Michiel Truin

Sudden cardiac death in dogs with remodeled hearts is associated with larger beat–to–beat variability of repolarization

AbstractIncreased proarrhythmia in dogs with chronic AV block (AVB) has been explained by ventricular remodeling causing a decrease in repolarization reserve. Beat–to–beat variability of repolarization (BVR) has been suggested to reflect repolarization reserve, in which high variability represents diminished reserve and larger propensity for repolarization–dependent ventricular arrhythmia. A subset of chronic AVB dogs (10%) suffers sudden cardiac death (SCD). With the assumption that repolarization defects constitute a potentially lethal proarrhythmic substrate, we hypothesized that BVR in SCD dogs are larger than in matched control chronic AVB dogs.From a population of 200 chronic AVB dogs, initially two groups were chosen retrospectively: 8 dogs that died suddenly (SCD) and 8 control dogs. Control dogs had a longer lifespan after AVB (10 to 18 weeks) than SCD dogs (5 to 10 weeks). All dogs had undergone electrophysiological testing under anesthesia where ECG, left and right ventricular endocardial monophasic action potentials (MAP) were recorded. BVR was assessed from 30 consecutive beats, illustrated by Poincaré plots and was the only parameter discriminating between SCD and control group. All other electrophysiological parameters (RR, QT and MAP durations) were comparable for the two groups. Extending the number of animals and groups confirmed a larger BVR in the SCD group (SCD: 5.1 ± 2.7; n = 11 versus control: 2.5 ± 0.4 ms; n = 61; P < 0.05) and showed reverse–use dependence of BVR. In comparison, dogs with acute AVB had low variability (1.3 ± 0.3 ms; n = 9; P < 0.05 versus chronic AVB).Cardiac electrical remodeling after AVB is associated with an increase in beat–to–beat variability of repolarization. Chronic AVB dogs displaying further elevated variability of repolarization are prone to arrhythmia–related SCD.

Differential GABAergic disinhibition during the development of painful peripheral neuropathy

An impaired spinal GABAergic inhibitory function is known to be pivotal in neuropathic pain (NPP). At present, data concerning time-dependent alterations within the GABAergic system itself and post-synaptic GABA(A) receptor-mediated inhibitory transmission are highly controversial, likely related to the experimental NPP model used. Furthermore, it is unknown whether the severity of NPP is determined by the degree of these GABAergic disturbances. In the present study we therefore examined in one experimental animal model whether anatomical changes within the spinal GABAergic system and its GABA(A) receptor-mediated inhibitory function are gradually aggravated during the development of partial sciatic nerve injury (PSNL)-induced NPP and are related to the severity of PSNL-induced hypersensitivity. Three and 16 days after a unilateral PSNL (early and late NPP, respectively), GABA-immunoreactivity (GABA-IR) and the number of GABA-IR neuronal profiles were determined in Rexed laminae 1-3 of lumbar spinal cord cryosections. Additionally, the efficiency of dorsal horn GABA(A) receptor-induced inhibition was examined by cation chloride cotransporter 2 (KCC2) immunoblotting. NPP-induced hypersensitivity was only observed at the ipsilateral side, both at early and late time points. During early NPP, a decrease in ipsilateral dorsal horn GABA-IR was observed without alterations in the number of GABA-IR neuronal profiles or KCC2 protein levels. In contrast, bilateral increases in spinal GABA-IR accompanied by an unchanged number of GABA-IR interneurons were observed during late NPP. This was furthermore attended with decreased ipsilateral KCC2 levels. Moreover, the degree of hypersensitivity was not related to disturbances within the spinal GABAergic system at all time points examined. In conclusion, our anatomical data suggest that a dysfunctional GABA production is likely to be involved in early NPP whereas late NPP is characterized by a combined dysfunctional GABA release and decreased KCC2 levels, the latter suggesting an impaired GABA(A) receptor-mediated inhibition.

Successful pain relief in non-responders to spinal cord stimulation: The combined use of ketamine and spinal cord stimulation

Although spinal cord stimulation (SCS) is an established therapy for chronic neuropathic pain, still 30% of patients do not respond adequately to trial stimulation. These so called “non‐responders” do not receive a permanent implantation for pain relief.

Decreased intracellular GABA levels contribute to spinal cord stimulation-induced analgesia in rats suffering from painful peripheral neuropathy: the role of KCC2 and GABA(A) receptor-mediated inhibition.

Elevated spinal extracellular γ-aminobutyric acid (GABA) levels have been described during spinal cord stimulation (SCS)-induced analgesia in experimental chronic peripheral neuropathy. Interestingly, these increased GABA levels strongly exceeded the time frame of SCS-induced analgesia. In line with the former, pharmacologically-enhanced extracellular GABA levels by GABA(B) receptor agonists in combination with SCS in non-responders to SCS solely could convert these non-responders into responders. However, similar treatment with GABA(A) receptor agonists and SCS is known to be less efficient. Since K⁺ Cl⁻ cotransporter 2 (KCC2) functionality strongly determines proper GABA(A) receptor-mediated inhibition, both decreased numbers of GABA(A) receptors as well as reduced KCC2 protein expression might play a pivotal role in this loss of GABA(A) receptor-mediated inhibition in non-responders. Here, we explored the mechanisms underlying both changes in extracellular GABA levels and impaired GABA(A) receptor-mediated inhibition after 30 min of SCS in rats suffering from partial sciatic nerve ligation (PSNL). Immediately after cessation of SCS, a decreased spinal intracellular dorsal horn GABA-immunoreactivity was observed in responders when compared to non-responders or sham SCS rats. One hour later however, GABA-immunoreactivity was already increased to similar levels as those observed in non-responder or sham SCS rats. These changes did not coincide with alterations in the number of GABA-immunoreactive cells. C-Fos/GABA double-fluorescence clearly confirmed a SCS-induced activation of GABA-immunoreactive cells in responders immediately after SCS. Differences in spinal dorsal horn GABA(A) receptor-immunoreactivity and KCC2 protein levels were absent between all SCS groups. However, KCC2 protein levels were significantly decreased compared to sham PSNL animals. In conclusion, reduced intracellular GABA levels are only present during the time frame of SCS in responders and strongly point to a SCS-mediated on/off GABAergic release mechanism. Furthermore, a KCC2-dependent impaired GABA(A) receptor-mediated inhibition seems to be present both in responders and non-responders to SCS due to similar KCC2 and GABA(A) receptor levels.

Increased efficacy of early spinal cord stimulation in an animal model of neuropathic pain

Although spinal cord stimulation (SCS) is an established treatment for chronic neuropathic pain, pain relief is still not successful in a large group of patients. We suggest that the success of SCS may be related to the timing of SCS during the development of chronic neuropathic pain. We therefore compared the effect of SCS applied after 24 h of neuropathic pain (early SCS) and after 16 days of neuropathic pain (late SCS).Although spinal cord stimulation (SCS) is an established treatment for chronic neuropathic pain, pain relief is still not successful in a large group of patients. We suggest that the success of SCS may be related to the timing of SCS during the development of chronic neuropathic pain. We therefore compared the effect of SCS applied after 24h of neuropathic pain (early SCS) and after 16days of neuropathic pain (late SCS). For early SCS, male Sprague-Dawley rats (n=13) were implanted with an SCS device, followed by a partial ligation of the sciatic nerve. Using von Frey monofilaments, tactile allodynia was assessed 24h after ligation. Animals with tactile allodynia received 30min of SCS. Withdrawal thresholds were assessed just before SCS, during SCS and until the return to pre-stimulation withdrawal threshold. Results were compared with the data from late SCS (n=29). Out of the 13 allodynic animals that received early SCS, 10 (77%) responded to SCS with significantly increased withdrawal thresholds, compared to 38% in the late SCS group. The increase of the withdrawal threshold in the early SCS group could still be noticed 90min after termination of SCS. In more than half of these animals, pre-stimulation withdrawal thresholds were reached only the next day. Early SCS resulted in an increased number of responders to SCS and furthermore an increased duration of the effect of SCS as compared to late SCS. Early SCS treatment of neuropathic rats is more effective as compared to the late SCS treatment.

The effect of Spinal Cord Stimulation in mice with chronic neuropathic pain after partial ligation of the sciatic nerve

ABSTRACT The effect of Spinal Cord Stimulation (SCS) in chronic neuropathic pain is inversely related to the severity of mechanical allodynia and the underlying mechanisms are poorly understood. To understand these mechanisms further we aimed to develop a model of SCS in a neuropathic mouse. Further, the CatWalk analysis, which is claimed to be an improved test for mechanical allodynia and therapeutic intervention, was used to analyze the effect of SCS on mechanical allodynia. Male C57BL/6 mice (N = 31) underwent partial ligation of the sciatic nerve. After 14 days an electrode was implanted and the effect of SCS (N = 22) on mechanical allodynia was tested. Unligated mice (N = 8) also received SCS. Behavioral analysis was performed using von Frey filaments and the CatWalk system. The withdrawal threshold showed a significant decrease which remained over time. Changes in CatWalk parameters were observed after 2 days, but tended to diminish during the next 14 days. Thirty minutes of SCS resulted in a 100% response and return to pre‐neuropathy levels of the withdrawal threshold. The effect of SCS on the withdrawal threshold was comparable for the most severe and milder allodynic animals. SCS did not affect any of the CatWalk parameters in all mice. In conclusion, we developed a model of SCS in a chronic neuropathic pain C57BL/6 mouse. The CatWalk gait analysis does not result in the detection of behavioral changes to SCS in mice with chronic neuropathic pain and control animals. This model allows future molecular‐genetic studies on the mechanisms of SCS in chronic neuropathic pain.

Spinal cord stimulation in a mouse chronic neuropathic pain model.

Objective.  Development of a spinal cord stimulation (SCS) system in a mouse model of chronic neuropathic pain.

167 SPINAL CORD STIMULATION IN A MOUSE CHRONIC NEUROPATHIC PAIN MODEL

Background and aims. We have previously demonstrated that gabapentin supraspinally activates the descending noradrenergic system to alleviate neuropathic pain. In this study, we investigated behaviorally and neurochemically whether pregabalin, an anticonvulsant and analgesic drug that is structurally and pharmacologically related to gabapentin, also exhibits similar analgesic effects involving the descending noradrenergic system. Methods. A chronic pain model was prepared by partially ligating the sciatic nerve in mice. The mice received intraperitoneal (i.p.), intracerebroventricular (i.c.v.) or intrathecal (i.t.) injections of pregabalin combined with either central noradrenaline (NA) depletion by 6hydroxydopamine or pharmacological blockade of spinal a2-adrenoceptors. Concentrations of spinal monoamines were also measured using high-performance liquid chromatography in mice after i.c.v. injection of pregabalin. Results. Systemically administered pregabalin (10 and 30 mg/kg, i.p.) reduced mechanical and thermal hypersensitivity in mice only after peripheral nerve injury. Similar analgesic effects were obtained when pregabalin (10 and 30 lg) was injected i.c.v. or i.t. Depletion of spinal NA or blockade of spinal a2-adrenoceptors with yohimbine (1 and 3 lg, i.t.) reduced the analgesic effects of pregabalin (i.p. or i.c.v.). Moreover, i.c.v. administered pregabalin increased the spinal MHPG concentration and the MHPG/NA ratio only in neuropathic pain mice, suggesting that supraspinal pregabalin resulted in an increase in spinal NA turnover. By contrast, the concentrations of NA, serotonin, 5-hydroxyindoleacetic acid and dopamine were unchanged. Conclusions. These results indicate that pregabalin supraspinally activates the descending noradrenergic pain inhibitory system to ameliorate neuropathic pain.