Critical assessment of noradrenaline uptake in synaptosomal preparations

Increased noradrenaline efflux induced by local infusion of fluoxetine in the rat frontal cortex

In microdialysis experiments in vivo, local infusion of either the selective serotonin reuptake inhibitor, fluoxetine, or the selective noradrenaline uptake inhibitor, desipramine, increased noradrenaline efflux in rat frontal cortex. Synaptosomal uptake of [3H]noradrenaline was used to test whether inhibition of uptake could contribute to this effect of fluoxetine. Low concentrations of fluoxetine were less effective than desipramine at inhibiting [3H]noradrenaline uptake; both compounds were more potent than the selective serotonin reuptake inhibitor, citalopram. To investigate whether this inhibition of uptake involved an action on noradrenergic neurones, experiments compared the effects of a noradrenergic lesion, induced by the neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4), on the inhibition of uptake by fluoxetine, desipramine and citalopram. The lesion reduced [3H]noradrenaline uptake in the presence of fluoxetine and citalopram but increased it in the presence of desipramine. The results suggest both that inhibition of noradrenaline uptake could contribute to the actions of fluoxetine and that a non-noradrenergic mechanisms is a target for this action.

Contrasting effects of the imidazol(in)e alpha 2-adrenoceptor agonists, medetomidine, clonidine and UK 14,304 on extraneuronal levels of noradrenaline in the rat frontal cortex: evaluation using in vivo microdialysis and synaptosomal uptake studies

1. In vivo microdialysis in halothane-anaesthetized rats and synaptosomal [3H]-noradrenaline uptake studies in vitro were used to evaluate the effects of imidazole (medetomidine) and imidazoline (clonidine and UK 14,304) alpha 2-adrenoceptor agonists on extraneuronal levels of noradrenaline in the frontal cortex. 2. Levels of noradrenaline in the dialysate were increased by a depolarizing concentration of K+ (60 mM for 20 min) and substantially attenuated by reducing Ca2+ supply in the perfusate. These results suggest that spontaneous efflux of noradrenaline in the cortex is regulated predominantly by cation-dependent exocytotic mechanisms. 3. At a low perfusion concentration (0.5 microM), medetomidine, clonidine and UK 14,304 all reduced the level of noradrenaline in cortical dialysates. Continuous perfusion of the selective alpha 2-adrenoceptor antagonist, atipamezole (0.5 microM) caused a sustained increase in noradrenaline efflux and reversed the inhibitory effects of medetomidine. All these changes are consistent with drug actions at presynaptic alpha 2-adrenoceptors. 4. Higher concentrations of medetomidine (5-50 microM), but not clonidine or UK 14,304, evoked a non-desensitizing increase in noradrenaline efflux. This effect was not antagonized by 0.5 microM atipamezole. 5. The tricyclic noradrenaline reuptake inhibitor, desmethylimipramine (0.5-50 microM), increased noradrenaline efflux in a concentration-dependent manner. 6. The specific uptake of [3H]-noradrenaline into cortical synaptosomes was inhibited by medetomidine and desmethylimipramine with IC50 values of approximately 7 microM and 8 microM respectively. Neither clonidine nor UK 14,304 inhibited [3H]-noradrenaline uptake. 7. These results indicate that micromolar concentrations of the selective alpha2-adrenoceptor agonist,medetomidine, can augment extraneuronal levels of noradrenaline in the rat frontal cortex; this effect seems to involve an inhibition of noradrenaline reuptake rather than an action at alpha2-adrenoceptors.

