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R Mark Wightman

Publications and source records attributed to R Mark Wightman.

At least 37 records · Page 2Linked to original sources
PubMed2004

Tuning emissive states in electrogenerated chemiluminescence.

Electrogenerated chemiluminescence (ECL) arising from the reaction of radical ions has previously be shown to arise from a variety of states including excited singlets, triplets, excimers, and exciplexes. In this work we describe two systems that form emissive states in ECL with different properties than those when formed with photoluminescence. The first system involves the reaction of the anthracene radical anion with the radical cation of 4,N,N-trimethylaniline. ECL from this system exhibited an exciplex whose energy and intensity relative to the emission from the anthracene singlet could be tuned by adjusting the solvent permittivity and ionic strength. Under conditions considered extreme for electrochemical experiments, no added electrolyte in dimethoxyethane, the relative intensity of the anthracene-related exciplex, formed from the encounter complex, was 8 times greater and red-shifted from that generated by photoluminescence in the same solution with 100-fold exciplex partner added. In the second system examined, the benzophenone radical anion reacted with the radical cation of either phenoxathiin or 4-methoxythioanisole; the ECL emission was from the benzophenone triplet state and an excimer. The excimer, a species not seen with photoluminescence, predominated as the benzophenone concentration was elevated into the low millimolar range. The results from these two simple systems clearly demonstrate that the radical ion annihilation pathway of ECL can generate different emissive states than those formed following photoexcitation.

Journal Article↗
PubMed2004

Cannabinoids enhance subsecond dopamine release in the nucleus accumbens of awake rats.

Dopaminergic neurotransmission has been highly implicated in the reinforcing properties of many substances of abuse, including marijuana. Cannabinoids activate ventral tegmental area dopaminergic neurons, the main ascending projections of the mesocorticolimbic dopamine system, and change their spiking pattern by increasing the number of impulses in a burst and elevating the frequency of bursts. Although they also increase time-averaged striatal dopamine levels for extended periods of time, little is known about the temporal structure of this change. To elucidate this, fast-scan cyclic voltammetry was used to monitor extracellular dopamine in the nucleus accumbens of freely moving rats with subsecond timescale resolution. Intravenous administration of the central cannabinoid (CB1) receptor agonist, R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-(1-naphthalenyl) methanone mesylate, dose-dependently produced catalepsy, decreased locomotion, and reduced the amplitude of electrically evoked dopamine release while markedly increasing the frequency of detected (nonstimulated) dopamine concentration transients. The CB1 receptor antagonist [N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methylpyrazole-3-carboxamide] reversed and prevented all agonist-induced effects but did not show effects on dopamine release when injected alone. These data demonstrate that doses of a cannabinoid agonist known to increase burst firing produce ongoing fluctuations in extracellular dopamine on a previously unrecognized temporal scale in the nucleus accumbens.

Animals↗
PubMed2004

Dynamic gain control of dopamine delivery in freely moving animals.

Activity changes in a large subset of midbrain dopamine neurons fulfill numerous assumptions of learning theory by encoding a prediction error between actual and predicted reward. This computational interpretation of dopaminergic spike activity invites the important question of how changes in spike rate are translated into changes in dopamine delivery at target neural structures. Using electrochemical detection of rapid dopamine release in the striatum of freely moving rats, we established that a single dynamic model can capture all the measured fluctuations in dopamine delivery. This model revealed three independent short-term adaptive processes acting to control dopamine release. These short-term components generalized well across animals and stimulation patterns and were preserved under anesthesia. The model has implications for the dynamic filtering interposed between changes in spike production and forebrain dopamine release.

Adaptation, Physiological↗
PubMed2004

Dopamine operates as a subsecond modulator of food seeking.

The dopamine projection to the nucleus accumbens has been implicated in behaviors directed toward the acquisition and consumption of natural rewards. The neurochemical studies that established this link made time-averaged measurements over minutes, and so the precise temporal relationship between dopamine changes and these behaviors is not known. To resolve this, we sampled dopamine every 100 msec using fast-scan cyclic voltammetry at carbon-fiber microelectrodes in the nucleus accumbens of rats trained to press a lever for sucrose. Cues that signal the opportunity to respond for sucrose evoked dopamine release (67 +/- 20 nm) with short latency (0.2 +/- 0.1 sec onset). When the same cues were presented to rats naive to the cue-sucrose pairing, similar dopamine signals were not observed. Thus, cue-evoked increases in dopamine in trained rats reflected a learned association between the cues and sucrose availability. Lever presses for sucrose occurred at the peak of the dopamine surges. After lever presses, and while sucrose was delivered and consumed, no further increases in dopamine were detected. Rather, dopamine returned to baseline levels. Together, the results strongly implicate subsecond dopamine signaling in the nucleus accumbens as a real-time modulator of food-seeking behavior.

