Nervous tissue thiamine metabolism in vivo. III. Influence of ethanol intake on the dynamics of thiamine and its phosphoesters in different brain regions and sciatic nerve of the rat

Experimental model of alcohol-related peripheral neuropathy

INTRODUCTION

The aim of this work was to determine the effect of chronic alcohol exposure on peripheral nerves in a nutritionally balanced rat model of alcoholism.

METHODS

Three different strains of adult male rats were pair-fed for 8 weeks with isocaloric liquid diets containing 0% or 37% ethanol. Nerve conduction studies (NCS) were performed. Peripheral nerve and muscle were examined histologically with morphometrics.

RESULTS

Ethanol exposure significantly slowed velocity in tibial and fibular nerves, but not in the plantar nerve in all 3 strains. Studies of the sciatic nerve revealed decreased fiber diameters and increased regenerative sprouts in peripheral nerves. There was muscle denervation of ethanol-exposed rats in all 3 strains.

CONCLUSIONS

Chronic ethanol exposure caused a polyneuropathy characterized by axonal degeneration despite adequate nutrition. These results suggest that ethanol exposure has direct neurotoxic effects on peripheral nerves. This model may be useful in understanding the underlying mechanism(s) of alcohol-related peripheral neuropathy.

Experimental alcohol-related peripheral neuropathy: role of insulin/IGF resistance

The mechanisms of alcohol-related peripheral neuropathy (ALPN) are poorly understood. We hypothesize that, like alcohol-related liver and brain degeneration, ALPN may be mediated by combined effects of insulin/IGF resistance and oxidative stress. Adult male Long Evans rats were chronically pair-fed with diets containing 0% or 37% ethanol (caloric), and subjected to nerve conduction studies. Chronic ethanol feeding slowed nerve conduction in the tibial (p = 0.0021) motor nerve, and not plantar sensory nerve, but it did not affect amplitude. Histological studies of the sciatic nerve revealed reduced nerve fiber diameters with increased regenerative sprouts, and denervation myopathy in ethanol-fed rats. qRT-PCR analysis demonstrated reduced mRNA levels of insulin, IGF-1, and IGF-2 polypeptides, IGF-1 receptor, and IRS2, and ELISAs revealed reduced immunoreactivity for insulin and IGF-1 receptors, IRS-1, IRS-4, myelin-associated glycoprotein, and tau in sciatic nerves of ethanol-fed rats (all p < 0.05 or better). The findings suggest that ALPN is characterized by (1) slowed conduction velocity with demyelination, and a small component of axonal degeneration; (2) impaired trophic factor signaling due to insulin and IGF resistance; and (3) degeneration of myelin and axonal cytoskeletal proteins. Therefore, ALPN is likely mediated by molecular and signal transduction abnormalities similar to those identified in alcoholic liver and brain degeneration.

Alcohol-related peripheral neuropathy: nutritional, toxic, or both?

Alcohol-related peripheral neuropathy (ALN) is a potentially debilitating complication of alcoholism that results in sensory, motor, and autonomic dysfunction. Unfortunately, ALN is rarely discussed as a specific disease entity in textbooks because it is widely assumed to primarily reflect consequences of nutritional deficiency. This hypothesis is largely based on observations first made over eight decades ago when it was demonstrated that thiamine deficiency (beriberi) neuropathy was clinically similar to ALN. In recent studies, failure of thiamine treatment to reverse ALN, together with new information demonstrating clinical and electrophysiological distinctions between ALN and nutritional deficiency neuropathies, suggests that alcohol itself may significantly predispose and enhance development of neuropathy in the appropriate clinical setting. We reviewed the evidence on both sides and conclude that ALN should be regarded as a toxic rather than nutritional neuropathy.

Alcohol consumption in the absence of adequate nutrition may lead to activation of the glyoxylate cycle in man

The consumption of alcohol prior to food intake results in alcohol metabolism occurring in the liver, and the liver is often damaged in chronic alcoholics. This paper highlights the possibility that alcohol consumption in the absence of adequate nutrition after an extended period of time may lead to activation of the glyoxylate cycle, an energy pathway associated with the conversion of fat into carbohydrate which until recently was thought to only exist in plants and bacteria.

