Compartmentation and regulation of acetylcholine synthesis at the synapse

Striatal cholinergic functional alterations in hypoxic neonatal rats: Role of glucose, oxygen, and epinephrine resuscitation

Molecular processes regulating cholinergic functions play an important role in the control of respiration under hypoxia. Cholinergic alterations and its further complications in respiration due to hypoxic insult in neonatal rats and the effect of glucose, oxygen, and epinephrine resuscitation was evaluated in the present study. Receptor binding and gene expression studies were done in the corpus striatum to analyse the changes in total muscarinic receptors, muscarinic M1, M2, M3 receptors, and the enzymes involved in acetylcholine metabolism, choline acetyltransferase and acetylcholinesterase. Neonatal hypoxia decreased total muscarinic receptors with reduced expression of muscarinic M1, M2, and M3 receptor genes. The reduction in acetylcholine metabolism is indicated by the downregulated choline acetyltransferase and upregulated acetyl cholinesterase expression. These cholinergic disturbances were reversed to near control in glucose-resuscitated hypoxic neonates. The adverse effects of immediate oxygenation and epinephrine administration are also reported. The present findings points to the cholinergic alterations due to neonatal hypoxic shock and suggests a proper resuscitation method to ameliorate these striatal changes.

Acetylcholine esterase activity and behavioral response in hypoxia induced neonatal rats: effect of glucose, oxygen and epinephrine supplementation

Brain damage due to an episode of hypoxia remains a major problem in infants causing deficit in motor and sensory function. Hypoxia leads to neuronal functional failure, cerebral palsy and neuro-developmental delay with characteristic biochemical and molecular alterations resulting in permanent or transitory neurological sequelae or even death. During neonatal hypoxia, traditional resuscitation practices include the routine administration of 100% oxygen, epinephrine and glucose. In the present study, we assessed the changes in the cholinergic system by measuring the acetylcholinesterase (AChE) activity and the behavioral responses shown by hypoxia induced neonatal rats and hypoxic rats supplemented with glucose, oxygen and epinephrine using elevated plus-maze and open-field test. The acetylcholine esterase enzyme activity showed a significant decrease in cerebral cortex, whereas it increased significantly in the muscle of experimental rats when compared to control. Hypoxic rats supplemented with glucose, glucose and oxygen showed a reversal to the control status. Behavioral studies were carried out in experimental rats with elevated plus-maze test and open-field test. Hypolocomotion and anxiogenic behavioral responses were observed in all experimental rats when compared to control, hypoxic rats supplemented with glucose, glucose and oxygen. Thus, our results suggest that brain damage due to hypoxia, oxygen and epinephrine supplementation in the neonatal rats cause acetylcholine-neuromuscular-defect leading to hypolocomotion and anxiogenic behavioral response. Glucose and glucose with oxygen supplementation to hypoxic neonates protect the brain damage for a better functional status in the later life.

Lactate spares glucose as a metabolic fuel in neurons and astrocytes from primary culture

The effect of lactate on glucose metabolism in neurons and astrocytes from primary culture has been studied. The rates of glucose metabolism through the pentose-phosphate shunt, the pyruvate dehydrogenase-catalyzed reaction, the tricarboxylic acid cycle, the total lipogenesis and the synthesis of glycerol-borne lipids in astrocytes were 2-3 fold higher than in neurons. However, the rate of glucose incorporation into sterols and esterified fatty acids was similar in both types of cells. Total glucose utilization was inhibited by lactate to the same extend in both neurons and astrocytes. Lactate strongly inhibited glucose oxidation through the pyruvate dehydrogenase-catalyzed reaction and the tricarboxylic acid cycle, in both neurons (60 and 44%, respectively) and astrocytes (64 and 62%, respectively). Glucose incorporation into sterols and fatty acids was also inhibited by lactate in both neurons and astrocytes (57 and 76%, respectively) while the oxidation of glucose in the pentose-phosphate shunt and the synthesis of glycerol-borne lipids was not significantly affected. These results suggest that in the presence of lactate both neurons and astrocytes can utilize lactate as the major metabolic substrate, sparing glucose for the synthesis of NADPH(H+), ribose-5-phosphate and/or glycerol-borne lipids. An interaction between glucose and lactate metabolism at the level of the pyruvate dehydrogenase-catalyzed reaction is suggested.

