Normal rats trained to circle show asymmetric caudate dopamine release

Caudate catecholamine release was monitored by bilateral $\text{in}$$\text{vivo}$ electrochemical electrodes in male Sprague-Dawley rats trained to circle for sucrose/water reward. Baseline release of dopamine was equal from both sides of caudate. When reinforced circling began, 33 ± 4 percent greater catechol release occured from the caudate contralateral to the circling direction. As turning subsided, differential release returned to basal levels. Further evidence that the catecholamine metabolism was affected by turning was obtained by direct measurement of caudate dopamine and DOPAC at selected time points. Concentration data showed relative increases in dopamine and DOPAC in the contralateral caudate. These data provide evidence that dopamine is released asymmetrically from caudate in unlesioned rats during voluntary behavior.     

Monitoring the Stimulated Release of Dopamine with In Vivo Voltammetry. I: Characterization of the Response Observed in the Caudate Nucleus of the Rat

Microvoltammetric electrodes were employed in the brain of an anesthetized rat to monitor chemical substances in extracellular fluid following electrical stimulation of the medial forebrain bundle. An increase in concentration of an easily oxidized substance is observed in the caudate nucleus and in the nucleus accumbens. A large amount of evidence suggests that the substance that is observed following stimulation is dopamine. (1) The location of the stimulating electrode must be in known dopaminergic tracts to induce release. (2) Release is most easily observed in brain regions that contain significant numbers of dopamine-containing neurons. (3) Two voltammetric electrodes with very different electrochemical responses provide voltammograms of the released species that are unique for catechols in one case and catecholamines in another case. (4) The amount of 3,4-dihydroxyphenylacetic acid found in striatal tissue by postmortem analysis correlates with the calculated amount of dopamine released. (5) Inhibition of tyrosine hydroxylase, and thus dopamine synthesis, decreases the observed release while inhibition of monoamine oxidase, and thus formation of dopamine metabolites, does not. (6) The dependence of release on stimulation parameters agrees with results obtained with perfusion techniques. Thus, a new method has been developed to characterize endogenous dopamine release in the rat brain and can be used on a time scale of seconds.     

Evidence of dopaminergic processing of executive inhibition

Inhibition of unwanted response is an important function of the executive system. Since the inhibitory system is impaired in patients with dysregulated dopamine system, we examined dopamine neurotransmission in the human brain during processing of a task of executive inhibition. The experiment used a recently developed dynamic molecular imaging technique to detect and map dopamine released during performance of a modified Eriksen's flanker task. In this study, young healthy volunteers received an intravenous injection of a dopamine receptor ligand ((11)C-raclopride) after they were positioned in the PET camera. After the injection, volunteers performed the flanker task under Congruent and Incongruent conditions in a single scan session. They were required to inhibit competing options to select an appropriate response in the Incongruent but not in the Congruent condition. The PET data were dynamically acquired during the experiment and analyzed using two variants of the simplified reference region model. The analysis included estimation of a number of receptor kinetic parameters before and after initiation of the Incongruent condition. We found increase in the rate of ligand displacement (from receptor sites) and decrease in the ligand binding potential in the Incongruent condition, suggesting dopamine release during task performance. These changes were observed in small areas of the putamen and caudate bilaterally but were most significant on the dorsal aspect of the body of left caudate. The results provide evidence of dopaminergic processing of executive inhibition and demonstrate that neurochemical changes associated with cognitive processing can be detected and mapped in a single scan session using dynamic molecular imaging.     

Changes in the kinetics of dopamine release and uptake have differential effects on the spatial distribution of extracellular dopamine concentration in rat striatum

The objective of this study was to examine whether the limited diffusion distance of dopamine in rat striatum produces spatial heterogeneity in the extracellular dopamine concentration on a dimensional scale of a few micrometers. Such heterogeneity would be significant because it would imply that the concentration of dopamine at a given receptor depends on the receptor's ultrastructural location. Spatially resolved measurements of extracellular dopamine were performed in the striatum of chloral hydrate-anesthetized rats with carbon fiber microdisk electrodes. Dopamine was monitored during electrical stimulation of the nigrostriatal pathway before and after administration of drugs that selectively affect the kinetics of evoked dopamine release and dopamine uptake. The effects of nomifensine (20 mg/kg), L-DOPA (250 mg/kg), and alpha-methyl-p-tyrosine (250 mg/kg) on the amplitude of the stimulation responses were examined. The outcome of these experiments was compared with predictions derived from a mathematical model that combines diffusion with the kinetics of release and uptake. The results demonstrate that the extracellular dopamine concentration is spatially heterogeneous on a micrometer scale and that changing the kinetics of dopamine release and uptake has different effects on this spatial distribution. The impact of these results on brain neurochemistry is considered.     

