y 20 % identical in their transmembrane segments and are in fact, more similar to receptors that bind acetylcholine and adrenergic ligands, respectively. Apparently, receptors have picked up the same ligand in different evolutionary branches, convergent evolution. DIAGRAM NOT AVAILABLEFigure Part of the evolutionary tree for rhodopsin-like 7TM receptors. All rhodopsin like receptors are at least 15 to 20 % homologous, the shaded area equals 70 % sequence identity which coveres most receptor subtypes (From Donally, D., Findlay, J.B.C., and Blundell, T.L., Receptors and Channels, 2,61,1994, and supplementary material from D. Donally) Figure 19 A dopaminergic neuron in which dopamine receptor subtypes can be seen at different locations on pre and post synaptic terminals.By using extremely advanced techniques it is possible to achieve computerised images pinpointing the distribution and location of certain receptor subtypes here subtypes have been imaged in the brain by using selective radioligands and advanced autoradiograpy, providing this remarkable imagery of the different distributions and locations of dopamine subtypes in the brain.These two sets of images (Fig. 19) can allow a clinician to diagnose a patient with an illness and treat more accurately, it also provides priceless research in psychoneuropharmacology in treating disease states such as schizophrenia and parkinsons disease in which dopamine receptor subtypes are heavily involved. In this image the majority of D2 receptor subtype can be located in the caudate putamen and the hippocampus and all subtypes apart from D1 can be found in the hippocampus. Not only can the actual location in the brain or body be different for subtypes but different receptor subtypes can be found on the presynaptic neurons as well as the postsynaptic neurons this may help regulate the transmission levels (Fig. 20) Figure 20. Images produced by autoradiography displaying the distribution and locati...
