Glutamate Receptors of a Quisqualate-Kainate Subtype are Involved in the Presynaptic Regulation of Dopamine Release in the Cat Caudate Nucleus in vivo

Role of excitatory amino acids in the direct and indirect presynaptic regulation of dopamine release from nerve terminals of nigrostriatal dopaminergic neurons

In vivo experiments carried out in halothane-anaesthetized cats implanted with push-pull cannulae demonstrated that glutamate (GLU) released from corticostriatal fibers triggers the release of dopamine (DA), even in the absence of activity in nigral DA cells. As shown in vitro, using rat striatal slices or synaptosomes or in vivo in the cat, both NMDA and AMPA receptors subtypes are involved in the GLU-induced release of DA. Beside this direct regulation, GLU also exert several indirect facilitatory and inhibitory controls on DA release, particularly through cholinergic and GABAergic striatal neurons. Indeed, as shown by numerous authors, the GLU-evoked release of DA is markedly reduced in the presence of tetrodotoxin, bicuculline or atropine or by previous kainate- or ibotenate-induced lesion of striatum. Differences in the presynaptic regulation of DA release in striosomal and matrix compartments have also been found with NMDA and acetylcholine. The effect of acetylcholine was of shorter duration in the matrix than in the striosomal-enriched areas. Two opposite indirect regulations of DA release could be demonstrated: one is facilitatory and involves nicotinic receptors, the other is inhibitory, involves muscarinic receptors and mediated, at least in the matrix by dynorphin containing neurons. The NMDA-evoked responses are of larger amplitude and more sensitive to tetrodotoxin in the matrix than in the striosomes. In conclusion, GLU released from corticostriatal fibers, is able to control the release of DA from terminals of nigrostriatal neurons through direct facilitatory mechanisms (NMDA and AMPA receptors), but also through indirect facilitatory and inhibitory local circuits involving cholinergic and GABAergic neurons.

Pharmacological interventions for spinal cord injury: where do we stand? How might we step forward?

Despite numerous studies reporting some measures of efficacy in the animal literature, there are currently no effective therapies for the treatment of traumatic spinal cord injuries (SCI) in humans. The purpose of this review is to delineate key pathophysiological processes that contribute to neurological deficits after SCI, as well as to describe examples of pharmacological approaches that are currently being tested in clinical trials, or nearing clinical translation, for the therapeutic management of SCI. In particular, we will describe the mechanistic rationale to promote neuroprotection and/or functional recovery based on theoretical, yet targeted pathological events. Finally, we will consider the clinical relevancy for emerging evidence that pharmacologically targeting mitochondrial dysfunction following injury may hold the greatest potential for increasing tissue sparing and, consequently, the extent of functional recovery following traumatic SCI.

Expression of amino acid receptors and neural peptides in the weaver mouse brain

In the present study, we conducted: (i) in situ hybridization in order to investigate the expression of kainate and GABA(A) receptor subunits and the pre-proenkephalin and prodynorphin peptides in the brain of weaver mouse (a genetic model of dopamine deficiency) and (ii) immunocytochemistry in order to study the somatostatin-positive cells in weaver striatum. Our results indicated: (i) increases in mRNA levels of KA2 and GluR6 kainate receptor subunits, of alpha(4) and beta(3) GABA(A) receptor subunits and of pre-proenkephalin and prodynorphin in 6-month-old weaver striatum; (ii) a decrease in alpha(1) and beta(2) GABA(A) subunit mRNAs in 6-month-old weaver globus pallidus; (iii) increases in KA2, alpha(4) and beta(3) and decreases in alpha(2) and beta(2) mRNAs in the 6-month-old weaver somatosensory cortex; and (iv) an increase in somatostatin-immunopositive cells in 3-month-old weaver striatum. We suggest that: (i) in striatum, the alterations are induced by the induction of the transcription factor DeltafosB (for GluR6, pre-proenkephalin and prodynorphin mRNAs) and the suppression of transcription factors like NGF-IB (nerve growth factor inducible B; for the KA2 mRNA), in response to dopamine depletion; (ii) in striatum and cortex, the alterations in the expression of the GABA(A) subunits indicate an increase of extrasynaptic versus a decrease of synaptic GABA(A) receptors; and (iii) in globus pallidus, the increased striatopallidal GABAergic transmission leads to a decrease in the number of GABA(A) receptors. Our results further clarify the regulatory role of dopamine in the expression of amino acid receptors and striatal neuropeptides.

Differential effect of dopamine deficiency on the expression of NMDA receptor subunits in the weaver mouse brain

The weaver mutant mouse is characterized by degeneration of the dopaminergic mesencephalic neurons. The role of the dopaminergic system in the regulation of N-methyl-d-aspartate (NMDA) receptor subunit expression was addressed in the present study. In situ hybridization experiments were conducted to determine the expression levels of the NMDA receptor subunit mRNAs, z1, epsilon1 and epsilon2, in striatum, nucleus accumbens, olfactory tubercle and cerebral cortical regions of 26-day-, 3- and 6-month-old weaver mice. Data indicated statistically significant increases in z1 and epsilon2 mRNA levels in 6-month-old weaver striatum, whereas at the same age epsilon1 mRNA expression was decreased in all striatal regions, as well as in the cortex. In the 26-day-old weaver striatum and nucleus accumbens, statistically significant increases were observed in epsilon1 mRNA levels, whereas no changes were observed in the other two subunits. In the somatosensory cortex of 26-day-old weaver brain an increased expression of all three subunits was observed. The upregulation of NMDA receptor subunit expression observed in the somatosensory cortex can be attributed to a decreased activity of the glutamatergic thalamocortical pathway, following the degeneration of the nigrostriatal dopaminergic fibres. In the striatum, the present results demonstrate a differential control on the expression of z1 and epsilon2 subunits on the one hand, and epsilon1 subunit on the other. It is suggested that dopamine exerts a negative control on the expression of z1 and epsilon2 subunits, through a downregulation of transcription factors associated with the AP1 regulatory site, which is mediated by the activation of striatal dopamine D2 receptors.

