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The ontogeny of antisocial behavior (ASB) is rooted in complex gene-environment (G×E) interactions. The best-characterized of these interplays occurs between: a) low-activity alleles of the gene encoding monoamine oxidase A (MAOA), the main serotonin-degrading enzyme; and b) child maltreatment. The purpose of this study was to develop the first animal model of this G×E interaction, to help understand the neurobiological mechanisms of ASB and identify novel targets for its therapy. Maoa hypomorphic transgenic mice were exposed to an early-life stress regimen consisting of maternal separation and daily intraperitoneal saline injections and were then compared with their wild-type and non-stressed controls for ASB-related neurobehavioral phenotypes. Maoa hypomorphic mice subjected to stress from postnatal day (PND) 1 through 7 - but not during the second postnatal week - developed overt aggression, social deficits and abnormal stress responses from the fourth week onwards. On PND 8, these mice exhibited low resting heart rate - a well-established premorbid sign of ASB - and a significant and selective up-regulation of serotonin 5-HT 2A receptors in the prefrontal cortex. Notably, both aggression and neonatal bradycardia were rescued by the 5-HT 2 receptor antagonist ketanserin (1-3 mg kg -1 , IP), as well as the selective 5-HT 2A receptor blocker MDL-100,907 (volinanserin, 0.1-0.3 mg kg -1 , IP) throughout the first postnatal week. These findings provide the first evidence of a molecular basis of G×E interactions in ASB and point to early-life 5-HT 2A receptor activation as a key mechanism for the ontogeny of this condition. This article is part of the Special Issue entitled 'The neuropharmacology of social behavior: from bench to bedside'., (Copyright © 2019. Published by Elsevier Ltd.)