Biological Models of Addiction

Biological models of addiction explain addictive behaviour in terms of brain chemistry, neural pathways, and genetic predisposition. These models are essential for understanding why addiction is so difficult to overcome and why pharmacological treatments can be effective. The AQA specification focuses on the role of the brain's reward system, the effects of specific substances on dopamine pathways, genetic vulnerability, and the processes of tolerance and withdrawal.

Key Definition: The mesolimbic dopamine pathway is the brain's reward circuit, running from the ventral tegmental area (VTA) through the nucleus accumbens to the prefrontal cortex. It is activated by pleasurable experiences and is hijacked by addictive substances and behaviours.

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The Brain's Reward System

All pleasurable experiences — eating, social interaction, sex — activate the mesolimbic dopamine pathway:

1. The ventral tegmental area (VTA) in the midbrain produces dopamine
2. Dopamine is released into the nucleus accumbens (the brain's "pleasure centre")
3. The prefrontal cortex evaluates the experience and forms memories and associations

This system evolved to reinforce survival behaviours — activities that promote survival and reproduction are experienced as pleasurable, making them more likely to be repeated.

How Drugs Hijack the Reward System

Addictive substances produce a dopamine surge in the nucleus accumbens that is far greater than natural rewards:

Stimulus	Approximate Dopamine Increase
Food	50% above baseline
Sex	100% above baseline
Nicotine	150–200% above baseline
Alcohol	40–360% (dose-dependent)
Cocaine	350% above baseline
Amphetamine	1,000% above baseline

This massive dopamine surge creates an intense feeling of pleasure (euphoria) that the brain "remembers" and seeks to repeat, driving the cycle of addiction.

graph LR
    A[Substance Taken] --> B[VTA Activated]
    B --> C[Dopamine Released into Nucleus Accumbens]
    C --> D[Intense Pleasure / Euphoria]
    D --> E[Brain Records Experience as Rewarding]
    E --> F[Craving and Seeking Behaviour]
    F --> A

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Specific Substances and the Dopamine System

Nicotine

Nicotine stimulates nicotinic acetylcholine receptors on dopamine neurons in the VTA, causing dopamine release into the nucleus accumbens. This produces feelings of relaxation, alertness, and mild euphoria. Nicotine is rapidly absorbed (reaching the brain within 10–20 seconds of inhalation) and is quickly metabolised, leading to frequent dosing (regular cigarettes throughout the day).

Alcohol

Alcohol has complex effects on multiple neurotransmitter systems:

• Enhances GABA (the brain's main inhibitory neurotransmitter) — producing relaxation, reduced anxiety, and sedation
• Inhibits glutamate (the main excitatory neurotransmitter) — further reducing neural activity
• Increases dopamine release in the nucleus accumbens — producing pleasure

These combined effects explain why alcohol produces both anxiolytic (anxiety-reducing) and euphoric effects, making it highly reinforcing.

Gambling

Although gambling does not involve ingesting a substance, it activates the same dopamine reward pathways. Research has shown:

• Intermittent reinforcement (unpredictable wins) produces strong dopamine responses
• Near misses (almost winning) activate the reward system almost as strongly as actual wins (Clark et al., 2009)
• The anticipation of a potential win produces dopamine release even before the outcome is known

Key Definition: Intermittent reinforcement is a partial reinforcement schedule in which rewards are delivered unpredictably. This produces the most resistant-to-extinction patterns of behaviour and is central to gambling addiction.

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Genetic Vulnerability

Twin Studies — Agrawal and Lynskey (2008)

Agrawal and Lynskey (2008) reviewed evidence from twin studies and concluded that the heritability of addiction is approximately 50–60% across different substances. This means that genetic factors account for roughly half of the variation in addiction risk, with the remainder attributable to environmental factors.

Key findings from twin research:

• MZ (identical) twins show higher concordance rates for addiction than DZ (fraternal) twins
• Heritability estimates vary by substance: alcohol (~50%), nicotine (~60%), cannabis (~55%)
• No single "addiction gene" has been identified — addiction is polygenic (influenced by many genes)

The DRD2 Gene — Noble et al. (1991)