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.

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