CGRP Modulation of Intracellular Calcium Levels in Trigeminal Ganglion Neurons and Glia

Elevated levels of calcitonin gene-related peptide (CGRP) are implicated in migraine and TMD pathology, but its effects on the excitability state of Aδ and C-fiber neurons and glia have not been investigated. The goal of this study was to determine changes mediated by CGRP on intracellular calcium levels in trigeminal ganglion neurons and glia in response to depolarizing stimulation. Intracellular calcium levels were determined in Aδ and C-fiber neurons of primary trigeminal ganglion cultures obtained from neonatal Sprague Dawley rats using Fura-2 and fluorescent microscopy. Cells were left untreated or preincubated for 2 hours with CGRP, then incubated with the depolarizing stimuli KCl or ATP. Data analysis was performed using Olympus CellSens Dimension software and JASP. CGRP greatly increased the calcium amplitude in response to 60 mM KCl and 100 µM ATP in Aδ neurons, while causing a smaller, similar response in C-fiber neurons. CGRP also increased the percentage of Aδ neurons responsive to 60 mM KCl and enhanced the magnitude of the calcium response. In glial cells, CGRP increased the magnitude of the 60 mM KCl-mediated response. However, CGRP suppressed the stimulatory calcium response to 15 mM KCl in neurons and glial cells and differentially modulated the calcium response to 30 mM KCl in Aδ and C-fiber neurons. These results provide evidence of a novel role of CGRP in differentially regulating the excitability state of Aδ and C-fiber neurons and glial cells implicated in pain signaling in migraine and TMD in response to the strength of the depolarizing stimulus.