We regard alcoholism as an option, but scientists are now proving that it's not exactly so.
A new study has found that individuals carrying a G allele of the A118G polymorphism of the mu-opioid receptor gene (OPRM1) are more prone to alcohol dependence than individuals with the common A allele.
"The more sensitive mu-opioid receptor in individuals carrying a G allele of the A118G polymorphism may lead to different behavioral responses due to a different physiological functioning of the receptor," explained Esther van den Wildenberg, a researcher at the University of Maastricht.
"Different receptor functioning could affect functions such as drug-induced euphoria and analgesia which might influence the subjective experience of the drug and subsequent use."
"We were interested whether heavy drinking individuals, not yet alcohol dependent, carrying a copy of the G allele would respond differently than 'standard' genotype individuals when exposed to alcohol cues," she
said. "It turns out that G allele carriers do crave significantly more for alcohol after alcohol exposure, compared with the A allele individuals", said Kent Hutchison, associate professor at the University of Boulder, Colorado.
"In this case, the study suggests that this genetic marker may be related to how much a person craves alcohol in a high-risk situation, that is, when someone hands him or her an alcoholic drink. At this point in time, this gene definitely seems to be one of the key genes related to alcohol dependence. There will undoubtedly be others."
108 Dutch heavy drinkers male subjects, carrying either homozygous for the A allele (n=84) or carrying at least one copy of the G allele (n=24), participated in a cue-reactivity test where they were exposed to water and beer in three-minute trials. The scientists checked subjective craving for alcohol, subjective arousal, and saliva production. "By looking at cue-elicited craving in heavy drinkers, we look more specifically into a so-called 'endophenotype", explained van den Wildenberg. "An endophenotype is like an intermediate phenotype. It lies "between" the genotype, referring to genes, and the phenotype, which refers to the disorder; reflecting a biological mechanism that underlies the disorder."
"Not only did G allele carriers report even significantly more craving for alcohol than the A allele individuals," said van den Wildenberg, "but G allele carriers reported more life-time drug use compared with the 'standard' genotype participants. Especially the use of cannabis and amphetamines."
"Individuals who are more prone to cue-induced craving may also be more prone to relapse in a high-risk situation," said Hutchison. "Alcoholics with the G allele who are trying to remain abstinent may have more difficulty than alcoholics without the G allele."
"This genetic marker may influence some of the variables that lead to drug use, but there are many other variables that are not related to this gene."
"Eventually, knowledge about the genetic variables that influence alcohol abuse may be used to help determine which treatment is most appropriate for a given individual," said Hutchison.
"For example, an individual who has a gene that confers some vulnerability in a particular system in the brain may benefit most from a treatment that targets this vulnerability."
"In this case", added van den Wildenberg, "patients with a G allele of the mu-opioid receptor gene might benefit from ...the anti-craving medication naltrexone, which blocks the mu-opioid receptor. Naltrexone has been found to work especially well in alcohol-dependent patients with a G allele, possibly due to a stronger decrease in craving in these individuals."
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