Contribution of endogenously formed arachidonic acid in the presynaptic facilitatory effects of NMDA and carbachol on dopamine release in the mouse striatum.

Arachidonic acid stimulated the release of [3H]-dopamine from striatal microdiscs in a concentration-dependent and partially calcium-dependent manner. Inhibitors of cytosolic and membrane-bound phospholipase A2 were used to determine whether endogenously formed arachidonic acid also contributes to the release of [3H]-DA (previously taken up in tissues or endogenously synthesized from [3H]-tyrosine) evoked by N-methyl-d-aspartate (NMDA) and carbachol alone or in combination. In the presence of magnesium, carbachol was found to remove the magnesium block of NMDA receptors and to facilitate the NMDA-evoked release of [3H]-DA from striatal microdiscs and synaptosomes. In addition, in the absence of magnesium, synergistic responses were induced by both agonists on microdiscs but not on synaptosomes. Responses induced by NMDA, carbachol or both agonists on microdiscs were reduced by phospholipase A2 inhibitors, the most striking effects being observed with mepacrine. Mepacrine was also shown to reduce the oxotremorine, but neither the nicotine- nor the potassium-evoked release of [3H]-DA. Tetrodotoxin decreased the release of [3H]-DA evoked by the co-application of NMDA and carbachol on microdiscs, but mepacrine still decreased this tetrodotoxin-resistant response. Similarly, mepacrine still decreased the release of [3H]-DA evoked by NMDA and carbachol on synaptosomes. Altogether, these results indicate that arachidonic acid which is formed in striatal neurons, and to a lesser extent in DA fibres, under stimulation of NMDA and muscarinic receptors, partially contributes to the presynaptic facilitation of DA release evoked by NMDA and carbachol.

DARPP-32: regulator of the efficacy of dopaminergic neurotransmission.

Dopaminergic neurons exert a major modulatory effect on the forebrain. Dopamine and adenosine 3',5'-monophosphate-regulated phosphoprotein (32 kilodaltons) (DARPP-32), which is enriched in all neurons that receive a dopaminergic input, is converted in response to dopamine into a potent protein phosphatase inhibitor. Mice generated to contain a targeted disruption of the DARPP-32 gene showed profound deficits in their molecular, electrophysiological, and behavioral responses to dopamine, drugs of abuse, and antipsychotic medication. The results show that DARPP-32 plays a central role in regulating the efficacy of dopaminergic neurotransmission.

Lack of autoreceptor-mediated inhibitory control of dopamine release in striatal synaptosomes of D2 receptor-deficient mice.

Mouse purified striatal synaptosomes were used to study the release of newly synthesised [3H]-dopamine ([3H]-DA) or of previously taken up [3H]-DA. Quinpirole (QP, 10 microM), a D2/D3 dopaminergic agonist, was found to reduce the release of newly synthesised [3H]-DA with a larger amplitude when 4-aminopyridine (100 microM) instead than veratridine (1 microM) or potassium (25 mM) was used to evoke DA release. Among the different D2/D3 dopaminergic agonists tested R(-)-propylnorapomorphine (NPA) and quinpirole were the most potent. These compounds reduced, in a concentration-dependent manner, the 4-aminopyridine-evoked release of [3H]-DA previously taken up by synaptosomes (50% maximal inhibition). In contrast, the D3 agonist PD-128,907 had little effect even when used at 100 nM. The QP (100 nM)-induced response was completely antagonised by sulpiride (1 microM). Strikingly, the NPA (100 nM) and PD-128,907 (100 nM)-evoked responses were completely suppressed in D2 receptor-deficient mice. This data strongly suggest that only D2 but not D3 receptors are involved in the autoreceptor-mediated inhibition of the evoked release of [3H]-DA. Interestingly, while amphetamine-induced release of [3H]-DA was not modified, a slight reduction of [3H]-DA efflux induced by the dopamine (DA) uptake inhibitor cocaine was observed in D2 receptor-deficient mice.

Direct and indirect presynaptic control of dopamine release by excitatory amino acids.

