Prolonged nicotine administration results in biphasic, brain-specific changes in kynurenate levels in the rat

Kynurenine Metabolites in CSF and Plasma in Healthy Males

In recent years, kynurenine metabolites generated by tryptophan catabolism have gained increasing attention in the context of brain diseases. The question of importance is whether there is a relationship between peripheral and central levels of these metabolites. Some of these compounds do not cross the blood-brain barrier; in particular, kynurenic acid, and most analyses of kynurenines from psychiatric patients have been performed using plasma samples. In the present study, we recruited 30 healthy volunteers with no history of psychiatric or neurological diagnosis, to analyze tryptophan, kynurenine, kynurenic acid, and quinolinic acid levels in CSF and plasma. In addition, kynurenic acid was analyzed in urine. The most important finding of this study is that CSF kynurenic acid levels do not correlate with those in plasma or urine. However, we found a correlation between plasma kynurenine and CSF kynurenic acid. Further, plasma kynurenine and plasma quinolinic acid were correlated. Our findings clarify the distribution of tryptophan and its metabolites in various body compartments and may serve as a guide for the analysis of these metabolites in humans. The most significant finding of the present study is that a prediction of brain kynurenic acid by of the analysis of the compound in plasma cannot be made.

Integrative Pharmacology in the Treatment of Substance Use Disorders

The detrimental physical, mental, and socioeconomic effects of substance use disorders (SUDs) have been apparent to the medical community for decades. However, it has become increasingly urgent in recent years to develop novel pharmacotherapies to treat SUDs. Currently, practitioners typically rely on monotherapy. Monotherapy has been shown to be superior to no treatment at all for most substance classes. However, many randomized controlled trials (RCTs) have revealed that monotherapy leads to poorer outcomes when compared with combination treatment in all specialties of medicine. The results of RCTs suggest that monotherapy frequently fails since multiple dysregulated pathways, enzymes, neurotransmitters, and receptors are involved in the pathophysiology of SUDs. As such, research is urgently needed to determine how various neurobiological mechanisms can be targeted by novel combination treatments to create increasingly specific yet exceedingly comprehensive approaches to SUD treatment. This article aims to review the neurobiology that integrates many pathophysiologic mechanisms and discuss integrative pharmacology developments that may ultimately improve clinical outcomes for patients with SUDs. Many neurobiological mechanisms are known to be involved in SUDs including dopaminergic, nicotinic, N-methyl-D-aspartate (NMDA), and kynurenic acid (KYNA) mechanisms. Emerging evidence indicates that KYNA, a tryptophan metabolite, modulates all these major pathophysiologic mechanisms. Therefore, achieving KYNA homeostasis by harmonizing integrative pathophysiology and pharmacology could prove to be a better therapeutic approach for SUDs. We propose KYNA-NMDA-α7nAChRcentric pathophysiology, the "conductor of the orchestra," as a novel approach to treat many SUDs concurrently. KYNA-NMDA-α7nAChR pathophysiology may be the "command center" of neuropsychiatry. To date, extant RCTs have shown equivocal findings across comparison conditions, possibly because investigators targeted single pathophysiologic mechanisms, hit wrong targets in underlying pathophysiologic mechanisms, and tested inadequate monotherapy treatment. We provide examples of potential combination treatments that simultaneously target multiple pathophysiologic mechanisms in addition to KYNA. Kynurenine pathway metabolism demonstrates the greatest potential as a target for neuropsychiatric diseases. The investigational medications with the most evidence include memantine, galantamine, and N-acetylcysteine. Future RCTs are warranted with novel combination treatments for SUDs. Multicenter RCTs with integrative pharmacology offer a promising, potentially fruitful avenue to develop novel therapeutics for the treatment of SUDs.

