Neuropathic pain is a poorly understood and severely debilitating condition (Yaksh, 1999). In the present study, a rat model of diabetic neuropathy (Courteix et al. 1993) has been used to investigate the molecular mechanisms underlying this condition.

In initial studies, adult male Sprague-Dawley rats (> 250 g) were divided into two groups of ten animals. One group received a single dose of 50 mg kg^-1 streptozocin (STZ, I.P.) whilst the second (control) group received a similar administration of isotonic saline. Two to four weeks after this procedure, both groups were tested for the development of static allodynia by measuring paw withdrawal thresholds using von Frey hairs (Field et al. 1999). After demonstrating the presence of static allodynia only in the STZ group, both groups of animal were killed by an approved Schedule 1 procedure. The lumbar spinal cords from each group were rapidly dissected and mRNA extracted to generate a normalised cDNA library containing partial fragments of genes potentially regulated in the lumbar spinal cord of hyperalgesic rats. Analysis of 500 sequences cloned from each library revealed that members of the extracellular signal-regulated kinase (ERK) cascade were present in the library containing up-regulated genes. Broad scale expression profiling carried out using an Atlas cDNA expression array (Clontech, Palo Alto, USA) demonstrated that the levels of ERK1 and ERK2 mRNA increase in the lumbar spinal cord of STZ-treated rats compared with control animals. The relative optical densities (ROD) measured for each gene were normalized to those of nine housekeeping genes. ROD for ERK1: control = 0.20 ± 0.06 (mean ± S.E.M.), STZ = 0.52 ± 0.07 (P < 0.05 using Student’s t test, n = 3). For ERK2: control = 0.36 ± 0.07, STZ = 0.57 ± 0.05 (P < 0.05, n = 3).

To investigate whether increased levels of mRNA correlated with increased ERK activity, non-radioactive ERK phosphorylation assays were undertaken. Analysis of the spinal cords of STZ-treated and control rats, revealed that STZ induced a time-dependent increase in the activity of both ERK1 and ERK2 correlating with the induction of hyperalgesia (ROD values for ERK1: control = 0.20 ± 0.06, STZ = 0.50 ± 0.07 (P < 0.05 using Student’s t test, n = 4)). For ERK2: control = 0.18 ± 0.11, STZ = 0.89 ± 0.20, P < 0.05; n = 4).

Having confirmed that there is an increase in ERK activity in the development of neuropathic pain, the effect of the ERK kinase (MEK) inhibitor PD 198306 was investigated in two animal models of neuropathic pain by assessing static allodynia with von Frey hairs. Intrathecal administration of PD 198306 (1-30 µg) significantly blocked static allodynia in both the STZ and the chronic constriction injury models of neuropathic pain (Bennett & Xie, 1988) in a dose-dependent manner with minimum effective doses (MED) of 3 and 10 µg, respectively (P < 0.05, Kruskall-Wallis followed by Mann-Whitney U test). The highest dose used (30 µg) blocked completely the maintenance of static allodynia, for up to 1 h after administration (P < 0.001, n = 6-9).

In conclusion, we demonstrate that there is an upregulation in the activity of the ERK cascade in the development of neuropathic pain and that pharmacological manipulation of this pathway may provide novel ways in which to treat this debilitating condition.

Bennett, G.J. & Xie, Y.K. (1988). Pain 3, 87-107.

Courteix, C., Eschalier, A. & Lavarenne, J. (1993). Pain 53, 81-88.

Field, M.J., McCleary, S., Hughes, J. & Singh, L. (1999). Pain 80, 391-398.

Yaksh, T.L. (1999). TiPS 20, 329-336.ndition.

Bennett, G.J. & Xie, Y.K. (1988). Pain 3, 87-107.

Courteix, C., Eschalier, A. & Lavarenne, J. (1993). Pain 53, 81-88.

Field, M.J., McCleary, S., Hughes, J. & Singh, L. (1999). Pain 80, 391-398.

Yaksh, T.L. (1999). TiPS 20, 329-336.