Long-term depression in striatal dopamine release monitored by in vivo voltammetry in free moving rats

It was proposed to monitor in free moving rats, by in vivo voltammetry, the effects of intracerebroventricular (i.c.v.) administration of drugs known to act on the synthesis of dopamine (DA), using an original multifiber carbon electrode which enables without-discontinuity long-term recordings in extracellular DA release. Results show that i.c.v. administration of alpha-methyl-p-tyrosine, gamma-butyrolactone, and apomorphine induced long-term depression in striatal DA release, over periods of time of more than 24 h. These results are in agreement with the dopaminergic hypothesis; and we conclude that i.c.v. administration of drugs and the use of the multifiber carbon electrode constitutes a valuable tool to monitor DA metabolism in chronically implanted animals.     

Measurement of 3-methoxytyramine by in vivo voltammetry: evidence for differences in central dopamine function in BALB/c and CBA mice

Differential pulse voltammetry (DPV) combined with carbon fibre electrodes allows selective detection of electroactive dopamine and serotonin metabolites in vivo. While usually employed in rats, we have now applied this in vivo technique in two inbred strains of mice: BALB/c and CBA. Three distinct oxidation peaks were recorded in vivo in the striatum of either BALB/c or CBA mice with a small shoulder occurring after the third peak at approximately +400 mV. Pargyline (150 mg/kg i.p.) potentiated this voltammetric shoulder into an easily measurable peak (Peak 4). In addition, Peak 4 was 2–3 times larger in BALB/c than in CBA mice. Homovanillic acid (HVA) and 3-methoxytyramine (3-MT), both catabolites of dopamine, oxidised at approximately +400 mV in vitro. Brain tissue levels of HVA and 3-MT, measured by high-performance liquid chromatography (HPLC) with electrochemical detection, demonstrated that pargyline treatment reduced striatal HVA, but increased 3-MT. These results support the view that Peak 4 recorded in the striatum of pargyline-treated mice in vivo is due to the oxidation of extracellular 3-MT. Thus, Peak 4 may be a useful index of dopamine release in situations where dopamine itself cannot be detected. Local infusion of KCl (2 μl, 0.1 M) further increased the size of Peak 4 in the striatum of both BALB/c and CBA mice. However, the increase was approx. 3 times greater in BALB/c mice, supporting previous evidence of greater dopaminergic function of BALB/c compared with CBA mice. In addition these two inbred strains of mice provide model systems for investigating the comparative functional roles of nigrostriatal pathways.     

Striatal dopamine terminals release serotonin after 5-HTP pretreatment: in vivo voltammetric data

Peripheral administration of 5-hydroxytryptophan (5-HTP) to rats causes 'wet dog' shakes and a parallel elevation of brain serotonin (5-HT) levels. The increase in 5-HT concentration does not, however, correlate with the endogenous 5-HT innervation raising the possibility that some 5-HTP is decarboxylated in non-serotonergic cells. In the present study we used in vivo voltammetry to establish whether 5-HTP treatment led to formation of 5-HT as a 'false transmitter' in striatal dopamine (DA) neurons. Fast cyclic voltammetry at carbon fibre microelectrodes (CFMs) was used to monitor striatal monoamine release following electrical stimulation of the median forebrain bundle (MFB). In the absence of any pretreatment DA was the sole compound released by stimulation. However, when DA release was abolished with alpha-methyl-p-tyrosine (AMPT), 5-HTP administration (after peripheral decarboxylase inhibition) caused a dose-dependent release of 5-HT, confirmed by the voltammetric characteristics. Central decarboxylase inhibition prevented release indicating that 5-HTP itself was not released. By monitoring reduction peaks it was possible to record DA and 5-HT release simultaneously at a single CFM. While DA and 5-HT oxidised at the same potential their reduction peaks were separated by approximately 450 mV. It was shown, using this means, that 5-HT was still detectable even when DA release was not abolished by AMPT. DA and 5-HT release showed a significant positive correlation suggesting that they were released from the same nerves. We conclude that, after 5-HTP treatment, 5-HT can be released as a false transmitter from striatal DA neurones.     

