Gabapentin suppresses ectopic nerve discharges and reverses allodynia in neuropathic rats.
Repetitive ectopic discharges from injured afferent nerves play an important role in initiation and maintenance of neuropathic pain. Gabapentin is effective for treatment of neuropathic pain but the sites and mechanisms of its antinociceptive actions remain uncertain. In the present study, we tested a hypothesis that therapeutic doses of gabapentin suppress ectopic afferent discharge activity generated from injured peripheral nerves. Mechanical allodynia, induced by partial ligation of the sciatic nerve in rats, was determined by application of von Frey filaments to the hindpaw. Single-unit afferent nerve activity was recorded proximal to the ligated sciatic nerve site. Intravenous gabapentin, in a range of 30 to 90 mg/kg, significantly attenuated allodynia in nerve-injured rats. Furthermore, gabapentin, in the same therapeutic dose range, dose-dependently inhibited the ectopic discharge activity of 15 injured sciatic afferent nerve fibers through an action on impulse generation. However, the conduction velocity and responses of 12 normal afferent fibers to mechanical stimulation were not affected by gabapentin. Therefore, this study provides electrophysiological evidence that gabapentin is capable of suppressing the ectopic discharge activity from injured peripheral nerves. This action may contribute, at least in part, to the antiallodynic effect of gabapentin on neuropathic pain. (+info)
Source of inappropriate receptive fields in cortical somatotopic maps from rats that sustained neonatal forelimb removal.
Previously this laboratory demonstrated that forelimb removal at birth in rats results in the invasion of the cuneate nucleus by sciatic nerve axons and the development of cuneothalamic cells with receptive fields that include both the forelimb-stump and the hindlimb. However, unit-cluster recordings from primary somatosensory cortex (SI) of these animals revealed few sites in the forelimb-stump representation where responses to hindlimb stimulation also could be recorded. Recently we reported that hindlimb inputs to the SI forelimb-stump representation are suppressed functionally in neonatally amputated rats and that GABAergic inhibition is involved in this process. The present study was undertaken to assess the role that intracortical projections from the SI hindlimb representation may play in the functional reorganization of the SI forelimb-stump field in these animals. The SI forelimb-stump representation was mapped during gamma-aminobutyric acid (GABA)-receptor blockade, both before and after electrolytic destruction of the SI hindlimb representation. Analysis of eight amputated rats showed that 75.8% of 264 stump recording sites possessed hindlimb receptive fields before destruction of the SI hindlimb. After the lesions, significantly fewer sites (13.2% of 197) were responsive to hindlimb stimulation (P < 0.0001). Electrolytic destruction of the SI lower-jaw representation in four additional control rats with neonatal forelimb amputation did not significantly reduce the percentage of hindlimb-responsive sites in the SI stump field during GABA-receptor blockade (P = 0.98). Similar results were obtained from three manipulated rats in which the SI hindlimb representation was silenced temporarily with a local cobalt chloride injection. Analysis of response latencies to sciatic nerve stimulation in the hindlimb and forelimb-stump representations suggested that the intracortical pathway(s) mediating the hindlimb responses in the forelimb-stump field may be polysynaptic. The mean latency to sciatic nerve stimulation at responsive sites in the GABA-receptor blocked SI stump representation of neonatally amputated rats was significantly longer than that for recording sites in the hindlimb representation [26.3 +/- 8.1 (SD) ms vs. 10.8 +/- 2.4 ms, respectively, P < 0.0001]. These results suggest that hindlimb input to the SI forelimb-stump representation detected in GABA-blocked cortices of neonatally forelimb amputated rats originates primarily from the SI hindlimb representation. (+info)
Expression of alpha2-adrenergic receptors in rat primary afferent neurones after peripheral nerve injury or inflammation.
