Preservation of acute pain and efferent functions following intrathecal resiniferatoxin-induced analgesia in rats

Abstract

Resiniferatoxin (RTX) is a potent agonist of TRPV1, which possesses unique properties that can be utilized to treat certain modalities of pain. In the present study, systemic intraperitoneal (i.p.) administration of RTX resulted in a significant decrease in acute thermal pain sensitivity, whereas localized intrathecal (i.t.) administration had no effect on acute thermal pain sensitivity. Both i.p. and i.t. administration of RTX prevented TRPV1-induced nocifensive behavior and inflammatory thermal hypersensitivity. There were no alterations in mechanical sensitivity either by i.p. or i.t. administration of RTX. In spinal dorsal horn (L4-L6), TRPV1 and substance P immunoreactivity were abolished following i.p. and i.t. administration of RTX. In dorsal root ganglia (DRG), TRPV1 immunoreactivity was diminished following i.p. administration, but was unaffected following i.t. administration of RTX. Following i.p. administration, basal and evoked calcitonin gene-related peptide release were reduced both in the spinal cord and peripheral tissues. However, following i.t. administration, basal and evoked calcitonin gene-related peptide release were reduced in spinal cord (L4-L6), but were unaffected in peripheral tissues. Both i.p. and i.t. RTX administration lowered the body temperature acutely, but this effect reversed with time. Targeting TRPV1-expressing nerve terminals at the spinal cord can selectively abolish inflammatory thermal hypersensitivity without affecting acute thermal sensitivity and can preserve the efferent functions of DRG neurons at the peripheral nerve terminals. I.t. administration of RTX can be considered as a strategy for treating certain chronic and debilitating pain conditions.

Perspective: Localized administration of RTX in spinal cord could be a useful strategy to treat chronic debilitating pain arising from certain conditions such as cancer and at the same time could maintain normal physiological peripheral efferent functions mediated by TRPV1.

Figure 1

Acute thermal and mechanical sensitivities after i.t. and i.p. RTX-treatment. A. B. Time course of the PWL to noxious heat and PWT to von Frey filament test following i.t. RTX treatment. C. D. Time course of the PWL to noxious heat and PWT to von Frey filament test following i.p. RTX administration. * denotes p<0.05 as compared to vehicle-treated group at respective time points.

Figure 2

Effect of i.t. and i.p. RTX administration on capsaicin-induced nocifensive behavior. A. B. C. Intraplantar capsaicin-induced guarding, licking and the total duration of nocifensive behavior after i.t. administration of RTX. D. E. F. Capsaicin-induced guarding, licking and the total duration of nocifensive behavior after i.p. administration of RTX. * denotes p< 0.05 as compared to vehicle-treated group.

Figure 3

Effect of i.t. and i.p. RTX administration on inflammatory thermal pain sensitivity. A. B. Capsaicin-induced thermal hypersensitivity following i.t. and i.p. RTX administration. C. D. Carrageenan-induced inflammatory thermal hypersensitivity following i.t and i.p. RTX administration. * denotes p < 0.05 as compared to vehicle-treated group at respective time point.

Figure 4

TRPV1 immunostaining 5 weeks after a single administration of i.t. and i.p. RTX. A. TRPV1 staining in lumbar DRG following vehicle, i.t. and i.p. RTX administration in rats. B. TRPV1 staining in dorsal horn of lumbar spinal cord sections following vehicle, i.t., and i.p. RTX administration in rats. C. SP staining in dorsal horn of lumbar spinal cord segment following vehicle, i.t. and i.p. RTX administration in rats.(Scale bar in Fig. 5A :50 μm; Scale bar in Fig. 5B and 5C: 200 μm)

Figure 5

Effect of i.t. and i.p. RTX administration on basal and evoked CGRP release from spinal cord segments. A. C. Basal (A) and capsaicin-evoked (C) CGRP release from cervical spinal cord segments following i.t and i.p. RTX administration. B. D. Basal (B) and capsaicin-evoked (D) CGRP release from lumbar spinal cord segments following i.t and i.p. RTX administration. * denotes p < 0.05 as compared to vehicle treated group. # denotes p< 0.05 as compared to capsaicin stimulated vehicle. + denotes p< 0.05 as compared to capsaicin-stimulated i.p.-RTX group.

Figure 6

Effect of i.t. and i.p. RTX administration on basal and evoked CGRP release from paw skin tissue (peripheral) following i.p. and i.t. RTX treatment. A. Basal CGRP release in paw skin following i.t and i.p. RTX administration. * denotes p<0.05 as compared to vehicle treated group. # denotes p<0.05 as compared to i.p. RTX-treated group. B. C. D. Capsaicin (B), anandamide (C) and inflammatory soup(D)-evoked CGRP release from paw skin tissue following i.t and i.p. RTX administration. * denotes ^p<0.05 as compared to vehicle treated group. # denotes p<0.05 as compared to capsaicin- or anandamide- or inflammatory soup-treated tissue.

Figure 7

Effect of i.t. and i.p. RTX administration on basal CGRP release from urinary bladder and colon tissue following i.t. and i.p. RTX administration. A. CGRP release from urinary bladder tissue. B. CGRP release from colon tissue. * denotes p<0.05 as compared to vehicle treated group. # denotes p<0.05 as compared to i.t. RTX treated group.

Figure 8

Changes in body temperature following i.t. and i.p. RTX administration. A. B. Acute (A) and chronic (B) body temperature measurement in rats treated with vehicle, i.t. and i.p. RTX treated rats. * denotes p<0.05 as compared to vehicle treated group. # denotes p<0.05 as compared to i.t. RTX-treated group.