An In Vivo Study of Dopamine Release and Metabolism in Rat Brain Regions Using Intracerebral Dialysis

Intracerebral dialysis was used with a specifically designed HPLC with electrochemical detection assay to monitor extracellular levels of endogenous 3,4-dihydroxyphenylethylamine (dopamine, DA) and its major metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in brain regions of the halothane-anesthetized rat. Significant amounts of DA, DOPAC, and HVA were detected in control perfusates collected from striatum and n. accumbens whereas the medial prefrontal cortex showed lower monoamine levels. The ratio of DA in perfusate to DA in whole tissue suggests that in f. cortex, compared to n. accumbens and striatum, there is a greater amount of DA in the extracellular space relative to the intraneuronal DA content. The DOPAC/HVA ratio in control perfusates varied between regions in accordance with whole tissue measurements. This ratio was highest in n. accumbens and lowest in f. cortex. The monoamine oxidase inhibitor pargyline (100 mg/kg i.p.) caused an exponential decline in DOPAC, but not of HVA, in regional perfusates, an effect that was associated with an increase in DA. The data indicated a higher turnover of extracellular DOPAC in n. accumbens than in striatum and the lowest DOPAC turnover in f. cortex. The rate of decline in extracellular DA metabolite levels was slow compared to whole tissue measurements. In the perfusates there was no statistical correlation between basal amounts of DA in the perfusates and DOPAC and HVA levels or DOPAC turnover for any of the areas, indicating that measurement of DA metabolism in the brain under basal conditions does not provide a good index of DA release. In summary, this study shows clear regional differences in basal DA release and metabolite levels, metabolite patterns, and DOPAC turnover rates in rat brain in vivo.     

Synaptosomal Uptake and Release of Dopamine in Rat Striatum After Hypoxia

Hypoxia induces alterations of central monoaminergic transmission and of behavior. We studied the effect of hypoxia on adult and newborn rats to obtain more information about long-lasting changes of dopamine (DA) transmission caused by neonatal hypoxia. One single exposure of adult rats to hypoxia leads to short-term alterations of DA uptake: decreased affinity of the uptake carrier to DA (Km, 269.5% versus control) and a sharp increase of Vmax up to 301.4% resulting in an increase of total uptake of DA into the striatum synaptosomes. The K+-evoked DA release decreased to 69.5%. After 1 week of recovery all parameters are normalized. Chronic postnatal hypoxia (postnatal day 2-11) caused long-lasting changes of DA release and uptake opposite to those observed in adult rats. Three months after hypoxia, the K+-stimulated DA release was enhanced (132% of control), and the uptake was reduced due to decreased affinity of the uptake carrier system for the substrate (Km, 187% of control value). In conclusion, the alterations observed after chronic postnatal hypoxia reflect special adaptive processes that are related to the high plasticity of the immature neonatal brain and contribute to an increased DA function in the nigrostriatal system.     

Dopamine Release in Rat Striatum: Physiological Coupling to Tyrosine Supply

Intracerebral microdialysis was used to monitor dopamine release in rat striatal extracellular fluid following the intraperitoneal administration of dopamine's precursor amino acid, L-tyrosine. Dopamine concentrations in dialysates increased transiently after tyrosine (50-100 mg/kg) administration. Pretreatment with haloperidol or the partial lesioning of nigrostriatal neurons enhanced the effect of tyrosine on dopamine release, and haloperidol also prolonged this effect. These data suggest that nigrostriatal dopaminergic neurons are responsive to changes in precursor availability under basal conditions, but that receptor-mediated feedback mechanisms limit the magnitude and duration of this effect.     

