Paul W. Wacnik

A cannabinoid agonist differentially attenuates deep tissue hyperalgesia in animal models of cancer and inflammatory muscle pain.

&NA; Pain associated with cancer and chronic musculoskeletal disorders can be difficult to control. We used murine models of cancer and inflammatory muscle pain to examine whether the cannabinoid receptor agonist WIN55,212–2 reduces hyperalgesia originating in deep tissues. C3H/He mice were anesthetized and implanted with osteolytic NCTC clone 2472 cells into the humeri or injected with 4% carrageenan into the triceps muscles of both forelimbs. At the time of peak hyperalgesia, WIN55,212–2 (1–30 mg/kg) or vehicle was administered intraperitoneally and forelimb grip force was measured 0.5–24 h later. WIN55,212–2 produced time‐ and dose‐related antihyperalgesia in both models. A 10 mg/kg dose of WIN55,212–2 fully reversed carrageenan‐evoked muscle hyperalgesia. However, 30 mg/kg of WIN55,212–2 attenuated tumor‐evoked hyperalgesia only ∼50%. After controlling for the difference in magnitude of hyperalgesia between the two models, WIN55,212–2 was still more potent at reducing hyperalgesia in the inflammatory model. In the cancer pain model, the antihyperalgesic effect of WIN55,212–2 was partially blocked by pretreatment with the selective CB1 (SR141716A) but not the CB2 (SR144528) receptor antagonist. In contrast, both antagonists blocked antihyperalgesic effects of WIN55,212–2 on carrageenan‐evoked muscle hyperalgesia. Catalepsy and loss of motor coordination, known side effects of cannabinoids, did not account for the antihyperalgesia produced by WIN55,212–2. These data show that cannabinoids attenuate deep tissue hyperalgesia produced by both cancer and inflammatory conditions. Interestingly, cannabinoids differentially modulated carrageenan‐ and tumor‐evoked hyperalgesia in terms of potency and receptor subtypes involved suggesting that differences in underlying mechanisms may exist between these two models of deep tissue pain.

Spinal Cord Stimulation-induced Analgesia Electrical Stimulation of Dorsal Column and Dorsal Roots Attenuates Dorsal Horn Neuronal Excitability in Neuropathic Rats

Background:The sites of action and cellular mechanisms by which spinal cord stimulation reduces neuropathic pain remain unclear. Methods:We examined the effect of bipolar electrical-conditioning stimulation (50 Hz, 0.2 ms, 5 min) of the dorsal column and lumbar dorsal roots on the response properties of spinal wide dynamic range (WDR) neurons in rats after L5 spinal nerve injury. The conditioning stimulation intensity was set at the lowest current that evoked a peak antidromic sciatic A&agr;/&bgr;-compound action potential without inducing an A&dgr;- or C-compound action potential. Results:Within 15 min of the dorsal column or root conditioning stimulation, the spontaneous activity rate of WDR neurons was significantly reduced in nerve-injured rats. Conditioning stimulation also significantly attenuated WDR neuronal responses to mechanical stimuli in nerve-injured rats and inhibited the C-component of the neuronal response to graded intracutaneous electrical stimuli applied to the receptive field in nerve-injured and sham-operated rats. It is noteworthy that dorsal column stimulation blocked windup of WDR neuronal response to repetitive intracutaneous electrical stimulation (0.5 Hz) in nerve-injured and sham-operated rats, whereas dorsal root stimulation inhibited windup only in sham-operated rats. Therefore, stimulation of putative spinal substrates at A-fiber intensities with parameters similar to those used by patients with spinal cord stimulators attenuated established WDR neuronal hyperexcitability in the neuropathic condition and counteracted activity-dependent increase in neuronal excitability (i.e., windup). Conclusions:These results suggest a potential cellular mechanism underlying spinal cord stimulation–induced pain relief. This in vivo model allows the neurophysiologic basis for spinal cord stimulation–induced analgesia to be studied.

