The neuropeptide arginine vasotocin (AVT) and its mammalian homologue arginine vasopressin (AVP) are neuromodulators known to be steroid sensitive and associated with social behaviors in a number of vertebrate taxa. However, the role of AVT/P in the regulation of aggression remains unclear and contrasting effects of this peptide on aggression are seen in differing species and contexts. In this study, we used immunohistochemistry to examine the effects of testosterone on the AVT system in male and female tree lizards, Urosaurus ornatus, and to determine whether AVT is related to territorial aggression in this species. Tree lizards are a free-living species that exhibit natural hormonal fluctuations across breeding seasons. We detected a male-biased sexual dimorphism in centrally projecting AVT fibers within the limbic system. Furthermore, changes with season, reproductive state, and hormonal treatment suggest that testosterone regulates AVT immunoreactivity in limbic brain regions, especially in the bed nucleus of the stria terminalis. Testosterone also affects AVT immunoreactivity in peripherally projecting cell clusters, as well as the size of AVT cell bodies in the paraventricular nucleus. Although higher testosterone levels alter AVT immunoreactivity, and are known to increase the frequency and intensity of male-male aggression in this species, no individual correlations between AVT immunoreactivity and aggression were detected.

Keywords:
Aggression, Lizard, Neuropeptide, Reptile, Seasonal plasticity, Steroid hormone, Testosterone, Vasopressin, Vasotocin
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