9: The reward pathway and addiction
Introduce the concept of reward. Humans, as well as other organisms engage in behaviors that are rewarding; the pleasurable feelings provide positive reinforcement so that the behavior is repeated. There are natural rewards as well as artificial rewards, such as drugs.

10: Natural rewards
Natural rewards such as food, water, sex, and nurturing allow the organism to feel pleasure when eating, drinking, procreating, and being nurtured. Such pleasurable feelings reinforce the behavior so that it will be repeated. Each of these behaviors is required for the survival of the species. Remind your audience that there is a pathway in the brain that is responsible for rewarding behaviors. This can be viewed in more detail in the next image.

11: The reward pathway
Tell your audience that this is a view of the brain cut down the middle. An important part of the reward pathway is shown and the major structures are highlighted: the ventral tegmental area (VTA), the nucleus accumbens, and the prefrontal cortex. The VTA is connected to both the nucleus accumbens and the prefrontal cortex via this pathway and it sends information to these structures via its neurons. The neurons of the VTA contain the neurotransmitter dopamine, which is released in the nucleus accumbens and in the prefrontal cortex (point to each of these structures). Reiterate that this pathway is activated by a rewarding stimulus. [Note: the pathway shown here is not the only pathway activated by rewards, other structures are involved too, but only this part of the pathway is shown for simplicity.]

12: Activation of the reward pathway by an electrical stimulus
The discovery of the reward pathway was achieved with the help of animals such as rats. Rats were trained to press a lever for a tiny electrical jolt to certain parts of the brain. Show that when an electrode is placed in the nucleus accumbens, the rat keeps pressing the lever to receive the small electrical stimulus because it feels pleasurable. This rewarding feeling is also called positive reinforcement. Point to an area of the brain close to the nucleus accumbens. Tell the audience that when the electrode is placed there, the rat will not press the lever for the electrical stimulus because stimulating neurons in a nearby area that does not connect with the nucleus accumbens does not activate the reward pathway. The importance of the neurotransmitter dopamine has been determined in these experiments because scientists can measure an increased release of dopamine in the reward pathway after the rat receives the reward. And, if the dopamine release is prevented (either with a drug or by destroying the pathway), the rat won't press the bar for the electrical jolt. So with the help of the rats, scientists figured out the specific brain areas as well as the neurochemicals involved in the reward pathway.

13: Addiction
Now that you have defined the concept of reward, you can define addiction. Addiction is a state in which an organism engages in a compulsive behavior, even when faced with negative consequences. This behavior is reinforcing, or rewarding, as you have just discussed. A major feature of addiction is the loss of control in limiting intake of the addictive substance. The most recent research indicates that the reward pathway may be even more important in the craving associated with addiction, compared to the reward itself. Scientists have learned a great deal about the biochemical, cellular, and molecular bases of addiction; it is clear that addiction is a disease of the brain. State that you will provide two examples of the interaction between drugs that are addictive, their cellular targets in the brain, and the reward pathway.