Distinct pharmacological regulation of evoked dopamine efflux in the amygdala and striatum of the rat in vivo

Conditional Regulation of Blood Pressure in Response to Emotional Stimuli by the Central Nucleus of the Amygdala in Rats

Humans and animals can determine whether a situation is favorable to them and act accordingly. For this, the autonomic tuning of the cardiovascular system to supply energy to active skeletal muscles through the circulatory system is as important as motor control. However, how the autonomic cardiovascular responses are regulated in dynamically changing environments and the neuronal mechanisms underlying these responses remain unclear. To resolve these issues, we recorded the blood pressure and heart rate of head-restrained rats during dynamically changing appetitive and aversive classical conditioning tasks. The rats displayed various associations between conditioned stimuli and unconditioned stimuli in appetitive (sucrose water), neutral (no outcome), and aversive (air puff) blocks. The blood pressure and heart rate in the appetitive block gradually increased in response to the reward-predicting cue and the response to the actual reward vigorously increased. The reward-predictive response was significantly higher than the responses obtained in the neutral and aversive condition blocks. To investigate whether the reward-predictive pressor response was caused by orofacial movements such as anticipatory licking behavior, we separately analyzed high- and low-licking trials. The conditioned pressor response was observed even in trials with low-licking behaviors. Blood pressure and heart rate responses to the air puff-predicting cue in the aversive block were not significantly different from the responses in the neutral block. The conditioned blood pressure response rapidly changed with condition block switching. Furthermore, to examine the contribution of the amygdala as an emotion center to these conditioned responses, we bilaterally microinjected a GABA_A receptor agonist, muscimol, into the central nucleus of the amygdala. Pharmacological inactivation of the central nucleus of the amygdala significantly decreased the reward-predictive pressor responses. These results suggest that the blood pressure is adaptively and rapidly regulated by emotional conditioned stimuli and that the central nucleus of the amygdala participates in regulating the pressor response in dynamically changing situations.

Diazepam attenuates the effects of cocaine on locomotion, 50-kHz ultrasonic vocalizations and phasic dopamine in the nucleus accumbens of rats

BACKGROUND AND PURPOSE

Currently, there is no effective drug to treat cocaine-use disorder, which affects millions of people worldwide. Benzodiazepines are potential therapeutic candidates, as microdialysis and voltammetry studies have shown that they can decrease dopamine concentrations in the nucleus accumbens of rodents and block the increase in dopamine levels and appetitive 50-kHz ultrasonic vocalizations (USVs) induced by amphetamine in rats.

EXPERIMENTAL APPROACH

Here, we tested whether administration of 2.5-mg·kg^-1 diazepam (i.p.) in adult male rats could block the effects of 20-mg·kg^-1 cocaine (i.p.) on electrically evoked phasic dopamine signals in the nucleus accumbens measured by fast-scan cyclic voltammetry, as well as 50-kHz USV and locomotor activity.

KEY RESULTS

Cocaine injection increased evoked dopamine signals up to threefold within 5 min, and the increase was significantly higher than baseline for at least 75 min. The injection of diazepam, 5 min after cocaine, attenuated the cocaine effect by nearly 50%, and this attenuation was maintained for at least 40 min. Behaviourally, cocaine increased the number of appetitive 50-kHz calls by about 12-fold. Diazepam significantly blocked this effect for the entire duration of the session. Also, cocaine-treated rats were more active than controls and diazepam significantly attenuated cocaine-induced locomotion, by up to 50%.

CONCLUSION AND IMPLICATIONS

These results suggest that the neurochemical and psychostimulant effects of cocaine can be mitigated by diazepam.

Distinct roles for dopamine clearance mechanisms in regulating behavioral flexibility

Dopamine plays a crucial role in adaptive behavior, and dysfunctional dopamine is implicated in multiple psychiatric conditions characterized by inflexible or inconsistent choices. However, the precise relationship between dopamine and flexible decision making remains unclear. One reason is that, while many studies have focused on the activity of dopamine neurons, efficient dopamine signaling also relies on clearance mechanisms, notably the dopamine transporter (DAT), which predominates in striatum, and catechol-O-methyltransferase (COMT), which predominates in cortex. The exact locus, extent, and timescale of the effects of DAT and COMT are uncertain. Moreover, there is limited data on how acute disruption of either mechanism affects flexible decision making strategies mediated by cortico-striatal networks. To address these issues, we combined pharmacological modulation of DAT and COMT with electrochemistry and behavior in mice. DAT blockade, but not COMT inhibition, regulated sub-second dopamine release in the nucleus accumbens core, but surprisingly neither clearance mechanism affected evoked release in prelimbic cortex. This was not due to a lack of sensitivity, as both amphetamine and atomoxetine changed the kinetics of sub-second release. In a multi-step decision making task where mice had to respond to reversals in either reward probabilities or the choice sequence to reach the goal, DAT blockade selectively impaired, and COMT inhibition improved, performance after reward reversals, but neither manipulation affected the adaptation of choices after action-state transition reversals. Together, our data suggest that DAT and COMT shape specific aspects of behavioral flexibility by regulating different aspects of the kinetics of striatal and cortical dopamine, respectively.

Organic cation transporter 3 and the dopamine transporter differentially regulate catecholamine uptake in the basolateral amygdala and nucleus accumbens

Regional alterations in kinetics of catecholamine uptake are due in part to variations in clearance mechanisms. The rate of clearance is a critical determinant of the strength of catecholamine signaling. Catecholamine transmission in the nucleus accumbens core (NAcc) and basolateral amygdala (BLA) is of particular interest due to involvement of these regions in cognition and motivation. Previous work has shown that catecholamine clearance in the NAcc is largely mediated by the dopamine transporter (DAT), but clearance in the BLA is less DAT-dependent. A growing body of literature suggests that organic cation transporter 3 (OCT3) also contributes to catecholamine clearance in both regions. Consistent with different clearance mechanisms between regions, catecholamine clearance is more rapid in the NAcc than in the BLA, though mechanisms underlying this have not been resolved. We compared the expression of DAT and OCT3 and their contributions to catecholamine clearance in the NAcc and BLA. We found DAT protein levels were ~ 4-fold higher in the NAcc than in the BLA, while OCT3 protein expression was similar between the two regions. Immunofluorescent labeling of the two transporters in brain sections confirmed these findings. Ex vivo voltammetry demonstrated that the magnitude of catecholamine release was greater, and the clearance rate was faster in the NAcc than in the BLA. Additionally, catecholamine clearance in the BLA was more sensitive to the OCT3 inhibitor corticosterone, while clearance in the NAcc was more cocaine sensitive. These distinctions in catecholamine clearance may underlie differential effects of catecholamines on behavioral outputs mediated by these regions.

Early weaning augments the spontaneous release of dopamine in the amygdala but not the prefrontal cortex: an in vivo microdialysis study of male rats

Our early weaning schedule was associated with the emergence of trait anxiety in male rodents performing an elevated plus maze but not an open-field test. We previously reported that early weaning weakened excitatory neurotransmission to the amygdala from the prefrontal cortex, where the mesocorticolimbic dopaminergic (DAergic) fiber terminates on each. In this study, we investigated DAergic transmission in both these brain regions. The extracellular levels of amygdalar DA in adulthood were two times higher in rats weaned at 16 days compared to those weaned at 30 days in both the home cage and the open-field. This difference in extracellular DA levels was not apparent in the prefrontal cortex. The concurrently measured locomotor and rearing behaviors did not vary according to the weaning period and the probe-implanted region, respectively. These results suggest that the effects of early weaning on DA tone appear to be specific to the amygdala and do not represent ubiquitous upregulation as these changes were not observed in the prefrontal cortex.

Dopamine manipulations drive changes in information sampling in healthy volunteers

BACKGROUND

Information sampling is the cognitive process of accumulating information before committing to a decision. Patients across numerous disorders show decreased information sampling relative to controls.

AIMS

Here, we used the Beads and the Best Choice Tasks to study the role of dopamine signaling in information sampling.

METHODS

Participants were given placebo, amisulpride, or ropinirole in each session, in a double-blind cross-over design.

RESULTS

We found that ropinirole (agonist) increased the number of beads drawn in the Beads Task specifically when participants faced a loss, and decreased the rank of the chosen option in the Best Choice Task.

