The threshold for eliciting the jaw opening reflex in rats is not increased by neonatal capsaicin

Neonatal capsaicin administration has previously been shown to increase the threshold for withdrawal and escape response to noxious thermal, chemical and mechanical stimuli. The reason for these effects is destruction of afferent C-fibers and associated reduction in central substance P levels. In contrast, neonatal capsaicin treatment did not eliminate the "jaw opening reflex" (JOR) or even significantly increases its threshold when tested in rats 2-8 months of age. There was no significant difference in the voltage required to elicit the JOR nor was there any difference in its latency, in the animal that had received an intraperitoneal injection of 50 mg/kg capsaicin on the second day of life. A reduction of 85% unmyelinated fibers was found as compared with untreated animals. This data support earlier indications that upon incisor tooth pulp chamber stimulation in rats, the JOR is affectively activated by low-threshold myelinated fibers in the tooth pulp and surrounding periodontal ligament. Destruction of tooth pulp C-afferents, therefore, does not affect jaw opening reflex threshold.