Objective: The purpose of this study was to investigate pulp responses after CO2 laser irradiation of rat molars using reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemical methods. Background data: Many kinds of lasers have been used for the treatment of dentin hypersensitivity. However, the immunohistochemical details of the responses of dentin-pulp complex cells after CO2 laser irradiation through the dentin are still not clear. Methods: Adult male Sprague–Dawley rats were used in this study. A CO2 laser (wavelength of 10.6 μm, 2 W in the super pulse mode, pulse 0.6 ms) and total laser energy of 4 J (density, 203.84 J/cm2) was used. The temperature change in the pulp was measured using a super fine sheath thermocouple. Results: The temperature at the pulp side after the CO2 laser irradiation increased 22.5°C. The expression of tumor necrosis factor-alpha (TNF-α) and interleukin (IL)-1-α mRNAs was significantly higher at 6, 12, and 24 h after laser irradiation than in the control group (p<0.05). Histologically, a slight degeneration of the pulp tissue was observed immediately after the laser irradiation. Immunohistochemically, heat shock protein (HSP)-70-positive cells in the pulp horn were observed immediately after irradiation and cells positive for vascular endothelial growth factor (VEGF) in the subodontoblast layer were observed after 5 days. There were no nestin-positive cells immediately, but after 5 days nestin-positive cells were recognized in the deeper pulp cells. Immediately after irradiation, neurofilament protein (NFP)-positive nerve fibers were observed but they disappeared in the pulp horn after 5 days. Conclusions: These results suggest that 203.84 J/cm2 CO2 laser irradiation of pulp tissue through the dentin, such as is used in the clinic, induces inflammatory and pathological cytokine pathways to repair the damaged pulp tissue.
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