Abstract
Expression of the neuropeptide calcitonin gene-related peptide (CGRP) in trigeminal ganglion is implicated in neurovascular headaches and temporomandibular joint disorders. Elevation of cytokines contributes to the pathology of these diseases. However, a connection between cytokines and CGRP gene expression in trigeminal ganglion nerves has not been established. We have focused on the effects of the cytokine tumor necrosis factor-alpha (TNF-alpha). TNFR1 receptors were found on the majority of CGRP-containing rat trigeminal ganglion neurons. Treatment of cultures with TNF-alpha stimulated CGRP secretion. In addition, the intracellular signaling intermediate from the TNFR1 receptor, ceramide, caused a similar increase in CGRP release. TNF-alpha caused a coordinate increase in CGRP promoter activity. TNF-alpha treatment activated the transcription factor NF-kappaB, as well as the Jun N-terminal kinase (JNK) and p38 mitogen-activated protein (MAP) kinase pathways. The importance of TNF-alpha induction of MAP kinase pathways was demonstrated by inhibiting MAP kinases with pharmacological reagents and gene transfer with an adenoviral vector encoding MAP kinase phosphatase-1 (MKP-1). We propose that selective and regulated inhibition of MAP kinases in trigeminal neurons may be therapeutically beneficial for inflammatory disorders involving elevated CGRP levels.