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.     

Stimulation of N-methyl-D-aspartate receptors, AMPA receptors or metabotropic glutamate receptors leads to rapid internalization of AMPA receptors in cultured nucleus accumbens neurons

In hippocampus and other regions, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors are inserted into synapses during long-term potentiation and removed during long-term depression. However, little is known about regulation of AMPA receptor trafficking in the nucleus accumbens (NAc), despite growing evidence that glutamate-dependent forms of plasticity in the NAc contribute to drug addiction. Using postnatal rat NAc cultures and an immunocytochemical method that selectively detects newly internalized GluR1, we studied the regulation of AMPA receptor internalization in NAc neurons by glutamate agonists. Newly internalized GluR1 was detected during 15 or 30 min of incubation at room temperature, indicating a basal rate of GluR1 turnover. The rate of GluR1 internalization was increased by glutamate (50 microM) within 5 min of its addition. Glutamate-induced GluR1 internalization was partially blocked by either an AMPA receptor antagonist (CNQX; 20 microM) or an N-methyl-D-aspartate (NMDA) receptor antagonist (APV; 50 microM). Both NMDA (50 microM) and AMPA (50 microM) increased GluR1 internalization in a Ca(2+)-dependent manner. The NMDA effect was blocked by APV while the AMPA effect was blocked by APV or CNQX. We interpret these findings to suggest that NMDA and AMPA ultimately trigger GluR1 internalization through the same NMDA receptor-dependent pathway. The effect of glutamate was also partially blocked by the group 1 metabotropic glutamate receptor antagonist N-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxamide (PHCCC; 50 microM), while the group 1 agonist 3,5-dihydroxyphenylglycine (DHPG; 50 microM) stimulated GluR1 internalization. These data suggest that AMPA receptors on NAc neurons may be subject to rapid regulation of their surface expression in response to changes in the activity of glutamate inputs from cortical and limbic regions.     

Differential mechanisms of Ca2+ responses in glial cells evoked by exogenous and endogenous glutamate in rat hippocampus

The mechanisms of Ca2+ responses evoked in hippocampal glial cells in situ, by local application of glutamate and by synaptic activation, were studied in slices from juvenile rats using the membrane permeant fluorescent Ca2+ indicator fluo-3AM and confocal microscopy. Ca2+ responses induced by local application of glutamate were unaffected by the sodium channel blocker tetrodotoxin and were therefore due to direct actions on glial cells. Glutamate-evoked responses were significantly reduced by the L-type Ca2+ channel blocker nimodipine, the group I/II metabotropic glutamate receptor antagonist (S)-alpha-methyl-4-carboxyphenylglycine (MCPG), and the N-methyl-D-aspartate (NMDA) receptor antagonist (+/-)2-amino-5-phosphonopentanoic acid (APV). However, glutamate-induced Ca2+ responses were not significantly reduced by the non-NMDA receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX). These results indicate that local application of glutamate increases intracellular Ca2+ levels in glial cells via the activation of L-type Ca2+ channels, NMDA receptors, and metabotropic glutamate receptors. Brief (1 s) tetanization of Schaffer collaterals produced increases in intracellular Ca2+ levels in glial cells that were dependent on the frequency of stimulation (> or =50 Hz) and on synaptic transmission (abolished by tetrodotoxin). These Ca2+ responses were also antagonized by the L-type Ca2+ channel blocker nimodipine and the metabotropic glutamate receptor antagonist MCPG. However, the non-NMDA receptor antagonist CNQX significantly reduced the Schaffer collateral-evoked Ca2+ responses, while the NMDA antagonist APV did not. Thus, these synaptically mediated Ca2+ responses in glial cells involve the activation of L-type Ca2+ channels, group I/II metabotropic glutamate receptors, and non-NMDA receptors. These findings indicate that increases in intracellular Ca2+ levels induced in glial cells by local glutamate application and by synaptic activity share similar mechanisms (activation of L-type Ca2+ channels and group I/II metabotropic glutamate receptors) but also have distinct components (NMDA vs. non-NMDA receptor activation, respectively). Therefore, neuron-glia interactions in rat hippocampus in situ involve multiple, complex Ca2+-mediated processes that may not be mimicked by local glutamate application.     