Compartmental model identification based on an empirical Bayesian approach: the case of thiamine kinetics in rats

Compartmental models are a very popular tool for the analysis of experiments in living systems. There are three main aspects that have to be taken into account: the degree of detail of the model, its a priori identifiability and the a posteriori (numerical) identifiability. In some cases, where standard approaches are adopted, the models can be either a priori or a posteriori unidentifiable. The paper proposes model identification within a Bayesian framework, to solve a posteriori unidentifiability problems. In particular, a stochastic simulation algorithm is proposed to perform a Bayesian identification of compartmental models, and an empirical Bayesian technique is proposed to propagate information among multiple experiments. The power of this methodology was demonstrated by evaluating the kinetics of thiamine under several experimental conditions. The complexity of the existing model (nine parameters) and limited experimental data (8/12 for each model) caused a posteriori identifiability problems when standard approaches were adopted. The application of the methodology identifies all 28 models (four tissues under seven different conditions).

Metabolic and permeability changes caused by thiamine deficiency in immortalized rat brain microvessel endothelial cells

The possible involvement of blood-brain barrier (BBB) breakdown in the pathogenesis of thiamine deficiency encephalopathy was investigated in RBE4 cells, an immortalized rat brain endothelial cell line. The effects of thiamine deficiency produced by addition of pyrithiamine and by reduction of thiamine in the culture medium, on the metabolism and permeability of the RBE4 monolayer was examined. Pyrithiamine treatment in low thiamine medium (M199) for 7 days caused cytotoxic effects on RBE4 cells at all concentrations (10-50 microg/ml). Pyrithiamine caused a concentration- and time-dependent decrease in MTT reduction and a significant increase in glucose consumption and lactate production compared to controls. Pyrithiamine treatment for 3 days caused a significant decrease in MTT reduction at 50 microg/ml only. In contrast, increased glucose consumption and lactate production by the RBE4 cells was observed after treatment for 3 days with concentrations of 25 microg/ml pyrithiamine and above. The permeability of RBE4 cell monolayers to [14C]sucrose (Mw 342), but not FITC-dextran (Mw 4000) was significantly increased by treatment with pyrithiamine concentrations of 25 microg/ml and above for 3 days. These effects were not accompanied by detectable changes in F-actin distribution or content, although F-actin content was significantly reduced by 7 days exposure to pyrithiamine. These results suggest that metabolic and permeability changes in thiamine-deficient RBE4 cells may be important early events in thiamine-deficiency encephalopathy. The relative role of the BBB in the pathogenesis of thiamine deficiency is discussed.

Can megadoses of thiamine prevent ethanol-induced damages of rat hippocampal CA1 pyramidal neurones?

The specific aim of this study was to evaluate whether high doses of thiamine can compensate or prevent alcohol-induced damages of rat hippocampus CA1 pyramids. Twenty weeks of ethanol consumption together with a dose of thiamine in the range of 1.19 mg/100 mg food induced significant enlargement (parameters measured were length of the whole spine and diameter of the end-bulb) of dendritic spines. Hypertrophy can be interpreted as a compensation process due to alcohol-induced cell death because viable spines are in search of new synaptic contacts. In contrast, dendritic spines of the alcohol group fed at the same time with a high dose of thiamine (119 mg/ 100 g food = megavitamintherapy) showed normal data concerning these parameters. From these results it may be concluded that a megavitamin therapy supports a neuron's carbohydrate metabolism and therefore could be able to prevent or reduce alcohol-induced damages of hippocampal CA1 pyramidal cells in rat central nervous system.

Reduced activities of thiamine-dependent enzymes in brains of alcoholics in the absence of Wernicke's encephalopathy

The relative roles of alcohol per se, thiamine deficiency, and liver disease in the pathogenesis of alcohol-related brain damage have not been fully elucidated. In particular, the extent to which alterations of brain thiamine metabolism contribute to cognitive dysfunction in alcoholism in the absence of Wernicke's encephalopathy has not been established. In the present study, thiamine diphosphate-dependent enzymes were measured using standard spectrophotometric techniques in homogenates of brain tissue obtained at autopsy from eight alcoholic patients, all of whom died in hepatic coma without clinical or neuropathological evidence of Wernicke's encephalopathy and six nonalcoholic, age-matched controls, matched for autopsy delay time and free, at the time of death, from gross malnutrition or other neurological or psychiatric disorders. Transketolase activities were reduced in cerebellum (by 35%, p < 0.01), thalamus (by 35%, p < 0.01), frontal cortex (by 22%, p < 0.01), temporal cortex (by 20%, p < 0.01), and prefrontal cortex (by 19%, p < 0.01). Activities of the pyruvate dehydrogenase complex were selectively reduced in prefrontal cortex by 25% (p < 0.01). Activities of alpha-ketoglutarate dehydrogenase were within normal limits in all brain regions of alcoholic patients. The generalized reductions of transketolase activity undoubtedly result from thiamine deficiency. Previous studies suggest that the presence of liver disease may exacerbate thiamine deficiency in alcoholics. A sustained loss of transketolase activity in brain could result in disruption of pentose shunt activity and concomitant reductions in reducing equivalents and lipid metabolism within the cell. The selective loss of pyruvate dehydrogenase activity in prefrontal cortex of alcoholic cirrhotics could relate to the phenomenon of hepatic coma.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of phenytoin on the in vivo kinetics of thiamine and its phosphoesters in rat nervous tissues

