Schizophrenia: The Dopamine Hypothesis

Dopamine, a major excitatory neurotransmitter, may play a key role in schizophrenia. According to the dopamine hypothesis, the symptoms of schizophrenia - particularly positive symptoms (e.g. delusions and hallucinations), are produced by over-activity of the dopamine in areas of the brain that regulate emotional expression, motivated behaviour and cognitive functioning.

 

Having "too much" of this neurotransmitter is probably too simplistic; the better term is a functional excess (Lieberman, 1990). This may be caused by a failure of any of the many processes involved in breaking down and re-creating the neurotransmitter, or disruption to the receptor system (such as the receptor functioning "too well"), or there may be problems with re-uptake into the presynaptic membrane.

 

Dopamine Pathways

People diagnosed with schizophrenia have more dopamine receptors on neuron membranes than do non-schizophrenics, and these receptors seem to be over-active to dopamine stimulation (Wong, 1986). Additional support comes from the finding that the effectiveness of antipsychotic drugs used to treat schizophrenia is positively related to their ability to reduce dopamine-produced synaptic activity (Green, 1997).

 

The hypothesis that dopamine and dopaminergic mechanisms are central to schizophrenia has been one of the most enduring ideas about the illness. It was not until the 1970's, however, that the dopamine hypothesis was finally crystallized with the finding that the clinical effectiveness of antipsychotic drugs was directly related to their affinity for dopamine receptors. To date, their have been more than 6800 articles on the topic of dopamine and schizophrenia since 1991.

 

In summary, molecular imaging studies show that presynaptic striatal dopaminergic function is elevated in patients with schizophrenia; however blockade of this heightened transmission, either by decreasing dopamine levels or blocking dopamine transmission, leads to a resolution of symptoms for most patients (Howes, 2009).  

Schizophrenia

Unlike the disruptions of mental tranquillity that disturb everyone from time to time, schizophrenic episodes represent a severe departure from normal mental functioning. The disorder has a distinctly biological character, suggesting that its fierce psychotic episodes reflect physiological alterations in normal brain function.

Schizophrenia is the diagnostic term for a family of severe mental disorders that involve psychotic features - a loss of contact with reality - and a widespread deterioration of the level of mental functioning affecting multiple psychological processes (Kandel, 1991). The disorder always involves delusions, hallucinations, or characteristic disturbances in the form of thought. By definition, schizophrenic disorders are relatively long lasting: brief, isolated psychotic episodes are not classified as schizophrenic. Schizophrenia, strictly defined, has an incidence of approximately 1 in 200. Rates of schizophrenia are generally similar from country to country - about 1 percent of the population. There are variations - but the variance is difficult to track due to differing measuring standards in many countries, etc. It is equally common in men and women.

Delusions are a major abnormality in the content of thought. Schizophrenic delusions - false beliefs about external reality - are often persecutory, as in the belief that a television newscaster is making fun of the viewing individual. Other typical delusions are more bizarre: The individual may believe that his or her thoughts are being broadcast so that everyone nearby can hear them, or that other people are inserting thoughts and their behaviour is controlled by others, perhaps by radio waves. Such delusional beliefs represent a marked failure in assessing reality.

Characteristic abnormalities in the form of thought also frequently occur. Most common is a loosening of associations , in which ideas shift from one topic to another in an apparently unrelated manner. When this is severe, speech becomes incoherent.

Hallucinations - perception without external stimulation of the sensory systems - are also characteristic of schizophrenia. Most hallucinations are auditory, involving voices that may make insulting statements or provide a continuing critical commentary on the individual's behaviour. Tactile and somatic hallucinations, such as the perception of snakes crawling inside the abdomen, also occur. However, visual hallucinations are less common.

The emotions of the schizophrenic patient are usually flattened or inappropriate. ''Flattened'' means a loss of emotional intensity: the patient speaks in a monotone, the face is expressionless, and the patient reports that normal feelings are no longer experienced. At other times, emotion may be present but is inappropriate to the circumstance.

The combination of symptoms leads to a gross distortion of the person's interactions with the real world. There is a deterioration in functioning, resulting in part from a preoccupation with internal thoughts and fantasies. In many cases, the acute active phase of florid schizophrenic symptoms persists for a prolonged period. It may be followed by a relative remission of symptoms, but a complete return to normal function is extremely unusual. In fact, such a recovery calls into question the original diagnosis of schizophrenia.

Despite the bizarre and florid nature of the schizophrenic symptoms, there is still considerable controversy as to the nature of the disorder. Many investigators believe that schizophrenia is not a single disease but forms a group of related psychotic disorders.

In schizophrenia, there seems to be an inheritable predisposition or susceptibility to the disorder. In the general population, the risk of schizophrenia is less than 1 percent. However, this risk is much greater for relatives of schizophrenics. The parents of a schizophrenic child have about a 5 percent risk of schizophrenia, the siblings of a schizophrenic have about a 10 percent risk, and the children of a schizophrenic parent have about a 14 percent chance of developing the disorder. If both parents are schizophrenic, the child has a risk factor of about 50 percent.

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''If you talk to God, you are praying; If God talks to you, you have schizophrenia.''
                                                                                                         ~ Thomas Szasz