Dopamine rises on the expectation of good things happening ((Wise RA, Wang B, You Z-B (2008). cocaine serves as a peripheral interoceptive conditioned stimulus for central glutamate and dopamine release. PLoS ONE 3(8): e2846. doi: 10.1371/journal.pone. 0002846)), and as Sapolsky says, Las Vegas is built on our dopamine response ((Comings DE, Blum K. Reward Deficiency syndrome: genetic aspects of behavioral disorders. Prog Brain Res 2000; 126: 325-41. abstract)) ((Comings DE, Gade R, Wu S et al. Studies of the potential role of the dopamine D1 receptor gene in addictive behaviors. Mol Psychiatry 1997 Jan; 2(1): 44-56. Abstract)) and here is why. In the experiment he is talking about, dopamine levels of the rats rose far more when the outcome of their lever pressing for food was uncertain than when they knew they would get their reward. And so it is posited that sometimes there are abnormalities in the dopamine receptor sites in the brain which predispose some to addictive behaviours – the uncertainty of gambling causes a huge hike in dopamine. As Sapolsky says, if the dopamine pathway is blocked, then you don’t get the work. So in the human, if there are abnormalities in the dopamine receptors reducing their effectiveness, then that person will feel very depressed and completely lacking in motivation. Depression and its causes are much more complex than shortage of Prozac.
This video is worth watching for the punchline at the end. I am a huge Sapolsky fan; he makes difficult topics not only understandable but also very funny.