Immunocytochemical analysis of serotonergic axons in laminae I and II of the lumbar spinal cord of the cat.

Serotonergic axons in the superficial dorsal horn were examined at the light and electron microscopic levels using an antibody specific for serotonin (5-HT). Immunoreactive 5-HT axons were most numerous in lamina I and fewest in lamina IIb. The 5-HT axons tended to orient rostrocaudally as they traveled long distances in the gray matter. Based on the size of the 5-HT varicosities along a strand of axon, at least three different types of 5-HT axons were observed at the light microscopic level. Ultrastructurally, 5-HT immunoreactive axonal endings contained either a mixture of flattened and small oval agranular vesicles or a relatively homogeneous population of oval vesicles. 5-HT endings synapsed primarily on small caliber dendritic shafts. They also were found synapsing on large caliber dendritic shafts, dendritic spines, and neuronal cell bodies. Based on the laminar location of 5-HT axosomatic synapses and the presence of 5-HT synapses on different morphological types of dendrites, we propose that 5-HT modulates the response properties of at least three different types of neurons in the superficial dorsal horn.