Such is the story of Dopamine. The average man on the street if asked would declare confidently, "Dopamine? Oh that's the pleasure chemical." Indeed he would have good reason for his confidence. Dopamine has been linked time and again to reward response, addiction, and mood; all pleasure associated concepts. Classic experiments involving animal lever presses causing Dopamine administration seem to clearly indicate that mammal brains will actively seek Dopamine to the exclusion of other stimuli. The most powerful and prevalent drugs generally act directly on the Dopaminergic system. Depressed patients generally exhibit lower Dopamine levels. In the face of all this evidence isn't it clear that Dopamine is closely tied to pleasure in humans?
Well, we really don't know.
It's true that Dopamine release is common during pleasurable experiences. However, it turns out it's also common during stressful experiences. So if Dopamine is released at times that most people would consider terrifying rather than pleasurable how can it be the so called pleasure chemical?
It's true that depressed patients often exhibit decreased Dopamine levels. However, no one attempts to make a case that depressed patients can't experience pleasure or even experience lesser degrees of pleasure than non-depressed individuals. Rather depressed patients generally find it difficult to make an effort towards anything, even activities they would generally enjoy.
It's true that drug addicts generally seek their chosen drug as if compelled. However, do they generally seem particularly happy when they achieve their goal of self medicating? My general perception of addicts is they do not achieve pleasure from getting their fix as much as they achieve relief. While relief is certainly something people seek it is not the same as pleasure.
There is a growing trend in neuroeconomics to view Dopamine not as related to pleasure, but as related to motivation. A growing body of work supports this idea and very little of the theory conflicts with previous studies. Studies in rats demonstrated that rats with lowered Dopamine levels would seek easier to achieve rewards over more substantial rewards. For example they would choose a smaller pile of food that was easy to obtain over a larger pile which require they climb over an obstruction. Rats with normal Dopamine levels would select the larger, more difficult reward the majority of the time. Thus we see a correlation between higher Dopamine levels and more willingness to exert effort to obtain reward.
Similarly we find in humans that more extroverted people generally have higher Dopamine levels. Extroverts by definition are more outgoing and willing to engage others. Does this indicate that they receive more enjoyment from being with others? It's possible. However, it's equally likely that they're simply more willing to put forth more effort than introverts.
So the question of Dopamine for the moment at least is do higher Dopamine levels mean you enjoy things more or are just more willing to work for the things you enjoy? It's difficult to separate these two concepts experimentally. Naturally, a person who enjoys an activity more would be willing to exert more effort to undertake that activity. However, a person who is willing to exert more effort innately would still exert more effort to participate in an activity that they enjoyed as much as another individual. So which is it then, is Dopamine about pleasure or motivation?
Well, we really don't know... yet.