Galantamine inhibits slowly inactivating K+ currents with a dual dose–response relationship in differentiated N1E-115 cells and in CA1 neurones

Does kynurenic acid act on nicotinic receptors? An assessment of the evidence

As a major metabolite of kynurenine in the oxidative metabolism of tryptophan, kynurenic acid is of considerable biological and clinical importance as an endogenous antagonist of glutamate in the central nervous system. It is most active as an antagonist at receptors sensitive to N-methyl-D-aspartate (NMDA) which regulate neuronal excitability and plasticity, brain development and behaviour. It is also thought to play a causative role in hypo-glutamatergic conditions such as schizophrenia, and a protective role in several neurodegenerative disorders, notably Huntington's disease. An additional hypothesis, that kynurenic acid could block nicotinic receptors for acetylcholine in the central nervous system has been proposed as an alternative mechanism of action of kynurenate. However, the evidence for this alternative mechanism is highly controversial, partly because at least eight earlier studies concluded that kynurenic acid blocked NMDA receptors but not nicotinic receptors and five subsequent, independent studies designed to repeat the results have failed to do so. Many studies considered to support the alternative 'nicotinic' hypothesis have been based on the use of analogs of kynurenate such as 7-chloro-kynurenic acid, or putatively nicotinic modulators such as galantamine, but a detailed analysis of the pharmacology of these compounds suggests that the results have often been misinterpreted, especially since the pharmacology of galantamine itself has been disputed. This review examines the evidence in detail, with the conclusion that there is no confirmed, reliable evidence for an antagonist activity of kynurenic acid at nicotinic receptors. Therefore, since there is overwhelming evidence for kynurenate acting at ionotropic glutamate receptors, especially NMDAR glutamate and glycine sites, with some activity at GPR35 sites and Aryl Hydrocarbon Receptors, results with kynurenic acid should be interpreted only in terms of these confirmed sites of action.

The kynurenine pathway and the brain: Challenges, controversies and promises

Research on the neurobiology of the kynurenine pathway has suffered years of relative obscurity because tryptophan degradation, and its involvement in both physiology and major brain diseases, was viewed almost exclusively through the lens of the well-established metabolite serotonin. With increasing recognition that kynurenine and its metabolites can affect and even control a variety of classic neurotransmitter systems directly and indirectly, interest is expanding rapidly. Moreover, kynurenine pathway metabolism itself is modulated in conditions such as infection and stress, which are known to induce major changes in well-being and behaviour, so that kynurenines may be instrumental in the etiology of psychiatric and neurological disorders. It is therefore likely that the near future will not only witness the discovery of additional physiological and pathological roles for brain kynurenines, but also ever-increasing interest in drug development based on these roles. In particular, targeting the kynurenine pathway with new specific agents may make it possible to prevent disease by appropriate pharmacological or genetic manipulations. The following overview focuses on areas of kynurenine research which are either controversial, of major potential therapeutic interest, or just beginning to receive the degree of attention which will clarify their relevance to neurobiology and medicine. It also highlights technical issues so that investigators entering the field, and new research initiatives, are not misdirected by inappropriate experimental approaches or incorrect interpretations at this time of skyrocketing interest in the subject matter. This article is part of the Special Issue entitled 'The Kynurenine Pathway in Health and Disease'.

Ion Fluxes through KCa2 (SK) and Cav1 (L-type) Channels Contribute to Chronoselectivity of Adenosine A1 Receptor-Mediated Actions in Spontaneously Beating Rat Atria

