Individual variability exists in infants’ socio-emotional stress regulation, in terms of behavioral response (i.e., negative emotionality) as well as magnitude and direction (i.e., increase or decrease) of hypothalamic-pituitary-adrenal (HPA) axis reactivity (i.e., salivary cortisol post-stress concentration). The catechol-O-methyltransferase polymorphism at codon 158 (COMTval158met) associates with stress regulation, but no evidence exists for infants. This study aimed to assess the association between COMTval158met and both negative emotionality and salivary cortisol reactivity to socio-emotional stress in 4-month-old infants. Sixty-nine infants were exposed to the Face-to-Face Still-Face (FFSF) paradigm. During the FFSF paradigm, socio-emotional stress is elicited by experimental manipulation of maternal responsiveness (i.e., Still-Face episode). A double-exposure FFSF with two Still-Face episodes was used to evaluate both behavioral response and HPA axis reactivity. Negative emotionality was 1-s microanalytically coded. Magnitude (i.e., area under the curve, AUC) and Direction (increase vs. decrease) of salivary cortisol post-stress concentration were assessed. COMTval158met genotype was categorized as val-homozygotes and met-carriers. Compared to val-homozygotes, met-carriers showed higher negative emotionality during the second Still-Face episode. AUC was greater in increaser met-carriers compared to val-homozygous infants. In addition, in the presence of an increasing HPA response, the met allele emerged as a specific risk condition in the face of repeated stress exposures. The present findings further extend previous studies conducted with children and adults suggesting that the COMT met allele might be involved in stress regulation during early infancy, especially in response to repeated socio-emotional stress exposure.
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