Dorsal root ganglion neurons respond with prolonged extracellular signal-regulated protein kinase phosphorylation following noxious heat and cold stimulation

Noxious, but not innocuous, thermal stimuli evoke pERK expression in dorsal horn neurons after spared nerve injury in adult rats

Noxious stimulation of sensory afferents evokes phosphorylated extracellular signal regulated kinase (pERK) expression in spinal cord neurons. This study investigated the expression of pERK in the dorsal horn neurons in response to innocuous and noxious cold stimuli in naïve versus spared nerve injury (SNI) rats. Noxious cold or hot stimuli (0 or 45°C) elicited pERK expression in laminae I-II whereas cooling stimuli from 32°C to 25, 15 or 5°C produced no or little pERK expression in dorsal horn neurons. Five days after SNI, a time when these animals showed heat hyperalgesia, cold and mechanical hypersensitivity, only noxious heat stimuli produced a significant increase in pERK expression compared to naïve rats in spinal cord neurons. Thus, pERK cannot be used as an activity marker for neurons responding to cooling stimuli or cold allodynia; however, these results confirm the role of pERK as an activity marker for heat hyperalgesia.

3-iodothyroacetic acid, a metabolite of thyroid hormone, induces itch and reduces threshold to noxious and to painful heat stimuli in mice

BACKGROUND AND PURPOSE

Itch is associated with increased sensitization to nociceptive stimuli. We investigated whether 3-iodothyroacetic acid (TA1), by releasing histamine, induces itch and increases sensitization to noxious and painful heat stimuli.

EXPERIMENTAL APPROACH

Itch was evaluated after s.c. administration of TA1 (0.4, 1.32 and 4 μg·kg(-1) ). Mice threshold to noxious (NHT) and to painful heat stimuli were evaluated by the increasing-temperature hot plate (from 45.5 to 49.5°C) or by the hot plate (51.5°C) test, respectively, 15 min after i.p. injection of TA1 (0.4, 1.32 and 4 μg·kg(-1) ). Itch, NHT and pain threshold evaluation were repeated in mice pretreated with pyrilamine. Itch and NHT were also measured in HDC(+/+) and HDC(-/-) following injection of saline or TA1 (1.32, 4 and 11 μg·kg(-1) ; s.c. and i.p.). pERK1/2 levels were determined by Western blot in dorsal root ganglia (DRG) isolated from CD1 mice 15 min after they received (i.p.): saline, saline and noxious heat stimulus (46.5°C), TA1 (0.1, 0.4, 1.32, 4 μg·kg(-1) ) or TA1 1.32 μg·kg(-1) and noxious heat stimulus.

KEY RESULTS

TA1 0.4 and 1.32 μg·kg(-1) induced itch and reduced NHT; pyrilamine pretreatment prevented both of these effects. TA1 4 μg·kg(-1) (i.p.) reduced pain threshold without inducing itch or modifying NHT. In HDC(-/-) mice, TA1 failed to induce itch and to reduce NHT. In DRG, pERK1/2 levels were significantly increased by noxious heat stimuli and by TA1 0.1, 0.4 and 1.32 μg·kg(-1) ; i.p.

CONCLUSIONS AND IMPLICATIONS

Increased TA1 levels induce itch and an enhanced sensitivity to noxious heat stimuli suggesting that TA1 might represent a potential cause of itch in thyroid diseases.

Butyrate-induced colonic hypersensitivity is mediated by mitogen-activated protein kinase activation in rat dorsal root ganglia

OBJECTIVE

Increased faecal butyrate levels have been reported in irritable bowel syndrome. Rectal instillation of sodium butyrate (NaB) increases visceral sensitivity in rats by an unknown mechanism. We seek to examine the signal transduction pathways responsible for the enhanced neuronal excitability in the dorsal root ganglion (DRG) following NaB enemas and demonstrate that this is responsible for the colonic hypersensitivity reported in this animal model.

