The neuropeptide arginine-vasopressin (AVP) has many peripheral functions, such as maintenance of blood pressure and antidiuresis (1, 2). In addition to these well-established physiological effects, AVP (and its non-mammalian analogue vasotocin) has long been implicated in the regulation of social behavior and communication in vertebrates, including humans (3–8). AVP acts on various brain regions to regulate social recognition (9–11), communication (3, 12, 13), aggression (14), maternal care (15), pair bonding (16), and cognition (17). Additionally, AVP contributes to avoidance and anxiety-like behavior in response to stressful situations (18, 19). In humans, AVP has been implicated in psychopathology, as variations in the vasopressin V1a receptor (V1aR) gene and AVP serum levels are associated with autism spectrum disorder (ASD) (20, 21). As such, there has been increasing interest in AVP mechanisms that regulate social behavior, thus opening up translational opportunities (8).

AVP-like-peptides contain conserved features that evolved over half a billion years ago and can be traced to similar molecules in invertebrates (22, 23). In non-mammalian vertebrates, vasotocin (AVT) is the most common ancestral AVP-like molecule and is produced in birds, fish, amphibians, and reptiles, while AVP is produced in most mammalian species. Other variants of AVP, such as lysine vasopressin and phenypressin, are found in marsupials and a limited number of eutherian mammals (24–26). In this review, we will use the term “AVP” for all these peptides. In mammals, AVP acts on three canonical receptor types: V1aR, V1bR, and V2R (27), as well as on the oxytocin receptor (OXTR) (28, 29), with V1aR and OXTR being the predominant receptors expressed in the nervous system. AVP acts on three canonical receptor types: V1aR, V1bR, and V2R (27), as well as on the oxytocin receptor (OXTR) (28, 29), with V1aR and OXTR being the predominant receptors expressed in the nervous system. Furthermore, AVP can have dual actions on cells in target zones via V1aR action. For example, AVP causes both direct excitation and indirect inhibition within the lateral septum (LS), in the latter case via excitation of inhibitory interneurons (30). In mammals, AVP is produced primarily in the paraventricular nucleus of the hypothalamus (PVN), supraoptic nucleus (SON), and the suprachiasmatic nucleus (SCN). AVP expressed in these nuclei regulate homeostatic functions, such as water/salt balance, blood pressure (i.e., PVN/SON), and circadian rhythms (i.e., SCN). Additional AVP-producing cells are found in the anterior hypothalamus (AH), nucleus circularis (NC), preoptic area (POA), as well as within sensory systems, such as the olfactory bulb (OB) and retina (31). Most mammals also produce AVP within the extended amygdala (bed nucleus of the stria terminalis (BNST), medial amygdala (MeA)), and do so in a steroid-dependent and sex-specific manner (32). Other non-mammalian species (e.g., roughskin newt) have AVP-expressing cells in other brain regions (33), and fish may lack the BNST/MeA AVP system altogether [but see (34)].

While the role of brain AVP on social and other behaviors has been well-described, the linkage of specific AVP sources to specific effects of AVP on social behavior has been, until recently, been based predominantly on circumstantial and correlational evidence, often with varying results sometimes in contradiction with each other. For example, AVP immunoreactivity, measured by immunohistochemical detection of AVP fiber density, and V1aR expression in the septum, detected by autoradiography, is positively correlated with aggression levels in California mice (Peromyscus) (35), but the density of septal AVP-immunoreactive (AVP-ir) fibers is negatively correlated with male aggression in laboratory mice (M. musculus) and rats (36, 37). Additionally, more aggressive rats release less (38) or more (39) AVP in the septum, and bouts of aggression in deer mice (Peromyscus) are positively correlated with BNST AVP-ir cell number (40). In addition, BNST AVP neurons of male laboratory mice increase Fos expression in response to sexual-, but not aggression-related stimuli (41). Direct comparisons of correlational measures and RNA-interference manipulations in birds have yielded different interpretations as to how AVP influences avian social behavior (42). These differences in the relationships between AVP expression, release, cell activity, and aggression, while possibly due to species and other differences, make it clear that studies that investigate the causal nature of AVP action are needed, such as those that delineate which AVP cell populations contribute to AVP effects on social behavior.

