D-amphetamine fails to increase extracellular dopamine levels in mice lacking alpha 1b-adrenergic receptors: relationship between functional and nonfunctional dopamine release.

It was found recently that locomotor and rewarding effects of psychostimulants and opiates were dramatically decreased or suppressed in mice lacking alpha1b-adrenergic receptors [alpha1b-adrenergic receptor knock-outs (alpha1bAR-KOs)] (Drouin et al., 2002). Here we show that blunted locomotor responses induced by 3 and 6 mg/kg d-amphetamine in alpha1bAR-KO mice [-84 and -74%, respectively, when compared with wild-type (WT) mice] are correlated with an absence of d-amphetamine-induced increase in extracellular dopamine (DA) levels in the nucleus accumbens of alpha1bAR-KO mice. Moreover, basal extracellular DA levels in the nucleus accumbens are lower in alpha1bAR-KO than in WT littermates (-28%; p < 0.001). In rats however, prazosin, an alpha1-adrenergic antagonist, decreases d-amphetamine-induced locomotor hyperactivity without affecting extracellular DA levels in the nucleus accumbens, a finding related to the presence of an important nonfunctional release of DA (Darracq et al., 1998). We show here that local d-amphetamine releases nonfunctional DA with the same affinity but a more than threefold lower amplitude in C57BL6/J mice than in Sprague Dawley rats. Altogether, this suggests that a trans-synaptic mechanism amplifies functional DA into nonfunctional DA release. Our data confirm the presence of a powerful coupling between noradrenergic and dopaminergic neurons through the stimulation of alpha1b-adrenergic receptors and indicate that nonfunctional DA release is critical in the interpretation of changes in extracellular DA levels. These results suggest that alpha1b-adrenergic receptors may be important therapeutic pharmacological targets not only in addiction but also in psychosis because most neuroleptics possess anti-alpha1-adrenergic properties.

5-HT2A and alpha1b-adrenergic receptors entirely mediate dopamine release, locomotor response and behavioural sensitization to opiates and psychostimulants.

Addictive properties of drugs of misuse are generally considered to be mediated by an increased release of dopamine (DA) in the ventral striatum. However, recent experiments indicated an implication of alpha1b-adrenergic receptors in behavioural responses to psychostimulants and opiates. We show now that DA release induced in the ventral striatum by morphine (20 mg/kg) is completely blocked by prazosin (1 mg/kg), an alpha1-adrenergic antagonist. However, morphine-induced increases in DA release in the ventral striatum were found to be similar in mice deleted for the alpha1b-adrenergic receptor (alpha1b-AR KO) and in wild-type (WT) mice, suggesting the presence of a compensatory mechanism. This acute morphine-evoked DA release was completely blocked in alpha1b-AR KO mice by SR46349B (1 mg/kg), a 5-HT2A antagonist. SR46349B also completely blocked, in alpha1b-AR KO mice, the locomotor response and the development of behavioural sensitization to morphine (20 mg/kg) and D-amphetamine (2 mg/kg). Accordingly, the concomitant blockade of 5-HT2A and alpha1b-adrenergic receptors in WT mice entirely blocked acute locomotor responses but also the development of behavioural sensitization to morphine, D-amphetamine or cocaine (10 mg/kg). We observed, nevertheless, that inhibitory effects of each antagonist on locomotor responses to morphine or D-amphetamine were more than additive (160%) in naïve WT mice but not in those sensitized to either drug. Because of these latter data and the possible compensation by 5-HT2A receptors for the genetic deletion of alpha1b-adrenergic receptors, we postulate the existence of a functional link between these receptors, which vanishes during the development of behavioural sensitization.

