Oxford scientists have implanted memories in the brains of fruitflies by activating brain cells directly with a flash of light. By targeting genetically-defined groups of cells, they have taken the first steps towards defining the precise neuronal circuitry that underlies this type of learning in the fly.
Even the simplest animals have to modify their behaviour according to their changing environment in order to survive. Fruit flies navigate a world of smells in which they learn to find food and sex, and avoid danger. Normal flies can be quickly trained to avoid odors that they previously favoured if they receive an electric shock each time they encounter the odor. This form of learning can only take place, however, if a part of the fly's brain called the mushroom body receives inputs carrying the neurotransmitter dopamine.
Dr Adam Claridge-Chang at the WTCHG, working in Professor Gero Miesenböck's group in the Department for Physiology, Anatomy and Genetics at Oxford and colleagues in the US, carried out an ingenious experiment to find the source of the dopamine that supports this form of aversive conditioning. They bred flies that had been engineered to carry a membrane receptor called P2X2 in either of two different sets of brain cell clusters. P2X2 reacts to the energy-carrying molecule adenosine triphosphate (ATP) by causing the cell to fire an impulse, and so release dopamine at its terminals.
Claridge-Chang and his colleagues were able to control when the cell fired by injecting the flies' brains with a form of ATP that was unable to interact with the receptor until it had been released by a flash of laser light. They then arranged for the laser to flash each time the fly strayed into an air stream containing a particular odor. As they report in the journal Cell, the procedure conditioned the flies to avoid the odor as effectively as electric shock, but only when the P2X2 receptor was active in four particular cell clusters. Only one of these, known as PPL1, sent connections to the relevant cells of the mushroom body. The researchers conclude that this tiny cluster of 12 cells sends the signals that train the fly to associate the odor with something bad.
'Now we hope to find the cells that lie upstream of these neurons that calculate some prediction of reward or punishment', says Claridge-Chang. Miesenböck's group is aiming to uncover the algorithm used to make this calculation, and to find out whether it resembles the temporal difference algorithm developed by researchers working on machine learning more than 20 years ago.
Nature News 15 October: www.nature.com/news/2009/091015/full/news.2009.1005.html
New Scientist 15 October: www.newscientist.com/article/dn17990-laser-creates-false-memories-in-fly-brains.html
CBC Canada 15 October: www.cbc.ca/technology/story/2009/10/15/tech-biology-fruit-fly-memory.html
BBC News Online 16 October: http://news.bbc.co.uk/1/hi/sci/tech/8310365.stm
New York Times 19 October: www.nytimes.com/2009/10/20/science/20fly.html
New Scientist 21 October: www.newscientist.com/article/mg20427315.400-laser-creates-false-memories-in-fly-brains.html