What do you mean by "mapping"--do you mean quantifying the distribution of the receptors in different parts of the brain? Or do you mean determining the three-dimensional structure of the receptor molecules themselves? I know that the first (low resolution!) molecular structures of the NMDA receptor have been released in the last year or so--I'm not aware of a recent study examining the distribution of NMDA receptors in different brain regions/nuclei, but NMDA would probably be a rather boring one in this respect, because it's found almost everywhere.
Much more interesting are receptors that are found in only a few regions of the brain, or that have many subtypes. For instance, GABA-A receptors have alpha, beta, and gamma subunits, of which there are (if I remember correctly) 6, 3, and 2 forms, respectively, for a total of 36 hypothetical combinations. However, we have little idea which of those combinations actually exist and where.
Plus, there are different resolutions of mapping. Knowing for instance that a particular receptor is abundant in the hippocampus or the striatum is much less specific than knowing that it is present specifically on GABA interneurons in one of these structures. I even saw a study of the CB1 receptor (the one responsible for most of the effects of THC) that looked microscopically in tissue slices to see on what specific synapses of each neuron the CB1 receptors were located at, whether they were pre- or postsynaptic, etc.
Finally, just because someone looked for a receptor and found it doesn't mean that the finding won't be later challenged. For instance, the receptor that responds to heat and hot peppers is also found in the brain, where it responds not to heat but to certain neurotransmitters, and plays a role in emotion. At least many scientists have very convincing indirect evidence that it's there, but looking for it directly didn't detect any for some reason.
12 Oct 2014, 12:02 pm
