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

Post-hypoxic recovery of acetylcholine release: different sensitivity of guinea pig neocortical and striatal slices

The release of endogenous acetylcholine (ACh) was measured in superfused guinea-pig cortical and striatal slices, kept at rest or electrically stimulated at different frequencies, before and during severe hypoxic conditions as well as after reoxygenation. In the cortex the basal release was unchanged by 30-60 min of hypoxia while it was inhibited in the striatum. The release evoked by short-term (2 min) stimulation at 0.5 Hz was moderately reduced (to 76%) by 30 min hypoxia in the cortex and in the striatum, but fully recovered after reoxygenation. The release evoked by continuous stimulation (from 5 to 10 to 20 Hz) was strongly inhibited (to 12-30%) in both areas after 30 min of hypoxia. After 30 min of reoxygenation, the recovery was complete in the cortex (mainly provided with cholinergic axons), but it was incomplete in the striatum (rich in cholinergic interneurones). The extent of the recovery in the latter area (i) was inversely related to stimulation frequency, (ii) did not depend on the depletion of neurotransmitter stores, because ACh tissue levels were fully restored by reoxygenation, and (iii) was consistently facilitated by excitatory aminoacid antagonists, slightly improved by the adenosine agonist R-phenylisopropyladenosine and unaffected by reducing the concentrations of radical species with catalase and superoxide dismutase or N omega-nitro-L-arginine. These results emphasize (i) the different vulnerability of the cortical and striatal cholinergic structures, (ii) the high sensitivity of the striatal interneurones to the frequency of stimulation during the posthypoxic recovery, and (iii) the relevant role played by endogenous glutamate on activity-dependent neurosecretory failure.

Fluorimetric determination of electrically evoked increase in intracellular calcium in cultured cerebellar granule cells

A technique is described to measure the electrically evoked increase in intracellular calcium in cerebellar granule cells cultured on glass coverslips and preloaded with FURA-2. To minimize light scattering, the coverslip containing the granules was placed in the fluorimeter cuvette at a 30 degrees angle to the exciting light beam. The cuvette was provided with 2 platinum electrodes so as to stimulate the neurons with a tangential field. The [Ca2+]i transients were maximized by omitting Mg2+. The fluorescence peaks were directly related to the pulse (1 ms, 100 mA) frequency and to the train length. The responses were completely tetrodotoxin- and [Ca2+]o-dependent and could be replicated 5-6 times at 5-min intervals. At the stimulation rate of 20 Hz for 5 s, a condition ensuring submaximal peaks, the [Ca2+]i rose from the basal levels of 41 +/- 2.7 nmol/l to 89.6 +/- 5.8 nmol/l. The participation of various membrane channels in the electrically induced [Ca2+]i increase was demonstrated. 4-Aminopyridine (1 mM) increased the height of the peaks to 240%. Both nifedipine (10 microM) and omega-conotoxin (1 microM) reduced the transients by about 25%. The residual response (in the absence of Mg2+) depended mostly on the release of endogenous glutamate as it proved sensitive to NMDA, AMPA and t-ACPD receptor antagonists. Since a technique to measure the electrically evoked release of D-[3H]aspartate is presently available, the parallel determination of release and of [Ca2+]i in twin populations of cultured granule cells is possible.

Effect of exogenous CDP-choline on choline metabolism in isolated adult rat ventricular myocytes under normoxic and hypoxic conditions

The purpose of this study was to examine the effect of exogenous CDP-choline on choline metabolism and phosphatidylcholine biosynthesis in adult rat ventricular myocytes. Choline uptake and metabolism were examined, using [methyl3H] choline. CDP-choline in the medium produced a concentration dependent reduction in the amount of radio-label in phosphocholine and phospholipid but it did not alter choline uptake into the myocytes. CDP-choline also did not antagonize the effect of hypoxia on phosphatidylcholine synthesis; rather it accentuated the hypoxia-induced reductions in cellular phosphocholine and phosphatidylcholine biosynthesis. These results indicate that the exogenous administration of CDP-choline alters choline metabolism in the heart by reducing the formation of phosphocholine and phosphatidylcholine without altering choline uptake and suggest an effect of a CDP-choline metabolite on choline metabolism which is not effective in opposing the effect of hypoxia on phosphatidylcholine biosynthesis.

