Simultaneous electrochemical and unit recording measurements: characterization of the effects of D-amphetamine and ascorbic acid on neostriatal neurons

Tracking tonic dopamine levels in vivo using multiple cyclic square wave voltammetry

For over two decades, fast-scan cyclic voltammetry (FSCV) has served as a reliable analytical method for monitoring dopamine release in near real-time in vivo. However, contemporary FSCV techniques have been limited to measure only rapid (on the order of seconds, i.e. phasic) changes in dopamine release evoked by either electrical stimulation or elicited by presentation of behaviorally salient stimuli, and not slower changes in the tonic extracellular levels of dopamine (i.e. basal concentrations). This is because FSCV is inherently a differential method that requires subtraction of prestimulation tonic levels of dopamine to measure phasic changes relative to a zeroed baseline. Here, we describe the development and application of a novel voltammetric technique, multiple cyclic square wave voltammetry (M-CSWV), for analytical quantification of tonic dopamine concentrations in vivo with relatively high temporal resolution (10 s). M-CSWV enriches the electrochemical information by generating two dimensional voltammograms which enable high sensitivity (limit of detection, 0.17 nM) and selectivity against ascorbic acid, and 3,4-dihydroxyphenylacetic acid (DOPAC), including changes in pH. Using M-CSWV, a tonic dopamine concentration of 120 ± 18 nM (n = 7 rats, ± SEM) was determined in the striatum of urethane anethetized rats. Pharmacological treatments to elevate dopamine by selectively inhibiting dopamine reuptake and to reduce DOPAC by inhibition of monoamine oxidase supported the selective detection of dopamine in vivo. Overall, M-CSWV offers a novel voltammetric technique to quantify levels and monitor changes in tonic dopamine concentrations in the brain to further our understanding of the role of dopamine in normal behavior and neuropsychiatric disorders.

Dysregulation of corticostriatal ascorbate release and glutamate uptake in transgenic models of Huntington's disease

SIGNIFICANCE

Dysregulation of cortical and striatal neuronal processing plays a critical role in Huntington's disease (HD), a dominantly inherited condition that includes a progressive deterioration of cognitive and motor control. Growing evidence indicates that ascorbate (AA), an antioxidant vitamin, is released into striatal extracellular fluid when glutamate is cleared after its release from cortical afferents. Both AA release and glutamate uptake are impaired in the striatum of transgenic mouse models of HD owing to a downregulation of glutamate transporter 1 (GLT1), the protein primarily found on astrocytes and responsible for removing most extracellular glutamate. Improved understanding of an AA-glutamate interaction could lead to new therapeutic strategies for HD.

RECENT ADVANCES

Increased expression of GLT1 following treatment with ceftriaxone, a beta-lactam antibiotic, increases striatal glutamate uptake and AA release and also improves the HD behavioral phenotype. In fact, treatment with AA alone restores striatal extracellular AA to wild-type levels in HD mice and not only improves behavior but also improves the firing pattern of neurons in HD striatum.

CRITICAL ISSUES

Although evidence is growing for an AA-glutamate interaction, several key issues require clarification: the site of action of AA on striatal neurons; the precise role of GLT1 in striatal AA release; and the mechanism by which HD interferes with this role.

FUTURE DIRECTIONS

Further assessment of how the HD mutation alters corticostriatal signaling is an important next step. A critical focus is the role of astrocytes, which express GLT1 and may be the primary source of extracellular AA.

Amphetamine's dose-dependent effects on dorsolateral striatum sensorimotor neuron firing

Amphetamine elicits motoric changes by increasing the activity of central neurotransmitters such as dopamine and serotonin, but how these neurochemical signals are transduced into motor commands is unclear. The dorsolateral striatum (DLS), a component of the cortico-subcortical reentrant motor loop, contains abundant neurotransmitter transporters that amphetamine could affect. It has been hypothesized that DLS medium spiny neurons contribute to amphetamine's motor effects. To study striatal activity contributing to amphetamine-induced movements, activity of DLS neurons related to vertical head movement was recorded while tracking head movements before and after acute amphetamine injection. Relative to saline, all amphetamine doses induced head movements above pre-injection levels, revealing an inverted U-shaped dose-response function. Lower doses (1 mg/kg and 2 mg/kg, intraperitoneal) induced a greater number of long (distance and duration) movements than the high dose (4 mg/kg), which induced stereotypy. Firing rates (FR) of individual head movement neurons were compared before and after injection during similar head movements, defined by direction, distance, duration, and apex. Changes in FR induced by amphetamine were co-determined by dose and pre-injection FR of the neuron. Specifically, all doses increased the FRs of slower firing neurons but decreased the FRs of faster firing neurons. The magnitudes of elevation or reduction were greater at lower doses, but less pronounced at the high dose, forming an inverted U function. Modulation of DLS firing may interfere with sensorimotor processing. Furthermore, pervasive elevation of slow firing neurons' FRs may feed-forward and increase excitability in thalamocortical premotor areas, contributing to the increased movement initiation rate.

