Addicted to Love
When prairie voles have sex, two hormones called oxytocin and vasopressin are released. If the release of these hormones is blocked, prairie-voles’ sex becomes a fleeting affair, like that normally enjoyed by their rakish montane cousins. Conversely, if prairie voles are given an injection of the hormones, but prevented from having sex, they will still form a preference for their chosen partner. When this magic juice is given to the montane vole it makes no difference. It turns out that the faithful prairie vole has receptors for oxytocin and vasopressin in brain regions associated with reward and reinforcement, whereas the montane vole does not.
As Larry Young explains, the brain has a reward system designed to make voles (and people and other animals) do what they ought to. Without it, they might forget to eat, drink and have sex — with disastrous results. That animals continue to do these things is because they make them feel good. And they feel good because of the release of a chemical called dopamine into the brain. When a female prairie vole mates, there is a 50% increase in the level of dopamine in the reward center of her brain.
Similarly, when a male rat has sex it feels good to him because of the dopamine. But, in contrast to the prairie vole, at no time do rats learn to associate sex with a particular female.
This is where the vasopressin and oxytocin come in. They are involved in parts of the brain that help to pick out the salient features used to identify individuals. If the gene for vasopressin or oxytocin is knocked out of a mouse before birth, that mouse will become a social amnesiac and have no memory of the other mice it meets.
Dr Young argues that prairie voles become addicted to each other through a process of sexual imprinting mediated by odour, and that the reward mechanism involved in this addiction has probably evolved in a similar way in other monogamous animals, humans included.
Sex stimulates the release of vasopressin and oxytocin in people, as well as voles, though the role of these hormones in the human brain is not yet well understood. Among those of Man’s fellow primates that have been studied, monogamous marmosets have higher levels of vasopressin bound in the reward centres of their brains than do non-monogamous rhesus macaques.
In 2000, Andreas Bartels and Semir Zeki located the areas of the brain activated by romantic love. They took students who said they were madly in love, put them into a brain scanner, and looked at their patterns of brain activity.
A relatively small area of the human brain is active in love, compared with that involved in, say, ordinary friendship. The brain areas active in love are different from the areas activated in other emotional states, such as fear and anger. Parts of the brain that are love-bitten include the one responsible for gut feelings, and the ones which generate the euphoria induced by drugs such as cocaine. So the brains of people deeply in love do not look like those of people experiencing strong emotions, but instead like those of people snorting coke. “We are literally addicted to love,” Dr Young observes.
The more receptors for vasopressin and oxytocin located in regions associated with reward, the more rewarding social interactions become.
Steven Phelps found great diversity in the distribution of vasopressin receptors between individual prairie voles and individual humans. He suggests that this variation contributes to individual differences in social behaviour — in other words, some will be more faithful than others.
Scientists can increase the expression of the relevant receptors in prairie voles, and thus strengthen the animals’ ability to attach to partners. In 1999, Dr Young took the prairie-vole receptor gene and inserted it into an ordinary (and therefore promiscuous) mouse. The transgenic mouse thus created was much more sociable to its mate.
Jim Pfaus says the aftermath of lustful sex is similar to the state induced by taking opiates. A heady mix of chemical changes occurs, including increases in the levels of serotonin, oxytocin, vasopressin and endogenous opioids (the body’s natural equivalent of heroin).
Researchers think humans develop a “love map” as they grow up — a blueprint that contains the many things that they have learnt are attractive. Research on the choices of partner made by identical twins suggests that the development of love maps takes time, and has a strong random component.
Dr Pfaus says that rats can be conditioned to prefer particular types of partner — for example by pairing sexual reward with some kind of cue, such as lemon-scented members of the opposite sex. Human fetishes develop early, and are almost impossible to change. The fetishist connects objects such as feet, shoes, stuffed toys and even balloons, that have a visual association with childhood sexual experiences, to sexual gratification.
Helen Fisher thinks that administering serotonin can help someone get over a bad love affair faster. She also suggests it is possible to trick the brain into feeling romantic love in a long-term relationship by doing novel things with your partner. Any arousing activity drives up the level of dopamine and can therefore trigger feelings of romance as a side effect. This is why holidays can rekindle passion.
“I get a kick out of you,” The Economist