According to fMRI images, much of the pleasure of addiction comes not from the fix, but from anticipation of the fix. In fact, the same pathways in the brain are activated when people anticipate a monetary reward as when cocaine addicts anticipate doing cocaine. A few years ago, scientists at Mass General mapped the brains of 12 non-pathological gamblers while they engaged in a betting game (with real money on the line). The results, published in Neuron in 2001, showed a pattern of brain activity in the gambling participants similar to patterns found in previous studies of drug addicts receiving cocaine or morphine. When the stakes increased, so did the brain's response.
The part of the brain triggered by both gambling and illicit drug use is known as the mesolimbic dopamine pathway. This is the same pleasure center involved in other feel-good activities, such as drinking alcohol, having sex, eating a delicious meal, or even receiving a compliment. The mesolimbic dopamine pathway is part of the limbic system, an area of the brain that has existed in vertebrates for 500 million years, and is driven by primal responses: if something makes you feel good, you do it again. That simple reaction has been integral to the propagation of many species, including humans, encouraging essential functions like eating and reproducing.
So you can thank the chemical dopamine, the chief neurotransmitter in the pleasure center, for those age-old feelings of bliss. When you start rubbing that scratch ticket or spinning a roulette wheel, the brain releases dopamine, which swims across the synaptic cleft and binds to receptors. While the dopamine is attached to these receptors, you experience pleasure. Once the dopamine is broken down or reabsorbed, the pleasure subsides.
In the case of gambling, the dopamine fires up even before you know whether Lady Luck is with you or against you. Why shouldn't you feel good in those glorious few seconds of hope, as you silently choose the upholstery for the seats inside the new Jaguar XKR convertible that you'll buy with your winnings, or weigh the benefits of vacationing in the Bahamas versus Amsterdam? If you win, the dopaminergic neurons maintain their gratifying activity, hugging those receptors for another few moments of glee. If you lose, the firing decreases, the fun dies down, and reality hits: no Jag convertible, no Dutch chocolate.
Simply thinking about gambling can also activate the brain's pleasure center. In a study by Marc Potenza, a professor of psychiatry at Yale University, pathological gamblers who were shown videos of people gambling and talking about gambling had altered brain activity in the same parts of the brain that light up when cocaine addicts see images of their precious nose candy. Of course, that response often leads addicts to seek out their vice, just as a hungry person might hit up the local pizzeria in response to beckoning smells.