Kinetics of dopamine and noradrenaline transport in synaptosomes from cerebellum, striatum and frontal cortex of normal and reeler mice

Recent evidence indicates that the cerebellum has a dopaminergic system. In order to elucidate further the dopaminergic system in the cerebellum, we investigated the transport of dopamine (DA) in synaptosomal preparations of normal and reeler mice. For comparative purposes we also studied DA transport in synaptosomal preparations from striatum and frontal cortex and compared DA transport to noradrenaline (NA) transport. [3H]-DA transport into cerebellar synaptosomes was found to be a Na(+)-dependent, two component system--a high affinity, low capacity and a low affinity, high capacity. In striatum [3H]-DA is transported by a similar high but different low affinity component. Maximal velocities of both transport components in the striatum were higher than the corresponding ones in the cerebellum. In the frontal cortex we also observed two [3H]-DA transport components with affinities significantly lower than those in cerebellum and striatum. [3H]-NA transport into synaptosomes, prepared from the three brain regions studied, showed two transport components with similar Kt and Vmax values, except for the high affinity component in striatum whose affinity is lower. In reeler mice [3H]-DA transport was different from normal only in the cerebellum where the maximal velocity for both transport components was significantly higher (2x). In contrast, no significant difference was observed in the transport of [3H]-NA. The accumulated [3H]-DA from cerebellar slices was found to be releasable by K+ stimulation, in a Ca(++)-dependent manner, and most of the released radioactivity was in the form of [3H]-DA. These results indicate that in the cerebellum there is a low-density dopaminergic system which is distinct from the corresponding noradrenergic system.

Inhibition of Uptake1 by dopexamine hydrochloride in vitro

1 Dopexamine hydrochloride, a compound under evaluation for the acute treatment of heart failure, was examined in vitro for its ability to prevent neuronal uptake of noradrenaline. 2 Despite possessing only weak beta 1-adrenoceptor agonist activity in paced guinea-pig left atria, dopexamine hydrochloride was only 23 times less potent than isoprenaline in augmenting responses of field-stimulated atrial preparations. 3 This potent effect was not observed in field-stimulated atria depleted of noradrenaline by reserpine and in the presence of cocaine was greatly reduced (1 microM) or abolished (50 microM). 4 Dopexamine hydrochloride (3 microM) potentiated the inotropic effect of exogenous noradrenaline in paced atria, thereby resembling cocaine (10 microM) and dopamine (30 microM), both of which are known inhibitors of Uptake. 5 The sodium-dependent uptake of [3H]-noradrenaline into rabbit brain synaptosomes was prevented by dopexamine hydrochloride (IC50 26 nM) and cocaine (IC50 108 nM), as well as by two other catecholamines used in the treatment of heart failure, dopamine (IC50 270 nM) and dobutamine (IC50 380 nM). 6 The cardiac stimulant effect of dopexamine hydrochloride reported in dogs and in patients with heart failure, may therefore be due in part to potentiation of endogenous catecholamines.

Examination of the relationship between the uptake site for 5-hydroxytryptamine and the high affinity binding site for [3H]imipramine. II. The role of sodium ions

Exogenous sodium ions stimulated both the high affinity binding of [3H]5-imipramine to membranes from the cortex of the rat and the high affinity accumulation of [3H]5-hydroxytryptamine (5-HT) into synaptosomes from the cortex of the rat with similar potencies. Imipramine and zimelidine inhibited synaptosomal uptake of [3H]5-HT potently in standard Tris-Krebs medium, but in a low-sodium medium their inhibitory potencies were significantly attenuated. The inhibitory potencies of panuramine and exogenous 5-HT on the uptake of [3H]5-HT were not significantly affected whether the uptake was measured in a normal or low-sodium Tris-Krebs. Imipramine and zimelidine were potent blockers of high affinity binding of [3H]imipramine whereas panuramine and 5-HT only inhibited the binding of [3H]imipramine at concentrations in excess of those required to inhibit the uptake of [3H]5-HT. It is suggested that imipramine inhibits the uptake of 5-HT by a sodium-dependent action probably at the high affinity binding site for [3H]imipramine, whereas panuramine and 5-HT inhibit the uptake of 5-HT by a sodium-independent mechanism at a site other than the binding site for [3H]imipramine.

