Spinal noradrenergic modulation and the role of the alpha-2 receptor in the antinociceptive effect of intrathecal nefopam in the formalin test

What do monoamines do in pain modulation?

PURPOSE OF REVIEW

Here, we give a topical overview of the ways in which brain processing can alter spinal pain transmission through descending control pathways, and how these change in pain states. We link preclinical findings on the transmitter systems involved and discuss how the monoamines, noradrenaline, 5-hydroxytryptamine (5-HT), and dopamine, can interact through inhibitory and excitatory pathways.

RECENT FINDINGS

Descending pathways control sensory events and the actions of the neurotransmitters noradrenaline and 5-HT in the dorsal horn of the spinal cord are chiefly implicated in nociception or antinociception according to the receptor that is activated. Abnormalities in descending controls effect central pain processing. Following nerve injury a noradrenaline-mediated control of spinal excitability is lost, whereas its restoration reduces neuropathic hypersensitivity. The story with 5-HT remains more complex because of the myriad of receptors that it can act upon; however the most recent findings support that facilitations may dominate over inhibitions.

SUMMARY

The monoaminergic system can be manipulated to great effect in the clinic resulting in improved treatment outcomes and is the basis for the actions of the antidepressant drugs in pain. Looking to the future, prediction of treatment responses will possible by monitoring a form of inhibitory descending control for optimized pain relief.

The role of inversely operating glutamate transporter in the paradoxical analgesia produced by glutamate transporter inhibitors

Controlling extracellular glutamate level in a physiological range is important to maintain normal sensory transmission. Here, we investigated the paradoxical action of glutamate transporters in the rat formalin test to elucidate a possible role of inversely operating transporters in its analgesic mechanism. The effects of glutamate transporter inhibitor on formalin-induced pain behavior were examined. Then we performed a microdialysis study to clarify the differential change in extracellular glutamate concentration by intrathecal administration of transportable and non-transportable blockers. And we further investigated the mechanism pharmacologically via pretreatment with antagonists of various receptors and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end labeling (TUNEL) staining. Intrathecally-injected glutamate transporter inhibitors, non-transportable DL-threo-β-benzyloxyaspartat (TBOA) and transportable trans-pyrrolidine-2,4-dicarboxylic acid (t-PDC), produced paradoxical antinociception in the formalin test. In normal rats, inhibition of the glutamate transporter increased extracellular glutamate. In the formalin model rats, TBOA suppressed while t-PDC enhanced glutamate release. When tPDC was pretreated 30min prior to formalin injection, glutamate release was blocked. Blocking α-2 adrenergic receptors reversed the tPDC analgesia. Increased apoptosis was not apparent in the spinal dorsal horn of tPDC-treated rats compared to the control group. These data suggest that glutamate transporters in a formalin-induced pain state work in a reverse mode and can be blocked from releasing glutamate by TBOA and preloaded tPDC. The analgesic mechanism of TBOA may be related to the blockade of inversely operating transporter, and that of tPDC may be associated with the activation of noradrenergic neurotransmission but not with dorsal horn neurotoxicity.

Anti-allodynic effect of intrathecal processed Aconitum jaluense is associated with the inhibition of microglial activation and P2X7 receptor expression in spinal cord

Background

For their analgesic and anti-arthritic effects, Aconitum species have been used in folk medicine in some East Asian countries. Although their analgesic effect is attributed to its action on voltage-dependent sodium channels, they also suppress purinergic receptor expression in dorsal root ganglion neurons in rats with neuropathic pain. In vitro study also demonstrated that the Aconitum suppresses ATP-induced P2X7 receptor (P2X7R)-mediated inflammatory responses in microglial cell lines. Herein, we examined the effect of intrathecal administration of thermally processed Aconitum jaluense (PA) on pain behavior, P2X7R expression and microglial activation in a rat spinal nerve ligation (SNL) model.

Methods

Mechanical allodynia induced by L5 SNL in Sprague-Dawley rats was measured using the von Frey test to evaluate the effect of intrathecal injection of PA. Changes in the expression of P2X7R in the spinal cord were examined using RT-PCR and Western blot analysis. In addition, the effect of intrathecal PA on microglial activation was evaluated by immunofluorescence.

Results

Intrathecal PA attenuated mechanical allodynia in a dose-dependent manner showing both acute and chronic effects with 65 % of the maximal possible effect. The expression and production of spinal P2X7R was increased five days after SNL, but daily intrathecal PA injection significantly inhibited the increase to the level of naïve animals. Immunofluorescence of the spinal cord revealed a significant increase in P2X7R expression and activation of microglia in the dorsal horn, which was inhibited by intrathecal PA treatment. P2X7R co-localized with microglia marker, but not neurons.

Conclusions

Intrathecal PA exerts anti-allodynic effects in neuropathic pain, possibly by suppressing P2X7R production and expression as well as reducing microglial activation in the spinal cord.

The Role of Spinal Dopaminergic Transmission in the Analgesic Effect of Nefopam on Rat Inflammatory Pain

Background

Nefopam has been known as an inhibitor of the reuptake of monoamines, and the noradrenergic and/or serotonergic system has been focused on as a mechanism of its analgesic action. Here we investigated the role of the spinal dopaminergic neurotransmission in the antinociceptive effect of nefopam administered intravenously or intrathecally.

