Veratridine-induced outward transport of 3H-noradrenaline from adrenergic nerves of the rat vas deferens

Dopamine release mediated by the dopamine transporter, facts and consequences

Spontaneous and/or stimulated neural activity of the nigrostriatal dopamine (DA) pathway makes amines run out from the neurons. This DA dynamic follows a rather complex path, running in or out the terminals, and flushing or diffusing into the extracellular space. The location of this leakage is not limited to the axon terminals; it also occurs from the cell bodies and dendrites. This molecular release mechanism was, for a long time, considered as being produced, in part, by the exocytosis of previously stored vesicles. The DA carrier protein (DAT, DA transporter) embedded in the DA cell membrane is known to clear previously released amines through an inward DA influx. The DAT also appears to be an active vector of amine release. Particular local conditions and the presence of numerous psychostimulant substances are able to trigger an outward efflux of DA through the DAT. This process, delivering slowly large amounts of amine could play a major regulatory role in extracellular DA homeostasis.

Zn2+ modulates currents generated by the dopamine transporter: parallel effects on amphetamine-induced charge transfer and release

The psychostimulant drug amphetamine increases extracellular monamines in the brain acting on neurotransmitter transporters, especially the dopamine transporter. Mediated by this plasmalemmal pump, amphetamine does not only induce release but also charge transfer which might be involved in the release mechanism. To study a potential link between the two phenomena, we used Zn(2+) as an acute regulatory agent which modulates dopamine uptake by a direct interaction with the transporter protein. Charge transfer was investigated in patch-clamp experiments on HEK 293 cells stably expressing the human dopamine transporter, release was studied in superfusion experiments on cells preloaded with the metabolically inert transporter substrate [(3)H]1-methyl-4-phenylpyridinium. Ten micromoles of Zn(2+) had only minor effects in the absence of amphetamine but stimulated release and inward currents induced by amphetamine depending on the concentration of the psychostimulant: the effect of 0.2 microM was not significantly modulated, whereas the effect of 1 and 10 microM amphetamine was stimulated, and the stimulation by Zn(2+) was significantly stronger at 10 microM than at 1 microM amphetamine. The stimulatory action of Zn(2+) on release and inward current was in contrast to its inhibitory action on dopamine uptake. This supports a release mechanism of amphetamine different from facilitated exchange diffusion but involving ion fluxes through the dopamine transporter.

Analysis of high intracellular [Na+]-induced release of [3H]noradrenaline in rat hippocampal slices

Our aim was to investigate the mechanisms involved in the high intracellular sodium-induced transmitter release in the CNS through the characterisation of the veratridine-evoked (40 microM) noradrenaline release from rat hippocampal slices. The response to veratridine was completely inhibited by tetrodotoxin (1 microM), indicating that the effect is due to the activation of sodium channels. Omission of Ca2+ from the superfusion fluid inhibited the veratridine-evoked release by 72%, showing that the majority of release results from external Ca2+-dependent exocytosis. The residual Ca2+-independent release was not blocked by the intracellular Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid acetoxymethyl ester (100 microM) suggesting that intracellular Ca2+ stores are not involved in this component of veratridine effect. The noradrenaline uptake blockers, desipramine (10 microM) and nisoxetine (10 microM), inhibited the external Ca2+-independent release by 50 and 46%, respectively, indicating that the release partly originates from the reversal of transporters (carrier-mediated release). In contrast to uptake blockers, lowering the temperature, another possibility to inhibit transporter function, completely inhibited the effect of veratridine in the absence of Ca2+. Further experiments revealed that low temperature (20 and 12 degrees C) reduces the veratridine-induced increase of intracellular sodium concentration ([Na+]i) in rat cortical synaptosomes (68 and 78% inhibition, respectively). The clinical relevance of our data is that during ischemia a massive release of transmitters occurs mainly due to the elevation of [Na+]i, which contributes to the development of ischemic brain injury. Our results show that low temperature may be a better therapeutic approach to the treatment of ischemia because it has a dual action on this process. Firstly, it inhibits the function of uptake transporters and hence reduces the carrier-mediated outflow of transmitters. Secondly, it inhibits the sodium influx and therefore prevents the unwanted elevation of [Na+]i. Our data also suggest that veratridine stimulation can be a suitable model for ischemic conditions.

