Loneliness can actually change the way people's brains work - making them feel vulnerable and aggressive, according to new research.

In experiments, mice kept in isolation for long periods produced more of a brain protein linked to fear.

The chemical - known as Tac2/NkB - causes animals to display much more persistent responses to threats, say scientists.

But the behaviour was reversed when the rodents were injected with a drug called osanetant that targets the molecule.

It offers hope of developing medications for bereavement, stress and even prisoners held in solitary confinement.

Osanetant was developed as a potential therapy for schizophrenia and major depression.

While it was safe and well tolerated in human studies, it failed to work for these disorders.

Professor David Anderson, of the California Institute of Technology, said: "Our study raises the possibility that this drug might be repurposed to treat other psychiatric disorders that are related to effects of social isolation in humans - not just in solitary confinement but perhaps in bereavement stress or other types of stress."

Last year Professor Helen Stokes-Lampard, chair of the Royal College of GPs, said loneliness is as dangerous as diabetes, high blood pressure and depression.

She said millions of OAPs in the UK are turning up a doctors' surgeries just for someone to talk to.

Prof Anderson said mice provide an excellent animal model for to analyse loneliness as social isolation is an intensely stressful experience for them.

When anxious, they generally become much more reactive to a variety of negative stimulants, and these behaviors persist for a long time.

The researchers said the findings published in Cell sheds light on one of the mechanisms by which chronic loneliness causes the brain to change in a profound way.

They found social isolation for two weeks resulted in an increase in the fear protein Tac2/NkB.

Study senior author Prof Anderson said: "We discovered Tac2/NkB is upregulated broadly throughout the mouse brain in multiple brain regions that are involved in different types of emotional coping behaviours and aggression."

His laboratory studies the neural circuitry related to emotional behaviours.

The team, led by Dr Moriel Zelikowsky, noted the diverse effects of Tac2/NkB on behaviour did not come from any particular spot.

Instead, the peptide is overproduced in multiple locations of the brain, each of which causes one of the changes that persist in social isolation.

Prof Anderson said: "We think Tac2/NkB is a chemical modulator that coordinates a whole inter-brain state that is caused by social isolation, one that operates in a distributed manner in the brain."

The researchers also discovered they could block the behavioural effects of loneliness in the mice using osanetant.

Injections of the medication blocked the aggressive or reactive behaviours when the mice were challenged with fearful or threatening stimuli.

The research follows earlier studies by the team that first found the protein controls the effects of social isolation by increasing aggression in fruit flies.

Prof Anderson said: "The fact there is some conservation of function from fruit flies to mice makes me think this peptide might have some role in some forms of stress and their effects on the brains of humans."

In the latest study, the researchers developed and applied a new way to artificially over- produce and release Tac2/NkB from the brains of mice in group-housing conditions.

Prof Anderson said: "When we did that, we can mimic many of the effects of social isolation."

He pointed out while socially isolated mice are so aggressive they normally cannot be returned to group housing, after treatment with osanetant they no longer attacked their cage mates.

Prof Anderson said: "To me, it brings up the question whether this drug could mitigate the well known deleterious effects of solitary confinement, such as increased violent behaviour in incarcerated individuals."

The study also underlines the brain's ability to adapt to environmental changes - a phenomenon known as brain plasticity.

Previous research by other US scientists has found prolonged loneliness can transform the brain in a way that makes people less able to relate to others.

Research showed adult mice kept in isolation for long periods produce less myelin - white matter - in parts of their brain crucial for complex emotional behaviour.

Changes in the brain's white matter have been seen before in psychiatric disorders, and demyelinating disorders like multiple sclerosis have also had an association with depression.