an example would be "Depression can be caused by a low level of serotonin in the brain".
Serotonin
You have it backward I believe. When serotonin is lower, people have more risk to experience depression.
Depression is due to alteration in the level of serotonin at the synaptic junctions of neurons in the limbic system.
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Personally I don't think it is possible to detect the levels of all neurotransmitters, because they are constantly being altered , except on a depressed person in which serotonin normal level drops to a lower one. But there is no needing to determine its level because a psychiatrist can tell when a person is depressed.
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The exact mechanism of Zoloft is unknown, but it is believed to increase the extracellular level of the neurotransmitter serotonin by limiting its reabsorption into the presynaptic cell.
Perhaps, but many patients with panic disorder are found to have a decrease in the number of receptors that serotonin binds to. The effect is equal to having a low serotonin level. I am a medical student currently doing research in this area.
The level of serotonin is lower in people who are sad, so it's the absence, rather than presence of a chemical. However, a certain level of sadness is normal. If it's long-lasting, then it's depression and should be treated with therapy and possibly medication.
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Overabundance of serotonin can cause serotonin syndrome.