Know your neurotransmitters
Imaging can be used to study neurotransmitters, too
Imaging can be used to measure the levels of a neurotransmitter, its receptors and its transporters (which remove neurotransmitters from the synapse after release). This is very useful because many neurological conditions involve the death of neurons that produce a specific neurotransmitter, or alterations in the activity or distribution of receptors or transporters within certain parts of the brain. The relationship between diseases and neurotransmitters is complex. For example, other neurotransmitters than those discussed below could also be involved in the conditions presented.
In Parkinson’s disease, dopamine neurons in the midbrain die. Alzheimer’s disease, depression and schizophrenia involve alterations in the transporter for the neurotransmitter serotonin. Attention deficit hyperactivity disorder (ADHD) has been associated with malfunctions of dopamine receptors and dopa decarboxylase, one of the enzymes needed for making dopamine, in the frontal cortex. ADHD, depression and schizophrenia can all involve disturbances in the function of receptors and transporters for the neurotransmitter noradrenaline.
The imaging methods used most often to measure neurotransmitter levels are positron emission tomography (PET) and single-photon emission computed tomography (SPECT). Both involve injecting a radioactive ‘tracer’ substance that binds to the molecule being studied. The scan reveals where the tracer binds, and the intensity of the radioactive signal in each brain area is related to the level of the molecule being studied.
Neurotransmitter receptors come in multiple forms, or subtypes, and each subtype has multiple variations. Some tracers bind to one particular subtype or more specifically to one variation of a subtype. To date, we know of at least five subtypes of receptor for dopamine – D1 to D5. Genetic mutations in the D4 subtype of the dopamine receptor (DRD4), for example, are linked to ADHD, Parkinson’s disease and schizophrenia, as well as drug use and personality traits such as aggression and impulsiveness. PET and SPECT can be used to examine exactly how alterations in the DRD4 receptor are linked to these conditions and behaviours.Lead image:
Arran Lewis/Wellcome Collection CC BY