Dopamine (DA) release from nerve terminals of the nigrostriatal DA neurons not only depends on the activity of nigral DA cells but also on presynaptic regulation. Glutamatergic neurons of cortical origin play a prominent role in these presynaptic regulations. The direct glutamatergic presynaptic control of DA release is mediated by N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate (AMPA) receptors, located on DA nerve terminals. In addition, by acting on striatal target cells, these glutamatergic neurons contribute also to indirect regulations of DA release involving several transmitters such as GABA, acetylcholine and neuropeptides. Diffusible messengers such as nitric oxide (NO) or arachidonic acid (AA) which are particularly formed under the stimulation of NMDA receptors may also participate to the regulation of DA release. In the present study, it will be shown that the co-application of NMDA and carbachol synergistically increases the release of [3H]-DA and that this effect is reduced by mepacrine or 4-bromophenacylbromide (10(-7) M), two inhibitors of PLA2. Therefore endogenously released AA induced by the co-stimulation of NMDA and cholinergic receptors seems to be involved, at least partly, in the release of DA.

Serotonin stimulation of 5-HT4 receptors indirectly enhances in vivo dopamine release in the rat striatum.

Serotonin (5-HT) applied at 1, 3, and 10 microM into the striatum of halothane-anesthetized rats by in vivo microdialysis enhanced dopamine (DA) outflow up to 173, 283, and 584% of baseline values, respectively. The 5-HT effect was partially reduced by 1 or 10 microM GR 125,487, a 5-HT4 antagonist, and by 100 microM DAU 6285, a 5-HT3/4 antagonist, whereas the 5-HT1/2/6 antagonist methiothepin (50 microM) was ineffective. In the presence of tetrodotoxin the effect of 1 microM 5-HT was not affected by 5-HT4 antagonists. In addition, tetrodotoxin abolished the increase in DA release induced by the 5-HT4 agonist (S)- zacopride (100 microM). In striatal synaptosomes, 1 and 10 microM 5-HT increased the outflow of newly synthesized [3H]DA up to 163 and 635% of control values, respectively. The 5-HT4 agonists BIMU 8 and (S)-zacopride (1 and 10 microM) failed to modify [3H]DA outflow, whereas 5- methoxytryptamine (5-MeOT) at 10 microM increased it (62%). In prelabeled [3H]DA synaptosomes, 1 microM 5-HT, but not (S)-zacopride (1 and 10 microM), increased [3H]DA outflow. DAU 6285 (10 microM) failed to modify the enhancement of newly synthesized [3H]DA outflow induced by 5-MeOT or 5-HT (1 microM), whereas the effect of 5-HT was reduced to the same extent by the DA reuptake inhibitor nomifensine (1 microM) alone or in the presence of DAU 6285. These results show that striatal 5-HT4 receptors are involved in the 5-HT-induced enhancement of striatal DA release in vivo and that they are not located on striatal DA terminals.

Stimulatory effect of arachidonic acid on the release of GABA in matrix-enriched areas from the rat striatum.

Arachidonic acid was shown to stimulate the release of preloaded [3H]GABA from microdiscs of tissue punched out in matrix-enriched areas of the rat striatum. This effect, which was calcium- and dose-dependent, persisted in the presence of inhibitors of arachidonic acid catabolism. Other fatty acids were less or not effective. Arachidonic acid also inhibited [3H]GABA uptake into purified striatal synaptosomes, however the arachidonic acid-evoked release of [3H]GABA persisted following inhibition of the GABA neuronal uptake process. The stimulatory effect of arachidonic acid on GABA release may largely result from the activation of a protein kinase C since the arachidonic acid response was reduced by several protein kinase C inhibitors. Arachidonic acid also dose-dependently stimulated the release of preloaded [3H]GABA from purified striatal synaptosomes. Similar results were obtained when synaptosomes were previously incubated with [3H]glutamine to study the release of endogenously synthesized [3H]GABA. Further indicating a direct action of the fatty acid on GABAergic neurons, the arachidonic acid-induced release of [3H]GABA from microdiscs was not modified in the presence of the D1 dopaminergic antagonist SCH23390 or of glutamatergic antagonists. Finally, the release of [3H]GABA evoked by the combined application of NMDA and carbachol (a treatment known to markedly stimulate arachidonic acid formation) was reduced by inhibitors of phospholipase A2 further indicating that endogenously formed arachidonic acid significantly facilitates the release of GABA in the striatum.

