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178 EFFECTS OF EARLY MALTREATMENT AND STRESSseparable actions of OT and AVP comes from the finding that the knockout mousefor the AVP V1a receptor shows reductions in various indications of anxiety(Bielsky et al., 2004), whereas OTKO mice tend to be more anxious than normalanimals (Amico, Mantella, Vollmer, & Li, 2004). It is possible that sex differencesin endogenous OT and AVP may be critical components of the sexual dimorphismin the capacity of males and females to cope with stressful experiencesthat has been reported in humans (Taylor et al., 2000).The functional effects of AVP are less easily summarized than those of OT,and may be dose dependent. For example, it is possible that low doses of AVP oracute exposure to this peptide may increase certain forms of social behavior (Choet al., 1999), possibly by reducing social anxiety (Dharmadhikari, Lee, Roberts,& Carter, 1997). However, higher doses or perhaps chronic or peripheral changesin AVP might have the opposite effect, creating visceral states that are more commonlyassociated with higher levels of anxiety (Landgraf & Wigger, 2002; Wiggeret al., 2004). The role of AVP may be especially context dependent, and probablyrelies on the presence or absence of other neurochemical changes, includingchanges in hormones of the HPA axis, which remain to be fully understood.Recent studies in voles also have identified a role for dopamine and the rewardsystem in many types of social behavior, including the formation of a pair bond(Aragona, Liu, Curtis, Stephan, & Wang, 2003; Gingrich, Liu, Cascio, Wang, &Insel, 2000; Liu & Wang, 2003; Wang et al., 1999) and the expression of parentalcare (Lonstein, 2002). Pair bonding behavior has even been compared to an “addiction”due to its use of the neural pathways implicated in substance abuse (Insel, 2003).Access to dopamine receptors is also necessary for formation of a pair bond (Aragonaand Wang, 2004). However dopamine, although necessary, does not appear to besufficient for pair bond formation. Species capable of forming social bonds mayexhibit a unique co-occurrence of dopamine and OT (particularly in the nucleusaccumbens) or dopamine and AVP (particularly in the ventral pallidum). Sex differencesin the neurochemistry of pair bond formation may be partially explainedby endogenous differences in OT (more abundant in female) and AVP (androgendependent and centrally more abundant in males). A sexual dimorphism in thesecentral neuropeptides could have a major impact on sex differences in social behaviorand the management of reactivity to stressors (Carter, 1998).Variation in Neonatal Experience Can AffectPeptide Levels and Later BehaviorOf importance to understanding the actions of neuropeptide hormones is the potentialfor these compounds to have long-lasting, epigenetic actions, includingchanges in neuropeptide receptors for OT or AVP (Carter, 2003). The epigeneticeffects of OT may be of particular medical relevance because this peptide is com-