Dopamine Release Process Research Articles

Synaptotagmins 1 and 7 Play Complementary Roles in Somatodendritic Dopamine Release.

The molecular mechanisms underlying somatodendritic dopamine (DA) release remain unresolved, despite the passing of decades since its discovery. Our previous work showed robust release of somatodendritic DA in submillimolar extracellular Ca2+ concentration ([Ca2+]o). Here we tested the hypothesis that the high-affinity Ca2+ sensor synaptotagmin 7 (Syt7), is a key determinant of somatodendritic DA release and its Ca2+ dependence. Somatodendritic DA release from SNc DA neurons was assessed using whole-cell recording in midbrain slices from male and female mice to monitor evoked DA-dependent D2 receptor-mediated inhibitory currents (D2ICs). Single-cell application of an antibody to Syt7 (Syt7 Ab) decreased pulse train-evoked D2ICs, revealing a functional role for Syt7. The assessment of the Ca2+ dependence of pulse train-evoked D2ICs confirmed robust DA release in submillimolar [Ca2+]o in wild-type (WT) neurons, but loss of this sensitivity with intracellular Syt7 Ab or in Syt7 knock-out (KO) mice. In millimolar [Ca2+]o, pulse train-evoked D2ICs in Syt7 KOs showed a greater reduction in decreased [Ca2+]o than seen in WT mice; the effect on single pulse-evoked DA release, however, did not differ between genotypes. Single-cell application of a Syt1 Ab had no effect on train-evoked D2ICs in WT SNc DA neurons, but did cause a decrease in D2IC amplitude in Syt7 KOs, indicating a functional substitution of Syt1 for Syt7. In addition, Syt1 Ab decreased single pulse-evoked D2ICs in WT cells, indicating the involvement of Syt1 in tonic DA release. Thus, Syt7 and Syt1 play complementary roles in somatodendritic DA release from SNc DA neurons.SIGNIFICANCE STATEMENT The respective Ca2+ dependence of somatodendritic and axonal dopamine (DA) release differs, resulting in the persistence of somatodendritic DA release in submillimolar Ca2+ concentrations too low to support axonal release. We demonstrate that synaptotagmin7 (Syt7), a high-affinity Ca2+ sensor, underlies phasic somatodendritic DA release and its Ca2+ sensitivity in the substantia nigra pars compacta. In contrast, we found that synaptotagmin 1 (Syt1), the Ca2+ sensor underlying axonal DA release, plays a role in tonic, but not phasic, somatodendritic DA release in wild-type mice. However, Syt1 can facilitate phasic DA release after Syt7 deletion. Thus, we show that both Syt1 and Syt7 act as Ca2+ sensors subserving different aspects of somatodendritic DA release processes.

Double target concept for smoking cessation

Tobacco use is estimated to be the largest single cause of premature death in the world. Nicotine is the major addictive substance in tobacco products. After cigarette smoking, nicotine quickly acts on its target, nicotinic acetylcholine receptors (nAChRs), which are widely distributed throughout the mammalian central nervous system and are expressed as diverse subtypes on cell bodies, dendrites and/or nerve terminals. Through the nAChRs in brain reward circuits, nicotine alters dopaminergic (DA) neuronal function in the ventral tegmental area (VTA) and increases dopamine release from VTA to nuclear accumbens (NA), which leads to nicotine reward, tolerance and dependence. After quitting smoking, smokers experience withdrawal symptoms, including depression, irritability, difficulty concentrating or sleeping, headache, and tiredness. Recently, evidence has been accumulated to reveal the molecular and cellular mechanisms of nicotine reward, tolerance and dependence. The outcomes of these investigations provide pharmacological basis for smoking cessation. Here, I briefly summarize recent advancements of our understanding of nicotine reward, tolerance and dependence. Based on these understandings, I propose a double target hypothesis, in which nAChRs and dopamine release process are two important targets for smoking cessation. Dysfunction of nAChRs (antagonism or desensitization) is crucial to abolish nicotine dependence and the maintenance of an appropriate level of extracellular dopamine eliminates nicotine withdrawal syndromes. Therefore, the medications simultaneously act on these two targets should have the desired effect for smoking cessation. I discuss how to use this double target concept to interpret recent therapies and to develop new candidate compounds for smoking cessation.

Extracting the basal extracellular dopamine concentrations from the evoked responses: Re-analysis of the dopamine kinetics

Fast-scan cyclic voltammetry in conjunction with carbon fiber microelectrode has been used to study dopamine (DA) release and uptake mechanisms in rat brains because of the smaller size of the electrode and the subsecond resolution. Current voltammetry data were analyzed by a DA kinetic model assuming a zero baseline, which is in conflict with existing microdialysis findings and a recent claim of the striatal extracellular DA concentration at micromolar levels. This work applied a new analysis approach based on a modified DA kinetic model to analyze the kinetics of electrically evoked DA overflow in the caudate-putamen of anesthetized rats. The DA uptake parameters were fitted from the electrical stimulation phase, and subsequently used to calculate theoretical DA uptake rates. Comparison of the theoretical uptake rates with experimental clearance rates allows for the study of the tonic DA release process following electrical stimulations. Analyses of DA voltammetry data suggest that the locally averaged basal level of extracellular DA in the rat striatum might be confined between 95 and 220 nM. The disparate time scales in the clearance kinetics of endogenous and exogenous DA were investigated. Long-distance diffusion could only partially explain the slow clearance time course of exogenous DA. Model simulations and parameter analyses on evoked DA responses indicate that suppression of the nonevoked DA release process immediately following electrical stimulation cannot completely account for the rapid clearance of the electrically evoked DA. Inconsistency in the measured uptake strengths in the literature studying endogenous and exogenous DA remains to be investigated in the future.

