Real time measurement of stimulated dopamine release in the conscious rat using fast cyclic voltammetry: dopamine release is not observed during intracranial self stimulation

Accumbal Dopamine Release Tracks the Expectation of Dopamine Neuron-Mediated Reinforcement

Dopamine (DA) transmission in the nucleus accumbens (NAc) facilitates cue-reward associations and appetitive action. Reward-related accumbal DA release dynamics are traditionally ascribed to ventral tegmental area (VTA) DA neurons. Activation of VTA to NAc DA signaling is thought to reinforce action and transfer reward-related information to predictive cues, allowing cues to guide behavior and elicit dopaminergic activity. Here, we use optogenetics to control DA neuron activity and voltammetry to simultaneously record accumbal DA release in order to quantify how reinforcer-evoked dopaminergic activity shapes conditioned mesolimbic DA transmission. We find that cues predicting access to DA neuron self-stimulation elicit conditioned responding and NAc DA release. However, cue-evoked DA release does not reflect the cost or magnitude of DA neuron activation. Accordingly, conditioned accumbal DA release selectively tracks the expected availability of DA-neuron-mediated reinforcement. This work provides insight into how mesolimbic DA transmission drives and encodes appetitive action.

Multimaterial and multifunctional neural interfaces: from surface-type and implantable electrodes to fiber-based devices

Development of neural interfaces from surface electrodes to fibers with various type, functionality, and materials.

Hitchhiker's Guide to Voltammetry: Acute and Chronic Electrodes for in Vivo Fast-Scan Cyclic Voltammetry

Fast-scan cyclic voltammetry (FSCV) has been used for over 20 years to study rapid neurotransmission in awake and behaving animals. These experiments were first carried out with carbon-fiber microelectrodes (CFMs) encased in borosilicate glass, which can be inserted into the brain through micromanipulators and guide cannulas. More recently, chronically implantable CFMs constructed with small diameter fused-silica have been introduced. These electrodes can be affixed in the brain with minimal tissue response, which permits longitudinal measurements of neurotransmission in single recording locations during behavior. Both electrode designs have been used to make novel discoveries in the fields of neurobiology, behavioral neuroscience, and psychopharmacology. The purpose of this Review is to address important considerations for the use of FSCV to study neurotransmitters in awake and behaving animals, with a focus on measurements of striatal dopamine. Common issues concerning experimental design, data collection, and calibration are addressed. When necessary, differences between the two methodologies (acute vs chronic recordings) are discussed. The topics raised in this Review are particularly important as the field moves beyond dopamine toward new neurochemicals and brain regions.

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.

Intracranial self-stimulation to evaluate abuse potential of drugs

Intracranial self-stimulation (ICSS) is a behavioral procedure in which operant responding is maintained by pulses of electrical brain stimulation. In research to study abuse-related drug effects, ICSS relies on electrode placements that target the medial forebrain bundle at the level of the lateral hypothalamus, and experimental sessions manipulate frequency or amplitude of stimulation to engender a wide range of baseline response rates or response probabilities. Under these conditions, drug-induced increases in low rates/probabilities of responding maintained by low frequencies/amplitudes of stimulation are interpreted as an abuse-related effect. Conversely, drug-induced decreases in high rates/probabilities of responding maintained by high frequencies/amplitudes of stimulation can be interpreted as an abuse-limiting effect. Overall abuse potential can be inferred from the relative expression of abuse-related and abuse-limiting effects. The sensitivity and selectivity of ICSS to detect abuse potential of many classes of abused drugs is similar to the sensitivity and selectivity of drug self-administration procedures. Moreover, similar to progressive-ratio drug self-administration procedures, ICSS data can be used to rank the relative abuse potential of different drugs. Strengths of ICSS in comparison with drug self-administration include 1) potential for simultaneous evaluation of both abuse-related and abuse-limiting effects, 2) flexibility for use with various routes of drug administration or drug vehicles, 3) utility for studies in drug-naive subjects as well as in subjects with controlled levels of prior drug exposure, and 4) utility for studies of drug time course. Taken together, these considerations suggest that ICSS can make significant contributions to the practice of abuse potential testing.

