Is dopamine required for natural reward?

Reward is fundamental to the organization of behavior, and the neurotransmitter dopamine (DA) is widely recognized to be critical to the neurobiology of reward, learning and addiction. Virtually all drugs of abuse, including heroin and other opiates, alcohol, cocaine, amphetamine and nicotine activate dopaminergic systems. So called "natural" rewards such as food, positive social interactions and even humor, likewise activate DA neurons and are powerful aids to attention and learning. Sweet solutions are a well-characterized natural reward. When a source of sugar is encountered, animals will consume substantial amounts, return to it preferentially, and will work to obtain access. Dopamine systems are activated in animals drinking sugar solutions, and lesions of dopaminergic neurons or pharmacological blockade of DA receptors seem to reduce the reward value of both sweet tastes and drugs of abuse. However, we have recently demonstrated that genetically modified mice that cannot make DA (DD mice) manifest normal sucrose preference. During preference tests, mutant mice initiated licking less frequently than did normal mice, but the rate of licking by DD mice for sweets was actually higher than that of normal mice, indicating that their motor ability to lick is intact. We conclude that DA is not required for the hedonic response to sweets nor for their discrimination. This brief and slightly humorous review discusses these findings in the context of current and historical answers to the question, "What is the role of DA in reward?"