Acoustic Stimulation↗
PubMed2004

Functional microcircuitry in the accumbens underlying drug addiction: insights from real-time signaling during behavior.

An understanding of the neurobiological basis of drug addiction requires examination of real-time (subsecond) cellular and chemical responses in the brain reward system during drug-seeking and drug-taking behavior. Electrophysiological and electrochemical studies in the rodent nucleus accumbens have examined changes in cell firing and rapid dopamine signaling during crucial periods of behavioral responding for drugs, and show the associative nature of those signals. These findings are considered with respect to the functional microcircuitry in the nucleus accumbens that underlies goal-directed behavior and the role of this circuit in drug addiction.

Action Potentials↗
PubMed2004

Nomifensine amplifies subsecond dopamine signals in the ventral striatum of freely-moving rats.

The present experiment examined the effect of the dopamine transporter blocker nomifensine on subsecond fluctuations in dopamine concentrations, or dopamine transients, in the nucleus accumbens and olfactory tubercle. Extracellular dopamine was measured in real time using fast-scan cyclic voltammetry at micron-dimension carbon fibers in freely-moving rats. Dopamine transients occurred spontaneously throughout the ventral striatum in the absence of apparent sensory input or change in behavioral response. The frequency of dopamine transients increased at the presentation of salient stimuli to the rat (food, novel odors and unexpected noises). Administration of 7 mg/kg nomifensine amplified spontaneous dopamine transients by increasing both amplitude and duration, consistent with its known action at the dopamine transporter and emphasizing the dopaminergic origin of the signals. Moreover, nomifensine increased the frequency of detected dopamine transients, both during baseline conditions and at the presentation of stimuli, but more profoundly in the nucleus accumbens than in the olfactory tubercle. This difference was not explained by nomifensine effects on the kinetics of dopamine release and uptake, as its effects on electrically-evoked dopamine signals were similar in both regions. These findings demonstrate the heterogeneity of dopamine transients in the ventral striatum and establish that nomifensine elevates the tone of rapid dopamine signals in the brain.

Animals↗
PubMed2003

Overoxidation of carbon-fiber microelectrodes enhances dopamine adsorption and increases sensitivity.

The voltammetric responses of carbon-fiber microelectrodes with a 1.0 V and a 1.4 V anodic limit were compared in this study. Fast-scan cyclic voltammetry was used to characterize the response to dopamine and several other neurochemicals. An increase in the adsorption properties of the carbon fiber leads to an increase in sensitivity of 9 fold in vivo. However the temporal response of the sensor is slower with the more positive anodic limit. Increased electron transfer kinetics also causes a decrease in the relative sensitivity for dopamine vs. other neurochemicals, and a change in their cyclic voltammograms. Stimulated release in the caudate-putamen was pharmacologically characterized in vivo using Ro-04-1284 and pargyline, and was consistent with that expected for dopamine.

Animals↗
PubMed2003

Subsecond dopamine release promotes cocaine seeking.

The dopamine-containing projection from the ventral tegmental area of the midbrain to the nucleus accumbens is critically involved in mediating the reinforcing properties of cocaine. Although neurons in this area respond to rewards on a subsecond timescale, neurochemical studies have only addressed the role of dopamine in drug addiction by examining changes in the tonic (minute-to-minute) levels of extracellular dopamine. To investigate the role of phasic (subsecond) dopamine signalling, we measured dopamine every 100 ms in the nucleus accumbens using electrochemical technology. Rapid changes in extracellular dopamine concentration were observed at key aspects of drug-taking behaviour in rats. Before lever presses for cocaine, there was an increase in dopamine that coincided with the initiation of drug-seeking behaviours. Notably, these behaviours could be reproduced by electrically evoking dopamine release on this timescale. After lever presses, there were further increases in dopamine concentration at the concurrent presentation of cocaine-related cues. These cues alone also elicited similar, rapid dopamine signalling, but only in animals where they had previously been paired to cocaine delivery. These findings reveal an unprecedented role for dopamine in the regulation of drug taking in real time.

