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Michael AC, Borland LM, editors. Electrochemical Methods for Neuroscience. Boca Raton (FL): CRC Press/Taylor & Francis; 2007.

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Chapter 8Using Fast-Scan Cyclic Voltammetry to Investigate Somatodendritic Dopamine Release

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What Is Somatodendritic Dopamine Release?

Midbrain Dopamine Neurons Release Dopamine from Their Somatodendrites

Midbrain dopamine (DA) neurons of the substantia nigra (SN, “A9”) and adjacent ventral tegmental area (VTA, “A10”) are critical to a range of CNS functions, including motor facilitation by the basal ganglia and the regulation of motivation by natural rewards as well as by drugs of addiction. A characteristic shared by DA cells in the SN and VTA is that they release DA locally from somatodendritic regions [1–4] as well as from their axonal projections. There is evidence for release from soma [5] as well as from dendrites [2,6]. Somatodendritic release of neurotransmitter is not restricted to DA neurons; rather, neurons found throughout the brain can signal via the somatodendritic release of neurotransmitters, including GABA and glutamate as well as neuropeptides [7,8]. Somatodendritic neurotransmission operates both at a synaptic level and by more paracrine/autocrine-like modes to offer neuronal cross-talk as well as self- or auto-feedback control [7,8]. This chapter will focus specifically on the somatodendritic release of DA within the midbrain and how voltammetric methods, particularly fast-scan cyclic voltammetry (FCV), have been used to explore its characteristics.

The Functions and Sites of Action of Somatodendritic Dopamine

What are the functions of somatodendritic DA? Somatodendritically released DA acts on somatodendritic DA D2-like autoreceptors to regulate subsequent somatodendritic DA release in the SN pars compacta (SNc) [9], DA neuron firing activity [10–13], as well as downstream axonal release in striatum [14,15]. Moreover, somatodendritically released DA in both SN and VTA can act at DA D1-like receptors presynaptic on GABAergic and glutamatergic terminals to modulate release of those transmitters [16–18]. For example, activation of D1 receptors on striatonigral terminals increases GABA inhibitory transmission to SN pars reticulata (SNr) output neurons, which decreases the inhibitory SNr output to the thalamus; [17] this would reinforce motor activation by the striatonigral pathway. In fact, the critical role of the nigrostriatal pathway in movement, as demonstrated by the motor deficits of Parkinson’s disease that accompany loss of nigrostriatal DA [19], may result from a loss of DA in striatal and midbrain regions because both axon terminal and somatodendritic release have been shown to be required for basal ganglia-mediated movement [20–23].

Where are DA receptors ultrastructurally located? There are few dendro-dendritic synapses in the SN/VTA [24,25]. DA receptors are, however, densely located at extrasynaptic sites throughout the length of the somatodendritic plasmamembrane and on afferent input membranes in SNc and SNr [12,26]. Moreover, the DA uptake transporter (DAT) is expressed at extrasynaptic sites remote from vesicles in the SN [27]. Together, the emerging picture of somatodendritic DA signaling is one where, unlike classical synaptic transmission (e.g., intrasynaptic release and receptor activation by glutamate), transmission by DA is mediated primarily by extrasynaptic receptors [28–30]. In fact, DA in both midbrain and striatum must act at its receptors via volume transmission [28,31]. As a consequence, a full understanding of DA transmission by somatodendritic DA release requires an understanding of local diffusion characteristics. This particular issue is discussed later in this chapter.

Methodological Approaches to the Study of Somatodendritic Dopamine Release

Electrochemical and Other Approaches to Dopamine Detection

A wide range of techniques have been used to investigate somatodendritic DA release. Initial studies used detection of [3H]-DA in vitro using midbrain slices [2,32] and in vivo using push–pull perfusion [4,33]. Development of more sensitive off-line detection methods, especially HPLC with electrochemical detection, permitted monitoring of endogenous DA release from midbrain slices in vitro [34]. Another advance was seen with the introduction of in vivo microdialysis, which permits evaluation of extracellular levels of either exogenous or endogenous DA when coupled with an appropriate off-line analytical method [14,35–39]. Microdialysis measurements have been helpful in elucidating factors that influence somatodendritic DA release. Like other in vivo methods, microdialysis affords the opportunity to study somatodendritic DA release after systemic drug administration or during behavior in freely moving animals [40]. Another feature of microdialysis is that the off-line dialysate analysis, usually after an HPLC separation step, permits selective detection of DA as well as the concurrent monitoring of DA metabolites or other transmitters. These strengths come with caveats, however. Firstly, the temporal and spatial resolutions of microdialysis and corresponding probes are limited. For example, control of release by discrete action potentials or bursts of patterned activity cannot be detected or explored on a subsecond basis. Spatially, neurotransmitter release cannot readily be attributed to discrete functional subterritories within any given nucleus, e.g., dorsal or ventral tiers of DA cells. Secondly, interpretation of in vivo studies to address the mechanism or factors regulating somatodendritic DA release are complicated by the unavoidable influence of the overall circuitry governing DA cell activity in the SN or VTA.

The use of voltammetric recording in vitro at carbon-fiber microelectrodes (CFMs) with fast-scan cyclic voltammetry (FCV) was introduced for the study of somatodendritic DA release over a decade ago [6]. FCV at CFMs is a high-speed, high-spatial-resolution detection method that is ideal for monitoring release of DA or other biogenic amines by discrete stimuli in discrete brain nuclei, including the SNc and VTA. Many insights into somatodendritic DA release in midbrain over the last decade have been obtained using this method. Major advantages of this method over those already described are that DA release can be monitored within and between discrete nuclei, e.g., SNc vs. VTA, with sub-second resolution, i.e., in “real-time,” and relatively independently from whole-brain circuitry.

Although considered a high-speed (or “real-time”) detection technique, the spatial resolution of FCV is finite. The types of scans used with FCV, typically bi-, tri- or quadri-phasic waveforms that are designed to include oxidizing as well as reducing scans, limit the number of FCV scans per second. In comparison, some other high-speed electrochemical detection methods suitable at microelectrodes, e.g., constant potential amperometry, offer the possibility to scan continuously at a fixed potential and thus offer continuous, temporally more precise data. And yet, temporal precision with constant potential amperometry can be at a cost of identification of electroactive species. Scan potentials fixed at the DA oxidation potential ( ~ 500–600 mV vs. Ag/AgCl) will detect DA but cannot readily discern DA from the possible multiple detectable events that accompany somatodendritic DA release in midbrain in situ, including changes in pH and local extracellular Ca2+ concentration [6,41,42], as well as the release of other monoamines, including 5-HT. Appreciation of the complex nature of these signals is imperative when undertaking electro-chemical recordings in midbrain, where these other current sources are significant [6,43–45]. The use of a scanning waveform that applies a reducing as well as an oxidizing phase, as used in FCV but not in constant potential amperometry, can circumvent some of these problems because these different components can be resolved, and, thus, the contribution that is selectively DA can be dissociated.

