Effects of noxious stimulation of orofacial tissues on rat licking behaviour

Objective: Since there is limited information of the effects of orofacial pain on oromotor behaviour, the aim of this study was to test the effects of injection of the algesic chemical glutamate into orofacial tissues on licking behaviour in rats. Design: Male Sprague-Dawley rats were trained to carry out stereotyped licking from a water spout connected to a force transducer, and once they met preset licking motor requirements (peak force of licking of 2-20 g, interlick interval of 0.11-0.19 s), their masseter muscle or tongue was injected with either isotonic saline (0.9%, 5 mu L) or glutamate (1.0 M, 5 mu L) and several parameters of their licking performance were re-assessed. Results: There were significant effects (P < 0.05; 1-way repeated measures ANOVA) of glutamate injection into the tongue on the number of clusters of licks, the number of licks per cluster, the intercluster period, the peak force of licking and the interlick interval, but there were no significant (P > 0.05) effects on licking of isotonic saline injection into the tongue or isotonic saline or glutamate injection into the masseter muscle. Conclusions: These findings provide the first documentation that noxious stimulation of the tongue, but not of the masseter muscle, has a modulatory effect on licking behaviour in the rat and suggest that the neural substrate for licking may be sensitive to selective nociceptive inputs from the orofacial region. (C) 2007 Elsevier Ltd. All rights reserved.