M.J. Besson

Dopamine: Spontaneous and drug-induced release from the caudate nucleus in the cat

(1) The spontaneous and pharmacologically induced release of3H-DA, synthesized from [3H]tyrosine, has been studied in vivo on the cat caudate nucleus. A new technique has been developed that enables to label and superfuse a limited area of the ventricular surface of the caudate nucleus. [3H]Tyrosine was applied for a short time (acute labelling) or superfused continuously (continuous labelling). Superfusates obtained after or during the labelling process were collected in serial fractions. (2) 3H-DA released spontaneously in the resting state was identified and estimated quantitatively in collected fractions. (3) Changes in the decline of3H-DA after acute labelling, observed in anaesthetized animals (N2OO2penthrane), are attributed to an inhibiting effect of the anaesthetic on the release of the newly synthesized3H-DA. (4) In unanaesthetized cats, marked and rapid increases in the3H-DA release (5–10 times the resting state level) were induced repetitively by K+, catron and amphetamine during continuous labelling of DA terminals with [3H]tyrosine. Bothd- andl-amphetamine were potently active when applied topically or injected intravenously. (5) Results obtained in continuous labelling experiments emphasize the importance of the newly synthesized transmitter in release studies.

The role of presynaptic receptors in the release and synthesis of 3H-dopamine by slices of rat striatum.

SummaryStriatal slices were continuously superfused with l-3,5-3H-tyrosine (50 μCi/ml) and 3H-H2O [index of 3H-dopamine (3H-DA) synthesis] and 3H-DA estimated in 0.5 ml (2.5 min) superfusate fractions. Depolarization with 50 mM K+ for 7.5 min induced a marked increase in 3H-DA release and a biphasic effect on synthesis (slight increase in the first fraction followed by a significant decrease in the third and fourth fractions). The decrease in the rate of 3H-H2O formation induced by K+ was not related to modifications of the specific activity of tyrosine in tissues. The possibility that the inhibition of synthesis was due to alterations in DA concentration in the synaptic cleft was examined. Benztropine in a concentration which produced inhibition of DA uptake (10−6 M) increased the K+ induced overflow of 3H-DA but failed to alter the inhibition of synthesis. On the other hand, when the powerful neuroleptic fluphenazine was added to the superfusion medium in a concentration which only weakly blocked 3H-DA uptake (10−6 M) it potentiated 3H-DA release and prevented the inhibition of synthesis both in the absence or presence of benztropine. A similar effect was seen following the in vivo treatment of rats with fluphenazine (2 mg/kg; 1 1/2 h before sacrifice). The addition of exogenous DA (0.6×10−6 M) or NA (10−6 M) to the superfusion medium increased 3H-DA outflow and reduced DA synthesis while isoproterenol (10−6 M) was without effect. The DA inhibitory effect on synthesis was still observed in the presence of benztropine (10−6 M) while the NA effect was prevented. This concentration of benztropine blocked both DA and NA uptake. The administration of fluphenazine (10−6 M) significantly prevented the decrease in 3H-DA synthesis induced by exogenous DA and partially prevented the effect of NA. In addition, the effect of exogenous DA on the inhibition of synthesis was still seen in the presence of 2-amino-4-hydroxy-6,7-dimethyl-5,6–7,8-tetrahydropteridine hydrochloride (DMPH4) (to protect against end-product inhibition). The present results provide direct support for the concept that activation of presynaptic DA receptors located on DA terminals in the striatum of the rat results in an inhibition of synthesis and release of the transmitter.

In vivo release of [3H]γ-aminobutyric acid in the rat neostriatum—II. Opposing effects of D1 and D2 dopamine receptor stimulation in the dorsal caudate putamen

The effects of several dopaminergic agonists and antagonists on the spontaneous release of [3H]gamma-aminobutyric acid were investigated in the dorsal striatum of halothane-anaesthetized rats. A push-pull cannula was implanted and the tissue was superfused continuously with a physiological medium containing [3H]glutamine, the precursor of [3H]GABA. Drugs were added to the superfusion medium. 2-Amino,6,7-dihydroxy,1,2,3,4-tetrahydro-naphtalene (ADTN, a mixed D1 and D2 receptor agonist) and D-amphetamine (a drug that enhances the release of endogenous dopamine) increased the release of 3H-GABA. The effect of ADTN was blocked by a D1 antagonist [R-(+),8-chloro, 7-hydroxy,2,3,4,5-tetrahydro,3-methyl,5-phenyl,1-H,3-benzazepine (SCH 23390)] but not by a D2 antagonist (S-sulpiride). Furthermore the stimulation of D1 receptors either by 2,3,4,5-tetrahydro,7,8-dihydroxy,1-phenyl,1-H,3-benzazepine or by D-amphetamine in the presence of S-sulpiride also enhanced the release of [3H]GABA. On the other hand, a selective D2 receptor agonist (3-(2-(N-3-hydroxy-phenylethyl)N-propylamino)ethyl-phenol) decreased the release of [3H]GABA. This effect was blocked in the presence of S-sulpiride. By itself the D1 receptor antagonist (SCH 23390) decreased the release of [3H]GABA whereas the D2 receptor antagonist (S-sulpiride) had no effect. It was concluded that stimulation of D1 and D2 receptors produces opposing effects on the spontaneous release of [3H]GABA in the dorsal striatum. Stimulation of D1 receptors facilitates the release of [3H]GABA whilst stimulation of D2 receptors inhibits it. The effect of D1 receptor stimulation appears to be predominant, and endogenous dopamine may activate tonically the release of GABA through these receptors in our experimental conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

