Credit: Nathan McClunie

Evolutionary mechanisms of recreational drug use have puzzled researchers for years. Many drugs, among them cocaine, function as natural insecticides that protect the plants in which they are produced. This creates a paradox: why would a substance that evolved to deter insects be attractive and rewarding to mammals? One proposed solution is that the brains of insects and mammals simply work differently, meaning that certain drugs trigger reward mechanisms in mammals but not in insects. Now, scientists from the University of Illinois (Urbana) and from the Australian National University (Canberra) refute this theory by showing that honeybees, like humans, seem to derive pleasure from ingesting cocaine.

The researchers, led by Gene Robinson of the University of Illinois, assessed bees' perception of reward by evaluating their performance in a 'waggle dance' (J. Exp. Biol. 212, 163–168; 2009). Bees use this sophisticated dance to communicate to their nestmates the precise location and quality of food resources ('rewards') they have discovered. Robinson and colleagues administered low doses of cocaine to honeybees as they fed at sucrose and pollen feeders. The researchers then recorded bees' movements upon returning to the hive. Compared with untreated bees, bees that were treated with cocaine danced more often and more rapidly after discovery of either sucrose or pollen. In a natural setting, a bee would intensify its dance this way to report a food resource of a higher value, meaning that cocaine-treated bees perceived a greater reward than they actually discovered. This suggests that contrary to previous assumptions, cocaine triggers reward mechanisms in bees. The effect was dose-dependent, and bees that were chronically treated with cocaine suffered withdrawal symptoms after treatment ceased.

To counter suggestions that cocaine merely triggered a motor response and not a reward response, the researchers showed that aside from changes to the dance, bees' motor function was normal. Furthermore, cocaine-treated bees that were separated from the hive did not spontaneously break out into dance; bees performed these movements only in the appropriate social context of returning to the hive after foraging and communicated clear, though exaggerated, information. “It's not like they're gyrating wildly on the dance floor out of control,” explained Robinson.

Cocaine kills herbivorous insects by disrupting motor control. Because the same neurochemical pathways that regulate motor control are involved in reward processing, Robinson and colleagues suggest that the rewarding properties of cocaine might be “side effects” of the drug's action as an insecticide.