Characterization of ultrananocrystalline diamond microsensors for in vivo dopamine detection

We show the technical feasibility of coating and micro patterning boron-doped ultrananocrystalline diamond (UNCD^®) on metal microwires and of applying them as microsensors for the detection of dopamine in vivo using fast-scan cyclic voltammetry. UNCD electrode surface consistently generated electrochemical signals with high signal-to-noise ratio of >800 using potassium ferrocyanide-ferricyanide redox couple. Parylene patterned UNCD microelectrodes were effectively applied to detect dopamine reliably in vitro using flow injection analysis with a detection limit of 27 nM and in the striatum of the anesthetized rat during electrical stimulation of dopamine neurons.

Mesolimbic dopamine D₂ receptor plasticity contributes to stress resilience in rats subjected to chronic mild stress

RATIONALE

Few studies have investigated neurobiological and biochemical differences between stress-resilient and stress-vulnerable experimental animals.

OBJECTIVES

We investigated alterations in mesolimbic dopamine D2 receptor density and mRNA expression level in stressed rats at two time points, i.e. after 2 and 5 weeks of chronic mild stress (CMS).

METHODS

We used the chronic mild stress paradigm because it is a well-established animal model of depression. Two groups of stressed rats were distinguished during CMS experiments: (1) stress reactive (70 %), which displayed a decrease in the drinking of a palatable sucrose solution during the stress regimen, and (2) stress resilient (30 %), which exhibited an unaltered drinking profile when compared with the unchallenged control group. [(3)H]Domperidone was used as a ligand to label dopamine D2 receptors, and a mixture of three specific oligonucleotides was used to evaluate dopamine D2 receptor mRNA changes in various regions of the rat brain.

RESULTS

CMS strongly affected the mesolimbic dopamine circuit in stress-resilient group after 2 weeks and stress-reactive group of rats after 5 weeks which exhibited a decrease in the level of dopamine D2 receptor protein without alterations in D2 mRNA expression. Stress-resilient animals, but not stress-reactive animals, effectively adapted to the extended stress and coped with it. The increase in D2 mRNA expression returned the dopamine D2 receptor density to control levels in stress-resilient rats after 5 weeks of CMS, but not in stress-reactive animals.

CONCLUSIONS

These results clearly demonstrate that, despite earlier blunting, the activation of dopamine receptor biosynthesis in the dopamine mesoaccumbens system in stress-resilient rats is involved in active coping with stressful experiences, and it exhibits a delay in time.

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.

N-acetylaspartylglutamate (NAAG) inhibits intravenous cocaine self-administration and cocaine-enhanced brain-stimulation reward in rats

Pharmacological activation of group II metabotropic glutamate (mGlu2 and mGlu3) receptors inhibits reward-seeking behavior and/or rewarding efficacy induced by drugs (cocaine, nicotine) or natural rewards (food, sucrose). In the present study, we investigated whether elevation of brain N-acetylaspartylglutamate (NAAG), an endogenous group II mGlu receptor agonist, by the NAAG peptidase inhibitor 2-PMPA attenuates cocaine's rewarding effects, as assessed by intravenous cocaine self-administration and intracranial electrical brain-stimulation reward (BSR) in rats. Systemic administration of 2-PMPA (10, 30, 100 mg/kg, i.p.) or intranasal administration of NAAG (100, 300 microg/10 microl/nostril) significantly inhibited intravenous cocaine self-administration under progressive-ratio (PR), but not under fixed-ratio 2 (FR2), reinforcement conditions. In addition, 2-PMPA (1, 10, 30 mg/kg, i.p) or NAAG (50, 100 microg/10 microl/nostril) significantly inhibited cocaine-enhanced BSR, but not basal BSR. Pretreatment with LY341495 (1 mg/kg, i.p.), a selective mGlu2/3 receptor antagonist, prevented the inhibitory effects produced by 2-PMPA or NAAG in both the self-administration and BSR paradigms. In vivo microdialysis demonstrated that 2-PMPA (10, 30, 100 mg/kg) dose-dependently attenuated cocaine-enhanced extracellular dopamine (DA) in the nucleus accumbens (NAc). 2-PMPA alone inhibited basal NAc DA release, an effect that was prevented by LY341495. These findings suggest that systemic administration of 2-PMPA or intranasal administration of NAAG inhibits cocaine's rewarding efficacy and cocaine-enhanced NAc DA - likely by activation of presynaptic mGlu2/3 receptors in the NAc. These data suggest a potential utility for 2-PMPA or NAAG in the treatment of cocaine addiction.

