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Delta-opioid receptor analgesia is independent of microglial activation in a rat model of neuropathic pain.

Mika J, Popiolek-Barczyk K, Rojewska E, Makuch W, Starowicz K, Przewlocka B - PLoS ONE (2014)

Bottom Line: Pre-emptive and repeated administration of minocycline (30 mg/kg, i.p.) over 7 days significantly reduced allodynia and hyperalgesia as measured on day 7 after CCI.Additionally, nerve injury-induced down-regulation of all types of opioid receptors in the spinal cord and dorsal root ganglia was not influenced by minocycline, which indicates that the effects of opioid ligands are dependent on other changes, presumably neuroimmune interactions.In summary, DOR analgesia is different from analgesia induced by MOR and KOR receptors because it does not dependent on injury-induced microglial activation.

View Article: PubMed Central - PubMed

Affiliation: Department of Pain Pharmacology, Institute of Pharmacology, Polish Academy of Sciences, Krakow, Poland.

ABSTRACT
The analgesic effect of delta-opioid receptor (DOR) ligands in neuropathic pain is not diminished in contrast to other opioid receptor ligands, which lose their effectiveness as analgesics. In this study, we examine whether this effect is related to nerve injury-induced microglial activation. We therefore investigated the influence of minocycline-induced inhibition of microglial activation on the analgesic effects of opioid receptor agonists: morphine, DAMGO, U50,488H, DPDPE, Deltorphin II and SNC80 after chronic constriction injury (CCI) to the sciatic nerve in rats. Pre-emptive and repeated administration of minocycline (30 mg/kg, i.p.) over 7 days significantly reduced allodynia and hyperalgesia as measured on day 7 after CCI. The antiallodynic and antihyperalgesic effects of intrathecally (i.t.) administered morphine (10-20 µg), DAMGO (1-2 µg) and U50,488H (25-50 µg) were significantly potentiated in rats after minocycline, but no such changes were observed after DPDPE (10-20 µg), deltorphin II (1.5-15 µg) and SNC80 (10-20 µg) administration. Additionally, nerve injury-induced down-regulation of all types of opioid receptors in the spinal cord and dorsal root ganglia was not influenced by minocycline, which indicates that the effects of opioid ligands are dependent on other changes, presumably neuroimmune interactions. Our study of rat primary microglial cell culture using qRT-PCR, Western blotting and immunocytochemistry confirmed the presence of mu-opioid receptors (MOR) and kappa-opioid receptors (KOR), further we provide the first evidence for the lack of DOR on microglial cells. In summary, DOR analgesia is different from analgesia induced by MOR and KOR receptors because it does not dependent on injury-induced microglial activation. DOR agonists appear to be the best candidates for new drugs to treat neuropathic pain.

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The presence of opioid receptors on microglia cells - in vitro studies.We analysed the presence of opioid receptors in primary microglial cell cultures. Using reverse transcriptase-polymerase chain reaction, we found that mRNA for MOR and KOR, but not DOR, is expressed in rat primary microglial cells cultures (A). Western blot analysis (B) detected proteins for MOR and KOR, but not DOR, in microglia. The presence of MOR and KOR, but not DOR, were confirmed by immunocytochemistry (C), and we show the colocalisation of MOR (left panel) and KOR (middle panel) antibodies (red) with OX/42 antibodies (green). The scale bar for all microphotographs is 25 µm. The data from 3–10 cell cultures are presented.
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pone-0104420-g005: The presence of opioid receptors on microglia cells - in vitro studies.We analysed the presence of opioid receptors in primary microglial cell cultures. Using reverse transcriptase-polymerase chain reaction, we found that mRNA for MOR and KOR, but not DOR, is expressed in rat primary microglial cells cultures (A). Western blot analysis (B) detected proteins for MOR and KOR, but not DOR, in microglia. The presence of MOR and KOR, but not DOR, were confirmed by immunocytochemistry (C), and we show the colocalisation of MOR (left panel) and KOR (middle panel) antibodies (red) with OX/42 antibodies (green). The scale bar for all microphotographs is 25 µm. The data from 3–10 cell cultures are presented.

Mentions: Using a reverse transcriptase-polymerase chain reaction, we found that mRNA for MOR and KOR, but not for DOR, is expressed in rat primary microglial cell cultures (Fig. 5A). In the Western blot analysis, we found that protein for MOR and KOR is present in microglia (Fig. 5B). The protein for DOR was undetectable (Fig. 5B). The expression of MOR and KOR, but not DOR, in microglial cells was confirmed by immunocytochemistry (Fig. 5C).