The effects of potassium-induced depolarization, glutamate receptor antagonists and N-methyl-D-aspartate on neuronal survival in cultured neocortex explants

The effects of elevating the potassium concentration of the growth medium of neocortical explants was studied. Under control conditions, 10 mM potassium resulted in ca 20% decrease in the number of surviving neurons. The same potassium concentration, however, was clearly neurotrophic in tetrodotoxin-grown cultures: tetrodotoxin-induced neuronal death was significantly reduced. Both effects could be mimicked by the addition of 10 microM N-methyl-D-aspartate (NMDA); lower concentrations were without effect; higher concentrations were neurotoxic under both control and tetrodotoxin conditions. The neurotoxic, as well as the neurotrophic effects of 10 mM potassium appear to be mediated through depolarization-induced glutamate release since they could be influenced by the application of glutamate receptor antagonists. The addition of the NMDA receptor antagonist D-2-amino-7-phosphonoheptanoate (APH) blocked the trophic effect of 10 mM potassium in tetrodotoxin-grown cultures, resulting in low survival. On the other hand, the addition of the non-NMDA antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) resulted in neuronal survival similar to control cultures, indicating that it blocked the toxic effects of glutamate, leaving the trophic effects on the NMDA receptor untouched. Under control (non-TTX) conditions, neither DNQX nor APH showed significant effects on 10 mM potassium-induced cell death, indicating that stimulation of the non-NMDA, as well as the NMDA receptors is neurotoxic. This differential effect of NMDA receptor stimulation on neuronal survival is discussed with respect to the maturational and/or functional state of the neurons in the culture.     

Biphasic modulation of evoked [3H]D-aspartate release by D-2 dopamine receptors in rat striatal slices

It has been hypothesized that dopamine (DA) inhibits glutamate release from corticostriatal fibers via presynaptically located D-2 DA receptors although the evidence presented in the literature has not been conclusive. In the present experiments, the effect of D-2 receptor ligands on K+-stimulated tritium release from rat striatal slices preloaded with the nonmetabolizable glutamate analog [3H]D-aspartate ([3H]ASP was measured. The D-2 receptor antagonist S-sulpiride increased stimulated [3H]ASP release by 75% (EC50 value = 240 nM) and the biologically less-active isomer R-sulpiride, although equally effective, was tenfold less potent. The D-2 receptor agonists pergolide and (+)-4-propyl-9-hydroxynapthoxazine (+PHNO) inhibited [3H]ASP release at nM concentrations; however, this effect was small (20%). This low efficacy of the exogenous agonists was apparently due to competition by high concentrations of endogenous DA since the effect of pergolide was increased in rats whose striatal DA levels were decreased by 97%. These data support the hypothesis that D-2 DA receptors modulate [3H]ASP release in an inhibitory fashion. However, when the agonists were tested at lower concentrations, [3H]ASP release was increased significantly by 20% in control rats and 60% in DA-depleted rats. Both the facilitory and inhibitory effects of pergolide were blocked by 10 microM S-sulpiride, suggesting D-2 receptor mediation. In addition, the facilitory effect of pergolide was blocked by tetrodotoxin (TTX) and by the GABAA antagonist bicuculline, implying mediation of this D-2 effect by an inhibitory GABAergic interneuron. The inhibitory effect of pergolide was decreased by the muscarinic antagonist atropine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of N-methyl-D-aspartate receptors in dopamine D1-, but not D2-, mediated changes in striatal and accumbens neurotensin systems.

