Formation of glucose from glucose 6-phosphate by rat brain slices.
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Animals↗
Biomedical subjects
P R Dodd
Publications and source records attributed to P R Dodd.
The metabolism of glucose 6-phosphate by mammalian cerebral cortex in vitro.
1. The metabolism of glucose 6-phosphate in rat cerebral-cortex slices in vitro was compared with that of glucose. It was found that a glucose 6-phosphate concentration of 25mm was required to achieve maximal oxygen uptake rates and ATP concentrations, whereas only 2mm-glucose was required. 2. When 25mm-[U-(14)C]glucose 6-phosphate was used as substrate, the pattern of labelling of metabolites was found to be quantitatively and qualitatively similar to the pattern found with 10mm-[U-(14)C]glucose, except that incorporation into [(14)C]lactate was decreased, and significant amounts of [(14)C]glucose and [(14)C]mannose phosphate and [(14)C]fructose phosphate were formed. 3. Unlabelled glucose (10mm) caused a tenfold decrease in the incorporation of 25mm-[U-(14)C]glucose 6-phosphate into all metabolites except [(14)C]glucose and [(14)C]mannose phosphate and [(14)C]fructose phosphate. In contrast, unlabelled glucose 6-phosphate (25mm) had no effect on the metabolism of 10mm-[U-(14)C]glucose other than to increase markedly the incorporation into, and amount of, [(14)C]lactate, the specific radioactivity of this compound remaining approximately the same. 4. The effect of glucose 6-phosphate in increasing lactate formation from glucose was found to occur also with a number of other phosphate esters and with inorganic phosphate. Further investigation indicated that the effect was probably due to binding of medium calcium by the phosphate moiety, thereby de-inhibiting glucose uptake. 5. Incubations carried out in a high-phosphate high-potassium medium gave a pattern of metabolism similar to that found when slices were subjected to depolarizing conditions. Tris-buffered medium gave similar results to bicarbonate-buffered saline, except that it allowed much less lactate formation from glucose. 6. Part of the glucose formed from glucose 6-phosphate was extracellular and was produced at a rate of 12mumol/h per g of tissue in Krebs tris medium when glycolysis was blocked. The amount formed was much less when 25mm-P(i) or 26mm-HCO(3) (-) was present, the latter being in the absence of tris. 7. Glucose 6-phosphate also gave rise to an intracellular glucose pool, whereas no intracellular glucose was detectable when glucose was the substrate.
Adenosine Triphosphate↗
Plasma GABA, GABA-like activity and the brain GABA-benzodiazepine receptor complex in rats with chronic hepatic encephalopathy.
Plasma gamma-aminobutyric acid (GABA)-like activity, plasma GABA and the brain GABA-benzodiazepine receptor complex were studied in rats with chronic hepatic encephalopathy. Portal vein ligation (after prior subcutaneous transposition of the spleen) results in complete portal bypass of splanchnic blood. In addition, significant hepatocellular damage was superimposed on this model of portosystemic shunting by ligation of the common bile duct. Plasma GABA-like activity (determined by radioreceptor assay) and true plasma GABA concentrations (determined by high-performance liquid chromatography) were found to be significantly increased in both portal vein-ligated and portal vein and bile duct-ligated rats, compared with controls. However, plasma GABA-like activity was consistently greater than the concentration of true GABA in the plasma of all rats. This suggested the presence of a GABA-like factor in plasma that can inhibit [3H]GABA binding, but is not GABA itself. The concentration of this GABA-like factor was significantly increased in the plasma of rats with chronic hepatic encephalopathy. Despite the significant increase in plasma GABA-like activity and plasma GABA concentrations, there was no alteration in the affinity or density of the physiologically relevant, low-affinity brain GABA binding site in the rats with portal vein ligation, with or without bile duct ligation. There was also no significant alteration in brain benzodiazepine binding in these rats. GABA enhancement of benzodiazepine binding was unchanged in the portal vein-ligated rats. However, the maximal enhancement of benzodiazepine binding was decreased in the rats with portal vein and bile duct ligation. There appears to be no substantial evidence that brain GABAergic neurotransmission is increased in these models of chronic hepatic encephalopathy.
Animals↗
Application of DNA microarrays to study human alcoholism.
An emerging idea is that long-term alcohol abuse results in changes in gene expression in the brain and that these changes are responsible at least partly for alcohol tolerance, dependence and neurotoxicity. The overall goal of our research is to identify genes which are differentially expressed in the brains of well-characterized human alcoholics as compared with non-alcoholics. This should identify as-yet-unknown alcohol-responsive genes, and may well confirm changes in the expression of genes which have been delineated in animal models of alcohol abuse. Cases were carefully selected and samples pooled on the basis of relevant criteria; differential expression was monitored by microarray hybridization. The inherent diversity of human alcoholics can be exploited to identify genes associated with specific pathological processes, as well as to assess the effects of concomitant disease, severity of brain damage, drinking behavior, and factors such as gender and smoking history. Initial results show selective changes in gene expression in alcoholics; of particular importance is a coordinated reduction in genes coding for myelin components.