Conserved dorsal–ventral gradient of dopamine release and uptake rate in mice,rats and rhesus macaques

Although the vast majority of research on the dopamine system has been performed in rodents, and it is assumed that this work will inform us about the human condition, there have been very few direct comparisons of presynaptic dopamine terminal function across multiple species. Because it is difficult to query rapid sub-second dopamine signaling in humans using voltammetric methods, we chose to compare dopamine signals across multiple striatal subregions in slices from C57BL/6J mice, Sprague–Dawley rats and rhesus macaques. We found a dorsal to ventral gradient of dopamine uptake rates with highest levels in the dorsal striatum and lowest levels in the nucleus accumbens shell, which is conserved across species. In addition to uptake rates, there was also a dorsal to ventral, high to low, gradient in the magnitude of stimulated DA release observed in monkeys, mice, and rats. These data demonstrate that there is considerable functional homology across striatal regions in non-human primates and rodents, lending support to the use of rodents as model systems to study dopamine-related circuitry and disorders that are clinically relevant to the human population.     

In vivo release of transmitters in the cat basal ganglia

The release of transmitters was studied in various structures of the basal ganglia in cats implanted with several push-pull cannulas. Local depolarization enhanced Met-enkephalin release in the globus pallidus. Activation of striatonigral substance P(SP) neutrons stimulated the transmitter release from terminals. Unilateral electrical stimulation of the caudate nucleus evoked GABA release in both substantia nigrae and pallidoentopeduncular nuclei. The unilateral facilitation or interruption of nigral SP transmission modified dopamine (DA) release in the ipsilateral caudate nucleus in contrast, modifications of GABAergic or glycinergic nigral transmissions induced bilateral symmetrical effects, whereas bilateral asymmetrical changes in DA release in the two caudate nuclei were seen during the unilateral modification of nigral DA transmission. Changes in the dendritic release of DA induced changes in serotonin release both in the substantia nigra and in the ipsilateral caudate nucleus. Finally, it will be shown that acetylcholinesterase can be released from the substantia nigra and the caudate nucleus through processes dependent on nerve activity.     

Brain antioxidant systems in human methamphetamine users

Animal data suggest that the widely abused psychostimulant methamphetamine can damage brain dopamine neurones by causing dopamine-dependent oxidative stress; however, the relevance to human methamphetamine users is unclear. We measured levels of key antioxidant defences [reduced (GSH) and oxidized (GSSG) glutathione, six major GSH system enzymes, copper-zinc superoxide dismutase (CuZnSOD), uric acid] that are often altered after exposure to oxidative stress, in autopsied brain of human methamphetamine users and matched controls. Changes in the total (n = 20) methamphetamine group were limited to the dopamine-rich caudate (the striatal subdivision with the most severe dopamine loss) in which only activity of CuZnSOD (+ 14%) and GSSG levels (+ 58%) were changed. In the six methamphetamine users with severe (- 72 to - 97%) caudate dopamine loss, caudate CuZnSOD activity (+ 20%) and uric acid levels (+ 63%) were increased with a trend for decreased (- 35%) GSH concentration. Our data suggest that brain levels of many antioxidant systems are preserved in methamphetamine users and that GSH depletion, commonly observed during severe oxidative stress, might occur only with severe dopamine loss. Increased CuZnSOD and uric acid might reflect compensatory responses to oxidative stress. Future studies are necessary to establish whether these changes are associated with oxidative brain damage in human methamphetamine users.     

Dopamine D1-stimulated adenylyl cyclase activity in cerebral cortex of autopsied human brain.