Inhibition of stimulus-triggered and spontaneous epileptiform activity in rat hippocampal slices by the Aconitum alkaloid mesaconitine

The aim of the present study was to investigate if the plant alkaloid, mesaconitine, which has been reported to have antinociceptive effects via stimulation of the noradrenergic system, inhibits epileptiform field potentials. The experiments were performed as extracellular recordings on rat hippocampal slices. Epileptiform activity was induced by omission of Mg2+ from the bathing medium or by addition of bicuculline and stimulus-evoked population bursts were recorded in the CA1 region. Spontaneous epileptiform activity was elicited by perfusing a nominally Mg2+-free bathing medium with high K+ concentration (5 mM). Both stimulus-triggered and spontaneous epileptiform activity was attenuated in a concentration-dependent manner by mesaconitine (30 nM-1 microM). The inhibitory effect was rather variable in appearance when lower concentrations (30 and 100 nM) of mesaconitine were applied. Pretreatment of the slices with the alpha-adrenoceptor antagonist yohimbine (1 microM) prevented the effect of mesaconitine. It is concluded that the inhibitory action of mesaconitine at low concentration is mediated via alpha-adrenoceptors.

Effects of the Aconitum alkaloid mesaconitine in rat hippocampal slices and the involvement of alpha- and beta-adrenoceptors

1 The effects of mesaconitine, the main alkaloid contained in Aconiti tuber, were investigated by use of extracellular recordings of stimulus-evoked population spikes and field excitatory postsynaptic potentials (e.p.s.ps) in the CA1 region of rat hippocampal slices. 2 At a concentration of 10 nM, mesaconitine evoked excitations, which were manifested as an increase in the amplitude of the orthodromic spike and the appearance of multiple spikes following the first postsynaptic spike, without affecting the magnitude of paired-pulse facilitation. The increase in spike amplitude was persistent and was not reversed by up to 90 min of washout. At concentrations of 30 and 100 nM, the alkaloid produced a biphasic effect, that is an excitation followed by an inhibition without having any effect upon the field e.p.s.p. At concentrations above 100 nM, mesaconitine suppressed the orthodromic population spike and the field e.p.s.p. 3 The excitatory effect was also observed when electrical stimulation was stopped completely during the application of mesaconitine (10 nM) and during the first 15 min of washout. 4 The enhancement of the population spike and the appearance of multiple spikes induced by mesaconitine (10-100 nM) were blocked by pretreatment with the beta-adrenoceptor antagonists propranolol (1 microM) and timolol (1 microM), whereas the inhibitory effect was blocked by the alpha-adrenoceptor antagonists yohimbine (1 microM) and phentolamine (10 microM). However, when the beta-adrenoceptor antagonist timolol was added 10 min after the application of mesaconitine, it failed to block the long-lasting enhancement of the spike amplitude and the appearance of multiple population spikes. 5 Application of the selective beta-adrenoceptor agonist isoprenaline (500 nM) to the hippocampal slices induced an increase in the amplitude of the orthodromic population spike and elicited 2-3 additional spikes. Mesaconitine (10 nM) did not further potentiate this enhancement of the spike amplitude when added after a 15 min pretreatment with isoprenaline. 6 Perfusion of forskolin, which directly activates adenylate cyclase, enhanced the population spike. Mesaconitine had no additional effect when applied after pretreatment with forskolin. 7 It is concluded that the excitatory effects evoked by lower concentrations of the plant alkaloid mesaconitine are mediated by stimulation of beta-adrenoceptors and the consequent activation of intracellular processes which lead to the long-lasting changes in excitability.

Effects of clorgyline on the release of noradrenaline from rat vas deferens

Release of endogenous noradrenaline (NA) and functional activity of presynaptic alpha 2-adrenoceptors were measured in isolated rat vas deferens following acute (2 mg/kg) or chronic (21 * 1 mg/kg daily, i.p.) treatment with clorgyline. Noradrenaline tissue content was higher in rats treated chronically with clorgyline than in those treated acutely. In vitro experiments done in the presence of 1 microM desipramine and 0.5 microM yohimbine showed that NA release, elicited by electrical field stimulation, was higher in chronic clorgyline rats than in controls, while no significant difference was found between acute clorgyline and control rats. Yohimbine enhanced evoked release of NA in all treatment groups, provided that desipramine was present in the Krebs solution, and the enhancement was non significantly higher in chronic clorgyline than in acute clorgyline and control rats. Efflux of 3,4-dihydroxyphenylglycol was lower in chronic clorgyline rats than in other groups. No difference was found between the treatment groups in a 1 min [3H]NA uptake into the tissue, nor in the ability of desipramine (1 microM) to block the uptake. The results indicate that following chronic treatment with clorgyline, evoked release of NA increased, and there was no reduction in the ability of the presynaptic alpha 2-adrenoceptor inhibitory mechanism to reduce nerve stimulation induced release of NA.