Comparative effects of organic and inorganic mercury on in vivo dopamine release in freely moving rats

The present study was carried out in order to compare the effects of administration of organic (methylmercury, MeHg) and inorganic (mercury chloride, HgCl 2 ) forms of mercury on in vivo dopamine (DA) release from rat striatum. Experiments were performed in conscious and freely moving female adult Sprague-Dawley (230-280 g) rats using brain microdialysis coupled to HPLC with electrochemical detection. Perfusion of different concentrations of MeHg or HgCl 2 (2 µL/min for 1 h, N = 5-7/group) into the striatum produced significant increases in the levels of DA. Infusion of 40 µM, 400 µM, or 4 mM MeHg increased DA levels to 907 ± 31, 2324 ± 156, and 9032 ± 70% of basal levels, respectively. The same concentrations of HgCl 2 increased DA levels to 1240 ± 66, 2500 ± 424, and 2658 ± 337% of basal levels, respectively. These increases were associated with significant decreases in levels of dihydroxyphenylacetic acid and homovallinic acid. Intrastriatal administration of MeHg induced a sharp concentration-dependent increase in DA levels with a peak 30 min after injection, whereas HgCl 2 induced a gradual, lower (for 4 mM) and delayed increase in DA levels (75 min after the beginning of perfusion). Comparing the neurochemical profile of the two mercury derivatives to induce increases in DA levels, we observed that the time-course of these increases induced by both mercurials was different and the effect produced by HgCl 2 was not concentration-dependent (the effect was the same for the concentrations of 400 µM and 4 mM HgCl 2 ). These results indicate that HgCl 2 produces increases in extracellular DA levels by a mechanism differing from that of MeHg.

Exogenous alpha-Synuclein decreases raft partitioning of Cav2.2 channels inducing dopamine release

alpha-Synuclein is thought to regulate neurotransmitter release through multiple interactions with presynaptic proteins, cytoskeletal elements, ion channels, and synaptic vesicles membrane. alpha-Synuclein is abundant in the presynaptic compartment, and its release from neurons and glia has been described as responsible for spreading of alpha-synuclein-derived pathology. alpha-Synuclein-dependent dysregulation of neurotransmitter release might occur via its action on surface-exposed calcium channels. Here, we provide electrophysiological and biochemical evidence to show that alpha-synuclein, applied to rat neurons in culture or striatal slices, selectively activates Cav2.2 channels, and said activation correlates with increased neurotransmitter release. Furthermore, in vivo perfusion of alpha-synuclein into the striatum also leads to acute dopamine release. We further demonstrate that alpha-synuclein reduces the amount of plasma membrane cholesterol and alters the partitioning of Cav2.2 channels, which move from raft to cholesterol-poor areas of the plasma membrane. We provide evidence for a novel mechanism through which alpha-synuclein acts from the extracellular milieu to modulate neurotransmitter release and propose a unifying hypothesis for the mechanism of alpha-synuclein action on multiple targets: the reorganization of plasma membrane microdomains.

Does intravenous Δ9-tetrahydrocannabinol increase dopamine release? A SPET study

Intravenous (IV) Δ9-tetrahydrocannabinol (THC) induces transient psychotic symptoms in healthy subjects and in schizophrenic patients, but the psychotomimetic mechanism is unknown. One possibility is that THC stimulates dopamine (DA) release in the striatum. In this study we tested whether IV THC led to an increase in striatal DA release compared to placebo. We also investigated whether DA release and positive psychotic symptoms were related. Eleven healthy male volunteers completed two 123I-iodobenzamide ([123I]IBZM) single photon emission tomography (SPET) sessions and received IV THC (2.5 mg) or placebo in a randomized counterbalanced order, under double-blind conditions. Analysable data were obtained from nine participants. The Positive and Negative Syndrome Scale (PANSS) was used to rate psychotomimetic effects. Striatal binding index values were calculated using the occipital cortex as a reference region. Both the PANSS positive and general symptoms increased significantly at 30 min following IV THC. There were no significant differences in binding index in the caudate or putamen under THC compared to placebo conditions. Positive psychotic symptoms and DA release were unrelated. THC did not lead to a significant increase in DA release even though the dose was sufficient for participants to have psychotic symptoms. These findings do not support a central role for striatal DA in THC-elicited psychosis.

Single photon emission computerized tomography imaging of amphetamine-induced dopamine release in drug-free schizophrenic subjects.

