Clearly, the pleasure circuits of the brain have not evolved just to be activated by implanted electrodes. Pleasure is central to survival. We must experience basic behaviors such as feeding, drinking, mating and care of offspring as pleasurable (rewarding) in order to survive and pass along our genes to the next generation. Of course, this consideration is not unique to humans. Indeed, rudimentary pleasure pathways appear quite early in evolutionary history. Even the roundworm C. elegans, which is a millimeter long and has only 302 neurons in its entire body has some basic pleasure circuitry. These worms typically feed on soil bacteria and they are very good at following odor cues to find clumps of them. However, when a group of 8 key neurons containing the neurotransmitter dopamine are silenced, the worms are totally indifferent to this favorite food source (even though they can still detect odors). The worms just don’t find eating bacteria to be that much fun anymore.
