"Mental Viagra" could become reality after scientists discovered that a hormone that surges during puberty can activate lust signals in the brain.

The hormone kisspeptin essentially switches on the desire to reproduce and is thought to be responsible for the sexual appetite of young people.

Now scientists at Imperial College London have found that injections of kisspeptin trigger chemicals in the brain that occur when people feel amorous.

They believe that pills containing the hormone could be used to treat sexual problems that are psychological, rather than physical.

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Asked if kisspeptin could be used as "mental Viagra", Professor Waljit Dhillo, the lead author, said: "Yes, exactly like that. Most of the research and treatment methods for infertility to date have focused on the biological factors that may make it difficult for a couple to conceive naturally.

"These of course play a huge part in reproduction, but the role that the brain and emotional processing play in this process is also very important, and only partially understood."

Kisspeptin, which is naturally occurring, stimulates the release of other reproductive hormones.

In the study, 29 healthy heterosexual young men were given injections of the hormone and asked to look at sexual and romantic pictures, as well as viewing control images that contained no people.

Magnetic Resonance Imaging (MRI) scans showed enhanced activity in regions of the brain linked to sexual arousal and romance. The effect did not occur when the volunteers viewed the control images.

The scientists believe that kisspeptin boosts brain circuits associated with sex and love, triggering reward centres and increasing desire. The study participants using kisspeptin also reported a reduction in negative mood in post-scan questionnaires and the team is now hoping to investigate whether the hormone could additionally be used for treating depression.

The team, whose findings are reported in the Journal of Clinical Investigation, now plans to study the effects of kisspeptin in a larger group that will include women as well as men.