Opposite presynaptic regulations by glutamate through NMDA receptors of dopamine synthesis and release in rat striatal synaptosomes

Presynaptic release-regulating NMDA receptors in isolated nerve terminals: A narrative review

The existence of presynaptic, release‐regulating NMDA receptors in the CNS has been long matter of discussion. Most of the reviews dedicated to support this conclusion have preferentially focussed on the results from electrophysiological studies, paying little or no attention to the data obtained with purified synaptosomes, even though this experimental approach has been recognized as providing reliable information concerning the presence and the role of presynaptic release‐regulating receptors in the CNS. To fill the gap, this review is dedicated to summarising the results from studies with synaptosomes published during the last 40 years, which support the existence of auto and hetero NMDA receptors controlling the release of transmitters such as glutamate, GABA, dopamine, noradrenaline, 5‐HT, acetylcholine and peptides, in the CNS of mammals. The review also deals with the results from immunochemical studies in isolated nerve endings that confirm the functional observations.

Memantine block depends on agonist presentation at the NMDA receptor in substantia nigra pars compacta dopamine neurones

NMDA glutamate receptors (NMDARs) have critical functional roles in the nervous system but NMDAR over-activity can contribute to neuronal damage. The open channel NMDAR blocker, memantine is used to treat certain neurodegenerative diseases, including Parkinson's disease (PD) and is well tolerated clinically. We have investigated memantine block of NMDARs in substantia nigra pars compacta (SNc) dopamine neurones, which show severe pathology in PD. Memantine (10 μM) caused robust inhibition of whole-cell (synaptic and extrasynaptic) NMDARs activated by NMDA at a high concentration or a long duration, low concentration. Less memantine block of NMDAR-EPSCs was seen in response to low frequency synaptic stimulation, while responses to high frequency synaptic stimulation were robustly inhibited by memantine; thus memantine inhibition of NMDAR-EPSCs showed frequency-dependence. By contrast, MK-801 (10 μM) inhibition of NMDAR-EPSCs was not significantly different at low versus high frequencies of synaptic stimulation. Using immunohistochemistry, confocal imaging and stereological analysis, NMDA was found to reduce the density of cells expressing tyrosine hydroxylase, a marker of viable dopamine neurones; memantine prevented the NMDA-evoked decrease. In conclusion, memantine blocked NMDAR populations in different subcellular locations in SNc dopamine neurones but the degree of block depended on the intensity of agonist presentation at the NMDAR. This profile may contribute to the beneficial effects of memantine in PD, as glutamatergic activity is reported to increase, and memantine could preferentially reduce over-activity while leaving some physiological signalling intact.

Timing is not Everything: Neuromodulation Opens the STDP Gate

Spike timing dependent plasticity (STDP) is a temporally specific extension of Hebbian associative plasticity that has tied together the timing of presynaptic inputs relative to the postsynaptic single spike. However, it is difficult to translate this mechanism to in vivo conditions where there is an abundance of presynaptic activity constantly impinging upon the dendritic tree as well as ongoing postsynaptic spiking activity that backpropagates along the dendrite. Theoretical studies have proposed that, in addition to this pre- and postsynaptic activity, a “third factor” would enable the association of specific inputs to specific outputs. Experimentally, the picture that is beginning to emerge, is that in addition to the precise timing of pre- and postsynaptic spikes, this third factor involves neuromodulators that have a distinctive influence on STDP rules. Specifically, neuromodulatory systems can influence STDP rules by acting via dopaminergic, noradrenergic, muscarinic, and nicotinic receptors. Neuromodulator actions can enable STDP induction or – by increasing or decreasing the threshold – can change the conditions for plasticity induction. Because some of the neuromodulators are also involved in reward, a link between STDP and reward-mediated learning is emerging. However, many outstanding questions concerning the relationship between neuromodulatory systems and STDP rules remain, that once solved, will help make the crucial link from timing-based synaptic plasticity rules to behaviorally based learning.

