Electrochemical measurement of release of dopamine and 5-hydroxytryptamine from synaptosomes

Dopamine and Dopamine Receptors in Alzheimer's Disease: A Systematic Review and Network Meta-Analysis

Background: The dopaminergic system has been associated with the progression of Alzheimer's disease. But previous studies found inconsistent results regarding the relationship between Alzheimer's disease and dopamine when looking at dopamine receptor concentrations. Objective: The aim of this review was to synthesize, using a random-effects model of meta-analysis, the link between the dopaminergic system and Alzheimer's disease. Methods: A detailed analysis protocol was registered at the PROSPERO database prior to data extraction (CRD42018110798). Electronic databases of PubMed, Embase, Web of Science, and Psyc-ARTICLES were searched up to December 2018 for studies that examined dopamine and dopamine receptors in relation to Alzheimer's disease. Standardized mean differences (SMD) were calculated to assess group differences in the levels of dopaminergic neurometabolites. Results: Seventeen studies met the eligibility criteria. Collectively, they included 512 patients and 500 healthy controls. There were significantly lower levels of dopamine in patients with Alzheimer's disease compared with controls (SMD = -1.56, 95% CI: -2.64 to -0.49). In addition, dopamine 1 receptor (SMD = -5.05, 95% CI: -6.14 to -3.97) and dopamine 2 receptor (SMD = -1.13, 95% CI: -1.52 to -0.74) levels were decreased in patients with Alzheimer's disease compared with controls. The results of network meta-analysis indicated that the rank of correlation with Alzheimer's disease from highest to lowest was dopamine (0.74), dopamine 2 receptor (0.49), dopamine 3 receptor (0.46), dopamine 4 receptor (0.33), dopamine 5 receptor (0.31), and dopamine 1 receptor (0.64). Conclusions: Overall, decreased levels of dopaminergic neurotransmitters were linked with the pathophysiology of Alzheimer's disease. Nonetheless, there is a clear need for more prospective studies to validate these hypotheses.

Electrochemical oxidation of catecholamines and catechols at carbon nanotube electrodes

The differences in the electrochemical oxidation of two commonly known catecholamines, dopamine and norepinephrine, and one catechol, dihydroxyphenylacetic acid (DOPAC), at three different types of carbon based electrodes comprising conventionally polished glassy carbon (GC), nitrogen-doped carbon nanotubes (N-CNTs), and non-doped CNTs were assessed. Raman microscopy and X-ray photoelectron spectroscopy (XPS) were employed to evaluate structural and compositional properties. Raman measurements indicate that N-CNT electrodes have ca. 2.4 times more edge plane sites over non-doped CNTs. XPS data show no evidence of oxygen functionalities at the surface of either CNT type. N-CNTs possess 4.0 at. % nitrogen as pyridinic, pyrrolic, and quaternary nitrogen functionalities that result in positively charged carbon surfaces in neutral and acidic solutions. The electrochemical behavior of the various carbon electrodes were investigated by cyclic voltammetry conducted in pH 5.8 acetate buffer. Semiintegral analysis of the voltammograms reveals a significant adsorptive character of dopamine and norepinephrine oxidation at N-CNT electrodes. Larger peak splittings, DeltaE(p), for the cyclic voltammograms of both catecholamines and a smaller DeltaE(p) for the cyclic voltammogram for DOPAC at N-CNT electrodes suggest that electrostatic interactions hinder oxidation of cationic dopamine and norepinephrine, but facilitate anionic DOPAC oxidation. These observations were supported by titrimetry of solid suspensions to determine the pH of point of zero charge (pH(pzc)) and estimate the number of basic sites for both CNT varieties. This study demonstrates that carbon purity, the presence of exposed edge plane sites, surface charge, and basicity of CNTs are important factors for influencing adsorption and enhancing the electrochemical oxidation of catecholamines and catechols.

