Nicotine is introduced into the body by smoking cigarettes, smoking cigars, using pipe tobacco, dipping snuff and chewing tobacco. Nicotine is readily absorbed from tobacco smoke in the lungs, and it does not matter whether the tobacco smoke is from cigarettes, cigars, or pipes. Nicotine also is absorbed readily when tobacco is chewed. With regular use of tobacco, levels of nicotine accumulate in the body during the day and persist overnight. Thus, daily smokers or chewers are exposed to the effects of nicotine 24 hours each day. Adolescents who chew tobacco are more likely than nonusers to eventually become cigarette smokers.
Nicotine acts on the nicotine acetylcholine receptors. In small concentrations it increases the activity of those receptors, among other things leading to an increased flow of adrenaline, a stimulating hormone. The release of adrenaline causes an increase in heart rate, blood pressure and respiration, as well as higher glucose levels in the blood. Cotinine is a break-down product of nicotine which remains in the blood for up to 48 hours and can be used as an indicator of a person’s exposure to smoke. In high doses, nicotine will cause a blocking of the nicotinic acetylcholine receptor, which is the reason for its toxicity and the need to sustain high dopamine levels.
In addition, nicotine increases dopamine levels in the reward circuits of the brain. Studies have shown that smoking tobacco inhibits monoamine oxidase (MAO), an enzyme responsible for breaking down monoaminergic neurotransmitters such as dopamine, in the brain. It is currently believed that nicotine by itself does not prohibit the production of monoamine oxidase, but that other ingredients in inhaled tobacco smoke are believed to be responsible for this activity. In this way, it generates feelings of pleasure, similar to those caused by cocaine and heroin, thus causing the addiction associated with the need to sustain high dopamine.
Several avenues of research now indicate that men and women differ in their smoking behavior. For instance, women smoke fewer cigarettes per day, tend to use cigarettes with lower nicotine content, and do not inhale as deeply as men. It is unclear whether this is due to differences in sensitivity to nicotine or other factors that affect women differently, such as social factors or the sensory aspects of smoking.