The dopamine hypothesis of schizophrenia proposes that hyperactivity of dopaminergic transmission is associated with this illness, but direct observation of abnormalities of dopamine function in schizophrenia has remained elusive. We used a newly developed single photon emission computerized tomography method to measure amphetamine-induced dopamine release in the striatum of fifteen patients with schizophrenia and fifteen healthy controls. Amphetamine-induced dopamine release was estimated by the amphetamine-induced reduction in dopamine D2 receptor availability, measured as the binding potential of the specific D2 receptor radiotracer [123I] (S)-(-)-3-iodo-2-hydroxy-6-methoxy-N-[(1-ethyl-2-pyrrolidinyl) methyl]benzamide ([123I]IBZM). The amphetamine-induced decrease in [123I]IBZM binding potential was significantly greater in the schizophrenic group (-19.5 +/- 4.1%) compared with the control group (-7.6 +/- 2.1%). In the schizophrenic group, elevated amphetamine effect on [123I]IBZM binding potential was associated with emergence or worsening of positive psychotic symptoms. This result suggests that psychotic symptoms elicited in this experimental setting in schizophrenic patients are associated with exaggerated stimulation of dopaminergic transmission. Such an observation would be compatible with an abnormal responsiveness of dopaminergic neurons in schizophrenia.

Circadian time-dependent effects of fencamfamine on inhibition of dopamine uptake and release in rat striatal slices

Fencamfamine (FCF) is a central stimulant that facilitates central dopaminergic transmission through inhibition of dopamine uptake and enhanced release of the transmitter. We evaluated the changes in the inhibition of uptake and the release of striatal [3H]-dopamine at 9:00 and 21:00 h, times corresponding to maximal and minimal behavioral responses to FCF, respectively. Adult male Wistar rats (200-250 g) maintained on a 12-h light/12-h dark cycle (lights on at 7:00 h) were used. In the behavioral experiments the rats (N = 8 for each group) received FCF (3.5 mg/kg, ip) or saline at 9:00 or 21:00 h. Fifteen minutes after treatment the duration of activity (sniffing, rearing and locomotion) was recorded for 120 min. The basal motor activity was higher (28.6 ± 4.2 vs 8.4 ± 3.5 s) after saline administration at 21:00 h than at 9:00 h. FCF at a single dose significantly enhanced the basal motor activity (38.3 ± 4.5 vs 8.4 ± 3.5 s) and increased the duration of exploratory activity (38.3 ± 4.5 vs 32.1 ± 4.6 s) during the light, but not the dark phase. Two other groups of rats (N = 6 for each group) were decapitated at 9:00 and 21:00 h and striata were dissected for dopamine uptake and release assays. The inhibition of uptake and release of [3H]-dopamine were higher at 9:00 than at 21:00 h, suggesting that uptake inhibition and the release properties of FCF undergo daily variation. These data suggest that the circadian time-dependent effects of FCF might be related to a higher susceptibility of dopamine presynaptic terminals to the action of FCF during the light phase which corresponds to the rats' resting period

Circadian time-dependent effects of fencamfamine on inhibition of dopamine uptake and release in rat striatal slices

Fencamfamine (FCF) is a central stimulant that facilitates central dopaminergic transmission through inhibition of dopamine uptake and enhanced release of the transmitter. We evaluated the changes in the inhibition of uptake and the release of striatal [ 3H]-dopamine at 9:00 and 21:00 h, times corresponding to maximal and minimal behavioral responses to FCF, respectively. Adult male Wistar rats (200-250 g) maintained on a 12-h light/12-h dark cycle (lights on at 7:00 h) were used. In the behavioral experiments the rats (N = 8 for each group) received FCF (3.5 mg/kg, ip) or saline at 9:00 or 21:00 h. Fifteen minutes after treatment the duration of activity (sniffing, rearing and locomotion) was recorded for 120 min. The basal motor activity was higher (28.6 ± 4.2 vs 8.4 ± 3.5 s) after saline administration at 21:00 h than at 9:00 h. FCF at a single dose significantly enhanced the basal motor activity (38.3 ± 4.5 vs 8.4 ± 3.5 s) and increased the duration of exploratory activity (38.3 ± 4.5 vs 32.1 ± 4.6 s) during the light, but not the dark phase. Two other groups of rats (N = 6 for each group) were decapitated at 9:00 and 21:00 h and striata were dissected for dopamine uptake and relase assays. The inhibition of uptake and release of [ 3H]-dopamine were higher at 9:00 than at 21:00 h, suggesting that uptake inhibition and the release properties of FCF undergo daily variation. These data suggest that the circadian time-dependent effects of FCF might be related to a higher susceptibility of dopamine presynaptic terminals to the action of FCF during the light phase which corresponds to the rats' resting period.

