Reward System

The reward system is a complex network of pathways in your brain, but it doesn’t just govern reward. It’s also involved with mediating pleasure, learning and memory. This is why addiction is strongly related to dysfunction in the reward system—it’s the site of pleasure and feedback and where unhealthy habits are reinforced.

The reward system consists of several parts of the brain, and new areas, such as the endocannabinoid system, are constantly being highlighted as potential parts of the reward system. However, when discussing the reward system and addiction, we usually talk about the mesolimbic pathway, which connects the ventral tegmental area, a part of the brain that deals with emotional processing, to the forebrain, where higher processing is done.

What is the reward system?

The central components of the brain’s reward system are areas that respond to pleasurable activities connected by pathways and neurotransmitters that carry messages along these pathways. These neurotransmitters are:

  • Dopamine: The reward system’s primary reward transmitter and a key component of pleasure and reinforcing behaviours
  • Norepinephrine: This is involved in arousal and alertness and plays a role in modulating reward-related behaviours. Stress triggers the release of norepinephrine, and stress can enhance the perceived value of a rewarding behaviour – you’re stressed, so drinking is more appealing. One theory of addiction is that it’s a reward deficit and stress surfeit disorder – your brain feels too little pleasure and too much stress, and you use substances to counterbalance this, which becomes self-reinforcing. The neurotransmitters that govern this process are dopamine and norepinephrine.
  • Serotonin: This neurotransmitter does many things, but its role in modulating mood and impulsivity and its ability to counteract the effects of dopamine are most relevant in terms of addiction.

Areas of the brain

The areas of the brain that the reward system encompasses are:

  • The Medial-Forebrain Bundle: This is a collection of fibres that run through the hypothalamus and cause intense feelings of pleasure when stimulated. A classic study revealed this, in which rats learned to press a lever to stimulate electrodes implanted in this area of their brains.
  • The Nucleus Accumbens: Often called the brain’s ‘pleasure centre,’ this area receives dopamine signals and plays a major role in reward, reinforcement and learning
  • The Ventral-Tegmental Area: This is the main source of neurons that create dopamine in the brain.
  • The prefrontal cortex: This is associated with decision-making, impulse control, and the modulation of behaviour in response to rewards.
  • The Hippocampus: This area will create memories associated with cues from rewards.
  • The Amygdala: This area is involved with the emotional aspects of rewards.

The dopamine reward system and addiction

Dopamine is often thought of as simply the reward chemical, something that ‘spikes’ when we get a reward and doesn’t when we don’t. But it’s more complicated than that.

Dopamine involves memory, movement, cognition, attention, sleep, mood, learning, and even lactation. Parkinson’s disease is caused by dopamine-producing neurons in your brain dying – and when they do, your ability to think, sleep and control your movement are all impaired.

We all have a baseline level of dopamine that differs from person to person. A pleasurable activity causes this baseline to rise – and a really pleasurable activity causes it to rise even further. Having a low dopamine baseline is genetically determined, and having a low baseline dopamine level is theorised to influence susceptibility to ADHD and addiction.

Drugs and alcohol cause dopamine to rise in the brain more than other activities. For example, chocolate increases baseline dopamine by 50%. Sex increases it by 100%. Alcohol increases it by 150%. Methamphetamine causes it to rise by 1000%. Normal activities can’t compete with these precipitous rises in baseline dopamine, and over time your brain will come to regard smaller pleasures as not ‘enough. These huge rises in baseline dopamine are also associated with larger crashes, causing mood disturbances, withdrawal, and a compulsion to escape the crash and chase the high.

The levels fall again after the rise in dopamine caused by a pleasurable activity. However, they don’t simply return to the baseline – they actually fall below it. This mechanism explains compulsive swiping on social media, the urge to reach for a piece of unhealthy food as soon as we’ve finished the first one, and the urge to take a substance again as the effects begin to diminish. This mechanism of dopamine rising then falling below baseline is one of the neurochemical basis of addiction.

The brain reward system and addiction – The link

A key component of the brain’s reward system is responding to both ‘natural’ stimuli, like food and social interactions, and ‘unnatural’ stimuli, like drugs and alcohol. It can’t differentiate between the two. In addition, drugs and alcohol offer a very high reward for little effort, hijacking the reward system.

Rising dopamine in the brain’s reward circuits is powerfully reinforcing, and if you repeat the behaviour, this reinforcement alters your brain. Over time, your dopamine receptors get less sensitive to these big rises in a process called downregulation. Now, normal activities don’t feel as pleasurable, and you need to take more to feel high.

When you try to stop, your amygdala activates, flooding you with anxiety and negative emotions. Over time, this part of the process gets more sensitive while your down regulates dopamine receptors are less sensitive. At this point, you’re taking drugs to stave off withdrawal, not just for the high, and addiction has hijacked your reward system.

How the reward system affects addiction

Susceptibility to addiction is about 50% genetic. A lot of this susceptibility comes from your biology – factors such as gene expression, which governs how your brain is structured. Having a reward system with certain structural quirks can mean you are vulnerable to developing an addiction.

This does not mean that you’re doomed to always struggle with addiction. Your genes influence the risk of addiction; they don’t guarantee it. The other 50% of your risk is environmental factors, and much more is in your control.

We don’t yet know all of the genetic factors that orient your reward system towards susceptibility to addiction, but there are some we do know.

Having fewer dopamine receptors, specifically D2 dopamine receptors. This leads to reduced dopamine signalling – you can’t feel the specifically D2 dopamine receptors. This leads to reduced dopamine signalling – you can’t feel the dopamine in your brain, and you are not like someone born with more receptors. Dopamine receptor density is genetically determined – if you’re born with less of them, you won’t be able to feel pleasure or reward as intensely. 

This leaves you susceptible to addiction, as you will find it harder to derive pleasure from smaller things and seek out bigger, more intense sensations. 

Other neurotransmitters, like norepinephrine and serotonin, are affected by gene expression too. Depending on your genes, mood regulation, stress response, and reward processing can work a little differently in ways that leave you more vulnerable to addiction.

The connectivity between different parts of your reward system also varies between people. Gene variations that affect the connectivity and functioning of these different parts have a role in addiction risk.

 

Understanding the intricate relationship between the brain’s reward system and addiction provides valuable insights into both the mechanisms of addictive behaviours and potential therapeutic approaches. The reward system, primarily involving the mesolimbic dopamine pathway, is crucial for reinforcing behaviours essential for survival. However, substances and activities that hijack this system can lead to addiction, characterised by compulsive behaviour and an inability to abstain despite adverse consequences.

Research has shown that addiction alters brain structure and function, diminishing self-control and increasing craving and dependency. These changes underscore the importance of viewing addiction as a chronic, relapsing brain disorder rather than a mere failure of willpower. Effective treatment strategies often combine behavioural therapies, medication, and support systems to address the multifaceted nature of addiction.

Fostering a deeper understanding of how the reward system operates and how addictive substances and behaviours can manipulate it can help us develop more effective prevention, intervention, and treatment strategies. This knowledge also highlights the need for ongoing research to continually improve our approaches to mitigating the impact of addiction on individuals and society.

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