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.     

Redox Modulation of N-Methyl-D-Aspartate-Stimulated Neurotransmitter Release from Rat Brain Slices

Rat brain cortical slices released tritiated norepinephrine ([3H]NA) during a 2-min stimulation with N-methyl-D-aspartate (NMDA). Dithiothreitol (DTT; 0.1-5 mM), present for 6 min prior to stimulation, dose-dependently increased the release of [3H]NA from cortical slices stimulated with a maximally effective concentration of NMDA (500 microM). Similar results were observed for [3H]NA release from hippocampal slices and tritiated and endogenous dopamine release from striatal slices. DTT treatment also markedly shifted the dose-response curve of NMDA to the left. Cortical slices released approximately the same amount of [3H]NA with 10 microM NMDA following DTT treatment (about 5%) as non-DTT-treated control slices did with 500 microM NMDA. The effects of DTT were fully reversed by subsequent treatment with 5,5'-dithio-bis(2-nitrobenzoic acid) (DTNB; 0.5 mM). DTT treatment did not significantly alter the ability of magnesium (1.3 mM) or the polyamine antagonist arcaine to block the NMDA-stimulated release of [3H]NA. In contrast, DTT treatment significantly attenuated the antagonist effects of the competitive glycine antagonist, 7-chlorokynurenic acid, and the competitive NMDA antagonist, 2-aminophosphonopentanoic acid. These results suggest that oxidation and reduction of disulfide bonds located within the NMDA receptor complex might regulate the activation of the NMDA receptor. This could have important consequences in vivo if endogenous oxidizing/reducing systems are found to have similar effects on NMDA-stimulated responses.     

Halothane Anesthesia Affects NMDA-Stimulated Cholinergic and GABAergic Modulation of Striatal Dopamine Efflux and Metabolism in the Rat In Vivo

Microdialysis of the striatum of halothane-anesthetized rats was used to study the participation of local cholinergic and GABAergic neurotransmission in NMDA receptor-modulated striatal dopamine release and metabolism. Reverse dialysis.of NMDA (1 mM) evoked a 10-fold increase in dopamine efflux and reduced DOPAC and HVA to > 20% of basal values. The effect of NMDA on dopamine efflux was abolished by atropine (10 microM) but unaffected by (+)-bicuculline (50 microM). NMDA-induced decrease in DOPAC (but not HVA) efflux was potentiated by atropine, whereas (+)-bicuculline attenuated the decrease in DOPAC and HVA. Compared to our previous studies in unanesthetised rats, our data suggest that halothane anesthesia alters the balance between NMDA-stimulated cholinergic and GABAergic influences on striatal dopamine release and metabolism. Differential sensitivity to halothane of NMDA receptors expressed by the neurones mediating these modulatory influences, or loss of specific NMDA receptor populations through voltage-dependent Mg2+ block under anesthesia, could underlie these observations.     

NMDA and Non-NMDA Ionotropic Glutamate Receptors Modulate Striatal Acetylcholine Release via Pre- and Postsynaptic Mechanisms

Abstract: The effects of NMDA and α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) on endogenous acetylcholine release from rat striatal slices and synaptosomes were investigated. Both agonists (1–300 µ M ) facilitated acetylcholine release from slices in a dose-dependent manner. NMDA (100–300 µ M ) and AMPA (30–300 µ M ), however, subsequently inhibited acetylcholine release. NMDA (100 µ M )-induced facilitation was antagonized by 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP) and dizocilpine (both 1–10 µ M ), whereas the 10 µ M AMPA effect was antagonized by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 1–30 µ M ). NMDA (100 µ M )-induced inhibition was counteracted by CPP, but not dizocilpine, and by the nitric oxide synthase inhibitor l -nitroarginine (1–100 µ M ). Tetrodotoxin (0.5 µ M ) prevented the facilitatory effect of 3 µ M NMDA and AMPA, but left unchanged that of 30 µ M NMDA and 100 µ M AMPA. Acetylcholine release from synaptosomes was stimulated by KCI (7.5–100 m M ) in a dose-dependent manner. NMDA and AMPA maximally potentiated the 20 m M KCl effect at 1 µ M and 0.01 µ M , but were ineffective at 100 µ M and 10 µ M , respectively. Inhibition of acetylcholine release was never found in synaptosomes. The effects of 1 µ M NMDA and 0.01 µ M AMPA were antagonized by CPP (0.0001–1 µ M ) or dizocilpine (0.0001–10 µ M ) and by CNQX (0.001–1 µ M ), respectively. These data suggest that glutamatergic control of striatal acetylcholine release is mediated via both pre- and post-synaptic NMDA and non-NMDA ionotropic receptors.     

