Children form a strong attachment to their caregiver-even when that caretaker is abusive. Paradoxically, despite the trauma experienced within this relationship, the child develops a preference for trauma-linked cues-a phenomenon known as trauma bonding. Although infant trauma compromises neurobehavioral development, the mechanisms underlying the interaction between infant trauma bonding (i.e., learned preference for trauma cues) and the long-term effects of trauma (i.e., depressive-like behavior, amygdala dysfunction) are unknown. We modeled infant trauma bonding by using odor-shock conditioning in rat pups, which engages the attachment system and produces a life-long preference for the odor that was paired with shock. In adulthood, this trauma-linked odor rescues depressive-like behavior and amygdala dysfunction, reduces corticosterone (CORT) levels, and exerts repair-related changes at the molecular level. Amygdala microarray after rescue implicates serotonin (5-HT) and glucocorticoids (GCs), and a causal role was verified through microinfusions. Blocking amygdala 5-HT eliminates the rescue effect; increasing amygdala 5-HT and blocking systemic CORT mimics it. Our findings suggest that infant trauma cues share properties with antidepressants and safety signals and provide insight into mechanisms by which infant trauma memories remain powerful throughout life.E arly-life trauma is associated with compromised neurobehavioral development and vulnerability to later-life psychiatric disorders like depression (1-5). Long-lasting effects of infant trauma relevant to depression include disruptions in social behavior, alterations in the serotonin (5-HT) system, and amygdala dysfunction (2, 5-8), which have been replicated by animal models (9-15). Paradoxically, trauma-linked cues-even those associated with abusive attachment-can elicit strong attraction and feelings of comfort in humans (16,17). Furthermore, a lifelong attraction to trauma-linked cues has also been demonstrated in rodent models of infant trauma, which suggests that sensory cues learned during infancy alter adult behaviors and amygdala activity (12,18,19).To explore the enduring effects of infant trauma and the mechanisms underlying their modulation by infant trauma-linked cues, we used an odor-learning paradigm during the sensitive period for attachment learning in rat pups, postnatal (PN) days 8-12, which is characterized by rapid and robust odor-preference learning that extends to aversive stimuli (14,(20)(21)(22). Specifically, we used infant olfactory classical conditioning in which a novel peppermint odor is paired with a mild shock (0.5 mA, 1s) to produce a preferred odor that functions behaviorally as a maternal odor and results in neural activation comparable with that induced by natural maternal odor (20,(23)(24)(25). During the sensitive period, low endogenous levels of corticosterone (CORT) and attenuated shock-induced CORT release prevent the infant amygdala from exhibiting the learning-induced plasticity required for avoidance learning (1...

Social support can attenuate the behavioral and stress hormone response to threat, a phenomenon called social buffering. The mother’s social buffering of the infant is one of the more robust examples; yet we understand little about the neurobiology. Using a rodent model, we explore the neurobiology of social buffering by assessing neural processing of the maternal odor, a major cue controlling social buffering in rat pups. We used pups before (postnatal day (PN) 7) and after (PN14, PN23) the functional emergence of social buffering. Pups were injected with 14C 2-deoxyglucose (2-DG) and presented with the maternal odor, a control preferred odor incapable of social buffering (acetophenone), or no odor. Brains were removed, processed for autoradiography and brain areas identified as important in adult social buffering were assessed, including the amygdala basolateral complex (Basolateral Amygdala [BLA]), medial prefrontal cortex (mPFC), and anterior cingulate cortex (ACC). Results suggest dramatic changes in the processing of maternal odor. PN7 pups show mPFC and ACC activation, although PN14 pups showed no activation of the mPFC, ACC, or BLA. All brain areas assessed were recruited by PN23. Additional analysis suggests substantial changes in functional connectivity across development. Together, these results imply complex nonlinear transitions in the neurobiology of social buffering in early life that may provide insight into the changing role of the mother in supporting social buffering.