NMDA-mediated release of glutamate and GABA in the subthalamic nucleus is mediated by dopamine: an in vivo microdialysis study in rats

The present study investigated the effects of N-methyl-D-aspartic acid.H2O (NMDA) on the dopamine, glutamate and GABA release in the subthalamic nucleus (STN) by using in vivo microdialysis in rats. NMDA (100 micromol/L) perfused through the microdialysis probe evoked an increase in extracellular dopamine in the STN of the intact rat of about 170%. This coincided with significant increases in both extracellular glutamate (350%) and GABA (250%). The effect of NMDA perfusion on neurotransmitter release at the level of the STN was completely abolished by co-perfusion of the selective NMDA-receptor antagonist MK-801 (10 micromol/L), whereas subthalamic perfusion of MK-801 alone had no effect on extracellular neurotransmitter concentrations. Furthermore, NMDA induced increases in glutamate were abolished by both SCH23390 (8 micromol/L), a selective D1 antagonist, and remoxipride (4 micromol/L), a selective D2 antagonist. The NMDA induced increase in GABA was abolished by remoxipride but not by SCH23390. Perfusion of the STN with SCH23390 or remoxipride alone had no effect on extracellular neurotransmitter concentrations. The observed effects in intact animals depend on the nigral dopaminergic innervation, as dopamine denervation, by means of 6-hydroxydopamine lesioning of the substantia nigra, clearly abolished the effects of NMDA on neurotransmitter release at the level of the STN. Our work points to a complex interaction between dopamine, glutamate and GABA with a crucial role for dopamine at the level of the STN.

Rationale for and use of NMDA receptor antagonists in Parkinson's disease

N-Methyl-d-aspartate (NMDA) glutamate receptors are a class of excitatory amino acid receptors, which have several important functions in the motor circuits of the basal ganglia, and are viewed as important targets for the development of new drugs to prevent or treat Parkinson's disease (PD). NMDA receptors are ligand-gated ion channels composed of multiple subunits, each of which has distinct cellular and regional patterns of expression. They have complex regulatory properties, with both agonist and co-agonist binding sites and regulation by phosphorylation and protein-protein interactions. They are found in all of the structures of the basal ganglia, although the subunit composition in the various structures is different. NMDA receptors present in the striatum are crucial for dopamine-glutamate interactions. The abundance, structure, and function of striatal receptors are altered by the dopamine depletion and further modified by the pharmacological treatments used in PD. In animal models, NMDA receptor antagonists are effective antiparkinsonian agents and can reduce the complications of chronic dopaminergic therapy (wearing off and dyskinesias). Use of these agents in humans has been limited because of the adverse effects associated with nonselective blockade of NMDA receptor function, but the development of more potent and selective pharmaceuticals holds the promise of an important new therapeutic approach for PD.

Halothane anesthesia affects NMDA-stimulated cholinergic and GABAergic modulation of striatal dopamine efflux and metabolism in the rat in vivo

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

Inhibition of NMDA-induced striatal dopamine release and behavioral activation by the neuroactive steroid 3alpha-hydroxy-5beta-pregnan-20-one hemisuccinate

Our laboratory has previously shown that the synthetic neuroactive steroid 3alpha-hydroxy-5beta-pregnan-20-one hemisuccinate (3alpha5betaHS) is a negative modulator of NMDA receptors in vitro. Similarly, 3alpha5betaHS exhibits rapid sedative, analgesic, anticonvulsive, and neuroprotective effects in vivo. Here we report a study designed to investigate whether a negatively charged neuroactive steroid, 3alpha5betaHS, modulates the action of NMDA receptors in vivo. Our results indicate that peripherally administered 3alpha5betaHS enters the CNS and inhibits NMDA-mediated motor activity and dopamine release in the rat striatum. The increase in motor activity induced by intrastriatal microinjection of NMDA was blocked by the systemic administration of 3alpha5betaHS and the NMDA-induced increase in extracellular dopamine in the striatum was also attenuated by both systemically administered and intrastriatally administered (by in vivo microdialysis) 3alpha5betaHS. These data indicate that 3alpha5betaHS acts through striatal NMDA receptors in vivo. When taken together, these results suggest that neuroactive steroids may prove to be effective in the treatment of neurological and psychiatric disorders involving over-stimulation of NMDA receptors in the mesotelencephalic dopamine system.

Involvement of intrinsic cholinergic and GABAergic innervation in the effect of NMDA on striatal dopamine efflux and metabolism as assessed by microdialysis studies in freely moving rats

