"Autocannibalism" of choline-containing membrane phospholipids in the pathogenesis of Alzheimer's disease-A hypothesis.

The selective vulnerability of certain cholinergic neurons in Alzheimer's disease could reflect a unique response of these neurons to a neurotoxic factor. Alternatively the etiologic factor causing the disease could affect the brain generally, and the selective death of the cholinergic neurons could occur because they have a biochemical property that makes them least able to withstand this factor. One such property might be their tendency to utilize choline-phospholipids both as a membrane constituent and as a source of free choline for acetylcholine synthesis: perhaps when choline levels in the brain's extracellular fluid are too low to sustain acetylcholine release, these neurons break down their choline-phospholipids more rapidly than they can synthesize them, thus changing membrane structure and, ultimately, neuronal viability.

Choline production from choline-containing phospholipids: a hypothetical role in Alzheimer's disease and aging.

Deficits in certain long-axon cholinergic brain neurons have been demonstrated both in senile dementia of the Alzheimer type (SDAT) and, to a lesser degree, in association with the cognitive and memory impairments sometimes seen with normal aging. Our studies support a hypothesis concerning the selective vulnerability of these neurons. In our superfused brain slice system, acetylcholine (ACh) release was dependent on the concentration of exogenous choline, at rest and during electrical stimulation. Decreases in intracellular levels of ACh and choline accounted for only a small fraction of the quantities of these compounds released into the superfusates, suggesting that some tissue pool of bound choline, such as the phospholipids (PL), might have liberated choline. In choline-free medium, the release and tissue content of ACh were sustained; thus, choline released from the putative endogenous pool can be utilized for ACh synthesis. We hypothesize that the use, by cholinergic neurons, of choline originating from the breakdown of membrane PL, may result in an impoverishment in certain PL. Since only cholinergic neurons use their membrane PL as a reservoir for their neurotransmitter's precursor, this relationship might explain the major deficits in long-axon cholinergic nerve terminals seen in SDAT and other age-related memory disorders.

Cholinesterases in cerebrospinal fluid. Correlations with clinical measures in Alzheimer's disease.

Acetylcholinesterase and butyrylcholinesterase activities in cerebrospinal fluid were measured in 17 patients with Alzheimer's disease and 6 control patients, as potential clinical measures of impaired cholinergic neurotransmission in Alzheimer's disease. The activity of butyrylcholinesterase was decreased in patients with Alzheimer's disease compared to that observed in control patients, but there was overlap between values in the 2 groups. Low butyrylcholinesterase activity was correlated with severity of dementia, memory impairment, and language disorder. Acetylcholinesterase activity was significantly correlated with visual contrast sensitivity, but not with dementia severity, and did not differentiate patients with Alzheimer's disease from control cases. These results suggest that cholinesterases in cerebrospinal fluid are related to brain cholinesterases, and indicate that the activities of acetylcholinesterase and butyrylcholinesterase should be distinguished in studies of cerebrospinal fluid.

Effects of diet-induced hyperthreoninemia. II). Tissue and extracellular amino acid levels in the brain.

Growing rats were fed graded levels of threonine (Thr, 0.4, 0.8, and 3.3 g/100 g diet). Free amino acid content was measured in plasma and brain. Extracellular amino acid levels were measured by microdialysis in brain slices. Large quantities of dietary Thr (3.3 g/100 g) raised plasma and brain Thr and glycine (Gly) levels. Brain and spinal cord extracellular levels of Thr were also raised, whereas the other amino acid levels remained unchanged. A moderate level of dietary Thr (0.8 g/100 g) raised plasma Thr and Gly levels and brain Thr but not Gly level. The diet raised cortical Thr extracellular levels but did not modify the levels of the other amino acids, including glutamate (Glu) and aspartate (Asp). These data suggest that brain neurochemical processes involving Gly, Glu, and Asp are safeguarded in rats fed high Thr levels.

Effects of diet-induced hyperthreoninemia. I). Amino acid levels in the central nervous system and peripheral tissues.

Rats were fed four levels of threonine (Thr, 0.4, 0.6, 0.8, and 5.8 g/100 g diet). After two weeks, Thr, serine (Ser), and glycine (Gly) levels were measured in plasma, liver, muscle, and central nervous system. The diet containing 5.8 g/100 g of Thr elevated Thr and Gly concentrations in plasma and nervous tissue in comparison with a standard diet. In muscle and liver, Thr concentrations were also raised but Gly levels did not change. The hepatic Thr dehydratase activity was enhanced. Diets containing moderate Thr quantities (0.6 and 0.8 g/100 g) induced slight elevations of Thr levels in all tissues. Gly concentrations were not modified. The activity of hepatic Thr dehydratase was diminished. Our results show that a high dietary content of Thr (15 times the normal levels) elevates Gly levels in various tissues, including the brain. On the contrary, diets containing 2 to 4 times the normal levels of Thr induce a weak hyperthreoninemia insufficient to modify brain Gly.

