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The Chinese Journal of Dental Research

Year 2002
Volume 5 , Issue 3

Pages: 30 - 35

C-fos Expression in the Caudal Brainstem, Rather than Face-rubbing Behavior, Reflects the Intensity of the Noxious Stimulus in the Formalin Orofacial Pain Model

Kai-Yuan Fu, DDS, PhD/Li Cao/Xu-Chen Ma/Zhen-Kang Zhang

Objective: The formalin pain animal model has been widely used, and was also applied to the study of orofacial pain. The aim of the study is to detect whether c-fos expression in the brainstem or face-rubbing behavior reflects the intensity of the noxious stimuli by injecting formalin subcutaneously into the orofacial region. Methods: All 25 Sprague-Dawley rats were divided into 5 treatment groups, with 5 rats in each group Group I to group III animals received respectively 5%, 2.5% and 1.5% formalin in solution, 50 µl being injected subcutaneously into the right upper lip. Group IV was the control (receiving 50 µl of a 0.9% saline solution), and group V animals were treated with 100 µl of a 1% articaine solution 10 minutes before the same volume of a 5% formalin injection. The animals’ nociceptive behavior was recorded 1 hour after injection, c-fos immunohistochemistry in the brainstem was performed after 2 hours, and c-fos expression was quantitatively analyzed. Results: The orofacial formalin injection produces a characteristic two-phase face-rubbing behavior which was related to pain, but higher concentrations of formalin could not induce more face-rubbing behavior. The degree of c-fos expression in the subnucleus caudalis (Vc) was, however, closely associated with the concentration of the injected formalin. Conclusion: The formalin test is a convenient method for the study of orofacial pain, and the face-rubbing behavior is one objective, being a reproducible and quantifiable response related to pain intensity, but 2.5% formalin or less was recommended. C-fos expression is a useful marker for nociceptive neurons, and the magnitude of expression is related to stimulus intensity.



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