It may be better to have loved and lost than never to have loved at all, but why is it so hard to find again? It may be that our brains are fixated on our former lovers, according to scientists.

Researchers at Florida State University examined the nature of love by studying the brains and behaviour of male prairie voles, picked for their habit of lifelong monogamy and aggression¹ towards other females once they have found a mate.

The scientists found that males became devoted² to females only after they had mated. The 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. Immediately, they lost interest in other females and spent all of their time with their chosen one. Further experiments showed that dopamine restructured a part of the vole's brain called the nucleus³ 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 neural⁴ circuit that made them go cold towards the new female.

"It seems that the first time they get together and the bond forms, it locks them into that monogamous behaviour ... You can take a female away from a male once he's formed a bond with her and two weeks later put him with a different female and he won't be remotely interested," said Dr Aragona, whose study appears in the journal Nature Neuroscience.

The researchers said that while the love lives of voles differ from those of humans, the same brain structures work in much the same ways across different species. "Things are always going to be more complicated in humans because we have larger brains and are under different pressures, but the basic mechanisms⁵ are there", said Dr Aragona. 

　　中文：

　　或许爱过以后再失去也比根本不再去爱强，但为什么许多人在失恋后却很难再爱一次？科学家最新的研究结果表明，这可能是人的大脑被一种叫做多巴胺的“痴情毒药”锁定在旧情人的身上。

　　据《卫报 》12月6日报道，美国佛罗里达州大学的研究人员通过研究雄性田鼠的大脑和行为来探索人类爱情的本质。和人一样，田鼠属于终生单配偶动物（即一夫一妻制），且一旦有了配偶后就本能地对其它异性产生排斥。

　　研究人员发现，雄性田鼠有了配偶后就会专注于对方，而有了配偶的田鼠开始大量分泌一种叫做多巴胺的化学物质（人脑也会分泌该物质）。负责此项研究的布兰登·阿拉戈纳博士证实，多巴胺就是让雄性田鼠痴情的“毒药”。

　　当尚未与雌鼠发生性关系的雄鼠的大脑被注入多巴胺后，这些雄鼠很快就失去了对其它异性的兴趣，而只是专注于自己心仪的雌鼠上。进一步实验表明，多巴胺改变了田鼠大脑中核团区域（大脑核团区域的功能是维持情绪和目的性行为，人也有）的结构。当已有配偶的雄鼠被介绍给新的雌鼠时，尽管此时雄鼠的脑中还在继续分泌多巴胺，但多巴胺会被导入另一个完全不同的神经中枢系统，使得雄鼠对新的雌鼠毫无兴趣。

　　阿拉戈纳博士指出：“研究发现，雄鼠与雌鼠一旦结合在一起，它们就成为了单配偶动物。若将雄鼠与配偶分开，两周后再让它与新的雌鼠接触，你会发现它对新的对象根本提不起兴趣。”

　　研究人员指出，尽管人的情感与田鼠的还有很大差别，但在不同的物种之间，相同的大脑结构也会有相似的运作方式。阿拉戈纳博士说：“人类的情感会更加复杂，因为人脑体积更大且所处的环境也更复杂，但人脑与田鼠大脑基本的运作方式是相同的。”

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