When we think about the reward circuits in the brain, heavily dependent on dopamine and now more evidence to suggest the involvement of endogenous opioids, you can begin to recognize that from an evolutionary standpoint, we really do need those circuits. We need to be motivated to seek out things that will allow us to survive and reproduce, and at the same time, we also need a mechanism by which to avoid pain and potentially dangerous or injurious situations. As we’ve looked at these two different aspects of our functioning that ensure our survival, we see that the same areas of the brain are active not necessarily at the same time, but sit alongside of each other, oftentimes only a cubic millimeter away from one another. So it’s not that pain and pleasure are the same, but that they exist along a continuum. The brain is wonderfully efficient and uses a small selection of chemicals to do a number of different things. Dopamine has more to do with wanting something or craving something, seeing it as something worth pursuing. So it has more or an aspect of motivation attached to it. Whereas the endogenous opioids really have not only to do with our own internal analgesia function but also that aspect of liking something–the likeability of a particular activity or substance. You can see how you need both of those things. You need to want something and then after you experience it, you need to like it.
Pain and pleasure come together and interact with one another in a number of different ways. We clearly need both processes. We need to experience pleasure, and we need to be able to experience pain. But the context is what really matters and you can think of a number of risky or painful situations, some rather mundane, some not so, that at their conclusion, convey a sense of enjoyment or accomplishment or satisfaction, having overcome a challenge or made it through a situation that was difficult. The relief from pain is contributing to a sense of pleasure. I think that when we see patients with chronic pain, one of the things that we are struck by is the suffering that they’re enduring. One of the hardest things to help patients do is to regain function in the face of that suffering. Part of what you hear patients describe is the loss of pleasure in their day-to-day existence. That may be because these circuits are turned off or inhibited because of pain or the presence of major depression. So as we’re working with patients, we want to be alert for depression, and we also want to be thinking about how to re-inject activities that will be enjoyable and may be able to jumpstart some of these pathways. There is evidence that you can reverse these changes that occur over time in the brains of patients with chronic pain and normalize those circuits again. That’s really what we’re after.