Dopaminergic neurons: simultaneous measurements of dopamine release and single-unit activity during stimulation of the medial forebrain bundle

Principles of voltammetry and microelectrode surface states

In vivo voltammetry is approaching the end of its second decade as a technique to explore extracellular concentrations in the brain. The issues of selectivity and sensitivity, which caused considerable discussion and confusion in the early 1980s, are now resolved. It is clear that in vivo voltammetry and dialysis are complimentary tools to understand neurotransmitter dynamics. The two chief advantages of voltammetry compared to dialysis, improved temporal resolution and reduced tissue damage, make this technique exceptionally well suited for providing information which is complementary to that obtained by single-unit recording and is uniquely capable of providing information on the short-term regulation of extracellular levels of biogenic amines.

Unilateral neostriatal kainate, but not 6-OHDA, lesions block dopamine agonist-induced ascorbate release in the neostriatum of freely moving rats

Unilateral kainate lesions of the neostriatum and 6-hydroxydopamine (6-OHDA) lesions of the medial forebrain bundle were used to assess the role of neostriatal and ascending dopaminergic neurons, respectively, on dopamine-agonist induced release of neostriatal ascorbate as measured voltammetrically in freely moving rats. Electrochemically modified, carbon-fiber electrodes recorded the effects of direct (a combination of 10 mg/kg SKF-38393 and 1.0 mg/kg quinpirole) as well as indirect (2.5 mg/kg D-amphetamine or 20.0 mg/kg GBR-12909) dopamine agonists. Relative to controls, kainate, but not 6-OHDA, lesions abolished the ability of both direct and indirect dopamine agonists to induce neostriatal ascorbate release. These results suggest that unlike dopaminergic afferents, neostriatal output pathways play a critical role in the modulation of neostriatal ascorbate levels.

Regulation of transient dopamine concentration gradients in the microenvironment surrounding nerve terminals in the rat striatum

Synaptic overflow of dopamine in the striatum has been investigated during electrical stimulation of the medial forebrain bundle in anesthetized rats. Dopamine has been detected with Nafion-coated, carbon-fiber electrodes used with fast-scan voltammetry. In accordance with previous results, dopamine synaptic overflow is a function of the stimulation frequency and the anatomical position of the carbon-fiber electrode. In some positions the concentration of dopamine is found to respond instantaneously to the stimulus when the time-delay for diffusion through the Nafion film is accounted for. In these locations the measured rates of change of dopamine are sufficiently rapid such that extracellular diffusion is not apparent. The rate of dopamine overflow can be described by a model in which each stimulus pulse causes instantaneous release, and cellular uptake decreases the concentration between stimulus pulses. Uptake is found to be described by a constant set of Michaelis-Menten kinetics at each location for concentrations of dopamine from 100 nM to 15 microM. The concentration of dopamine released per stimulus pulse is found to be greatest at low frequency (< or = 10 Hz) with stimulus trains, and with single-pulse stimulations in nomifensine-treated animals. The frequency dependence of release is not an effect of dopamine receptor activation; haloperidol (2.5 mg/kg) causes a uniform increase in release at all frequencies. The absence of diffusional effects in the measurement locations means that the constants determined with the electrode are those operant inside intact striatal tissue during stimulated overflow. These values are then extrapolated to the case where a single neuron fires alone. The extrapolation shows that while the transient concentration of dopamine may be high (200 nM) at the interface of the synapse and the extrasynaptic region, it is normally very low (< 6 nM) in the bulk of extracellular fluid.