Amphetamine-Induced Dopamine Release in the Rat Striatum: An In Vivo Microdialysis Study

The effects of a number of biochemical and pharmacological manipulations on amphetamine (AMPH)-induced alterations in dopamine (DA) release and metabolism were examined in the rat striatum using the in vivo brain microdialysis method. Basal striatal dialysate concentrations were: DA, 7 nM; dihydroxyphenylacetic acid (DOPAC), 850 nM; homovanillic acid (HVA), 500 nM; 5-hydroxyindoleacetic acid (5-HIAA), 300 nM; and 3-methoxytyramine (3-MT), 3 nM. Intraperitoneal injection of AMPH (4 mg/kg) induced a substantial increase in DA efflux, which attained its maximum response 20-40 min after drug injection. On the other hand, DOPAC and HVA efflux declined following AMPH. The DA response, but not those of DOPAC and HVA, was dose dependent within the range of AMPH tested (2-16 mg/kg). High doses of AMPH (greater than 8 mg/kg) also decreased 5-HIAA and increased 3-MT efflux. Depletion of vesicular stores of DA using reserpine did not affect significantly AMPH-induced dopamine efflux. In contrast, prior inhibition of catecholamine synthesis, using alpha-methyl-p-tyrosine, proved to be an effective inhibitor of AMPH-evoked DA release (less than 35% of control). Moreover, the DA releasing action of AMPH was facilitated in pargyline-pretreated animals (220% of control). These data suggest that AMPH releases preferentially a newly synthesised pool of DA. Nomifensine, a DA uptake inhibitor, was an effective inhibitor of AMPH-induced DA efflux (18% of control). On the other hand, this action of AMPH was facilitated by veratrine and ouabain (200-210% of control). These results suggest that the membrane DA carrier may be involved in the actions of AMPH on DA efflux.     

Characterisation of Inhibitory 5-Hydroxytryptamine Receptors That Modulate Dopamine Release in the Striatum

Abstract: The effect of a series of indoleamines on the potassium-evoked tritium release of previously accumulated [³H]dopamine from rat striatal slices has been investigated. The indoleamines 5-hydroxytryptamine, 5-methoxy-tryptamine, 5-methoxy- N, N' -dimethyltryptamine and tryptamine (10⁻⁷ to 10⁻³ M) all reduced potassium-evoked release of tritium, to a maximum of 50%. The uptake of [³H]dopamine was unaffected by these compounds. A series of 5-hydroxytryptamine antagonists were examined for their ability to reduce the inhibition of potassium-evoked tritium release induced by 5-methoxytryptamine. The relative order of antagonist potency obtained was methysergide > metergoline > methiothepin > cinanserin > cyproheptadine > mianserin, and was consistent with an action on 5-hydroxytryptamine receptors. It is concluded that there are inhibitory 5-hydroxytryptamine receptors located on the terminals of dopaminergic neurones in the striatum.     

In Vitro Release of Endogenous Dopamine from the Striatum of the Weaver Mutant Mouse

The weaver mutant mouse has a genetically determined defect in the nigrostriatal dopaminergic system. The present study was undertaken to test the hypothesis that in the weaver mutant mouse, striatal nerve terminals undergo compensatory changes in response to this deficiency. To test this hypothesis, we studied the basal and stimulated release of dopamine from striatal slices of weaver mutant mice and matched controls. By using a superfusion system and concentrating the superfusate by passage over alumina, resting dopamine release could be determined in the weaver mutant despite the fact that striatal tissue content of dopamine in these mice is reduced by greater than 75% compared with control mice. Fractional resting release of dopamine in weaver striatal slices was significantly elevated compared with that in controls, suggesting that the release mechanisms in the weaver may be adapting to overcome the dopamine deficit. Potassium-evoked release (24 and 48 mM potassium) was not significantly different between the two genotypes. In contrast, amphetamine-evoked release (1 microM) was significantly greater in the weaver mice than in controls. In both genotypes, release evoked by amphetamine was completely inhibited by cocaine, implicating the dopamine uptake carrier in this release process. These findings suggest that fundamental differences in dopamine release mechanisms exist between weaver and control mice and support the hypothesis that compensatory mechanisms may develop in neurons in response to dopamine deficits.     

