Describe and Evaluate Biological Explanations for Schizophrenia DiathesisStress

Describe and Evaluate Biological Explanations for Schizophrenia

Diathesis-Stress Model A theory that explains behaviour as both a result of biological and genetic Factors ("nature"), and life experiences ("nurture"). This model thus assumes that a disposition towards a certain disorder may result from a combination of one's genetics and early learning. The term "diathesis" is used to refer to a genetic predisposition toward an abnormal or diseased condition. According to the model, this predisposition, in combination with certain kinds of environmental stress, results in abnormal behaviour. This theory is often used to describe the pronunciation of mental disorders, like schizophrenia that are produced by the interaction of a vulnerable hereditary predisposition, with precipitating events in the environment.

We can think of it like this….

Vulnerability • In the diathesis–stress model, a biological or genetic vulnerability or predisposition (diathesis) interacts with the environment and life events (stressors) to trigger behaviours or psychological disorders. • The greater the underlying vulnerability, the less stress is needed to trigger the behaviour or disorder. • Conversely, where there is a smaller genetic contribution greater life stress is required to produce the particular result. • Even so, someone with a diathesis towards a disorder does not necessarily mean they will ever develop the disorder. • Both the diathesis and the stress are required for this to happen. • This theory was created by Holmes & Rahe.

Here comes the Science!

• DNA contains a set of instructions. It is the carrier of information. • Half of their genes come from their maternal line, and half from their paternal. • The DNA is composed of a series of genes, each of which ‘codes’ for a particular protein. • Minute differences in the DNA code create different shaped / functions in the proteins made.

• As each persons DNA is different from anyone else’s, this is their specific Genotype. • This results in a completely unique Phenotype (the characteristic show, as a result of both the genes and the way in which it interacts with the environment).

• Dizygotic twins (non-identical twins) share similar characteristics, much like a siblings. • Monozygotic twins (identical twins) are the result of an embryo viably splitting early on in development. They share almost exactly the same Chromosomal DNA.

• Some genes always lead to certain characteristics; these are known as dominant genes. To produce a characteristic, dominant genes need to be on only one pair of chromosomes (one copy). • Some genes need more than one copy to produce a characteristic, there are known as recessive genes – if a Recessive gene is present on only one chromosome, the characteristic will not appear. • HOWEVER, they may be passed on and appear in a future generation. • Characteristics can be aspects of appearance, personality, physical health and behaviour.

Genes and Schizophrenia • Researchers have looked for a particular ‘Schizophrenia Gene’ without success. • It is now thought that combinations of certain genes might make people more vulnerable to Schizophrenia, but this does not necessarily mean that they will develop the symptoms. • The evidence shows that people who have a parent with Schizophrenia are more likely to develop it themselves. • Biochemical research has been centred on Dopamine, which is one of the chemicals that carry messages between brain cells. • The theory is that an excess of dopamine, or dopamine receptors may be involved in the development of Schizophrenia. • Look at your AS textbook at the Gottesmann and Sheilds research.

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Pass the Pen Activity A Genetic Explanation – The Synapse How does this explain the Dopamine Hypothesis

DOPAMINE HYPOTHESIS The Dopamine hypothesis states that the brain of schizophrenic patients produces more dopamine than normal brains. –Evidence comes from –studies with drugs –post mortems –pet scans

Disturbance in the Neurochemistry • The first discovery in the mid 1950 s was that chronic usage of large daily doses of Amphetamines could produce a psychosis that was virtually indistinguishable from schizophrenia. • It was found that Amphetamine could enhance neurotransmission of Dopamine, Norepinephrine and (to a lesser extent) Serotonin Synapses. • The second discovery was that Chlorpromazine could improve symptoms for schizophrenia. • It was also discovered that Chlorpromazine could prevent Dopamine from activating it’s D 2 receptor subtype. • The knowledge that Chlorpromazine improves symptoms of schizophrenia while blocking D 2 receptors for Dopamine has led to the development of drugs that have similar pharmacological properties to chlorpromazine.

Normal Level of Dopamine In The Human Brain Elevated Level of Dopamine In The Brain of a Schizophrenic Patient (specifically the D 2 receptor) o Neurons that use the transmitter ‘dopamine’ fire too often and transmit too many messages or too often. o Certain D 2 receptors are known to play a key role in guiding attention. o Lowering DA activity helps remove the symptoms of schizophrenia

ROLE OF DRUGS –Amphetamines (agonists) lead to increase in DA levels –Large quantities lead to delusions and hallucinations –If drugs are given to schizophrenic patients their symptoms get worse

Parkinson’s disease • Parkinson’s sufferers have low levels of dopamine • L-dopa raises DA activity • People with Parkinson's develop schizophrenic symptoms if they take too much L-dopa –Chlorphromazine (given to schizophrenics) reduces the symptoms by blocking D 2 receptors

POST MORTEM Falkai et al 1988 o Autopsies have found that people with schizophrenia have a larger than usual number of dopamine receptors. o Increase of DA in brain structures and receptor density (left amygdala and caudate nucleus putamen) • Concluded that DA production is abnormal for schizophrenia

PET SCANS Lindstroem et al (1999) • Radioactively labelled a chemical L-Dopa • administered to 10 patients with schizophrenia and 10 with no diagnosis • L-Dopa taken up quicker with schizophrenic patients • Suggests they were producing more DA than the control group

Chickens hatch from eggs, but a mother chicken must keep an egg warm in order for it to hatch Which Came First? The Chicken or the Egg? Schizophrenia or Faulty Chemicals? Faulty chemicals cause schizophrenia but schizophrenia may cause faulty chemicals Drugs may influence other systems that impact on schizophrenia so cant be 100% sure about their effects

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Enlarged Ventricle Hypothesis

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