6.3 Psychoactive effects of nicotine
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As well as the range of physical effects outlined in the previous section, nicotine also has acute effects on mental state.
Nicotine has biphasic effects of both stimulation and blockade, meaning that its use can provide perceptions of relaxation or alertness. The mental and physical state of the smoker, and the situation in which smoking occurs, can influence the way in which a particular cigarette will affect psychological perceptions and physiological response.1 Tobacco provides the average daily smoker with a convenient and socially acceptable way of self-dosing a psychoactive drug more than 100 times a day18(about 140 puffs per day for the 14-a-day smoker).19[1]
For nicotine to provide a strong psychoactive reaction, it must be delivered rapidly to the brain. The inhalation of tobacco smoke into the lungs is a highly optimised method of nicotine delivery to the brain via the blood stream. On reaching the brain it immediately acts upon neuronal nicotinic receptors located throughout. There, in common with all addictive drugs, it causes a complex range of biochemical changes that together create dependency, reinforced by withdrawal.2
6.3.1The neurobiology of the positive reinforcing properties of nicotine
Pleasant or rewarding effects, which have only a partially understood neurobiology, act positively to reinforce usage. Knowledge of the neurobiology of abstinence symptoms is also understood to only a limited extent, particularly the biological basis of the abstinence syndrome which facilitates continued drug use in order to avoid onset of withdrawal symptoms.2
Like several other addictive drugs, nicotine activates dopamine neurotransmission in the mesocorticolimbic dopamine system, which is an integral part of the brain reward system. Acute and repeated nicotine exposure stimulates the release of dopamine in the nucleus accumbens, a part of the brain that is understood to play an important role in addiction. This region of the brain is also recognised to be involved in the regulation of emotions induced by music, as well as the processing of rewards such as food and sex. It also has a role in the actions of other drugs of abuse, such as amphetamines and cocaine. Nicotine exerts these effects by activating specific sites called receptor proteins, which trigger the release of dopamine in the nucleus accumbens and the secretion of other nerve-stimulating chemicals such as acetycholine and glutamate in the hippocampus and cerebral cortex, which act to improve vigilance, attention and cognition, benefits sometimes attributed to smoking by smokers. Evidence does suggest low doses of nicotine may improve memory, information processing and attentiveness, although the overall increase is small and can be compared to the effects of drinking coffee or other drinks containing caffeine.2
Repeated administration of nicotine greatly enhances the release of dopamine in a specific region of the nucleus accumbens called the accumbal core. There is evidence that the enhancement of dopamine release is central to the development of a Pavlovian or classical conditioned response to nicotine driven by cues present both in the environment and in the tobacco smoke that is inhaled by the smoker. The development of a response to cues strongly linked to a drug is considered pivotal to the transition to addiction in which the addict can no longer control cravings for the drug.2
According to smokers, nicotine use helps them when they are depressed, stressed, embarrassed, bored, irritable, or in a bad mood. Alleviating these unpleasant feelings with a cigarette helps reinforce the psychological aspects of tobacco addiction. However research has not found any intrinsic reason why tobacco use should serve this purpose, other than that smoking a cigarette satisfies the physiological and psychological discomfort of withdrawal.2 For more information on tobacco withdrawal, see Section 6.9.
[1] According to the National Drug Strategy Household Survey 2004 (Detailed Findings, Table 4.6), average weekly consumption for current smokers aged 14 years and over is 99 cigarettes per week, or approximately 14 cigarettes per day. Assuming that a smoker takes about 10 puffs (or 'doses' per cigarette), then the average smoker self-doses about 140 times per day.
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