Aggression is an adaptive behavioral trait that is crucial for competition for territories, food, and mating partners and the establishment of social hierarchies. Although the regulation of aggression in vertebrates involves many different neural mechanisms, particular attention has been given to two hormonal systems in the brain: sex steroids and nonapeptides (Kelly and Wilson, 2020).

Sex steroids are peripherally derived or produced in the brain and act on neural circuits to modulate behavior, primarily through binding to specific nuclear receptors that serve as ligand-gated transcription factors (Yang and Shah, 2014). Androgens, among other sex steroids, play a central role in facilitating aggression, and males are typically more aggressive than females due to their androgen-dominated steroid milieu (Hashikawa et al., 2018; Lischinsky and Lin, 2020). In rodents, the stimulatory effects of androgens on aggression are largely mediated by the activation of estrogen receptors (ESRs) after their conversion to estrogens in the brain (Yang and Shah, 2014). Recent studies have revealed that activation of the ESR subtype ESR1 in the ventromedial hypothalamus (VMH) is particularly important for the expression of aggressive behavior (Chen and Hong, 2018; Hashikawa et al., 2018; Lischinsky and Lin, 2020). However, the transcriptional targets of ESR1 that mediate aggressive behavior remain elusive, and the specific role of sex steroids in the regulation of aggression is unclear. Furthermore, and importantly, it is unlikely that the findings in rodents apply to other vertebrates including primates and teleost fish, where androgens act directly on behaviorally relevant neural circuits via the androgen receptor (AR) without conversion to estrogens (Okubo et al., 2019, 2022).

Nonapeptides, namely, vasotocin (VT, also called vasopressin in mammals) and oxytocin (OT), are evolutionarily conserved neuropeptides that have been associated with a wide range of social behaviors, including aggression (Theofanopoulou et al., 2021; Mennigen et al., 2022). The largest population of neurons expressing VT and that expressing OT both lie in the paraventricular nucleus (PVN), where they project throughout the brain, as well as to the pituitary, to modulate behavior (Rigney et al., 2022). In many terrestrial vertebrates, additional neuronal populations expressing VT in an androgen-dependent, and hence male-biased, manner have been identified in the extended amygdala, specifically in the bed nucleus of the stria terminalis (BNST) and the medial amygdala (MeA) (Kelly and Goodson, 2014; Aspesi and Choleris, 2022; Rigney et al., 2022, 2023). These neurons serve as the main regulators of male-typical social behaviors, and elicit pro- or anti-aggressive behavioral responses in males, depending on species and social context (Aspesi and Choleris, 2022).

Notably, however, teleost fish lack VT-expressing neurons in the extended amygdala, even though their aggression—like that of terrestrial vertebrates—seems to depend on VT and is generally more prevalent in males (Godwin and Thompson, 2012; Rigney et al., 2023). This suggests that, in teleosts, VT may function within another neural circuit to elicit high levels of aggression in males. In line with this idea, teleosts have several populations of VT-expressing neurons in the tuberal hypothalamus, in addition to the major population that spans the brain nucleus homologous to the PVN and its immediate surroundings (Godwin and Thompson, 2012; Oldfield et al., 2015). Our previous findings further revealed that, in medaka fish (Oryzias latipes), the populations in the posterior tuberal nucleus (NPT) and the posterior part of the ventral tuberal nucleus (pNVT) are confined to males (Kawabata et al., 2012). The NPT and pNVT are considered homologous to the ventral tegmental area/substantia nigra and the anterior hypothalamus, respectively (Forlano and Bass, 2011; Loveland and Hu, 2018), both of which have been implicated in nonapeptide-regulated social behavior (Rigney et al., 2022).

To our knowledge, no information is available on the regulation or role of VT-expressing neuronal populations in the NPT and pNVT, but it has been reported in pupfish (Cyprinodon nevadensis amargosae) that VT expression in the hypothalamus is higher in socially dominant, highly aggressive males (Lema et al., 2015). Taken together, the above observations led us to hypothesize that VT expression in either or both of these neuronal populations is induced exclusively in males in an androgen-dependent manner and contributes to the high levels of aggression typical of males. Here we tested this hypothesis by investigating the regulatory mechanisms and physiological roles of male-specific VT expression in the tuberal hypothalamus of medaka.