Mouse study reveals part of the brain that reacts to the 'love hormone'

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According to researchers, "Oxytocin, the body’s natural love potion, helps couples fall in love, makes mothers bond with their babies, and encourages teams to work together. Now new research at Rockefeller University reveals a mechanism by which this prosocial hormone has its effect on interactions between the sexes, at least in certain situations."

The discovery of a small group of neurons that respond to oxytocin in mice has shed new light on how the so-called "love hormone" actually causes females to become sexually interested in a potential mate, claims a new study appearing in the October 9 edition of .

The discovery was made by researchers from Rockefeller University, who had initially set out to locate a new type of interneuron, a neuron that relays messages to other neurons across relatively short distances. The study authors started creating profiles of the genes expressed in interneurons using a technique known as translating ribosome affinity purification (TRAP) when they found receptors that respond to oxytocin in the prefrontal cortex.

According to BBC News science reporter Melissa Hogenboom, the neurons control sexual behavior in mice. When the scientists switched off the neurons, meaning that they were no longer receptive to oxytocin, the female mice lost interest in mating during their sexually-active period and were "no more attracted to a mate than to a block of Lego," Hogenboom added.

Co-author Dr. Nathaniel Heintz told BBC News that the results were "pretty fascinating because it was a small population of cells that had such a specific effect." He explained that the researchers used toxins that block the ability of cells to transmit signals to other neurons in order to observe the impact it had on the female mice.

"This internal hormone gets regulated in many different contexts; in this particular context, it works through the prefrontal cortex to help modulate social and sexual behavior in female mice," Dr. Heintz added. "It doesn't mean it's uniquely responsible because the hormone acts in several important places in the brain but it does show that this particular cell type is required for this aspect of female social behavior."

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The Rockefeller University research team, led by doctoral student Miho Nakajima, dubbed the neurons "oxytocin receptor interneurons" (OxtrINs). In one experiment, she and her colleagues blocked the receptor's ability to detect the hormone in some females, then gave the rodents a choice between exploring a room containing a male mouse or one containing an inanimate object - which, in this case, was the aforementioned Lego block.

Surprisingly, sometimes the mice with silenced OxtrINs demonstrated an abnormally high interest in the building block, while sometimes they responded normally. This caused Nakajima to suspect that the female reproductive cycle played a role, so she began recording whether or not the mice were in their sexually receptive phase (estrus) or their period of sexual inactivity (diestrus).

"Estrus, it turned out, was key," the university explained in a statement. "Female mice in this phase showed an unusual lack of interest in the males when their receptors were inactivated. They mostly just sniffed at the Lego. There was no effect on mice is diestrus, and there was no effect if the male love interest was replaced with a female."

When the experiments were repeated in male mice, there was no effect, the researchers noted. While oxytocin has similar effects for humans as for mice, the authors are not yet certain if the hormone would influence the human equivalent of this behavior, or if it functions through a similar network of interneurons.

"We don't yet understand how, but we think oxytocin prompts mice in estrus [heat] to become interested in investigating their potential mates," Nakajima explained, according to Victoria Woollaston of the . "This suggests that the social computation going on in a female mouse's brain differs depending on the stage of her reproductive cycle."

"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," added Dr. Heintz. "In a general sense, this new research helps explain why social behavior depends on context as well as physiology."