Search

The influence of lanthanoids on exocytosis was investigated. It was shown that gadolinium increases the spontaneous release of the glutamate nonmetabolizing analogue [3H]D-aspartate. It was established using the fluorescent dye acridine orange that gadolinium and lanthanum induce exocytosis. The effect was dose-dependent and was maximum at 300 microM Gd3+. The exocytosis induced by gadolinium was calcium-independent. It is suggested that lanthanides induce a vesicular release of neurotransmitters by the mechanisms common for all polyvalent cations.

The effect of calcium ionophore A23187 on the release of nonmetabolizable glutamate analogues [3H]D-aspartate and the exocytosis registered by fluorescent dyes in synaptosomes was investigated. It was shown that A23187 is able to induce neurotransmitter release both in calcium-containing and calcium-free medium, the effect in the latter case being more pronounced. Calcium ionophore is able to induce exocytosis registered by acridine orange and FM 2-10. The influence of A23187 on the fluorescence of acridine orange was mainly calcium-independent, whereas the change in the fluorescence of FM 2-10 was calcium-dependent. It was suggested that the calcium-independent increase in acridine orange fluorescence is related to the dissipation of pH gradient in synaptic vesicles. Probably, the calcium-independent release of D-aspartate is also associated with the dissipation of pH gradient and subsequent leakage of neurotransmitters.

Nitric oxide (NO) modulates processes of synaptic transmission at pre- and postsynaptic levels. In the present work we studied the mechanisms of action of NO on [gamma-14C]amino-n-butyric acid ([14C]GABA) release in rat cortical synaptosomes. NO donors--S-nitroso-L-cysteine and hydroxylamine (but not sodium nitroprusside)--inhibited the neurotransmitter efflux in a concentration range from 10 microM to 1 mM. Nitrosocysteine completely and selectively suppressed the Ca2+-dependent (vesicular) [14C]GABA release, while not affecting the Ca2+-independent component of the [14C]GABA transport. The influence of NO donors was not related to activation of guanylyl cyclase, since the membrane-permeable cGMP analog dibutyryl-cGMP did not mimic and the guanylyl cyclase inhibitor methylene blue did not change the NO effects. In contrast, the membrane-permeable SH-reagent N-ethylmaleimide (NEM) resembled the effects of NO donors on the Ca2+-dependent [14C]GABA release. The degree of inhibition of the release by nitrosocysteine, hydroxylamine, and NEM correlated with their ability to oxidize intra-synaptosomal SH-groups. These data suggest that synaptosomal sulfhydryl groups are the target for NO action at the presynaptic level. The NO-induced oxidation of thiols may be involved in physiological and, especially, pathological effects of nitric oxide in the central nervous system.

We studied the influence of plasma membrane depolarization on cAMP content in presynaptic nerve endings (synaptosomes) isolated from brain hemispheres (HS) and cerebellum (CS). Depolarization by elevated [K+]o decreased basal cAMP level in both types of synaptosomes; reduced cAMP content in HS and increased cAMP in CS in the presence of IBMX; and lowered forskolin-stimulated cAMP accumulation in both the HS and the CS. Similar results were obtained when depolarization was induced by veratrine or when [Ca2+]i was elevated by treatment of the synaptosomes with the ionophore A23187. In Ca2+-free media, depolarization was not able to affect the synaptosomal cAMP levels. These data suggest that in brain synaptosomes intracellular cAMP pathway is modulated by alterations in [Ca2+]i.

The origin of calcium responsible for earlier observed acidosis-induced decrease in ATP content and inhibition of respiration in rat brain synaptosomes was studied. Acidosis (pH 6.0) inhibits both basal and potassium-stimulated 45Ca2+ uptake (60 mM KCl). Calcium channel blockers verapamil (100 microM) and 45Ca2+ (100 microM) have no effect on the level of ATP and respiration rate at pH 6.0. Theophylline (10 mM) releasing calcium from intracellular stores lowered ATP and O2 consumption rate at pH 7.4 but not at pH 6.0 being effective only in calcium-containing medium. Inhibitor of calcium transport in mitochondria ruthenium red (10 microM) prevented acidosis-induced ATP decrease. It is suggested that acidosis inhibits oxidative phosphorylation by releasing calcium from cytoplasmic stores with its subsequent transport into intrasynaptosomal mitochondria.

