An Operant Conditioning Model to Assess Changes in Feeding Behavior Associated with Temporomandibular Joint Inflammation in the Rat

Aims: To develop and validate a model in which to assess a loss of function associated with temporomandibular joint (TMJ) inflammation in awake, freely moving rats. Methods: The dependent variable in the model was the time between food rewards (pellets), or interfeeding interval (IFI). IFI was quantified after rats were trained to "bar-press" for food. To validate use of the IFI as a surrogate for temporomandibular disorder (TMD) pain, we determined the impact of several manipulations, including changes in pellet size, the presence and severity of inflammation of the TMJ, masseter muscle, or skin (induced with complete Freund's adjuvant [CFA]), and the influence of preadministration of the non-steroidal anti-inflammatory drug indomethacin (4 mg/kg). Furthermore, in order to determine whether a change in IFI reflected an increase in the time rats spent eating, rats were videotaped, and the amount of time spent eating, grooming, and exploring was analyzed. Results: Inflammation of the TMJ or masseter muscle resulted in significant dose- and pellet size-dependent increases in the IFI. Inflammation of the skin overlying the TMJ had no effect on IFI. Pre-administration of indomethacin reversed the inflammation-induced shift in the IFI. An inflammation-induced increase in IFI was associated with an increase in feeding time. Conclusions: Our model constitutes a relatively fast and sensitive method with which to assess changes in feeding behavior associated with TMJ inflammation. Only 2 days of training are required to obtain a stable baseline IFI. It is possible to detect changes in IFI as small as 40% with 12 rats per group. Keywords: inflammation, nociception, pain, temporomandibular joint disorders