Methyl-4-phenylpyridinium (MPP(+))-evoked dopamine release from rat striatal slices: possible roles of voltage-dependent calcium channels and reverse dopamine transport.

We examined the properties of voltage-dependent Ca(2+) channels (VDCCs) mediating 1-methyl-4-phenylpyridinium (MPP(+))-evoked [3H]DA release from rat striatal slices. In some cases, the Ca(2+)-independent efflux of neurotransmitters is mediated by the high-affinity neurotransmitter-uptake systems. To determine whether such a mechanism might be involved in MPP(+)-evoked [3H]DA release. MPP(+) (1,10 and 100 microM) evoked the release of [3H]DA from rat striatal slices in a concentration-dependent manner. In the absence of Ca(2+), MPP(+) (10 and 100 microM)-evoked [3H]DA release was significantly decreased to approximately 50% of control (a physiological concentration of Ca(2+)). In the presence of Ca(2+), nomifensine (0.1,1 and 10 microM) dose-dependently and significantly inhibited the MPP(+)-evoked release of [3H]DA. Nomifensine (1 and 10 microM) also dose-dependently and significantly inhibited the MPP(+)-evoked release of [3H]DA under Ca(2+)-free conditions. MPP(+)-evoked [3H]DA release was partly inhibited by nicardipine (1 and 10 microM), an L-type Ca(2+) channel blocker. On the other hand, the N-type Ca(2+) channel blocker omega-conotoxin-GVIA (omega-CTx-GVIA) (1 and 3 microM) did not affect this release. omega-agatoxin-IVA (omega-Aga-IVA) at low concentrations (0.1 microM), which are sufficient to block P-type Ca(2+) channels alone, also had no effect. On the other hand, MPP(+)-evoked [3H]DA release was significantly decreased by high concentrations of omega-Aga-IVA (0.3 microM) that would inhibit Q-type Ca(2+) channels. In addition, application of the Q-type Ca(2+) channel blocker omega-conotoxin-MVIIC (omega-CTx-MVIIC) (0.3 and 1 microM) also significantly inhibited MPP(+)-evoked [3H]DA release. These results suggest that MPP(+)-evoked [3H]DA release from rat striatal slices is largely mediated by Q-type Ca(2+) channels, and the Ca(2+)-independent component is mediated by reversal of the DA transport system.     

Autoreceptor Regulation of Glutamate and Aspartate Release from Slices of the Hippocampal CA1 Area

Slices of hippocampal area CA1 were employed to test the hypothesis that the release of glutamate and aspartate is regulated by the activation of excitatory amino acid autoreceptors. In the absence of added Mg2+, N-methyl-D-aspartate (NMDA)-receptor antagonists depressed the release of glutamate, aspartate, and gamma-aminobutyrate evoked by 50 mM K+. Conversely, the agonist NMDA selectively enhanced the release of aspartate. The latter action was observed, however, only when the K+ stimulus was reduced to 30 mM. Actions of the competitive antagonists 3-[(+/- )-2-carboxypiperazin-4-yl]-propyl-l-phosphonic acid (CPP) and D-2-amino-5-phosphonovalerate (D-AP5) differed, in that the addition of either 1.2 mM Mg2+ or 0.1 microM tetrodotoxin to the superfusion medium abolished the depressant effect of CPP without diminishing the effect of D-AP5. These results suggest that the activation of NMDA receptors by endogenous glutamate and aspartate enhances the subsequent release of these amino acids. The cellular mechanism may involve Ca2+ influx through presynaptic NMDA receptor channels or liberation of a diffusible neuromodulator linked to the activation of postsynaptic NMDA receptors. (RS)-alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, a selective quisqualate receptor agonist, and kainate, an agonist active at both kainate and quisqualate receptors, selectively depressed the K(+)-evoked release of aspartate. Conversely, 6-cyano-7-nitro-quinoxaline-2,3-dione, an antagonist active at both quisqualate and kainate receptors, selectively enhanced aspartate release. These results suggest that glutamate can negatively modulate the release of aspartate by activating autoreceptors of the quisqualate, and possibly also of the kainate, type. Thus, the activation of excitatory amino acid receptors has both presynaptic and postsynaptic effects.     