CONCLUSIONS

These effects are likely driven by a combination of effects at presynaptic D₂ receptors, which affect dopamine release, and post-synaptic D₂ receptors. Increased D₂ relative to D₁ receptor activation in the striatum leads to increased sampling in the loss condition in the Beads Task. It also leads to choice of a poorer ranked option in the Best Choice Task. Decreased D₂ relative to D₁ receptor activation leads to decreased sampling in the Beads Task in the loss condition.

Comparing phasic dopamine dynamics in the striatum, nucleus accumbens, amygdala, and medial prefrontal cortex

Midbrain dopaminergic neurons project to and modulate multiple highly interconnected modules of the basal ganglia, limbic system, and frontal cortex. Dopamine regulates behaviors associated with action selection in the striatum, reward in the nucleus accumbens (NAc), emotional processing in the amygdala, and executive functioning in the medial prefrontal cortex (mPFC). The multifunctionality of dopamine likely occurs at the individual synapses, with varied levels of phasic dopamine release acting on different receptor populations. This study aimed to characterize specific aspects of stimulation-evoked phasic dopamine transmission, beyond simple dopamine release, using in vivo fixed potential amperometry with carbon fiber recording microelectrodes positioned in either the dorsal striatum, NAc, amygdala, or mPFC of anesthetized mice. To summarize results, the present study found that the striatum and NAc had increased stimulation-evoked phasic dopamine release, faster dopamine uptake (leading to restricted dopamine diffusion), weaker autoreceptor functioning, greater supply levels of available dopamine, and increased dopaminergic responses to DAT blockade compared to the amygdala and mPFC. Overall, these findings indicate that phasic dopamine may have different modes of communication between striatal and corticolimbic regions, with the first being profuse in concentration, rapid, and synaptically confined and the second being more limited in concentration but longer lasting and spatially dispersed. An improved understanding of regional differences in dopamine transmission can lead to more efficient treatments for disorders related to dopamine dysfunction.

Carbon Nanotube-Based Microelectrodes for Enhanced Neurochemical Detection

Carbon nanotube (CNT) fiber microelectrodes have been developed as electrode materials for the detection of neurotransmitters using fast scan cyclic voltammetry (FSCV). We have used acid-wet spinning to create "neat" carbon nanotube fibers and utilized them as electrode materials. Thirty-forty micron diameter acid spun CNT fiber microelectrodes were more sensitive than PEI-CNT fiber microelectrodes, with a 3 nM limit of detection. They also had faster electron transfer kinetics and a greater reversibility for the oxidation of dopamine using FSCV than CFMEs and other carbon nanomaterials. The acid spun CNT fiber microelectrodes also displayed a frequency independent response for the peak oxidation current of dopamine. This property was also seen in other CNT materials such as PEI-CNT fiber microelectrodes and CNT-Yarn microelectrodes. Upon varying the frequency from 10 Hz to 100 Hz, there was no decrease in sensitivity. When scanning at 2,000 V/s, there was no decrease in sensitivity upon changing the frequency from 10 Hz to 500 Hz. This could potentially allow for a 2 ms sampling rate for FSCV, comparable to those used with amperometry as opposed to 100 ms temporal resolution of traditional FSCV, an almost two orders of magnitude difference. Since the frequency independent response is seen with many CNT fibers/yarns, it suggests it is a fundamental property of the CNTs shared by many types of CNT fibers and yarns.

Motivational neural circuits underlying reinforcement learning

Reinforcement learning (RL) is the behavioral process of learning the values of actions and objects. Most models of RL assume that the dopaminergic prediction error signal drives plasticity in frontal-striatal circuits. The striatum then encodes value representations that drive decision processes. However, the amygdala has also been shown to play an important role in forming Pavlovian stimulus-outcome associations. These Pavlovian associations can drive motivated behavior via the amygdala projections to the ventral striatum or the ventral tegmental area. The amygdala may, therefore, play a central role in RL. Here we compare the contributions of the amygdala and the striatum to RL and show that both the amygdala and striatum learn and represent expected values in RL tasks. Furthermore, value representations in the striatum may be inherited, to some extent, from the amygdala. The striatum may, therefore, play less of a primary role in learning stimulus-outcome associations in RL than previously suggested.

Aptamer-functionalized neural recording electrodes for the direct measurement of cocaine in vivo

Cocaine is a highly addictive psychostimulant that acts through competitive inhibition of the dopamine transporter. In order to fully understand the region specific neuropathology of cocaine abuse and addiction, it is unequivocally necessary to develop cocaine sensing technology capable of directly measuring real-time cocaine transient events local to different brain regions throughout the pharmacokinetic time course of exposure. We have developed an electrochemical aptamer-based in vivo cocaine sensor on a silicon based neural recording probe platform capable of directly measuring cocaine from discrete brain locations using square wave voltammetry (SWV). The sensitivity of the sensor for cocaine follows a modified exponential Langmuir model relationship and complete aptamer-target binding occurs in < 2 sec and unbinding in < 4 sec. The resulting temporal resolution is a 75X increase from traditional microdialysis sampling methods. When implanted in the rat dorsal striatum, the cocaine sensor exhibits stable SWV signal drift (modeled using a logarithmic exponential equation) and is capable of measuring real-time in vivo response to repeated local cocaine infusion as well as systemic IV cocaine injection. The in vivo sensor is capable of obtaining reproducible measurements over a period approaching 3 hours, after which signal amplitude significantly decreases likely due to tissue encapsulation. Finally, aptamer functionalized neural recording probes successfully detect spontaneous and evoked neural activity in the brain. This dual functionality makes the cocaine sensor a powerful tool capable of monitoring both biochemical and electrophysiological signals with high spatial and temporal resolution.

Striatal dopamine neurotransmission: regulation of release and uptake

Dopamine (DA) transmission is governed by processes that regulate release from axonal boutons in the forebrain and the somatodendritic compartment in midbrain, and by clearance by the DA transporter, diffusion, and extracellular metabolism. We review how axonal DA release is regulated by neuronal activity and by autoreceptors and heteroreceptors, and address how quantal release events are regulated in size and frequency. In brain regions densely innervated by DA axons, DA clearance is due predominantly to uptake by the DA transporter, whereas in cortex, midbrain, and other regions with relatively sparse DA inputs, the norepinephrine transporter and diffusion are involved. We discuss the role of DA uptake in restricting the sphere of influence of DA and in temporal accumulation of extracellular DA levels upon successive action potentials. The tonic discharge activity of DA neurons may be translated into a tonic extracellular DA level, whereas their bursting activity can generate discrete extracellular DA transients.

Microdialysis in the rat striatum: effects of 24 h dexamethasone retrodialysis on evoked dopamine release and penetration injury

The power of microdialysis for in vivo neurochemical monitoring is a result of intense efforts to enhance microdialysis procedures, the probes themselves, and the analytical systems used for the analysis of dialysate samples. Our goal is to refine microdialysis further by focusing attention on what happens when the probes are implanted into brain tissue. It is broadly acknowledged that some tissue damage occurs, such that the tissue nearest the probes is disrupted from its normal state. We hypothesize that mitigating such disruption would refine microdialysis. Herein, we show that the addition of dexamethasone, an anti-inflammatory drug, to the perfusion fluid protects evoked dopamine responses as measured by fast-scan cyclic voltammetry next to the probes after 24 h. We also show that dexamethasone stabilizes evoked dopamine responses measured at the probe outlet over a 4-24 h postimplantation interval. The effects of dexamethasone are attributable to its anti-inflammatory actions, as dexamethasone had no significant effect on two histochemical markers for dopamine terminals, tyrosine hydroxylase and the dopamine transporter. Using histochemical assays, we confirmed that the actions of dexamethasone are tightly confined to the immediate, local vicinity of the probe.