Naloxone reduces the neurochemical and behavioral effects of amphetamine but not those of cocaine

The specific opioid receptor antagonist naloxone modifies the effects of amphetamine in a wide variety of behavioral paradigms. Naloxone also attenuates the amphetamine-induced increase in extracellular dopamine in the brain of rats. Therefore, these experiments were designed to replicate the neurochemical and behavioral interactions between naloxone and amphetamine, and to extend these observations to interactions between naloxone and cocaine. Microdialysis was performed on adult male rats of Sprague-Dawley descent. Rats were pretreated with a subcutaneous injection of 5.0 mg/kg naloxone or vehicle, which was followed 30 min later by cumulative doses of subcutaneous d-amphetamine (0.0, 0.1, 0.4, 1.6, 6.4 mg/kg) or intraperitoneal cocaine (0, 3, 10, 30, 56 mg/kg) at 30 min intervals. The microdialysis probes were perfused at a flow rate of 0.6 microliter/min with artificial cerebrospinal fluid. Dialysate samples were collected every 10 min from either the nucleus accumbens or striatum and analyzed for dopamine content by high-performance liquid chromatography (HPLC). Locomotor activity (photobeam breaks) was monitored simultaneously with the collection of dialysate samples. Amphetamine and cocaine dose-dependently increased extracellular dopamine in both the nucleus accumbens and striatum. Naloxone pretreatment significantly reduced the amphetamine-induced increase in extracellular dopamine in both brain regions and also attenuated the increase in locomotor activity elicited by amphetamine. Naloxone pretreatment had no effect, however, on the cocaine-induced increase in extracellular dopamine or locomotor activity. These findings suggest that endogenous opioid systems play a role in mediating the neurochemical and behavioral effects of amphetamine, but not those of cocaine.

Outflow and overflow of picogram levels of endogenous dopamine and DOPAC from rat striatal slices: improved methodology for studies of stimulus-evoked release and metabolism

An improved method for the rapid and sensitive determination of endogenous dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) in superfusates of single rat striatal slices, using high-performance liquid chromatography (HPLC) coupled with 'coulometric' electrochemical detection (EC), is described. Superfusates are directly injected into an HPLC-EC system following addition of a small aliquot of solubilized ascorbate-oxidase. DA and DOPAC are both separated and quantitated in 3-5 min. Twelve to 15 samples can be analyzed each hour with a nominal detection limit of 1.0 pg per injection for each compound or 10-20 pg/ml of superfusate. The present method was used to study changes in DA and DOPAC outflow and overflow in superfusates of single striatal slices following electrical field stimulation, both in the absence and presence of the catecholamine uptake inhibitor nomifensine. Studies of 1 min superfusate collections clearly showed that electrical field stimulation produced a latent increase in DOPAC as compared to DA. The routine sensitivity and sample throughout of the method allows for studies of both outflow and overflow of DA and DOPAC, as well as studies involving time-dependent overflow of these compounds.

Neurochemical and behavioural changes induced by ascorbic acid and d-amphetamine in the rat

In male Wistar rats, ascorbic acid (AA) (1 gr/kg i.p.) significantly (P less than 0.05) decreased the rate of CAR in the shuttle box avoidance test and antagonized the d-amphetamine (d-A) (1 mg/kg s.c.)-induced increase of CAR. Shuttle box test reduced striatal dopamine (DA) levels by about 86% (with a consistent 2-3 fold increase of DOPAC levels). DA levels were greatly reduced also in hypothalamus (HYP) and in hippocampus (HIP). AA further decreased striatal DA but significantly antagonized the reduction of DA levels in HYP and HIP. d-A antagonized the reduction of DA levels in all the above brain regions. Shuttle box test reduced noradrenaline (NA) levels mainly in HIP (-52%) and again d-A showed a significant protective effect in all regions; AA did not interfere with NA level reduction in striatum (ST) and HYP, but antagonized it in HIP. It is concluded that striatal DA system plays an important role in rat operant active avoidance behaviour. AA seems to exert a negative modulatory activity which can be counteracted by d-A.

