Chemical Transfer of Fear

Can learning be transferred chemically from one animal to another? Scientists have been arguing the question since 1962, when University of Michigan Psychologist James McConnell reported that untrained flatworms could acquire knowledge by feasting on trained worms. Using rats and mice, some researchers have achieved experimental results that seem to prove statistically that learning, or memory, can indeed be transferred by injecting the brain extract of one animal into the brain of another. But since the tests were difficult to duplicate, the results could never be properly validated. Now a Baylor University scientist, writing in Nature, has reported an experiment that could help resolve the controversy.

Neurochemist Georges Ungar based his work on the natural preference of rats and mice for the dark; given a choice between entering a lighted or a dark enclosure, the rat will almost invariably enter the dark one. After constructing a short passageway between a translucent plastic box and a black box with an electrical grid for a floor, Ungar placed rats one at a time into the light box. As is their nature, the rats scurried into the dark box. They were in for a rude shock. Dropping a gate that prevented them from running back into the tunnel, Ungar sent electric current through the grid floor for five seconds, giving them a painful jolt. When the gate was lifted, the rats usually were only too happy to return to the lighted box. The procedure was repeated five times a day for eight days, enough to give even the most slow-witted rat a fear of the dark.

Different Intervals. Meanwhile in a nonshocking version of his dark-box-light-box apparatus, Ungar tested the preference of untrained mice for darkness. Given their choice between a light or dark box, the test animals spent an average of 138 seconds of a 180-second test period in the dark.

Ungar then decapitated his fear-trained rats, and prepared an extract from their brains. He injected the extract into the brains of the untrained animals and found that the untrained mice began to shun darkness. The average time that members of one group spent in the dark box declined to 98 seconds when each was injected with three-tenths of a gram of extract. It went down to 67 seconds when the injection was increased to six-tenths of a gram, and to only 24 seconds when a full gram was administered. Other groups injected with extracts from the brains of rats that had been shock-trained for different intervals of time acquired similar fear of the dark.

New Connections. After many such tests, Ungar concluded that the fear had indeed been transferred and that the degree of transfer depended on the amount of extract injected. It was also affected by the training of the donor rats--longer training produced better transfer--and the interval between training and removal of the donor-rat brain; brain removal too soon after training apparently prevented the transfer material from fully developing.

Further analysis of the extract led Ungar to believe that the actual transfer agents are short amino-acid chains called peptides. The peptides, he suggests, are synthesized in the brain of the donor rats after they are shocked. According to theory, they then produce new connections between the brain's nerve cells, the process that many scientists associate with learning. Unlike previous memory-transfer research, Ungar says, his experiments are "easily and rapidly reproducible and yield unequivocal results, which clearly demonstrate the possibility of a purely chemical transfer of some types of acquired information."

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