The love hormone oxytocin which triggers feelings of love, sexual arousal, bonding between parent and child - appears to also have a dark side too.

The new findings, by researchers from Northwestern University, come from two experiments detailed in the journal Nature Neuroscience. Oxytocin appears to be the reason stressful social situations, such as being bullied at school or tormented by a boss, remain with us long past the event and can trigger fear and anxiety in the future.

While testing the effects of oxytocin as an anti-anxiety drug, Northwestern Medicine research team found that oxytocin can also cause emotional pain.

As part of the experiment, the reaction of three different groups of mice was observed as they faced a stressful experience. One group was missing its oxytocin receptors, which is essentially the plug by which the hormone accesses brain cells. The lack of receptors means oxytocin couldn't enter the mice's brain cells. The second group had an increased number of receptors so their brain cells were flooded with the hormone. The third control group had a normal number of receptors.

Six hours later, the mice were returned to cages with the aggressive mice. The mice that were missing their oxytocin receptors didn't appear to remember the aggressive mice and show any fear. On the contrary, when mice with increased numbers of oxytocin receptors were reintroduced to the aggressive mice, they showed an intense fear reaction and avoided the aggressive mice.

In the second experiment, the three groups of mice were again exposed to the stressful experience of social defeat in the cages of other more aggressive mice, according to the study. This time, six hours after the social stress, the mice were put in a box in which they received a brief electric shock, which startles them but is not painful. Then 24 hours later, the mice were returned to the same box but did not receive a shock.

The mice missing their oxytocin receptors did not show any enhanced fear when they re-entered the box in which they received the shock. The second group, which had extra oxytocin receptors showed much greater fear in the box. The third control group exhibited an average fear response.

The findings surprised the researchers, who were expecting oxytocin to modulate positive emotions in memory, based on its long association with love and social bonding. "Oxytocin is usually considered a stress-reducing agent based on decades of research," said researcher Yomayra Guzman. "With this novel animal model, we showed how it enhances fear rather than reducing it."

 "By understanding the oxytocin system's dual role in triggering or reducing anxiety, depending on the social context, we can optimize oxytocin treatments that improve well-being instead of triggering negative reactions," said Jelena Radulovic, the senior author of the study and the Dunbar Professsor of Bipolar Disease at Northwestern University Feinberg School of Medicine.