Microdialysis perfusion was used to study the participation of striatal cholinergic and gamma-aminobutyric acid-ergic (GABAergic) neurotransmission in basal and N-methyl-D-aspartate (NMDA) receptor-modulated dopamine release and metabolism in the striatum of the freely moving rat. Reverse dialysis of atropine (1-50 microM) induced a concentration-related increase in dopamine efflux and decrease in 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) efflux. (+)-Bicuculline (10-100 microM) similarly increased dopamine efflux, but was without consistent effect on metabolite efflux. Reverse dialysis of NMDA (1 mM) evoked an approximately twofold increase in dopamine efflux and decreased DOPAC and HVA efflux to 30-40% of basal levels. The effect of NMDA on dopamine efflux was completely abolished by coadministration of tetrodotoxin (TTX; 1 microM) or atropine (10 microM), and markedly potentiated (approximately fourfold) by coadministration of (+)-bicuculline (50 microM). The NMDA-induced decrease in dopamine metabolite efflux was inhibited by coadministration of TTX or (+)-bicuculline, but was unaffected by atropine. Our data suggest that dopamine release in the striatum is subject to both cholinergic and GABAergic tonic inhibitory mechanisms mediated through muscarinic and GABAA receptors, respectively. Furthermore, NMDA-stimulated dopamine release also involves obligatory cholinergic facilitation and an inhibitory GABAergic component mediated through these respective receptors.

Increased extracellular DA and normal evoked DA release in the rat striatum after a partial lesion of the substantia nigra

After injection of 6-hydroxydopamine into the lateral part of the rat substantia nigra, tissue dopamine (DA), dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were reduced in the corresponding lateral part of the ipsilateral caudate/putamen (CP) complex (13, 40 and 56% of controls, respectively). In this region, tyrosine hydroxylase (TH, the rate limiting enzyme of the DA synthesis) immunoautoradiography decreased by more than 80% as was the case for the binding of tritiated GBR12935 (a specific marker of the DA-carrier protein). In the medial region of the CP, only very moderate reductions of DA, DOPAC and HVA (77, 76 and 84% of controls, respectively) were observed. In this region, TH immunoautoradiography and GBR12935 binding were only reduced by about 20% reflecting weak DA denervation. However, using in vivo voltammetry, extracellular basal DA levels were found to be particularly high in the medial region of CP complex when compared to unoperated animals (up to 235%). In the medial region, TH activity was also significantly increased (161%) but the electrical stimulation of DA fibers produced the same DA overflow in control and lesioned animals. From these results, it may be concluded that elevated basal DA levels in this region cannot be attributed to the reduced DA uptake and/or to an increased ability of DA neurons to release DA in response to impulse flow.

The reverse transport of DA, what physiological significance?

It is well established that midbrain dopamine neurons innervating the striatum, release their neurotransmitter through an exocytotic process triggered by the neural firing and involving a transient calcium entry in the terminals. Long ago, it had been proposed, however, that another mechanism of release could co-exist with classical exocytosis, involving the reverse-transport of the cytosolic amine by the carrier, ordinarily responsible for uptake function. This atypical mode of release could be evoked directly at the preterminal level by multiple environmental endogenous factors involving transient alterations of the sodium gradient. It cannot be excluded that this mode of release participates in the firing-induced release. In contrast with the classical exocytosis of a preformed DA pool, the reverse-transport of DA requires simultaneous alterations of intraterminal amine metabolism including synthesis and displacement from storage compartment. The concept of a reverse-transport of dopamine is coming from the observations that releasing substances, such as amphetamine-related molecules, actually induce this type of transport. A large set of arguments advocates that reverse-transport plays a role in the maintenance of basal extracellular DA concentration in striatum. It was also often evoked in physiopathological situations including ischemia, neurodegenerative processes, etc. The most recent studies suggest that this release could occur mainly outside the synapses, and thus could constitute a major feature in the paracrine transmission, sometimes evoked for DA.

Regulation of tyrosine hydroxylase activity and phosphorylation at Ser(19) and Ser(40) via activation of glutamate NMDA receptors in rat striatum

The activity of tyrosine hydroxylase, the rate-limiting enzyme in the biosynthesis of dopamine, is stimulated by phosphorylation. In this study, we examined the effects of activation of NMDA receptors on the state of phosphorylation and activity of tyrosine hydroxylase in rat striatal slices. NMDA produced a time-and concentration-dependent increase in the levels of phospho-Ser(19)-tyrosine hydroxylase in nigrostriatal nerve terminals. This increase was not associated with any changes in the basal activity of tyrosine hydroxylase, measured as DOPA accumulation. Forskolin, an activator of adenylyl cyclase, stimulated tyrosine hydroxylase phosphorylation at Ser(40) and caused a significant increase in DOPA accumulation. NMDA reduced forskolin-mediated increases in both Ser(40) phosphorylation and DOPA accumulation. In addition, NMDA reduced the increase in phospho-Ser(40)-tyrosine hydroxylase produced by okadaic acid, an inhibitor of protein phosphatase 1 and 2A, but not by a cyclic AMP analogue, 8-bromo-cyclic AMP. These results indicate that, in the striatum, glutamate decreases tyrosine hydroxylase phosphorylation at Ser(40) via activation of NMDA receptors by reducing cyclic AMP production. They also provide a mechanism for the demonstrated ability of NMDA to decrease tyrosine hydroxylase activity and dopamine synthesis.

