Dopamine hypothesis

• Explanation of tolerance:


  Dopamine receptors (D2) may become less sensitive to dopamine over time, resulting in a less rewarding feeling (tolerance). Person requires higher levels of drug/addictive behaviour to stimulate receptors to keep same level of pleasure & same sensations.
• Olds & Milner (1954) :

  Placed electrode in specific region of rat's brain & stimulated it every time rat went into certain corner of box. Rats kept returning to same corner, so region of brain known as 'pleasure centre'. Further studies: rats will repeatedly press a lever to experience stimulation in this region, even forgoing other behaviours like eating.
• What is the Mesolimbic Pathway?


  Addictive behaviours & substances trigger release of dopamine in ventral tegmental area of brain, leading to sense of pleasure in the nucleus accumbens (NAc). Reward pathway has evolved in humans to be the usual adaptive response to behaviours good for us, e.g, eating. Activated when engaging in rewarding behaviours. In case of addictive behaviours, system responding to more harmful actions & becomes maladaptive. VTA->NAc->PFC
• What is dopamine?


  Neurotransmitter associated with rewards, risk-taking & goal-driven behaviour.
• Explain withdrawal symptoms:


  Actual number of D2 receptors decrease over time so less dopamine over time. If addict stops, experience unpleasant withdrawal symptoms, so addiction becomes more about avoiding these than experiencing initial pleasure (Volkow et al, 1997).
• Explanation of relapse (Volkow, 1992) :


  Changes in frontal cortex turn engaging in behaviour into addiction. Frontal cortex linked to higher cognitive functions like memory & decision-making. Addicts have altered brain circuitry that controls attention- salience to behaviour & cues associated.
• Robinson & Berridge (2003) :

  Dopamine system makes individual want to engage in behaviour rather than simply making them like it. Why many addicts no longer enjoy experience but can't overcome cravings associated with it.
• What does the frontal cortex control?

  Decision-making, behavioural/impulse control, memory, attention & other higher cognitive functions.
• Wang et al (1999) :

  Addicts show increase in activity in frontal cortex when exposed again to the drugs or cues associated with them.
• Volkow et al (1992) :

  Cocaine addicts show abnormalities in frontal cortex & impaired performance in tasks that would usually use frontal cortex, such as decision-making. (Boila et al, 2003).
• Boileau et al (2003) :

  Alcohol increases dopamine levels in brain.
• Joutsa et al (2012) :

  Dopamine levels increased during gambling task, & these levels were raised regardless of whether the outcome was a win or not.
• Yoder et al (2007) :

  Found no consistent increase in dopamine across participants who were given alcohol.
• Stokes et al (2009) :

  Found no significant increase in dopamine levels in volunteers on cannabis.
• Use of non-human animal studies with dopamine:

  Studies on dopamine & frontal cortex involve animals. Can study effects of drugs in ways not possible with humans (e.g, depleting dopamine neurons to see effect, or making rats self-stimulate with drugs). May not give true insight on how humans become addicted. Misses social context & can't replicate gambling, etc. May not generalise to humans or all addiction types.
• Nutt et al (2015) :


  Criticised methodology used by research in dopamine. Samples small & when participants given substance to measure effects, often given it in different form (e.g, nicotine inhalators rather than in a cig) & in lab- measured effects may be inaccurate.
• Bell (2013) :


  Dopamine has range of other functions- fashionable to blame dopamine for causing range of problems when brain systems involved are much more complicated than media presents.
• Liberzon et al (1999) :


  Dopamine levels increase in presence of aversive stimuli as well. Brain systems not only motivate us to seek pleasure but also to avoid unpleasant things. Increased activation in NAc in war veterans (PTSD) when reminded of battle through combat sounds.
• Neuroadaptation:


  Brain seeks balance. Drugs disturb balance, e.g, lots of dopamine released. Brain adapts to the effects to keep the balance.
• What is Varenicline?


  Drug prescribed to help those trying to quit smoking-works by affecting dopamine release in brain. Binds to nicotine receptors, tricking brain into thinking it's had nicotine & leading to a small release of dopamine, which alleviates craving & withdrawal symptoms. Drug reduces rewarding effects of nicotine by blocking ability of nicotine to access the receptors & activate dopamine release. If someone smokes cig, they'll get less pleasure from it.