Glutamate stimulation of tyrosine hydroxylase is mediated by NMDA receptors in the rat striatum.

We have studied the action of glutamate on striatal tyrosine hydroxylase activity and determined which type of glutamate receptors are involved. Glutamate stimulated (EC50 = 4 +/- 2 microM) the activity of tyrosine hydroxylase in slices of rat neostriatum. The selective N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovalerate (10 microM) blocked the stimulation; however, both the non-NMDA receptor antagonist glutamate diethyl ester (10 microM) and the general excitatory amino acid antagonist kynurenate (10 microM) had no effect. NMDA was even more potent than glutamate in stimulating tyrosine hydroxylase activity. Quisqualate (100 microM) only slightly stimulated the enzyme, and kainate had practically no effect. Omission of Mg2+ from the incubation medium potentiated the glutamate stimulation. Neither tetrodotoxin nor atropine prevented the stimulation. These results suggest that glutamate stimulates striatal tyrosine hydroxylase activity via NMDA receptors. The lack of effect of tetrodotoxin and atropine suggests that glutamate acts on NMDA receptors located on the dopaminergic nigrostriatal terminal. The stimulation may involve the entry of Ca2+ into the terminal through the NMDA receptor ionophore, since a Ca(2+)-free medium or cadmium totally blocked the stimulation of the enzyme by glutamate.     

NMDA receptor mediates dopamine release in the striatum of unanesthetized rats as measured by brain microdialysis.

We have studied the characteristics associated with the activation of the N-methyl-D-aspartate (NMDA) subtype of the glutamate receptor on the release of dopamine (DA) in the striatum of awake rats as measured by brain microdialysis technique. NMDA dose-dependently stimulated the striatal DA release in Mg(2+)-free Ringer's solution. The stimulation was significant at 90 microM and the maximum observed effect was at the highest concentration tested (800 microM). The selective NMDA receptor antagonist, 2-amino-5-phosphonovalerate (AP5; 300 microM), blocked the stimulatory effect of NMDA. The NMDA-induced release of DA was reduced by 1.2 mM Mg2+ and totally blocked by 2.5 mM of the cation. Glycine (200 microM) potentiated the response evoked by 300 microM NMDA while 7-chloro-kynurenate (100 microM), an antagonist of the glycine site, reduced markedly this response. Neither atropine (100 microM) nor tetrodotoxin (TTX) (5 microM) prevented the stimulatory effect of NMDA. These results suggest that glutamate released from corticostriatal terminals presynaptically stimulates the release of DA via an NMDA receptor.     

Serotonin release in human cerebral cortex and its modulation via serotonin receptors

Human cerebral cortex slices were prepared from brain tissue which had to be removed in order to gain access to deep-seated tumours. Subsequent to incubation with [3H]serotonin, the slices were superfused with physiological salt solution containing paroxetine, and 3H overflow was evoked by electrical field stimulation. The evoked tritium overflow (86% of which was accounted for by unmetabolized [3H]serotonin) was abolished by tetrodotoxin or omission of calcium from the superfusion fluid. Unlabelled serotonin decreased, and the serotonin receptor antagonist metitepin increased, the evoked overflow. The inhibition produced by serotonin was antagonized by metitepin. It is concluded that serotonin release in human cerebral cortex is modulated by inhibitory serotonin receptors, which may be localized presynaptically on the serotoninergic nerve fibers themselves. There are marked similarities between human and rat brain cortex with respect to action potential-induced, Ca2+-dependent serotonin release and its modulation via serotonin receptors.     

Inhibition of fast phase calcium uptake and endogenous norepinephrine release in rat brain region synaptosomes by ethanol