Non-NMDA receptor-mediated mechanisms are involved in levodopa-induced motor response alterations in Parkinsonian rats

Chronic dopaminomimetic administration to parkinsonian animal models or Parkinson's disease patients leads to characteristic alteration in motor response. Previous studies suggested that the nonphysiologic stimulation of dopaminergic receptors on striatal medium spiny neurons enhances the synaptic efficacy of juxtaposed glutamate receptors of the N-methyl-D-aspartate (NMDA) subtype. Resultant NMDA receptor sensitization due to differential changes in subunit phosphorylation appears to favor alterations in striatal output in ways that influence motor function. To detail the involvement of NMDA receptors further as well as to determine whether similar functional changes might develop in alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors, the effects of selective antagonist of AMPA receptors (6-nitro-7-sulfamoyl-benzo[f]-quinoxaline-2,3 (1H,4H)-dione sodium salt, NBQX, 10 mg/kg) on levodopa-induced response alterations in 6-hydroxydopamine (6-OHDA) lesioned rats were compared with drugs which act competitively (3-(+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonicacid, CPP, 6.25 mg/kg) or noncompetitively (dextromethorphan, 40 mg/kg) to block NMDA receptors, or a nonselective inhibitor of glutamatergic transmission (2-amino-6-trifluoromethoxy benzothiazole, riluzole, 5 mg/kg). We found that the shortened duration of the motor response to levodopa, which underlies human wearing-off fluctuations, was reversed to a similar degree by the acute coadministration of CPP, NBQX, or riluzole (n = 4-6) but dextromethorphan did not. These observations strengthen the possibility that a reduction in levodopa-associated changes in motor response by inhibitors of glutamatergic transmission acting generally or selectively at the glutamate binding-sites may relate to their ability to attenuate pathologic gain in striatal glutamatergic function. The capacity of NBQX to reverse these altered responses suggests that an enhanced synaptic efficacy of striatal AMPA receptors may also participate in the generation of these motor response changes in levodopa-treated parkinsonian rats.

Regional localization of dopamine and ionotropic glutamate receptor subtypes in striatolimbic brain regions

Localization of dopamine (D(1)-, D(2)-like, and D(4)) and ionotropic glutamate (NMDA, AMPA, and KA) receptor subtypes within the striatolimbic forebrain remains incomplete, but basic to understanding the functional organization of this important brain region. We found that frontal cortical ablation supported colocalization of D(4) and NMDA receptors on corticostriatal afferents to caudate-putamen and nucleus accumbens in rat forebrain. Local injection of kainic acid into caudate-putamen, nucleus accumbens, or hippocampus produced massive local postsynaptic losses of D(1)- and D(2)-like, as well as NMDA, AMPA, and KA receptors, and kainic acid ablation of hippocampal-striatal projections indicated the selective expression of presynaptic NMDA and KA autoreceptors. Degeneration of nigrostriatal dopamine projections with 6-hydroxydopamine showed that all three glutamatergic subtypes exist as heteroceptors on nigrostriatal dopaminergic terminals. Our findings suggest common interactions between excitatory glutamatergic and inhibitory dopaminergic receptors in rat forebrain. Further localization of these receptor subtypes in striatolimbic forebrain should help to clarify their contributions to the pathophysiology of neuropsychiatric disorders and their treatment.

Reciprocal dopamine-glutamate modulation of release in the basal ganglia

Dopaminergic and glutamatergic transmissions have long been known to interact at multiple levels in the basal ganglia to modulate motor and cognitive functions. One important aspect of their interactions is represented by the reciprocal modulation of release. This topic has been the object of interest since the late 70's, particularly in the striatum and in midbrain dopaminergic areas (substantia nigra and ventral tegmental area). Analysis of glutamate-dopamine interactions in the control of each other's release is complicated by the fact that both glutamate and dopamine act on multiple receptor subtypes which can exert different effects. Therefore, glutamatergic modulation of dopamine release has been reviewed by analyzing the effects of glutamatergic selective receptor agonists and antagonists in the striatum (both motor and limbic portions) and in midbrain dopaminergic areas, as revealed by in vitro (slices, cell cultures, synaptosomes) and in vivo (push-pull, microdialysis and voltammetry techniques) experimental approaches. The same approach has been followed for dopaminergic modulation of glutamate release. The facilitatory nature of glutamate modulating both presynaptic and dendritic dopamine release has clearly emerged from in vitro studies. However, evidence is presented that, at least in the striatum and in the nucleus accumbens of awake rats, glutamate-mediated inhibitory effects may also occur. In vitro and in vivo experiments in the striatum and midbrain dopaminergic areas mainly depict dopamine as an inhibitory modulator of glutamate release. However, in vivo studies reporting dopamine D1 receptor mediated facilitatory effects are also considered. Therefore, the general notion that glutamate and dopamine act oppositely to regulate each other's release, is only partly supported by the available data. Conversely, the nature of the interaction between the two neurotransmitters seems to vary depending on the experimental approach, the brain area considered and the subtype of receptor involved.

Localization of ionotropic glutamate receptors in caudate-putamen and nucleus accumbens septi of rat brain: comparison of NMDA, AMPA, and kainate receptors

Changes in binding of selective radioligands at NMDA ([3H]MK-801), AMPA ([3H]CNQX), and kainate ([3H]kainic acid) glutamate (GLU) ionotropic receptors in rat caudate-putamen (CPu) and nucleus accumbens (NAc) were examined by quantitative autoradiography following: 1) unilateral surgical ablation of frontal cerebral cortex to remove descending corticostriatal GLU projections, 2) unilateral injection of kainic acid (KA) into CPu or NAc to degenerate local intrinsic neurons, or 3) unilateral injections of 6-hydroxydopamine (6-OH-DA) into substantia nigra to degenerate ascending nigrostriatal dopamine (DA) projections. Cortical ablation significantly decreased NMDA receptor binding in ipsilateral medial CPu (20%), and NAc (16%), similar to previously reported losses of DA D4 receptors. KA lesions produced large losses of NMDA receptor labeling in CPu and NAc (both by 52%), AMPA (41% and 45%, respectively), and kainate receptors (40% and 45%, respectively) that were similar to the loss of D2 receptors in CPu and NAc after KA injections. Nigral 6-OH-DA lesions yielded smaller but significant losses in NMDA (17%), AMPA (12%), and kainate (11%) receptor binding in CPu. The results indicate that most NMDA, AMPA, and kainate receptors in rat CPu and NAc occur on intrinsic postsynaptic neurons. Also, some NMDA, but not AMPA or kainate, receptors are also found on corticostriatal projections in association with D4 receptors; these may, respectively, represent excitatory presynaptic NMDA autoreceptors and inhibitory D4 heteroceptors that regulate GLU release from corticostriatal axons in medial CPu and NAc. Conversely, the loss of all three GLU receptor subtypes after lesioning DA neurons supports their role as excitatory heteroceptors promoting DA release from nigrostriatal neurons.

Differences in D2 dopamine receptor binding in the neostriatum between cats hemidecorticated neonatally or in adulthood

In order to study differences in response to neocortical injury sustained at different ages at the neurotransmitter level, we examined the density in D2 dopamine receptors in the neostriatum of cats hemidecorticated neonatally (N = 4) or in adulthood (N = 4), as well as in intact brains (N = 6). Receptor densities were measured using quantitative autoradiography and [3H]-spiperone binding in 12 regions of the neostriatum and nucleus accumbens septi. We found that the anterior lateral caudate nucleus on both sides of the brain contained a higher D2 receptor density in neonatal-lesioned as compared to adult-lesioned brains. Ipsilateral to the lesion, the increase was 101% (P < 0.05) and contralaterally it amounted to 77% (P < 0.05). Moreover, this region of the ipsilateral caudate nucleus of neonatal-lesioned cats tended to be more densely labeled than that of intact brain by 58% (P < 0.1). D2 receptor densities in adult-lesioned cats did not differ from that of intact controls. Comparison of these data with those of a former morphological study using the same animals suggested that this bilateral elevation of D2 receptor density in neonatally lesioned brains represents a higher mean density of binding sites per neuron. The elevation in the neonatal-lesioned cats might be a response of the striatum to neuroplastic changes in the striatal neuropil, including the corticostriatal afferents, since such changes are different in neonatal- as compared to adult-lesioned cats.

