Monogamous cheaters

In my previous post I wrote about vasopressin and oxytocin, so-called “love molecules” that promote attachment and pair-bonding in voles and humans. These molecules act on receptors found in the dopamine-reward system to enhance the dopamine “pleasure” response thereby re-enforcing monogamous activities. However, socially monogamous voles, like humans, are prone to “mistakes” or “slip-ups” with males and females frequency engaging in uncommitted sexual behaviors. Indeed, using genetic testing of offspring, researchers have found that many of the species first thought to be “monogamous” like birds and gibbons actually participate in extra-pair copulations. It’s now more exciting to find a species that is actually sexually monogamous (such as the recent discovery of a monogamous frog). Evolutionarily speaking this is not surprising; we want to get it on with as many people as possible to pass on our genes and increase genetic diversity, while at the same time having enough resources to take care of our young. Therefore, it is possible that two competing systems co-evolved in humans: one that promotes monogamy (vasopressin, oxytocin and others) and another previously uncharacterized system that promotes sleeping around with as many people as possible. This predicts that our tendencies towards infidelity and sexual promiscuity could also be genetically encoded.

Is unfaithfulness really all in the genes? A new study published recently in PLoS ONE suggests that it might be, at least in part. The authors linked a certain variant of the dopamine D4 receptor gene to the propensity towards one-night stands (but not the actual number) and the number of sexual partners in those that were unfaithful (but not to unfaithfulness per se, although there was a trend towards significance). Interestingly, people with this variant have less dopamine D4 receptors in the reward centers of the brain and these receptors show less binding to dopamine, suggesting that these individuals might need more dopamine floating around to reach the same feel-good mood. It’s not a surprise then, that this variant has also been associated with a slew of other behaviors that increase dopamine release including addictions, risky behavior and novelty-seeking.

So, what does this mean? The authors are careful to point out that the dopamine D4 receptor gene should not be labeled the “cheating gene” or the “promiscuity gene” (although it already has) since having the variant does not necessarily mean that you will sleep around or be unfaithful. Many other genetic, environmental (alcohol) and cultural influences are likely to play into an individual’s decision to sleep around or cheat. In addition, the variant may be associated with a third confounding variable like being more honest about sexual behavior, more attractive, etc. Or it may simply be associated with risky behavior and novelty-seeking, increasing the likelihood of wanting uncommitted sex.

For now, I’d hold off on sending your significant other for genetic testing.

What rodents are telling us about human behavior and autism

A tales of two voles

Prairie voles are extremely social creatures, preferring to spend the majority of their time with other prairie voles. In contrast, the closely related montane voles are extremely asocial creatures, choosing a solitary lifestyle over one where they’d have to be tied down. Eerily, prairie voles and humans share many other social behaviors: after a male and female prairie vole decide to spend the night together, they fall madly in love (called pair-bonding), move into a shared nest and raise their children together (‘Till death do us part: 75% of prairie voles stay together until one partner dies). Like with humans, affairs are fairly common, jealousy is rampant and some males (>40%) just never settle down (known in the field as “wanderers”). Prairie voles are even known to enjoy an alcoholic drink every once in a while. With these similarities, it’s no wonder the prairie vole has been the model system of choice to study the physiological basis of social behavior, and why the montane vole has served as the perfect “asocial” control.

Researchers have focused much of their attention on the role of two neuropeptide hormones, oxytocin and vasopressin, in mediating these social behaviors. Oxytocin and vasopressin are well-known regulators of peripheral tissues involved in birth, lactation and water homeostasis. However, their receptors (the cell-surface molecules which bind to these hormones and transmit their signals to the inside of cells) are also found scattered throughout the brain. Turns out, these hormones are also released by prairie voles when they “spend the night”, suggesting that these hormones may induce the post-mating behaviors of prairie voles: partner preference, aggression towards intruders and parental care (characteristics of social monogamy… referred to as monogamy for the rest of the article). Indeed, researchers can induce partner preference and aggression in virgin male prairie voles by injecting them with vasopressin and can block these behaviors by blocking the vasopressin receptor during mating. The same can be done in female prairie voles, except with oxytocin and an oxytocin receptor blocker, reflecting gender-specific differences in how these hormones affect social behaviors. Only thing is, montane voles also release oxytocin and vasopressin following mating and giving them additional vasopressin fails to induce monogamy—what then results in the difference in post-mating behaviors seen between prairie and montane voles? Surprisingly, when researchers looked at the brains of montane and prairie voles, they found striking differences in the location of vasopressin and oxytocin receptors. Prairie voles had more receptors in the ventral pallidum and nucleus accumbens, areas associated with reward and reinforcement, whereas montane voles lacked receptors in these areas. Importantly, other monogamous species, such as marmosets and California mice, show a similar distribution of receptors to the prairie voles. These results suggest that monogamous species may perceive social attachments as pleasurable and rewarding, therefore reinforcing these behaviors, whereas these behaviors are not reinforced in non-monogamous species.

Of course, the goal of all these studies is to understand and possibly treat some human behaviors. Autism can be a particularly devastating neurodevelopmental disorder characterized by severe social deficits such as lack of eye contact, empathy and social attachment. It is therefore not surprising that researchers are looking to see whether oxytocin and vasopressin are dysregulated in autistic patients. Indeed, some cases of autism are associated with reduced levels of circulating oxytocin or complete deletion or mutations in the oxytocin or vasopressin receptor genes. Can administration of oxytocin or vasopressin help with the symptoms of autism? Surprisingly, administration of oxytocin to humans increases social behaviors such as eye contact, trust and empathy and reduces social anxiety. Initial trials with oxytocin and high-functioning autistics showed improvements in their ability to make eye contact and other social behaviors. Therefore, although a link between autism and the oxytocin or vasopressin systems is tenuous and much research remains to be done, research on voles may surprisingly hold the key to understanding human social and asocial behaviors.

For more information:
Oxytocin, vasopressin and autism (free full-text): http://rstb.royalsocietypublishing.org/content/361/1476/2187.long

Those binge-drinking voles:
http://www.oregonlive.com/health/index.ssf/2010/07/voles_a_party_animal_sheds_lig.html