Nicotine, when smoked, sniffed or chewed, is absorbed into the bloodstream through the mucous membranes of the throat, nose or mouth as well as through the mucosal lining of the airways. It can also be absorbed into the bloodstream through the skin.

Once in the bloodstream, nicotine circulates around the body and crosses over the blood-brain barrier to enter the brain. In the brain, nicotine binds to and activates receptors called cholinergic receptors.

These cholinergic receptors are abundant in the brain as well as in other areas of the body such as the muscles, heart, adrenal glands and other vital organs. Normally, these receptors are activated by the neurotransmitter acetylcholine which is produced at nerve endings in the brain and in the nerves of the peripheral nervous system.

Since nicotine has a similar structure to acetylcholine, it can activate the cholinergic receptors. However, unlike acetylcholine, nicotine enters the brain and disrupts its normal functioning, causing chemical changes and addiction to the habit.

Nicotine stimulates the reward centres in the brain, providing pleasure and euphoria when taken. On binding to receptors present in the brain, nicotine causes release of the neurotransmitter dopamine, amongst others, a chemical involved in reward sensations.

Furthermore, the number and sensitivity of the nicotinic acetylcholine receptors in the brain significantly reduces. To compensate, the brain increases production of a number of receptors for important neurotransmitters. Therefore, the reward pathways of the brain become more sensitive, with an increased number of receptors. This increases a smoker's craving to smoke and leads to addiction.With continued use, nicotine leads to a decrease in the release of dopamine at a dose the body is used to. There is also a down regulation or decreased production of other stimulatory neurotransmitters in the brain.

Stopping smoking or intake of nicotine may lead to unpleasant symptoms called withdrawal symptoms. Some of the withdrawal symptoms of nicotine include:

• Nicotine cravings
• Irritability, frustration, anger and mood swings
• Depression and anxiety
• Weight gain

Treatment of nicotine withdrawal symptoms

Nicotine replacement systems deliver measured doses of nicotine to help relieve the cravings caused by withdrawal. Approved forms of nicotine replacement include nicotine patches which are worn on the skin, nicotine gum and lozenges, nicotine nasal sprays and inhalers.

Some medications may also be used to treat nicotine withdrawal symptoms including bupropion which is an antidepressant. Another new drug called varenicline can also help smokers and tobacco users to quit.