Although the cerebral cortical dopamine D(1) receptor is considered to play a role in normal and abnormal brain function, little information is available on its characteristics in human brain. We compared dopamine-stimulated adenylyl cyclase (AC) activity in homogenates of cerebral cortex (frontal, temporal, parietal, occipital and cingulate cortex) of autopsied brain of neurologically normal subjects to that in striatum. Cerebral cortical AC activity was modestly and dose-dependently stimulated by dopamine (maximal 20-30%) with low microM EC50s and such stimulation was inhibited by the selective dopamine D1 receptor antagonist SCH23390. The magnitude of the maximal stimulation by dopamine was similar in autopsied and biopsied cerebral cortex. The extent of maximal stimulation was similar to that in dopamine-rich striatum (caudate, putamen and nucleus accumbens), despite much lower density of dopamine D1 receptors in cerebral cortex vs. striatum. The EC50 for dopamine stimulation in cerebral cortex (approximately 1 microM) was lower than that for caudate and putamen (approximately 3 microM). No detectable dopamine stimulation was observed in cerebellar cortex, thalamus or hippocampus. Dopamine stimulation in both cerebral cortex and striatum was independent of calcium activation. We conclude that dopamine stimulated AC can be measured in cerebral cortex of human brain allowing for the possibility that this process can be examined in human brain disorders in which dopaminergic abnormalities are suspected.     

Effects of Amphetamine on Carrier-Mediated and Electrically Stimulated Dopamine Release in Slices of Rat Caudate Putamen and Nucleus Accumbens

Abstract: The effects of (+)-amphetamine on carrier-mediated and electrically stimulated dopamine release were investigated using fast cyclic voltammetry in rat brain slices incorporating the nucleus accumbens, and in the caudate putamen. In the caudate putamen, dopamine release either increased with increasing frequency of local electrical stimulation (hot spots) or did not increase significantly (cold spots); dopamine release increased with increasing frequency of electrical stimulation in the nucleus accumbens. Local pressure application of (+)-amphetamine from a micropipette caused dopamine efflux at all sites examined, and this was not affected by sulpiride, indicating that efflux of dopamine caused by (+)-amphetamine is not regulated by dopamine D₂ autoreceptors. (+)-Amphetamine reduced single-pulse electrically stimulated dopamine release at all sites; sulpiride reversed this decrease, indicating that endogenous dopamine released by (+)-amphetamine activates dopamine D₂ autoreceptors. In nucleus accumbens and hot spots, (+)-amphetamine did not affect 20-pulse 50-Hz-stimulated dopamine release, whereas in cold spots it potentiated 20-pulse 50-Hz-stimulated dopamine release. We conclude that (+)-amphetamine modifies electrically stimulated dopamine release by uptake inhibition or by indirect activation of D₂ autoreceptors; the precise mechanism is determined by site and duration of electrical stimulation.     

Selective decrease of immunoreactive tyrosine hydroxylase in nigrostriatum of adult male rats after N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment

Summary Several laboratories have reported that N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine causes damage to the nigral dopamine neurons of man, monkey, and mouse. Controversial data suggest that a rat model of Parkinsonism may be possible. Although loss of dopamine cells has not been detected in the rat brain, our immunocytochemical studies show that immunoreactive tyrosine hydroxylase, the rate-limiting enzyme which synthesizes dopamine, is significantly reduced in concentration, or its antigenicity altered, in substantia nigra/pars compacta as well as the caudate nucleus. Optical density measurements demonstrate the reduction or alteration of immunoreactive tyrosine hydroxylase in nigro-striatal neurons, indicating that axonal terminals, as well as parent perikarya, may be sensitive to the drug. After treatment, abnormal morphological remodelling may result in the affected neuronal processes, perhaps indicating sublethal toxicity, followed by slow recovery. Despite the lack of nigral cell death, it is proposed that the present data support the use of the rat as a model to investigate the early effects of Parkinsonism induced by this agent, and the biological mechanisms of cellular recovery.     

