Both ethanol and ifenprodil inhibit NMDA-evoked release of various neurotransmitters at different, yet proportional potency: potential relation to NMDA receptor subunit composition

Superfused slices of the rat cerebral cortex and corpus striatum were used to investigate the effect of ethanol on the N-methyl-D-aspartate (NMDA)-evoked release of various [³H]neurotransmitters. To obtain information on the subunit composition of the NMDA receptors involved, the influence of ifenprodil (a drug which preferentially inhibits NMDA receptors containing the NMDAR2B subunit) on the NMDA-evoked [³H]neurotransmitter release was also determined.Ethanol inhibited the NMDA-evoked release of all neurotransmitters investigated in a concentration-dependent manner. Irrespective of the brain region, ethanol was 2.6–5.1 times more potent in inhibiting the release of [³H]noradrenaline, [³H]5-hydroxytryptamine and [³H]GABA than the release of [³H]acetylcholine and [³H]dopamine. Ifenprodil inhibited the NMDA-evoked release of these [³H]neurotransmitters in both brain regions at virtually the same potency order as ethanol. Comparison of the potencies of ethanol and ifenprodil in inhibiting NMDA-evoked [³H]neurotransmitter release revealed an excellent correlation (r = 0.96; P<0.001). After blockade of the highly ifenprodil-sensitive fraction of the NMDA receptors by 100 nM ifenprodil, the potency of ethanol in inhibiting NMDA-evoked [³H]noradrenaline release in the cerebral cortex was decreased by a factor of 6. In contrast, 100 nM ifenprodil did not affect the potency of ethanol in inhibiting [³H]dopamine and [³H]acetylcholine release in the corpus striatum.It is concluded that ethanol predominantly inhibits NMDA receptors containing a high proportion of the NMDAR2B subunit (as reflected by high sensitivity to ifenprodil), i.e. the NMDA receptors involved in stimulation of noradrenaline, 5-hydroxytryptamine and GABA release. In view of the ability of ifenprodil to interact with ethanol at NMDA receptors, which are important sites of action of ethanol, ifenprodil may be assumed to influence neuronal effects of ethanol in vivo.     

N-methyl-d-aspartate (NMDA)-stimulated noradrenaline (NA) release in rat brain cortex is modulated by presynaptic H3-receptors

In superfused rat brain cortex slices and synaptosomes preincubated with [³H]noradrenaline the effect of agonists or antagonists at presynaptic H₃ receptors on NMDA-evoked [³H]noradrenaline release was investigated. In experiments on slices, histamine and the preferential H₃ receptor agonist R-(–)--methylhistamine inhibited NMDA-evoked tritium overflow (IC₂₀ values 0.27 mol/l or 0.032 mol/l, respectively); S-(+)--methylhistamine (up to 10 mol/l) as well as the selective H₁ receptor agonist (2-(2-thiazolyl)ethylamine) and the selective H₂ receptor agonist dimaprit (each up to 10 mol/l) were ineffective. The H₃ receptor antagonist thioperamide abolished the inhibitory effect of histamine whereas the preferential H₁ receptor antagonist dimetindene and the preferential H₂ receptor antagonist ranitidine were ineffective. In experiments on synaptosomes, histamine and R-(–)--methylhistamine inhibited NMDA-evoked tritium overflow, whereas 2-(2-thiazolyl)ethylamine or dimaprit had no effect. The inhibitory effect of histamine was abolished by thioperamide. When tritium overflow was stimulated by NMDA in the presence of -conotoxin GVIA (which by itself decreased the response to NMDA by about 55%), R-(–)--methylhistamine did not inhibit NMDA-evoked overflow. It is concluded that NMDA-evoked noradrenaline release in the cerebral cortex can be modulated by inhibitory H₃ receptors. NMDA receptors and H₃ receptors are both located presynaptically and may interact at the same noradrenergic varicosity. An unimpaired function of the N-type voltage-sensitive calcium channel probably is a prerequisite for the inhibition of NMDA-evoked noradrenaline release by H₃ receptor stimulation. Correspondence to: M. Göthert at the above address     

Functional interactions between presynaptic NMDA receptors and metabotropic glutamate receptors co-expressed on rat and human noradrenergic terminals