Mechanisms for the release of dopamine from turtle retina

The retinal circuitry underlying the release of dopamine was examined in the turtle, Pseudemys scripta elegans, using neurochemical release studies, anatomical techniques, and biochemistry. There was a dose- and calcium-dependent release of dopamine from turtle retinas incubated in $\sp3$H-dopamine after perfusion of the GABA antagonist bicuculline. This indicated that dopamine release was tonically inhibited by GABA. Other putative retinal transmitters were examined. Glutamate antagonists selective for hyperpolarizing bipolar cells, such as 2,3-piperidine dicarboxylic acid (PDA), caused dose- and calcium-dependent release of dopamine from the retina. In contrast, release was not observed after perfusion with 4-aminophosphonobutyric acid, a specific antagonist of depolarizing bipolar cells. This indicated that depolarizing bipolar cells were not involved in retinal circuitry underlying the release of dopamine in the turtle retina. The release produced by PDA was blocked by bicuculline, indicating a polysynaptic mechanism of release. None of the other agents tested, which included carbachol, strychnine, dopamine uptake inhibitors, serotonin, tryptamine, muscimol, melatonin, or dopamine itself produced release.^ The cells capable of the release of dopamine were identified using both uptake autoradiography and immunocytochemical localization with dopamine antisera. The simplest circuitry based on these findings is signal transmission from photoreceptors to hyperpolarizing bipolar cells then to GABAergic cells, and finally to dopaminergic amacrine cells. ^

Nitric oxide inhibits the release of norepinephrine and dopamine from the medial basal hypothalamus of the rat.

Previous research indicates that norepinephrine and dopamine stimulate release of luteinizing hormone (LH)-releasing hormone (LHRH), which then reaches the adenohypophysis via the hypophyseal portal vessels to release LH. Norepinephrine exerts its effect via alpha 1-adrenergic receptors, which stimulate the release of nitric oxide (NO) from nitricoxidergic (NOergic) neurons in the medial basal hypothalamus (MBH). The NO activates guanylate cyclase and cyclooxygenase, thereby inducing release of LHRH into the hypophyseal portal vessels. We tested the hypothesis that these two catecholamines modulate NO release by local feedback. MBH explants were incubated in the presence of sodium nitroprusside (NP), a releaser of NO, and the effect on release of catecholamines was determined. NP inhibited release of norepinephrine. Basal release was increased by incubation of the tissue with the NO scavenger hemoglobin (20 micrograms/ml). Hemoglobin also blocked the inhibitory effect of NP. In the presence of high-potassium (40 mM) medium to depolarize cell membranes, norepinephrine release was increased by a factor of 3, and this was significantly inhibited by NP. Hemoglobin again produced a further increase in norepinephrine release and also blocked the action of NP. When constitutive NO synthase was inhibited by the competitive inhibitor NG-monomethyl-L-arginine (NMMA) at 300 microM, basal release of norepinephrine was increased, as was potassium-evoked release, and this was associated in the latter instance with a decrease in tissue concentration, presumably because synthesis did not keep up with the increased release in the presence of NMMA. The results were very similar with dopamine, except that reduction of potassium-evoked dopamine release by NP was not significant. However, the increase following incubation with hemoglobin was significant, and hemoglobin, when incubated with NP, caused a significant elevation in dopamine release above that with NP alone. In this case, NP increased tissue concentration of dopamine along with inhibiting release, suggesting that synthesis continued, thereby raising the tissue concentration in the face of diminished release. When the tissue was incubated with NP plus hemoglobin, which caused an increase in release above that obtained with NP alone, the tissue concentration decreased significantly compared with that in the absence of hemoglobin, indicating that, with increased release, release exceeded synthesis, causing a fall in tissue concentration. When NO synthase was blocked by NMMA, the release of dopamine, under either basal or potassium-evoked conditions, was increased. Again, in the latter instance the tissue concentration declined significantly, presumably because synthesis did not match release. Therefore, the results were very similar with both catecholamines and indicate that NO acts to suppress release of both amines. Since both catecholamines activate the release of LHRH, the inhibition of their release by NO serves as an ultra-short-loop negative feedback by which NO inhibits the release of the catecholamines, thereby reducing the activation of the NOergic neurons and decreasing the release of LHRH. This may be an important means for terminating the pulses of release of LHRH, which generate the pulsatile release of LH that stimulates gonadal function in both male and female mammals.

