Cytoplasm Of Nerve Endings Research Articles

Intracellular magnesium does not antagonize calcium-dependent acetylcholine secretion.

1. The effects of intracellular application of Ca and Mg ions on evoked acetylcholine secretion at frog motor nerve terminals were studied. Ca and Mg were applied to the nerve-ending cytoplasm using liposomes as a vehicle. 2. Under conditions in which intracellular application of Ca produced many-fold increased in evoked acetylcholine release. 3. When Mg was applied to the nerve-ending cytoplasm concurrently with Ca, acetylcholine release was further increased above the level produced by introducing Ca alone. 4. The results suggest that intracellular Mg does not antagonize depolarization-secretion coupling and that antagonism of transmitter release by extracellular Mg occurs only at the external surface of the nerve ending.

Compartmentation of catecholamines in rat brain: Effects of agonists and antagonists

The subsynaptosomal distributions of dopamine (DA) in striatum and of norepinephrine (NE) in hypothalamus and cerebral cortex were examined. Isolated nerve-endings from each region were osmotically disrupted and subfractionated into a soluble cytoplasmic fraction (end supernatant, Se) and a synaptic vesicle fraction (P2V). DA and NE were measured in the crude homogenate and in subcellular fractions by a radioenzymatic assay. Levels of NE and DA were 3--5 times higher in the nerve-ending cytoplasm than in the synaptic vesicles, suggesting that catecholamines within the nerve-endings are predominantly in soluble form. Amphetamine increased DA levels in the tissue homogenate and in the nerve-ending cytoplasm but not in synaptic vesicles. Pargyline and gamma-butyrolactone (GBL) increased DA levels in all fractions with the greatest increase occurring in the cytoplasmic fraction. Both 6-hydroxydopamine (6-OHDA) and alpha-methyltyrosine (AMT) caused uniform DA decreases in all fractions. Hypothalamic levels of NE in the two nerve-ending compartments were also reduced to a similar extent after AMT. Reserpine produced uniform depletions of striatal DA in both nerve-ending fractions while the rate of DA repletion was more rapid in the vesicular compartment. Levels of hypothalamic NE were also uniformly depleted by reserpine at the times examined. The cytoplasmic storage compartment is discussed in terms of a possible anatomical correlate such as the smooth endoplasmic reticulum.

Choline Kinase and Ethanolamine Kinase Activity in the Cytoplasm of Nerve Endings from Rat Forebrain

In brain tissue there are two pathways by which choline and ethanolamine can be incorporated into a lipid-bound form. The first is the cytidine pathway originally described by Kennedy and Weiss (1956) for liver and the second, though it is probably a minor pathway, is a calciummediated exchange where the base is exchanged with that of an intact phospholipid (Ansell & Spanner, 1966; Porcellati, et al., 1971; Kanfer, 1972). In this account the first step of the cytidine pathway will be discussed in detail and in particular, this step in the nerve endings of brain.

Histamine as a transmitter in brain

Histamine is probably a neurotransmitter in mammalian brain. It is synthesized by a specific decarboxylase localized in the cytoplasm of nerve-endings, partly stored in synaptic vesicles, and during depolarization it is released and its synthesis accelerated. Specific receptors to this amine as evidenced by electrophysiological and behavioural studies, as well as by the activation of cyclic AMP formation, are present in brain. Lesion studies indicate that histamine-containing neurones constitute an ascending bundle arising from the brain-stem, passing through the lateral hypothalamus, and projecting diffusely into the whole telencephalon. This disposition, together with neuropharmacological data, suggest that histaminergic neurones might be involved, like the monoaminergic ones, in the control of states of wakefulness and sleep. Additionally, as indicated by histological and biochemical studies, a fraction of cerebral histamine is held in mast-cells and, when released from the latters, might also play a “transmitter” role in immune or inflammatory processes.

