Wednesday, December 19, 2007
What is Drug Craving?
Exploring the engine of drug relapse.
“In terms of treatment, you can’t just attack the rewarding features of the drug. In the case of alcohol, we already have a perfect drug to make alcohol aversive--and that’s Antabuse. But people don’t take it. Why don’t they take it? Because they still crave. And so they stop taking it. You have to attack the other side, and hit the craving.”
--Dr. Ting-Kai Li, 1990 interview
It causes relapses and treatment failure. It leads good people to break good promises and do harm to themselves and others. What is this thing called craving? Isn’t it just another word for lack of will power?
Scientists have gained a much deeper understanding of how and why addicts crave. For years, craving was represented by the tortured tremors and sweaty nightmares of extreme heroin and alcohol withdrawal. Significantly, however, the symptom common to all forms of withdrawal and craving is anxiety. This prominent manifestation of craving plays out along a common set of axes: depression/dysphoria, anger/irritability, and anxiety/panic. These biochemical states are the result of the “spiraling distress” (George Koob’s term) and “incomprehensible demoralization” (AA’s term) produced by the addictive cycle. The mechanism driving this distress and demoralization is the progressive dysregulation of brain reward systems, leading to biologically based craving. The chemistry of excess drives the engine of addiction, which in turn drives the body and the brain to seek more of the drug.
Whatever the neuroscientists wanted to call it, addicts knew it as “jonesing,” from the verb “to jones,” meaning to go without, to crave, to suffer the rigors of withdrawal. Most doctors don’t get it, and neither did many of the therapists, and least of all the public policy makers. Drug craving is ineffable to the outsider.
Drug craving itself is mediated by glutamate receptor activity in the hippocampus—the seat of learning and memory. A fundamental branch of what we might dub the “relapse pathway” runs through the glutamate-rich areas of the hippocampus. The puzzling matter of craving and relapse began to come into focus only when certain researchers began to rethink the matter of memory and learning as it applies to the addictive process. This led back to the role of glutamate, and it gradually became clear that the drug high and the drug craving were, in a manner of speaking, stored in separate places in the brain. Research at the National Institute for Drug Abuse (NIDA) strongly supports the hypothesis that drug memories induced by environmental triggers originate primarily in the hippocampus. And glutamate may be the substance out of which the brain fashions “trigger” memories that lead certain addicts down the road to relapse.
Glutamate is the most common neurotransmitter in the brain. (In sodium salt form, as monosodium glutamate, it is a potent food additive.) About half of the brain’s neurons are glutamate-generating neurons. Glutamate receptors are dense in the prefrontal cortex, indicating an involvement with higher thought processes like reasoning and risk assessment. The receptor for glutamate is called the N-methyl-D-aspartate (NMDA) receptor. And unfortunately, as the gifted science writer Constance Holden related in Science 292, NMDA antagonists, which might have proven to be potent anti-craving drugs, cannot be used because they induce psychosis. Dissociative drugs like PCP and ketamine are glutamate antagonists.
However, drugs that play off receptors for glutamate are already available, and more are in the pipeline. As a precursor for the synthesis of GABA, glutamate has lately become a tempting new target for drug research. Ely Lilly and others have been looking into glutamate-modulating antianxiety drugs, which might also serve as effective anti-craving medications for abstinent drug and alcohol addicts.
Photo credit: Changing Lives Foundation