Investigation of dopamine content, synthesis, and release in the rabbit retina in vitro: I. Effects of dopamine precursors, reserpine, amphetamine, and L-DOPA decarboxylase and monoamine oxidase inhibitors

The basal catecholamine content of rabbit retina was determined by liquid chromatography with electrochemical detection (LC-EC) and 3,4-dihydroxyphenylethylamine (dopamine, DA) found to be the major catecholamine. The immediate DA precursor, 3,4-dihydroxyphenylalanine (L-DOPA), and the metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), were also detected at about 2.8% and 17% of DA levels, respectively. When added exogenously, L-tyrosine did not increase the rate of DA synthesis over the basal level. In contrast, exogenous L-DOPA led to a 3.5-fold increase in DA, and to a 20-fold increase in DOPAC content. The monoamine oxidase inhibitors pargyline and (-)-deprenyl differentially affected the degradation of DA, since 100 microM pargyline was apparently more effective than 100 microM (-)-deprenyl. Reserpine and (+/-)-amphetamine each induced a Ca2+-independent decrease of DA stores. The separate actions of reserpine and (+/-)-amphetamine in lowering tissue DA levels were additive, suggesting two separate pools of DA available for release from presynaptic stores. The present study demonstrates that the LC-EC technique may be used to investigate the modulation of the synthesis and release of retinal DA in vitro, without the prior uptake of radiolabelled transmitter.

Differences between the release of radiolabelled and endogenous dopamine from superfused rat brain slices: effects of depolarizing stimuli, amphetamine and synthesis inhibition

Direct comparisons between radiolabelled and endogenous dopamine (DA) release from superfused rat brain slices have been made. Striatal slices were prelabelled with [3H]dopamine ([3H]DA), then superfused at 0.5 ml/min and the released catecholamines analyzed by HPLC with electrochemical detection and the radioactivity present in superfusate fractions also counted. Two successive 50 mM K+ pulses released similar amounts of endogenous DA from striatal slices, but the second pulse released 50% less [3H]DA than the first. A K+ gradient (5-53 mM) released relatively more [3H]DA compared to endogenous DA at lower K+ than at higher K+ concentrations. Blockade of DA synthesis in vitro by 50 microM a-methyl-p-tyrosine greatly reduced K+-induced endogenous DA release without any major effect on [3H]DA release. Amphetamine (10 microM) greatly increased both basal DA release and release induced by a 5 microM veratrine pulse, but its effects were 3-4 times greater on endogenous than on [3H]DA release. Although a-methyl-p-tyrosine reduced both basal and veratrine-stimulated endogenous DA release from non-prelabelled tissue by over 50% in either the presence or absence of amphetamine, it did not decrease endogenous DA release from prelabelled tissue. These studies indicate that labelled and endogenous amine release do not always occur in parallel, and that major causes of discrepancy between them may include the presence of a large newly-synthesized component in endogenous release and the uneven distribution of labelled amine within endogenous releasable pools. The results also suggest that the prelabelling process itself may alter the pools contributing to subsequent endogenous release. In the light of these studies, the assumption that labelled amine release provides an accurate marker for endogenous release should be reconsidered.

Release of endogenous dopamine, 3,4-dihydroxyphenylacetic acid, and amino acid transmitters from rat striatal slices

The release of endogenous dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) was measured in superfused striatal slices of the rat and the results compared with data obtained for the release of endogenous (a) DA and DOPAC in the cerebral cortex, nucleus accumbens and thalamus; (b) 5-hydroxytryptamine (5-HT), 5-hydroxyindoleacetic acid (5-HIAA), GABA, and glutamate in the striatum; and (c) GABA, glutamate and 5-HT in the cerebral cortex. In superfused slices of all four CNS regions, there appeared to be a Ca2+-dependent, K+-stimulated release of endogenous DA. In addition, in slices of the striatum and nucleus accumbens there also appeared to be a Ca2+ -dependent, 60 mM K+ stimulated release of endogenous DOPAC. In the striatum, 16 mM Mg2+ was as effective as 2.5 mM Ca2+ in promoting the 60 mM K+-stimulated release of DOPAC. In addition, 16 mM Mg2+ appeared to function as a weak Ca2+ agonist since it also promoted the release of DA to approximately 40% of the level attained with Ca2+ in the presence of 60 mM K+. On the other hand, in the striatum, 16 mM Mg2+ inhibited the Ca2+-dependent, 60 mM K+-stimulated release of GABA and glutamate. Similar Mg2+-inhibition was observed in the cerebral cortex not only for GABA and glutamate but also for DA and 5-HT. With the use of alpha-methyl rho-tyrosine (tyrosine hydroxylase inhibitor), cocaine (uptake inhibitor) and pargyline (monoamine oxidase inhibitor), it was determined that most of the released DA and DOPAC was synthesized in the slices during the superfusion; DOPAC was not formed from DA which had been released and taken up; and DA and DOPAC were released from DA nerve terminals. In addition, the data indicate a difference in the release process between the amino acids and the monoamines from striatal slices since Mg2+ inhibited the Ca2+-dependent, K+-stimulated release of GABA and glutamate and appeared to promote the release of DA and 5-HT.