The effects of ethanol on fast phase calcium (Ca2+) uptake and endogenous norepinephrine release were assessed simultaneously in KCl-depolarized synaptosomes isolated from rat hypothalamus, brainstem and cerebellum. Incubation of brain regional synaptosomes with ethanol resulted in a concentration-dependent inhibition of Ca2+ uptake after 1 s of depolarization. Hypothalamic synaptosomes were most sensitive to the inhibitory effect of ethanol on voltage-dependent Ca2+ uptake and brainstem synaptosomes were least sensitive. Endogenous norepinephrine release from synaptosomes was not altered by addition of ethanol in vitro at any of the concentrations examined (25-200 mM). Chronic ethanol administration resulted in an adaptation to the inhibitory effect of ethanol on Ca2+ uptake into hypothalamic synaptosomes but did not alter the inhibitory effect of ethanol on Ca2+ uptake into brainstem or cerebellar synaptosomes. Fast phase, voltage-dependent norepinephrine release was inhibited by ethanol added in vitro but only in synaptosomes isolated from hypothalami and cerebella of chronically treated animals. Brain regional norepinephrine concentrations were unaltered by chronic ethanol administration. These results suggest that chronic ethanol treatment may alter the coupling of Ca2+ entry with norepinephrine release in some noradrenergic neurons. Effects of ethanol on synaptosomal Ca2+ entry and norepinephrine release differ depending on the brain region.     

Role of calcium on the modulation of spontaneous acetylcholine efflux by the D2 dopamine receptor subtype in rat striatal synaptosomes

The role of calcium in the modulation of spontaneous [3H]acetylcholine ([3H]ACh) efflux through presynaptic D2 dopamine hetero-receptors was investigated in rat striatal synaptosomes. The kinetic studies of [3H]ACh efflux in the presence or absence of Ca2+ were carried out in nonstimulating conditions. When Ca2+ was omitted from the superfusion medium, a notable and significant (P<0.001) decrease of tritium efflux (39%) was obtained. While [3H]ACh efflux was insensitive to tetrodotoxin (TTX) 1 microM, cadmium (10 microM), a nonselective antagonist of calcium channels, significantly reduced the tritium efflux by 24% (P<0.001), while the L-type calcium antagonist, nifedipine, (30 microM) inhibited the tritium efflux by only 10% (P<0.02). 2-(4-Fenylpiperidine)cyclohexanol (vesamicol), an inhibitor of the vesicular [3H]ACh carrier, significantly depressed the spontaneous tritium efflux in the presence of Ca2+ (60%; P<0.001) and in a low-calcium medium (20%; P<0.001). Although 1 microM of 7-hydroxy-N,N-di-n-propyl-2-aminotetraline (7-OH-DPAT) inhibited spontaneous [3H]ACh efflux in the presence of calcium, this dopaminergic agonist did not modify the neurotransmitter release in either the low-Ca2+ medium or in the presence of vesamicol. These results suggest that the spontaneous [3H]ACh efflux is a process involving a Ca2+-dependent component (39%), sensitive to calcium channel-blockers and vesamicol, in rat striatal synaptosomes. In addition, activation of the D2 dopamine hetero-receptor only modulates the calcium-dependent component of spontaneous [3H]ACh efflux.     

Glutamate-induced inhibition of paired pulse facilitation of monosynaptic excitatory post-synaptic potentials in frog spinal motoneurons.

To evaluate actions of glutamate on excitatory synaptic transmission in the central nervous system, we examined glutamate-induced changes in the paired pulse facilitation of monosynaptic excitatory post-synaptic potentials evoked by stimulation of the lateral column fibers (LC-EPSPs) on lumbar motoneurons in the frog spinal cord. Glutamate (1 mM) depolarized motoneurons both in the presence and absence of Mg2+. In most cells perfused with Mg(2+)-free or high Ca(2+)-Mg2+ solutions, the glutamate potential was accompanied by a reduction in peak amplitude of EPSPs, although the degree of change varied with the cells. Glutamate enhanced the EPSP amplitude in a few cells with Mg(2+)-free and high Ca(2+)-Mg2+ solutions, and in most cells with high Mg2+ medium. In 3/5 cells tested, the paired pulse facilitation of EPSPs was reduced by glutamate when the EPSP amplitude either increased or decreased. NMDA (50 microM), kainate (50-100 microM), quisqualate (5-50 microM) and L-2-amino-4-phosphonobutyrate (L-AP4, 1 mM) also decreased the facilitation in about half of the cells tested. The glutamate-induced decrease in the facilitation was observed in both the presence and absence of Mg2+ and was not affected by the concomitant application of glutamate and antagonists for non-NMDA or NMDA receptors, such as 6-cyano-7-nitro-quinoxalinediones (CNQX, 60 microM) or 2-amino-5-phosphonovalerate (APV, 250 microM). Glutamate reduced the facilitation of excitatory post-synaptic currents (EPSCs) recorded at a constant membrane potential under voltage clamp, when the EPSC amplitude either increased or decreased and when the input conductance either increased or decreased.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characteristics of NMDA receptors that stimulate release of hypothalamic alpha-MSH.

