title: Immunocytochemical analysis of serotonergic axons in laminae I and II of the lumbar spinal cord of the cat.
abstract: 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.
mesh_terms: D000818:Animals; D001369:Axons / Q000648:ultrastructure*; D002415:Cats; D008161:Lumbosacral Region; D009412:Nerve Fibers / Q000648:ultrastructure; D012701:Serotonin / Q000032:analysis*; D013116:Spinal Cord / Q000166:cytology* / Q000648:ultrastructure; D013569:Synapses / Q000648:ultrastructure
chemical_list: D012701:Serotonin