Higher sensitivity dopamine measurements with faster-scan cyclic voltammetry

Fast-scan cyclic voltammetry (FSCV) with carbon-fiber microelectrodes has been successfully used to detect catecholamine release in vivo. Generally, waveforms with anodic voltage limits of 1.0 or 1.3 V (vs Ag/AgCl) are used for detection. The 1.0 V excursion provides good temporal resolution but suffers from a lack of sensitivity. The 1.3 V excursion increases sensitivity but also increases response time, which can blur the detection of neurochemical events. Here, the scan rate was increased to improve the sensitivity of the 1.0 V excursion while maintaining the rapid temporal response. However, increasing scan rate increases both the desired faradaic current response and the already large charging current associated with the voltage sweep. Analog background subtraction was used to prevent the analog-to-digital converter from saturating from the high currents generated with increasing scan rate by neutralizing some of the charging current. In vitro results with the 1.0 V waveform showed approximately a 4-fold increase in signal-to-noise ratio with maintenance of the desired faster response time by increasing scan rate up to 2400 V/s. In vivo, stable stimulated release was detected with an approximate 4-fold increase in peak current. The scan rate of the 1.3 V waveform was also increased, but the signal was unstable with time in vitro and in vivo. Adapting the 1.3 V triangular wave into a sawhorse design prevented signal decay and increased the faradaic response. The use of the 1.3 V sawhorse waveform decreased the detection limit of dopamine with FSCV to 0.96 ± 0.08 nM in vitro and showed improved performance in vivo without affecting the neuronal environment. Electron microscopy showed dopamine sensitivity is in a quasi-steady state with carbon-fiber microelectrodes scanned to potentials above 1.0 V.

How addictive drugs disrupt presynaptic dopamine neurotransmission

The fundamental principle that unites addictive drugs appears to be that each enhances synaptic dopamine by means that dissociate it from normal behavioral control, so that they act to reinforce their own acquisition. This occurs via the modulation of synaptic mechanisms that can be involved in learning, including enhanced excitation or disinhibition of dopamine neuron activity, blockade of dopamine reuptake, and altering the state of the presynaptic terminal to enhance evoked over basal transmission. Amphetamines offer an exception to such modulation in that they combine multiple effects to produce nonexocytic stimulation-independent release of neurotransmitter via reverse transport independent from normal presynaptic function. Questions about the molecular actions of addictive drugs, prominently including the actions of alcohol and solvents, remain unresolved, but their ability to co-opt normal presynaptic functions helps to explain why treatment for addiction has been challenging.

Acquiring local field potential information from amperometric neurochemical recordings

Simultaneous acquisition of in vivo electrophysiological and neurochemical information is essential for understanding how endogenous neurochemicals modulate the dynamics of brain activity. However, up to now such a task has rarely been accomplished due to the major technical challenge of operating two independent recording systems simultaneously in real-time. Here we propose a simpler solution for achieving this goal by using only a standard electrochemical technique--amperometry. To demonstrate its feasibility, we compared amperometric signals with simultaneously recorded local field potential (LFP) signals. We found that the high frequency component (HFC) of the amperometric signals did not reflect neurochemical fluctuations, but instead it resembled LFPs in several aspects, including: (1) coherent spectral fluctuations; (2) clear characterization of different brain states; (3) identical hippocampal theta depth profile. As such, our findings provide the first demonstration that both LFP and local neurochemical information can be simultaneously acquired from electrochemical sensors alone.

Ascorbate modulation of sensorimotor processing in striatum of freely moving rats

The striatum, which receives projections from the entire cortical mantle, is highly responsive to sensorimotor activity. Because either systemic or intra-striatal injections of ascorbate (AA) influence behavior known to involve striatal circuits, it is possible that the level of striatal AA, which is known to fluctuate with behavioral activation, directly alters striatal neuronal processing. To test this hypothesis, we recorded the activity of 94 presumed medium spiny striatal neurons in behaving rats treated with AA or vehicle and examined firing rate during periods of quiescence and sensorimotor stimulation (e.g., stroking of the whiskers, mid-back, and rump). Slow-scan voltammetry was used in separate rats to determine the extent to which AA treatment elevated striatal AA. Vehicle-treated rats had relatively slow basal firing rates at rest that routinely increased during sensorimotor stimulation. Comparable results were obtained in rats treated with 100 mg/kg AA, which failed to alter AA levels in striatum. Dose-dependent increases in striatal AA, however, occurred after injection of 500 and 1000 mg/kg AA, and at these doses, there was a significant decrease in the number of sensorimotor-related excitations. In fact, treatment with 1000 mg/kg AA reversed a significant proportion of excitations to inhibitions. Our results substantiate the role of the striatum in sensorimotor processing and emphasize extracellular AA as a modulator of striatal neuronal function.

Combined treatment of ascorbic acid or alpha-tocopherol with dopamine receptor antagonist or nitric oxide synthase inhibitor potentiates cataleptic effect in mice

RATIONALE

Drugs like haloperidol (Hal) that decrease dopamine (DA) neurotransmission in the striatum induce catalepsy in rodents and Parkinson disease-like symptoms in humans. Nitric oxide synthase (NOS) inhibitors interfere with motor activity, disrupting rodent exploratory behavior and inducing catalepsy. Catalepsy induced by NOS inhibitors probably involves striatal DA-mediated neurotransmission. Antioxidants such as ascorbic acid (vitamin C) and alpha-tocopherol (vitamin E) have also been shown to interfere with movement modulation and the DA system.

OBJECTIVE

The objective of the study is to investigate if the antioxidants vitamins C and E would influence the catalepsy produced by Hal and NOS inhibitors.

METHODS

The effects of the following treatments on catalepsy were examined using the hanging-bar test on male Swiss mice (25-30 g): (1) vitamin C (30-1,000 mg/kg)xHal (1 mg/kg); (2) vitamin C (90-1,000 mg/kg)xN (G)-nitro-L: -arginine (LNOARG, 10 and 40 mg/kg); (3) vitamin C (300 mg/kg)xN (G)-nitro-L: -arginine methylester (LNAME, 20-80 mg/kg); (4) vitamin C (300 mg/kg) x 7-nitroindazole (7NI, 3-50 mg/kg); (5) vitamin C (90 mg/kg i.p.) x LNOARG [40 mg/kg twice a day during 4 days (subchronic treatment)]; (7) vitamin E (3-100 mg/kg) x Hal (1 mg/kg); and (6) vitamin E (3-100 mg/kg) x LNOARG (40 mg/kg).