N-methyl-D-aspartic acid (NMDA) 10(-4) M stimulated release of immunoreactive alpha-melanocyte-stimulating hormone (alpha-MSH) from superfused slices of rat hypothalamus through receptors which shared common features with other central NMDA-type glutamate receptors. The receptors possessed inhibitory sites for both Mg2+ and ketamine; basal and NMDA-stimulated alpha-MSH release was reduced by high (5 mM) Mg2+ ion concentrations and by 10(-4) M ketamine, whilst use of Mg(2+)-free media led to a prolongation of the NMDA-stimulated response. The receptors were also shown to possess an allosteric glycine site. The glycine site agonist D-serine 10(-4) M potentiated basal and NMDA-stimulated alpha-MSH release whilst the antagonist, 7-chlorokynurenic acid 10(-4) M, reduced NMDA-stimulated release, an effect which was partially reversed by 10(-4) M D-serine.     

Cellular mechanisms underlying the rhythmic bursts induced by NMDA microiontophoresis at the apical dendrites of CA1 pyramidal neurons

This article reports the cellular mechanisms underlying a form of intracellular "theta-like" (theta-like) rhythm evoked in vitro by microiontophoresis of N-methyl-D-aspartate (NMDA) at the apical dendrites of CA1 pyramidal neurons. Rhythmic membrane potential (Vm) oscillations and action potential (AP) bursts (approximately 6 Hz; approximately 20 mV; approximately 2-5 APs) were evoked in all cells. The response lasted approximately 2 s, and the initial oscillations were usually small (< 20 mV) and below AP threshold. Rhythmic bursts were never evoked by imposed depolarization in the absence of NMDA. Block of Na+ conductance with tetrodotoxin (TTX) (1.5 microM), of non-NMDA receptors with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) (20 microM) and of synaptic inhibition by bicuculline (50 microM) and picrotoxin (50 microM) did not prevent NMDA oscillation. Inhibition of the voltage dependence of the NMDA conductance in Mg2+-free Ringer's solution blocked oscillations. Preventing Ca2+ influx with Ca2+-free and Co2+ (2-mM) solutions and block of the slow Ca2+-dependent afterhyperpolarization (sAHP) by carbamilcholine (5 microM), isoproterenol (10 microM), and intracellular BAPTA blocked NMDA oscillations. Inhibition of L-type Ca2+ conductance with nifedipine (30 microM) reduced oscillation amplitude. Block of tetraethylammonium (TEA) (10 mM) and 4AP (10 mM)-sensitive K+ conductance increased the duration and amplitude, but not the frequency, of oscillations. In conclusion, theta-like bursts relied on the voltage dependence of the NMDA conductance and on high-threshold Ca2+ spikes to initiate and boost the depolarizing phase of oscillations. The repolarization is initiated by TEA-sensitive K+ conductance and is controlled by the sAHP. These results suggest a role of interactions between NMDA conductance and intrinsic membrane properties in generating the CA1 theta-rhythm.     

Excitotoxicity Plays a Role in the Death of Tyrosine Hydroxylase- Immunopositive Nigral Neurons Cultured in Serum-Free Medium

We studied the effects of different amino acid receptor antagonists and a calcium (Ca2+) channel blocker on the survival of embryonic tyrosine hydroxylase (TH)-immunopositive nigral neurons grown under serum-free culture conditions. Ventral mesencephalic neurons were cultivated for 2 or 7 days. Following serum withdrawal on day 2, some cultures were treated with different concentrations of the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine hydrogen maleate (MK-801), the competitive NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid, the competitive kainate/quisqualate receptor antagonist 6,7-dinitroquinoxaline-2, 3-dione, and the Ca2+ channel blocker flunarizine. Treatment with MK-801 or flunarizine increased the survival of TH-positive neurons after serum deprivation. These findings suggest a possible role for excitotoxicity in dopaminergic cell death which can be prevented by blocking the NMDA receptor or by inhibiting Ca2+ entry through voltage-gated channels.     