BACKGROUND AND PURPOSE: Electrophysiological studies described potentiation of NMDA receptor function by metabotropic glutamate receptors (mGluRs) of group I occurring postsynaptically. Since release-enhancing NMDA receptors exist on noradrenergic terminals and group I mGluRs have recently been identified on these nerve endings, we have investigated if NMDA receptor-mGluR interactions also can occur at the presynaptic level. EXPERIMENTAL APPROACH: Rat hippocampus and human neocortex synaptosomes were labelled with [(3)H]noradrenaline and superfused with mGluR agonists and antagonists. NMDA-evoked [(3)H]noradrenaline release was produced by removal of external Mg(2+) or by simultaneous application of NMDA and AMPA in Mg(2+)-containing solutions. KEY RESULTS: The mGluR1/5 agonist 3,5-DHPG, inactive on its own, potentiated both the release of [(3)H]noradrenaline elicited by AMPA/NMDA/glycine and that evoked by NMDA/glycine following Mg(2+) removal. The effect of 3,5-DHPG on the AMPA/NMDA/glycine-induced release was insensitive to the mGluR1 antagonist CPCCOEt, but it was abolished by the mGluR5 antagonist MPEP; moreover, it was potentiated by the mGluR5 positive allosteric modulator DFB. When NMDA receptors were activated by Mg(2+) removal, both mGluR5 and mGluR1 contributed to the evoked release, the mGluR-mediated release being blocked only by CPCCOEt and MPEP in combination. Experiments with human neocortex synaptosomes show NMDA receptor-mGluR interactions qualitatively similar to those observed in rodents. CONCLUSIONS AND IMPLICATIONS: Group I mGluRs, both of the mGluR1 and mGluR5 subtypes, co-localize with NMDA receptors on noradrenergic terminals of rat hippocampus and human neocortex. Depending on the mode of activation, NMDA receptors exert differential permissive roles on the activation of presynaptic mGluR1 and mGluR5.     

Increased secretion of glandular-kallikrein in the bronchial washings induced by intravenous injection of leukotriene C4 in guinea-pigs.

1. The effects of N-methyl-D-aspartate (NMDA), KCl, and veratridine on the release of endogenous adenosine and its metabolites, inosine and hypoxanthine, from the rat hippocampus have been studied by in vivo microdialysis. 2. In the hippocampus of rats anaesthetized with urethane the adenosine level reached a stable state estimated at 0.93 microM during the first 2 h after the implantation of the dialysis probe. NMDA (50 microM to 25 mM) in the perfusate evoked a concentration-dependent release of adenosine, inosine and hypoxanthine with an EC50 of 180 microM. The release was reduced by 93% by the specific NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (2-AP5) at 200 microM, indicating an NMDA receptor-mediated process. In addition, the 100 mM KCl-evoked release of adenosine was also substantially reduced by 77% by 2-AP5, suggesting that a large component of the K(+)-evoked release is NMDA-receptor-mediated. 3. Perfusion with zero-Ca2+ artificial cerebrospinal fluid attenuated the NMDA-evoked release of adenosine only by 16% (not significant) but depressed the K(+)-evoked release by 62%, indicating that most of the NMDA-evoked release is directly receptor-mediated, whereas a large component of the K(+)-evoked release could be via the release of an excitatory amino acid acting at the NMDA receptors.     

Preferential location of N-methyl-D-aspartate (NMDA) receptors on postsynaptic membranes and on non-noradrenergic nerve terminals of the rat brain cortex

Summary The effect of DSP4-induced destruction of noradrenergic neurones on ³H-3-(2-carboxypiperazine-4-yl)propyl-1-phosphonic acid (³H-CPP) binding to N-methyl-D-aspartate (NMDA) receptors and on ³H-desipramine (³H-DMI) binding to the neuronal noradrenaline carrier was investigated in rat brain cortex buffy coat membranes. ³H-DMI bound with high affinity to a single site at the neuronal noradrenaline carrier (K_D = 5.26 ± 1.67 nmol/l) whereas the binding of ³H-CPP to the NMDA receptor was of intermediate affinity (K_D = 274 ± 45 nmol/l). Fourteen days after a single-dose treatment with DSP4 (1) the B_max value for ³H-DMI binding was reduced by 74%, (2) the B_max value for ³H-CPP binding only tended to be decreased (by 24%; not statistically significant), (3) the endogenous noradrenaline content was reduced by 70% compared to untreated controls and, (4) the absolute amount of the NMDA-evoked ³H-noradrenaline overflow but not the fractional release was reduced by 55%. It is concluded that in the rat cerebral cortex presynaptic NMDA-receptors on noradrenergic nerve endings, which have previously been detected in release experiments with NMDA on cortical synaptosomes preincubated with ³H-noradrenaline, cannot be identified in radioligand binding experiments. Obviously, the cerebral cortical NMDA receptors are predominantly located on postsynaptic neuronal membranes and potentially on non-noradrenergic nerve terminals as well.Send offprint requests to M. Gothert at the above address     

Stimulation of noradrenaline release in human cerebral cortex mediated by N-methyl-D-aspartate (NMDA) and non-NMDA receptors.