Brain stimulation reward is integrated by a network of electrically coupled GABA neurons

The neural substrate of brain stimulation reward (BSR) has eluded identification since its discovery more than a half-century ago. Notwithstanding the difficulties in identifying the neuronal integrator of BSR, the mesocorticolimbic dopamine (DA) system originating in the ventral tegmental area (VTA) of the midbrain has been implicated. We have previously demonstrated that the firing rate of a subpopulation of gamma-aminobutyric acid (GABA) neurons in the VTA increases in anticipation of BSR. We show here that GABA neurons in the VTA, midbrain, hypothalamus, and thalamus of rats express connexin-36 (Cx36) gap junctions (GJs) and couple electrically upon DA application or by stimulation of the internal capsule (IC), which also supports self-stimulation. The threshold for responding for IC self-stimulation was the threshold for electrical coupling between GABA neurons, the degree of responding for IC self-stimulation was proportional to the magnitude of electrical coupling between GABA neurons, and GJ blockers increased the threshold for IC self-stimulation without affecting performance. Thus, a network of electrically coupled GABA neurons in the ventral brain may form the elusive neural integrator of BSR.

Detecting subsecond dopamine release with fast-scan cyclic voltammetry in vivo

BACKGROUND

Dopamine is a potent neuromodulator in the brain, influencing a variety of motivated behaviors and involved in several neurologic diseases. Measurements of extracellular dopamine in the brains of experimental animals have traditionally focused on a tonic timescale (minutes to hours). However, dopamine concentrations are now known to fluctuate on a phasic timescale (subseconds to seconds).

APPROACH

Fast-scan cyclic voltammetry provides analytical chemical measurements of phasic dopamine signals in the rat brain.

CONTENT

Procedural aspects of the technique are discussed, with regard to appropriate use and in comparison with other methods. Finally, examples of data collected using fast-scan cyclic voltammetry are summarized, including naturally occurring dopamine transients and signals arising from electrical stimulation of dopamine neurons.

SUMMARY

Fast-scan cyclic voltammetry offers real-time measurements of changes in extracellular dopamine concentrations in vivo. With its subsecond time resolution, micrometer-dimension spatial resolution, and chemical selectivity, it is the most suitable technique currently available to measure transient concentration changes of dopamine.

The interpretation of the measurement of nucleus accumbens dopamine by in vivo dialysis: the kick, the craving or the cognition?

Psychopharmacological studies have implicated the dopaminergic innervation of the nucleus accumbens (NAC) in reward and reinforcement, in the actions of addictive drugs, and in the control of the symptoms of schizophrenia. Recent developments in in vivo dialysis, and other in vivo neurochemical techniques have permitted a more direct analysis of the behavioural correlates of increased dopamine release in rats, and have largely confirmed these findings in relation to reward, and drugs of abuse potential. However, dopamine release has also been found to be increased by many other stimuli/situations including aversive stimuli, stimuli conditioned to aversive stimuli, complex novel stimuli, and in the process of conditioning itself. These results contrast with electrophysiological data obtained in the behaving monkey, where rewarding stimuli, or stimuli predictive of reward are associated with increased firing of presumptive dopamine neurones projecting to the NAC (and indeed to the striatum), but mild aversive stimuli are not, leading to the suggestion that this system subserves a more purely reward function, or indeed that it provides a reward error signal. Further exploration of these issues will depend upon a comparison of increased dopamine cell firing and increased dopamine release, and an analysis of the behavioural effects of blocking these increases in dopamine transmission. One suggestion, deriving from work on latent inhibition, is that the significance of dopamine release by salient stimuli is to allow learning about stimuli which would otherwise be excluded on the basis of familiarity. This suggests that in addition to a role in some types of learning about salient stimuli, dopamine release in NAC may have a role in controlling the attention paid to familiar stimuli. Since it is difficult to see a connection between simple learning about rewards, and the symptoms of schizophrenia, this provides a more convincing link between the dopamine theory of schizophrenia, and the attentional difficulties held by many theorists to underlie schizophrenic symptoms.

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.

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.