Distinct presynaptic regulation of dopamine release through NMDA receptors in striosome- and matrix-enriched areas of the rat striatum.

Striosome- and matrix-enriched striatal zones were defined in coronal and sagittal brain sections of the rat, on the basis of 3H-naloxone binding to mu-opiate receptors (a striosome-specific marker). Then, using a new in vitro microsuperfusion device, the NMDA (50 microM)-evoked release of newly synthesized 3H-dopamine (3H-DA) was examined in these four striatal areas under Mg(2+)-free conditions. The amplitudes of the responses were different in striosomal (171 +/- 6% and 161 +/- 5% of the spontaneous release) than in matrix areas (223 +/- 6% and 248 +/- 12%), even when glycine (1 or 100 microM) was coapplied (in the presence of 1 microM strychnine). In the four areas, the NMDA-evoked release of 3H-DA was blocked completely by Mg2+ (1 mM) or (+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine maleate (MK-801; 1 microM) and almost totally abolished by kynurenate (100 microM). Because the tetrodotoxin (TTX)-resistant NMDA-evoked release of 3H-DA was similar in striosome- (148 +/- 5% and 152 +/- 6%) or matrix-enriched (161 +/- 5% and 156 +/- 7%) areas, the indirect (TTX-sensitive) component of NMDA-evoked responses, which involves striatal neurons and/or afferent fibers, seems more important in the matrix- than in the striosome-enriched areas. The modulation of DA release by cortical glutamate and/or aspartate-containing inputs through NMDA receptors in the matrix appears thus to be partly distinct from that observed in the striosomes, providing some functional basis for the histochemical striatal heterogeneity.     

Epileptiform activity induced by lowering extracellular [Mg2+] in combined hippocampal-entorhinal cortex slices: modulation by receptors for norepinephrine and N-methyl-D-aspartate

Reduction of extracellular Mg2+ concentration induced spontaneous and evoked epileptiform activity in the entorhinal cortex (EC) and dentate gyrus (DG) of combined hippocampus (HC)-EC slices. Extracellular field potentials, as well as changes in extracellular Ca2+ and K+ concentrations, were measured in EC and DG with ion-selective/reference electrodes during both repetitive and single stimuli. In the EC, lowering extracellular [Mg2+] induces both spontaneous and single stimulus evoked ictal events consisting of extracellular negative potential shifts (up to 5 mV, 30 sec), decreases in [Ca2+]0 and increases in [K+]0. In the DG, spontaneous events were much shorter, but similar changes in [Ca2+]0, [K+]0 and field potentials (FPs) could be evoked by brief high-frequency stimulation. In both areas, the N-methyl-D-aspartate (NMDA) receptor antagonist 2-aminophosphonovalerate (2-APV) completely blocked spontaneous as well as stimulus evoked epileptiform events. The neurotransmitter norepinephrine (NE), which has previously been shown to modulate long-term potentiation in the DG, was found to exhibit differential modulation of epileptiform activity in the EC and DG. In the EC, NE, acting via alpha 1-receptors, completely blocked low Mg2+-induced epileptiform activity. In contrast, in the DG, NE exhibited a beta-receptor mediated prolongation of the low Mg2+-induced ictal events, and enhanced the stimulus-induced ionic and field potential changes. From these results, we conclude that lowering extracellular [Mg2+], acting in large part through the removal of the Mg2+ voltage-dependent blockade of NMDA receptors, leads to induction of epileptiform activity in both the EC and DG.(ABSTRACT TRUNCATED AT 250 WORDS)     

Spontaneous synchronized calcium oscillations in neocortical neurons in the presence of physiological [Mg(2+)]: involvement of AMPA/kainate and metabotropic glutamate receptors

