8: Reward: drug self-administration
Introduce the concept of positive reinforcement or reward. Explain that rats will press a lever to self-administer an injection of cocaine or heroin that is inserted into either the peripheral bloodstream (left image) or into specific brain regions (right image). The rat keeps pressing to get more cocaine or heroin because the drugs make the rat feel so good. This is called positive reinforcement, or reward. Natural rewards include food, water, and sex - each is required to maintain survival of our species. Animals and people will continue to exhibit a behavior that is rewarding, and they will cease that behavior when the reward is no longer present. Explain that there is actually a part of the brain that is activated by natural rewards and by artificial rewards such as addictive drugs. This part of the brain is called the reward system. Neuroscientists have been able to pinpoint the exact parts of the brain involved, with the help of the rats. Point to the cartoon on the right and explain that rats will also self-administer addictive drugs directly into their brains, but only into a specific area of the reward system. If the injection needle is moved less than a millimeter away from this crucial area, the rat won't press the lever for more drug. So based on information from working with the rats, scientists have drawn a map of the brain, and located the structures and pathways that are activated when an addictive drug is taken voluntarily. Tell the students that you will show them this "map."

9: The reward pathway
Tell students that this is a view of the brain cut down the middle. An important part of the reward system is shown and the major structures are highlighted: the ventral tegmental area (VTA), the nucleus accumbens (nuc. acc.) and the prefrontal cortex. Also, the pathway connecting these structures is highlighted. The information travels from the VTA to the nucleus accumbens and then up to the prefrontal cortex. Reiterate that this pathway is activated by a rewarding stimulus. [Note to scientists - this is not the only pathway activated by reward, other structures are involved too, but only this part of the pathway is shown for simplicity.]

10: Injection of cocaine into the nucleus accumbens
Demonstrate how scientists located the structures important for the addictive nature of drugs. Show that a rat will self-administer cocaine directly into the nucleus accumbens (or the VTA) to activate the pathway. Point to an area close to the nucleus accumbens or VTA and state that if the injection is placed in this other area, the rat will not press the lever to receive the drug. Indicate that scientists know a lot more than where the drug acts to produce rewarding effects - they also know how the drugs work. Show examples with cocaine, heroin, and marijuana.