Methodology for coupling local application of dopamine and other chemicals with rapid in vivo electrochemical recordings in freely-moving rats

Intrauterine Growth Restriction Modifies the Accumbal Dopaminergic Response to Palatable Food Intake

Intrauterine growth restriction (IUGR) associates with increased preference for palatable foods and altered insulin sensitivity. Insulin modulates the central dopaminergic response and changes behavioral responses to reward. We measured the release of dopamine in the accumbens during palatable food intake in IUGR rats both at baseline and in response to insulin. From pregnancy day 10 until birth, gestating Sprague-Dawley rats received either an ad libitum (Control), or a 50% food restricted (FR) diet. In adulthood, palatable food consumption and feeding behavior entropy was assessed using an electronic food intake monitor (BioDAQ®), and dopamine response to palatable food was measured by chronoamperometry recordings in the nucleus accumbens (NAcc). FR rats eat more palatable foods during the dark phase, and their eating pattern has a higher entropy compared to control rats. There was a delayed dopamine release in the FR group in response to palatable food and insulin administration reverted this delayed effect. Western blot showed a decrease in suppressor of cytokine signaling 3 protein (SOCS3) in the ventral tegmental area (VTA) and an increase in the ratio of phospho-tyrosine hydroxylase to tyrosine hydroxylase (pTH/TH) in the NAcc of FR rats. Administration of insulin also abolished this latter effect in FR rats. FR rats showed metabolic alterations and a delay in the dopaminergic response to palatable foods. This could explain the increased palatable food intake and behavioral entropy found in FR rats. IUGR may lead to binge eating, obesity and its metabolic consequences by modifying the central dopaminergic response to sweet food.

Contrasting Regulation of Catecholamine Neurotransmission in the Behaving Brain: Pharmacological Insights from an Electrochemical Perspective

Catecholamine neurotransmission plays a key role in regulating a variety of behavioral and physiologic processes, and its dysregulation is implicated in both neurodegenerative and neuropsychiatric disorders. Over the last four decades, in vivo electrochemistry has enabled the discovery of contrasting catecholamine regulation in the brain. These rapid and spatially resolved measurements have been conducted in brain slices, and in anesthetized and freely behaving animals. In this review, we describe the methods enabling in vivo measurements of dopamine and norepinephrine, and subsequent findings regarding their release and regulation in intact animals. We thereafter discuss key studies in awake animals, demonstrating that these catecholamines are not only differentially regulated, but are released in opposition of each other during appetitive and aversive stimuli.

Phasic dopamine signals: from subjective reward value to formal economic utility

Although rewards are physical stimuli and objects, their value for survival and reproduction is subjective. The phasic, neurophysiological and voltammetric dopamine reward prediction error response signals subjective reward value. The signal incorporates crucial reward aspects such as amount, probability, type, risk, delay and effort. Differences of dopamine release dynamics with temporal delay and effort in rodents may derive from methodological issues and require further study. Recent designs using concepts and behavioral tools from experimental economics allow to formally characterize the subjective value signal as economic utility and thus to establish a neuronal value function. With these properties, the dopamine response constitutes a utility prediction error signal.

Electrochemical Analysis of Neurotransmitters

Chemical signaling through the release of neurotransmitters into the extracellular space is the primary means of communication between neurons. More than four decades ago, Ralph Adams and his colleagues realized the utility of electrochemical methods for the study of easily oxidizable neurotransmitters, such as dopamine, norepinephrine, and serotonin and their metabolites. Today, electrochemical techniques are frequently coupled to microelectrodes to enable spatially resolved recordings of rapid neurotransmitter dynamics in a variety of biological preparations spanning from single cells to the intact brain of behaving animals. In this review, we provide a basic overview of the principles underlying constant-potential amperometry and fast-scan cyclic voltammetry, the most commonly employed electrochemical techniques, and the general application of these methods to the study of neurotransmission. We thereafter discuss several recent developments in sensor design and experimental methodology that are challenging the current limitations defining the application of electrochemical methods to neurotransmitter measurements.