As most of our foundational knowledge of AVP’s influence on social behavior has been gained from pharmacological targeting of AVP receptors within different brain regions as well as from microdialysis measurements of local AVP release during social behavior (12, 15, 43–45), we have incomplete understanding about which AVP cell populations are directly regulating social behavior. Indeed, the diverse anatomy of AVP projections across species suggests that AVP control of social behavior is complex (42). An additional complication is that distinct AVP sources overlap in their projections to some terminal zones. For example, the LS receives AVP input from both the BNST and PVN (46, 47) ( Figure 1 ), and so pharmacological manipulations within the LS may influence multiple AVP pathways, each with potentially different functions. This may be one reason for inconsistent results of manipulations of V1aR in the LS on intermale aggression and anxiety (38, 48) ( Figure 1 ). Similarly, microdialysis measurements of AVP release in LS similarly cannot distinguish between AVP derived from different sources (38, 39, 49, 50).

In addition to the adjacent or overlapping innervation of target structures from different AVP sources, complications arise from primarily focusing on targets of AVP action and not on the sources of AVP. For example, somatodendritic release of AVP from magnocellular PVN and SON neurons may generate both local autocrine/paracrine effects and as well as influencing distal regions (51), but see (52). Thus, AVP action on a particular brain region can originate from multiple sources, both synaptic and extrasynaptic. Moreover, the known cross-talk between AVP and OXT, a closely related peptide (29, 53), on their cognate receptors suggests that AVP may have some of its behavioral action via OXTR, further complicating the analysis of sources and sites of AVP action (12, 23, 45, 54). Consequently, this review will assess the role of specific AVP cell groups in social behavior by focusing on experiments that directly manipulate these cell populations. Specifically, we will examine the functional role of sexually dimorphic AVP cells within the BNST and MeA, AVP cells within the hypothalamus, and AVP cells in sensory systems, such as the retina and olfactory bulb. For more in-depth information on AVP action within specific target brain regions, see: (3, 12, 22, 44, 49).

The BNST and MeA, which is where the sexually-dimorphic and steroid-sensitive AVP cell populations are found, belong to the “extended amygdala” (55, 56) and are key nodes connecting the Social Behavioral Neural Network (SBNN) and the Social Decision-Making Network (57, 58). Work by de Vries and others demonstrated that these cells are greater in number and express more AVP per cell in males than in females (59, 60). They are also steroid-dependent and express only AVP in the presence of gonadal hormones (61), which may act directly on these cells, as they express both androgen receptor (AR) and estrogen receptor alpha (ERα) (62, 63). AVP-expressing cells in the posterior BNST/MeA are the most likely sources of sexually-dimorphic and steroid-dependent AVP innervation of specific brain regions that regulate various aspects of social behavior (e.g., the lateral septum, ventral pallidum, dorsal raphe, lateral habenula, and other areas; see Figure 2 ) (32). Although the BNST and MeA are complex structures with numerous subdivisions (65, 66), AVP cell populations are generally limited to the intermediate and principal posterior sections of BNST and the posterodorsal part of MeA (46, 47). Steroidal effects on AVP peptide and AVP mRNA expression occur during early development as well as in adulthood, reflecting organizational and activational effects of steroid hormones, respectively (67, 68). Additionally, phenotypically female mice with XY sex chromosomes (compared to XX) have denser AVP-ir fibers in the LS compared to XX female mice, demonstrating that genes on sex chromosomes, independent of gonadal development, partially regulate the sexually differentiation of BNST/MeA AVP expression (69).

Despite the substantial indirect evidence implicating the sexually-dimorphic BNST AVP cells in the control of male-typical social behavior, direct evidence for their involvement has been limited. To address this issue, we used intersectional genetic techniques to selectively delete BNST AVP cells in mice and found that such lesions strongly reduced male-male social investigation, modulated aspects of male social communication (i.e., urine marking), and impaired female sexual behavior, without altering resident-intruder aggression, ultrasonic vocalizations, male copulation, or anxiety-related behaviors (106). Similarly, shRNA knockdown of AVP within the BNST also reduced male-male social investigation but, unlike BNST AVP cell ablations, decreased consummatory aspects of male copulatory behavior (107). Some of these effects align with the effects of similar manipulations in birds (42). For example, in territorial Angolan blue Waxbills, knockdown of BNST AVP reduces social contact primarily between males but does not affect anxiety-like behavior (108). However, in the more social zebra finch, knockdown of BNST AVP reduces preference for larger flocking groups (gregariousness), and increases anxiety-like behavior (109). In housing situations that promote mate-guarding, BNST AVP knockdown in zebra finches increases aggression and reduces courtship communication, primarily in males (7, 109). Thus, although species/context differences are apparent, reducing BNST AVP seems to primarily influence male social interactions, mostly (but not exclusively) in the context of male-male competition, communication, and courtship behavior. This suggests that BNST AVP neurons normally play a prominent role in driving specific aspects of male-male competitive behavior and investigation as well as affiliation (i.e., copulatory behavior; gregariousness in birds). In addition, BNST AVP cells are also critical for male social recognition, as deletion of these cells reduces social recognition in males, but not females (93). This is consistent with AVP effects within LS on social recognition (see above) and the recent finding that stimulation of BNST AVP terminals in LS restores social recognition in male mutant mice (Magel2-KO) with pre-existing social recognition deficits (110).

