Oxytocin, or the body's "natural love potion," can have a significant influence on interactions between males and females.
The new research observed a brain cell-based mechanism that causes this prosocial hormone phenomenon, Rockefeller University reported.
"By identifying a new population of neurons activated by oxytocin, we have uncovered one way this chemical signal influences interactions between male and female mice," said Nathaniel Heintz, James and Marilyn Simons Professor and head of the Laboratory of Molecular Biology.
Using a technique known as translating ribosome affinity purification (TRAP) the researchers observed a brain receptor that responds to oxytocin in the outer layer of the brain, known as the cortex.
"This raised the question: What is this small, scattered population of interneurons doing in response to this important signal, oxytocin?" said doctorate student Miho Nakajima. "Because oxytocin is most involved in social behaviors of females, we decided to focus our experiments on females."
To determine how these oxytocin receptor interneurons (OxtrINs) affected behavior when triggered by oxytocin the researchers silenced the OxtrINs; in a second experiment blocked the receptor's ability to detect oxytocin in rodent females. These females were then given a social behavior test in which they were given the choice to explore a room containing a male mouse or an inanimate object. Usually female mice are much more interested in a potential mate than Lego, but the mice with the silenced OxtrINs showed an abnormally high level of interest in the object.
These findings suggest the female reproductive cycle has an influence over behavior. In a second study Nakajima looked at whether the mice were in a sexually reproductive phase or in a period of inactivity. She found female mice in estrus (a sexually reproductive phase) showed an unusual lack of interest in males when their receptor was inactivated.
"In general, OxtrINs appear to sit silently when not exposed to oxytocin," said Andreas Görlich, a postdoc in the lab who recorded the electrical activity of these neurons with and without the hormone. "The interesting part is that when exposed to oxytocin these neurons fire more frequently in female mice than they do in male mice, possibly reflecting the differences that showed up in the behavioral tests."
The researchers are still not sure how oxytocin triggers mice in estrus to become interested in potential mates. Oxytocin has a similar effect in humans, but it is not yet clear if the human version of this interaction is influenced by the hormone.
"Oxytocin responses have been studied in many parts of the brain, and it is clear that it, or other hormones like it, can impact behavior in different ways, in different contexts and in response to different physiological cues," Heintz said. "In a general sense, this new research helps explain why social behavior depends on context as well as physiology."
The findings were published Oct. 9 in the journal Cell.