Methods

The effects of intravenously and intrathecally administered nefopam were examined using the rat formalin test. Then we performed a microdialysis study to confirm the change of extracellular dopamine concentration in the spinal dorsal horn by nefopam. To determine whether the changes of dopamine level are associated with the nefopam analgesia, its mechanism was investigated pharmacologically via pretreatment with sulpiride, a dopaminergic D2 receptor antagonist.

Results

When nefopam was administered intravenously the flinching responses in phase I of the formalin test were decreased, but not those in phase II of the formalin test were decreased. Intrathecally injected nefopam reduced the flinching responses in both phases of the formalin test in a dose dependent manner. Microdialysis study revealed a significant increase of the level of dopamine in the spinal cord by intrathecally administered nefopam (about 3.8 fold the baseline value) but not by that administered intravenously. The analgesic effects of intrathecally injected nefopam were not affected by pretreatment with sulpiride, and neither were those of the intravenous nefopam.

Conclusions

Both the intravenously and intrathecally administered nefopam effectively relieved inflammatory pain in rats. Nefopam may act as an inhibitor of dopamine reuptake when delivered into the spinal cord. However, the analgesic mechanism of nefopam may not involve the dopaminergic transmission at the spinal level.

Opioid sparing effect and safety of nefopam in patient controlled analgesia after laparotomy: A randomized, double blind study

OBJECTIVES

A double-blind randomised study to evaluate the opioid sparing effect and safety of nefopam when administered via intravenous patient controlled analgesia (PCA) with fentanyl.

METHODS

Patients planned for elective open laparotomy, were randomly assigned to receive into fentanyl 25 µg/ml (SF group) or nefopam 2.4 mg/ml plus fentanyl 25 µg/ml (NF group). Patients were assessed before surgery and for 24 h postoperatively.

RESULTS

Total PCA fentanyl consumption was significantly lower in the NF group (n = 35) than the SF group (n = 36). Pain scores were significantly lower and patients' satisfaction with treatment significantly better in the NF group than the SF group. Dry mouth and dizziness were significantly more frequent in the NF group than the SF group. There were no other statistically significant between-group differences in the incidence of adverse events.

CONCLUSIONS

Intravenous PCA using nefopam + fentanyl following laparotomy has an opioid sparing effect and is associated with a low incidence of some of the typical opioid related adverse events.

TRIAL REGISTRY

Clinicaltrials.gov Registration No: NCT02596269.

Expression of the spinal 5-HT7 receptor and p-ERK pathway in the carrageenan inflammatory pain of rats

BACKGROUND

Although the inhibitory role of the 5-hydroxytrypatmine receptor 7(5-HT7R) on nociceptive processing is generally recognized, an excitatory effect associated with a reduced 5-HT7R expression has also been observed in the nerve injury model. In the carrageenan model, no significant effect is produced by the 5-HT7R activation, but the change in 5-HT7R expression has not been examined. Lesioning of the spinal serotonergic pathway enhances allodynia in the carrageenan model, but it also relieves several other pain states, including in the formalin model. While lesioning suppresses the activation of the extracellular signal-regulated kinase (ERK) of the spinal cord in the formalin model, its role in the carrageenan model has not been reported.

METHODS

Following intraplantar injections of carrageenan, the spinal 5-HT7R expression was examined using Western blotting in male Sprague-Dawley rats. The effect of serotonergic pathway lesioning with intrathecal 5,7-dihydroxytryptamine (5,7-DHT) on the expression of the phospho-ERK was measured.

RESULTS

The expression of the 5-HT7R in the carrageenan model was not significantly different from that of naive animals. The expression of the spinal p-ERK in the carrageenan model was significantly increased, but returned to the level of a naive rat 1 hour after the carrageenan injection. However, it remained significantly higher 1 hour after the injection in the animals treated with 5,7-DHT than in the naive and control rats.

CONCLUSIONS

The expression of the spinal 5-HT7R is not altered by peripheral inflammation with carrageenan, suggesting that the lack of antinociceptive effect of the 5-HT7R activation is partly attributable to the absence of changes in the expression of the 5-HT7R in the spinal cord. The extended increase of the spinal p-ERK might be related to the enhanced pain behavior in the animals with lesions of the spinal serotonergic pathway in the carrageenan model.

Intrathecal nefopam-induced antinociception through activation of descending serotonergic projections involving spinal 5-HT7 but not 5-HT3 receptors

We examined the involvement of spinal 5-HT(5-hydroxytryptamine) receptor 3(5-HT3R) and 7(5-HT7R) as well as the overall role of descending serotonergic projections in the analgesic effects of intrathecal(i.t.) nefopam for two rat models of formalin and paw incision test. I.t. nefopam produced an antinociceptive effect in a dose-dependent manner in both tests. Lesioning the spinal serotonergic projections using i.t. 5,7-dihydroxytryptamine(5,7-DHT) did not influence the intensity of allodynia in the paw incision test, but i.t. 5,7-DHT abolished the effect of nefopam. In the formain test, i.t. 5,7-DHT alone significantly diminished the flinches, but the effect of nefopam was not affected by i.t. 5,7-DHT. Antagonism study showed that i.t. 5-HT7R antagonist, SB269970 significantly blocked the antinociceptive effect of nefopam in both tests, but i.t. 5-HT3R antagonist, ondansetron has no influence on the effect of nefopam. The present study demonstrates that descending spinal serotonergic projections play a vital role in antinociceptive effect of i.t. nefopam in the paw incision test, but indeterminate in the formalin test. In both tests, the antinociceptive effect of i.t. nefopam involves the spinal 5-HT7R, but not 5-HT3R.