The Deep Evolutionary Roots of 'Addiction'

Abstract

Addiction is now recognized as a phenomenon with exceedingly deep evolutionary roots. Addictive plant alkaloids, as secondary metabolites, evolved primarily to counter insect herbivory. Although some views regard addiction as a human/mammal specific, cognitive phenomenon, we suggest that its roots are found in much more fundamental biological mechanisms for learning and motivation, mechanisms which are shared by taxa since the early evolution of bilateral metazoans. According to this view, addiction is fundamentally an invertebrate phenomenon and humans can be viewed as collateral damage in this coevolutionary arms race. Results from a variety of invertebrate preparations demonstrate behavioral and neural consequences of drug exposure, ranging from psychostimulant properties and sensitization, to conditioned cue learning and operant self-administration. This mini review focuses attention on our recent work in crayfish, but there is certainly evidence for the presence of addictionlike phenomena in more ancient invertebrate taxa.