n this assignment the Author intends to describe the dopamine hypothesis and its relationship to Schizophrenia, describe the biological and/or physiological theory of depression, evaluate the theories and offer alternative explanations for both schizophrenia and depression and finally conclude by writing an essay identifying three psychological disorders. The Dopamine Hypothesis According to the biochemical theory of schizophrenia and /or the dopamine hypothesis the causes of schizophrenia like symptoms is essentially an excess of the neurotransmitter dopamine. (Coordination Group Publications 2009).
‘This was based on observations that an overdose of amphetamine causes schizophrenia like symptoms in normal participants (e.g., Snyder et al., 1974) and when given to schizophrenic patients, amphetamines tend to exaggerate their symptoms (Snyder et al., 1974). Amphetamines enhance the synaptic activity of dopamine.’ (Snyder, 1976). (Keiron Walsh 2008). The primary evidence for this hypothesis comes from three main sources; the first is post mortems on schizophrenics which have shown unusually high levels of dopamine, especially in the limbic system (Iverson, 1979).
The second is anti-schizophrenic drugs such as chlorpromazine which are thought to work by inhibiting the ability of the dopamine D2 receptors to respond to dopamine thus reducing dopamine activity; these drugs produce side effects similar to Parkinson’s disease which is known to be caused by low levels of dopamine in particular nerve tracts. The third is high doses of L-dopa used in the treatment of Parkinson’s disease; this treatment can often produce symptoms very similar to the psychomotor disorders seen in certain types of schizophrenia. (Coordination Group Publications 2009; Richard Gross 2010).
The Biological Theory of Depression
‘According to the monoamine oxidase hypothesis or MAOH a depletion of serotonin, noradrenaline and/or dopamine underlies the melancholic symptoms of depression; Serotonin (5-HT), norepinephrine (noradrenaline) and dopamine are collectively known as monoamine transmitters, the greatest number of studies on these have involved serotonin.’ (Claridge and Davies, 2003). (Richard Gross 2010).
The biochemical factors contributing to depression are essentially low levels of serotonin in the brain, Kety (1975) developed the permissive amine theory which states that serotonin controls the levels of the neurotransmitter noradrenaline, a low level of serotonin causes the level of noradrenaline to fluctuate; low levels of noradrenaline cause depression whilst high levels cause mania. (Coordination Group Publications 2009).
Evaluation of Theories
In order to evaluate the biochemical factors of schizophrenia one would have to acknowledge that antipsychotic drugs appear to reduce the symptoms of schizophrenia by blocking dopamine receptors, this would suggest that it’s the overactive receptors causing the symptoms whereas antipsychotic drugs only seem to work on the positive aspects of schizophrenia such as hallucinations; this means that increased dopamine function doesn’t explain negative symptoms such as social withdrawal.
Drugs like amphetamines which increase dopamine function can sometimes cause schizophrenia-like symptoms in people without schizophrenia therefore the link with dopamine is correlational, it doesn’t display cause and effect; it may be that increased dopamine function is a symptom of schizophrenia rather than a cause of it. . (Coordination Group Publications 2009; Richard Gross 2010). Overall the evidence for a biochemical theory of schizophrenia is inconclusive, there exists no consistent difference in dopamine levels between drug-free schizophrenics and non-schizophrenics, similarly, there is no evidence of higher levels of other metabolites indicating any greater dopamine activity. (Richard Gross 2010).
In respect of biochemical factors for the treatment of depression anti-depressant drugs work by increasing the availability of serotonin at the synapses by preventing its reuptake or breakdown suggesting that it’s the low levels of serotonin that lead to depressive disorders, however because anti depressants relieve some of the symptoms associated with depression doesn’t mean they treat the cause; low levels of serotonin could be a symptom of depression. Post mortems carried out on people who have committed suicide have revealed abnormally low levels of serotonin suggesting that this may have caused their depression, however psychological research has discovered that there are alternative and often complex explanations for the cause of depression. (Coordination Group Publications 2009; Richard Gross 2010).