1. Immunocytochemistry with polyclonal antibodies directed against specific fragments of intracellular loops of alpha2A- and alpha2C-adrenergic receptors (alpha2A-AR, alpha2C-AR) was used to explore the possibility that expression of these receptors in dorsal root ganglion (DRG) neurones of rat alters as a result of peripheral nerve injury or localized inflammation. 2. Small numbers of neurones with positive alpha2A-AR immunoreactivity (alpha2A-AR-IR) were detected in DRG from normal animals or contralateral to nerve lesions. In contrast, after complete or partial sciatic nerve transection the numbers of ipsilateral L4 and L5 DRG somata expressing alpha2A-AR-IR sharply increased (>5-fold). There was no discernible change in the number of DRG neurones exhibiting alpha2A-AR-IR innervating a region in association with localized chemically induced inflammation. 3. After nerve injury, double labelling with Fluoro-Gold, a marker of retrograde transport from transected fibres, or by immunoreactivity for c-jun protein, an indicator of injury and regeneration, suggested that many of the neurones expressing alpha2A-AR-IR were uninjured by the sciatic lesions. 4. In general the largest proportionate increase in numbers of neurones labelled by alpha2A-AR-IR after nerve lesions appeared in the medium-large diameter range (31-40 microm), a group principally composed of cell bodies of low threshold mechanoreceptors. The number of small diameter DRG neurones labelled by alpha2A-AR-IR, a category likely to include somata of nociceptors, also increased but proportionately less. 5. Relatively few DRG neurones exhibited alpha2C-AR-IR; this population did not appear to change after either nerve lesions or inflammation. 6. These observations are considered in relation to effects of nerve injury on excitation of primary afferent neurones by sympathetic activity or adrenergic agents, sympathetically related neuropathy and reports of sprouting of sympathetic fibres in DRG. (+info)
Hypothermic neuroprotection of peripheral nerve of rats from ischaemia-reperfusion injury.
Although there is much information on experimental ischaemic neuropathy, there are only scant data on neuroprotection. We evaluated the effectiveness of hypothermia in protecting peripheral nerve from ischaemia-reperfusion injury using the model of experimental nerve ischaemia. Forty-eight male Sprague-Dawley rats were divided into six groups. We used a ligation-reperfusion model of nerve ischaemia where each of the supplying arteries to the sciatic-tibial nerves of the right hind limb was ligated and the ligatures were released after a predetermined period of ischaemia. The right hind limbs of one group (24 rats) were made ischaemic for 5 h and those of the other group (24 rats) for 3 h. Each group was further divided into three and the limbs were maintained at 37 degrees C (36 degrees C for 5 h of ischaemia) in one, 32 degrees C in the second and 28 degrees C in the third of these groups for the final 2 h of the ischaemic period and an additional 2 h of the reperfusion period. A behavioural score was recorded and nerve electrophysiology of motor and sensory nerves was undertaken 1 week after surgical procedures. At that time, entire sciatic-tibial nerves were harvested and fixed in situ. Four portions of each nerve were examined: proximal sciatic nerve, distal sciatic nerve, mid-tibial nerve and distal tibial nerve. To determine the degree of fibre degeneration, each section was studied by light microscopy, and we estimated an oedema index and a fibre degeneration index. The groups treated at 36-37 degrees C underwent marked fibre degeneration, associated with a reduction in action potential and impairment in behavioural score. The groups treated at 28 degrees C (for both 3 and 5 h) showed significantly less (P < 0.01; ANOVA, Bonferoni post hoc test) reperfusion injury for all indices (behavioural score, electrophysiology and neuropathology), and the groups treated at 32 degrees C had scores intermediate between the groups treated at 36-37 degrees C and 28 degrees C. Our results showed that cooling the limbs dramatically protects the peripheral nerve from ischaemia-reperfusion injury. (+info)
Injury-induced gelatinase and thrombin-like activities in regenerating and nonregenerating nervous systems.
It is now widely accepted that injured nerves, like any other injured tissue, need assistance from their extracellular milieu in order to heal. We compared the postinjury activities of thrombin and gelatinases, two types of proteolytic activities known to be critically involved in tissue healing, in nonregenerative (rat optic nerve) and regenerative (fish optic nerve and rat sciatic nerve) neural tissue. Unlike gelatinases, whose induction pattern was comparable in all three nerves, thrombin-like activity differed clearly between regenerating and nonregenerating nervous systems. Postinjury levels of this latter activity seem to dictate whether it will display beneficial or detrimental effects on the capacity of the tissue for repair. The results of this study further highlight the fact that tissue repair and nerve regeneration are closely linked and that substances that are not unique to the nervous system, but participate in wound healing in general, are also crucial for regeneration or its failure in the nervous system. (+info)
A role for insulin-like growth factor-I in the regulation of Schwann cell survival.