1. Human brain cortical slices from patients undergoing neurosurgery for treatment of epilepsy resistant to antiepileptic drugs were used to identify and characterize N-methyl-D-aspartate (NMDA) and non-NMDA receptors mediating stimulation of noradrenaline release. The slices preincubated with [3H]-noradrenaline were superfused with Krebs-Henseleit solution with or without Mg2+ (1.2 mmol l-1) and were stimulated by 2-min exposure to NMDA, kainic acid or (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). 2. In slices superfused without Mg2+, NMDA induced a concentration-dependent tritium overflow. 3. The NMDA-evoked tritium overflow was almost abolished by tetrodotoxin (TTX), Mg2+ or by omission of Ca2+ from the superfusion fluid. 2-Amino-5-phosphonopentanoic acid (AP5; a competitive NMDA receptor antagonist) or dizocilpine (formerly MK-801; an antagonist at the phencyclidine receptor within the NMDA-gated ion channel) inhibited the NMDA-evoked tritium overflow. The stimulatory effect of NMDA was not significantly enhanced by glycine added to the superfusion fluid but was reduced by 7-chlorokynurenic acid (an antagonist at the glycine site coupled to the NMDA receptor). 4. In slices superfused with solution containing Mg2+, kainic acid or AMPA induced a concentration-dependent tritium overflow which was susceptible to blockade by TTX. 5. The kainic acid-evoked tritium overflow was not affected by DL-(E)-2-amino-4-methyl-5-phosphono-3-pentanoic acid (CGP37849; a competitive NMDA receptor antagonist), but was inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; an antagonist at glutamate receptors of the non-NMDA type). 6. The AMPA-evoked tritium overflow was also inhibited by CNQX.2ń     

Stimulation of serotonin release in the rat brain cortex by activation of ionotropic glutamate receptors and its modulation via α2-heteroreceptors

Rat brain cortex slices were used to study (1) the release of 5-hydroxytryptamine (5-HT) induced by activation of N-methyl-D-aspartate (NMDA) or non-NMDA receptors and (2) the ₂-adrenoceptor-mediated modulation of NMDA-evoked 5-HT release.Cortical slices were preincubated with [³H]5-HT in the presence of the selective noradrenaline reuptake inhibitor, maprotiline (to avoid false labelling of noradrenergic axon terminals), and then superfused with solution containing the 5-HT reuptake inhibitor, 6-nitroquipazine. In slices superfused with Mg²⁺-free medium, NMDA and L-glutamate, in a concentration-dependent manner, elicited an overflow of tritium. The NMDA-evoked tritium overflow was abolished by omission of Ca²⁺ ions, almost completely suppressed by 1.2 mM Mg²⁺ and only partly (by about 60%) inhibited by tetrodotoxin. Dizocilpine (formerly MK801), an antagonist at the phencyclidine site within the NMDA-gated channel, also decreased the NMDA-evoked overflow. The competitive NMDA receptor antagonist DL-(E)-2-amino-4-methyl-5-phosphono-3-pentanoic acid (CGP 37849) caused a parallel shift of the NMDA concentration-response curve to the right. The NMDA-induced tritium overflow was not affected by addition of exogenous glycine but was inhibited by 5,7-dichlorokynurenic acid, an antagonist at the glycine site of the NMDA receptor. Spermidine slightly increased the NMDA-induced tritium overflow whereas arcaine, an antagonist at the polyamine site of the NMDA-receptor, caused a decrease. Ifenprodil and eliprodil, which exhibit different affinities for NMDA receptors composed of different subunits were highly potent (in the nanomolar range) in inhibiting the NMDA-evoked tritium overflow. Noradrenaline reduced, whereas the 2-adrenoceptor antagonist idazoxan facilitated, the NMDA-evoked overflow. Idazoxan shifted the concentration-response curve of noradrenaline to the right. In slices superfused with solution containing 1.2 mM Mg²⁺, kainic acid or (RS)--amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) also caused a concentration-dependent overflow of tritium, which again was not completely (by about 75 and 50%, respectively) inhibited by tetrodotoxin. The kainate-evoked tritium overflow was inhibited by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) but not affected by CGP 37849 or arcaine. The AMPA-evoked tritium overflow was also decreased by CNQX.It is concluded that activation of NMDA or non-NMDA receptors elicits a release of 5-HT in the rat brain cortex. The receptors are at least partly located on the serotoninergic nerve terminals. The results with ifenprodil and eliprodil are compatible with the view that the NMDA receptor involved contains the NR2B subunit. The NMDA-evoked 5-HT release is modulated by presynaptic 2-adrenoceptors.     