RESULTS

Vitamin C enhanced the catalepsy produced by NOS inhibitors and Hal. Treatment with vitamin C did not affect tolerance to LNOARG cataleptic effect induced by subchronic treatment. Vitamin E potentiated the catalepsy induced by LNOARG at all doses tested; in contrast, catalepsy induced by Hal was enhanced only by the dose of 100 mg/kg.

CONCLUSIONS

Results support an involvement of dopaminergic and nitrergic systems in motor behavior control and provide compelling evidence that combined administration of the antioxidants vitamins C and E with either Hal or NOS inhibitors exacerbates extrapyramidal effects. Further studies are needed to assess possible clinical implications of these findings.

Ascorbate modulates pentylenetetrazol-induced convulsions biphasically

Ascorbate is an antioxidant vitamin that is found in high concentrations in the brain which seems to have neuroprotective properties in some experimental models of excitotoxic neurological disorders, including convulsive behavior and reactive species-related damage. In this study we tested whether ascorbate (30, 100 or 300 mg/kg, i.p.) protects against the convulsions, protein carbonylation and inhibition of Na(+),K(+)-ATPase activity induced by pentylenetetrazol (PTZ; 1.8 micromol/striatum), a classical convulsant agent that has been fairly used for the study of epilepsy and screening of new compounds with antiepileptic activity. The intrastriatal injection of PTZ caused convulsive behavior in a dose-dependent manner and an increase in the total protein carbonyl content of the injected striatum. However, duration of PTZ-induced convulsive episodes did not correlate with protein carbonyl content of the injected striatum. Ascorbate, at high doses (300 mg/kg), protected against PTZ-induced convulsions, protein carbonylation and inhibition of Na(+),K(+)-ATPase activity in the rat striatum, further suggesting a anticonvulsant and neuroprotective role for this vitamin. Conversely, intermediate doses of ascorbate (100 mg/kg) potentiated the duration of the convulsive episodes, but had no additive effects on protein carbonylation or Na(+),K(+)-ATPase activity inhibition induced by PTZ. Low doses of ascorbate (30 mg/kg) prevented PTZ-induced increase of total striatal carbonyl protein content, but did not alter PTZ-induced convulsions and Na(+),K(+)-ATPase activity inhibition. Collectively, these data indicate that the anticonvulsant activity of ascorbate is not related to its antioxidant action and support a dual role for this compound as a neuroprotective agent, since while it protects against PTZ-induced cellular oxidative damage, it has a biphasic effect on PTZ-induced convulsions.

Opposite effects of sulpiride and SCH 23390 on ethanol-induced striatal ascorbic acid release in intact and 6-hydroxydopamine lesioned rats

The effects of L-sulpiride and SCH 23390 on ethanol-induced striatal ascorbic acid (AA) release in normal and 6-hydroxydopamine-lesioned rats were studied by using microdialysis coupled to high performance liquid chromatography with electrochemical detection. Ethanol (3.0 g/kg i.p.) significantly stimulated striatal AA release by 200% above the baseline in normal, 6-hydroxydopamine-lesioned, and reserpine-treated rats. L-Sulpiride, a dopamine D(2) antagonist, at the dose of 100 mg/kg i.p., decreased basal ascorbic acid release and showed an inhibitory tendency on ethanol-induced ascorbic acid release. However, at the higher dose of 200 mg/kg i.p., L-sulpiride significantly inhibited ethanol-induced ascorbic acid release in both normal and 6-hydroxydopamine-treated rats. SCH 23390, a dopamine D(1) antagonist, at the doses of 0.5 and 1.0 mg/kg i.p., potentiated ethanol-induced ascorbic acid release in normal rats. However, the potentiation of SCH 23390 on ethanol effect was not significant in 6-hydroxydopamine-treated rats at the dose of 1.0 mg/kg i.p. The present study demonstrates that opposite actions exist in the regulation of ethanol-induced ascorbic acid release in the striatum by dopamine D(1) and D(2) receptor blockade. It also suggests that the postsynaptic dopamine receptors are involved in mediation of ethanol-induced ascorbic acid release in rat striatum.

Modulatory effects of ascorbate, alone or with haloperidol, on a lever-release conditioned avoidance response task

Pretreatment with ascorbate, a modulator of dopamine transmission in the striatum, enhances the ability of haloperidol, a dopamine antagonist, to induce catalepsy and block the motor-activating effects of amphetamine. The present study extended this line of work to a lever-release version of the conditioned avoidance response (CAR) task, which is highly sensitive to changes in striatal dopamine. Adult male rats were trained to avoid footshock by releasing a lever within 500 ms of tone onset. Ascorbate (100 and 1000 mg/kg, IP) or vehicle was tested either alone or in conjunction with haloperidol (0.01 and 0.05 mg/kg, SC). Compared to vehicle pretreatment, 1000 mg/kg ascorbate alone or in combination with haloperidol impaired CAR performance by increasing avoidance latency. Latency to escape footshock was not impaired, ruling out a generalized motor deficit. In contrast, 100 mg/kg ascorbate alone or in combination with haloperidol had no consistent effects on CAR performance, even at a haloperidol dose (0.005 mg/kg, SC) known to potentiate dopamine transmission by preferentially blocking autoreceptors. Collectively, these results support an antidopaminergic action of ascorbate on striatal function, but suggest that this effect requires relatively high systemic doses.