Primary cultures of neocortical neurons exhibit spontaneous Ca(2+) oscillations under zero or low extracellular [Mg(2+)] conditions. We find that mature murine neocortical neurons cultured for 9 days also produce spontaneous Ca(2+) oscillations in the presence of physiological [Mg(2+)]. These Ca(2+) oscillations were action potential mediated inasmuch as tetrodotoxin eliminated their occurrence. AMPA receptors were found to regulate the frequency of Ca(2+) oscillations. In contrast, Ca(2+) oscillations were independent of activation of L-type Ca(2+) channels, and NMDA receptors provided only a minor contribution. Release of intracellular Ca(2+) stores was involved in the oscillatory activity since thapsigargin reduced the amplitude and frequency of the oscillations. S-4-carboxyphenylglycine (S)-4CPG), an antagonist of group I metabotropic glutamate receptor (mGluR), also reduced the amplitude of oscillations. In addition, 1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD), a group I mGluR agonist, increased the oscillation frequency, suggesting a critical role for mGluR in the generation of Ca(2+) oscillations. The mGluR-mediated release of intracellular Ca(2+) stores appeared to be mediated by phospholipase C (PLC) since the PLC inhibitor U73122 eliminated the Ca(2+) oscillations. These results indicate that Ca(2+) oscillations in neocortical cultures in the presence of physiologic [Mg(2+)] are primarily initiated by excitatory input from AMPA receptors and involve mobilization of intracellular Ca(2+) stores following activation of mGluR.     

Group-II metabotropic glutamate receptors negatively modulate NMDA transmission at striatal cholinergic terminals: role of P/Q-type high voltage activated Ca++ channels and endogenous dopamine

Striatal cholinergic nerve terminals express functional group-II metabotropic (mGlu) and NMDA glutamate receptors. To investigate whether these receptors interact to regulate ACh release, LY354740 (a group-II mGlu receptor agonist) and NMDA were co-applied in striatal synaptosomes and slices. LY354740 prevented the NMDA-evoked [3H]-choline release from synaptosomes and ACh release from slices. In synaptosomes, this modulation was prevented by omega-agatoxin IVA, suggesting that it was mediated by P/Q-type high voltage activated Ca++ channels. In slices, LY341495 (a group-II mGlu receptor antagonist) enhanced the NMDA-induced ACh release, suggesting that group-II mGlu receptor activation by endogenous glutamate inhibits NMDA transmission. Co-immunoprecipitation studies excluded direct group-II mGlu-NMDA receptor interactions. Finally, group-II mGlu negative modulation of NMDA transmission was abolished in dopamine-depleted synaptosomes and slices, suggesting that it relied on endogenous dopamine. We conclude that group-II mGlu receptors attenuate NMDA inputs at striatal cholinergic terminals via Ca++ channel modulation and dopamine-sensitive pathways.     

N-methyl-d-aspartate receptor-mediated processing of beta-amyloid precursor protein in rat hippocampal slices: in vitro--superfusion study

Abnormal proteolytic degradation of the beta amyloid precursor protein (beta-APP) may result in accumulation of potentially neurotoxic beta amyloid (betaA). The role of various receptors in the regulation of beta-APP processing has been suggested. This study aimed to determine how NMDA receptors and Ca2+ ions regulate proteolysis of beta-APP in rat hippocampus in vitro. Adult rat hippocampal slices were superfused with NMDA-containing media, and immunoreactivity of soluble beta-APP derivatives was detected in dialysates. Application of 100 microM and 250 microM NMDA for 20 min in Ca2+-containing medium induced dose-dependent release of aminoterminal beta-APP derivatives, and a fragment of Abeta sequence, whereas carboxy-terminal fragments of beta-APP were only slightly detected. This indicates activation of beta-APP processing, and release of its soluble cleavage products. This effect was inhibited by NMDA receptor antagonist 1 microM MK-801 and 100 microM CPP in Ca2+-free medium, thus indicating that NMDA receptors and calcium ions mediate proteolytic non-amyloidogenic degradation of the beta-APP.     

Role of nitric oxide in NMDA-evoked release of [3H]-dopamine from striatal slices.