Cooperative contributions of cholinergic and NMDA receptors in the presynaptic control of dopamine release from synaptosomes of the rat striatum.

In the presence of magnesium, although ineffective alone, N-methyl-D-aspartate (NMDA, 10(-3) M plus glycine 10(-6) M) stimulated the release of [3H]-dopamine ([3H]-DA) continuously synthesized from [3H]-tyrosine when applied with ACh, the amplitude of the NMDA response being dependent on the ACh concentration. Experiments performed with nicotine, oxotremorine and the antagonists hexamethonium and atropine indicated that both muscarinic and nicotinic receptors are involved in the permissive effect of ACh on the NMDA response. Data obtained in the absence of magnesium or with increasing concentrations of magnesium revealed that the permissive effect of ACh on the NMDA-evoked release of [3H]-DA results from removal of the magnesium block of NMDA receptors. The NMDA-evoked release of [3H]-DA observed in the presence of ACh, nicotine or oxotremorine (10(-3) M) was blocked by either of the protein kinase C inhibitors staurosporine (10(-8) M) and chelerythrine (5 x 10(-7) M). However, these drugs were without effect on responses induced by ACh, nicotine or oxotremorine alone and by NMDA (10(-3) M, in the absence of magnesium). Supporting further the involvement of a protein kinase C activation in the permissive effects of ACh or the cholinergic agonists, NMDA (10(-3) M) stimulated the release of [3H]-DA in the presence of both magnesium and phorbol 12-myristate 13-acetate (10(-6) M) or 1 -oleoyl-2-acetyl-glycerol (10(-4) M), and the NMDA response was markedly potentiated by ionomycin (10(-7) M) used at a concentration that stimulated [3H]-DA release to about the same degree as ACh (10(-4) M). Therefore, besides their depolarizing action, ACh, nicotine and oxotremorine could eliminate the magnesium block of NMDA receptors by activation of protein kinase C.

Effects of arachidonic acid on dopamine synthesis, spontaneous release, and uptake in striatal synaptosomes from the rat.

Arachidonic acid (AA) markedly stimulated, in a dose-dependent manner, the spontaneous release of [3H]dopamine ([3H]DA) continuously synthesized from [3H]tyrosine in purified synaptosomes from the rat striatum. As estimated by simultaneous measurement of the rate of [3H]H2O formation (an index of [3H]tyrosine conversion into [3H]DOPA), the AA response was associated with a progressive and dose-dependent reduction of [3H]DA synthesis. In contrast to AA, arachidonic acid, oleic acid, and the methyl ester of AA (all at 10(-4) M) did not modify [3H]DA release. The AA (3 x 10(-5) M)-evoked release of [3H]DA was not affected by inhibiting AA metabolism, with either 5,8,11,14-eicosatetraynoic acid or metyrapone, suggesting that AA acts directly and not through one of its metabolites. AA also inhibited in a dose-dependent manner [3H]DA uptake into synaptosomes, with a complete blockade observed at 10(-4) M. However, AA (10(-4) M) still stimulated [3H]DA spontaneous release in the presence of either nomifensine or other DA uptake inhibitors, indicating that AA both inhibits DA reuptake and facilitates its release process. Finally, the AA (10(-4) M)-evoked release of [3H]DA was not affected by protein kinase A inhibitors (H-89 or Rp-8-Br-cAMPS) but was markedly reduced in the presence of protein kinase C inhibitors (Ro 31-7549 or chelerythrine).

Presynaptic control of dopamine synthesis and release by excitatory amino acids in rat striatal synaptosomes.

