Ultra-Low-Dose Naloxone Restores the Antinociceptive Effect of Morphine and Suppresses Spinal Neuroinflammation in PTX-Treated Rats

Abstract

The aim of the present study was to examine the effect of ultra-low-dose naloxone on pertussis toxin (PTX)-induced thermal hyperalgesia in rats and its underlying mechanisms. Male Wistar rats, implanted with an intrathecal catheter with or without a microdialysis probe, received a single intrathecal injection of PTX (1 mg in 5 ml saline). Four days after PTX injection, they were randomly given a different dose of naloxone (either 15 mg or 15 ng in 5 ml saline), followed by a morphine injection (10 mg in 5 ml saline) after 30 min. The results found that PTX injection induced thermal hyperalgesia and increasing excitatory amino acid (EAA; L-glutamate and L-aspartate) concentration in the spinal CSF dialysates. Ultra-low-dose naloxone not only preserved the antinociceptive effect of morphine but also suppressed the PTX-evoked EAA release as well. Moreover, ultra-low-dose naloxone plus morphine administration inhibited the downregulation of L-glutamate transporters (GTs) and the L-glutamate-metabolizing enzyme glutamine synthetase (GS), and, moreover, inhibited microglial activation and suppressed cytokine expression in PTX-treated rat spinal cords. These results show that ultra-low-dose naloxone preserves the antinociceptive effect of morphine in PTX-treated rats. The mechanisms include (a) inhibition of pro-inflammatory cytokine expression, (b) attenuation of PTX-evoked EAA release, and (c) reversion of the downregulation of GT expression.

Phosphorylation of the GluN1 subunit in dorsal horn neurons by remifentanil: a mechanism for opioid-induced hyperalgesia

“…One study has demonstrated the efficacy of NMDAR antagonists for preventing OIH (Minville et al, 2010). Furthermore, a number of investigations found that clinical doses of naloxone antagonize opioid effects, whereas ultra-low doses enhance the antinociceptive effects of morphine (Tsai et al, 2008;Yang et al, 2011). In this study, we investigated the relationships among the GluN1 subunit, opioids, and OIH in search of an effective method to treat OIH.…”

Ultra-low dose naloxone restores the antinocicepitve effect of morphine in PTX-treated rats: Association of IL-10 upregulation in the spinal cord

“…Ultra-low dose naloxone suppresses microglia activation and induces IL-10 expression in spinal neuron and microglia in PTX-treated rats As in our previous study (Tsai et al, 2008(Tsai et al, , 2009b, weak immunostaining was observed in resting microglia (Fig. 1A).…”

“…In our previous studies, intrathecal administration of ultra-low dose naloxone restored the antinociceptive effect of morphine, revered the coupling of μ-opioid receptors from Gs-protein to Gi-protein (Tsai et al, 2009a), suppressed microglia activation, inhibited the expression of IL-1β, IL-6, and phosphorylated p38 (P-p38) mitogen-activated protein kinase (MAPK) (Tsai et al, 2008(Tsai et al, , 2009b in PTX-treated rats. However, the anti-inflammation mechanisms of ultra-low dose naloxone remain unclear.…”

“…Opioids are the therapeutic mainstay for pain alleviation, but are less effective in treating neuropathic pain (Bettoni et al, 2008). Several studies have indicated that neuropathic pain activates the neuroinflammatory cascade (Ji and Suter, 2007), downregulates glutamate transporter expression in spinal cord (Tai et al, 2007;Tsai et al, 2008), and downregulates μ-opioid receptor after nerve injury (Kohno et al, 2005).…”

Purinergic P2X Receptor Regulates N -Methyl-d-aspartate Receptor Expression and Synaptic Excitatory Amino Acid Concentration in Morphine-tolerant Rats

“…The concentrations of EAAs were measured by phenylisothiocyanate derivatization using a high-performance liquid chromatography (Agilent 1100; Agilent Technologies, Santa Clara, CA) with a reversedphase ZORBAX Eclipse amino acid analysis column (4.6 ϫ 150 mm 2 , 3.5 M) and fluorescent detector (Gilson model 121, set at 428 nm), as described previously. 25 External standards (authentic amino acids at concentrations of 156.25, 312.5, 625, 1,250, and 2,500 M) were run at the beginning and end of each sample group. Peak heights were normalized to the standard peaks and quantified based on the linear relationship between peak height and the amount of the corresponding standard.…”