The Histochemistry of Tachykinin Systems in the Brain

After more than 70 years of research it seems an appropriate moment to summarize the progress in the substance P (SP)/tachykinin field, not at least because of the recent FDA approval of an SP (NK1) antagonist for treatment of chemotherapy-induced emesis/nausea. In the present review a wide range of histochemical studies based especially on immunohistochemistry and in situ hybridization, but also ligand binding autoradiography have been compiled to give an overview of the distribution of the three cloned tachykinins, SP, neurokinin A (NKA) and neurokinin B (NKB), as well as their three receptors (as so far known), NK1, NK2 and NK3. The main focus is on the rat brain, and to a lesser extent on the human brain, but several other species are also briefly mentioned. All tachykinins and their receptors are widely distributed in the brain, often with a strong similarity between species. The SP/NKA and NKB systems are often complementary, for example NKB is strongly expressed in cortical areas, where SP and NKA are comparatively less abundant. In addition the receptors show complementary distribution patterns, and there is often an overlap between ligands and receptors. However, cases of apparent mismatch can also be found. The wide distribution suggests that tachykinin systems can be involved in a large number of brain functions. It will be an important task to further correlate such functions with distribution patterns, in particular in the human brain and especially with regard to possible involvements of tachykinin systems in brain disease. Here further detailed histochemical analyses can provide important information to better understand such processes.