Axons which take up [3H]serotonin are presynaptic to enkephalin immunoreactive neurons in cat dorsal horn.

Through combined ultrastructural localization of ENK immunoreactivity and [3H]5-HT uptake sites, it was found that 5-HT radiolabeled axons contact ENK immunoreactive cell bodies and small dendrites in the cat superficial dorsal horn. While some contacts displayed both pre- and postsynaptic specializations, the majority lacked a definitive synaptic cleft and postsynaptic density. These results suggest that 5-HT-containing axons, presumably derived from the medullary nucleus raphe magnus, directly influence spinal opiod antinociceptive activity.