Behavioral study of non-evoked orofacial pain following different types of infraorbital nerve injury in rats

• Face grooming after different types of infraorbital nerve ligation was compared.
• Tight ligation of the infraorbital nerve has long lasting effects on face grooming.
• Formalin grooming does not predict grooming after infraorbital nerve ligation.
• Repeated testing increases statistical power and reduces the number of animals.

Directed isolated face grooming following unilateral chronic constriction injury to the infraorbital nerve (IoN-CCI) is a unique measure of spontaneous neuropathic pain. Variability between rats and the limited duration of the increased face grooming behavior has hampered its usefulness. We studied three possible sources of variability: variations in surgery, pre-existing differences in nocifensive behavior between the rats and variation in time. Three different types of IoN lesion were performed: loose ligation (CCI), tight ligation (CCI-T) and partial tight ligation (PTL, Seltzer method); the latter two offer greater surgical standardization. Face grooming behavior following IoN injury, on the one hand, and during the orofacial formalin test, on the other hand, was analyzed and correlated. Significant differences in isolated face grooming behavior were found between the IoN groups. Interestingly, CCI-T rats continued to show significantly increased isolated face grooming for the duration of the experiment, i.e., up to 32 days post-operative, whereas CCI animals were no longer significantly different from sham animals after two weeks. Surprisingly, PTL operated rats only showed minor effects. Variability was not smaller in the CCI-T or PTL group. Face grooming behavior after IoN lesion was poorly correlated to that in the orofacial formalin test. It is therefore unclear if pre-existing behavioral differences between animals are a major cause of variability in the IoN-CCI model. Finally, repeated testing showed significant variability in time. It is concluded that tight ligation of the IoN nerve has long-lasting effects on face grooming behavior and that part of the variability in face grooming behavior may be reduced by performing repeated testing.