the same neurons in the mPOA. To do this, using fluorescent immunohistochemistry, we examined the extent to which MOR and D1 receptors co-localize in mPOA neurons and the degree to which photoperiod and the sex-steroid hormone testosterone alter co-localization. We found that MOR and D1 receptors co-localize throughout the mPOA and the bed nucleus of the stria terminalis, a region also implicated in the control of sexual behavior. Numbers of single and co-labeled MOR and D1 receptor labeled cells were higher in the rostral mPOA in photosensitive males (a condition observed just prior to the breeding season) compared to photosensitive males treated with testosterone (breeding season condition). In the caudal mPOA co-localization of MOR and D1 receptors was highest in photosensitive males compared to photorefractory males (a post-breeding season condition). Seasonal shifts in the degree to which neurons in the mPOA integrate signaling from opioids and dopamine may underlie seasonal changes in the production of sexual behavior.

How Song Experience Affects Female Mate-Choice, Male Song, and Monoaminergic Activity in the Auditory Telencephalon in Lincoln's Sparrows

A sexual signal can indicate not only the signaler's attractiveness as a potential mate but also the signaler's competitiveness relative to rivals. As the attractiveness or competitiveness of the prevailing signaling environment increases, individuals prospecting for mates should change their choice threshold, whereas competing individuals should shift resources toward elevating their own competitiveness. Previous studies show that experimental elevations of song competition increase male competitive behavior in Lincoln's sparrows (Melospiza lincolnii) and European starlings (Sturnus vulgaris). Through a series of experimental manipulations using laboratory-housed Lincoln's sparrows, we have also discovered that females change the strength of their song preferences depending on the attractiveness of the song environment to which they have recently been exposed; compared to a less-attractive environment, a highly-attractive environment elevates the threshold for releasing phonotaxis behavior toward male song. These behavioral adjustments are associated with changes in forebrain monoaminergic activity that are triggered by experimental manipulations of the quality of the song environment. Findings from these studies suggest possible neural mechanisms for the regulation of adaptive behavioral plasticity associated with dynamic sexual signaling environments.

Song practice as a rewarding form of play in songbirds

In adult songbirds, the primary functions of song are mate attraction and territory defense; yet, many songbirds sing at high rates as juveniles and outside these primary contexts as adults. Singing outside primary contexts is critical for song learning and maintenance, and ultimately necessary for breeding success. However, this type of singing (i.e., song "practice") occurs even in the absence of immediate or obvious extrinsic reinforcement; that is, it does not attract mates or repel competitors. Here we review studies that support the hypothesis that song practice is stimulated and maintained by intrinsic reward mechanisms (i.e., that it is associated with a positive affective state). Additionally, we propose that song practice can be considered a rewarding form of play behavior similar to forms of play observed in multiple young animals as they practice sequences of motor events that are used later in primary adult reproductive contexts. This review highlights research suggesting at least partially overlapping roles for neural reward systems in birdsong and mammalian play and evidence that steroid hormones modify these systems to shift animals from periods of intrinsically rewarded motor exploration (i.e., singing in birds and play in mammals) to the use of similar motor patterns in primary reproductive contexts.

CNTNAP2 is a direct FoxP2 target in vitro and in vivo in zebra finches: complex regulation by age and activity

Mutations of FOXP2 are associated with altered brain structure, including the striatal part of the basal ganglia, and cause a severe speech and language disorder. Songbirds serve as a tractable neurobiological model for speech and language research. Experimental downregulation of FoxP2 in zebra finch Area X, a nucleus of the striatal song control circuitry, affects synaptic transmission and spine densities. It also renders song learning and production inaccurate and imprecise, similar to the speech impairment of patients carrying FOXP2 mutations. Here we show that experimental downregulation of FoxP2 in Area X using lentiviral vectors leads to reduced expression of CNTNAP2, a FOXP2 target gene in humans. In addition, natural downregulation of FoxP2 by age or by singing also downregulated CNTNAP2 expression. Furthermore, we report that FoxP2 binds to and activates the avian CNTNAP2 promoter in vitro. Taken together these data establish CNTNAP2 as a direct FoxP2 target gene in songbirds, likely affecting synaptic function relevant for song learning and song maintenance.

Song-associated reward correlates with endocannabinoid-related gene expression in male European starlings (Sturnus vulgaris)

Vocal communication is required for successful social interactions in numerous species. During the breeding season, songbirds produce songs that are reinforced by behavioral consequences (e.g., copulation). However, some songbirds also produce songs not obviously directed at other individuals. The consequences maintaining or reinforcing these songs are less obvious and the neural mechanisms associated with undirected communication are not well-understood. Previous studies indicate that undirected singing is intrinsically rewarding and mediated by opioid or dopaminergic systems; however, endocannabinoids are also involved in regulating reward and singing behavior. We used a conditioned place preference paradigm to examine song-associated reward in European starlings and quantitative real-time PCR to measure expression of endocannabinoid-related neural markers (CB₁, FABP7, FABP5, FAAH, DAGLα), in brain regions involved in social behavior, reward and motivation (ventral tegmental area [VTA], periaqueductal gray [PAG], and medial preoptic nucleus [POM]), and a song control region (Area X). Our results indicate that starlings producing high rates of song developed a conditioned place preference, suggesting that undirected song is associated with a positive affective state. We found a significant positive relationship between song-associated reward and CB₁ receptors in VTA and a significant negative relationship between song-associated reward and CB₁ in PAG. There was a significant positive relationship between reward and the cannabinoid transporter FABP7 in POM and a significant negative relationship between reward and FABP7 in PAG. In Area X, FABP5 and DAGLα correlated positively with singing. These results suggest a role for endocannabinoid signaling in vocal production and reward associated with undirected communication.