Incubation of rat brain synaptosomes at pH 6.0 in Ca2+-containing medium is associated with a decrease in the ATP content and the rate of oxygen consumption. ATP/ADP ratio decreased from 6.6 +/- 0.24 at pH 7.4 to 3.2 +/- 0.17 at pH 6.0. The content of 86Rb+ and [3H]tetraphenylphosphonium measured at pH 7.4 did not change after preincubation at pH 6.0, indicating the absence of lesion of synaptosomal plasma membranes and intrasynaptosomal mitochondria. Incubation with 1 mM EGTA in Ca2+-free medium as well as addition of 1 mM ouabain or 10 microM ruthenium red prevents the effect of acidosis. Similar results were obtained when 5 mM pyruvate was used as a mitochondrial substrate instead of glucose. It is suggested that acidosis-induced decrease in the ATP level is associated with the increase in Ca2+ concentration in the cytoplasm and its transport into mitochondria. Ouabain reverses this process due to activation of Na+/Ca2+ exchange.

Influence of hypotonic swelling on Ca2+ (45Ca2+) uptake in rat brain synaptosomes was studied. A decrease in medium osmolality from 310 to 260-180 mOsm led to a progressive stimulation of 45Ca2+ accumulation. The effect was blocked by verapamil (IC50 = 5 microM), CoCl2 (IC50 = 58 microM) and retained at a fixed concentration of external sodium indicating the involvement of Ca2+ channels rather than Na+/Ca2+ exchange in swelling-induced Ca2+ influx. The populations of calcium channels observed in hypoosmotic and depolarizing conditions are different in three aspects: (i) kinetics of 45Ca2+ entry; (ii) insensitivity to dihydropyridines and omega-conotoxin GVIA; (iii) insensitivity to preliminary depolarization by high potassium. The effects of swelling and depolarization on Ca2+ uptake were additive. No change in membrane potential monitored with diS-C3-(5) was recorded during synaptosome hypotonic swelling. The results suggest the existence in synaptosomal plasma membrane of volume-dependent calcium-permeable channels with properties distinct from those of the voltage-dependent calcium channels. Activation of these channels may constitute an early event in volume regulation of nerve terminals in anisoosmotic conditions.

The influence of acidosis on the transmembrane potential, sodium pump and membranous systems of calcium transport was studied on isolated presynaptic nerve terminals (synaptosomes) from rat brain. It is established that acidic shift causes a decrease of membrane potential, a large inhibition of the sodium pump (by three times at pH 6.0). All the systems controlling both inward- and outward-directed calcium fluxes are partially blocked by low pH. At pH 6.0 the basal influx and calcium pump are reduced two-fold while the voltage-sensitive calcium channels and Na+/Ca2+ exchanger are inhibited by three and four to five times, respectively. We have no found any evidence of acidosis-induced net flux of calcium directed inwards.

Accumulation of TBA-reactive substances after a 40 min incubation of rat brain synaptosomes at 37 degrees C was analysed. A lowering of pH to 6.5-5.5 or arachidonic acid (0.1-1.0 mM) increased lipid peroxidation which was blocked by antioxidants. Acidosis (pH 6.0) and arachidonic acid used in combination had a strong synergic effect. Depolarisation of plasma membranes or intrasynaptosomal mitochondria were without influence on lipid peroxidation at neutral or acid pH. The results support a leading role of acidosis and phospholipases in stimulation of peroxidation under ischaemia.

Enzymatic proteolysis of the proteins of synaptic membranes has been found to be accompanied by the promotion of lipid peroxidation probably mediated by the liberation of membrane-bound iron. As fluorescent probes pyrene and diphenylhexatriene show, the microviscosity and micropolarity of membrane lipid phase rise as a result of lipid peroxidation. Different structural changes induced by proteolysis are displayed under inhibition of lipid peroxidation. Thus, the microviscosity of the bulk lipid phase appears to be lowered and the annular lipid microviscosity raised. Another explanation of the fluorescent data for annular lipids is the exclusion of pyrene molecules from this lipid pool, leading to a reduction of the probe local concentration. The changes observed in membrane lipid phase are considered as primary structural effects of proteolysis, not mediated by a phospholipase activation.