Neurotoxicology and amino acid intake during development: the case of threonine.

The development of the central nervous system is highly dependent on an adequate supply of nutrients. In particular, protein and amino acid availability is of major concern during gestation and in early postnatal life. Numerous data have been published on some amino acids directly involved in brain functions as neurotransmitters or indirectly as precursors of neurotransmitters, but scant information is available on the possible consequences of hyperthreoninemia, a phenomenon repeatedly noted in clinical reports. The results of neurochemical and behavioral studies in the developing rat suggest that despite numerous possible effects of threonine on brain constituents, moderate hyperthreoninemia does not impair markedly the development of the central nervous system.

Effect of threonine on the behavioural development of the rat.

Rats received different levels of threonine (Thr), one, 1.7 and four times the normal dietary intake, from conception to adulthood. The mothers were fed the experimental diets before and during pregnancy. Their offspring received a daily oral load of Thr or placebo until weaning. Thereafter, the juveniles were fed the same diet as their mothers. Morphologic development, ingestive behaviour, homing, and locomotion were observed before weaning. Exploration and spontaneous alternation were studied thereafter. Animals exposed during gestation to 1.7 times the normal Thr intake consumed more food during the test of independent ingestion. Grooming showed inconsistent variations between days 12 and 29 in pups fed 1.7 times the normal Thr intake. Rats performed equally well on the other behavioural tasks independently of the dietary treatment. We conclude that Thr intake as much as four times higher than the levels found in normal diets does not impair the behavioural ontogenesis of the rat.

Release of inorganic phosphate during activity in mammalian non-myelinated nerve fibres.

1. The efflux of labelled phosphate was measured in desheathed rabbit vagus nerve at rest and during activity.2. In solutions with 2 mM-phosphate and 1 mM-K the rate constant of the resting efflux was 2.7 x 10(-3) min(-1); stimulation caused an extra fractional loss of 2.8 x 10(-6) impulse(-1).3. Lowering the phosphate concentration decreased the resting and the stimulated efflux; with 0.2 mM-phosphate the corresponding values were 1.9 x 10(-3) min(-1) and 1.8 x 10(-6) impulse(-1), respectively.4. Increasing the K to 5.6 mM decreased both resting and stimulated efflux.5. Lowering the temperature decreased the resting efflux with a Q(10) of 2.9 and the stimulated efflux with a Q(10) of 8.1.6. Chromatography of the effluent showed that at rest and during activity at least 96% of the radiophosphate was in the orthophosphate fraction.7. Replacing the Na of the solution by Li lowered the rate constant of the resting efflux to 0.8 x 10(-3) min(-1) and abolished the extra release during activity, without reduction of the action potential.8. The presence of ouabain did not affect the resting efflux, except at 100 muM, when a transient reduction was found. The extra fractional loss was not affected with 0.001 muM; with 0.01-0.5 muM, it was reduced without much change in the action potential, and abolished at higher concentrations.9. The results agree with the hypothesis that the extra release results from an increase in internal inorganic phosphate caused by increased break-down of ATP during recovery.10. Comparison with the O(2) consumption shows that about 1% of the inorganic phosphate liberated at the inside of the axons escapes to the outside.

Effects of electrical stimulation and choline availability on the release and contents of acetylcholine and choline in superfused slices from rat striatum.

The presence of 5 or 20 microM choline in the eserinized medium superfusing striatal slices enhanced the spontaneous release of acetylcholine (ACh) at both concentrations and, at 20 microM, the release of transmitter evoked by electrical field stimulation. Neither the electrical stimulation nor the addition of choline altered choline acetyltransferase activity. These results show that ACh release is dependent on the availability of extracellular choline. The rate of choline efflux was 7 times higher than the rate of ACh release, was not affected by stimulation, and was increased by 40% when hemicholinium-3 (HC-3), an inhibition of choline uptake, was present. The muscarinic antagonist atropine (1 microM) increased the evoked release of ACh into both the choline-free medium and that containing 20 microM choline. An adenosine receptor antagonist, 1,3-diethyl-8-phenyl xanthine (10 microM), failed to affect ACh release or the enhancement of release produced by atropine. In medium containing HC-3, stimulation of the slices elicited ACh release for the first 20 min of the 30 min stimulation period (15 Hz); thereafter, although stimulation was continued, the rate of release decreased to that associated with spontaneous release. Tissue ACh contents were not modified by the addition of choline or atropine to the medium, but were depressed by HC-3. Neither atropine nor HC-3 altered tissue choline content. The total amount of ACh + choline released during an experiment was 5-15 times higher than the decrease in tissue levels of these two compounds during the same period of time.(ABSTRACT TRUNCATED AT 250 WORDS)

Release of adenosine, inosine and hypoxanthine from rabbit non-myelinated nerve fibres at rest and during activity.

