Recently an increasing number of studies have highlighted

Recently, an increasing number of studies have highlighted the crucial role of nociceptive factors produced by microglia under neuropathic pain conditions (Mika et al., 2013; Popiolek-Barczyk and Mika, 2016). iNOS is an important pronociceptive factor that is strictly expressed by microglia/macrophages (Makuch et al., 2013, Possel et al., 2000). Zaprinast significantly decreased the spinal level of iNOS after nerve injury, which may be one reason for its antinociceptive effects. In nociceptive transmission, interleukins also play an important role. The IL-1 family consists of pro- and antinociceptive interleukins, such as IL-1beta/IL-1RA (Pilat et al., 2015) and IL-18/IL18BP (Pilat et al., 2016). The CCI-induced development of hypersensitivity was associated with the upregulation of pronociceptive factors IL-1beta, IL-6 and IL-18 in the spinal cord and DRG, which is in agreement with previous reports (Pilat et al., 2015, Pilat et al., 2016, DeLeo et al., 1996, Rojewska et al., 2014a; Lee et al., 2010). We demonstrated for the first time that zaprinast prevented the CCI-induced upregulation of IL-1beta in the DRG, as well as IL-6 and IL-18 in the spinal cord, which may be another reason for its strong antinociceptive properties. Our results correspond well with those of Haddad et al. (2002), who reported that zaprinast diminished IL-6 NVP 231 synthesis in an inflammatory model. However, zaprinast does not influence the expression of antinociceptive interleukins, such as IL-1RA (Pilat et al., 2015), IL-18BP (Pilat et al., 2016) and IL-10 (Sawada et al., 1999; Shen et al., 2013).
Conclusions
Acknowledgements This work was supported by the National Science Centre, Poland (SONATA 2015/17/D/NZ4/02284) and by statutory funds from the Institute of Pharmacology Polish Academy of Sciences, Department of Pain Pharmacology. A Piotrowska is a Ph.D. students funded by a scholarship from the National Centre of Scientific Leading sponsored by the Ministry of Science and Higher Education, Republic of Poland. A. Piotrowska supported by the Fundation for Polish Sciences (FNP) and L'Oréal Foundation and UNESCO For Women in Science..
Introduction G-protein-coupled receptors (GPCRs) are major targets of drug discovery and have been studied in detail with respect to structure and function. However, several members of GPCRs remain in the orphan status because they have not been matched with suitable endogenous activators. Thus, the true therapeutic potential of such orphan GPCRs remains unclear. The G-protein-coupled receptor 35 (GPR35) is one of the orphan receptors, originally discovered during a human genomic DNA screen [1] and has been found predominantly in immune and gastrointestinal tissues [2]. The discovery that kynurenic acid (KYNA), an endogenous tryptophan metabolite, previously known to inhibit NMDA receptors and α7 nicotinic acetylcholine receptors [3], acts as a GPR35 agonist [2] raised the possibility that it could be the endogenous activator of GPR35. However, the observations that high concentrations (7–66μM) are needed to activate rat and human GPR35 challenged the notion that KYNA is the endogenous agonist of this receptor [2], [4]. It is also assumed that GPR35 is an unlikely target for KYNA in the brain [5], [6], because different brain regions contain undetectable or very low levels of GPR35 protein [1], [2], [7]. Activation of GPR35 in neurons of the peripheral nervous system results in inhibition of cellular mechanisms that are known to facilitate synaptic transmission [5], [8]. For instance, activation of GPR35 heterologously expressed in sympathetic neurons results in inhibition of voltage-gated N-type Ca2+ channels [5]. Likewise, activation of GPR35 natively expressed by dorsal root ganglion neurons suppresses forskolin-induced cyclic-AMP (cAMP) production [8]. Therefore, if GPR35s were present in CNS neurons, they could affect neuronal excitability and synaptic transmission. Over the past decade, numerous surrogate ligands have been identified as GPR35 agonists. Their apparent potencies vary with the assay and the animal species from which the receptors originate [9]. For instance, the apparent potency (EC50) for agonists to activate rat GPR35 increases in the order amlexanox (23nM), zaprinast (98nM), dicumarol (2μM), cromolyn sodium (4.4μM), and pamoic acid (>1000μM) [9], [10]. In contrast, the EC50 for pamoic acid to activate human GPR35 is <100nM and that of zaprinast is 1μM [11]. Likewise, ML145 is a potent antagonist at the human GPR35 (IC50 close to 20nM) and a weak antagonist at the rat GPR35, while having no effect at the mouse ortholog [12], [13].