Combined electrochemical and electrophysiological studies of monoamine overflow in rat hippocampal slices

In Vivo Electrochemical Biosensors: Recent Advances in Molecular Design, Electrode Materials, and Electrochemical Devices

Electrochemical biosensors provide powerful tools for dissecting the dynamically changing neurochemical signals in the living brain, which contribute to the insight into the physiological and pathological processes of the brain, due to their high spatial and temporal resolutions. Recent advances in the integration of in vivo electrochemical sensors with cross-disciplinary advances have reinvigorated the development of in vivo sensors with even better performance. In this Review, we summarize the recent advances in molecular design, electrode materials, and electrochemical devices for in vivo electrochemical sensors from molecular to macroscopic dimensions, highlighting the methods to obtain high performance for fulfilling the requirements for determination in the complex brain through flexible and smart design of molecules, materials, and devices. Also, we look forward to the development of next-generation in vivo electrochemical biosensors.

Instrumentation for fast-scan cyclic voltammetry combined with electrophysiology for behavioral experiments in freely moving animals

Fast-scan cyclic voltammetry is a unique technique for sampling dopamine concentration in the brain of rodents in vivo in real time. The combination of in vivo voltammetry with single-unit electrophysiological recording from the same microelectrode has proved to be useful in studying the relationship between animal behavior, dopamine release and unit activity. The instrumentation for these experiments described here has two unique features. First, a 2-electrode arrangement implemented for voltammetric measurements with the grounded reference electrode allows compatibility with electrophysiological measurements, iontophoresis, and multielectrode measurements. Second, we use miniaturized electronic components in the design of a small headstage that can be fixed on the rat's head and used in freely moving animals.

Implantable microelectrode arrays for simultaneous electrophysiological and neurochemical recordings

Implantable microfabricated microelectrode arrays represent a versatile and powerful tool to record electrophysiological activity across multiple spatial locations in the brain. Spikes and field potentials, however, correspond to only a fraction of the physiological information available at the neural interface. In urethane-anesthetized rats, microfabricated microelectrode arrays were implanted acutely for simultaneous recording of striatal local field potentials, spikes, and electrically evoked dopamine overflow on the same spatiotemporal scale. During these multi-modal recordings we observed (1) that the amperometric method used to detect dopamine did not significantly influence electrophysiological activity, (2) that electrical stimulation in the medial forebrain bundle (MFB) region resulted in electrochemically transduced dopamine transients in the striatum that were spatially heterogeneous within at least 200 microm, and (3) following MFB stimulation, dopamine levels and electrophysiological activity within the striatum exhibited similar temporal profiles. These neural probes are capable of incorporating customized microelectrode geometries and configurations, which may be useful for examining specific spatiotemporal relationships between electrical and chemical signaling in the brain.

Neural Probes for Concurrent Detection of Neurochemical and Electrophysiological Signals in vivo

Electrochemical sensing with microelectrode arrays provides a means for monitoring neurotransmitter dynamics across multiple locations within a micro-scale region of brain tissue. Here we present a multi-modal neural probe design for concurrent recording of neurochemical and electrophysiological signals in vivo. Prior to implantation, platinum sites on each array underwent platinum-black electroplating and Nafion electropolymerization, which increased sensitivity to dopamine by 74% and decreased sensitivity to common interferents by at least 89%. In a series of three rats, we applied various electrochemical waveforms to platinum sites and monitored neural activity on adjacent iridium sites. We found that chronoamperometry and constant-potential amperometry did not alter firing rates at +0.25, +0.50, and +0.75 V. In addition, we have demonstrated multi-modal recordings of striatal neurons in response to medial forebrain bundle stimulation.

Targeted screening for biogenic amine transporters: Potential applications for natural products

The biogenic amine transporters (BATs) are integral membrane proteins that terminate the actions of dopamine (DA), serotonin (5-HT) and norepinephrine (NE) by pumping these substrates from the extracellular space back into the nerve terminal. Numerous drugs and medications target BATs, acting as inhibitors or substrates. This paper will review some of the methods used to measure the activity of test drugs at the BATs. These methods include traditional uptake inhibition assays and transporter binding assays, as well as methods developed in our lab to determine if a test agent is a BAT substrate or inhibitor. Newer methods, developed in our lab, are used to determine the potency of test drugs as BAT substrates in a relatively high throughput manner. The potential application of these methods to characterizing natural products will be discussed in reference to results obtained with "purified" natural products, such as ephedrine stereoisomers.

