Molecular brain imaging of psychedelic action

Molecular brain imaging by positron emission tomography (PET) and single photon emission computer-tomography (SPECT) entails the mapping of the cerebral distribution of radiopharmaceuticals that track physiological processes such as blood perfusion and glucose metabolism, or the abundance in brain of specific molecular targets such as neuroreceptors. PET and SPECT emerged as useful in vivo research technologies in the 1980s, finding early application in the study of psychostimulant drugs. The past decade has seen growing use of molecular imaging methods in the study of psychedelic action, although the published literature remains comparatively small. The preponderance of publications cited in this review are SPECT studies of cerebral perfusion and PET studies of metabolism and neuroreceptors, the latter mainly focusing on the 5-hydroxytryptamine (serotonin) 5-HT_2A receptors, which are largely responsible for the psychedelic action of classical psychedelic substances. There is some documentation of interactions of psychedelics at dopamine D_2/3 receptors in the striatum, but many other plausible molecular targets of psychedelic action await investigation by molecular brain imaging. The emerging role of psychedelics as treatments for neurological and psychiatric disorders calls for a broader and systematic investigation of their effects on brain function.