The latter is metabolized to CO2 and water, while the former is “trapped”

in neurons long enough to be imaged, if radiolabeled. The long measurement period of this method (a 20- to 30-min scan carried out 30 to 40 min postinjection, when CMRGlu is assumed to have reached a steady state) limits its temporal resolution and sensitivity to cognitive and acute pharmacological activations; it is, therefore, best suited to providing detailed, quantitative maps Inhibitors,research,lifescience,medical of trait-like brain functional characteristics, as opposed to mental states. Measurement of neurochemical systems Using PET to image and quantify the functional activity of various neurochemical system components (eg, neuro-receptors and enzymes) has much in common with autoradiography and in vitro receptor-binding techniques. A Small molecule library manufacturer specific ligand (or binding agent) is labeled with a positron emitter and injected into the subject and the anatomical distribution of the radioligand in the brain is determined with PET. A quantitative estimate of specific receptor binding can be achieved Inhibitors,research,lifescience,medical Inhibitors,research,lifescience,medical by compartimental modeling to account for the kinetic behavior of the ligand between extra- and intracerebral plasma and tissue, as well as nonspecific binding and extraneuronal concentration. Alternatively, and more simply, the radioligand concentration in a brain area known to have little or no specific binding (eg, the cerebellum for dopamine receptors)

can be used to estimate nonspecific binding. PET ligands are available for dopamine,

opiate, serotonin, benzodiazepine, and other receptors. Cerebral concentration and distribution of neurotransmitter turnover and enzymes Inhibitors,research,lifescience,medical can also be measured using ligands, such as [11C]clorgiline and L-[11C]deprenyl, irreversible inhibitors of monoamine oxidase (MAO) for mapping MAOA and MAOB, respectively, and [18F]dihyroxyphenylalanine ([18F]DOPA), an analog of the dopamine precursor. Distribution and kinetics of pharmacological agents such as [11C]chlorpromazine, [11C]benztropine, Inhibitors,research,lifescience,medical and [11C]cocaine can also be determined. Ligands specific to gonadal steroid hormones that cross the blood-brain barrier have yet to be developed. Magnetic resonance spectroscopy (MRS) is a chemical assay technique Urease for measuring chemical moieties in the living brain. 31P spectroscopy measures high energy compounds and phospholipids (eg, phosphomonoesters and phosphodiesters, ATP, phosphocreatine), which reflect the energy state of neurons and constituents such as membrane precursors. 1H spectroscopy can detect amino acids, energy substrates, and membrane and myelin metabolites. Its greatest application has been to measure N-acetyl aspartate, an intracellular neuronal marker and sensitive indicator of neuronal pathology. Available methods for functional brain imaging Positron emission tomography PET involves the administration of cyclotron-produced radioisotopes such as 1F, 15O, and 11C.