Now we know - love is the drug you can't forget!

According to researchers at Florida State University who were examining the nature of love it may really, as the poets say, be better to have loved and lost than never to have loved at all.

They also suggest that the reason some find it hard to love again may be because their brains are fixated on former lovers.

The scientists came to these conclusions by studying the brains and behaviour of male prairie voles, chosen for their habit of lifelong monogamy and aggression towards other females once they have found a mate.

The team found that males only became devoted to females after they had mated, and that bond coincided with a huge release of the feelgood chemical dopamine inside their brains.

Brandon Aragona, who led the study, demonstrated that dopamine was the voles' love drug by injecting the chemical into the brains of males who had not yet had sex with female companions.

He found that the voles immediately lost interest in other females and spent all of their time with their chosen one.

Further experiments went on to show that dopamine restructured a part of the vole's brain called the nucleus accumbens, a region that many animals have, including humans.

The change was so drastic that when paired-up males were introduced to new females, although their brains still produced dopamine on sight, the chemical was channelled into a different neural circuit that made them cold towards the new female.

Aragona says that it seems that the first time they get together and the bond forms, it locks them into that monogamous behaviour.

He says that when a female was taken away from a male once he had formed a bond with her, and then two weeks later put him with a different female, he was totally disinterested.

The researchers say that despite the fact the love lives of voles differ from those of humans, the same brain structures work in much the same ways across different species.

Dr Aragona says that although things are always more complicated in humans due to the larger brains and different pressures, the basic mechanisms are still there.

The study appears in the journal Nature Neuroscience.