Delta-opioid receptor analgesia is independent of microglial activation in a rat model of neuropathic pain.

Mika J, Popiolek-Barczyk K, Rojewska E, Makuch W, Starowicz K, Przewlocka B - PLoS ONE (2014)

The presence of opioid receptors on microglia cells - in vitro studies.We analysed the presence of opioid receptors in primary microglial cell cultures. Using reverse transcriptase-polymerase chain reaction, we found that mRNA for MOR and KOR, but not DOR, is expressed in rat primary microglial cells cultures (A). Western blot analysis (B) detected proteins for MOR and KOR, but not DOR, in microglia. The presence of MOR and KOR, but not DOR, were confirmed by immunocytochemistry (C), and we show the colocalisation of MOR (left panel) and KOR (middle panel) antibodies (red) with OX/42 antibodies (green). The scale bar for all microphotographs is 25 µm. The data from 3–10 cell cultures are presented.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4126741&req=5

pone-0104420-g005: The presence of opioid receptors on microglia cells - in vitro studies.We analysed the presence of opioid receptors in primary microglial cell cultures. Using reverse transcriptase-polymerase chain reaction, we found that mRNA for MOR and KOR, but not DOR, is expressed in rat primary microglial cells cultures (A). Western blot analysis (B) detected proteins for MOR and KOR, but not DOR, in microglia. The presence of MOR and KOR, but not DOR, were confirmed by immunocytochemistry (C), and we show the colocalisation of MOR (left panel) and KOR (middle panel) antibodies (red) with OX/42 antibodies (green). The scale bar for all microphotographs is 25 µm. The data from 3–10 cell cultures are presented.
Mentions: Using a reverse transcriptase-polymerase chain reaction, we found that mRNA for MOR and KOR, but not for DOR, is expressed in rat primary microglial cell cultures (Fig. 5A). In the Western blot analysis, we found that protein for MOR and KOR is present in microglia (Fig. 5B). The protein for DOR was undetectable (Fig. 5B). The expression of MOR and KOR, but not DOR, in microglial cells was confirmed by immunocytochemistry (Fig. 5C).

Bottom Line: Pre-emptive and repeated administration of minocycline (30 mg/kg, i.p.) over 7 days significantly reduced allodynia and hyperalgesia as measured on day 7 after CCI.Additionally, nerve injury-induced down-regulation of all types of opioid receptors in the spinal cord and dorsal root ganglia was not influenced by minocycline, which indicates that the effects of opioid ligands are dependent on other changes, presumably neuroimmune interactions.In summary, DOR analgesia is different from analgesia induced by MOR and KOR receptors because it does not dependent on injury-induced microglial activation.

View Article: PubMed Central - PubMed

Affiliation: Department of Pain Pharmacology, Institute of Pharmacology, Polish Academy of Sciences, Krakow, Poland.

ABSTRACT
The analgesic effect of delta-opioid receptor (DOR) ligands in neuropathic pain is not diminished in contrast to other opioid receptor ligands, which lose their effectiveness as analgesics. In this study, we examine whether this effect is related to nerve injury-induced microglial activation. We therefore investigated the influence of minocycline-induced inhibition of microglial activation on the analgesic effects of opioid receptor agonists: morphine, DAMGO, U50,488H, DPDPE, Deltorphin II and SNC80 after chronic constriction injury (CCI) to the sciatic nerve in rats. Pre-emptive and repeated administration of minocycline (30 mg/kg, i.p.) over 7 days significantly reduced allodynia and hyperalgesia as measured on day 7 after CCI. The antiallodynic and antihyperalgesic effects of intrathecally (i.t.) administered morphine (10-20 µg), DAMGO (1-2 µg) and U50,488H (25-50 µg) were significantly potentiated in rats after minocycline, but no such changes were observed after DPDPE (10-20 µg), deltorphin II (1.5-15 µg) and SNC80 (10-20 µg) administration. Additionally, nerve injury-induced down-regulation of all types of opioid receptors in the spinal cord and dorsal root ganglia was not influenced by minocycline, which indicates that the effects of opioid ligands are dependent on other changes, presumably neuroimmune interactions. Our study of rat primary microglial cell culture using qRT-PCR, Western blotting and immunocytochemistry confirmed the presence of mu-opioid receptors (MOR) and kappa-opioid receptors (KOR), further we provide the first evidence for the lack of DOR on microglial cells. In summary, DOR analgesia is different from analgesia induced by MOR and KOR receptors because it does not dependent on injury-induced microglial activation. DOR agonists appear to be the best candidates for new drugs to treat neuropathic pain.

Show MeSH
Related in: MedlinePlus