Medical Use and Behavioral Effects
Evidence indicates that people smoke primarily to experience the psychopharmacological properties of nicotine and that the majority of smokers eventually become dependent on nicotine if they start as adolescents. Numerous preclinical animal studies have demonstrated nicotine’s reinforcing properties in many species. In humans, nicotine produces positive reinforcing effects, including mild euphoria, increased energy, heightened arousal, reduced stress, reduced anxiety, and reduced appetite. Cigarette smokers report that smoking produces arousal, particularly with the first cigarette of the day, but also relaxation when under stress (see Behavioral Mechanism of Action).
Nicotine reduces pain in humans and raises pain thresholds. Nicotine nasal spray or transdermal nicotine at low doses has been shown to reduce postoperative pain or reduce postoperative opiate requirements (see also Behavioral Mechanism of Action below).
The positive reinforcing effects of acute nicotine administration through tobacco smoking are considered to be critically important in the initiation and maintenance of tobacco smoking that ultimately leads to dependence. Nevertheless, factors other than nicotine contribute to smoking in human smokers, including sensory and conditioned reinforcing effects. For example, cigarette smoke has monoamine oxidase-inhibiting properties, similar to monoamine oxidase inhibitor antidepressants, which might contribute to its psychotropic effects and addiction potential.
Nicotine also decreases appetite, particularly the desire for sweet-tasting food and carbohydrates in both rats and humans, and this has been linked to the motivation to continue smoking in women and their higher rates of relapse compared with men. This appetite suppression is accompanied by decreases in blood insulin and changes in serotonin function. Nicotine also increases metabolism and fat oxidation.
Nicotine is well known to improve attention, learning, reaction time, and problem solving abilities in abstinent smokers and is particularly effective in enhancing selective attention and vigilance when performing repetitive tasks, a classic stimulant effect. Nicotine has also been shown to enhance cognitive performance in non-smokers.
Nicotine has numerous physiological effects that are mainly attributed to its ability to activate ganglionic receptors in the autonomic nervous system, including the adrenal medulla. However, this activation is only short-lasting, followed by persistent depression of all autonomic ganglia, depending on the dose and history of nicotine intake. In humans, cigarette smoking increases heart rate and blood pressure. It also stimulates the gastrointestinal tract, followed by inhibition, indicating parasympathetic activation. It causes bronchial dilation and stimulates the salivary glands.
Nicotine in a nicotine-naive individual can cause vomiting through activation of an emetic chemoreceptor trigger zone in the area postrema of the medulla and activation of vagal and spinal nerves that form the sensory component of the vomiting reflex. Thus, nicotine has numerous acute activating effects, reinforcing effects, and anxiolytic effects in humans.