Activation of Meningeal 5‐HT2B Receptors: An Early Step in the Generation of Migraine Headache?

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Abstract
Several pharmaceuticals are frequently dispensed to prevent or reduce the occurrence of migraine attacks. The prophylactic effect of these drugs has been suggested to be caused through blockade of serotonin (5-HT) receptors of type 5-HT2B or 5-HT2C. To elucidate which of these receptors is involved, we first used radioligand binding assays to determine the pharmacological profile of the human and rat 5-HT2B receptor. Furthermore, the potency of drugs used in migraine prophylaxis to stimulate or inhibit 5-HT2B or 5-HT2C receptor-mediated phosphatidyl inositol hydrolysis was measured. All these drugs were found to block both human receptors. Correlation of the receptor affinities with the potencies used in migraine prophylaxis showed significant correlations, which were better for the 5-HT2B (P= 0.001) than for the 5-HT2C receptor (P= 0.005). Migraine headache is thought to be transmitted by the trigeminal nerve from the meninges and their blood vessels. Using the reverse transcription–polymerase chain reaction, the expression patterns of all cloned G-protein-coupled serotonin receptors were analysed in various human meningeal tissues. All tissues expressed 5-HT1Dβ, 5-HT2A, 5-HT2B, 5-HT4 and 5-HT7 mRNAs. Only trace amounts of 5-HT2C receptor mRNA were found. With organ bath experiments we showed that the 5-HT2B receptor stimulated the relaxation of the pig cerebral artery via the release of nitric oxide. Our data support the hypothesis that 5-HT2B receptors located on endothelial cells of meningeal blood vessels trigger migraine headache through the formation of nitric oxide.