Vasopressin and Aggression in Cross-Fostered California Mice ( Peromyscus californicus ) and White-Footed Mice ( Peromyscus leucopus ) Janet K. Bester-Meredith and Catherine A. Marler Department of Psychology, University of Wisconsin, 1202 W. Johnson Street, Madison, Wisconsin 53706 Received November 17, 2000; revised January 26, 2001; accepted March 20, 2001 To examine how developmental experiences alter neural pathways associated with adult social behavior, we cross- fostered pups between the more aggressive and monog- amous California mouse ( Peromyscus californicus) and the less aggressive and polygamous white-footed mouse ( P. leucopus). Cross-fostered males became more like their foster parents when tested as adults. Male white-footed mice became more aggressive only in an aggression test in a neutral arena, whereas the territorial California mice became less aggressive in resident–intruder aggression test, as measured by attack latency. Only the species that displayed a change in resident–intruder aggression showed a change in arginine vasopressin (AVP) levels: cross-fostered California mice had significantly lower lev- els of AVP-immunoreactive (AVP-ir) staining than controls in the bed nucleus of the stria terminalis (BNST) and the supraoptic nucleus (SON) and a nonsignificant trend to- ward lower levels in the medial amygdala (MA). Neither species showed changes in AVP-ir staining in a control area, the paraventricular nucleus (PVN). The changes in AVP-ir staining in the BNST and SON may not be caused by stress because cross-fostering was not associated with changes in adult plasma concentrations of two steroid hormones, corticosterone and testosterone, that have been associated with stress-related alterations in AVP pathways. These results suggest that manipulating the early parental environment can directly alter both a neuro- transmitter system and species-typical patterns of social behavior, but that these effects may vary between species and under different social contexts. © 2001 Academic Press Key Words: vasopressin; aggression; cross-fostering; Peromyscus; resident–intruder; bed nucleus of the stria terminalis; medial amygdala. A large body of work indicates a critical role of environmental conditions and experience in the devel- opment of the central nervous system and various behavioral processes. Lacking in this work has been the systematic examination of the impact of the post- natal environment on neural pathways associated with adult social behaviors such as aggression. Al- though adult stress responsiveness is clearly affected by maternal behaviors like pup grooming (Liu, Diorio, Tannenbaum, Caldji, Francis, Freedman, Sharma, Pearson, Plotsky, and Meaney, 1997), it is unknown how the postnatal environment alters the develop- ment of these brain pathways more directly associated with social behavior than stress responsiveness. While recent attention has focused on the association be- tween neurotransmitters like vasopressin (AVP) and adult male social behavior (reviewed in De Vries and Villalba, 1997; De Vries and Miller, 1998), the role of the early rearing environment in shaping this associ- ation in adults has received less consideration. For example, studies involving administration of AVP and antagonists demonstrate a positive rela- tionship between neurotransmission and aggression and/or dominance behaviors in prairie voles (Win- slow, Hastings, Carter, Harbaugh, and Insel, 1993), golden hamsters (Ferris, Melloni, Koppel, Perry, Fuller, and Delville, 1997), and Syrian hamsters (Bamshad and Albers, 1996). In humans, aggressive- ness correlates with levels of AVP in cerebrospinal fluid (Coccaro, Kavoussi, Hauger, Cooper, and Fer- ris, 1998). Differences in AVP-immunoreactive (AVP-ir) staining also are associated with differ- ences in attack latency between species of Pero- myscus mice (Bester-Meredith, Young, and Marler, 1999) and within different strains of house mice (Compaan, Koolhaas, Bujis, Pool, de Ruiter, and van Oortmerssen, 1992; Compaan, Bujis, Pool, De Ruiter, and Koolhaas, 1993). Despite this association between AVP and adult attack latency, it remains unclear how the early envi- Hormones and Behavior 40, 51–64 (2001) doi:10.1006/hbeh.2001.1666, available online at http://www.idealibrary.com on 0018-506X/01 $35.00 Copyright © 2001 by Academic Press All rights of reproduction in any form reserved. 51