Effect of ethyl pyruvate on Paclitaxel-induced neuropathic pain in rats

Abstract

Background: Although paclitaxel is a widely used chemotherapeutic agent for the treatment of solid cancers, side effects such as neuropathic pain lead to poor compliance and discontinuation of the therapy. Ethyl pyruvate (EP) is known to have analgesic effects in several pain models and may inhibit apoptosis. The present study was designed to investigate the analgesic effects of EP on mechanical allodynia and apoptosis in dorsal root ganglion (DRG) cells after paclitaxel administration.

Methods: Rats were randomly divided into 3 groups: 1) a control group, which received only vehicle; 2) a paclitaxel group, which received paclitaxel; and 3) an EP group, which received EP after paclitaxel administration. Mechanical allodynia was tested before and at 7 and 14 days after final paclitaxel administration. Fourteen days after paclitaxel treatment, DRG apoptosis was determined by activated caspase-3 immunoreactivity (IR).

Results: Post-treatment with EP did not significantly affect paclitaxel-induced allodynia, although it tended to slightly reduce sensitivities to mechanical stimuli after paclitaxel administration. After paclitaxel administration, an increase in caspase-3 IR in DRG cells was observed, which was co-localized with NF200-positive myelinated neurons. Post-treatment with EP decreased the paclitaxel-induced caspase-3 IR. Paclitaxel administration or post-treatment with EP did not alter the glial fibrillary acidic protein IRs in DRG cells.

Conclusions: Inhibition of apoptosis in DRG neurons by EP may not be critical in paclitaxel-induced mechanical allodynia.

Keywords: allodynia; apoptosis; dorsal root ganglion; ethyl pyruvate; paclitaxel.

Fig. 1

The effect of ethyl pyruvate (EP) on paclitaxel-induced mechanical allodynia in rats. Groups of rats were injected i.p. with either vehicle (control) or paclitaxel (2 mg/kg on 4 alternate days). Another group of rats was treated i.p. with EP (50 mg/kg on 6 consecutive days) after final paclitaxel administration. Mechanical allodynia was tested before and at 7 and 14 days after final paclitaxel administration by application of von Frey filaments to the surface of the hind paw. The vertical bars indicate the standard error of the mean. The number of animals used for each group was 7 (^*P < 0.05 and ^**P < 0.01 compared with the control group).

Fig. 2

The representative double immunofluorescence staining and quantitative analysis of activated caspase-3 and neurofilament 200 (NF200) in the rat dorsal root ganglion (DRG). Groups of rats were injected i.p. with either vehicle (control) or paclitaxel (2 mg/kg on 4 alternate days). Another group of rats was treated i.p. with either vehicle or ethyl pyruvate (EP, 50 mg/kg on 6 consecutive days) after final paclitaxel administration. Fourteen days after paclitaxel treatment, L5 DRG samples were immunostained with caspase-3, an indicator of apoptosis, and NF200, a marker of myelinated neurons. The co-localization of caspase-3 was visualized in a merged image. The arrow heads indicate where activated caspase-3 is co-localized with NF200. The scale bar represents 50 µm (A). The NF200 positive neurons that co-localized with caspase-3 were counted and are expressed as a percentage of the total neurons per section (^***P < 0.001 compared with the control group, and ^†††P < 0.001 compared with paclitaxel group). Error bars represent SEM (B).

Fig. 3

The representative double immunofluorescence staining of activated caspase-3 and glial fibrillary acidic protein (GFAP) in the rat dorsal root ganglion (DRG). Groups of rats were injected i.p. with either vehicle (control) or paclitaxel (2 mg/kg on 4 alternate days). Another group of rats was treated i.p. with either vehicle or ethyl pyruvate (EP, 50 mg/kg on 6 consecutive days) after final paclitaxel administration. Fourteen days after paclitaxel treatment, L5 DRG samples were immunostained with caspase-3, an indicator of apoptosis, and GFAP, a marker of activated astrocytes. Neither paclitaxel nor post-treatment with EP affected GFAP- or caspase-3 IRs in DRG. The scale bar represents 50 µm.