When you’re physically attracted to someone, there is this indefinable element to your interactions that many people simply chalk up to “chemistry.” Scientists have long tried to understand the precise mechanism of this interaction, and now a new study is helping shed some light on the way that pheromones could enhance human sexual behavior. The study, which was carried out in mice, shows how different brain circuits in females and males can turn chemical signals into sexual behavior or aggression.
Much of this is tied to animals’ sense of smell, which plays a major role in regulating the instinctive responses to not only predators and competitors but also potential mates.
The study at the University of Tokyo’s Graduate School of Agricultural and Life Sciences set out to determine how male pheromones work to enhance the sexual behavior of female mice. The team looked at the male pheromone known as ESP1, which previous studies have identified as a key driver of female sexual behavior and male aggression in mice. This pheromone is secreted from tear glands, and because it is a single chemical that corresponds to just one receptor, it is fairly easy for scientists to track.
After infecting ESP1 receptor neurons with a virus, the researchers used a fluorescent protein to mark the infected brain cells so they could get a clear picture of the neural circuit used by ESP1. This allowed them to see just how the ESP1 signals are conveyed within the brain when neurons send electrical impulses across synapses to other neurons.
Different circuits used depending on gender
The scientists noted that the ESP1 signal in the amygdala took different circuits in males and females. Within the amygdala, they found a sub-area that served as something of a “switch” to relay ESP1 information to a certain part of the hypothalamus according to the gender of the mouse in question. This is significant because the amygdala is the part of the brain that is responsible for emotional behavior, motivation and emotions in general.
When male mice detect the ESP1, they show aggressive behavior toward the other male mice around them and the MPA subregion comes into play; female mice display sexual behavior when they detect ESP1 and a subregion known as VMHd is used.
The researchers also discovered that sexual activity in female mice could be boosted merely by activating the receptor neurons for ESP1 in their brain’s hypothalamus, even if ESP1 itself was not actually present. This is accomplished by controlling neural activities either optically or chemically using a process known as the TRAP method wherein neurons that respond to a particular stimulus are selectively manipulated.
Past studies have shown that a sex pheromone in fruit flies also enhances male aggression and female sexual behavior through different neural circuits depending on gender.
It is believed that these findings could lead to further studies into sex-specific innate behaviors. For example, a deeper understanding of the neural basis for female sexual behavior could help shed new light on sexual dysfunction.