DOPAMINE
HYPOTHESIS
LEARNING OBJECTIVES
By the end of the lesson your goal is to
be able to
• Describe the role of Dopamine (DA)
in schizophrenia
and you should be able to
• Outline evidence to support this
hypothesis
You could even be able to
• Evaluate the Dopamine Hypothesis
Lets remind ourselves how
neurotransmitters work
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Lets remind ourselves how
neurotransmitters work
DOPAMINE HYPOTHESIS
The Dopamine hypothesis states that the
brain of schizophrenic patients produces
more dopamine than normal brains.
Evidence for this comes from
studies with drugs
post mortems
pet scans
Dopamine Hypothesis
Although the original
Dopamine Hypothesis
states that the brain of
schizophrenic patients
produces more dopamine
than the brain of a
“normal” person.
It is now thought that
schizophrenics have an
abnormally high number of
D2 receptors.
Normal Level of
Dopamine In The
Human Brain
Elevated Level of Dopamine
In The Brain of a
Schizophrenic Patient
(specifically the D2 receptor)
 Neurons that use the transmitter ‘dopamine’ fire too often and
transmit too many messages.
 There may be an excess of DA receptors at the synapse in
schizophrenics
 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 these drugs are given to schizophrenic patients
their symptoms get worse
Randrup et al
Rats given
amphetamines
developed
schizophrenia
type symptoms
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 D2 receptors
Post-mortems
Seidman (1990)
Post-mortem (after death) examinations have
found that people with schizophrenia have a
larger than usual number of dopamine
receptors.
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
PET Scans
Gjedde and Wong (1987)
There are more than twice as many dopamine
receptors in schizophrenics compared to controls.
Farde et al. (1990)
There is no difference in the number of dopamine
receptors between schizophrenics and controls.
What is the main strength of a PET scan?
What conclusions can be drawn from the research
findings?
Which Came First?
Chickens
hatch
The Chicken or the Egg?
from eggs,
but a
mother
chicken
must keep
an egg
warm in
Schizophrenia or Faulty
order for
Chemicals?
it to hatch
Faulty chemicals cause
schizophrenia but schizophrenia
may cause faulty chemicals
Drugs may influence other systems that impact on
schizophrenia so we cannot be 100% sure about their
effects
There are lots of problems with
the dopamine hypothesis!
• Read your handout carefully.
• There is a lack of correspondence between
taking phenothiazines and signs of clinical
effectiveness. It takes 4 weeks to see any
sign that the drugs are working when they
begin to block dopamine immediately. We
cannot seem to explain this time difference.
• It could be that the development of receptors
in one part of the brain may inhibit their
development in another.
Other EVALUATION POINTS
• Type 1 cases respond well to conventional anti-psychotic drugs.
Drugs such as CHLOPROMAZINE: Only effective at relieving the
Positive Symptoms of the Illness.
• Not good at explaining negative symptoms. Therefore suggested that
Type 2 is related to a different kind of abnormality such as brain
structure.
• PET scans have suggested that drugs did not reduce symptoms of
patients diagnosed with disorder for 10 yrs or more
• There may be other neurotransmitters involved.
• Possible that social and environmental factors trigger the condition.
ACTIVITY
• Describe and evaluate the dopamine
hypothesis as an explanation of
schizophrenia
• You must comment on how the evidence you use supports or
challenges the DA hypothesis.
• You should comment on evidence both for and against the
hypothesis.
• You could use your own skills and knowledge to make
additional critical and evaluative points.