Impulse generation in supraventricular tissue is inhibited by adenosine and acetylcholine via the activation of A₁ and M₂ receptors coupled to inwardly rectifying GIRK/K_IR3.1/3.4 channels, respectively. Unlike M₂ receptors, bradycardia produced by A₁ receptors activation predominates over negative inotropy. Such difference suggests that other ion currents may contribute to adenosine chronoselectivity. In isolated spontaneously beating rat atria, blockade of K_Ca2/SK channels with apamin and Ca_v1 (L-type) channels with nifedipine or verapamil, sensitized atria to the negative inotropic action of the A₁ agonist, R-PIA, without affecting the nucleoside negative chronotropy. Patch-clamp experiments in the whole-cell configuration mode demonstrate that adenosine, via A₁ receptors, activates the inwardly-rectifying GIRK/K_IR3.1/K_IR3.4 current resulting in hyperpolarization of atrial cardiomyocytes, which may slow down heart rate. Conversely, the nucleoside inactivates a small conductance Ca²⁺-activated K_Ca2/SK outward current, which eventually reduces the repolarizing force and thereby prolong action potentials duration and Ca²⁺ influx into cardiomyocytes. Immunolocalization studies showed that differences in A₁ receptors distribution between the sinoatrial node and surrounding cardiomyocytes do not afford a rationale for adenosine chronoselectivity. Immunolabelling of K_IR3.1, K_Ca2.2, K_Ca2.3, and Ca_v1 was also observed throughout the right atrium. Functional data indicate that while both A₁ and M₂ receptors favor the opening of GIRK/K_IR3.1/3.4 channels modulating atrial chronotropy, A₁ receptors may additionally restrain K_Ca2/SK activation thereby compensating atrial inotropic depression by increasing the time available for Ca²⁺ influx through Ca_v1 (L-type) channels.

m-Cresol affects the lipid bilayer in membrane models and living neurons

A preferential interaction of m-cresol with high dipole-potential cholesterol/sphingomyelin-enriched lipid domains jeopardizes membrane integrity, explaining the toxicity of m-cresol-containing insulin formulations.

The role of intracellular zinc release in aging, oxidative stress, and Alzheimer's disease

Brain aging is marked by structural, chemical, and genetic changes leading to cognitive decline and impaired neural functioning. Further, aging itself is also a risk factor for a number of neurodegenerative disorders, most notably Alzheimer’s disease (AD). Many of the pathological changes associated with aging and aging-related disorders have been attributed in part to increased and unregulated production of reactive oxygen species (ROS) in the brain. ROS are produced as a physiological byproduct of various cellular processes, and are normally detoxified by enzymes and antioxidants to help maintain neuronal homeostasis. However, cellular injury can cause excessive ROS production, triggering a state of oxidative stress that can lead to neuronal cell death. ROS and intracellular zinc are intimately related, as ROS production can lead to oxidation of proteins that normally bind the metal, thereby causing the liberation of zinc in cytoplasmic compartments. Similarly, not only can zinc impair mitochondrial function, leading to excess ROS production, but it can also activate a variety of extra-mitochondrial ROS-generating signaling cascades. As such, numerous accounts of oxidative neuronal injury by ROS-producing sources appear to also require zinc. We suggest that zinc deregulation is a common, perhaps ubiquitous component of injurious oxidative processes in neurons. This review summarizes current findings on zinc dyshomeostasis-driven signaling cascades in oxidative stress and age-related neurodegeneration, with a focus on AD, in order to highlight the critical role of the intracellular liberation of the metal during oxidative neuronal injury.

Functional expression of large-conductance Ca2+-activated potassium channels in lateral globus pallidus neurons

The presence of large-conductance Ca(2+)-activated potassium (BK) channels, which are considered to play an important role in the excitability of neurons, in the highly-excitable lateral globus pallidus (LGP) neurons has yet to be confirmed. In this study, we confirmed the functional expression of BK channels in mouse LGP neurons and investigated the characteristics of their single-channel currents using inside-out patch-clamp recordings. These BK channels had a conductance of 276 pS, were activated by the elevation of both the transmembrane potential and intracellular calcium concentration ([Ca(2+)](i)), and were completely blocked by the BK channel-specific blocker paxilline (100 nM). In addition, the channel currents were sensitive to high-energy phosphate compounds and low internal pH. The cellular function of these BK channels was then investigated by nystatin-perforated whole-cell recording. Paxilline (100 nM) had no effect on the frequency and half-width of the action potential (AP) in LGP neurons under control conditions, but significantly attenuated the hyperpolarization that was caused by carbonyl cyanide m-chlorophenylhydrazone (CCCP), an inhibitor of ATP synthesis. In addition, the pancreatic beta-cell type ATP-sensitive potassium channel (K(ATP) channel) blocker tolbutamide (0.25 mM) also attenuated the hyperpolarization, in a manner similar to paxilline. The voltage-dependent potassium channel blocker tetraethylammonium (TEA, 2 mM) significantly decreased the frequency and increased the half-width of the AP in LGP neurons under control conditions, and attenuated CCCP-induced hyperpolarization to an extent close to that of paxilline. The results presented here suggest that functional BK channels are present in LGP neurons, and may behave as partners of K(ATP) channels in the regulation of neuronal activity under metabolic stress conditions.