Evidence that excitatory amino acids act via N-methyl-D-aspartate (NMDA) receptors to evoke the release of catecholamines from axonal terminals and synaptosomes has been used to argue for the presence of pre-synaptic NMDA receptors. NMDA receptor agonists also generate nitric oxide (NO) which rapidly diffuses through neural tissue. We find that exogenously applied NO evokes [3H]-dopamine release from cultured neurons. This release is not blocked by the NMDA antagonist MK-801 nor by tetrodotoxin. Both NG-nitroarginine which inhibits NO synthesis, and hemoglobin which binds extracellular NO, block NMDA-evoked [3H]-dopamine release from striatal slices. A major role of endogenously-synthesized NO may be to evoke neurotransmitter release in local volumes of neural tissue.     

The bradykinin receptor--a putative receptor-operated channel in PC12 cells: studies of neurotransmitter release and inositol phosphate accumulation.

Bradykinin (BK) induced [3H]norepinephrine [( 3H]NE) release and phosphatidylinositol turnover were investigated in PC12 cells. Induction of [3H]NE release by BK is mediated by activation of BK-B2-receptors, as determined using type specific BK receptor antagonists. BK induces [3H]NE release with a half maximal effective concentration of 30 +/- 0.5 nM, and reaches maximal net fractional release of 9.0 +/- 1% with 200 nM BK. The BK-induced release is Ca2+ dependent, reaching maximal release at 1.0 mM Ca2+, is pertussis toxin insensitive (1 microgram/ml), slightly increased by a dibutyryl cAMP (1 mM) and not affected by inhibitors of the cyclooxygenase or lipoxygenase pathways. Voltage-sensitive Ca2+ channel blockers, verapamil (10 microM), nifedipine (10 microM), and omega-conotoxin (CgTx 10 nM), do not block the BK-induced release. However, a considerable inhibitory effect was obtained by divalent cations Co2+ (ED50 = 0.2 mM) and Ni2+ (ED50(2)+ = 1 mM). These results indicate the involvement of a Ca2+ channel in the BK-mediated release which is different from the L- or N-type voltage sensitive calcium channels. Whereas [Ca2+]ex is essential for the BK-induction of catecholamine release, the rise in level of InsP's induced by BK in the presence or in the absence of [Ca2+]ex is similar up to concentration of 1 microM. This indicates that the rise in InsP's induced by BK is not sufficient to cause neurotransmitter release. Moreover, subsequent addition of Ca2+ to BK-stimulated cells in Ca(2+)-free medium yields no release. Hence, no activity triggered by BK alone could be further stimulated by Ca2+ for induction of release.(ABSTRACT TRUNCATED AT 250 WORDS)     

N-methyl-D-aspartate autoreceptors respond to low and high agonist concentrations by facilitating, respectively, exocytosis and carrier-mediated release of glutamate in rat hippocampus

Presynaptic NMDA autoreceptors regulating glutamate release have rarely been investigated. High-micromolar N-methyl-D-aspartate (NMDA) was reported to elicit glutamate release from hippocampal synaptosomes in a Ca(2+)-independent manner by reversal of excitatory amino acid transporters. The aim of this work was to characterize excitatory amino acid release evoked by low-micromolar NMDA from glutamatergic axon terminals. Purified rat hippocampal synaptosomes were prelabelled with [(3)H]D-aspartate ([(3)H]D-ASP) and exposed in superfusion to varying concentrations of NMDA in the presence of 1 microM glycine. The release of [(3)H]D-ASP and also that of endogenous glutamate provoked by 10 microM NMDA were external Ca(2+) dependent and sensitive to the NMDA channel blocker MK-801 but insensitive to the glutamate transporter inhibitor DL-TBOA, which, on the contrary, prevented the Ca(2+)-independent release evoked by 100 microM NMDA. The NMDA (10 microM) response was blocked by 1 nM Zn(2+) and 1 microM ifenprodil, compatible with the involvement of a NR1/NR2A/NR2B assembly, although the presence of two separate receptor populations, i.e., NR1/NR2A and NR1/NR2B, cannot be excluded. This response was strongly antagonized by submicromolar (0.01-1 microM) concentrations of kynurenic acid and was mimicked by quinolinic acid (1-100 microM) plus 1 microM glycine. Finally, the HIV-1 protein gp120 potently mimicked the NMDA co-agonists glycine and D-serine, being significantly effective at 30 pM. In conclusion, glutamatergic nerve terminals possess NMDA autoreceptors mediating different types of release when activated by different agonist concentrations: low-micromolar glutamate would potentiate glutamate exocytosis, whereas higher glutamate concentrations would also provoke carrier-mediated release.     