Tumor implantation in mouse humerus evokes movement-related hyperalgesia exceeding that evoked by intramuscular carrageenan.

&NA; In this paper we compare two innovative models of movement‐related pain: tumor‐induced nociception following implantation of fibrosarcoma cells into bone and muscle inflammation‐induced nociception following injection of the irritant carrageenan into muscle. Importantly, using the grip force test, an assay of movement‐related hyperalgesia, both non‐malignant and malignant pain are examined in parallel. Movement‐related hyperalgesia, known clinically as a specific type of ‘breakthrough pain’, is a common feature of bone cancer and is thought to be a predictor of poor response to conventional analgesic pharmacotherapy (Bruera et al., 1995, J. Pain Symptom. Manage. 10 (1995) 348; Mercadaute et al., 1992, Pain 50 (1992) 151; Pain 81 (1999) 129). Implantation of NCTC 2472 sarcoma cells in both humeri or injection of carrageenan (4%) in both triceps of C3H/He mice produced apparent forelimb hyperalgesia that was not associated with mechanical hyperalgesia in the forepaw, whereas carrageenan at 6 and 8% did evoke significant cutaneous hyperalgesia of the forepaw as well. Control groups receiving implants of vehicle or no treatment at all did not manifest this forelimb hyperalgesia. B6C3/F1 mice implanted with non‐lysis‐inducing G3.26 melanoma cells or vehicle did not manifest significant hyperalgesia when compared to B6C3/F1 mice receiving fibrosarcoma cells, indicating a dependence on bone involvement for induction of hyperalgesia in this model. Histological examination at days 3, 7, and 10 post‐implantation showed a clear correlation of tumor growth‐induced bone destruction with behavioral hyperalgesia. Morphine was more potent in decreasing the maximal hyperalgesia induced by carrageenan than that induced by tumor implantation. Acutely administered morphine (3–100 mg/kg, i.p.) attenuated peak hyperalgesia of carrageenan‐injected mice (ED50 6.9 mg/kg) and tumor‐bearing mice (ED50 23.9 mg/kg) in a dose‐related manner with a difference in potency of 3.5. Tumor‐implanted mice with a level of hyperalgesia comparable to that induced by carrageenan required almost three times more morphine (ED50 18.5 mg/kg) for equivalent attenuation of forelimb hyperalgesia. These animal models of movement‐related hyperalgesia may aid in discerning the peripheral and central mechanisms underlying pain that accompanies bone metastases and distinguishing it from the pain associated with muscular inflammation. Importantly, they may also aid in predicting differences in analgesic efficacy in different types of musculoskeletal pain.

Conventional and kilohertz-frequency spinal cord stimulation produces intensity-and frequency-dependent inhibition of mechanical hypersensitivity in a rat model of neuropathic pain

Background:Spinal cord stimulation (SCS) is a useful neuromodulatory technique for treatment of certain neuropathic pain conditions. However, the optimal stimulation parameters remain unclear. Methods:In rats after L5 spinal nerve ligation, the authors compared the inhibitory effects on mechanical hypersensitivity from bipolar SCS of different intensities (20, 40, and 80% motor threshold) and frequencies (50, 1 kHz, and 10 kHz). The authors then compared the effects of 1 and 50 Hz dorsal column stimulation at high- and low-stimulus intensities on conduction properties of afferent A&agr;/&bgr;-fibers and spinal wide-dynamic–range neuronal excitability. Results:Three consecutive daily SCS at different frequencies progressively inhibited mechanical hypersensitivity in an intensity-dependent manner. At 80% motor threshold, the ipsilateral paw withdrawal threshold (% preinjury) increased significantly from pre-SCS measures, beginning with the first day of SCS at the frequencies of 1 kHz (50.2 ± 5.7% from 23.9 ± 2.6%, n = 19, mean ± SEM) and 10 kHz (50.8 ± 4.4% from 27.9 ± 2.3%, n = 17), whereas it was significantly increased beginning on the second day in the 50 Hz group (38.9 ± 4.6% from 23.8 ± 2.1%, n = 17). At high intensity, both 1 and 50 Hz dorsal column stimulation reduced A&agr;/&bgr;-compound action potential size recorded at the sciatic nerve, but only 1 kHz stimulation was partially effective at the lower intensity. The number of actions potentials in C-fiber component of wide-dynamic–range neuronal response to windup-inducing stimulation was significantly decreased after 50 Hz (147.4 ± 23.6 from 228.1 ± 39.0, n = 13), but not 1 kHz (n = 15), dorsal column stimulation. Conclusions:Kilohertz SCS attenuated mechanical hypersensitivity in a time course and amplitude that differed from conventional 50 Hz SCS, and may involve different peripheral and spinal segmental mechanisms.