Comparison of release of endogenous dopamine and gamma-aminobutyric acid from rat caudate synaptosomes

Release of endogenous dopamine (DA) and gamma-aminobutyric acid (GABA) from superfused rat caudate synaptosomes was monitored with liquid chromatography with electrochemical detection. Dopamine was analyzed by oxidative detection following alumina extraction while GABA was analyzed with reductive detection following pre-column derivatization with trinitrobenzenesulfonic acid and extraction. Both spontaneous and K+-stimulated (40 mM) release were examined as well as the effect of several possible neuromodulatory agents (DA, GABA, muscimol, ascorbic acid, acetylcholine). The content of GABA in the sample and the amount released by K+ were approximately fifty times those of DA although the relative amounts released by repetitive K+ stimulations were similar. Muscimol and DA significantly attenuated both the spontaneous and stimulated release of GABA while ascorbate and acetylcholine had no effect. Acetylcholine significantly increased both the stimulated and spontaneous release of DA while the other agents had no effect. Dopamine showed an absolute dependence on calcium for stimulated release while GABA exhibited a significant calcium-independent release. These results indicate that profound differences exist in the factors which modulate the release of endogenous DA and GABA.

Reserpine labels the catecholamine transporter in synaptic vesicles from bovine caudate nucleus

Tritiated reserpine binds to synaptic vesicles from bovine caudate with high affinity (Kappd = 1.25 nM, Bmax = 3.3 pmol/mg protein). This interaction is both ATP-dependent and sensitive to the protonophores CCCP and nigericin, suggesting that a proton electrochemical gradient is required for binding. Dopamine, epinephrine, norepinephrine and serotonin all inhibit reserpine binding at concentrations similar to those required for inhibition of dopamine uptake. Treatment with saponin to release vesicle contents results in complete loss of accumulated dopamine but retention of bound reserpine. These results indicate that reserpine binds to the catecholamine transport system of synaptic vesicles with high affinity and specificity.

In vivo voltammetry with electrodes that discriminate between dopamine and ascorbate

Untreated carbon-fiber voltammetric electrodes have been employed as chemical sensors of easily oxidized compounds in the brains of rats anesthetized with chloral hydrate. These electrodes can be used to distinguish dopamine from ascorbate and dihydroxyphenylacetate by the shape of the voltammograms. The electrodes are shown to provide a reproducible response to different neuronal stimuli. The rapid release of dopamine in the caudate nucleus can be measured following a local application of potassium chloride. Intraperitoneal injections of amphetamine also induce an increase of easily oxidized compounds; however, the voltammetry suggests that ascorbic acid, rather than dopamine, is the primary substance detected. Measurements in the cortex or in the caudate nucleus of animals lesioned by prior injection of 6-hydroxydopamine show that a substance with voltammetric properties identical to those of ascorbic acid also increases in concentration in these areas as a result of amphetamine administration.