The MeA is a heterogeneous structure that is implicated in the control of defensive responses, aggression, parental behavior, play behavior, social communication, sexual behavior, as well as social recognition (65, 111). The posterior dorsal part of the MeA (MeApd) contains the steroid-sensitive and sexually dimorphic AVP cell population (32, 47). The MeA and BNST may form an integrated system originating from a singular structure as there is developmental continuity between these two cell groups around the internal capsule (55, 112) and AVP expression is similarly modulated by sex-steroids in these two structures (46). Despite having been described several decades ago (113, 114), few studies have tested the functional role of this AVP cell population. Most of the existing work has focused on the role of MeA AVP cells in regulating defensive responses to stressors, such as predator odors, as well as reproductive behavior. For example, MeApd AVP cell activation (Fos expression) in rats positively correlates with male investigation of female conspecifics and copulatory behavior (115) and reduced methylation of the Avp promoter in MeA/BNST, which promotes gene transcription, correlates with reduced aversion to predator odors (116). Similarly, ablation of MeA AVP cells increases aversion to predator odors whereas overexpression of AVP in MeA reduces aversion to predator odors (117). Counter-intuitively, over-expression of AVP in MeA increases activation of PVN-AVP neurons following predator odor exposure, suggesting that MeA AVP may facilitate PVN AVP cell responses during an acute stressor (118). However, while the functional nature of this interaction is unknown, deletion of PVN AVP cells increases anxiety-like behavior in male mice ((119); see below), suggesting that activation of PVN AVP cells may be normally anxiolytic, and therefore anti-defensive, in males. Taken together, this data suggests that MeA AVP cells normally suppress defensive behavior, perhaps in situations where defensive behavior would be counter-productive, such as in reproductive contexts or during competitive behavior (120). More experiments targeting these cells are clearly needed to determine the social role of MeA AVP cells, their projections, and neurochemical interactions.

AVP is produced in magnocellular PVN and SON neurons that project to the posterior pituitary where they release AVP into the periphery to regulate, e.g., blood pressure and water balance (121, 122). AVP is also expressed in parvocellular PVN neurons that project to the median eminence, where AVP is released into a portal system between the median eminence and the anterior pituitary to regulate adrenocorticotropic hormone (ACTH) from the corticotrophs, and thereby indirectly release of glucocorticoids from the adrenal cortex (123). A separate group of parvocellular neurons project centrally to brain regions to modulate physiology as well as social and other motivated behaviors (8, 12, 124). PVN AVP neurons, while not differing in cell number in males and females (46), do display projection patterns that partially differ by sex (125). In females, PVN AVP cells send stronger projections to striatal areas (i.e., nucleus accumbens; NAcc) whereas males have denser innervation of mid- and hind-brain regions (125). Moreover, PVN AVP cell projections are generally denser in females compared to males (46). As many of these regions contain both V1aR and OXTR (126), AVP released from PVN cells may act on both V1aR and OXTR-expressing neurons to drive behavior, given the known cross-talk between OXT/AVP receptors (29). Not only do PVN AVP cells project to a variety of regions, they also receive input from brain regions that regulate aspects of social and emotional behavior, such as the ventral tegmental area (VTA), NAcc, dorsal raphe (DR), BNST, LS, VP, and preoptic area (POA) (127) ( Figure 3 ). Even though PVN and BNST/MeA AVP cell projections sometimes overlap, regions that regulate prediction of aversive and reward outcomes (i.e., lateral habenula and lateral septum) receive much stronger AVP input from the BNST/MeA compared to the PVN (46, 128) ( Figure 1B ).

Although the peripheral effects of SON AVP on physiology are well known (176), much less is known about the behavioral function of these cells. Changes in measures of cellular activity in SON AVP cells suggest that they may modulate aggressive behavior or aggression-related physiology in rats and hamsters (169, 177, 178). More directly, non-selective lesions of the medial SON impaired odor-induced flank marking in hamsters (179), suggesting that a subpopulation of SON AVP cells may drive aspects of social communication.