Prodynorphin and kappa opioid receptor mRNA expression in the brain relates to social status and behavior in male European starlings

Numerous animal species display behavioral changes in response to changes in social status or territory possession. For example, in male European starlings only males that acquire nesting sites display high rates of sexual and agonistic behavior. Past studies show that mu and delta opioid receptors regulate behaviors associated with social ascension or defeat. Opioids also act at kappa receptors, with dynorphin binding with the highest affinity; however, the role of these opioids in social behavior has not been well studied. We observed flocks of male starlings during the breeding season and ran quantitative real-time polymerase chain reaction (qPCR) to measure expression of kappa opioid receptors (OPRK1) and prodynorphin (PDYN) in brain regions involved in social behavior and motivation (ventral tegmental area [VTA], medial preoptic nucleus [mPOA]) and vocal behavior (Area X). Males with nesting territories displayed more sexual/agonistic behavior than males without nesting territories. They also had lower OPRK1 expression in VTA and mPOA. OPRK1 expression in VTA correlated negatively with sexual/agonistic behaviors, consistent with past studies showing kappa receptors in VTA to inhibit sociosexual behaviors. PDYN in mPOA correlated negatively with a measure of nesting behavior that may also reflect sexual motivation. PDYN in Area X related positively to song. Distinct patterns of OPRK1 and PDYN expression in VTA, mPOA, and Area X related to gonad volume, suggesting that breeding condition may modify (or be modified by) OPRK1 and PDYN expression. Studies are now needed to further characterize the role of OPRK1 and PDYN in status-appropriate social behaviors.

Song in an Affiliative Context Relates to the Neural Expression of Dopamine- and Neurotensin-Related Genes in Male European Starlings

Some animals, including songbirds, vocalize at high rates when alone or in large groups. In songbirds, vocal behavior in these contexts is important for song learning and group cohesion. It is not obviously targeted at any particular individual and is described as 'undirected'. Studies suggest a role for dopamine (DA) in undirected song. The neuropeptide neurotensin (NT) can enhance dopaminergic signaling upon binding to the NT receptor 1 (NTR1) and is found in regions where DA can influence song, including the ventral tegmental area (VTA), septum, and the song control nucleus Area X. To begin to test the hypothesis that NT and DA in these regions interact to influence undirected song, we used quantitative real-time PCR to relate undirected singing to mRNA expression of NT, NTR1, tyrosine hydroxylase (TH; a synthetic enzyme for DA) and D1 and D2 receptors in male European starlings. TH and NT expression in VTA, and NT and D1 expression in Area X, positively correlated with song. NT markers also correlated positively with DA markers in VTA. Given the role of VTA projections to Area X in song learning, these results suggest that interactions between NT and DA in these regions may contribute to vocal learning. In septum, NTR1 expression positively correlated with song and NT and DA markers were correlated, suggesting that NT in this region may influence dopaminergic transmission to facilitate undirected vocalizations. Overall, these findings implicate interactions between NT and DA in affiliative communication.

Social and physical environments as a source of individual variation in the rewarding effects of testosterone in male California mice (Peromyscus californicus)

Despite extensive research revealing the occurrence of testosterone (T) pulses following social encounters, it is unclear how they lead to varied behavioral responses. We investigated the influence of residency (home versus unfamiliar environment) and social/sexual experience (pair-bonded, isolated or housed with siblings) on the plasticity of T's rewarding effects by measuring the development of conditioned place preferences (CPPs), a classical paradigm used to measure the rewarding properties of drugs. For pair-bonded males, T-induced CPPs were only produced in the environment wherein the social/sexual experience was accrued and residency status had been achieved. For isolated males, the T-induced CPPs only occurred when the environment was unfamiliar. For males housed with a male sibling, the T-induced CPPs were prevented in both the home and unfamiliar chambers. Our results reveal the plasticity of T's rewarding effects, and suggest that the behavioral functions of T-pulses can vary based on social/sexual experience and the environment in which residency was established. The formation of CPPs or reward-like properties of drugs and natural compounds can therefore exhibit malleability based on past experience and the current environment.

Neural endocannabinoid CB1 receptor expression, social status, and behavior in male European starlings

Many species modify behavior in response to changes in resource availability or social status; however, the neural mechanisms underlying these modifications are not well understood. Prior work in male starlings demonstrates that status-appropriate changes in behavior involve brain regions that regulate social behavior and vocal production. Endocannabinoids are ubiquitously distributed neuromodulators that are proposed to play a role in adjusting behavior to match social status. As an initial step to provide insight into this hypothesis we observed flocks of male starlings in outdoor aviaries during the breeding season. We used quantitative real-time PCR to measure expression of endocannabinoid CB1 receptors in brain regions involved in social behavior and motivation (lateral septum [LS], ventral tegmental area [VTA], medial preoptic nucleus [POM]) and vocal behavior (Area X and robust nucleus of the arcopallium; RA). Males with nesting sites sang to females and displaced other males more than males without nesting sites. They also had higher levels of CB1 receptor expression in LS and RA. CB1 expression in LS correlated positively with agonistic behaviors. CB1 expression in RA correlated positively with singing behavior. CB1 in VTA also correlated positively with singing when only singing birds were considered. These correlations nicely map onto the well-established role of LS in agonistic behavior and the known role of RA in song production and VTA in motivation and song production. Studies are now needed to precisely characterize the role of CB1 receptors in these regions in the production of status-appropriate social behaviors.

Endogenous opiates and behavior: 2014

This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants). This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants).