Gamma-aminobutyric acid is released in the dorsal horn by electrical spinal cord stimulation: an in vivo microdialysis study in the rat

A Narrative Review of the Dorsal Root Ganglia and Spinal Cord Mechanisms of Action of Neuromodulation Therapies in Neuropathic Pain

Neuropathic pain arises from injuries to the nervous system in diseases such as diabetes, infections, toxicity, and traumas. The underlying mechanism of neuropathic pain involves peripheral and central pathological modifications. Peripheral mechanisms entail nerve damage, leading to neuronal hypersensitivity and ectopic action potentials. Central sensitization involves a neuropathological process with increased responsiveness of the nociceptive neurons in the central nervous system (CNS) to their normal or subthreshold input due to persistent stimuli, leading to sustained electrical discharge, synaptic plasticity, and aberrant processing in the CNS. Current treatments, both pharmacological and non-pharmacological, aim to alleviate symptoms but often face challenges due to the complexity of neuropathic pain. Neuromodulation is emerging as an important therapeutic approach for the treatment of neuropathic pain in patients unresponsive to common therapies, by promoting the normalization of neuronal and/or glial activity and by targeting cerebral cortical regions, spinal cord, dorsal root ganglia, and nerve endings. Having a better understanding of the efficacy, adverse events and applicability of neuromodulation through pre-clinical studies is of great importance. Unveiling the mechanisms and characteristics of neuromodulation to manage neuropathic pain is essential to understand how to use it. In the present article, we review the current understanding supporting dorsal root ganglia and spinal cord neuromodulation as a therapeutic approach for neuropathic pain.

Spinal cord stimulation for the symptomatic treatment of rigidity and painful spasm in a case of stiff person syndrome

BACKGROUND

Stiff person syndrome (SPS) is a rare neuroimmunological disorder characterized by rigidity and painful spasm primarily affecting the truncal and paraspinal musculature due to autoimmune-mediated neuronal hyperexcitability. Spinal cord stimulation (SCS) is an approved therapy for managing painful neuropathic conditions, including diabetic peripheral neuropathy and refractory angina pectoris. We describe the novel use of SCS for the treatment of spasm and rigidity in a 49-year-old man with seropositive stiff person syndrome (SPS). The patient was treated with intravenous immunoglobulin (IVIG) and oral medications over a 13-month period with minimal improvement, prompting consideration of SCS. To our knowledge, this is the first report of the successful use of SCS in SPS with the demonstration of multifaceted clinical improvement.

METHODS

Following a successful temporary SCS trial, permanent implantation was performed. Spasm/stiffness (Distribution of Stiffness Index; Heightened Sensitivity Scale; Penn Spasm Frequency Scale, PSFS), disability (Oswestry Disability Index, ODI; Pain Disability Index, PDI), depression (Patient Health Questionnaire-9, PHQ-9), sleep (Pittsburgh Sleep Quality Index, PSQI), fatigue (Fatigue Severity Scale, FSS), pain (Numerical Pain Rating Scale, NPRS), quality of life (EuroQoL 5 Dimension 5 Level, EQ-5D-5L), and medication usage were assessed at baseline, 6-month, and 10-month postimplantation.

RESULTS

ODI, PHQ-9, FSS, NPRS, PSQI, and EQ-5D-5L scores showed a notable change from baseline and surpassed the defined minimal clinically important difference (MCID) at 6-month and 10-month follow-up. Oral medication dosages were reduced.

CONCLUSIONS

The novel use of SCS therapy in seropositive SPS resulted in functional improvement and attenuation of symptoms. We present possible mechanisms by which SCS may produce clinical response in patients with SPS and aim to demonstrate proof-of-concept for a future comprehensive pilot study evaluating SCS-mediated response in SPS.

Cervical Spinal Cord Stimulation for the Treatment of Headache Disorders: A Systematic Review

OBJECTIVES

Chronic headache remains a major cause of disability and pain worldwide. Although the literature has extensively described pharmacologic options for headache treatment and prophylaxis, there remains a paucity of data on the efficacy of neuromodulation interventions for treatment of headache unresponsive to conventional pharmacologic therapy. The primary aim of this review was to appraise the literature for the efficacy of cervical spinal cord stimulation (cSCS) in treating any intractable chronic headache, including migraine headaches (with or without aura), cluster headache, tension headache, and other types of headaches.

MATERIALS AND METHODS

In accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analysis guidelines, we performed a systematic review by identifying studies in PubMed, Embase (Scopus), Web of Science, and Cochrane Central Register of Controlled Trials that assessed cSCS to treat chronic headache. Data were synthesized qualitatively, with primary outcomes of headache intensity and frequency. The secondary outcome was adverse effects.

RESULTS

In total, 16 studies comprising 107 patients met the inclusion criteria. Findings were presented based on type of headache, which included migraine headache with or without aura, cluster headache, trigeminal neuropathy, occipital neuralgia, posttraumatic headache, cervicogenic headache, short-lasting unilateral neuralgiform headache with autonomic symptoms, and poststroke facial pain. Per the Grading of Recommendations, Assessment, Development and Evaluations criteria, there was very low-quality evidence that cSCS is associated with a decrease in migraine headache frequency, migraine headache intensity, and trigeminal neuropathy intensity. Placement for cSCS leads ranged from C1 to C4.

CONCLUSIONS

Our review suggests promising data from observational studies that cSCS may be helpful in decreasing frequency and intensity of chronic intractable headache. Future well-powered, randomized controlled trials are needed.

Research hotspots and trends on spinal cord stimulation for pain treatment: a two-decade bibliometric analysis

Background

Chronic pain poses a significant social burden. Spinal cord stimulation (SCS) is considered to be the most promising treatment for refractory pain. The aim of this study was to summarize the current research hotspots on SCS for pain treatment during the past two decades and to predict the future research trends by bibliometric analysis.

Methods

The literature over the last two decades (2002-2022) which was related to SCS in pain treatment was obtained from the Web of Science Core Collection. Bibliometric analyses were conducted based on the following aspects: (1) Annual publication and citation trends; (2) Annual publication changes of different publication types; (3) Publications and citations/co-citations of different country/institution/journal/author; (4) Citations/co-citation and citation burst analysis of different literature; and (5) Co-occurrence, cluster, thematic map, trend topics, and citation burst analysis of different keywords. (6) Comparison between the United States and Europe. All analyses were performed on CiteSpace, VOSviewer, and R bibliometrix package.

Results

A total of 1,392 articles were included in this study, with an increasing number of publications and citations year by year. The most highly published type of literature was clinical trial. United States was the country with the most publications and citations; Johns Hopkins University was the institution with the most publications; NEUROMODULATION published the most papers; the most published author was Linderoth B; and the most cited paper was published in the PAIN by Kumar K in 2007. The most frequently occurring keywords were "spinal cord stimulation," "neuropathic pain," and "chronic pain," etc.

Conclusion

The positive effect of SCS on pain treatment has continued to arouse the enthusiasm of researchers in this field. Future research should focus on the development of new technologies, innovative applications, and clinical trials for SCS. This study might facilitate researchers to comprehensively understand the overall perspective, research hotspots, and future development trends in this field, as well as seek collaboration with other researchers.

Activity-dependent plasticity and spinal cord stimulation for motor recovery following spinal cord injury

Spinal cord injuries lead to permanent physical impairment despite most often being anatomically incomplete disruptions of the spinal cord. Remaining connections between the brain and spinal cord create the potential for inducing neural plasticity to improve sensorimotor function, even many years after injury. This narrative review provides an overview of the current evidence for spontaneous motor recovery, activity-dependent plasticity, and interventions for restoring motor control to residual brain and spinal cord networks via spinal cord stimulation. In addition to open-loop spinal cord stimulation to promote long-term neuroplasticity, we also review a more targeted approach: closed-loop stimulation. Lastly, we review mechanisms of spinal cord neuromodulation to promote sensorimotor recovery, with the goal of advancing the field of rehabilitation for physical impairments following spinal cord injury.

Sex-specific characteristics of cells expressing the cannabinoid 1 receptor in the dorsal horn of the lumbar spinal cord

It is becoming increasingly clear that robust sex differences exist in the processing of acute and chronic pain in both rodents and humans. However, the underlying mechanism has not been well characterized. The dorsal horn of the lumbar spinal cord is the fundamental building block of ascending and descending pain pathways. It has been shown that numerous neurotransmitter and neuromodulator systems in the spinal cord, including the endocannabinoid system and its main receptor, the cannabinoid 1 receptor (CB₁ R), play vital roles in processing nociceptive information. Our previous findings have shown that CB₁ R mRNA is widely expressed in the brain in sex-dependent patterns. However, the sex-, lamina-, and cell-type-specific characteristics of CB₁ R expression in the spinal cord have not been fully described. In this study, the CB₁ R-iCre-EGFP mouse strain was generated to label and identify CB₁ R-positive (CB₁ R^GFP ) cells. We reported no sex difference in CB₁ R expression in the lumbar dorsal horn of the spinal cord, but a dynamic distribution within superficial laminae II and III in female mice between estrus and nonestrus phases. Furthermore, the cell-type-specific CB₁ R expression pattern in the dorsal horn was similar in both sexes. Over 50% of CB₁ R^GFP cells were GABAergic neurons, and approximately 25% were glycinergic and 20-30% were glutamatergic neurons. The CB₁ R-expressing cells also represented a subset of spinal projection neurons. Overall, our work indicates a highly consistent distribution pattern of CB₁ R^GFP cells in the dorsal horn of lumbar spinal cord in males and females.

Conventional Dorsal Root Ganglion Stimulation in an Experimental Model of Painful Diabetic Peripheral Neuropathy: A Quantitative Immunocytochemical Analysis of Intracellular γ-Aminobutyric Acid in Dorsal Root Ganglion Neurons

Background and Objective

The sensory cell somata in the DRG contain all equipment necessary for extensive GABAergic signaling and are able to release GABA upon depolarization. With this study, we hypothesize that pain relief induced by conventional dorsal root ganglion stimulation (Con‐DRGS) in animals with experimental painful diabetic peripheral neuropathy is related to the release of GABA from DRG neurons. With use of quantitative immunocytochemistry, we hypothesize DRGS to result in a decreased intensity of intracellular GABA‐immunostaining in DRG somata.

Materials and Methods

Female Sprague‐Dawley rats (n = 31) were injected with streptozotocin (STZ) in order to induce Diabetes Mellitus. Animals that developed neuropathic pain after four weeks (Von Frey) were implanted with a unilateral DRGS device at L4 (n = 14). Animals were then stimulated for 30 min with Con‐DRGS (20 Hz, pulse width = 0.2 msec, amplitude = 67% of motor threshold, n = 8) or Sham‐DRGS (n = 6), while pain behavior (von Frey) was measured. DRGs were then collected and immunostained for GABA, and a relation to size of sensory cell soma diameter (small: 12–26 μm, assumed to be C‐fiber related sensory neurons; medium: 26–40 μm, assumed to be Aδ related sensory neurons; and large: 40–54 μm, assumed to be Aβ related sensory neurons) was made.

Results

DRGS treated animals showed significant reductions in STZ‐induced mechanical hypersensitivity. No significant differences in GABA immunostaining intensity per sensory neuron cell soma type (small‐, medium‐, or large‐sized) were noted in DRGs of stimulated (Con‐DRGS) animals versus Sham animals. No differences in GABA immunostaining intensity per sensory cell soma type in ipsi‐ as compared to contralateral DRGs were observed.

Conclusion

Con‐DRGS does not affect the average intracellular GABA immunofluorescence staining intensity in DRG sensory neurons of those animals which showed significant pain reduction. Similarly, no soma size related changes in intracellular GABA immunofluorescence were observed following Con‐DRGS.

Efficacy of Simultaneous Usage of Spinal Cord Stimulation and Intrathecal Therapy for Nonmalignant Chronic Neuropathic Pain

BACKGROUND

Some patients with chronic pain and implanted spinal cord stimulators or intrathecal (IT) pumps fail to obtain significant pain relief. The use of dual modality treatment with both therapies is understudied. This study evaluated comprehensive outcomes in this patient population and reported outcomes primarily using IT ziconotide.

METHODS

We retrospectively analyzed 11 patients with chronic pain treated with both spinal cord stimulation and IT therapy. When a primary treatment failed to achieve significant pain relief, a secondary device was trialed and implanted. Pain severity (measured by a numeric rating scale) was assessed by the change from baseline to after the first and second intervention. In a subset of patients (n = 6), quality-of-life metrics were also assessed. Outcome measures were analyzed closest to the 1-year follow-up date after implantation of the first modality and then at the most recent follow-up after implantation of the second modality.

RESULTS

Spinal cord stimulation leads were percutaneous (n = 2) or paddles (n = 9) and commonly covered T8-10. IT medication included ziconotide (n = 8), baclofen (n = 1), hydromorphone (n = 1), and morphine/clonidine (n = 1). There was a mean of 19.64 ± 3.17 months between primary and secondary intervention. There was a significant improvement in pain severity from baseline to implantation of the second modality (P = 0.032) at a mean follow-up of 50.18 ± 11.83 months.

CONCLUSIONS

Dual modality therapy is a potential treatment option in patients who have lost efficacy with a single neuromodulation modality. Further study is required to identify potential responders and nonresponders.

The Effect of Clinically Controllable Factors on Neural Activation During Dorsal Root Ganglion Stimulation

OBJECTIVE

Dorsal root ganglion stimulation (DRGS) is an effective therapy for chronic pain, though its mechanisms of action are unknown. Currently, we do not understand how clinically controllable parameters (e.g., electrode position, stimulus pulse width) affect the direct neural response to DRGS. Therefore, the goal of this study was to utilize a computational modeling approach to characterize how varying clinically controllable parameters changed neural activation profiles during DRGS.

MATERIALS AND METHODS

We coupled a finite element model of a human L5 DRG to multicompartment models of primary sensory neurons (i.e., Aα-, Aβ-, Aδ-, and C-neurons). We calculated the stimulation amplitudes necessary to elicit one or more action potentials in each neuron, and examined how neural activation profiles were affected by varying clinically controllable parameters.

RESULTS

In general, DRGS predominantly activated large myelinated Aα- and Aβ-neurons. Shifting the electrode more than 2 mm away from the ganglion abolished most DRGS-induced neural activation. Increasing the stimulus pulse width to 500 μs or greater increased the number of activated Aδ-neurons, while shorter pulse widths typically only activated Aα- and Aβ-neurons. Placing a cathode near a nerve root, or an anode near the ganglion body, maximized Aβ-mechanoreceptor activation. Guarded active contact configurations did not activate more Aβ-mechanoreceptors than conventional bipolar configurations.

CONCLUSIONS

Our results suggest that DRGS applied with stimulation parameters within typical clinical ranges predominantly activates Aβ-mechanoreceptors. In general, varying clinically controllable parameters affects the number of Aβ-mechanoreceptors activated, although longer pulse widths can increase Aδ-neuron activation. Our data support several Neuromodulation Appropriateness Consensus Committee guidelines on the clinical implementation of DRGS.

Mechanisms and mode of action of spinal cord stimulation in chronic neuropathic pain

Tonic spinal cord stimulation (SCS) has been used as a treatment for chronic neuropathic pain ever since its discovery in late 1960s. Despite its clinical successes in a subset of chronic neuropathic pain syndromes, several limitations such as insufficient pain relief and uncomfortable paresthesias have led to the development of new targets, the dorsal root ganglion, and new stimulation waveforms, such as burst and high frequency. The aim of this review is to provide a brief overview of the main mechanisms behind the mode of action of the different SCS paradigms. Tonic SCS mainly acts via a segmental spinal mechanism where it induces GABA-release from inhibitory interneurons in the spinal dorsal horn. Tonic SCS concurrently initiates neuropathic pain modulation through a supraspinal-spinal feedback loop and serotonergic descending fibers. Mechanisms of stimulation of the DRG as well as those related to new SCS paradigms are now under investigation, where it seems that burst SCS not only stimulates sensory, discriminative aspects of pain (like Tonic SCS) but also emotional, affective, and motivational aspects of pain. Initial long-term study results on closed-loop SCS systems hold promise for improvement of future SCS treatment.

Early high-frequency spinal cord stimulation treatment inhibited the activation of spinal mitogen-activated protein kinases and ameliorated spared nerve injury-induced neuropathic pain in rats

BACKGROUND

Neuromodulation therapies offer a treatment option that has minimal side effects and is relatively safe and potentially reversible. Spinal cord stimulation (SCS) has been used to treat various pain conditions for many decades. High-frequency SCS (HFSCS) involves the application of a single waveform at 10,000 Hz at a subthreshold level, therefore providing pain relief without any paresthesia.

METHODS

We tested whether early HFSCS treatment attenuated spared nerve injury (SNI)-induced neuropathic pain. The phosphorylation profile of mitogen-activated protein kinases (MAPKs), i.e., extracellular signal-regulated kinases (ERKs), c-Jun N-terminal kinases (JNKs), and p38, was evaluated to elucidate the potential underlying mechanism.

RESULTS

SNI of rat unilateral sciatic nerves induced mechanical hyperalgesia in the ipsilateral hind paws. Rats were assigned to SCS sessions with HFSCS (frequency 10 kHz; pulse width 30 μs; pulse shape of charge-balanced, current controlled; delivered continuously for 72 h), or sham stimulation immediately after SNI. Tissue samples were examined at 1, 3, 7, and 14 days after SNI. Behavioral studies showed that HFSCS applied to the T10/T11 spinal cord significantly attenuated SNI-induced mechanical hyperalgesia compared with the sham stimulation group. Moreover, western blotting revealed a significant attenuation of the activation of ERK1, ERK2, JNK1, and p38 in the dorsal root ganglia and the spinal dorsal horn.

CONCLUSION

Application of HFSCS provides an effective treatment for SNI-induced persistent mechanical hyperalgesia by attenuating ERK, JNK, and p38 activation in the dorsal root ganglia and the spinal dorsal horn.

Dorsal Root Ganglion Stimulation in Experimental Painful Diabetic Polyneuropathy: Delayed Wash-Out of Pain Relief After Low-Frequency (1Hz) Stimulation

OBJECTIVE

Up until now there is little data about the pain relieving effect of different frequency settings in DRGS. The aim of this study was to compare the pain relieving effect of DRGS at low-, mid-, and high-frequencies and Sham-DRGS in an animal model of painful diabetic neuropathy (PDPN).

MATERIAL AND METHODS

Diabetes mellitus was induced by an intraperitoneal injection of streptozotocin in 8-week-old female Sprague-Dawley rats (n = 24; glucose ≥15 mmol/L: n = 20; mechanical hypersensitivity: n = 15). Five weeks later, a DRGS device was implanted at the L5 DRG. Ten animals were included for stimulation, alternating 30 minutes of low (1 Hz)-, mid (20 Hz)-, and high (1000 Hz)-frequencies and Sham-DRGS during four days, with a pulse width of 0.2 msec (average amplitude: 0.19 ± 0.01 mA), using a randomized cross-over design. The effect on mechanical hypersensitivity of the hind paw to von Frey filaments was evaluated.

RESULTS

All DRGS frequencies resulted in a complete reversal of mechanical hypersensitivity and "a clinically relevant reduction" was achieved in 70-80% of animals. No significant differences in maximal pain relieving effect were found between the different frequency treatments (p = 0.24). Animals stimulated at 1000 and 20 Hz returned to baseline mechanical hypersensitivity values 15 and 30 min after stimulation cessation, respectively, while animals stimulated at 1 Hz did not.

CONCLUSIONS

These results show that DRGS is equally effective when applied at low-, mid-, and high-frequency in an animal model of PDPN. However, low-frequency-(1 Hz)-DRGS resulted in a delayed wash-out effect, which suggests that this is the most optimal frequency for pain therapy in PDPN as compared to mid- and high-frequency.

Burst and Tonic Spinal Cord Stimulation Both Activate Spinal GABAergic Mechanisms to Attenuate Pain in a Rat Model of Chronic Neuropathic Pain

BACKGROUND

Experimental and clinical studies have shown that tonic spinal cord stimulation (SCS) releases gamma-aminobutyric acid (GABA) in the spinal dorsal horn. Recently, it was suggested that burst SCS does not act via spinal GABAergic mechanisms. Therefore, we studied spinal GABA release during burst and tonic SCS, both anatomically and pharmacologically, in a well-established chronic neuropathic pain model.

METHODS

Animals underwent partial sciatic nerve ligation (PSNL). Quantitative immunohistochemical (IHC) analysis of intracellular GABA levels in the lumbar L4 to L6 dorsal spinal cord was performed after 60 minutes of burst, tonic, or sham SCS in rats that had undergone PSNL (n = 16). In a second pharmacological experiment, the effects of intrathecal administration of the GABA_A antagonist bicuculline (5 μg) and the GABA_B antagonist phaclofen (5 μg) were assessed. Paw withdrawal thresholds to von Frey filaments of rats that had undergone PSNL (n = 20) were tested during 60 minutes of burst and tonic SCS 30 minutes after intrathecal administration of the drugs.

RESULTS

Quantitative IHC analysis of GABA immunoreactivity in spinal dorsal horn sections of animals that had received burst SCS (n = 5) showed significantly lower intracellular GABA levels when compared to sham SCS sections (n = 4; P = 0.0201) and tonic SCS sections (n = 7; P = 0.0077). Intrathecal application of the GABA_A antagonist bicuculline (5 μg; n = 10) or the GABA_B antagonist phaclofen (5 μg; n = 10) resulted in ablation of the analgesic effect for both burst SCS and tonic SCS.

CONCLUSIONS

In conclusion, our anatomical and pharmacological data demonstrate that, in this well-established chronic neuropathic animal model, the analgesic effects of both burst SCS and tonic SCS are mediated via spinal GABAergic mechanisms.

Mechanism of dorsal root ganglion stimulation for pain relief in painful diabetic polyneuropathy is not dependent on GABA release in the dorsal horn of the spinal cord

AIMS

It is hypothesized that dorsal root ganglion stimulation (DRGS), sharing some of the mechanisms of traditional spinal cord stimulation (SCS) of the dorsal columns, induces γ-aminobutyric acid (GABA) release from interneurons in the spinal dorsal horn.

METHODS

We used quantitative immunohistochemical analysis in order to investigate the effect of DRGS on intensity of intracellular GABA-staining levels in the L4-L6 spinal dorsal horn of painful diabetic polyneuropathy (PDPN) animals. To establish the maximal pain relieving effect, we tested for mechanical hypersensitivity to von Frey filaments and animals received 30 minutes of DRGS at day 3 after implantation of the electrode. One day later, 4 Sham-DRGS animals and four responders-to-DRGS received again 30 minutes of DRGS and were perfused at the peak of DRGS-induced pain relief.

RESULTS

No significant difference in GABA-immunoreactivity was observed between DRGS and Sham-DRGS in lamina 1-3 of the spinal levels L4-6 neither ipsilaterally nor contralaterally.

CONCLUSIONS

Dorsal root ganglion stimulation does not induce GABA release from the spinal dorsal horn cells, suggesting that the mechanisms underlying DRGS in pain relief are different from those of conventional SCS. The modulation of a GABA-mediated "Gate Control" in the DRG itself, functioning as a prime Gate of nociception, is suggested and discussed.

Functional dissection of astrocyte-secreted proteins: Implications in brain health and diseases

Astrocytes, which are homeostatic cells of the central nervous system (CNS), display remarkable heterogeneity in their morphology and function. Besides their physical and metabolic support to neurons, astrocytes modulate the blood-brain barrier, regulate CNS synaptogenesis, guide axon pathfinding, maintain brain homeostasis, affect neuronal development and plasticity, and contribute to diverse neuropathologies via secreted proteins. The identification of astrocytic proteome and secretome profiles has provided new insights into the maintenance of neuronal health and survival, the pathogenesis of brain injury, and neurodegeneration. Recent advances in proteomics research have provided an excellent catalog of astrocyte-secreted proteins. This review categorizes astrocyte-secreted proteins and discusses evidence that astrocytes play a crucial role in neuronal activity and brain function. An in-depth understanding of astrocyte-secreted proteins and their pathways is pivotal for the development of novel strategies for restoring brain homeostasis, limiting brain injury/inflammation, counteracting neurodegeneration, and obtaining functional recovery.

And yet it moves: Recovery of volitional control after spinal cord injury

Preclinical and clinical neurophysiological and neurorehabilitation research has generated rather surprising levels of recovery of volitional sensory-motor function in persons with chronic motor paralysis following a spinal cord injury. The key factor in this recovery is largely activity-dependent plasticity of spinal and supraspinal networks. This key factor can be triggered by neuromodulation of these networks with electrical and pharmacological interventions. This review addresses some of the systems-level physiological mechanisms that might explain the effects of electrical modulation and how repetitive training facilitates the recovery of volitional motor control. In particular, we substantiate the hypotheses that: (1) in the majority of spinal lesions, a critical number and type of neurons in the region of the injury survive, but cannot conduct action potentials, and thus are electrically non-responsive; (2) these neuronal networks within the lesioned area can be neuromodulated to a transformed state of electrical competency; (3) these two factors enable the potential for extensive activity-dependent reorganization of neuronal networks in the spinal cord and brain, and (4) propriospinal networks play a critical role in driving this activity-dependent reorganization after injury. Real-time proprioceptive input to spinal networks provides the template for reorganization of spinal networks that play a leading role in the level of coordination of motor pools required to perform a given functional task. Repetitive exposure of multi-segmental sensory-motor networks to the dynamics of task-specific sensory input as occurs with repetitive training can functionally reshape spinal and supraspinal connectivity thus re-enabling one to perform complex motor tasks, even years post injury.

Burst and Tonic Spinal Cord Stimulation Differentially Activate GABAergic Mechanisms to Attenuate Pain in a Rat Model of Cervical Radiculopathy

OBJECTIVE

Spinal cord stimulation (SCS) is widely used to treat neuropathic pain. Burst SCS, an alternative mode of stimulation, reduces neuropathic pain without paresthesia. However, the effects and mechanisms of burst SCS have not been compared to conventional tonic SCS in controlled investigations. This study compares the attenuation of spinal neuronal activity and tactile allodynia, and the role of γ-aminobutyric acid (GABA) signaling during burst or tonic SCS in a rat model of cervical radiculopathy.

METHODS

The effects of burst and tonic SCS were compared by recording neuronal firing before and after each mode of stimulation at day 7 following a painful cervical nerve root compression. Neuronal firing was also recorded before and after burst and tonic SCS in the presence of the GABAB receptor antagonist, CGP35348.

RESULTS

Burst and tonic SCS both reduce neuronal firing. The effect of tonic SCS, but not burst SCS, is blocked by CGP35348. In a separate study, spinal cord stimulators were implanted to deliver burst or tonic SCS beginning on day 4 after painful nerve root compression; allodynia and serum GABA concentration were measured through day 14. Burst and tonic SCS both reduce allodynia. Tonic SCS attenuates injury-induced decreases in serum GABA, but GABA remains decreased from baseline during burst SCS.

CONCLUSION AND SIGNIFICANCE

Together, these studies suggest that burst SCS does not act via spinal GABAergic mechanisms, despite its attenuation of spinal hyperexcitability and allodynia similar to that of tonic SCS; understanding other potential spinal inhibitory mechanisms may lead to enhanced analgesia during burst stimulation.

Effects of Spinal Cord Stimulation on Pain Thresholds and Sensory Perceptions in Chronic Pain Patients

OBJECTIVE

Spinal cord stimulation (SCS) has been in clinical use for nearly four decades. In earliest observations, researchers found a significant increase in pain threshold during SCS therapy without changes associated with touch, position, and vibration sensation. Subsequent studies yielded diverse results regarding how SCS impacts pain and other sensory thresholds. This pilot study uses quantitative sensory testing (QST) to objectively quantify the impact of SCS on warm sensation, heat pain threshold, and heat pain tolerance.

MATERIALS AND METHODS

Nineteen subjects with an indwelling SCS device for chronic pain were subjected to QST with heat stimuli. QST was performed on an area of pain covered with SCS-induced paresthesia and an area without pain and without paresthesia, while the SCS was turned off and on. The temperature at which the patient detected warm sensation, heat pain, and maximal tolerable heat pain was used to define the thresholds.

RESULTS

We found that all three parameters, the detection of warm sensation, heat pain threshold, and heat pain tolerance, were increased during the period when SCS was on compared with when it was off. This increase was observed in both painful and non-painful sites.

CONCLUSION

The observed pain relief during SCS therapy seems to be related to its impact on increased sensory threshold as detected in this study. The increased sensory threshold on areas without pain and without the presence of SCS coverage may indicate a central (spinal and/or supra-spinal) influence from SCS.

Advanced technologies and novel neurostimulation targets in trigeminal autonomic cephalalgias

The trigeminal autonomic cephalalgias (TACs) are a group of rare but disabling primary headache disorders. Their management is challenging, since only few effective treatments are available and high doses may be required to control the headache, compromising patients' adherence to treatments. A significant minority of patients, who fail to respond to or tolerate established treatments, are left with enormous level of disability and disruption to their quality of life. A growing body of evidence demonstrates the efficacy of central and peripheral neuromodulation approaches for management of patients with refractory TACs. In view of the potential risks related to deep brain stimulation of the posterior hypothalamic region, occipital nerve stimulation is currently considered the first treatment option for refractory chronic TACs. However, in view of the presence of paraesthesia induced by the stimulator, no robust controlled trials have been possible so far. Additionally, the equipment used for occipital nerve stimulation is not designed specifically for peripheral nerve stimulation, thus a significant proportion of patients experience device-related complications that often require surgical revisions. To overcome these issues, new neurostimulation technologies using less invasive or non-invasive approaches and modulating different neuroanatomical targets have been recently studied.

The appropriate use of neurostimulation of the spinal cord and peripheral nervous system for the treatment of chronic pain and ischemic diseases: the Neuromodulation Appropriateness Consensus Committee

INTRODUCTION

The Neuromodulation Appropriateness Consensus Committee (NACC) of the International Neuromodulation Society (INS) evaluated evidence regarding the safety and efficacy of neurostimulation to treat chronic pain, chronic critical limb ischemia, and refractory angina and recommended appropriate clinical applications.

METHODS

The NACC used literature reviews, expert opinion, clinical experience, and individual research. Authors consulted the Practice Parameters for the Use of Spinal Cord Stimulation in the Treatment of Neuropathic Pain (2006), systematic reviews (1984 to 2013), and prospective and randomized controlled trials (2005 to 2013) identified through PubMed, EMBASE, and Google Scholar.

RESULTS

Neurostimulation is relatively safe because of its minimally invasive and reversible characteristics. Comparison with medical management is difficult, as patients considered for neurostimulation have failed conservative management. Unlike alternative therapies, neurostimulation is not associated with medication-related side effects and has enduring effect. Device-related complications are not uncommon; however, the incidence is becoming less frequent as technology progresses and surgical skills improve. Randomized controlled studies support the efficacy of spinal cord stimulation in treating failed back surgery syndrome and complex regional pain syndrome. Similar studies of neurostimulation for peripheral neuropathic pain, postamputation pain, postherpetic neuralgia, and other causes of nerve injury are needed. International guidelines recommend spinal cord stimulation to treat refractory angina; other indications, such as congestive heart failure, are being investigated.

CONCLUSIONS

Appropriate neurostimulation is safe and effective in some chronic pain conditions. Technological refinements and clinical evidence will continue to expand its use. The NACC seeks to facilitate the efficacy and safety of neurostimulation.

Spinal cord stimulation in the management of pain from brachial plexus avulsion

This is a retrospective study of four patients suffering from brachial plexus root avulsion of traumatic origin. Spinal cord stimulation was used to treat pain in all patients. A seven-contact electrode was percutaneously introduced in the epidural cervical space and under fluoroscopic control, advanced to the level were stimulation provoked a tingling sensation in the painful region. A stimulation trial was performed for 2 weeks and during this period the patients showed significant pain relief, so the system was permanently implanted. A significant difference of more than three points in the pain between the first and the last follow-up (0-9 months) on the Visual Analog Scale was obtained with a steady and progressive decrease of the pain scores.

Physiology of spinal cord stimulation: review and update

Spinal cord stimulation (SCS) was an outgrowth of the well-known gate control theory presented by Melzack and Wall in 1965. Although the method has been used to treat chronic severe pain for more than three decades, very little was known about the physiological and biochemical mechanisms behind the beneficial effects until recently. We now know that SCS activates several different mechanisms to treat different types of pain such as neuropathic and ischemic. In general, these mechanisms seem most dependent on activation of only a few segments of the spinal cord. However, both animal studies and human observations have indicated that supraspinal circuits may contribute as well. In the treatment of neuropathic pain, intermittent SCS may give several hours of pain relief after cessation of the stimulation. This protracted effect indicates long-lasting modulation of neural activity involving changes in the local transmitter systems in the dorsal horns. In ischemic pain, animal experiments demonstrate that inhibition of afferent activity in the spinothalamic tracts, long-term suppression of sympathetic activity, and antidromic effects on peripheral reflex circuits may take part in the pain alleviation. Moderate SCS intensities seem to evoke sympathetic inhibition, but higher stimulation intensities may induce antidromically mediated release of vasoactive substances, eg, the calcitonin gene-related peptide (CGRP), resulting in peripheral vasodilation. The anti-ischemic effect of SCS in angina pectoris due to intermittent coronary ischemia probably occurs because application of SCS appears to result in a redistribution of cardiac blood supply, as well as a decrease in tissue oxygen demand. Recent studies indicate that SCS modulates the activity of cardiac intrinsic neurons thereby restricting the arrythmogenic consequences of intermittent local coronary ischemia. The present state of knowledge is briefly reviewed and recent research directions outlined.

Experimental spinal cord stimulation and neuropathic pain: mechanism of action, technical aspects, and effectiveness

Spinal cord stimulation (SCS) is a valuable treatment for chronic intractable neuropathic pain. Although SCS has gone through a technological revolution over the last four decades, the neurophysiologic and biochemical mechanisms of action have only been partly elucidated. Animal experimental work has provided some evidence for spinal as well as supraspinal mechanisms of neuropathic pain relief of SCS. A SCS computer model of the electrical properties of the human spinal cord revealed many basic neurophysiologic principles that were clinically validated later on. The main question in clinical SCS is how to further improve the effectiveness of SCS as there is still a significant failure rate of 30%. In this context, experimental studies are needed to elucidate which target pain neuron(s) are involved, as well as with what exact electrical stimulation this target neuron can be influenced to produce an optimal supapression of neuropathic pain. This article reviews the basic clinical and experimental technical aspects in relation to the effectiveness of SCS in view of recent understanding of the dorsal horn pain circuit involved. These data may then result in experiments needed for an improved understanding of the mechanisms underlying SCS and consequently lead to improvement and increased effectiveness of SCS in neuropathic pain as a clinical therapy.

Sacral nerve modulation in the treatment of chronic pelvic pain

BACKGROUND

Chronic pelvic pain is a common condition that significantly compromises the quality of life of affected patients. Unfortunately, despite treatment procedures, the results are often ineffective and symptoms persist for years. For these reasons, the search for less aggressive treatment options with fewer negative consequences leading to minimally invasive techniques was conducted.

OBJECTIVE

The aim of the present study was to evaluate the efficacy of sacral nerve modulation in the treatment of chronic pelvic pain. Moreover, we aimed to identify potential predictors of positive results of sacral neuromodulation through the comparison between failed and successful patients.

PATIENTS

From January 2004 to December 2009, all consecutive patients suffering from chronic pelvic pain and tested for sacral nerve modulation in three pelvic floor dedicated centers were evaluated. Severity of symptoms were analyzed by a visual analog scale (VAS) RESULTS: Twenty-seven patients (2 males; mean age, 53 years) were tested for sacral nerve modulation in the screening period and were included in the present study. The mean duration of pain was 51 months (range, 10-132 months). The mean preoperative VAS was 7.8 (range, 5-10). Previous pelvic surgery was reported in 18 patients (66.5%). Sixteen patients (59%) fulfil the successful criteria and were definitively implanted. The mean follow-up was 37 months (range, 12-71 months). The mean preoperative VAS was 8.1 (range, 6-8) and decreased to 2.1 ± 1.2 at 6-month follow-up (p < 0.0001), to 2.1 ± 1.1 at 12 months (16 patients), to 2.0 ± 1.2 at 24 months (13 patients), to 2.3 ± 1.4 at 36 months (9 patients), to 2.1 ± 1.5 at 48 months (5 patients), and to 1.9 ± 1.3 at 60 months (3 patients).

CONCLUSIONS

Sacral neuromodulation proved to be effective in the treatment of some patients affected by chronic pelvic pain, and the effect persists over time. A positive screening phase and a positive response to gabapentin or pregabalin showed to be predictors of a successful response. Multiple localizations of pelvic pain and pain occurred after stapler surgery seem to be negative factors for the success of the treatment.

Contemporary insights into painful diabetic neuropathy and treatment with spinal cord stimulation

A substantial body of literature is available on the natural history of diabetes, but much less is understood of the natural history of painful diabetic peripheral neuropathy (PDPN), a pervasive and costly complication of diabetes mellitus. Multiple mechanisms have been proposed, including polyol pathway activation, advanced glycosylation end-product formation, and vasculopathic changes. Nevertheless, specific treatment modalities addressing these basic issues are still lacking. The mainstay of treatment includes pharmacological management with antidepressants, anticonvulsants, and opioids, but these drugs are often limited by unfavorable side-effect profiles. For over 30 years, spinal cord stimulation (SCS) has been used extensively for the management of various chronic neuropathic pain states. In the past decade, interest in the use of SCS for treatment of PDPN has increased. This article reviews pathophysiological mechanisms of PDPN, proposed mechanisms of SCS, and the role of SCS for the treatment of PDPN.

Intrathecal baclofen as adjuvant therapy to enhance the effect of spinal cord stimulation in neuropathic pain: a pilot study

Only about 60-70% of well selected patients with neuropathic pain syndromes of peripheral origin enjoy sufficient pain relief with spinal cord stimulation (SCS). Since recent animal experiments have demonstrated that the GABA-B receptor is pivotal in the effect of SCS on certain neuropathic symptoms, the use of baclofen as an adjunct to stimulation emerged as an option in patients not responding satisfactorily to SCS. Forty-eight patients with neuropathic pain of peripheral origin responding poorly to SCS were enrolled in a study with intrathecal baclofen; in a few cases adenosine was also tried. Twenty patients reported significant pain reduction at bolus trials and were offered implantation of a drug pump. Seven patients subsequently had pumps implanted together with SCS and four had pumps alone. Three patients had only peroral baclofen therapy as an adjunct to SCS. The 14 patients continuing with baclofen therapy as an adjunct to SCS, or alone, were followed for an average of 35 months after pump implant. The group with SCS+pump n=5; 2 explanted) reported an average decrease of pain ratings from VAS 82 to 33. The group with i.t. baclofen only had a pain decrease from VAS 63 to 33, while the three patients with peroral baclofen+SCS had less benefit from drug therapy. Adjunctive drug therapy for patients with unsatisfactory pain relief by SCS may offer a possibility to enhance pain alleviation.

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.

Effects of sub-perception threshold spinal cord stimulation in neuropathic pain: a randomized controlled double-blind crossover study

BACKGROUND

Recent studies suggest that perception of the paraesthesia elicited by spinal cord stimulation (SCS) is not necessarily required for the pain relieving effect.

OBJECTIVE

The purpose of the study was to determine the effect of sub-perception threshold SCS in patients with neuropathic pain.

METHODS

Ten patients with implanted SCS systems underwent continuous sub-threshold stimulation and no stimulation in a blinded randomized crossover design. Pain scores under these treatment modalities were compared with usual supra-threshold stimulation.

RESULTS

Sub-threshold stimulation elicited significantly lower pain relief than supra-perception threshold SCS. Mean pain scores were 3.6 [max 6.3, min 1.9, standard deviation (SD) 1.3] under supra-threshold stimulation, 5.6 (max 9.0, min 2.4, SD 1.9) under sub-threshold stimulation and 6.4 (max 10.0, min 4.0, SD 2.0) without stimulation.

CONCLUSION

Sub-threshold stimulation under otherwise conventional stimulation parameters has a measurable but not clinically sufficient effect. Thus, the pain relieving effect elicited by SCS is not necessarily linked to the perceptibility of stimulation but may instead be attributed to the intensity of the electric field.

High cervical spinal cord stimulation for chronic cluster headache

BACKGROUND

Cluster headache (CH) is the most painful and debilitating primary headache syndrome. Conventional treatment combines acute and prophylactic drugs. Also with maximal therapy a substantial proportion of patients do not experience a meaningful prevention or pain relief. Recent case series and early trials have suggested that occipital nerve stimulation can be very effective in the management of intractable CH.

METHODS

Seven patients with medically intractable chronic cluster headache were implanted with high cervical epidural electrodes. After a median test phase of 10 days (range 4-19 days) an impulse generator was implanted subcutaneously. Mean follow up was 23 months (median 12 months, range 3-78 months).

RESULTS

All patients showed significant treatment effects. In all patients, improvement occurred immediately after electrode implantation. The mean attack frequency decreased, as well as the mean duration and intensity of attacks. Also, depression, anxiety, and pain-related impairment scores decreased and medication intake was markedly reduced.

CONCLUSIONS

In this prospective series, high cervical spinal cord stimulation shows an effect size equal or larger than occipital nerve stimulation with immediate onset after surgery and may serve as a valuable additional treatment option of intractable cluster headache in the future.

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. The induction and maintenance of central sensitization plays a pivotal role in (chronic) neuropathic pain and is thought to be the resultant of the activation of the N-methyl-d-aspartate (NMDA) receptor in the dorsal horn. Blocking the NMDA receptor through the use of the non-competitive blocker ketamine has shown to attenuate neuropathic pain, although the undesirable side effects limit its use. The present study was performed to examine whether the combination of SCS with an individually determined sub-effective dose of intrathecal (i.t.) ketamine could convert non-responders into responders in rats with chronic neuropathic pain. Rats received a partial ligation of the sciatic nerve for the induction of neuropathic pain. Animals with tactile hypersensitivity to von Frey monofilaments (n=15) received 30 min of SCS. Non-responders to SCS (n=8) received their individually determined sub-effective i.t. dose of ketamine followed by 30 min of SCS. No side effects of the sub-effective dose of ketamine could be noted. The combined treatment of SCS and sub-effective dose of i.t. ketamine in non-responders resulted in a significant reduction of the withdrawal threshold in all previous non-responders to SCS, thereby converting them into responders to SCS.

What does the mechanism of spinal cord stimulation tell us about complex regional pain syndrome?

Spinal cord stimulation (SCS) can have dramatic effects on painful, vascular, and motor symptoms of complex regional pain syndrome (CRPS), but its precise mechanism of action is unclear. Better understanding of the physiologic effects of SCS may improve understanding not only of this treatment modality but also of CRPS pathophysiology. Effects of SCS on pain perception are likely to occur through activation of inhibitory GABA-ergic and cholinergic spinal interneurons. Increased release of both neurotransmitters has been demonstrated following SCS in animal models of neuropathic pain, with accompanying reductions in pain behaviors. Effects of SCS on vascular symptoms of CRPS are thought to occur through two main mechanisms: antidromic activation of spinal afferent neurons and inhibition of sympathetic efferents. Cutaneous vasodilation following SCS in animal models has been shown to involve antidromic release of calcitonin gene-related peptide and possibly nitric oxide, from small-diameter sensory neurons expressing the transient receptor potential V1 (TRPV1) receptor. The involvement of sympathetic efferents in the effects of SCS has not been studied in animal models of neuropathic pain, but has been demonstrated in models of angina pectoris. In conclusion, SCS is of clinical benefit in CRPS, and although its mechanism of action merits further elucidation, what little we do know is informative and can partially explain some of the pathophysiology of CRPS.

Spinal cord stimulation induces c-Fos expression in the dorsal horn in rats with neuropathic pain after partial sciatic nerve injury

Spinal cord stimulation (SCS) is an established treatment for intractable neuropathic pain, especially CRPS-1. The mechanisms of action of SCS have only been partly elucidated and include suppression of the hyper-excitability of the Wide Dynamic Range neurons and a GABA increase in the dorsal horn. In the present study we demonstrate an increase of c-Fos immunoreactive cells in the dorsal horn after SCS, suggesting early cellular activation that may preclude earlier described electrophysiological and biochemical changes in the dorsal horn after SCS. In a rat model of neuropathic pain, allodynia was induced and quantified using the von Frey test. In 11 rats a SCS device was implanted and spinal cord stimulation performed. Withdrawal threshold were measured every 15 min up to 90 min. A sham group (n=6) also had a SCS device implanted, but did not receive SCS. After SCS the animals were perfused and histology was performed for quantification of c-Fos immunoreactivity in the dorsal horns. We found a significant increase in c-Fos in the SCS group compared to our sham group and control tissue, indicating late cellular activity in the dorsal horn after SCS.

Cholinergic mechanisms involved in the pain relieving effect of spinal cord stimulation in a model of neuropathy

The mechanisms underlying the pain relieving effect of spinal cord stimulation (SCS) on neuropathic pain remain unclear. We have previously demonstrated that suppression of tactile hypersensitivity produced by SCS may be potentiated by i.t. clonidine in a rat model of mononeuropathy. Since the analgesic effect of this drug is mediated mainly via cholinergic mechanisms, a study exploring the possible involvement of the spinal cholinergic system in SCS was undertaken. The effect of SCS was assessed with von Frey filaments in rats displaying tactile hypersensitivity after partial ligation of the sciatic nerve and both SCS-responding and non-responding as well as normal rats were subjected to microdialysis in the dorsal horn. Acetylcholine (ACh) was analyzed with HPLC before, during and after SCS. SCS produced significantly increased release of ACh in the dorsal horn in rats responding to SCS whereas the release was unaffected in the non-responding animals. Furthermore, the basal release of ACh was significantly lower in nerve lesioned than in normal rats. In another group of rats it was found that the response to SCS was completely eliminated by i.t. atropine and a muscarinic M(4) receptor antagonist while a partial attenuation was produced by M(1) and M(2) antagonists. Blocking of nicotinic receptors did not influence the SCS effect. In conclusion, the attenuating effect of SCS on pain related behavior is associated with the activation of the cholinergic system in the dorsal horn and mediated via muscarinic receptors, particularly M(4,) while nicotinic receptors appear not to be involved.

State-dependent changes in glutamate, glycine, GABA, and dopamine levels in cat lumbar spinal cord

Recent studies have indicated that the glycine receptor antagonist strychnine and the gamma-aminobutyric acid type A (GABA A) receptor antagonist bicuculline reduced the rapid-eye-movement (REM) sleep-specific inhibition of sensory inflow via the dorsal spinocerebellar tract (DSCT). These findings imply that the spinal release of glycine and GABA may be due directly to the REM sleep-specific activation of reticulospinal neurons and/or glutamate-activated last-order spinal interneurons. This study used in vivo microdialysis and high-performance liquid chromatography analysis techniques to provide evidence for these possibilities. Microdialysis probes were stereotaxically positioned in the L3 spinal cord gray matter corresponding to sites where maximal cerebellar-evoked field potentials or individual DSCT and nearby spinoreticular tract (SRT) neurons could be recorded. Glutamate, glycine, and GABA levels significantly increased during REM sleep by approximately 48, 48, and 14%, respectively, compared with the control state of wakefulness. In contrast, dopamine levels significantly decreased by about 28% during REM sleep compared with wakefulness. During the state of wakefulness, electrical stimulation of the nucleus reticularis gigantocellularis (NRGc) at intensities sufficient to inhibit DSCT neuron activity, also significantly increased glutamate and glycine levels by about 69 and 45%, respectively, but not GABA or dopamine levels. We suggest that the reciprocal changes in the release of glutamate, glycine, and GABA versus dopamine during REM sleep contribute to the reduction of sensory inflow to higher brain centers via the DSCT and nearby SRT during this behavioral state. The neural pathways involved in this process likely include reticulo- and diencephalospinal and spinal interneurons.

Spinal cord stimulation: an update

Spinal cord stimulation has been used in the treatment of many chronic pain disorders since 1967. In this update, the indications for spinal cord stimulation are reviewed with attention to recent publications. A focused review of the literature on abdominal and visceral pain syndromes is also provided. Furthermore, the technology has evolved from the use of monopolar electrodes to complex electrode arrays. Similarly, the power source has changed from a radio frequency-driven system to a rechargeable impulse generator. These topics are covered, along with a short discussion of implant technique. Finally, we include a review of complications of such therapy. SCS as a technology and therapy continues to evolve.

Acetylcholine release from fetal tissue homotopically grafted to the motoneuron-depleted lumbar spinal cord. An in vivo microdialysis study in the awake rat

Grafts of spinal cord (SC) tissue can survive and develop into the severed SC, but no conclusive data are available concerning the functional activity of transplanted neurons. In the present study, suspensions of prelabeled embryonic ventral SC tissue were grafted to the lumbar SC of rats with motoneuron loss induced by perinatal injection of volkensin. Eight to ten months post-grafting, acetylcholine (ACh) release was measured by microdialysis in awake rats, under either basal or stimulated conditions. In normal animals, baseline ACh output averaged 1.6 pmol/30 microl, it exhibited a 4-fold increase after KCl-induced depolarization or handling, and it was completely inhibited by tetrodotoxin administration. Moreover, ACh levels did not change following acute SC transection performed under anesthesia during ongoing dialysis, suggesting an intrinsic source for spinal ACh. Treatment with volkensin produced a severe (>85%) motoneuronal loss accompanied by a similar reduction in baseline ACh release and almost completely abolished effects of depolarization or handling. In transplanted animals, many motoneuron-like labeled cells were found within and just outside the graft area, but apparently in no case were they able to extend fibers towards the denervated muscle. However, the grafts restored baseline ACh output up to near-normal levels and responded with significantly increased release to depolarization, but not to handling. The present findings indicate that spinal neuroblasts can survive and develop within the motoneuron-depleted SC and release ACh in a near-normal, but apparently non-regulated, manner. This may be of importance for future studies involving intraspinal stem cell grafts.

