This chronic phase is characterized by the presence of numerous cysts and inflammation in the brain, with activation of astrocytes and microglia (Hermes et al., 2008;Wilson and Hunter, 2004), as confirmed here by histological means. these KP metabolites were observed in the brain at 28 days post-infection. Notably, the anti-parasitic drugs pyrimethamine and sulfadiazine, a standard treatment of toxoplasmosis, significantly reduced 3-HK and KYNA levels in the brain of infected mice when applied between 28 and 56 days post-infection. In Prulifloxacin (Pruvel) summary,T. gondiiinfection, probably by activating microglia and astrocytes, enhances the production of KP metabolites in the brain. However, during the first two months after infection, the KP changes in these mice do not reliably duplicate abnormalities seen in the brain of individuals with schizophrenia. Keywords:Astrocytes, 3-Hydroxykynurenine, Kynurenic acid, Microglia, Quinolinic acid == 1. Introduction == Toxoplasma gondii, an obligate intracellular protozoan parasite, infects approximately one-third of the human population worldwide (Desmonts and Couvreur, 1974;Hofhuis et al., 2011). Initial transmission occurs orally, PDK1 and the parasite then starts its cycle of replication and dissemination with the tachyzoite stage (Dubey et al., 1998). During acute infection, the parasite is mainly present in peripheral tissues and blood, but also has access to the brainviacirculating cells (Courret et al., 2006;Da Gama et al., 2004). One early feature ofT. gondii-induced brain infection is the activation of glial cells, in particular astrocytes (Wilson and Hunter, 2004) and microglia (Hermes et al., 2008). The next stage is defined by the presence of bradyzoite tissue cysts in skeletal muscle and, most notably, in the brain. These dormant cysts are characteristic of chronic, latent infection (Dubey et al., 1998). In individuals suffering from acquired immune deficiencies, e.g. AIDS, or undergoing prolonged immune suppressive treatments, reactivation of the infection can lead to lethal toxoplasmic encephalitis (Dubey et al., 1998). Latent toxoplasmosis, although frequently dismissed as asymptomatic and clinically unimportant, alters host behavior in both humans and rodents (Burkinshaw et al., 1953;Flegr, 2007;Kannan and Pletnikov, 2012;Webster et al., 2006;Wilson et al., 1980). In addition and possibly related, a growing number of epidemiological studies associate persistentT. gondiiinfection with an increased likelihood of developing schizophrenia (Dickerson et al., 2007;Torrey et al., 2012). Notably, elevated anti-T. gondiiIgG antibody levels have been reported in patients with first-onset schizophrenia, suggesting an involvement of the parasite in the etiology of the disease (Torrey et al., 2007;Wang et al., 2006). In immunocompetent hosts, infection withT. gondiileads to the production of interferon- (IFN-) and, consequently, the induction of indoleamine 2,3-dioxygenase (IDO), which converts the essential amino acid tryptophan to kynurenine and inhibitsT. gondiigrowthin vitro(Dai et al., 1994;Dubener and MacKenzie, 1999) andin vivo(Silva et al., 2002). Kynurenine, in turn, is further degraded via a metabolic cascade – the kynurenine pathway (KP) -, which contains several neuroactive metabolites (kynurenines), such as 3-hydroxykynurenine (3-HK), a free radical generator, quinolinic acid (QUIN), an agonist of N-methyl-D-aspartate (NMDA) receptor, and kynurenic Prulifloxacin (Pruvel) acid (KYNA), an endogenous antagonist of 7 nicotinic acetylcholine and NMDA receptors (Fig. 1). These compounds play distinct roles in brain physiology and have also been linked to the etiology of schizophrenia, as well as other major brain diseases (seeSchwarcz et al., 2012, for review). == Fig. 1. == The kynurenine pathway of tryptophan degradation. As KP metabolites in the brain are synthesized primarily in microglial cells and astrocytes (Espey et al., 1997;Guillemin et al., 2001;Heyes et al., 1996), glial activation that occurs duringT. gondiiinfection may affect KP metabolism and thus provide a mechanistic link to the pathophysiology of schizophrenia (Schwarcz and Hunter, 2007). In susceptible mice, increased IDO mRNA expression afterT. gondiiinfection is accompanied by Prulifloxacin (Pruvel) elevations in the concentration of kynurenine in plasma, peripheral organs and brain (Engin et al., 2012;Fujigaki et al., 2002;Silva et al., 2002), and the presence of the parasite, IDO expression and kynurenine content in the brain were shown to peak approximately one month after the infection (Fujigaki et al., 2002;Silva et al., 2002). In these studies, no information was provided with regard to the fate of neuroactive downstream kynurenines, namely 3-HK, QUIN and KYNA, in either periphery or brain. We therefore designed experiments to fill this void and report here that substantial changes in KP metabolism take place inT. gondii-infected C57BL/6 mice over time. These changes turned out to be qualitatively and quantitatively distinct, and did not occur simultaneously, in the periphery and the brain. In addition, we tested the effect of a standard treatment of toxoplasmosis, the combination of pyrimethamine and sulfadiazine, on KP metabolism. == 2. Materials and methods == == 2.1 Chemicals == L-Tryptophan, DL-3-hydroxykynurenine (3-HK), kynurenic acid (KYNA), quinolinic acid (QUIN), pentafluoropropionic anhydride, 2,2,3,3,3-pentafluoro-1-propanol, [2H6]L-kynurenine were purchased from Sigma-Aldrich (St. Louis, MO, USA). L-Kynurenine sulfate (kynurenine; purity: 99.4%) was obtained from Sai Advantium (Hyderabad, India). [2H3]Quinolinic acid was purchased from Synfine Research (Richmond Hill, Ontario, Canada), and [2H5]L-tryptophan was obtained from.
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