Are hormones addictive?

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http://www.oxytocin.org/oxytoc/love-science.html

 

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The details of what is going on — the vole story, as it were — is a

fascinating one. 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. In other words, researchers can make prairie

voles fall in love — or whatever the vole equivalent of this is — with

an injection.

 

A clue to what is happening — and how these results might bear on the

human condition — was found when this magic juice was given to the

montane vole: it made 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. The question is, do humans (another species in the 3%

of allegedly monogamous mammals) have brains similar to prairie voles?

 

To answer that question you need to dig a little deeper. As Larry

Young, a researcher into social attachment at Emory University, in

Atlanta, Georgia, 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. Sure enough,

when a female prairie vole mates, there is a 50% increase in the level

of dopamine in the reward centre of her brain.

 

Similarly, when a male rat has sex it feels good to him because of the

dopamine. He learns that sex is enjoyable, and seeks out

more of it based on how it happened the first time. But, in contrast

to the prairie vole, at no time do rats learn to associate sex with a

particular female. Rats are not monogamous.

 

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 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. The same is true if the

vasopressin gene is knocked out.

 

The salient feature in this case is odour. Rats, mice and voles

recognise each other by smell. Christie Fowler and her colleagues at

Florida State University have found that exposure to the opposite sex

generates new nerve cells in the brains of prairie voles — in

particular in areas important to olfactory memory. Could it be that

prairie voles form an olfactory " image " of their partners — the rodent

equivalent of remembering a personality — and this becomes linked with

pleasure?

 

Dr Young and his colleagues suggest this idea in an article published

last month in the Journal of Comparative Neurology. They argue that

prairie voles become addicted to each other through a process of

sexual imprinting mediated by odour. Furthermore, they suggest that

the reward mechanism involved in this addiction has probably evolved

in a similar way in other monogamous animals, humans included, to

regulate pair-bonding in them as well.