Co-detection of DA and 5-HT in Midbrain

FCV Waveform Choice

FCV offers an ideal approach for the study of somatodendritic DA release. Nonetheless, its own limitations must also be appreciated. In particular, voltammetric studies with FCV of somatodendritic DA release in the SN of some rodents, including rats [45–48] and mice [49] have been hindered by the concomitant or even predominant detection of 5-HT. The SN receives a major 5-HT innervation density (see Figure 8.1) from raphe nuclei projections, which provide direct, asymmetric synaptic inputs to both dopaminergic and non-dopaminergic dendrites in SNc and SNr in primates and rodents (SNr > SNc > VTA) [50–54]. Despite the theoretical ability to resolve DA and 5-HT on the basis of their reduction currents during a reducing scan (Figure 8.2), this task is not always simple analytically. The problem is compounded by the typical 3- to 10-fold greater sensitivity of CFMs to 5-HT than DA [45].

FIGURE 8.1. ( See color insert following page 272.

FIGURE 8.1

( See color insert following page 272.) Dopamine and 5-HT immunoreactivity in substantia nigra. (a) Low-power image of tyrosine hydroxylase-immunoreactivity (TH-IR) (grey/brown profiles) of soma and dendrites in SN pars compacta (SNc) and dendrites in (more...)

FIGURE 8.2. Voltammograms for dopamine (DA) and 5-HT.

FIGURE 8.2

Voltammograms for dopamine (DA) and 5-HT. (a) Fast-scan cyclic voltammetry voltage waveform, 800 V/s scan rate, applied against an Ag/AgCl reference electrode. Scan switched out of circuit between scans. (b) DA voltammogram indicates oxidation and reduction (more...)

Historically, different scan waveforms have been used for the preferential detection of DA or 5-HT. A biphasic triangular waveform has largely been used for detection of DA release and a triphasic “N” waveform for 5-HT. Triangular waveforms beginning and ending at negative potentials that include the reduction potentials for DA are typically less problematic for DA than 5-HT, due to the relatively lower propensity of the catecholamine DA to adsorb to the recording electrode and foul the surface. Furthermore, the triangular waveform has fewer changes in polarity of d V /dt and, therefore, fewer artifacts that can frequently occur with these switches in polarity, particularly upon electrical stimulation of tissue [6,43]. These artifacts can cause problems when quantifying neurotransmitter oxidation/reduction peaks, especially if they overlap with or distort a peak of interest. The indoleamines, such as 5-HT, have a much greater propensity than DA to adsorb to the electrode and foul the surface, potentially leading to artificially enhanced oxidation peaks [55]. For example, on calibration of 5-HT, using the triangular waveform and a resting potential of − 0.4 V, the concentration-time profile is not rectangular as expected from a species that does not adsorb but displays an ever-rising profile characteristic of species that do adsorb [55]. To minimize this adsorption, a higher rest potential (between scans) for the triangular waveform can be employed, typically 0 or + 0.2 V. To continue to be able to observe the reduction peak, which allows discrimination of the substance as 5-HT vs. DA, the reducing scan must be ramped below this potential before returning to rest; hence, the triphasic “N” waveform, e.g., scanning from (i) + 0.2 to + 1.0 V, (ii) + 1.0 to − 0.1 V, and (iii) − 0.1 back to + 0.2 V. Net adsorption is thought to be reduced by this procedure while the reduction peak essential for identification of the signal is retained [55].

Alternatively, our procedure of choice when recording in midbrain, where DA and 5-HT can both occur, is to use a biphasic triangular waveform (from − 0.7 V) that uses a minimal number of switches in polarity of d V /dt and which is switched out of circuit (or to 0 V) between scans to minimize adsorption. The scan range we use contains potentials for oxidation and reduction of both DA and 5-HT and thus leads to a relative ease of signal identification. Using this waveform, 5-HT exhibits dual reduction peaks and DA a single reduction peak at a potential between those for 5-HT (Figure 8.2), while the fewer changes in polarity of d V /dt (compared to the “N”) generate less interference from capacitance-associated electrical artifacts. We have taken this approach in our studies for the detection of DA and 5-HT in SN [43–45,56].

Attempts to improve the distinction between DA and 5-HT currents have recently been made in related FCV studies in striatum [57] based on the temporal profiles of current decay due to a more rapid reuptake of DA by DATs and more rapid clearance of DA than 5-HT from the CFM. A similar approach has not been explored in midbrain where current decay is already difficult due to both slower DA uptake [58] and the accompanying current shifts that usually accompany any release-evoking stimulus [43]. One of the most convincing ways to differentiate between 5-HT and DA in midbrain remains to be the use of the extended triangular waveform ( − 0.7 to + 1.3 V) switched out of circuit between scans, with companion studies using pharmacological tools for confirmation.

Species Variation in 5-HT Interference

Contamination of somatodendritic DA voltammograms with interference from 5-HT varies with the species of rodent. The guinea pig SN receives a less dense 5-HT innervation than the rat SN so that pure somatodendritic DA release can be monitored in guinea pig SNc but not in rat SNc [45]. Interestingly, there are species differences in 5-HT receptor binding profiles as well, e.g., 5HT4 receptors [59], with the pattern in guinea pig better resembling that in human SN. Thus, the guinea pig is the rodent species of choice for the characterization of somatodendritic DA release in SN and VTA using FCV [45] while the rat is preferable over the guinea pig for exploring nigral 5-HT [56]. The problematic co-detection of 5-HT is not a concern for microdialysis because of the separate off-line separation step usually used for DA detection. Nonetheless, this problem is worse yet for amperometric detection methods because, without a reducing scan, no attempt at distinction between DA and 5-HT can be made at all.

Characteristic Voltammetric and Pharmacological Differences

Several groups have studied 5-HT release in SN using FCV [6,45,47,48,56]. Release of 5-HT in SN may exhibit a different characteristic frequency response from that of DA. Reports to date suggest that DA release in SNc is frequency-dependent up to 10 Hz, beyond which no further increase in [DA]0 is observed [43], whereas 5-HT release in SNr is frequency dependent up to 50 Hz [47,48,56]. Nonetheless, train lengths were not constant across these studies; thus, frequency responses have not yet been systematically compared within a given study using a single experimental paradigm.

Is it possible to differentiate between 5-HT and DA pharmacologically in SN? There are indeed a few key features of the pharmacology of these two species in SN that differ. Firstly, evoked extracellular concentrations of 5-HT are extremely sensitive to blockade of the 5-HT uptake transporter (SERT) but not to blockade of the DAT [47,48,56], whereas evoked extracellular concentrations of DA are modified by the DAT but not the SERT [44]. Also apparent is the control by DA D2 receptors of somatodendritic release of DA but not 5-HT release in SNr [47].