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.     

Effect of MK-801 on dopamine release evoked by hypoxia combined with hypoglycemia.

[3H]dopamine ([3H]DA) release was measured from rat striatal slices under normoxic and hypoxic conditions. In some experiments hypoxia was combined with glucose withdrawal. Hypoxia increased the evoked release of dopamine without affecting resting release. Hypoglycemia itself increased only the resting release of [3H]DA. In the absence of glucose hypoxia provoked a dramatic rise in both resting and stimulation-evoked release of dopamine. This effect was partly reduced by Ca2+ withdrawal, and was abolished in the presence of tetrodotoxin (1 microM). The NMDA-receptor antagonist MK-801 (3 microM) attenuated the effect of hypoxia and hypoglycemia on [3H]DA release. It was suggested that activation of NMDA receptors is involved in dopamine release during hypoxia and energy deprivation.     

Comparison of dopamine uptake and release in vitro in sheep and rat striatum.

Cocaine inhibits tritium-labeled dopamine ([3H]DA) uptake in rat (IC50 approximately 400 nM) and sheep (IC50 approximately 1 microM) striatum. GBR 12909, a selective DA uptake inhibitor, potently inhibits [3H]DA uptake in rat (IC50 less than 10 nM), but is less effective (only 60% of the uptake is inhibited at a concentration of 10 microM) and less potent (IC50 approximately 300 nM) in sheep. [3H]DA release from slices of rat or sheep striatum is stimulated by potassium (15-50 mM). In the presence of nomifensine (10 microM), cocaine (10 microM) had no effect on potassium-stimulated [3H]DA release in either species. [3H]DA release is increased by N-methyl-D-aspartate (NMDA) (10-1000 microM) in rat striatum but NMDA did not stimulate [3H]DA release in sheep striatum. These findings suggest that NMDA receptors either are absent from or do not regulate release of preloaded [3H]DA in sheep striatum.     

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.     

Long-Term Survival of Intrastriatal Dopaminergic Grafts: Modulation of Acetylcholine Release by Graft-Derived Dopamine

The nigrostriatal dopaminergic system of rats was unilaterally lesioned with 6-hydroxydopamine. Part of the animals was grafted 2 weeks later with fetal dopaminergic cells on the lesioned side; untreated rats of the same strain served as controls. Both 3 and 12-14 months after surgery the striatal dopamine (DA) content and the in vivo rotational response following injection of D-amphetamine showed significant changes in grafted as compared to lesioned animals. At 12-14 months after transplantation, the electrically evoked release of tritiated DA and acetylcholine (ACh) in slices (preincubated with [3H]DA or [3H]choline, respectively) of striata of intact, lesioned, or grafted animals was also investigated. Electrical field stimulation of striatal slices of the lesioned side did not evoke any significant [3H]DA overflow, whereas a marked [3H]DA release was observed in slices of grafted and control striata. Moreover, both DL-amphetamine (3 microM) and nomifensine (10 microM) strongly enhanced basal 3H outflow in these slices. Electrically evoked [3H]ACh release was significantly reduced in slices from all striatal tissues by 0.01 microM apomorphine. In slices from denervated striata a clearcut hypersensitivity for this action of apomorphine was present, indicating supersensitivity of DA receptors on cholinergic terminals; this hypersensitivity was significantly reduced in graft-bearing striata. Furthermore, because this hypersensitivity was unchanged in slices of lesioned striata under stimulation conditions (four pulses/100 Hz) avoiding inhibition by endogenously released DA, it is concluded that lesion-induced DA receptor supersensitivity is caused by an increase in receptor density or efficacy rather than by a decreased competition between endogenous and exogenous agonists. Both reuptake blockade of DA with nomifensine (10 microM) and release of endogenous DA by DL-amphetamine (3 microM) potently reduced [3H]ACh release only in control and grafted but not in lesioned tissue. In experiments using potassium-evoked [3H]ACh release, tetrodotoxin had no effect on the inhibitory activity of amphetamine and nomifensine, indicating that the DA receptors involved in their indirect inhibitory action are located directly on the cholinergic terminals.     

