Neuroanatomy, neurobiology, and pharmacology
Substance dependence is a disorder of altered brain function brought on by the use of psychoactive substances. These substances affect normal perceptual, emotional and motivational processes in the brain. However, as with any disorder specific to an organ or system, one must first understand the normal function of that organ or system to understand its dysfunction. Because the output of the brain is behaviour and thoughts, disorders of the brain can result in highly complex behavioural symptoms. The brain can suffer many types of diseases and traumas, from neurological conditions such as stroke and epilepsy, to neurodegenerative diseases such as Parkinson disease and Alzheimer disease, and infectious or traumatic brain injuries. In each of these cases, the behavioural output is recognized as being part of the disorder.
Similarly, with dependence, the behavioural output is complex, but is mostly related to the short-term or long-term effects of substances on the brain. The tremors of Parkinson disease, the seizures of epilepsy, even the melancholy of depression are widely recognized and accepted as symptoms of underlying brain pathology.
Substance dependence has not previously been recognized as a disorder of the brain, in the same way that psychiatric and mental illnesses were previously not viewed as such. However, with recent advances in neuroscience, it is clear that substance dependence is as much a disorder of the brain as any other neurological or psychiatric illness. New technologies and research provide a means to visualize and measure changes in brain function from the molecular and cellular levels, to changes in complex cognitive processes that occur with short-term and long-term substance use.
Major advances in neuroscience research on substance dependence have come from the development and use of techniques that allow the visualization of brain function and structure in the living human brain, known as neuroimaging techniques. Using these techniques, researchers can see what happens from the level of receptors to global changes in metabolism and blood flow in various brain regions. Images can be observed when substances are administered, to see where they act in the brain, and also following long-term substance use to observe the effects on normal brain functions. One example of an imaging technique is magnetic resonance imaging (MRI), which uses magnetic fields and radio waves to produce high-quality two- or three-dimensional images of brain structures (10-12). The brain can be imaged with a high degree of detail. Although MRI gives only static pictures of brain anatomy, functional MRI (fMRI) can provide functional information about brain activity by comparing oxygenated and deoxygenated blood.
Another important and useful imaging technique is positron emission tomography (PET) (10-12). PET scans provide information about the metabolic activity in a certain brain region. Most commonly, a person is injected with a radioactive compound that can be followed through the bloodstream in the brain. This can be visualized as two- or three-dimensional images, with different colours on a PET scan indicating different levels of radioactivity (blues and greens indicating areas of lower activity, and yellows and reds indicating areas of higher activity). Using different compounds, PET scans can be used to show blood flow, oxygen and glucose metabolism, and drug concentrations in the tissues of the living brain.