Reproducibility of 5-HT and DA voltammetric signals over time also aids in species identification in the SN. Once DA has been detected in a single site, it can be difficult to evoke concentrations of the same magnitude again from the same site, even after a considerable pause after initial stimuli. In contrast to the release of DA, 5-HT release in SN is extremely robust [56], and single sites can reproducibly release for up to eight hours (Threlfell and Cragg, unpublished observation). Thus, strong re-releasability from a recording site is likely to further indicate signal identity as 5-HT.

Further points worth noting are empirical observations of the profiles of the oxidation peaks and the concentrations of species expected in the SN. In our hands, using a triangular waveform ( − 0.7 to + 1.3 V), 5-HT oxidation peaks are notably narrower (span a smaller range of potentials) than those for DA that tend to have a much broader profile. Detected concentrations of each electroactive species in the SN vary significantly from one another by up to one order of magnitude ([DA]0 > [5-HT]0), e.g., evoked [DA] in SNc can reach ~ 500 nM while evoked [5-HT] (in rat SN) are in the range of 50–120 nM. However, the enhanced sensitivity of the CFMs to 5-HT (typically three to five times greater than that for DA) can lead to similar overall detected currents. Where DA and 5-HT occur together at these levels, e.g., in rat SNc [45], reduction currents comprise multiple irresolvable peaks, which precludes accurate signal identification and quantification.

In Vitro Methods for FCV in Midbrain

Slice Preparations and Species

To date, successful voltammetric approaches to explore somatodendritic DA release have used coronal brain slice preparations (300–400 μm) from a range of rodent species, including guinea pig, rat, and mouse. Acute slices are prepared by conventional methods used for electrophysiology preparations, e.g., over ice using commercially available vibratomes. Coronal slices offer a choice of recording sites in VTA, SNc, and SN pars reticulata (SNr), with readily identifiable anatomical landmarks to delineate each region, e.g., the myelinated accessory optic tract neatly divides the VTA from the SNc. The coordinates of midbrain slices containing the SN/VTA in guinea pig correspond to A8.0–A8.7 mm anterior (A) to the interaural line, according to the atlas of Smits et al. [60]. In adult rat, the equivalent slice range corresponds to the coordinates A3.2–A4.2 mm, according to the atlas of Paxinos and Watson [61], i.e., between rostral interpeduncular nucleus and caudal mammillary bodies. Somatodendritic DA release can be detected from early postnatal ages onwards (e.g., postnatal day two) in guinea pig and rat [45] at ages prior to the development of innervation by 5-HT projection axons. If dissecting multiple brain regions for voltammetry from one brain, e.g., midbrain and forebrain, optimal success with midbrain recordings in our hands has been achieved when the midbrain block is sectioned first, i.e., given priority over any other. On a further technical note, the large network of blood vessels present on the ventral aspect of the adult rat brain must be removed as quickly as possible. These elastic vessels may otherwise be poorly cut by even the best commercially available sectioning instruments and thus cause tissue snagging, stretching, and subsequent damage that limits slice viability.

Solutions

To date, published methods for preparing animals for midbrain slices for FCV have used inhalation or intraperitoneal anesthesia (halothane or pentobarbital, respectively), and, after brain removal, coronal slices have typically been cut on vibratomes in ice-cold, HEPES-buffered artificial CSF (ACSF) containing (in mM): 120 NaCl, 5 KCl, 1.25 KH2 PO4, 20 NaHCO3, 6.7 HEPES acid, 3.3 HEPES salt, 2 CaCl2, 2 MgSO4, and 10 glucose (saturated with 95% O2 /5% CO2) [43,44,62]. After cutting, slices are allowed to recover in HEPES-buffered ACSF for at least one hour at room temperature before being transferred to a submersion recording chamber. Once in the recording chamber, slices are equilibrated for an additional thirty minutes with a physiological bicarbonate-buffered aCSF, which usually contains (in mM): 124 NaCl, 3.7 KCl, 26 NaHCO3, 1.5–2.4 CaCl2, 1.3 MgSO4, 1.3 KH2 PO4, and 10 glucose (saturated with 95% O2 /5% CO2). Chamber temperatures used to date are typically 32°C.

During recording, viewing through a low-power binocular microscope is usually sufficient to locate VTA, SNc, and SNr within a coronal slice; higher magnification optics and infra red difference interference contrast (IR-DIC) microscopy can offer more precise electrode placements to tiers of DA neurons within the SNc, if required. Carbon-fiber electrodes are only 7–8 μm in diameter and thus sample from highly discrete regions; as a consequence, the accurate placement of electrodes to within specific nuclei is important. Nonetheless, the typical electrode tip lengths (e.g., 30–100 μm tip length) required to generate currents of appropriate signal to noise ratios for voltammetry entail that electrodes sample from a local population rather than single release sites. In turn, the concentration of DA release detected will thus depend on the local packing density of somatodendritic release sites and correlates well with the density of immunoreactivity to tyrosine hydroxylase (TH-ir) (Figure 8.3). Other factors that govern the detected concentration of DA release include stimulation protocol, Ca2+ availability, DA uptake, extracellular volume fraction, and autoreceptor and heteroreceptor regulation.

FIGURE 8.3. Rostral-caudal range of SN and corresponding voltammograms obtained in SNc after electrical stimulation.

FIGURE 8.3

Rostral-caudal range of SN and corresponding voltammograms obtained in SNc after electrical stimulation. (a) Coronal sections extending from rostral (A8.7 mm) to mid-caudal SN (A7.5) stained for tyrosine hydroxylase immunoreactivity (TH-ir). (Coordinates (more...)

Scan Waveforms

A variety of scan waveforms, scan rates, and sample frequencies have successfully been used to detect and experiment with somatodendritic DA release in VTA and SNc, ranging from so-called “N”-shape waveforms (scans from 0.0 to + 1.1 V to − 0.5 to 0.0 V vs. Ag/AgCl) at 660 V/s using sample frequencies of 2–4 Hz [6] to “triangular” waveforms (scans from − 0.7 to + 1.3 V to − 0.7 V vs. Ag/AgCl, switched out of circuit between scans) at 800 V/s at 8–10 Hz [9,43,62] where the scan duration is ~ 5 ms. Arguably, the biphasic triangular scan is more simple to interpret than a triphasic N scan due to the fewer number of switches in scan rate polarity and consequent artifacts that are associated with each switch; these can be particularly problematic during stimulated release in SN and VTA [6,43,44]. The scan potentials for the oxidation and reduction of DA are well within such scan potential ranges (occurring at ~ 500 to − 600 mV and − 200 to − 300 mV), and these generous scan potential ranges have been used in midbrain to increase the separation of oxidation/reduction currents from any artifacts occurring at the switches between + δ V / δ t and − δ V / δ t. Both DA and 5-HT can be resolved with such scans (Figure 8.2). Given the strong innervation density of 5-HT and DA in midbrain (Figure 8.1), it is imperative to use a scan that has the capability to resolve both amines.