The effect of hypoosmolality of incubation medium on the rat of ouabain-sensitive 86Rb+ transport in rat brain synaptosomes was studied. A decreased osmolality from 310 to 250 mOsm increased the rate of 86Rb+ uptake from 3.72 to 6.23 nmol/mg of protein min. To evaluate the involvement of cytoplasmic sodium in sodium pump stimulation inhibitors of ion channels and transport pathways able to increase [Na+]in were used. Tetrodotoxin (1 microM), amiloride (0.5 mM) and verapamil (0.1 mM) had no influence on the osmotic response of the sodium pump. The decrease of sodium concentration in incubation medium to 15 mM, leading to a practical loss of its transmembrane gradient, did not abolish stimulation of pump. No increase in 22Na+ influx or intrasynaptosomal sodium content was registered at hypotonic conditions. It is suggested that osmotic regulation of Na+,K(+)-ATPase is not connected with an increase of internal sodium through opening of sodium channels, or with activation of other membrane sodium-transporting systems.

In the rat brain synaptosomes the furosemide-sensitive component of 86Rb+ uptake constituted 30.8% of the total uptake in the medium containing 132 mM Na+. A decrease in the medium tonicity from 310 to 230 mOsm increased the rate of 86Rb+ uptake from 2.38 +/- 0.58 to 7.12 +/- 0.52 nMoles/mg of protein/min.

Effect of medium osmolarity on 3H-ouabain binding and the rate of ouabain-sensitive 86Rb+ transport in the rat brain synaptosomes was studied. A decrease in tonicity to 230 mOsm increases both parameters indicating the activation of the sodium pump upon synaptosome swelling. The effect is retained in the absence of inside-oriented Na+ gradient, i. e. a rise in Na(in) is not responsible for hypoosmotic activation. Colchicine (5mM) decreased and cytochalasin B (40 microM) increased the ouabain binding. In the presence of cytochalasin B the inhibition of binding observed under hypotonic conditions was shifted to higher osmolarity values. It is suggested that volume regulation of the sodium pump is controlled by the cytoskeleton elements.

Effect of neurotoxins veratrine (100 micrograms/ml) and tetrodotoxin (1 microM) on the binding of 3H-ouabain (10(-8) M) with Na,K-ATPase of intact synaptosomes and isolated synaptic membranes was studied. The persistent opening of sodium channels in synaptosomes by veratrine results in an increase of specific binding of the labeled ligand by 20%. A similar effect was caused by Na/H exchanger monensin. Destruction of microtubules with vinblastine and colchicine has no influence on veratrine action, while depolymerization of microfilaments with cytochalasin B reverses the neurotoxin effect. In isolated synaptic membranes veratrine and tetrodotoxin stimulate ouabain binding, the absolute veratrine-induced increment being several times higher in the presence of ATP than in its absence. Since the closed vesicles of any type are not permeable to ATP and ouabain, it means that in the isolated membranes an interaction between sodium channels and Na,K-ATPase molecules takes place. In intact nerve endings such a mechanism may be operative along with the known ways of control of sodium pump and its ouabain-binding site.

Studies are presented of the effect of procaine group anesthetics on rat brain synaptosome stability to dodecyl sulfate and on catalytic properties of the membrane bound alkaline phosphatase. The dose curves of detergent stability are characterized by two maxima, one at 3.10(-4) M for tetracaine, 2.10(-6) M for lidocaine and 5.10(-5) M for procaine; the other being at 10(-3) M for all anesthetics. The curves of Vmax and KM versus procaine concentration to exhibit the minimum at 5.10(-7) M and maximum at 3,2.10(-6) M. Procaine at 5.10(-7) M increases enthalpy and enthropy of membraneous alkaline phosphatase. It is suggested that interactions between anesthetics and centers of high affinity lead to synaptosome structural rearrangements, which affect the properties of membraneous enzymes.