Nimodipine prevents the in vivo decrease in hippocampal extracellular acetylcholine produced by hypobaric hypoxia

Hypoxia decreases acetylcholine (ACh) synthesis and release in vitro, and ACh synthesis in vivo; however, its effect on extracellular concentration of ACh in vivo is not known. The calcium channel blocker nimodipine is a cerebrovascular dilator which also increases extracellular ACh in vivo. Therefore, it may provide protection from the effects of hypobaric hypoxia on the cholinergic system either via its effects on vascular function or by direct action on the nervous system. This study examined the effect of hypobaric hypoxia on extracellular ACh and choline levels, as measured by microdialysis, as well as the effects of nimodipine under hypoxia. Microdialysis guide cannulae were implanted into the hippocampal region of male Fischer rats so that probes would sample from the CA1 and DG regions. Animals were then exposed for eight hours to a simulated altitude of 5,500 m (18,000 ft) or tested at sea level for an equivalent duration. HPLC with electrochemical detection was used for analysis of the dialysates. At 5,500 m extracellular ACh levels in the placebo-treated group were significantly lower than the sea level group values. This decrement was reversed by nimodipine administered i.p. immediately preceding altitude ascent (10 mg/kg) and 250 min post-altitude ascent (10 mg/kg). These data suggest that nimodipine may provide protection from the detrimental effects of hypoxia on hippocampal cholinergic function.

Derangements in acetylcholine metabolism in brain synaptosomes in chronic renal failure

Chronic renal failure causes abnormalities in the central nervous system function and in norepinephrine metabolism of brain synaptosomes. The present study examined the effect of renal failure on the metabolism of another neurotransmitter, acetylcholine, which is involved in the modulation of behavioral and motor function. We measured acetylcholine content and release, choline content, uptake and release and activity of choline kinase in synaptosomes from rats with renal failure with various duration, renal failure-parathyroid-ectomized rats maintained normocalcemic, renal failure and normal rats treated with verapamil. Acetylcholine content increased while choline content decreased proportionally and significantly (P < 0.01) with the duration of renal failure; choline kinase activity was reduced (P < 0.01). These derangements were prevented by parathyroidectomy of renal failure rats or by their treatment with verapamil. Choline uptake and release were elevated in renal failure and these abnormalities were not corrected by parathyroidectomy or verapamil therapy. Acetylcholine release was elevated in renal failure and parathyroidectomy prevented this derangement. Verapamil reduced acetylcholine release in both normal and renal failure rats. The data show that: (a) renal failure causes significant derangements in acetylcholine metabolism leading to its accumulation in and an increase in its release from brain synaptosomes; (b) this is mainly due to reduced activity of choline kinase, most likely, mediated by the state of secondary hyperparathyroidism of renal failure; (c) blocking the parathyroid hormone-induced calcium influx into synaptosomes by verapamil prevented the abnormalities in acetylcholine metabolism; and (d) the derangement in choline uptake and release in CRF is not related to excess parathyroid hormone since parathyroidectomy or verapamil treatment did not correct them.

Somatic Acetylcholine Release in Rabbit Nodose Ganglion

In the rabbit, as in various other species, the presence of a cholinergic vagal afferent contingent has been demonstrated previously using biochemical and immunohistological approaches at the nodose ganglion level, where vagal afferent cell bodies are located. This structure is completely devoid of synaptic contacts. In the present study, somatic acetylcholine release is demonstrated on different types of in vitro rabbit nodose ganglion preparations (fragments of nodose tissue or isolated cell bodies) using chemiluminescent detection. Acetylcholine endogenous content was measured and was shown to be greater in the right nodose ganglion compared to the left. This difference was also observed when spontaneous and potassium chloride-evoked acetylcholine release was measured in extracellular fluid after a 15-min incubation of nodose ganglion fragments. Calcium removal totally blocked this somatic release. A kinetic study of acetylcholine release was also performed by placing the samples (nodose ganglion fragments or isolated cell bodies) directly in front of the photomultiplier, allowing the direct monitoring of (acetylcholine + choline) and choline effluxes. The net acetylcholine release was then deduced by subtraction. Identical kinetics was obtained with the two different nodose ganglion preparations used. This somatic release is calcium-dependent. The occurrence of acetylcholine release at the nodose ganglion level is discussed in comparison with the events occurring in the cholinergic nerve endings. These mechanisms could be implicated in the premodulation of the vagal afferent messages conveyed from the periphery to the central nervous system.

