Which hormone stimulates milk release from mammary glands and is involved in social bonding?

Oxytocin is the hormone responsible for stimulating milk release from mammary glands, a process known as milk ejection reflex or let-down. When a baby suckles at the breast, sensory signals from this action trigger the release of oxytocin from the posterior pituitary gland. This hormone causes the smooth muscle around the mammary glands to contract, leading to the expulsion of milk.

In addition to its role in lactation, oxytocin is also closely associated with social bonding and attachment. It is often termed the "love hormone" because of its involvement in forming connections between individuals, including between mothers and their infants. This dual function of oxytocin as both a facilitator of milking and a promoter of social interactions highlights its importance in fostering nurturing behaviors in mammals.

The other hormones listed do not share these specific roles. Orexin is primarily involved in regulating arousal, wakefulness, and appetite. Prolactin plays a critical role in milk production but does not directly stimulate the release of milk. Cortisol is a stress hormone that influences metabolism and immune responses but is not involved in lactation or bonding.