Effect of 6-fluoro-m-tyrosine on dopamine release and metabolism in rat striatum using in vivo microdialysis

6-[¹⁸F]Fluoro-m-tyrosine (FMT) is a positron emission tomography (PET) imaging agent for the aromatic -amino acid decarboxylase enzyme. Its parent compound, -m-tyrosine (LMT) induces behavioral effects in rodents via dopamine release. To assess the potential pharmacologic effect of FMT, its role in dopamine release and metabolism in rat striatum was compared with LMT and -DOPA using in vivo microdialysis. Results indicate that FMT will not have the same dopamine-induced behavioral effects as LMT.     

Blockade of both D1- and D2-dopamine receptors inhibits amphetamine-induced ascorbate release in the neostriatum

In vivo recordings with electrochemically modified microvoltammetric electrodes revealed that several neuroleptic drugs, including haloperidol, clozapine, and thioridazine, blocked the rise in extracellular ascorbate produced by amphetamine in the neostriatum of urethane-anesthetized rats. This effect was also observed in animals that received a combined injection of Sch-23390 and sulpiride, but not when either of these drugs were administered alone or in combination with the 5-HT₂ blocker, ritanserin. These results indicate that a combined blockade of D₁- and D₂-dopamine receptors blocks amphetamine-induced ascorbate release.     

Blockade of both D1- and D2-dopamine receptors inhibits amphetamine-induced ascorbate release in the neostriatum

In vivo recordings with electrochemically modified microvoltammetric electrodes revealed that several neuroleptic drugs, including haloperidol, clozapine, and thioridazine, blocked the rise in extracellular ascorbate produced by amphetamine in the neostriatum of urethane-anesthetized rats. This effect was also observed in animals that received a combined injection of Sch-23390 and sulpiride, but not when either of these drugs were administered alone or in combination with the 5-HT₂ blocker, ritanserin. These results indicate that a combined blockade of D₁- and D₂-dopamine receptors blocks amphetamine-induced ascorbate release.     

Extracellular dopamine in the rat striatum during ischemia and reperfusion as measured by in vivo electrochemistry and in vivo microdialysis

The effects of transient global forebrain ischemia and reperfusion on striatal extracellular dopamine levels were analyzed using both in vivo electrochemistry and in vivo microdialysis in urethane-anesthetized rats. Electrochemical records showed that extracellular dopamine levels increased once during the period of ischemia, and a second time during reperfusion. This biphasic pattern was not detected by microdialysis, probably because of the relatively low time resolution of this technique. Microdialysis provided evidence that the voltammetric signal was a measure of dopamine, and also allowed measurement of the metabolites dihydroxyphenylacetic acid and homovanillic acid, both of which decreased during ischemia. The biphasic dopamine pattern seen in rats is similar to that reported previously in gerbils, suggesting that it is a phenomenon common to transient ischemia and reperfusion across different species and models of transient global ischemia. This phenomenon may have important implications for therapeutic intervention in cerebral ischemia.     

Sub-second striatal dopamine release measured by in vivo voltammetry

High speed cyclic voltammetry was used to measure electrically stimulated striatal dopamine release in vivo with 25 ms time resolution. Dopamine release after 1 s stimulations was readily detectable and could be manipulated by drugs known to influence dopaminergic neurones. Using data-averaging dopamine release could be detected following stimulus trains as short as 100 ms. The use of carbon fibre microelectrodes gave a spatial resolution better than 5 micron.     

Reserpine attenuates d-amphetamine and MDMA-induced transmitter release in vivo: a consideration of dose, core temperature and dopamine synthesis

