Tomorrow is Valentine’s Day, the most romantic day of the year. Couples will stroll down the lane, gaze into each other’s eyes, and experience one of the most enigmatic feelings of all: love. But we won’t be Cupid’s only targets; prairie voles will be falling in love, too! This Valentine’s Day eve, let’s discuss what we have learned about this crazy little thing called love from these little cute animals.
Like many human relationships, prairie voles form monogamous commitments, called pair-bonds. Prairie vole couples mate and affiliate exclusively with each other, share nests, and even raise their offspring together! Male prairie voles often display intense aggression toward strangers for defense of territory, nest, and mate. Only 3% of all mammals exhibit these behaviors, so prairie voles are valuable models for investigating the neurobiological mechanisms for pair-bonding – the ability to form intense, exclusive social attachments with a mate.
But not all voles exhibit pair-bonding. The close relatives of prairie voles, montane voles, prefer a rather free approach to relationships. They are more promiscuous, do not pair-bond, and do not exhibit aggressive behaviors to defend a mate or their offspring. Intrigued by this difference, neuroscientists compared neuropeptide expression, receptor distribution, and nerve pathways between prairie voles and montane voles. Two molecules were hypothesized to be implicated in pair-bonding behavior: oxytocin and vasopressin.
In a classic experiment by researchers from the National Institute of Mental Health, male prairie voles were injected with an antagonist (a chemical that binds to the receptor but does not activate it) of either the oxytocin or vasopressin receptor. They found that blocking the prairie voles’s oxytocin receptors with the antagonist had no effect in mating-induced aggression and partner preference over other new potential mates (they still exhibited pair-bonding). However, male prairie voles injected with the vasopressin antagonist failed to exhibit aggression to defend the female partner, and did not spend as much time with the original mate as with new potential mates (they no longer pair-bonded). When researchers administered vasopressin to these male prairie voles, the effects were reversed; they became aggressive toward other males and preferred to spend time with their partner than with strangers. It’s almost as if he fell in love with her all over again!
This study implicated vasopressin as the neuropeptide that facilitates the social cues necessary for protective aggression and pair-bonding in males. In her previous post, JL mentioned that oxytocin promotes bonds and trust between parents and children and between couples in committed, monogamous relationships. Together, oxytocin and vasopressin are part of the molecular and neural mechanisms that allow for the social cues necessary for individual recognition and behavior that we describe as loving. In the larger neural architecture of pair-bonding and attachment, vasopressin and oxytocin act in the reward centers of the brain, similar to dopamine! The convergence of the social and reward circuits may be what’s truly behind the adage that “love is an addiction.”
Winslow J.T., Hastings N., Carter C.S., Harbaugh C.R. & Insel T.R. (1993). A role for central vasopressin in pair bonding in monogamous prairie voles, Nature, 365 (6446) 545-548. DOI: 10.1038/365545a0
Young L.J. & Wang Z. (2004). The neurobiology of pair bonding, Nature Neuroscience, 7 (10) 1048-1054. DOI: 10.1038/nn1327
Images adapted from Corbis (pink silk, heart, syringe); McGraw L.A. & Young L.J. (2010). The prairie vole: an emerging model organism for understanding the social brain, Trends in Neurosciences, 33 (2) 103-109. DOI: 10.1016/j.tins.2009.11.006; and Edwards S. & Self D.W. (2006). Monogamy: dopamine ties the knot, Nature Neuroscience, 9 (1) 7-8. DOI: 10.1038/nn0106-7.