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Why are drugs addictive?

The brain is made up of multiple neuronal connections.The nucleus accumbens is the part of the brain most associated with pleasant feelings like calmness, satiety, happiness and pleasure. Reinforcing connections in the nucleus accumbens can strengthen certain reward pathways, creating an addiction.

Drugs that are addictive often interfere with neurotransmitters, which are chemicals compounds produced by a neurone, and are responsible for transmitting the signal from one neurone to the next. Normally neurotransmitters remain in the synaptic cleft, the gap between two neurones, for only a short amount of time before they are taken up by the neurones and recycled. However, addiction-causing drugs will interfere with the re-uptake process, leading to continued activation of the reward pathway.

Over stimulation of the neuronal pathways in the nucleus accumbens is what leads to the initial feeling of euphoria after taking a drug. Taking the drug over long periods reinforces the reward pathways by overstimulating the neurones. After a while, the brain adapts, and becomes less and less sensitive to the reinforcer (drug). Higher doses of drug are then required to achieve the same euphoric effect, and an addiction cycle starts, that is often hard to break.

Brain images showing decreased dopamine2 receptors in the brain of a person addicted to cocaine versus a nondrug user. The dopamine system is important for conditioning and motivation, and alterations such as this are likely responsible, in part, for the diminished sensitivity to natural rewards that develops with addiction.Brain images showing decreased dopamine (D2) receptors in the brain of a person addicted to cocaine versus a nondrug user. The dopamine system is important for conditioning and motivation, and alterations such as this are likely responsible, in part, for the diminished sensitivity to natural rewards that develops with addiction. Source (image and caption): http://www.drugabuse.gov/publications/research-reports/cocaine-abuse-addiction

 

Useful links:

The reward pathway and addiction, NIH website, January 2014

Understand Addiction, Harvard Health Publications, January 2014

Neuroanatomy and Physiology of the “Brain Reward System” in Substance Abuse, Institute for Behavioural Genetics, January 2014

 

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Imaging the brain with fMRI

fMRI is a technique for measuring brain activity, by looking at the flow of oxygenated blood to different areas of a brain during a particular activity.

fMRI uses powerful electromagnetic radiation to align the proton in the nucleus of a hydrogen atom in water, either with or against the magnetic field. When enough protons align in one direction, they produce a signal, which can be picked up by the machine.

The more active the neurones, the more oxygen they need. Oxygen is delivered to neurones via haemoglobin. Oxygenated haemolgobin is dimagnetic i.e. it can disturb the magnetic field of the neighbouring protons. Oxygenated haemoglobin produces a signal in fMRI but deoxygenated haemoglobin does not. Thus areas of the brain with high activity can be picked out easily with fMRI scans.

fRMI looking at differences in brain activity among professional music players and non-music players. Source: http://misc.karger.com/gazette/70/altenmueller/images/fig01.jpg

fMRI has gained popularity in recent years, because it is minimally invasive but still gives lots of useful information.
On the other hand, skeptics argue that it is hard to get any meaningful information from ‘blobs’. This is because when the signal strength is low, it can be hard to pick out a signal from the background noise. There is ongoing research focused on improving signal:noise ratios

Useful links:
Kerri Smith (2012) Brain Imaging: fMRI 2.0 At: http://www.nature.com/news/brain-imaging-fmri-2-0-1.10365

Hannah Devlin (2013) What is functional magnetic resonance imaging? At: http://psychcentral.com/lib/what-is-functional-magnetic-resonance-imaging-fmri/0001056