The role of N-methyl-D-aspartate (NMDA) receptors in specific D1 and D2 regulation of striatal and accumbens neurotensin (NT) systems was investigated. As demonstrated previously, stimulation of D1 receptors with multiple administrations of SKF 38393 significantly increased striatal and accumbens NT content to approximately 145% of control. These responses were completely blocked by coadministration of the non-competitive NMDA antagonist, MK 801. Previous studies have documented that D2 receptors tonically regulate striatal NT systems. Thus, multiple doses of sulpiride, a D2 antagonist, increased striatal NT content to 167% of control while quinpirole, a D2 agonist, decreased striatal NT content to 58% of control. MK 801 did not alter either striatal NT response to D2 manipulation. As previously reported, levels of accumbens NT changed only in response to D2 blockade and not to D2 stimulation. Thus, sulpiride increased accumbens NT content to 138% of control; this was not blocked by the coadministration of MK 801. NT content also significantly increased after stimulation of glutamate receptors with NMDA. To determine if D1 receptors participate in this NMDA-mediated change, the D1 antagonist SCH 23390 was coadministered. Blockade of D1 receptors did not significantly alter the response of striatal NT systems to NMDA. However, in both striatum and nucleus accumbens, the NMDA effect on NT systems appeared to be lessened. In summary, expression of D1-, but not D2-mediated changes in striatal and accumbens NT systems are markedly dependent on NMDA receptor activity. In comparison, expression of the NMDA-mediated changes in the same NT systems do not appear to be as dependent on D1 receptor activity.     

Blockade of NMDA receptors by MK-801 reverses the changes in striatal glutamate immunolabeling in 6-OHDA-lesioned rats.

A lesion of the dopamine (DA)-containing nigrostriatal pathway with 6-hydroxydopamine (6-OHDA) results in an increase in the density of nerve terminal glutamate immunolabeling and in the mean percentage of asymmetrical synapses containing a discontinuous postsynaptic density [Meshul et al. (1999) Neuroscience 88:1-16]. Similar alterations in striatal glutamate synapses have been reported following blockade of striatal DA D-2 receptors with subchronic haloperidol treatment [Meshul et al. (1994) Brain Res 648:181-195]. The haloperidol-induced change in glutamate synapses was blocked by coadministration of the N-methyl-D-aspartate (NMDA) noncompetitive receptor antagonist MK-801. In order to determine if blockade of NMDA receptors could alter the density of nerve terminal glutamate immunolabeling following a 6-OHDA lesion of the nigrostriatal pathway, MK-801 was administered to lesioned animals for 14 days. In addition, the number of apomorphine-induced contralateral rotations was determined prior to and following the administration of MK-801. MK-801 administration reversed the increase in the density of nerve terminal glutamate immunolabeling due to a 6-OHDA lesion. There was a small but significant decrease in the number of apomorphine-induced contralateral rotations following administration of MK-801 compared to the number of rotations prior to treatment with the NMDA antagonist. These results demonstrate that blockade of postsynaptic NMDA receptors affects the density of presynaptic glutamate immunolabeling and that this change in nerve terminal glutamate density is associated with a decreased behavioral response to direct DA receptor stimulation. Whether the effect of MK-801 is directly on the striatum or acts through other excitatory pathways of the basal ganglia remains unclear.     

Long-term synaptic depression in the striatum: physiological and pharmacological characterization.