Inhibition of N-methyl-D-aspartate-induced noradrenaline release by alcohols is related to their hydrophobicity

In rat brain cortex slices, ethanol was three times more potent in inhibiting [3H]noradrenaline ([3H]NA) release evoked by N-methyl-D-aspartate (NMDA) than in inhibiting the release evoked by kainic acid. Methanol, 1-propanol, 1-butanol, 1-pentanol and 1-hexanol shared the inhibitory property of ethanol on NMDA-evoked [3H]NA release. The inhibitory potency of the alcohols, as expressed by their IC50 values, was correlated with their membrane/buffer partition coefficient. It is concluded that the inhibitory effect of the alcohols may be due to a hydrophobic interaction with the NMDA receptor system.     

Ethanol inhibits the N-methyl-D-aspartate (NMDA)-induced attenuation of the NMDA-evoked noradrenaline release in the rat brain cortex: interaction with NMDA-induced desensitization

Summary The influence of ethanol, AP5 (DL-2-amino-5-phosphonopentanoic acid) and dizocilpine (MK-801) ((+)-5-methyl-10, 11-dihydro-5H-dibenzo (a, b)-cyclohept-5,10-imine hydrogen maleate) on the NMDA-induced attenuation of the NMDA-evoked noradrenaline release was examined in rat brain cortex slices preincubated with ³H-noradrenaline. The slices were superfused with Mg²⁺-free Krebs-Henseleit solution. Tritium overflow (corresponding to ³H-noradrenaline release) was stimulated by 300 mol/l NMDA for 2 min.Presence of 10–100 mol/l NMDA from 20 to 2 min before stimulation concentration-dependently inhibited the NMDA (300 mol/l)-evoked ³H overflow, suggesting an agonist-induced desensitization attributable to the modification of events at the NMDA receptor itself and/or distal to this receptor system. The desensitizing effect of 100 mol/l NMDA was almost complete and was not diminished when the time of preexposure was decreased to 10 min, and when NMDA was removed from the superfusion fluid for up to 5 min before the stimulus; however, the densitizing effect was reduced after further decrease in the duration of preexposure to NMDA or further prolongation of the interval between preexposure and stimulation. Ethanol inhibited the NMDA-induced ³H overflow (IC₅₀ 45 mmol/l); this effect was almost abolished when ethanol was omitted from the superfusion fluid from 2 min before stimulation onward. Ethanol, when simultaneously present with 100 mol/l NMDA in the superfusion fluid from 20 to 2 min before the NMDA stimulus, concentration-dependently (IC₅₀ 112 mol/l) decreased the inhibitory effect of NMDA. The effect of ethanol was identical to that of the competitive NMDA receptor antagonist AP5 both with respect to the rapid reversibility of the inhibition of NMDA induced ³H overflow (i.e. within 2 min) and to the decrease of the desensitizing effect of NMDA. The pattern of effects obtained with dizocilpine, an inhibitor of the ion channel coupled to the NMDA receptor, was different from that obtained with ethanol and AP5: the inhibitory effect of dizocilpine on the NMDA-evoked ³H overflow was only partly reversible within 8 min after withdrawal, and the inhibitory effect of NMDA on NMDA-evoked ³H overflow was not decreased by dizocilpine, but was more pronounced than with either NMDA or dizocilpine alone.It is concluded that ethanol mimics AP5 in that in inhibits the NMDA induced desensitization, possibly by preventing the NMDA receptor from being activated by NMDA applied during the desensitization period. In contrast, dizocilpine produces no such effect. The potency of ethanol in inhibiting the NMDA-induced desensitization was about 3 times less than in inhibiting NMDA-evoked noradrenaline release. These results are compatible with the suggestion that the NMDA recognition site itself may be one of the sites of action of ethanol and that ethanol does not modify the function of the NMDA receptor system by a machanism comparable to that of dizocilpine. Send offprint requests to M. Göthert at the above address     

Modulation of N-methyl-d-aspartate (NMDA)-stimulated noradrenaline release in rat brain cortex by presynaptic α2-adrenoceptors