Deltamethrin-induced changes in synaptosomal transport of 3H-epinephrine in the squid optic lobes

In fresh synaptosomal preparation from the squid optic lobe 3H-epinephrine transport was significantly affected by deltamethrin at greater than 10(-9) M. Under the experimental conditions such an effect of deltamethrin manifested as either a reduction (at nondepolarized state) or an increase (at depolarized state) in the final level of 3H-epinephrine accumulated in the synaptosomes. Only a part of such a deltamethrin effect was due to its effect on the sodium channel. The remainder of the deltamethrin effect was strongly influenced by agents or treatments which are known to influence internal Ca2+ concentration, suggesting a possible involvement of Ca2+ regulatory mechanisms in the process of stimulation of transmitter release by deltamethrin.

Time course of the metabolite patterns of intraventricularly injected [3H]noradrenaline in rat brain regions

In the hypothalamus, septum, pons with medulla, and hippocampus regions of rat brain, the level of radioactivity of [3H]noradrenaline and of five of its metabolites were determined up to 6 h after intraventricular injection of the tritiated amine. The following main results were found: In anterior hypothalamus and septum, the [3H]noradrenaline level declined in two phases. Similar turnover curves were obtained for the primary deaminated metabolites, with almost the same final half-lives as for [3H]noradrenaline. The level of the initial methylation product, normetanephrine, also showed a biphasic decline, which did not correspond to that of [3H]noradrenaline but rather was faster throughout the experiment. The final metabolites (i.e., the glycol sulfates) reached maximal levels in hypothalamus and septum earlier than in other regions. Thereafter, their levels declined with almost similar rates in all areas tested, but always faster than the [3H]noradrenaline level. The following conclusions were drawn: In areas rich in catecholaminergic nerve terminals, there seems to be a site, in addition to the vesicular storage pool, that accumulates exogenous noradrenaline and then releases it with relatively short half-lives. The contents of primary deaminated metabolites followed the turnover of [3H]noradrenaline at both sites. Exogenous [3H]noradrenaline seems to be methylated at two extraneuronal sites, which are distinguished by the rates of subsequent deamination. The size of the pool of slowly deaminated [3H]normetanephrine that is formed immediately after [3H]noradrenaline injection determined the apparent turnover of this product throughout the experiment and, thus, like the final metabolites, reflects for several hours the initial degradation of the unstored [3H]noradrenaline, rather than the metabolism of the stored amine.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterisation of sodium-dependent uptake of 5-hydroxytryptamine into guinea-pig brain

The uptake of 5-hydroxytryptamine (5-HT) was studied using isolated nerve endings (synaptosomes) prepared from the corpus striatum of the guinea pig. Analysis of temperature-dependent uptake revealed that two processes were involved, a sodium-dependent and sodium-independent mechanism. Kinetic studies indicated that the affinity of 5-HT (Ku) and the transport capacity (Vu) of the uptake process were dependent upon the external concentration of sodium. The transport capacity increased linearly with increasing concentrations of sodium up to 140 mmol/l, whereas the affinity for 5-HT was enhanced at concentrations of sodium up to 100 mmol/l, but reduced at 130 mmol/l sodium. The findings of this study suggest that uptake of 5-HT occurs as a result of a mobile carrier transport system which, depending upon the external concentration of sodium proceeds by a single or double displacement mechanism.

Identification of multiple binding sites for [3H]5-hydroxytryptamine in the rat CNS