The effect of tetanic activation of corticostriatal glutamatergic fibers was studied in striatal slices by utilizing extracellular and intracellular recording techniques. Tetanic stimulation produced a long-term synaptic depression (LTD) (> 2 h) of both extracellularly recorded field potentials and intracellularly recorded EPSPs. LTD was not coupled with changes of intrinsic membrane properties of the recorded neurons. In some neurons, repetitive cortical activation produced a short-term posttetanic potentiation (1-3 min). Subthreshold tetanic stimulation, which under control condition did not cause LTD, induced LTD when associated with membrane depolarization. Moreover, LTD was not expressed in cells in which the conditioning tetanus was coupled with hyperpolarization of the membrane. Bath application of aminophosphonovalerate (30-50 microM), an antagonist of NMDA receptors, did not affect the amplitude of the synaptic potentials and the expression of LTD. Striatal LTD was significantly reduced by the pretreatment of the slices with 30 microM 2-amino-3-phosphonopropionic acid, an antagonist of glutamate metabotropic receptors. LTD was not blocked by bicuculline (30 microM), a GABA(A) receptor antagonist. Scopolamine (3 microM), an antagonist of muscarinic receptors, induced a slight, but significant, increase of the amplitude of LTD. Both SCH 23390 (3 microM), an antagonist of D1 dopamine (DA) receptors, and I-sulpiride (1 microM), an antagonist of D2 DA receptors, blocked LTD. LTD was also absent in slices obtained from rats in which the nigrostriatal DA system was lesioned by unilateral nigral injection of 6-hydroxydopamine. In DA-depleted slices, LTD could be restored by applying exogenous DA (30 microM) before the conditioning tetanus. In DA-depleted slices, LTD could also be restored by coadministration of SKF 38393 (3-10 microM), a D1 receptor agonist, and of LY 171555 (1-3 microM), a D2 receptor agonist. Application of a single class of DA receptor agonists failed to restore LTD. These data show that striatal LTD requires three main physiological and pharmacological conditions: (1) membrane depolarization and action potential discharge of the postsynaptic cell during the conditioning tetanus, (2) activation of glutamate metabotropic receptors, and (3) coactivation of D1 and D2 DA receptors. Striatal LTD may alter the output signals from the striatum to the other structures of the basal ganglia. This form of synaptic plasticity can influence the striatal control of motor activity.     

The Cerebral Cortex and Parafascicular Thalamic Nucleus Facilitate In vivo Acetylcholine Release in the Rat Striatum through Distinct Glutamate Receptor Subtypes

Electrical stimulation (ten pulses of 0.5 ms, 10 V applied over 10 s at 10 Hz, 140 pA) delivered bilaterally to the prefrontal cortex or the parafascicular thalamic nucleus of freely moving rats facilitated acetylcholine release in dorsal striata, assessed by trans-striatal microdialysis. The facilitatory effects were blocked by coperfusion with 5 μM tetrodotoxin, suggesting that the release was of neuronal origin. The response of the striatal cholinergic neurons to prefrontal cortical stimulation was short-lived and required a longer period of stimulation (20 min) than the response to thalamic stimulation (4 min) to reach maximal effect. The α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate glutamatergic receptor antagonist 6,7-dinitroquinoxaline-2,3-dione [DNQX; 12 nmol per side, intracerebroventricularly (i.c.v.)] and the AMPA antagonist 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2,3-dione (NBQX; 12 nmol per side, i.c.v. or 12.8 μM infused into the striatum), but not the NMDA-type receptor antagonist MK-801 (0.2 mg/kg, i.p.), abolished the facilitatory effect on striatal acetylcholine release evoked by stimulation of the prefrontal cortex. By contrast, DNQX or NBQX did not prevent the increase in striatal acetylcholine release evoked by parafascicular nucleus stimulation, but MK-801, in accordance with previous results, did so. MK-801 by itself lowered striatal acetylcholine output while DNQX and NBQX did not. The results provide in vivo evidence that the cerebral cortex facilitates cholinergic activity in the dorsal striatum apparently through the non-tonic activation of AMPA-type glutamatergic receptors while the parafascicular nucleus does this through tonic activation of NMDA receptors. Both glutamate receptor types are probably located in the striatum. The overall results suggest that the two pathways operate independently to regulate striatal cholinergic activity through distinct mechanisms.     

Electrophysiological characterization of laminar synaptic inputs to the olfactory tubercle of the rat studied in vitro: modulation of glutamatergic transmission by cholinergic agents is pathway-specific

