A Behavioral Model for Spontaneous Orofacial Pain in Rats

Objectives: One of the most common clinically reported symptoms of pain is spontaneous, chronic pain. Few studies have explored spontaneous pain-related behaviors after pulpal insult in animals. We aimed to determine novel behavioral indicators of dental pain and to determine transitions from acute to chronic pain following pulp exposure in rats. Methods: We have extended previous studies by including novel indicators of pain-related behavior, significantly increasing the quantity of analyzed pre- and post-treatment behavioral data and conducted our experiments without the placement of pulpal dyes or restorations. We made the following observations in adult, male rats prior to and following mechanical tooth pulp exposure: grooming, resting, standing, ultrasonic vocalizations, time spent exploring, exploratory distance and cold versus room temperature water preference. Pre- and post treatment observations were made by continuously recording audio and video in 30 minute sessions for four days prior to and four days following treatment. Profound anesthesia was obtained with 3% isoflurane and pulp exposures were made with a ¼ round bur on the first and second maxillary molars and the first mandibular molar until pulpal blood was observed (n=8). Naïve animals received anesthesia, their mouths were opened but no pulp exposures were performed (n=8). Results: Exploring and standing times significantly decreased 1-2 days following treatment in both naïve and pulp exposed animals. However, pulp exposed animals demonstrated significant increases in exploring and standing times on days 3-4 when compared to their naïve counterparts (unpaired t-tests; p<0.05). Interestingly, ultrasonic vocalizations significantly increased in both groups on the day of the treatment but no significant differences in vocalizations existed in pulp exposed rats on post-treatment days 1-4. Conclusions: Our observations may provide behavioral indicators of the transition from acute to chronic pain as well as a means to test the efficacy of possible therapeutics directed at nociceptive channels.