Advances in studying phasic dopamine signaling in brain reward mechanisms

The last sixty years of research has provided extraordinary advances of our knowledge of the reward system. Since its discovery as a neurotransmitter by Carlsson and colleagues (1), dopamine (DA) has emerged as an important mediator of reward processing. As a result, a number of electrochemical techniques have been developed to measure DA in the brain. Together, these techniques have begun to elucidate the complex roles of tonic and phasic DA signaling in reward processing and addiction. In this review, we will first provide a guide for the most commonly used electrochemical methods for DA detection and describe their utility in furthering our knowledge about DA's role in reward and addiction. Second, we will review the value of common in vitro and in vivo preparations and describe their ability to address different types of questions. Last, we will review recent data that has provided new mechanistic insight of in vivo phasic DA signaling and its role in reward processing and reward-mediated behavior.

Enhanced dopamine transporter activity in middle-aged Gdnf heterozygous mice

Glial cell line-derived neurotrophic factor (GDNF) supports the viability of midbrain dopamine (DA) neurons that degenerate in Parkinson's disease. Middle-aged, 12 month old, Gdnf heterozygous (Gdnf(+/-)) mice have diminished spontaneous locomotor activity and enhanced synaptosomal DA uptake compared with wild type mice. In this study, dopamine transporter (DAT) function in middle-aged, 12 month old Gdnf(+/-) mice was more thoroughly investigated using in vivo electrochemistry. Gdnf(+/-) mice injected with the DAT inhibitor, nomifensine, exhibited significantly more locomotor activity than wild type mice. In vivo electrochemistry with carbon fiber microelectrodes demonstrated enhanced clearance of DA in the striatum of Gdnf(+/-) mice, suggesting greater surface expression of DAT than in wild type littermates. Additionally, 12 month old Gdnf(+/-) mice expressed greater D(2) receptor mRNA and protein in the striatum than wild type mice. Neurochemical analyses of striatal tissue samples indicated significant reductions in DA and a faster DA metabolic rate in Gdnf(+/-) mice than in wild type mice. Altogether, these data support an important role for GDNF in the regulation of uptake, synthesis, and metabolism of DA during aging.

Chronic microsensors for longitudinal, subsecond dopamine detection in behaving animals

Neurotransmission operates on a millisecond timescale but is changed by normal experience or neuropathology over days to months. Despite the importance of long-term neurotransmitter dynamics, no technique exists to track these changes in a subject from day to day over extended periods of time. Here we describe and characterize a microsensor that can detect the neurotransmitter dopamine with subsecond temporal resolution over months in vivo in rats and mice.

Differential involvement of ventral tegmental GABA(A) and GABA(B) receptors in the regulation of the nucleus accumbens dopamine response to stress

Evidence indicates that dopamine (DA) transmission in nucleus accumbens (NAcc) is modulated by glutamate (GLUT) projections from medial prefrontal cortex (PFC) to NAcc and the ventral tegmental area (VTA). Local NMDA receptor blockade in NAcc has previously been shown to enhance the DA stress response in this region as well as in the VTA. This raises the possibility that the NAcc DA stress response is regulated by GLUT acting at NMDA receptors located on NAcc GABA output neurons that project to the VTA where GABA is known to regulate DA cell activity. Thus, in the present study, we used voltammetry to examine the effects of intra-VTA administration of GABA(A) and GABA(B) agonists and antagonists on restraint stress-induced increases in NAcc DA. The results show that local VTA GABA(B) receptor activation with baclofen (0.01, 0.1 and 1.0 nmol) dose-dependently inhibited the NAcc DA stress response whereas GABA(B) receptor blockade with phaclofen had the opposite effect, resulting in a dose-dependent potentiation of the stress response. A similar potentiation of the NAcc DA stress response was observed following VTA GABA(A) receptor blockade with bicuculline, but only at the highest dose (1.0 nmol). Interestingly, intra-VTA injection of the GABA(A) receptor agonist, muscimol, at the lowest dose (0.01 nmol) but not at the higher doses (0.1 or 1.0 nmol) also potentiated the NAcc DA stress response, suggesting an action mediated primarily at GABA(A) receptors located on non-DA neurons. These results indicate that the NAcc DA stress response is regulated by GABA afferents to VTA DA cells and that this action is differentially mediated by GABA(A) and GABA(B) receptors. The data suggest that the relevant GABA(B) receptors are located on DA neurons whereas the GABA(A) receptors are located on GABA interneurons and perhaps also on DA cells. The present findings are also consistent with the idea that the corticofugal GLUT input to NAcc indirectly regulates stress-induced DA release in this region through the GABA feedback pathway to VTA.

