17.3.1 Nicotine Addiction & Brain Neurochemistr

• The desensitisation hypothesis of nicotine addiction suggests that prolonged exposure to nicotine leads to reduced sensitivity of nicotinic acetylcholine receptors
• The desensitisation hypothesis is based on the action of nicotine in the brain, and its effect on the neurotransmitter dopamine
• During smoking, the nicotine in tobacco enters the body and stimulates the nicotinic acetylcholine receptors in the brain and this increases the production of acetylcholine
• The activation of the nicotinic acetylcholine receptors also leads to the production of dopamine in the ventral tegmental area (VTA) of the brain which is part of the dopaminergic pathway - also known as the main reward pathway - transporting dopamine within the limbic system
• This dopamine is transported to the Nucleus accumbens , generating pleasure, reducing anxiety and increasing alertness by triggering activity in the frontal cortex
• The increased dopamine in the limbic system leads to pleasurable feelings
• Nicotine also stimulates the release of glutamate which increases and speeds up the release of dopamine, thus increasing the rewarding effects
• However, the nicotinic acetylcholine receptors become desensitised after constant stimulation, which lowers the signal in the main rewards pathway and leads to more nicotine being needed to achieve the same effect
• The more nicotine that is used, the more tolerance increases, which leads to even more nicotine being needed and addiction is the result 

The brain regions involved in nicotine addiction.

• If nicotine is not used for some time, then it is metabolised by the body and excreted by the kidneys
• This metabolisation and excretion of the nicotine leads the nicotinic acetylcholine receptors to become sensitised again
• This sensitisation produces withdrawal symptoms of agitation and anxiety
• The withdrawal symptoms are worse if nicotine use has been prolonged, as long-term use of nicotine increases the number of nicotinic acetylcholine receptors, increasing the sensitisation and the subsequent agitation and anxiety

Nicotine withdrawal symptoms make it hard to quit smoking.

Research which investigates nicotine addiction and brain neurochemistry

• Dani & Basi (2001) found that part of the reason nicotine is so addictive is that it acts upon the dopaminergic systems, which helps reinforce rewarding behaviour, showing a direct relationship between the neural mechanisms in the brain and the rewarding effects of smoking
• Domino (2004) used fMRI scans to track the blood flow in the nucleus accumbens, amygdala, and hippocampus immediately after smoking the first cigarette in the morning and found that the first cigarette had the largest effect, supporting the desensitisation hypothesis
• Cosgrove et al. (2014) used PET scans to compare the brains of men and women while smoking and found that the dopamine effect took place in different regions of the brain, suggesting that men and women might smoke for different reasons. 

Evaluation of nicotine addiction and brain neurochemistry

Strengths

• The role of dopamine could explain how there appears to be a genetic link to addiction, as the release of dopamine may lead some individuals to feel more reward from using nicotine than others, and this difference in the dopaminergic pathway may be inherited
• The link between the use of nicotine and dopamine is also supported by research showing that smokers are less likely to develop Parkinson’s disease (which is due to the loss of dopamine-producing cells) than non-smokers (Chen, 2010)

Weaknesses

• The research evidence in this area often comes from animal experiments, meaning the findings may not be generalisable to humans
• Research into nicotine addiction and brain neurochemistry cannot explain individual differences in reasons for smoking and cannot explain why some people become addicted very quickly and others do not

Link to Issues & Debates: 

Theories of addiction and risk factors seem to be seen as universal. This could be an example of beta bias. For example, Cosgrove et al (above) found that the dopamine effect took place in different regions of the brain for men and women and therefore we cannot assume men and women become addicted in the same way.

The neurochemical explanation for addiction is purely biological and, as a consequence, the psychological aspects of addiction are largely ignored. This makes the dopamine explanation biologically reductionist.