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The fight or flight response is well known, but now scientists have discovered brain cells that control the desire to fight – and mate.

While researchers found that innate social behaviours like mating and aggression are closely related long ago, this is the first time they have pinpointed specific neurons deep in the brain that trigger them.

They discovered that it is possible to switch the behaviour of a mouse from 'sniffing and mounting' to attacking by increasing how strongly the neurons were stimulated.

Brainy breakthrough: For the first time researchers have pinpointed the specific neutrons deep in the hypothalamus of a mouse's brain (circled in orange) that trigger fighting and mating. Here, nerve fibres (the yellow lines) emanate from estrogen receptor-expressing neurons in the hypothalamus (yellow dots) that control aggression

Biologist David J Anderson led a team at the California Institute of Technology (Caltech) that genetically identified neurons that control aggressive behaviour in the mouse hypothalamus - a structure that lies deep in the brain.

They managed this by using pulses of light to stimulate the neurons in a method called optogenetics.

Optogenetics uses light emitted by LEDS mounted to the skull, or deeply implanted optical fires, to control neurons that have been genetically sensitised to light.

The scientists found that when the neurons were strongly activated by pulses of light a male mouse attacked another male or female.

However, weaker activation of the same neurons triggered 'sniffing and mounting,' or mating behaviours.

A love and a fighter: If the neurons were strongly activated by pulses of light, a male mouse (pictured) attacked another male or female. However, weaker activation of the same neurons triggered 'sniffing and mounting,' or mating behaviours

They found that they could switch the behaviour of a single animal from mounting to attack by gradually increasing the strength of neuronal stimulation during a social encounter.

The results of the study, which was published in Nature, suggest that the level of activity within the population of neurons may control the decision between mating and fighting.  

The neurons were initially identified because they express a protein receptor for the hormone oestrogen - reinforcing the view that oestrogen plays an important role in the control of male aggression.

Because the human brain contains a hypothalamus that is structurally similar to that in the mouse, these results may be relevant to human behaviour too.