We have exploited the complementary arrangement of afferents in a coronal slice (300-400 microm) of the rat olfactory tubercle (OT) maintained in vitro to investigate transmission in two separate synaptic pathways. We recorded extracellular responses within the OT dense cell layer in slices and stimulated either the outermost layer to activate primary olfactory fibres or deeper to activate secondary input. Superficial stimulation produced a synaptic potential with superimposed population spike. This interpretation was based on blockade by calcium removal from the bathing medium and the use of the glutamate antagonist DNQX (10 microM); the spike was found to be selectively suppressed by tetrodotoxin applied near the cells. The spike, but not the synaptic wave, was depressed by 12 mM Ca2+ and enhanced by 1 mM Ba2+ in the bathing medium. Deep stimulation to activate association and intrinsic fibres elicited a nerve volley followed by a later response, also blocked by Ca2+ removal or 10 microM DNQX. It was unaffected by high Ca2+ or Ba2+, hence resulting from synaptic and not action current flow. Removal of Mg2+ from the bathing medium revealed an NMDA component of synaptic transmission at both loci that was selectively blocked by D-AP-5. The deep synaptic response, only, was depressed by carbachol IC50 7 microM or muscarine IC50 13 microM. This depression was also induced by AChE inhibitors eserine or tacrine and was antagonized by 1 microM atropine or 5-10 microM clozapine. These results characterize transmission in the OT and demonstrate a role for muscarinic modulation of deeper synapses in the OT that is influenced by psychotherapeutic drugs.     

Evidence for a striatal NMDA receptor modulation of nigral glutamate release. A dual probe microdialysis study in the awake freely moving rat

Dual probe microdialysis was employed to characterize dialysate glutamate levels from the substantia nigra pars reticulata of awake freely moving rats, and to test its sensitivity to alterations in striatal neurotransmission including striatal N-methyl-d -aspartic acid (NMDA) receptor stimulation and blockade. Intranigral perfusion with low (0.1 mm ) Ca²⁺ medium (60 min) did not affect nigral glutamate levels, whereas intranigral perfusion with tetrodotoxin (10 μm , 60 min) increased nigral glutamate levels. Perfusion of KCl (100 mm , 10 min) in the dorsolateral striatum transiently stimulated nigral glutamate levels (maximal increase + 60%), whereas intrastriatal perfusion (60 min) with low Ca²⁺ medium and tetrodotoxin gradually increased nigral glutamate levels. Intrastriatal perfusion with NMDA (0.1–100 μm , 10 min) dose-dependently stimulated glutamate levels in the substantia nigra pars reticulata. The NMDA (1 μm )-induced increase in nigral glutamate release was transient and maximal (+60% within 20 min), whereas that for NMDA (10 μm ) had a slow onset but was long lasting (+35% after 60 min). Lower (0.1 μm ) and higher (100 μm ) NMDA concentrations were ineffective. The effect of intrastriatal NMDA (1 μm ) was prevented by coperfusion with MK-801 (1 μm ). Intrastriatal MK-801 (10 μm ) alone gradually increased glutamate levels up to +50% after 60 min of perfusion. The present results suggest that glutamate levels in the substantia nigra pars reticulata are sensitive to changes in neuronal transmission in the dorsolateral striatum, and that striatal NMDA receptors regulate nigral glutamate release in both a tonic and phasic fashion.     

Increased responsivity of glutamate release from the substantia nigra pars reticulata to striatal NMDA receptor blockade in a model of Parkinson's disease. A dual probe microdialysis study in hemiparkinsonian rats

Dual probe microdialysis was employed in freely moving 6-hydroxydopamine (6-OHDA) hemilesioned rats to investigate the effects of blockade of N-methyl-D-aspartate (NMDA) receptors in the dorsolateral striatum on glutamate (Glu) release from the ipsilateral substantia nigra pars reticulata (SNr). Perfusion for 60 min with the NMDA antagonist dizocilpine (0.1 and 1 microM) in the dopamine (DA)-denervated striatum stimulated nigral Glu release (peak effect of 139 +/- 7% and 138 +/- 9%, respectively). The lower (0.01 microM) and higher (10 microM) concentrations were ineffective. In sham-operated rats, dizocilpine failed to affect nigral Glu release up to 1 microM but induced a prolonged stimulation at 10 microM (153 +/- 9% at the end of perfusion). The present results show that DA-deficiency in the striatum of hemiparkinsonian rats is associated with increased responsivity of nigral Glu release to striatal NMDA receptor blockade. This suggests that changes of NMDA receptor mediated control of the striatofugal pathways occur during Parkinson's disease (PD).     