Choline metabolism in the cerebral cortex of guinea pigs. Phosphorylcholine and lipid choline.

1. The labelling of phosphorylcholine and choline-containing phospholipids in the subcellular fractions of guinea-pig cerebral cortex after the intraventricular injection of [N-Me-(3)H]choline into conscious animals has been studied. Special emphasis was placed upon the synaptosome fraction and early time-periods after administration. 2. The labelling of phosphorylcholine was rapid compared with that of phospholipid and was confined to two distinct subcellular fractions: the soluble cytoplasmic fraction and the synaptosome fraction. Most of the labelled phosphorylcholine of the synaptosome fraction was readily released by osmotic rupture indicating location in the nerve-ending cytoplasm. The two pools of phosphorylcholine had similar specific radioactivities at all observed times. 3. (3)H-labelled phospholipid was found in all membranous fractions. The labelling was confined to choline-containing phospholipids, notably phosphatidylcholine. 4. The labelling of the different membranous fractions was similar. 5. The half-life of the choline-containing phospholipids in the synaptic vesicle fraction was very much greater than the acetylcholine in this fraction. 6. Evidence is presented that synthesis de novo of phosphatidylcholine at nerve terminals occurs in vivo.

Subcellular distribution of taurine and cysteinesulphinate decarboxylase in developing rat brain.

The concentration of taurine and the activities of cysteinesulphinate decarboxylase and glutamate decarboxylase have been measured in rat brain. During development, taurine exhibited a decrease in concentration unrelated to the activity of cysteinesulphinate decarboxylase which increased during the same period. The distribution of taurine in subcellular fractions of adult and 7-day-old rat brain was typical of most amino acids, whereas half of the cysteinesulphinate decarboxylase activity was found in the nerve-ending cytoplasm. In anatomical distribution, taurine displayed great regional heterogeneity but both cysteinesulphinate decarboxylase and glutamate decarboxylase were more evenly distributed. Hypertaurinaemia was shown to have no effect on the entry of glycine into the brain or on its utilization in protein synthesis.

Choline Acetyltransferase from Rat Brain

Abstract The localization, purification, and some enzymatic properties of choline acetyltransferase from rat brain were studied. Most assays were performed with a specific radiometric micromethod. Solubilization of the enzyme was examined after homogenization of cerebral cortices by means which disintegrate nerve endings. In isotonic KCl the enzyme was recovered in solution. In more dilute KCl the enzyme was adsorbed progressively but reversibly to particles, especially at low pH. The subcellular localizations of the enzyme and of acetylcholine formed from choline-14C in vivo were examined further after gentle homogenization of cortices. Subcellular fractions were prepared in density gradients and were sedimented in isotonic NaCl to desorb transferase from microsomes. From isotonic homogenates, 56% of the total enzyme and 60% of acetylcholine-14C were found in nerve endings. From hypotonic homogenates (in which nerve endings are lysed) the enzyme was recovered in solution, whereas 60% of the acetylcholine-14C was found with microsomes. 3H-Acetylcholine added to the homogenizing medium remained in solution. It is concluded that acetylcholine is synthesized in the cytoplasm and is then incorporated into synaptic vesicles. The enzyme was purified from whole brains to a specific activity (Vmax) of 0.727 µmole of product min-1 mg of protein-1 at 38°. The final preparation was free of deacylases, acetylcholinesterase, and carnitine acetyltransferase. By gel filtration the molecular weight of the enzyme was about 50,000; other studies indicated that the enzyme possesses essential —SH groups, that the protein is quite cationic, and that it is soluble in 0.1 mm buffers. Fifteen salts appeared to activate the soluble enzyme. Michaelis constants for the forward reaction were determined; each substrate reduced the affinity for the other substrate. The enzyme-catalyzed reaction was reversible with an apparent equilibrium constant of 514. It is possible that the amount of acetylcholine synthesized in the cytoplasm of nerve endings is regulated by mass action.