Have you ever been in a relationship? Do you remember when you met? Did your feelings change over a period of time? My guess is, probably. What causes this shift from initial attraction to long-lasting fidelity? Scientists at Florida State University sought to answer this question by examining the neurobiological changes that occur in the formation and maintenance of pair bonding in prairie voles.
Prairie voles are monogamous creatures and are a good model for examining the neurological changes associated with pair bonding. The maturation of this bond is marked by a shift from passive acceptance to aggressive rejection of all unfamiliar voles (including potential mates). Both positive and negative behaviors associated with this change are controlled by dopamine (DA) activity in the nucleus accumbens (NAcc). Researchers blocked and enhanced activity in this brain region to observe the effect on social behavior. They found opposing effects in pair bond formation when observing two different DA receptors, D1 and D2. These receptors are specifically located in the frontal portion of the NAcc shell- a region which controls appetitive behaviors.
Partner preference is required for pair bonding. Scientists found that activating D2-like receptors in the NAcc causes the formation of partner preference even in the absence of mating. Conversely, blocking these receptors prevented partner preference even when mating was available. Activation of D1-like receptors in this region blocked partner preference, which suggested that the different receptors were working in opposition. Researchers examined this through combinations of blocking or activating either receptor. D1-like receptors blocked both pharmacologically induced and naturally occurring partner preference, whereas D2-like receptors facilitated preference formation.
After uncovering the mechanism behind initial bond formation, scientists then explored how this bond endures over time, and whether there are coinciding neurological changes. They monitored the amount of receptor binding in pair-bonded and bachelor males with varying exposure to females. This revealed much greater D1-like receptor binding in the NAcc shell and core in pair-bonded males after two weeks with a female. This change was only observed in D1-like receptors, and was not observed after the initial 24 hours, suggesting development over time. Indeed, researchers suggest that this shift plays a key role in bond maintenance; specifically, preventing the male from forming other preferences or bonds. To test this, they examined the behavior of males in the presence of unfamiliar females and found pair-bonded males exhibited aggression, while bachelors did not. However, if D1-like receptors were blocked, this aggression disappeared (confirming the hypothesis).
Finally, these receptors were examined in a related but much less social (and non-monogamous) species of meadow vole. These creatures had much higher baseline D1-like binding, which is consistent with their known social behavior. When D1-like receptors were blocked, the animals displayed more social interaction, but unlike the prairie voles, this did not induce partner preference. Taken together, the findings from these experiments suggest there is a specific neurobiological foundation for monogamous relationships, which causes structural reconfiguration and can account for observable differences in mating practices among different species.