Basal forebrain glutamatergic modulation of cortical acetylcholine release

The mediation of cortical ACh release by basal forebrain glutamate receptors was studied in awake rats fitted with microdialysis probes in medial prefrontal cortex and ipsilateral basal forebrain. Repeated presentation of a stimulus consisting of exposure to darkness with the opportunity to consume a sweetened cereal resulted in a transient increase in cortical ACh efflux. This stimulated release was dependent on basal forebrain glutamate receptor activity as intrabasalis perfusion with the ionotropic glutamate receptor antagonist kynurenate (1.0 mM) markedly attenuated darkness/cereal-induced ACh release. Activation of AMPA/kainate receptors by intrabasalis perfusion of kainate (100 microM) was sufficient to increase cortical ACh efflux even under basal (nonstimulated) conditions. This effect of kainate was blocked by coperfusion with the antagonist DNQX (0.1-5.0 mM). Stimulation of NMDA receptors with intrabasalis perfusion of NMDA (50 or 200 microM) did not increase basal cortical ACh efflux. However, perfusion of NMDA in rats following exposure to the darkness/cereal stimulus resulted in a potentiation of both the magnitude and duration of stimulated cortical ACh efflux. Moreover, intrabasalis perfusion of the higher dose of NMDA resulted in a rapid increase in cortical ACh efflux even before presentation of the darkness/cereal stimulus, suggesting an anticipatory change in the excitability of basal forebrain cholinergic neurons. These data demonstrate that basal forebrain glutamate receptors contribute to the stimulation of cortical ACh efflux in response to behavioral stimuli. The specific roles of basal forebrain glutamate receptor subtypes in mediating cortical ACh release differ and depend on the level of activity of basal forebrain cholinergic neurons.     

Long-term Potentiation in the Striatum is Unmasked by Removing the Voltage-dependent Magnesium Block of NMDA Receptor Channels

We have studied the effects of tetanic stimulation of the corticostriatal pathway on the amplitude of striatal excitatory synaptic potentials. Recordings were obtained from a corticostriatal slice preparation by utilizing both extracellular and intracellular techniques. Under the control condition (1.2 mM external Mg2+), excitatory postsynaptic potentials (EPSPs) evoked by cortical stimulation were reversibly blocked by 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an antagonist of dl-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) ionotropic glutamate receptors, while they were not affected by 30 - 50 microM 2-amino-5-phosphonovalerate (APV), an antagonist of N-methyl-d-aspartate (NMDA) glutamate receptors. In the presence of 1.2 mM external Mg2+, tetanic activation of cortical inputs produced long-term depression (LTD) of both extracellularly and intracellularly recorded synaptic potentials. When Mg2+ was removed from the external medium, EPSP amplitude and duration increased. In Mg2+-free medium, cortically evoked EPSPs revealed an APV-sensitive component; in this condition tetanic stimulation produced long-term potentiation (LTP) of synaptic transmission. Incubation of the slices in 30 - 50 microM APV blocked striatal LTP, while it did not affect LTD. In Mg2+-free medium, incubation of the slices in 10 microM CNQX did not block the expression of striatal LTP. Intrinsic membrane properties (membrane potential, input resistance and firing pattern) of striatal neurons were altered neither by tetanic stimuli inducing LTD and LTP, nor by removal of Mg2+ from the external medium. These findings show that repetitive activation of cortical inputs can induce long-term changes of synaptic transmission in the striatum. Under control conditions NMDA receptor channels are inactivated by the voltage-dependent Mg2+ block and repetitive cortical stimulation induces LTD which does not require activation of NMDA channels. Removal of external Mg2+ deinactivates these channels and reveals a component of the EPSP which is potentiated by repetitive activation. Since the striatum has been involved in memory and in the storage of motor skills, LTD and LTP of synaptic transmission in this structure may provide the cellular substrate for motor learning and underlie the physiopathology of some movement disorders.     

Presynaptic site of action underlying the ethanol-induced inhibition of norepinephrine release evoked by stimulation of N-methyl-D-aspartate (NMDA) receptors in rat cerebral cortex.

