Orofacial pain has been well-characterized clinically, but evaluation of orofacial pain in animals has not kept pace. The objective of this study was to describe behavioral responses to facial thermal stimulation and inflammation with/without an analgesic using a novel operant paradigm. Animals were trained to voluntarily place their face against a stimulus thermode (37.7–57.2 °C) providing access to positive reinforcement. These contingencies present a conflict between positive reward and tolerance for nociceptive stimulation. Inflammation was induced and morphine was provided as an analgesic in a subset of animals. Six outcome measures were determined: reward intake, reward licking contacts, stimulus facial contacts, facial contact duration, ratio of reward/stimulus contacts, and ratio of facial contact duration/event.
Animals displayed aversive behaviors to the higher temperatures, denoted by a significant decrease in reward intake, total facial contact duration, and reward licking events. The number of facial contacts increased with increasing temperature, replacing long drinking bouts with more frequent short drinks, as reflected by a low ratio of facial contact duration/event. The number of reward licking/facial contact events was significantly decreased as the thermal stimulus intensity increased, providing another pain index derived from this operant method. These outcomes were significantly affected in the direction of increased nociception following inflammation, and these indices of hyperalgesia were reversed with morphine administration. These data reflect an orofacial pain behavior profile that was based on an animal's responses in an operant escape paradigm. This technique allows evaluation of nociceptive processing and modulation throughout the neuraxis.
aDepartment of Orthodontics, College of Dentistry, University of Florida, 1600 SW Archer Road, P.O. Box 100444, Gainesville, FL 32610-0444, USA
bDepartment of Oral Surgery, College of Dentistry, University of Florida, Gainesville, FL, USA
cDepartment of Neuroscience, College of Medicine, University of Florida, Gainesville, FL, USA
dEvelyn F. and William L. McKnight Brain Institute, University of Florida, Gainesville, FL, USA
*Corresponding author. Tel.: +1 352 392 1126; fax: +1 352 846 0459.
Received 4 February 2005; received in revised form 18 April 2005; accepted 3 May 2005.