Dizocilpine (MK-801) and tetrodotoxin influence accumbal dopamine release evoked by intrapallidal morphine

It has been hypothesized that the intrapallidal morphine-induced dopamine release in the nucleus accumbens may be mediated by thalamocorticostriatal or mesolimbic pathways. In order to challenge the above hypothesis, we examined whether changes in accumbal dopamine and its metabolites produced by intrapallidal morphine a) are associated with local excitatory amino acid neurotransmission b) are determined by impulse propagation in dopamine neurons and c) are observed both ipsi- and contralateral to the morphine administration site. In vivo microdialysis was used to assess dopamine release and metabolism in the right and the left nucleus accumbens separately of awake, unrestrained rats. Vehicle or morphine hydrochloride (10 microliters/26.0 mM) was applied unilaterally into the pallidum alone or in combination with ipsilateral application of the N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine (MK-801) into the nucleus accumbens or the sodium channel blocker tetrodotoxin into the medial forebrain bundle. Drugs' application was performed via reverse dialysis. Concentrations of dopamine, 3,4-dihydroxyphenyl acetic acid (DOPAC) and homovanillic acid (HVA) in the collected dialysate were measured by high performance liquid chromatography with electrochemical detection. Morphine administration resulted in elevated levels of dopamine in the ipsilateral and of DOPAC and HVA in both the ipsi- and contralateral nucleus accumbens. Dizocilpine (MK-801) (0.3 mM) did not influence the basal levels of dopamine, DOPAC or HVA in the nucleus accumbens. Ipsilaterally, dizocilpine (MK-801) inhibited the effect of morphine on dopamine release, whereas it increased significantly the effect of the drug on DOPAC and HVA. Tetrodotoxin (3 microM) reversed the effect of intrapallidal morphine on dopamine, DOPAC or HVA in the ipsilateral nucleus accumbens. The results show that the intrapallidal morphine-induced dopaminergic activation in the ipsilateral nucleus accumbens is dependent upon both phasic and tonic activation of dopaminergic neurons. They suggest that both the thalamocorticostriatal and the mesolimbic dopamine pathways may mediate the investigated effect of morphine.

Apomorphine-induced hypoattention in rats and reversal of the choice performance impairment by aniracetam

Aging-, disease- and medication-related imbalance of central dopaminergic neurons causes functional impairment of cognition and neuropsychological delirium in humans. We attempted to develop a new delirium model using the direct dopamine agonist, apomorphine, and a choice reaction performance task performed by middle-aged rats. The psychological properties of the model were assessed by determining behavioral measures such as choice reaction time, % correct and % omission. Apomorphine (0.03-0.3 mg/kg s.c.) produced a dose-dependent impairment of task performance. The dose of 0.1 mg/kg prolonged choice reaction time, decreased % correct and increased % omission, indicating that rats had attentional deficits and a reduced arousal or vigilance but no motor deficits or reduced food motivation. This psychological and behavioral impairment of performance resembled that of clinically defined delirium. In this model, the cholinomimetic, aniracetam (10 mg/kg p.o.), reversed the performance impairment induced by apomorphine. Its two metabolites, 2-pyrrolidinone (10 and 30 mg/kg p.o.) and N-anisoyl-gamma-aminobutyric acid (GABA, 10 mg/kg p.o.), effectively reversed the performance impairment as the intact drug did. Another pyrrolidinone derivative, nefiracetam (10 and 30 mg/kg p.o.), tended to worsen the apomorphine effect. The cholinesterase inhibitor, tacrine (10 mg/kg p.o.), markedly worsened all of the behavioral measures. Neuroleptics, haloperidol (0.025 mg/kg s.c.), tiapride (30 mg/kg p.o.) and sulpiride (10 and 30 mg/kg p.o.), antagonized the apomorphine effect. The present results suggest that apomorphine-induced behavioral disturbances in the choice reaction performance task seems to be a useful delirium model and aniracetam may improve delirium through the action of 2-pyrrolidinone and N-anisoyl-GABA, presumably by facilitating dopamine release in the striatum by acting as an AMPA or metabotropic glutamate receptor agonist.

Electrically-evoked dopamine and acetylcholine release from rat striatal slices perfused without magnesium: regulation by glutamate acting on NMDA receptors

1. Rat striatal slices, preincubated with [3H]-dopamine and [14C]-choline, were continuously superfused and electrically stimulated. Electrically evoked release of [3H]-dopamine and [14C]-acetylcholine (ACh) was not significantly changed by elimination of Mg2+ from superfusion buffer, but the basal release of [3H]-dopamine was doubled. 2. Kynurenic acid (100-800 microM) caused, in the absence but not presence of Mg2+, a concentration-dependent decrease in the evoked release of these two transmitters. The addition of glycine reversed the inhibition of the evoked release of both transmitters caused by kynurenic acid (400 microM) in a concentration-dependent manner. In addition, glycine increased the evoked release of [3H]-dopamine via a site inhibitable by strychnine (1 microM). 3. Another two antagonists at N-methyl-D-aspartate (NMDA) receptors, 2-amino-5-phosphonovaleric acid and dizocilpine, also decreased significantly the evoked release of the two transmitters in a concentration-dependent manner in the absence, but not presence of Mg2+. By contrast, an antagonist of non-NMDA receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM) significantly decreased the evoked release of the two transmitters in the presence, but not in the absence of Mg2+. 4. Electrical field stimulation evoked release of endogenous adenosine, and this release tended to be higher in the absence of Mg2+. However, the addition of a selective adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (200 nM) did not influence the evoked release of the two transmitters, showing that the released adenosine is of little importance in controlling ACh and dopamine release from striatal slices. Non-NMDA receptors may play a similar role when Mg2+ ions are present. 5. The results indicate that NMDA receptors activated in the absence of Mg2+ participate in the electrically-evoked release of [3H]-dopamine and [14C]-ACh from the striatum.