Dual synaptic sites of D(1)-dopaminergic regulation of ethanol sensitivity of NMDA receptors in nucleus accumbens

Regulation of NMDAreceptor-mediated synaptic transmission onto accumbal medium spiny neurons (MSN) may constitute an important site in drug reward and reinforcement in mesolimbic structures. Previously, we reported that D(1)-like dopamine receptors activate a postsynaptic cAMP/PKA/DARPP-32 signaling cascade culminating in phosphorylation of SER897-NR1 subunits and a reduction in the sensitivity to ethanol of NMDA receptor-mediated synaptic transmission. Here, we use a detailed electrophysiological analysis of D(1)-like receptor regulation of the ethanol sensitivity of accumbal NMDA receptors (NMDARs) through recordings of quantal Sr(2+)-supported NMDA miniature synaptic currents (mEPSCs) in reduced Mg(2+) (0.6 mM) and report dual presynaptic and postsynaptic components of D(1)-like regulation of ethanol sensitivity of NMDARs. Ethanol inhibited NMDA mEPSC amplitude and frequency in a dose-dependent manner (25-75 mM), indicating inhibitory effects on presynaptic and postsynaptic components NMDA receptor-mediated synaptic transmission. The presynaptic inhibitory effect was corroborated by analysing the ratio of paired-pulse facilitation (PPF) of Ca(2+)-supported NMDA EPSCs. Activation of D(1) receptors with the agonist, SKF 38393 (25 microM), reversed ethanol suppression of NMDA mEPSC frequency and amplitude. Furthermore, the Mg(2+)-dependent decay off-rate of NMDA mEPSCs was substantially reduced by ethanol in a manner strongly reversed by the D(1) agonist. D(1) receptor-mediated attenuation of both the presynaptic and postsynaptic actions of ethanol was completely blocked by a D(1) selective antagonist (SCH 23390). These data suggest that D(1)-like receptors modulate both the presynaptic and postsynaptic effects of ethanol on NMDA receptor-mediated synaptic transmission in nucleus accumbens (NAc) and that these interactions may contribute to ethanol-induced neuroadaptation of the reward pathway.     

NMDA antagonists and potentiation of NMDA-induced motoneuron depolarizations in the isolated frog spinal cord

The action of N-methyl-D-aspartate (NMDA) antagonists on motoneurons was studied in the isolated, hemisected frog spinal cord using sucrose gap techniques. NMDA-evoked motoneuron depolarizations were depressed by application of APV, APH, kynurenate, Mg2+ ions, ketamine, and MK-801. Upon returning to normal Ringer's solution after exposure to all antagonists (except MK-801). NMDA responses were significantly potentiated. Kainate- and quisqualate-induced depolarizations were unchanged. The facilitation appeared to result, at least in part, from a direct action on motoneuron membranes since it persisted in the presence of tetrodotoxin which eliminated interneuronal firing. However, indirect actions involving interneurons also contributed to the potentiation because NMDA-evoked changes in K+ release were increased following exposure to NMDA antagonists and return to normal medium. Reduction of temperature (7 degrees C) which should reduce amino acid uptake did not affect results with APV. In addition, desensitization of NMDA responses was not altered by application of APV. The results indicate that NMDA antagonists have complex and long-lasting effects on the function of the NMDA receptor complex.     

The importance of striatal interneurons in age-related effects upon potassium- and amphetamine-stimulated dopamine release

The ability of superfused corpus striatal tissue fragments to release endogenous dopamine (DA) in response to potassium (K+ 30 mM) stimulation was significantly attenuated in 18- to 24-month- compared to 2- to 4-month-old male rats. These age-related effects on K+ stimulation were completely abolished with the addition of tetrodotoxin (1 microM) to the superfusion medium. Moreover, no difference in stimulated DA release was obtained between these two age groups following amphetamine stimulation (10 microM).