A. Chéramy

Glutamatergic Control of Dopamine Release in the Rat Striatum: Evidence for Presynaptic N-Methyl-D-Aspartate Receptors on Dopaminergic Nerve Terminals

Abstract: The N‐methyl‐D‐aspartate (NMDA) receptor‐mediated regulation of the release of newly synthesized [3H]dopamine ([3H]DA) was studied in vitro, both on rat striatal slices using a new microsuperfusion device and on rat striatal synaptosomes. Under Mg2+‐free medium conditions, the NMDA (5 × 10−5M)‐evoked release of [3H]DA from slices was found to be partly insensitive to tetrodotoxin (TTX). This TTX‐resistant stimulatory effect of NMDA was blocked by either Mg2+ (10−3M) or the noncompetitive antagonist MK‐801 (10−6M). In addition, the TTX‐resistant NMDA‐evoked response could be potentiated by glycine (10−6M) in the presence of strychnine (10−6M). The coapplication of NMDA (5 × 10−5M) and glycine (10−6M) stimulated the release of [3H]DA from striatal synaptosomes. This effect was blocked by Mg2+ (10−3M) or MK‐801 (10−5M). These results indicate that some of the NMDA receptors involved in the facilitation of DA release are located on DA nerve terminals. These presynaptic receptors exhibit pharmacological properties similar to those described in electro‐physiological studies for postsynaptic NMDA receptors.

In vivo presynaptic control of dopamine release in the cat caudate nucleus—II. Facilitatory or inhibitory influence ofl-glutamate

The local effects of various concentrations of L-glutamate (from 10(-8) M up to 10(-3) M) on the release of [3H]dopamine synthesized continuously from [3H]tyrosine were examined in the caudate nucleus of halothane-anaesthetized cats implanted with push-pull cannulae. When used at a concentration of 10(-8) M or 10(-7) M, L-glutamate stimulated the release of [3H]dopamine from nerve terminals of the nigrostriatal dopamine neurons. This effect was still observed in the presence of tetrodotoxin (5 X 10(-7) M) but it was antagonized by 2-amino 6-trifluoromethoxy benzothiazole (PK 26124) (10(-5) M), an antagonist dopamine nerve terminals. While no significant change in the release of [3H]dopamine was observed with 10(-6) M L-glutamate, higher concentrations (from 10(-5) M to 10(-3) M) of the amino acid produced a long-lasting reduction in the [3H]transmitter release. This latter effect was also antagonized by PK 26124 (10(-5) M) but, unlike that observed with 10(-8) M L-glutamate, it did not persist in the presence of tetrodotoxin (5 X 10(-7) M). On the contrary, a marked stimulation of the release of [3H]dopamine was seen in the presence of this neurotoxin. The reduction in the release of [3H]dopamine produced by 10(-4) M L-glutamate was also antagonized by bicuculline (10(-5) M) and moreover a marked stimulation of [3H]dopamine release took place in the presence of this gamma-aminobutyric acid (GABA) antagonist. Therefore, high concentrations of L-glutamate exerted an inhibitory presynaptic control on [3H]dopamine release which seemed to be indirect and mediated partly by GABAergic neurons. Since a sustained reduction in the spontaneous release of [3H]dopamine was seen in the presence of PK 26124, the corticostriatal glutamatergic neurons appeared to exert a tonic facilitatory presynaptic influence on dopamine release. This effect was important since it represented 40% of the tetrodotoxin-sensitive release of the [3H]transmitter. The direct (stimulatory) and indirect (inhibitory) presynaptic controls on dopamine release mediated by corticostriatal glutamatergic fibres are discussed in light of previous findings and of the anatomical organization of the caudate nucleus.

Release of dopamine evoked by electrical stimulation of the motor and visual areas of the cerebral cortex in both caudate nuclei and in the substantia nigra in the cat.

The effects of unilateral stimulation of the cerebral motor cortex and of the visual cortical area on the activity of the nigrostriatal dopaminergic neurons were examined in halothane-anesthetized cats. For this purpose, one push-pull cannula was inserted in each caudate nuclei and another one in the substantia nigra ipsilateral to the stimulated side. In all cases, the release of [3H]dopamine ([3H]DA) continuously formed from L-[3,5-3H]tyrosine was estimated in superfusates. Unilateral electrical stimulation of the cerebral motor cortex (area 4) induced a long-lasting and similar activation of [3H]DA release in both caudate nuclei. The activation of [3H]DA release in the contralateral side was selectively abolished after acute transection of the rostral part of the corpus callosum. This transection also suppressed the flexion of the contralateral forelimb induced by the stimulation. The activation of [3H]DA release could be related to the stimulation of corticostriatal neurons which may interact directly or indirectly with dopaminergic terminals in both caudate nuclei. Unilateral electrical stimulation of the visual cortex (areas 18 and 19) markedly stimulated the release of [3H]DA in the ipsilateral caudate nucleus. A slight effect was seen in the contralateral structure 20 min after the stimulation. These results are consistent with the existence of a main ipsilateral pathway originating from the visual cortex and projecting directly to the striatum. Both types of electrical stimulation immediately activated the release of [3H]DA in the ipsilateral substantia nigra. These effects were still seen 20 min after the stimulations. The activation of the dendritic release of [3H]DA could be related to the stimulation of a corticonigral projection. These results further indicate that the nigrostriatal dopaminergic neurons may be involved in sensory motor integration.

