Long-term changes in sciatic-evoked A-fiber dorsal horn field potentials accompany loose ligation of the sciatic nerve in rats.

The goal of the present study was to examine whether loose ligation of the sciatic nerve was associated with long-term changes in neuronal excitability in the spinal dorsal horn in urethane-anesthetized rats. The sciatic nerve was stimulated with 0. 1 ms long pulses at 1 stimulus/5 min, and the evoked dorsal horn field potentials remained stable in the absence of tetanic stimulation. In one set of control and ligated animals, high-frequency tetanic stimulation was applied to the nerve at 50 Hz (one 400 ms train of twenty 0.1 ms pulses), and the field potentials were recorded again (1 stimulus/5 min) for up to 4 h post-tetanus. In control animals, this protocol produced significant increases in field potential amplitudes at 15, 30 and 60 min post-tetanus. Interestingly, after this time the evoked field potentials began to decrease, and attained less than 50% of their pre-tetanic values at 240 min post-tetanus. In contrast, in ligated rats the pattern of post-tetanic potentiation was significantly different as the increases in amplitude persisted, and at 240 min post-tetanus the field potentials were almost twice their baseline values. In another set of control and ligated animals, low-frequency tetanic stimulation was applied at 5 Hz (one 400 ms train of two 0.1 ms pulses). Again a differential pattern of post-tetanic responses between control and ligated rats was seen. In control animals, a significant decrease in amplitude was evident within 30 min, and the depression became progressively more pronounced as the field potentials attained about a quarter of their baseline values at 180 min, and remained at these low levels at 240 min post-tetanus. On the other hand, in ligated animals, the depression was not significant, and at 240 min post-tetanus the field potentials were still at about 80% of their baseline values. These data demonstrate that long-term changes in spinal dorsal horn neuronal excitability accompany sciatic ligation to perhaps contribute to the development of neuropathic pain. These changes may result from a lessening of normally strong inhibitory processes in the spinal dorsal horn to generate conditions which favor post-tetanic potentiation over depression of dorsal horn neuronal responses.

Contribution of GABA-A receptors to metaplasticity in the spinal dorsal horn.

Metaplasticity is a higher-order form of synaptic plasticity that is induced by synaptic or cellular activity, which by itself may not produce changes in synaptic strength, but which modifies subsequent changes in synaptic efficacy. In this description of metaplasticity in the spinal dorsal horn, we report that a 50 Hz high-frequency tetanus, previously shown to elicit a potentiation of sciatic-evoked A-fiber spinal dorsal horn potentials, caused a depression when coupled with a more rapid rate of repetitive stimulation. This depression appeared to be dependent upon GABA(A) receptor activation because the 50 Hz tetanus elicited a persistent potentiation when the GABA(A) antagonist bicuculline was injected at 1 mg/kg (but not at 0.5 mg/kg) prior to tetanic stimulation. These data suggest the presence of strong inhibitory inputs in the spinal dorsal horn that are activated by an increased rate of primary afferent firing. The activation of these inputs may be necessary to prevent prolonged bursts of afferent activity from modifying synaptic strength because the latter may contribute to the development of persistent pain following peripheral nerve injury.

Changes in post-tetanic potentiation of A-fiber dorsal horn field potentials parallel the development and disappearance of neuropathic pain after sciatic nerve ligation in rats.

Significant plastic changes in spinal nociceptive processing appear to accompany peripheral nerve injury or inflammation. Using a well-established model of neuropathic pain, we have recently reported that loose ligation of the sciatic nerve was accompanied by a long-lasting post-tetanic potentiation of sciatic-evoked A-fiber superficial dorsal horn field potentials. In the present study we demonstrate that the typical disappearance of thermal hyperalgesia as a behavioral sign of neuropathic pain several weeks after loose sciatic nerve ligation is accompanied by the loss of the long-lasting potentiation. These data suggest that a significant but reversible shift in the processing of sensory information in the spinal dorsal horn follows peripheral nerve injury, and lend further support to the notion that long-lasting synaptic plasticity may contribute to the development of neuropathic pain.

Loose ligation of the rat sciatic nerve is accompanied by changes in the subcellular content of protein kinase C beta II and gamma in the spinal dorsal horn.

This study examined whether loose ligation of the sciatic nerve was accompanied by specific changes in protein kinase C (PKC) betaII and gamma isozymes in the spinal dorsal horn. The isozyme staining pattern was visualized with immunocytochemistry. Their content in subcellular fractions was estimated from Western immunoblots. In control animals, PKC betaII immunoreactivity extended from lamina I into lamina III, while PKC gamma immunoreactivity was concentrated within laminae II and III. In ligated animals exhibiting thermal hyperalgesia, the content of both PKC betaII and gamma in the synaptosomal membrane fraction, but not crude cytosolic fraction, was significantly greater by an average of 40% from their respective controls. These data support suggestions that peripheral nerve injury engenders plastic changes in the dorsal horn to contribute to the development of persistent pain.

The effect of continuous morphine analgesia on chronic thermal hyperalgesia due to sciatic constriction injury in rats.

We employed hindfoot withdrawal latencies to radiant heat to assess the analgesic effect of prolonged morphine infusion on thermal hyperalgesia induced by chronic constriction injury (CCI) of the rat sciatic nerve. All CCI rats developed thermal hyperalgesia while sham-operated animals did not. Continuous systemic infusion of morphine dose-dependently reversed the thermal hyperalgesia in the CCI rats. In contrast, thermal hyperalgesia persisted in saline-treated CCI rats. Tolerance to morphine's analgesic effect did not develop over a period of seven days of morphine infusion, which is considered long-term for animal models. These data suggest that morphine acts rapidly and effectively to reduce behavioral signs of hyperalgesia in rats with sciatic CCI, without the concomitant development of tolerance. Scheduled administration of morphine might be an appropriate treatment regimen for relief of neuropathic pain, and the infrequent use of opioids in equivalent human clinical pain syndromes due to fear of opioid unresponsiveness and tolerance might need to be re-evaluated.