Enhancement of Dopamine Release In Vivo from the Rat Striatum by Dialytic Perfusion of 6R-l-erythro-5,6,7,8-Tetrahydrobiopterin

We have previously reported that intracerebroventricular administration of 6R-L-erythro-5,6,7,8-tetrahydrobiopterin (6R-BH4), a cofactor for tyrosine hydroxylase, enhances biosynthesis of 3,4-dihydroxyphenylethylamine (dopamine) in the rat brain. In the present study, we have more precisely examined the effects of 6R-BH4 on dopamine release in vivo from the rat striatum using brain microdialysis. The amount of dopamine collected in striatal dialysates was determined using HPLC with electrochemical detection after purification with an alumina batch method. When the striatum was dialyzed with Ringer solution containing various concentrations of 6R-BH4 (0.25, 0.5, and 1.0 mM), dopamine levels in striatal dialysates increased in a concentration-dependent manner. Biopterin had little effect on dopamine levels in dialysates. The 6R-BH4-induced increase in dopamine levels in dialysates was abolished after pretreatment with tetrodotoxin (50 microM) added to the perfusion fluid, but after pretreatment with nomifensine (100 mg/kg, intraperitoneal injection), an inhibitor of dopamine uptake mechanism, a larger increase was observed. After inhibition of tyrosine hydroxylase by pretreatment with alpha-methyl-p-tyrosine (250 mg/kg, intraperitoneal injection), most of the increase persisted. These results suggest that 6R-BH4 has a dopamine-releasing action, which is not dependent on biosynthesis of dopamine.     

Ontogeny of Dopamine D1 Receptors in Rat Striatum

The development of dopamine D1 receptors in rat striatum during the early postnatal period is described, using [3H]piflutixol as ligand. Dopamine D1 receptors increase in number from day of birth until about 21 days of age, when they reach adult levels. This increase in number parallels the increase in several other dopamine markers in striatum during the same time period. The increase is reflected in an increase in Bmax of ligand binding to D1 receptors. All other properties of D1 receptors that were examined do not change throughout this developmental period and are essentially the same as those found in adult tissue. These include association and dissociation rates, affinity for piflutixol as determined by kinetic and saturation studies, and pharmacology. These studies provide a biochemical and pharmacological basis for further studies on the ontogeny of dopamine receptors and of striatum and on factors regulating development of this region.     

Influence of Cannabinoids on Electrically Evoked Dopamine Release and Cyclic AMP Generation in the Rat Striatum

Abstract: Using the endogenous cannabinoid receptor agonist anandamide, the synthetic agonist CP 55940 {[1α,2β( R )5α]-(−)-5-(1,1-dimethylheptyl)-2-[5-hydroxy-2-(3-hydroxypropyl)cyclohexyl]phenol}, and the specific antagonist SR 141716 [ N -(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1 H -pyrazole-3-carboxamide hydrochloride], second messenger activation of the central cannabinoid receptor (CB1) was examined in rat striatal and cortical slices. The effects of these cannabinoid ligands on electrically evoked dopamine (DA) release from [³H]dopamine-prelabelled striatal slices were also investigated. CP 55940 (1 µ M ) and anandamide (10 µ M ) caused significant reductions in forskolin-stimulated cyclic AMP accumulation in rat striatal slices, which were reversed in the presence of SR 141716 (1 µ M ). CP 55940 (1 µ M ) had no effect on either KCl- or neurotransmitter-stimulated ³H-inositol phosphate accumulation in rat cortical slices. CP 55940 and anandamide caused significant reductions in the release of dopamine after electrical stimulation of [³H]dopamine-prelabelled striatal slices, which were antagonised by SR 141716. SR 141716 alone had no effect on electrically evoked dopamine release from rat striatal slices. These data indicate that the CB1 receptors in rat striatum are negatively linked to adenylyl cyclase and dopamine release. That the CB1 receptor may influence dopamine release in the striatum suggests that cannabinoids play a modulatory role in dopaminergic neuronal pathways.     