In Vitro Evidence for Increased Facilitation of Striatal Acetylcholine Release via Pre- and Postsynaptic NMDA Receptotors in Hemiparkinsonian Rats

Abstract : The NMDA-evoked acetylcholine release from striatal slices and synaptosomes was investigated in rats subjected to unilateral injection of 6-hydroxydopamine into the substantia nigra. In slices prepared from the striatum contralateral to the lesion, the NMDA-evoked endogenous acetylcholine release was not significant at 10 μ M NMDA and maximal at 100 μ M NMDA (124 ± 19%). Conversely, in slices taken from the dopamine-depleted striatum, NMDA was effective even at 10 μ M (41 ± 4%), and at 100 μ M (196 ± 24%) efficacy was nearly doubled. In synaptosomes prepared from the contralateral striatum, NMDA maximally stimulated 20 m M KCl-induced endogenous acetylcholine release at 1 μ M (66 ± 5.1%), with lower concentrations (0.01-0.1 μ M ) being ineffective. Conversely, in synaptosomes prepared from the dopamine-depleted striatum, NMDA maximally enhanced the K⁺-evoked acetylcholine release at 0.1 μ M (118 ± 12.4%). Concentration-response curves of NMDA-evoked acetylcholine release in sham-operated rats could be superimposed on those observed in the contralateral striatum of the 6-hydroxydopamine-lesioned animals. The present data support the view of an increased glutamatergic regulation of striatal acetylcholine release via pre- and postsynaptic NMDA receptors during Parkinson's disease.     

Pharmacological evidence for N-methyl-D-aspartate receptors on nigrostriatal dopaminergic nerve terminals.

The efflux of tritium from rat striatal synaptosomes labelled with [3H]dopamine was utilized as an index of dopamine (DA) release for the purpose of characterizing the receptors underlying the effects of L-glutamate. N-Methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA), and kainate each induced DA release in the absence of Mg2+, through NMDA was much more efficacious and only the NMDA response was inhibited by Mg2+. The response to L-glutamate was potentiated in a concentration-dependent manner by glycine. Further, it was completely inhibited by the competitive NMDA antagonist 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid and by the NMDA channel blocker phencyclidine. Finally, the response to L-glutamate was unaffected by either tetrodotoxin or the kainate-AMPA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione. These data demonstrate the presence of NMDA receptors on dopaminergic nerve terminals that mediate the ability of L-glutamate to release DA and suggest an additional mechanism by which information from the nigrostriatal and corticostriatal pathways may be integrated.     

Plasticity of glutamatergic control of striatal acetylcholine release in experimental parkinsonism: opposite changes at group-II metabotropic and NMDA receptors

To investigate whether adaptive changes of glutamatergic transmission underlie dysfunction of the cholinergic system in experimental parkinsonism, the effects of group-II metabotropic glutamate and NMDA receptor ligands on acetylcholine release was studied in striatal slices and synaptosomes obtained from naive rats, 6-hydroxydopamine hemi-lesioned rats and 6-hydroxydopamine hemi-lesioned rats chronically treated with levodopa (L-DOPA) plus benserazide (non-dyskinetic). Group-II metabotropic glutamate receptor agonists LY354740, DCG-IV and L-CCG-I inhibited the electrically-evoked endogenous acetylcholine release from slices, while NMDA facilitated it. LY354740 also inhibited K+-evoked acetylcholine release from synaptosomes. LY354740-induced inhibition was prevented by the group-II metabotropic glutamate receptor antagonist LY341495. In hemi-parkinsonian rats, sensitivity towards LY354740 was reduced while that to NMDA was enhanced in the lesioned (denervated) compared with unlesioned striatum. Moreover, dizocilpine inhibited acetylcholine release in the lesioned compared with unlesioned striatum. Chronic treatment with L-DOPA normalized sensitivity towards glutamatergic agonists. We conclude that striatal dopamine denervation results in plastic changes at group-II metabotropic glutamate and NMDA receptors that may shift glutamatergic control of acetylcholine release towards facilitation. From a clinical perspective, L-DOPA and NMDA antagonists appear effective in counteracting overactivity of striatal cholinergic interneurones associated with Parkinson's disease.     