In Vivo Release of Endogenous Amino Acids from the Rat Striatum: Further Evidence for a Role of Glutamate and Aspartate in Corticostriatal Neurotransmission

By means of the push‐pull cannula method, the outflow of endogenous amino acids was studied in the striatum of halothane‐anesthetized rats. Addition of K + ions (30 mM for 4 min) to the superfusion fluid increased the release of aspartate (+116%), glutamate (+ 217%), taurine (+109%), and γ‐aminobutyric acid (GABA) (−429%) whereas a prolonged decrease in the outflow of glutamine (−28%) and a delayed reduction in the efflux of tyrosine (−25%) were observed. In the absence of Ca2‐, the K+‐induced release of aspartate, glutamate, and GABA was blocked whereas the K + ‐induced release of taurine was still present. Under these conditions, the decrease in glutamine efflux was reduced and that of tyrosine was abolished. Local application of tetrodotoxin (5 μM) decreased only the outflow of glutamate (‐25%). One week following lesion of the ipsilateral sensorimotor cortex the spontaneous outflow of glutamine and of tyrosine was enhanced. Despite the lack of change in their spontaneous outflow, the K +‐evoked release of aspartate and glutamate was less pronounced in lesioned than in control animals, whereas the K + ‐evoked changes in GABA and glutamine efflux were not modified. Our data indicate that the push‐pull cannula method is a reliable approach for the study of the in vivo release of endogenous amino acids. In addition, they provide further evidence for a role for glutamate and aspartate as neuro‐transmitters of corticostriatal neurons.

Increased synthesis and release of dopamine in the striatum of the rat after amantadine treatment.

Abstract Synthesis and release of 3 H-dopamine (DA) were estimated by incubating slices of rat striatum with 3,5- 3 H-L-tyrosine and by measuring the 3 H-amine accumulation in slices and incubating medium at the end of the incubation. Absolute 3 H-DA synthesis was estimated by measuring the 3 H-water quantities formed during the conversion of 3,5- 3 H-L-tyrosine to 3 H-dopa. Amantadine pretreatment increased both 3 H-DA synthesis and release. This drug effect on striatal dopaminergic terminals is discussed with respect to its utilization in human Parkinsonism.

Stimulation of dopamine release by GABA in rat striatal slices.

Abstract The effect of GABA on the release of dopamine (DA) from dopaminergic terminals was examined in striatal slices of the rat. The slices were continuously superfused with L -[3H]tyrosine and [3H]DA released was estimated in serial 2.5 min superfusate fractions. GABA from 10−5M to 10−3M stimulated the spontaneous release of [3H]DA. The intensity of the effect appeared to be concentration dependent. The GABA stimulatory effect on [3H]DA release persisted during the entire application of GABA for periods as long as 50 min. At a much higher concentration (10−1M) GABA inhibited the spontaneous release of [3H]DA. At a concentration (10−4M) which stimulated [3H]DA release, GABA had no effect on [3H]DA uptake or [3H]tyrosine initial transport in striatal sucrose homogenates. Furthermore it did not affect DA synthesis as indicated by the estimation of the conversion index of tyrosine into DOPA (3H2O formed from L -[3,5-3H]tyrosine/tyrosine specific activity in tissues). The stimulatory effect of GABA (5 × 10−5M) on the spontaneous release of [3H]DA was not detectable when slices were superfused in the absence of calcium or in the presence of tetrodotoxin (5 × 10−7M). It was markedly reduced in the presence of picrotoxin (10−5M), a GABA antagonist. Finally, like GABA, GABA agonists such as muscimol and compounds structurally related to GABA such as lioresaal and gammahydroxybutyrate stimulated [3H]DA spontaneous release. Gamma-hydroxylioresal, an inactive metabolite of lioresal, was without effect. These various results suggest that GABA stimulates specifically the release of DA endogenously synthesized in dopaminergic terminals. The blockade of the GABA stimulatory effect by tetrodotoxin (5 × 10−7M) suggests that the GABA receptors mediating the action of GABA are not located presynaptically on dopaminergic terminals but on neurons or neuronal afferences within the striatum.

