Distribution of serotonin (5-HT) neurons in the central nervous system (CNS) of various vertebrates was investigated with a highly sensitive immunohistochemical technique. Antibodies were raised in rabbits against an antigen prepared by coupling 5-HT to bovine thyroglobulin. 5-HT neurons were found to be distributed more widely and densely than has been heretofore described. Serotonergic neuronal somata are organized according to certain basic patterns, but there are interspecific differences with regard to the distribution of 5-HT fibers. The processes of 5-HT neurons form a dense plexus by ramification and anastomosis in almost all areas of the CNS, including the ventricular surfaces. In the light of our observations, Golgi's reticular theory may have to be revised.
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