Recent studies indicate that there may be multiple subtypes of [3H]5-hydroxytryptamine ([3H]5-HT) binding sites. Mianserin and spiperone inhibited the specific binding of [3H]5-HT (2-3 nM) to rat brain cortical membranes with shallow displacement curves. The displacement data for spiperone were best described by the presence of three independent binding sites, for which spiperone had high, medium, and low affinities. The displacement data for mianserin were best fitted by two independent, high- and low-affinity sites. The inclusion of mianserin (250 nM) to inhibit [3H]5-HT binding to the mianserin-sensitive site selectively blocked one of the sites discriminated by spiperone. These results suggest the presence of three binding sites for [3H]5-HT, one blocked by low concentrations of spiperone (5-HT1A), one blocked by low concentrations of mianserin (5-HT1C), and one blocked only by high concentrations of both mianserin and spiperone (5-HT1B). Regional differences in the relative densities of the three sites were observed. The hippocampus was rich in 5-HT1A sites, whereas the striatum contained mainly 5-HT1B and 5-HT1C sites. Selective degeneration of 5-HT-containing nerve terminals induced by the neurotoxin 5,7-dihydroxytryptamine increased binding to all three sites in the cerebral cortex. Binding of [3H]5-HT to the three sites was differentially modulated by CaCl2 and guanylimidodiphosphate. The present data suggest the presence of three independent 5-HT1 binding sites having different affinities for mianserin and spiperone and having different regional distributions.

Examination of the relationship between the uptake system for 5-hydroxytryptamine and the high-affinity [3H]imipramine binding site--I. Inhibition by drugs

The relationship between the binding site for imipramine and the uptake system for 5-hydroxytryptamine was examined. This was determined from the interaction between various drugs (including tricyclic antidepressants) and the high affinity accumulation of [3H]5-hydroxytryptamine in cortical synaptosomes from the rat, and with the high affinity binding of [3H]imipramine to cortical membranes of the rat. Imipramine and clomipramine, but not desipramine, were potent inhibitors of both binding of [3H]imipramine and the uptake of [3H]5-hydroxytryptamine. However, ouabain, panuramine and 5-hydroxytryptamine itself, all inhibited the binding of [3H]imipramine only at concentrations greater than those required to inhibit the uptake of [3H]5-hydroxytryptamine. Kinetic analysis revealed that inhibitors of the uptake system for 5-hydroxytryptamine produced inhibition by different mechanisms, but this did not account for their differential potency against uptake and binding. It is concluded that the binding site for [3H]imipramine and the uptake site for 5-HT are not directly linked and that drugs may inhibit the uptake of 5-HT at sites other than the binding site for [3H]imipramine.

Assessment of noradrenaline and 5-hydroxytryptamine uptake in intact rat brain

A method for the measurement of 3H-monoamine [noradrenaline (NA) and 5-hydroxytryptamine (5-HT)] uptake into intact brain following intracerebroventricular injection is described. Most of the accumulated monoamine was associated with nerve terminals (synaptosomes). Radioactivity in the synaptosomal fraction was retained by 0.45-micron filters and was osmotically sensitive, features indicative of accumulation within particles rather than binding to membranes. [3H]5-HT associated with synaptosomes was reduced in animals pretreated with the neurotoxin 5,7-dihydroxytryptamine, in parallel with a reduction in endogenous 5-HT levels. Oral administration of tricyclic antidepressants including clomipramine, desipramine, and imipramine inhibited the synaptosomal accumulation of 3H-monoamines. Overall, the features of the accumulation of 3H-monoamines, after intracerebroventricular administration, displayed many of the characteristics expected of a physiological monoamine reuptake system.

Characterization of sodium-dependent, high-affinity serotonin uptake in rat spinal cord synaptosomes

Synaptosomal accumulation of [3H]serotonin was used to determine if the rat spinal cord possesses a high-affinity neuronal uptake system for serotonin. Two temperature-dependent accumulation processes were found, one sodium-dependent, the second sodium-independent. Sodium-dependent [3H]serotonin accumulation was linear with sodium concentrations up to 143 mM, was associated with the purified synaptosomal fraction (P2B), and decreased 76% by osmotic lysis, 88% by sonication, and 96% by 0.1% Triton X-100. Drug inhibition studies demonstrated fluoxetine to be the most potent inhibitor of this system (IC50 0.075 microM) while desipramine (IC50 0.43 microM) and nomifensine (IC50 0.95 microM) were less potent. Kinetic analysis revealed that sodium-dependent accumulation in purified synaptosomes was saturable at low [3H]serotonin concentrations (Ku = 50 nM, Vmax = 4 pmol/mg protein/min). Sodium-independent [3H]5-HT accumulation was substantially less sensitive to fluoxetine, desipramine and nomifensine. While sodium-independent accumulation was not significantly affected by osmotic lysis, it was markedly increased by prior sonication of tissue. Also, in contrast to sodium-dependent accumulation, sodium-independent accumulation was evenly distributed in all tissue fractions, and was not saturable at low [3H]serotonin concentrations. It is concluded that sodium-dependent [3H]serotonin accumulation reflects uptake into spinal serotonergic nerve terminals while sodium-independent accumulation probably reflects a temperature-sensitive binding to membrane fragments. Comparison to brain uptake of serotonin and the necessity for using 37 degrees C sodium-free blanks rather than 0 degree C blanks in spinal cord homogenates is discussed.