Cholecystokinin Facilitates Methamphetamine-Induced Dopamine Overflow in Rat Striatum and Fetal Ventral Mesencephalic Grafts

The purpose of this study was to investigate the interactions of sulfated (S) and unsulfated (US) cholecystokinin (CCK) and methamphetamine (MA)-induced dopamine (DA) overflow in rat striatum. High-speed chronoamperometric recording techniques, using Nafion-coated carbon fiber electrodes, were used to evaluate extracellular DA concentration. CCK-8S, CCK-8US, MA, and DA were locally applied directly to the striatum of urethane-anesthetized Sprague-Dawley rats. We found that CCK potentiated MA-induced DA release in the anterior striatum. This response is probably mediated through CCK-A receptors because CCK-8S, but not CCK-8US, enhanced MA-induced responses. Replacement of Ca 2+ with Mg 2+ in the drug barrel antagonized this reaction, suggesting that the modulation of MA-induced DA release by CCK is Ca 2+ dependent. Both MA-induced DA release and CCK modulatory effects disappeared in the striatum after unilaterally lesioning the medial forebrain boundle with 6-hydroxydopamine (6-OHDA). We had previously found that the zone of normalized dopamine clearance in 6-OHDA-lesioned rats was considerably larger than that of normalized release in the anterior striatum after fetal ventral mesencephalic (VM) transplantation, which may be a result of partial reinnervation from the transplant. In the present study, we found that the modulation of DA release by CCK was restored only in the zone of normalized release after fetal nigral transplantation; CCK did not increase MA-induced DA release throughout the larger partially innervated area. In conclusion, these findings suggest that not only DA release processes but also CCK modulatory mechanisms are restored in the anterior striatum after fetal VM transplantation.

Effect of Nitroprusside (Nitric Oxide) on Endogenous Dopamine Release from Rat Striatal Slices

It is becoming apparent that the synthesis of nitric oxide (NO) from L-arginine not only explains endothelium-dependent vascular relaxation, but is a widespread mechanism for the regulation of cell function and communication. We examined the role of NO on the endogenous dopamine (DA) release from rat striatum. Nitroprusside, in the concentration range of 3-100 microM, induced a dose-dependent increase in the endogenous DA release from rat striatal slices. The maximal response was 330% over the baseline release. A higher concentration of nitroprusside (300 microM) produced an inhibitory effect on the spontaneous release of DA. L-Arginine (10 and 100 microM), a substrate in the NO-forming enzyme system, also produced an elevation of DA release. L-Arginine-induced DA release was attenuated by NG-monomethyl-L-arginine, an inhibitor of NO synthase. NADPH (1 microM), a cofactor of NO synthase, enhanced L-arginine-induced DA release. These results suggest a possible involvement of NO in the DA release process in rat striatum.

Behavioral sensitization following a single apomorphine pretreatment — selective effects on the dopamine release process

Once daily subchronic pretreatments with the dopamine (DA) agonist apomorphine (APO) increase striatal DA synthesis and metabolism. Such changes imply that adaptation to APO do not dissipate completely with in 24 h. In the present report we evaluated the effects of a single APO treatment 24 h prior to euthanasia on behavior and on striatal DA synthesis, metabolism, release and receptor binding. The single APO pretreatment reduced DA release from striatal synaptosomes. In contrast, striatal DA synthesis, metabolism, and the high-affinity binding of DA to the D 2 receptor were unaltered 24 h after agonist pretreatment. At this time the stereotypic response to a subsequent APO challenge was enhanced. This adaptive pattern is different from that observed 60 min following an acute APO pretreatment, when high-affinity D 2 binding is reduced. The pattern 24 h following a single APO pretreatment is also different from that observed following subchronic agonist dosing, when stereotypic behavior is enhanced, while basal DA synthesis and metabolism are increased.

Characterization of dopamine release in the rat medial prefrontal cortex as assessed by in vivo microdialysis: Comparison to the striatum

Using the technique of perfusion microdialysis combined with a small-bore liquid chromatography system we have measured the basal and drug-induced fluxes of extracellular dopamine in the medial prefrontal cortex of chloral hydrate-anesthetized rats and have compared our findings in the cortex to that observed in the striatum. The results were as follows. (1) At a flow rate of 2 μ1/min, the basal level of dopamine in the medial prefrontal cortex was 0.28 ±0.1 ( n = 32) fmol/μl perfusate, which was nearly an order of magnitude less than that obtained from the striatum. (2) alpha-Methyl-para-tyrosine (150 mg/kg, i.v.) significantly decreased the extracellular levels of striatal and cortical dopamine. The magnitude and duration of the responses were similar in both regions. (3) Local perfusion with 30 mM K + had a more profound effect on dopamine release in the striatum than in the medial prefrontal cortex. The K +-induced release in both regions was significantly attenuated in the absence of Ca 2+. (4) The anxiogenic beta carboline FG 7142 (15 mg/kg, i.p.) enhanced the release of cortical dopamine by about 50% while it was without an effect in the striatum. (5) Amphetamine (1 mg/kg, i.v.) significantly elevated, while reserpine (5 mg/kg, i.p.) rapidly attenuated, the dopamine level in the medial prefrontal cortex. These studies demonstrate that perfusion microdialysis, in conjunction with small-bore liquid chromatography with electrochemical detection, can be used to measure the basal release of dopamine in the rat medial prefrontal cortex and that the dopamine release process in this region, as has been shown in the striatum, is sensitive to stimulation conditions and pharmacological manipulations. The significance of variations in the pharmacological responsiveness of the dopamine released in the medial prefrontal cortex and striatum is discussed.