Evoked neuronal activity accompanied by transmitter release increases oxygen concentration in rat striatum in vivo but not in vitro

Dopamine and oxygen (O2) were measured in the caudate nucleus of anesthetized rats and in striatal slices during electrical stimulation. Simultaneous electrochemical detection of dopamine and O2 was accomplished with fast-scan cyclic voltammetry at a Nafion-coated carbon-fiber microelectrode. Stimulation of the medial forebrain bundle resulted in synaptic overflow of dopamine in the caudate nucleus. At the same time, O2 concentration increased in the extracellular fluid with two separate phases. The amplitude of the initial increase directly correlated with the frequency of the stimulus, with the time of maximum concentration reproducible across a range of frequencies. The second increase occurred at later times with a more random amplitude and with a broad, variable shape. Agents which blocked vasodilation affected both phases: atropine attenuated the initial increase, while the second feature was nearly absent after theophylline. Yohimbine and alpha-methyl-p-tyrosine did not affect the O2 responses. Local electrical stimulation of the slice preparation also resulted in dopamine overflow, but a prolonged decrease in O2 concentration accompanied this event. Striatal field stimulation in vivo produced changes in O2 concentration dependent on the relative position of the stimulating and working electrodes, but none of the responses resembled that seen in the caudate slice. Thus, while measurements in brain slices show O2 consumption as a result of stimulated neuronal activity, an apparent elevation of local cerebral blood flow during and after stimulation dominate the in vivo response.

Regional differences in the regulation of dopamine and noradrenaline release in medial frontal cortex, nucleus accumbens and caudate-putamen: a microdialysis study in the rat

Dopamine (DA) and noradrenaline (NA) extracellular levels have been measured by microdialysis in the medial frontal cortex (MFC), nucleus accumbens (NAc) and caudate-putamen (CP) under baseline conditions in awake and halothane-anaesthetized rats, and after application of three types of stimuli which are likely to activate the brainstem catecholaminergic systems: mild stressors (handling and tail pinch), rewarded behavior (eating palatable food without prior food deprivation) and electrical stimulation of the lateral habenular nucleus. Changes were studied with and without uptake blockade (10 microM nomifensine in the perfusion fluid). The influence of calcium concentration (1.2 or 2.3 mM in the perfusion fluid) on DA and NA overflow was tested in some cases. Handling and tail pinch stimulated both DA and NA overflow in MFC, and enhanced NA overflow in NAc. By contrast, these mildly stressful stimuli had only marginal effects on DA overflow in NAc and no effects on either DA or NA overflow in CP. Eating behavior was accompanied by increased DA and NA overflow in MFC but had no effect in NAc. These regional differences were similar also when the manipulations were applied under uptake blockade, which indicates that the more pronounced changes seen in MFC did not simply reflect a more sparse innervation (i.e. lower density of uptake sites) in the MFC compared to the more densely innervated NAc and CP areas. Stimulation of the lateral habenula induced a 2-3-fold increase in NA overflow in both MFC, NAc and CP but had no consistent effect on DA overflow in any region. The effect on NA release was abolished by a transection of the ipsilateral fasciculus retroflexus (which carries the efferent output of the lateral habenula). The results show that the forebrain DA and NA projections to cortical and striatal targets are differentially regulated during ongoing behavior, that the mesocortical and mesostriatal DA systems respond quite differently to stressful and rewarding stimuli; and that the NA projection to MFC (like the dopaminergic one) is more responsive to stressful and rewarding stimuli than the ones innervating the striatum (NAc and CP). The results support the view that environmental stimuli evoking emotional arousal (whether aversive or non-aversive) are accompanied by increased DA and NA release above all in the MFC and only to a minor extent in limbic and striatal areas.

A comparison of axonal and somatodendritic dopamine release using in vivo dialysis

The release of endogenous dopamine from the axon terminal field in the nucleus accumbens and from the A10 dopamine cell bodies of conscious rats was measured using intracranial dialysis. Release of dopamine from both areas was calcium-dependent and markedly inhibited by the presence of the D2 agonist, quinpirole, in the dialysis buffer. However, the addition of tetrodotoxin to the buffer produced less of a decrease in dopamine in the A10 region than in the nucleus accumbens. When dopamine release was examined by substituting K+ for Na+ or by adding amphetamine to the buffer, the evoked release was significantly less in the A10 region than in the nucleus accumbens. Likewise, enhanced extracellular dopamine following blockade of reuptake by nomifensine addition to the dialysis buffer was not as great in the A10 region as in the nucleus accumbens. These data argue that, in general, axonal and somatodendritic dopamine release are regulated by similar factors, although somatodendritic release is less influenced by action potential generation and less responsive to some releasing agents.