Ascorbic acid antagonizes ethanol-induced locomotor activity in the open-field

It has been reported that ascorbic acid (AA) antagonizes the physiological and behavioral effects of dopamine (DA). AA reduces locomotor activity induced by dopaminergic agonist drugs. Also, AA amplifies the action of antidopaminergic drugs. Ethanol, like other drugs, produces a release of DA in the mesolimbic pathway, and at some doses, induces locomotor activity in mice. The ethanol-induced locomotor activity could be dopamine-dependent because it can be reduced by antidopaminergic drugs. In the present study, we investigated whether an acute administration of AA reduces ethanol-induced locomotor behavior. AA, at doses (0.0, 21.85, 87.5, 175, 350. and 1400 mg/kg) was injected i.p. into mice, 0, 30, 60, or 90 min before an i.p. injection of ethanol (0.0, 0.8, 1.6, 2.4, and 3.2 g/kg). Locomotor activity was evaluated in open-field chambers. Our results showed that AA (350 and 1400 mg/kg) reduced ethanol-induced locomotor activity when injected 30 min before ethanol treatment. This effect was lost when ethanol was administered 90 min after AA injection. AA also reduced locomotor activity produced by d-amphetamine and methanol. The results support a pro-dopaminergic action of ethanol, and suggest a common dopaminergic pathway for the drugs of abuse in locomotor activity.

Iontophoresis in the neostriatum of awake, unrestrained rats: differential effects of dopamine, glutamate and ascorbate on motor- and nonmotor-related neurons

The neostriatum and its major afferent transmitters, dopamine and glutamate, play a critical role in behavior, but relatively little information is available on their postsynaptic effects in behaving animals. As a first step in addressing this shortcoming, single-unit electrophysiology was combined with iontophoresis in the neostriatum of awake, unrestrained rats. Relative to periods of quiet rest, most neurons (58 of 77) changed discharge rate in close temporal association with movement, while the remainder showed no such relationship. When animals resumed a resting posture, iontophoretic current-response curves were established for dopamine and glutamate as well as for ascorbate, a modulator of neostriatal function released from glutamatergic terminals. Application of either glutamate or ascorbate produced current-dependent increases in activity in all neurons, although this effect was somewhat less pronounced for nonmotor cells. In both types of neurons, the excitatory effect of ascorbate either diminished or shifted to an inhibition at high ejection currents. Dopamine, on the other hand, routinely excited motor-related, but inhibited nonmotor-related neurons. Further assessment of motor-related neurons revealed that in most cases the excitatory effects of either glutamate or dopamine alone were supra-additive when these compounds were either administered together or co-administered with ascorbate. Our results suggest that the response of neostriatal neurons to glutamate or dopamine depends, at least in part, on the motor responsiveness of these cells. Motor-related neurons, moreover, respond to the co-administration of glutamate and dopamine with synergistic increases in firing rate. Ascorbate also influences neostriatal activity, but the postsynaptic action of this substance cannot be explained as a simple interaction with either glutamatergic or dopaminergic mechanisms.

A vitamin as neuromodulator: ascorbate release into the extracellular fluid of the brain regulates dopaminergic and glutamatergic transmission

Ascorbate is an antioxidant vitamin that the brain accumulates from the blood supply and maintains at a relatively high concentration under widely varying conditions. Although neurons are known to use this vitamin in many different chemical and enzymatic reactions, only recently has sufficient evidence emerged to suggest a role for ascorbate in interneuronal communication. Ascorbate is released from glutamatergic neurons as part of the glutamate reuptake process, in which the high-affinity glutamate transporter exchanges ascorbate for glutamate. This heteroexchange process, which also may occur in glial cells, ensures a relatively high level of extracellular ascorbate in many forebrain regions. Ascorbate release is regulated, at least in part, by dopaminergic mechanisms, which appear to involve both the D1 and D2 family of dopamine receptors. Thus, amphetamine, GBR-12909, apomorphine, and the combined administration of D1 and D2 agonists all facilitate ascorbate release from glutamatergic terminals in the neostriatum, and this effect is blocked by dopamine receptor antagonists. Even though the neostriatum itself contains a high concentration of dopamine receptors, the critical site for dopamine-mediated ascorbate release in the neostriatum is the substantia nigra. Intranigral dopamine regulates the activity of nigrothalamic efferents, which in turn regulate thalamocortical fibers and eventually the glutamatergic corticoneostriatal pathway. In addition, neostriatonigral fibers project to nigrothalamic efferents, completing a complex multisynaptic loop that plays a major role in neostriatal ascorbate release. Although extracellular ascorbate appears to modulate the synaptic action of dopamine, the mechanisms underlying this effect are unclear. Evidence from receptor binding studies suggests that ascorbate alters dopamine receptors either as an allosteric inhibitor or as an inducer of iron-dependent lipid peroxidation. The applicability of these studies to dopamine receptor function, however, remains to be established in view of reports that ascorbate can protect against lipid peroxidation in vivo. Nevertheless, ample behavioral evidence supports an antidopaminergic action of ascorbate. Systemic, intraventricular, or intraneostriatal ascorbate administration, for example, attenuates the behavioral effects of amphetamine and potentiates the behavioral response to haloperidol. Some of these behavioral effects, however, may be dose-dependent in that treatment with relatively low doses of ascorbate has been reported to enhance dopamine-mediated behaviors. Ascorbate also appears to modulate glutamatergic transmission in the neostriatum. In fact, by facilitating glutamate release, ascorbate may indirectly oppose the action of dopamine, though the nature of the neostriatal dopaminergic-glutamatergic interaction is far from settled. Ascorbate also may alter the redox state of the NMDA glutamate receptor thus block NMDA-gated channel function.(ABSTRACT TRUNCATED AT 400 WORDS)

A system for measuring electrophysiological multiple unit activity and extracellular dopamine concentration at single electrodes

A technique is described for measuring electrophysiological multiple-unit activity and extracellular dopamine concentration with in vivo voltammetry at an array of 8 electrodes. In addition, new procedures in the construction of the voltammetric electrode that reduce the effects of capacitance and improve sensitivity are outlined. A system of relays controlling the connections of the electrode and associated systems is also detailed. Both in vitro and in vivo voltammetric selectivity tests indicate an almost exclusive response to dopamine. A large increase in extracellular dopamine concentration was detected in response to 2.5 mg/kg D-amphetamine. Both prestimulus and cortical stimulation evoked increases in striatal multiple-unit activity were measured.

