A3 adenosine receptor agonist attenuates neuropathic pain by suppressing activation of microglia and convergence of nociceptive inputs in the spinal dorsal horn.

Peripheral nerve injuries cause glial activation and neuronal hyperactivity in the spinal dorsal horn. These changes have been considered to be involved in the underlying mechanisms for the development and maintenance of neuropathic pain. Using double immunofluorescence labeling, we previously demonstrated that spinal microglial activation induced by nerve injury enhanced convergence of nociceptive inputs in the spinal dorsal horn from uninjured afferents. The adenosine A3 receptor (A3AR) agonists have been shown to have antinociceptive activities in several experimental neuropathic pain models. However, the mechanisms underlying these antinociceptive actions of the A3AR agonist are still not fully explored. In this study, the effects of the A3AR agonist (i.e., IB-MECA) on microglial activation, enhancement of convergent nociceptive inputs, and nocifensive behaviors were examined after tibial nerve injury. Injury to the tibial nerve initially caused hyposensitivity to touch stimulus at 3 days, and then resulted in tactile allodynia at 14-day post-injury. The daily systemic administration of IB-MECA (0.1 mg/kg/day) for 8 days in a row starting on the day of nerve injury or 7 days after nerve injury prevented the development of behaviorally assessed hypersensitivities, and spinal microglial activation induced by nerve injury. These treatments also suppressed anomalous convergence of nociceptive primary inputs in the spinal dorsal horn. The present findings indicate that the A3AR agonist attenuates neuropathic pain states by suppressing enhanced microglial activation, and anomalous convergence of nociceptive inputs in the spinal dorsal horn from uninjured afferents after injury to the peripheral nerve.

Differential induction of c-Fos and phosphorylated ERK by a noxious stimulus after peripheral nerve injury.

PURPOSE: In this study, we compared induction of c-Fos and phosphorylated extracellular signal-regulated kinase (p-ERK) in the spinal dorsal horn after peripheral nerve injury. MATERIALS AND METHODS: We examined the spinal dorsal horn for noxious heat-induced c-Fos and p-ERK protein-like immunoreactive (c-Fos- and p-ERK-IR) neuron profiles after tibial nerve injury. The effect of administration of a MEK 1/2 inhibitor (PD98059) on noxious heat-induced c-Fos expression was also examined after tibial nerve injury. RESULTS: A large number of c-Fos- and p-ERK-IR neuron profiles were induced by noxious heat stimulation to the hindpaw in sham-operated animals. A marked reduction in the number of c-Fos- and p-ERK-IR neuron profiles was observed in the medial 1/3 (tibial territory) of the dorsal horn at 3 and 7 days after nerve injury. Although c-Fos-IR neuron profiles had reappeared by 14 days after injury, the number of p-ERK-IR neuron profiles remained decreased in the tibial territory of the superficial dorsal horn. Double immunofluorescence labeling for c-Fos and p-ERK induced by noxious heat stimulation to the hindpaw at different time points revealed that a large number of c-Fos-IR, but not p-ERK-IR, neuron profiles were distributed in the tibial territory after injury. Although administration of a MEK 1/2 inhibitor to the spinal cord suppressed noxious heat-induced c-Fos expression in the peroneal territory, this treatment did not alter c-Fos induction in the tibial territory after nerve injury. CONCLUSIONS: ERK phosphorylation may be involved in c-Fos induction in normal nociceptive responses, but not in exaggerated c-Fos induction after nerve injury.

Differential Changes in Neuronal Excitability in the Spinal Dorsal Horn After Spinal Nerve Ligation in Rats.

Previous studies demonstrated that peripheral nerve injury induced excessive neuronal response and glial activation in the spinal cord dorsal horn, and such change has been proposed to reflect the development and maintenance of neuropathic pain states. The aim of this study was to examine neuronal excitability and glial activation in the spinal dorsal horn after peripheral nerve injury. We examined noxious heat stimulation-induced c-Fos protein-like immunoreactivity (Fos-LI) neuron profiles in fourth-to-sixth lumbar (L4-L6) level spinal dorsal horn neurons after fifth lumbar spinal nerve ligation (L5 SNL). Immunofluorescence labeling of OX-42 and GFAP was also performed in histological sections of the spinal cord. A significant increase in the number of Fos-LI neuron profiles in the spinal dorsal horn at the L4 level was found at 3 days after SNL, but returned to a level similar to that in sham-operated controls by 14 days after injury. As expected, a decrease in the number of Fos-LI neuron profiles in the spinal dorsal horn at the L5 level was found at 3 days after SNL. However, these profiles had reappeared in large numbers by 14 and 21 days after injury. Immunofluorescence labeling of OX-42 and GFAP indicated sequential activation of microglia and astrocytes in the spinal dorsal horn. We conclude that nerve injury causes differential changes in neuronal excitability in the spinal dorsal horn, which may coincide with glial activation. These changes may play a substantial role in the pathogenesis of neuropathic pain after peripheral nerve injury.