In vitro microdialysis: a novel technique for stimulated neurotransmitter release measurements

A novel microdialysis technique suitable for parallel measurements of neurotransmitter release and single unit recordings from brain slices is presented. The effects of 3,4-methylenedioxymethamphetamine (MDMA) on slices of dorsal raphe nucleus and frontal cortex in a perfusion chamber for electrophysiological measures were studied. MDMA caused measurable release of serotonin which, in the case of the dorsal raphe, was of similar duration as the period of reduced cell firing induced by MDMA. Tryptophan potentiated the action of MDMA. Possible additional applications of this technique are discussed.

Phasic versus tonic dopamine release and the modulation of dopamine system responsivity: a hypothesis for the etiology of schizophrenia

A novel mechanism for regulating dopamine activity in subcortical sites and its possible relevance to schizophrenia is proposed. This hypothesis is based on the regulation of dopamine release into subcortical regions occurring via two independent mechanisms: (1) transient or phasic dopamine release caused by dopamine neuron firing, and (2) sustained, "background" tonic dopamine release regulated by prefrontal cortical afferents. Behaviorally relevant stimuli are proposed to cause short-term activation of dopamine cell firing to trigger the phasic component of dopamine release. In contrast, tonic dopamine release is proposed to regulate the intensity of the phasic dopamine response through its effect on extracellular dopamine levels. In this way, tonic dopamine release would set the background level of dopamine receptor stimulation (both autoreceptor and postsynaptic) and, through homeostatic mechanisms, the responsivity of the system to dopamine in these sites. In schizophrenics, a prolonged decrease in prefrontal cortical activity is proposed to reduce tonic dopamine release. Over time, this would elicit homeostatic compensations that would increase overall dopamine responsivity and thereby cause subsequent phasic dopamine release to elicit abnormally large responses.

Interference by DOPAC and ascorbate during attempts to measure drug-induced changes in neostriatal dopamine with Nafion-coated, carbon-fiber electrodes

The selectivity of Nafion-coated, carbon-fiber electrodes was evaluated for the voltammetric detection of dopamine in the presence of 3,4-dihydroxyphenylacetic acid (DOPAC) and ascorbate, which are interferant anions in the brain. Nafion coating was applied to both polished carbon-disk electrodes and electrochemically modified electrodes. During in vitro testing, polished disk electrodes showed the greatest selectivity for dopamine at the fastest scan rate tested (300 V s-1), but the drift in signal made slow changes in dopamine difficult to determine. Electrochemically modified electrodes already provide an ascorbate wave distinct from that for dopamine, but Nafion coating actually decreased dopamine selectivity with respect to DOPAC. In vivo testing was carried out in the neostriatum of urethane-anesthetized rats in response to drug- or stimulation-induced increases in dopamine transmission. Administration of haloperidol (0.5 mg kg-1) followed by GBR 12909 (20 mg kg-1), which is known to cause a 10-fold increase in extracellular dopamine, failed to produce a selective signal for dopamine when measured voltammetrically. Comparable results were obtained following administration of amphetamine (2.5 mg kg-1), which also increases dopamine overflow. Voltammetric detection of dopamine was possible, however, during electrical stimulation of dopaminergic afferents in the medial forebrain bundle. Thus, voltammetry with Nafion-coated electrodes is best suited to the measurement of transient changes in extracellular dopamine rather than the relatively prolonged changes in dopamine overflow produced by various drugs.