Reduced dopamine transporter expression in the amygdala of subjects diagnosed with schizophrenia

A disruption of dopaminergic transmission in the amygdala of subjects with schizophrenia was proposed as a main contributor to pathophysiological and clinical manifestations of this disorder. We tested the hypothesis that the expression of the dopamine transporter (DAT) is decreased in the amygdala of subjects with schizophrenia. In normal control, schizophrenic subjects and bipolar disorder subjects, we measured numerical density of axon varicosities immunoreactive (IR) for DAT in the lateral (LN), basal, accessory basal (ABN), and cortical (CO) nuclei and intercalated cell masses (ITCM) of the amygdala. Tyrosine hydroxylase (TH)-IR and dopamine beta-hydroxylase (DBH)-IR varicosities were measured to test for potential loss of varicosities and serotonin transporter (5HTT)-IR for involvement of the serotoninergic system. Among several potential confounding variables tested, particular emphasis was placed on exposure to therapeutic drugs. In schizophrenic subjects, DAT-IR varicosities were decreased in LN (P = .0002), ABN (P = .013), and CO (P = .0001) in comparison with controls, and in comparison with bipolar disorder subjects in LN (P = .004) and CO (P = .002). DBH-IR varicosities were decreased in ABN (P = .008) and ITCM (P = .017), compared with controls. TH- and 5HTT-IR varicosities were not altered. No changes were detected in bipolar disorder. Taken together with TH and DBH findings, reductions of DAT-IR varicosities point to decreased DAT expression in dopaminergic terminals in the amygdala of subjects with schizophrenia. This DAT decrease may disrupt dopamine uptake, leading to increased dopaminergic synaptic transmission and spillage into the extracellular space with activation of extrasynaptic dopamine receptors. Concurrent decrease of noradrenaline in the ABN may disrupt memory consolidation.

Pharmacological mitigation of tissue damage during brain microdialysis

Microdialysis sampling in the brain is employed frequently in the chemical analysis of neurological function and disease, but implanting the probes, which are substantially larger than the size and spacing of brain cells and blood vessels, is injurious and triggers ischemia, gliosis, and cell death at the sampling site. The nature of the interface between the brain and the microdialysis probe is critical to the use of microdialysis as a neurochemical analysis technique. The objective of the work reported here was to investigate the potential of two compounds, dexamethasone, a glucocorticoid anti-inflammatory agent, and XJB-5-131, a mitochondrially targeted reactive oxygen species scavenger, to mitigate the penetration injury. Measurements were performed in the rat brain striatum, which is densely innervated by axons that release dopamine, an electroactive neurotransmitter. We used voltammetry to measure electrically evoked dopamine release next to microdialysis probes during the retrodialysis of dexamethasone or XJB-5-131. After the in vivo measurements, the brain tissue containing the microdialysis probe tracks was examined by fluorescence microscopy using markers for ischemia, neuronal nuclei, macrophages, and dopamine axons and terminals. Dexamethasone and XJB-5-131 each diminished the loss of evoked dopamine activity, diminished ischemia, diminished the loss of neuronal nuclei, diminished the appearance of extravasated macrophages, and diminished the loss of dopamine axons and terminals next to the probes. Our findings confirm the ability of dexamethasone and XJB-5-131 to mitigate, but not eliminate, the effects of the penetration injury caused by implanting microdialysis probes into brain tissue.

Restricted diffusion of dopamine in the rat dorsal striatum

Recent evidence has shown that the dorsal striatum of the rat is arranged as a patchwork of domains that exhibit distinct dopamine kinetics and concentrations. This raises the pressing question of how these distinct domains are maintained, especially if dopamine is able to diffuse through the extracellular space. Diffusion between the domains would eliminate the concentration differences and, thereby, the domains themselves. The present study is a closer examination of dopamine's ability to diffuse in the extracellular space. We used voltammetry to record dopamine overflow in dorsal striatum while stimulating the medial forebrain bundle over a range of stimulus currents and frequencies. We also examined the effects of drugs that modulated the dopamine release (raclopride and quinpirole) and uptake (nomifensine). Examining the details of the temporal features of the evoked profiles reveals no clear evidence for long-distance diffusion of dopamine between fast and slow domains, even though uptake inhibition by nomifensine clearly prolongs the time that dopamine resides in the extracellular space. Our observations support the conclusion that striatal tissue has the capacity to retain dopamine molecules, thereby limiting its tendency to diffuse through the extracellular space.

A comparison of the subsecond dynamics of neurotransmission of dopamine and serotonin

The neuromodulators dopamine (DA) and serotonin (5-hydroxytryptamine; 5-HT) are similar in a number of ways. Both monoamines can act by volume transmission at metabotropic receptors to modulate synaptic transmission in brain circuits. Presynaptic regulation of 5-HT and DA is governed by parallel processes, and behaviorally, both exert control over emotional processing. However, differences are also apparent: more than twice as many 5-HT receptor subtypes mediate postsynaptic effects than DA receptors and different presynaptic regulation is also emerging. Monoamines are amenable to real-time electrochemical detection using fast scan cyclic voltammetry (FSCV), which allows resolution of the subsecond dynamics of release and reuptake in response to a single action potential. This approach has greatly enriched understanding of DA transmission and has facilitated an integrated view of how DA mediates behavioral control. However, technical challenges are associated with FSCV measurement of 5-HT and understanding of 5-HT transmission at subsecond resolution has not advanced at the same rate. As a result, how the actions of 5-HT at the level of the synapse translate into behavior is poorly understood. Recent technical advances may aid the study of 5-HT in real-time. It is timely, therefore, to compare and contrast what is currently understood of the subsecond characteristics of transmission for DA and 5-HT. In doing so, a number of areas are highlighted as being worthy of exploration for 5-HT.

Amphetamine augments vesicular dopamine release in the dorsal and ventral striatum through different mechanisms

Amphetamine has well-established actions on pre-synaptic dopamine signaling, such as inhibiting uptake and degradation, activating synthesis, depleting vesicular stores, and promoting dopamine-transporter reversal and non-exocytotic release. Recent in vivo studies have identified an additional mechanism: augmenting vesicular release. In this study, we investigated how amphetamine elicits this effect. Our hypothesis was that amphetamine enhances vesicular dopamine release in dorsal and ventral striata by differentially targeting dopamine synthesis and degradation. In urethane-anesthetized rats, we employed voltammetry to monitor dopamine, electrical stimulation to deplete stores or assess vesicular release and uptake, and pharmacology to isolate degradation and synthesis. While amphetamine increased electrically evoked dopamine levels, inhibited uptake, and up-regulated vesicular release in both striatal sub-regions in controls, this psychostimulant elicited region-specific effects on evoked levels and vesicular release but not uptake in drug treatments. Evoked levels better correlated with vesicular release compared with uptake, supporting enhanced vesicular release as an important amphetamine mechanism. Taken together, these results suggested that amphetamine enhances vesicular release in the dorsal striatum by activating dopamine synthesis and inhibiting dopamine degradation, but targeting an alternative mechanism in the ventral striatum. Region-distinct activation of vesicular dopamine release highlights complex cellular actions of amphetamine and may have implications for its behavioral effects.

Harmine augments electrically evoked dopamine efflux in the nucleus accumbens shell

Harmine is a β-carboline alkaloid and major component of ayahuasca, a traditional South American psychoactive tea with anecdotal efficacy for treatment of cocaine dependence. Harmine is an inhibitor of monoamine oxidase A (MAO-A) and interacts in vitro with several pharmacological targets which modulate dopamine (DA) neurotransmission. In vivo studies have demonstrated dopaminergic effects of harmine, attributed to monoamine oxidase inhibitor (MAOI) activity, however none have directly demonstrated a pharmacological mechanism. This study investigated the acute effects, and pharmacological mechanism(s), of harmine on electrically evoked DA efflux parameters in the nucleus accumbens both in the absence and presence of cocaine. Fast cyclic voltammetry in rat brain slices was used to measure electrically evoked DA efflux in accumbens core and shell. Harmine (300 nM) significantly augmented DA efflux (148±8% of baseline) in the accumbens shell. Cocaine augmented efflux in shell additive to harmine (260±35%). Harmine had no effect on efflux in the accumbens core or on reuptake in either sub-region. The effect of harmine in the shell was attenuated by the 5-HT(2A/2C) antagonist ketanserin. The MAOI moclobemide (10 µM) had no effect on DA efflux. These data suggest that harmine augments DA efflux via a novel, shell-specific, presynaptic 5-HT(2A) receptor-dependent mechanism, independent of MAOI activity. A DA-releasing 'agonist therapy' mechanism may thus contribute to the putative therapeutic efficacy of ayahuasca for cocaine dependence.