The tipsy terminal: presynaptic effects of ethanol

Considerable evidence suggests that the synapse is the most sensitive CNS element for ethanol effects. Although most alcohol research has focussed on the postsynaptic sites of ethanol action, especially regarding interactions with the glutamatergic and GABAergic receptors, few such studies have directly addressed the possible presynaptic loci of ethanol action, and even fewer describe effects on synaptic terminals. Nonetheless, there is burgeoning evidence that presynaptic terminals play a major role in ethanol effects. The methods used to verify such ethanol actions range from electrophysiological analysis of paired-pulse facilitation (PPF) and spontaneous and miniature synaptic potentials to direct recording of ion channel activity and transmitter/messenger release from acutely isolated synaptic terminals, and microscopic observation of vesicular release, with a focus predominantly on GABAergic, glutamatergic, and peptidergic synapses. The combined data suggest that acute ethanol administration can both increase and decrease the release of these transmitters from synaptic terminals, and more recent results suggest that prolonged or chronic ethanol treatment (CET) can also alter the function of presynaptic terminals. These new findings suggest that future analyses of synaptic effects of ethanol should attempt to ascertain the role of presynaptic terminals and their involvement in alcohol's behavioral actions. Other future directions should include an assessment of ethanol's effects on presynaptic signal transduction linkages and on the molecular machinery of transmitter release and exocytosis in general. Such studies could lead to the formulation of new treatment strategies for alcohol intoxication, alcohol abuse, and alcoholism.

Differences in norepinephrine clearance in cerebellar slices from low-alcohol-sensitive and high-alcohol-sensitive rats

High-alcohol-sensitive (HAS) and low-alcohol-sensitive (LAS) rats were bred for sensitivity and insensitivity, respectively, to the sedative/hypnotic effects of ethanol. These rats also display differential sensitivity to the depressant effects of locally applied ethanol on cerebellar Purkinje neurons in vivo. We have found that LAS animals exhibit a greater influence of endogenous beta-adrenergic activity on neuronal responses to gamma-aminobutyric acid (GABA) and ethanol than do HAS animals. In the current study, we investigated the possibility that the regulation of synaptic norepinephrine levels by norepinephrine transporters could contribute to a differential beta-adrenergic influence on GABA and ethanol sensitivity between HAS and LAS rats. We locally applied norepinephrine from a glass micropipette into the various layers of cerebellar brain slices prepared from LAS and HAS rats, and recorded the levels of norepinephrine clearance by using Nafion-coated carbon-fiber microelectrodes. Norepinephrine clearance was significantly faster by approximately 64% in the Purkinje cell layer of HAS rats. No differences in norepinephrine clearance were found in the molecular or the granule layer between LAS and HAS rats. The catecholamine uptake inhibitor nomifensine reduced norepinephrine clearance in both rat lines. These findings support the hypothesis that regulation of synaptic norepinephrine levels by norepinephrine transporter activity in the Purkinje cell layer may contribute to the differential sensitivity of Purkinje neurons to ethanol and GABA in LAS and HAS rats.

In vivo electrochemical measurements of serotonin clearance in rat striatum: effects of neonatal 6-hydroxydopamine-induced serotonin hyperinnervation and serotonin uptake inhibitors

Diffusion and clearance of extracellular serotonin (5-HT) was examined using in vivo chronoamperometry with "delayed-pulse" recordings after pressure ejections of 1 to 60 picomoles 5-HT into rat striatum at a fixed distance from a Nafion-coated carbon fiber electrode. Signals obtained were identified based on the signal characteristics to consist of 5-HT. Clearance times of 5-HT decreased, while amplitudes and rise times increased with serotonergic hyperinnervation induced by neonatal 6-hydroxydopamine (6-OHDA) lesions of dopamine (DA) neurons. Local applications of the 5-HT uptake inhibitors zimelidine or fluoxetine, in conjunction with 5-HT ejections, produced increased clearance times in both normal and 6-OHDA-treated animals. Thus, direct in vivo evidence was obtained for the importance of high affinity nerve terminal uptake as a key mechanism for clearance of 5-HT from the extracellular space. Inhibitors of 5-HT uptake appear to prolong the extracellular presence of 5-HT by increasing its clearance time.