Fast cyclic voltammetry: measuring transmitter release in 'real time'

Fast cyclic voltammetry (FCV) differs from all other voltammetric methods in the type of waveform used, the speed of measurement and the applications for which it is best suited. The main assets of FCV are its high spatial and temporal resolution, the use of a reduction scan and the ease with which it can be combined with unit activity recording at the same electrode. The drawbacks of the method are the inability to measure basal extracellular amine levels or changes in multicomponent signals, and the modest sensitivity of the technique. Principal applications of FCV are the monitoring of ionophoresed or pressure-ejected monoamines, direct measurement of stimulated amine release and uptake, and the study of brain ascorbic acid function. The high spatial (5 microns) resolution of FCV is such that the anatomical heterogeneity of brain nuclei can be measured in great detail. The temporal resolution (25 ms) means that transmitter release and uptake can be measured essentially in 'real time'.

Application of in vivo electrochemistry to the measurement of changes in dopamine release during intracranial self-stimulation

Stearate-modified graphite paste recording electrodes were acutely or chronically implanted into the nucleus accumbens along with bipolar stimulating electrodes in the ipsilateral ventral tegmental area (VTA). Chronoamperometry was used to monitor changes in electrochemical signals that may correspond to the oxidation of dopamine (DA) during experimenter-administered stimulation (EAS) and intracranial self-stimulation (ICS). Application of EAS to stimulating electrodes in the VTA produced increases in the electrochemical signal in both the anesthetized and conscious preparation. The magnitude of both effects increased as a function of current intensity. Initiation of ICS was also accompanied by an immediate increase in the electrochemical signal. Rate-intensity experiments revealed a corresponding increase in both the ICS rates and the electrochemical signal with successive increases or decreases in current intensity. In subsequent experiments, intraperitoneal injections of DA uptake blockers nomifensine and GBR-12909 produced significant increases in the amplitude of the chronoamperometric signal which corresponded to drug-induced increases in bar press rates. The noradrenergic uptake blocker desipramine had no significant effect on either ICS rates or oxidation current. These data indicate that ICS of the VTA may produce concurrent increases in DA neurotransmission in the nucleus accumbens. The pharmacological studies are consistent with a dopaminergic substrate of brain stimulation reward at electrode sites in the VTA.

Enhanced dopamine receptor activation in accumbens and frontal cortex has opposite effects on medial forebrain bundle self-stimulation

This study was undertaken to investigate the effects of activating dopamine receptors in accumbens and prefrontal cortex on self-stimulation behavior in the medial forebrain bundle. The experiments were carried out in rats chronically implanted with one stimulating electrode in medial forebrain bundle and two bilaterally-placed cannulas for giving injections into accumbens or prefrontal cortex. After completion of training, animals classified as responders and non-responders were given drug tests. The non-responders were tested to determine the effects of the treatment on motor activity. The self-stimulation task involved the depression of a lever to obtain a stimulus of 0.25 s duration, 60 Hz sine waves applied to the medial forebrain bundle. Dopamine receptor activation in accumbens or prefrontal cortex was induced with bilateral injections in these structures of a mixture containing 5 mg dopamine, 10 mg d-amphetamine sulfate and 5 mg pargyline mixed in 0.5 ml saline containing 0.1% ascorbic acid (dopamine + d-amphetamine sulfate + pargyline, the cocktail). Each injection was of 2 microliters/side, yielding a concentration of 20 micrograms of dopamine, 40 micrograms of d-amphetamine sulfate and 20 micrograms of pargyline/injection. The bilateral injections were given immediately before the self-stimulation session which lasted 12 h, starting in late afternoon. The effects of saline containing the ascorbate were determined in control sessions. Saline injected bilaterally in accumbens or prefrontal cortex of self-stimulators or non-self-stimulators had no effects on the response-rate of self-stimulators or on the gross motor activity of non-responders. In contrast, the cocktail of dopamine + d-amphetamine sulfate + pargyline injected in accumbens of self-stimulators induced a complex response which included first a facilitation, then a prolonged suppression and then again one or two episodes of facilitation interspersed with periods of suppression of self-stimulation and then a return to baseline rats. The same cocktail of dopamine + d-amphetamine sulfate + pargyline injected bilaterally in accumbens of non-self-stimulators resulted also in a complex response including as a first component a facilitation of responding, but the complex effect was of shorter duration and lower magnitude, never raising the rate of lever-pressing to levels meeting self-stimulation criteria. The same cocktail of dopamine + d-amphetamine sulfate + pargyline injected in prefrontal cortex of self-stimulators simply attenuated or suppressed responding, and the effect lasted for most of the session. The same effect was seen in non-self-stimulators indicating a decrease in gross motor activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurochemical correlates of brain-stimulation reward measured by ex vivo and in vivo analyses

