Nicotine and Acetylcholine
Nicotine is able to get into the brain by traveling through the blood and penetrating a barrier called the blood-brain barrier. As NIDA for Teens explains, one of the ways that nicotine affects the brain is by mimicking neurotransmitters. Neurotransmitters are chemicals produced by nerve cells to generate or inhibit the activity of other nerve cells. Nerve cells (neurons) have special proteins called receptors that bind to neurotransmitters based on their shape. Nicotine is similar in shape to the neurotransmitter acetylcholine. As a result, nicotine can increase acetylcholine signaling throughout the brain.
Nicotine and Pleasure
Nicotine also can have effects on the parts of the brain that sense pleasure. When nicotine gets into the brain, it is able to stimulate the release of a neurotransmitter called dopamine. Dopamine activates some of the neurons in the part of the brain that register pleasure. As a result, nicotine triggers a chemical sensation of pleasure, which causes people to associate tobacco use with a feeling of pleasure.
Nicotine and Addiction
As Stop Smoking Foundation explains, nicotine alters the chemistry in the brain. Over time, the brain attempts to compensate for the increased chemical signaling that nicotine causes. The neurons that respond to nicotine also reduce the number of receptors for acetylcholine that they have to help normalize their activity. This means that nicotine has less of an effect than it did previously, a phenomenon known as tolerance. If a person stops using tobacco, the affected neurons will not receive enough signaling, leading to withdrawal symptoms and cravings.