Alcohol-Related Disorders↗
The nature of d,l-fenfluramine-induced 5-HT reuptake transporter loss in rats.
The administration of the anorexigenic drug d,l-fenfluramine (Ponderax) to laboratory animals results in a dose-dependent reduction in presynaptically located serotonergic reuptake transporter protein. This long-term effect may represent an altered mechanism of synthesis of the transporter (downregulation). Alternatively, fenfluramine may destroy the serotonergic terminals on which 5-HT transporters are located. To distinguish between these two alternatives, we applied an assay of neurotransmitter-specific nerve endings (alpha) to brain tissue from two animal models of reduced 5-HT transporter density. In Model 1, serotonergic nerve terminals were destroyed (rats received 5,7-dihydroxytryptamine [5,7-DHT] intracisternally); in Model 2, there was a loss of 5-HT transporter per se on otherwise intact serotonergic nerve terminals. The manner in which alpha declined as transporter density was decreased (reducing Vmax values) in animal Models 1 and 2 was found to be significantly different. In rats treated with fenfluramine, the association between 5-HT transporter density and alpha was the same as in the neurotoxic model.
5,7-Dihydroxytryptamine↗
Mechanism of degradation of LH-RH and neurotensin by synaptosomal peptidases.
The products of degradation of LH-RH and neurotensin by synaptosomes isolated from rat hypothalamus and cortex have been identified. LH-RH is cleaved at Tyr5-Gly6 and Pro9-Gly10 giving rise to LH-RH (1-5), LH-RH (6-10) and LH-RH (1-9). Neurotensin is cleaved at Arg8-Arg9, Pro10-Tyr11 and Ile12-Leu13, giving neurotensin (1-8), neurotensin (1-10), neurotensin (1-12) and neurotensin (9-13) as major products. While most of the peptidase activity is localized in the cytoplasmic fraction, a small but significant proportion is membrane bound. For LH-RH, the specificity of the membrane-bound activity is similar to that in the cytosol fraction; for neurotensin, the membrane fraction preferentially gives rise to the (1-10) and (1-11) peptides. The most potent inhibitors of the LH-RH and neurotensin degrading enzymes in synaptosomes are heavy metal ions (mercury and copper), p-chloromercuribenzoate and 1,10 phenanthroline.
Amino Acids↗
Glutamate-mediated transmission, alcohol, and alcoholism.
Glutamate-mediated neurotransmission may be involved in the range of adaptive changes in brain which occur after ethanol administration in laboratory animals, and in chronic alcoholism in human cases. Excitatory amino acid transmission is modulated by a complex system of receptors and other effectors, the efficacy of which can be profoundly affected by altered gene or protein expression. Local variations in receptor composition may underlie intrinsic regional variations in susceptibility to pathological change. Equally, ethanol use and abuse may bring about alterations in receptor subunit expression as the essence of the adaptive response. Such considerations may underlie the regional localization characteristic of the pathogenesis of alcoholic brain damage, or they may form part of the homeostatic change that constitutes the neural substrate for alcohol dependence.
Alcoholism↗
New evidence for a loss of serotonergic nerve terminals in rats treated with d,l-fenfluramine.
Fenfluramine has been classified as a neurotoxin because animals treated with this anorectic lose 5-HT uptake sites located on serotonergic nerve terminals. However, there are two possible bases for this finding: either uptake sites are lost because the terminals themselves have been destroyed (neurotoxicity); or uptake sites are lost from otherwise intact terminals. To distinguish between these possibilities, we established an animal model in which male Wistar rats were injected (intraperitoneally) with an irreversible 5-HT uptake site antagonist (EEDQ). Since their 5-HT sites were inhibited (blocked) non-competitively, by this agent, such animals had effectively lost 5-HT uptake sites from intact serotonergic terminals. Synaptosomes prepared from such animals showed the predicted reduction in the Bmax of [3H]paroxetine binding to the 5-HT uptake site, and a reduction in the Vmax of [14C]5-HT uptake. However, they showed no significant reduction in maximal [14C]5-HT loading (alpha) compared with synaptosome from sham-injected controls. In contrast, fenfluramine-treated animals showed reduced [3H]paroxetine binding, reduced maximal [14C]5-HT uptake and significantly (P < 0.02) reduced synaptosomal [14C]5-HT loading. Therefore, the results suggest that fenfluramine does indeed cause the destruction of serotonergic nerve terminals.
Animals↗