Evidence from ex vivo analyses of dopaminergic function following self-stimulation behavior is reviewed and compared to in vivo analyses of extracellular dopamine measured by chronoamperometry during self-stimulation. Both data bases provide strong support for a dopaminergic substrate for brain-stimulation reward obtained by electrical stimulation of the ventral tegmental area (VTA). Data obtained from in vivo measures of dopamine release are particularly compelling as a positive correlation was observed between the rate/intensity function for self-stimulation and increments in the oxidation current for dopamine. An examination of the effects of the dopamine uptake blockers, cocaine and GBR 12909 on self-stimulation and stimulated release of dopamine revealed a facilitation of both measures. In contrast, the noradrenaline uptake blocker desipramine had no effect on either self-stimulation or extracellular dopamine. These pharmacological experiments also are consistent with a dopaminergic substrate of brain-stimulation reward at electrode sites in the VTA.

Heterogeneity of stimulated dopamine overflow within rat striatum as observed with in vivo voltammetry

Overflow of dopamine has been measured in the striatum of anesthetized rats with 60-Hz, 300-microA electrical stimulations of the medial forebrain bundle. Electrodes were placed in the region of the caudate-putamen or the nucleus accumbens. The elliptical electrodes were fabricated from carbon fibers and had a major radius of 35 microns. Overflow was detected with fast-scan voltammetry repeated at 100-ms intervals. In both brain regions the detected substance had the voltammetric properties of dopamine. Measurements were made at different vertical electrode positions, varied by 100-microns intervals. Significant differences in stimulated overflow were observed between each position in both brain regions. The dimensions over which overflow was observed compare to that of previous descriptions of the patch and matrix topology of the striatum. When the electrode was moved in 10-microns intervals the variance was considerably less. The observed rate of overflow during stimulation exactly correlated with the maximal amount of dopamine observed during a stimulation. In addition, the overflow rate and the disappearance rate also correlated. This suggests that in both brain regions uptake and release sites are anatomically close to one another. The major difference between overflow curves measured in the two different regions was the appearance of an apparent mass transfer barrier to the electrode in the caudate-putamen.

Synthesis and release of dopamine in rat brain: comparison between substantia nigra pars compacts, pars reticulata, and striatum

Dopamine (DA) is synthesized and released not only from the terminals of the nigrostriatal dopaminergic neuronal pathway, but also from the dendrites in the substantia nigra. We have investigated the regulation of the DA turnover, the DA synthesis rate, and the DA release in the substantia nigra pars compacts (SNpc) and pars reticulata (SNpr) in vivo. As a measure of DA turnover, we have assessed the concentrations of 3,4-dihydroxyphenylacetic acid and homovanillic acid. As a measure of the DA synthesis rate, we have determined the 3,4-dihydroxyphenylalanine accumulation after inhibition of aromatic L-amino acid decarboxylase by 3-hydroxybenzylhydrazine. As a measure of DA release, we have investigated the disappearance rate of DA after inhibition of its synthesis by alpha-methyl-p-tyrosine and the 3-methoxytyramine accumulation following monoamine oxidase inhibition by pargyline. Both the DA turnover and the DA synthesis rate increased following treatment with the DA receptor antagonist haloperidol and decreased following treatment with the DA receptor agonist apomorphine in the SNpc and in the SNpr, but the effects of the drugs were less pronounced than in the striatum. gamma-Butyrolactone treatment, which suppresses the firing of the dopaminergic neurons, increased the DA synthesis rate in the striatum (165%), but had no such effect in the SNpc or SNpr. Haloperidol, apomorphine, and gamma-butyrolactone increased, decreased, and abolished, respectively, the DA release in the striatum, but the drugs had no or only slight effects on the alpha-methyl-p-tyrosine-induced DA disappearance and on the pargyline-induced 3-methoxytyramine accumulation in the SNpc or SNpr. Taken together, these results indicate that the DA synthesis rate, but not the DA release, are influenced by DA receptor activity and neuronal firing in the SNpc and SNpr. This is in contrast to the situation in the striatum, where both the DA synthesis rate and the DA release are under such control.

