| Nitric oxide (NO) is an important neurotransmitter to regulate pain. Nitric oxide synthase (NOS) is the key enzyme catalyzing the synthesis of NO. Three isoforms of NOS has been identified due to their different mechanism of activation and function. They are neuronal NOS (nNOS), inducible NOS (iNOS) and endothelial NOS (eNOS), which are the production of different gene and they have different biological effects. It has been reported that the expression of nNOS increased during formalin-induced inflammatory pain, but it is unknown whether the expression of iNOS increase. More and more evidence show that glia play an important role in developing and maintaining pain and hyperalgesia. A variety of noxious stimulation, such as subcutaneous irritants, peripheral nerve trauma, spinal nerve trauma, partial sciatic ligation, spinal nerve inflammation, can activate astrocyte and microglia of the dorsal horn of the spinal cord. In vitro, glia are also activated by such pain related substances as: ATP, calcitonin gene-related peptide (CGRP), excitatory amino acid, prostaglandins, and substance P. Activated glia are known to release a variety of neuroactive substance, including NO, arachidonic acid, prostaglandins, leukotrienes, excitatory amino acid(including glutamate, aspartate, cysteine), nerve growth factors, tumor necrosis factor and so on. Activated glia also enhance the release of substance P and excitatory amino acid from primary afferents in the spinal cord. Although the expression of iNOS in glia increased during many pain states, such as spinal cord injury, it is remained to be clearify whether glia contribute to the up-regulation of NOS during formalin-induced inflammatory pain. Thereby, the present study was undertaken to observe and compare the changes of nNOS and iNOS in the rat dorsal horn of the spinal cord during formalin-induced inflammatory pain , and further to examine the effect of fluorocitrate (FC), a glial inhibitor, on the of up-regulation of NOS in the spinal dorsal horn during this process. Our experiments may provide evidence which isoform of NOS is involved during this inflammatory pain and confirm whether glia contribute to the up-regulation of NOS during formalin-induced inflammatory pain.1 The changes in expression of nNOS and iNOS of the spinal cord during formalin-induced inflammatory painThirty-five male SD rats were divided randomly into 5groups: control group and formalin 12h, 24h, 48h, 72h groups. The rats in formalin 12h, 24h, 48h, 72h groups were sacrificed at corresponding time after formalin injection to the right hind paw, and the tissues of L5 segment were obtained to observe the expression of nNOS and iNOS by immunohistochemical staining.The results were as follows: nNOS immunoreactive (IR) cells in laminae â… -â…¡ of L5 segment of spinal cord were round, ellipse, and rhombus and most of them were in small shape. Compared with control group, the number and immunostaining density of IR cells in ipsilateral and contralateral to the formalin injection increased significantly in the spinal cord in formalin 12h, 24h, 48h, 72h groups (p<0.05), but the most obvious increase was in 24h group, which was also increased in contrast to that of 12h, 48h, and 72h groups (p<0.01). In comparison between the two sides of the dorsal horn, the increases of number and density of IR cells were greater on the injected side than the contralateral side (p<0.05). The density of IR processes in laminae â… -â…¡ of the spinal dorsal horn increased significantly in 12h, 24h, 48h, 72h groups than in the control group(p<0.05). Futhermore, it was observed that the immunostaining density of IR processes in laminae â… -â…¡ in ipsilateral was greater than that in contralateral of the spinal cord in each group after formalin injection (p<0.05).iNOS IR cells of L5 segment of spinal cord were rhombus, round and ellipse scattering in the dorsal horn and the white matter. IR substance in filiform shape was found in the spinal dorsal horn and the white matter. Compared with control group, the num...
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