Amphetamine releases dopamine through a transporter-mediated mechanism. The purpose of this report was to further our understanding of the intracellular pool from which amphetamine releases dopamine: the cytoplasmic pool, the vesicular pool, or both. Rats were treated with -amphetamine (AMPH) (1.0 or 10.0 mg/kg) or an amphetamine analog, methylenedioxymethamphetamine (MDMA) (2.0, 5.0, or 10.0 mg/kg). Pre-treatment with 10.0 mg/kg reserpine (18 h prior to AMPH or MDMA) attenuated dopamine release for high and low AMPH doses; however the low-dose effect showed borderline significance. Pre-treatment with 10.0 mg/kg reserpine attenuated dopamine and serotonin release induced by MDMA. The dopamine effect was seen at all three MDMA doses; the effect on serotonin was only measured at the 10.0 mg/kg dose. Reserpine pre-treatment caused reductions in core body temperature; heating the rats to normal body temperature for 3 h prior to AMPH or MDMA, and during the 4 h post-treatment period partially reversed the reserpine-induced attenuation of dopamine release. However, the intermediate level of dopamine release for the reserpinized-heated animals was not significantly different from either the reserpine group (not heated) or the AMPH or MDMA alone groups. In a separate group of rats, the effects of reserpine and reserpine+heat on dopamine synthesis were measured. DOPA accumulation after treatment with the aromatic acid decarboxylase inhibitor NSD-1015 (100 mg/kg, 30 min before sacrifice), was greater in rats treated with reserpine compared to controls; heating the reserpinized rats did not significantly alter the amount of DOPA accumulation; however there was a trend towards further increase. These results suggest that -amphetamine releases dopamine that is stored in both vesicles and the cytoplasm. Cooling may contribute to the attenuation of AMPH or MDMA-induced dopamine release observed after reserpine; however, AMPH or MDMA dependence upon vesicular stores most likely explains the diminished release after reserpine. The attenuation of AMPH or MDMA-induced transmitter release by reserpine is thought to be counteracted by a reserpine-induced replenishment of stores. Therefore, all doses of -amphetamine may use vesicular stores; the degree to which new synthesis counteracts the vesicular depletion may be the variable which differentiates low from high doses of -amphetamine.     

Spreading depression in the cortex differently modulates dopamine release in rat mesolimbic and nigrostriatal terminal fields

The effects of cortical spreading depression (SD) on evoked dopamine release in mesolimbic (nucleus accumbens) and nigrostriatal (nucleus caudatus) terminal fields were studied by in vivo voltammetry in anesthetized rats. Dopamine release was evoked by electrical stimulation of medial forebrain bundle (20 Hz, 100 pulses). Local application of 3 M KCl on the dura initiated SD in the cortex. It was found that SD modulated evoked dopamine release in subcortical structures at the same time when the wave of depression of cortical activity reached reciprocally connected subcortical areas. This cortical depression increased stimulated dopamine release in the nucleus accumbens and decreased dopamine release in the nucleus caudatus. In agreement with these results, electrical stimulation of the prefrontal cortex at 20 Hz, synchronized with medial forebrain bundle stimulation, decreased evoked dopamine release in the nucleus accumbens. Areas of the cortex which modulated dopamine release in these two terminal fields were spatially separated by at least 5 mm from each other. It is proposed that depression and activation of evoked dopamine release in the nucleus caudatus and nucleus accumbens following SD are indicative of tonic activation of the nigrostriatal and tonic inhibition of the mesolimbic dopaminergic terminals by cortex in normal conditions. SD in the cortex, modulating neurotransmitter release in subcortical structures, may have a general impact on redistribution of oxygen supply in these subcortical areas and on behavior associated with brain trauma, migraine, insult or seizures, i.e. the kind of neuropathology which may cause SD type phenomena also in human brain.     

In vivo electrochemical analysis of cholecystokinin-induced inhibition of dopamine release in the nucleus accumbens

In vivo electrochemical techniques were used to study the effects of the sulfated (CCK8-S) and unsulfated (CCK8-US) forms of the cholecystokinin octapeptide on dopamine (DA) release in the nucleus accumbens. A dose-dependent inhibition of DA release was observed only with CCK8-S. This inhibitory effect was blocked by the CCK receptor antagonist proglumide, and was reversed by systemic injections of apomorphine. Given that apomorphine can hyperpolarize DA neurons, these data indicate that CCK may inhibit the release of DA by a process of depolarization block and suggest a mechanism by which CCK may regulate overactive mesolimbic DA transmission.     