The nucleus circularis (NC) is a small cluster of cells that contain AVP located within the anterior hypothalamus between the SON and PVN and has been historically implicated in osmotic thirst (180, 181). The discovery that AVP release within the anterior hypothalamus (AH) drives social communication (i.e., flank marking) in male hamsters (182) and that non-selective NC lesions reduces flank marking, led to the suggestion that these cells could be one of the AVP sources important for flank marking (179). Indeed, in male hamsters, the number of NC AVP-ir neurons positively correlates with offensive aggression, social dominance status, and flank marking (167, 177, 183), and, in prairie voles, the number of cells correlates with the level of mating-induced aggression (50, 168, 184). This may explain why PVN and BNST AVP cell lesions in mice did not affect aggressive behavior in resident-intruder settings (106, 119). However, as lesions to NC were non-specific, damaging both NC and the nearby AH, i.e., the location where AVP drives flank marking in males (182, 185), it is difficult to discern whether AVP production within the NC drives flank marking. Specific manipulations of NC AVP cells, such as those used to assess BNST AVP function (106), will be necessary for determining their role in social behavior.

AVP is also produced within the suprachiasmatic nucleus (SCN), a key driver of circadian rhythms in behavioral and physiological regulation (186–188). The AVP cells, located in the dorsomedial SCN, receive their inhibitory inputs from the ventrolateral SCN, which, in turn, is targeted by a subpopulation of retinal ganglion cells some of which also express AVP (189). SCN AVP release, as well as the expression of genes involved in photo-entrainment of biological rhythms, likely mediate aspects of jetlag (189, 190). SCN AVP neurons project locally as well as to the PVN, the subparaventricular zone, MPOA, BNST, PVT, ARC, dorsal medial hypothalamus, NTS, and into the cerebrospinal fluid (CSF) (46, 186, 188, 191).

Although it has been suggested that SCN AVP may interact with sex hormones to influence sexual preference in humans (192), the direct influence of SCN AVP on social behavior is much less clear. Non-specific SCN lesions in male Syrian hamsters did not influence AVP-sensitive social communication (i.e., flank marking) (193). Lesions of AVP cells in SCN of mice did not influence their social behavior but did increase anxiety-like behavior and sucrose consumption in both sexes (194). However, since these lesions only reduced the number of SCN AVP cells by about 50%, greater reductions in the population of SCN AVP cells may reveal additional effects on social behavior.

In rats, the main (MOB) and accessory (AOB) olfactory bulbs contain projection neurons that express AVP (195, 196). These AVP cells are non-bursting, tufted cells (31), which extend their primary dendrites toward glomeruli, thereby receiving information from olfactory sensory neurons (195). Some of these AVP cells also co-express V1bR and may therefore be strongly autoregulated (196). In the AOB, AVP cells project to MePD, which may influence kisspeptin release within this region (which also contains AVP cells) (197). In addition, AVP neurons have also been found in the olfactory cortex (anterior olfactory nucleus, piriform cortex), indicating their involvement in complex odor recognition (31, 195).

In most mammals, olfactory processing is critical for appropriate social communication via chemosignals produced by a conspecific (198). Pharmacological work suggests that AVP is released in the OB to modulate early stages of sensory processing and social recognition (195) via feedback inhibition, a key component in sensory processing (199). As with other OB projection neurons, AVP cells are modulated by cholinergic systems to ultimately regulate social odor processing (200). More work is needed to determine the specific excitation, inhibition, or disinhibition pathways within which OB AVP cells function to modulate social interactions.

Like humans, many non-human primates are highly social and capable of complex social cognition, making them desirable models for studies of social competence. As in other species, AVP and V1aR is expressed in social brain regions of primates (201–204) and is likely to influence their social behavior. For example, AVP concentration in CSF, but not CSF OXT or blood AVP concentration, predicts social functioning in Rhesus monkeys and positively correlates with time spent in social grooming (21, 205). In humans, manipulation of AVP and V1aR systems have been explored as therapies for ASD, schizophrenia, and drug-abuse (21, 53, 206, 207). Indeed, CSF AVP is linked to ASD symptom severity in children (208) and blood AVP concentration may be useful in predicting social development outcomes in newborns (21). Intranasal AVP also increases risky cooperative behavior in men (209) and enhanced social skills in autistic children (210). Additionally, intranasal AVP has been shown to modulate emotional responses to faces (211, 212), improve memory for emotional faces, and identification of social words (5, 21, 213). Although intranasal administration of AVP may offer a promising future therapeutic, the mechanisms by which AVP acts to modulate social behavior in humans remains to be seen. This will be especially important because of sex differences in ASD diagnoses (21) and sex-specific effects observed in animal models. In none of these cases, it is clear which source of AVP contributes to AVP effects on social behavior.

