Objective validation of central sensitization in the rat UVB and heat rekindling model

Abstract

Background: The UVB and heat rekindling (UVB/HR) model shows potential as a translatable inflammatory pain model. However, the occurrence of central sensitization in this model, a fundamental mechanism underlying chronic pain, has been debated. Face, construct and predictive validity are key requisites of animal models; electromyogram (EMG) recordings were utilized to objectively demonstrate validity of the rat UVB/HR model.

Methods: The UVB/HR model was induced on the heel of the hind paw under anaesthesia. Mechanical withdrawal thresholds (MWTs) were obtained from biceps femoris EMG responses to a gradually increasing pinch at the mid hind paw region under alfaxalone anaesthesia, 96 h after UVB irradiation. MWT was compared between UVB/HR and SHAM-treated rats (anaesthetic only). Underlying central mechanisms in the model were pharmacologically validated by MWT measurement following intrathecal N-methyl-d-aspartate (NMDA) receptor antagonist, MK-801, or saline.

Results: Secondary hyperalgesia was confirmed by a significantly lower pre-drug MWT {mean [±standard error of the mean (SEM)]} in UVB/HR [56.3 (±2.1) g/mm(2) , n = 15] compared with SHAM-treated rats [69.3 (±2.9) g/mm(2) , n = 8], confirming face validity of the model. Predictive validity was demonstrated by the attenuation of secondary hyperalgesia by MK-801, where mean (±SEM) MWT was significantly higher [77.2 (±5.9) g/mm(2) n = 7] in comparison with pre-drug [57.8 (±3.5) g/mm(2) n = 7] and saline [57.0 (±3.2) g/mm(2) n = 8] at peak drug effect. The occurrence of central sensitization confirmed construct validity of the UVB/HR model.

Conclusions: This study used objective outcome measures of secondary hyperalgesia to validate the rat UVB/HR model as a translational model of inflammatory pain.

Fig 1

Figures showing pre-drug mechanical withdrawal threshold (MWT) to a pinch of increasing magnitude applied to the mid-region (i.e., area of secondary hyperalgesia) of the ipsilateral hind paw in UVB and heat rekindled (UVB/HR) and SHAM-treated rats. An example of raw data from a pre-drug recording is demonstrated in (Ai) from a UVB/HR rat and (Aii) from a SHAM-treated rat. Scale bars correlate to both (Ai) and (Aii). (B) Scatter plot showing overall pre-drug MWT in UVB/HR (n = 15) and SHAM-treated rats (n = 8). Each point represents data from an individual animal 4 days after initial UVB treatment or SHAM treatment. Horizontal lines represent mean ± standard error of the mean. **p < 0.01, unpaired t-test.

Fig 2

Figures showing the effect of intrathecal administration of N-methyl-d-aspartate receptor antagonist, MK-801 or saline on mechanical withdrawal threshold (MWT) in response to a pinch of increasing magnitude in the mid-region (i.e., area of secondary hyperalgesia) of the ipsilateral hind paw. An example of raw data from a UVB/HR rat is demonstrated in (Ai) showing a pre-drug MWT response and (Aii) MWT 30 min after MK-801 administration in the same rat. Scale bars correlate to both (Ai) and (Aii). (B) Time course of drug action in all groups [UVB/HR + MK-801 = 7 (squares), UVB/HR + saline =8 (circles), SHAM + MK-801 = 8 (triangles)] where MK-801 or saline was administered immediately after obtaining pre-drug responses. Post-drug responses were recorded every 15 min for 60 min, where each data point represents mean ± standard error of the mean 4 days after initial UVB or SHAM treatment. **p < 0.01 within group, ##p < 0.01 between groups, two-way analysis of variance following by Bonferroni post hoc test.