Ascorbic acid in the brain

Ascorbic acid is highly concentrated in the central nervous system. Measurement of the extracellular concentration of ascorbate in animals, mainly by the technique of voltammetry in vivo, has demonstrated fluctuation in release from neuropil, both spontaneously and in response to physical stimulation of the animal and to certain drugs. Although in the adrenal medulla ascorbate is co-released with catecholamines, release of ascorbate from brain cells is associated principally with the activity of glutamatergic neurones, mainly by glutamate-ascorbate heteroexchange across cell membranes of neurones or glia. This phenomenon is discussed in relation to a possible role of ascorbate as a neuromodulator or neuroprotective agent in the brain.

Effect of ethanol on ascorbate release in the nucleus accumbens and striatum of freely moving rats

An in vivo voltammetry technique was used to monitor the extracellular ascorbate (AA) concentration in the nucleus accumbens and striatum of unanesthetized, freely moving rats. A single injection of ethanol, 1.0 g/kg intraperitoneally (IP), induced a significant increase in extracellular AA concentration in both the nucleus accumbens and striatum. This effect was dose dependent within a dose range from 0.5-2.0 g/kg. 4-Methylpyrazole (50 mg/kg, IP), which inhibits alcoholdehydrogenase, could not prevent the increase in AA concentration, evoked by ethanol. Furthermore, systemic administration of acetaldehyde (20 mg/kg, IP), the main metabolite of ethanol, did not have any effect on the level of AA in the nucleus accumbens or striatum. These results show that ethanol can alter the brain extracellular AA levels and that this effect seems to be attributed to ethanol itself and not to acetaldehyde. Consequently, these results indicate that a role for AA in the action of ethanol in the brain should be considered.

Dopamine-, NMDA- and sigma-receptor antagonists exert differential effects on basal and amphetamine-induced changes in neostriatal ascorbate and DOPAC in awake, behaving rats

Amphetamine and other dopamine agonists elevate the extracellular level of neostriatal ascorbate, which has been shown to modulate neuronal function. To assess the receptor mechanisms underlying neostriatal ascorbate release, drug-induced changes in both basal and amphetamine-induced ascorbate release were monitored voltammetrically in the neostriatum of freely moving rats. A variety of dopamine receptor antagonists decreased basal ascorbate and reversed the increase induced by 2.5 mg/kg D-amphetamine. Thus, compared to vehicle treatment, administration of classical (haloperidol) and atypical (clozapine) neuroleptics or selective D1 (SCH-23390) and D2 (sulpiride) antagonists completely reversed the amphetamine-induced rise in ascorbate and also lowered basal levels by 20-40%. These same effects occurred following injection of dizocilpine (MK-801), a non-competitive NMDA antagonist, whereas BMY-14802, a sigma ligand, reversed the amphetamine-induced rise without altering basal levels. Simultaneous measurements of extracellular DOPAC, a major dopamine metabolite, revealed that haloperidol, clozapine, sulpiride and BMY-14802 elevated basal levels and reversed the amphetamine-induced decline. Dizocilpine also increased basal DOPAC but failed to alter the DOPAC response to amphetamine, whereas both basal and amphetamine-induced changes in DOPAC were unaffected by SCH-23390. A combination of subthreshold doses of SCH-23390 and sulpiride, however, reversed both the amphetamine-induced release of ascorbate and the corresponding decline in DOPAC. Collectively, these results suggest that whereas dopamine, sigma, and NMDA receptors modulate neostriatal ascorbate release, they exert an opposing influence on extracellular DOPAC. All drugs attenuated at least some components of the amphetamine behavioral response, suggesting a role for multiple mechanisms in the behavioral effects of this drug.

Ascorbate neurotoxicity in cortical cell culture

Ascorbate (vitamin C) is believed to act as a neuromodulator that facilitates the release of neurotransmitters and inhibits neurotransmitter binding to receptors, including dopamine and N-methyl-D-aspartate receptors. Extracellular levels of ascorbate are known to reach the low millimolar range after ischemic brain injury. This study shows that treatment of cultured cortical neurons with micromolar to low millimolar ascorbate first inhibits total protein synthesis and then results in late neuronal death. Astrocytes are much less vulnerable to ascorbate than neurons. Ascorbate may exacerbate neuronal and glial damage after brain ischemia, and it may play a pathological role in other neurological diseases.

Chronic treatment with ascorbic acid inhibits the morphine withdrawal response in guinea-pigs

The effects of ascorbic acid (AA) were investigated on the morphine withdrawal response of guinea-pigs, a species which shares with man the inability to synthesize AA. Chronic pretreatment of guinea-pigs with AA, 1 g/l, in drinking water for 3 days, or AA 200 mg/kg subcutaneously (s.c.) 3 times daily for 3 days, markedly reduced the locomotor and behavioural withdrawal responses of guinea-pigs given naloxone hydrochloride, 15 mg/kg s.c. 2 h after a single dose of morphine sulphate, 15 mg/kg s.c. AA, 1 g/kg given intraperitoneally (i.p.) 30 min before morphine had no significant effect on morphine withdrawal. However, intracerebroventricular injection of AA, 1 mumol, 30 min before naloxone significantly enhanced morphine withdrawal. It is concluded that chronic but not acute administration of AA inhibits opiate withdrawal.