In vivo comparison of norepinephrine and dopamine release in rat brain by simultaneous measurements with fast-scan cyclic voltammetry

Brain norepinephrine and dopamine regulate a variety of critical behaviors such as stress, learning, memory, and drug addiction. In this study, we demonstrate differences in the regulation of in vivo neurotransmission for dopamine in the anterior nucleus accumbens (NAc) and norepinephrine in the ventral bed nucleus of the stria terminalis (vBNST) of the anesthetized rat. Release of the two catecholamines was measured simultaneously using fast-scan cyclic voltammetry at two different carbon-fiber microelectrodes, each implanted in the brain region of interest. Simultaneous dopamine and norepinephrine release was evoked by electrical stimulation of a region where the ventral noradrenergic bundle, the pathway of noradrenergic neurons, courses through the ventral tegmental area/substantia nigra, the origin of dopaminergic cell bodies. The release and uptake of norepinephrine in the vBNST were both significantly slower than for dopamine in the NAc. Pharmacological manipulations in the same animal demonstrated that the two catecholamines are differently regulated. The combination of a dopamine autoreceptor antagonist and amphetamine significantly increased basal extracellular dopamine whereas a norepinephrine autoreceptor antagonist and amphetamine did not change basal norepinephrine concentration. α-Methyl-p-tyrosine, a tyrosine hydroxylase inhibitor, decreased electrically evoked dopamine release faster than norepinephrine. The dual-microelectrode fast-scan cyclic voltammetry technique along with anatomical and pharmacological evidence confirms that dopamine in the NAc and norepinephrine in the vBNST can be monitored selectively and simultaneously in the same animal. The high temporal and spatial resolution of the technique enabled us to examine differences in the dynamics of extracellular norepinephrine and dopamine concurrently in two different limbic structures.

Nicotine increases dopamine clearance in medial prefrontal cortex in rats raised in an enriched environment

Environmental enrichment results in differential behavioral and neurochemical responsiveness to nicotine. The present study investigates dopamine clearance (CL(DA) ) in striatum and medial prefrontal cortex (mPFC) using in vivo voltammetry in rats raised in enriched (EC) or impoverished conditions (IC) and administered nicotine (0.4 mg/kg) or saline. Baseline CL(DA) in striatum or mPFC was not different between EC and IC. Across repeated DA application, striatal CL(DA) increased in saline-control EC and IC. CL(DA) increased in mPFC in saline-control IC; CL(DA) did not change in saline-control EC. Thus, enrichment differentially alters dynamic responses of the dopamine transporter (DAT) to repeated DA application in mPFC, but not in striatum. In EC, nicotine increased mPFC CL(DA) compared to saline-control, but had no effect on CL(DA) in IC; nicotine had no effect in striatum in EC or IC. Compared to respective saline-controls, nicotine increased dihydroxyphenylacetic acid content in striatum and mPFC in EC, but not in IC. Nicotine also had no effect on DA content in striatum or mPFC in EC or IC. Results indicate that enrichment eliminated the dynamic response of mPFC DAT to repeated DA application in saline-control and augmented the nicotine-induced increase in DAT function in mPFC, but not in striatum.

Demon voltammetry and analysis software: analysis of cocaine-induced alterations in dopamine signaling using multiple kinetic measures

The fast sampling rates of fast scan cyclic voltammetry make it a favorable method for measuring changes in brain monoamine release and uptake kinetics in slice, anesthetized, and freely moving preparations. The most common analysis technique for evaluating changes in dopamine signaling uses well-established Michaelis-Menten kinetic methods that can accurately model dopamine release and uptake parameters across multiple experimental conditions. Nevertheless, over the years, many researchers have turned to other measures to estimate changes in dopamine release and uptake, yet to our knowledge no systematic comparison amongst these measures has been conducted. To address this lack of uniformity in kinetic analyses, we have created the Demon Voltammetry and Analysis software suite, which is freely available to academic and non-profit institutions. Here we present an explanation of the Demon Voltammetry acquisition and analysis features, and demonstrate its utility for acquiring voltammetric data under in vitro, in vivo anesthetized, and freely moving conditions. Additionally, the software was used to compare the sensitivity of multiple kinetic measures of release and uptake to cocaine-induced changes in electrically evoked dopamine efflux in nucleus accumbens core slices. Specifically, we examined and compared tau, full width at half height, half-life, T₂₀, T₈₀, slope, peak height, calibrated peak dopamine concentration, and area under the curve to the well-characterized Michaelis-Menten parameters, dopamine per pulse, maximal uptake rate, and apparent affinity. Based on observed results we recommend tau for measuring dopamine uptake and calibrated peak dopamine concentration for measuring dopamine release.

Evaluation of the D3 dopamine receptor selective agonist/partial agonist PG01042 on L-dopa dependent animal involuntary movements in rats

The substituted 4-phenylpiperazine D3 dopamine receptor selective antagonist PG01037 ((E)-N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)but-2-enyl)-4-(pyridin-2-yl)benzamide) was reported to attenuate L-dopa-associated abnormal involuntary movements (AIMs) in unilaterally lesioned rats, a model of L-dopa-dependent dyskinesia in patients with Parkinson's Disease (Kumar et al., 2009a). We now report that PG01042 (N-(4-(4-(2,3-dichlorophenyl)piperazin-1-yl)butyl)-4-(pyridin-3-yl)benzamide), which is a D3 dopamine receptor selective agonist for adenylyl cyclase inhibition and a partial agonist for mitogenesis, is also capable of attenuating AIMs scores. The intrinsic activity of PG01037 and PG01042 were determined using a) a forskolin-dependent adenylyl cyclase inhibition assay and b) an assay for agonist-associated mitogenesis. It was observed that the in vivo efficacy of PG01042 increased when administered by intraperitoneal (i.p.) injection simultaneously with L-dopa/benserazide (8 mg/kg each), as compared to a 60 min or 30 min pretreatment. PG01042 was found to attenuate AIM scores in these animals in a dose dependent manner. While PG01042 did not effectively inhibit SKF 81297-dependent AIMs, it inhibited apomorphine-dependent AIM scores. Rotarod studies indicate that PG01042 at a dose of 10 mg/kg did not adversely affect motor coordination of the unilaterally lesioned rats. Evaluation of lesioned rats using a cylinder test behavioral paradigm indicated that PG01042 did not dramatically attenuate the beneficial effects of L-dopa. These studies and previously published studies suggest that both D3 dopamine receptor selective antagonists, partial agonists and agonists, as defined by an adenylyl cyclase inhibition assay and a mitogenic assay, are pharmacotherapeutic candidates for the treatment of L-dopa-associated dyskinesia in patients with Parkinson's Disease.

The dopamine metabolite 3-methoxytyramine is a neuromodulator

Dopamine (3-hydroxytyramine) is a well-known catecholamine neurotransmitter involved in multiple physiological functions including movement control. Here we report that the major extracellular metabolite of dopamine, 3-methoxytyramine (3-MT), can induce behavioral effects in a dopamine-independent manner and these effects are partially mediated by the trace amine associated receptor 1 (TAAR1). Unbiased in vivo screening of putative trace amine receptor ligands for potential effects on the movement control revealed that 3-MT infused in the brain is able to induce a complex set of abnormal involuntary movements in mice acutely depleted of dopamine. In normal mice, the central administration of 3-MT caused a temporary mild hyperactivity with a concomitant set of abnormal movements. Furthermore, 3-MT induced significant ERK and CREB phosphorylation in the mouse striatum, signaling events generally related to PKA-mediated cAMP accumulation. In mice lacking TAAR1, both behavioral and signaling effects of 3-MT were partially attenuated, consistent with the ability of 3-MT to activate TAAR1 receptors and cause cAMP accumulation as well as ERK and CREB phosphorylation in cellular assays. Thus, 3-MT is not just an inactive metabolite of DA, but a novel neuromodulator that in certain situations may be involved in movement control. Further characterization of the physiological functions mediated by 3-MT may advance understanding of the pathophysiology and pharmacology of brain disorders involving abnormal dopaminergic transmission, such as Parkinson's disease, dyskinesia and schizophrenia.

