|35287. Sun Nov 27, 2005 5:25 am
|Dopamine, Dopamine, Dopamine....where to begin...
A shortage of dopamine in the basal gangal motor loop causes Parkinson's disease, Dopamine's involved in motor skills.
Dopamine in controls the flow of information from other areas of the brain to the frontal lobe, thus imbalances of Dopamine here are related to cognitive problems.
But perhaps the most interesting aspect of Dopamine is its still not entirely clear role in pleasure and motivation. Here's wikipedia's entry on the topic:
|Dopamine is commonly associated with the pleasure system of the brain, providing feelings of enjoyment and reinforcement to motivate us to do, or continue doing, certain activities. Dopamine is released (particularly in areas such as the nucleus accumbens and striatum) by naturally-rewarding experiences such as food, sex, use of certain drugs and neutral stimuli that become associated with them. This theory is often discussed in terms of drugs (such as cocaine and amphetamines), which seem to be directly or indirectly related to the increase of dopamine in these areas, and in relation to neurobiological theories of addiction, which argue that these dopamine pathways are pathologically altered in addicted persons. The mechanisms of cocaine and amphetamine are different, however. Cocaine acts as a dopamine transporter blocker, competively inhibiting dopamine uptake to increase the lifetime of dopamine. On the other hand, amphetamines act as dopamine transporter substrates to competitively inhibit dopamine uptake and increase the dopamine efflux via a dopamine transporter.
However, the idea that dopamine is the 'reward chemical' of the brain, a view held by many during early stages of its research, seems too simple as more evidence has been gathered. Dopamine is known to be released when unpleasant or aversive stimuli are encountered, suggesting that it is not only associated with 'rewards' or pleasure. Recent research has begun to examine whether or not the firing of dopamine neurons might function as a reward-prediction error signal, based on evidence that, when a reward is greater than expected, there is an increase in the firing of certain dopaminergic neurons (in contrast to when there is a lesser-than-expected reward, and there is a marked decrease in the firing of the same neurons). Some argue that dopamine may be involved in desire rather than pleasure. Drugs that are known to reduce dopamine activity (e.g., antipsychotics) have been shown to reduce people's desire for pleasurable stimuli, despite the fact that they will rate them as just as pleasurable when they actually encounter or consume them. It seems that these drugs reduce the wanting but not the liking, providing more evidence for the desire theory.
Other theories suggest that the crucial role of dopamine may be in predicting pleasurable activity. Related theories argue that dopamine function may be involved in the salience ('noticeableness') of perceived objects and events, with potentially important stimuli (including rewarding things, but also things that may be dangerous or a threat) appearing more noticeable or more important. This theory argues that dopamine's role is to assist decision-making by influencing the priority of such stimuli to the person concerned.
However, the above theories are based on correlational, rather than causal, experimental evidence. The available experimental evidence that examined causal rather than correlational relationships between dopamine and motivation does not seem to agree with any of above-stated theories. For example, pharmacological blockade of brain dopamine receptors increases rather than decreases the rate of drug-taking behavior. The theories viewing dopamine as the mediator of 'desire/wanting,' 'predicting pleasurable activity,' 'noticeableness' or 'decision-making' cannot adequately explain this experimental evidence. Thus, the functional role of dopamine in motivation remains to be the topic of controversy.