Long-term attachment, which includes pair bonding with a sexual partner and parental bonding with offspring, are naturally rewarding behaviors in some species of mammals.
The hypothalamus is a critical region for the formation of social bonds. Magnocellular neurosecretory cells, the larger type of neurosecretory cell compared to parvocellular neurons, send axons from the hypothalamus down to pituitary stalk where they terminate on capillaries of the general circulation located within the posterior pituitary. Therefore, unlike the control of stress and gonadal hormones, where the hypothalamic neurons release hormones onto anterior pituitary endocrine cells, release of hormones from the posterior pituitary comes directly from hypothalamic neurons.
The magnocellular neurons synthesize and release oxytocin and vasopressin, two neuropeptides, into the blood. Oxytocin, often referred to as the love hormone, promotes social bonding. It is released during reproduction and alsocauses uterine contractions during labor and the milk letdown reflex after birth. Vasopressin, also called antidiuretic hormone, plays a role in regulating salt concentration in the blood by acting on the kidneys to promote water retention and decrease urine production. Vasopressin has also been shown to be involved in bonding, parenting, territoriality, and mate guarding in some animals.
Much of the research on social attachment has been done using voles as the animal model. Voles are useful because there are closely related species that display considerably different reproductive behavior. The prairie vole is a monogamous rodent, with males and females displaying strong pair bonds and both sexes showing parental behavior. The montane vole, on the other hand, is a non-social species. Pair bonds are not formed, and only the female cares for the young. Differences in brain and behavior can be studied between these species.
In social voles, oxytocin and vasopressin are released by the hypothalamus in response to mating and act on regions of the reward and limbic systems. Female prairie voles express higher levels of oxytocin receptors in the nucleus accumbens compared to montane voles, whereas male prairie voles express higher levels of vasopressin receptors in the ventral pallidum compared to montane voles. The nucleus accumbens (also called ventral striatum) and ventral pallidum are both located in the basal ganglia and are involved in the limbic loop, which is responsible for processing of emotions, rewards, and motivation.
Oxytocin, vasopressin, and the reward system also appear to be important for bonding in humans. When presented with pictures of either their own children or partners, subjects in an fMRI show increased activation in regions like the ventral tegmental area and striatum compared to when viewing pictures of friends. These regions are also known to express oxytocin and vasopressin receptors, and the hormones are released during times of bond formation, like breastfeeding and intercourse.