Carbon fibre micro-electrodes for concomitant in vivo electrophysiological and voltammetric measurements: no reciprocal influences

Differential pulse voltammetry and more recently cyclic voltammetry have been successfully used to monitor basal levels of endogenous chemicals by means of treated carbon fibre microbiosensors inserted in specific brain regions. In this study, feasibility of concomitant in vivo recordings of stable electrophysiological signals and basal ascorbate, catecholaminergic and indolaminergic voltammetric peaks at the same cerebral site by means of a single electrically treated carbon fibre micro electrode (microbiosensor) is presented. The results indicate that these two independent techniques can be combined in vivo at a single electrode, and that voltammetric measurements of unstimulated levels of extracellular compounds do not alter concomitant basal cell firing for a period long enough (more than 6 h) to allow pharmacological manipulations.

Ethanol inhibits the uptake of exogenous norepinephrine from the extracellular space of the rat cerebellum

Rapid chronoamperometric recordings using nafion-coated carbon fiber electrodes coupled with pressure-ejection of drugs were used to investigate the effects of ethanol on norepinephrine (NE)-containing nerve terminals in the urethane-anesthetized Fischer 344 rat. Local application of ethanol from a double-barrel micropipette did not produce detectable changes in extracellular levels of NE in the rat cerebellar cortex. However, when ethanol was applied prior to local application of NE, it was seen to inhibit the uptake of NE from the extracellular space. These results were compared to the effects seen from the local application of a known high-affinity uptake inhibitor, nomifensine. Nomifensine was found to inhibit the extracellular uptake of NE in rat cerebeller cortex similar to ethanol. Our results support the hypothesis that one effect of ethanol on the noradrenergic system of the rat cerebellum is an alteration in the uptake of NE into NE-containing nerve endings. In addition, the present data concerning ethanol-induced inhibition of NE clearance or uptake support our previous electrophysiological studies in which we found that ethanol can potentiate the modulatory effects of beta-agonists on GABA responses of cerebellar Purkinje neurons.

Simultaneous "real-time" electrochemical and electrophysiological recording in brain slices with a single carbon-fibre microelectrode

Many previous studies have demonstrated the value of carbon-fibre microelectrodes (CFMs) for single-unit activity recording and for fast cyclic voltammetry. In this report we show that these two independent methodologies can be combined at a single CFM and used to study simultaneous electrochemical and electrophysiological events in brain slices. In superfused slices of rat locus coeruleus, dorsal raphe and substantia nigra, we were able to record stable electrophysiological signals and stimulated monoamine efflux for periods of at least 2 h, thereby allowing quantitative pharmacological interventions. The simultaneous recording of amine efflux and unit activity at the same locus facilitates comparison of drug effects at pre- and post-synaptic sites. Furthermore, the system described here uses commercially available instrumentation. The circuitry is described and examples of its application are shown.

Overview of chemical sampling techniques

Although many of the ideas for sampling the chemical microenvironment of the brain were present, at least in nascent form, three decades ago or more, the last 10 years have witnessed a particularly spectacular surge of development, refinement, and use. We are now able to measure virtually any endogenous brain chemical in vivo at commendable levels of sensitivity, selectivity, and speed. The long-dreamt-of goal of being able to correlate neurochemical events with ongoing behavior and/or presentation of salient environmental cues and stimuli has already been largely achieved. Further refinements of existing techniques may well lead to levels of analysis inconceivable even a few years ago. The implications for theory-building and hypothesis-testing are enormous, particularly within such essentially virgin domains as behavioral neuroscience and biological psychiatry. These are truly exciting times.

