ATP Tooth Pulp Stimulation Evokes Brainstem Reflexes and ERK Phosphorylation

OBJECTIVE: Extracellular ATP is involved in peripheral and central nociceptive transmission via P2X receptors. It has also been reported that P2X₃ and P2X_2/3 receptors as well as phosphorylated extracellular signal-regulated kinase (pERK) mechanisms may be involved in trigeminal central sensitization following acute inflammation of the tooth pulp. The aim of this study was to determine if stimulation of pulpal P2X receptors evokes brainstem reflexes and if brainstem pERK processes might contribute to these effects.

METHODS: Adult male Sprague-Dawley rats (280-350g) were anesthetized with halothane and EMG electrodes were implanted in anterior digastric (AD), genioglossus (GG) and masseter (MA) muscles bilaterally. The P2X₃ and P2X_2/3 receptor agonist α,β-meATP (100mM: n=11, 10mM: n=4) was applied to the exposed maxillary first molar tooth pulp to test for reflexly evoked EMG activities from these muscles. In some animals, the P2X₃ and P2X_2/3 receptor antagonist TNP-ATP (100μM: n=6, 1mM: n=6) was applied along with α,β-meATP. PBS was used as a vehicle control (n=7). We also analyzed expression of pERK-like immunoreactive (pERK-LI) cells in trigeminal subnucleus caudalis (Vc) and adjacent brainstem regions following tooth pulp application of α,β-meATP.

RESULTS: GG activity was significantly greater following application of 100mM α,β-meATP compared with PBS (p<0.05, Dunnett's test), an effect that was antagonized by co-application of TNP-ATP dose-dependently (100μM and 1mM: p<0.05, Tukey-Kramer test). Following α,β-meATP application, pERK-LI cells were mainly expressed in the dorsal half of the trigeminal subnucleus interpolaris, Vc/interpolaris transition zone, middle part of Vc, and Vc/ upper cervical cord transition zone . The number of pERK-LI cells was smaller in rats receiving α,β-meATP and TNP-ATP compared with rats receiving only α,β-meATP.

CONCLUSION: The findings suggest that tooth pulp pain may involve peripheral P2X receptor mechanisms and associated brainstem pERK processes.

Supported by grants NIH DE04786 & Japan-Canada Joint Health Research Program 07033211-000165.