The release of endogenous dopamine from rat striatum during incubation in vitro

Increasing dopamine synthesis in nigrostriatal circuits increases phasic dopamine release and alters dorsal striatal connectivity: implications for schizophrenia

One of the most robust neurochemical abnormalities reported in patients with schizophrenia is an increase in dopamine (DA) synthesis and release, restricted to the dorsal striatum (DS). This hyper functionality is strongly associated with psychotic symptoms and progresses in those who later transition to schizophrenia. To understand the implications of this progressive neurobiology on brain function, we have developed a model in rats which we refer to as EDiPs (Enhanced Dopamine in Prodromal schizophrenia). The EDiPs model features a virally mediated increase in dorsal striatal (DS) DA synthesis capacity across puberty and into adulthood. This protocol leads to progressive changes in behaviour and neurochemistry. Our aim in this study was to explore if increased DA synthesis capacity alters the physiology of DA release and DS connectivity. Using fast scan cyclic voltammetry to assess DA release we show that evoked/phasic DA release is increased in the DS of EDiPs rats, whereas tonic/background levels of DA remain unaffected. Using quantitative immunohistochemistry methods to quantify DS synaptic architecture we show a presynaptic marker for DA release sites (Bassoon) was elevated within TH axons specifically within the DS, consistent with the increased phasic DA release in this region. Alongside changes in DA systems, we also show increased density of vesicular glutamate transporter 1 (VGluT1) synapses in the EDiPs DS suggesting changes in cortical connectivity. Our data may prove relevant in understanding the long-term implications for DS function in response to the robust and prolonged increases in DA synthesis uptake and release reported in schizophrenia.

Dopaminergic-like neurons derived from oral mucosa stem cells by developmental cues improve symptoms in the hemi-parkinsonian rat model

Achieving safe and readily accessible sources for cell replacement therapy in Parkinson’s disease (PD) is still a challenging unresolved issue. Recently, a primitive neural crest stem cell population (hOMSC) was isolated from the adult human oral mucosa and characterized in vitro and in vivo. In this study we assessed hOMSC ability to differentiate into dopamine-secreting cells with a neuronal-dopaminergic phenotype in vitro in response to dopaminergic developmental cues and tested their therapeutic potential in the hemi-Parkinsonian rat model. We found that hOMSC express constitutively a repertoire of neuronal and dopaminergic markers and pivotal transcription factors. Soluble developmental factors induced a reproducible neuronal-like morphology in the majority of hOMSC, downregulated stem cells markers, upregulated the expression of the neuronal and dopaminergic markers that resulted in dopamine release capabilities. Transplantation of these dopaminergic-induced hOMSC into the striatum of hemi-Parkinsonian rats improved their behavioral deficits as determined by amphetamine-induced rotational behavior, motor asymmetry and motor coordination tests. Human TH expressing cells and increased levels of dopamine in the transplanted hemispheres were observed 10 weeks after transplantation. These results demonstrate for the first time that soluble factors involved in the development of DA neurons, induced a DA phenotype in hOMSC in vitro that significantly improved the motor function of hemiparkinsonian rats. Based on their neural-related origin, their niche accessibility by minimal-invasive procedures and their propensity for DA differentiation, hOMSC emerge as an attractive tool for autologous cell replacement therapy in PD.

Mass transport at rotating disk electrodes: effects of synthetic particles and nerve endings

An unstirred layer (USL) exists at the interface of solids with solutions. Thus, the particles in brain tissue preparations possess a USL as well as at the surface of a rotating disk electrode (RDE) used to measure chemical fluxes. Time constraints for observing biological kinetics based on estimated thicknesses of USLs at the membrane surface in real samples of nerve endings were estimated. Liposomes, silica, and Sephadex were used separately to model the tissue preparation particles. Within a solution stirred by the RDE, both diffusion and hydrodynamic boundary layers are formed. It was observed that the number and size of particles decreased the following: the apparent diffusion coefficient excluding Sephadex, boundary layer thicknesses excluding silica, sensitivity excluding diluted liposomes (in agreement with results from other laboratories), limiting current potentially due to an increase in the path distance, and mixing time. They have no effect on the detection limit (6 ± 2 nM). The RDE kinetically resolves transmembrane transport with a timing of approximately 30 ms.

Rotating disk electrode voltammetric measurements of dopamine transporter activity: an analytical evaluation

Rotating disk electrode (RDE) voltammetry at glassy carbon electrodes in 300- to 500-microL volumes has been shown in the literature by this laboratory and others to be useful in measuring dopamine and norepinephrine inward transport and release in preparations from rat brain and in expression systems. However, an analytical evaluation of the technique has not been made, and it is presented here in studies in striatal suspensions and human embryonic kidney cells expressing the human transporter for dopamine. The RDE was found to be able to measure dopamine and its metabolites with linear responses over the range of physiologically relevant concentrations with practical limits of quantification in the 10 to 50 nM range without signal and/or signal to noise enhancements. Response times of the technique were found to vary between 20 and 60 ms depending on rotation rate. Release and inward transport velocities were shown to be kinetically resolved. The glassy carbon electrode was found to be useful for several years at physiological temperatures without significant changes in electrochemical surface area, residual current, or response factors. Some sources of error in the measurement of release and inward transport values of velocity were noted and described.