During postnatal development in the peripheral nerve, differentiating Schwann cells are susceptible to apoptotic death. Schwann cell apoptosis is regulated by axons and serves as one mechanism through which axon and Schwann cell numbers are correctly matched. This regulation is mediated in part by the provision of limiting axon-derived trophic molecules, although neuregulin-1 (NRG-1) is the only trophic factor shown to date to support Schwann cell survival. In this report, we identify insulin-like growth factor-I (IGF-I) as an additional trophin that can promote Schwann cell survival in vitro. We find that IGF-I, like NRG-1, can prevent the apoptotic death of postnatal rat Schwann cells cultured under conditions of serum withdrawal. Moreover, we show that differentiating Schwann cells in the rat sciatic nerve express both the IGF-I receptor (IGF-I R) and IGF-I throughout postnatal development. These results indicate that IGF-I is likely to control Schwann cell viability in the developing peripheral nerve and, together with other findings, raise the interesting possibility that such survival regulation may switch during postnatal development from an axon-dependent mechanism to an autocrine and/or paracrine one. (+info)
Krox-20 controls SCIP expression, cell cycle exit and susceptibility to apoptosis in developing myelinating Schwann cells.
The transcription factors Krox-20 and SCIP each play important roles in the differentiation of Schwann cells. However, the genes encoding these two proteins exhibit distinct time courses of expression and yield distinct cellular phenotypes upon mutation. SCIP is expressed prior to the initial appearance of Krox-20, and is transient in both the myelinating and non-myelinating Schwann cell lineages; while in contrast, Krox-20 appears approximately 24 hours after SCIP and then only within the myelinating lineage, where its expression is stably maintained into adulthood. Similarly, differentiation of SCIP-/- Schwann cells appears to transiently stall at the promyelinating stage that precedes myelination, whereas Krox-20(-/-) cells are, by morphological criteria, arrested at this stage. These observations led us to examine SCIP regulation and Schwann cell phenotype in Krox-20 mouse mutants. We find that in Krox-20(-/-) Schwann cells, SCIP expression is converted from transient to sustained. We further observe that both Schwann cell proliferation and apoptosis, which are normal features of SCIP+ cells, are also markedly increased late in postnatal development in Krox-20 mutants relative to wild type, and that the levels of cell division and apoptosis are balanced to yield a stable number of Schwann cells within peripheral nerves. These data demonstrate that the loss of Krox-20 in myelinating Schwann cells arrests differentiation at the promyelinating stage, as assessed by SCIP expression, mitotic activity and susceptibility to apoptosis. (+info)
Sorbitol accumulation in rats kept on diabetic condition for short and prolonged periods.
AIM: To study the influence of the course of diabetes, aging, and glycemia on the sorbitol accumulation in diabetic rats. METHODS: Streptozocin (Str) diabetic rats were obtained by Str i.v. (35 mg.kg-1). Glycemia and sorbitol levels from sciatic nerve and lens were measured after 1 d, 2, 5, and 8 months of diabetes. Sorbitol concentrations in serum, heart, diaphragm, small intestine, and kidney after 8 months of diabetes were measured. RESULTS: Diabetic rats after Str injection showed hyperglycemia (> 1.7 g.L-1), hyperphagia, polyuria, polydipsia, and loss of body weight. Sorbitol levels in lens and sciatic nerve increased in normal and diabetic rats; the increase was higher in diabetic rats. No relationship was shown between glycemia and sorbitol levels. An increased sorbitol level after 8 months of diabetes was found in small intestine and kidney. CONCLUSION: The sorbitol levels increased in lens and sciatic nerve with aging and this process was accelerated by diabetes. (+info)