Purified striatal synaptosomes were continuously superfused with L,3,5[3H]tyrosine in order to estimate the synthesis ([3H]water) and release of newly formed [3H]dopamine. In the presence of magnesium, L-glutamate, D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate (AMPA) and kainate, but not N-methyl-D-aspartate (NMDA) and 1-aminocyclopentane-1S,3R-dicarboxylate (t-ACPD), stimulated the release of [3H]dopamine, in a dose-dependent manner. When magnesium was omitted or in the presence of AMPA, NMDA also increased the release of [3H]dopamine. The effects of AMPA and kainate were competitively inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or 6,7-dinitro-quinoxaline-2,3-dione (DNQX), whereas those of NMDA were reduced by 2-amino-5-phosphonovalerate (APV) or (+)-5-methyl-10,11-dihydro-5-H-dibenzo(a,d)cyclo-hepten-5,10-imine maleate (MK801). The stimulation of [3H]dopamine release by a high concentration of glutamate resulted from the concomitant activation of AMPA and NMDA receptors since this effect was potentiated by glycine and reduced by 2-amino-5-phosphonovalerate or MK801. This reduction was almost complete in the combined presence of DNQX and MK801. Surprisingly, glutamate and NMDA (in the absence of magnesium) reduced the efflux of [3H]water. The reduction of [3H]dopamine synthesis was blocked by 2-amino-5-phosphonovalerate indicating the involvement of NMDA receptors. Neither AMPA nor kainate affected dopamine synthesis. The inhibition of [3H]dopamine synthesis resulting from the stimulation of NMDA receptors was prevented when synaptosomes were continuously superfused with adenosine deaminase and quinpirole, a combined treatment known to markedly reduce the phosphorylation of tyrosine hydroxylase by cAMP-dependent protein kinase. The opposite effects of a high concentration of glutamate on [3H]dopamine synthesis and release were mimicked by ionomycin. As a working hypothesis, it is proposed that the NMDA-triggered calcium influx could lead to a reduction of tyrosine hydroxylase phosphorylation, possibly through an activation of calcineurin.

NMDA and carbachol but not AMPA affect differently the release of [3H]GABA in striosome- and matrix-enriched areas of the rat striatum.

The effects of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA; 10(-3) M), N-methyl-D-aspartate (10(-3) M, in the absence of magnesium or presence of AMPA) and carbachol (10(-3) M) on the release of preloaded [3H]gamma-aminobutyric acid ([3H]GABA) from microdiscs of tissue punched out from sagittal brain slices in striosome- or matrix-enriched areas of the rat striatum have been compared. Although AMPA stimulated similarly the release of [3H]GABA in both striatal compartments, the release of [3H]GABA evoked by either N-methyl-D-aspartate (in the presence of AMPA) or carbachol was more pronounced in matrix- than in striosome-enriched areas. AMPA- and N-methyl-D-aspartate- (in the absence of magnesium) evoked responses were reduced but not abolished in the presence of tetrodotoxin (10(-6) M) in both compartments while the carbachol-evoked release of [3H]GABA was decreased by tetrodotoxin only in the matrix. The interruption of cholinergic transmission by the combined application of atropine (10(-5) M) and pempidine (10(-4) M) was without effect on the AMPA-evoked release of [3H]GABA, but it reduced the N-methyl-D-aspartate- (in the absence of magnesium or presence of AMPA) evoked release of [3H]GABA in both compartments, these reductions being of similar amplitude than those observed with tetrodotoxin.

Opposite presynaptic regulations by glutamate through NMDA receptors of dopamine synthesis and release in rat striatal synaptosomes.

Purified striatal synaptosomes were superfused continuously with L-[3,5-3H]tyrosine to measure simultaneously the synthesis ([3H]water formed during the conversion of [3H]tyrosine into [3H]DOPA) and the release of [3H]dopamine ([3H]DA). Glutamate (10(-3) M) and NMDA (10(-3) M, in the absence of Mg2+) stimulated the release of [3H]DA, but they reduced the efflux of [3H]water. This reduction of [3H]DA synthesis was blocked by 2-amino-5-phosphonovalerate indicating the involvement of NMDA receptors. Although D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate (AMPA) and kainate stimulated the release of [3H]DA, they did not affect its synthesis. The glutamate-evoked inhibition of [3H]DA synthesis was prevented when synaptosomes were superfused continuously with adenosine deaminase plus quinpirole, a treatment which markedly reduces the phosphorylation of tyrosine hydroxylase by cAMP dependent protein kinase. The opposite effects of glutamate on [3H]DA synthesis and release were mimicked by ionomycin (10(-6) M). It is proposed that both an activation of a cyclic nucleotide phosphodiesterase and a dephosphorylation of tyrosine hydroxylase linked to the influx of calcium through NMDA receptors is responsible for the inhibition of dopamine synthesis by glutamate and that calcineurin could play a critical role in these processes.