Catalytic determination of nanogram amounts from iron(III) using its catalytic effect on the oxidation of sodium pyrogallol-5-sulphonate by hydrogen peroxide.

A kinetic method is described for the determination of iron(III) based on its catalytic effect on the oxidation of sodium pyrogallol-5-sulphonate by hydrogen peroxide. The reaction was followed spectrophotometrically by measuring the rate of change in the absorbance of the coloured product at 436.8 nm. Nanogram amounts of iron(III) (2.0-75.0 ng cm(-3)) can be determined with good accuracy and reproducibility. The influence of foreign ions on the results was investigated and the method was found to be adequately selective. It has been applied satisfactorily to the determination of iron(III) in tap water samples. The kinetic parameters of both the catalysed and uncatalysed reactions are reported.

Kinetic determination of traces of iodide by its catalytic effect on oxidation of sodium pyrogallol-5-sulfonate by hydrogen peroxide.

The kinetic method is based on a catalytic effect on the oxidation of sodium pyrogallol-5-sulfonate by hydrogenperoxide. The reaction is followed spectrophotometrically at 436.8 nm. The kinetic parameters of the reaction are reported and a rate equation is suggested. The calibration graph is linear in the range 10-200 ng cm(-3). The effects of certain foreign ions upon the reaction rate were determined for the assessment of the selectivity of the method. This method has high sensitivity and good selectivity when anions are concerned as well. That is why it can be successfully applied to determination of iodide in real samples (mineral water and soil) directly after the elimination of cations, which interfere. The method was applied to determine iodide in natural waters and soil.

Changes in calcineurin message, enzyme activity and protein content in the spinal dorsal horn are associated with chronic constriction injury of the rat sciatic nerve.

Plasticity in the spinal dorsal horn is thought to underlie the development of neuropathic pain. Calcineurin (protein phosphatase 3) plays an important role in plasticity in the brain. Here we examined whether chronic constriction injury (CCI) of the sciatic nerve modifies calcineurin expression in the spinal dorsal horn. Male rats were assigned to control (uninjured), sham-operated or CCI groups. CCI animals exhibited both a shift in weight bearing and a reduction in paw withdrawal latencies as signs of pain behavior. At 3 days (3D) the pain behavior was associated with a significant increase in calcineurin gene expression, enzyme activity and content of its Aα isoform in the ipsilateral spinal dorsal horn. In contrast, while the pain behavior persisted at 7 days (7D) calcineurin gene expression returned to control levels and activity and protein content decreased. A single intrathecal injection of MK-801 15 min before the ligation attenuated both signs of pain behavior in 3D but not 7D CCI animals. The same pre-treatment also prevented the CCI-associated increases in calcineurin in these animals. These data suggested an involvement of calcineurin in CCI-elicited neuropathic pain. The time-dependent divergent changes in calcineurin expression may underlie the different phases of neuropathic pain development.

La infusión sistémica continua de lidocaína proporciona analgesia en un modelo animal de dolor neuropático / Continual systemic infusion of lidocaine provides analgesia in an animal model of neuropathic pain

La finalidad de este estudio fue determinar si la infusión continua a velocidad constante de lidocaína proporcionaba analgesia durante la fase inicial posterior a la lesión en el modelo de dolor neuropático causado por lesión por constricción crónica. Ratas Sprague-Dawley macho se dividieron en grupos de control y ligadura y recibieron una infusión con solución salina fisiológica o lidocaína (0,15, 0,33, 0,67 y 1,3 mg . k g- 1. h- 1) por medio de minibombas osmóticas Alzet® implantadas subcutáneamente. Se obtuvieron las latencias de retirada ante estímulos términos antes (día 0) y 3 días después de realizar una ligadura floja del nervio ciático e implantar la bomba. Los animales sometidos a ligadura que estaban recibiendo lidocaína con una velocidad de infusión de 0,67 ó 1,3 mg.kg- 1. h- 1 no exhibieron ningún cambio en la latencia de retirada el día 3 después de la cirugía, lo que indica que la lidocaína a esas dosis previno la aparición de hiperalgesia térmica como manifestación de dolor neuropático. Por el contrario, los animales sometidos a ligadura y tratados con solución salina fisiológica o lidocaína con una velocidad de infusión de 0,15 ó 0,33 mg.kg- 1. h- 1 exhibieron hiperalgesia el día 3 después de la cirugía, lo que indica que esas dosis más pequeñas de lidocaína no proporcionaron analgesia. Los animales de control tratados con solución salina fisiológica o con cualquiera de las dosis de lidocaína no exhibieron ningún cambio en las latencias de retirada entre el día 0 y el día 3. En otro grupo de animales sometidos a ligadura, la infusión de lidocaína (0,67 mg.kg- 1. h- 1) iniciada 24 horas después de la intervención quirúrgica para ligar el nervio ciático suprimió la hiperalgesia térmica ya presente. Las concentraciones plasmáticas medidas de lidocaína fueron de 0,11, 0,36 y 0,45 µg.ml- 1 en los animales que recibieron 0,33, 0,67 y 1,3 mg- 1. k g . h- 1 de lidocaína, respectivamente. Estos resultados sugieren que la infusión sistémica continua de lidocaína previene o revierte la aparición de dolor neuropático después de una lesión crónica por constricción. Estos resultados se suman a las evidencias cada vez más numerosas del valor terapéutico de la administración preventiva y postoperatoria de lidocaína para el alivio del dolor neuropático. (AU)