The composition of the efflux from desheathed rabbit vagus nerve, loaded with radioactivity by incubation in [3H]adenosine, was studied at rest and during electrical activity and after application of inhibitors of ecto-enzymes and modifications of intermediary metabolism. In addition, the degradation of externally applied ATP and adenosine was examined. [3H]ATP applied to the incubation medium was degraded to ADP, AMP, adenosine and inosine. The hydrolysis to nucleosides was inhibited by alpha, beta-methylene ADP; the appearance of AMP and nucleosides was slowed by beta, gamma-methylene ATP. Deamination of [3H]adenosine was blocked by 2-deoxycoformycin. The effluent from resting and stimulated preparations showed the presence of large amounts of inosine and hypoxanthine, smaller amounts of adenosine and adenine and traces of nucleotides. The composition of the effluent was not significantly altered by addition of alpha, beta-methylene ADP; beta, gamma-methylene ATP or 2-deoxycoformycin. Application of glucose-free solutions caused a large release of adenosine instead of inosine and hypoxanthine and a small increase in resting and stimulated efflux of 3H. Addition of 2-deoxyglucose produced a large increase in resting efflux and increased liberation of adenosine. Cyanide, 2,4-dinitrophenol, arsenate or salicylate increased the resting efflux of adenosine, inosine and hypoxanthine, and the effect of activity. It is concluded that electrical activity leads to release of adenosine, inosine and hypoxanthine, in various proportions depending on metabolic state, and that there is practically no liberation of nucleotides from nerve axons.

Uptake of adenosine and release of adenine derivatives in mammalian non-myelinated nerve fibres at rest and during activity.

1. Influx of adenosine into rabbit non-myelinated nerve fibres was measured using [2-(3)H]adenosine. The uptake of radioactivity increased linearly with duration of incubation for up to 60 min and adenosine concentration up to 200 mum. The uptake at different adenosine concentrations showed a saturable component with a half-maximal activation at 17.1 mum and a linear part.2. The radioactivity taken up was rapidly incorporated into AMP, ADP and ATP. Isotopic equilibrium between the nucleotides was achieved within 15 min.3. The uptake of (3)H from 0.2 mum-adenosine was almost completely inhibited by addition of 200 mum-adenosine and to a similar extent by 200 mum-tubercidin and AMP; a 70% inhibition was found with ATP and ADP; alpha, beta methylene-ADP had no effect.4. ATP, ADP and AMP added to the extracellular medium of a desheathed vagus were slowly hydrolysed.5. In preparations loaded with [2-(3)H]adenosine and then washed with adenosine and label-free solution there was a steady efflux of radioactivity amounting to 0.18 x 10(-3)/min. Addition of adenosine or tubercidin transiently increased the efflux.6. Electrical stimulation caused an extra release of radioactivity. The extra fractional loss was 21.8 x 10(-6)/impulse in preparations that had rested for several hours; it decreased to 2.3 x 10(-6)/impulse when stimulation was applied after a 30 min rest.7. The radioactivity of the resting efflux and of the extra efflux after stimulation was found mostly in inosine and hypoxanthine; adenosine and adenine accounted for only 3%, and the nucleotides for less than 1% of the efflux.8. Adenosine added to the external medium of a desheathed nerve was slowly deaminated.9. It is concluded that inosine and hypoxanthine found in the effluent from desheathed vagus nerve trunk result from release of these compounds from nerve fibres and not from extracellular breakdown of released ATP or adenosine.10. Electrical activity in non-myelinated nerve fibres of the nerve trunk thus causes the release of metabolites (inosine and hypoxanthine) together with small amounts of adenosine and adenine, while release of ATP and other nucleotides is almost completely absent.

Failure of tryptophan deficiency to reduce specifically serum levels of transthyretin or albumin in rats.

Because transthyretin (TTR) is a tryptophan-rich molecule and a sensitive nutritional marker, tryptophan deficiency might markedly influence the circulating level of TTR. The effect of severe tryptophan (Trp) deficiency on serum TTR, as well as on albumin and transferrin levels, was studied in growing rats for 8 d. The animals were then refed a control diet for 12 d. The Trp-deficient and control diets contained 0.008 and 0.34% Trp, respectively. A loss of body weight and a dramatic reduction in food intake were observed in the Trp-deficient rats. Although serum total Trp concentration was significantly less in these rats than in pair-fed controls, serum TTR declined to the same extent in both groups compared to control rats fed ad libitum. Albumin concentrations were not altered, but transferrin levels declined slightly in the Trp-deficient rats compared to both the pair-fed group and the controls fed ad libitum. Refeeding the control diet to Trp-deficient rats restored total and free Trp concentrations, as well as TTR and transferrin levels, but body weight and food intake remained lower than in the control group. To examine the effect of moderate Trp restriction, rats were fed for 2 wk a diet whose Trp content was 50% less than that of the control diet. Although total and free Trp concentrations were significantly lower in the rats fed the Trp-deficient diet than in the control group, body weight, food intake and TTR levels were similar in both groups. The results suggest that acute and severe Trp deficiency per se does not modify TTR and albumin levels.