There are some psychologists who believe that dopamine is not released during pleasurable activities but before them; my understanding is that a little dopamine is released pre-reward in order to motivate us to complete the action and then a large amount is released when we consume/get the reward. Can you please explain this? Their argument is that dopamine release tells the brain that it is about to feel good and then the brain releases opioids to make us feel good. This doesn't make sense if when addicts take opioids, such as heroin, the brain is flooded with dopamine and the person feels good. In that case we could say that dopamine is responsible for the euphoria. Do you believe this to be correct?
This is an ongoing discussion, and entire textbooks are written in this field, but I will try to clear up some misconceptions and point you toward some further reading that may help.
Is dopamine pleasure?
"Pleasure" is an emotional phenomenon; dopamine is a neurotransmitter. Dopamine can be involved in the perception of pleasure, but there was a now-outdated hypothesis that "dopamine==pleasure".
More recent evidence says that this is not the case, because endogenous dopamine release occurs in some brain regions during the anticipation of a pleasurable event, and really doesn't correlate with pleasure itself at all. Instead, dopamine is currently thought to reflect a "reward prediction error": a signal that tells the brain "hey, we just got something better than we expected!"
Why is a prediction error useful?
This signal can then be used by the brain to upregulate circuits that were recently active, effectively using the heuristic: "I just got something I didn't expect, so whatever I did just before must have been a better thing to do than I previously thought! I should do that more often!"
There are 'degrees' of unexpectedness, however, which is why you get a dopamine signal when a positive event is expected. Let's say a cue, like a bell, signals a coming reward. The reward is now expected, so it won't produce an error signal, but the bell itself produces an error signal: prior to the bell, you didn't know the bell was coming, so you didn't know the reward was coming. Now you update your brain circuits to try to do things to make bells ring!
This is a powerful learning algorithm that is also used, in a form, in artificial neural networks.
Context in drug abuse
If a drug of abuse causes a release of dopamine, that's a signal to the rest of the brain "Hey, something good just happened, let's try to do that again in the future!" Dopamine is the message, not the experience.
The two bolded works below are a recent review and a highly relevant original paper, respectively.
Peter Dayan, Wolfram Schultz, and Ray Dolan were recently awarded for their work in this field and you can find lots of popular press that describes their work, as well.
Schultz, W. (2013). Updating dopamine reward signals. Current opinion in neurobiology, 23(2), 229-238.
Schultz, W., Dayan, P., & Montague, P. R. (1997). A neural substrate of prediction and reward. Science, 275(5306), 1593-1599.
Fiorillo, C. D., Tobler, P. N., & Schultz, W. (2003). Discrete coding of reward probability and uncertainty by dopamine neurons. Science, 299(5614), 1898-1902.
Glimcher, P. W. (2011). Understanding dopamine and reinforcement learning: the dopamine reward prediction error hypothesis. Proceedings of the National Academy of Sciences, 108(Supplement 3), 15647-15654.
Pessiglione, M., Seymour, B., Flandin, G., Dolan, R. J., & Frith, C. D. (2006). Dopamine-dependent prediction errors underpin reward-seeking behaviour in humans. Nature, 442(7106), 1042-1045.