DHE Repression of ATP-Mediated Sensitization of Trigeminal Ganglion Neurons

Migraine is a prevalent painful headache disorder that can be divided into 2 major subtypes, migraine without aura and migraine with aura. In migraine with aura, the headache phase is preceded or accompanied by transient neurological symptoms, most typically visual.1,2 It has been proposed that cortical spreading depression (CSD), which is implicated in migraine visual aura, leads to release of protons, potassium ions (K+), nitric oxide, prostaglandins, and adenosine 5′-triphosphate (ATP) in cerebrospinal fluid.3,4 The increased level of these molecules can cause sensitization and activation of sensory primary afferent fibers of trigeminal ganglion (TG) neurons that transmit nociceptive information from the meninges to second order neurons in the trigeminal nucleus caudalis (TNC). Thus, CSD and the subsequent release of pro-inflammatory molecules are thought to play an important role in the initiation of headache in migraine with aura. In response to hyperexcitable cortical activity that precedes CSD, ATP levels are known to increase greatly in the extracellular fluid.4,5 Elevated levels of ATP are implicated in acute, inflammatory, as well as neuropathic pain mechanisms by mediating either sensitization or activation of nociceptive neurons depending on its concentration. The inflammatory and nociceptive effects of ATP are known to involve activation of P2X and P2Y receptors, which are expressed by primary sensory afferents.6–9 P2X receptors are a member of a family of ligand gated ion channels, expressed in peripheral sensory afferents, and preferentially on nociceptive C-fibers. Importantly, many trigeminal nociceptive neurons that provide sensory innervation of the dura and release calcitonin gene-related peptide (CGRP) are reported to express P2X3 receptors.10–12 Thus, activation of P2X3 receptors is likely to play an important role in the underlying pathology of migraine with aura by facilitating release of CGRP within the dura to promote an inflammatory response as well as CGRP release in the TNC that activates second order neurons involved in pain transmission. Dihydroergotamine (DHE) is a semisynthetic ergot alkaloid derivative that has been used extensively in the acute treatment of migraine and has been reported to be superior to sumatriptan in preventing migraine recurrence.13 DHE exhibits high affinity for several serotonergic, adrenergic, and dopaminergic receptors.14 It is thought that the therapeutic benefit of DHE in treating migraine involves 5-HT1B and 5-HT1D receptors and leads to the repression of trigeminal neuron activation15,16 and constriction of meningeal blood vessels via the 5-HT1B receptor.17,18 However, results from more recent studies provide evidence that activation of the α2-adrenoreceptors, which are expressed on TG neurons, had an inhibitory effect on nociceptive transmission in the TG system at the level of primary afferent terminals.19 α2-adrenoceptors have been shown to be involved in inhibition of calcium influx through voltage-gated calcium channels via a G-protein dependent pathway in neuronal cells.17,20–22 α2-adrenoceptors also couple to the inhibitory Go/Gi-protein to inhibit adenylyl cyclase or to interact directly with calcium channels.23–26 Thus, DHE activation of α2-adrenoreceptors may also be involved in repressing stimulated CGRP release from trigeminal neurons in response to inflammatory stimuli. In this study, we used primary cultures of TG neurons to investigate the mechanism by which ATP causes sensitization of trigeminal neurons and DHE represses this modulatory effect. Data from our study provide evidence that ATP, via P2X3 receptors, lowers the threshold of activation of trigeminal neurons to K+ and that DHE activation of α2-adrenoceptors represses ATP-mediated sensitization of trigeminal neurons involves down regulating membrane expression of P2X3 receptors.