The heterogeneity of the neuronal distribution of exogenous noradrenaline in the rat vas deferens

After loading of the incubated rat vas deferens with 0.2 mumol/l 3H-noradrenaline (followed by 100 min of wash-out with amine-free solution), the efflux of endogenous and exogenous compounds was determined by HPLC with electrochemical detection and by column chromatography with scintillation counting. Two different types of heterogeneity of labelling were found. The first one is due to the preferential labelling of varicosities close to the surface of the tissue, the second one to the preferential labelling of vesicles close to the surface of loaded varicosities. As diffusion distances within the tissue and within varicosities are then longer for endogenous than for exogenous amine and metabolites, the composition of spontaneous efflux of exogenous compounds differed from that for endogenous compounds. Because of preferential neuronal and vesicular re-uptake of endogenous noradrenaline, the percentage contribution by noradrenaline to overall efflux was: endogenous less than exogenous. While 3H-DOPEG was the predominant exogenous metabolite, DOPEG and MOPEG equally contributed to the "endogenous" efflux. Desipramine abolished the consequences of the first heterogeneity of labelling, i.e., it increased the efflux more for endogenous than for exogenous noradrenaline; moreover it decreased the efflux of 3H-DOPEG, but increased that of 3H-MOPEG. The reserpine-like compound Ro 4-1284, on the other hand, abolished the consequences of the second type of heterogeneity; it reduced the specific activity of "total efflux" (i.e., of the sum of noradrenaline + DOPEG + MOPEG) to the specific activity of the tissue noradrenaline. The degree of heterogeneity of labelling was reduced after inhibition of monoamine oxidase and also when the tissues were loaded with 2 or 20 mumol/l 3H-noradrenaline. It is proposed that the various "compartments" and "pools" of noradrenaline described in the literature reflect the two heterogeneities described here.

The release of 3H-noradrenaline by p- and m-tyramines and -octopamines, and the effect of deuterium substitution in alpha-position

The 3H-noradrenaline-releasing effects of p- and m-tyramines and -octopamines, either deuterated or not, were studied in isolated vasa deferentia of the rat (COMT inhibited and calcium-free solution in all experiments). Km for uptake1 was higher for octopamines than for tyramines, but not increased by the introduction of deuterium in alpha-position, except for (probably contaminated) deuterated p-octopamine. Other tissues were preloaded with 3H-noradrenaline. After inhibition of vesicular uptake and MAO equi-releasing concentrations of the eight amines were strictly correlated with Km, they were 6 to 7 times higher for unsubstituted octopamines than for corresponding tyramines. When only MAO (but not vesicular uptake) was inhibited, this difference decreased to about 4-fold, but the releasing potency of the deuterated amines (relative to their parent amines) remained unchanged (except for p-octopamine). When vesicular uptake and MAO were intact, unsubstituted octopamines were only 1.5 to 2.2 times less potent than the corresponding tyramines. Analysis of the efflux of 3H-DOPEG confirmed that this gain in the relative potencies of octopamines is due to their increased ability to mobilize vesicular 3H-noradrenaline; moreover, deuterated amines as well were then better mobilizers than were their parent amines. It is concluded that, provided vesicular uptake is intact, the introduction of a beta-OH-group enhances the ability of indirectly acting sympathomimetic amines to mobilize vesicular noradrenaline; the introduction of deuterium in alpha-position, on the other hand, enhances this mobilizing effect exclusively when MAO is intact.

Sodium dependent 3H-noradrenaline release from rat neocortical slices in the absence of extracellular calcium: presynaptic modulation by mu-opioid receptor and adenylate cyclase activation

In Ca2+-free EGTA (1 mmol/l)-containing medium veratrine (3 mumol/l) and ouabain (100 mumol/l) strongly enhanced the efflux of 3H-noradrenaline from superfused rat brain neocortical slices prelabelled with the radioactive amine. In both cases 3H-noradrenaline release was prevented by tetrodotoxin (1 mumol/l). These effects of veratrine and ouabain were virtually additive and independent of whether the noradrenaline uptake carrier was blocked with 1 mumol/l desipramine or not. The adenylate cyclase activator forskolin (10 nmol/l - 10 mumol/l) strongly enhanced veratrine- and ouabain-induced 3H-noradrenaline release, without affecting spontaneous tritium efflux. The release induced by both stimuli was profoundly inhibited by the selective mu-opioid receptor agonist [D-Ala, MePhe4, Gly-ol5]enkaphalin (DAGO, 3 nmol/l - 1 mumol/l) in a concentration-dependent manner. The inhibitory effects of 1 mumol/l DAGO were abolished by 1 mumol/l naloxone. On the other hand, preincubation of the slices for 1 h with the delta-opioid receptor-selective irreversible ligand fentanyl isothiocyanate (1 mumol/l) did not change the inhibitory effects of DAGO. These data show that veratrine- and ouabain-induced 3H-noradrenaline release from central noradrenergic nerve terminals is facilitated by increasing intracellular cyclic AMP levels and reduced by activation of presynaptic mu-opioid receptors, indicating the involvement of exocytotic neurotransmitter release. The results provide further evidence for the hypothesis that under these conditions neurotransmitter release from central noradrenergic neurons is triggerred by a Na+-induced efflux of Ca2+ ions from intracellular stores.

