Objective.— The goal of this study was to determine whether the physiological effects of carbon dioxide (CO2) 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% CO2 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 CO2 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 CO2 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, CO2 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, CO2 treatment of cultures under isohydric conditions, which prevents extracellular acidification while allowing changes in PCO2 values, significantly repressed the stimulatory effects of KCl, capsaicin, and nitric oxide on CGRP secretion. We found that CO2 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 CO2 inhibits sensory nerve activation, and subsequent neuropeptide release. Furthermore, the observed inhibitory effect of CO2 on CGRP secretion likely involves modulation of calcium channel activity and changes in intracellular pH.