Contrasting effects of dopamine antagonists and frequency reduction on Fos expression induced by lateral hypothalamic stimulation

To help further identify the reward-relevant regions activated by electrical stimulation of the lateral hypothalamus, Fos expression was quantified in 23 brain regions in naïve, awake rats following non-contingent stimulation with a frequency that supports self-stimulation (100 Hz), a frequency that supports only minimal responding (50 Hz) and a frequency that does not support self-stimulation (25 Hz). Fos expression was also examined in stimulated and unstimulated rats pretreated with SCH 23390 (a dopamine D1 antagonist) or spiperone (a D2-like antagonist), at doses known to greatly inhibit responding for self-stimulation. Lowering the stimulation frequency from 100 to 50 Hz reduced Fos labelling in all areas, except for a few cells immediately surrounding the electrode tip. No differences were observed between unstimulated rats and those receiving 25 Hz stimulation. This suggests that a critical threshold of stimulation is required before other reward-relevant regions in the midbrain and forebrain are recruited with higher frequency stimulation. Pretreatment with SCH 23390 (0.1 mg/kg) inhibited stimulation-induced Fos expression in some key dopamine terminal areas, such as the nucleus accumbens (core and shell) and medial caudate-putamen, but not in directly driven neurons near the stimulation site. In contrast, spiperone (0.1 mg/kg) did not affect the pattern of stimulation-induced Fos expression, but induced immunolabelling in the dorsolateral caudate-putamen, an area associated with the extrapyramidal side-effects of antipsychotic drugs. These results reveal the utility of Fos immunohistochemistry to show how different treatments that alter the rewarding impact of electrical brain stimulation achieve their effects at the neural level.

Responses of ventral tegmental area GABA neurons to brain stimulation reward

Dopamine neurons in the ventral tegmental area (VTA) have been implicated in rewarded behaviors, including intracranial self-stimulation (ICSS). We demonstrate, in unrestrained rats, that the discharge activity of a homogeneous population of presumed VTA GABA neurons, implicated in cortical arousal, increases before ICSS of the medial forebrain bundle (MFB). These findings suggest that VTA GABA neurons may be involved in the attentive processes related to brain stimulation reward (BSR).

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.

Extracellular dopamine dynamics in rat caudate-putamen during experimenter-delivered and intracranial self-stimulation

Intracranial self-stimulation is an operant behavior whereby animals are conditioned to press a lever in order to receive an electrical stimulation of their dopamine neurons. This paradigm is thought to stimulate brain reward pathways and, as such, has been used to clarify the role of dopamine in reward. Striatal extracellular dopamine concentrations were monitored during the acquisition and maintenance of self-stimulation and compared to dopamine release generated by experimenter-delivered and yoked stimulation. Fast-scan cyclic voltammetry in conjunction with carbon-fiber microelectrodes was used to monitor evoked dopamine release in the caudate-putamen during electrical stimulation of the substantia nigra/ventral tegmental area. The sub-second temporal resolution of fast-scan cyclic voltammetry coupled with the micron spatial resolution of the microelectrodes allowed for the measurement of dopamine neurotransmission in real-time. Single experimenter-delivered stimulations, identical to those used during self-stimulation, evoked dopamine release in the caudate-putamen both before and after the self-stimulation sessions. Likewise, yoked stimulations of the substantia nigra/ventral tegmental area delivered to animals untrained to perform self-stimulation resulted in an increase in extracellular dopamine levels. During training sessions, experimenter-delivered stimulations evoked dopamine release. However, as the animals began lever-pressing, extracellular dopamine levels subsequently declined. Taken together, these results suggest that dopamine functions as an alerting device, wherein increases in extracellular dopamine are obtained by unpredicted or novel rewarding stimuli, but not by those which can be anticipated.

Dissociation of dopamine release in the nucleus accumbens from intracranial self-stimulation

Mesolimbic dopamine-releasing neurons appear to be important in the brain reward system. One behavioural paradigm that supports this hypothesis is intracranial self-stimulation (ICS), during which animals repeatedly press a lever to stimulate their own dopamine-releasing neurons electrically. Here we study dopamine release from dopamine terminals in the nucleus accumbens core and shell in the brain by using rapid-responding voltammetric microsensors during electrical stimulation of dopamine cell bodies in the ventral tegmental area/substantia nigra brain regions. In rats in which stimulating electrode placement failed to elicit dopamine release in the nucleus accumbens, ICS behaviour was not learned. In contrast, ICS was acquired when stimulus trains evoked extracellular dopamine in either the core or the shell of the nucleus accumbens. In animals that could learn ICS, experimenter-delivered stimulation always elicited dopamine release. In contrast, extracellular dopamine was rarely observed during ICS itself. Thus, although activation of mesolimbic dopamine-releasing neurons seems to be a necessary condition for ICS, evoked dopamine release is actually diminished during ICS. Dopamine may therefore be a neural substrate for novelty or reward expectation rather than reward itself.