AVP in the nervous system originates from several distinct sources which are, in turn, regulated by different inputs and regulatory factors. Based on both direct and indirect evidence, we can begin to define the specific roles of AVP cell populations in social behavior ( Figure 4 ). The cell population that has been most directly linked to social behavior is the sexually differentiated, steroid-sensitive BNST AVP cells. This population appears to be a likely driver of male-typical social investigation (of potential competitors) and social recognition, as well as regulators of affiliative responses (pair bonding, flocking, sexual behavior). Although much less is known about the other sex-different, steroid-sensitive AVP population in the MeA, it too may be involved in social investigation as well as defensive behavior. The BNST and MeA AVP cells send projections to several regions known to regulate social behavior (32); however, it is unknown if these projections originate from the same or different AVP cell populations within the BNST/MeA. Paradoxically, one of the largest AVP cell populations, the PVN, has the least amount of direct evidence for its role in social behavior. The indirect evidence that is available presents a rather inconsistent picture of the role of PVN AVP cells in social behavior and may, at best, suggest a modest role for PVN AVP cells in the behavioral responses to stress. In contrast, direct manipulations of PVN AVP cells in mice suggest that these cells normally suppress inappropriate social/emotional behavior: social investigation in females and anxiety-like behavior in males. However, the specific role that PVN AVP cells play in social behavior may depend heavily on species- and social system- differences. For example, PVN AVP may promote social affiliation in gregarious bird species of both sexes as well as suppressing inappropriate (mate-directed) male aggression. While the available evidence suggests a minor role for PVN, BNST, and MeA AVP cells in aggressive behavior, the reputation of AVP as a pro-aggressive neuropeptide in males rests almost exclusively on AVP action within the anterior hypothalamus, with the NC and parts of the SON likely contributors of behaviorally-relevant AVP. More recently, AVP expression within early segments of the olfactory systems has been shown to facilitate odor-based social recognition. Consequently, AVP may facilitate social odor memory by both biasing social odor processing (olfactory bulb and cortex) and by modulating memory consolidation through more central circuits (BNST-LS, PVN-hippocampus). Even though AVP in the SCN plays an important role in adjusting circadian rhythms, it does not appear to play a critical role in social behavior but may, instead, regulate reward and anxiety-like behavior. How (or whether) these different AVP systems interact in influencing social behavior is completely unknown.

Several themes and conclusions emerge from examining the evidence implicating different AVP sources in social and emotional behavior. First, because different AVP systems project to overlapping or contiguous brain regions, limiting analysis to sites of AVP action prevents a complete understanding of how AVP regulates social behavior (42, 214). Similarly, as indirect measures of AVP action often show contradictory results (particularly in the PVN), direct tests of AVP cell population function are needed to clarify how different systems function across the neural axis. Second, the few studies that have tested the function of specific AVP cell projection zones have only examined a limited set of social behaviors (social investigation, communication, and recognition). Consequently, the role of AVP sources in social processing must be expanded to include other aspects of social behavior, different contexts, and other model organisms, preferably using comparable and robust measures of function. Lastly, it is clear that sex differences in function may be apparent in both sexually-dimorphic structures (BNST/MeA) as well as ones without prominent structural differences (PVN). For example, PVN and BNST AVP cells in mice control social and emotional behavior differently in the two sexes (106, 107, 119) ( Figure 5 ), such that BNST AVP cell ablations decrease male social investigation of other males (106) ( Figure 5A ), whereas PVN AVP cell ablations increase only female social investigation of males and females (119) ( Figure 5B ). This suggests that the PVN may contribute to baseline sex differences in social investigation, since males show higher levels of investigation than do females (90, 215–217) and removal of these cells removes the sex difference in social investigation (119).

The application of modern molecular approaches will greatly increase our basic understanding of how and where AVP acts to regulate social behavior, much in the same way as it has transformed research on oxytocin systems (126). This increasingly precise understanding of how AVP systems are organized and function may ultimately lead to better therapeutic interventions for psychiatric disorders characterized by substantial social deficits.

NR wrote the review article, GD and AP provided feedback and edited the article. All authors contributed to the article and approved the submitted version.