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.

Modulation of GABA release by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate and N-methyl-D-aspartate receptors in matrix-enriched areas of the rat striatum.

Using a new in vitro superfusion device, the release of preloaded [3H]GABA was examined in microdiscs of tissues taken from sagittal slices in matrix-enriched areas of the rat striatum. Potassium (9 mM, 15 mM) stimulated the release of [3H]GABA in a concentration- and calcium-dependent manner and the veratridine (1 microM)-evoked release of [3H]GABA was completely abolished in the presence of tetrodotoxin (1 microM). The selective glutamatergic agonist alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (1 mM) enhanced the potassium-evoked release of [3H]GABA as well as the basal outflow of [3H]GABA. This latter effect was found to be calcium-dependent, partially diminished by tetrodotoxin (1 microM), completely blocked by 6,7-dinitro-quinoxaline-2,3-dione (0.1 mM), which is generally used as an antagonist of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors, but not affected by (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK801, 10 microM), a specific antagonist of N-methyl-D-aspartate receptors. Similarly, N-methyl-D-aspartate (1 mM) enhanced both the potassium (9 mM) and the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (1 mM)-evoked release of [3H]GABA but when used alone, due to the presence of magnesium in the superfusion medium, was ineffective on the basal efflux of [3H]GABA. A stimulatory effect of N-methyl-D-aspartate (1 mM) on the basal outflow of [3H]GABA was observed, however, when magnesium was omitted from the superfusion medium. The stimulatory effect of N-methyl-D-aspartate (1 mM) observed in the presence of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate was not potentiated by glycine (1 microM, in the presence of strychnine 1 microM) and the N-methyl-D-aspartate-evoked response seen in the absence of magnesium was not enhanced by D-serine (1 mM), suggesting that endogenous glycine is already acting on N-methyl-D-aspartate receptors. In fact, in the absence of magnesium, 7-chloro-kynurenate (1 mM) completely abolished the stimulatory effect of N-methyl-D-aspartate on the release of [3H]GABA confirming that under our conditions, the glycine site of the N-methyl-D-aspartate receptor is saturated. N-methyl-D-aspartate-evoked responses were all blocked by MK801 (10 microM). Finally, the N-methyl-D-aspartate-evoked response seen in the absence of magnesium was markedly reduced in the presence of tetrodotoxin (1 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Anesthetic properties of riluzole (54274 RP), a new inhibitor of glutamate neurotransmission.

It has been suggested that some anesthetic agents could exert their hypnotic/anesthetic effects by selectively blocking receptors involved in the central excitatory neurotransmission mediated by glutamate. In the present study, we analyzed whether riluzole (54274 RP), a novel compound that inhibits both the release and some postsynaptic effects of glutamate in some brain structures, has anesthetic properties in rats. For this purpose, we investigated whether 1) riluzole administered intraperitoneally (ip) at doses ranging from 2.5 to 45 mg/kg induces loss of righting reflex (LRR); 2) riluzole (2.5 and 5 mg/kg) prolongs sleep-times induced by either ketamine (30 or 80 mg/kg ip) or thiopental (25 or 35 mg/kg ip); 3) a 5-mg/kg subanesthetic riluzole dose affects the minimum alveolar concentration of halothane (MACh). Onset of drug action was defined as the period of time from the ip injection to LRR. Sleep-time was considered the period of time from LRR to restoration of righting reflex. Riluzole at doses greater than 15 mg/kg was able to induce LRR (riluzole dose for which LRR was achieved in 50% of the rats [ED50 = 25.6 mg/kg]). A positive correlation was found between the dose of riluzole and sleep-time (r = 0.92, P less than 0.001). A 5-mg/kg (but not 2.5-mg/kg) riluzole dose significantly prolonged sleep-times induced by both ketamine (30 and 80 mg/kg) and thiopental (25 but not 35 mg/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

In vitro and in vivo inhibition of N-acetyl-L-aspartyl-L-glutamate catabolism by N-acylated L-glutamate analogs.

