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Comparison of gene expression profiles in neuropathic and inflammatory pain

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Molecular mechanisms underlying the differences between chronic neuropathic and inflammatory pain are still poorly understood. Identifying those differences should provide insight into the molecular mechanism underlying features unique for neuropathic pain, such as allodynia. We have performed screening for differentially expressed genes in the spinal cord in the rat models of neuropathic and inflammatory pain. Using BD Atlas Rat 4K arrays we found several differences in expression of secretion-related genes between inflammatory and neuropathic pain. Development of the latter was characterized by up-regulated expression of genes associated with immune response and microglia activation and also, to a lesser extent, with cytoskeleton rearrangement. The relative increase in abundance of four genes, intercellular adhesion molecule 1 (ICAM-1), calcitonin gene related peptide (CGRP), tissue inhibitor of metalloproteinase 1 (TIMP-1), chemokine-like receptor 1 was confirmed by reverse transcription Real-Time PCR (qPCR) validation in the spinal cord in neuropathic pain. Levels of transcripts corresponding to ICAM-1 and TIMP-1 were also increased in the dorsal root ganglia (DRG) of neuropathic rats. Our data point at the importance of immune response- and microglia activation-related genes in the development of chronic neuropathic pain, and suggest that expression of CGRP gene in the dorsal horn of the spinal cord could be involved in persistence of its symptoms.
Conantokin G (CTG), isolated from the venom of the marine cone snail Conus geographus, is an antagonist of N-methyl-d-aspartate receptors (NMDARs), the activation of which, especially those located on the central afferent terminals and dorsal horn neurons, leads to hypersensitivity and pain. Thus, CTG blocking of NMDARs, has an antinociceptive effect, particularly in the case of neurogenic pain treatment. As many urinary bladder disorders are caused by hyperactivity of sensory bladder innervation, it seems useful to estimate the influence of CTG on the plasticity of sensory neurons supplying the organ. Retrograde tracer Fast Blue (FB) was injected into the urinary bladder wall of six juvenile female pigs. Three weeks later, intramural bladder injections of CTG (120 μg per animal) were carried out in all animals. After a week, dorsal root ganglia of interest were harvested from all animals and neurochemical characterization of FB+ neurons was performed using a routine double-immunofluorescence labeling technique on 10-μm-thick cryostat sections. CTG injections led to a significant decrease in the number of FB+ neurons containing substance P (SP), pituitary adenylate cyclase activating polypeptide (PACAP), somatostatin (SOM), calbindin (CB) and nitric oxide synthase (NOS) when compared with healthy animals (20% vs. 45%, 13% vs. 26%, 1.3% vs. 3%, 1.2 vs. 4% and 0.9% vs. 6% respectively) and to an increase in the number of cells immunolabelled for galanin (GAL, 39% vs. 6.5%). These data demonstrated that CTG changed the chemical coding of bladder sensory neurons, thus indicating that CTG could eventually be used in the therapy of selected neurogenic bladder illnesses.
In the present study the ELISA test was used to investigate the influence of chemically-induced ileitis on the dorsal root ganglia (DRG) neurons in the pig. The preliminary retrograde fluorescent tracing study revealed that ileum-projecting sensory neurones (IPN) are located in the thoracic ganglia (Th; Th₈–Th₁₃). The ileum wall in experimental (E) pigs was subjected to multiple injection with 4% paraformaldehyde to induce inflammation, while in the control (C) animals the organ was injected with 0.1 M phosphate buffer. Three days later the DRGs (Th₈–Th₁₃) collected from all the animals were evaluated for VIP, SP, CGRP, NPY, GAL and SOM content with an ELISA test. It was found that the inflammation increased clearly the tissue level of SP, GAL and SOM.
Although resiniferatoxin (RTX) becomes more often used in experimental therapies of sensory system disorders, so far there is no data concerning the influence of RTX on the chemical coding of neurons in dorsal root ganglia (DRG) supplying the urinary bladder in the pig, an animal species considered as a reliable animal model for investigation dealing with human lower urinary tract disorders. Retrograde tracer Fast Blue (FB) was injected into the wall of the right half of the urinary bladder in six juvenile female pigs, and three weeks later, bladder instillation of RTX (500 nmol per animal) was carried out in all the animals. After a week, DRGs were harvested from all the pigs and the neurochemical characterization of FB+ neurons was performed using routine single-immunofluorescence labeling technique on 10-μm-thick cryostat sections. RTX instillation resulted in a distinct decrease in the numbers of FB+ cells containing calcitonin gene-related peptide (CGRP), nitric oxide synthase (NOS), somatostatin (SOM) and calbindin (CB) when compared with those found in the healthy animals (18% vs. 36%, 1% vs. 6%, 0.8% vs. 4% and 0.5% vs. 3%, respectively), and an increase in the number of pituitary adenylate cyclase-activating polypeptide (PACAP)- and galanin (GAL)-immunoreactive (IR) nerve cells (51% vs. 26% and 47% vs. 6.5%). The results obtained suggest that RTX could be taken into consideration when the neuroactive agents are planned to be used in experimental therapies of selected neurogenic bladder illnesses.
The vanilloid receptor (VR1) is a molecular integrator of various painful stimuli, including capsaicin, acid and high temperature. VR1 protein functions both as a receptor for capsaicin and a transducer of noxious thermal stimuli. In addition, VR1 is well characterised at the terminals of sensory nerves involved in the pain pathway. VR1 is also expressed in a capsaicin-sensitive and peptide-containing sub-population of primary sensory nerves. Indirect immunohistochemistry was used to examine the distribution of nerves immunoreactive (ir) for VR1 in the base of the urinary bladder and in the neurones of the lumbosacral dorsal root ganglia (L1-L2 and L6-S1) of young adult (3 months) and aged (24 months) male rats. Semi-quantitative estimations of nerve densities were assessed and quantitative studies were also used to examine the effects of age on the percentage of VR1-ir dorsal root ganglion neurones. The bladder base in young adults showed dense VR1-ir fibres within the urothelium and in the subepithelium and fibres ranging from sparse to moderate in number in the muscle coat. In comparison to the young animals, the aged rats showed sparse to moderate densities of VR1-ir nerves in the subepithelium and sparse fibres in the muscle layers. In the lumbosacral dorsal root ganglia the percentage of VR1-ir neuronal profiles showed a significant reduction from (mean ± SEM) 17.8 ± 2% in the young adult to 12 ± 1.6 in the aged rats. The present findings suggest that the effects of VR1 on bladder function (nociception and reflex micturition) are influenced by age and the reduction with age of VR1-ir neurones in the dorsal root ganglia could also have important implications for the micturition reflex.
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