Protection of ischaemic synaptosomes from calcium overload by addition of exogenous lactate

In depolarised anoxic synaptosomes, in which lactate production was significantly raised compared with normoxic conditions, calcium uptake, net acetylcholine release, and the intrasynaptosomal calcium concentration were all significantly lowered. In contrast, lactate production in synaptosomes incubated under aglycaemic- and ischaemic-type conditions was significantly lower and basal calcium uptake, acetylcholine release, and intrasynaptosomal calcium concentration were elevated compared with normoxia. In addition, the increase in intrasynaptosomal calcium concentration under the ischaemic-type condition appeared to be greater than could be accounted for by the rise in calcium uptake alone. Intrasynaptosomal pH reflected the lactate production under each condition investigated. Addition of exogenous lactate to normoxic synaptosomes mimicked the effects observed in anoxia, suggesting that lactate itself may have blocked the calcium uptake, inhibiting the rise in intrasynaptosomal calcium and acetylcholine release occurring in depolarised anoxic synaptosomes. When lactate was added to ischaemic synaptosomes, the large rise in intrasynaptosomal calcium concentration, calcium uptake, and acetylcholine release were decreased, suggesting that lactate may have a protective role in preventing cell death by calcium overload under ischaemic-type conditions. Evidence is presented to suggest that the effect of L-lactate was due to the lactate moiety itself rather than the associated acidosis.

Conditions restricting depolarization-dependent calcium influx in synaptosomes reveal a graded response of P96 dephosphorylation and a transient dephosphorylation of P65

Temporal changes in the phosphorylation level of synaptosomal phosphoproteins following depolarization of synaptosomes were investigated under conditions restricting calcium influx. High-K+ depolarization in media of low [Na+]o (32 mM during preincubation and depolarization) at pH 6.5 resulted in a pronounced fall in the cytosolic free calcium concentration transient, and in a reduction in the initial K(+)-stimulated 45Ca2+ uptake and endogenous acetylcholine release relative to the values obtained with control synaptosomes (preincubated and depolarized in Na(+)-based media). This reduction was paralleled by a decrease in the rate of dephosphorylation of the synaptosomal protein P96. A slower dephosphorylation of P96 also was observed on exposure to 20 microM veratridine at 0.5 mM external calcium. Our results indicate that, similar to synapsin I phosphorylation, P96 dephosphorylation shows a graded response to the amount of calcium entering the presynaptic terminal. Depolarization of synaptosomes under conditions restricting the influx of calcium revealed a transient dephosphorylation (reversed within 10 s) of the phosphoprotein P65. The possible significance of this finding to the process of neurotransmitter release is discussed.

Involvement of lactic acidosis in anoxia-induced perturbations of synaptosomal function

L-Lactate (4-32 mM) added exogenously to resting or depolarised rat forebrain synaptosomes led to a significant decrease in intrasynaptosomal pH. Similarly depolarisation-induced increases in intrasynaptosomal calcium, calcium uptake, and acetylcholine release were all inhibited. These effects mimicked those previously observed in synaptosomes under anoxic conditions and suggest that lactate may be involved in limiting the damage due to calcium accumulation occurring during ischaemia. D-Lactate (added exogenously up to 32 mM) did not produce similar effects on these parameters even though the concentrations of intrasynaptosomal D-lactate reached levels comparable to those obtained with L-lactate (at 8-16 mM exogenous concentration). The results suggest that the mechanism of action of lactate on these parameters is stereospecific for the L-enantiomer. The effect of glucose availability on lactate production was assessed to explore the role of substrate availability on ischaemia/anoxic events. When exogenous glucose was increased (10-60 mM), there was no further increase in lactate production in normoxic synaptosomes, which suggests that glucose is not limiting under these conditions. When glucose was removed, as may occur in complete ischaemia, there was a significant decrease in lactate production after 60 min under anoxic or normoxic conditions. It would seem likely therefore that the mechanism underlying the changes observed in synaptosomes incubated under conditions reflecting complete ischaemia does not involve lactate.