The in vivo effects of chronic (30 days) and subchronic (10 days) intragastric treatment with phenytoin (PHT) (500 mg/kg) b.wt., suspended in 10% arabic gum water solution) on the uptake and metabolism of thiamine (T), T monophosphate (TMP) and T pyrophosphate (TPP) were evaluated in rat nervous regions (cerebral cortex, brainstem, cerebellum and sciatic nerve) by determining the radioactivity of T and its phosphoesters in plasma and tissues at fixed time intervals (0.25-240 h) after an i.p. injection of thiazole-[2-14C]thiamine (30 micrograms: 1.25 microCi). A nutritionally adequate diet containing T in excess was given to the animals in order to produce a virtually stable content of T compounds in the tissues. Analytical data were processed by using a compartmental model which allowed the calculation of fractional rate constants (FRC), turnover rates (TR) and turnover times. Compared with vehicle-treated controls, animals treated chronically with PHT exhibited lower levels of radiolabelled T compounds in all nervous regions except for the cerebral cortex. These alterations were not found in animals receiving subchronic treatment. Evaluation of FRC values indicated that PHT-induced effects on T metabolism differed depending on the length of PHT treatment and the nervous region considered. Overall, PHT appeared to interfere mainly with T and TMP uptake, TPP dephosphorylation to TMP and TPP turnover times, these effects being particularly prominent in the cerebellum and in the brainstem of chronically treated animals. Since all changes in T uptake and metabolism were observed in the absence of overt behavioural toxicity, these findings may have potential clinical relevance in highlighting possible mechanisms by which PHT therapy can alter brain metabolism.

Thiamine-dependent enzyme changes in the brains of alcoholics: relationship to the Wernicke-Korsakoff syndrome

Chronic alcoholism results in thiamine deficiency as a consequence of poor nutrition, impaired absorption, and decreased phosphorylation to the enzyme cofactor form of the vitamin, thiamine pyrophosphate (TPP). Results of this study demonstrate significant reductions of TPP-dependent enzymes [pyruvate dehydrogenase complex, alpha-ketoglutarate dehydrogenase (alpha KGDH), and transketolase] in autopsied cerebellar vermis samples from alcoholic patients with the clinical and neuropathologically confirmed diagnosis of Wernicke-Korsakoff Syndrome (WKS). Enzyme activities in brain samples from alcoholics without WKS were within normal limits and activities of a nonthiamine-dependent enzyme, glutamate dehydrogenase, were not significantly different from control values in brain samples from alcoholics with or without WKS. These findings provide evidence, for the first time, of a direct implication of TPP-related metabolic processes in the pathogenesis of WKS. Decreased activities of alpha KGDH could be the trigger for a sequence of metabolic events resulting in energy compromise, and ultimately neuronal death in this syndrome.

Reversible inhibition of acetylcholine synthesis and behavioural effects caused by 3-bromopyruvate

3-Bromopyruvate inhibits pyruvate decarboxylase in brain homogenates and causes a 90% drop in acetylcholine tissue content at a concentration of 2 mM. Stereotaxic injection of 3-bromopyruvate into the basal forebrain causes after 7 days a 40% drop of acetylcholine concentration and pyruvate decarboxylase activity in the cortex and hippocampus, and greater decreases at the site of injection. However, values return to normal 18 days after injection. Choline acetyltransferase is partially inhibited only at the site of injection after 7 days. Choline transport and choline concentration are not affected at either 7 or 18 days after injection. Impairments in spontaneous alternation and in retention of passive avoidance were seen only 7 days after the injection. The results suggest that stereotaxic injection of bromopyruvate can induce discrete reversible cholinergic lesions on a time scale useful for behavioural experiments and for comparison with neurodegeneration.