The in vivo modulation of dopamine synthesis by calcium ions: influences on the calcium independent release

To investigate the contribution of the dopamine (DA) synthesis to both the calcium-dependent and the carrier-mediated, mechanisms of DA release in the striatum, anaesthetized rats were locally superfused in the striatum with a push pull cannula supplied with an artificial CSF containing tritiated tyrosine. DA, dihydroxyphenylacetic acid (DOPAC) and their respective specific activity were measured in effluent and used to evaluate changes in the DA synthesizing rate. Excluding calcium ions from the CSF only partially reduced spontaneous DA release (70%) still leaving a possible carrier-mediated DA release. This effect was not additive with a local superfusion with 0.1 mM a-methyl-p-tyrosine, a blocker of DA synthesis, suggesting that synthesis could already be reduced by calcium-free superfusion. Local superfusion with 100 microM cadmium in the presence or not of calcium ions, increased the DA release (220 and 350%, respectively), simultaneously reducing DA synthesis. Local application of 1 microM calcium ionophore (A23187) was without effect on the basal release of DA but enhanced DA synthesis and increased the amphetamine-evoked and carrier-mediated amine release. We conclude that DA synthesis can be a modulatory process of the firing-independent and carrier-mediated amine release while it weakly affects the classical calcium-dependent release.

N-methyl-D-aspartate receptor 1 in the caudate-putamen nucleus: ultrastructural localization and co-expression with sorcin, a 22,000 mol. wt calcium binding protein

Entry of calcium through N-methyl-D-aspartate-type glutamate receptors in the caudate-putamen nucleus is essential for normal motor activity, but can produce cytotoxicity with continued stimulation and subsequent release of intracellular calcium. To determine potential functional sites for N-methyl-D-aspartate receptor activation in this region, we examined the ultrastructural localization of the R1 subunit of the N-methyl-D-aspartate receptor (NMDAR1) in rat brain. In addition, we comparatively examined the localization of NMDAR1 and sorcin, a 22,000 mol. wt calcium binding protein present in certain striatal neurons and involved in calcium-induced calcium release. NMDAR1-like immunoreactivity was seen at synaptic and non-synaptic sites on neuronal plasma membranes. Of 1514 NMDAR1-labeled profiles, 62% were dendrites and dendritic spines and the remainder were mainly unmyelinated axons and axon terminals. Sorcin-like immunoreactivity was present in 39% of the profiles that contained NMDAR1 labeling, most (533/595) of which were dendrites and dendritic spines. Of 1807 sorcin-labeled profiles, 42% were identified, however, as small processes including spine necks and unmyelinated axons or axon terminals. These profiles also occasionally contained NMDAR1 or showed synaptic or appositional contacts with other NMDAR1-immunoreactive neurons. The results of this study suggest that in the caudate-putamen nucleus, activation of NMDA receptors permits calcium influx at plasmalemmal sites mainly on dendrites where sorcin may play a role in calcium-induced calcium release. The presence of sorcin in some, but not all NMDA-containing neurons in the caudate-putamen nucleus has potential implications for the known differential vulnerability of certain striatal neurons to excitotoxins.

Excitatory amino acids act on the median eminence nerve terminals to induce gonadotropin-releasing hormone release in female rats

The present study is designed to examine the terminal regulation of gonadotropin-releasing hormone (GnRH) release by excitatory amino acids in the median eminence of ovariectomized (OVX) rats. In in vitro experiments, median eminence tissues were superfused in the medium containing glutamate or excitatory amino acid agonists, such as N-methyl-d,l-aspartate or kainate. These drugs induced a Ca2+-dependent GnRH release from median eminence fragments. The agonists also stimulated GnRH release from superfused synaptosome prepared from the median eminence tissues in a Ca2+-dependent manner. In the immunocytochemical study, immunoreactivity for glutamate or its ionotropic receptor subtypes, such as NR1, GluR1, GluR2/3, GluR6/7, and KA2, was examined in the median eminence of OVX rats under electron microscopy. Immunoreactivities for glutamate or its receptor subtypes were observed on the nerve terminals, most of which were located in close proximity to the other nerve terminals without forming synaptic contacts. In addition, quite a few synaptic contacts which were immunopositive for GluR1, GluR2/3, KA2, or glutamate were found in this area. The present results indicate that excitatory amino acids stimulate GnRH release by acting at the nerve terminals of the median eminence in a Ca2+-dependent manner in the absence of gonadal steroid. The effect of excitatory amino acids in this area might be mediated by glutamate receptors mainly in nonsynaptic fashion, such as by volume transmission.