Excitatory amino acid receptor agonists stimulate membrane inositol phospholipid hydrolysis and increase cytoplasmic free Ca2+ in primary cultures of retinal neurons

A variety of neurotransmitters are believed to elicit effects through receptor-stimulated inositol phospholipid metabolism. It appears that most major types of retinal neurons receive a direct glutamatergic input. The aim of the present studies was to characterize excitatory amino acid (EAA) receptor-mediated breakdown of inositol phospholipids and changes in Ca2+ homeostasis in primary avian retinal cell cultures. Cell monolayers, prepared from 8-day-old chick embryo neural retina, were labelled with [3H]inositol for 48 h, and used after 7 days in vitro. Kainic acid stimulated the accumulation of inositol phosphates in a time- and dose-dependent manner (ED50 = 30 microM). The EAA receptor agonists glutamate, N-methyl-D-aspartate (NMDA), ibotenate and quisqualate were all active, with the rank order: glutamate greater than kainate greater than NMDA much greater than ibotenate approximately quisqualate. External Ca2+ was required for these effects. Agonist actions were inhibited by type-specific antagonists, and also Mg2+ in the case of glutamate and NMDA. Glutamate, NMDA and kainate also elevated cytosolic free Ca2+ in individual retinal cells loaded with the Ca2(+)-sensitive dye Fura-2, as assessed by digital fluorescence ratio imaging microscopy. The agonist-induced increases in [Ca2+]i were largely dependent on extracellular Ca2+, independent of membrane depolarization and were blocked by Mg2+ for glutamate and NMDA. These results demonstrate that vertebrate retinal cells possess EAA receptors coupled to intracellular signal transduction pathways.     

Transmitter-Like Release of Endogenous 3,4-Dihydroxyphenylalanine from Rat Striatal Slices

Biphasic electrical field stimulation (0.5-5 Hz, 2 ms, 25 V, 3 min) and high K+ (10-30 mM, 5 min) released endogenous 3,4-dihydroxyphenylalanine (DOPA) from superfused rat striatal slices. Characteristics of the DOPA release were compared with those of 3,4-dihydroxyphenylethylamine (dopamine, DA). Electrical stimulation at 2 Hz evoked DOPA and DA over similar time courses. alpha-Methyl-p-tyrosine (0.2 mM) markedly reduced release of DOPA but not of DA. Maximal release (0.3 pmol) of DOPA was obtained at 2 Hz and at 15 mM K+. The impulse-evoked release of DOPA and DA was completely tetrodotoxin (0.3 microM) sensitive and Ca2+ dependent and the 15 mM K+-evoked release was also Ca2+ dependent. On L-[3,5-3H]tyrosine (1 microM) superfusion, high K+ (15 and 60 mM) released DOPA and DA together with concentration-dependent decreases in tyrosine 3-monooxygenase (EC 1.14.16.2) activity as indicated by [3H]H2O formation, followed by concentration-dependent increases after DOPA and DA release ended. These findings suggest that striatal DOPA is released by a Ca2+-dependent excitation-secretion coupling process similar to that involved in transmitter release.     

Regional release of [3H]dopamine from rat brain in vitro: effects of opioids on release induced by potassium, nicotine, and L-glutamic acid

