Fish oil concentrate delays sensitivity to thermal nociception in mice

Abstract

Fish oil has been used to alleviate pain associated with inflammatory conditions such as rheumatoid arthritis. The anti-inflammatory property of fish oil is attributed to the n-3 fatty acids docosahexaenoic acid and eicosapentaenoic acid. Contrarily, vegetable oils such as safflower oil are rich in n-6 fatty acids which are considered to be mediators of inflammation. This study investigates the effect of n-3 and n-6 fatty acids rich oils as dietary supplements on the thermally induced pain sensitivity in healthy mice. C57Bl/6J mice were fed diet containing regular fish oil, concentrated fish oil formulation (CFO) and safflower oil (SO) for 6 months. Pain sensitivity was measured by Plantar test and was correlated to the expression of acid sensing ion channels (ASICs), transient receptor potential vanilloid 1 (TRPV1) and c-fos in dorsal root ganglion cells. Significant delay in sensitivity to thermal nociception was observed in mice fed CFO compared to mice fed SO (p<0.05). A significant diminution in expression of ion channels such as ASIC1a (64%), ASIC13 (37%) and TRPV1 (56%) coupled with reduced expression of c-fos, a marker of neuronal activation, was observed in the dorsal root ganglion cells of mice fed CFO compared to that fed SO. In conclusion, we describe here the potential of fish oil supplement in reducing sensitivity to thermal nociception in normal mice.

Figure 1

Plantar assay of (A) C57Bl/6J mice and (B) BW mice after 6 months on diets. Data represent mean ± SEM (n=6). CFO significantly delayed the pain sensitivity in both the strains of mice compared to SO fed mice at a level of significance p<0.05 by Student’s t-test.

Figure 2

c-fos immuno-staining of the DRGs isolated from mice fed CFO and SO. L3–L5 Dorsal root ganglion was isolated from mice fed CFO and SO and processed for, A) H&E staining for structural verification (original magnification, 20×). B) Section of a DRG was immuno-stained with rabbit anti-human c-fos antibody (original magnification, 40×). Development was carried out using the appropriate Vector Stain kit (Vector Labs, Burlingame, CA), and counterstained with hematoxylin. Photomicrographs were recorded using a light microscope. Brown staining represents the presence and intensity of c-fos. C) Quantitation of relative c-fos immuno-reactive area in DRG of mice fed diets (mean±SE, n=6). Relative c-fos immuno-reactive area was found to be significantly lower (p<0.05 by Student’s t-test) in DRG of CFO fed mice indicating reduced expression of c-fos.

Figure 3

Quantitative RT-PCR analysis of ASIC1a, ASIC3 and TRPV1 in dorsal root ganglion of C57Bl/6J mice (n=6) fed CFO and SO enriched diets. DRGs were isolated from L3–L5 region of mice fed experimental diets and processed for RNA isolation. Relative quantification of mRNA was expressed as fold change with respect to SO diet using 2^−ΔΔCt after normalization of the C_t data to that of the housekeeping gene, GAPDH. Data represent the mean ± S.E.M. A significant reduction in the expression of ASIC1a, ASIC3 and TRPV1 was observed in DRGs of mice fed CFO compared to that in SO fed mice. *p<0.05; **p<0.001 vs SO fed mice, according to Unpaired Student’s t-test.