Inflammation-induced up-regulation of protein kinase Cgamma immunoreactivity in rat spinal cord correlates with enhanced nociceptive processing

Inhibition of spinal protein kinase C reduces nerve injury-induced tactile allodynia in neuropathic rats

We investigated the effects of inhibiting spinal protein kinases including PKC, PKA and PKG on tactile allodynia in rats with a unilateral tight ligation on L5/L6 spinal nerves (Chung model). The intrathecal (IT) delivery of GF109203X, a PKC inhibitor, produced a potent and long lasting anti-allodynic effect. The effect was dose-dependent and stereospecific. Bisindolymaleimide V, an inactive homologue of GF, had no effect. Additionally, two other PKC inhibitors, PKC19-31 and chelerythrine, displayed significant anti-allodynic action. Spinal PKA, but not PKG, is likely involved in Chung tactile allodynia, since H89 (a PKA inhibitor) showed anti-allodynic activity, while KT5823 (a PKG inhibitor) had only a minor effect. These data emphasize that spinal PKC plays an important role in nerve injury-induced tactile allodynia. Other protein kinases such as PKA may also contribute to this phenomenon.

Second messengers, the substantia gelatinosa and injury-induced persistent pain

Although there is now unequivocal evidence that the circuitry within the substantia gelatinosa is a major contributor to the transmission and control of nociceptive messages, this was not known 35 years ago, when Pat Wall first focussed attention on this region. In addition to being the target of neurochemically distinct nociceptors, this region contains a heterogeneous population of excitatory and inhibitory interneurons. This review focuses on the contribution of second messenger systems that are found in the substantia gelatinosa. In particular the review highlights their critical contribution to the development of persistent pain conditions in the setting of tissue and nerve injury. Several of the studies used animals with deletions of genes that encode major second messenger molecules, including protein kinase A, C and nitric oxide synthase. Our laboratory has shown that mice with a deletion of the gene that encodes the gamma isoform of protein kinase C (which is almost exclusively expressed in a population of interneurons of the inner part of the substantia gelatinosa) have completely normal acute pain responses. However, the allodynia that characteristically develops after injury does not occur in these mice, particularly when it is generated by partial sciatic nerve injury. By contrast, deletion of genes that encode protein kinase A subunits only show deficits in the development of tissue inflammation-induced pain. These differences highlight the selectivity that characterizes the contribution of different second messenger molecules. Because of the restricted distribution of these molecules, it is likely that they are activated by different populations of primary afferent nociceptor and under very different conditions of injury. Understanding the circuitry within the substantia gelatinosa is thus critical to elucidating the mechanisms through which these second messenger molecules contribute to the development of persistent pain in the setting of injury.

Distinct neurochemical features of acute and persistent pain

To address the neurochemistry of the mechanisms that underlie the development of acute and persistent pain, our laboratory has been studying mice with deletions of gene products that have been implicated in nociceptive processing. We have recently raised mice with a deletion of the preprotachykinin-A gene, which encodes the peptides substance P (SP) and neurokinin A (NKA). These studies have identified a specific behavioral phenotype in which the animals do not detect a window of "pain" intensities; this window cuts across thermal, mechanical, and chemical modalities. The lowered thermal and mechanical withdrawal thresholds that are produced by tissue or nerve injury, however, were still present in the mutant mice. Thus, the behavioral manifestations of threshold changes in nociceptive processing in the setting of injury do not appear to require SP or NKA. To identify relevant neurochemical factors downstream of the primary afferent, we are also studying the dorsal horn second messenger systems that underlie the development of tissue and nerve injury-induced persistent pain states. We have recently implicated the gamma isoform of protein kinase C (PKCgamma) in the development of nerve injury-induced neuropathic pain. Acute pain processing, by contrast, is intact in the PKCgamma-null mice. Taken together, these studies emphasize that there is a distinct neurochemistry of acute and persistent pain. Persistent pain should be considered a disease state of the nervous system, not merely a prolonged acute pain symptom of some other disease conditions.

The types of neuron which contain protein kinase C gamma in rat spinal cord

Protein kinase C (PKC) is thought to have a role in sensitization of dorsal horn neurons in certain pain states, and a recent study has reported that mice which lack the gamma isoform (PKCgamma) show reduced neuropathic pain after peripheral nerve injury. Although PKCgamma is present at high levels in the ventral part of lamina II we have limited information concerning the types of neuron in which it is located. In this study we have used immunocytochemistry to characterise the neurons which contain PKCgamma. Immunoreactive neurons were concentrated in ventral lamina II, but were also present in lamina III. Some weakly-immunoreactive neurons were located in the dorsal part of lamina II and in lamina I. The great majority (92%) of cells with PKCgamma were not GABA-immunoreactive, and these cells are likely to be excitatory interneurons. Dual-immunofluorescence labelling showed that PKCgamma was not randomly distributed amongst non-GABAergic neurons, since it was present in 76% of cells with neurotensin and 45% of those with somatostatin, but only 5% of those with the mu-opioid receptor (MOR-1). Cells with the neurokinin 1 receptor are found in lamina I and lamina III, and PKCgamma was present in 22% and 37% of these populations, respectively. These results suggest that excitatory interneurons in laminae II and III which lack the micro-opioid receptor may have a significant role in generating neuropathic pain.