Acute and chronic continuous methamphetamine have different long-term behavioral and neurochemical consequences

We compared two different methamphetamine dosing regimens and found distinct long-term behavioral and neurochemical changes. Adult rats were treated with 1-day methamphetamine injection (3x5 mg/kg s.c., 3 h apart) or 7-day methamphetamine minipump (20 mg/kg/day s.c.). The minipump regimen models the sustained methamphetamine plasma levels in some human bingers whereas the 1-day regimen models a naive user overdose. On withdrawal days 7 and 28, rats were acutely challenged with cocaine to test for behavioral sensitization and subsequently sacrificed for caudate and accumbens dopamine tissue content. Other rats were analyzed on withdrawal days 3, 7 or 28 using voltammetry in caudate slices. On withdrawal days 7 and 28, the methamphetamine injection but not the minipump rats showed behavioral cross-sensitization to cocaine. There was no change in baseline dopamine release, reuptake or sensitivity to quinpirole in any treatment group on either withdrawal day. However, consistent with the behavioral sensitization, cocaine had a greater effect in potentiating dopamine release and in blocking dopamine reuptake in methamphetamine injection versus saline irrespective of withdrawal day. The minipump group showed tolerance to the dopamine releasing effect of cocaine on withdrawal day 28 and had lower dopamine tissue content in the caudate versus the methamphetamine injection group. Dopamine turnover as measured by the DOPAC/dopamine ratio tended to be higher in the minipump-treated rats. These data suggest that the behavioral cross-sensitization seen in the methamphetamine injection rats could be in part due to the increased potency of cocaine in blocking dopamine reuptake and in increasing dopamine release. The decreased potency of cocaine in the caudate slices from the minipump-treated group may be related to decreased dopamine tissue content.     

Galpha(olf) is necessary for coupling D1 and A2a receptors to adenylyl cyclase in the striatum

In the brain, dopamine and adenosine stimulate cyclic AMP (cAMP) production through D1 and A2a receptors, respectively. Using mutant mice deficient in the olfactory isoform of the stimulatory GTP-binding protein alpha subunit, Galpha(olf), we demonstrate here the obligatory role of this protein in the adenylyl cyclase responses to dopamine and adenosine in the caudate putamen. Responses to dopamine were also dramatically decreased in the nucleus accumbens but remained unaffected in the prefrontal cortex. Moreover, in the caudate putamen of mice heterozygous for the mutation, the amounts of Galpha(olf) were half of the normal levels, and the efficacy of dopamine- and CGS 21680 A(2) agonist-stimulated cAMP production was decreased. Together, these results identify Galpha(olf) as a critical parameter in the responses to dopamine and adenosine in the basal ganglia.     

Real-time decoding of dopamine concentration changes in the caudate-putamen during tonic and phasic firing

The fundamental process that underlies volume transmission in the brain is the extracellular diffusion of neurotransmitters from release sites to distal target cells. Dopaminergic neurons display a range of activity states, from low-frequency tonic firing to bursts of high-frequency action potentials (phasic firing). However, it is not clear how this activity affects volume transmission on a subsecond time scale. To evaluate this, we developed a finite-difference model that predicts the lifetime and diffusion of dopamine in brain tissue. We first used this model to decode in vivo amperometric measurements of electrically evoked dopamine, and obtained rate constants for release and uptake as well as the extent of diffusion. Accurate predictions were made under a variety of conditions including different regions, different stimulation parameters and with uptake inhibited. Second, we used the decoded rate constants to predict how heterogeneity of dopamine release and uptake sites would affect dopamine concentration fluctuations during different activity states in the absence of an electrode. These simulations show that synchronous phasic firing can produce spatially and temporally heterogeneous concentration profiles whereas asynchronous tonic firing elicits uniform, steady-state dopamine concentrations.     

Identification of a noradrenaline-rich subdivision of the human nucleus accumbens

The nucleus accumbens, situated at the junction between rostral pre-commissural caudate and putamen, is now considered to be critically involved in rewarding and motivational functions mediated by the neurotransmitter dopamine. However, in the human, the precise anatomical boundaries of this nucleus are still undetermined and controversy exists as to the extent to which nucleus accumbens activity is controlled by noradrenaline, a related neurotransmitter now much neglected (in favor of dopamine) by the scientific community. Here we resolve the question of noradrenaline in the human nucleus accumbens and identify, in autopsied brain of normal subjects, a small subdivision of the caudomedial portion of this nucleus that selectively contains strikingly high levels of noradrenaline and thus represents the only area in human brain having equally high levels of both noradrenaline and dopamine. The presence of very high, localized noradrenaline concentrations in the caudomedial nucleus accumbens implies a special biological role for this neurotransmitter in human brain motivational processes.     

