Ergosterol Interestingly zaprinast administration not only p

Interestingly, zaprinast administration not only prevents the development of hypersensitivity but also enhances opioid effectiveness. Opioids have long been considered among the most effective drugs used to treat pain (Watkins et al., 2005; Mika et al., 2013; Szczudlik et al., 2014a, Szczudlik et al., 2014b); however, the changes that contribute to neuropathic pain development are responsible for the reduction in their antinociceptive properties. The mechanism underlying this phenomenon is still poorly understood. We demonstrated that the antinociceptive properties of intrathecally injected morphine and buprenorphine are similar in CCI-exposed rats but lower than in naïve rats (Kwiatkowski et al., 2016, Pilat et al., 2016). In the present study, we have shown for the first time that administration of zaprinast enhances the antinociceptive effects of both opioids. Similarly, in an inflammatory pain model, zaprinast enhanced morphine effectiveness (Yoon et al., 2006, Heo et al., 2005). Based on our results, we believe that the stronger analgesic effect of opioids co-administered with zaprinast involves mechanisms other than a direct influence on opioid Ergosterol biosynthesis. Undoubtedly, preemptive and repeated administration of zaprinast strongly inhibits the activation of microglia and, consequently, reduces the level of pronociceptive factors. There is evidence from the literature that iNOS, IL-1beta, IL-6, and IL-18 directly diminish opioid effectiveness under neuropathy (Ferreira et al., 1991, Granados-Soto et al., 1997, Hervera et al., 2012, Johnston et al., 2004, Makuch et al., 2013, Pilat et al., 2015, Pilat et al., 2016). Pentoxifylline has been shown to potentiate the effects of morphine in neuropathic pain models and inhibit microglial activation and the synthesis of IL-1beta and IL-6 (Liu et al., 2007, Lundblad et al., 1995, Mika et al., 2007, Okimura et al., 1996). We and others have shown that repeated administration of minocycline enhances the effectiveness of morphine by diminishing microglial activation and the spinal level of iNOS, IL-1beta, IL-6, and IL-18 in CCI-exposed animals (Johnston et al., 2004, Makuch et al., 2013, Mika et al., 2014, Osikowicz et al., 2008, Piotrowska et al., 2017, Popiolek-Barczyk et al., 2014, Popiolek-Barczyk et al., 2015, Rojewska et al., 2014a, Zhang et al., 2015). Similarly, repeated administration of zaprinast enhanced morphine and buprenorphine effectiveness by attenuating microglial activation and, consequently, reducing pronociceptive factors IL-1beta, IL-18, IL-6 and iNOS in the spinal cord and/or DRG in a neuropathic pain model. Most importantly, already known microglial modulators (pentoxifylline, minocycline) enhance the analgesic properties of morphine only when they are administered repeatedly for several days (Mika et al., 2007, Rojewska et al., 2014a, Rojewska et al., 2014b). Zaprinast is an extremely interesting substance because it enhances morphine analgesia even after a single injection on day 7 after CCI, when neuropathic pain symptoms are fully developed. However, the molecular mechanisms of this effect need to be studied further. In the literature, there is a lack of information about co-treatment with zaprinast and non-opioid analgesic drugs. The analgesic effect of zaprinast alone is similar to that of amitriptyline, doxepin, milnacipran, venlafaxine and fluoxetine in the CCI model (Zychowska et al., 2015); however, the analgesic effect of zaprinast is weaker than that of gabapentin and pregabalin (Kaul et al., 2018, Nakai et al., 2014). In our opinion, examining the influence of zaprinast on the analgesic effects of antiepileptics and antidepressant drugs, which are commonly used to treat neuropathic pain in the clinic, would be extremely interesting. Gabapentin and pregabalin are frequently used as antiepileptics; these drugs bind to the voltage-dependent calcium channel auxiliary subunit α2δ and exert an analgesic effect by suppressing the presynaptic calcium influx and inhibiting the release of excitatory neurotransmitters (Nishikawa and Nomoto, 2017). The antidepressants cause a balanced inhibition of serotonin/noradrenaline reuptake (Hatch et al., 2018). A combination of drugs with different mechanisms of action seems to be a way to enhance the effectiveness of pharmacotherapy; for example, minocycline enhances antidepressant (amitriptyline and milnacipran) analgesia under neuropathic pain (Zychowska et al., 2015). In our opinion, zaprinast, which has a broad spectrum of action, is an interesting, biologically active substance that has potential therapeutic use as a co-analgesic. However, the involvement of GPR35 receptor and phosphodiesterase in the mechanism of antinociceptive action of zaprinast is not fully clarified and requires future studies.