Prevention of paclitaxel-evoked painful peripheral neuropathy by acetyl-L-carnitine: effects on axonal mitochondria, sensory nerve fiber terminal arbors, and cutaneous Langerhans cells

Abstract

Prophylactic treatment with acetyl-L-carnitine (ALCAR) prevents the neuropathic pain syndrome that is evoked by the chemotherapeutic agent, paclitaxel. The paclitaxel-evoked pain syndrome is associated with degeneration of the intraepidermal terminal arbors of primary afferent neurons, with the activation of cutaneous Langerhans cells, and with an increased incidence of swollen and vacuolated axonal mitochondria in A-fibers and C-fibers. Previous work suggests that ALCAR is neuroprotective in other nerve injury models and that it improves mitochondrial dysfunction. Thus, we examined whether the prophylactic efficacy of ALCAR was associated with the prevention of intraepidermal terminal arbor degeneration, the inhibition of Langerhans cell activation, or the inhibition of swelling and vacuolation of axonal mitochondria. In animals with a confirmed ALCAR effect, we found no evidence of a neuroprotective effect on the paclitaxel-evoked degeneration of sensory terminal arbors or an inhibition of the paclitaxel-evoked activation of Langerhans cells. However, ALCAR treatment completely prevented the paclitaxel-evoked increase in the incidence of swollen and vacuolated C-fiber mitochondria, while having no effect on the paclitaxel-evoked changes in A-fiber mitochondria. Our results suggest that the efficacy of prophylactic ALCAR treatment against the paclitaxel-evoked pain may be related to a protective effect on C-fiber mitochondria.

Figure 1

The effect of ALCAR on paclitaxel-evoked pain. The expected mechano-allodynia (A) and mechano-hyperalgesia (B) developed in vehicle-treated rats. Prophylactic treatment with ALCAR completely prevented both mechano-allodynia and mechano-hyperalgesia. Baseline values are the average of tests done on three separate days prior to paclitaxel treatment. Paclitaxel effects were measured in vehicle-treated (n = 6) and ALCAR-treated (n = 8) rats on D25-D27. Means ± S.E.M. Note that the ordinates differ. # p < 0.01 compared to baseline; ** p < 0.01 compared to vehicle-treated group.

Figure 2

Immunocytochemical demonstration of intraepidermal nerve fibers (A) and Langerhans cells (A, B) in glabrous skin stained with the anti-PGP9.5 antisera. (A) Section from a naïve animal showing a lightly stained LC and an IENF arising from a cutaneous nerve fascicle (CNF) and ascending into the epidermis. (B) Section showing the intensely stained LCs typically observed after paclitaxel-treatment.

Figure 3

ALCAR effect on the number of intraepidermal nerve fibers and Langerhans cells. Counts shown are mean ± S.E.M. per centimeter of epidermal border for naïve rats (n = 7), paclitaxel-treated rats receiving vehicle treatment (n = 6), and paclitaxel-treated rats receiving ALCAR treatment (n = 8). Paclitaxel treatment significantly reduced the number of IENFs (A) and increased the number of LCs (B) in vehicle-treated rats compared to naïve rats. ALCAR treatment had no statistically significant effect on the IENF loss or on LC activation. * p < 0.05 relative to the naïve group.

Figure 4

Atypical and normal mitochondria in C-fibers (A) and A-fibers (B). Swollen and vacuolated mitochondria shown at arrows; barred arrows point to normal mitochondria. Note the pooling of electron dense semi-linear structures (believed to be collapsed cristae) at one pole of the swollen mitochondria. As shown here, a mixture of normal and swollen and vacuolated mitochondria were often found within the same axon. These examples are from a paclitaxel-treated rat that received ALCAR injections. 44,400X.

Figure 5

Effects of ALCAR on C-fiber mitochondria (A) and A-fiber mitochondria (B). Bars show the proportion (means ± S.E.M) of atypical mitochondria relative to the total number of mitochondria in each fiber type. # p < 0.05, # # p < 0.01 compared to naïve group; * p < 0.05 compared to the vehicle-treated group; NS: non-significant.