Elsevier

Neuroscience

Volume 159, Issue 4, 10 April 2009, Pages 1244-1256
Neuroscience

Behavioural Neuroscience
Ultra-low dose naloxone restores the antinociceptive effect of morphine in pertussis toxin–treated rats and prevents glutamate transporter downregulation by suppressing the p38 mitogen-activated protein kinase signaling pathway

https://doi.org/10.1016/j.neuroscience.2009.01.058Get rights and content

Abstract

We previously demonstrated that ultra-low dose naloxone restores the antinociceptive effect of morphine in rats with pertussis toxin (PTX)–induced thermal hyperalgesia by reversing the downregulation of glutamate transporter (GT) expression and suppressing spinal neuroinflammation. In the present study, we examined the underlying mechanisms of this anti-inflammatory effect in PTX-treated rats, particularly on the expression of GTs. Male Wistar rats were implanted with an intrathecal catheter and, in some cases, with a microdialysis probe. All rats were injected intrathecally with saline (5 μl) or PTX (1 μg), then, 4 days later, were randomly assigned to receive a single injection of saline, ultra-low dose naloxone (15 ng), or the p38 mitogen-activated protein kinase (MAPK) inhibitor SB203580 (5 μg), followed by morphine injection (10 μg) 30 min later. Our results showed that PTX injection induced activation of microglia and a significant increase in P-p38 MAPK expression in the spinal cord. Ultra-low dose naloxone plus morphine significantly inhibited the effect of PTX on P-p38 MAPK expression in the spinal cord, while the p38 MAPK inhibitor SB203580 attenuated the PTX-induced mechanical allodynia, thermal hyperalgesia, increase in spinal cerebrospinal fluid excitatory amino acids, and downregulation of GTs. These results show that the restoration of the antinociceptive effect of morphine and GT expression in PTX-treated rats by ultra-low dose naloxone involves suppression of the p38 MAPK signal transduction cascade.

Section snippets

Animal preparation and intrathecal drug delivery

One hundred twenty-three 8-week-old male Wistar rats (320–370 g) were anaesthetized with phenobarbital (65 mg/kg, intraperitoneally; Sigma, MO, USA) and implanted with an intrathecal catheter; in 33, a microdialysis probe was also inserted via the atlanto-occipital membrane down to spinal cord segments L5–S3 for CSF dialysate collection. After catheterization, the rats were housed individually in their home cages for recovery and maintained on a 12-h light/dark cycle, with food and water freely

Ultra-low dose naloxone reverses PTX-induced mechanical allodynia

Fig. 1 shows the time course of the mechanical PWT test (A) and the area under the curve (AUC) for the results in A (B). The baseline value for the PWT in naïve rats was 49.32±0.23 g (n=5). Intrathecal morphine injection had no effect on the normal baseline. Four days after intrathecal PTX injection, the rats displayed a significant reduction in the mechanical PWT, with an average PWT of 27.73±3.96 g. Intrathecal injection of morphine (10 μg) resulted in a significant reversal of the reduction

Discussion

In the present study, PTX was found to induce p38 MAPK phosphorylation in the rat spinal cord dorsal horn, and this was attenuated by ultra-low dose naloxone. In addition, the p38 MAPK inhibitor SB203580 partially reversed the reduction of the antinociceptive effect of morphine, increase in CSF EAA levels, and the activation of microglia seen in PTX-treated rats. These results show that the suppressive effect of ultra-low dose naloxone on neuroinflammation and the increase in CSF EAAs in

Conclusion

Taking together our previous study (Tsai et al., 2008) and the present results, the possible cellular mechanisms of ultra-low dose of naloxone in treating neuropathic pain of PTX-treated rats are described in Fig. 11. PTX-treatment (1) inhibited glutamate metabolism enzyme glutamate dehydrogenase (GDH) and glutamine synthase (GS), which results in an increasing of EAAs accumulation in the synaptic junction, (2) increased P-p38 MAPK and pro-inflammatory cytokines expression, (3) subsequently

Acknowledgments

This study was supported by grants from the National Health Research Institutes, Taiwan (NHRI-EX97-9401NP), Tri-Service General Hospital (TSGH-C97-71), and the Chi-Mei Medical Center (CMNDMC9719). It was performed at the Nociception Signal Transduction Laboratory, Department of Anesthesiology, Tri-service General Hospital and the National Defense Medical Center.

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