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.

Topographical distribution of dopaminergic innervation and of dopaminergic receptors in the rat striatum. I. Microestimation of [3H]dopamine uptake and dopamine content in microdiscs

Topographical variations in the uptake of [3H] dopamine (DA) and in the endogenous content of DA were estimated in the striatum of the rat. For this purpose, microdiscs were punched out in serial 500 mum sections. [3H] DA uptake was measured in 0.25 M sucrose homogenates prepared from microdiscs punched out from frozen slices (--7C). This uptake was similar to that observed in fresh tissues. It was unaffected by desmethylimipramine (5 X 10(-7) M), inhibited by benztropine (10(-6) M) and no longer detectable after 6-hydroxydopamine-induced degeneration of the nigrostriatal dopaminergic pathway. Both [3H] DA uptake and DA content decreased regularly from the rostral to the caudal part of the structure. In contrast, no important differences could be found in the dorso-ventral plane. These results suggest that the extent of dopaminergic innervation is heterogenous within the structure.

l-Glutamate-evoked release of dopamine from synaptosomes of the rat striatum: Involvement of AMPA and N-methyl-d-aspartate receptors

Previously, using purified synaptosomes from the rat striatum, we have shown that agonists of D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors stimulate the release of [3H]dopamine continuously synthesized from [3H]tyrosine. Similar results were obtained with N-methyl-D-aspartate in the absence of magnesium. In the present study, using the same approach, attempts were made to determine whether in the presence of magnesium, the combined stimulation of AMPA receptors allows us to demonstrate the presynaptic facilitation of [3H]dopamine release through N-methyl-D-aspartate receptors. L-Glutamate (10(-3) M) markedly stimulated the release of [3H]dopamine from synaptosomes, this effect being about twice that found with AMPA (10(-3) M) while N-methyl-D-aspartate (10(-3) M) even in the presence of glycine (10(-6) M) was ineffective. In agreement with previous results, a stimulatory effect of N-methyl-D-aspartate and glycine was only observed in the absence of magnesium. This response was blocked by 6,7-dinitro-quinoxaline-2,3-dione (3 x 10(-5) M), confirming that this compound, generally used as an AMPA antagonist, also blocks N-methyl-D-aspartate receptors. The AMPA (10(-3) M)-evoked release of [3H]dopamine was markedly potentiated by the combined application of N-methyl-D-aspartate (10(-3) M) and glycine (10(-6) M) in the presence of strychnine, indicating that the concomitant activation of AMPA receptors removes the voltage-dependent magnesium block of N-methyl-D-aspartate receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Riluzole inhibits the release of glutamate in the caudate nucleus of the cat in vivo

When applied locally to the caudate nucleus of the halothane-anaesthetized cat, riluzole (10(-5) M) markedly reduced (-57%) the spontaneous release of glutamate. This effect seems to be specific, since the efflux of the other amino acids, including aspartate was not affected. Indicating further its selective inhibitory effect on the spontaneous release of glutamate, the prolonged (90 min) application of riluzole (10(-5) M) enhanced the size of the potassium-releasable pool of glutamate, but not that of aspartate. This effect of riluzole was not noticed with classical anti-glutamatergic drugs, tested in the same conditions.

Gabaergic processes involved in the control of dopamine release from nigrostriatal dopaminergic neurons in the cat.