Synapsins differentially control dopamine and serotonin release

Synapsins are a family of synaptic vesicle proteins that are important for neurotransmitter release. Here we have used triple knock-out (TKO) mice lacking all three synapsin genes to determine the roles of synapsins in the release of two monoamine neurotransmitters, dopamine and serotonin. Serotonin release evoked by electrical stimulation was identical in substantia nigra pars reticulata slices prepared from TKO and wild-type mice. In contrast, release of dopamine in response to electrical stimulation was approximately doubled in striatum of TKO mice, both in vivo and in striatal slices, in comparison to wild-type controls. This was due to loss of synapsin III, because deletion of synapsin III alone was sufficient to increase dopamine release. Deletion of synapsins also increased the sensitivity of dopamine release to extracellular calcium ions. Although cocaine did not affect the release of serotonin from nigral tissue, this drug did enhance dopamine release. Cocaine-induced facilitation of dopamine release was a function of external calcium, an effect that was reduced in TKO mice. We conclude that synapsins play different roles in the control of release of dopamine and serotonin, with release of dopamine being negatively regulated by synapsins, specifically synapsin III, while serotonin release appears to be relatively independent of synapsins. These results provide further support for the concept that synapsin function in presynaptic terminals varies according to the neurotransmitter being released.

Genetic variants within the dopaminergic system interact to modulate endocrine stress reactivity and recovery

Catecholamines modulate endocrine stress reactivity by affecting regulatory influences of extra-hypothalamic brain structures on hypothalamus-pituitary-adrenal (HPA)-axis. Therefore, we aimed to investigate combined effects of functional allelic variations that affect dopamine availability in both cortical (COMT Val¹⁵⁸Met polymorphism) and subcortical (DAT1 VNTR) brain regions on HPA-axis reactivity to psychosocial stress. By using a standardized laboratory stress task (public speaking) we obtained saliva cortisol samples during stress exposure and an extended recovery period in 100 healthy male adults. We report for the first time significant epistasis between COMT Val¹⁵⁸Met and DAT1 VNTR on cortisol response patterns. Subjects homozygous for both the Met¹⁵⁸ and the 10-repeat allele of DAT1 VNTR were characterized by markedly elevated cortisol reactivity and impaired stress recovery compared to all other groups. Our results indicate a crucial role of functional genetic variants within the dopaminergic system in the modulation of HPA-axis response patterns and highlight the need to investigate combined effects of specific candidate genes on stress-related endophenotypes.

Blockade of the acquisition, but not expression, of associative learning by pre-session intra-amygdala R(+) 7-OH-DPAT

RATIONALE

Two issues were addressed regarding the effects of amygdala dopamine manipulations on associative learning: first, an apparent contradiction between the effects of post- vs. pre-session dopaminergic manipulations and second, the ability of dopaminergic infusions to affect association formation vs. its expression following extended training.

OBJECTIVES

The ability of pre-session infusions of a dopamine receptor agonist (R(+) 7-OH-DPAT) to inhibit acquisition of a conditioned approach response was examined and compared with the same manipulation following overtraining. Further experiments extended these findings.

MATERIALS AND METHODS

Experiment 1 infused pre-session intra-amygdala R(+) 7-OH-DPAT (0, 0.1, 1 nmol) during conditioned approach acquisition. Experiment 2 applied pre-session intra-amygdala R(+) 7-OH-DPAT (0, 0.01, 0.1, 1 nmol) during expression of the same response, once well learned. Experiment 3 required the inhibition of a conditioned approach response following unconditioned stimulus (US) removal. Experiment 4 examined the ability of animals with prior drug experience to acquire a conditioned response to a novel stimulus.

RESULTS

Experiments 1-3 showed that pre-session amygdala R(+) 7-OH-DPAT impaired acquisition of either excitatory or inhibitory conditioned responding, but was ineffective following overtraining. Drug-induced impairments in acquisition of a specific conditioned stimulus (CS)-US relationship continued well beyond the cessation of drug treatment, but were found not to transfer to an alternate CS in Experiment 4.

CONCLUSIONS

Pre-session dopamine receptor activation within the amygdala may impair the acquisition, but not expression, of CS-US associations. Enhanced learning reported earlier following post-session dopamine receptor activation may occur indirectly through reduced interference with the consolidation of recent learning.

Reuptake of L-DOPA-derived extracellular DA in the striatum of a rodent model of Parkinson's disease via norepinephrine transporter

To determine the role of norepinephrine transporter in reuptake of L-DOPA-derived extracellular DA in the DA-denervated Parkinsonian striatum, we examined extracellular DA levels in the striatum of 6-hydroxyDA-lesioned rats that received L-DOPA (50 mg/kg with 12.5 mg/kg of benserazide) and L-DOPA plus desipramine (25 mg/kg), a selective norepinephrine reuptake inhibitor, using in vivo microdialysis. The pretreatment with desipramine increased levels of extracellular DA derived from administrated L-DOPA in the DA-denervated striatum. This study provides evidence that L-DOPA-derived DA is taken up by the norepinephrine transporter, instead of the dopamine transporter, in the striatum with dopaminergic denervation. This result suggests that the norepinephrine transporter could be a promising target in the treatment for Parkinson's disease.

Site-specific role of catechol-O-methyltransferase in dopamine overflow within prefrontal cortex and dorsal striatum

Accumulating evidence from clinical and preclinical studies shows that catechol-O-methyltransferase (COMT) plays a significant role in dopamine metabolism in the prefrontal cortex, but not in the striatum. However, to what extent dopamine overflow in the prefrontal cortex and striatum is controlled by enzymatic degradation versus reuptake is unknown. We used COMT deficient mice to investigate the role of COMT in these two brain regions with in vivo voltammetry. A real-time analysis of evoked dopamine overflow showed that removal of dopamine was twofold slower in the prefrontal cortex of mice lacking COMT than in wild-type mice, indicating that half of the dopamine decline in this brain region results from COMT-mediated enzymatic degradation. Lack of COMT did not influence dopamine overflow/decline in the dorsal striatum. COMT-deficient mice demonstrated a small (20-25%) but consistent increase in evoked dopamine release in the prefrontal cortex, but not in the dorsal striatum. Cocaine affected equally dopaminergic neurotransmission in the prefrontal cortex in both genotypes by prolonging 3-4 times dopamine elimination from extracellular space. Paradoxically, this happened without increase of the peak levels of evoked dopamine release. The present findings represent the first demonstration of the significant contribution of COMT in modulating the dynamics of dopamine overflow in the prefrontal cortex and underscore the therapeutic potential of manipulating COMT activity to alter dopaminergic neurotransmission in the prefrontal cortex.

Treatment of cognitive deficits associated with schizophrenia: potential role of catechol-O-methyltransferase inhibitors

In the last two decades, understanding of the dynamics of dopamine function in the prefrontal cortex and its role in prefrontal cortex physiology has opened up new avenues for therapeutic interventions in conditions in which prefrontal cortex function is compromised. Neuropsychological and imaging studies of prefrontal information processing have confirmed specific cognitive and neurophysiological abnormalities in individuals with schizophrenia. Because such findings are also observed in the healthy siblings of patients with schizophrenia, they may represent intermediate phenotypes related to schizophrenia susceptibility genes.Catechol-O-methyltransferase (COMT) represents an important candidate as a susceptibility gene for cognitive dysfunction in schizophrenia because of the unique role this enzyme plays in regulating prefrontal dopaminergic function. A functional COMT polymorphism (Val158Met) predicts performance in tasks of prefrontal executive function and the neurophysiological response measured with electroencephalography and functional magnetic resonance imaging in tasks assessing working memory. In fact, individuals with the Val/Val genotype, which encodes for the high-activity enzyme resulting in lower dopamine concentrations in the prefrontal cortex, perform less well and are less efficient physiologically than Met/Met individuals. These findings raise the possibility of new pharmacological interventions for the treatment of prefrontal cortex dysfunction and of predicting outcome based on COMT genotype. One strategy consists of the use of CNS-penetrant COMT inhibitors such as tolcapone. A second strategy is to increase extracellular dopamine concentrations in the frontal cortex by blocking the noradrenaline (norepinephrine) reuptake system, a secondary mechanism responsible for the disposal of dopamine from synaptic clefts in the prefrontal cortex. A third possibility involves the use of modafinil, a drug with an unclear mechanism of action but with positive effects on working memory in rodents. The potential of these drugs to improve executive cognitive function by selectively increasing dopamine load in the frontal cortex but not in subcortical territories, and the possibility that response to them may be modified by a COMT polymorphism, provides a novel genotype-based targeted pharmacological approach without abuse potential for the treatment of cognitive disorder in schizophrenia and in other conditions involving prefrontal cortex dysfunction.