Dynamic analysis of ethanol effects on NMDA-evoked dopamine overflow in rat striatum

This study was undertaken to dynamically examine the effects of ethanol on the striatal dopaminergic transmission, in terms of N-methyl-D-aspartate (NMDA)-evoked dopamine release and dopamine uptake. In the striatum of urethane-anesthetized Sprague-Dawley rats, extracellular dopamine was measured using in vivo electrochemical detection coupled with a nafion-coated carbon fiber working electrode. Micro-ejection of NMDA evoked a transient dopamine release from the dopamine-containing nerve terminals in striatum. Local application of ethanol by pressure ejection did not elicit significant changes in spontaneous dopamine release. However, with ethanol pretreatment, the time course of NMDA-induced dopamine release was markedly prolonged while the magnitude and the rate of clearance were significantly reduced. These effects were compared to those of nomifensine, a dopamine uptake blocker. Nominfensine pretreatment was found to augment the time course of NMDA-evoked dopamine release analogous to those by ethanol pretreatment. Furthermore, pretreatment with ethanol did not increase the time course parameters of dopamine signals if dopamine releases were induced by co-application of NMDA and nominfensine. These data suggest that in addition to the attenuation of NMDA-evoked dopamine release, ethanol inhibits dopamine uptake in a similar fashion to that observed with nomifensine in situ in the striatum. Indeed, ethanol altered the uptake of exogenous dopamine from the extracellular space of striatal cortex. The time course of dopamine signals was prolonged and the rate of clearance was reduced after ethanol treatment. Taken together, our data demonstrate that ethanol simultaneously inhibits NMDA-evoked dopamine release and dopamine uptake in the striatum, suggesting the importance of the interplay between release and uptake in ethanol effects on striatal dopaminergic transmission.

Factors influencing the calculation of results from studies of the release of tritiated neurotransmitters from superfused slices of guinea pig striata

Slices of guinea pig striata were incubated with tritiated choline, dopamine, or serotonin and the release of radioactive transmitter was studied in a superfusion system. Some experiments were also done on the release of [3H]acetylcholine from rat striatal slices. For analysis of the results, various parameters of the system were determined in order to establish the most reliable method of assessing drug effects on transmitter release. Peaks of radioactivity (S1 and S2) above basal release of radioactivity (B1 and B2) were observed after two depolarizations of the nerve endings with high K+ buffer. Each stimulation was for 2 min, and S2 occurred 20 min after S1. There was a highly significant correlation between S1 and the protein content of the slices for acetylcholine release from guinea pig striata. Basal release of radioactivity, and the ratio S2/S1, were not sensitive to minor changes in the experimental conditions. It was concluded that S2/S1, rather than S1 or S1/B1, should be used as a measure of drug effects on release. The experiments also demonstrated that re-uptake of released neurotransmitter is operative in the superfusion system for dopamine, but not for serotonin. Differences were observed between the rat and the guinea pig with respect to the release of [3H]acetylcholine.

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.

Ethanol enhances the calcium-dependent stimulus-induced release of endogenous dopamine from slices of rat striatum and nucleus accumbens in vitro

When slices of striatum from the rat were preincubated for 40 min in calcium-free medium in vitro, the subsequent release of endogenous dopamine (DA) induced by 40 mM KCl was completely calcium-dependent, showing a maximal response in the presence of 1.5 mM CaCl2, and a half-maximal response in the presence of 0.5 mM CaCl2. In this calcium-dependent preparation, ethanol, at concentrations of 80-120 mM, significantly increased the KCl-induced release of endogenous DA from the striatum by 19-29%. This effect was not reproducible in calcium-replete incubation medium (2.0 mM CaCl2), suggesting that the phenomenon was calcium-dependent. Preliminary studies in the nucleus accumbens of the rat showed a similar increase (30%) in the calcium-dependent, KCl-induced release of endogenous DA in the presence of 100 mM ethanol.

The effects of 1-methyl-4-phenylpyridinium ion (MPP+) on the efflux and metabolism of endogenous dopamine in rat striatal slices

1-Methyl-4-phenylpyridinium ion (MPP+) was shown to accumulate concentration-dependently in slices from rat striatum. At 10 microM, MPP+, the tissue concentration was found to be 118 +/- 9 microM following 75 min of incubation. The accumulation of MPP+ was reduced in the presence of 10 microM of the selective dopamine uptake inhibitor GBR 12909 (-50%) or by destruction of the dopaminergic terminals by complete hemisection of the forebrain 4 days before the experiments (-75%). Accumulation of MPP+ in the catecholamine-poor occipital cortex and cerebellum was only 25% of that obtained in striatum. Reserpine pretreatment of the rats in-vivo did not modify the accumulation of MPP+ in the striatal slices. MPP+ (1-10 microM) increased the net efflux of dopamine and reduced the efflux of the dopamine metabolite DOPAC from the striatal slices. The effect on dopamine was readily diminished if MPP+, after a 15 min incubation, was then omitted from the medium. In contrast, the DOPAC efflux was reduced for 75 min even though MPP+ was present in the incubation medium only for the first 15 min. In the presence of the monoamine oxidase inhibitor, pargyline (350 microM), MPP+ also produced an increase in dopamine efflux. In normal medium, the presence of the dopamine uptake inhibitor GBR 12909 (10 microM), or the absence of calcium, failed to modify the MPP+-induced increase in dopamine efflux. MPP+ also increased dopamine efflux from slices from reserpinized rats.(ABSTRACT TRUNCATED AT 250 WORDS)