[The relationship between diffuse noxious inhibitory control effects and the magnitude of noxious stimuli]

Diffuse noxious inhibitory control (DNIC) is a phenomenon by which the effect of noxious stimuli is inhibited by heterotopic noxious stimulation. We investigated whether heterotopic conditioning stimuli could modify the second component of EMG activities of the biceps femoris reflex (C-fiber reflex) with different levels of intensity. Forty-two adult male anesthetized Wistar rats were divided into 6 groups with 6 or 8 rats in each. Three groups (8 rats in each group) were stimulated with 3 intensity levels of the electrical test stimuli, 1.2 T, 2.0 T, or 3.0 T (times the C-fiber reflex threshold), respectively. Conditioning stimuli (pinch) were delivered to the upper lip of each rat. Test stimuli were delivered to the digits of the hindpaw. Another 3 groups (6 rats in each group) as control groups were given similar test stimuli without conditioning stimuli. The inhibitory magnitudes of C-fiber reflex activities between the control and lip pinch were 215.5 +/- 105.5, 277.7 +/- 159.1, 462.3 +/- 407.9 microV x ms in group 1.2 T, 2.0 T, and 3.0 T, respectively. The inhibitory magnitudes of C-fiber reflex activities in group 3.0 T at pinch were significantly increased compared with group 1.2 T and 2.0 T. The present study demonstrated that the C-fiber reflexes were inhibited by conditioning stimuli with 3 different levels of test stimuli under sevofurane anesthesia. Though it has been considered that DNIC effect does not depend on test stimulus intensity, the present study suggested that DNIC effect might be affected by the intensity of test stimuli.