Nicotine is the drug in tobacco. Its shape is similar to acetylcholine (ACh), so it acts as an agonist at one type of ACh receptor, which is named after it: the nicotinic acetylcholine receptor. These receptors are found at neuromuscular junctions, in the autonomic nervous system and in the brain.When nicotine binds to nicotinic receptors on a postsynaptic membrane:• the receptor's ligand-gated sodium ion channel opens
• Na+ diffuses into the postsynaptic neurone and depolarises it
• if threshold is reached, an action potential is producedSo nicotine stimulates postsynaptic neurones, mimicking ACh. Unlike ACh, nicotine is not broken down by acetylcholinesterase, so it stays in the synapse longer and its effect lasts longer.In the brain, nicotinic receptors are found on neurones of the reward pathway. Stimulating them causes the release of dopamine, which produces feelings of pleasure and alertness. This is why nicotine is so addictive: the brain links smoking with reward.With repeated use, the nervous system adapts. The nicotinic receptors become less responsive and more of them are made, so the smoker needs more nicotine to get the same effect (tolerance). Without nicotine, dopamine release falls, causing withdrawal symptoms such as irritability and craving.Nicotine also stimulates the release of adrenaline from the adrenal glands, raising heart rate and blood pressure. Nicotine replacement therapy, such as patches or gum, supplies nicotine without the tar and carbon monoxide in tobacco smoke, and the dose can be reduced gradually.
• Na+ diffuses into the postsynaptic neurone and depolarises it
• if threshold is reached, an action potential is producedSo nicotine stimulates postsynaptic neurones, mimicking ACh. Unlike ACh, nicotine is not broken down by acetylcholinesterase, so it stays in the synapse longer and its effect lasts longer.In the brain, nicotinic receptors are found on neurones of the reward pathway. Stimulating them causes the release of dopamine, which produces feelings of pleasure and alertness. This is why nicotine is so addictive: the brain links smoking with reward.With repeated use, the nervous system adapts. The nicotinic receptors become less responsive and more of them are made, so the smoker needs more nicotine to get the same effect (tolerance). Without nicotine, dopamine release falls, causing withdrawal symptoms such as irritability and craving.Nicotine also stimulates the release of adrenaline from the adrenal glands, raising heart rate and blood pressure. Nicotine replacement therapy, such as patches or gum, supplies nicotine without the tar and carbon monoxide in tobacco smoke, and the dose can be reduced gradually.