Scientists at Emory University have become the cupids of the vole world by discovering that by activating specific brain circuits in the female prairie vole they could cause them to fall in love with certain males.
Prairie voles are highly sociable and form long-term monogamous relationships, a behaviour known as pair bonding which is relatively rare in the natural world – occurring in fewer than 5% of mammals. This makes prairie voles the perfect candidate for a small mammal model to learn more about the brain circuitry that underpins these bonds, circuits that are also believed to be implicated in human pair bonding.
Finding the love circuit
First, electrodes were inserted into the brains of female prairie voles to identify the brain circuitry that’s activated when a pair bond is formed. The researchers found that rhythmic oscillations of a group of neurons in the prefrontal cortex controlled the strength of oscillations in neurons of the anatomically linked area the ‘nucleus accumbens’, and that the connections in this circuit became stronger after mating and when the females huddled close to a male. The nucleus accumbens is involved in pleasure, reward and addiction, while the prefrontal cortex is mainly involved in decision-making and executive functions, suggesting that the partners themselves had started to act as rewards to the female.
Cupid’s pulses of light
Scientists then exploited a technique known as optogenetics to trigger the love circuit that they’d found between the prefrontal cortex and the nucleus accumbens. First, genes were inserted into the neurons of the prefrontal cortex via a viral vector. These genes were designed to act as switches, turning the neurons on when exposed to light. Light could then be pulsed into that brain region via a fine optical fibre to activate the genes. A male and a female were then put together for a bonding period of an hour without being allowed to mate, while their cortex neurons were stimulated to connect with the reward system at the same oscillatory frequency observed during mating (mimicking what happens to naturally strengthen the pair bond).
Female voles were then given the choice of mating with the male they had been artificially bonded with or a male stranger, with 10 of the 12 females choosing the latter compared to only 3 of the 10 control females who had an irrelevant gene simulated.
Treating social disorders
“We are not doing these studies to uncover the secrets of love, or to make people fall in love,” Robert Liu, lead author of the paper, told Quartz. Instead, scientists are hoping to develop new psychiatric treatments. Neurochemicals such as oxytocin (colloquially known as the ‘love hormone’) are known to play a role in social bonding, but their exact function is not fully understood. Studies of voles offer an important way to learn more about the effect neurochemicals have on bonding. For example, it has been shown that monogamous prairie voles have an abundance of oxytocin receptors in the nucleus accumbens and the prefrontal cortex (the areas involved in the ‘love circuit’) while non-monogamous montane voles do not. It has even been found that the oxytocin receptor density of prairie voles predicts how resilient female voles are to early life neglect. The ultimate aim of studies like these is to develop new treatments for social disorders such as schizophrenia or sociopathy that might include circuit-level targeting of the oxytocin system, which could massively advance psychiatry.
Image: Jonathan Ridley via Unsplash