Of Mice and Men: Could new findings about oxytocin change the way scientists understand love?

For the past year, I have been involved with a laboratory that exclusively uses mice to study the brain. This is extremely common in the world of neuroscience — mice and other rodents boast a neural structure that is similar enough to ours and a physiological system that is different enough that we can justify tinkering pretty heavily with their neural function in a laboratory setting. A few months ago, my lab attended a talk by another lab that studied the role of dopamine in the brain.

Now, the state of neuroscience and genetics are such that it is pretty standard to be able to turn on and shut off specific neurons or neurotransmitters at will. That is why it didn’t surprise me that the lab giving the talk on this particular day told us that they’d attempted to completely silence dopamine in the brain in their experimental mice population. The result was something that would stick with me for a long time — whatever the state of a brain without dopamine is, it was so traumatic to the mice that they completely refused to enter the experimental setup again after they’d been exposed to it. It was implied that the mice had literally seen hell.

This is what was on my mind while I was reading the new study from "Neuron" by Berendzen et. al. — “Oxytocin receptor is not required for social attachment in prairie voles” — that used oxytocin deprivation to test whether or not oxytocin is a required neurotransmitter for “love.” Oxytocin is a hormone produced in the hypothalamus, and has been linked to physical behaviors like uterine contractions and lactation as well as social behaviors like trust and the formation of attachments. Basically, whether you’re cuddling with someone or in labor, oxytocin is believed to be a key player in all things reproduction.

The study in question defined the reproductive cycle by pair bonding, a very quantitative and clear measurement which falls slightly short of our human definition of love as a highly emotional and complex force. Knowing what I do about oxytocin — that’s it's one of the main neurotransmitters that makes us feel good, specifically during the mating or courting process — I instinctively felt really bad for the poor prairie voles who were deprived of oxytocin during the process of the study. Regardless of what neuron, neuron group, or neurotransmitter is toyed with in a neuroscience study, the implication is usually that there will be some kind of change in the behavior of the experimental animal — we like to think that everything going on in our brains has a carefully evolved purpose.

Evidently, though, it didn’t make much difference to them. Voles typically form long-term mating bonds, which are indicative of a more complex social bond than random heterosexual mating between “stranger” animals (this is what makes them such a good choice for this study). The study followed two groups of voles — one normal control group and one group that could not produce oxytocin. From the process of choosing a mating partner to birthing pups to weaning them, the mutant (no oxytocin) voles displayed not a single difference. They continued to prefer mating with a chosen long-term partner over strangers, they birthed normal pups, and they cared for them and raised them as normal. Even gathering wandering pups back to the nest, yet another oxytocin-linked behavior, was still present in the mutant voles. The study concluded that, as pair bonding had proceeded as normal in the mutant voles, oxytocin must not be as critical as we think in mating.

This, to state it mildly, was a huge shock to all. Prior studies, opposingly, have focused on the utmost importance of oxytocin in all of the behaviors listed above. Additionally, the Berendzen et. al. study mentions that the type of suppression they perform on voles is actually not possible in some other animals. In mice, an oxytocin drought is a death sentence, as mothers become incapable of producing milk for their pups.

In prairie voles, then, the question becomes: “If oxytocin is not required for successful reproduction, what does it do? And what is responsible for the reproductive behaviors we see?” The study does not answer either of these questions, but it certainly provides food for thought. Berendzen et. al. identify the possibility that there may be other neurotransmitter pathways that have evolved separately in voles to facilitate pair-bonding, which explains why only voles have so far been identified as not really needing oxytocin.

Given how essential these hormones are to the survival of a given species, I expect more studies to surface on this topic. Certainly, it is possible that human evolution has also given rise to a variation on the traditional oxytocin pathway, and it would be incredibly useful to know what those variations are. Because of the complexity of human society as compared with that of voles, “love” is typically considered to be much more nuanced in humans. We develop bonds with people for very specific reasons and in very specific ways. People no longer mate only to reproduce. What, then, are the implications of oxytocin in our world? Our only answer for now: leave it to beaver.