An in vivo voltammetric comparison of the effects of three psychomotor stimulants on electrically evoked neostriatal dopamine release

The effects of three psychomotor stimulants (mazindol, beta-phenylethylamine and D-phenmetrazine) on electrically evoked neostriatal dopamine release were studied by in vivo voltammetry. Mazindol (10 mg/kg) enhanced release and this effect persisted after dopamine synthesis inhibition by alpha-methyl-p-tyrosine. beta-Phenylethylamine (100 mg/kg) caused a large decrease in stimulated dopamine release and exerted no effect after dopamine synthesis inhibition. D-Phenmetrazine (45 mg/kg) enhanced dopamine release on the first post-drug stimulation and also restored release after dopamine synthesis inhibition. Disruption of vesicular dopamine storage by Ro 4-1284 abolished electrically stimulated dopamine release. Only D-phenmetrazine was able to cause dopamine release following Ro 4-1284. These results imply different biochemical modes of action of these three stimulants.     

Systemic administration of thyrotropin-releasing hormone enhances striatal dopamine release in vivo

We examined the effects of systemically administered thyrotropin-releasing hormone (TRH) on the release of dopamine (DA), as assessed by brain microdialysis within the corpus striatum of anesthetized rats. A single dose (10 micrograms i.v.) elevated DA levels in brain extracellular fluid (ECF) by 240% above baseline levels after 150 min. Systemic tyrosine ([TME] 20 mg/kg i.v.) also increased DA release (by 190% after 150 min), while combined treatment with both agents was associated with significant potentiation of the DA response (to 640% after 150 min). None of the treatments significantly altered striatal tissue levels of DA or its metabolites. A large dose of TRH (50 micrograms i.v.) significantly increased DA release (by 1150%) whether or not animals had received an active or denatured prolactin (PRL) antiserum prior to the experiment, suggesting that the TRH effect is not mediated by PRL. Although TRH is rapidly metabolized in plasma and penetrates the blood-brain barrier only poorly, our results suggest that even relatively small doses of the hormone can affect striatal dopaminergic neurotransmission.     

Effect of dopamine content in rat brain striatum on anesthetic requirement: an in vivo microdialysis study

This study investigated the changes in anesthetic requirement caused by administration of two different concentrations (0.1 μM and 1.0 μM/h) of dopamine into the rat striatum. During the measurement of minimum alveolar concentration (MAC), each dopamine solution was continuously injected directly into rat striatum by microdialysis technique. During perfusion of the lower dose, MAC did not change. During the higher dose of dopamine, MAC decreased by approximately 30%.     

Functional meaning of tryptophan-induced increase of 5-HT metabolism as clarified by in vivo voltammetry

Differential pulse voltammetry with carbon fiber electrodes was used to study serotonin (5-HT) metabolism in freely moving rats. The electrodes implanted in the striatum recorded the extracellular 5-hydroxyindoleacetic acid (5-HIAA) oxidation peak after oral tryptophan (150 mg/kg). This 5-HT precursor did not modify the 5-HIAA peak in any rat tested, but it raised 5-HIAA levels determined in total tissue by a classical biochemical method (HPLC). The administration of 5-hydroxytryptophan (5-HTP) (25 mg/kg i.p.) induced an increase of 5-HIAA detectable both in the extracellular medium by voltammetry and in tissue samples. As previously shown, dorsal raphe electrical stimulation raises extracellular 5-HIAA in the striatum and this effect is enhanced by pretreatment with tryptophan. The results suggest that tryptophan in 'normal' conditions enhances 5-HT metabolism without affecting 5-HT release unless such release is stimulated. 5-HTP increases 5-HT metabolism and release.     

Selective inhibition of mesolimbic dopamine release following chronic administration of clozapine: involvement of α1-noradrenergic receptors demonstrated by in vivo voltammetry

The release of dopamine (DA) in vivo was compared in the striatum and nucleus accumbens following chronic (21 day) administration of clozapine (CLOZ) and repeated coadministration of haloperidol (HAL) and the α₁-noradrenergic (NE) receptor antagonist prazosin. Treatment with HAL reduced basal DA release in both brain regions, whereas treatment with CLOZ decreased basal DA release only in the accumbens. Chronic coadministration of HAL and prazosin resulted in decreased DA release in accumbens but not striatum. These results suggest that the α₁-NE receptor blocking properties of CLOZ may, in part, mediate its differential actions on nigrostriatal and mesolimbic DA release, an effect which may in addition contribute to its paucity of extrapyramidal side effects.     