Calcium-stimulated adenosine triphosphatases in synaptic membranes

We have investigated the properties of several ATPases present in synaptic membrane preparations from the cerebral cortex of rat. In addition to the intrinsic (Na+ + K+)-ATPase and a low level of contaminating Mg2+-ATPase of mitochondrial origin, both of which could be controlled by the addition of ouabain and azide, respectively, four activities were studied: (1) a Mg2+-ATPase; (2) a Mg2+-independent activity requiring Ca2+ ions at high concentrations; (3) a (Ca2+ + Mg2+)-ATPase with a high affinity for Ca2+, which were enhanced further (4) by the inclusion of calmodulin (33 nM for half-maximal activity). In the presence of 0.5 mM-EGTA in the buffer used, half saturation for these respective metal ions was observed at 0.9 mM for (1), 1.0 mM for (2), and approximately 0.3 mM for (3) and (4); the latter values correspond to concentrations of free Ca2+ of 0.38 and 0.18 microM for (3) and (4), respectively. The level of activities observed, all in nmol X min-1 X mg-1, under optimal conditions of 37 degrees C, was in a number of preparations (n in parenthesis): for (1) 446 +/- 19 (19); for (2) 362 +/- 18 (3) for (3) 87 +/- 13 (12); and for (4) 161 +/- 29 (12). The (Ca2+ + Mg2+)-ATPase, both in the presence and absence of calmodulin, could be inhibited specifically by a number of agents (approximate I0.5 in parentheses) which, at these concentrations, showed little or no potency against the other activities; among them were vanadate (less than or equal to 10 microM), La3+ (75 microM), trifluoperazine, and other phenothiazines (50 microM). These properties suggest that the (Ca2+ + Mg2+)-ATPase described may be responsible for calcium transport across one (or more) of the several membranes present in nerve endings and contained in the preparation used.

In vitro and in vivo depolarization coupled efflux of ascorbic acid in rat brain preparations

The depolarization-coupled efflux of endogenous ascorbate is demonstrated using rat synaptosome preparations and a rat cortical cup method. Ascorbate and catecholamines (monitored as a methodological control) were quantitated by high pressure liquid chromatography with electrochemical detection. In vitro potassium ion induces ascorbate efflux in a concentration-dependent, although calcium ion-dependent, manner. Veratridine also induces ascorbate efflux and its effect can be blocked by tetrodotoxin. In vivo, ascorbate efflux was likewise stimulated by increased potassium ion and by veratridine. In addition, electrical stimulation of medial lemniscus was accompanied by an increased efflux of ascorbate from somatosensory cortex. These results are intriguing in light of the recent evidence for the interaction of ascorbate with several neurotransmitter systems.

Release of endogenous catecholamines from slices of hypothalamus of rats

The release of endogenous catecholamines from superfused slices of rat hypothalamus was studied under basal conditions and during release evoked by 40 mM K+. Catecholamines in superfusates, and in extracts of the tissue after stimulation, were isolated by column chromatography and quantitated by liquid chromatography with electrochemical detection. Norepinephrine (NE) was not consistently demonstrable in superfusate collected under basal conditions, but 40 mM K+ caused the release of from 2 to 4 ng/g of tissue per min. The addition of cocaine to the superfusate caused increases in basal and evoked release of NE. Epinephrine (E) could be measured in superfusates of slices from male but not female rats and then only when cocaine was added to the superfusate. Accordingly, the concentration of E in hypothalamus was greater in male rats than in female rats. Dopamine (DA) was not consistently measurable in the spontaneous overflow from slices either in the presence or absence of cocaine. K+-evoked release of DA could be demonstrated in slices from female rats. The addition of cocaine increased the evoked release of DA from slices from both sexes. Corticosterone, added to cocaine, had no effects on the efflux of any of the catecholamines. The experiments suggest that neuronal reuptake of all catecholamines is very efficient in the hypothalamus both under basal conditions and during evoked release.

In vivo dopamine release from the anterior hypothalamus of the rat

The release of dopamine from the anterior hypothalamic/preoptic region of the anesthetized rat was investigated in vivo using a superfusion technique with a push-pull cannula. Dopamine was measured electrochemically after separation by liquid chromatography. The spontaneous release of dopamine was very low but detectable in some experiments. An inhibitor of monoamine oxidase (pargyline) and the immediate precursor of dopamine (L-DOPA) were added to synthetic cerebrospinal fluid superfusing the area. When these substances were present dopamine release was increased considerably and appeared to be stable for a long period of time. Mechanisms contributing to the formation of newly synthetized dopamine are discussed in relation to the releasing effect of d-amphetamine and the inhibiting effect of calcium-free medium. The functional significance of dopamine release was shown by the increased release of dopamine following an increase in blood pressure obtained by an intraarterial injection of blood. Finally, ventral noradrenergic bundle lesion on the same side of the superfusion site considerably enhanced dopamine release which may indicate an inhibitory control of dopamine release by noradrenergic neurons. Furthermore, this experimental procedure provides valuable means for analyzing the effects of pharmacological as well as other manipulations on the dopamine released from a superfused brain area in vivo.