Previous studies have suggested that the release of dopamine (DA) in the rat brain may be sensitive to modulation by opioid agents, including the endogenous opioid peptides (enkephalins and endorphins). The present study examined the effects of morphine and the enkephalin analogue D-Ala2-Met5-enkephalinamide (DALA) on the release of radiolabeled DA from superfused slices of rat brain regions. The release of preloaded [3H]DA was evoked from slices of the caudate-putamen (CP) by application of potassium (K+), nicotine (NIC), or L-glutamic acid (L-GLU). The release of [3H]DA from slices of the nucleus accumbens (NA), olfactory tubercle (OT), and substantia nigra (SN) was evoked by L-GLU. Both K+ and NIC evoked a concentration-related release of [3H]DA from CP slices. K+-induced release was only partially dependent on calcium (Ca2+), while NIC-evoked release was completely Ca2+ independent. Neither morphine nor DALA influenced the release of [3H]DA evoked by K+ or NIC. L-GLU produced a concentration-dependent release of [3H]DA from slices of CP, NA, OT, and SN. In all four brain regions, this release was (a) Ca2+-dependent, (b) strongly inhibited by low concentrations of magnesium (Mg2+), (c) greater than the release evoked by D-GLU, (d) attenuated by the putative L-GLU receptor antagonist glutamic acid diethylester (GDEE), and (e) insensitive to tetrodotoxin (TTX) except in the SN. Morphine produced a significant inhibition of L-GLU-evoked [3H]DA release from all four regions. Naloxone, which by itself had no significant effect on the L-GLU-evoked release of [3H]DA, blocked the inhibitory effect of morphine on this release in the CP but not in the other regions. Levorphanol and dextrorphan were equipotent in reducing the glutamate-stimulated release of [3H]DA from CP slices. DALA had no effect on L-GLU-induced release in any of the brain regions examined. The results indicate that L-GLU provokes regional release of DA by acting at a Mg2+-sensitive glutamate receptor. This release is selectively modified by morphine through a mechanism which is insensitive to naloxone.     

Glutamate Receptor Subtypes in Cultured Cerebellar Neurons: Modulation of Glutamate and γ-Aminobutyric Acid Release

Using cerebellar, neuron-enriched primary cultures, we have studied the glutamate receptor subtypes coupled to neurotransmitter amino acid release. Acute exposure of the cultures to micromolar concentrations of kainate and quisqualate stimulated D-[3H]aspartate release, whereas N-methyl-D-aspartate, as well as dihydrokainic acid, were ineffective. The effect of kainic acid was concentration dependent in the concentration range of 20-100 microM. Quisqualic acid was effective at lower concentrations, with maximal releasing activity at about 50 microM. Kainate and dihydrokainate (20-100 microM) inhibited the initial rate of D-[3H]aspartate uptake into cultured granule cells, whereas quisqualate and N-methyl-DL-aspartate were ineffective. D-[3H]Aspartate uptake into confluent cerebellar astrocyte cultures was not affected by kainic acid. The stimulatory effect of kainic acid on D-[3H]aspartate release was Na+ independent, and partly Ca2+ dependent; the effect of quisqualate was Na+ and Ca2+ independent. Kynurenic acid (50-200 microM) and, to a lesser extent, 2,3-cis-piperidine dicarboxylic acid (100-200 microM) antagonized the stimulatory effect of kainate but not that of quisqualate. Kainic and quisqualic acid (20-100 microM) also stimulated gamma-[3H]-aminobutyric acid release from cerebellar cultures, and kynurenic acid antagonized the effect of kainate but not that of quisqualate. In conclusion, kainic acid and quisqualic acid appear to activate two different excitatory amino acid receptor subtypes, both coupled to neurotransmitter amino acid release. Moreover, kainate inhibits D-[3H]aspartate neuronal uptake by interfering with the acidic amino acid high-affinity transport system.     

Pharmacology of putative glutamate receptors from insect skeletal muscles, insect central nervous system and rat brain

Binding of [3H]glutamate to housefly brain and honeybee brain and thoracic muscle membranes as well as to the American cockroach nerve cord was measured in Na+-free Tris-citrate buffer, 2.5 mM CaCl2, pH 7.4. The dissociation constants (KDS) ranged from 0.16 to 1.36 microM, and thoracic muscles had 2-4-fold higher density of receptors than brain tissue. The potent inhibitors of housefly brain binding were in decreasing order of effectiveness: L-glutamate greater than L-aspartate = L-cysteate = ibotenate greater than quisqualate greater than L-homocysteate greater than L-APB greater than L-APV greater than NMDA greater than D-APB greater than D-glutamate, with no inhibition by 100 microM of GDEE, dihydrokainate, D-APV, D-homocysteate or D-aspartate. The drug specificity of [3H]glutamate binding sites in housefly brain was generally similar to that of binding sites in housefly muscle, except that the former had a slightly higher affinity for L-APB, L-homocysteate and NMDA. [3H]Glutamate binding to insect tissues differed in its drug sensitivity from binding to rat brain. Binding to insect membranes was much less sensitive to L-APB, D-APB, APV, homocysteate, L-cysteate, quisqualate and ibotenate. However, the insect binding site was much more stereoselective for the L than D isomers of glutamate and aspartate, while the rat brain site was more stereoselective for APB. It is suggested that the observed [3H]glutamate binding to insect tissue is not to NMDA or kainate receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