Tumor-induced mechanical hyperalgesia involves CGRP receptors and altered innervation and vascularization of DsRed2 fluorescent hindpaw tumors.

Functional and anatomical relationships among primary afferent fibers, blood vessels, and cancers are poorly understood. However, recent evidence suggests that physical and biochemical interactions between these peripheral components are important to both tumor biology and cancer‐associated pain. To determine the role of these peripheral components in a mouse model of cancer pain, we quantified the change in nerve and blood vessel density within a fibrosarcoma tumor mass using stereological analysis of serial confocal optical sections of immunostained hind paw. To this end we introduced the Discoma coral‐derived red fluorescent protein (DsRed2) into the NCTC 2472 fibrosarcoma line using the Sleeping Beauty transposon methodology, thus providing a unique opportunity to visualize tumor–nerve–vessel associations in context with behavioral assessment of tumor‐associated hyperalgesia. Tumors from hyperalgesic mice are more densely innervated with calcitonin gene related peptide (CGRP)‐immunoreactive nerve fibers and less densely vascularized than tumors from non‐hyperalgesic mice. As hyperalgesia increased from Day 5 to 12 post‐implantation, the density of protein gene product 9.5 (PGP9.5)‐immunoreactive nerves and CD31‐immunoreactive blood vessels in tumors decreased, whereas CGRP‐immunoreactive nerve density remained unchanged. Importantly, intra‐tumor injection of a CGRP1 receptor antagonist (CGRP 8–37) partially blocked the tumor‐associated mechanical hyperalgesia, indicating that local production of CGRP may contribute to tumor‐induced nociception through a receptor‐mediated process. The results describe for the first time the interaction among sensory nerves, blood vessels and tumor cells in otherwise healthy tissue, and our assessment supports the hypothesis that direct tumor cell‐axon communication may underlie, at least in part, the occurrence of cancer pain.

Nociceptive characteristics of tumor necrosis factor-α in naive and tumor-bearing mice

A nociceptive role for tumor necrosis factor-alpha (TNF-alpha) in naive mice and in mice with fibrosarcoma tumor-induced primary hyperalgesia was investigated. The presence of TNF-alpha mRNA was confirmed in tumor site homogenates by reverse transcription-polymerase chain reaction (RT-PCR), and examination of TNF-alpha protein levels in tumor-bearing mice indicated a significantly higher concentration of this cytokine in tumor microperfusates and tumor site homogenates compared with that obtained from a similar site on the contralateral limb or in naive mice. Intraplantar injection of TNF-alpha into naive or fibrosarcoma tumor-bearing mice induced mechanical hypersensitivity, as measured by withdrawal responses evoked by von Frey monofilaments. This hypersensitivity suggests that TNF-alpha can excite or sensitize primary afferent fibers to mechanical stimulation in both naive and tumor-bearing mice. In addition, the hyperalgesia produced by TNF-alpha was completely eliminated when the injected TNF-alpha was pre-incubated with the soluble receptor antagonist TNFR:Fc. Importantly, pre-implantation systemic as well as post-implantation intra-tumor injection of TNFR:Fc partially blocked the mechanical hyperalgesia, indicating that local production of TNF-alpha may contribute to tumor-induced nociception.

