Pleasure reward pathways

While activation of the reward pathways explains the pleasurable sensations associated with acute substance use, chronic use of abused substances resulting in both addiction and withdrawal may be related to neuroadaptive effects occurring within the brain. 

These dopaminergic reward pathways are critical for normal survival, since they provide the pleasure drives for eating, drinking and reproduction. However, this system produces similar sensations of pleasure with alcohol, cocaine, heroin, nicotine,... 

FIGURE 10-9. The dopamine hypothesis of psychosis. Hyperactivity of dopamine neurons in the mesolimbic dopamine pathway theoretically mediates the positive symptoms of psychosis, such as delusions and hallucinations. This pathway is also involved in pleasure, reward, and reinforcing behavior, and many drugs of abuse interact here. 

There is a reward pathway, or circuit, in the brain that positively reinforces pleasurable behavior (Figure 4.3). The reward circuit includes three areas of the brain that are interconnected with one another the ventral tegmental area (or VTA), the nucleus accumbens, and the prefrontal cortex. Axons from neurons in the VTA communicate with neurons in the other two areas via the reward pathway. 

When the brain receives a rewarding stimulus, a chemical messenger called dopamine is released by VTA neurons. Dopamine affects neurons of the nucleus accumbens and the prefrontal cortex, and its release is associated with a feeling of pleasure and reward. Scientists can measure an increased release of dopamine in the reward pathway after a rat receives a reward. ... 

Figure 4.3 The reward pathway is the pathway in the brain that positively reinforces pleasurable behavior. When the brain receives a rewarding stimulus, the neurons in the ventral tegmental area release dopamine. The dopamine, in turn, affects neurons in the nucleus accumbens and the prefrontal cortex.

Rimonabant may also alter reward pathways that promote addictive behaviors such as overeating. This drug has been shown to attenuate the rewarding effects of alcohol in a strain of alcohol-preferring rat s (161) and may similarly diminish the pleasurable effects of food and overeating that can often play important roles in the etiology of obesity. 

Dopamine is the neurotransmitter that is most implicated in addictive behaviors. Drug addictions occur more readily in people who have inherited an abnormal dopamine receptor, for example. It is also a major transmitter in the reward pathway of the brain. Pleasurable activities such as eating and sex, release dopamine, so in terms of evolution, behaviors that promote human survival activate our built-in reward system. 

These natural reward centers have developed over the course of evolution to reinforce useful behaviors (e.g., pleasure, sexual satisfaction, eating, and drinking). It is believed that drugs such as cocaine and amphetamine directly stimulate these centers, while opiates free the pathways from inhibitory control. Nicotine, on the other hand, reaches the brain in as little as 10-20 seconds, where it stimulates nicotine receptors to cause dopaminergic neurons to release large quantities of dopamine. After a few hours, dopamine levels decline, causing withdrawal symptoms to readily appear (e.g., anxiety, irritability, and inattentiveness). When cigarette smokers say they need a smoke to steady their nerves, what they really mean is that they have to contend with nicotine withdrawal. 

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