Thursday, May 15, 2008

Neuroaddiction and the Reward Pathway

How addictive drugs fool Mother Nature

"The addicted brain is distinctly different from the nonaddicted brain,” writes Alan Leshner, the former director of the National Institute of Drug Abuse (NIDA). “Changes in brain structure and function is what makes it, fundamentally, a brain disease. A metaphorical switch in the brain seems to be thrown as a result of prolonged drug use.

Addiction is both a cause and a consequence of these fundamental alterations in brain function. If physical abnormalities in the brain are at the root of the problem, then any treatment program worth its weight ought to be dealing—directly or indirectly--with these differences in brain state. Writing in Lancet, researcher Charles O’Brien has suggested a similar orientation: “Addiction must be approached more like other chronic illnesses--such as diabetes and chronic hypertension--than like an acute illness, such as a bacterial infection or a broken bone."

All of this suggests that we are not likely to win a war on drugs, achieve zero tolerance, or become chemical-free any time soon. The drug problem is an artifact of the basic design of the mammalian brain. Humankind is extraordinarily susceptible to drug abuse anywhere and everywhere certain drugs are widely available—and all because of a “design quirk” in the reward pathways of the central nervous system.

Any sufficiently powerful receptor-active drug is, in its way, fooling Mother Nature. This deceit means, in a sense, that all such drugs are illicit. They are not natural, however organic they may be. Yet, the human drive to use them is all-pervasive. We have no real built-in immunity to drugs that directly target specific receptors in the limbic and cortical pleasure pathways. The act of “liking” something is controlled by the forebrain and brain stem. If you receive a pleasant reward, your reaction is to “like” it.

If, however, you are anticipating a reward, and are, in fact, engaging in behaviors motivated by that anticipation, it can be said that you “want” it. The wholly different act of wanting something strongly is a mesolimbic dopamine-serotonin phenomenon. We like to receive gifts, for example, but we want food, sex, and drugs. As Nesse and Berridge put it, “The ‘liking’ system is activated by receiving the reward, while the ‘wanting’ system anticipates reward and motivates instrumental behaviors. When these two systems are exposed to drugs, the “wanting” system motivates persistent pursuit of drugs that no longer give pleasure, thus offering an explanation for a core paradox in addiction."

Under the biochemical paradigm, a runaway appetite for non-stop stimulation of the reward pathway is a prescription for disaster. The harm is physical, behavioral, and psychological--as are the symptoms. Peer pressure, disciplinary difficulties, contempt for authority--none of these conditions is necessary for drug addiction to blossom. What the drug itself does to people who are biologically vulnerable is enough. No further inducements are required.

Even this brief summation of the ways in which addictive drugs alter neurotransmission should serve to demonstrate that these substances have more in common than we ordinarily assume. All these drugs are of course rewarding, so it is perhaps not too surprising, for all their differences, that they work the limbic reward pathways. All these drugs share common mechanisms of action, which is why they are addictive.

--Excerpted from The Chemical Carousel: What Science Tells Us About Beating Addiction © Dirk Hanson 2008, 2009.

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