Animals↗
PubMed2003

Presynaptic dopaminergic function is largely unaltered in mesolimbic and mesostriatal terminals of adult rats that were prenatally exposed to cocaine.

Fast-scan cyclic voltammetry in brain slices and postmortem tissue content assessment were used to evaluate presynaptic dopaminergic function in the caudate putamen and nucleus accumbens of adult male rats (180+ days old) that were prenatally treated with either cocaine or saline. Experiments were carried out to test whether there were differences in dopamine release, reuptake, autoreceptor function or the tissue levels of dopamine and its metabolites between cocaine- and saline-exposed rats. We report that presynaptic dopaminergic function remains largely intact in adult rats that were prenatally exposed to cocaine. The ability of terminals in the caudate putamen and nucleus accumbens to release and regulate dopamine is unaltered by prenatal cocaine exposure. However the tissue content of dopamine in the caudate putamen was decreased, representing a diminution in the dopamine storage pool. We conclude, therefore, that behavioral changes that have previously been observed in rats that were prenatally exposed to cocaine are not mediated through alteration of presynaptic dopaminergic mechanisms in these brain regions.

3,4-Dihydroxyphenylacetic Acid↗
PubMed2003

Correlation of local changes in extracellular oxygen and pH that accompany dopaminergic terminal activity in the rat caudate-putamen.

Terminal activity causes an increase in local cerebral blood flow that can be quantified by measuring the accompanying increase in tissue oxygen. Alkaline pH changes can also follow neuronal activation. The purpose of these studies was to determine whether these changes in extracellular oxygen and pH correlate. Fast-scan cyclic voltammetry was used to detect changes in dopamine, pH and oxygen levels simultaneously in the caudate-putamen after electrical stimulation of the substantia nigra in anesthetized rats. The biphasic increases in oxygen and pH followed similar time courses, and were delayed a few seconds from the immediate release and uptake of dopamine. The changes following administration of neurotransmitter receptor antagonists as well as agents that modulate blood flow were identical for oxygen and pH. Two distinct mechanisms were identified that give rise to the oxygen and pH changes: blood vessel dilatation caused by nitric oxide synthesis after muscarinic receptor activation and adenosine receptor activation. We conclude that changes in blood flow accompanying terminal activity cause alkaline pH shifts by the rapid removal of carbon dioxide, a component of the extracellular brain buffering system.

Animals↗
PubMed2003

Real-time decoding of dopamine concentration changes in the caudate-putamen during tonic and phasic firing.

The fundamental process that underlies volume transmission in the brain is the extracellular diffusion of neurotransmitters from release sites to distal target cells. Dopaminergic neurons display a range of activity states, from low-frequency tonic firing to bursts of high-frequency action potentials (phasic firing). However, it is not clear how this activity affects volume transmission on a subsecond time scale. To evaluate this, we developed a finite-difference model that predicts the lifetime and diffusion of dopamine in brain tissue. We first used this model to decode in vivo amperometric measurements of electrically evoked dopamine, and obtained rate constants for release and uptake as well as the extent of diffusion. Accurate predictions were made under a variety of conditions including different regions, different stimulation parameters and with uptake inhibited. Second, we used the decoded rate constants to predict how heterogeneity of dopamine release and uptake sites would affect dopamine concentration fluctuations during different activity states in the absence of an electrode. These simulations show that synchronous phasic firing can produce spatially and temporally heterogeneous concentration profiles whereas asynchronous tonic firing elicits uniform, steady-state dopamine concentrations.

Action Potentials↗
PubMed2003

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

BACKGROUND: Dopamine is a potent neuromodulator in the brain, influencing a variety of motivated behaviors and involved in several neurologic diseases. Measurements of extracellular dopamine in the brains of experimental animals have traditionally focused on a tonic timescale (minutes to hours). However, dopamine concentrations are now known to fluctuate on a phasic timescale (subseconds to seconds). APPROACH: Fast-scan cyclic voltammetry provides analytical chemical measurements of phasic dopamine signals in the rat brain. CONTENT: Procedural aspects of the technique are discussed, with regard to appropriate use and in comparison with other methods. Finally, examples of data collected using fast-scan cyclic voltammetry are summarized, including naturally occurring dopamine transients and signals arising from electrical stimulation of dopamine neurons. SUMMARY: Fast-scan cyclic voltammetry offers real-time measurements of changes in extracellular dopamine concentrations in vivo. With its subsecond time resolution, micrometer-dimension spatial resolution, and chemical selectivity, it is the most suitable technique currently available to measure transient concentration changes of dopamine.

