Mechanism Underlying Analgesia Induced By Small-Dose Morphine Intra-Articular Injection In The Arthritic Rat

As the major symptom of arthritis,pain seriously affects patient’s life quality.Thus,relieving this kind of pain has become the top priority when treating arthritis.Previous studies have confirmed that opioid receptors are distributed in the central nervous system(CNS)and peripheral tissues.Opioids can exert analgesic effects when administrated in the peripheral neuropathy disease,including arthritis,however,the mechanisms underlying this peripheral analgesic effects are still not clear.Considering these,herein we combined morphological,molecular,and behavioral methods to explore the distribution of μ-opioid receptors(MORs)in synovial tissues,on the basis of which the mechanisms underlying intra-articular injection of small-dose morphine induced analgesia were investigated.1.Morphological study of MOR-immunopositive nerve fibers distributed in synoviumThe distribution of MOR-immunopositive(MOR-IP)nerve fibers in synovium was observed by double-labeling immunofluorescence and immunoelectron microscopy.The results showed that:(1)In synovium,MOR-IPs are located in primary afferent fibers,mainly concentrated in calcitonin gene-related peptide(CGRP)-IP peptidergic fibers.(2)The MOR-IP products are mainly located in unmyelinated nerve fibers in synovium,with a small amount in myelinated nerve fibers.These morphological study results indicate:(1)MOR-IP fibers are distributed in synovial tissue,which might be the target where morphine exerts its analgesic effects when administrating into the joint cavity.(2)Synovial fibers which express MORs may originate from the dorsal root ganglion neurons,through which peripheral noxious information is transmitted into the superficial dorsal horn of the spinal cord.2.The effects of intra-articular injection of morphine on the expression of MOR in synovium and on the pain behavior in arthritic ratThe arthritis model was established by intra-articular injection of complete Freund’s adjuvant(CFA).The changes of the MOR-expression in the synovium,dorsal root ganglion and spinal dorsal horn and nociceptive behaviors after intra-articular injection small-dose morphine were investigated by immunofluorescence histochemistry,behavioral test and molecular biological methods.The results showed that:(1)After the animal arthritis model was established,arthritic rats showed significant mechanical allodynia and impaired locomotor ability compared with normal rats.(2)Intra-articular morphine injection in arthritic rats can effectively alleviate the mechanical allodynia and overcome locomotor disability.(3)The expression of MORs in the synovium,dorsal root ganglion and spinal dorsal horn of arthritic rats was significantly increased compared with normal group.(4)After arthritic rats were administrated with intra-articular injection of small-dose morphine,the expression of MORs in synovium was significantly reduced compared with those treated with same-dose of saline.(5)After arthritic rats were given intra-articular injection of small-dose morphine,the expression of MORs in the dorsal root ganglion and spinal dorsal horn was significantly reduced compared with arthritic rats treated with same-dose saline.The expression of CGRP was also significantly reduced.These results indicate that:(1)MOR-IP synovial fibers are the target where morphine exerts its analgesic effects when administrating in the joint cavity.(2)Intra-articular injection of small-dose morphine produces significant analgesic effects.(3)The expressions and changes of MOR-IP fibers in the synovium,dorsal root ganglion and spinal dorsal horn is closely related to the pain behaviors and their changes in arthritic rats,suggesting that MOR is involved in the transmission and regulation of pain signals in the condition of inflammatory pain.In summary,the results of this study indicate that:(1)MOR-IP fibers are abundantly distributed on the peripheral terminals and branches of primary afferent fibers,especially on peptidergic afferent fibers.(2)The expression of MORs in the synovium,dorsal root ganglion and spinal dorsal horn is increased significantly in arthritic rats.(3)Intra-articular injection of small-dose morphine produces significant analgesic effects and relieves the locomotion impairment.(4)The analgesic effect produced by injection of small-dose morphine in arthritic rats is related to the expression changes of MORs.After administrated with intra-articular injection of small-dose morphine,the expression of MORs in the synovium,dorsal root ganglion and spinal dorsal horn of arthritic rats is significantly reduced,and the expression of CGRP is also significantly reduced,suggesting that MOR and peptides might be associated with peripheral opioid analgesia.