Intraepidermal nerve fiber loss corresponds to the development of taxol-induced hyperalgesia and can be prevented by treatment with minocycline

Abstract

Loss of intraepidermal nerve fibers (IENFs) has been speculated to play a critical role in the development of various neuropathies. In this study, the density of IENFs were studied over time during the induction of Taxol (Bristol-Myers Squibb, NY, USA)-induced chemoneuropathy and compared with the changes in IENFs in animals co-treated with Taxol plus the protective agent minocycline. Rats were injected (intraperitoneally) with 2mg/kg of Taxol every other day for four injections (day 1, 3, 5, and 7). Minocycline (25mg/kg) was given in a separate group of rats 24h prior to the first dose of Taxol and every day for the next 9days (day 0 through 9). Animals were tested for mechanical paw withdrawal thresholds prior to any drug administrations and again on day 7, 14, and 30. Immunohistochemistry using the pan-neuronal marker protein gene product 9.5 was performed on glabrous skin of the hind-paw foot pad to stain for IENFs also on day 7, 14, and 30. The results show that Taxol-treated animals developed mechanical sensitivity and corresponding IENF loss. Animals receiving minocycline plus Taxol showed no hyperalgesia or loss of IENFs. This study confirms, for the first time, that a loss of IENFs occurs as a neuropathy develops, and further shows a protection against both IENF loss and hyperalgesia with minocycline treatment. The progression of Taxol-induced mechanical hypersensitivity coincides with loss of intraepidermal nerve fibers, and the hyperalgesia and nerve fiber loss were prevented with minocycline treatment.

Figure 1

Taxol treated animals had significantly lower mechanical threshold in a test of paw withdrawal, a finding that was prevented by treatment with minocycline. Animals treated with Taxol (vehicle/Taxol; n=11) had a significant decrease in threshold that began on Day 14 when compared to vehicle treated animals (vehicle/vehicle; n=10). This effect was also found on Day 30. Rats receiving both minocycline and Taxol (mino/Taxol; n=12) were not significantly different from vehicle treated animals at any time point, indicating a protective effect of minocycline. (**:p<.001)

Figure 2

Immunohistochemisty results showing staining of intraepidermal nerve fibers by PGP9.5 (red). Nerve fibers can clearly be seen crossing the collagen-stained basement membrane (green), and extending into the epidermis. On Day 7, vehicle/Taxol treated rats (b) had fewer, although not significantly so, IENFs compared to both vehicle/vehicle (a) and minocycline/Taxol (c) treated animals. By Day 14, there was a significant decrease in the number of fibers in vehicle/Taxol treated animals (e) compared to vehicle/vehicle (d) and minocycline/Taxol (f) treated rats. This was again seen on Day 30 (h-i).

Figure 3

Taxol treatment (vehicle/Taxol) results in decreased intraepidermal nerve fibers within the footpad of the hind paw on Day 14 and Day 30, but not on Day 7. Rats receiving Taxol with minocycline pretreatment (mino/Taxol) were not significantly different from vehicle treated animals (vehicle/vehicle) in regards to IENF counts, indicating a protective effect of minocycline against Taxol-induced nerve fiber loss. (*:p=.05; **:p=.01)