Oxytocin secreted from the pituitary gland cannot re-enter the brain because of the blood-brain barrier. Instead, the behavioral effects of oxytocin are thought to reflect release from centrally projecting oxytocin neurons, different from those that project to the pituitary gland, or that are collaterals from them.[47] Oxytocin receptors are expressed by neurons in many parts of the brain and spinal cord, including the amygdala, ventromedial hypothalamus, septum, nucleus accumbens, and brainstem.
Sexual arousal. Oxytocin injected into the cerebrospinal fluid causes spontaneous erections in rats,[36] reflecting actions in the hypothalamus and spinal cord. Centrally administrated oxytocin receptor antagonists can prevent non-contact erections, which is a measure of sexual arousal. Studies using oxytocin antagonists in female rats provide data that oxytocin increases lordosis behavior, indicating an increase in sexual receptivity.[48]
Bonding. In the Prairie Vole, oxytocin released into the brain of the female during sexual activity is important for forming a monogamous pair bond with her sexual partner. Vasopressin appears to have a similar effect in males.[49] Oxytocin has a role in social behaviors in many species, and so it seems likely that it also does in humans. In 2003, a study showed that in both humans and dogs oxytocin levels in the blood rose after five to twenty-four minutes of a petting session. It is possible that this plays a role in the emotional bonding between humans and dogs.[50]