Presynaptic Glutamate Receptors Regulate Noradrenaline Release from Isolated Nerve Terminals

The wide-ranging neuronal actions of excitatory amino acids, such as glutamate, are thought to be mediated mainly by postsynaptic N-methyl-D-aspartate (NMDA) and non-NMDA receptors. We now report the existence of presynaptic glutamate receptors in isolated nerve terminals (synaptosomes) prepared from hippocampus, olfactory bulb, and cerebral cortex. Activation of these receptors by NMDA or non-NMDA agonists, in a concentration-dependent manner, resulted in Ca(2+)-dependent release of noradrenaline from vesicular transmitter stores. The NMDA-stimulated release was potentiated by glycine and was blocked by Mg2+ and selective NMDA antagonists. In contrast, release stimulated by selective non-NMDA agonists was blocked by 6-cyano-7-nitroquinoxaline-2,3- dione, but not by Mg2+ or NMDA antagonists. Our data suggest that the presynaptic glutamate receptors can be classified pharmacologically as both the NMDA and non-NMDA types. These receptors, localized on nerve terminals of the locus ceruleus noradrenergic neurons, may play an important role in interactions between noradrenaline and glutamate.     

N-methyl-D-aspartate receptors mediating hippocampal noradrenaline and striatal dopamine release display differential sensitivity to quinolinic acid, the HIV-1 envelope protein gp120, external pH and protein kinase C inhibition

NMDA receptors regulating hippocampal noradrenaline (NA) and striatal dopamine (DA) release have been compared using superfused synaptosomes prelabelled with the [(3)H]catecholamines. Both receptors mediated release augmentation when exposed to NMDA plus glycine. Quinolinic acid (100 microM or 1 mM) plus glycine (1 microM)-elicited [(3)H]NA, but not [(3)H]DA release. The NMDA (100 microM)-evoked release of [(3)H]NA and [(3)H]DA was similar and concentration-dependently enhanced by glycine or D-serine (0.1-1 microM); in contrast, the HIV-1 envelope protein gp120 potently (30-100 pM) enhanced the NMDA-evoked release of [(3)H]NA, but not that of [(3)H]DA. Gp120 also potentiated quinolinate-evoked [(3)H]NA release. Ifenprodil (0.1-0.5 microM) or CP-101,606 (0.1-10 microM) inhibited the NMDA plus glycine-evoked release of both [(3)H]catecholamines. Zinc (0.1-1 microM) was ineffective. Lowering external pH from 7.4 to 6.6 strongly inhibited the release of [(3)H]NA elicited by NMDA plus glycine, whereas the release of [(3)H]DA was unaffected. The protein kinase C inhibitors GF 109203X (0.1 microM) or H7 (10 microM) selectively prevented the effect of NMDA plus glycine on the release of [(3)H]NA. GF 109203X also blocked the release of [(3)H]NA induced by NMDA or quinolinate plus gp120. It is concluded that the hippocampal NMDA receptor and the striatal NMDA receptor are pharmacologically distinct native subtypes, possibly containing NR2B subunits but different splice variants of the NR1 subunit.     

