Protection against oxaliplatin-induced mechanical hyperalgesia and intraepidermal nerve fiber loss by minocycline

Abstract

Treatment with the chemotherapeutic agent oxaliplatin produces a robust painful neuropathy similar to various other neuropathic conditions which result in loss of nerve fibers innervating the skin. This loss of intraepidermal nerve fibers (IENFs) appears to play an important role in neuropathy, but has yet to be investigated in oxaliplatin-induced neuropathic pain. For this study, mechanical hyperalgesia and IENF density were measured in rats receiving oxaliplatin, given at a dosage of 2 mg/kg every other day for four injections. The immunomodulatory agent minocycline (25 mg/kg) was also administered and was given 24 h prior to the first dose of oxaliplatin and continued throughout oxaliplatin treatment. Immunohistochemistry using the pan-neuronal marker PGP9.5 was used to investigate IENF densities in hind paw skin on Day 15 and Day 30. The results show that a robust mechanical sensitivity developed in oxaliplatin treated animals, as did a pronounced decrease in epidermal nerve fibers, and these outcomes were effectively prevented by minocycline treatment. This is the first study to show changes in IENF density in oxaliplatin treated animals, and confirm not only a relationship between IENF loss and hypersensitivity but also prevention of both with minocycline treatment.

Figure 1

Oxaliplatin produces a decrease in mechanical paw withdrawal threshold that is prevented by treatment with minocycline. Animals treated with oxaliplatin (vehicle/oxali; n=10) had a significant decrease in threshold that began on Day 6 when compared to vehicle treated animals (vehicle/vehicle; n=6). Rats receiving oxaliplatin with minocycline pretreatment (mino/oxali; n=8) were not significantly different from vehicle treated animals at any timepoint, indicating a protective effect of minocycline. Minocycline treatment did not alter mechanical responding (mino/vehicle; n=6). (***=p<.0001)

Figure 2

Animals receiving minocycline (mino/vehicle; mino/oxali) gained significantly less weight than animals treated with vehicle (vehicle/vehicle) starting on Day 4 and continuing throughout the experiment. Oxaliplatin treatment alone (vehicle/oxali) also lead to less weight gain on Days 6, 8, and 12; however, these animals had recovered and were not different from vehicle treated rats on Day 30. (***=p<.0001)

Figure 3

Immunohistochemisty results showing staining of intraepidermal nerve fibers by PGP9.5. Fibers can clearly be seen crossing the basement membrane, in green, and extending as long lines into the epidermis. A through d show epidermal innervation on Day 15 from animals treated with the vehicle (a), vehicle/oxaliplatin (b), minocycline/oxaliplatin (c), and minocycline/vehicle (d). Similar results were found on Day 30 for vehicle (e), vehicle/oxaliplatin (f), minocycline/oxaliplatin (g), and minocycline/vehicle (h) treated animals.

Figure 4

Oxaliplatin treatment (vehicle/oxaliplatin) results in decreased intraepidermal nerve fibers within the footpad of the hind paw at Day 15 and Day 30. Rats receiving oxaliplatin with minocycline pretreatment (mino/oxali) did not have altered IENF density compared to vehicle treated animals (vehicle/vehicle), indicating that minocycline protected against oxaliplatin-induced nerve fiber loss. Minocycline treatment alone (mino/vehicle) did not significantly change nerve fiber density. (**=p<.001; ***=p<.0001)