Effect Of Intrathecal Administration Recombinant Rat Interleukin-18on Pain Threshold And Morphine Analgesic Efficiency In Normal Rats

Objective:To evaluate the effect and possible underlying mechanisms of intrathecal administration recombinant rat interleukin-18(rrIL-18) on the formation of morphine tolerance in rats.Methods:Forty-eight male Sprague-dawley rats weighing150-180g were randomly divided into four groups(n=12each). After intrathecal catheterized, the rats in group I received20μl PBS and the others in group Ⅱ to Ⅳ received0.2,2and10μg rrIL-18(soluted in20μl PBS) once a day for consecutive7days respectively. Paw withdrawal thermal latency (PWTL) before and45min after injection of morphine(4mg/kg) via tail vein was measured on1day before intrathecal administration and8,10,12day after intrathecal administration. The percentage of maximum possible efficiency (MPE%) of morphine was calculated according to the equation (MPE%=(after IT administration morphine-basic values/cutoff-basic values) X100%). The animals were sacrificed at day12after behavioral testing, the lumbar spinal cord was removed for determination of the expression of the glial fibrillary acidc protein(GFAP) by immunofluorescence(IF) and the phosphorylated extracellular signaling-regulated kinases42/44(p-ERK42/44) by western blot respectively. All data were expressed as mean+/-standard deviation (x±s), then analysed by SPSS18.0statistical package. The behavioral results were compared using analysis of variance of repeated measurement design and the molecular semiquantitative data were compared using the single factor analysis of variance. Then following comparisons between two groups were analysed using LSD testing. P<0.05for the difference was statistically significant. Results:Compared with those in group Ⅰ, the PWTL before morphine injection and MPE%of8,10,12days was significantly decreased in group Ⅲ, Ⅳ. Furthemore the immunofluorescence intensity of GFAP in lumbar spinal cord was significantly enhanced and the astrocytes showed a significant increase in the number, the cell body, protuberant coarsens in group III and IV at day12in comparsion with those in group I. Lastly, the expression of p-ERK42/44was markedly elevated in the group III and IV compared with the in the group I at day12.Conclusions:Intrathecal administration2and10μg IL-18for seven days could result in the formation of morphine tolerance. This effect might be due to IL-18binding with IL-18receptor on the astrocytes, subsequently promoting the ERK phosphorylation and activation of astrocyte.