Depolarization Stimulates Initial Calcitonin Gene-Related Peptide Expression by Embryonic Sensory NeuronsIn Vitro

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Abstract
The neuropeptide calcitonin gene-related peptide (CGRP) is expressed by one-third of adult rat lumbar dorsal root ganglion (DRG) neurons, many of which mediate pain sensation or cause vasodilation. The factors that regulate the developmental expression of CGRP are poorly understood. Embryonic DRG neurons initially lack CGRP. When these neurons were stimulated in culture by serum or persistent 50 mmKCl application, the same percentage of CGRP-immunoreactive (CGRP-IR) neurons developedin vitroas was seen in the adult DRGin vivo. The addition of the L-type calcium channel blockers, 5 μmnifedipine or 10 μmverapamil, dramatically decreased the proportion of CGRP-IR neurons that developed, although the N-type calcium channel blocker, 2.5 μmω-conotoxin, was less effective. By contrast, the sodium channel blocker 1 μmtetrodotoxin had no effect on CGRP expression after depolarization. Fura-2 ratiometric imaging demonstrated that mean intracellular free calcium levels increased from 70 to 135 nmwith chronic depolarization, and the addition of nifedipine inhibited that increase. Only a subpopulation of neurons had elevated calcium concentrations during chronic depolarization, and they were correlated with CGRP expression. Key signal transduction pathways were tested pharmacologically for their role in CGRP expression after depolarization; the addition of the CaM kinase inhibitor KN-62 reduced the proportion of CGRP-IR neurons to basal levels. By contrast, protein kinase A and protein kinase C were not implicated in the depolarization-induced CGRP increases. These data suggest that depolarization and the subsequent Ca2+-based signal transduction mechanisms play important roles in thede novoexpression of CGRP by specific embryonic DRG neurons.