The game of cat and mouse will never seem the same again. Scientists have created a mutant mouse which has lost its instinctive fear of felines.
A genetic change to the mouse's sense of smell has caused it to lose the ability to associate the smell of a cat with an innate fear of a potential predator.
The result is a mouse that will happily approach a cat without showing any of the natural alarm that the rodents normally display when they catch the tiniest whiff of a feline foe.
Hitoshi Sakano, who led the study at the University of Tokyo, said his research was important because it showed it was possible to override the innate aversion that all mammals – including humans – have to certain smells. "The mammalian olfactory system mediates various responses, including aversive behaviours to spoiled foods and fear response to predator odours," Professor Sakano said.
By inserting a gene for the diphtheria toxin into the mouse, scientists were able selectively to remove certain nerve cells in the mouse's olfactory bulb, which is situated at the base of the brain and receives nerve impulses from olfactory receptor cells in the nasal cavity.
The olfactory bulb is made of glomeruli, or nerve cells, and Professor Sakano's study, published in the journal Nature, showed there are two sorts of glomeruli – those which deal with innate responses and those dealing with responses that are learnt.
The reason Professor Sakano's findings were important, he said, was that the fearless mouse showed that the two nerve circuits in the olfactory bulb – the learnt and the innate – are quite separate from one another.
"The mutant mice lacked innate responses to aversive odorants, even though they were capable of detecting them and could be conditioned for aversion with the remaining glomeruli," he added. "These results indicate that, in mice, aversive information is received in the olfactory bulb by separate sets of glomeruli – those dedicated for innate and those for learnt responses."
The next stage of the study will investigate whether similar dual circuits are present in other parts of the mammalian sensory system. In the first tests, an added precaution was taken before introducing the mouse to the pet cats. "The cats were fed well before taking photos. Otherwise, they would kill our precious mutant mice," Professor Sakano said.
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