Genetic variation in catechol-O-methyltransferase (COMT) and obesity in the prostate, lung, colorectal, and ovarian (PLCO) cancer screening trial

Catechol-O-methyltransferase (COMT) is an important modulator in the catabolism of extraneural dopamine, which plays an important role in drug reward mechanisms. It is hypothesized that genetic variations in the COMT gene, which can result in a three to fourfold difference in COMT enzyme activity, may be associated with several reward-motivated behaviors. The aim of our study was to examine the relationship between COMT polymorphisms with smoking, obesity and alcohol. Three single nucleotide polymorphisms (SNPs) in COMT were genotyped in 2,371 participants selected randomly from the screening arm of the PLCO Cancer Screening Trial after stratifying by sex, age, and smoking status. Smoking, obesity, and alcohol consumption were assessed by questionnaire. SNP and haplotype associations were estimated using odds ratios (ORs) and 95% confidence intervals (CIs) derived from conditional logistic regression models, adjusted for race/ethnicity. The COMT Ex4-76C > G (Leu136Leu) polymorphism was statistically significantly associated with individuals who had >30% increases in BMI from ages 20 to 50 years, compared to those with 0-5% increase in BMI (0-5%) over the same age period: (CC is referent; OR(CG )= 1.42, OR(GG )= 1.46, P (trend )= 0.06). By sex, the increased risk was further pronounced among females (OR(CG )= 1.50, OR(GG )= 2.10, P (trend )= 0.03). Consistent with our analyses of single polymorphisms, individuals whose BMI increased >30% from ages 20 to 50 years were more likely than individuals with 0-5% increases in BMI to possess COMT haplotypes [COMT Ex3-104C > T-COMT Ex4-76 C > G-COMT Ex4-12 A > G] that included the variant allele for COMT Ex4-76 C > G: C-G-G (T-C-A is referent: OR(C-G-G )= 1.33, 95% CI 1.01-1.77) and C-G-A (OR(C-G-A )= 1.79, 95% CI 0.72-4.49). We observed no association between any of the COMT polymorphisms with smoking behavior or alcohol intake. The COMT Ex4-76C > G (Leu136Leu) polymorphism appears to play a role in large increases in BMI. The null association with smoking and alcohol and the pronounced association with increasing BMI among women further implicates COMT's role in estrogen metabolism as a potentially culpable pathway. Our results support a need for comprehensive evaluation of COMT variations and their functional relevance as COMT may be an important molecular target to evaluate for new treatments regarding obesity.

Substrates and inhibitors display different sensitivity to expression level of the dopamine transporter in heterologously expressing cells

The use of heterologous expression systems for studying dopamine (DA) transporter (DAT) function has provided important information corroborating and complementing in situ obtained knowledge. Preliminary experiments with human embryonic kidney cells (HEK293) heterologously expressing varying amounts of DAT suggested fluctuations in the potency of cocaine in inhibiting DA uptake and led to the present systematic assessment of the impact of the density of DAT on its function. Transiently expressing intact HEK293 cells, transfected with increasing amounts of DAT cDNA, displayed increasing levels of surface DAT, binding of the cocaine analog [(3)H]2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane ([(3)H]CFT), and uptake of [(3)H]DA, [(3)H]N-methyl-4-phenylpyridinium ([(3)H]MPP(+)), [(3)H]norepinephrine, and [(3)H]serotonin. However, the amount of DAT cDNA and the DAT expression level required to produce 50% of maximal activity was threefold higher for CFT binding than for DA uptake. Increased DAT expression was accompanied by weakened potency in inhibiting [(3)H]DA uptake for cocaine, CFT, benztropine, and its analog JHW025, GBR 12909 and mazindol; their potency in inhibiting [(3)H]CFT binding was unaffected. Inhibition of uptake by the substrates DA, m-tyramine, d-amphetamine, or MPP(+) was also unaffected. Increasing DAT in stably expressing HEK293 cells by stimulation of gene expression with sodium butyrate also decreased the uptake inhibitory potency of a number of the above blockers without affecting the interaction between substrates and DAT. The present results prompt discussion of models explaining how factors regulating DAT expression at the plasma membrane can regulate DAT function and pharmacology.

Pharmacogenetic tools for the development of target-oriented cognitive-enhancing drugs

The identification of the anatomical and physiological substrates involved in the regulation of the dorsolateral prefrontal cortex function in humans provided the basis for the understanding of mechanisms involved in cognitive and executive function under normal as well as pathological conditions. In this context, substantial evidence indicates that alterations in monaminergic function in the dorsolateral prefrontal cortex significantly contributes to the cognitive impairments present in schizophrenia, attention deficit disorders, and other neuropsychiatric conditions. The development of a number of compounds that selectively increase extracellular dopamine (DA) concentrations in the dorsolateral prefrontal cortex but not in subcortical areas by either blocking its metabolism or reuptake, or increasing its release, or that directly activate postsynaptic DA-1 receptor mechanisms provided powerful pharmacotherapeutic tools to mitigate the cognitive deficits brought about by the dopaminergic alterations of the prefrontal cortex. More recently, the findings that polymorphisms of the catecholamine-O-methyl-transferase gene may also modify the effect of these drugs on the prefrontal cortex points toward a more specific genotype-based neuropsychopharmacology for the treatment of cognitive deficits in schizophrenia as well as in a number of other neuropsychiatric conditions. The ability of these compounds to increase DA load selectively in the frontal cortex and not on subcortical systems allows a targeted intervention without the stimulant-like effects observed with older drugs used to treat those conditions.

Differential effects of amphetamine isomers on dopamine release in the rat striatum and nucleus accumbens core

RATIONALE

Current medications for attention-deficit/hyperactivity disorder (ADHD) include some single isomer compounds [dextroamphetamine (D: -amphetamine, dexedrine) and dexmethylphenidate (Focalin)] and some racemic compounds [methylphenidate and mixed-salts amphetamine (Adderall)]. Adderall, which contains approximately 25% L: -amphetamine, has been successfully marketed as a first-line medication for ADHD. Although different clinical effects have been observed for D: -amphetamine, Adderall, and benzedrine; potential psychopharmacological differences on the level of neurotransmission between D: -amphetamine and L: -amphetamine have not been well characterized.

OBJECTIVES

To evaluate potential differences in the isomers, we used the technique of high-speed chronoamperometry with Nafion-coated single carbon-fiber microelectrodes to measure amphetamine-induced release of dopamine (DA) in the striatum and nucleus accumbens core of anesthetized male Fischer 344 rats. Amphetamine solutions were locally applied by pressure ejection using micropipettes.

RESULTS

The presence of L: -amphetamine in the D: ,L: -amphetamine solutions did not cause increased release of DA but did change DA release kinetics. The D: ,L: -amphetamine-evoked signals exhibited significantly faster rise times and shorter signal decay times. This difference was also observed in the nucleus accumbens core. When L: -amphetamine was locally applied, DA release was not significantly different in amplitude, and it exhibited the same rapid kinetics of D: ,L: -amphetamine.

CONCLUSIONS

These data support the hypothesis that amphetamine isomers have different effects on release of DA from nerve endings. It is possible that L: -amphetamine may have unique actions on the DA transporter, which is required for the effects of amphetamine on DA release from nerve terminals.

