The ultrastructural localization of serotonin immunoreactivity in myelinated and unmyelinated axons within the medial forebrain bundle of rat and monkey.

Myelinated and unmyelinated serotonin-containing axons were demonstrated in the medial forebrain bundle of rats and monkeys with an antibody against serotonin (5-HT) conjugated to hemocyanin. The level of 5-HT in the brain was increased by pretreatment of animals with pargyline and L-tryptophan. The addition of nickel salts to the diaminobenzidine-peroxidase reaction mixture increased the intensity of the reaction product. Specific 5-HT immunoreactivity was observed to be localized to membrane-enclosed and membrane-attached compartments. 5-HT immunoreactivity was seen predominantly in unmyelinated axons (0.2 to 1.25 micron in diameter) surrounded by unlabeled processes. More than half of these 5-HT-immunoreactive fibers were in apposition with unreactive myelinated axons in the hypothalamus of both the rat (55%) and the monkey (54%). These appositions may provide the anatomical substrate for the observations that sprouting serotonergic fibers in the adult brain use myelinated axons for contact guidance. In addition to the unmyelinated fibers, intensely labeled myelinated axons (1.0 to 2.1 micron) were seen in the medial forebrain bundle of both rats and monkeys. The percentage of 5-HT-immunoreactive myelinated axons appeared greater in the monkey than in the rat (25.4% versus 0.7% of the total number of 5-HT-immunoreactive fibers, respectively). the observation of an increased number of myelinated 5-HT-immunoreactive axons in the monkey hypothalamus as compared to the rat supports the general hypothesis of a trend toward increased myelination during evolution.