Changes in [3H]AMPA and [3H]kainate binding in rat caudate-putamen and nucleus accumbens after 6-hydroxydopamine lesions of the medial forebrain bundle: an autoradiographic study

The binding parameters of [3H] alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and [3H]kainate binding were examined in caudate-putamen and nucleus accumbens of rat striatum after unilateral lesions of the right medial forebrain bundle (MFB) using in vitro receptor autoradiography. Lesioning of the dopaminergic fibres in the MFB with 6-hydroxydopamine (6-OHDA) resulted, after one or four weeks, in a significant decrease in the levels of [3H]GBR 12935 (1-[2-diphenylmethoxy)-ethyl]-4-(3-phenylpropyl) piperazine) in ipsilateral caudate-putamen and nucleus accumbens (62 and 43%, respectively). A comparison of the dissociation constants (Kd) of [3H]AMPA and [3H]kainate binding in caudate-putamen and nucleus accumbens between control and MFB-lesioned side did not indicate any significant change. However, the maximum number of [3H]AMPA and [3H]kainate binding sites (Bmax) were significantly decreased in caudate-putamen and nucleus accumbens of the MFB-lesioned side of the brain. This decrease was between 17 and 26%. Our results suggested that at least one-fourth to one-fifth of AMPA and kainate receptors in rat caudate-putamen and nucleus accumbens are localized on the presynaptic endings of dopamine fibres that follow the MFB. A role of non-NMDA glutamate receptors in the presynaptic regulation of dopamine release in rat striatum is therefore supported.

Distinct regulations by septide and the neurokinin-1 tachykinin receptor agonist [pro9]substance P of the N-methyl-D-aspartate-evoked release of dopamine in striosome- and matrix-enriched areas of the rat striatum

The effects of septide (a short substance P C-terminal analogue) and of the neurokinin-1 receptor agonist [Pro9]substance P on the N-methyl-D-aspartate (50 microM)-evoked release of [3H]dopamine (continuously synthesized from [3H]tyrosine) were investigated in the absence or the presence of the selective neurokinin-1 receptor antagonist RP 67580 in selected striosome- and matrix-enriched areas of the rat striatum. Experiments were performed in vitro using a microsuperfusion procedure described previously. At a concentration of 0.1 microM, septide and [Pro9]substance P stimulated the spontaneous release of [3H]dopamine in striosome-enriched areas similarly. However, in this compartment, these peptides induced larger and opposite effects on the N-methyl-D-aspartate (50 microM)-evoked release of [3H]dopamine (estimated in the absence of magnesium). Indeed, septide markedly enhanced the N-methyl-D-aspartate response, while [Pro9]substance P largely reduced the N-methyl-D-aspartate-evoked release of [3H]dopamine. Septide also enhanced the N-methyl-D-aspartate response in the matrix, but [Pro9]substance P was without effect. When used alone, at 0.1 or 1 microM, RP 67580 reduced by about 33% the N-methyl-D-aspartate-evoked release of [3H]dopamine in striosome-enriched areas. In contrast, in the matrix, the N-methyl-D-aspartate response was enhanced in the presence of a low concentration of the antagonist, while the higher concentration was ineffective. In striosomes, the reducing effect of [Pro9]substance P and the enhancing action of septide on the N-methyl-D-aspartate response were respectively blocked in the presence of low and high concentrations of RP 67580, while the stimulatory effect of septide on the N-methyl-D-aspartate response in the matrix was prevented with both concentrations of the neurokinin-1 receptor antagonist. Finally, the co-application of [Pro9]substance P (0.1 microM) with septide (0.1 microM) abolished the enhancing effect of septide on the N-methyl-D-aspartate-evoked release of [3H]dopamine in both striatal compartments. Altogether, these results suggest that substance P and eventually one of its metabolites, substance P(6-11) or another endogenous tachykinin released under the action of N-methyl-D-aspartate, contribute to the regulation of [3H]dopamine release in both striatal compartments. They also extend previous observations which allowed us to demonstrate that the local circuits contributing to the presynaptic regulation of [3H]dopamine release differ in striosome- and matrix-enriched areas. Furthermore, in agreement with observations made in some peripheral tissues, the present results support the existence of "septide-sensitive" tachykinin receptors in the rat striatum or alternatively of septide sensitive sites on tachykinin neurokinin-1 receptors distinct from those sensitive to neurokinin-1 receptor agonists, coupled to distinct transducing systems, and thus leading to biological responses which differ from those evoked by neurokinin-1 receptor agonists.

Functional neuroanatomy of the nigrostriatal and striatonigral pathways as studied with dual probe microdialysis in the awake rat--II. Evidence for striatal N-methyl-D-aspartate receptor regulation of striatonigral GABAergic transmission and motor function

In the present study we used the dual probe approach to investigate striatal N-methyl-D-aspartate receptor regulation of GABA release from the substantia nigra pars reticulata of the awake, freely moving rat. One microdialysis probe of concentric design was implanted in the dorsolateral striatum and another in the ipsilateral substantia nigra pars reticulata. Perfusion with N-methyl-D-aspartate (100 microM) in the dorsolateral striatum decreased local dopamine release (-25%) and increased both glutamate (+40%) and GABA (+35%) release. Moreover, perfusion with N-methyl-D-aspartate (100 microM) in the dorsolateral striatum increased GABA release (+20%) in the substantia nigra pars reticulata. Perfusion with the lower (10 microM) N-methyl-D-aspartate concentration in the dorsolateral striatum did not affect striatal dopamine, glutamate and GABA release or nigral GABA release. Intrastriatal perfusion with the N-methyl-D-aspartate receptor antagonist dizocilpine maleate (10 microM), at a dose which by itself did not affect basal striatal or nigral neurotransmitter levels, prevented the effects of striatal perfusion with N-methyl-D-aspartate on both striatal and nigral neurotransmitter release. Intrastriatal dizocilpine maleate was also perfused concurrently with intranigral tetrodotoxin (10 microM) (see accompanying paper). Intrastriatal perfusion with dizocilpine maleate prevented the tetrodotoxin-induced rise in both striatal and nigral GABA levels and profoundly reduced the tetrodotoxin-induced contralateral turning. In addition, intrastriatal dizocilpine maleate delayed the increase in striatal glutamate release evoked by intranigral tetrodotoxin without affecting the associated decrease in striatal dopamine release. The present study demonstrates that N-methyl-D-aspartate receptors in the dorsolateral striatum regulate GABA release in the substantia nigra pars reticulata of the awake rat and provides evidence that this regulation plays a key role in motor function.

Glutamate receptor agonist injections into the dorsal striatum cause contralateral turning in the rat: involvement of kainate and AMPA receptors

Unilateral stimulation of glutamate receptors in the dorsal striatum of intact rats resulted in contralateral turning. Turning behavior was recorded for 20 min following unilateral intrastriatal injections (0.5 microliter) in chronically cannulated rats. Kainate injections caused a dose-dependent increase in contralateral rotation that was blocked by the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), the action potential blocker tetrodotoxin, and by increasing doses of the dopamine receptor antagonist cis-flupenthixol. Injections of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) also caused rotation that was blocked with co-injections of CNQX, tetrodotoxin or cis-flupenthixol. Neither CNQX nor tetrodotoxin injected alone caused turning. This effect is dopamine-dependent, and may result from a kainate or AMPA-induced increase in dopamine release. Glutamate receptor agonist injections into the striatum may cause contralateral turning by degrading information in ascending cortical projections and may further influence locomotion via basal ganglia output nuclei projections to the brainstem.