Associations between expression of indoleamine 2, 3-dioxygenase enzyme and inflammatory cytokines in patients with first-episode drug-naive Schizophrenia

The indoleamine 2,3-dioxygenase (IDO) enzyme is the first rate-limiting enzyme of the tryptophan degradation pathway in which dysfunction of neuroactive metabolites has been implicated in the pathophysiology of schizophrenia. Inflammatory molecules such as pro-inflammatory cytokines could enhance the activity of IDO. There are few studies on the expression of IDO levels and its correlation with levels of inflammatory cytokines in first-episode drug-naive patients with schizophrenia. One hundred inpatients (female = 33, male = 67) with first-episode drug-naive schizophrenia entered a 6-week, double-blind, randomized, placebo-controlled clinical trial. All individuals were assigned celecoxib or placebo combined with risperidone. Serum levels of IDO and six inflammatory cytokines (IL-1β, IL-6, TNF-α IL-17, IL-4, and INF-γ) were measured. The Positive and Negative Syndrome Scale (PANSS) was used to assess the severity of psychotic symptoms. Compared to healthy subjects, patients had significantly elevated levels of IDO and six cytokines at baseline. Over the 6-week treatment period, the decrease in the levels of IDO and TNF-α and the improvement in the PANSS total score, positive scores, and negative scores in the celecoxib group were significantly greater than in the placebo group. There was a significantly positive correlation between IDO levels and the PANSS negative scores and between IDO levels and TNF-α and IFN-γ levels in the celecoxib group. These findings showed abnormal expression of IDO levels which correlated with negative symptoms and pro-inflammatory cytokine levels in patients with first-episode drug-naive schizophrenia, suggesting the important role of IDO in the pathological mechanism of schizophrenia. Registration number: ChiCTR2000041403.

Influence of plasma cytokines on kynurenine and kynurenic acid in schizophrenia

Abnormalities in the kynurenine pathway (KP) of tryptophan degradation, leading to the dysfunction of neuroactive KP metabolites in the brain, have been implicated in the pathophysiology of schizophrenia (SZ). One plausible mechanism involves dysregulation of various pro-inflammatory cytokines associated with the disease, which affect indoleamine-2,3-dioxygenase (IDO), a key enzyme for tryptophan to kynurenine conversion. In order to test this hypothesis directly, we measured plasma levels of the major KP metabolites kynurenine and kynurenic acid (KYNA), as well as four major cytokines, in a sample of 106 SZ patients and 104 control participants. In contrast to the replicable findings of elevation of KYNA in the central nervous system in SZ, plasma levels of KYNA were significantly lower in SZ compared to controls (p = .004). Kynurenine levels were significantly correlated with levels of interferon-γ (p < .001), which is involved in the regulation of IDO, in both patients and controls. However, although patients had higher levels of interleukin-6 (IL-6) compared to controls (p = .012), IL-6 levels were not correlated with kynurenine or KYNA, and did not explain group differences in KYNA. Based on the lack of evidence that pro-inflammatory cytokines were significantly related to the KP abnormality in SZ despite an adequate sample size, further studies must consider alternative hypotheses to identify the origins of the KP abnormalities in SZ.

Attenuating Nicotine Reinforcement and Relapse by Enhancing Endogenous Brain Levels of Kynurenic Acid in Rats and Squirrel Monkeys

The currently available antismoking medications have limited efficacy and often fail to prevent relapse. Thus, there is a pressing need for newer, more effective treatment strategies. Recently, we demonstrated that enhancing endogenous levels of kynurenic acid (KYNA, a neuroinhibitory product of tryptophan metabolism) counteracts the rewarding effects of cannabinoids by acting as a negative allosteric modulator of α7 nicotinic receptors (α7nAChRs). As the effects of KYNA on cannabinoid reward involve nicotinic receptors, in the present study we used rat and squirrel monkey models of reward and relapse to examine the possibility that enhancing KYNA can counteract the effects of nicotine. To assess specificity, we also examined models of cocaine reward and relapse in monkeys. KYNA levels were enhanced by administering the kynurenine 3-monooxygenase (KMO) inhibitor, Ro 61-8048. Treatment with Ro 61-8048 decreased nicotine self-administration in rats and monkeys, but did not affect cocaine self-administration. In rats, Ro 61-8048 reduced the ability of nicotine to induce dopamine release in the nucleus accumbens shell, a brain area believed to underlie nicotine reward. Perhaps most importantly, Ro 61-8048 prevented relapse-like behavior when abstinent rats or monkeys were reexposed to nicotine and/or cues that had previously been associated with nicotine. Ro 61-8048 was also effective in monkey models of cocaine relapse. All of these effects of Ro 61-8048 in monkeys, but not in rats, were reversed by pretreatment with a positive allosteric modulator of α7nAChRs. These findings suggest that KMO inhibition may be a promising new approach for the treatment of nicotine addiction.