The dipeptide N-acetyl-aspartyl-glutamic acid (NAAG) present in brain and spinal cord tissues may act as a neurotransmitter at excitatory synapses in the central nervous system. However, pharmacological and biochemical studies of NAAG are hampered by its rapid inactivation in vivo and in vitro by an enzyme that cleaves NAAG into N-acetyl-aspartate and glutamate. This enzyme has been previously named N-acetylated alpha-linked acidic dipeptidase (NAALADase). Based upon our earlier studies on the specificity of this enzyme, we have now designed new competitive inhibitors of this peptidase. N-Succinyl-glutamic acid, 4, was almost as potent as N-acetyl-beta-aspartyl-glutamic acid (beta-NAAG), 2, in inhibiting the hydrolysis of [Glu-3H]NAAG by rat brain membranes, with an IC50 value in the micromolar range. The analogous affinities of the substrate NAAG and of N-succinyl-glutamic acid suggest that the N-acetyl moiety is not an absolute requirement for entry into the active site of the enzyme. Therefore, the acronym NAALADase seems to be incorrect, and peptidase activity against NAAG will be used throughout this manuscript when referring to the enzyme that cleaves NAAG and whose activity is inhibited by quisqualate and beta-NAAG. Two N-acylated glutamic acid analogs, 5 and 6, were also found to be effective inhibitors of the in vitro degradation of NAAG, with Ki values in the micromolar range. Compounds 5 and 6 possess two free carboxylic functions on the N-acyl moiety, one of which could interact with the S1 subsite of the enzyme; the other could chelate the Zn++ cation involved in the catalytic hydrolysis of NAAG.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Presynaptic facilitation of dopamine release through D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors on synaptosomes from the rat striatum.

Purified synaptosomes from the rat striatum were superfused continuously with [3H]tyrosine in order to estimate the release of newly synthesized [3H]dopamine. When tested from 10(-6) to 10(-3) M, several excitatory amino acids or their analogues markedly stimulated the release of [3H]dopamine, their apparent rank order of potency being kainate greater than glutamate = D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) greater than homocysteate greater than quisqualate greater than aspartate greater than ibotenate. N-acetyl-aspartyl-glutamate was without effect. In addition, in the range of concentrations of 10(-6) to 10(-3) M, the maximal response of glutamate was higher than that of kainate, AMPA or homocysteate, whereas the effects of quisqualate, aspartate and ibotenate, particularly, were of lower amplitude. In favor of the existence of glutamate receptors of the AMPA type on dopaminergic nerve terminals, the stimulatory effect of AMPA (5 x 10(-5) M) on [3H]dopamine release was antagonized by 6,7-dinitroquinoxaline-2,3-dione, 6-cyano-7-nitro-quinoxaline-2,3-dione, tau-D-glutamyl-amino-methyl-sulphonate and tau-D-glutamyl-glycine tested at 10(-4) M. 6,7-Dinitroquinoxaline-2,3-dione was the most potent, whereas L-glutamate diethylester was without effect. As expected D-2-amino-5-phosphonovalerate did not affect the AMPA-evoked response. Further experiments indicated that kainate and quisqualate stimulate the release of [3H]dopamine by acting on quisqualate/kainate or AMPA receptors. The quisqualate-evoked desensitization of AMPA receptors was prevented by concanavalin A (10(-7) M).(ABSTRACT TRUNCATED AT 250 WORDS)

Specific role of N-acetyl-aspartyl-glutamate in the in vivo regulation of dopamine release from dendrites and nerve terminals of nigrostriatal dopaminergic neurons in the cat.

