[Show abstract][Hide abstract] ABSTRACT: Spinal cord stimulation (SCS) is a useful neuromodulatory technique for treatment of certain neuropathic pain conditions. However, the optimal stimulation parameters remain unclear.
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 Aα/β-fibers and spinal wide-dynamic-range neuronal excitability.
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 Aα/β-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.
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.
[Show abstract][Hide abstract] ABSTRACT: Peripherally acting opioids are potentially attractive drugs for the clinical management of certain chronic pain states due to the lack of centrally-mediated adverse effects. However, it remains unclear whether tolerance develops to peripheral opioid analgesic effects under neuropathic pain conditions. We subjected rats to L5 spinal nerve ligation (SNL) and examined the analgesic effects of repetitive systemic and local administration of loperamide hydrochloride, a peripherally acting opioid agonist. We found that the inhibition of mechanical hypersensitivity, an important manifestation of neuropathic pain, by systemic loperamide (1.5 mg/kg, s.c.) decreased after repetitive drug treatment (tolerance-inducing dose: 0.75-6.0 mg/kg, s.c). Similarly, repeated intraplantar injection of loperamide (150 μg/50 μl, i.pl.) and DAMGO (300 μg/50 μl), a highly selective mu-opioid receptor (MOR) agonist, also resulted in decreased inhibition of mechanical hypersensitivity. Pretreatment with naltrexone hydrochloride (5 mg/kg, i.p.) and MK-801 (0.2 mg/kg, i.p.) attenuated systemic loperamide tolerance. Western blot analysis showed that repetitive systemic administration of morphine (3 mg/kg, s.c.), but not loperamide (3 mg/kg, s.c.) or saline, significantly increased MOR phosphorylation in the spinal cord of SNL rats. In cultured rat dorsal root ganglion (DRG) neurons, loperamide dose-dependently inhibited KCl-induced increases in [Ca(2+)]i. However, this drug effect significantly decreased in cells pretreated with loperamide (3 μM, 72 hours). Intriguingly, in "loperamide-tolerant" cells, the delta-opioid receptor (DOR) antagonist, naltrindole, restored loperamide's inhibition of KCl-elicited [Ca(2+)]i increase. Our findings indicate that animals with neuropathic pain may develop acute tolerance to the anti-allodynic effects of peripherally-acting opioids after repetitive systemic and local drug administration.
[Show abstract][Hide abstract] ABSTRACT: The analgesic properties and mechanisms of loperamide hydrochloride, a peripherally acting opioid receptor agonist, in neuropathic pain warrant further investigation.
We examined the effects of systemic or local administration of loperamide on heat and mechanical hyperalgesia in rats after an L5 spinal nerve ligation (SNL).
(1) Systemic loperamide (0.3-10 mg/kg, subcutaneous in the back) dose dependently reversed heat hyperalgesia in SNL rats, but did not produce thermal analgesia. Systemic loperamide (3 mg/kg) did not induce thermal antinociception in naïve rats; (2) systemic loperamide-induced anti-heat hyperalgesia was blocked by pretreatment with intraperitoneal naloxone methiodide (5 mg/kg), but not by intraperitoneal naltrindole (5 mg/kg) or intrathecal naltrexone (20 μg/10 μL); (3) local administration of loperamide (150 μg), but not vehicle, into plantar or dorsal hind paw tissue induced thermal analgesia in SNL rats and thermal antinociception in naïve rats; (4) the analgesic effect of intraplantar loperamide (150 μg/15 μL) in SNL rats at 45 min, but not 10 min, post-injection was blocked by pretreatment with an intraplantar injection of naltrexone (75 μg/10 μL); (5) systemic (3.0 mg/kg) and local (150 μg) loperamide reduced the exaggerated duration of hind paw elevation to noxious pinprick stimuli in SNL rats. Intraplantar injection of loperamide also decreased the frequency of pinprick-evoked response in naïve rats.
These findings suggest that both systemic and local administration of loperamide induce an opioid receptor-dependent inhibition of heat and mechanical hyperalgesia in nerve-injured rats, but that local paw administration of loperamide also induces thermal and mechanical antinociception.
Full-text · Article · Aug 2012 · European journal of pain (London, England)
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: The neurophysiological basis by which neuromodulatory techniques lead to relief of neuropathic pain remains unclear. We investigated whether electrical stimulation at different peripheral sites induces unique profiles of A-fiber afferent activation in nerve-injured rats. At 4-6weeks after subjecting rats to L5 spinal nerve injury (SNL) or sham operation, we recorded the orthodromic compound action potential (AP) at the ipsilateral L4 dorsal root in response to (1) transcutaneous electrical nerve stimulation (TENS, a patch electrode placed on the dorsum of the foot), (2) subcutaneous electrical stimulation (SQS, electrode inserted subcutaneously along the dorsum of the foot), (3) peroneal nerve stimulation (PNS, electrode placed longitudinally abutting the nerve), and (4) sciatic nerve stimulation (SNS). The area under the Aα/β compound AP was measured as a function of the bipolar, constant-current stimulus intensity (0.02-6.0 mA, 0.2 ms). In both nerve-injured and sham-operated groups, the stimulus-response (S-R) functions of the Aα/β compound APs differed substantially with the stimulation site; SNS having the lowest threshold and largest compound AP waveform, followed by PNS, SQS, and TENS. The S-R function to PNS was shifted to the right in the SNL group, compared to that in the sham-operated group. The Aα/β-threshold to PNS was higher in the SNL group than in the sham-operated group. The S-R functions and Aα/β-thresholds to TENS and SQS were comparable between the two groups. Electrical stimulation of different peripheral targets induced distinctive profiles of A-fiber afferent activation, suggesting that the neuronal substrates for the various forms of peripheral neuromodulatory therapies may differ.
Full-text · Article · Nov 2011 · Neuroscience Letters
[Show abstract][Hide abstract] ABSTRACT: The sites of action and cellular mechanisms by which spinal cord stimulation reduces neuropathic pain remain unclear.
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 Aα/β-compound action potential without inducing an Aδ- or C-compound action potential.
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).
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.
[Show abstract][Hide abstract] ABSTRACT: The relationship between pain severity and the extent of injury to a peripheral nerve remains elusive. In this study, we compared the pain behavior resulting from partial (1/3-1/2 thickness) and full L5 spinal nerve ligation (SNL) in rats. The decrease in paw withdrawal threshold (PWT) to mechanical stimuli in the hindpaw ipsilateral to the injury was comparable in the two groups on days 3-21 post-injury. However, the decreased PWT recovered earlier in the partial SNL group than in the full SNL group. These observations suggest that the duration of neuropathic pain behavior, but not the early development of mechanical allodynia, is dependent on the extent of nerve injury. On days 6 and 15 post-injury, when the mechanical allodynia was similar in the two groups, systemic morphine induced a greater reduction of mechanical allodynia in the partial SNL group than in the full SNL group. Furthermore, in partial SNL rats, at post-injury time points when they had largely recovered from the neuropathic pain state, systemic administration of naloxone hydrochloride (day 53) or naloxone methiodide (a non-selective peripherally acting opioid receptor antagonist; day 64) or intra-plantar injection of naloxone methiodide rekindled mechanical pain hypersensitivity in the ipsilateral hindpaw, suggesting a prolonged activation of endogenous opioidergic pain-inhibition. Therefore, partial SNL in rats may represent an efficient model for studying the mechanisms of neuropathic pain, testing effects of analgesic/antihyperalgesic drugs, and understanding endogenous pain-inhibitory mechanisms that lead to reversal of the pain behavior with time.
Full-text · Article · Feb 2010 · Neuroscience Letters