The purposes of today's lecture are as follows. First to relate dopamine to natural activities that are rewarding. Second, to use sexual activity as an example. Third, to show how dopamine release occurs for both drugs and certain natural activities. Fourth, to discuss the power of drugs over behavior. And fifth, to caution everyone that dopamine is not the entire story of reward, but an important part of drugs and reward. So now, let's consider dopamine as a mediator, or modulator, of reward for a natural activity. The natural activity that we're going to examine as an example, is sexual activity or mating behavior. Here is the image shown previously that includes data from hundreds of experiments on reward centers in brain. It shows that one of the dopamine-containing systems, the mesolimbic dopamine system, is at least part of the reward system. And we know from the last lecture that we can measure dopamine release and levels from these regions. We mentioned that cocaine administration results in an elevation of synaptic dopamine in the nucleus accumbens. Here again, we show a laboratory rat, with a very small probe placed in the brain, that helps us measure dopamine levels. This arrangement was used in a previous experiment, to verify that cocaine blocks the dopamine transporter, and results in a massive synaptic increase in dopamine levels. Now we're going to look at the effect of a rewarding natural activity. An activity that makes one feel good and rewarded, and it's effect on dopamine levels in reward circuitry. In this interesting experiment, we have a probe implanted into the brain of a male rat. And we're going to follow the levels of dopamine in the nucleus accumbens during the exposure of this male rat to a female rat. Dopamine levels are shown in relative terms, and in the first half hour or so, with the rat in its home cage, dopamine levels are approximately at the arbitrary level of 100. When the rat is moved into the test chamber, a novel environment, dopamine levels rise a little, and when the animal is exposed to the female and allowed to mate with the female, dopamine levels rise dramatically. After the female's removed, dopamine levels gradually fall back to normal levels. These data show that engaging in sexual activity causes an increase of synaptic dopamine in the nucleus accumbens. Remember this data? This shows that synaptic dopamine levels rise dramatically in the nucleus accumbens when cocaine is given. How interesting. Dopamine levels rise when engaging in sexual activity or after taking cocaine. From the point of view of dopamine receptors, and the downstream parts of the brain, the brain doesn't know whether you've just had cocaine or whether you've just engaged in sexual activity. The motivation to engage in sexual activity is very strong. It may very well be that one of the reasons that drugs have such power over our behavior in some cases, is because they affect the same systems as powerful, natural activities. Drugs can be as powerful as sexual activity, and many drug users say this. This seems to be an important discovery. Throughout this course, I've been dopamine a great deal. I've shown dopamine in the context of its being a neurotransmitter, talked about its five receptors, and now about its involvement in reward. However, I want to point out that dopamine is not simply the entire story. Dopamine is an important modulator of reward, but not the only factor in reward. There are many different neurotransmitters found in reward centers in the brain. Dopamine does not simply equal reward, but most likely something more complicated. It may be that dopamine is involved in attracting attention to a variety of stimuli. Nevertheless, again, dopamine is a very important part of the addiction picture. Let me summarize what we've talked about. Reward systems in the brain contribute to survival and are powerful because they have to be. The dopamine-containing mesolimbic circuit is one of the pathways involved in reward. Dopamine is related to both drug reward and natural rewards. Therefore, it is profitable to explore dopamine systems and drugs further. As I mentioned in the last lecture, this is what we plan to do. Please examine the review questions for this lecture.
