Characterization of a Novel Assay for Orofacial Hyperalgesia and Allodynia

Objectives: The aim of this study was to characterize behavioral responses to mechanical and thermal stimuli to the vibrissal pad region of rats using a new model to evaluate orofacial nociception. Methods: Animals were trained in 20 min intervals to voluntarily place their face against either thermal or mechanical stimuli to provide access to a reward, a bottle with condensed milk solution. Copper tubing shaped to contact the vibrissal pad region and warmed by heated water circulating through it was used to deliver the thermal test stimulus; animals were tested over a temperature range of 40-63ºC. The mechanical stimuli was provided by an insert with 0-32 nickel titanium wires attached to a plastic ring which made contact with the vibrissal pad region when the animal accessed the reward. Experimental testing sessions were 10 minutes and endpoints that were measured were the duration of feeding, measured by accumulated breakage of an infrared beam which crosses the opening to the reward, and number of feeding attempts, measured by the number of times the infrared beam was broken. Results: A significant decrease in feeding time and increase in number of feeding attempts was observed with increasing temperature. Similar to the thermal stimulus response a decrease in feeding time was also detected with increasing mechanical force induced by increased numbers of nickel titanium wires, although the number of beam breaks did not correspond to an increase in force. Conclusions: A decrease in feeding time was evaluated as an increase in nociception. Feeding duration was seen to decrease with increasing thermal and mechanical stimuli. This behavioral model will be a useful index of animal nociception and possible therapeutics in the orofacial region. This work was supported by NIDCR grant DE14318 for CO STAR.