Real-time monitoring of endogenous noradrenaline release in rat brain slices using fast cyclic voltammetry: 1. Characterisation of evoked noradrenaline efflux and uptake from nerve terminals in the bed nucleus of stria terminalis, pars ventralis

Fast cyclic voltammetry (FCV) at carbon fibre microelectrodes was used to monitor endogenous noradrenaline (NA) efflux in superfused slices of bed nucleus of stria terminalis pars ventralis (BSTV) in 'real time'. NA efflux was evoked by local electrical stimulation at bipolar tungsten stimulating electrodes. Confirmation of the identity of the released species as NA was made on the basis of anatomical, electrochemical and pharmacological proofs. Firstly, the signal matched the NA innervation density; efflux of monoamine was greater in BSTV than in the pars dorsalis of the nucleus. Secondly, the voltammogram of the released species was indistinguishable from those of the catecholamines NA and dopamine (DA) but dissimilar to that of the indoleamine serotonin (5-hydroxytryptamine, 5-HT). Thirdly, amine efflux was influenced in a predictable fashion by the drugs tested. Tetrodotoxin (10(-6) M) or omission of Ca2+ from the superfusate reversibly reduced amine efflux by 90.2 and 88.0% respectively. Ro 4-1284 (10(-6) M) decreased amine efflux by 75.8%. Desipramine (5 x 10(-8) M), the selective NA uptake blocker, significantly increased amine efflux and uptake half-life (to 214.3 and 389.5% of control respectively). Fluvoxamine (5 x 10(-7) M) and GBR 12909 (3 x 10(-7) M), blockers of 5-HT and DA uptake respectively, had no effect on amine efflux, although fluvoxamine caused a modest (91.0%) increase in the uptake half-life. Pargyline (2 x 10(-6) M) affected neither efflux nor uptake. The combined anatomical, electrochemical and pharmacological data confirm that the monoamine detected in BSTV by local electrical stimulation was NA. Stimulated NA efflux was stable and reproducible over at least 2.5 h (longest period tested). This study demonstrates the ability of FCV to selectively monitor endogenous NA efflux and uptake in 'real time' and with high spatial resolution.

Electrically-evoked release of norepinephrine in the rat cerebellum: an in vivo electrochemical and electrophysiological study

Norepinephrine (NE) release from the locus coeruleus (LC) afferents to the cerebellar cortex of urethane anesthetized rats was achieved by electrical stimulation of the locus coeruleus or by local administration of potassium into the cerebellum. Both methods evoked an overflow of NE-like electroactive species. Electrically-evoked and potassium-induced overflow of NE-like responses were found to be reversible and reproducible. Releases were not observed in cerebellar white matter, an area which is relatively devoid of monoamine containing terminals. Systemic administration of desipramine, a potent and selective norepinephrine re-uptake blocker, significantly augmented the electrically-evoked electrochemical responses. Measurements of evoked release taken using high-speed chronoamperometry support the idea that a predominant contributor to electrically-induced signals was NE. Electrophysiological recordings of single Purkinje cells were performed with the same Nafion-coated single carbon fiber electrodes used for electrochemical recordings. Electrical stimulation of the LC was seen to depress Purkinje cell firing rates; an increase in electroactive species was detected at the same site that paralleled the time course of the electrophysiological response. These studies provide further direct evidence that the LC norepinephrine-containing cells have a direct inhibitory effect on Purkinje cells in the cerebellum, and that both pre- and postsynaptic events can be measured with the same recording sensor.

Age-induced changes in single locus coeruleus brain transplants grown in oculo: an in vivo electrochemical study

Brain stem tissue from fetal Sprague-Dawley rats containing the nucleus locus coeruleus (LC) was transplanted into the anterior chamber of the eye of young adult host rats and was studied at 4-6 months (young control) or 24-28 months after grafting (old). High-speed in vivo electrochemical measurements were used to characterize the potassium-evoked synaptic overflow of norepinephrine (NE) in both young and aged LC brain grafts. The amplitudes of potassium-evoked NE overflow were attenuated in the aged grafts as compared to the young LC grafts. In addition, the rise times of potassium-evoked responses were longer in the old LC grafts than in the young transplants. In contrast, the NE content of aged LC grafts, as determined by high-performance liquid chromatography coupled with electrochemical detection (HPLC-EC), was only slightly diminished and not significantly different from the NE levels seen in young LC grafts. However, light microscopical evaluation using tyrosine-hydroxylase immunocytochemistry revealed pyknotic cell bodies and fluorescent accumulations in aged locus coeruleus transplants which were indicative of degeneration in these grafts. The present data demonstrate a significant age-related decline in the presynaptic function of NE-containing neurons in intraocular locus coeruleus transplants of Sprague-Dawley rats.