DESIGN

Colorectal distention (CRD) studies were performed in rats treated with NaB rectal instillation with/without intrathecal or intravenous administration of mitogen-activated protein (MAP) kinase kinase inhibitor U0126. Western blot analysis and immunocytochemistry studies elucidated intracellular signalling pathways that modulate IA. Patch-clamp recordings were performed on isolated DRG neurons treated with NaB, with/without U0126.

RESULTS

Visceromotor responses (VMR) were markedly enhanced in NaB-treated rats. Western blot analysis of DRG neurons from NaB-treated rats showed a 2.2-fold increase in phosphorylated ERK1/2 (pEKR1/2) and 1.9-fold increase in phosphorylated voltage-gated potassium channel subunit 4.2 (pKv4.2). Intrathecal or intravenous administration of U0126 reduced VMR to CRD in NaB-treated rats and prevented increases in pERK1/2 and pKv4.2. Patch-clamp recordings of isolated DRG neurons showed that NaB caused a reduction in IA to 48.9%±1.4% of control and an increase in neuronal excitability, accompanied by a twofold increase in pERK1/2 and pKv4.2. Concurrent U0126 administration prevented these changes.

CONCLUSIONS

Visceral hypersensitivity induced by colonic NaB treatment is mediated by activation of the MAP kinase-ERK1/2 pathway, which phosphorylates Kv4.2. This results in a reduction in IA and an enhancement of DRG neuronal excitability.

Phosphorylation of ERK1/2 in dorsal root ganglia following sequential mustard oil and thermal stimulation of the rat hind paw

The effect of a sequential stimulation by mustard oil and noxious heat or cold on the phosphorylation of ERK1/2 in sensory afferents was investigated. The stimuli were applied to the rat hind paw and phospho-ERK1/2 (p-ERK1/2) was measured by fluorescence-immunohistochemistry in the lumbar dorsal root ganglia (DRGs) neurons. All stimuli lead to a significant increase in the number of small size DRG cells displaying cytoplasmic staining for p-ERK1/2. The combination of mustard oil with cold significantly increased the number of cells with cytoplasmic staining above the level obtained with cold stimulus alone, however this was not observed with the combination of mustard oil and heat. Nuclear staining was weak and was found increased by mustard oil combined with cold stimulation. Mustard oil is known to activate TRPA1 and TRPV1 channels, heat TRPV1, and cold TRPA1 and TRPM8. The present findings shed new light on the DRG cell populations reacting with cytoplasmic and nuclear staining for p-ERK1/2 following sequential irritant chemical and thermal stimuli to the skin.

A fluorescence-immunohistochemical study on phosphorylation of ERK1/2, p38, and STAT3 in rat dorsal root ganglia following noxious stimulation of hind paw sensory neurons

A fluorescence-immunohistochemical investigation was performed in lumbar dorsal root ganglia (DRGs) neurons of the rat with regard to ERK1/2-, p38- and STAT3-phosphorylation in response to nociceptor activation in the rat. The stimuli applied were perineural capsaicin treatment of the sciatic nerve, mustard oil application to the hind paw and heat or cold stimulation of the hind paw. The time points of investigations were 15 min/30 min after perineural capsaicin, 30 min/2 h/4 h for mustard oil, 10 min/4 h for cold and 30 min/2 h/8 h for the heat stimulus. All four stimuli lead to a time-dependent, significant 2-3 fold increase in the number of small and medium size DRG cells displaying cytoplasmic staining for p-ERK1/2, but to no activation of satellite cells. Phosphorylated p38 immunoreactivity was increased in the cytoplasma of DRG cells at 2 h after the mustard oil treatment of the hind paw and 30 min after the perineural capsaicin application to the sciatic nerve axons, but not following heat or cold stimuli to the hind paws. Phospho-STAT3 staining was characteristically observed as nuclear and cytoplasmic staining. It was found increased after the perineural capsaicin application to the sciatic nerve axons, however, no marked increase was found with the other 3 noxious stimuli. The present results show that sensory neurons respond with a selective long-lasting increase in p-ERK1/2 in small and medium-size DRG cells, when their axons or axon terminals are stimulated by capsaicin, mustard oil, noxious heat or noxious cold.