| Objicetive To investigate the interaction of TrkB receptor, NMDA receptor and opiate mu receptor and the effect of fentanyl to the expression of these kinds of receptor through observation of the change of coexistence between these kinds of receptors in the spinal dorsal horn and dorsal root ganglion following peripheral inflammatory hyperalgesia in rats.Method Fifty four adult male SD rats which were successfully intrathecal catheterized (IT) weighing 200-250g were randomly divided into 3 groups (n=18 each): group A normal animal received IT normal saline 25μl once a day for 8 days starting from the 1st day after normal saline injection; group B arthritic animal received IT normal saline 25μl once a day for 8 days starting from the 1st day after CFA injection; group C arthritic animal received IT fentanyl 1μg in 25μl once a day for 7 days starting from the 1st day after CFA injection. Every group were divided into three subgroup determined by the timespan after CFA or normal saline injection: 2nd subgroup, 4th subgroup, 8th subgroup. Arthritis was induced by injection of complete Freund's adjuvant (CFA) 50μl into the right ankle joint and was confirmed by swelling redness and decreased spontaneous activity of the joint. Latency for removal of left and right hind paw to noxious thermal stimulation was measured in hot plate test every day for 8 days starting from the 1st day after CFA injection, and hyperalgesia score was calculated: hyperalgesia score (second) =left foot latency -right foot latency. Movement score, standing score, joint flexion and expansion score was measured at the same time. The animals were anesthetized with intraperitoneal 10% chloral hydrate and the L5-L7 lumbar segment of spinal cord and the unilateral dorsal root ganglion of the same segment was removed for determination of expression and coexistence of the three kinds of receptors (by double-labled immunofluorescence) in spinal dorsal horn and dorsal root ganglion on the 2nd, 4th, 8th day after CFA or normal saline injection abide by subgroup. The spinal cord and dorsal root ganglion tissues were taken to be cut into sections (10μm) which were then treated with double immuno-fluorescent labeling techniques. The number of double-labelled positive cells was calculated and analyzed by using fluorescent microscope and image analysis software.Results Compared with group A, Movement score, standing score, joint flexion and expansion score and hyperalgesia score were significantly higher in group B. In comparison with group B, the above-mentioned index were significantly lower except movement score, the 2nd stand score and the 8th joint flexion and expansion score. The number of double-labelled cells indicating TrkB receptor and NMDA receptor: in spinal dorsal horn, compared with group A, the number was higher in group B; compared with group B, the number decreased in group C.The number of double-labelled cells indicating TrkB receptor and opiateμreceptor: in spinal dorsal horn, compared with group B, the number was higher in group C; in dorsal root ganglion, in comparison with group A, the number was higher in group B all the time.The number of double-labelled cells indicating NMDA receptor and opiateμreceptor: in spinal dorsal horn, compared with group A, the number was higher in group B;, compared with group B, the number was higher in group C. In dorsal root ganglion, in comparison with group A, the number decreased on the 2nd and 4th day in group B. On the 8th day, the number increased to the normal level in group B. Conclusion There are wide coexistences of TrkB receptor, NMDA receptor and opiateμreceptor in spinal dorsal horn and dorsal root ganglion. The three kinds of receptors will be changed in the same direction induced by inflammatory pain. There are cross-talks in the signal transduction level between the three kinds of receptors. Fentanyl not only acts on opiateμreceptor but NMDA receptor, TrkB receptor as well to enduce its pharmacological action. The action on NMDA receptor will induce the development of opioid tolerance. The action on TrkB receptor will reinforce analgesic effect of fentanyl.
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