Carbon-Fiber Microelectrodes: Preparation, Calibration

Carbon-fiber microelectrodes (CFMs) for use with FCV in the study of somatodendritic DA release have typically been prepared from 7- to 8-μm diameter untreated carbon fibers in a 2 mm glass capillary tube pulled to a taper. The electrodes we prepared ourselves, as well as those purchased from MPB electrodes (London), were prepared to final tip lengths of 30–60 μm (length of carbon-fiber exposed beyond the glass insulation), using either cutting or spark-etching followed by beveling to a point of tip diameter of 2–4 μm [63]. Full instructions for electrode preparation using the method of Millar and colleagues as used by us, are detailed at http://www.qmw.ac.uk/~physiol/makingCFE.html.

Electrode calibrations for sensitivity to DA (or 5-HT) were performed in the recording chamber at 32°C using known concentrations of DA (or where appropriate 5-HT), made up in aCSF solutions to amine concentrations similar to those seen in situ (0.5–2 μM). Electrodes should respond linearly to concentration over this range (data not illustrated). Stock amines (1000–2500x) were kept in solutions of 0.1 M HClO4 and made up in aCSF buffer immediately before use. Electrode sensitivity during recordings was assessed by calibration immediately after recording. Typically, electrodes had sensitivities to DA in the order of 2–10 nA/μM, and detection limits (~ 2x noise) were 30–50 nM.

Choice of Stimulation Parameters

Because electrochemical current responses are proportional to the concentration of electroactive species, optimal signal strength of DA will occur following synchronous release from a population of local release sites. With FCV to date, data acquisition has been most successful using electrical stimulation at local bipolar electrodes [9,43,44,62,64–66]. Spontaneous somatodendritic release has not been reported with FCV. However, this could be due to limitations of sensitivity rather than a paucity of spontaneous DA release events because DA neurons are spontaneously active within midbrain slice preparations and, furthermore, amperometry measurements at single cells cleared of adjacent tissue by micropipette suction do reveal spontaneous exocytotic-like events from DA-like neurons, albeit at very low frequencies ( ~ 0.3 to 3 events per minute) [5]. Nonetheless, if there is spontaneous somatodendritic DA release in slices, it is not sufficient to activate release-regulating autoreceptors [9].

Prior to the introduction of electrical stimulation to study discrete and repeated observations of DA release with FCV, the chemical secretagogue veratrine (mixed alkaloids) [6,43] or high potassium [6] were used to evoke DA release. Voltammetric signals following veratrine superfusion were attributable to DA but were frequently accompanied by the progressive distortion of the DA voltammogram by 5-HT as well as changes in background currents in VTA and SNc [6]. These artifacts were particularly problematic with high potassium [6]. These additional shifts can correspond to changes in Ca2+, pH, O2 tension, and “switching artifacts.” [6] Repeat measures in a single slice cannot readily be achieved following veratrine superfusion, thus limiting data collection with this approach. We have previously experimented with iontophoresis of high K+ concentrations or glutamate (Cragg and Rice, unpublished data) but with notably less success than electrical stimulation.

Chemical stimulation of the subthalamic nucleus (STN) has been used to remotely activate the SN in a study using amperometry [13]. In that study, changes in electroactive species were not detected following electrical stimulation of the STN, but, rather, local pressure application (36 s, 20 psi) of carbachol (10 mM) was apparently the most effective means of stimulating currents in SN. However, the speculated identity of the signals detected in SN as DA in that study remains unresolved and has not been explored with FCV.

By using electrical stimulation at local bipolar electrodes, we have found FCV recordings within SN or VTA to have a higher data yield, to be spatially discrete, repeatable at several loci within a slice, and less confounded by accompanying stimulation artifacts than with local veratrine [43,44], as well being more readily controlled in intensity, duration, and location than was possible with the depolarization induced by veratrine. However, electrically eliciting DA release from the SN can prove difficult, as noted by others [46], and is not reliably repeatable. Typically, we have constructed bipolar stimulating electrodes from Teflon-coated platinum wire (Clark Electromedical Instruments) of either (1) bare diameter 125 μm, coated diameter 175 μm, and tip separation of 100 μm or (2) bare diameter 50 μm, coated diameter 75 μm, tip separation 50 μm. However, stimuli generated with the larger electrodes readily evoked release in a manner that was independent of voltage-gated Na+ channels, i.e., TTX-insensitive [43,44], whereas stimulation with the smaller electrodes (presumably higher resistance and lower applied currents) permitted DA release via recruitment of voltage-gated Na+ channels, i.e., TTX-sensitive release [9]. In all cases, we superficially position the tips of the bipolar electrode for surface stimulation and insert the CFM 50–100 μm into the slice between the stimulator poles. Constant voltage (10–20 V, 0.1 ms pulses) [9] and constant current (0.4–0.8 mA, 0.1–1.0 ms pulses) [62,64] stimuli have been used to evoke TTX-sensitive somatodendritic DA release. Commercially available, e.g., concentric bipolar electrodes, would likely be equally successful.

Several factors will govern the choice of electrical stimulation parameters for studies of neuro-transmitter release. Experimenters should decide whether they want to optimize signal to noise ratio (relative signal amplitude), local synaptic input by other neurotransmitters, independence from other neurotransmitter inputs, or repeatable release at a single loci. For example, long stimulation trains will have the disadvantages (or advantages) of poor reproducibility in a short timeframe and engaging the neuromodulation of somatodendritic DA release by other neurotransmitters released during the stimulus that will subsequently control DA release at successive pulses. In contrast, release by single stimulus pulses may be lower in concentration but more reproducible (see below) and will be governed only by factors that regulate intrinsic somatodendritic release probability as well as any spontaneously active sources of neuromodulatory inputs.

In SNc and VTA, the most routinely reported stimulation paradigms are trains of pulses at 10 Hz (e.g., 0.1 ms pulses in three second trains) [9,62,64,65] (although up to ten seconds have been used) [43,44], or single pulses (when longer pulse durations are required, 1 ms) [62,66]. In our hands, 10 Hz trains for three seconds most reliably evokes the greatest single increase in [DA]0 [43]. Typically, however, electrical stimulation evokes the greatest [DA]0 during the first of any stimulation train at a given SNc or VTA recording site [43,44,62]. Although detectable release can again be elicited after several minutes, subsequent stimulations are rarely as effective. This failure at successive stimuli has been assessed most rigorously following 10 Hz stimulation trains (of 3–10 s duration) when maximal detected [DA]0 decline significantly at subsequent trains, even after 20–30 min recovery between trains [43,44]. However, we have found that stimulation trains of shorter duration and higher frequency (e.g., ten pulses of 0.1 ms duration at 100 Hz) [9,67] can be relatively successful at evoking reproducible [DA]0 at a given recording site when using 5–10 min sampling intervals [67]. Nonetheless, concentrations of DA detected with this paradigm are low, in the order of ~ 100–200 nM DA, which are at least five-fold less than concentrations detected at single 10 Hz trains only.

