Most drugs that affect the brain do so by changing the way specific types of chemical signals are sent or received. Nutt has just shown that the brains of people who suffer from anxiety attacks have fewer receptors of the type that are affected by benzodiazapine drugs. This suggests that they should have a reduced capacity to respond to the signals that keep them calm under stress. PET scans are also providing new and more efficient ways of detecting diseases in people who are known to be susceptible. Brooks has found that Parkinson's disease can be detected about six years before the appearance of any clinical signs by doing PET scans using a radioactive precursor of the chemical signalling molecule dopamine.
Dopamine is the signal used by the neurones in the brain area most affected by the disease.
PET scans can also be made using less specific markers that show how much energy is being taken up in the brain, or how much blood is flowing. A variant of the MRI scan known as fMRI can also be used to detect blood oxygen. By looking at the difference between scans made when subjects are doing particular tasks, like remembering words, with those made when they are resting, researchers can work out which areas of the brain are used in those tasks. The two types of scan can provide very similar information, says Peter Morris, of the Magnetic Resonance Centre at Nottingham University. PET has the advantage that it can be used to probe specific chemical messenger systems.