Summary Rat brain cortex slices and synaptosomes preincubated with [³H]noradrenaline were used to investigate whether the NMDA-evoked noradrenaline release is modulated by agonists or antagonists at presynaptic ₂-adrenoceptors.In experiments on slices, noradrenaline and the preferential ₂-adrenoceptor agonists talipexole (former B-HT 920) and clonidine inhibited the NMDA evoked tritium overflow whereas the selective ₁-adrenoceptor agonists cirazoline and methoxamine were ineffective. The ₂-adrenoceptor antagonists rauwolscine and idazoxan facilitated the NMDA-evoked tritium overflow whereas the preferential ₁-adrenoceptor antagonist prazosin was ineffective. The concentration-response curve of talipexole for its inhibitory effect on NMDA-evoked overflow was shifted to the right by idazoxan (apparent pA₂ = 7.5). The EC₅₀ of NMDA (97 mol/l) for its stimulating effect on tritium overflow was not substantially changed by blockade of ₂-autoreceptors with 1 mol/l rauwolscine (EC₅₀ of NMDA in the presence of the ₂-adrenoceptor antagonist, 155 mol/l), but the maximal overflow of tritium was increased 2.5 fold by this rauwolscine concentration. In experiments on synaptosomes, talipexole and noradrenaline inhibited the NMDA-evoked tritium overflow. The inhibitory effect of talipexole was abolished by idazoxan which, given alone, was ineffective, as was prazosin. Talipexole did also not produce an inhibition when tritium overflow was evoked by NMDA in the presence of -conotoxin GVIA 0.1 mol/l; the latter, by itself, decreased the response to NMDA by about 55%. It is concluded that the NMDA-evoked noradrenaline release in the cerebral cortex is modulated via presynaptic ₂-adrenoceptors on the noradrenergic neurones. Stimulation of these autoreceptors in slices by endogenous noradrenaline does not result in a decreased potency of NMDA, but in a decreased maximum effect, in stimulating noradrenaline release. The inhibitory effect of ₂-adrenoceptor agonists on the NMDA-evoked release is at least partially due to a functional interaction between the NMDA receptors and ₂-autoreceptors at the level of the same varicosities. The results obtained with -conotoxin GVIA suggest that Ca²⁺ influx via the N-type voltage-sensitive calcium channel (VSCC) occurs in response to NMDA receptor stimulation and contributes substantially to the induction of NMDA-evoked noradrenaline release. The inhibitory effect of ₂-adrenoceptor stimulation on this release appears to be ultimately due to an inhibition of the influx of Ca²⁺ via the N-type VSCC. Correspondence to: M. Göthert at the above address     

Ionotropic glutamate receptor types leading to adenosine-mediated inhibition of electrically evoked [3H]-noradrenaline release in rabbit brain cortex slices.

1. Glutamate inhibits the electrically evoked release of noradrenaline in rabbit brain cortex slices; the inhibition is mediated by adenyl compounds, presumably adenosine. The aim of the present study was to identify the receptors involved in this indirect inhibitory effect of glutamate. Slices of the occipitoparietal cortex were preincubated with [3H]-noradrenaline and then superfused and stimulated by trains of 6 pulses, 100 Hz. 2. The ionotropic glutamate receptor agonists alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AM-PA; 10-100 microM), kainate (10-100 microM) and N-methyl-D-aspartate (NMDA; 30-300 microM) but not the metabotropic glutamate receptor agonist, 1-amino-1,3-cyclopentanedicarboxylate (ACPD; 10-100 microM) reduced the electrically evoked overflow of tritium. 3. The effects of AMPA, kainate and NMDA were attenuated or abolished by the adenosine A1-receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) as well as by adenosine A1-receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) as well as by adenosine deaminase but not by the alpha 2-adrenoceptor antagonist yohimbine, the gamma-aminobutyric acid (GABA) receptor antagonists, bicuculline and 2-hydroxysaclofen and the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME). 4. The NMDA receptor antagonist, 2-amino-5-phosphonopentanoate (AP5) blocked the inhibitory effect of NMDA but not that of AMPA and kainate. The non-NMDA-receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocked the effect of AMPA but not of kainate and NMDA. 5. In addition to decreasing the electrically evoked overflow of tritium, AMPA, kainate and NMDA but not ACPD caused a steep but transient rise of basal tritium efflux.(ABSTRACT TRUNCATED AT 250 WORDS)     

Suppression of presynaptic responses to adenosine by activation of NMDA receptors.