Role of dopamine uptake in NMDA-modulated K(+)-evoked dopamine overflow in rat striatum: an in vivo electrochemical study

An involvement of dopamine uptake in the N-methyl-D-aspartate (NMDA)-modulated dopaminergic transmission in rat striatum was studied using the technique of in vivo electrochemical detection. Microinjection of potassium (K+) evoked dopamine overflows from the dopamine-containing nerve terminals in the striatum. While application of NMDA did not evoke any dopamine overflow, co-application of NMDA and K+ induced larger dopamine overflows than those by K+ alone. Furthermore, dynamic analysis showed that the rate of clearance (Tc) was reduced by NMDA. Indeed, our uptake study demonstrated an NMDA-induced inhibition of dopamine clearance. The time course of electrochemical signals evoked by microinjection of exogenous dopamine was increased and Tc was reduced following NMDA application. In order to delineate the effects of NMDA on K(+)-evoked dopamine overflows and/or on dopamine uptake, nomifensine, a dopamine uptake inhibitor was used. Application of nomifensine potentiated K(+)-evoked dopamine overflows. Co-administration of NMDA further augmented dopamine overflows by the K+ and nomifensine mixture. Taken together, our data suggest that NMDA concomitantly potentiated dopamine overflows in response to depolarizing stimuli and attenuated dopamine uptake. The increment by NMDA of K(+)-evoked dopamine overflows may partially result from an attenuated dopamine uptake in rat striatum.

Differential regulation of dopamine release by N-methyl-D-aspartate receptors in rat striatum after partial and extreme lesions of the nigro-striatal pathway

The participation of N-methyl-d-aspartate (NMDA) receptors on dopamine (DA) efflux in the striatum of anaesthetized rats, which had their DA nigrostriatal pathway previously lesioned with different doses of 6-hydroxydopamine (6-OH-DA), was assessed by in vivo microdialysis methodology. In addition, the in vivo basal DA and dihydroxy-phenyl-acetic acid (DOPAC) effluxes and the effect of local K+-depolarization on DA release were also evaluated in the striatum of these 6-OH-DA treated rats. Lesioned rats were divided in three groups corresponding to animals with 25-75%, 75-95% and >95% of striatum tissue DA depletion, respectively. Striatal DA tissue depletion between 25-75% occurred in parallel with a 30% reduction in DA extracellular levels, with a moderate 10% increase in basal fractional DA efflux, and with no statistical changes in the fractional DA efflux induced by NMDA (500 microM) receptor stimulation by reverse dialysis. Rats with higher DA tissue depletion (between 75-95%) exhibited a 60% reduction in DA extracellular levels in the striatum and this reduction occurred in parallel with a modest rise in basal fractional DA efflux, but with a striking decrease in the NMDA-induced fractional DA efflux. In rats with extreme or >95% of striatal DA tissue depletion, basal fractional DA efflux in the striatum increased quite substantially along with a recovery in the ability of NMDA receptor stimulation to induce fractional DA release. The >95% striatal DA-depleted rats also exhibited a significant decrease in tissue and extracellular DOPAC/DA ratio when compared to sham and partially DA-depleted rats. In contrast to the previous results, fractional DA efflux induced by reverse dialysis with K+ (40 mM) remained the same in the striatum of sham and all groups of DA-tissue depleted rats. The present findings suggest the existence of at least three features associated to the regulation of basal and NMDA-induced extracellular levels of DA in the striatum of rats as a function of striatal tissue DA depletion produced by 6-OH-DA. They also support the view that a differential regulation of basal and NMDA-induced DA extracellular levels occur in partial and extreme DA-depleted striatum after 6-OH-DA treatment. Such findings may have implications as regard to the participation of the NMDA receptor in the compensatory mechanisms associated to the progress of Parkinson's disease, as well as in the therapeutic treatment of this neurological disorder.

