The Evolutionary Roots of Addiction

By E.J. Mundell

NEW YORK (Reuters) Evolution may dictate that humans will never win the
war against drugs, two experts conclude.

Drug addiction is "a problem that is likely to persist because it is rooted
in the fundamental design of the human nervous system," write Drs. Randolph
Nesse and Kent Berridge, psychiatric investigators with the Institute for
Social Research at the University of Michigan in Ann Arbor. Their report
was published recently in the journal Science.

Theories on the underlying causes of drug addiction whether social,
psychological, or economic have risen and fallen in prominence for as
long as the problem has plagued us. Nesse and Berridge's perspective is
based on the new science of 'Darwinian medicine.' In an interview with
Reuters, Nesse described this theory as "the enterprise of trying to figure
out why humans are vulnerable to diseases, in terms of how natural
selection has shaped our bodies."

Drug addiction presented a special puzzle for the two researchers: why do
addicts compulsively pursue their drug of choice, long after its
pleasuregiving ability has waned?

The secret may lie in ancient neurochemical mechanisms shared by organisms
ranging from mollusks to man. These mechanisms involve brain "transmitter
molecules" such as dopamine, serotonin and norepinephrine chemicals
linked to two behaviorconditioning responses Nesse labels "liking" and
"wanting."

The "liking" response helps direct humans toward behaviors that ensure
survival and procreation it rewards 'good' actions with pleasurable
sensations. Nesse points out that an organism will usually experience these
sensations most powerfully "while it's getting in stride, while it's
learning how to do what works." Once the behavior is repeated, initial
levels of pleasure diminish.

If "liking" is the reward, then the brain's "wanting" mechanism provides
the motivation to pursue the reward. The "wanting" system is stored in a
separate part of the brain from the "liking" mechanism, however, say the
researchers.

With many stimulants (chocolate, sex, money), received pleasure ("liking")
reaches a certain plateau, while "wanting" is regulated so that behaviors
do not become excessive.

But "drugs essentially shortcircuit these mechanisms," Nesse explains.
"The aspect of the drug that acts on the pleasurable mechanism fades very
quickly, with repeated use, while the effect of the drugs on the
behaviorregulation mechanism sticks permanently." So compulsive
drugseeking continues, long after enjoyment recedes.

This fits with most patterns of drug abuse. Addicts feel good when "they do
drugs the first time, and maybe the tenth time," Nesse points out, "but
after that there's a divergence of these two mechanisms."

Nesse believes Darwinian explanations of substance abuse contradict
longcherished social mores. First of all, he says, evolutionary rationales
imply that drug addicts are not simply "defective" members of the species.
"That's not correct. We're all at risk because of our basic design."

And he says Darwinian theory runs counter to the notion that drug use is
necessarily tied to social decay. Instead, the two authors believe there
is, and always will be, a "mismatch between our bodies and our modern
environment." Nesse says "it's not surprising at all that people use drugs.
In fact, from our point of view, its quite remarkable that more people
don't." SOURCE: Science, (197;278:6365)