Mast Cell Mediators as Pain Triggers in Migraine: Comparison of Histamine and Serotonin in the Activation of Primary Afferents in the Meninges in Rats

The meninges around the brain are characterized by an abundant blood supply, a high density of sensory nerves, and large numbers of mast cells. In migraine, the commonest neurological disorder, activation of trigeminal nerve fibers in the meninges is the initial trigger mechanism for generating pain signals. The recently suggested concept of the neuroimmune synapse suggests that mast cell transmitters can activate receptor proteins in close-lying nerve endings, leading to the generation of nociceptive spike activity. Serotonin and histamine, presumptive triggers for migraine, are classical transmitters released on activation of mast cells. Our recent research has identifi ed powerful activation of primary afferents by serotonin, mediated mainly via 5-HT3 receptors. However, the role of histamine in meningeal neuroimmune synapses has received little study. The present study therefore used recording of spike activity from primary afferents in the meninges in rats to study the role of histamine as a possible trigger for pain in migraine. Results from testing a wide range of histamine concentrations identified only a minimal (about 12%) effect with 10 μM histamine on the nociceptive activity of the trigeminal nerve. More detailed cluster analysis showed that the proportion of fibers reacting to histamine was no more than 29%, increases in spike activity in these fibers being significantly lower than on exposure to serotonin. Longer (4 h) exposure to histamine also produces no significant change in trigeminal nerve activity. The results do not exclude a stimulatory role for histamine in migraine but suggest that this mast cell transmitter has an action other than activation of the trigeminal nerve.