Most psychiatric disorders are classified as complex, meaning that they cannot be easily explained by a single genetic or environmental component. One of the most hindering of these disorders is schizophrenia, which affects about one percent of the entire world population. Once the symptoms of schizophrenia occur, usually beginning in early adult hood, they continue for the entire lifetime of the patient and are almost totally disabling (Sawa & Snyder, 2002).
Schizophrenia is a severe psychopathology which is categorized by positive and negative symptoms (Breedlove, Watson & Rosenzweig, 2010). This disorder distorts thoughts and perceptions. It is described as a loss of contact ...view middle of the document...
Dopaminergic pathways begin in the midbrain, a small central part of the brain stem, in regions such as the ventral tegmental area (VTA) and the substantia nigra, and travel to different parts of the brain. A shortage of dopamine in the cortex is thought to result in the stimulation of D1 receptors, the largest dopamine receptor in this area, and as a result, negative and cognitive symptoms arise. Positive symptoms, on the other hand, arise from an excess of dopamine subcortically that causes the stimulation of D2 receptors (Abi-Dargham, 2004).
The dopamine hypothesis of schizophrenia began in the 1960's through the increase of antipsychotic drugs such as chlorpromazine and other amphetamines that were believed to be able to induce psychosis. It was found that the effect of these drugs was due to their ability to block dopamine receptors, which are responsible for motivation, pleasure, cognition, memory, learning, and fine motor control (Birtwistle & Baldwin, 1998). The dopamine hypothesis of schizophrenia suggests that an excess of dopamine in the subcortex is linked with the positive symptoms of schizophrenia. It was observed that a substantial exposure to D2 receptor agonist brought upon positive symptoms of schizophrenia (Abi-Dargham, 2004).
Patients who demonstrate positive symptoms of schizophrenia tend to have had a minor onset of the disorder. When their symptoms are decreasing, their social functioning tends to be relatively good. Rather than searching for brain abnormalities in these patients, it has been suggested that neurochemical abnormalities are the prime cause of these positive symptoms. It has been shown in previous studies that these patients respond well to the antipsychotic drugs which block dopamine receptors while their symptoms are aggravated by drugs that enhance dopamine function (Philippot, Coats & Feldman, 2003).
Chlorpromazine was the first drug that was constructed with specific antipsychotic action, and was successfully tested on schizophrenia patients in 1952, with results emerging. It was widely used for the treatment of schizophrenia and thus, many other antipsychotic drugs evolved from this. Positive and negative symptoms of schizophrenia respond differently when treated with antipsychotic drugs. The positive symptoms of schizophrenia respond very well when treated with these drugs. All of the antipsychotic drugs that have been developed function the same way. The drugs are designed to antagonize the effects of dopamine at the D2 receptor in the brain (Iversen, et al., 2009).
Although dopamine levels are unable to be measured in humans, there has been a development of techniques which provide indirect indices of dopamine synthesis and release and synaptic dopamine levels (Howes & Kapur, 2009). By using imaging methods such as positron emission tomography (PET), it is possible to measure the ability that antipsychotic drugs have to transfer a D2 receptor in the human brain with the...