Characteristics of extravascular serotonin receptors in the brain.

At least three types of extravascular 5-HT receptors are present in the brain. Peripheral 5-HT antagonists mimic the inhibitory effects but block the excitatory effects 5-HT in the brain. Although the forebrain serotonergic system is apparently not involved in nociception, the MPR may be an important link in an antinociceptive system; this system may be activated by morphine. A decrease in the amount of 5-HT released by projections from the MPR to the trigeminal nucleus could be partially responsible for the pain experienced in migraine headache. Drugs useful in treating migraine headache may mimic 5-HT at synapses in the trigeminal nucleus that are associated with projections from the MPR. This possibility remains to be tested experimentally.