Remote astrocytic and microglial activation modulates neuronal hyperexcitability and below-level neuropathic pain after spinal injury in rat.

In this study, we evaluated whether astrocytic and microglial activation mediates below-level neuropathic pain following spinal cord injury. Male Sprague-Dawley (225-250 g) rats were given low thoracic (T13) spinal transverse hemisection and behavioral, electrophysiological and immunohistochemical methods were used to examine the development and maintenance of below-level neuropathic pain. On postoperation day 28, both hind limbs showed significantly decreased paw withdrawal thresholds and thermal latencies as well as hyperexcitability of lumbar (L4-5) spinal wide dynamic range (WDR) neurons on both sides of spinal dorsal horn compared to sham controls (* P<0.05). Intrathecal treatment with propentofylline (PPF, 10 mM) for 7 consecutive days immediately after spinal injury attenuated the development of mechanical allodynia and thermal hyperalgesia in both hind limbs in a dose-related reduction compared to vehicle treatments (* P<0.05). Intrathecal treatment with single injections of PPF at 28 days after spinal injury, attenuated the existing mechanical allodynia and thermal hyperalgesia in both hind limbs in a dose related reduction (* P<0.05). In electrophysiological studies, topical treatment of 10 mM PPF onto the spinal surface attenuated the neuronal hyperexcitability in response to mechanical stimuli. In immunohistochemical studies, astrocytes and microglia in rats with spinal hemisection showed significantly increased GFAP and OX-42 expression in both superficial and deep dorsal horns in the lumbar spinal dorsal horn compared to sham controls (* P<0.05) that was prevented in a dose-related manner by PPF. In conclusion, our present data support astrocytic and microglial activation that contributes to below-level central neuropathic pain following spinal cord injury.

Effects of iontophoretically applied substance P, calcitonin gene-related peptide on excitability of dorsal horn neurones in rats.

Spontaneous pain, allodynia and hyperalgesia are well known phenomena following peripheral nerve or tissue injury, and it is speculated that secondary hyperalgesia and allodynia, are generally thought to depend on a hyperexcitability (sensitization) of neurons in the dorsal horn. It is supposed that the sensitization may be due to various actions of neurotransmitters (SP, CGRP, excitatory amino acids) released from the primary afferent fibers. In this study, we examined effects of the iontophoretically applied SP and CGRP on the response to EAA receptor agonists (NMDA and non-NMDA) in the WDR dorsal horn neurones and see if the effects of SP or CGRP mimic the characteristic response pattern known in various pain models. The main results are summarized as follows: 1) SP specifically potentiated NMDA response. 2) CGRP non-specifically potentiated both NMDA and AMPA responses. Potentiation of NMDA response, however, was significantly greater than that of AMPA response. 3) 50% of SP applied cells and 15.8% of CGRP applied cells showed reciprocal changes(potentiation of NMDA response and suppression of AMPA response). These results are generally consistent with the sensitization characteristics in diverse pain models and suggests that the modulatory effects of SP and CGRP on NMDA and non-NMDA (AMPA) response are, at least in part, contribute to the development of sensitization in various pain models.

Oxidative damage in tissues of rats exposed to cigarette smoke.

Cigarette smoke is known to contain high concentrations of free radicals and oxidants. To examine the oxidative effect of cigarette smoking, we subjected rats to inhalation of cigarette smoke, and measured cellular free glutathione, the degree of protein S-thiolation, and 8-oxo-2'-deoxyguanosine (oxo8dG) in DNA. Inhalation of the cigarette smoke for 30 days, three times a day, resulted in a significant decrease of the total free glutathione contents in tissues, especially in the lung. Elevated levels of oxidized glutathione and protein S-thiolation were observed in the lung but not in other tissues. Increased contents of oxo8dG in DNA were found in all tissues analyzed. When rats were treated with buthionine sulfoximine (BSO, 80 mg/kg/day) to deplete glutathione, the oxidative effect of cigarette smoking was greatly potentiated. The effect of glutathione depletion was most evident in the lung. Cigarette smoking for only 7 days resulted in extreme depletion of the glutathione both in the lungs and in the liver of BSO-treated rats. Furthermore, oxo8dG in DNA increased markedly, especially in lung. The results verified that the lung is a primary target of cigarette smoke-induced oxidative damage, and cigarette smoke exerts its oxidative effects on the rest of the entire organs eventually. Our results indicate that glutathione plays crucial roles in protecting proteins and DNA from oxidation caused by cigarette smoking.

Involvement of alpha2-adrenoceptors in mediating sympathetic excitation of injured dorsal root ganglion neurons in rats with spinal nerve ligation.

The present study examined the effects of sympathetic stimulation on the activity of primary afferent neurons that had peripheral axons being injured previously by a spinal nerve ligation. About 22% of afferents with injured fibers that showed spontaneous discharge were excited by sympathetic stimulation or systemic injection of adrenaline. Most sympathetically-excited afferent neurons had axons that conducted in the A-fiber range. This sympathetically-evoked afferent excitation was not affected by cutting the spinal nerve at a place close to the dorsal root ganglion (DRG). Yohimbine, alpha2-antagonist, suppressed sympathetically-evoked afferent excitation which was not affected by alpha1-antagonist prazosin. Clonidine, alpha2-agonist, exerted an excitatory effect, whereas alpha1-agonist phenylephrine had no effect on the activity of afferents with injured fibers. No afferent fibers in control preparations responded to sympathetic stimulation. The results suggest that after a spinal nerve ligation, injured DRG neurons with fast-conducting fibers become sensitive to sympathetic activity via activation of alpha2-adrenoceptors.