Inhibition by noncompetitive NMDA receptor antagonists of apomorphine-induced climbing behavior in mice

The N-methyl-D-aspartate (NMDA) subtype of glutamate receptors is an important mediator of several forms of neural and behavioral plasticity. In the present study, we examined the potential role of NMDA receptors in the glutamatergic modulation of dopaminergic function at the postsynaptic dopamine receptor by determining the effects of NMDA antagonists on apomorphine-induced climbing behavior in mice. The noncompetitive NMDA receptor antagonists, MK-801, ketamine, dextrorphan, and dextromethorphan attenuated the apomorphine-induced climbing behavior at does well below those that produce untoward side effects. These results suggest that the NMDA receptors play important roles in the glutamatergic modulation of dopaminergic function at the postsynaptic dopamine receptors that mediate the apomorphine-induced climbing behavior in mice.

Transmitter release associated with long-term synaptic depression in rat corticostriatal slices

Using a corticostriatal slice preparation, we have recently shown that tetanic stimulation of the corticostriatal pathway produces long-term depression (LTD) of striatal excitatory synaptic transmission. In the present study we have analysed the relationship between LTD and the striatal release of different endogenous transmitters. Samples of perfusate were collected via a small cannula placed just above the surface of the striatal slice close to the recording electrode, and were analysed by HPLC. The high-frequency stimulation (100 Hz, three trains, 3 s duration, 20 s interval) used to induce LTd caused a significant but transient increase in the release of both excitatory (aspartate and glutamate) and inhibitory (glycine and GABA) amino acid transmitters. Tetanic stimulation also produced a significant, but transient increase in the release of endogenous dopamine. We conclude that the tetanic stimulation of the corticostriatal pathway is able to induce a large but transient release of excitatory amino acids and of dopamine, whose participation in the induction of striatal LTD has been demonstrated previously. Moreover, the maintenance of this form of synaptic plasticity does not seem to require a sustained change in transmitter release.

Ascorbate potentiates amphetamine-induced conditioned place preference and forebrain dopamine release in rats

In order to evaluate the effects of ascorbate, which is known to modulate dopamine neurotransmission, on the reinforcing effects of amphetamine, we coadministered ascorbate and amphetamine during the acquisition of conditioned place preference (CPP) in rats. Our results indicate that 100 mg/kg ascorbate potentiates the CPP induced by 0.5 mg/kg, but not 1.0 mg/kg, amphetamine. A higher dose of ascorbate (500 mg/kg) did not influence the CPP induced by either dose of amphetamine. In vitro release assays revealed that, whereas ascorbate alone (0.01-1.0 mM) did not influence striatal dopamine levels, this vitamin potentiated amphetamine-induced dopamine release in both the nucleus accumbens and neostriatum. Collectively, these results raise the possibility that ascorbate potentiates amphetamine-induced CPP by increasing the ability of this psychostimulant to release dopamine.

Conditioned dopamine release: dependence upon N-methyl-D-aspartate receptors

The study have investigated the effect of a conditioned emotional response using a contextual cue on dopamine release in the rat nucleus accumbens, measured with in vivo microdialysis, and its inhibition by N-methyl-D-aspartate antagonist dizocilpine maleate. The extracellular level of dopamine in the medial nucleus accumbens markedly increased for up to 40 min when rats were given mild footshock in the testing box. When the rats were returned to the testing box, but not given footshock (conditioned emotional response), there was an immediate and long-lasting (80 min) increase in extracellular dopamine. Dizocilpine maleate (50 mumol/l) administered into the nucleus accumbens through the dialysis probe had no significant effect on the immediate increase in dopamine induced by conditioned emotional response but completely prevented the later phase. Dizocilpine maleate had no effect on basal dopamine release in control rats but decreased basal dopamine in rats exposed to footshock 2 h previously. The alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate/kainate antagonist 6-cyano-7-nitroquinnoxaline-2,3-dione (100 mumol/l) had no effect on the increase in dopamine release in response to conditioned emotional response. The results indicate that the acquisition of conditioned emotional response causes long-lasting changes in the mechanisms involved in the glutamatergic control of dopamine release in the nucleus accumbens. Furthermore glutamate inputs into the nucleus accumbens may also regulate the delayed phase of conditioned dopamine release during expression of conditioned emotional response to a contextural cue through activation of N-methyl-D-aspartate receptors.

The role of N-methyl-D-aspartate receptors in the regulation of physiologically released dopamine

In vivo voltammetry was used to measure changes in ascorbate, which are an index of changes in the release of glutamate, and microdialysis was used to measure changes in dopamine in the striatum of freely moving rats. A 5 min tail pinch produced a rapid rise in striatal ascorbate paralleled by an increase in motor activity and a slower, more prolonged rise in dopamine. Systemic administration of ketamine or dizocilpine maleate, non-competitive antagonists of the N-methyl-D-aspartate glutamate receptor, produced an increase in the basal level of ascorbate but not dopamine; however, the tail pinch-evoked rise in both ascorbate and dopamine was completely abolished by these drugs. The rise in dopamine was also abolished by local infusion of dizocilpine maleate into the striatum. Local application of N-methyl-D-aspartate produced a dose-dependent increase in dopamine, which was partially reduced in the presence of tetrodotoxin. The results show that the tail pinch-evoked increase in motor activity involves an increase in the release of striatal dopamine which requires the activation of N-methyl-D-aspartate receptors in the striatum. This suggests that phasic increases in striatal dopamine release are triggered by the action of glutamate on dopaminergic nerve terminals.

Recent basic findings in support of excitatory amino acid hypotheses of schizophrenia

1. Several clinical and post-mortem tissue findings have suggested a role for excitatory amino acid neuronal systems in the pathophysiology of schizophrenia. 2. These include the ability of NMDA antagonists, phencyclidine and ketamine, to cause both negative and positive symptoms in healthy subjects, and abnormalities in the densities of some types of excitatory amino acid receptors in the postmortem tissue of schizophrenic brains. 3. The present review describes recent basic findings that have examined the involvement of excitatory amino acids in the mechanism of action of antipsychotic drugs. These include studies on the functional links between glutamatergic and dopaminergic systems, effect of acute and chronic antipsychotic drug treatment on excitatory amino acid function, and stress-induced activation of excitatory amino acid release, in particular in the prefrontal cortex.