Striatal dopamine release in the rat during a cued lever-press task for food reward and the development of changes over time measured using high-speed voltammetry

Substantia nigra dopamine neuronal activity in the primate is thought to be related to the error in predicting reward delivery. Dopamine release in rat nucleus accumbens has been shown to increase in relation to drug/food-seeking behaviour. It is not known how the release of dopamine in the striatum corresponds to the many distinct steps of a rewarded, cued task (e.g. recognizing the cue, executing the behaviour, anticipating the reward, receiving the reward) and how dopamine release then changes over time as task performance improves. To investigate dopamine release during a rewarded, cued task and the development of changes in dopamine release over time, changes in extracellular striatal dopamine concentration during a rewarded, cued lever-press task were measured a few days every week for 5 months using high-speed in vivo voltammetry. Rats were trained to press a lever after a tone to obtain a food reward. The reaction time for the lever press decreased gradually as training continued. Changes in dopamine concentration were measured in the anterior striatum (ventral portion) during the task performance after an initial 6-day familiarization period, in which the animals learned that a lever press yielded food, and a 5-week period for surgery, recovery, and electrode preparation. During the task performance, dopamine concentration started to increase just after the cue, peaked near the time of the lever press, and returned to basal levels 1-2 s after the lever press. This pattern of changes in dopamine concentration was observed over the 5 months of testing, the peak dopamine concentration increasing steadily until peaking at week 7, at which time the task performance had not yet improved significantly from week 2. By week 13, task performance had significantly improved and peak dopamine concentration had begun to subside. Thus, the increase in dopamine concentration after the cue was highest while the task was not yet perfected and subsided toward the end of the learning process. It was concluded that striatal dopamine release during a cued lever-press task is relevant to the novelty of the conditions.

D-amphetamine responses in catechol-O-methyltransferase (COMT) disrupted mice

RATIONALE

We have earlier found that 1). COMT inhibitors did not enhance amphetamine-induced dopamine efflux into striatal extracellular, that 2). they did not increase dopamine levels in striatal tissue and that 3). they did not potentiate amphetamine-induced turning behavior of hemiparkinsonian rats. Further, when COMT knockout mice were challenged with l-dopa or a dopamine transporter (DAT) inhibitor, an accumulation of dopamine occurred and the neurochemical and locomotor effects of l-dopa and GBR 12909 were modified accordingly.

OBJECTIVE

Since DAT inhibitors and amphetamine apparently have different mechanisms of action, we were interested to see how COMT knockout mice would react to d-amphetamine treatment.

METHODS

We measured the effects of d-amphetamine on locomotor activity and on the levels of catecholamines and their metabolites in striatal microdialysis fluid and in striatal, hypothalamic and cortical brain regions of COMT gene disrupted mice. Striatal dopamine receptor binding was also determined. RESULTS. After d-amphetamine administration, the DOPAC content in homozygous mice was 3-fold in the striatum, 17- to 18-fold in the cortex and 7- to 8-fold in the hypothalamus higher than in wild-type control mice, and there were no indications of genotypexsex interactions. However, the lack of COMT did not potentiate d-amphetamine-induced dopamine levels in brain tissue or in striatal extracellular fluid. D-amphetamine-induced (10 mg/kg) hyperlocomotion was less suppressed in male COMT knockout mice than in their wild-type counterparts. Striatal dopamine D(1) and D(2) receptor levels in male mice were not altered by COMT gene disruption.

