Know Your Brain: Oxytocin — The “Love Hormone”

Oxytocin is a hormone that functions in the brain as a neurotransmitter. We often think of it as the “hug hormone” or “love hormone,” the chemical signaling in the brain that draws couples to each other; in other words, the chemical that makes people miss each other. We may also know the disturbing stories of the mother lab rat deprived of oxytocin, who no longer cared for her infants. While oxytocin plays a significant role in the regulation of human emotions, its role to the body is much more complex.


Humans of both genders release oxytocin during hugging, touching, and orgasms. It is secreted by the pituitary gland at the base of the brain. Once secreted, it is unable to reenter the brain due to the blood-brain barrier, a protective lining of endothelial cells. The chemical is then expressed by neurons throughout the brain and into the spinal cord, in particular, the amygdala, ventromedial hypothalamus, septum, and brainstem.

In addition to pair bonding, particularly among people who declare themselves to be in love, it also affects sexual arousal as it moves through the hypothalamus and spinal cord. The compound first drew the attention of scientists when they observed that mothers who breastfed their infants tended to be more relaxed, and reported lower stress levels overall than mothers who were bottle-feeding.

In the first group of mothers, oxytocin acts at the mammary glands, allowing the milk to release in a collecting chamber. This stimulation drives neurons to produce oxytocin in sudden bursts. Low levels of oxytocin can also cause contractions to stop or slow during labor. When the infant sucks at the mother’s nipple, a chemical signal is relayed by spinal nerves toward the hypothalamus, the region of the brain responsible for emotional activity as well as for regulating body temperature, thirst, and hunger.

The connection with the hypothalamus is interesting for yet another reason — levels of oxytocin have been shown to play a role in the human capacity for empathy as well as trust, as its presence may lower the amygdala’s fear response when exposed to new stimuli. A study performed by the neuroeconomist Thomas Baumgartner and colleagues at the University of Zurich in Switzerland investigated the nature of trust by involving his test subjects to play a game while hooked up with functional MRI equipment.

In Baumgartner’s “trust game,” players act as investors. They decide whether or not to keep a small amount of money or to invest it with the other players, who may become trustees. Should the investor choose to share their $10, their investment is tripled. The trustee then must either pay back each investor with an increase ($15 per participant) or keep the money for himself, violating the trust of the investors. The players were either given oxytocin as a nasal spray, or a placebo, before playing. Participants who were given the placebo, became suspicious of the other players who did not pay them back, while the other group continued to play and invest money as they had been, and also showed decreased activity in both the amygdala and caudate nucleus. The test subjects also played a “risk game” against a computer, and were told after playing several rounds if their trust had been betrayed or not. The effects of oxytocin were not apparent when playing against the computer, suggesting that the chemical only acts this way when interacting with other people.

Just a simple drive through the country may seem tedious, but whether or not the traffic is manageable, your brain is dealing with a deluge of stimuli — all of it happening rapidly. The trees and foliage, the billboards and exit signs, the smoky smell of autumn or of freshly cut grass, the rush of the wind on your face as you drive across the winding highway — it’s a lot to take in. There’s probably a lot you might not remember about your trip, like details about the surrounding landscape, regardless of how many times you’ve been there before. One thing the brain rarely gets credit for is its ability to separate the important details — the right exit; the landmarks that come before it and when you know if you’ve gone too far — from the mundane ones. It may owe this ability, at least in part, to oxytocin.

Catherine Dulac, the Higgins Professor of Molecular and Cellular Biology at Harvard University, conducted a research effort last year that suggests oxytocin may actually serve as a modulator for the brain — capable of sorting through a number of social signals. According to Dulac and her team of researchers, the compound hones in on certain stimuli while filtering others out, as though adjusting the picture on a television set, altering the frequency constantly at each moment.

For their project, Dulac and her colleagues observed lab mice, following a well-known pattern of behavior seen in labs — the way in which male mice prefer to congregate with females. This behavior isn’t just due to these little rodents trying to break the ice, this mannerism is encoded into their brains. They need the interaction almost as much as they need food and water.

The female mice give off chemical signals known as pheromones. The males pick up on this and, as found in the study, the neurons in their medial amygdala spike with activity. The number of times mice made nose to nose contact was an indicator of rewarding social interaction. The same group of mice were then exposed to the pheromones produced by other males, and the medial amygdala indicated very little stimulation.

Following these trials, Dulac was able to isolate the gene needed to produce oxytocin, and switch the gene off. Once the signal was turned off, the neural signaling disappeared — as did their desire to socialize. Dulac’s team also found that the signaling effect occurs on and off, from moment to moment. A failure in the switch could potentially be one of the factors that causes depression in humans, when depressed people no longer find joy from interacting with other people.

“There may be many different regulators,” says Dulac, reflecting on the study. “Oxytocin might be one of a whole realm of modulators, each of which are important in a particular circumstance. That therefore gives the animal a great deal of plasticity in terms of engaging in a particular behavior, so it’s not the case that each time the animal encounters a particular stimulus it will react in exactly the same way. Depending on the state of the brain and the release of these neurotransmitters, the animal can boost its behavior toward the stimulus or ignore it.”

She hopes that the team’s observations could be used to develop further treatments in depression if similar patterns are seen in humans. Yet another study, published this year in the Journal of Medicinal Chemistry, reports the discovery of a new compound the first known variation of a chemical that rewards the mouse’s brain for interacting with other mice. Some of the mice in the sample group exhibited symptoms similar to people with autism spectrum disorders (repetitive behaviors, deficient communication skills, and low levels of social interaction).

The study’s lead researcher, Marcel Hibert, knew of oxytocin’s ability to help improve the lives of some patients with autism. Because the oxytocin cannot be administered by oral tablets and quickly breaks down when given by injection before it can reach the brain, Hibert and his colleagues sought another compound in the brain that could mimic the abilities of oxytocin — just enough to activate the brain’s receptors and set them active.


The candidate, LIT-001, binds to oxytocin as well as vasopressin, which serves as an antidiuretic hormone. Hibert and researchers established that they both share a similar component, known as benzoyl benzazepine. After several molecular models were tested, they found one that offered benefits similar to oxytocin but none of the drawbacks that made it difficult to synthesize as medicine.

This article was originally published in the Winter 2019 issue of Brain World Magazine.


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