PACAP stimulates dopamine neuronal activity in the medial basal hypothalamus and inhibits prolactin

In sheep intracerebroventricular injection of PACAP (10 nmol) significantly (P < 0.01) stimulated the levels of the dopamine metabolite DOPAC within the medial basal hypothalamus las measured by in vivo microdialysis) and this effect was temporally correlated with a significant (P < 0.05) suppression in peripheral prolactin concentrations. This result is in accord with the hypothesis that PACAP suppresses prolactin secretion from the anterior pituitary gland by stimulating dopamine release from tuberoinfundibular dopaminergic neurons. (C) 1998 Elsevier Science B.V.

Leptin modulation of locomotor and emotional behaviors : the role of STAT3 signaling in dopamine neurons

La leptine circule en proportion de la masse graisseuse du corps et la transduction de son signal à travers la forme longue de son récepteur via un certain nombre de voies neurales , y compris MAPK, PI3-K ,AMPK et JAK2 - STAT3 . Il faut noter que STAT3 constitue une voie clée au récepteur de la leptine par laquelle la leptine module l'expression des gènes impliqués dans la régulation du bilan énergétique. La plupart des recherches ont porté sur la fonction du récepteur de la leptine au sein de l' hypothalamus, en particulier la fonction du récepteur de la leptine dans le noyau arqué. Toutefois, les récepteurs de la leptine sont également exprimés sur les neurones dopaminergiques de l'aire tégmentale ventrale et la leptine agit sur cette région du cerveau pour influencer la prise alimentaire, la motivation, la locomotion, l'anxiété et la transmission de la dopamine. De plus, la leptine active la STAT3 dans les dopaminergiques et GABAergiques populations neuronales. Bien que ces résultats contribuent à notre compréhension des multiples actions de la leptine dans le système nerveux central, il reste à résoudre les cellules et la signalisation du récepteur de la leptine qui sont responsables des effets neurocomportementaux de la leptine dans le mésencéphale. Visant à déterminer la contribution de la voie de signalisation STAT3 dans les neurones dopaminergiques du mésencéphale, nous avons généré une lignée de souris knockout conditionnel dans lequel l'activation du gène de STAT3 sur son résidu tyrosine 705 ( Tyr 705 ) est absent spécifiquement dans les neurones dopaminergiques. Avec l'utilisation de ce modèle de souris génétique, nous avons évalué l'impact de l'ablation de la signalisation STAT3 dans les neurones dopaminergiques sur un certain nombre de fonctions liées à la dopamine, y compris l'alimentation, la locomotion, les comportements liés à la récompense, l'émotion et la libération de dopamine dans le noyau accumbens. Fait intéressant, nous avons observé un dimorphisme sexuel dans le phénotype des souris STAT3DAT-KO. L'activation de la voie de signalisation STAT3 dans les neurones dopaminergiques est responsable de l'action de la leptine dans la réduction de la locomotion, récompense liée à l'activité physique, et de l'augmentation de la libération et de la disponibilité de la dopamine chez les souris mâles. Cependant, il ne module pas le comportement émotionnel. D'autre part, les souris femelles STAT3DAT-KO augmentent les niveaux d'anxiété et les niveaux plasmatiques de corticostérone, sans provoquer de changements de la dépression. Cependant, la perte d'activation de STAT3 dans les neurones dopaminergiques ne module pas le comportement locomoteur chez les souris femelles. Notamment, les actions de la leptine dans le mésencéphale pour influencer le comportement alimentaire ne sont pas médiées par l'activation de STAT3 dans les neurones dopaminergiques, considérant que les souris mâles et femelles ont un comportement alimentaire normal. Nos résultats démontrent que la voie de signalisation STAT3 dans les neurones dopaminergiques est responsable des effets anxiolytiques de la leptine, et soutient l'hypothèse que la leptine communique l'état d'énergie du corps (i.e. la relation entre la dépense et les apports énergétiques) pour les régions mésolimbiques pour atténuer les effets de motivation et de récompense de plusieurs comportements qui servent à réhabiliter ou à épuiser les réserves d'énergie. En outre, ce travail souligne l'importance d'étudier la modulation de la signalisation de la leptine dans différente types de cellules, afin d'identifier les voies de signalisation et les mécanismes cellulaires impliqués dans les différentes fonctions neuro-comportementales de la leptine.