A comparison of the distribution of neurotransmitters in brain regions of the rat and guinea-pig using a chemiluminescent method and HPLC with electrochemical detection

Six brain areas of rats and guinea-pigs, killed by microwave irradiation, were used for the concomitant measurement of the levels and regional distribution of cholinergic, biogenic amine, and amino acid neurotransmitters and metabolites. Acetylcholine (ACh) and choline (Ch) were quantified by chemiluminescence; noradrenaline (NA), dopamine (DA), 5-hydroxytryptamine (5-HT), and their metabolites by HPLC with electrochemical detection (HPLC-EC); and six putative amino acid neurotransmitters by HPLC-EC following derivatisation. The levels and regional distribution of these transmitters and their metabolites in the rat were similar to those reported in previous studies, except that biogenic amine transmitter levels were higher and metabolite concentrations were lower. The guinea-pig showed a similar regional distribution, but the absolute levels of ACh were lower in striatum and higher in hippocampus, midbrain-hypothalamus, and medulla-pons. In all areas, the levels of Ch were higher and those of NA, 5-HT, and taurine were lower than in the rat. The most marked differences between the rat and guinea-pig were in the relative proportion of DA metabolites and 5-HT turnover, as estimated by metabolite/transmitter ratios. This study can be used as a basis for a comprehensive understanding of the central effects of drugs on the major neurotransmitter systems.

The immunolysis, isolation, and properties of subpopulations of mammalian brain synaptosomes

Five subpopulations of mammalian brain synaptosomes can be selectively damaged by complement-mediated immunolysis employing antibodies to specific surface markers for each subpopulation. This allows the size of these subpopulations to be estimated. Employing antibodies alone, it has proved possible to isolate three of these subpopulations in very pure preparations which are metabolically viable. The immunoaffinity technique involved (immunomagnetophoresis) uses magnetic microspheres and produces mg (protein) quantities of synaptosomes.

2-Oxoglutarate: oxidation and role as a potential precursor of cytosolic acetyl-CoA for the synthesis of acetylcholine in rat brain synaptosomes

The possibility that 2-oxoglutarate may supply acetyl units for the cytosolic synthesis of acetylcholine in rat brain synaptosomes was investigated. The contribution of [14C]2-oxoglutarate to the synaptosomal synthesis of [14C]acetylcholine was found to be negligible despite evidence for its uptake and oxidation. The activity of the enzymes NADP-isocitrate dehydrogenase (EC 1.1.1.42), aconitate hydratase (EC 4.2.1.3), and ATP citrate-lyase (EC 4.1.3.8) were measured in the synaptosol. NADP-isocitrate dehydrogenase and aconitate hydratase are present at three- to 1.5-fold higher activities than ATP citrate-lyase. It seems likely that these enzymes contribute to the metabolism of citrate and prevent detectable formation of cytosolic acetyl-CoA from exogenously added 2-oxoglutarate (or citrate). The data further suggest that ATP citrate-lyase may in part be associated with the mitochondrial fraction.

Effects of lactic acidosis on the function of cerebral cortical synaptosomes

Synaptosomes exposed to anoxic insult produce lactate at a slow rate (measured over 60 min). No measurable damaging effects were produced by prolonged depolarisation, anoxic insult, or exogenous lactate (2-32 mM) either on the synaptic plasma membrane (as judged by release of lactate dehydrogenase and soluble proteins), or on synaptosomal phospholipases (as judged by choline release from membrane phospholipids). Potassium-stimulated acetylcholine release was decreased by incubation in the presence of lactate (2-32 mM), as was potassium- and veratrine-stimulated calcium uptake and the calcium content of depolarised synaptosomes. The intrasynaptosomal pH was also reduced but there was no stimulation of oxygen radical production (as judged by H2O2 generation) by exogenous lactate. The role that lactic acidosis may play in giving rise to the altered calcium homeostasis and decreased acetylcholine release from synaptosomes exposed to anoxic insult is discussed.

Menadione-treated synaptosomes as a model for post-ischaemic neuronal damage

Menadione bisulphite increased endogenous oxygen-radical production by rat brain synaptosomes, as indicated by H2O2 generation. Increased oxygen-radical production was also demonstrated in synaptosomes prepared from menadione-treated rats and synaptosomes reoxygenated after an anoxic insult. Acetylcholine synthesis de novo was inhibited in synaptosomes incubated with menadione in vitro, in synaptosomes prepared from menadione-treated animals in vivo, and in depolarized post-anoxic synaptosomes. Intrasynaptosomal free Ca2+ was increased by menadione in vitro (50 microM), but this increase was not due to stimulation of Ca2+ entry into the nerve terminals. Acetylcholine release was stimulated by menadione in vitro, possibly as a consequence of the elevated intrasynaptosomal Ca2+ content. The Ca2+ contents of synaptosomes prepared from menadione (10 mg/kg)-treated animals in vivo and synaptosomes reoxygenated after anoxia were unchanged. In synaptosomes prepared from menadione-treated animals, acetylcholine release was no longer significantly stimulated by K+, whereas it was unchanged from control (normoxic) values in synaptosomes reoxygenated after anoxia. None of these treatments caused any measurable damage to the synaptic plasma membrane (as judged by the release of lactate dehydrogenase), or to synaptosomal phospholipases (as judged by choline release from membrane phospholipids). Synaptosomes prepared from menadione-treated rats were found to be a good model for the study of post-anoxic damage to nerve-terminal function.