CONCLUSIONS

Changes in COMT activity modulates dopamine metabolism but the behavioral effects of d-amphetamine in male mice only to a small extent, and this action does not seem to depend on the actual extracellular dopamine concentration. Nor is it mediated through compensatory changes in dopamine D(1) and D(2) receptor levels. In dopaminergic neurons, the contribution of intracellular COMT remains secondary in conditions when dopamine is released by d-amphetamine.

Differential effects of post-implantation time on potassium- versus D-amphetamine-evoked dopamine overflow in the striatum of F344 rats

Effects of post-implantation time on potassium (K+)- versus D-amphetamine (D-AMPH)-evoked striatal dopamine (DA) overflow were measured using microdialysis in freely moving young and aged Fischer 344 rats. In one group, samples were collected on the day of probe insertion (Day 1 group). In a second group, samples were collected 24 h after probe insertion (Day 2 group). While analyses revealed no significant differences between the two age groups, the 100 mM K+ stimulus evoked a significantly greater amount of DA overflow in the Day 1 group compared to the Day 2 group. The decrease in 3,4-dihydroxyphenylacetic acid (DOPAC) produced by K+ stimulation was not influenced by post-implantation time. The effect of the 250 microM D-AMPH stimulus on DA overflow did not differ between the Day 1 and Day 2 groups, nor did the decrease in DOPAC that accompanied D-AMPH stimulation. These results support the hypothesis that under some stimulus conditions, post-implantation time is an important variable in microdialysis studies.

Chloral hydrate and ethanol, but not urethane, alter the clearance of exogenous dopamine recorded by chronoamperometry in striatum of unrestrained rats

Several general anesthetics reduce dopamine (DA) neuronal activity and release. However, anesthetic-induced alterations in DA transporter (DAT) function are unclear. We used high-speed chronoamperometry to examine how urethane, chloral hydrate and ethanol affected clearance of locally-applied DA in the dorsal striatum of unrestrained rats. Maximal DA signal amplitudes (A(max)) and signal decay rate constants (k) were monitored continuously from the awake to anesthetized state. Urethane did not significantly change A(max) or k. Chloral hydrate and ethanol potentiated A(max) by 32% and 37%, respectively, over baseline, indicative of DAT inhibition. These effects were not additive. Only ethanol increased k, an effect blocked by chloral hydrate. These results suggest differential effects of anesthetics on striatal DAT function, with chloral hydrate and ethanol significantly altering DAT activity but urethane producing only minimal effects.

Individual differences in cocaine-induced locomotor sensitization in low and high cocaine locomotor-responding rats are associated with differential inhibition of dopamine clearance in nucleus accumbens

Behavioral sensitization to cocaine reflects neuroadaptive changes that intensify drug effects. However, repeated cocaine administration does not induce behavioral sensitization in all male Sprague-Dawley rats. Because cocaine inhibits the dopamine (DA) transporter (DAT), we investigated whether altered DAT function contributes to these individual differences. Freely moving rats had electrochemical microelectrode/microcannulae assemblies chronically implanted in the nucleus accumbens so that exogenous DA clearance signals were recorded simultaneous with behavior. The peak DA signal amplitude (A(max)) and efficiency of clearance (k) were used as indices of in vivo DAT function. Low and high cocaine responders (LCRs and HCRs, respectively) were identified based on their locomotor responsiveness to an initial injection of cocaine (10 mg/kg i.p.). Consistent with DAT inhibition, cocaine elevated A(max) and reduced k in HCRs, but not in LCRs. The same dose of cocaine was administered for six additional days and after a 7-day withdrawal. Baseline behavioral and dopamine clearance indices were unaltered by repeated cocaine or after withdrawal. Only LCRs expressed cocaine-induced sensitized locomotor activation, and this was accompanied by cocaine-induced elevations in A(max) and reductions in k. These sensitized responses to cocaine persisted in LCRs after withdrawal. In contrast, neither locomotor nor electrochemical responses were altered by repeated saline administration or a saline challenge after repeated cocaine administration, suggesting that conditioning did not significantly contribute. Our results suggest that increased DAT inhibition by cocaine is associated with locomotor sensitization and that DAT serves as a common substrate for mediating both the initial and sensitized locomotor responsiveness to cocaine.

