Nicotion Addiction & Brain Chemistry (AQA A-Level Psychology): Revision Notes
Nicotine Addiction & Brain Chemistry
Neurochemistry refers to chemicals in the brain that regulate psychological functioning, whilst dopamine is a neurotransmitter with excitatory effects that creates sensations of pleasure. When dopamine levels are unusually high, this links to schizophrenia, and when unusually low, to Parkinson's disease.
Nicotine addiction affects approximately one-third of the global adult population, making it one of the most widespread and dangerous addictions. Understanding the neurochemistry behind nicotine addiction reveals how the neurotransmitter dopamine plays a central role in the development and maintenance of this dependency.
The brain's reward system operates through complex neurochemical pathways that can be hijacked by addictive substances. Nicotine's ability to manipulate these natural pleasure and reward mechanisms makes it particularly addictive compared to many other substances.
The desensitisation hypothesis
John Dani and Steve Heinemann (1996) proposed the desensitisation hypothesis, which examines how brain chemistry changes when nicotine enters the system. This theory focuses specifically on neurons that produce dopamine and their interaction with acetylcholine receptors.
Acetylcholine receptors and nicotine binding
Like many neurons in the central nervous system, dopamine-producing neurons have receptors on their surfaces that bind with acetylcholine (ACh). When ACh molecules bind with these receptors, electrical impulses can travel from one neuron (presynaptic) to another (postsynaptic), enabling neurotransmission.
A specific subtype called the nicotinic acetylcholine receptor (nAChR) can bind with both ACh and nicotine. When nicotine molecules attach to nAChRs, the neuron becomes stimulated and dopamine transmission occurs. However, within milliseconds of this binding, the nicotinic receptor shuts down temporarily and becomes unresponsive to neurotransmitters.
This process is called desensitisation and leads to downregulation - a reduction in the number of active neurons available. This rapid shutdown mechanism is crucial to understanding how nicotine tolerance develops so quickly in users.
Dopamine transmission pathways
The nAChRs concentrate heavily in the ventral tegmental area (VTA) of the brain. When nicotine stimulates nAChRs in the VTA, dopamine travels along two key pathways:
- Mesolimbic pathway: dopamine transmits from the VTA to the nucleus accumbens (NA), then releases dopamine into the frontal cortex
- Mesocortical pathway: dopamine transmits directly to the frontal cortex
These pathways form part of the brain's reward and pleasure centre. The activation creates pleasurable effects similar to those seen in addictions to cocaine, heroin and amphetamines, including mild euphoria, increased alertness and reduced anxiety.
The nicotine regulation model
This model explains how nicotine addiction develops and maintains itself through cycles of receptor sensitivity changes.
Withdrawal and receptor availability
When individuals smoke regularly, nicotinic receptors remain desensitised and withdrawal symptoms stay suppressed. However, when smokers go without nicotine for extended periods (such as overnight sleep), nicotine disappears from the body. This allows nicotinic receptors to become functional again, causing more dopamine neurons to become available through upregulation.
During withdrawal, more nAChRs become available and overstimulated by acetylcholine, contributing to acute withdrawal symptoms including anxiety and agitation. At this point, nAChRs reach their most sensitive state, explaining why smokers often describe the first cigarette of the day as the most enjoyable - it reactivates the dopamine reward system.
Dependence and tolerance
The model explains how nicotine dependence develops through motivation to avoid unpleasant physiological and psychological withdrawal states by smoking another cigarette. The repeated cycle of daytime downregulation and night-time upregulation creates chronic desensitisation of nAChRs.
Continuous exposure of nAChRs to nicotine causes permanent changes to brain neurochemistry, decreasing the number of active receptors. Tolerance develops as smokers need to smoke more cigarettes to achieve the same pleasurable effects.
Supporting research evidence
Research provides both indirect and direct support for the role of neurochemistry in nicotine addiction.
Research Study: McEvoy et al. (1995)
Joseph McEvoy et al. studied smoking behaviour in people with schizophrenia. Haloperidol is a dopamine antagonist that blocks dopamine receptors in the brain and is used to treat schizophrenia.
Findings: The study found that haloperidol treatment increased smoking in participants, suggesting this was a form of self-medication - an attempt to achieve the nicotine effect by increasing dopamine release.
Significance: This provides indirect evidence that dopamine pathways are central to nicotine addiction mechanisms.
Brain imaging studies (Ray et al., 2008) provide more direct evidence for the importance of the dopamine reward system in the mesolimbic pathway, showing actual brain activity during nicotine use.
Real-world applications
Understanding neurochemistry has led to practical developments in treating nicotine addiction. Nicotine replacement therapy (NRT) in the form of patches and inhalers directly applies this knowledge. Research is exploring the possibility of nicotine immunisation.
The practical benefits extend beyond nicotine addiction itself. Several mental health conditions show high co-morbidity with nicotine use - they occur together frequently. Examples include schizophrenia, depression and alcoholism, all strongly associated with continued smoking. More effective treatments for nicotine addiction could lead to advances in treating these co-occurring disorders.
Evaluation
Limited explanation
Explanations focusing solely on dopamine's role are limited because research increasingly shows many other neurochemical mechanisms are involved. The current understanding reveals complex interactions between several neurochemical systems, including neurotransmitter pathways such as GABA and serotonin (5-HT), plus endogenous opioids (endorphins - the brain's natural painkillers).
However, Fernando Berrendero et al. (2010) emphasise that the dopamine system remains central to nicotine addiction neurochemistry, with other systems interacting with it to produce their effects.
Reductionist approach
Neurochemical explanations are reductionist accounts that explain addiction at the most basic level of neurotransmitter molecule activity, rather than considering social and psychological influences. Only about 50% of people who experiment with cigarette smoking become dependent on nicotine.
Research Study: Choi et al. (2003)
Won Choi et al. found that adolescents most likely to become dependent were not committed to abstaining, had friends who smoked, and perceived themselves as underachieving at school.
Significance: These are all psychological factors rather than neurochemical ones, highlighting the limitations of purely biological explanations.
The reductionist approach risks oversimplifying complex behaviour by focusing on biological processes and structures whilst failing to consider social context or psychological meaning. This may explain how addiction occurs but not why it develops.
However, reductionist theories allow scientific testing by measuring the effect of one variable on another, potentially uncovering causal explanations.
Individual differences
Research Study: Shiffman et al. (1995)
Saul Shiffman et al. studied 'chippers' - people who regularly smoke for decades without becoming dependent on nicotine.
Findings: Those smoking an average of five cigarettes daily showed no withdrawal syndrome signs when they abstained and performed better on cognitive tasks than dependent smokers.
Implications: This suggests non-chemical factors protect some smokers from addiction, though these protective factors remain unclear.
One possibility is that some people smoke through learned modelling behaviour, meaning their motivation has nothing to do with nicotine itself. Neurochemical theories struggle to explain these individual differences, representing a limitation of such approaches.
Key Points to Remember:
- The desensitisation hypothesis explains how nicotine binds to acetylcholine receptors, causing dopamine release followed by receptor shutdown and downregulation
- The mesolimbic and mesocortical pathways carry dopamine from the VTA to the nucleus accumbens and frontal cortex, creating rewarding effects
- Withdrawal and tolerance develop through cycles of receptor desensitisation and upregulation, leading to chronic brain chemistry changes
- Research support comes from studies with schizophrenia patients and brain imaging showing dopamine pathway activation
- Reductionist explanations are limited as they focus only on brain chemistry while ignoring social and psychological factors that influence individual differences in addiction development