Depolarization-induced release of propranolol and atenolol from rat cortical synaptosomes

The accumulation and release of [3H]-propranolol and [3H]-atenolol were examined in synaptosomes from rat cerebral cortex. Synaptosomes accumulated 20 pmol propranolol and 0.6 pmol atenolol mg-1 protein when incubated at 30 degrees C with radiolabelled drugs (0.1 microM). Exposure of propranolol-loaded synaptosomes to elevated K+, Rb+ or Cs+ evoked a concentration-dependent increase in propranolol efflux. The action of these ions in releasing propranolol was highly correlated with their ability to produce synaptosomal membrane depolarization, as estimated with the voltage-sensitive dye diS-C3-(5). Elevated K+ also promoted atenolol release from synaptosomes in a concentration-dependent manner. Veratridine (10 microM) released propranolol and atenolol from synaptosomes and these effects were antagonized by tetrodotoxin (1 microM). Under Ca2+-free conditions, K+-induced release of propranolol was reduced by 37% and atenolol release was diminished by 68%. The results support the concept that both polar and non-polar beta-adrenoceptor blocking drugs may be accumulated by nerve endings for release by membrane depolarization and suggest that neural storage and release of these molecules may influence their concentrations at localized sites of action.

Panuramine, a selective inhibitor of uptake of 5-hydroxytryptamine in the brain of the rat

The neurochemical profile of the novel inhibitor of uptake of 5-hydroxytryptamine (5-HT) panuramine (Wy 26002) has been investigated in the rat. In vitro, panuramine was found to be a potent and selective inhibitor of uptake of 5-HT with an IC50 of 22 +/- 4 nM. The IC50 for inhibition of uptake of noradrenaline was 848 nM and that for uptake of dopamine greater than 10 micron. Panuramine, in concentrations up to 10 micron did not displace the specific binding of either [3H]spiroperidol or [3H]5-HT and had no effect on the spontaneous or potassium-evoked release of 5-HT, suggesting that the compound had little effect on serotonergic transmission other than the inhibition of uptake of 5-HT. Panuramine also produced a dose-related antagonism of the depletion of 5-HT in brain induced by p-chloroamphetamine, confirming the ability of the drug to inhibit uptake of 5-HT in vivo.

Preferential uptake of norepinephrine into dopaminergic terminals of a synaptosomal preparation from rat cerebral cortex

In a synaptosomal preparation from male rat cerebral cortex only 34% of total norepinephrine (NE) uptake could be inhibited by nanomolar concentrations of desmethylimipramine (DMI) with an apparent IC50 value of 0.37 nM. The residual uptake was efficiently inhibited by micromolar concentrations of DMI (IC50 = 4.0 microM). In synaptosomes from the hypothalamus, 74% of total NE uptake could be blocked by DMI with an IC50 of 0.1 nM whereas in synaptosomes from the striatum the IC50 for DMI inhibition was 3.8 microM. It is concluded that in synaptosomes from rat cerebral cortex only 34%, and in synaptosomes from the hypothalamus 74% of total NE are taken up by noradrenergic nerve terminals whereas the residual NE uptake occurs in dopaminergic nerve endings.