In vivo interaction of cocaine with the dopamine transporter as measured by voltammetry

The goal of this review is to describe what the voltammetry technique tells us about cocaine-dopamine transporter (DAT) interactions and the subsequent changes in extracellular dopamine levels in the brain. The primary advantage of voltammetry, in this regard, is the capability for kinetic analysis in situ. Analysis of electrically evoked dynamics suggests that cocaine competitively inhibits dopamine uptake in the caudate-putamen and nucleus accumbens with a similar efficacy. The preferential increase in accumbal dopamine following systemic cocaine administration was found to be related not to a unique cocaine-DAT interaction, but rather to a unique combination of dopamine release and uptake rates. Similar enhancement occurs in sub-regions of the caudate-putamen exhibiting this release and uptake combination. Other factors such as diffusion and whether dopaminergic signaling is tonic or phasic also determine the effects of cocaine on striatal dopamine levels.

Modeling fast dopamine neurotransmission in the nucleus accumbens during behavior

Recent advances in electrophysiology and voltammetry permit monitoring of dopamine (DA) neuronal activity in real time in the brain of awake animals. Studies using these approaches demonstrate that behaviorally relevant events elicit characteristic patterns of electrical activity in midbrain DA neurons as well as large, transient changes in extracellular DA in the nucleus accumbens (NAc). In addition to providing insight into the role of the DA system in the processing of motor, motivational, and sensory information, the new findings also shed light on fast DA neurotransmission in a behavioral context. This report, (1). summarizes the information obtained by electrophysiological and real-time voltammetric approaches and (2). describes a general model of phasic DA signaling in the NAc that links the observed changes in DA electrical activity and extracellular dynamics. The analysis demonstrates that the behaviorally evoked DA transients are governed by similar mechanisms as those produced by short trains of electrical stimulation. Thus, action potential-dependent release and presynaptic uptake are primary determinants of functional DA levels in the brain during behavior. Interestingly, the model predicts that the same burst of electrical activity generated at DA cell bodies produces markedly different DA dynamics in forebrain projection fields. The distinct changes result from heterogeneous release and uptake rates and may underlie region-specific effects of DA. Auto- and heteroreceptors, as well as other sites of presynaptic control, could further modulate the DA transients.

Brief, repeated exposure to substrates down-regulates dopamine transporter function in Xenopus oocytes in vitro and rat dorsal striatum in vivo

In heterologous expression systems, dopamine transporter (DAT) cell-surface localization is reduced after relatively prolonged exposure to d-amphetamine (AMPH) or dopamine (DA), suggesting a role for substrate-mediated regulation of transporter function. Here, we investigated whether brief, repeated periods of substrate exposure modulated transporter function, first, in an in vitro model system and, second, in intact rat brain. In human DAT-expressing Xenopus laevis oocytes, repeated exposure to low micromolar concentrations of DA, AMPH or tyramine markedly reduced transport-mediated currents. This functional down-regulation was attenuated by inclusion of a protein kinase C (PKC) inhibitor and probably reflects DAT redistribution, as cell-surface [3H]WIN 35 428 binding was significantly lower following DA exposure. High-speed chronoamperometry was used to measure clearance of exogenously applied DA in dorsal striatum (STR) and nucleus accumbens (NAc) of anesthetized rats. In STR, frequent (every 2 min) applications of DA altered DA clearance parameters in a manner consistent with profound down-regulation of DAT function. Similar changes were not observed in NAc or after repeated vehicle (ascorbic acid) application. Together, our results suggest that brief, repeated periods of substrate exposure lead to rapid down-regulation of DAT activity and that this type of regulation can occur in vivo in STR, but not NAc.

Dopamine autoreceptor regulation of release and uptake in mouse brain slices in the absence of D(3) receptors

The effects of the dopamine D(3) receptor, a putative autoreceptor, have been investigated by comparing behavioral and neurochemical properties of wild-type mice and mice with a genetic deletion of the D(3) receptor. The D(3) knock-out mice were modestly hyper-responsive to a novel environment relative to wild-type mice, and, consistent with this, quantitative in vivo microdialysis revealed elevated striatal dopamine extracellular levels. The dynamic actions of autoreceptors on electrically evoked dopamine release were examined in striatal brain slices from these animals and monitored with fast scan cyclic voltammetry at carbon-fiber microelectrodes. Quinpirole, a dopamine receptor agonist with potency at both D(2) and D(3) receptors, inhibited evoked dopamine in a dose-dependent manner with a slightly higher dose required in the knock-out animals (EC(50) of 60+/-10 nM in wild-type animals and 130+/-40 in D(3) knock-out animals; both curves had a Hill slope near 2). Dopamine synthesis inhibition with alpha-methyl-p-tyrosine caused released dopamine levels to decrease in each genotype. However, regulation of secretion by autoreceptors was still operant. Dose-response curves to quinpirole were unchanged in D(3) knock-out tissue, but secretion-regulated release exhibited a Hill slope decreased to 1 in the wild-type animals. In both genotypes, similar quinpirole-evoked increases in uptake rate were evident following synthesis inhibition. These data are consistent with the D(3) receptor having a small but significant role as a dopamine autoreceptor that partially regulates secretion, but not synthesis, in the caudate-putamen.

Blockade of sensitisation-induced facilitation of appetitive conditioning by post-session intra-amygdala nafadotride

Prior D-amphetamine experience has been reported to enhance appetitive Pavlovian conditioning. The present study assessed the involvement of the mesoamygdaloid dopamine projection in this effect. Bilateral post-session intra-amygdala infusions of the D3 dopamine receptor antagonist, L-nafadotride, or vehicle were given during acquisition of a Pavlovian association in sensitised and unsensitised rats. During these sessions, subjects received presentations of a stimulus (CS(+)) paired with 10% sucrose availability. A second stimulus (CS(-)) was also presented but never paired with sucrose. Sensitised animals infused post-session with vehicle acquired a Pavlovian conditioned approach response during CS(+) presentations more rapidly than controls, as we have shown previously. However, post-session intra-amygdala L-nafadotride selectively retarded conditioned responding to the CS(+) in both groups of animals, abolishing the difference between sensitised and unsensitised rats. These results, therefore, extend the evidence for the involvement of the mesoamygdaloid dopamine projection in Pavlovian conditioning, and the facilitation of associative learning following sensitisation.

Concurrent autoreceptor-mediated control of dopamine release and uptake during neurotransmission: an in vivo voltammetric study

Receptor-mediated feedback control plays an important role in dopamine (DA) neurotransmission. Recent evidence suggests that release and uptake, key mechanisms determining brain extracellular levels of the neurotransmitter, are governed by presynaptic autoreceptors. The goal of this study was to investigate whether autoreceptors regulate both mechanisms concurrently. Extracellular DA in the caudate-putamen and nucleus accumbens, evoked by electrical stimulation of the medial forebrain bundle, was monitored in the anesthetized rat by real-time voltammetry. Effects of the D2 antagonist haloperidol (0.5 mg/kg, i.p.) on evoked DA levels were measured to evaluate autoreceptor control mechanisms. Two strategies were used to resolve individual contributions of release and uptake to the robust increases in DA signals observed after acute haloperidol challenge in naive animals: pretreatment with 3beta-(p-chlorophenyl)tropan-2beta-carboxylic acid p-isothiocyanatophenylmethyl ester hydrochloride (RTI-76; 100 nmol, i.c.v.), an irreversible inhibitor of the DA transporter, and kinetic analysis of extracellular DA dynamics. RTI-76 effectively removed the uptake component from recorded signals. In RTI-76-pretreated rats, haloperidol induced only modest increases in DA elicited by low frequencies and had little or no effect at high frequencies. These results suggest that D2 antagonism alters uptake at all frequencies but only release at low frequencies. Kinetic analysis similarly demonstrated that haloperidol decreased V(max) for DA uptake and increased DA release at low (10-30 Hz) but not high (40-60 Hz) stimulus frequencies. We conclude that presynaptic DA autoreceptors concurrently downregulate release and upregulate uptake, and that the mechanisms are also independently controlled during neurotransmission.

