Immunohistochemical studies on the processes of serotonin neurons and their ramification in the central nervous system-with regard to the possibility of the existence of golgi's rete nervosa diffusa.

Abstract

By using the modified PAP [peroxidase-antiperoxidase] method on various vertebrates [rat, cat and chicken] from teleost to macaque, the serotonin neurons that are distributed mainly in the raphe nuclei area were studied, focusing on their process ramifications. Neuron shapes varied; they were multipolar, with many long, thick dendrites. In most neurons, > 2 axons originated either from the stem of a dendrite or from its tip. The axons contained spherical or spindle-shaped varicosities of varied sizes (diameter 0.5-6 .mu.m). Rarely, they became large, Herring body-like droplets. The axons were classified into 4 groups according to the shape and size of the varicosities they contained: stem fibers, A-fibers (tract fibers), B-fibers (branching fibers) and C-fibers (ground fibers). The axon not only branched frequently but formed a true network by way of frequent anastomosis at the level of B- and C-fibers. Serotonin fibers do not have endings that correspond to telodendrons but conduct impulses through en passant type synapses. A supraependymal plexus is formed over large areas of the ventricular system. This vast network is considered to be formed by the syncytial continuity of axonal reticulum of many serotonin neurons. Serotonin neurons, and probably noradrenaline [norepinephrine] neurons as well, should be classified as a 3rd type of neuron, which belong to neither the Deiters type nor the Golgi type neuron. Their characteristic feature is the long, reticular axon, and it is highly probable that this neuron system forms the rete nervosa diffusa proposed by Golgi.