Abstract Push-pull cannulae were used to study the release of 3 H-dopamine ( 3 H-DA) continuosly synthetized from L-3,5- 3 H-tyrosine in the caudate nucleus and the substantia nigra of the “encephale isole” cat. GABA (10 −5 M), muscimol (10 −6 M), γ-hydroxybutyrate (GHB, 10 −5 M), GABA-choline (GABA-CH, 10 −5 M) and amino-oxyacetic acid (AOAA, 10 −5 M) stimulated the release of 3 H-DA in the caudate nucleus when introduced for 15 min in the superfusing medium. The initial stimulation of 3 H-DA release was followed by an inhibition of transmitter release when GABA was introduced for a longer time (60 min) into the caudate nucleus. GABA (10 −5 M) had no effect on the release of 3 H-DA in the substantia nigra. However, muscimol (10 −6 M), GHB (10 −5 M) and baclofen (10 −6 M) stimulated the dendritic release of 3 H-DA when each was introduced for 15 min in the substantia nigra. The introduction for 15 min of GABA (10 −5 M), muscimol (10 −6 M), GHB (10 −5 M), GABA-Ch (10 −5 M) or baclofen (10 −6 M) into the substantia nigra simultaneously induced a stimulation of 3 H-DA release in the ipsilateral caudate nucleus. A marked post stimulatory effect was seen with muscimol. When GABA (10 −5 M) was applied for 75 min into the substantia nigra, the initial stimulation of 3 H-DA release (30 min) seen in the caudate nucleus was followed by a short-lasting inhibitory phase (15 min). This inhibition of 3 H-DA release was followed by a second stimulation period. This multiphasic phenomenon was not seen with other compounds. The stimulation of 3 H-DA release in the caudate nucleus induced by a 15 min nigral application of GABA (10 −5 M) was no longer seen when picrotoxin (10 −5 M) was added with GABA in the substantia nigra. On the basis of these results it is proposed that at least two types of GABAergic regulatory processes are involved in the control of the activity of the dopaminergic neurons in the substantia nigra. On the other hand, the interaction of GABA or GABA-related compounds with GABAergic receptors in the striatum may facilitate DA release from dopaminergic terminals through an intrastriatal interneuronal process. The secondary inhibition of 3 H-DA release induced by GABA or GHB could be mediated by a non-GABAergic striato-nigral pathway.

In vivo presynaptic control of dopamine release in the cat caudate nucleus—III. Further evidence for the implication of corticostriatal glutamatergic neurons

In confirmation of previous results, experiments in halothane-anaesthetized cats implanted with push-pull cannulae showed that the unilateral application of GABA (10(-5) M for 30 min) into the left thalamic motor nuclei (either ventralis medialis, or ventralis lateralis) markedly stimulated the release of [3H]dopamine continuously synthesized from [3H]tyrosine in both caudate nuclei and in the contralateral substantia nigra. Three types of experiments confirmed that the changes in [3H]dopamine release evoked in both caudate nuclei resulted from a presynaptic facilitation mediated by the bilateral corticostriatal glutamatergic projection: The constant delivery of 2-amino 6-trifluoromethoxy benzothiazole (PK 26124) (10(-5) M) to the left caudate nucleus prevented the increased release of [3H]DA evoked by application of gamma-aminobutyric acid (GABA) (10(-5)M) into ventralis medialis-ventralis lateralis while an enhanced release of [3H]dopamine still occurred in the contralateral caudate nucleus. Since PK 26124 is an antagonist of glutamatergic transmission, the presynaptic facilitation may involve glutamatergic neurons. Single unit recordings of dopamine cells in the contralateral substantia nigra indicated that the increased release of [3H]dopamine from dendrites evoked by the application of GABA (10(-5)M) into ventralis medialis-ventralis lateralis was associated with a reduction in the firing rate of dopamine cells. Thus, the enhanced release of [3H]dopamine in the contralateral caudate nucleus may involve a presynaptic facilitatory process. Finally, the unilateral lesion of the sensory motor cortex made prior to the superfusion of caudate nucleus with [3H]tyrosine prevented the responses evoked in the two caudate nuclei by the application of GABA (10(-4) M) into ventralis medialis-ventralis lateralis.(ABSTRACT TRUNCATED AT 250 WORDS)

Release of dopamine in both caudate nuclei and both substantia nigrae in response to unilateral stimulation of cerebellar nuclei in the cat

The effects of unilateral focal electrical stimulation of the deep cerebellar nuclei on the activity of the nigrostriatal dopaminergic neurons on both sides of the brain were examined in halothane anaesthetized cats. For this purpose, push-pull cannulae were inserted into both caudate nuclei and both substantia nigrae, and the release of [3H] dopamine ([3H]DA) continuously formed from [3,5-3H]L-tyrosine was estimated in superfusates. The unilateral electrical stimulation of the right cerebellar dentate nucleus induced a long-lasting increase in the release of [3H]DA in the left caudate nucleus and a simultaneous decrease in the release of [3H]transmitter in the right caudate nucleus. These changes were associated with opposite fluctuations in the release of [3H]DA from the corresponding substantia nigrae. Thus, the electrical stimulation of the right dentate nucleus induced a pronounced decrease in the release of the [3H]-amine in the [3H]transmitter in the corresponding substantia nigra, whereas the activity of the contralateral substantia nigra, whereas the release in the ipsilateral substantia nigra was simultaneously increased. In contrast, the unilateral electrical stimulation of the right cerebellar fastigial nucleus resulted only in an increased release of [3H]DA in the ipsilateral (right) caudate nucleus, associated with a decreased release of the [3H]transmitter in the corresponding substantia nigra, whereas the activity of the contralateral (left) dopaminergic system was not significantly affected. These results support a direct functional interaction between the cerebellum and the basal ganglia. They also suggest that the release of DA from dopaminergic axonal terminals is inversely correlated to the extent of the transmitter release from dendrites.