Because optimal [DA]0 are released by longer pulse trains, after which release is not fully reproducible at any given recording site, a suitable experimental design that can be used to assess the control of somatodendritic release before and after manipulation of a parameter of interest (ion channel, neurotransmitter receptor, ligand, etc.), is one involving paired observations in two hemispheres. For example, single measurements can be taken at several recording sites in VTA and SNc, then the parameter of interest can be suitably modulated (e.g. by drug or ligand application), and then measurements can be taken in the opposite hemisphere by single measurements at matching, i.e. paired recording sites in VTA and SNc. Caution should be taken if currents are reproducible at a given recording site because 5-HT release is readily reproducible at recording sites in rat SNc and SNr; e.g. [56], the identity of the reproducible component of these voltammograms should be appropriately analyzed.

Another useful consideration when using electrical stimulation in conjunction with any electro-chemical scan is to prevent the stimulus from occurring during the limited duration of the scan in order to avoid distortion of the electrochemical currents with a stimulus artifact. This can be achieved by either electronically blanking stimuli during the voltammetric scans or by the introduction of a simple electronic timing circuit to offset the depolarizing stimulus at a fixed interval out of phase with FCV scans.

Concerns in Signal Identification

Among the advantages of FCV as a technique for real-time detection of somatodendritic DA release is its ability to discriminate between DA and the other sources of apparent currents that constant potential scans or non-cyclic scans cannot. Nonetheless, these other sources of currents can either obscure redox currents due to DA or can contribute to the current measured at the DA oxidation potential. Thus, due caution should be taken in attributing solely to DA the electrochemical currents at the DA oxidation potential. Where necessary, baseline adjustments may need to be made to correctly determine currents due to DA. Such interferents can also greatly hinder the apparent decay of currents at the DA oxidation potential after release, thereby often precluding determination of uptake rates [44]. This issue is particularly important in the midbrain where these other current sources, e.g., due to changes in pH, extracellular Ca2+, O2 tension [6,41,42,68] and 5-HT seem to be particularly significant [6,43–45].

As with the use of FCV anywhere in the brain, the identity of DA-like electrochemical signals must be validated by supplementary data in addition to electrochemical information provided by the scan, e.g., through anatomic verification and pharmacological manipulations. Clearly, a comparison of the electrochemical signals with calibration voltammograms should indicate that the released substance is a catechol, like DA or DOPAC, rather than the indole 5-HT. The contribution of electroactive DOPAC must be considered; nonetheless the low sensitivity of our electrodes for DOPAC (1:70, DOPAC:DA) suggest that any contribution of DOPAC to the catechol signal will likely not be appreciable. We have confirmed this experimentally: the monoamine oxidase inhibitor pargyline does not modify the amplitude of the release signal in SNc [6]. Another possible interferent, ascorbic acid (ascorbate), is also unlikely to contribute; previous studies indicate that 95% of tissue ascorbate content is lost from slices [69]. Even when present, the ascorbate voltammogram is readily distinguished from that of a catechol [70]. Importantly, DA-like signals in SN and VTA should be anatomically specific, i.e., restricted to regions that are positive for TH-immunoreactivity [43,44] with, furthermore, a general correlation between the amplitude of the responses and density of TH-ir at these locations [44] (Figure 8.3). In addition, these signals should be indistinguishable (except in amplitude) from those obtained in striatum, a region for which DA release has been extensively characterized with FCV.

Additional pharmacological manipulations can identify whether Ca2+-free media reversibly eliminate DA-like signals in an expected manner and which uptake transporter (DAT, NET, or SERT) plays the most prominent role in regulating amine-like signals. Primarily, somatodendritic DA is governed by the DAT rather than the NET or SERT [44,58], but monoaminergic neurotransmitter uptake systems can be highly promiscuous with respect to neuroamine substrates [44,58,71–73]. Thus, care should be taken in identifying amine-like signals in SN and VTA based solely on the effects of uptake transport inhibitors. Although the voltammogram of DA is similar to that of the catecholamine norepinephrine (NE), anatomical observations of the restricted localization of immunoreactivity to the NE synthesizing enzyme, DA-β-hydroxylase, in conjunction with the limited effects of NE uptake inhibitors on the catechol signals, together suggest that NE does not contribute to the release signal in VTA or SNc [44].

What Is the Mechanism of Release?

Anatomical Evidence

Whether or not somatodendritic DA release is exocytotic remains a subject of continuing speculation and investigation [13,43,44,62,66]. Anatomical evidence for exocytosis is certainly limited in SN; synaptic sites where vesicle fusion might occur are rare [25]. Vesicles are also rare in DA cells of the SNc. Whereas vesicle density is high in DA terminals in striatum [27,74], there are few vesicles in DA somata or dendrites [24,25,27], implying a limited source for exocytotic release [27].

Storage of somatodendritic DA has been proposed to be in saccules of smooth endoplasmic reticulum [75,76] as well as in vesicles [24,25]. Consistent with dual storage sites, the vesicular mono-amine transporter, VMAT2, is expressed in saccules (so-called tubulovesicles) and, less commonly, in vesicles [77]. Whether both storage sites contribute to the releasable pool of DA is not known. Both would be susceptible to DA depletion by reserpine, an irreversible inhibitor of VMAT2, which limits the information provided by reserpine sensitivity about vesicular release or otherwise [37].

Mechanistic Evidence

The non-axonal source for somatodendritic DA release and paucity of dendritic synapses has led to speculation over whether the mechanism of somatodendritic release might also differ from that seen from axons. The limited numbers of vesicles and abundance of DATs in DA dendrites led several groups to speculate that somatodendritic release might be mediated by vesicle-independent reverse DAT transport [24,27,77,78]. Indeed, this is a primary mode for release following certain pharmacological manipulations, including veratridine-induced depolarization [34] or amphetamine-induced DA displacement from intracellular stores [79–81]. Somatodendritic DA release elicited by veratridine requires voltage-dependent Na+ channel opening and subsequent Na+ loading to activate reverse DA transport; release can subsequently be blocked by the DAT inhibitor GBR-12909 [34]. Under normal conditions, it has been suggested that cytoplasmic DA concentrations are not sufficient to reverse the DAT unless increased by pharmacological agents, e.g., by amphetamine [80]. However, Falkenburger et al. [13] reported electrophysiological evidence that carbachol stimulation of the subthalamic nucleus could also induce dendritic DA release in the SN by a mechanism that could be prevented by low concentrations of GBR-12909. This finding suggests that under some physiological conditions, somatodendritic release might be mediated by reversal of the DAT. It is not yet clear how the data of Falkenburger et al. can be reconciled with other in vitro and in vivo studies of somatodendritic DA release, including those with FCV that typically find an increase, not a decrease, in [DA]0 in the presence of an uptake blocker [14,44,58,62,82–85].