The interactions between adenosine and NMDA receptors has been investigated using the paired-pulse paradigm in hippocampal slices. This technique allows the study of drug effects specifically at presynaptic terminals. The inhibitory effect of adenosine on population spikes, and the decrease of paired-pulse inhibition assessed using either population spikes or population excitatory postsynaptic potentials, were suppressed by performing the experiments in magnesium-free medium, or by superfusion of the slices with N-methyl-D-aspartate (NMDA) at a concentration (4 microM) which did not itself affect potential size. The suppressant effect of NMDA was prevented by 2-amino-5-phosphonopentanoic acid. All these interactions were still seen in the presence of bicuculline methobromide, 30 microM. Neither alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) nor kainate produced a suppression of adenosine responses. The presence of NMDA did not modify the effects of baclofen on population potentials or paired-pulse inhibition. Activating NMDA receptors by the induction of long-term potentiation or by superfusion with glycine also reduced significantly the effects of adenosine on population spikes and paired-pulse interactions. Increasing population potential size by a mechanism which did not involve the activation of NMDA receptors (increasing stimulus strength) did not change sensitivity to adenosine. When adenosine receptor-selective agonists were tested, it was found that NMDA did not modify the inhibitory effect of the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine, but did enhance the excitatory effect of the adenosine A(2A) receptor agonist 2-[p-(2-carboxyethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680). The combined response to NMDA and CGS21680 was prevented by the adenosine A(2A) receptor selective antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM241385). It is concluded that NMDA receptor activation can suppress neuronal sensitivity to adenosine by acting at presynaptic sites, and that this interaction results from an increase in the excitatory action of adenosine A(2A) receptors, rather than a depression of A(1) receptor function.     

ATP modulates the release of noradrenaline through two different prejunctional receptors on the adrenergic nerves of rat prostate

1. The effects of adenosine and ATP receptor agonists on the release of endogenous noradrenaline from electrically stimulated (2 Hz, 0.1 msec) rat prostate were examined in order to clarify the pharmacological properties of prejunctional receptors for adenosine and ATP on the adrenergic nerve varicosities in the prostate. Noradrenaline was quantified by HPLC coupled with electrochemical detection techniques. 2. Both adenosine and ATP receptor agonists (1 micromol/L) inhibited noradrenaline release and the relative order of inhibitory effect was N(6)-cyclopentyl-adenosine (CPA) > 5'-N-ethylcarboxamidoadenosine > 2-chloroadenosine > adenosine > 2-methylthio-ATP (2mSATP) > AMP > ATP. 3. The adenosine receptor agonist CPA (1 nmol/L-1 micromol/L) and the ATP receptor agonist 2mSATP (100 nmol/L-100 micromol/L) inhibited the stimulation-induced release of noradrenaline in a concentration-dependent manner. The concentrations of CPA and 2mSATP that produced 50% inhibition of noradrenaline release were 9.6 nmol/L and 1.4 micromol/L, respectively. 4. 1,3-Dipropyl-8-cyclopentylxanthine, an adenosine A(1) receptor antagonist, significantly reduced the inhibitory effects of not only CPA, but also 2mSATP. 5. Suramin, an ATP receptor antagonist, significantly reduced the inhibitory effects of 2mSATP, but not those of CPA. 6. Pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid, another ATP receptor antagonist, had no effect on the inhibitory action of either agonist. 7. These results suggest that, in the sympathetic nerve terminals of rat prostate, adenosine and ATP induce inhibition of noradrenaline release via the activation of adenosine A(1) and/or xanthine-sensitive ATP receptors, which play an inhibitory regulatory role in adrenergic neurotransmission in the prostate.     

Inhibitory and potentiating influences of glycine on N-methyl-D-aspartate-evoked dopamine release from cultured rat mesencephalic cells.

In the presence of 1.2 mM Mg2+, glycine (30-100 microM) inhibited [3H]dopamine ([3H]DA) release stimulated by N-methyl-D-aspartate (NMDA), in fetal rat mesencephalic cell cultures. Strychnine (1 microM) blocked the inhibitory effect of 100 microM glycine, indicating an action via strychnine-sensitive inhibitory glycine receptors. A higher concentration of strychnine (100 microM), by itself, inhibited NMDA-evoked [3H]DA release in the presence or absence of Mg2+. Spontaneous [3H]DA release and [3H]DA release stimulated by kainate and quisqualate were unaffected by glycine (less than or equal to 100 microM) or strychnine (less than or equal to 100 microM), indicating that glycine and strychnine modulatory effects are only associated with the NMDA receptor subtype. [3H]DA release evoked by K+ (56 mM) was unaffected by glycine (less than or equal to 100 microM) but was attenuated by a high concentration of strychnine (100 microM). In the absence of exogenous Mg2+, glycine (30-100 microM) potentiated NMDA-evoked [3H]DA release by a strychnine-insensitive mechanism. A selective antagonist of the NMDA-associated glycine receptor, 7-chlorokynurenate (10 microM), attenuated NMDA-evoked [3H]DA release in the absence of Mg2+. The effect of 10 microM 7-chlorokynurenate was overcome by 1 microM glycine. Also, when tested in the presence of 1.2 nM Mg2+ and 1 microM strychnine, 100 microM 7-chlorokynurenate inhibited NMDA-evoked [3H]DA release, and this antagonism was overcome by 30 to 100 microM glycine. These results indicate that two distinct glycine receptors modulate NMDA-stimulated [3H]DA release from mesencephalic cells in culture. Manipulation of extracellular Mg2+ permits the differentiation of a strychnine-sensitive glycine response (inhibition of NMDA-evoked [3H]DA release) from a strychnine-insensitive glycine response (potentiation of NMDA-evoked [3H]DA release). It is suggested that voltage-dependent Mg2+ blockade of the NMDA response may allow for the expression of these opposing effects of glycine.     

