title: Two mechanisms involved in trigeminal CGRP release: implications for migraine treatment.
abstract: OBJECTIVE
  The goal of this study was to better understand the cellular mechanisms involved in proton stimulation of calcitonin gene-related peptide (CGRP) secretion from cultured trigeminal neurons by investigating the effects of 2 antimigraine therapies, onabotulinumtoxinA and rizatriptan. Stimulated CGRP release from peripheral and central terminating processes of trigeminal ganglia neurons is implicated in migraine pathology by promoting inflammation and nociception. Based on models of migraine pathology, several inflammatory molecules including protons are thought to facilitate sensitization and activation of trigeminal nociceptive neurons and stimulate CGRP secretion. Despite the reported efficacy of triptans and onabotulinumtoxinA to treat acute and chronic migraine, respectively, a substantial number of migraineurs do not get adequate relief with these therapies. A possible explanation is that triptans and onabotulinumtoxinA are not able to block proton-mediated CGRP secretion.
  
  
  METHODS
  CGRP secretion from cultured primary trigeminal ganglia neurons was quantitated by radioimmunoassay while intracellular calcium and sodium levels were measured in neurons via live cell imaging using Fura-2 AM and SBFI AM, respectively. The expression of acid-sensing ion channel 3 (ASIC3) was determined by immunocytochemistry and Western blot analysis. In addition, the involvement of ASICs in mediating proton stimulation of CGRP was investigated using the potent and selective ASIC3 inhibitor APETx2.
  
  
  RESULTS
  While KCl caused a significant increase in CGRP secretion that was significantly repressed by treatment with ethylene glycol tetraacetic acid (EGTA), onabotulinumtoxinA, and rizatriptan, the stimulatory effect of protons (pH 5.5) was not suppressed by EGTA, onabotulinumtoxinA, or rizatriptan. In addition, while KCl caused a transient increase in intracellular calcium levels that was blocked by EGTA, no appreciable change in calcium levels was observed with proton treatment. However, protons did significantly increase the intracellular level of sodium ions. Under our culture conditions, ASIC3 was shown to be expressed in most trigeminal ganglion neurons. Importantly, proton stimulation of CGRP secretion was repressed by pretreatment with the ASIC3 inhibitor APETx2, but not the transient receptor potential vanilloid-1 antagonist capsazepine.
  
  
  CONCLUSIONS
  Our findings provide evidence that proton regulated release of CGRP from trigeminal neurons utilizes a different mechanism than the calcium and synaptosomal-associated protein 25-dependent pathways that are inhibited by the antimigraine therapies, rizatriptan and onabotulinumtoxinA.
authors.affiliation: From the Center for Biomedical & Life Sciences, Missouri State University, Springfield, MO, USA.
authors.forename: Paul L
authors.initials: PL
authors.lastname: Durham
authors.affiliation: From the Center for Biomedical & Life Sciences, Missouri State University, Springfield, MO, USA.
authors.forename: Caleb G
authors.initials: CG
authors.lastname: Masterson
mesh_terms: D000700:Analgesics / Q000494:pharmacology*; D000818:Animals; D015153:Blotting, Western; D019274:Botulinum Toxins, Type A / Q000494:pharmacology; D015740:Calcitonin Gene-Related Peptide / Q000378:metabolism*; D002478:Cells, Cultured; D007150:Immunohistochemistry; D008881:Migraine Disorders / Q000378:metabolism*; D009474:Neurons / Q000187:drug effects* / Q000378:metabolism*; D011863:Radioimmunoassay; D051381:Rats; D017207:Rats, Sprague-Dawley; D014230:Triazoles / Q000494:pharmacology; D012668:Trigeminal Ganglion / Q000187:drug effects / Q000378:metabolism*; D014363:Tryptamines / Q000494:pharmacology
chemical_list: D000700:Analgesics; D014230:Triazoles; D014363:Tryptamines; C093622:rizatriptan; D019274:Botulinum Toxins, Type A; D015740:Calcitonin Gene-Related Peptide