Long-term effects of a high-dose methamphetamine regimen on subsequent methamphetamine-induced dopamine release in vivo

Rats were treated with a high-dose methamphetamine (METH) regimen (40 mg/kg/injection, four times at 2-h intervals) or a saline regimen (four injections at 2-h intervals). Temperature related measures taken during the high-dose METH treatment were maximum core temperature and minimum chamber temperature. Fourteen rats (METH N=7; Saline N=7) were implanted with in-vivo dialysis probes 4-7 weeks post-regimen (average=6 weeks). The next day, they received a challenge dose of METH (4.0 mg/kg) and dopamine release was measured. Results showed a significant decrease in challenge-induced dopamine release in rats previously treated with the high-dose METH regimen. These findings demonstrate a functional deficit in the dopamine system 6 weeks after high-dose METH treatment. Temperature-related measures taken during the high-dose regimen were not correlated with METH-induced dopamine release 6 weeks later. An additional group of rats were sacrificed 6 weeks after the high-dose regimen (METH N=12; Saline N=10), and their brains was analyzed for dopamine and serotonin concentrations. Tissue concentrations of dopamine were significantly depleted in striatum and nucleus accumbens/olfactory tubercle, but not septum, hypothalamus, or ventral mid-brain 6 weeks after the high-dose regimen. Tissue concentrations of serotonin were also significantly depleted in striatum, nucleus accumbens/olfactory tubercle, hippocampus, somatosensory cortex, but not septum, hypothalamus or ventral mid-brain. Significant correlations between the temperature-related measures and post-mortem neurotransmitter tissue concentrations were region and transmitter dependent.     

Influence of anaesthetics on rat striatal dopamine metabolism in vivo

The effects of 4 anaesthetics, halothane, α-chloralose, chloral hydrate and pentobarbitone, on rat striatal extracellular dihydroxyphenylacetic acid (DOPAC) levels were determined using in vivo voltammetry. Stable baseline levels of DOPAC were maintained under halothane/N ₂O and α-chloralose while the extracellular levels of DOPAC gradually declined under chloral hydrate or pentobarbitone anaesthesia. Administration of haloperidol (0.3 mg/kg i.p.) significantly increased DOPAC using halothane/N ₂O, α-chloralose or chloral hydrate but not under pentobarbitone anaesthesia. The greatest increase in DOPAC was seen in rats anaesthetized with α-chloralose>halothane/N₂O>chloral hydrate > pentobarbitone. Apomorphine (0.5 mg/kg s.c.) given 2 h after haloperidol partially reversed the increase in DOPAC produced by haloperidol. The results suggest care needs to be exercised in the choice of anaesthetic used for voltammetric studies with pentobarbitone being the least recommended.     

Ceruletide, a cholecystokinin-related peptide, attenuates haloperidol-induced increase in dopamine release from the rat striatum: an in vivo microdialysis study

The effects of ceruletide diethylamine (CLT), a cholecystokinin (CCK)-related peptide, on the spontaneous and haloperidol (HPD)-induced release of striatal dopamine (DA) were investigated with the in vivo microdialysis method. The striatum was perfused with Ringer solution containing different concentrations of K+. (1) When the dialysis tube was perfused with 4 mM K(+)-containing Ringer solution, CLT exerted no influence on the spontaneous and HPD-induced release of DA. (2) Increasing the K+ concentration in the perfusate from 4 to 15 mM failed to change the spontaneous and HPD-induced DA release. In this perfusion condition, the HPD-induced increase in DA release was significantly attenuated by CLT. (3) Perfusion of the striatum with the 20 mM K+ significantly reduced both the spontaneous and HPD-induced output of DA. (4) Even under the condition of perfusing the dialysis tube with the 4 mM K+, CLT significantly decreased the HPD-stimulated DA release in rats given HPD alone for the first 7 days and with CLT for the last 3 days. (5) Sixty consecutive daily administrations of HPD alone markedly reduced HPD-induced DA release from the striatum perfused with the Ringer solution containing 4 mM K+. From these results, we suggest that CLT, under the appropriate depolarization, can facilitate or induce depolarization inactivation of the A9 DA cells and/or nigrostriatal DA terminals, and consequently, produce significant inhibition of HPD-induced DA release from the rat striatum.