Influence of the malate-aspartate shuttle on oxidative metabolism in synaptosomes

beta-Methyleneaspartate, a specific inhibitor of aspartate aminotransferase (EC 2.6.1.1.), was used to investigate the role of the malate-aspartate shuttle in rat brain synaptosomes. Incubation of rat brain cytosol, "free" mitochondria, synaptosol, and synaptic mitochondria, with 2 mM beta-methyleneaspartate resulted in inhibition of aspartate aminotransferase by 69%, 67%, 49%, and 76%, respectively. The reconstituted malate-aspartate shuttle of "free" brain mitochondria was inhibited by a similar degree (53%). As a consequence of the inhibition of the aspartate aminotransferase, and hence the malate-aspartate shuttle, the following changes were observed in synaptosomes: decreased glucose oxidation via the pyruvate dehydrogenase reaction and the tricarboxylic acid cycle; decreased acetylcholine synthesis; and an increase in the cytosolic redox state, as measured by the lactate/pyruvate ratio. The main reason for these changes can be attributed to decreased carbon flow through the tricarboxylic acid cycle (i.e., decreased formation of oxaloacetate), rather than as a direct consequence of changes in the NAD+/NADH ratio. Malate/glutamate oxidation in "free" mitochondria was also decreased in the presence of 2 mM beta-methyleneaspartate. This is probably a result of decreased glutamate transport into mitochondria as a result of low levels of aspartate, which are needed for the exchange with glutamate by the energy-dependent glutamate-aspartate translocator.

Effects of in vitro anoxia and low pH on acetylcholine release by rat brain synaptosomes

Acetylcholine and choline release from rat brain synaptosomes have been measured using a chemiluminescent technique under a variety of conditions set up to mimic anoxic insult, including conditions of low pH (6.2) and the presence of lactate plus pyruvate as substrate. Lactate plus pyruvate as substrate consistently gave higher respiration rates than glucose alone, but with either substrate (glucose or lactate plus pyruvate) the omission of Ca2+ caused an increase in respiration whereas a low pH caused a decreased respiration. Acetylcholine release under control conditions (glucose, pH 7.4) was Ca2+-dependent, stimulated by high K+ concentrations, and decreased significantly during anoxia but recovered fully after a period of postanoxic oxygenation. Low pH (6.2) suppressed K+ stimulation of acetylcholine release, and after a period of anoxia at low pH the recovery of acetylcholine release was only partial. With lactate plus pyruvate as substrate, the effects of anoxia and/or low pH on acetylcholine release and its subsequent recovery were exacerbated. Choline release from synaptosomes, however, was not affected by anoxic/ionic conditions in the same way as acetylcholine release. At low pH (6.2) there was a marked reduction in choline release both under aerobic and anoxic conditions. These results suggest that acetylcholine release per se from the nerve is very sensitive to anoxic insult and that the low pH occurring during anoxia may be an important contributory factor.

Effects of 1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine and its metabolite 1-methyl-4-phenylpyridine on acetylcholine synthesis in synaptosomes from rat forebrain

1-Methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP) and its metabolite, 1-methyl-4-phenylpyridine (MPP+), have been shown to cause a number of lesions in dopaminergic pathways of the nigro-striatal region of the brain. However, data on the effects of these neurotoxins on other aspects of brain metabolism are scarce. The data presented here show that MPTP and MPP+ inhibit glucose oxidation via the tricarboxylic acid cycle, and acetylcholine synthesis in synaptosomal preparations from rat forebrain. Monoamine oxidase B inhibitors (e.g., pargyline, MDL 72145) relieve the inhibition caused by MPTP but not MPP+. The inhibitory effects of MPP+ on glucose oxidation and acetylcholine synthesis are a consequence of the decreased glucose metabolism in synaptosomes and are consistent with its role as an inhibitor of the Complex I (NADH-CoQ reductase) of the mitochondrial respiratory chain.