A PET scan of a GABA-A receptor subtype [the alpha5]
PET = positron emission tomography and its close cousin of SPECT = single photon emission computerised tomography – are used to explore brain activity and drug binding and thereby receptor localization.
These techniques use radiolabelled tracers such as 11Carbon or 18Flourine for PET or 99Technecium metastable (99Tcm) or 123Iodine for SPECT. These unstable radionuclides decompose in the brain giving out 2 gamma rays in the case of PET or a single photon in the case of SPECT and these electromagnetic rays are detected by sensitive photocells arrange in a ring around the head. These techniques can be used to explore changes in either blood flow [eg with 15O water or 99TmHMPAO] or in cellular metabolic activity [using the uptake of 18F-deoxyglucose that enters cells in proportion to the usage of glucose in them].
In some cases it is possible to attach a radioactive nuclide to a drug and so measure its entry and distribution in the brain. If the drug has high affinity for a receptor/reuptake site/enzyme then the density of these can be estimated in discrete brain regions from the radioactive signal in each – fig 1. Currently we have good tracers for estimating the density of a number of receptors for the dopamine 5-HT and GABA-A systems; these are described in more detail in part 2. The time resolution of the PET/SPECT signal is in the order of minutes.