Stereospecific effects of ascorbic acid and analogues on D1 and D2 agonist binding

Ascorbic acid inhibited the specific binding of both the D1 agonist, [3H] SKF 38393, and the D2 agonist, [3H] N-0437 at physiologically relevant concentrations. This inhibition was both stereospecific and receptor selective. Using ligand concentrations approximating their KD's, the IC50's for ascorbate and two structural analogues, isoascorbate and D-glucoascorbate, were determined. The rank order of IC50's at both D1 and D2 were D-glucoascorbate greater than isoascorbate greater than ascorbate. However, the IC50 for each compound was greater at D1 than D2. Evaluation of the relationship between the IC50 for ascorbate and the ligand concentration using both the D1 and the D2 ligand yielded data inconsistent with competitive inhibition models. Preliminary experiments were conducted to evaluate the site and type of inhibition with results consistent with an allostearic effect at the level of the receptor.

Simultaneous polarographic and electrophysiological in vivo measurements through optoelectronic interconnection

It is shown that simultaneous pickup of polarographic and electrophysiological signals, which is usually prevented by a severe electrical interference, becomes possible when the polarograph is connected to its electrodes via an optoelectronic link. The sources of interference in simultaneous measurements are discussed, as well as the advantage of breaking the ground loop. Reported experimental data show that the simultaneous signals have the same quality as if separately detected.

Regional distribution of ascorbate and 3,4-dihydroxyphenylacetic acid (DOPAC) in rat striatum

In vivo voltammetry was used to study the regional distribution of extracellular ascorbate (AA) and 3,4-dihydroxyphenylacetic acid (DOPAC), a major dopamine metabolite, in the striatum of the rat. An electrochemically-modified carbon-fiber electrode, which provides distinct oxidation curves for each of these substances, was lowered in 1-mm increments through one of four striatal regions selected to sample the entire extent of this structure, including the nucleus accumbens. In anteromedial striatum, the level of AA was highest in the most dorsal and ventral aspects and lowest in the middle, whereas DOPAC levels generally showed the opposite pattern. This inverse relationship between AA and DOPAC was not evident in either lateral or posterior areas. To the extent that AA and DOPAC are released from different axon terminals, as mounting evidence suggests, regional differences in the extracellular concentration of these compounds may reflect the different and, in some cases, reciprocal distributions of two neuronal populations.

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

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

Chronic ascorbate potentiates the effects of chronic haloperidol on behavioral supersensitivity but not D2 dopamine receptor binding

Ample behavioral evidence suggests that ascorbate parallels the action of haloperidol, a widely used neuroleptic. To determine the extent to which this parallel extends to chronic treatment, 21 days of exposure to ascorbate (100 or 500 mg/kg) alone or combined with haloperidol (0.5 mg/kg) were assessed on stereotyped behavior and neostriatal D2 dopamine receptor binding in rats. Our results indicate that when challenged with the dopamine agonist, apomorphine (0.5 mg/kg), animals chronically treated with haloperidol or high-dose ascorbate alone display a supersensitive sniffing response relative to controls, while animals chronically treated with the combination of haloperidol and high-dose ascorbate display a further potentiation of sniffing relative to the haloperidol groups. In addition, [3H]spiperone saturation studies showed, as expected, an up-regulation of striatal D2 dopamine receptors in rats treated with haloperidol as reflected by a change in receptor density (Bmax) but not affinity (KD). Ascorbate treatment, however, had no effect on D2 receptor density or the distribution of [3H]apomorphine in whole brain. Even though chronic treatment with the haloperidol-high-dose-ascorbate combination produced an up-regulation of striatal D2 dopamine receptors, this treatment did not cause a further up-regulation relative to haloperidol alone nor did it have any effect on [3H]apomorphine distribution. Taken together, these findings indicate that although chronic ascorbate produces behavioral supersensitivity to apomorphine through central mechanisms, they appear to differ from those induced by chronic haloperidol.

Stimulation of both D1 and D2 dopamine receptors increases behavioral activation and ascorbate release in the neostriatum of freely moving rats

Electrochemically modified carbon-fiber electrodes were used to assess the effects of indirect (amphetamine and GBR-12909) as well as direct D1 (SKF-38393) and D2 (quinpirole) dopamine agonists on extracellular ascorbate in the neostriatum of awake, behaving rats. Relative to controls, 2.5 mg/kg d-amphetamine and 20.0 mg/kg GBR-12909 produced marked behavioral activation concomitant with a significant increase in ascorbate. Comparable effects were observed following the combined administration of 10.0 mg/kg SKF-38393 and 1.0 mg/kg quinpirole, but not after either of these drugs alone. Thus, behavioral activation and release of neostriatal ascorbate were closely related to the concurrent stimulation of both D1 and D2 dopamine receptors.

Miniaturized optoelectronic system for telemetry of in vivo voltammetric signals

In vivo voltammetry is an electrochemical technique that uses carbon fiber microelectrodes stereotaxically implanted in brain areas to monitor monoamine metabolism and release continuously, in freely moving animals. Electric wires connect the polarograph to the animal. A wire-less transmission system (optoelectronic transmission, OPT) of voltammetric signals is described here. It uses infrared diffused light, exploiting the diffusion of the transmitted light over walls and ceiling towards a receiver. The transmission system consists of a main unit and a satellite unit (40 x 30 x 5 mm) positioned on the animal's back. Voltammetric recordings obtained by the classical system (with wires) and by OPT are well defined and almost identical in shape. The power supply is provided by two thin lithium batteries (+/- 3V) that can record for up to 20 h. OPT permits detailed behavioral observations since the animal can be left free to move in a spacious environment. Voltammetry using OPT allows simultaneous recording of neuronal firing activity as well as electroencephalographic recordings (EEG) since there is no cross-talk between the circuits used. The results illustrate the reliability and usefulness of this wire-less transmission system for studying relationships between neurochemical, behavioral and electrophysiological activities.