In vivo release of [3H]γ-aminobutyric acid in the rat neostriatum—I. Characterization and topographical heterogeneity of the effects of dopaminergic and cholinergic agents

The release of [3H]gamma-aminobutyric acid continuously synthesized from [3H]glutamine was studied in the striatum of halothane-anaesthetized rats superfused with a push-pull cannula. The levels of spontaneously released [3H]GABA were identical in all striatal regions examined, but were found to be higher at the junction between the striatum and the globus pallidus. Superfusion with a medium enriched in K+ ions induced a concentration-dependent increase in [3H]GABA release. Superfusion with a Ca2+-free medium did not affect the spontaneous outflow of [3H]GABA but sharply reduced the release of [3H]GABA evoked by 30 mM K+. Locally applied tetrodotoxin (50 microM) decreased slightly the spontaneous release of [3H]GABA (-22%). When acetylcholine (50 or 500 microM) was added to a superfusion medium containing eserine (50 microM), the spontaneous release of [3H]GABA was enhanced in the ventral but not in the dorsal region of the striatum. The local application of 2,3,4,5-tetrahydro, 7,8,-dihydroxy, 1-phenyl, 1-H, 3-benzazepine (10 microM), a dopaminergic agonist acting preferentially on D1 receptors increased the release of [3H]GABA in the dorsal striatum (+32%) but decreased it slightly (-19%) in the ventral striatum. 3-(2-(N-3 hydroxyphenylethyl)N-propylamino)ethyl-phenol (50 microM), a preferential D2 receptor agonist, decreased [3H]GABA release when it was applied dorsally (-23%) but not ventrally in the striatum. It is concluded that the regulation of the release of [3H]GABA by acetylcholine and dopaminergic drugs is different in the dorsal and ventral regions of the striatum. These differences may be related to the existence of subpopulations of GABA neurons and may well have functional implications as suggested by behavioural studies.

Three-dimensional organization of the striosomal compartment and patchy distribution of striatonigral projections in the matrix of the cat caudate nucleus

Acetylcholinesterase staining on successive frontal or sagittal sections was used to determine the three-dimensional organization of the striosomal and matrix compartments in the adult cat caudate nucleus. Reconstruction drawings of the acetylcholinesterase-poor zones (striosomes) indicated that the striosomal compartment is a labyrinthine network organized in the rostrocaudal and mediolateral axis which is reproducible from one animal to another. Four main anteroposterior channels converging in the mediorostral pole of the caudate nucleus were distinguished. Seven to eight diagonally oriented channels crossing the previous ones were seen also in the mediolateral axis on the central core of the caudate nucleus. The pattern of organization of the numerous and tortuous striosomal channels was more complicated medially, while the lateral part of the caudate nucleus was represented mainly by the matrix compartment. In addition, a sub-compartmentation of the matrix was demonstrated by retrograde tracing studies made by injecting either horseradish peroxidase-wheat germ agglutinin, [14C]amino acids or a mixture of horseradish peroxidase-wheat germ agglutinin and [14C]amino acids in several areas of the substantia nigra pars reticulata. Labelled patches were seen with both tracers, their topographical localization depended on the nigral injection site but reconstruction analysis indicated that the populations of cells which innervate the substantia nigra pars reticulata originate in the two third lateral parts of the caudate nucleus all along its rostrocaudal extension. Examination of horseradish peroxidase-wheat germ agglutinin labelled cells indicated that not all cells were labelled in patches suggesting a further sub-compartmentation of these patches. Finally, a comparison of the topographical distributions of labelled patches and of striosomes revealed that most patches were located in the extrastriosomal matrix.

Increased release of dopamine from striatal dopaminergic terminals in the rat after treatment with a neuroleptic: Thioproperazine

Abstract The effects of thioproperazine (TZ) pretreatment on 3 H-DA synthesis and release were examined by measuring the simultaneous accumulation of 3 H-DA in tissues and incubating medium after incubation of isolated cut striatum with 3 H-tyrosine. TZ treatment in intact animals produced an almost immediate sustained increase in the synthesis of 3 H-DA; this effect was associated with a markedly enhanced release of newly synthesized 3 H-amine. Various data suggest that the drug indirectly activates nigro-striatal DA containing neurons through “DA receptor blockade”. TZ added directly onto isolated striatum did not affect release or synthesis of 3 H-DA. TZ potentiated amphetamine induced release of 3 H-DA but it blocked the other pharmacological effects of amphetamine occurring in animals.

Effects of amphetamine and desmethylimipramine on amines synthesis and release in central catecholamine-containing neurons.

Abstract Amphetamine and DMI pretreatment in rats increased 3 H-CA quantities in incubating medium of medulla oblongata slices simultaneously incubated with 3 H-tyrosine. Amphetamine but not DMI induced a similar effect on 3 H-DA newly synthesized and released from striatal slices. Thus both drugs enhanced the synthesis of 3 H CA in the two structures examined.