In the Carribean, St. Kitts Island is serving as a small, vacation like laboratory to observe the alcohol addiction tendency’s of the green vervet monkey.
Like humans, many of these monkeys appear to enjoy the intoxicating effects of alcohol—at least, most do. Remarkably, the number of monkeys in vervet colonies (who steal patrons drinks) and drink heavily, drink lightly, or choose to abstain from alcohol resembles the same ratios in humans (in cultures where alcohol use is prevalent). Even teetotaler monkeys who love sweet drinks will turn up their noses if alcohol is added. The green vervet monkeys of the St. Kitts Islands have practically unlimited access to sugary alcoholic beverages; they either lay in wait for the dregs of deserted daiquiris, or swoop in take a blended sugary concoction from unsuspecting beach goers.
The situation in St. Kitts individual differences in alcohol preferences occurring “naturally” in a population has provided a unique opportunity for researchers to examine the natural environment of susceptibility to alcohol addiction. Here scientists selected monkeys based on individual differences in observed alcohol consumption over a period of two years in St. Kitts. These researchers then examined the dopamine system in the brains of these animals by measuring the distribution and density of a transporter protein called the Dopamine Transporter specifically, they compared levels of this protein in alcohol-preferring versus alcohol-abstaining monkeys.
The Dopamine Transporter is a protein that acts by clearing dopamine, the brain’s main “reward” chemical, from the communication space between neurons (the synapse). Essentially, a higher amount of Dopamine Transporter means a lower amount of dopamine is available for signaling; changes in this “clean-up” aspect of the dopamine system are associated with ADHD and depression in humans. Results of the vervet monkey study by Mash demonstrated that monkeys who preferred to drink the most alcohol also had a significantly higher amount of Dopamine Transporter in a brain region called the stratium when compared to their alcohol-avoidant kin, but ONLY when these former-drunk monkeys had been denied access to alcohol for some time. Sure enough, when allowed to drink, the density and distribution of Dopamine Transporter decreased as alcohol-preferring monkeys were allowed to drink; levels of DAT rebounded again during alcohol withdrawal.
Similar alterations in Dopamine Transporter have been observed in human alcoholism, and interestingly, Major Depressive Disorder is also associated with higher availability of Dopamine Transporter. Perhaps this individual difference in the brain dopamine’s system can help explain why many individuals who suffer from depression appear to self-medicate with alcohol, and provide a biological target for pharmacological and cognitive therapy in these deadly disorders.