Bipolar spinal cord stimulation attenuates mechanical hypersensitivity at an intensity that activates a small portion of A-fiber afferents in spinal nerve-injured rats.

Spinal cord stimulation (SCS) is used clinically to treat neuropathic pain states, but the precise mechanism by which it attenuates neuropathic pain remains to be established. The profile of afferent fiber activation during SCS and how it may correlate with the efficacy of SCS-induced analgesia are unclear. After subjecting rats to an L5 spinal nerve ligation (SNL), we implanted a miniature quadripolar electrode similar to that used clinically. Our goal was to determine the population and number of afferent fibers retrogradely activated by SCS in SNL rats by recording the antidromic compound action potential (AP) at the sciatic nerve after examining the ability of bipolar epidural SCS to alleviate mechanical hypersensitivity in this model. Notably, we compared the profiles of afferent fiber activation to SCS between SNL rats that exhibited good SCS-induced analgesia (responders) and those that did not (nonresponders). Additionally, we examined how different contact configurations affect the motor threshold (MoT) and compound AP threshold. Results showed that three consecutive days of SCS treatment (50 Hz, 0.2 ms, 30 min, 80-90% of MoT), but not sham stimulation, gradually alleviated mechanical hypersensitivity in SNL rats. The MoT obtained in the animal behavioral study was significantly less than the Aα/β-threshold of the compound AP determined during electrophysiological recording, suggesting that SCS could attenuate mechanical hypersensitivity with a stimulus intensity that recruits only a small fraction of the A-fiber population in SNL rats. Although both the MoT and compound AP threshold were similar between responders and nonresponders, the size of the compound AP waveform at higher stimulation intensities was larger in the responders, indicating a more efficient activation of the dorsal column structure in responders.

Default Mode Network Functional Connectivity Altered in Failed Back Surgery Syndrome

UNLABELLED The purpose of this study was to identify alterations in the default mode network of failed back surgery syndrome patients as compared to healthy subjects. Resting state functional magnetic resonance imaging was conducted at 3 Tesla and data were analyzed with an independent component analysis. Results indicate an overall reduced functional connectivity of the default mode network and recruitment of additional pain modulation brain regions, including dorsolateral prefrontal cortex, insula, and additional sensory motor integration brain regions, including precentral and postcentral gyri, for failed back surgery syndrome patients. PERSPECTIVE This article presents alterations in the default mode network of chronic low back pain patients with failed back surgery syndrome as compared to healthy participants.

Mathematical Model of Nerve Fiber Activation During Low Back Peripheral Nerve Field Stimulation: Analysis of Electrode Implant Depth

The lower back is the most common location of pain experienced by one‐fifth of the European population reporting chronic pain. A peripheral nerve field stimulation system, which involves electrodes implanted subcutaneously in the painful area, has been shown to be efficacious for low back pain. Moreover, the predominant analgesic mechanism of action is thought to be via activation of peripheral Aβ fibers. Unfortunately, electrical stimulation also might coactivate Aδ fibers, causing pain or unpleasantness itself. The aim of this study was to investigate at which implant depth Aβ‐fiber stimulation is maximized, and Aδ‐fiber minimized, which in turn should lead to therapy optimization.

Comparison of intensity-dependent inhibition of spinal wide-dynamic range neurons by dorsal column and peripheral nerve stimulation in a rat model of neuropathic pain

Spinal cord stimulation (SCS) and peripheral nerve stimulation (PNS) are thought to reduce pain by activating a sufficient number of large myelinated (Aβ) fibres, which in turn initiate spinal segmental mechanisms of analgesia. However, the volume of neuronal activity and how this activity is associated with different treatment targets is unclear under neuropathic pain conditions.