Animals↗
PubMed2002

Frequency of dopamine concentration transients increases in dorsal and ventral striatum of male rats during introduction of conspecifics.

Transient, elevated concentrations of extracellular dopamine were characterized in the dorsal and ventral striatum of male rats during solitude, brief interaction with a conspecific, and copulation. Conspecific rats were systematically presented to male rats and allowed to interact for 30 sec; the males were kept in solitude between each presentation. During these episodes, 125 dopamine concentration transients from 17 rats were detected with fast-scan cyclic voltammetry at carbon-fiber microelectrodes (peak amplitude, 210 +/- 10 nm; duration, 530 +/- 20 msec). The frequency of dopamine transients increased sixfold during conspecific episodes compared with solitude. However, the phasic dopamine activity habituated on the second presentation of the conspecifics. When males were allowed to copulate with receptive females, additional dopamine transients were observed at frequencies approximately 20% of those during the previous interaction episodes. A subset of these transients immediately preceded intromission. Overall, phasic dopamine activity appeared to be associated with input from multiple sensory modalities and was followed by a variety of approach and appetitive behaviors, consistent with electrophysiological observations of dopaminergic neuron burst-firing. In summary, (1) dopamine concentration transients occur in awake rats during solitude, in the absence of overt external cues; (2) dopamine transients are significantly more frequent in the presence of a conspecific, although this effect habituates; and (3) dopamine transients are less frequent during copulation than during brief conspecific episodes. These results establish for the first time that transient dopamine fluctuations occur throughout the dorsal and ventral striatum and demonstrate that they are more frequent with salient stimuli that elicit a response behavior.

Animals↗
PubMed2002

Dopamine transport into a single cell in a picoliter vial.

The analysis of chemical events in small volumes requires careful manipulation of samples and sensitive detection methods. Here, we describe the measurement of the neurotransmitter dopamine in a picoliter vial with electrochemical techniques. The vials were fabricated from fused-silica capillaries that provided a transparent container suitable for the observation and manipulation of a biological cell, sample solutions, and electrodes. Evaporation of the sample was prevented with a mineral oil layer, allowing for experiments lasting for several minutes. The small volume of these vials (100-200 pL) allows rapid mixing of all of the solution reagents. Similarly, the small volume allows exhaustive electrolysis of the vial contents with a 3-microm radius, disk-shaped carbon fiber microelectrode within 60 s. Fast-scan cyclic voltammetry at carbon fiber microelectrodes was used to monitor the concentration of analyte in the vial without depleting its contents. The concentration of dopamine introduced by pneumatic injection remained stable when sampled by cyclic voltammetry, and no evidence for adsorption to the walls was observed. However, when the vial contained a single HEK-293 cell transfected to express the dopamine transporter, the dopamine concentration decreased with time at a rate consistent with the uptake kinetics mediated by the transporter located on the cell membrane.

Cell Line↗
PubMed2002

Release and uptake of catecholamines in the bed nucleus of the stria terminalis measured in the mouse brain slice.

The release and clearance of electrically evoked catecholamine (CA) in the ventral portion of the bed nucleus of the stria terminalis (BSTV) in mouse brain slices was evaluated with fast-scan cyclic voltammetry at carbon-fiber microelectrodes (CFME). Uptake in this region was observed to be markedly slower than in the caudate putamen (CPu). Clearance rates were reduced in the BSTV in both norepinephrine transporter knockout (NET KO) and dopamine transporter knockout (DAT KO) mice when compared to results in wild-type (WT) mice. However, uptake was faster in the BSTV in both the DAT and NET KO mice than in the CPu of DAT KO mice. This indicates that both transporters play a role in CA clearance in the BSTV. The transporters restrict extracellular CA to the general area of the BSTV, as revealed by the diminished signal as the CFME is moved sequentially further and further from the site where CA release is evoked. However, in slices from the DAT KOs and NET KOs, CA release could be observed outside of the BSTV region during such experiments. These results show that the low rate uptake in the BSTV facilitates extrasynaptic diffusion of catecholamine, but that uptake still regulates and limits the range of the transmitter to the region. Slower clearance from the extracellular fluid allows the released CA to act as a volume transmitter and diffuse to distant sites within the region to exert its neurochemical action.

Alkaline Phosphatase↗