Rat brain cortex synaptosomes pre-incubated with [3H]norepinephrine were used (1) to provide evidence that part of the NMDA receptors mediating stimulation of norepinephrine (NE) release are located on the noradrenergic varicosities themselves, (2) to characterize these receptors and (3) to examine whether ethanol specifically inhibits the NMDA-evoked NE release via a presynaptic site of action. In synaptosomes superfused with Mg(2+)-free Krebs-Henseleit solution, NMDA (2-min exposure) stimulated tritium overflow in a concentration- and glycine-dependent manner. The stimulatory effect of NMDA was not altered by tetrodotoxin but was abolished by omission of Ca2+ from the superfusion fluid and was considerably reduced in the presence of 1.2 mM Mg2+. DL-(E)-2-Amino-4-methyl-5-phosphono-3-pentanoic acid (CGP 37849; a competitive NMDA receptor antagonist) produced a parallel shift of the concentration-response curve for NMDA to the right, whereas dizocilpine (MK-801; an antagonist at the phencyclidine, PCP, recognition site of the NMDA-gated ion channel) reduced the maximum effect of NMDA. Ethanol inhibited the NMDA-evoked tritium overflow in a concentration-dependent manner. In contrast, in synaptosomes superfused with Ca(2+)-free Krebs-Henseleit solution containing 15 mM K+ throughout, ethanol did not affect the tritium overflow evoked by 2 min introduction of 75 microM Ca2+ into the superfusion fluid. This Ca(2+)-evoked overflow was also not altered by tetrodotoxin and dizocilpine, but was inhibited by the inorganic Ca2+ channel antagonist Cd2+.(ABSTRACT TRUNCATED AT 250 WORDS)     

Long-lasting facilitation by dehydroevodiamine. HClof synaptic responses evoked in the CA1 region of rat hippocampal slices

We tested the effects of dehydroevodiamine.Cl (DHED) on field excitatory postsynaptic potentials (fEPSPs) evoked by the electrical stimulation of Schaffer collaterals-commissural fibres in the CA1 region of rat hippocampal slices. Bath applications of 10 microM DHED for 20 or 40 min induced long-lasting facilitation of fEPSPs, which outlasted the presence of DHED. A 10 min treatment with a higher concentration (100 microM) also induced long-lasting facilitation. The long-lasting facilitation was blocked either by 10 microM atropine, the muscarinic receptor antagonist, or by 50 microM D-2-amino-5-phosphonopentanoic acid (D-AP5), an NMDA receptor antagonist. These results show that DHED produces long-lasting facilitation of synaptic transmission, and that this facilitation depends upon the activation of both the muscarinic and NMDA receptors.     

2-Phenylaminoadenosine stimulates dopamine synthesis in rat forebrain in vitro and in vivo via adenosine A2 receptors

The adenosine agonist 2-phenylaminoadenosine (PAD) stimulated tyrosine hydroxylase activity in rat striatum in vitro. This effect was selectively blocked by the A₂ antagonist 8-chlorostyrylcaffeine (CSC), suggesting an A₂ receptor-mediated mechanism. PAD also produced a corresponding increase in striatal adenylyl cyclase activity. Using an in vivo model that measures presynaptic effects of drugs at dopamine nerve terminals, intracerebroventricular administration of PAD to rats stimulated tyrosine hydroxylase activity in striatum in a manner that was selectively blocked by CSC. These results suggest that PAD stimulates adenylyl cyclase and tyrosine hydroxylase activity, with a corresponding increase in dopamine synthesis, by activation of presynaptic A₂-type receptors in mammalian forebrain.     

Carbachol induces a form of long-term potentiation in lateral amygdala

We examined the effects of carbachol, a muscarinic acetylcholine receptor agonist, on excitatory synaptic transmission at thalamo-amygdala synapses in rat brain slices. The application of a low concentration of carbachol (0.25 microM) produced a form of long-term potentiation (cLTP) and a transient suppression of synaptic responses, which was blocked by a muscarinic receptor antagonist, atropine (10 microM). M2 receptor agonist produced only a transient suppression, whereas M1 receptor agonist induced both a transient suppression and a long-term potentiation. Induction of cLTP required simultaneous low-frequency afferent stimulation, and was also dependent upon the activation of NMDA receptors. SQ22536 (50 microM), an adenylyl cyclase inhibitor completely blocked cLTP. Consistently, pretreatment with a maximal concentration of forskolin (10 microM) reduced cLTP.     

