Depression of primate spinothalamic tract neurons by iontophoretic application of 5-hydroxytryptamine.

The effects of iontophoretic applications of 5-hydroxytryptamine (5-HT) were tested upon primate spinothalamic tract neurons recorded extracellularly in the spinal cord of anesthetized monkeys. The activity of most high threshold and wide dynamic range spinothalamic tract cells was depressed. 5-HT also reduced the responses of the cells to glutamate pulses which by themselves had a powerful excitatory action. It is concluded that 5-HT has a depressant action upon the postsynaptic membranes of spinothalamic tract cells, although the action has a slow time course. The observations are consistent with, but by no means prove, the hypothesis that serotonergic pathways descending from the brain stem produce a postsynaptic inhibiton of spinothalamic tract neurons.