In Vivo Mechanisms Underlying Dopamine Release from Rat Nigrostriatal Terminals: II. Studies Using Potassium and Tyramine

The brain microdialysis technique has been used to examine the in vivo effects of potassium and tyramine on dopamine (DA) release and metabolism in the striatum of halothane-anaesthetised rats. Increasing the concentration of potassium perfusing the dialysis probe (30-120 mM) induced a dose-related efflux of DA. A dose-related release of DA was also observed following addition of tyramine (1-100 microM) to the perfusing buffer. High concentrations of potassium were found to reduce the dialysate content of the DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid and the serotonin metabolite 5-hydroxyindoleacetic acid. No such effect was observed even when using the highest concentration of tyramine tested. Potassium-evoked DA release was facilitated by pretreatment with the DA uptake inhibitor nomifensine, was inhibited by depletion of extracellular calcium, and was not significantly affected by tetrodotoxin (TTX). The effect of tyramine on DA efflux was inhibited by nomifensine and was insensitive to both TTX and calcium depletion. These data suggest that potassium and tyramine induce release of DA via different mechanisms. Potassium-induced DA release involves a carrier-independent process and may utilise an exocytotic release mechanism. On the other hand, tyramine-induced DA release would appear to involve a carrier-dependent process. Depletion of vesicular stores of DA by pretreatment with reserpine did not significantly affect potassium-induced DA release, whereas a marked inhibition of the effects of tyramine was noted. However, in reserpinised animals the potassium-induced release of DA was inhibited by nomifensine, a result suggesting that a carrier-dependent release mechanism operates in the absence of vesicular DA.(ABSTRACT TRUNCATED AT 250 WORDS)     

In Vivo Microdialysis Evidence for Transient Dopamine Release by Benzazepines in Rat Striatum

Abstract: The effects of benzazepine derivatives on extracellular levels of dopamine (DA) and l -3,4-dihydroxyphenylacetic acid (DOPAC) in the dorsal striatum of freely moving rats were studied using in vivo microdialysis. Direct injection of SKF-38393 (0.5 or 1.5 µg/0.5 µl), a selective D₁ receptor agonist, into the striatum through a cannula secured alongside a microdialysis probe produced a rapid dose-dependent transient increase in striatal DA efflux and a more gradual reduction in efflux of DOPAC. The rapid increase in DA efflux was not affected by infusion of tetrodotoxin (TTX; 2 µ M ) or Ca²⁺-free Ringer's solution and occurred after either enantiomer of SKF-38393. A TTX-insensitive increase in DA level similar to that induced by SKF-38393 was also seen after other benzazepines acting as agonists (SKF-75670 and SKF-82958, each 1.5 µg in 0.5 µl) and antagonists (SCH-23390, 1.5 µg in 0.5 µl) at the D₁ receptor and after (+)-amphetamine. These effects were inhibited by infusion of nomifensine (100 µ M ). It is concluded that the transient increases in striatal DA efflux seen after intrastriatal injection of SKF-38393 and other benzazepines are not mediated by presynaptic D₁ receptors but by an amphetamine-like action on the dopamine transporter.     

Effect of Quinine on Autoreceptor-Regulated Dopamine Release in the Rat Striatum

In vivo brain microdialysis was used to examine the role of potassium channel activation in dopamine (DA) autoreceptor function in the striatum of freely moving rats. Local application of the D2 receptor agonists quinpirole or N-0437 through the dialysis probe significantly reduced extracellular concentrations of DA. Local application of the D2 antagonist (-)-sulpiride produced significant increases in DA. Local perfusion with quinine, a K+ channel blocker, completely blocked the (-)-sulpiride-induced increases in DA but did not affect the DA agonist-induced decreases. (-)-Sulpiride completely blocked the effect of quinpirole on DA both in control and in quinine-treated animals. At the highest dose used, quinine caused a large transient increase in extracellular DA. Local application of tetrodotoxin or infusion of Mg2+ in the absence of Ca2+ did not prevent this quinine-induced transient increase in extracellular DA. These results demonstrate that DA autoreceptors in the striatum regulate DA release in awake, behaving animals. Local application of (-)-sulpiride increases DA levels by blocking the tonic activation of autoreceptors by endogenous DA. Quinine blocks the neuroleptic-induced increase in DA, perhaps by preventing the K+ channel opening that would normally accompany endogenous autoreceptor activation. The fact that exogenously applied DA receptor agonists can decrease extracellular DA levels in the presence of quinine suggests that they may be acting at extrasynaptic autoreceptors that are not tonically active in vivo. The effect of DA agonists on this site is via a DA receptor because it is blocked by (-)-sulpiride. However, this receptor does not appear to be coupled to a quinine-sensitive potassium channel.     

