Effect of carbon dioxide on calcitonin gene-related peptide secretion from trigeminal neurons.
OBJECTIVE
The goal of this study was to determine whether the physiological effects of carbon dioxide (CO(2)) involve regulation of CGRP secretion from trigeminal sensory neurons.


BACKGROUND
The neuropeptide calcitonin gene-related peptide (CGRP) is implicated in the pathophysiology of allergic rhinosinusitis and migraine. Recent clinical evidence supports the use of noninhaled intranasal delivery of 100% CO(2) for treatment of these diseases. Patients report 2 distinct physiological events: first, a short duration stinging or burning sensation within the nasal mucosa, and second, alleviation of primary symptoms.


METHODS
Primary cultures of rat trigeminal ganglia were utilized to investigate the effects of CO(2) on CGRP release stimulated by a depolarizing stimulus (KCl), capsaicin, nitric oxide, and/or protons. The amount of CGRP secreted into the culture media was determined using a CGRP-specific radioimmunoassay. Intracellular pH and calcium levels were measured in cultured trigeminal neurons in response to CO(2) and stimulatory agents using fluorescent imaging techniques.


RESULTS
Incubation of primary trigeminal ganglia cultures at pH 6.0 or 5.5 was shown to significantly stimulate CGRP release. Similarly, CO(2) treatment of cultures caused a time-dependent acidification of the media, achieving pH values of 5.5-6 that stimulated CGRP secretion. In addition, KCl, capsaicin, and a nitric oxide donor also caused a significant increase in CGRP release. Interestingly, CO(2) treatment of cultures under isohydric conditions, which prevents extracellular acidification while allowing changes in PCO(2) values, significantly repressed the stimulatory effects of KCl, capsaicin, and nitric oxide on CGRP secretion. We found that CO(2) treatment under isohydric conditions resulted in a decrease in intracellular pH and inhibition of the KCl- and capsaicin-mediated increases in intracellular calcium.


CONCLUSIONS
Results from this study provide the first evidence of a unique regulatory mechanism by which CO(2) inhibits sensory nerve activation, and subsequent neuropeptide release. Furthermore, the observed inhibitory effect of CO(2) on CGRP secretion likely involves modulation of calcium channel activity and changes in intracellular pH.
Missouri State University-Biology, Department of Biology, Springfield, MO 65897, USA.
Carrie
C
Vause
Elizabeth
E
Bowen
Egilius
E
Spierings
Paul
P
Durham
D000818:Animals; D015740:Calcitonin Gene-Related Peptide / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D002211:Capsaicin / Q000494:pharmacology; D002245:Carbon Dioxide / Q000494:pharmacology*; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D006863:Hydrogen-Ion Concentration; D008297:Male; D009475:Neurons, Afferent / Q000187:drug effects*; D020030:Nitric Oxide Donors / Q000494:pharmacology; D010396:Penicillamine / Q000031:analogs & derivatives / Q000494:pharmacology; D011189:Potassium Chloride / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D013997:Time Factors; D012668:Trigeminal Ganglion / Q000166:cytology*
D020030:Nitric Oxide Donors; C110881:S-nitro-N-acetylpenicillamine; D002245:Carbon Dioxide; D011189:Potassium Chloride; D010396:Penicillamine; D015740:Calcitonin Gene-Related Peptide; D002211:Capsaicin; D002118:Calcium