Apart from the study by Falkenburger et al. [13] there is little evidence to contradict the original proposal by Geffen et al. [2] that DA release in the SN is vesicular and mediated by exocytosis, as it is in axon terminals in striatum. Consistent with the ionic and pharmacologic characteristics of exocytosis, dendritic DA release is depolarization-, Na+ channel-, and Ca2+ -dependent [2,6,9,33,43,62], is sensitive to DA depletion by reserpine [6,37], and is not prevented by DAT blockers [14,44,62,82,84]. Additionally, other manipulations that alter [DA]0 in striatum generally also modify [DA]0 in SN [9,14,37,44,62,86] although there is evidence for less similarity between DA release in VTA vs. nucleus accumbens [9,87]. As detected by FCV, the dependence of somatodendritic DA release on voltage-gated Na+ channels (TTX-sensitivity) [9,62] is in keeping with the presence of regenerative Na+ currents in DA neuron dendrites [88], as in axons. And yet, how somatodendritic DA release depends on action potential vs. other regulation of membrane excitability, e.g., local afferent input, remains unknown. Thus, it remains unclear how somatodendritic DA release reflects the characteristic repertoire of tonic and phasic firing patterns of DA neuron activity [89–92].

Evidence for Ca2+ dependence is often taken as confirmatory of vesicular release because Ca2+ entry is typically required for exocytosis [93]. Voltammetric studies of somatodendritic DA release in SN have been particularly informative. Somatodendritic DA release in SN requires Ca2+ ; incubation in Ca2+ -free media with EGTA inhibits evoked DA release by > 90% [6,43]. However, in contrast to striatal release, DA release in SNc persists in low-Ca2+ media; [6,43,62,86] inclusion of EGTA is required to eliminate release [62], suggesting a limited regulation of somatodendritic DA release by Ca2+ entering the neuron [66]. Furthermore, blockers of voltage-gated Ca2+ -channels (N-, P/Q-, T-, R-, L-types) have undetectable to modest effects on [DA]0 in the SN contrasting with their various, strong roles in the regulation of DA release in striatum [66]. Together, these findings suggest a difference in requirement for Ca2+ entry, intracellular Ca2+ availability, Ca2+ sensitivity or some other related aspect in the mechanisms of somatodendritic vs. axonal release. For example, ready somatodendritic release in low [Ca2+ ]0 might reflect differential sensitivity of Ca2+ -dependent release mechanisms, including fusion proteins, between these compartments. For example, the varying forms of synaptotagmin, a key Ca2+ -sensing protein responsible for triggering synaptic transmitter release, can differ in Ca 2+ affinity by 10- to 20-fold [94,95]. Differential expression of fusion proteins among cell compartments is not without precedent; Bergquist et al. [39] recently suggested that different synaptobrevin isoforms underlie axonal DA release in striatum vs. somatodendritic release in the SN.

A possible confounding factor in voltammetric studies of the Ca2+ -dependence of somatodendritic DA release was that the pulse-train stimulation used to evoke DA release simultaneously elicits release of other local transmitters, including GABA and glutamate, that strongly modulate somatodendritic DA release in the SNc [65]. Furthermore, comparison with striatal DA release is complicated by the strong dependence of striatal release on endogenous ACh [96]. This raises the concern that the true Ca2+ -dependence of somatodendritic and striatal DA release might be masked by the Ca2+ -dependence of GABA, glutamate, and ACh release in each region that could subsequently differently govern evoked [DA]0. This concern has been addressed to some extent in SNc by studies that reexamined the Ca2+ -dependence of DA release using single-pulse stimulation, which elicits DA release independently of ionotropic glutamate or GABA receptor modulation [66,97]. Importantly, DA release varied with [Ca2+]0 in a manner similar to that observed with pulse trains [97] indicating that the Ca2+ -dependence reported previously was intrinsic rather than extrinsic. Full comparison of the Ca2+ -dependence, as well as voltage-gated Ca2+ channel control of somatodendritic compared to striatal DA release during conditions similarly independent of afferent neurotransmitter input (e.g., during ACh receptor block), has not yet been assessed and requires further delineation.

Regulation of Somatodendritic Dopamine Signals by Extracellular Geometry and Uptake

Volume Transmission and Uptake

Since DA transmission is mediated primarily by extrasynaptic receptors [28–30] and thus must act via volume transmission [28,31], a full understanding of DA transmission by somatodendritic DA release requires an understanding of local extracellular diffusion characteristics. Key features that define extracellular concentrations of DA and its extracellular diffusion are the extracellular volume fraction, extracellular tortuosity, and plasmamembrane re-uptake [30,98]. Together, these factors define the sphere of influence of a released neurotransmitter.

We have defined these variables using FCV as well as using ion-selective microelectrode recordings. Strikingly, the extracellular volume fraction (α) available for DA diffusion in the SN and VTA is 0.30 [58], compared to values of ~ 0.20 that are typical for forebrain structures, including striatum [99]. This means that the extracellular concentration of DA ([DA]0) after release of a given number of molecules will be > 30% lower in the SN/VTA than in striatum in the absence of other regulatory mechanisms. This has obvious implications for concentration-dependent receptor activation in SN/VTA vs. striatum as well as for experimental observations of [DA]0 in these regions. The tortuosity factor, λ, which governs the apparent diffusion coefficient of a diffusing substance, is similar between midbrain and striatum [58].

Electrophysiological studies indicate a physiological role for uptake in the modulation of somatodendritic DA signaling [100,101]. In keeping with this and with the high density of DAT-expressing cells in these areas [27,102–104], in vitro experiments with FCV (and high-speed chronoamperometry) indicate that there is uptake of DA via the DAT after evoked release or application of exogenous DA in SNc or VTA [44,58,84]. Nonetheless, the DAT is not the only transporter that can transport somatodendritic DA. Uptake of DA by the NE transporter (NET), albeit modest, is more prominent in VTA than SNc [44,58] and appears to be mediated by a few sparsely packed, en passant rostral norepinephrinergic processes [44]. Uptake of DA in SNr is much less avid than in SNc or VTA, enabling DA to diffuse over larger distances without encountering uptake sites [58].

An important point to be made about uptake is in the context of DA as a volume transmitter. Dopamine transporters (DATs) are often considered to participate in this process by “gating” spillover from release sites. Indeed, although there is competition between DATs and diffusion in sculpting extracellular DA transients after release [30], it should be noted that a role for the DAT or any uptake process is not at odds with the notion that a neurotransmitter acts by volume transmission, in contrast to the recent publication of this notion [85]. DATs are expressed only by dopaminergic neurons, at perisynaptic and non-synaptic sites on DA dendrites and axons [27,105]. Neither striatal nor somatodendritic DATs are empowered anatomically or kinetically to prevent initial spillover from a release site [30]. This is in contrast to the situation seen for glutamate at glutamate synapses that are enriched in synaptic and extrasynaptic glutamate uptake transporters (GluTs) on multiple presynaptic and postsynaptic cells, including astrocytes, and not only occur at densities at least two orders of magnitude greater than for DATs surrounding DA release sites but, furthermore, operate with transport numbers one order of magnitude greater than for DA at DATs [30]. Thus, GluTs are better positioned than DATs, anatomically, numerically, and kinetically, to limit spillover and active lifetime of transmitter.