Dopamine Agonist-Mediated Inhibition of Acetylcholine Release in Rat Striatum Is Modified by Thyroid Hormone Status

Abstract: K⁺-evoked acetyl[³H]choline ([³H]ACh) release was inhibited in a concentration-dependent manner by apomorphine and the D₂ agonist quinpirole in striatal slices prepared from euthyroid and hypothyroid rats. However, there was a significant increase in the maximum inhibition observed with both agonists in the hypothyroid compared with the euthyroid group, which paralleled the increased D₂ agonist sensitivity reported for stereotyped behavior. The D₂ antagonist raclopride decreased, and the D, antagonist SCH 23390 increased, the inhibition of [³H]ACh release by apomorphine, confirming an inhibitory role for D₂ receptors and an opposing role for D₁ receptors. Because there is no difference in D₁ or D₂ receptor concentration between the euthyroid and hypothyroid groups, it is suggested that thyroid hormone modulation of D₂ receptor sensitivity affects a receptor-mediated event. Following intrastriatal injection of pertussis toxin (PTX), apomorphine no longer inhibited [³H]ACh release. In fact, increased [³H]- ACh release was observed, an effect reduced by SCH 23390, providing evidence that D₁ receptors enhance [³H]- ACh release, and confirming that a PTX-sensitive G protein mediates the D₂ response. As it has been reported that thyroid hormones modulate G protein expression, this mechanism may underlie their effect on dopamine agonist- mediated inhibition of ACh.     

A Comparison of Axonal and Somatodendritic Dopamine Release Using In Vivo Dialysis

The release of endogenous dopamine from the axon terminal field in the nucleus accumbens and from the A10 dopamine cell bodies of conscious rats was measured using intracranial dialysis. Release of dopamine from both areas was calcium-dependent and markedly inhibited by the presence of the D2 agonist, quinpirole, in the dialysis buffer. However, the addition of tetrodotoxin to the buffer produced less of a decrease in dopamine in the A10 region than in the nucleus accumbens. When dopamine release was examined by substituting K+ for Na+ or by adding amphetamine to the buffer, the evoked release was significantly less in the A10 region than in the nucleus accumbens. Likewise, enhanced extracellular dopamine following blockade of reuptake by nomifensine addition to the dialysis buffer was not as great in the A10 region as in the nucleus accumbens. These data argue that, in general, axonal and somatodendritic dopamine release are regulated by similar factors, although somatodendritic release is less influenced by action potential generation and less responsive to some releasing agents.     

Acute Effects of Typical and Atypical Antipsychotic Drugs on the Release of Dopamine from Prefrontal Cortex, Nucleus Accumbens, and Striatum of the Rat: An In Vivo Microdialysis Study

In vivo microdialysis has been used to study the acute effects of antipsychotic drugs on the extracellular level of dopamine from the nucleus accumbens, striatum, and prefrontal cortex of the rat. (-)-Sulpiride (20, 50, and 100 mg/kg i.v.) and haloperidol (0.1 and 0.5 mg/kg i.v.) enhanced the outflow of dopamine in the striatum and nucleus accumbens. In the medial prefrontal cortex, (-)-sulpiride at all doses tested did not significantly affect the extracellular level of dopamine. The effect of haloperidol was also attenuated in the medial prefrontal cortex; 0.1 mg/kg did not increase the outflow of dopamine and the effect of 0.5 mg/kg haloperidol was of shorter duration in the prefrontal cortex than that observed in striatum and nucleus accumbens. The atypical antipsychotic drug clozapine (5 and 10 mg/kg) increased the extracellular concentration of dopamine in all three regions. In contrast to the effects of sulpiride and haloperidol, that of clozapine in the medial prefrontal cortex was profound. These data suggest that different classes of antipsychotic drugs may have distinct effects on the release of dopamine from the nigrostriatal, mesolimbic, and mesocortical terminals.     