Dopaminergic modulation at the olfactory nerve synapse

Dopamine can change the membrane potential, regulate cyclic nucleotides, and modulate transmitter release in central neurons. In the olfactory bulb (OB), the dopamine synthetic enzyme, tyrosine hydroxylase, is largely confined to neurons in the glomerular layer. After demonstrating dopamine D2 receptors in the glomerular and olfactory nerve (ON) layers, Nickell et al. [W.T. Nickell, A.B. Norman, L.M. Wyatt, M.T. Shipley, Olfactory bulb DA receptors may be located on terminals of the olfactory nerve, NeuroReport, 2 (1991) 9-12.] proposed that these receptors may reduce transmitter release due to their localization to ON presynaptic boutons. We have previously demonstrated that olfactory receptor neurons use glutamate to excite OB neurons through activation of glutamate receptors subtypes, NMDA and AMPA/kainate [D.A. Berkowicz, P.Q. Trombley, G.M. Shepherd, Evidence for glutamate as the olfactory receptor cell neurotransmitter. J. Neurophysiol., 71 (1994) 2557-2561]. Here, we used a hemisected turtle OB preparation and patch-clamp recording techniques to assess dopamine modulation of the ON/OB neuron synapse. We found that dopamine (10-300 microM) reversibly decreased the excitatory postsynaptic response to ON stimulation. This effect could be overcome by recruiting additional nerve fibers by increasing the intensity of ON stimulation. Quinpirole (10 microM), a D2 agonist, mimicked the effects of dopamine. Conversely, sulpiride (300 microM), a D2 antagonist, prevented the inhibitory effects of dopamine on synaptic transmission. Whereas dopamine appeared to equally affect the NMDA and AMPA/kainate receptor-mediated components of the synaptically evoked response, it had no direct effect on membrane currents evoked by exogenous glutamate, kainate or NMDA applied to cultured OB neurons. Our data, therefore, support the notion that dopamine modulates synaptic transmission between olfactory receptor neurons and OB neurons via a presynaptic mechanism involving D2 receptor activation. Our abstract (Berkowicz et al. (1994) Neuroscience Abs. 20:328) is the first report of these results.     

Mechanism of NMDA receptor contribution to axon-to-glia signaling in the crayfish medial giant nerve fiber

Electrical stimulation of crayfish giant axons at high frequency activates group II metabotropic and NMDA glutamate receptors on adjacent glial cells via release of N-acetylaspartylglutamate and glutamate formed upon its hydrolysis. This produces a transient depolarization followed by a prolonged hyperpolarization of glial cells that involves nicotinic acetylcholine receptor activation. The hyperpolarization is nearly completely blocked by antagonists of metabotropic glutamate receptors but only slightly reduced by inhibition of NMDA receptors. We report that the NMDA-induced hyperpolarization of glial cells is reduced by decreased calcium in the solution bathing the giant nerve fiber, while removal of sodium ions or block of voltage-dependent calcium channels completely prevents the glial response to NMDA. Inhibition of nicotinic acetylcholine receptors or removal of extracellular Cl(-) converts the glial response from a hyperpolarization to a depolarization that is sensitive to NMDA receptor antagonist. We propose that NMDA receptor activation by glutamate, formed from extracellular N-acetylaspartylglutamate during nerve stimulation, contributes to glial hyperpolarization by increasing intracellular Ca(2+) via opening of voltage-sensitive Ca(2+) channels. Based on our previous work, we propose further that the added Ca(2+) supplements that produced by N-acetylaspartylglutamate and glutamate acting on group II metabotropic glutamate receptors to cause an increased release of acetylcholine and a larger hyperpolarization.