NMDA modulation of dopamine dynamics is diminished in the aged striatum: an in vivo voltametric study

The technique of in vivo voltametry and a paired recording paradigm were employed to study the age-related changes in N-methyl-d-aspartate (NMDA) function in regulating the striatal dopaminergic transmission in male Sprague-Dawley rats. Microinjection of NMDA (100pmol) consistently elicited larger striatal dopamine (DA) overflows from young rats (3-4 months old) than from aged rats (27-28 months old). Furthermore, the rate of clearance (T(c)) of the NMDA-evoked dopamine release was lower in the aged rats. Local application of dopamine evoked reversible electrochemical signals with similar amplitudes in both young and aged rats. However, T(c) was reduced and time course parameters were prolonged in the aged rats. While microejection of NMDA (1pmol) did not induce any dopamine overflow, simultaneous administration of NMDA and K(+) evoked larger dopamine releases than K(+) alone in the young striatum. Concomitant application of NMDA did not potentiate the K(+)-evoked dopamine release in the aged striatum. Taken together, with the reduced dopamine release in response to depolarizing stimuli, our in vivo electrochemical data suggest that age-related changes in NMDA function contribute to the impaired dopaminergic dynamics, including an attenuation of NMDA-evoked dopamine release and a diminished augmentation by K(+) of NMDA-induced dopamine release during the normal aging process.     

Neurotensin Regulation of Endogenous Acetylcholine Release from Rat Striatal Slices Is Independent of Dopaminergic Tone

The effects of neurotensin (NT) alone or in combination with the dopamine antagonist sulpiride were tested on the release of endogenous acetylcholine (ACh) from striatal slices. NT enhanced potassium (25 mM)-evoked ACh release from striatal slices in a dose-dependent manner. This effect was tetrodotoxin-insensitive, suggesting an action directly on cholinergic elements. The dopamine antagonist sulpiride (5 x 10(-5) M) significantly increased (63%) potassium-evoked ACh release from striatal slices; potassium-evoked ACh release was further increased (90%) in the presence of NT (10(-5) M) and sulpiride (5 x 10(-5) M). The second set of experiments tested the effects of 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra on NT-induced increases of potassium-evoked ACh release. These lesions did not alter the NT regulation of potassium-evoked ACh release from striatal slices, but did significantly increase spontaneous (33%) and potassium-evoked (40%) ACh release from striatal slices. Striatal choline acetyltransferase activity was not affected by 6-OHDA lesions. In addition, following 6-OHDA lesions, sulpiride was ineffective in altering ACh release from striatal slices. Furthermore, evoked ACh release in the presence of the combination of NT and sulpiride was not different from that in the presence of NT alone. These results suggest that in the rat striatum, NT regulates cholinergic interneuron activity by interacting with NT receptors associated with cholinergic elements. Moreover, the NT modulation of cholinergic activity is independent of either an interaction of NT with D2 dopamine receptors or the sustained release of dopamine.     

Presynaptic Glutamate Receptors Modulate Dopamine Release from Striatal Synaptosomes

The wide-ranging neuronal actions of glutamate are thought to be mediated by postsynaptic N-methyl-D-aspartate (NMDA) and non-NMDA receptors. The present report demonstrates the existence of presynaptic glutamate receptors in isolated striatal dopaminergic nerve terminals (synaptosomes). Activation of these receptors, by NMDA in the absence of Mg2+ and presence of glycine and by non-NMDA agonists in the presence of Mg2+, results in Ca(2+)-dependent release of dopamine from striatal synaptosomes. The release stimulated by NMDA is blocked by Mg2+ and by selective NMDA antagonists, whereas the release stimulated by selective non-NMDA agonists is blocked by a non-NMDA antagonist but not by Mg2+ or NMDA antagonists. Thus, these presynaptic glutamate receptors, localized on dopaminergic terminals in the striatum, appear to be pharmacologically similar to both the NMDA and the non-NMDA postsynaptic receptors. By modulating the release of dopamine, these presynaptic receptors may play an important role in transmitter interactions in the striatum.     