Intrastriatal infusions of ascorbate antagonize the behavioral response to amphetamine

Compared to saline, bilateral infusions of ascorbate (AA) into the neostriatum of freely moving rats attenuated rearing, head bobbing, and sniffing at various times after systemic amphetamine administration. Comparable AA infusions into overlying cerebral cortex failed to alter the amphetamine behavioral response. Intrastriatal AA also enhanced the ability of haloperidol to antagonize amphetamine-induced forepaw shuffling and locomotion. Voltammetric measurements in separate animals revealed a linear increase in neostriatal AA that remained within reasonable physiological limits over the course of the AA infusion. Thus, endogenous AA may modulate behavior via mechanisms intrinsic to the neostriatum.

Trauma-induced increase of extracellular ascorbate in rat cerebral cortex

Extracellular (EC) ascorbate concentrations were measured in microdialysates from the cerebral cortex in rats subjected to cortical compression-contusion trauma. The trauma induced a transient, dramatic increase in EC ascorbate compared to the basal level before the insult and compared to control animals. The data support the presence of a releasable intracellular pool of ascorbate in the neocortex. The possibility that ascorbate may influence traumatic brain damage by its proposed neuromodulatory property and/or by its ability to induce lipid peroxidation is considered.

Corticostriatal and thalamic regulation of amphetamine-induced ascorbate release in the neostriatum

Lesions of cerebral cortex and ventromedial nucleus (VM) of the thalamus were made in rats to investigate the contribution of these structures to amphetamine (AMPH)-induced ascorbate (AA) release in the neostriatum as measured by in vivo voltammetry. Following a recovery period of at least one week, rats were anesthetized, and electrochemically modified, carbon-fiber electrodes were lowered into the neostriatum. Compared to data obtained from sham-operated and unoperated controls, bilateral aspiration lesions of cerebral cortex significantly lowered both the basal level of AA and the amount of AA released by AMPH in the neostriatum. Similar results were obtained after bilateral, but not unilateral electrolytic lesions of the VM thalamus. Collectively, these results suggest that the corticostriatal pathway and the VM thalamic nuclei participate in the regulation of basal and AMPH-induced AA release in the neostriatum.

Increased extracellular levels of ascorbate in the striatum after middle cerebral artery occlusion in the rat monitored by intracerebral microdialysis

Extracellular (EC) ascorbate concentrations were measured in microdialysates from the striatum bilaterally in rats subjected to unilateral middle cerebral artery occlusion (MCAO). The focal cerebral ischemia induced a dramatic increase in ascorbate on the ipsilateral (operated) side while the levels remained at the preocclusion level in the striatum of the contralateral (control) hemisphere. The possibility that ascorbate may aggravate ischemic neuronal damage by its proposed neuromodulatory properties and/or by its ability to induce lipid peroxidation is discussed.

The effects of serotonin depletion on the voltammetric response to amphetamine

In vivo voltammetry with carbon paste electrodes reliably produces two oxidation peaks. Previous research suggests that in caudate peak 1 (P1) monitors ascorbic acid and peak 2 (P2) monitors uric acid. To provide additional evidence that P2 monitors uric acid rather than indoles, the effects of the serotonin synthesis inhibitor p-chlorophenylalanine (PCPA) were studied in caudate (serotonin-poor) and globus pallidus (serotonin-rich). In both caudate and globus pallidus PCPA had little effect on P2 and pretreatment with PCPA failed to inhibit the amphetamine-induced increase in P2. In general, P2 recorded from globus pallidus was always very similar to P2 recorded from caudate. These data are consistent with the hypothesis that P2 represents uric acid even in serotonin-rich areas of the brain. Pretreatment with PCPA dramatically enhanced the amphetamine-induced increase in P1 in caudate but not in globus pallidus. This finding is interesting in light of reports that PCPA enhances certain behavioral effects of amphetamine.

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

Simultaneous electrical and chemical recordings have been made of dopamine neuronal activity in the rat brain during electrical stimulation of the medial forebrain bundle. Tungsten recording electrodes were placed at the level of the substantia nigra and carbon-fiber, Nafion-coated, voltammetric electrodes were placed in the neostriatum. Dopamine units, verified by histology to be in the zona compacta of the substantia nigra, were identified by previously established electrophysiological criteria. Dopamine release was detected by fast-scan cyclic voltammetry, a technique which allows dopamine to be determined in vivo on a sub-second time scale. The majority of dopamine cells examined (7 out of 10) were antidromically activated by 60 Hz stimulation of the medial forebrain bundle. The same stimulus also elicits dopamine overflow in the caudate nucleus. Following stimulation, dopamine concentrations in the extracellular fluid of the neostriatum rapidly declined to prestimulus levels. In addition, impulse flow in dopaminergic neurons was inhibited for 20 s following stimulation. These measurements represent the first direct observation from a neuronal tract of simultaneous unit activity and chemical release of a neurotransmitter in real time.

Neurochemical and behavioural changes induced by ascorbic acid and d-amphetamine in the rat

In male Wistar rats, ascorbic acid (AA) (1 gr/kg i.p.) significantly (P less than 0.05) decreased the rate of CAR in the shuttle box avoidance test and antagonized the d-amphetamine (d-A) (1 mg/kg s.c.)-induced increase of CAR. Shuttle box test reduced striatal dopamine (DA) levels by about 86% (with a consistent 2-3 fold increase of DOPAC levels). DA levels were greatly reduced also in hypothalamus (HYP) and in hippocampus (HIP). AA further decreased striatal DA but significantly antagonized the reduction of DA levels in HYP and HIP. d-A antagonized the reduction of DA levels in all the above brain regions. Shuttle box test reduced noradrenaline (NA) levels mainly in HIP (-52%) and again d-A showed a significant protective effect in all regions; AA did not interfere with NA level reduction in striatum (ST) and HYP, but antagonized it in HIP. It is concluded that striatal DA system plays an important role in rat operant active avoidance behaviour. AA seems to exert a negative modulatory activity which can be counteracted by d-A.