Presynaptic control of dopamine synthesis and release by excitatory amino acids in rat striatal synaptosomes

Purified striatal synaptosomes were continuously superfused with L,3,5[3H]tyrosine in order to estimate the synthesis ([3H]water) and release of newly formed [3H]dopamine. In the presence of magnesium, L-glutamate, D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate (AMPA) and kainate, but not N-methyl-D-aspartate (NMDA) and 1-aminocyclopentane-1S,3R-dicarboxylate (t-ACPD), stimulated the release of [3H]dopamine, in a dose-dependent manner. When magnesium was omitted or in the presence of AMPA, NMDA also increased the release of [3H]dopamine. The effects of AMPA and kainate were competitively inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or 6,7-dinitro-quinoxaline-2,3-dione (DNQX), whereas those of NMDA were reduced by 2-amino-5-phosphonovalerate (APV) or (+)-5-methyl-10,11-dihydro-5-H-dibenzo(a,d)cyclo-hepten-5,10-imine maleate (MK801). The stimulation of [3H]dopamine release by a high concentration of glutamate resulted from the concomitant activation of AMPA and NMDA receptors since this effect was potentiated by glycine and reduced by 2-amino-5-phosphonovalerate or MK801. This reduction was almost complete in the combined presence of DNQX and MK801. Surprisingly, glutamate and NMDA (in the absence of magnesium) reduced the efflux of [3H]water. The reduction of [3H]dopamine synthesis was blocked by 2-amino-5-phosphonovalerate indicating the involvement of NMDA receptors. Neither AMPA nor kainate affected dopamine synthesis. The inhibition of [3H]dopamine synthesis resulting from the stimulation of NMDA receptors was prevented when synaptosomes were continuously superfused with adenosine deaminase and quinpirole, a combined treatment known to markedly reduce the phosphorylation of tyrosine hydroxylase by cAMP-dependent protein kinase. The opposite effects of a high concentration of glutamate on [3H]dopamine synthesis and release were mimicked by ionomycin. As a working hypothesis, it is proposed that the NMDA-triggered calcium influx could lead to a reduction of tyrosine hydroxylase phosphorylation, possibly through an activation of calcineurin.

Excitatory amino acid receptors mediate the orofacial stereotypy elicited by dopaminergic stimulation of the ventrolateral striatum

The present experiments examined the role of excitatory amino acid receptors in the orofacial stereotypy induced by direct amphetamine microinjection into the ventrolateral striatum. In these experiments, the influence of prior intra-ventrolateral striatum treatment with various excitatory amino acid antagonists on the expression of amphetamine-stimulated oral stereotypy was observed. In all experiments, behavioral observations were conducted in the home cage using a time-sampling procedure. In the first experiment, different groups of rats received bilateral microinfusions of either kynurenic acid, 2-amino-5-phosphonopentanoic acid, 6,7-dinitroquinoxaline or dizocilpine maleate. The excitatory amino acid antagonists were administered immediately prior to bilateral microinfusions of d-amphetamine. Both N-methyl-D-aspartate and non-N-methyl-D-aspartate antagonists dose-dependently attenuated or blocked the expression of dopamine-mediated stereotypy. 2-Amino-5-phosphonopentanoic acid was the most potent of these compounds, totally suppressing stereotypy at a dose of 0.3 micrograms (equivalent to 1.5 nmol). In the second experiment, the same compounds were tested for their ability to suppress physostigmine-induced mouth movements. Cholinergic stimulation of the ventrolateral striatum has previously been shown to elicit non-directed mouth movements, quite distinguishable from stimulus-directed, amphetamine-induced biting. Excitatory amino acid antagonists were administered in the same doses prior to bilateral infusion of physostigmine (2.5 micrograms/0.5 microliters). The expression of physostigmine-induced mouth movements was for the most part not affected by excitatory amino acid antagonists, although dizocilpine maleate slightly reduced this oral behavior. In a third experiment, behavior was observed following infusion of the antagonists alone, using the same doses as in the previous experiments. No behavioral alterations were observed with the exception of a small increase in nonspecific mouth movements induced by kynurenic acid and 2-amino-5-phosphonopentanoic acid. These findings indicate that the expression of dopamine-mediated oral stereotypy, induced by amphetamine stimulation of the ventrolateral striatal region, is highly dependent on activation of striatal excitatory amino acid receptors. In contrast, oral behavior induced by cholinergic stimulation of the ventrolateral region is not mediated by glutamate input. These results are discussed in relation to the synaptic organization of neuronal elements within the striatum. Moreover, the relevance to further understanding of orofacial dyskinesias is noted.

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

Purified striatal synaptosomes were superfused continuously with L-[3,5-3H]tyrosine to measure simultaneously the synthesis ([3H]water formed during the conversion of [3H]tyrosine into [3H]DOPA) and the release of [3H]dopamine ([3H]DA). Glutamate (10(-3) M) and NMDA (10(-3) M, in the absence of Mg2+) stimulated the release of [3H]DA, but they reduced the efflux of [3H]water. This reduction of [3H]DA synthesis was blocked by 2-amino-5-phosphonovalerate indicating the involvement of NMDA receptors. Although D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate (AMPA) and kainate stimulated the release of [3H]DA, they did not affect its synthesis. The glutamate-evoked inhibition of [3H]DA synthesis was prevented when synaptosomes were superfused continuously with adenosine deaminase plus quinpirole, a treatment which markedly reduces the phosphorylation of tyrosine hydroxylase by cAMP dependent protein kinase. The opposite effects of glutamate on [3H]DA synthesis and release were mimicked by ionomycin (10(-6) M). It is proposed that both an activation of a cyclic nucleotide phosphodiesterase and a dephosphorylation of tyrosine hydroxylase linked to the influx of calcium through NMDA receptors is responsible for the inhibition of dopamine synthesis by glutamate and that calcineurin could play a critical role in these processes.

Does bicuculline antagonize NMDA receptors? Further evidence in the rat striatum

In two areas of the rat striatum, the in vitro N-methyl-D-aspartate (NMDA, 50 microM)-evoked release of [3H]dopamine was studied in the presence of bicuculline (5 and 50 microM), an antagonist of GABAA receptors. The responses observed with the higher concentration (50 microM) is compatible with an antagonistic activity of bicuculline on NMDA receptor, as recently reported by Wright and Nowak.