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.

Kinetic analysis of striatal clearance of exogenous dopamine recorded by chronoamperometry in freely-moving rats

Previously, we developed technology that coupled high-speed chronoamperometry with microejections of dopamine (DA) to measure DA clearance in the brains of freely-behaving rats. Here, by varying the ejection volumes of DA across a 200-fold difference, the kinetics of striatal clearance were analyzed as a function of time and DA volume from 289 chronoamperometric signals (n=20 rats). Each DA clearance trace was fitted to a first-order exponential decay function to determine the rate constant for DA clearance (k). Additionally, the apparent Michaelis-Menten V(max) and K(m) kinetic constants were determined in freely-moving rats, enabling quantitative comparison of our values with other models of reuptake. The first-order rate constant for DA clearance, which reflects the V(max)/K(m) ratio or clearance efficiency, did not vary significantly when small volumes of DA were ejected resulting in peak DA signal amplitudes (A(max)) of <5 microM. However, following nomifensine-induced DAT inhibition, A(max) was increased and k was attenuated simultaneously with behavioral activation; and A(max) and behavior remained elevated beyond the initial period. Our results indicate that the analysis of kinetic parameters from chronoamperometric DA signals may be useful for investigating drug-induced regulation of DAT kinetics in relation to the behavior of freely-moving rats.

Acute cocaine differentially alters accumbens and striatal dopamine clearance in low and high cocaine locomotor responders: behavioral and electrochemical recordings in freely moving rats

Behavioral responses of rodents to cocaine are characterized by marked individual variability. Here, outbred male Sprague-Dawley rats were profiled based on concomitant recording of behavioral and electrochemical responses. Rats were categorized as either low or high cocaine responders (LCRs or HCRs, respectively) based on their differential locomotor responsiveness to an acute, low-dose injection of cocaine (10 mg/kg i.p.). LCRs and HCRs also differed in other cocaine-induced behaviors. The role of the dopamine transporter (DAT) in mediating the behavioral differences in cocaine responsiveness in LCRs and HCRs was investigated by high-speed chronoamperometric recording of exogenous dopamine (DA) clearance signals in nucleus accumbens (NAc) and dorsal striatum (dSTR). Higher volumes of DA were required in NAc of HCRs, than of LCRs, to produce equivalent peak DA signal amplitude (A(max)) responses. In HCRs, systemic cocaine administration evoked an immediate and prolonged 2-fold augmentation in A(max) in both brain regions, coincident with locomotor activation. The cocaine-induced decrease in the efficiency of DA clearance (k) in NAc of HCRs was more immediate and prolonged than in dSTR, where the transient decrease coincided with maximal stereotypic behavior. In contrast, in LCRs, A(max) was not altered by cocaine, and decay rate constant (k) was transiently attenuated only in dSTR. Correlation analyses of individual responses revealed that cocaine-induced changes in DA clearance signal parameters accounted for 20 to 40% of the variation in behavioral responsiveness to cocaine. Overall, our findings emphasize the importance of characterizing individual responses to understand more fully the range of functional consequences resulting from DAT inhibition.

