Expression of distinct Mas-related gene (Mrg/Mrgpr) receptors in dental pulp

Tooth pain is a common form of acute pain experienced by humans, yet there is a lack of detailed knowledge about the signaling pathways that produce it. Interactions between sensory nerve endings and other cells, including fibroblasts, odontoblasts and immune cells, are important in mediating inflammation and pain processing in dental pulp. Members of the Mas-related gene (Mrg/Mrgpr) family of receptors are selectively expressed in small-diameter sensory neurons, and are implicated in modulating nociceptive responses in skin; however, their role in dental pain pathways is unknown. Our working hypothesis is that Mrg receptors modulate nociceptive responses in dental pulp. Objectives: i) to identify the type/s of Mrg receptors expressed in human dental pulp and ii) to test whether carious pulp is associated with increased expression of Mrgs compared to healthy pulp. Methods: Dental pulp from individual tooth samples was isolated for analysis. mRNA was isolated from individual pulp tissue samples (N=8), and subjected to reverse transcription (RT) followed by quantitative PCR (QPCR) using Mrg specific primers. Amplification products from selected samples were analyzed by sequencing. Expression of Mrg proteins was determined by immunostaining (N=6) and Western blot analysis (N=6). Results: cDNA fragments of the correct size and sequence, corresponding to selected regions from three pharmacologically distinct Mrg receptors were successfully amplified using primers specific for each Mrg.  MrgF transcripts showed the highest expression in healthy pulp (N=4), and also showed the highest increase in expression in carious pulp (N=4). MrgX1 and MrgE immunoreactivity was present in pulp tissue, and was colocalized with nerve fibers. Conclusions: The presence of distinct Mrg receptors in human dental pulp (N=20), together with their predicted role in pain modulation strongly suggest that Mrg receptors have specific functions in dental nociception, and are potential targets for development of novel dental analgesics.