Release of GABA and activation of GABA(A) in the spinal cord mediates the effects of TENS in rats

Transcutaneous electrical nerve stimulation (TENS) is a commonly utilized non-pharmacological, non-invasive treatment for pain. GABA is a neurotransmitter in the dorsal horn of the spinal cord that mediates analgesia locally, and also through activation of supraspinal sites. TENS reduces hyperalgesia through activation of receptor-mediated pathways at the level of the spinal cord, and supraspinally. The current study tested the hypothesis that either high or low frequency TENS applied to the inflamed knee joint increases GABA in the spinal cord dorsal horn and activates GABA receptors spinally. We utilized microdialysis to sample the extracellular fluid before, during and after TENS and analyzed GABA in dialysates with high performance liquid chromatography. We analyzed the extracellular GABA concentrations in animals with and without knee joint inflammation induced by intra-articular injection of kaolin and carrageenan. We further tested if spinal blockade of GABA receptors prevents the antihyperalgesia produced by TENS in rats with joint inflammation. We show that high frequency TENS increases extracellular GABA concentrations in the spinal cord in animals with and without joint inflammation. The increases in GABA do not occur in response to low frequency TENS, and there are no increases in glycine in response to low or high frequency TENS. However, the reduction in primary hyperalgesia by both high and low frequency TENS is prevented by spinal blockade of GABA(A) receptors with bicuculline. Thus, high frequency TENS increases release of GABA in the deep dorsal horn of the spinal cord, and both high and low frequency TENS reduce primary hyperalgesia by activation of GABA(A) receptors spinally.

Effect of spinal cord stimulation in an animal model of neuropathic pain relates to degree of tactile “allodynia”

Spinal cord stimulation (SCS) is an established treatment for chronic neuropathic pain. However, in recent studies conflicting results regarding the effect of SCS were noted in a selected group of patients suffering from complex regional pain syndrome and mechanical allodynia. In the present study we investigated the pain relieving effect of SCS in a rat experimental model of neuropathic pain as related to the severity of mechanical allodynia. Adult male rats (n=45) were submitted to a unilateral sciatic nerve ligation. The level of allodynia was tested using the withdrawal response to tactile stimuli with the von Frey test. A portion of these rats developed marked tactile hypersensitivity in the nerve-lesioned paw (von Frey test), similar to "tactile allodynia" observed after nerve injury in humans. Prior to SCS treatment the rats were subdivided into three groups based on the level of allodynia: mild, moderate and severe. All allodynic rats were treated with SCS (n=27) for 30 min (f=50 Hz; pulse width 0.2 ms and stimulation at 2/3 of motor threshold) at 16 days post-injury. Our data demonstrate a differential effect of SCS related to the severity of the mechanical allodynia. SCS leads to a faster and better pain relief in mildly allodynic rats as compared with the more severely allodynic rats. Thus, we suggest that the selection and subdivision of patient groups similar to those defined in our experimental setting (mild, moderate and severe allodynic) may provide better pre-treatment prediction of possible therapeutic benefits of SCS.

Response to spinal cord stimulation in variants of the spared nerve injury pain model

Spinal cord stimulation (SCS) is a treatment given to patients with drug-resistant neuropathic pain, in particular pain resulting from peripheral nerve injury. However, the reasons why some patients develop neuropathic pain and why SCS is not effective in all patients with this chronic pain are not fully understood. The present study compares the response to SCS and the yield of neuropathic animals in variants of the spared nerve injury (SNI) model introduced by Decosterd and Woolf (I. Decosterd, C.J. Woolf, Spared nerve injury: an animal model of persistent peripheral neuropathic pain, Pain 87 (2000) 149-158). Sprague-Dawley rats were prepared with various types of lesions of different branches of the sciatic nerve and then tested for paw mechanical hypersensitivity. A miniature electrode system for SCS was implanted at the T10-T11 vertebral level. Stimulation was applied in awake, freely moving animals with parameters comparable to those employed clinically. Suppression of paw hypersensitivity was considered a positive response to SCS. The incidence of mechanical hypersensitivity ("allodynia") in the different models was: SNI 53%; peroneal axotomy 45%; tibial axotomy 68%; tibial tight ligation 73% and partial tibial tight ligation 50%. "Mirror phenomena" with contralateral paw hypersensitivity was present in about 20% of the animals. The response to SCS differed between models with the lowest response rate in the original SNI model (8%) while the others demonstrated rates in the order of 40-50%. There was a tendency that the efficacy of SCS in suppressing allodynia was inversely related to the severity of hypersensitivity. In conclusion, modifications of the SNI model provide a reproducible incidence of neuropathic hypersensitivity and an increased responsiveness to SCS. These variants may prove suitable for future research on the mechanisms involved in pain relief with SCS.

[Cost-benefit evaluation of spinal cord stimulation treatment for failed-back surgery syndrome patients]

BACKGROUND AND PURPOSE

Spinal cord stimulation is a well-known treatment of rigorously selected failed-back surgery syndrome patients. Efficacy levels over 50% of pain relief have been reported in long-term studies. The objective of this multicenter prospective evaluation was to analyze the cost to benefit ratio of spinal cord stimulation treatment for failed back surgery syndrome patients.

METHODS

Nine hospitals (pain evaluation and treatment centers) were involved in the study. Forty-three patients were selected and implanted between January 1999 and January 2000. For each patient, pre- and post-operative evaluations (6, 12 and 24 months after implantation) were performed to assess pain relief and economical impact on pain treatment costs.

RESULTS

After 24 months, mean 60% pain relief was achieved as assessed with the neuropathic pain score using a Visual Analog Scale (success rate=70%), whereas low-back pain was moderately reduced (29%). The Oswestry Disability questionnaire score was improved by a mean 39%. Costs of pain treatment (medication, consultation, other) are reduced by a mean 64% (1705 Euro) per patient per year.

CONCLUSIONS

This study confirms a clear analgesic effect on neuropathic sciatalgia, and moderate attenuation of low-back pain. One particular interest of this study is the medico-economic prospective evaluation showing that the initial cost of the implanted device is compensated by a significant, early, and stable reduction in the cost of associated pain therapies.

Spinal cord stimulation in chronic pain management

As a general rule, even though it is always difficult to predict the efficacy of a method ina single patient, we consider SCS in every non-malignant chronic pain patient when other conservative treatments have failed. After three decades of clinical experience with SCS, we have learned a lot about its efficacy indifferent pain conditions and have made great technical progress with the materials and surgical procedures. Acceptance of the technique was slow at the beginning; however, we must be aware of the problems related to the application of a therapy that cannot be shamed, and thus the necessity of performing studies that include large numbers of patients. This is even more complicated when dealing with pain patients because of the well-known multifactoriality of pain. Nowadays, every algorithm for the treatment of different pain conditions includes SCS; consequently, every pain center should be able to offer this therapy in its treatment program. This article discusses what has been learned so far with regard to SCS, but there is a lot more to learn about this technique as well as about other types of neuromodulation procedures. As mentioned in the introduction of this article and discussed in the section on the effects of SCS, particularly in clinical applications like peripheral vascular disease and angina, the results of the interaction with the function of the nervous system can be observed in other systems in the body affecting pathologic conditions that are of interest to different specialists. Only the strict cooperation of different medical disciplines can provide substantial help in acquiring knowledge about the mechanisms put into play by SCS and the possible extension of its clinical applications. The complexity of the procedures of neuromodulation and the theoretic background needed for safe and proficient clinical use and for progress raise the issue for medical schools of offering courses in this new discipline.