Neurotransmitter imaging: Basic concepts and future perspectives

Neurochemistry of human cognition remains uninvestigated because neuroimaging techniques have limited ability to detect neurochemical changes associated with cognitive processing. In recent years investigators have used molecular imaging to develop methods for detection, mapping and measurement of neurotransmitters released acutely during cognitive processing in the live human brain. Most of these methods exploit the competition between endogenous neurotransmitter and a radiolabeled receptor ligand. Because of the competition, the ligand concentration decreasesin the brain areas where neurotransmitter is released endogenously during a task performance. The decreased concentration is detected by measuring the activity of intravenously injected radioligand using a PET camera. The PET data acquired dynamically isapplied to a receptor kinetic model, which estimates kinetic parameter values at multiple time point. Based on these values dopamine released during performance of a task is detected, mapped and measured. By using different ligands, dopamine released inside or outside the striatum can be detected using this technique. The neurotransmitter imaging technique at present can detect acute changes only in dopamine neurotransmission. Since it significantly limits the scope, there is a need to develop methods and ligands for detection of acute changes in the levels of other neurochemicals.