Effects of electrical stimulation of brain reward sites on release of dopamine in rat: an in vivo electrochemical study

Behavioral studies suggest that mesencephalic dopamine neurons mediate the rewarding effects of electrical stimulation of the medial forebrain bundle. Yet there is little direct evidence that rewarding electrical stimulation actually activates dopamine-containing neurons. The purpose of the present study was to determine, using in vivo electrochemistry, if electrical stimulation applied to lateral hypothalamic or ventral tegmental reward sites would elicit changes in extracellular levels of dopamine. In vivo high speed chronoamperometric recordings were performed in anesthetized rats that had been previously trained to respond for rewarding electrical stimulation of the medial forebrain bundle or ventral tegmental area. We found that a single 500 msec train of pulses elicited a small transient electrochemical signal, the magnitude of which was dependent on the pulse duration and frequency. This signal was potentiated by inhibition of dopamine reuptake. Prolonged electrical activation with a self-stimulation-like regimen resulted in the gradual accumulation of an electroactive compound, tentatively identified as dihydroxyphenylacetic acid (DOPAC). Taken together, the data reported here support the idea that rewarding electrical stimulation causes the release of dopamine.

Diffusion and uptake of dopamine in rat caudate and nucleus accumbens compared using fast cyclic voltammetry

Fast cyclic voltammetry was used in the caudate and nucleus accumbens of anaesthetised rats to study the release and reuptake of dopamine following stimulation of the median forebrain bundle. Dopamine uptake was significantly slower in accumbens than caudate, indicating a lower number of functional uptake sites. This implies that dopamine may be able to diffuse further from its sites of release in nucleus accumbens than in caudate and thus may have a neuromodulator role in this region.

Real-time characterization of dopamine overflow and uptake in the rat striatum

The rate of overflow and disappearance of dopamine from the extracellular fluid of the rat striatum has been measured during neuronal stimulation. Overflow of dopamine was induced by electrical stimulation of the medial forebrain bundle with biphasic pulse trains. The instantaneous concentration of dopamine was measured with a Nafion-coated, carbon fiber microelectrode implanted in the brain. The measurement technique, fast-scan cyclic voltammetry, samples the concentration of dopamine in less than 10 ms at 100 ms intervals. Identification of dopamine is made with cyclic voltammetry. Stimulated overflow was measured as a function of electrode position, stimulation duration, stimulation frequency, and after administration of L-DOPA and nomifensine. The observed concentration during a 2-s, 60-Hz stimulation was found to alter with position of the carbon fiber electrode. For stimuli of 3 s or less the amount of overflow was found to be a linear function of stimulus duration at a fixed electrode position. The observed overflow was found to be steady-state at a frequency of 30 Hz, suggesting a balance between uptake and synaptic overflow under these conditions. The experimental data was found to be successfully modelled when the balance of uptake and stimulated overflow was considered. It was assumed that each stimulus pulse releases a constant amount of dopamine (125 nM), and that uptake follows a Michaelis-Menten model for a single uptake site with Km = 200 nM and Vmax = 5 microM/s. The increase in stimulated overflow observed after L-DOPA (250 mg/kg) could be modelled by a 1.6-fold increase in the amount of dopamine release with no alteration of the uptake parameters. The increase in modelled by an increase in Km. In addition, the fit of the modelled data to the experimental data was improved when diffusion from the release and uptake sites was considered.