Not having that receptor might explain my brother's ability to smoke a cigarette maybe once in three months. He did that for 30 years or so.

There is no way I could have done that. The reason I stopped when I did is that I was smoking more and more heavily and getting more and more obsessive about it. In the old days, if I had a mostly full pack in my purse I was comfortable leaving home. By the time I quit 25 years later, I was lugging up to three extra packs.