We examined the effect of p38 mitogen-activated protein kinase (MAPK) inhibitors in models of nociception and correlated this effect with localization and expression levels of p38 MAPK in spinal cord. There was a rapid increase in phosphorylated p38 MAPK in spinal cord following intrathecal administration of substance P or intradermal injection of formalin. Immuncytochemisty revealed that phosphorylated p38 MAPK-immunoreactive cells were predominantly present in laminae I-IV of the dorsal horn. Double-staining with markers for neurons, microglia, astrocytes and oligodendrocytes unexpectedly revealed co-localization with microglia but not with neurons or other glia. Pretreatment with p38 MAPK inhibitors (SB20358 or SD-282) had no effect on acute thermal thresholds. However, they attenuated hyperalgesia in several nociceptive models associated with spinal sensitization including direct spinal activation (intrathecal substance P) and peripheral tissue inflammation (intraplantar formalin or carrageenan). Spinal sensitization, manifested by enhanced expression of cyclo-oxygenase-2 and inflammation-induced appearance of Fos-positive neurons, was blocked by pretreatment, but not post-treatment, with p38 MAPK inhibitors. Taken together, these results indicate that spinal p38 MAPK is involved in inflammation-induced pain and that activated spinal microglia play a direct role in spinal nociceptive processing.

Aims/hypothesis. Sensory neuropathy in diabetic patients frequently presents itself as progressive loss of thermal perception, while some patients describe concurrent spontaneous pain, allodynia or hyperalgesia. Diabetic rats develop thermal hypoalgesia and tactile allodynia by unknown mechanisms. We investigated whether sensory disorders in rats were related to glucose metabolism by aldose reductase. We also explored the therapeutic potential of exogenous neurotrophic factors. Methods. Behavioural assessments of thermal and tactile sensitivity were performed in normal rats and in rats with streptozotocin-induced diabetes. Some of the rats were treated with insulin, aldose reductase inhibitors, ciliary neurotrophic factor or brain-derived neurotrophic factor. Results. Thermal hypoalgesia was present after 8 weeks of diabetes and was prevented by insulin treatment, which maintained normoglycaemia, by the aldose reductase inhibitor Statil or by ciliary neurotrophic factor. Brain-derived neurotrophic factor did not have an effect. When diabetic rats were tested after shorter durations of diabetes, they showed transient thermal hyperalgesia after 4 weeks which progressed to thermal hypoalgesia after 8 weeks. The aldose reductase inhibitor IDD 676 (Lidorestat), given from the onset of diabetes, prevented the development of thermal hyperalgesia and also stopped progression to thermal hypoalgesia when delivered in the last 4 weeks of an 8-week period of diabetes. Tactile allodynia was not prevented by neurotrophic factor or aldose reductase inhibitor treatment. Conclusions/interpretation. Transient thermal hyperalgesia and subsequent progressive thermal hypoalgesia occur in diabetic rats secondary to exaggerated flux through the polyol pathway. A depletion of ciliary neurotrophic factor mediated by the polyol pathway may be involved in the aetiology of thermal hypoalgesia.

IntroductionThe mammalian hedgehog (Hh) protein family (sonic, desert, and indian) are relatives of a Drosophila hedgehog protein that regulates segmental polarity during development and are expressed at many sites in the developing mouse embryo (1). Within the nervous system, sonic hedgehog protein (SHh) is associated with development and patterning of the central nervous system (2-5). In contrast, the morphogenic effects of desert hedgehog protein (DHh) are restricted to the peripheral nervous system and appear to focus specifically on cellular elements of the epi-and perineurial sheaths (6). Deletion of the dhh gene that codes for DHh in mice results in disruption of the fascicular structure of peripheral nerves, with development of a thin and disorganized perineurial sheath and an increase in blood-nerve barrier permeability that is associated with decreased connexin-43 expression by perineurial cells (6). A similar modification of nerve structure was associated with a homozygous missense mutation of the DHH gene in a human diagnosed with "minifascicular neuropathy" (7). These findings point to a role for Hh proteins in the development of the peripheral nervous system.Recent findings in acute peripheral-nerve injury models have revealed a novel function for hedgehog proteins in adult nerve regeneration and repair. The shh mRNA was strikingly upregulated in the peripheral nerve after crush injury (8), and exogenous SHh protein administration enhanced the speed of nerve recovery (9). The Hh signaling pathway is also upregulated soon after injury in a model of surgically induced hind limb ischemia, and exogenous SHh protein administration augments blood-flow recovery and limb salvage (10). Emerging evidence therefore suggests that signaling pathways initiated by Hh proteins are involved in the response to a range of peripheral-nerve lesions and participate in the repair and recovery process. Hedgehog proteins modulate development and patterning of the embryonic nervous system. As expression of desert hedgehog and the hedgehog receptor, patched-1, persist in the postnatal and adult peripheral nerves, the hedgehog pathway may have a role in maturation and maintenance of the peripheral nervous system in normal and disease states. We measured desert hedgehog expression in the peripheral nerve of maturing diabetic rats and found that diabetes caused a significant reduction in desert hedgehog mRNA. Treating diabetic rats with a sonic hedgehog-IgG fusion protein fully restored motor-and sensory-nerve conduction velocities and maintained the axonal caliber of large myelinated fibers. Diabetes-induced deficits in retrograde transport of nerve growth factor and sciatic-nerve levels of calcitonin gene-related product and neuropeptide Y were also ameliorated by treatment with the sonic hedgehog-IgG fusion protein, as was thermal hypoalgesia in the paw. These studies implicate disruption of normal hedgehog function in the etiology of diabetes-induced peripheral-nerve dysfunction and indicate that delivery of exogenous hedge...

