Nucleotide receptors in trigeminal satellite glial cells: new targets for the pharmacological control of migraine pain

According to the WHO, migraine is a rather common disorder affecting 15% of adults in the Western World. It has been included in the list of the 20 more disabling pathologies, due to the dramatic reduction of the patient’s quality of life, and also to massive personal and social costs, in terms of both medical expenses and lost workdays. Despite the recent introduction to the market of new, potent and effective anti-migraine drugs (e.g., triptans), still a significant number of migraineurs is insensitive to the currently available pharmacological approaches, suggesting that there are yet-to-be identified molecular and cellular players at the basis of the disease, which are not targeted by currently utilized drugs. For example, it is now believed that the trigeminal (TG)-brainstem sensory system plays a central role in the development and maintenance of migraine pain. Currently available pharmacological approaches to migraine mostly modulate neuronal activity, but it is now clear that TG neurons act in strict synergy with non-neuronal cells, in particular “satellite” glial cells (SGCs) that envelop neuronal bodies within the ganglion, to promote and maintain migraine-associated pain. Therefore, others and we have become interested in verifying whether SGCs might represent innovative pharmacological targets to migraine pain, and we have focused our research on the purinergic system, based on the knowledge that the crosstalk between neurons and SGCs is at least partly mediated by the activation of purinergic receptors responding to endogenously released nucleotides, like ATP and UTP. We are also interested in studying the cross-talk between the purinergic system and other known pro-algogenic signals, in order to test whether the modulation of purinergic receptors is involved in the mechanism of action of known pain-killers currently utilized by patients. To test our hypotheses, we utilized both mixed neuron-glia and purified glial cultures form mouse TG. We previously showed that the algogenic mediator bradykinin (BK) potentiates G protein-coupled purinergic P2Y-receptors on SGCs in primary trigeminal cultures (Ceruti et al., Cell Calcium 43:576-90, 2008), through the neuronal release of the pro-algogenic mediator calcitonin gene-related peptide (CGRP; Ceruti et al., J Neurosci 31:3638-49, 2011). Interestingly, the anti-migraine drug sumatriptan fully inhibited both CGRP release and glial P2Y-receptor potentiation, therefore suggesting a possible role for receptors activated by adenine and uracil nucleotides in the mechanism of action of this drug. Indeed, exposure to BK led to an increased production of PGE2, which was fully inhibited by the predominant COX-1 inhibitor acetyl salicylic acid (ASA), sometimes utilized to abort migraine attacks. The latter also blocked neuronal CGRP release, thus highlighting the existence of a complex cross-talk between arachidonic acid metabolites, the CGRP system and purinergic receptors, which modulate glial and neuronal cell functions in the TG. By a pharmacological approach employing selective antagonists, we have identified the P2Y receptor subtypes that are up-regulated in TG glial cells by BK and CGRP exposure, namely the ADP-responsive P2Y1 and the UTP-sensitive P2Y2 subtypes. Our data also suggest that their increased activity is both due to an increased receptor protein expression, but also, mostly for the P2Y1 subtype, to the modulation of their subcellular localization to membrane lipid rafts. Studies on the possible pro- or anti-algogenic role of these receptor subtypes are currently in progress. Interestingly, this complex molecular cross-talk between the purinergic system and known pro-algogenic substances was further potentiated in cell cultures from CaV2.1 a1 R192Q mutant knock-in (KI) mice expressing a human mutation causing familial hemiplegic migraine type 1, further suggesting the possible involvement of glial purinergic receptors in the onset and maintenance of migraine-associated pain. Overall, our findings suggest, for the first time, that P2Y receptors on glial cells act might represent new targets for the development of innovative therapeutic agents against migraine pain. The financial contribution of the Italian Comitato Telethon (project #GGP10082A) and of the Fondazione Cariplo (project #2011-0505) is gratefully acknowledged.