Dopamine-dependent responses to morphine depend on glucocorticoid receptors

Previous work has shown that glucocorticoid hormones facilitate the behavioral and dopaminergic effects of morphine. In this study we examined the possible role in these effects of the two central corticosteroid receptor types: mineralocorticoid receptor (MR), and glucocorticoid receptor (GR). To accomplish this, specific antagonists of these receptors were infused intracerebroventricularly and 2 hr later we measured: (i) locomotor activity induced by a systemic injection of morphine (2 mg/kg); (ii) locomotor activity induced by an infusion of morphine (1 μg per side) into the ventral tegmental area, which is a dopamine-dependent behavioral response to morphine; (iii) morphine-induced dopamine release in the nucleus accumbens, a dopaminergic projection site mediating the locomotor and reinforcing effects of drugs of abuse. Blockade of MRs by spironolactone had no significant effects on locomotion induced by systemic morphine. In contrast, blockade of GRs by either RU38486 or RU39305, which is devoid of antiprogesterone effects, reduced the locomotor response to morphine, and this effect was dose dependent. GR antagonists also reduced the locomotor response to intraventral tegmental area morphine as well as the basal and morphine-induced increase in accumbens dopamine, as measured by microdialysis in freely moving rats. In contrast, spironolactone did not modify dopamine release. In conclusion, glucocorticoids, via GRs, facilitate the dopamine-dependent behavioral effects of morphine, probably by facilitating dopamine release. The possibility of decreasing the behavioral and dopaminergic effects of opioids by an acute administration of GR antagonists may open new therapeutic strategies for treatment of drug addiction.

The prospects for immunotherapy in smoking cessation

Context Smoking is a major preventable cause of death and disability that is maintained by dependence on nicotine. Smoking cessation reduces mortality and morbidity. Although existing pharmacological aids to smoking cessation and relapse prevention (nicotine replacement therapy and bupropion) improve on unassisted quitting and behavioural methods, they are only modestly effective. More effective pharmacological methods are required that improve compliance, reduce side-effects, and can be used in combination with existing cessation methods. Starting point A nicotine vaccine is a promising immunotherapeutic approach to smoking cessation and relapse prevention. Such a vaccine would induce the immune system to form specific antibodies to nicotine to prevent it from crossing the blood-brain barrier to act on receptor sites in the central nervous system. Recent studies in rats provide proof of principle by showing that nicotine-specific antibodies can prevent the reinstatement of nicotine self-administration (N Lindblom et al, Respiration 2002; 69: 254–60) and block dopamine release in the shell of the nucleus accumbens (Sde Villiers et al, Respiration 2002; 69: 247–53). A phase 1 trial of a human cocaine vaccine has also recently been successfully completed (T Kosten et al, Vaccine 2002; 20: 1196–204). A safe and effective human nicotine vaccine would potentially have fewer side-effects and better compliance than existing smoking-cessation pharmacotherapies. It could also be used in combination with some of them (eg, bupropion). Where next? The most promising clinical application of a human nicotine vaccine is likely to be in relapse prevention in abstinent smokers. A vaccine may also have a role in preparing smokers to quit. Clinical trials of safety and efficacy in human smokers and ex-smokers are warranted. If a nicotine vaccine proves to be safe and effective, the health-care system will need to ensure that it is registered for clinical use and that the poorer members of the community (among whom smoking prevalence is now highest in developed countries) have access to the vaccine. The community will need to be appropriately informed about the role of a nicotine vaccine to ensure that it is not prematurely used for preventive purposes in children and adolescents.