Voles are addicted to love.

It's said that love is a drug. In a rodent species, scientists may now have proof. They've collected evidence that love, or more accurately, the choice of a mate, stimulates the brain in the same way as do drugs such as cocaine and heroin. "This type of social attachment uses the same reward pathway," says Thomas R. Insel of Emory University in Atlanta.

For many years, Insel has examined the brain chemistry behind the monogamy of one strain of voles (SN: 11/27/93, p. 360). When these rodents mate, their brains release chemicals that prompt the animals to form enduring partnerships. In females, it's a compound called oxytocin that drives the vole's attachment to a partner. In fact, injections of oxytocin into the brain of a female vole can duplicate the effects of mating and induce pair bonding. Drugs blocking oxytocin's action prevent females from pairing with males after mating.

Hoping to explain oxytocin's effects, Insel and his colleagues Brenden S. Gingrich and Carissa Cascio have studied the role of another brain chemical, the neurotransmitter dopamine. Recent research indicates that the addictive nature of many drugs, including nicotine, depends on dopamine. The drugs' pleasant sensations are stimulated by the neurotransmitter's release within a brain region called the nucleus accumbens (SN: 7/20/96, p. 38). Over time, the brain appears to become more and more dependent on this dopamine, causing people to crave the drugs.

Insel's group has now found that this reward mechanism may drive the monogamy of voles as well. Brain injections of dopaminelike drugs induce female voles to prefer a single male, without mating, while dopamine-blocking agents inhibit the usual bonding that occurs after mating. The researchers also measured the natural production of the neurotransmitter within the brains of female voles as they mated. Within the nucleus accumbens of mating females, the scientists found dramatic dopamine increases, which lasted for several hours.

Insel and his colleagues have discovered that brain cells in the nucleus accumbens sport the cell-surface proteins that oxytocin activates. Blocking these receptors made it less likely that female voles would pair after mating. From such evidence, the researchers hypothesize that mating induces oxytocin production, which in turn stimulates the release of dopamine.