Effects of haloperidol on amphetamine-induced increases in ascorbic acid and uric acid as determined by voltammetry in vivo

Amphetamine produces dramatic changes in extracellular ascorbic acid (AA) and uric acid (UA) in rat caudate; the origin of extracellular AA and UA is being widely investigated. In this study, linear sweep voltammetry with carbon paste electrodes was used to monitor extracellular AA and UA levels in conscious behaving rats. Amphetamine (2 and 4 mg/kg) produced a dose-related increase in UA; the increase in AA was very similar at both doses. Haloperidol (0.2 mg/kg) blocked the amphetamine-induced increase in UA but reduced the AA increase only by about 20%. Thus the amphetamine-induced increase in AA is only partly dependent on dopamine (DA) receptor stimulation whereas the amphetamine-induced increase in UA is completely dependent upon DA receptor stimulation.

Ascorbic acid-deficient condition alters central effects of methamphetamine

Effects of multiple doses of methamphetamine (METH) were examined in normal and ascorbic acid-deficient (scorbutic) guinea pigs. METH-induced decreases of striatal serotonin concentrations were completely antagonized in scorbutic animals. Elevations of nigral substance P-like immunoreactivity also differed significantly in METH-treated scorbutic compared to METH-treated normal animals. Various lines of evidence indicate that dopamine is an essential mediator of METH-induced effects in both serotonergic and substance P systems in the brain areas examined; however, results from the present study indicate that, along with dopamine, ascorbic acid also plays a role in mediating the effects of METH in the central nervous system.

Effect of ascorbic acid on the synaptosomal uptake of [3H]MPP+, [3H]dopamine, and [14C]GABA

The effects of ascorbic acid on the synaptosomal uptake of [3H]MPP+, [3H]dopamine, and [14C]GABA were examined in attempts to understand the mechanism of ascorbic acid attenuation of MPTP neurotoxicity. [3H]Dopamine uptake was increased at lower levels (0.01 and 0.1 mM) and decreased at higher levels (10 mM) of ascorbic acid. Ascorbic acid inhibited [3H]MPP+ uptake (IC50 = 0.1 mM) and [14C]GABA uptake (IC50 = 10 mM). Washout of ascorbic acid restored uptake of [3H]dopamine and [3H]MPP+, suggesting that ascorbic-acid-induced lipid peroxidation was not involved in the effect on uptake. In addition to the possible involvement of antioxidant mechanisms in the in vivo attenuation of the neurotoxicity of MPTP by ascorbic acid, the present results indicate a direct effect of ascorbic acid in inhibiting the uptake of MPP+ into dopaminergic nerve terminals.

Unilateral, intranigral infusions of amphetamine produce differential, bilateral changes in unit activity and extracellular levels of ascorbate in the neostriatum of the rat

Simultaneous recordings of single-unit activity and oxidation current were obtained bilaterally from the neostriatum of rats in response to a unilateral infusion of D-amphetamine (2.0 microliters of 10 micrograms/microliter) into the substantia nigra. Whereas neuronal activity increases ipsilaterally and decreases contralaterally, the electrochemical signals, which reflect extracellular ascorbate, increase bilaterally. In each case, changes in unit activity precede the change in ascorbate release. Systemic administration of haloperidol (0.2 mg/kg) blocks the increase in oxidation current bilaterally, but reverses the neuronal activity response only on the ipsilateral side. These results lend further support to the view that intranigral amphetamine facilitates neostriatal ascorbate release, but this effect is not correlated with changes in single-unit activity.

Crus cerebri lesions abolish amphetamine-induced ascorbate release in the rat neostriatum

An increase in the concentration of ascorbate in the extracellular fluid of the rat striatum following systemic amphetamine administration previously has been demonstrated with the use of in vivo electrochemistry, push-pull cannula and in vivo dialysis. In this report, the effect of infusions of amphetamine into the substantia nigra on extracellular ascorbate levels in the striatum is further investigated by in vivo electrochemistry. Electrolytic lesions in the crus cerebri of the striatonigral pathway abolish this effect. Furthermore, these lesions abolish the effect of systemic amphetamine in the striatum ipsilateral to the lesion but not on the contralateral side. This phenomenon is independent of neostriatal dopamine or gamma-aminobutyric acid since levels were shown to be unchanged relative to the intact side. These results lend further support to the hypothesis that ascorbate secretion in the striatum after amphetamine is mediated in the central nervous system, and indicate that a pathway that courses through the crus cerebri is necessary for this release.

Potentiation of haloperidol-induced catalepsy by ascorbic acid in rats and nonhuman primates

Ascorbic acid was examined for potentiative effects on the catalepsy induced by haloperidol in rats and squirrel monkeys. In both animal species pretreatment with ascorbic acid (1000 mg/kg) markedly potentiated catalepsy induced by haloperidol. It is suggested that vitamin binds to, and inactivates, some brain dopamine receptors and in so doing potentiates an otherwise minimally cataleptogenic dose of haloperidol.

Multiple amphetamine injections reduce the release of ascorbic acid in the neostriatum of the rat

Voltammetric signals were recorded bilaterally with unmodified or electrochemically-modified carbon-fiber electrodes in the neostriatum of rats that sustained a unilateral depletion of dopamine by pretreatment with 6-hydroxydopamine. In both hemispheres, single daily injections of D-amphetamine (5.0 mg/kg or steadily increasing doses beginning with 6.0 mg/kg) for 6 consecutive days significantly reduced the rise in oxidation current produced by a challenge injection of 2.5 mg/kg D-amphetamine compared to the response obtained from saline-treated controls. Voltammetric scans indicated that the oxidized substance shared the electrochemical characteristics of ascorbic acid. It appears, therefore, that multiple amphetamine injections reduce the release of ascorbic acid in the neostriatum and that this effect does not depend on the integrity of dopaminergic neurons.