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by Sigalit Hoffman
Prolonged drug use changes the structure of the brain, making our efforts
to quit for good more difficult. That’s the essence of the message
delivered in a public lecture at the downtown Faculty Club on September
21 by Psychology Professor Jane Stewart.
In her work as a senior researcher with the Centre for Studies in Behavioural
Neurobiology (CSBN), Stewart has been able to show that sensitivity to
drugs — and thus, the chance of relapse — is at its highest
some time after drug use has ceased. This brings into question the common
belief that craving and relapse are strongest shortly after quitting.
She explained that chronic drug use eventually leads to changes in the
brain that increase the individual’s sensitivity to drugs. On re-exposure
to the drug, this heightened sensitivity induces renewed craving and a
high probability of relapse. Stewart admitted, “This is really quite
pessimistic for drug treatment. Just being away from it is not good enough.”
Sensitization to drugs could
be prevented
She shed some light on the biological basis of the changes in brain structure.
She noticed that the brains of rats sensitized to the stimulant drug amphetamine
had four times the normal amount of a neurotrophic factor known to be
associated with growth and repair.
Neurotrophic factors are compounds that are present in the brain at birth
and are responsible for the growth, extension, and guiding of neurons
to their targets. They are also present in adult brains and may be increased
in brains undergoing trauma. By giving rats an agent that neutralized
the effect of the neurotrophic factor, she found that sensitization to
drugs could be prevented.
Stewart illustrated the long-term effects of brain changes in her conditioning
experiments. Rats were taught to associate cocaine with a particular cage,
and then this association was reduced by replacing the drug injections
by injection of saline.
As soon as the animals were re-administered the drug, they preferred the
cage that they had previously associated with cocaine. These data emphasize
the importance of conditioned stimuli, in this case the environment, in
relapse. She explained how an initially aversive stimulus can be made
attractive by pairing it with a pleasurable effects of a drug, such as
alcohol. “Think of the initially nasty taste of pure malt whiskey,”
she said.
Drugs may not initially be attractive, but when associated with the feelings
they create, they become highly sought. In addition, Stewart pointed to
stress as another potential cause of relapse. Stressed animals, she discovered,
exhibit a greater desire for drugs.
In her lab animals, Stewart has observed that the drugs that have effects
on the brain similar to those of important natural stimuli — such
as food and sexual partners — are the same ones that animals, including
humans, will self-administer.
She spoke of the bi-directional effects of her research approach. She
said, for example, that “by studying the motivational systems of
the brain activated by natural incentives, we found something about how
drugs work, but [at the same time,] we can use drugs as tools to find
out how the motivational systems work.”
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