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, we compared the inhibitory effects on mechanical hypersensitivity from bipolar SCS of different intensities (20%, 40%, 80% motor threshold) and frequencies (50-Hz, 1-kHz, and 10-kHz). We then compared the effects of 1-kHz 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. 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), while 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-kHz 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. 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.

Activation of spinal astrocytes may contribute to neuropathic pain. Adjacent astrocytes can make direct communication through gap junctions formed by connexin 43 (Cx43) in the central nervous system. Yet, the role of spinal astroglial gap junctions in neuropathic pain is not fully understood. Since Cx43 is the connexin isoform expressed preferentially in astrocytes in the spinal cord, we used a small interfering RNA (siRNA) approach to examine whether suppression of spinal Cx43 expression inhibits mechanical hypersensitivity in rats after an L5 spinal nerve ligation (SNL). SNL rats were administered intrathecal Cx43 siRNA (3 μg/15 μl, twice/day) or an equal amount of mismatch siRNA (control) on days 14 through17 post-SNL. Cx43 siRNA, but not mismatch siRNA, alleviated mechanical hypersensitivity in SNL rats. Furthermore, Western blot analysis showed that the pain inhibition induced by Cx43 siRNA correlated with downregulation of Cx43 expression, but not that of Cx36 (the neuronal gap junction protein) or glial fibrillary acidic protein (GFAP, a marker for reactive astrocytes) in the spinal cord of SNL rats. Western blot analysis and immunohistochemistry also showed that SNL increased GFAP expression, but decreased Cx43 expression, in spinal cord. Our results provide direct evidence that selective suppression of spinal Cx43 after nerve injury alleviates neuropathic mechanical hypersensitivity. These findings suggest that in the spinal cord, the enhanced function of astroglial gap junctions, especially those formed by Cx43, may be important to neuropathic pain in SNL rats.

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 subcutaneously) decreased after repetitive drug treatment (tolerance-inducing dose: 0.75 to 6.0 mg/kg subcutaneously). Similarly, repeated intraplantar injection of loperamide (150 µg/50 µL intraplantarly) and D-Ala2-MePhe4-Glyol5 enkephalin (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 intraperitoneally) and MK-801 (0.2 mg/kg intraperitoneally) attenuated systemic loperamide tolerance. Western blot analysis showed that repetitive systemic administration of morphine (3 mg/kg subcutaneously), but not loperamide (3 mg/kg subcutaneously) or saline, significantly increased MOR phosphorylation in the spinal cord of SNL rats. In cultured rat dorsal root ganglion neurons, loperamide dose-dependently inhibited KCl-induced increases in [Ca2+]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 antagonist naltrindole restored loperamide’s inhibition of KCl-elicited [Ca2+]i increase. Our findings indicate that animals with neuropathic pain may develop acute tolerance to the antiallodynic effects of peripherally acting opioids after repetitive systemic and local drug administration.