Aluminum fluoride inhibits phospholipase D activation by a GTP-binding protein-independent mechanism

Aluminum fluoride (AlF4-) inhibited guanine nucleotide-activated phospholipase D (PLD) in rat submandibular gland cell-free lysates in a concentration-dependent response. This effect was consistent in permeabilized cells with endogenous phospholipid PLD substrates. Inhibition was not caused by either fluoride or aluminum alone and was reversed by aluminum chelation. Inhibition of PLD by aluminum fluoride was not mediated by cAMP, phosphatases 1, 2A or 2B, or phosphatidate phosphohydrolase. AlF4- had a similar inhibitory effect on rArf-stimulated PLD, but did not block the translocation of Arf from cytosol to membranes, indicating a post-GTP-binding-protein site of action. Oleate-sensitive PLD, which is not guanine nucleotide-dependent, was also inhibited by AlF4-, supporting a G protein-independent mechanism of action. A submandibular Golgi-enriched membrane preparation had high PLD activity which was also potently inhibited by AlF4-, leading to speculation that the known fluoride inhibition of Golgi vesicle transport may be PLD-mediated. It is proposed that aluminum fluoride inhibits different forms of PLD by a mechanism that is independent of GTP-binding proteins and that acts via a membrane-associated target which may be the enzyme itself.     

Amperozide, a putative anti-psychotic drug: uptake inhibition and release of dopamine in vitro in the rat brain

The effects of amperozide (a diphenylbutylpiperazinecarboxamide derivative) on the uptake and release of 3H-dopamine in vitro were investigated. Amperozide inhibited the amphetamine-stimulated release of dopamine from perfused rat striatal tissue in a dose-dependent manner. With 1 and 10 microM amperozide there was significant inhibition of the amphetamine-stimulated release of dopamine, to 44 and 36% of control. In contrast, 10 microM amperozide significantly strengthened the electrically stimulated release of dopamine from perfused striatal slices. Amperozide 1-10 microM had no significant effect on the potassium-stimulated release of dopamine. 10 microM amperozide also slightly increased the basal release of 3H-dopamine from perfused striatal tissue. These effects on various types of release are similar to those reported for uptake inhibitors (Bowyer et al, 1984). The uptake of dopamine in striatal tissue was inhibited by amperozide with IC50 values of 18 microM for uptake in chopped tissue and 1.0 microM for uptake in synaptosomes. Amperozide also inhibited the uptake of serotonin in synaptosomes from frontal cortex, IC50 = 0.32 microM and the uptake of noradrenaline in cortical synaptosomes, IC50 = 0.78 microM. In conclusion, amperozide shows uptake-inhibiting properties in both release and uptake studies done in vitro on the rat. In the in vivo studies, however, amperozide differs from dopamine uptake inhibitors.     

Dopamine Inhibition of the Release of Endogenous Acetylcholine from Corpus Striatum and Cerebral Cortex in Tissue Slices and Synaptosomes: A Presynaptic Response?

Abstract: The effect of dopamine on the release of endogenous acetylcholine from striatal slices and synaptosomes and from cerebral cortex synaptosomes was studied. K⁺ (56 m M ) and veratrine (75 μM ) increased the release of acetylcholine from striatal slices by 3.7 and 3.3 times the resting release, respectively. The effect of veratrine was completely abolished by tetrodotoxin (1 μM ). Dopamine (10⁻⁶ to 10⁻³ M ) reduced the K⁺-evoked release of acetylcholine from striatal slices in a dose-dependent manner. The resting release of acetylcholine was also significantly reduced by dopamine. Apomorphine (20 μM ) significantly reduced the K⁺-evoked release of acetylcholine, and both this effect and the inhibition due to dopamine (1 m M ) were significantly antagonised by chlorpromazine (20 μM ). Dopamine had a similar effect on the release of acetylcholine from striatal synaptosome beds; the resting release was depressed 32% by the presence of dopamine (1 m M ). A greater effect of dopamine was seen on the release of acetylcholine from cerebral cortex synaptosome beds, the resting release being reduced by 54% and the K⁺-evoked release by 29%. These results are discussed in terms of the possible role of presynaptic dopamine receptors in controlling the release of acetylcholine and the magnitude of their contribution compared with that of the postsynaptic dopamine receptor.     

