GABAA-Receptor blockade reverses the injury-induced sensitization of nociceptor-specific (NS) neurons in the spinal dorsal horn of the rat.

Single-unit electrical activity was recorded from 80 nociceptor-specific (NS) neurons in the dorsal horn of the lumbar spinal cord of pentobarbital anesthetized rats. Their responses to low- and high-intensity mechanical stimulation of their receptive fields (RFs) were recorded before and after the application of irritant agents [capsaicin (CAP) or mustard oil (MO)] to the RF. Before the applications of the irritants the neurons responded only to high-intensity stimuli, but after this procedure 20 of 28 neurons tested were sensitized, i.e., gave increased responses to high-intensity stimuli and showed novel responses to low-intensity mechanical stimulation as well as an Abeta-fiber afferent drive. CAP was more likely to induce sensitization than MO and the majority of sensitized neurons were located in the superficial dorsal horn. No relationship was found between the magnitude of the response to the sensitizing agent and the presence or absence of sensitization. Cumulative doses of two gamma-aminobutyric acid type A (GABA(A))-receptor antagonists, picrotoxin and bicuculline, were administered systemically or applied directly over the spinal cord. The GABA(A) antagonists reversed the sensitization of the neurons by reducing the novel low-threshold responses. These results show that NS neurons in the spinal dorsal horn can be sensitized by a sustained afferent discharge in peripheral nociceptors and that this sensitization can be reduced or reversed by low doses of GABA(A)-receptor antagonists. This provides evidence for a mechanism in which an enhanced GABAergic transmission can lead to hyperexcitability and sensitization of NS neurons in the dorsal horn.

Extracellular signaling-regulated kinase-1 and -2 (ERK 1/2) mediate referred hyperalgesia in a murine model of visceral pain.

We have investigated the role of spinal extracellular signaling-regulated kinase-1 and -2 (ERK1/2) in a model of visceral pain and hyperalgesia induced by intracolonic instillation of irritants in adult mice. Instillation of either capsaicin or mustard oil induced a significant activation of lumbosacral spinal ERK1/2, measured by immunoblot, with a peak 2.4-fold increase over control levels between 45 and 90 min post-treatment. Intracolonic saline did not produce significant activation of lumbosacral spinal ERK1/2, and none of the treatments evoked ERK1/2 activation in thoracic or cervical spinal cord. These studies suggested a preferential nuclear localization, which was explored by subcellular fractionation. Both mustard oil and capsaicin produced a redistribution of phosphorylated ERK1/2 from cytosol into the nucleus that was statistically significant at 45 min after treatment. Spinal ERK1/2 activation with capsaicin treatment correlated with the development of prolonged referred hyperalgesia. The upstream inhibitor of ERK phosphorylation, U0126 (100-400 microg/kg, i.v., 10 min pre-capsaicin), dose-dependently inhibited referred hyperalgesia 3-6 h after capsaicin. Treatment with U0126 did not affect spontaneous pain behavior or colon inflammation. Our data show that ERK activation plays a specific role in maintaining prolonged referred (secondary) hyperalgesia in visceral pain. The time course and subcellular localization of the effects observed suggest that ERK is involved in transcriptional events underlying the maintenance of secondary hyperalgesia.

Deficits in visceral pain and referred hyperalgesia in Nav1.8 (SNS/PN3)-null mice.

The tetrodotoxin-resistant sodium channel alpha subunit Nav1.8 is expressed exclusively in primary sensory neurons and is proposed to play an important role in sensitization of nociceptors. Here we compared visceral pain and referred hyperalgesia in Nav1.8-null mice and their wild-type littermates in five tests that differ in the degree to which behavior depends on spontaneous, ongoing firing in sensitized nociceptors. Nav1.8-null mice showed normal nociceptive behavior provoked by acute noxious stimulation of abdominal viscera (intracolonic saline or intraperitoneal acetylcholine). However, Nav1.8-null mutants showed weak pain and no referred hyperalgesia to intracolonic capsaicin, a model in which behavior is sustained by ongoing activity in nociceptors sensitized by the initial application. Nav1.8-null mice also showed blunted pain and hyperalgesia to intracolonic mustard oil, which sensitizes nociceptors but also provokes tissue damage. To distinguish between a possible role for Nav1.8 in ongoing activity per se and ongoing activity after sensitization in the absence of additional stimuli, we tried a visceral model of tonic noxious chemical stimulation, cyclophosphamide cystitis. Cyclophosphamide produces cystitis by gradual accumulation of toxic metabolites in the bladder. In this model, Nav1.8-null mice showed normal responses. There were no differences between null mutants and their normal littermates in tissue damage and inflammation evoked by any of the stimuli tested, suggesting that the behavioral differences are not secondary to impairment of inflammatory responses. We conclude that there is an essential role for Nav1.8 in mediating spontaneous activity in sensitized nociceptors.