NMDA and AMPA Receptors Evoke Transmitter Release from Noradrenergic Axon Terminals in the Rat Spinal Cord

N-methyl-D-aspartate (NMDA) stimulated release of [³H]noradrenaline (NA) from prelabelled rat spinal cord slices. The release was partially insensitive to tetrodotoxin (TTX) and was inhibited by the NMDA antagonist MK-801. Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) also evoked release of [³H]NA, which was enhanced by blocking AMPA receptor desensitization with cyclothiazide. AMPA-evoked release was inhibited by the non-NMDA antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)-quinoxaline (NBQX) but was not affected by TTX. NMDA and AMPA showed synergistic effects, indicating co-existence of NMDA and AMPA receptors on noradrenergic terminals. Kainate evoked [³H]NA release only at high concentrations and the release was not potentiated by blocking kainate receptor desensitization with concanavalin A. Thus, the results indicate that there are stimulatory presynaptic NMDA and AMPA receptors on noradrenergic axon terminals in the spinal cord and that they interact synergistically to evoke release of [³H]NA.     

Quisqualic Acid Modulates Kainate Responses in Cultured Cerebellar Granule Cells

The activation of kainic acid and quisqualic acid receptors in cultured cerebellar granule cells stimulated the release of preaccumulated D-[3H]aspartate. The effect of kainate could be distinguished from that of quisqualate by its sensitivity to the antagonists kynurenic acid and 2,3-cis-piperidine dicarboxylic acid. At a concentration of kainic acid (50 microM) close to its half-maximal releasing effect, simultaneous addition of quisqualic acid (10-50 microM) resulted in a significant dose-dependent inhibition of the kainate-induced component of D-[3H]aspartate release, which was monitored by the progressive decrease in sensitivity of the evoked release to kynurenic acid. In contrast, when kainic acid was used at a subeffective concentration (10 microM), addition of low doses of quisqualate (2-5 microM) resulted in a synergistic effect on D-[3H]aspartate release. Under these conditions, the effect of the two agonists was sensitive to kynurenic acid. Kainic acid (50-100 microM) also caused a dose-dependent, kynurenic acid-sensitive accumulation of cyclic GMP (cGMP) in granule cell cultures. Quisqualic acid was, by itself, ineffective and prevented, in a dose-dependent manner, the kainate-induced cGMP formation (IC50 = 5 microM). Finally, the guanylate cyclase activator sodium nitroprusside greatly enhanced cGMP formation but had no effect on D-[3H]aspartate release. Together, these results demonstrate the existence of complex interactions between quisqualic and kainic acids and indicate that the effects of the two glutamate agonists on D-[3H]aspartate release and on cGMP accumulation are independent.     

Interleukin-2 Modulates Evoked Release of [3H]Dopamine in Rat Cultured Mesencephalic Cells

Abstract: Mesencephalic cell cultures were used as a model to investigate the effects of interleukin-2 (IL-2) on evoked release of [³H]dopamine ([³H]DA) and γ-[³H]-aminobutyric acid ([³H]GABA). At low concentrations (10^?13-10^?12M), IL-2 potentiated [³H]DA release evoked by the excitatory amino acids N-methyl-D-aspartate (NMDA) and kainate, whereas higher IL-2 concentrations (10^?9-10^?8M) had no effect. IL-2 (10^?14-10^?8M) modulated K⁺-evoked [³H]DA release in a biphasic manner, with low concentrations (10^?12-10^?11M) of IL-2 potentiating and higher concentrations (10^?9-10^?8M) inhibiting K⁺-induced [³H]DA release. IL-2 (10^?14-10^?8M) by itself failed to alter spontaneous [³H]DA release. The inhibition by IL-2 of K⁺-evoked [³H]DA release was reversible and not due to neurotoxicity, as preexposure to IL-2 (10^?8M) had no significant effect on the subsequent ability of dopaminergic cells to take up and to release [³H]DA. Under our experimental conditions, IL-2 (10^?8 M) did not alter Ca²⁺-independent [³H]GABA release evoked by either K⁺ or NMDA. The results of this study indicate that IL-2 is able to potentiate [³H]DA release evoked by a number of different stimuli, including K⁺ depolarization and activation of both NMDA and non-NMDA receptor subtypes in mesencephalic cell cultures. IL-2 is active at very low concentrations, a finding that indicates a potent effect of IL-2 on dopaminergic neurons and implicates a physiological role for this cytokine in the modulation of DA release.     