Effects of Selective Monoamine Oxidase Inhibitors on the In Vivo Release and Metabolism of Dopamine in the Rat Striatum

Brain microdialysis was used to examine the in vivo efflux and metabolism of dopamine (DA) in the rat striatum following monoamine oxidase (MAO) inhibition. Relevant catecholamines and indoleamines were quantified by HPLC coupled with a electrochemical detection system. The MAO-B inhibitor selegiline only affected DA deamination at a dose shown to inhibit partially type A MAO. Alterations in DA and metabolite efflux were not observed when using the MAO-B-selective dose of 1 mg/kg of selegiline. At 10 mg/kg, selegiline reduced the efflux of DA metabolites to approximately 70% of basal values without affecting DA efflux. K(+)- and veratrine-stimulated DA efflux was not affected by selegiline. Experiments using amphetamine and the DA uptake inhibitor nomifensine demonstrated that the effect of selegiline on DA metabolism was unlikely to be mediated either by inhibition of DA uptake or by an indirect effect of its metabolite amphetamine. The possibility that the effect of selegiline is mediated via a nonspecific inhibition of MAO is discussed. In contrast, the MAO-A inhibitor clorgyline inhibited basal DA metabolism and increased basal and depolarisation-induced DA efflux. A 1 mg/kg dose of clorgyline reduced basal DA metabolite efflux (40-60% of control values) without affecting DA efflux. At 10 mg/kg of clorgyline, DA efflux increased to 253 +/- 19% of basal values, whereas efflux of DA metabolites was reduced to between 15 and 26% of control values. The release of DA induced by K+ and veratrine was not affected by 1 mg/kg of clorgyline but was increased by approximately 200% following pretreatment with 10 mg/kg of clorgyline. The nonselective MAO inhibitor pargyline caused similar but more pronounced alterations in these parameters.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of the Cerebral Tryptaminergic System on the Turnover of Dopamine in the Striatum of the Rat

The effect of the cerebral 5-hydroxytryptamine system on the turnover of striatal 3,4-dihydroxyphenyl-ethylamine (dopamine) was investigated by measuring the level of dopamine and one of its metabolites in rats depleted of cerebral 5-hydroxytryptamine or treated with a 5-hydroxytryptamine receptor blocker. Treatment with p-chlorophenylalanine induced, in addition to a reduction in striatal 5-hydroxytryptamine and 5-hydroxyindol-3-ylacetic acid, an increase in the striatal concentration of dopamine, a diminution in the concentration of homovanillic acid in the same cerebral area, and a reduction in the rise of this acid after the administration of a butyrophenone derivative or tetrabenazine. Treatment with methysergide also reduced the increase of homovanillic acid induced by the butyrophenone. When probenecid was given to rats treated with p-chlorophenylalanine, homovanillic acid failed to accumulate, whereas the accumulation of 5-hydroxyindol-3-ylacetic acid was unaffected. The decay of dopamine after alpha-methyl-p-tyrosine administration was normal for the first 6 h but was later reduced in rats given p-chlorophenylalanine or methysergide. The results suggest that the lack of activation of 5-hydroxytryptamine receptors leads to a reduction in the turnover of dopamine in the nigrostriatal pathway.     