Chronic nicotine causes functional upregulation of ionotropic glutamate receptors mediating hippocampal noradrenaline and striatal dopamine release

It has been proposed that (-)-nicotine can activate release-stimulating presynaptic nicotinic acetylcholine receptors (nAChRs) on glutamatergic nerve terminals to release glutamate, which in turn stimulates the release of noradrenaline (NA) and dopamine (DA) via presynaptic ionotropic glutamate receptors on catecholaminergic terminals. The objective of this study was to compare the function of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazide-4-propionic acid (AMPA) glutamate receptors in synaptosomes of rat hippocampus and striatum following acute and chronic (-)-nicotine administration. In hippocampal synaptosomes, prelabeled with [3H]NA, both the NMDA- and AMPA-evoked releases were higher in (-)-nicotine-treated (10 days) than in (-)-nicotine-treated (1 day) or vehicle-treated (1 or 10 days) rats. In striatal synaptosomes prelabeled with [3H]DA, the NMDA-evoked, but not the AMPA-evoked, release of [3H]DA was higher in (-)-nicotine-treated (10 days) than in nicotine-treated (1 day) or vehicle-treated (1 or 10 days) animals. Chronic (-)-nicotine did not affect catecholamine uptake, basal release and release evoked by high-K+ depolarization. Thus, chronic exposure to nicotine enhances the function of ionotropic glutamate receptors mediating noradrenaline release in the hippocampus and dopamine release in the striatum.     

Evidence that [3H]Dopamine Is Taken Up and Released from Nondopaminergic Nerve Terminals in the Rat Substantia Nigra In Vitro

Potassium chloride (25 mM) and (+)-amphetamine (100 microM) both stimulated the release of radioactivity from slices of substantia nigra preincubated with [3H]3,4-dihydroxyphenylethylamine [( 3H]dopamine). Potassium chloride (25 mM) released radioactivity from slices of both zona compacta and zona reticulata. Prior 6-hydroxydopamine (6-OHDA) lesions of one nigrostriatal pathway did not reduce the spontaneous release of radioactivity, or the potassium chloride- or amphetamine-induced release of radioactivity from slices of nigra ipsilateral to the lesion after preincubation with [3H]dopamine. The accumulation of radioactivity following incubation of nigral slices from 6-OHDA-lesioned animals with [3H]dopamine was increased when compared to uptake into slices from intact tissue. In synaptosomal preparations of striatum, nomifensine but not desipramine or fluoxetine inhibited [3H]dopamine uptake. In contrast, nomifensine, desipramine, and fluoxetine all inhibited [3H]dopamine uptake in nigral synaptosomal preparations. Following 6-OHDA lesions of one nigrostriatal pathway the uptake of [3H]dopamine into nigral synaptosomal preparations was unchanged but uptake into striatal preparations was substantially decreased. In contrast, bilateral electrolesions of the dorsal and medial raphe nuclei reduced [3H]dopamine uptake into nigral preparations but not into striatal synaptosomes. The uptake of [3H]5-hydroxytryptamine ([3H]5-HT) into synaptosomal preparations of substantia nigra was abolished by fluoxetine and reduced by desipramine, but was unaffected by nomifensine. In contrast, fluoxetine, desipramine, and nomifensine all inhibited [3H]5-HT uptake into striatal synaptosomal preparations. Following 6-OHDA lesions of one nigro-striatal pathway the uptake of [3H]5-HT into nigral synaptosomal preparations was unchanged but uptake into striatal preparations was reduced.(ABSTRACT TRUNCATED AT 250 WORDS)     

Diacylglycerol-Induced Stimulation of Neurotransmitter Release from Rat Brain Striatal Synaptosomes

These studies were undertaken to test the hypothesis that alterations in phosphatidylinositol metabolism can modulate neurotransmitter release in the central nervous system. The effects of 1,2-diacylglycerols (DAGs) on dopamine release in the rat central nervous system were determined by measuring dopamine release from rat striatal synaptosomes in response to two DAGs (sn-1,2-dioctanoylglycerol and 1-oleoyl-2-acetylglycerol) that can activate protein kinase C and one DAG (deoxydioctanoylglycerol) that does not activate this kinase. Dioctanoylglycerol and 1-oleoyl-2-acetylglycerol, at a concentration of 50 micrograms/ml, stimulated the release of labeled dopamine from striatal synaptosomes by 35-50 and 17%, respectively. Dioctanoylglycerol-induced release was also demonstrated for endogenous dopamine. In contrast, deoxydioctanoylglycerol (50 micrograms/ml) did not stimulate dopamine release. Dioctanoylglycerol-induced dopamine release was independent of external calcium concentration, indicating a utilization of internal calcium stores. Dioctanoylglycerol (50 micrograms/ml) also produced a 38% increase in labeled serotonin release from striatal synaptosomes. The addition of dioctanoylglycerol to the striatal supernatant fraction increased protein kinase C activity. These results are consistent with the concept that an increase in phosphatidylinositol metabolism can stimulate neurotransmitter release in the central nervous system via an increase in DAG concentration. The data suggest an involvement of protein kinase C in the DAG-induced release, but other sites for DAG action are also possible.     

