Bengt Linderoth

Neuropathic pain may be effectively relieved by electric stimulation of the spinal cord (SCS). However, the underlying mechanisms for the ensuing pain relief are poorly understood. In a rat model of neuropathy displaying hypersensitivity to innocuous tactile stimuli, (allodynia), we have earlier demonstrated that SCS may normalise withdrawal response thresholds. In the present study, using microdialysis, it is shown that SCS induces a decreased release of the dorsal horn excitatory amino acids (EAA), glutamate and aspartate, concomitant with an increase of the GABA release. Local perfusion with a GABA(B)-receptor antagonist in the dorsal horn transiently abolishes the SCS-induced suppression of the EAA release. Thus, the effect of SCS on neuropathic pain and allodynia may be due to an activation of local GABAergic mechanisms inhibiting the EAA release which is chronically elevated in such conditions.

Peripheral hypersensitivity (hyperalgesia and allodynia) are common phenomena both in inflammatory and in neuropathic pain conditions. Several rat models of mononeuropathy (Bennett, Seltzer and Gazelius models) display such symptoms following partial injury to the sciatic nerve. Using immunohistochemistry and behavioral tests, we investigated inflammatory cell and cytokine responses in the sciatic nerve 14 days after injury created in these different models as well as after axotomy. Tactile hypersensitivity ('allodynia') was present in all Gazelius model rats whereas only 38 and 29% of the Bennett and Seltzer models, respectively, displayed this sign of neuropathy. The inflammatory reactions in rats with and without tactile allodynia were compared. Monocytes/macrophages (ED-1), natural killer cells, T lymphocytes, and the pro-inflammatory cytokines tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6), were significantly upregulated in all nerve injured rats in comparison to sham-operated controls. Interestingly, ED-1-, TNF-alpha- and IL-6-positive cells increased more markedly in allodynic Bennett and Seltzer rats than in non-allodynic ones. The magnitude of the inflammatory response does not seem to relate to the extent of damage to the nerve fibers because axotomized rats displayed much lower upregulation. Our findings indicate that the considerable increase in monocytes/macrophages induced by a nerve injury results in a very high release of IL-6 and TNF-alpha. This may relate to the generation of tactile allodynia/hyperalgesia, since there was a clear correlation between the number of ED-1 and IL-6-positive cells and the degree of allodynia. It is possible that measures to reduce monocyte/macrophage recruitment and the release of pro-inflammatory interleukins after nerve damage could influence the development of neuropathic pain.

OBJECTIVE: The aim of the present study is to monitor the extracellular gamma-aminobutyric acid (GABA) levels in the lumbar dorsal horn of allodynic rats, which respond to spinal cord stimulation (SCS) with a normalization of the tactile withdrawal threshold. In addition, we monitored the GABA levels in nonresponding and sham-stimulated rats. METHODS: Partial constriction injury of the sciatic nerve was performed, and a permanent electrode for SCS was inserted into the spinal canal. The response to SCS was assessed with von Frey hairs in awake animals. Later, microdialysis was performed in the dorsal horn of the spinal cord under halothane anesthesia. The concentration of GABA in the microdialysate was assessed by high-performance liquid chromatography. RESULTS: Extracellular GABA levels in rats with sciatic nerve lesions and allodynia (2.3 +/- 0.5 nmol/L) were significantly lower (P < 0.001) than in control rats with intact sciatic nerves (8.1 +/- 1.0 nmol/L), whereas only slightly decreased GABA levels (5.7 +/- 1.1 nmol/L) were detected in nonallodynic rats with sciatic nerve lesions. In the allodynic rats, which respond to SCS by a normalization of the tactile withdrawal threshold, significantly (P < 0.001) increased GABA levels (6.7 +/- 2.3 nmol/L) were detected after SCS. In contrast, neither the allodynic rats, which did not respond to SCS, nor the sham-stimulated allodynic rats displayed increased GABA levels in response to stimulation. CONCLUSION: Our results indicate that the development of allodynia, a common symptom in neuropathic pain states, may be linked to a decreased spinal release of GABA. We suggest that an SCS-induced release of GABA could be important for the suppression of allodynia observed in rats after SCS. Similar mechanisms could also be involved in the SCS-induced alleviation of pain in patients with peripheral neuropathy.