A new research at Rockefeller University has discovered a mechanism by which the Oxytocin hormone has some crucial effect on interactions between the sexes – at least in certain situations. The key is in fact a newly discovered class of brain cells. The findings have been published in the journal Cell on October 9th.
Oxytocin is the body's natural love potion. It is what makes couples fall in love; it is responsible for the bond between mothers and babies. Nathaniel Heintz, James and Marilyn Simons Professor and head of the Laboratory of Molecular Biology said that by identifying a new population of neurons that is activated by oxytocin, they have discovered a way through which this chemical influences interactions between male and female mice.
The study was prompted by the search for a new type of interneuron which is a specialized neuron that relays messages to other neurons across relatively short distances. For this, Miho Nakajima started creating profiles of the genes expressed in interneurons using a technique known as translating ribosome affinity purification (TRAP). This was a part of her doctoral thesis. The technique TRAP was developed by the Heintz lab and Paul Greengard’s Laboratory of Molecular and Cellular Neuroscience at Rockefeller. In some of the profiles of genes that were collected from the cortex, she noticed an intriguing protein. It was a receptor that responded to oxytocin.
That was what raised the question what this scattered population of interneurons doing in response to this important signal, oxytocin remarked Nakajima. Since, it is known that oxytocin is most involved in social behaviors of females; it was decided to focus on females for the experiment.
In order to find out how these neurons affected behavior when activated by oxytocin, Nakajima silenced only this class of interneurons and, in separate experiments, blocked the receptor’s ability to detect oxytocin in some females. Post this social behavior test was conducted on them.
The mice were given the choice between exploring a room with a male mouse or a room with an inanimate object like a plastic Lego block. Typically, Legos just aren’t that interesting to rodents. However, Nakajima’s results showed that sometimes the mice with the silenced OxtrINs showed an abnormally high interest in the Lego, and at other times they behaved normally. This confusing behavior led her to suspect the influence of the female reproductive cycle. In the next round of experiment, she also took into account if the female mice were in estrus which is the sexually receptive phase, or diestrus, a period of sexual inactivity. It was found that Estrus was of prime importance.
Those female mice in this sexually active phase showed a lack of interest in the males when their receptor was inactivated. However, there was no effect on mice is diestrus, and there was no effect if the male love interest was replaced with a female. Nakajima then tried the same alteration in males, and found that there was no effect.
Andreas GÃ¶rlich, a postdoc in the lab, said that typically OxtrINs appear to sit silently when not exposed to oxytocin. The interesting thing to note here is that when exposed to oxytocin these neurons fire more frequently in female mice than they do in male mice. Nakajima remarked that it is not yet understood how oxytocin prompts mice in estrus to become interested in investigating their potential mates but surely it does. The results are an indication that the social computation going on in a female mouse’s brain differs depending on the stage of her reproductive cycle.
We know that Oxytocin has similar effects for humans as for mice. But, it is not yet clear if the hormone influences the human version of this mouse interaction. Further studies are needed for that.