Dopamine Transporter-Dependent and -Independent Endogenous Dopamine Release from Weaver Mouse Striatum In Vitro

Abstract: The weaver mutant mouse (wv/wv) has an ～70% loss of nigrostriatal dopamine (DA) neurons, but the fractional DA release evoked by amphetamine (but not a high potassium level) has been shown to be greater from striatal slices of the weaver compared with +/+ mice. In the present work we tested the hypothesis that fractional DA release from weaver striatum would be greater when release was mediated by the DA transporter. Serotonin (5-HT)-stimulated fractional DA release was greater from weaver than from +/+ striatum. The release evoked by 5-HT in the presence of 10 µM nomifensine (an antagonist of the DA transporter) was less than in its absence, but the difference between weaver and +/+ striatum remained. In the presence of nomifensine, 1-(m-chlorophenyl)biguanide, classified as a 5-HT₃ agonist, also induced a greater fractional release from weaver compared with +/+ striatum. When veratridine was used at a low concentration (1 µM), the fractional evoked release of DA was higher from the weaver in the presence and absence of nomifensine. These findings suggest that the reason for the difference in the responsiveness of the two genotypes to these release-inducing agents is not related to DA transporter function.     

Role of Adenylate Cyclase in the Modulation of the Release of Dopamine: A Microdialysis Study in the Striatum of the Rat

In the present study, we have applied the brain microdialysis technique to investigate the effect of the stimulation of adenylate cyclase on the extracellular levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) in the striatum of freely moving rats. Infusion of 8-bromo-adenosine 3',5'-cyclic monophosphate (8-Br-cAMP), 3-isobutyl-1-methylxanthine, or forskolin produced a significant increase in the release of DA. The effect of 8-Br-cAMP was tetrodotoxin, Ca2+, and dose dependent and was saturable. 8-Br-cAMP also caused an increase in the output of DOPAC and HVA. No effects were seen on the output of 5-HIAA, except at the highest 8-Br-cAMP concentration studied. Infusion of 8-Br-cAMP (25 microM, 1.0 mM, and 3.3 mM) together with infusion of (-)-sulpiride (1 microM) or systemic administration of (+/-)-sulpiride (55 mumol/kg i.p.) produced an additive effect on the release of DA. Infusion or peripheral administration of (-)-N-0437 (1 microM or 1 mumol/kg) both decreased the 8-Br-cAMP-induced increase in the release of DA. These results demonstrate that cyclic AMP may stimulate the release of DA, but it is unlikely that this second messenger is linked to presynaptic D2 receptors controlling the release of DA.     

Synthesis and Release of Dopamine in Rat Brain: Comparison Between Substantia Nigra Pars Compacta, Pars Reticulata, and Striatum

Dopamine (DA) is synthesized and released not only from the terminals of the nigrostriatal dopaminergic neuronal pathway, but also from the dendrites in the substantia nigra. We have investigated the regulation of the DA turnover, the DA synthesis rate, and the DA release in the substantia nigra pars compacts (SNpc) and pars reticulata (SNpr) in vivo. As a measure of DA turnover, we have assessed the concentrations of 3,4-dihydroxyphenylacetic acid and homovanillic acid. As a measure of the DA synthesis rate, we have determined the 3,4-dihydroxyphenylalanine accumulation after inhibition of aromatic L-amino acid decarboxylase by 3-hydroxybenzylhydrazine. As a measure of DA release, we have investigated the disappearance rate of DA after inhibition of its synthesis by alpha-methyl-p-tyrosine and the 3-methoxytyramine accumulation following monoamine oxidase inhibition by pargyline. Both the DA turnover and the DA synthesis rate increased following treatment with the DA receptor antagonist haloperidol and decreased following treatment with the DA receptor agonist apomorphine in the SNpc and in the SNpr, but the effects of the drugs were less pronounced than in the striatum. gamma-Butyrolactone treatment, which suppresses the firing of the dopaminergic neurons, increased the DA synthesis rate in the striatum (165%), but had no such effect in the SNpc or SNpr. Haloperidol, apomorphine, and gamma-butyrolactone increased, decreased, and abolished, respectively, the DA release in the striatum, but the drugs had no or only slight effects on the alpha-methyl-p-tyrosine-induced DA disappearance and on the pargyline-induced 3-methoxytyramine accumulation in the SNpc or SNpr. Taken together, these results indicate that the DA synthesis rate, but not the DA release, are influenced by DA receptor activity and neuronal firing in the SNpc and SNpr. This is in contrast to the situation in the striatum, where both the DA synthesis rate and the DA release are under such control.