1.   The idea is to home in on certain types of nicotine receptors in the brain, he said.

2.   They speculate these people have fewer nicotine receptors in their brains or their receptors are less sensitive than in other people.

3.   Levin and his colleagues found another compound that stimulates nicotine receptors and can improve learning and memory in brain-damaged rats.

4.   They also worked on the nicotine receptors on nerve cells.