Stress-induced dopamine release in the neostriatum: evaluation of the role of action potentials in nigrostriatal dopamine neurons or local initiation by endogenous excitatory amino acids

It has been hypothesized that excitatory amino acids can initiate dopamine release in neostriatum. We examined whether the increase in extracellular dopamine in neostriatum produced by acute stress reflects presynaptic initiation of dopamine release by endogenous excitatory amino acids. Thirty minutes of intermittent tail-shock stress significantly elevated extracellular concentrations of dopamine, glutamate, aspartate, and gamma-aminobutyric acid in neostriatum of freely moving rats as measured with in vivo microdialysis. Local infusion of the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovalerate or the non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione via the dialysis probe did not attenuate the stress-induced increase in extracellular dopamine. In fact, the increase was prolonged in rats treated with specific excitatory amino acid receptor antagonists. Infusion of tetrodotoxin into medial forebrain bundle increased extracellular glutamate and aspartate in neostriatum yet reduced basal dopamine in extracellular fluid to below the limit of detection of the assay and eliminated the stress-induced increase in extracellular dopamine. These findings fail to support the hypothesis that the stress-induced increase in extracellular dopamine in neostriatum is initiated locally by excitatory amino acids. Rather, the effects of stress on extracellular dopamine seem to be determined by impulse propagation in dopamine neurons.

Prefrontal cortical dopamine systems and the elaboration of functional corticostriatal circuits: implications for schizophrenia and Parkinson's disease

The dopaminergic innervation of the prefrontal cortex is able to transsynaptically regulate the activity of subcortical dopamine innervations. Disruption of the prefrontal cortical DA innervation results in the enhanced biochemical responsiveness of the dopamine innervation of the nucleus accumbens. We present recent data indicating that distinct prefrontal cortical dopamine innervations can be functionally dissociated on the basis of responsiveness to stress. The ventral striatal projection target (nucleus accumbens shell) of the prefrontal cortical region that is stress sensitive is also responsive to stress. In this manner interconnected cortico-striato-pallido-mesencephalic loops can be defined on the basis of the biochemical responsive of local dopamine systems to stress and on the basis of responsiveness to antipsychotic drugs. These data suggest the functional derangement of a distinct corticofugal loops in schizophrenia and in certain aspects of Parkinson's disease.

In vivo regulation of extracellular dopamine in the neostriatum: influence of impulse activity and local excitatory amino acids

It has been suggested that dopamine release can be evoked by excitatory amino acids acting on dopaminergic terminals, as well as by the classical process of impulse-evoked exocytosis. We used in vivo microdialysis to examine whether endogenous excitatory amino acids locally evoked dopamine efflux under basal conditions. Infusion of N-methyl-d-aspartate (NMDA) or kainate into the neostriatum increased extracellular dopamine, and this effect was blocked by co-infusion of 2-amino-5-phosphonovalerate (APV) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), respectively. However, neither these antagonists nor kynurenate decreased extracellular dopamine when administered alone. In contrast, infusion of tetrodotoxin into the medial forebrain bundle reduced extracellular dopamine to below the limit of detection of our assay. These and other findings reviewed in this report suggest to us that extracellular dopamine in the neostriatum is not stimulated locally by endogenous excitatory amino acids.

Effect of riluzole on focal cerebral ischemia in rats

The effects of riluzole, a putative inhibitor of glutamate release, on the histological and neurobehavioral consequences of middle cerebral artery occlusion were tested in Sprague-Dawley rats. Neurobehavioral studies (neurological examination, passive avoidance task) were carried out with sham-operated and occluded rats. Riluzole 4 and 8 mg/kg administered 30 min after occlusion reduced (P < 0.01) the cortical infarct (respectively 94 +/- 12 mm3 and 73 +/- 15 mm3 versus 139 +/- 8 mm3 for control rats). Striatum necrosis was not modified by the low dosage (46 +/- 3 mm3 versus 49 +/- 3 mm3 for control rats), whereas the high dosage increased it (61 +/- 3 mm3, P < 0.05). The ischemia-induced neurological and memory impairments were not improved by riluzole. Our results indicate that a drug depressing glutamatergic neurotransmission without blocking the glutamate receptors exerts anti-ischemic activity. Moreover, the results highlight the need for carrying out histological and neurobehavioral studies in parallel in this model.

Riluzole inhibits the release of glutamate in the caudate nucleus of the cat in vivo

When applied locally to the caudate nucleus of the halothane-anaesthetized cat, riluzole (10(-5) M) markedly reduced (-57%) the spontaneous release of glutamate. This effect seems to be specific, since the efflux of the other amino acids, including aspartate was not affected. Indicating further its selective inhibitory effect on the spontaneous release of glutamate, the prolonged (90 min) application of riluzole (10(-5) M) enhanced the size of the potassium-releasable pool of glutamate, but not that of aspartate. This effect of riluzole was not noticed with classical anti-glutamatergic drugs, tested in the same conditions.

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.

The depolarization block hypothesis of neuroleptic action: implications for the etiology and treatment of schizophrenia

Antipsychotic drugs are known to block dopamine receptors soon after their administration, resulting in an increase in dopamine neuron firing and dopamine turnover. Nonetheless, antipsychotic drugs must be administered repeatedly to schizophrenics before therapeutic benefits are produced. Recordings from dopamine neurons in rats have revealed that chronic antipsychotic drug treatment results in the time-dependent inactivation of dopamine neuron firing via over-excitation, or depolarization block. Furthermore, the clinical profile of the response to antipsychotic drugs appears to correspond to the dopamine system affected: antipsychotic drugs that exert therapeutic actions in schizophrenics inactivate dopamine neuron firing in the limbic-related ventral tegmental area, whereas drugs that precipitate extrapyramidal side effects cause depolarization block of the motor-related substantia nigra dopamine cells. One factor that remains unresolved with regard to the actions of antipsychotic drugs is the relationship between dopamine turnover and depolarization block--i.e., why does a significant level of dopamine release or turnover remain after antipsychotic drug treatment if dopamine cells are no longer firing? We addressed this question using an acute model of neuroleptic-induced depolarization block. In this model, dopamine cells recorded in rats one month after partial dopamine lesions could be driven into depolarization block by the acute administration of moderate doses of haloperidol. However, similar doses of haloperidol, which were effective at increasing dopamine levels in the striatum of intact rats, failed to change dopamine levels in lesioned rats. This is consistent with a model in which neuroleptic drugs exert their therapeutic effects in schizophrenics by causing depolarization block in DA cells, thereby preventing further activation of dopamine neuron firing in response to external stimuli. Thus, attenuating the responsivity of the dopamine system to stimuli may be more relevant to the therapeutic actions of antipsychotic drugs than receptor blockade or decreases in absolute levels of dopamine, which could presumably be circumvented by homeostatic adaptations in this highly plastic system.