Levels of N-acetyl-aspartyl-glutamate measured by high-pressure liquid chromatography were found to be very high in the cat substantia nigra, particularly in the pars compacta, while those in the caudate nucleus were much lower. In halothane-anaesthetized cats implanted with push-pull cannulae, N-acetyl-aspartyl-glutamate (10(-8) M) induced a marked and prolonged release of newly synthesized [3H]dopamine, when infused into the posterior but not into the anterior part of the caudate nucleus. In contrast, in the presence of tetrodotoxin (10(-6) M), N-acetyl-aspartyl-glutamate (10(-8) M) reduced the residual release of [3H]dopamine; this effect was also more pronounced in the posterior than in the anterior part. In the conditions used, as indicated by experiments with [3H]N-acetyl-aspartyl-glutamate no glutamate was formed from the infused N-acetyl-aspartyl-glutamate. Ibotenate (10(-5) M) induced changes in [3H]dopamine release in both the absence and presence of tetrodotoxin, which were closely similar to those observed with N-acetyl-aspartyl-glutamate. Responses induced by either N-acetyl-aspartyl-glutamate or ibotenate were not mediated by N-methyl-D-aspartate receptors since N-methyl-D-aspartate stimulated the release of [3H]dopamine only when used in a high concentration (10(-4) M) and applied in a magnesium-free superfusion medium in both the presence of glycine (10(-6) M) and strychnine (10(-6) M). In addition, the stimulatory effect of N-methyl-D-aspartate persisted in the presence of tetrodotoxin; it was of similar amplitude in both parts of the caudate nucleus and of shorter duration than that evoked by either N-acetyl-aspartyl-glutamate or ibotenate alone. N-Acetyl-aspartyl-glutamate interacted with dopaminergic neurons not only presynaptically in the caudate nucleus but also in the substantia nigra since a marked increase in [3H]dopamine release was observed both from local dendrites and from nerve terminals in the ipsilateral caudate nucleus when N-acetyl-aspartyl-glutamate (10(-7) M) was infused locally into the substantia nigra pars compacta. No effect could be seen in contralateral structures. The isomer of natural N-acetyl-aspartyl-glutamate, beta-N-acetyl-aspartyl-glutamate (10(-7) M), had no effect on [3H]dopamine release when applied similarly in the substantia nigra, thus confirming the specificity of the action of N-acetyl-aspartyl-glutamate.

Glutamatergic control of dopamine release in the rat striatum: evidence for presynaptic N-methyl-D-aspartate receptors on dopaminergic nerve terminals.

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 X 10(-5) M)-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(-3) M) or the noncompetitive antagonist MK-801 (10(-6) M). In addition, the TTX-resistant NMDA-evoked response could be potentiated by glycine (10(-6) M) in the presence of strychnine (10(-6) M). The coapplication of NMDA (5 X 10(-5) M) and glycine (10(-6) M) stimulated the release of [3H]DA from striatal synaptosomes. This effect was blocked by Mg2+ (10(-3) M) or MK-801 (10(-5) M). 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 electrophysiological studies for postsynaptic NMDA receptors.

Role of excitatory amino acids in the direct and indirect presynaptic regulation of dopamine release from nerve terminals of nigrostriatal dopaminergic neurons.

In vivo experiments carried out in halothane-anaesthetized cats implanted with push-pull cannulae demonstrated that glutamate (GLU) released from corticostriatal fibers triggers the release of dopamine (DA), even in the absence of activity in nigral DA cells. As shown in vitro, using rat striatal slices or synaptosomes or in vivo in the cat, both NMDA and AMPA receptors subtypes are involved in the GLU-induced release of DA. Beside this direct regulation, GLU also exert several indirect facilitatory and inhibitory controls on DA release, particularly through cholinergic and GABAergic striatal neurons. Indeed, as shown by numerous authors, the GLU-evoked release of DA is markedly reduced in the presence of tetrodotoxin, bicuculline or atropine or by previous kainate- or ibotenate-induced lesion of striatum. Differences in the presynaptic regulation of DA release in striosomal and matrix compartments have also been found with NMDA and acetylcholine. The effect of acetylcholine was of shorter duration in the matrix than in the striosomal-enriched areas. Two opposite indirect regulations of DA release could be demonstrated: one is facilitatory and involves nicotinic receptors, the other is inhibitory, involves muscarinic receptors and mediated, at least in the matrix by dynorphin containing neurons. The NMDA-evoked responses are of larger amplitude and more sensitive to tetrodotoxin in the matrix than in the striosomes. In conclusion, GLU released from corticostriatal fibers, is able to control the release of DA from terminals of nigrostriatal neurons through direct facilitatory mechanisms (NMDA and AMPA receptors), but also through indirect facilitatory and inhibitory local circuits involving cholinergic and GABAergic neurons.