Neuroendocrine signatures of urban aggression: Arginine vasotocin (AVT) receptor distribution and expression in male song sparrow (Melospiza melodia) brain.

Urbanization drastically alters environments, presenting wildlife with novel challenges to which they must adjust to persist. Often, an animal's first response to environmental change is to shift behavior, which requires processing environmental cues into neural signals that can govern adaptive behavior. However, the neural mechanisms underpinning behavioral responses to urbanization are largely unresolved. During the breeding season, urban songbirds of several species are more aggressive than their rural counterparts. Among the many neural mechanisms that regulate aspects of aggression, arginine vasotocin (AVT) is a neuropeptide highly implicated in the regulation of sociality, including territorial aggression. Prior work has shown that the abundance of AVT peptide is associated with behavioral responses to urbanization. However, social behavior is also influenced by AVT receptor availability, making it important to compare receptor distribution and expression between urban and rural birds. To address this gap in understanding of how AVT receptors could contribute to behavioral responses to urbanization, we used in situ hybridization to describe the distribution of one AVT receptor known to influence behavior, AVT3R, throughout the brain of adult urban and rural male song sparrows (Melospiza melodia). Additionally, we used quantitative PCR (qPCR) to compare the relative expression of mRNA for AVTI1 (AVT), and two of its receptors (AVT3R and AVT4R) in the hypothalamus. We predicted that urban males would have a higher abundance of AVT3R transcripts in the lateral septum, greater mRNA expression of AVT receptors in the hypothalamus, and a positive correlation of AVT gene targets with aggression compared to rural males. We observed high AVT3R mRNA signal in the lateral septum, medial bed nucleus of the stria terminalis, hippocampus, nidopallium, cerebellum and optic tectum, with diffuse signal in the lateral hypothalamus and nucleus rotundus. There were no significant differences in the number of AVT3R transcripts in the lateral septum between urban and rural males. When we quantified relative mRNA expression of AVT and two of its receptors using qPCR, we found that urban males had higher relative mRNA expression of AVT3R in the hypothalamus compared to rural males, and aggression immediately before capture was negatively associated with the expression of AVT, AVT3R, and AVT4R. Thus, expression of AVT and its receptors has the potential to contribute to higher aggression in male song sparrows but may not explain habitat-related variation.