l-Glutamic acid,a neuromodulator of dopaminergic transmission in the rat corpus striatum

A superfusion system was used to study the effects of neuroexcitatory amino acids upon spontaneous and depolarization-evoked release of exogenously taken up and newly synthesized [³H]dopamine by rat striatal slices. Neither l-glutamate nor other aminoacids such as l-aspartate and d-glutamate (5 × 10^?5 M) modified the spontaneous release of exogenous [³H]dopamine from rat striatal slices. In contrast, these neuroexcitatory aminoacids did potentiate spontaneous release of striatal [³H]dopamine newly synthesized from [³H]tyrosine. A different pattern of effects emerged when depolarization-evoked release of dopamine was studied. Only l-glutamate (5 × 10^?6-1 × 10^?4 M) potentiated dopamine release under these experimental conditions in a rather specific and stereoselective manner. In addition, similar results were obtained regardless of whether depolarization-induced release of exogenous or newly synthesized [³H]dopamine was studied. The effect of l-glutamate on depolarization-induced release depended both upon the degree of neuronal depolarization and upon the presence of external Ca²⁺ in the superfusion medium and it was blocked by l-glutamate diethylester. Furthermore, this effect of l-glutamate seemed quite specific with regard to regional localization within the brain as it was only demonstrated in slices from striatum and not in slices from olfactory tubercle or hippocampus. It is suggested that during depolarization a Ca²⁺-dependent event occurs at the striatal membrane level which changes the sensitivity of the dopamine release process to neuroexcitatory aminoacids in such a way as to render it relatively more specific and stereoselective towards l-glutamate stimulation. The findings reported have led us to propose that l-glutamic acid could play a role as a neuromodulator of dopaminergic transmission in the rat corpus striatum.     

Effects of dextromethorphan on dopamine dependent behaviours in rats

Dextromethorphan, a noncompetitive blocker of N-methyl-D- aspartate (NMDA) type of glutamate receptor, at 7.5-75 mg/kg, ip did not induce oral stereotypies or catalepsy and did not antagonize apomorphine stereotypy in rats. These results indicate that dextromethorphan at 7.5-75 mg/kg does not stimulate or block postsynaptic striatal D2 and D1 dopamine (DA) receptors. Pretreatment with 15 and 30 mg/kg dextromethorphan potentiated dexamphetamine stereotypy and antagonised haloperidol catalepsy. Pretreatment with 45, 60 and 75 mg/kg dextromethorphan, which release 5-hydroxytryptamine (5-HT), however, antagonised dexamphetamine stereotypy and potentiated haloperidol catalepsy. Apomorphine stereotypy was not potentiated or antagonised by pretreatment with 7.5-75 mg/kg dextromethorphan. This respectively indicates that at 7.5-75 mg/kg dextromethorphan does not exert facilitatory or inhibitory effect at or beyond the postsynaptic striatal D2 and D1 DA receptors. The results are explained on the basis of dextromethorphan (15-75 mg/kg)-induced blockade of NMDA receptors in striatum and substantia nigra pars compacta. Dextromethorphan at 15 and 30 mg/kg, by blocking NMDA receptors, activates nigrostriatal dopaminergic neurons and thereby potentiates dexampetamine stereotypy and antagonizes haloperidol catalepsy. Dextromethorphan at 45, 60 and 75 mg/kg, by blocking NMDA receptors, releases 5-HT and through the released 5-HT exerts an inhibitory influence on the nigrostriatal dopaminergic neurons with resultant antagonism of dexampetamine stereotypy and potentiation of haloperidol catalepsy.