Milk as the proximal mechanism for behavioral change in the newborn.

Early experience in the context of suckling at the nipple is a crucial determinant of neurobehavioral development in mammals. In particular, milk has been recognized by developmental psychobiologists as an ecologically relevant sensory stimulus and should be viewed by clinicians as more than a source of nutrition in early human development. Gaining access to and processing milk during suckling serves as a primary focus of activity in the newborn. Because the rat fetus lacks prior exposure to milk or other suckling stimuli, it provides an excellent model system for investigating the development of milk-related responses. Fetal exposure to milk results in a cascade of behavioral, physiological and neurochemical consequences. Milk-induced activation of the endogenous opioid system plays an important role in reorganizing fetal motor activity, altering sensory responsiveness and supporting associative learning in the fetus. Subtle changes in contextual stimulation alter the ability of milk to engage different neurochemical systems. These findings suggest that the infant's first experiences with milk and other suckling stimuli may have lasting consequences for neurobehavioral development.