Abnormal kynurenine pathway of tryptophan catabolism in cardiovascular diseases

Kynurenine pathway (KP) is the primary path of tryptophan (Trp) catabolism in most mammalian cells. The KP generates several bioactive catabolites, such as kynurenine (Kyn), kynurenic acid (KA), 3-hydroxykynurenine (3-HK), xanthurenic acid (XA), and 3-hydroxyanthranilic acid (3-HAA). Increased catabolite concentrations in serum are associated with several cardiovascular diseases (CVD), including heart disease, atherosclerosis, and endothelial dysfunction, as well as their risk factors, including hypertension, diabetes, obesity, and aging. The first catabolic step in KP is primarily controlled by indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO). Following this first step, the KP has two major branches, one branch is mediated by kynurenine 3-monooxygenase (KMO) and kynureninase (KYNU) and is responsible for the formation of 3-HK, 3-HAA, and quinolinic acid (QA); and another branch is controlled by kynurenine amino-transferase (KAT), which generates KA. Uncontrolled Trp catabolism has been demonstrated in distinct CVD, thus, understanding the underlying mechanisms by which regulates KP enzyme expression and activity is paramount. This review highlights the recent advances on the effect of KP enzyme expression and activity in different tissues on the pathological mechanisms of specific CVD, KP is an inflammatory sensor and modulator in the cardiovascular system, and KP catabolites act as the potential biomarkers for CVD initiation and progression. Moreover, the biochemical features of critical KP enzymes and principles of enzyme inhibitor development are briefly summarized, as well as the therapeutic potential of KP enzyme inhibitors against CVD is briefly discussed.

Kynurenic acid inhibits glutamatergic transmission to CA1 pyramidal neurons via α7 nAChR-dependent and -independent mechanisms

Glutamatergic hypofunction and elevated levels of kynurenic acid (KYNA) in the brain are common features of patients with schizophrenia. In vivo studies indicate that in the hippocampus KYNA decreases glutamate levels, presumably via inhibition of α7 nicotinic receptors (nAChRs). Here we tested the hypothesis that basal synaptic glutamate activity in the hippocampus is regulated by tonically active α7 nAChRs and is sensitive to inhibition by KYNA. To this end, spontaneous excitatory postsynaptic currents (EPSCs), sensitive to AMPA receptor antagonist CNQX (10 μM), were recorded from CA1 pyramidal neurons at -70 mV in rat hippocampal slices. The α7 nAChR antagonists α-bungarotoxin (α-BGT, 100 nM) and methyllycaconitine (MLA, 1-50 nM), and the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid (APV, 50 μM) reduced the frequency of EPSCs. MLA and α-BGT had no effect on miniature EPSCs (mEPSCs). The effect of MLA decreased in the presence of APV (50 μM), with 1 nM MLA becoming completely ineffective. KYNA (1-20 μM) suppressed the frequency of EPSCs, without affecting mEPSCs. The effect of KYNA decreased in the presence of MLA (1 nM) or α-BGT (100 nM), with 1 μM KYNA being devoid of any effect. In the presence of both MLA (10 nM) and APV (50 μM) higher KYNA concentrations (5-20 μM) still reduced the frequency of EPSCs. These results suggest that basal synaptic glutamate activity in CA1 pyramidal neurons is maintained in part by tonically active α7 nAChRs and NMDA receptors and is inhibited by micromolar concentrations of KYNA, acting via α7 nAChR-dependent and -independent mechanisms.

