The effects of methylglyoxal on central synaptic transmission in the isolated nerve cord of the cockroach. (Periplaneta americana L)

The Type 2 Diabetes Factor Methylglyoxal Mediates Axon Initial Segment Shortening and Alters Neuronal Function at the Cellular and Network Levels

Recent evidence suggests that alteration of axon initial segment (AIS) geometry (i.e., length or location along the axon) contributes to CNS dysfunction in neurological diseases. For example, AIS length is shorter in the prefrontal cortex of type 2 diabetic mice with cognitive impairment. To determine the key type 2 diabetes-related factor that produces AIS shortening we modified levels of insulin, glucose, or the reactive glucose metabolite methylglyoxal in cultures of dissociated cortices from male and female mice and quantified AIS geometry using immunofluorescent imaging of the AIS proteins AnkyrinG and βIV spectrin. Neither insulin nor glucose modification altered AIS length. Exposure to 100 but not 1 or 10 μm methylglyoxal for 24 h resulted in accumulation of the methylglyoxal-derived advanced glycation end-product hydroimidazolone and produced reversible AIS shortening without cell death. Methylglyoxal-evoked AIS shortening occurred in both excitatory and putative inhibitory neuron populations and in the presence of tetrodotoxin (TTX). In single-cell recordings resting membrane potential was depolarized at 0.5-3 h and returned to normal at 24 h. In multielectrode array (MEA) recordings methylglyoxal produced an immediate ∼300% increase in spiking and bursting rates that returned to normal within 2 min, followed by a ∼20% reduction of network activity at 0.5-3 h and restoration of activity to baseline levels at 24 h. AIS length was unchanged at 0.5-3 h despite the presence of depolarization and network activity reduction. Nevertheless, these results suggest that methylglyoxal could be a key mediator of AIS shortening and disruptor of neuronal function during type 2 diabetes.

In Vitro Testing of Neurotoxicity

Many of the toxic compounds that are at large in the environment represent a risk to our neuronal functions. Chemicals may have a direct or indirect effect on the nervous system and they may interfere with general biochemical properties or specific neuronal structures and processes. In this review, a brief presentation of the major neurotoxicological targets is given, together with a discussion of some aspects of the use of different in vitro models for screening purposes and mechanistic studies. It is believed that in vitro methods offer special opportunities for the development of new neurotoxicological assays, and that this development will mainly involve cultured model systems. Therefore, a presentation of nerve and glia tissue culture methods is given, followed by an overview of how information on the action of mercury and mercurials, excitotoxins and acrylamide has been obtained through the use of cultured cell models. It is concluded that the developmental potential in cell neurotoxicology lies within the areas of separation and identification of cells representative for different structures in the nervous system, co-cultivation of different cell types, in vivo/in vitro (ex vivo) procedures, chemically defined media, metabolic competent cultures of human cells and improved physiological conditions for cultivation and exposure.

Possible mechanism for the effect of ketogenic diet in cases of uncontrolled seizures

High fat and low carbohydrate diet, designated ketogenic diet, has been successfully used for the treatment of intractable epilepsy in a portion of patients, particularly in children. Several hypotheses have been worked out for the explanation of its beneficial effects, among others acetone has been raised as a causative factor. However, despite the endeavours the mode of action of the diet is still an enigma. And it is also not known why the diet is effective in some patients, while in others it is without any effect. Here a possible way for the effect of ketogenic diet is proposed in which the central point of seizure preventing effect of the diet is the metabolism of acetone. The proposal nominates S-D-lactoylglutathione, an intermediate of acetone metabolism, as a factor being responsible for seizure controlling effect of ketogenic diet and on the other hand, a sequence of evidence is given for the support of suggestion. S-D-lactoylglutathione would exert its effect by the influence of K(+) channels. The proposal also gives an explanation of why only a portion of patients benefit while on the diet. A proposition is also made for the future directions of research involving the measurement of intermediates of S-D-glutathione breakdown in the cerebrospinal fluid and clinical trials with selected groups of patients. The establishment of this specific mechanism for the ketogenic diet may lead to novel strategies for the investigation and the treatment of intractable epilepsies.

Methylglyoxal in living organisms: chemistry, biochemistry, toxicology and biological implications

Despite the growing interest towards methylglyoxal and glyoxalases their real role in metabolic network is still obscure. In the light of developments several reviews have been published in this field mainly dealing with only a narrow segment of this research area. In this article a trial is made to present a comprehensive overview of methylglyoxal research, extending discussion from chemistry to biological implications by reviewing some important characteristics of methylglyoxal metabolism and toxicity in a wide variety of species, and emphasizing the action of methylglyoxal on energy production, free radical generation and cell killing. Special attention is paid to the discussion of alpha-oxoaldehyde production in the environment as a potential risk factor and to the possible role of this a-dicarbonyl in diseases. Concerning the interaction of methylglyoxal with biological macromolecules (DNA, RNA, proteins) an earlier review (Kalapos, Toxicology Letters, 73, 1994, 3-24) means a supplementation to this paper, thus hoping the avoidance of unnecessary bombast. The paper arrives at the conclusion that since the early stage of evolution the function of methylglyoxalase pathway has been related to carbohydrate metabolism, but its significance has been changed over the thousands of years. Namely, at the beginning of evolution methylglyoxalase path was essential for the reductive citric acid cycle as an anaplerotic route, while in the extant metabolism it concerns with the detoxification of methylglyoxal and plays some regulatory role in triose-phosphate household. As there is a tight junction between methylglyoxal and carbohydrate metabolism its pathological role in the events of the development of diabetic complications emerges in a natural manner and further progress is hoped in this field. In contrast, significant advancement cannot be expected in relation to cancer research.

Possible developments in neurotoxicity testing in vitro

1. Many different chemical compounds produce a range of neurotoxic effects. They do not share a common mechanism and there is no single universally applicable test for neurotoxicity. 2. Testing for neurotoxicity in vivo is not always completely reliable, and many neurotoxic effects are still discovered following accidental exposure to humans. 3. Given the complexity of neurotoxins and their effects in vivo and the difficulties of elucidation of such effects in vivo, there is a place for the use of simple in vitro model systems in neutrotoxicity testing. 4. Various preparations of invertebrate neurones have been used in toxicological studies because of the similarities in basic neuronal properties between invertebrates and higher species. Mammalian nerve cells can be grown in culture and they are suitable for biochemical and electrophysiological experiments. Such neuronal cultures could also be adapted for toxicological studies. Cultures of continuous cell lines would probably be more convenient than primary cultures. 5. Future developments should concentrate on the establishment of a battery of human hybrid cell lines and of a range of biochemical assays. After validation by experiments with a series of known neurotoxins, such cultures would be suitable for neurotoxicological tests.

Effects of methylglyoxal on central and peripheral cholinergic responses

Methylglyoxal (MG) has been shown to have a depolarizing effect on the giant interneurones of the isolated 6th abdominal ganglion of the cock-roach. This effect of MG was inhibited by concentrations of nicotine, d-tubocurarine and physostigmine which blocked transmission at the cholinergic cercal nerve-giant interneurone synapse. MG (5 X 10(-5) to 5 X 10(-4) M) produced concentration-dependent contractures of the isolated frog rectus abdominis muscle which were inhibited by d-tubocurarine (10(-4) M) and physostigmine (10(-6) M). MG enhanced the action of acetylcholine and inhibited KCl-evoked contractures whereas it had no effect on the response to carbachol. It is concluded that MG appears to act as a cholinomimetic in both the peripheral and central nervous systems.