By Robert Thorn
As Valentine’s Day approaches, excitement starts to fill the air in anticipation of Cupid’s visit. The key to the potency and strength of love has been a mystery to scientists for some time. It may surprise you to know that the key to unlocking the secrets of love came from studies of common prairie voles. Prairie voles are unique in the fact that when they get together during mating season they generally mate with one partner and this connection lasts for life. This is in stark contrast to the prairie vole’s evolutionary cousin, the montane vole which is a more promiscuous species of vole. By comparing the different between these monogamous and promiscuous voles, scientists were able to get an idea of what hormones and brain regions may be involved in pair bonding, or as we humans call it, relationships!
Two hormones that were identified in early studies of the prairie and montane voles are oxytocin and vasopressin. If you have heard of these before, it is probably because they are important hormones in different human systems. Oxytocin is used primarily during childbirth, acting to start muscle contractions during birth, as well as being involved in lactation during breastfeeding. Vasopressin is important for a healthy functioning cardiovascular system, as well as maintaining blood pressure by helping to regulating the amount of water filtered out in the kidney. While both of these hormones seem unrelated to each other, and definitely unrelated to love, they were thought to be important for pair bonding because compared to the montane voles, prairie voles have more oxytocin and vasopressin receptors in their brains.
Once two prairie voles mate, vasopressin and oxytocin are released and pair bonding occurs. To show the importance of these two hormones, scientists manipulated the action of one or the other in prairie and montane voles. When the action of either vasopressin or oxytocin was blocked, prairie voles began displaying more promiscuous behaviors, similar to those seen in montane voles. On the other hand, when scientists increase the activity of vasopressin in the brains of montane voles, by increasing the number of vasopressin receptors located in the brain, they began to adopt a more monogamous relationship with their mating partners.
To further examine the role of these hormones in pair bonding, the location of their receptors were mapped to different areas of the brain. Oxytocin receptors were seen in areas of the brain that are involved in dopamine reward system (nucleus accumbens) and emotional memory formation (amygdaloid complex). Vasopressin is seen in different areas of the brain that are involved in motivation and the dopamine reward system (ventral palladium). The role of the dopamine reward system in pair bonding was further confirmed by experiments, which showed that blocking dopamine blocks pair bonding, and adding extra dopamine artificially creates pair bonding without mating (i.e. without the release of oxytocin).
It is interesting to note that the dopamine reward system has been implicated in the addictive nature of some drugs. Keep this in mind if you get struck by one of Cupid’s arrows this Valentine’s Day – You might just get hooked on love.