Enhanced striatal dopamine release during food stimulation in binge eating disorder

Subjects with binge eating disorder (BED) regularly consume large amounts of food in short time periods. The neurobiology of BED is poorly understood. Brain dopamine, which regulates motivation for food intake, is likely to be involved. We assessed the involvement of brain dopamine in the motivation for food consumption in binge eaters. Positron emission tomography (PET) scans with [(11)C]raclopride were done in 10 obese BED and 8 obese subjects without BED. Changes in extracellular dopamine in the striatum in response to food stimulation in food-deprived subjects were evaluated after placebo and after oral methylphenidate (MPH), a drug that blocks the dopamine reuptake transporter and thus amplifies dopamine signals. Neither the neutral stimuli (with or without MPH) nor the food stimuli when given with placebo increased extracellular dopamine. The food stimuli when given with MPH significantly increased dopamine in the caudate and putamen in the binge eaters but not in the nonbinge eaters. Dopamine increases in the caudate were significantly correlated with the binge eating scores but not with BMI. These results identify dopamine neurotransmission in the caudate as being of relevance to the neurobiology of BED. The lack of correlation between BMI and dopamine changes suggests that dopamine release per se does not predict BMI within a group of obese individuals but that it predicts binge eating.     

The regional distribution of dopamine and serotonin uptake and transmitter concentrations in the human brain

The regional distribution of the dopamine and serotonin uptake sites in human brain have been assessed and compared with the distribution of the transmitters and their metabolites measured in the same brains and also with a limited regional distribution of the uptake sites in rat and sheep brain. The affinity of the uptake sites for both transmitters was determined and found to be c. 0.2 μ M in all 3 species. Most dopamine uptake in all species was in caudate and putamen samples. Many regions of the human brain showed no dopamine uptake and little dopamine uptake was seen in sheep cortex or nigral preparations. Dopamine and metabolite concentrations were highest in the caudate, putamen and substantia nigra. Most serotonin uptake was seen in the hypothalamus in all 3 species; less was observed in the striatal regions; the cortical and nigral preparations of sheep brain showed little serotonin uptake though cortical preparations of rat brain had high levels of uptake. In the human brain, other regions did not show serotonin uptake. Highest concentrations of serotonin were found in the substantia nigra and medulla, intermediate concentrations in the putamen, globus pallidus, hypothalamus, olfactory tubercle and thalamus; very low concentrations of serotonin were found in other regions. The use of the human uptake site for pharmacological studies and as a marker for monoaminergic afferents in human health and disease is discussed.     

Attenuated Tonic and Enhanced Phasic Release of Dopamine in Attention Deficit Hyperactivity Disorder

It is unclear whether attention deficit hyperactive disorder (ADHD) is a hypodopaminergic or hyperdopaminergic condition. Different sets of data suggest either hyperactive or hypoactive dopamine system. Since indirect methods used in earlier studies have arrived at contradictory conclusions, we directly measured the tonic and phasic release of dopamine in ADHD volunteers. The tonic release in ADHD and healthy control volunteers was measured and compared using dynamic molecular imaging technique. The phasic release during performance of Eriksen’s flanker task was measured in the two groups using single scan dynamic molecular imaging technique. In these experiments volunteers were positioned in a positron emission tomography (PET) camera and administered a dopamine receptor ligand ¹¹C-raclopride intravenously. After the injection PET data were acquired dynamically while volunteers either stayed still (tonic release experiments) or performed the flanker task (phasic release experiments). PET data were analyzed to measure dynamic changes in ligand binding potential (BP) and other receptor kinetic parameters. The analysis revealed that at rest the ligand BP was significantly higher in the right caudate of ADHD volunteers suggesting reduced tonic release. During task performance significantly lower ligand BP was observed in the same area, indicating increased phasic release. In ADHD tonic release of dopamine is attenuated and the phasic release is enhanced in the right caudate. By characterizing the nature of dysregulated dopamine neurotransmission in ADHD, the results explain earlier findings of reduced or increased dopaminergic activity.     