Rather, DATs particularly govern the sphere of influence, lifetime, and, thus, net extracellular concentration of DA beyond a release site [30]. This can readily be demonstrated both experimentally and theoretically even for axonally released DA where DAT density is at its highest. There is abundant experimental evidence from voltammetric studies at extrasynaptic CFMs that DA signals readily appear in the extracellular space following evoked action potentials [106] and during motivational and reward-associated behaviors [107–109]. Furthermore, the extrasynaptic sphere of influence of DA can be modeled using a modified version of the equation for three-dimensional diffusion in brain tissue; [30,58,110] this model has been used to predict the sphere of influence of DA at its receptors in the striatum where DA uptake kinetics are at least one order of magnitude greater than in midbrain [30,58]. Such modeling readily predicts that in striatum, despite avid DA uptake, e.g., V max of 4–5 μM/s defined from studies with FCV [111], the release of a single vesicle of 2000–14000 DA molecules would reach an estimated 20–100 synapses, or a volume approaching 2000 μm3, at concentrations greater than the EC50 value for D2 receptors [30]. These spheres of influence and synapse numbers would be up to twice these values with lower rates of DA uptake [30]. Thus, volume transmission readily occurs for DA whether released from somatodendrites or striatal synapses.

Regional Kinetic Distinctions

Within the midbrain, there is a regional variation in the regulation of [DA]0 by uptake. The role of the DAT on [DA]0 after somatodendritic release may be more marked in SNc than VTA [44], but see [58] reflecting differential DAT expression in these regions. Ventral tier DA neurons (found in the SNc only) have greater mRNA and protein levels of DA transporter (and D2 DA receptor) than dorsal tier DA cells (dorsal SNc, VTA) [102,103,112–114]. In particular, in FCV studies of evoked somatodendritic DA release, a role for DA uptake was less apparent in VTA than SNc [44]. Subsequently, we used FCV in conjunction with exogenously applied DA to eliminate the contribution of local depolarization as a possible hindrance to DA uptake by the electrogenic DA transporter; in turn, uptake of somatodendritic DA was also identified in VTA [58]. It is relevant to note that ventral tier SNc cells are more susceptible to degeneration in Parkinson’s disease than dorsal tier cells in either VTA or SNc [115–117]. This pattern of susceptibility is paralleled in the vulnerability to the DA uptake substrate and toxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) [118], which can be prevented by DA uptake inhibition. Together, such findings have implicated DA uptake activity or other regionally specific DA handling mechanisms as possible risk factors in Parkinsonian degeneration [119–121]. DA uptake within the midbrain, therefore, is crucial not only for normal DA neuron physiology but may also contribute to the differential vulnerability of DA cells to pathophysiology.

Although uptake via the DAT is an important mechanism of regulating [DA]0 in midbrain [44,58], it plays an apparently lesser role on the regulation of [DA]0 in the midbrain than in striatum [44], as shown empirically by a greater increase in [DA]0 during local electrical stimulation in striatum than in SNc when the DAT is blocked. Approximations to linear uptake constants for each region have been estimated at ~ 0.1 s −1 in SNc [58] and ~ 20 s −1 in striatum [30]. This difference is in keeping with the lower density of DAT protein in somatodendritic than axon terminal regions [102,103,122]. It can be speculated that a consequence of this apparently less avid regulation of dendritic compared to axon terminal [DA]0 will be a greater sphere of influence of dendritic than axonal DA [30,58].

Neuromodulation of Somatodendritic Release

Autoreceptor Regulation

Inhibition of DA release from striatal axon terminals via presynaptic D2-like autoreceptors plays a powerful role in the regulation of striatal [DA]0 [9,123–125]. D2 receptors are also expressed and located directly on DA dendrites in VTA and SN [12,26,113,126]. However, particular care with experimental protocol is needed when using FCV to evaluate whether evoked neurotransmitter release is controlled by neurotransmitter receptors. Without a large background or basal level of [DA]0, the control of DA release by a tonic DA activation of D2 receptors is unlikely to be detected. The detection of autoreceptor activation by discretely released endogenous neurotransmitter requires that concentrations of release as well as durations of pulse trains are appropriate. D2-receptor tone may need to be generated by stimulated release (e.g., in slices where spontaneous release is minimal). Furthermore, a time delay for activation may occur: short stimulus trains ( < 100 ms) are unable to reveal DA D2-receptor control of DA release at either somatodendrites or axon terminals in striatum [9,106,125,127]. Rather, longer stimulus trains ( > 100 ms) are required to permit sufficient time for DA released early in the train to activate D2 autoreceptors such that the effects of D2 antagonists can be observed later in the train [9]. With due consideration to this issue, D2-receptor control of somatodendritic DA release has been documented in SNc indicating that D2 receptors can operate as an autoinhibitory mechanism to regulate somatodendritic release [9]. The modulation of [DA]0 by D2 receptors is less marked in VTA than in SNc, consistent with the higher expression and protein levels of the D2 receptor in ventral tier (SNc only) DA neurons [113]. As for differential DAT function discussed above, regional variation in the management of somatodendritic DA transmission in VTA and SNc by autoreceptor regulation might also contribute to differential susceptibility of these cell groups to degeneration in Parkinson’s disease.

Notably, the degree of control of somatodendritic [DA]0 by D2 autoreceptors is less avid than for striatal DA. Less effective autoinhibition of release, in conjunction with less avid DA reuptake via the DAT [44,128] and other Ca2+ -dependent processes, could result in altogether different regulation of [DA]0 in somatodendritic and axon terminal regions.

Heteroreceptor and Other Neuromodulatory Regulation

DA neurons receive significant non-DA synaptic input and concomitantly express somatodendritic receptors for corresponding major classes of neurotransmitters, including GABA, glutamate, 5-HT, and acetylcholine (ACh), as well as co-transmitters like dynorphin and ATP. The role of each of these inputs in the direct heteroreceptor regulation of DA release within the SN and VTA is increasingly being revealed.

Strong inhibitory control of somatodendritic DA release by GABA has been identified in experiments with FCV, especially in the SNc where both GABAA and GABAB receptors control release [65]. This control is particularly evident during somatodendritic release elicited by pulse trains (10 Hz for three seconds), rather than single pulses [97]. This difference, therefore, presumably reflects insufficient spontaneous GABA release in coronal midbrain slices to modify DA release as measured at an isolated stimulus but that GABA released by an electrical stimulus can sub-sequently modulate DA release by ongoing depolarization, i.e., in a stimulus train. This issue readily highlights an important methodological strength of FCV. FCV can be used to dissociate intrinsic vs. extrinsic (input) control of neurotransmitter release; its sophisticated time resolution permits resolution of release by instantaneous (single pulse) as well as longer events (bursts of action potentials). In turn, a related point must be made that the effects of the local stimulation on the release of other non-electroactive neurotransmitters must always be considered in evaluating the regulation of the neuro-transmitter of study (here, somatodendritic DA). If the focus of experimental investigation is the regulation of somatodendritic DA release by mechanisms that are intrinsic to DA neurons and not those that concomitantly regulate the release of other local neurotransmitters then either appropriate short stimulus trains and/or appropriate synaptic blockers should be incorporated into the experimental design.