Excitatory amino acids inhibit stimulated phosphoinositide hydrolysis in the rat prefrontal cortex

In rat prefrontal cortical slices, the excitatory amino acids N-methyl-D-aspartate (NMDA), ibotenate, L-aspartate, quisqualate, kainate and L-glutamate inhibit carbachol-induced phosphoinositide hydrolysis as measured by the accumulation of [3H]inositol-1-phosphate ([3H]IP1). NMDA dose-dependently inhibited the carbachol response (IC50 = 14.4 microM), and this inhibition was blocked by the NMDA receptor antagonist D,L-aminophosphonovaleric acid. Lowering medium Na+ concentration to 10 mM or exposing slices to pertussis toxin alleviated the inhibitory effect of NMDA on carbachol-induced [3H]IP1 formation. Serotonin-induced stimulation of [3H]IP1 was also inhibited by NMDA; in contrast, stimulation by norepinephrine, epinephrine or dopamine was unaffected. The results suggest that excitatory amino acids, besides their traditional role as stimulatory substances, can also act to inhibit the production of 2nd messengers activated by certain neurotransmitters in the brain.     

Chronic Nicotine Administration Differentially Affects Neurotransmitter Release from Rat Striatal Slices

Abstract: The objective of these experiments was to determine whether the chronic administration of nicotine, at a dose regimen that increases the density of nicotine binding sites, alters the nicotine-induced release of [³H]dopamine ([³H]DA), [³H]norepinephrine ([³H]NE), [³H]serotonin ([³H]5-HT), or [³H]acetylcholine ([³H]ACh) from rat striatal slices. For these experiments, rats received subcutaneous injections of either saline or nicotine bitartrate [1.76 mg (3.6 µmol)/kg, dissolved in saline] twice daily for 10 days, and neurotransmitter release was measured following preloading of the tissues with [³H]DA, [³H]NE, [³H]5-HT, or [³H]choline. Chronic nicotine administration did not affect the accumulation of tritium by striatal slices, the basal release of radioactivity, or the 25 mM KCl-evoked release of neurotransmitter. Superfusion of striatal slices with 1, 10, and 100 µM nicotine increased [³H]DA release in a concentration-dependent manner, and release from slices from nicotine-injected animals was significantly (p < 0.05) greater than release from saline-injected controls; release from the former increased to 132, 191, and 172% of release from the controls following superfusion with 1, 10, and 100 µM nicotine, respectively. Similarly, [³H]5-HT release increased in a concentration-related manner following superfusion with nicotine, and release from slices from nicotine-injected rats was significantly (p < 0.05) greater than that from controls. [³H]5-HT release from slices from nicotine-injected rats evoked by superfusion with 1 and 10 µM nicotine increased to 453 and 217%, respectively, of release from slices from saline-injected animals. The nicotine-induced release of [³H]NE from striatal slices was also concentration dependent but was unaffected by chronic nicotine administration. [³H]ACh release from striatal slices could not be detected when samples were superfused with nicotine but was measurable when tissues were incubated with nicotine. The release of [³H]ACh from slices from nicotine-injected rats was significantly (p < 0.05) less than release from controls and decreased to 36, 83, and 77% of control values following incubation with 1, 10, or 100 µM nicotine, respectively. This decreased [³H]ACh release could not be attributed to methodological differences because slices from nicotine-injected rats incubated with nicotine exhibited an increased [³H]DA release, similar to results from superfusion studies. In addition, it is unlikely that the decreased release of [³H]ACh from striatal slices from nicotine-injected rats was secondary to increased DA release because [³H]ACh release from slices from hippocampus, which is not tonically inhibited by DA, also decreased significantly (p < 0.05) in response to nicotine; hippocampal slices from nicotine-injected rats incubated with 1 and 10 µM nicotine decreased to 42 and 70%, respectively, of release from slices from saline-injected animals. Results indicate that the chronic administration of nicotine increases the ability of nicotine to induce the release of [³H]DA and [³H]5-HT and decreases the ability of nicotine to evoke the release of [³H]ACh but does not alter the nicotine-induced release of [³H]NE from brain slices.     

