tration therefore did not influence the effect of DOR selective agonists. The above results indicate not only that minocycline potentiates analgesia after MOR and KOR agonists but also that DOR is a potentially important target in the search for new drugs that would be effective against neuropathic pain. doi:10.1371/journal.pone.0104420.g006 11 DOR Analgesia Is Microglia-Independent in Neuropathy Green), which is not sufficiently selective and can give false positive results. The TaqMan assay that we used is standardised and highly specific, and we confirmed the lack of DOR in microglia cell cultures by using Western blot and immunocytochemistry, therefore, our results are not consistent with those that were obtained by immunostaining by Turchan-Cholewo et al. and Thorlin et al.. Moreover studies by Turchan-Cholewo et al. used commercial antibodies against DOR, which are no longer recommended for such studies. Due to the lack of selective DOR antibody a unique genetic mouse model was developed in order to investigate the distribution of DOR in the nervous system. Opioid receptors have high degrees of homology, and antibodies may recognise other subtypes within the same family. DOR expression has also been observed on glial-like cells in the dentate gyrus and rat cervical spinal cord, but none of these authors distinguished between astrocytes and microglia. Our results concerning the absence of DOR in microglia are consistent with recently published results of pharmacological studies that were conducted by Merighi et al.. These authors have shown in primary PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19664276/ microglia cell cultures that DPDPE, a DOR agonist, does not change the level of protein for PKC after LPS stimulation, while morphine and DAMGO up-regulate this kinase. The inGS 1101 effectiveness of DPDPE in this study may be explained by the lack of DOR receptors in microglial cells. Horvath and DeLeo, showed that selective agonists of DOR1 DPDPE had no effect on microglial migration, which confirms our results that DOR, in contrast to MOR and KOR, is not present in microglial cells. DOR shows a functional profile that is distinct from that of MOR or KOR and plays an important role in chronic pain, for example DOR knockout mice showed augmented neuropathic pain. DOR agonists are poor analgesics in acute pain, but are 
highly effective following inflammatory or neuropathic pain. Happel et al. suggested in 2008 that morphine and DAMGO influence the immune system. For example, opioids alter macrophage functions and they modulate cytokine production and chemokine and chemokine receptor expression. Activation of proinflammatory chemokine receptors is known to downregulate the analgesic functions of opioid receptors, and this enhances the perception of pain. Horvath et al. showed that morphine increases microglial migration by means of an interaction between MOR and P2X4 receptors. This interaction is dependent on PI3K/Akt pathway activation. Under neuropathic pain, the phosphorylation of p38 MAPK in microglia results in increased synthesis of the proinflammatory cytokines IL1b, IL-6, and TNF-a. Spinal blockade of these cytokines is known to attenuate neuropathic pain and morphine tolerance. Therefore, in cases where MOR and KOR agonists target microglial signalling by inhibiting the actions of chemokines, ATP receptors, MMP-9, p38 MAPK, or/and proinflammatory cytokines improve their effectiveness. However, this is not the case with DOR agonists. This difference deserves future exploration. MOR ag