Transient changes in mesolimbic dopamine and their association with 'reward'

Mesolimbic dopaminergic neurons modulate complex circuitry in the ventral forebrain involved in reward processing, although the precise function of the dopaminergic input is debated. Electrophysiological measurements have revealed that mesolimbic dopaminergic neurons can fire in either tonic or phasic modes, and that phasic firing accompanies the alerting or anticipatory phases of reward. However, the neurochemical relevance of this rapid neuronal discharge within the reward processing circuitry is not yet clear, in part because of difficulty in interpretation of extracellular dopamine measurements. Herein, the nature of the information provided by different neurochemical techniques is critically discussed. Classical methods of monitoring dopamine reveal changes in extracellular dopamine resulting from tonic neuronal activity, but do not have the temporal resolution to distinguish concentration transients. However, recent advances in dopamine sensors now enable transient dopamine concentrations resulting from phasic firing to be positively identified and followed on a physiologically relevant timescale. This has enabled demonstrations of discrete, phasic dopamine signals accompanying rewarding or alerting stimuli. Thus, enhanced dopamine release at terminals appears to be coincident with phasic electrical activity at cell bodies. These accumulating data promise to help unravel the precise role of phasic dopamine transmission in reward processing.

In vivo voltammetry with removable carbon fibre electrodes in freely-moving mice: dopamine release during intracranial self-stimulation

The advantages of in vivo voltammetry at carbon fibre electrodes cannot be fully realised without the registration of neurotransmitter release in freely moving animals. Here we describe an approach to record electrically evoked dopamine release in freely-moving mice. A description of a simple in-house made micromanipulator (0.4 g) and preamplifier (0.6 g) is given. This system was used to record electrochemical signal in the nucleus accumbens four to eight times during 2 weeks following intracranial self-stimulation (ICSS) of the median forebrain bundle. High-speed chronoamperometry was found to be the best choice for recording. The most efficient parameters of electrical stimulation for training and self-stimulation (50 Hz, 0.2 s, 60 microA) were insufficient to induce a measurable voltammetric signal. Increasing the strength of stimulation (0.5 s, 160 microA) significantly decreased the rate of self-stimulation and allowed the registration of separate peaks of dopamine overflow on each stimulation and a tonic increase of electrochemical signal following higher rates of ICSS. Due to exhaustion of the readily releasable dopamine pool, the length of recording the electrochemical signal depended on the rate of self-stimulation. A fixed ratio schedule of ICSS decreased the rate of electrical stimulation and permitted the maintenance of stable peaks of dopamine overflow at a high rate of nose-poking.

Phasic inhibition of dopamine uptake in nucleus accumbens induced by intravenous cocaine in freely behaving rats

A new approach combining fast-scan cyclic voltammetry with iontophoretic dopamine delivery was used in freely behaving rats to evaluate the time-course of dopamine uptake inhibition in nucleus accumbens induced by intravenous cocaine at a dose (1.0mg/kg) known to maintain self-administration behavior. Cocaine significantly increased the decay time of the dopamine response without altering its magnitude or time to peak. An increase in decay time was evident at 2 min, peaked at 6 min (+87%), and decreased to baseline at 18 min after a single cocaine injection. The change in decay time was similar in all rats and remained essentially the same, albeit slightly larger, for subsequent cocaine injections both within a session and over repeated sessions. The change in dopamine decay time did not correlate with cocaine-induced motor activation, which was maximal during the first minute after injection and decreased slowly over the next 20 min. Our data provide direct evidence for a phasic change in dopamine uptake induced by intravenous cocaine under behaviorally relevant conditions. The relatively slow and gradual development of dopamine uptake inhibition, which peaks at times when behaving rats self-inject cocaine, is inconsistent with the suggested role of this mechanism in the acute rewarding (euphoric) effects of self-injected cocaine, but supports its role in the activational and motivational aspects of drug-seeking and drug-taking behavior. Because intravenous cocaine enters the brain rapidly and peaks in neural tissue (1-2 min) long before it effectively inhibits dopamine uptake (6 min), it appears that some of the acute psychoemotional ("rush"), behavioral, autonomic, and neuronal effects of this drug, which are apparently resistant to dopamine receptor blockade, are mediated via rapid central or peripheral mechanisms independent of monoamine uptake.