Lack of autoreceptor-mediated inhibitory control of dopamine release in striatal synaptosomes of D2 receptor-deficient mice

Mouse purified striatal synaptosomes were used to study the release of newly synthesised [3H]-dopamine ([3H]-DA) or of previously taken up [3H]-DA. Quinpirole (QP, 10 microM), a D2/D3 dopaminergic agonist, was found to reduce the release of newly synthesised [3H]-DA with a larger amplitude when 4-aminopyridine (100 microM) instead than veratridine (1 microM) or potassium (25 mM) was used to evoke DA release. Among the different D2/D3 dopaminergic agonists tested R(-)-propylnorapomorphine (NPA) and quinpirole were the most potent. These compounds reduced, in a concentration-dependent manner, the 4-aminopyridine-evoked release of [3H]-DA previously taken up by synaptosomes (50% maximal inhibition). In contrast, the D3 agonist PD-128,907 had little effect even when used at 100 nM. The QP (100 nM)-induced response was completely antagonised by sulpiride (1 microM). Strikingly, the NPA (100 nM) and PD-128,907 (100 nM)-evoked responses were completely suppressed in D2 receptor-deficient mice. This data strongly suggest that only D2 but not D3 receptors are involved in the autoreceptor-mediated inhibition of the evoked release of [3H]-DA. Interestingly, while amphetamine-induced release of [3H]-DA was not modified, a slight reduction of [3H]-DA efflux induced by the dopamine (DA) uptake inhibitor cocaine was observed in D2 receptor-deficient mice.

Differential effects of NMDA and non-NMDA antagonists on the activity of aromatic L-amino acid decarboxylase activity in the nigrostriatal dopamine pathway of the rat

This study examined the acute effects of a variety of NMDA and non-NMDA antagonists on the activity of aromatic l-amino acid decarboxylase (AADC) in the corpus striatum (CS) and substantia nigra (SN) of the rat. Sixty min pretreatment with the high affinity NMDA receptor-channel blockers MK 801 (0.01, 0.1 and 1 mg/kg) and phencyclidine (4 mg/kg) elevated AADC activity in both the CS and SN (2- to 3-fold). Even more striking increases in AADC were noted with 40 mg/kg amantadine (3.8-fold for CS, 9.0-fold for SN), 40 mg/kg memantine (3.4-fold for CS, 3.1-fold for SN; 20 mg/kg no effect) and 40 mg/kg dextromethorphan (3.4-fold for CS, 6.2-fold for SN, in 6/10 'responders'). Similarly pronounced increases in AADC activity in CS (1.9-fold) and SN (2.8-fold) were detected after administering clonidine (2 mg/kg). R-HA 966 (5 mg/kg, not 1 mg/kg) modestly raised AADC activity in CS (0.45-fold) and not SN. Other drugs had no effect on the activity of the decarboxylase enzyme, including CGP 40116 (1 and 5 mg/g), eliprodil (10 mg/kg), NBQX (10 mg/kg, 30 min pretreatment) and atropine (1 mg/kg). These experiments indicate that blocking the NMDA receptor-channel (and to a lesser extent the glycine site) or stimulating alpha2-adrenoceptors, profoundly increases AADC activity, more especially in the SN than CS. By contrast, inhibiting the NMDA glutamate recognition or polyamine sites, AMPA or muscarinic receptors is without effect on AADC in either brain region. The ability of amantadine and memantine to potentiate the antiparkinsonian actions of l-DOPA in the clinic, may be due to facilitated decarboxylation of l-DOPA by the brain.

Upregulation of (+)-7-hydroxy-N,N-di-n-[3H]propyl-2-aminotetralin binding following intracerebroventricular administration of a nitric oxide generator

Nitric oxide modulation of dopamine D2 and D3 receptor binding was examined using [125I]epidepride (D2) and (+)7-hydroxy-N,N-di-n-[3H]propyl-2-aminotetralin ([3H](+)-7-OH-DPAT, D3). Nitric oxide, generated by i.c.v. injection of S-nitroso-N-acetyl-penicillamine (SNAP; 5 microg or 10 microg), significantly increased the density of [3H](+)-7-OH-DPAT binding sites (39% and 134%, respectively) in the striatum 24 hours post-injection in the absence of Gpp(NH)p, representing an upregulation of either D3 receptors or high affinity D2 receptors. In the presence of 10 microM Gpp(NH)p, D3 receptor upregulation was maintained in both the 5 microg (increased 35%) and 10 microg SNAP (increased 44%) groups. [3H](+)-7-OH-DPAT binding was reduced in both striatum and nucleus accumbens in the presence of 10 microM Gpp(NH)p compared to binding in the absence of Gpp(NH)p, suggesting an upregulation of D3 receptors. Administration of SNAP did not alter total specific [125I]epidepride binding in either brain region. These data suggest that; 1) D3 receptor density is modified following nitric oxide generation, and 2) the density of high affinity D2 receptors identified by [3H](+)-7-OH-DPAT increases in the striatum, but decreases in the nucleus accumbens.