Exposure to kynurenic acid during adolescence produces memory deficits in adulthood

The glia-derived molecule kynurenic acid (KYNA) is an antagonist of α7 nicotinic acetylcholine receptors and the glycine(B) binding site on n-methyl-d-aspartateglutamate receptors, both of which have critical roles in neural plasticity as well as learning and memory. KYNA levels are increased in the brains and cerebral spinal fluid of persons with schizophrenia, leading to the notion that changes in KYNA concentration might contribute to cognitive dysfunction associated with this disorder. Indeed, recent studies indicate that increasing endogenous KYNA concentration by administering l-kynurenine (L-KYN, the precursor of KYNA) impairs spatial as well as contextual learning and memory in adult rats. In the present study, rats were treated with L-KYN (100 mg/kg) throughout adolescence to increase endogenous KYNA concentration during this critical time in brain development. Rats were then tested drug-free as adults to test the hypothesis that exposure to elevated levels of KYNA during development may contribute to cognitive dysfunction later in life. Consistent with prior studies in which adult rats were treated acutely with L-KYN, juvenile rats exposed to increased KYNA concentration during adolescence exhibited deficits in contextual fear memory, but cue-specific fear memory was not impaired. In addition, rats treated with L-KYN as adolescents were impaired on a novel object recognition memory task when tested as adults. The memory deficits could not be explained by drug-induced changes in locomotor activity or shock sensitivity. Together, these findings add to the growing literature supporting the notion that exposure to increased concentration of KYNA may contribute to cognitive deficits typically observed in schizophrenia.

Mammalian nicotinic acetylcholine receptors: from structure to function

The classical studies of nicotine by Langley at the turn of the 20th century introduced the concept of a "receptive substance," from which the idea of a "receptor" came to light. Subsequent studies aided by the Torpedo electric organ, a rich source of muscle-type nicotinic receptors (nAChRs), and the discovery of alpha-bungarotoxin, a snake toxin that binds pseudo-irreversibly to the muscle nAChR, resulted in the muscle nAChR being the best characterized ligand-gated ion channel hitherto. With the advancement of functional and genetic studies in the late 1980s, the existence of nAChRs in the mammalian brain was confirmed and the realization that the numerous nAChR subtypes contribute to the psychoactive properties of nicotine and other drugs of abuse and to the neuropathology of various diseases, including Alzheimer's, Parkinson's, and schizophrenia, has since emerged. This review provides a comprehensive overview of these findings and the more recent revelations of the impact that the rich diversity in function and expression of this receptor family has on neuronal and nonneuronal cells throughout the body. Despite these numerous developments, our understanding of the contributions of specific neuronal nAChR subtypes to the many facets of physiology throughout the body remains in its infancy.

A systematic review of off-label uses of memantine for psychiatric disorders

Recent data points to glutamatergic dysfunction in mood disorders, anxiety disorders, obsessive-compulsive disorder, and schizophrenia. Memantine, a drug approved by the FDA for the treatment of moderate to severe Alzheimer's disease that acts at the N-methyl-d-aspartate receptor, has been used off-label for various psychiatric disorders. Although promising, the available data for the use of memantine in these disorders is limited. Given this data, the routine use of memantine for depression, schizophrenia, obsessive-compulsive disorder, substance abuse, pervasive developmental disorders, bipolar disorder, and binge eating disorder cannot be recommended at this time.