Diabetic rats display exaggerated hyperalgesic behavior in response to noxious stimuli that may model aspects of painful diabetic neuropathy. This study examined the contribution of spinal prostaglandin production to this exaggerated hyperalgesic behavior. Rats were implanted with spinal dialysis probes and received noxious stimulation to the hind paw by subcutaneous injection of 0.5% formalin solution. Prostaglandin E 2 (PGE 2 ) was measured in dialysates of lumbar spinal cerebrospinal fluid concurrent with behavioral responses to formalin injection. In separate experiments, formalin-evoked behavioral responses were measured after intrathecal delivery of either a cyclooxygenase inhibitor or an EP 1 receptor antagonist, and cyclooxygenase protein was measured in spinal cord homogenates. Diabetic rats exhibited exaggerated behavioral responses to paw formalin injection and a concurrent prolongation of formalin-evoked PGE 2 release. Formalin-evoked behavioral responses were dose-dependently reduced in diabetic rats by spinal delivery of a cyclooxygenase inhibitor or an EP 1 receptor antagonist. Protein levels of cyclooxygenase-2 were elevated in the spinal cord of diabetic rats, whereas cyclooxygenase-1 protein was reduced. Hyperalgesic behavior in diabetic rats is associated with both increased cyclooxygenase-2 protein and cyclooxygenase-mediated PGE 2 release. Spinal delivery of selective inhibitors of cyclooxygenase-2 or antagonists of prostaglandin receptors may have therapeutic potential for treating painful diabetic neuropathy. Diabetes 51: 2249 -2255, 2002 A proportion of patients with diabetic neuropathy report aberrant sensations that may range from exaggerated perception of sensory stimuli to spontaneous paresthesias and pain. Morphometric analyses of peripheral nerves from patients with painful diabetic neuropathy have been unable to find clear associations with either nerve fiber degeneration or regeneration (1,2). In the absence of simple structural correlates in the peripheral nerves, it is plausible that neurochemical abnormalities at the peripheral, spinal, or supraspinal levels could contribute to painful diabetic neuropathy.Diabetic rats also display evidence of altered sensory processing, as illustrated by behavioral studies in which nocifensive responses to normally nonpainful stimuli (allodynia) and exaggerated responses to stimuli that usually produce mild nocifensive responses (hyperalgesia) were reported. Thus, allodynia to light touch (3) or mechanical pressure (4) and hyperalgesia after paw formalin injection (5,6) all develop within weeks of the onset of hyperglycemia and can be corrected by instituting a protracted period of tight glycemic control (3,7). Rats with short-term (4 weeks) experimental diabetes do not exhibit marked fiber degeneration or regeneration in their peripheral (8) or cutaneous (9) nerves. They may therefore allow investigation of the contribution of neurochemical disorders to the behavioral indexes of hyperalgesia and also provide a model for the assessment of p...

To test the osmoregulatory rules of Schwann cell aldose reductase (AR) and myo-inositol, JS1 Schwann cells were grown under control and hyperosmotic conditions with and without excess glucose or galactose. JS1 cells cultured in control conditions possessed AR protein and activity that were not altered by the inclusion of 25 mM glucose or galactose. Following culture with 100 mM NaCl, there was a decline in cell number accompanied by an increase in AR activity, both of which were attenuated by the addition of 25 mM glucose or galactose. Sorbitol was not detected in JS1 Schwann cells following culture in control, glucose-supplemented, or hyperosmotic medium, and dulcitol accumulated only following culture with galactose. However, both polyols were dramatically increased in JS1 cells cultured in hyperosmotic medium supplemented with 25 mM glucose or galactose. In contrast, myo-inositol levels were elevated only during hyperosmotic exposure but decreased when glucose or galactose was also present. These data are consistent with the use of polyol formation by JS1 Schwann cells as a means of responding to osmotic stress.