Glutamate receptor agonists decrease extracellular dopamine in the rat nucleus accumbens in vivo

Intracerebral microdialysis was used to investigate the effects of local application of L-glutamate, N-methyl-D-aspartate, and the glutamate uptake inhibitor 1-trans-pyrrolidine-2,4-dicarboxylic acid (PDC) in the nucleus accumbens (NAc) on extracellular dopamine (DA) concentrations. The effects of locally applied PDC on extracellular glutamate concentrations were also examined. Glutamate produced a concentration-dependent decrease in extracellular DA that could be blocked by concurrent, local application of the broad spectrum ionotropic glutamate receptor antagonist kynurenic acid (KYN:1 mM). N-Methyl-D-aspartate had a concentration-dependent effect on DA release, with a low concentration (0.1 mM) producing a decrease and a higher concentration (1.0 mM) resulting in an increase. Both effects were blocked by KYN. PDC (1 mM) increased extracellular glutamate concentrations to 102% above baseline. The same concentration of PDC decreased extracellular DA concentrations, and coapplication of KYN attenuated this effect. These results indicate that glutamate receptor agonists can have both facilitatory and inhibitory effects on extracellular DA concentrations. However, the effects of PDC indicate that inhibition of DA release is the more physiologically relevant effect. Furthermore, the results of these and other experiments suggest that glutamate's inhibitory effects on DA release in the NAc are not due to direct actions of this excitatory amino acid on DA terminals. A multisynaptic model that accounts for glutamate's actions on DA release is proposed.

MK-801 increases locomotor activity without elevating extracellular dopamine levels in the nucleus accumbens

In vivo microdialysis was used in freely moving rats to determine whether the locomotor stimulant effects of dizocilpine maleate (MK-801) were related to increased dopamine (DA) release within the nucleus accumbens (N. Acc.). Each experiment began with a baseline period of at least 2 h (starting 15-20 h after insertion of concentric, removable dialysis probes), during with activity records and dialysate samples were collected every 20 min. Rats in the first experiment then were injected with MK-801 (0.125, 0.25, or 0.50 mg/kg, i.p.) or saline, and activity and extracellular levels of DA, dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) were measured for a further 160 min post-injection. In a second experiment, rats were given 1.5 mg/kg d-amphetamine sulphate 40 min after receiving the same doses of MK-801, and testing was continued for 120 min. Rats in a third experiment were given low, autoreceptor-preferring doses of apomorphine hydrochloride (25 or 50 micrograms/kg, s.c.) or its vehicle 40 min after injection of 0.25 mg/kg MK-801 and then monitored for 120 min. MK-801 produced strong and consistent increases in locomotor activity that were augmented by amphetamine and greatly reduced by the low doses of apomorphine. MK-801 did not increase extracellular DA levels within the N. Acc. when given alone, and it failed to influence the changes in extracellular DA produced by d-amphetamine and apomorphine. MK-801 did produce consistent, dose-related increases in DOPAC and HVA that were probably not related to transmitter release. These results indicate that the increases in locomotor activity seen following MK-801 do not arise from a drug-induced increase in DA levels within the N. Acc.

Differential regulation, by MK-801, of dopamine receptor gene expression in rat nigrostriatal and mesocorticolimbic systems

Glutamate agonists have been shown to stimulate striatal dopamine release, but less is known about dopamine-glutamate interactions at the receptor level. We treated rats with 0.3, 1.0, or 3.0 mg/kg of MK-801, an NMDA antagonist, daily for 1 week and, using in situ hybridization, measured dopamine receptor mRNA levels in cortical and subcortical structures. MK-801 caused a significant increase of D1 and D2 mRNA in the dorsal and ventral striatum, a significant decrease of D3 mRNA in the nucleus accumbens, and a significant decrease of D1 mRNA in the limbic cortex. Dopamine autoreceptor expression, reflected by D2 mRNA in the midbrain, was increased in the ventral tegmental area, but not in the substantia nigra. Thus, MK-801 appears to differentially regulate the mesocorticolimbic and nigrostriatal dopamine systems.