Preferential increases in nucleus accumbens dopamine after systemic cocaine administration are caused by unique characteristics of dopamine neurotransmission

In vivo voltammetry was used to investigate the preferential increase of extracellular dopamine in the nucleus accumbens relative to the caudate-putamen after systemic cocaine administration. In the first part of this study, cocaine (40 mg/kg, i.p.) was compared with two other blockers of dopamine uptake, nomifensine (10 mg/kg, i.p.) and 3beta-(p-chlorophenyl)tropan-2beta-carboxylic acid p-isothiocyanatophenylmethyl ester hydrochloride (RTI-76; 100 nmol, i.c.v.), to assess whether the inhibitory mechanism of cocaine differed in the two regions. All three drugs robustly increased electrically evoked levels of dopamine, and cocaine elevated dopamine signals to a greater extent in the nucleus accumbens. However, kinetic analysis of the evoked dopamine signals indicated that cocaine and nomifensine increased the K(m) for dopamine uptake whereas the dominant effect of RTI-76 was a decrease in V(max). Under the present in vivo conditions, therefore, cocaine is a competitive inhibitor of dopamine uptake in both the nucleus accumbens and caudate-putamen. Whether the preferential effect of cocaine was mediated by regional differences in the presynaptic control of extracellular DA that are described by rates for DA uptake and release was examined next by a correlation analysis. The lower rates for dopamine release and uptake measured in the nucleus accumbens were found to underlie the preferential increase in extracellular dopamine after cocaine. This relationship explains the paradox that cocaine more effectively increases accumbal dopamine despite identical effects on the dopamine transporter in the two regions. The mechanism proposed for the preferential actions of cocaine may also mediate the differential effects of psychostimulant in extrastriatal regions and other uptake inhibitors in the striatum.

Dopamine attenuates prefrontal cortical suppression of sensory inputs to the basolateral amygdala of rats

The basolateral complex of the amygdala (BLA) plays a significant role in affective behavior that is likely regulated by afferents from the medial prefrontal cortex (mPFC). Studies suggest that dopamine (DA) is a necessary component for production of appropriate affective responses. In this study, prefrontal cortical and sensory cortical [temporal area 3 (Te3)] inputs to the BLA and their modulation by DA receptor activation was examined using in vivo single-unit extracellular recordings. We found that Te3 inputs are more capable of driving BLA projection neuron firing, whereas mPFC inputs potently elicited firing from BLA interneurons. Moreover, mPFC stimulation before Te3 stimulation attenuated the probability of Te3-evoked spikes in BLA projection neurons, possibly via activation of inhibitory interneurons. DA receptor activation by apomorphine attenuated mPFC inputs, while augmenting Te3 inputs. Additionally, DA receptor activation suppressed mPFC-induced inhibition of Te3-evoked spikes. Thus, the mPFC may attenuate sensory-driven amygdala-mediated affective responses via recruitment of BLA inhibitory interneurons that suppress sensory cortical inputs. In situations of enhanced DA levels in the BLA, such as during stress and after amphetamine administration, mPFC regulation of BLA will be dampened, leading to a disinhibition of sensory-driven affective responses.

Utility of a tripolar stimulating electrode for eliciting dopamine release in the rat striatum

The present study evaluated tripolar stimulating electrodes for eliciting dopamine release in the rat brain in vivo. Stimulating electrodes were placed either in the medial forebrain bundle or in the ventral mesencephalon associated with the ventral tegmental area and substantia nigra. The concentration of extracellular dopamine was monitored in dopamine terminal fields at 100-ms intervals using fast-scan cyclic voltammetry at carbon-fiber microelectrodes. To characterize the stimulated area, recordings were collected in several striatal regions including the caudate putamen and the core and shell of the nucleus accumbens. The tripolar electrode was equally effective in stimulating dopamine release in medial and lateral regions of the striatum. In contrast, responses evoked by a bipolar electrode were typically greater in one mediolateral edge versus the other. The added size of the tripolar electrode did not appear to cause complications as signals were stable over the course of the experiment (3 h). Subsets of mesostriatal dopamine neurons could also be selectively activated using the tripolar electrode in excellent agreement with previously described topography. Taken together, these results suggested that the tripolar stimulating electrode is well suited for studying the regulation of midbrain dopamine neurons in vivo.

In vivo electrochemical studies of dopamine clearance in subregions of rat nucleus accumbens: differential properties of the core and shell

The dopamine (DA) uptake/clearance properties of the DA transporter (DAT) in the core and shell of the nucleus accumbens were measured using in vivo electrochemical recordings. Calibrated amounts of a DA solution were pressure-ejected from a micropipette/electrode assembly placed in the core or shell of the nucleus accumbens in anesthetized male Fischer 344 rats. Initial studies in the two brain regions revealed that the core and shell have different DA clearance properties as measured by the extracellular DA signal amplitudes, clearance times, and clearance rates. Although the same number of picomoles of DA were applied, DA clearance signals recorded in shell had significantly greater amplitudes but faster clearance rates than those recorded in the core. Systemic administration of 20 mg/kg cocaine, a monoamine transporter inhibitor, greatly increased the signal amplitude from the locally applied DA in both the core and shell. Signal amplitudes were increased to a greater extent in the shell, compared with the core, after cocaine administration. However, cocaine affected the clearance time of DA only in the core and the DA clearance rate only in the shell. Taken together with previously reported data, these studies further support differential activity of the DAT in the core versus shell subregions of the nucleus accumbens. In addition, these data indicate that DATs are more sensitive to the effects of psychomotor stimulants, such as cocaine, in the shell of the nucleus accumbens.

Mediation of the discriminative stimulus properties of cocaine by mesocorticolimbic dopamine systems

This paper provides a brief review of the scientific evidence implicating the mesocorticolimbic dopamine (DA) system in modulating the discriminative stimulus properties of cocaine in rats. Briefly, systemic administration of DA releasers, reuptake inhibitors, and DA D1, D2, and putative D3 receptor agonists engendered partial to full substitution for the discriminative stimulus effects of cocaine. Dopamine D1 and D2 receptor antagonists attenuate this behavioral property of cocaine. Intracranial microinjection studies have indicated certain key limbic nuclei us loci of action for DA in mediating the discriminative stimulus effects of cocaine. Microinjections of cocaine into either DA cell body (i.e., ventral tegmental area, substantia nigra) or DA terminal regions (i.e., prefrontal cortex, central amygdala, caudate putamen) have failed to reproduce the systemic cocaine discriminative stimulus. Only infusion of cocaine into the nucleus accumbens has been demonstrated to substitute fully for the systemic effects of this psychostimulant. Interestingly, microinjections of the DA D1 receptor antagonist SCH 23390 into either the prefrontal cortex, nucleus accumbens, or central or basolateral amygdala have been demonstrated to block the discriminative stimulus properties of cocaine. Although a determination of the antagonism of the cocaine discriminative stimulus following intra-accumbens microinjection of DA D2 receptor antagonists has not been made, intra-accumbens administration of the DA D2 receptor antagonist sulpiride blocked the discriminative stimulus effects of another psychostimulant, amphetamine. 6-Hydroxydopamine lesions of DA terminals in the nucleus accumbens also attenuated the dose-effect curve for systemic administration of cocaine. Taken together, this intracranial evidence suggests that DA D1 and D2 receptors in the mesocorticolimbic system are involved in modulating the discriminative stimulus properties of psychostimulants and that the nigrostriatal DA system is not primarily involved.

Microelectrodes for the measurement of catecholamines in biological systems

Many of the molecules involved in biological signaling processes are easily oxidized and have been monitored by electrochemical methods. Temporal response, spatial considerations, and sensitivity of the electrodes must be optimized for the specific biological application. To monitor exocytosis from single cells in culture, constant potential amperometry offers the best temporal resolution, and a low-noise picoammeter improves the detection limits. Smaller electrodes, with 1-micron diameters, provided spatial resolution sufficient to identify the locations of release sites on the surface of single cells. For the study of neurotransmitter release in vivo, larger cylindrical microelectrodes are advantageous because the secreted molecules come from multiple terminals near the electrode, and the greater amounts lead to a larger signal that emerges from the Johnson noise of the current amplifier. With this approach, dopamine release elicited by two electrical stimulus pulses at 10 Hz was detected with fastscan cyclic voltammetry in vivo. Nafion-coated elliptical electrodes have previously been shown to be incapable of detecting such concentration changes without extensive signal averaging. In addition, we demonstrate that high-pass filtering (200 Hz) of cyclic voltammograms recorded at 300 V/s decreases the background current and digitization noise at these microelectrodes, leading to an improved signal. Also, high-pass filtering discriminated against ascorbic acid, DOPAC, and acidic pH changes, three common interferences in vivo.