Kainate receptors are involved in the glutamate-induced indirect,purinergic inhibition of [3H]-noradrenaline release in rabbit brain cortex

Activation of ionotropic but not of metabotropic glutamate receptors causes an indirect inhibition of the release of noradrenaline in slices of rabbit brain cortex. The inhibition is mediated by adenosine which activates presynaptic adenosine A₁-receptors. The present study characterizes the ionotropic receptor types through which glutamate itself produces this indirect inhibition. Rabbit brain cortex slices were preincubated with [³H]-noradrenaline, superfused with medium containing desipramine (1 M) and stimulated electrically by trains of 6 pulses at 100 Hz.Glutamate (100–3000 M) reduced the electrically evoked overflow of tritium by up to 58 %. The effect did not differ 20 min and 60 min after addition of glutamate. Adenosine deaminase (1 U ml^-1) as well as 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 30 M) and d--glutamylamino-methanesulfonate (GAMS; 30 M), both of which block kainate receptors, attenuated the glutamate-induced inhibition. The NMDA receptor antagonist 2-amino-5-phosphonopentanoate (AP5; 100 M) and the AMPA receptor antagonist 6-nitro-7-sulfamoylbenzo(f)quinoxaline-2,3-dione (NBQX; 30 M) did not change the effect of glutamate. Given alone, CNQX and GAMS, but not AP5 and NBQX, slightly increased the evoked overflow of tritium; the increases were abolished in the presence of adenosine deaminase.The results indicate that activation of kainate but not NMDA and AMPA receptors is involved in the indirect, adenosine-mediated inhibition by exogenous glutamate of the release of noradrenaline in rabbit brain cortex slices. Moreover, as shown by the increase caused by CNQX and GAMS, endogenous excitatory amino acids inhibit the release of noradrenaline through the kainate receptor-adenosine mechanism and thus contribute to the purinergic inhibitory control of noradrenaline release in the brain.     

Functional Interaction Between NMDA and mGlu5 Receptors: Effects on Working Memory, Instrumental Learning, Motor Behaviors, and Dopamine Release.

Pharmacological manipulation of N-methyl-D-aspartate (NMDA) receptors may be critical for the treatment of many neurological and psychiatric disorders. Metabotropic glutamate (mGlu5) receptors are abundant in corticolimbic circuitry, where they modulate NMDA receptor-mediated signal transduction. Therefore, pharmacological manipulation of mGlu5 receptor may provide a treatment strategy for cognitive disorders that are associated with NMDA receptor dysfunction. We sought to determine whether the recently described molecular and cellular interactions between NMDA and mGlu5 receptors coregulate higher order behaviors. We examined the interaction of the selective mGlu5 receptor antagonist, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), and the use-dependent NMDA antagonist MK-801, on locomotion, stereotypy, working memory, instrumental learning, and corticolimbic dopamine release. MPEP, at 10 mg/kg, but not 3 mg/kg, impaired working memory and instrumental learning, transiently increased dopamine release in prefrontal cortex and nucleus accumbens, and augmented the effect of MK-801 on cortical dopamine release, locomotion, and stereotypy. Pretreatment with 3 mg/kg of MPEP enhanced the detrimental effects of MK-801 on cognition. These results demonstrate that an mGlu5 receptor antagonist can potentiate the motoric, cognitive, and dopaminergic effects of an NMDA receptor antagonist. Thus, mGlu5 receptors appear to play a major role in regulating NMDA receptor-dependent cognitive functions such as learning and working memory. By extension, these results suggest that pharmacological potentiation of mGlu5 receptors may ameliorate the cognitive and other behavioral abnormalities associated with NMDA receptor deficiency.     

Effect of 1-aminocyclopropanecarboxylic acid on N-methyl-D-aspartate-stimulated [3H]-noradrenaline release in rat hippocampal synaptosomes.