Examination of Rapid Dopamine Dynamics with Fast Scan Cyclic Voltammetry During Intra-oral Tastant Administration in Awake Rats

Rapid, phasic dopamine (DA) release in the mammalian brain plays a critical role in reward processing, reinforcement learning, and motivational control. Fast scan cyclic voltammetry (FSCV) is an electrochemical technique with high spatial and temporal (sub-second) resolution that has been utilized to examine phasic DA release in several types of preparations. In vitro experiments in single-cells and brain slices and in vivo experiments in anesthetized rodents have been used to identify mechanisms that mediate dopamine release and uptake under normal conditions and in disease models. Over the last 20 years, in vivo FSCV experiments in awake, freely moving rodents have also provided insight of dopaminergic mechanisms in reward processing and reward learning. One major advantage of the awake, freely moving preparation is the ability to examine rapid DA fluctuations that are time-locked to specific behavioral events or to reward or cue presentation. However, one limitation of combined behavior and voltammetry experiments is the difficulty of dissociating DA effects that are specific to primary rewarding or aversive stimuli from co-occurring DA fluctuations that mediate reward-directed or other motor behaviors. Here, we describe a combined method using in vivo FSCV and intra-oral infusion in an awake rat to directly investigate DA responses to oral tastants. In these experiments, oral tastants are infused directly to the palate of the rat – bypassing reward-directed behavior and voluntary drinking behavior – allowing for direct examination of DA responses to tastant stimuli.     

[3H]GBR-12935 Binding to the Dopamine Transporter Is Decreased in the Caudate Nucleus in Parkinson''s Disease

The specific binding of [3H]GBR-12935 to membranes prepared from human caudate nucleus is saturable (Bmax 1.36 +/- 0.18 pmol/mg protein), sodium dependent and of high affinity (KD 2.34 +/- 0.18 nM). Freezing of tissue from rat brain, or refrigeration followed by freezing, results in a small but significant (less than or equal to 20%) decrease in specific [3H]GBR-12935 binding when compared to the binding observed in fresh (nonfrozen) tissue, and this decrease may account, in part, for the differences in specific binding between rat and human brain membranes. Despite small differences in binding site density between fresh and frozen tissue there is a good correlation (r = 0.98; p less than 0.01) between the potencies of a series of drugs in displacing specific [3H]GBR-12935 binding to human caudate membranes and rat striatum as well as in inhibiting dopamine uptake in rat striatal synaptosomes (r = 0.96; p less than 0.01). The specific binding of [3H]GBR-12935 to membranes prepared from the caudate nuclei of patients with Parkinson's disease is decreased compared to membranes prepared from age- and sex-matched controls. These data suggest that [3H]GBR-12935 binds in a sodium-dependent fashion to the dopamine transport complex in human brain and that specific binding is decreased by a pathological degeneration of dopaminergic neurons to the caudate nucleus.     

Chronic nicotine modifies the effects of morphine on extracellular striatal dopamine and ventral tegmental GABA

Previously, we have shown that 7-week oral nicotine treatment enhances morphine-induced behaviors and dopaminergic activity in the mouse brain. In this study, we further characterized the nicotine-morphine interaction in the mesolimbic and nigrostriatal dopaminergic systems, as well as in the GABAergic control of these systems. In nicotine-pretreated mice, morphine-induced dopamine release in the caudate putamen and nucleus accumbens was significantly augmented, as measured by microdialysis. Chronic nicotine treatment did not change basal extracellular concentrations of dopamine and its metabolites in the caudate putamen and nucleus accumbens, nor did it affect the rate of dopamine synthesis, as assessed by 3-hydroxybenzylhydrazine dihydrochloride-induced DOPA accumulation. GABAergic control of dopaminergic activity was studied by measuring extracellular GABA in the presence of nipecotic acid, an inhibitor of GABA uptake. Acute (0.3 mg/kg or 0.5 mg/kg i.p.) and chronic nicotine, as well as morphine (15 mg/kg s.c.) in control mice decreased nipecotic acid-induced increase in extracellular GABA in the ventral tegmental area/substantia nigra (VTA/SN). In contrast, in nicotine-treated mice, morphine increased GABA levels in the presence of nipecotic acid. We did not find any alterations in GABA(B)-receptor function after chronic nicotine treatment. Thus, our data show that chronic nicotine treatment sensitizes dopaminergic systems to morphine and affects GABAergic systems in the VTA/SN.