We investigated the involvement of extracellular signal-regulated protein kinases (ERK) within spinal neurons in producing pain hypersensitivity. Within a minute of an intense noxious peripheral or C-fiber electrical stimulus, many phosphoERK-positive neurons were observed, most predominantly in lamina I and IIo of the ipsilateral dorsal horn. This staining was intensity and NMDA receptor dependent. Low-intensity stimuli or A-fiber input had no effect. Inhibition of ERK phosphorylation by a MEK inhibitor reduced the second phase of formalin-induced pain behavior, a measure of spinal neuron sensitization. ERK signaling within the spinal cord is therefore involved in generating pain hypersensitivity. Because of its rapid activation, this effect probably involves regulation of neuronal excitability without changes in transcription.

To clarify whether inhibitory transmission in the superficial dorsal horn of the spinal cord is reduced after peripheral nerve injury, we have studied synaptic transmission in lamina II neurons of an isolated adult rat spinal cord slice preparation after complete sciatic nerve transection (SNT), chronic constriction injury (CCI), or spared nerve injury (SNI). Fast excitatory transmission remains intact after all three types of nerve injury. In contrast, primary afferent-evoked IPSCs are substantially reduced in incidence, magnitude, and duration after the two partial nerve injuries, CCI and SNI, but not SNT. Pharmacologically isolated GABA(A) receptor-mediated IPSCs are decreased in the two partial nerve injury models compared with naive animals. An analysis of unitary IPSCs suggests that presynaptic GABA release is reduced after CCI and SNI. Partial nerve injury also decreases dorsal horn levels of the GABA synthesizing enzyme glutamic acid decarboxylase (GAD) 65 kDa ipsilateral to the injury and induces neuronal apoptosis, detected by terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling staining in identified neurons. Both of these mechanisms could reduce presynaptic GABA levels and promote a functional loss of GABAergic transmission in the superficial dorsal horn.

There is accumulating evidence to implicate the importance of N-methyl-D-aspartate (NMDA) receptors to the induction and maintenance of central sensitization during pain states. However, NMDA receptors may also mediate peripheral sensitization and visceral pain. NMDA receptors are composed of NR1, NR2 (A, B, C, and D), and NR3 (A and B) subunits, which determine the functional properties of native NMDA receptors. Among NMDA receptor subtypes, the NR2B subunit-containing receptors appear particularly important for nociception, thus leading to the possibility that NR2B-selective antagonists may be useful in the treatment of chronic pain.