1. The effect of 1-aminocyclopropanecarboxylic acid (ACPC), a partial agonist at the glycine site of the N-methyl-D-aspartate (NMDA) receptor complex that exhibits neuroprotective, anxiolytic and antidepressant-like actions, was investigated in a functional assay for presynaptic NMDA receptors. 2. NMDA (100 microM) produced a 36% increase of tritium efflux above basal efflux in rat hippocampal synaptosomes preincubated with [3H]-noradrenaline ([3H]-NA), reflecting a release of tritiated noradrenaline. This effect was prevented by 10 microM 7-chlorokynurenic acid, an antagonist of the glycine site of the NMDA receptor. 3. Glycine enhanced the effect of NMDA with Emax and EC50 values of 84 +/- 11% and 1.82 +/- 0.04 microM, respectively. ACPC potentiated the effect of NMDA on tritium overflow with a lower EC50 (43 +/- 6 nM) and a lower maximal effect (Emax = 40 +/- 9%) than glycine. Furthermore, ACPC (0.1 microM) shifted the EC50 of glycine from 1.82 microM to > or = 3 mM. 4. These results show that ACPC can reduce the potentiation by glycine of NMDA-evoked [3H]-NA release and hence, may act as an antagonist at the glycine site of presynaptic hippocampal NMDA receptors when the concentration of glycine is high.     

Adenosine: an endogenous modulator of hippocampal noradrenaline release

In slices of hippocampus from the rabbit, preincubated with [3H]noradrenaline and then continuously superfused, the modulation of the release of noradrenaline by adenosine receptors was studied. Electrical field stimulation of the slices elicited a release of [3H]noradrenaline which was inhibited in a concentration-dependent manner by various adenosine receptor agonists. From the order of potency: cyclohexyladenosine greater than (-)phenylisopropyladenosine [(-)PIA] greater than 5'-N-ethylcarboxamide-adenosine (NECA) greater than 2-chloro-adenosine greater than adenosine (+)phenylisopropyladenosine greater than ATP, the inhibitory adenosine receptor was classified as A1- (Ri-) receptor. The effect of the agonist was strongly reduced by adenosine receptor antagonists, the methylxanthines. A role for endogenous adenosine in the modulation of hippocampal noradrenaline release is supported by these findings: (1) that blockade of adenosine receptors by methylxanthines, especially by 8-phenyltheophylline, increased, whereas (2) inhibition of the uptake of adenosine decreased the evoked release of noradrenaline and (3) that deamination of endogenous extracellular adenosine by addition of adenosine deaminase to the medium enhanced the evoked transmitter release. Inhibitors of endogenous adenosine deaminase and 5'-nucleotidase were without effect. It is concluded that release of noradrenaline in the hippocampus is inhibited at the level of the noradrenergic nerve terminals by endogenous adenosine via A1 (or Ri) receptors.     

Modulation of the NMDA receptor by D-serine in the cortex and the spinal cord, in vitro

We present a comparative study of the modulation of the N-methyl-D-aspartate (NMDA) receptor at the strychnine-insensitive glycine site in the spinal cord and in the cortex. The excitatory effect of NMDA was potentiated by D-serine (a glycine mimetic) in the hemisected rat spinal cord. The non-competitive NMDA antagonists 7-chlorokynurenic acid (7-Cl KYNA; 10 microM) and 3-amino-1-hydroxypyrrolid-2-one (HA-966; 100 or 200 microM) antagonized the effect of NMDA in the spinal cord and cortical wedge preparation. The antagonism was reversed by the addition of D-serine. This effect was strychnine-insensitive and hence not related to the inhibitory glycine receptor known to be present in the spinal cord. Our results suggest strongly that glycine positively modulates the NMDA system not only at a supraspinal level but also at the spinal level. As the positive modulation of NMDA responses by D-serine was also seen in the presence of tetrodotoxin, we conclude that the NMDA/glycine complex is (also) located on motoneurones in addition to the known glycine-mediated inhibitory system.     

Tetrodotoxin reveals action of spermine at presynaptic N-methyl-D-aspartate receptors.

By testing the effect of spermine on N-methyl-D-aspartate (NMDA)-evoked [3H]noradrenaline release in rat hippocampal slices, we revealed a presynaptic spermine-sensitive NMDA receptor most probably localized on noradrenergic terminals. Spermine (1 mM), per se totally inactive, was able to reverse the tetrodotoxin-induced blockade of the NMDA effect, restoring tritium efflux to approximately 50% of the NMDA-evoked release measured in the absence of tetrodotoxin. This effect was partially antagonized by D(-)-2-amino-7-phosphonoheptanoic acid. Our results may help to elucidate the role that spermine exerts on excitatory amino acids receptors.