Potential sites of action of monoamines.

5-HT, NA and DA may exert actions on one or many of the following locations: 1) presynaptically directly inhibiting afferent transmission to the spinal cord, 2) inhibiting synaptic transmission to putative interneurons or altering their excitability so that fewer are recruited to produce PAD, or 3) inhibiting last order GABAergic transmission to the afferents producing PAD. 4) Monoamines also usually act to increase SPN excitability.