Changes in Excitatory Amino Acid Receptor Binding in the Intact and Decorticated Rat Neostriatum Following Insulin-Induced Hypoglycemia

An involvement of excitatory amino acid (EAA) transmitter-receptor interactions in the development of hypoglycemia-induced neuronal damage has been suggested. We report here on the binding to EAA receptors in the rat caudate nucleus and cerebral cortex, during and following severe insulin-induced hypoglycemia with an isoelectric EEG of 10 or 30 min duration. The binding of alpha-[3H]amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [( 3H]AMPA) to quisqualate receptors, [3H]kainic acid (KA) to kainate receptors, and [3H]glutamate to N-methyl-D-aspartate (NMDA)-sensitive sites was determined by quantitative autoradiography. During EEG isoelectricity, AMPA binding was reduced by approximately 40%, which could represent quisqualate receptor desensitization. One hour following glucose-induced recovery, AMPA binding was no longer different from control level. As the recovery period was prolonged to 1 or 4 weeks, AMPA binding decreased. The decrease was more pronounced in the dorsolateral than in the ventromedial part of the striatum. This correlates with the distribution of neuronal damage, and probably reflects loss of receptor binding sites due to cell death. During the period of EEG silence there was a tendency toward an increase in NMDA displaceable glutamate binding. Following 4 weeks of recovery, binding to NMDA receptors was significantly decreased. Glutamate binding to NMDA-sensitive sites was remarkably resistant to neuronal necrosis and was not significantly different from control values in the dorsolateral caudate 1 week following the hypoglycemic coma. No changes in KA binding were found until 1 week posthypoglycemia, when a significant reduction in binding was noted in the lateral striatum.(ABSTRACT TRUNCATED AT 250 WORDS)     

Homocysteic Acid as a Putative Excitatory Amino Acid Neurotransmitter: I. Postsynaptic Characteristics at N-Methyl-D-Aspartate-Type Receptors on Striatal Cholinergic Interneurons

The actions of the stereoisomers of homocysteic acid (HCA) were characterized at N-methyl-D-aspartate (NMDA)-type receptors which mediate excitatory amino acid-evoked [3H]acetylcholine ([3H]ACh) release from striatal cholinergic interneurons. Like NMDA, L-HCA and D-HCA evoked the release of [3H]ACh formed from [3H]choline in striatal slices. The concentration-response curve for L-HCA was virtually superimposable on that for NMDA, yielding an equal EC50 value (56.1 microM) and maximal response. However, D-HCA was weaker, with an EC50 value of 81.1 microM, and an apparently smaller maximal response. L-HCA-evoked [3H]ACh release was inhibited by the same categories of compounds which inhibit NMDA-evoked [3H]ACh release: the divalent ion Mg2+ (IC50 = 25.8 microM); competitive NMDA antagonists 2-amino-7-phosphonoheptanoate (IC50 = 51.2 microM) and 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (IC50 = 20.1 microM); and the dissociative anesthetics tiletamine (IC50 = 0.59 microM) and MK-801 (IC50 = 0.087 microM). Like NMDA, L-HCA produced a tachyphylaxis in this system. Tachyphylaxis to NMDA resulted in a decrease response to L-HCA, and conversely, tachyphylaxis to L-HCA resulted in a decrease response to NMDA. The results suggest that L-HCA is an agonist at the NMDA-type receptor and may represent an endogenous ligand for this excitatory amino acid receptor.