The contribution of Ca2+ channels and Na+/Ca2+ exchange to Ca2+ uptake in rat brain synaptosomes upon long- (t greater than or equal to 30 s) and short-term (t less than 30 s) depolarization by high K+ was studied by measuring the 45Ca content and free Ca2+ concentration (from Quin-2 fluorescence). At 37 degrees C, the system responsible for the K+-stimulated uptake of 45Ca (t greater than or equal to 30 s) and the Na+/Ca+ exchanger are characterized by a similar concentration dependence of external Ca2+ (Ca0(2+] and K0+ as well as by an equal sensitivity to verapamil (Ki = approximately 20-40 microM) and La2+ (Ki = approximately 50 microM). These data and the results from predepolarization suggest that the 45Ca entry into synaptosomes at t greater than or equal to 30 s is due to the activation of Na+/Ca+ exchange caused by its electrogenic component, while the insignificant contribution of Ca2+ channels can be accounted for by their inactivation. At low temperatures (2-4 degrees C) which decelerate the inactivation, the initial phase of 45Ca uptake is fully provided for by Ca2+ channels, showing a lower (as compared to the exchanger) affinity for Ca0(2+) (K0.5 greater than 1 mM)m a greater sensitivity to La3+ (Ki = approximately 0.2-0.3 microM) and verapamil (Ki = approximately 2-3 microM); these channels are fully inactivated by predepolarization with K0+, ouabain and batrachotoxin. The Ca2+ channels can be related to T-type channels, since they are not blocked by nicardipine and niphedipine.

Using a radioactive permeant cation 3H-tetraphenylphosphonium, the sensitivity of rat brain synaptosomes to depolarizing action of veratrine, which specifically opens the sodium channels, was compared before and after destruction of microtubules and microfilaments. Depolymerization of microtubules with colchicin and vinblastine decreased an apparent affinity of veratrine to its receptor in the channel, while destruction of microfilaments with cytochalasin B had the opposite effect. Colchicine did not change allosteric interactions between the receptor for veratrine and that for scorpion venom in the sodium channel evaluated by the ability of scorpion venom to facilitate veratrine-induced depolarization of synaptosomes. It is suggested that two main cytoskeleton subsystems control the state of sodium channels in the nerve ending.

Studies were carried out on structural state of synaptosomes and synaptic plasma membranes from brain of rats and guinea pigs at 5--50C. In membranes from guinea pig brain there were two breaks at 8 and at 18--20C in the curves of temperature dependence of light scattering. At the same temperatures adhesive properties of membranes had changed in jump-like manner when evaluating the degree of aggregation at pH4. The degree of aggregation in membranes from rat brain as well as rate of solubilization by 0.2% SDS (stoped-flow measurements) are characterized by single break point at 27C in Arrhenius curves. The breaks were interpreted as phase transitions with membrane surface involvement. Chlorpromazine and tetraciane (10(4)--10(3) M) abolished or modified these transitions. Because the number of negative charges and tryptophanyl and ANS fluorescence parameters were unchanged it was suggested that redistribution of ionogenic and hydrophobic goups took place at membrane surface during phase transitions.

General anesthetic ketamine (1 mM) decreases relative sodium permeability (PNa+/PK+) of synaptosomes measured by transmembrane potential--external potassium concentration curves. Since tetrodotoxin (10(-7) M) does not affect the value PNa+/PK+ it is concluded that potassium permeability of the membrane increases in the presence of ketamine.

Depolarization of synaptosomes with veratrine (0.1 mg/ml) or 50 mmol/l of KCl results in the release of radioactivity from 14C-choline loaded synaptosomes with partial dependence on external Ca2+. Like tetrodotoxin, tetracaine prevented veratrine but not KCl action, with a half-maximal effect at approximately 10(-5) mol/l of anesthetic. A similar half-maximal value was obtained for tetracaine blockade of veratrine-stimulated tetrodotoxin-sensitive 22Na influx into synaptosomes, complete blockage being achieved at 10(-4) mol/l. At this concentration tetracaine failed to modify Ca2+ channels measured by KCl-induced 45Ca uptake. Microviscosity of a lipid bilayer in synaptic membranes evaluated with 5- and 16-doxylstearate spin labels decreased at tetracaine concentrations exceeding 10(-3) mol/l. It is suggested that Na+ channels of synaptosomes are blocked by direct action of anesthetic or through changes in the channel annular lipids.