The role of GABA in the VTA is less clear. Although measurements in vivo using microdialysis show that GABAB receptors tonically inhibit DA release in VTA [129], in vivo data can be complicated by concomitant activation of long-loop circuits. Local measurements in slices with FCV suggest that GABAA and GABAB receptor control of release in VTA is limited and may depend on the strength and timing of other synaptic input [65].

Glutamate receptor regulation of somatodendritic DA release has been assessed with several methods and can take the form of direct excitation by ionotropic receptors [65,130–133], particularly in the VTA [65], or heterosynaptic inhibition via AMPA-receptor activation on GABA afferents in the SNc [65]. As discussed previously for GABA, evaluation of glutamate receptor control with FCV requires careful consideration of experimental protocol to ensure local glutamate synapses are activated by the stimulation protocol [97]. Possible differences between SNc and VTA are consistent with the dominance of glutamate input to VTA DA neurons (70% of axodendritic synapses) and the dominance of GABA input to SNc DA cells (70% of axodendritic synapses) [134]. Whether local glutamate input enhances or inhibits somatodendritic DA release (via GABA afferents) therefore depends on the balance between excitatory and inhibitory inputs at a given moment [65] and, as such, can vary according to brain region, the pattern of local input circuitry, and experimental conditions.

In keeping with the expression of nicotinic ACh receptors by DA neurons (primarily α 4 β 2-subunit-containing) [135,136], nicotine/ACh facilitatory control of somatodendritic DA release has been identified in VTA using microdialysis following systemic nicotine administration [137] and more directly in a dendrosomal preparation from SN/VTA using a [3H]DA release assay [138]. Although the effect of nicotinic ACh receptor activation on DA neuron activity is typically excitatory, inhibitory effects can also occur by subsequent activation of Ca2+ -activated K+ channels [139], and a complex multisynaptic activation can occur via nAChR desensitization on synaptic inputs [140,141]. However, measurements with FCV have not yet been exploited to dissect the roles of direct and indirect actions of ACh on any control of endogenous somatodendritic DA release.

Somatodendritic DA release may also be regulated by other classes of molecules, including reactive oxygen species; FCV has been used to reveal the real-time control of somatodendritic DA by endogenously generated hydrogen peroxide (H2O2 ). H2O2 can suppress DA release in the SNc, although not in the VTA [64], on a time course of a few seconds. This feature is not unique to dendrites; striatal synaptic DA release is similarly governed by the generation of endogenous peroxide [62,142]. Whether other inputs or neuromodulatory molecules regulate somatodendritic DA release, e.g., as suggested by microdialysis studies for ATP [143], has yet to be explored directly with the superior spatial and temporal resolution offered by FCV at CFMs.

Thus, in addition to the role that major synaptic inputs play in the regulation of DA neuron activity, these neuromodulatory inputs can also powerfully and variously gate the release of somatodendritic DA. As a consequence, these inputs will govern the action of somatodendritic DA release at D1 and D2 receptors in VTA and SN on the final signal integration and the ultimate activity of all dopaminoceptive output neurons of the VTA, SNc, and SNr.

Summary and Conclusions

FCV at CFMs offers a powerful approach for the study of endogenous somatodendritic DA release in real-time. With attention to appropriate scan and stimulation waveforms, pharmacological characterizations, and choice of species, DA can readily be detected with sub-second resolution and also discerned from possible contaminants and artifacts (including 5-HT, NE, DOPAC, pH, Ca2+, O2) that other electrochemical measurements with greater temporal (and possible spatial) resolution cannot, e.g., constant potential amperometry. Despite the greater possible resolutions of amperometry, FCV is actually better suited to electrochemical detection of somatodendritic DA release in the slice environment due to the likelihood of interference with DA signals by artifacts and contaminants that amperometry would not readily discern from DA.

Evidence from FCV has not contradicted the original proposal by Geffen et al. [2] that DA release in the SN is mediated by vesicular exocytosis as it is in axon terminals in striatum. In fact, somatodendritic DA release, like synaptic release in striatum, is regulated by depolarization, Ca2+, D2-like autoreceptors, and DA uptake via the DAT (and occasionally also the NET) as well as local neurotransmitters and other neuromodulators. Nonetheless, the extent or manner to which these features regulate somatodendritic release can differ from the control of synaptic DA release in striatum; however, whether these differences are merely quantitative or more qualitative remains to be clarified. Locally within the midbrain, the good spatial resolution of FCV at a CFM has also permitted comparison of the regulation of somatodendritic DA release in SNc with the adjacent cell body region, the VTA. Several discrete differences have already been noted between these two key DA cell groups (e.g., control by afferent input, degree of regulation by D2 receptors, uptake transporters), reinforcing the principle that these two regions and projections differ functionally and should always be considered separately. Moreover, the many functional differences in DA signals between these regions may yet have ramifications for the differential vulnerability of DA cells to pathophysiology, e.g., in Parkinson’s disease, where regionally specific DA handling mechanisms may pose differential risks to neurodegeneration.

Several key questions regarding the mechanisms that regulate somatodendritic DA remain to be resolved. Fundamental mechanisms underlying release continue to be questioned. Not only is it unclear what might constitute, ultrastructurally, the presynaptic active zone or other site for release or which “presynaptic” proteins are localized to dendritic plasmamembranes but also the possible conditions under which alternative mechanisms proposed for release to Ca2+ -dependent exocytosis, i.e., DAT reversal, could operate. It is also still unknown how somatodendritic DA is released in response to the tonic vs. phasic firing patterns and frequencies of activity of DA neurons and/or whether dendritic release of DA is under discrete control by the local excitability of remote dendrites or whether sub-second processes akin to short-term synaptic dynamics seen at DA synapses [144] operate in dendrites.

Somatodendritic DA release in the SN and VTA serves a number of critical functions in the regulation of animal behavior, from motivation to motion. Released DA in midbrain acts at predominantly extrasynaptic DA receptors to mediate autoreceptor control of DA cell activity and heteroreceptor regulation of synaptic input that is also reciprocal. In the thirty years since somatodendritic DA release was proposed, much has been learned about its role and how it is regulated, as summarized in this chapter. Excitingly, there are many remaining questions to address.

Acknowledgments

We acknowledge support from Novartis Pharma AG (ST) and the Paton Fellowship, University of Oxford (SJC).

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