Genetic Variation in the Maternal Oxytocin System Affects Cortisol Responsiveness to Breastfeeding in Infants and Mothers

Objectives: The neuropeptide oxytocin regulates milk let-down during breastfeeding and maternal behavior in mammals. Oxytocin has also been shown to reduce stress through inhibitory effects on hypothalamic-pituitary-adrenal (HPA) reactivity. However, it remains unknown whether and how infant cortisol levels are affected by breastfeeding and what role the oxytocin system plays in this process. In the current study, we examined whether genetic variation in the oxytocin system impacts the cortisol response to breastfeeding in 84 mother-infant dyads. Methods: Salivary cortisol was measured before and after a breastfeeding session. Mothers and infants were genotyped for a single-nucleotide polymorphism (SNP) in the CD38 gene (rs3796863). We compared between CC carriers and CA/AA carriers, as the CC genotype has been associated with reduced release of oxytocin and higher rates of autism in prior studies. Results: Our results show that differences in infant and maternal cortisol responses to breastfeeding were associated with variation in maternal CD38. Specifically, CA/AA mothers displayed a significantly greater reduction in cortisol after breastfeeding than mothers with the CC genotype. Moreover, infants of CA/AA mothers showed significantly reduced cortisol levels after breastfeeding, as compared to infants of CC mothers. Conclusions: The current findings demonstrate that maternal cortisol responses to breastfeeding vary as a function of their genetic capacity to release oxytocin, and this may also impact their infant’s stress regulation. This suggests a potential mechanism by which breastfeeding contributes to the development of HPA reactivity in infancy.