Nicotine and nicotinic system in hypoglutamatergic models of schizophrenia

Schizophrenia is a complex neuropsychiatric disorder with devastating consequences. It is characterized by thought fragmentation, hallucination and delusion, collectively referred to as positive symptoms. In addition, mood changes or affective disorders, referred to as negative symptoms, as well as cognitive impairments can be manifested in these patients. Arguably, modeling such a disorder in its entirety in animals might not be feasible. Despite this limitation, various models with significant construct, predictive and some face validity have been developed. One such model, based on hypoglutamatergic hypothesis of schizophrenia, makes use of administering NMDA receptor antagonists and evaluating behavioral paradigms such as sensorimotor gating. Because of very high incidence of smoking among schizophrenic patients, it has been postulated that some of these patients may actually be self medicating with tobacco's nicotine. Research on nicotinic-glutamatergic interactions using various animal models has yielded conflicting results. In this review, some of these models and possible confounding factors are discussed. Overall, a therapeutic potential for nicotinic agonists in schizophrenia can be suggested. Moreover, it is evident that various experimental paradigms or models of schizophrenia symptoms need to be combined to provide a wider spectrum of the behavioral phenotype, as each model has its inherent limitations.

Increased concentration of cerebral kynurenic acid alters stimulus processing and conditioned responding

Kynurenic acid (KYNA) is a tryptophan metabolite synthesized and released by glia and recently shown to be a non-competitive antagonist of alpha7 nicotinic acetylcholine receptors at physiologically relevant concentrations, and NMDA receptors at higher concentrations. KYNA concentration is elevated in individuals with schizophrenia and those with Alzheimer's disease, two populations exhibiting cholinergic-related cognitive impairments. The present study investigated the effects of elevated KYNA concentration on conditioned stimulus processing in rats. For the first 2 days of the experiment, a subset of rats received intracerebroventricular infusions of either KYNA (0.1 microM) or vehicle and were either returned to the home cage or received non-reinforced presentations of a visual stimulus. All rats subsequently received presentations of the same visual stimulus followed by food reward during a 6-day training phase. In vehicle-treated rats, pre-exposure to the visual stimulus reduced orienting behaviour to the light (standing on the hind legs and orienting towards the visual stimulus) when it was later reinforced (i.e., conditioned orienting). In contrast, pre-exposure to the visual cue or 2 days of KYNA pretreatment reduced conditioned orienting behaviour. Finally, the reduction of orienting in KYNA-treated rats following pre-exposure was not as robust as in vehicle-treated rats. These results suggest that elevated KYNA levels can alter specific aspects of attentional processing of environmental stimuli and are discussed in terms of the potential contribution of KYNA to cognitive function and dysfunction.

Spontaneous alternation behaviour in rats: Kynurenic acid attenuated deficits induced by MK-801

The present study was undertaken to investigate the effects of pharmacological modulation of the NMDA receptors on spontaneous alternation behaviour. The performance of rats treated with MK-801 and kynurenic acid (KYNA) was assessed in the cross-arm-maze. We evaluated: (a) the total number of arm entries representing locomotor activity, (b) spontaneous variation of different arms thought to reflect alternation performance. In the first experiment, MK-801 (0.01, 0.025, 0.05, 0.1 and 0.2 mg/kg, i.p.) was given 30 min prior to the testing. Beginning the dose of 0.05 mg/kg the drug increased locomotion and impaired alternation performance. An ability of animals to enter subsequently three or four different arms was reduced significantly. In the second experiment, the dose of 0.05 mg/kg was chosen as the lowest possible dose of MK-801 producing marked behavioural impairment. KYNA (0.3, 3 and 30 mg/kg, s.c.) was administered 60 min prior to the MK-801. While all KYNA doses prevented hyperlocomotion, only the highest dose (30 mg/kg) maintained alternation score at the control levels, i.e. the KYNA plus MK-801 treated animals alternated regularly three or four different arms. The results suggest different sensitivity of the two behavioural systems, i.e. locomotion and space orientation, towards pharmacological insult. In conclusion, the study confirmed protective behavioural effects of KYNA given in sufficient amounts and sufficiently long prior MK-801.