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.

Ca(2+)-independent release of glutamate during in vitro anoxia in isolated nerve terminals

The effects of in vitro anoxia on the release of glutamate in isolated nerve terminals were studied. The extra-synaptosomal concentration of glutamate ([Glu]ext) under aerobic conditions was 2.3 microM and increased to 4.9 microM after 10 min of anoxia. However, when synaptosomes were incubated in the presence of lactate plus pyruvate instead of glucose, to prevent anaerobic glycolysis, anoxia induced an eightfold increase in the [Glu]ext. The accumulation of glutamate in the external medium during anoxia was Ca2+ independent and insensitive to a significant reduction of the Ca(2+)-dependent release of the amino acid. These results indicate that a Ca(2+)-independent efflux of cytoplasmic glutamate occurs during in vitro anoxia in isolated nerve terminals.

Output, tissue levels, and synthesis of acetylcholine during and after transient forebrain ischemia in the rat

Biochemical changes in the rat brain cholinergic system during and after 60 min of ischemia were studied using a four-vessel occlusion model. Extracellular acetylcholine (ACh) concentrations in the unanesthetized rat hippocampus markedly increased during ischemia and reached a peak (about 13.5 times baseline levels) at 5-10 min after the onset of ischemia. At 2-5 h after reperfusion, extracellular ACh concentrations were reduced to 64-72% of the levels of controls. ACh levels in the hippocampus, striatum, and cortex decreased significantly during ischemia and exceeded their control values just after reperfusion. A significant increase in hippocampal ACh level after 2 days of reperfusion and a decrease in [14C]ACh synthesis from [14C]glucose in hippocampal slices excised at 2 days after reperfusion were observed. The extracellular concentrations and tissue levels of choline markedly increased after ischemia. These results show that ACh is markedly released into the extracellular space in the hippocampus during ischemia, and they suggest that ACh synthesis is activated just after reperfusion and that cholinergic activity is reduced after 2-48 h of reperfusion in the hippocampus.

Failure of the cholinergic modulation of norepinephrine release during acute myocardial ischemia in the rat

The effect of ischemia on cholinergic presynaptic inhibition of exocytotic norepinephrine release was studied in the innervated perfused rat heart. In normoxic hearts, vagal nerve stimulation significantly reduced exocytotic norepinephrine overflow to 75% of control values. This inhibitory effect was not affected by 3 minutes of low-flow ischemia (67% of control overflow values), but was attenuated or abolished by 10-minute low-flow ischemia or by 1-, 3-, and 5-minute stop-flow ischemia (107%, 85%, 101%, and 120% of control overflow values, respectively). The alpha-adrenergic antagonist phentolamine could completely or partly restore the failure of vagally induced inhibition of norepinephrine overflow in hearts with 1-, 3-, and 5-minute stop-flow ischemia (72%, 73%, and 85% of control overflow values, respectively). The muscarinic agonist methacholine substantially inhibited norepinephrine overflow to 18% of control overflow values in normoxic hearts. This effect was also significantly attenuated by 10-minute low-flow ischemia or by 1-, 3-, and 5-minute stop-flow ischemia (46%, 38%, 53%, and 55% of control overflow values, respectively). The cholinesterase inhibitor physostigmine did not restore the methacholine-induced inhibition of norepinephrine overflow after 3-minute stop-flow ischemia to normoxic level (55% vs. 17%). These results indicate that myocardial ischemia interferes with endogenous and exogenous cholinergic presynaptic inhibition of norepinephrine overflow in the rat heart. The extent of this attenuation depends on the severity and duration of ischemia. Reduced exocytotic acetylcholine release, which is at least in part due to an enhanced adrenergic presynaptic modulation, and dysfunction of presynaptic muscarinic receptors are suggested as two possible mechanisms.

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.