Concentration-dependent dual action of locally applied N-methyl-D-aspartate on extracellular dopamine in the rat prefrontal cortex in vivo

Using microdialysis, the glutamate agonist N-methyl-D-aspartate (NMDA) was perfused for 20 min through the medial prefrontal cortex of freely moving rats, and its effects on extracellular concentrations of dopamine (DA) were determined. NMDA (1 mM) increased DA to 170-1500%, depending on the intensity and duration of the clonic forelimb jerks and convulsions that were induced. NMDA (0.1 mM), however, decreased DA to 61%. Metabolites of DA were decreased after both concentrations of NMDA. The effects of both 0.1 mM and 1 mM NMDA were blocked by 0.5 mM of the competitive NMDA-antagonist D-AP-5. The NMDA-induced decrease in release and metabolism possibly results from an indirect action via an inhibitory local interneuron or polysynaptic circuit.

Alterations in striatal dopamine release and reuptake under conditions of mild, moderate, and severe cerebral ischemia

Cerebral ischemia can result in varying degrees of tissue damage. Conditions of severe ischemia can produce extensive areas of irreversible injury, whereas in conditions of moderate ischemia, tissue damage may be reversible, as in the region of the ischemic penumbra. The reversibility of tissue damage in the penumbral region is of clinical interest, because the characterization of conditions underlying this reversible state may provide information needed for the development of new therapeutic approaches for treatment. Our previous studies demonstrated neurochemical alterations in the levels of dopamine (DA) within the striatum after cerebral ischemia. In the present study, we postulate that these changes may be caused, in part, by alterations in transmitter release and reuptake. To test this hypothesis, forebrain ischemia was induced in Sprague-Dawley rats (Harlan, Indianapolis, IN) by means of bilateral common carotid artery occlusion and hemorrhagic hypotension. Cerebral blood flow (CBF) in the striatum was measured by the method of hydrogen clearance, and the extracellular DA ([DA]e) levels were measured by in vivo microdialysis. Varied reductions of CBF were induced and maintained for 5 hours. Three subgroups were established retrospectively according to the degree of CBF reduction: 67.7, 35.6, and 13.2% of normal CBF in the mild, moderate, and severe ischemic groups, respectively. The induction of ischemia resulted in 1.9-, 9.3-, and 122.3-fold increases in [DA]e above baseline in the mild, moderate, and severe ischemia groups, respectively. At 3 hours after the induction of ischemia, high potassium (100 mmol/L) or Nomifensin (Sigma, St. Louis, MO) (10 mmol/L), a DA uptake blocker, was administrated via a microdialysis probe to stimulate DA release while reductions in CBF were maintained continuously. Thirteen rats were used in the study of the release of DA by potassium or Nomifensin in nonischemic conditions. The administration of high potassium or Nomifensin stimulated DA release in conditions of mild and moderate ischemia. The increase in DA release by potassium stimulation was higher in rats with mild ischemia (106.6-fold) than that in normal rats (22.3-fold). This suggests a hyperexcitability of DA terminals under mild ischemia, as compared with nonischemic conditions. On the other hand, Nomifensin increased [DA]e levels more in moderately ischemic brains than in control brains, suggesting that DA uptake is up-regulated in the former case. The increased release of DA by potassium and Nomifensin was sustained after stimulation in conditions of mild and moderate ischemia. The high level of [DA]e with severe ischemia after ischemic induction was sustained throughout the period of study and was not stimulated by potassium or Nomifensin. We conclude that under conditions of mild and moderate ischemia, DA terminals become highly excitable and reuptake mechanisms are compromised. These changes of DA metabolism during mild and moderate ischemia may explain the sustainability of neurons in the "penumbra" condition of cerebral ischemia and the transformation of the ischemic penumbra to a necrotic core.

Direct in vivo comparison of two mechanisms releasing dopamine in the rat striatum

A push-pull cannula supplied with artificial CSF was implanted in the striatum of anaesthetized rats, and the basal extracellular DA and DOPAC was assayed in the superfusates using HPLC and electrochemical detection. Simultaneously, a carbon fibre electrode was implanted in close proximity of the cannula and the evoked DA release was detected by differential pulse amperometry during stimulation of the DA axons. Local treatments with cadmium (100 microM) blocked the evoked DA release (-90%), but substantially increased the basal extracellular DA (+125%). The effects of glutamate agonists NMDA (1 mM) and kainate (0.1 mM), known to increase basal extracellular DA were confirmed (+150% and +60% respectively). It was, however, simultaneously observed that the evoked DA release was inhibited (-80% and -50%, respectively). Amphetamine (1 microM) released DA (+150%) and produced also an increase (+100%) of the evoked DA release. These results, apparently conflicting, show that the two mechanisms releasing dopamine (firing-dependent and not) can be directly and simultaneously observed. These two releasing processes appear to be not strictly antagonist. They are also differently and independently modulated by calcium and by local influences such those conveyed by glutamate.