The organization of serotonin fibers in the anterior column of the mammalian spinal cord. An immunohistochemical study.

Detailed comparative analysis of the organization of serotonin fibers in the anterior column of the mammalian spinal cord (rat, guinea pig, cat, dog and monkey) was carried out by use of the indirect antibody peroxidase-antiperoxidase (PAP) method. The plexus formation of serotonin-containing varicose fibers around the alpha-motoneurons in the monkey anterior horn was in much closer apposition to the cell bodies in comparison with the spinal cords of the rodents and carnivores. The results may suggest that anterior horn motoneurons in the simian spinal cord are intimately innervated by serotonin fibers in a manner different from that of rodents and carnivores. Furthermore, the small cell groups endowed with particularly dense networks of serotonin fibers were demonstrable in the anterior horn of L1-L2 segments of rats, and L3-L4 of guinea pigs and monkeys; however, in the lumbar levels of the carnivores this was not the case. Hence it seems doubtless that there exists in the lumbar anterior horn of the rodent and primate spinal cords a cell group with an unknown specialized function.