The mechanism of the 3H-noradrenaline releasing effect of various substrates of uptake1: multifactorial induction of outward transport

The mechanism of action of indirectly acting sympathomimetic amines was studied in the rat vas deferens, after inhibition of vesicular uptake (by reserpine), of MAO (by pargyline) and of COMT (by U-0521). 1. Km-values for the neuronal uptake of 12 substrates were determined as the IC50 of the unlabelled substrate inhibiting the initial rate of neuronal uptake of 0.2 mumol/l 3H-(-)-noradrenaline. The IC50 ranged from 0.35 mumol/l (for(+)-amphetamine) to 44.3 mumol/l (for 5-HT). The Vmax (determined for 8 substrates) was substrate-dependent. 2. Tissues were loaded with 0.2 mumol/l 3H-(-)-noradrenaline and then washed out with amine-free solution. All 12 substrates of uptake1 induced an outward transport of 3H-noradrenaline, and equieffective concentrations were positively correlated with Km. Moreover, the EC50 for release greatly exceeded Km. It is proposed that this discrepancy between EC50 and Km is indicative of the fact that at least four factors (each one in strict dependence on Km) contribute to the initiation of outward transport of 3H-noradreanline: a) the appearance of the carrier on the inside of the axonal membrane (facilitated exchange diffusion), b) the co-transport of Na+, c) the co-transport of Cl- (both lowering the Km for 3H-noradrenaline at the inside carrier), and d) inhibition of the re-uptake of released 3H-noradrenaline (through competition for the outside carrier). 3. At least for amezinium, Vmax appears to limit the maximum rate of outward transport. 4. For some substrates (especially for the highly lipophilic ones) bell-shaped concentration-release curves were obtained.(ABSTRACT TRUNCATED AT 250 WORDS)

Simulation of outward transport of neuronal 3H-noradrenaline with the help of a two-compartment model

In order to simulate the outward transport of 3H-noradrenaline induced by veratridine from adrenergic varicosities, a mathematical two-compartment model was developed in which the two compartments (representing axoplasm and storage vesicles) are arranged in series. Simulated results were compared with experimental results obtained with 100 mumol/l veratridine + 1 mmol/l ouabain and rat vasa deferentia kept in calcium-free solution (Bönisch and Trendelenburg 1987). As in experiments, the time course of efflux of 3H-noradrenaline had a pronounced and early peak under RPU-conditions, a minor peak under PU-conditions, and solely a plateau under U-conditions (where R stands for pretreatment with reserpine, P for pretreatment with pargyline, and U for inhibition of COMT by U-0521). From the width of the peak of release, it was deduced that--under RPU-conditions--about 40% of neuronal 3H-noradrenaline are distributed into the axoplasm, about 60% into the storage vesicle. However, this estimate represents an average value; the results are compatible with the view that the ratio "axoplasmic/vesicular 3H-noradrenaline" is quite variable from rat to rat. Under U-conditions, calculations confirm that reserpine-like compounds induce an efflux of tritium that consists predominantly of deaminated 3H-metabolites. The stimulation of outward transport, on the other hand, causes an efflux of tritium that consists predominantly of 3H-noradrenaline; indeed, the efflux of deaminated 3H-metabolites declines (as it did in experiments). Simulations showed further that the highest rates of outward transport of 3H-noradrenaline were achieved when there was a simultaneous induction of outward transport of 3H-noradrenaline and a reserpine-like effect (as it is known to occur when tissues are exposed to veratridine; Bönisch and Trendelenburg 1987). While there was satisfactory agreement between simulated and experimental results under various conditions, there were also two discrepancies that may be caused by a) inhomogeneous labelling of the storage vesicles in individual varicosities (RPU less than PU less than U) and b) saturation of outward transport of 3H-noradrenaline when a reserpine-like compound greatly increases the axoplasmic level of total noradrenaline (under U-conditions).