Imaging of receptors in clinical neurosciences

This article deals with the question why should one determine receptors in the brain with positron and single photon emission tomography (PET and SPECT, respectively). Radiopharmaceuticals for a wide variety of receptors are available now. Receptors studies with PET and SPECT have thus far focused on the following issues: occupancy during drug treatment, quantification in neuropsychiatric diseases and visualizing specific pathology. Far most studies on receptor occupancy are concerned with antipsychotic treatment in schizophrenic patients. During treatment with classical antipsychotic drugs more than 65% of the receptors are occupied in both responding and drug therapy resistant patients. High occupancy appeared to be linked to frequent occurrence of extrapyramidal symptoms (parkinsonism). Atypical antipsychotic drugs, producing no parkinsonism, are effective even when less than 50% dopamine D2 receptors are blocked. Quantification of receptor density in vivo depends very much on the mathematical models used, and is often very difficult and thus far of little clinical use. Finally, receptors can be used as markers for specific neurons or other cells. In the caudate nucleus dopamine receptors are localized in neurons that degenerate in Huntington's disease, so early in the disease decreases in receptor binding can be shown. In Alzheimer disease decreases in neural receptor and increases in omega-3 receptors are observed indicating degeneration and inflammation, respectively.