Modulation of GABA release by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate and N-methyl-D-aspartate receptors in matrix-enriched areas of the rat striatum

Using a new in vitro superfusion device, the release of preloaded [3H]GABA was examined in microdiscs of tissues taken from sagittal slices in matrix-enriched areas of the rat striatum. Potassium (9 mM, 15 mM) stimulated the release of [3H]GABA in a concentration- and calcium-dependent manner and the veratridine (1 microM)-evoked release of [3H]GABA was completely abolished in the presence of tetrodotoxin (1 microM). The selective glutamatergic agonist alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (1 mM) enhanced the potassium-evoked release of [3H]GABA as well as the basal outflow of [3H]GABA. This latter effect was found to be calcium-dependent, partially diminished by tetrodotoxin (1 microM), completely blocked by 6,7-dinitro-quinoxaline-2,3-dione (0.1 mM), which is generally used as an antagonist of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptors, but not affected by (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK801, 10 microM), a specific antagonist of N-methyl-D-aspartate receptors. Similarly, N-methyl-D-aspartate (1 mM) enhanced both the potassium (9 mM) and the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (1 mM)-evoked release of [3H]GABA but when used alone, due to the presence of magnesium in the superfusion medium, was ineffective on the basal efflux of [3H]GABA. A stimulatory effect of N-methyl-D-aspartate (1 mM) on the basal outflow of [3H]GABA was observed, however, when magnesium was omitted from the superfusion medium. The stimulatory effect of N-methyl-D-aspartate (1 mM) observed in the presence of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate was not potentiated by glycine (1 microM, in the presence of strychnine 1 microM) and the N-methyl-D-aspartate-evoked response seen in the absence of magnesium was not enhanced by D-serine (1 mM), suggesting that endogenous glycine is already acting on N-methyl-D-aspartate receptors. In fact, in the absence of magnesium, 7-chloro-kynurenate (1 mM) completely abolished the stimulatory effect of N-methyl-D-aspartate on the release of [3H]GABA confirming that under our conditions, the glycine site of the N-methyl-D-aspartate receptor is saturated. N-methyl-D-aspartate-evoked responses were all blocked by MK801 (10 microM). Finally, the N-methyl-D-aspartate-evoked response seen in the absence of magnesium was markedly reduced in the presence of tetrodotoxin (1 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

Pertussis toxin pretreatment abolishes the inhibitory effect of riluzole and carbachol on D-[3H]aspartate release from cultured cerebellar granule cells

The release of D-[3H]aspartate from cultured cerebellar granule cells evoked by glutamic acid can be inhibited by riluzole and the muscarinic agonist carbachol. The combined application of maximally efficacious concentrations of riluzole and carbachol produces no greater inhibition than that seen with either agent alone, indicating that a common mechanism is involved. The effects of both agents are abolished when the cells have been pretreated with pertussis toxin, which suggests that this mechanism may involve a GTP-binding protein. The effect of pertussis toxin pretreatment is not mimicked by cholera toxin, nor does pertussis toxin pretreatment interfere with the inhibitory effect of the competitive excitatory amino acid receptor antagonist D-alpha-aminoadipic acid.

NMDA receptors mediate the behavioral effects of amphetamine infused into the nucleus accumbens

The present experiments examined glutamate-dopamine interactions within the nucleus accumbens in rats. It has been hypothesized that dopaminergic nerve terminals exert a modulatory influence on glutamate-mediated signals from corticolimbic areas. In the present studies, the effect of the selective NMDA (n-methyl-d-aspartate) antagonist AP5 (2-amino-5-phosphonopentanoic acid) on amphetamine-mediated behaviors was observed. In two behavioral paradigms, AP5 (0, 0.05, 0.5, 1.0 micrograms bilaterally) was microinjected immediately prior to amphetamine (5 micrograms bilaterally) in the nucleus accumbens. In the first experiment, the influence of AP5 on amphetamine-induced motor activity was examined. AP5 dose-dependently reduced the effectiveness of amphetamine in stimulating motor behavior. AP5 alone, paradoxically, tended to increase motor activity. In the second experiment, the effects of AP5 on amphetamine-potentiated responding (lever pressing) for conditioned reward (CR) were investigated. Normally, when amphetamine is infused into the nucleus accumbens, a marked potentiation of CR responding occurs. Prior infusion of AP5 also attenuated this behavioral effect of amphetamine. The results demonstrate that NMDA receptors within the nucleus accumbens mediate the behavioral consequences of increased dopamine release. They provide additional evidence for the involvement of limbic-striatal connections in the activating and reinforcing effects of psychostimulant drugs.

Extracellular dopamine in striatum: Influence of nerve impulse activity in medial forebrain bundle and local glutamatergic input

Microdialysis probes were used to measure dopamine in, and to administer glutamate receptor antagonists and agonists to, the striatum of unanesthetized rats. Antagonists used were: kynurenate, 2-amino-5-phosphonovalerate and 6-cyano-7-nitroquinoxaline-2,3-dione. Agonists used were: N-methyl-D-aspartate and kainate. In some rats an additional dialysis probe was implanted in medial forebrain bundle for infusion of tetrodotoxin (10 microM) to block action potential propagation along dopaminergic axons in this pathway. The latter treatment reduced dopamine in striatal dialysate to below detectable levels (less than 0.5 pg). The quantity of dopamine in striatal dialysate was not reduced by the local application of glutamate receptor antagonists. At lower concentrations, the receptor antagonists failed to alter significantly the quantity of dopamine, whereas the highest concentration of each antagonist increased the amount of dopamine in the dialysate. At the highest concentration tested (0.75 mM or 1.0 mM), as well as at a lower concentration (0.1 mM), 2-amino-5-phosphonovalerate and 6-cyano-7-nitroquinoxaline-2,3-dione blocked the dopamine-releasing effects of exogenously applied N-methyl-D-aspartate (1.0 mM) or kainate (0.1 mM), respectively. Thus, concentrations of glutamate receptor antagonists that produced effective pharmacological blockade of the respective receptors had no effect on the basal amount of dopamine in striatal extracellular fluid. Finally, N-methyl-D-aspartate and kainate produced a significant elevation in extracellular dopamine during the infusion of tetrodotoxin into the medial forebrain bundle, indicating that impulse activity in this pathway is not necessary for dopamine release produced by glutamate receptor agonists.(ABSTRACT TRUNCATED AT 250 WORDS)

L-glutamate-evoked release of dopamine from synaptosomes of the rat striatum: involvement of AMPA and N-methyl-D-aspartate receptors

Previously, using purified synaptosomes from the rat striatum, we have shown that agonists of D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors stimulate the release of [3H]dopamine continuously synthesized from [3H]tyrosine. Similar results were obtained with N-methyl-D-aspartate in the absence of magnesium. In the present study, using the same approach, attempts were made to determine whether in the presence of magnesium, the combined stimulation of AMPA receptors allows us to demonstrate the presynaptic facilitation of [3H]dopamine release through N-methyl-D-aspartate receptors. L-Glutamate (10(-3) M) markedly stimulated the release of [3H]dopamine from synaptosomes, this effect being about twice that found with AMPA (10(-3) M) while N-methyl-D-aspartate (10(-3) M) even in the presence of glycine (10(-6) M) was ineffective. In agreement with previous results, a stimulatory effect of N-methyl-D-aspartate and glycine was only observed in the absence of magnesium. This response was blocked by 6,7-dinitro-quinoxaline-2,3-dione (3 x 10(-5) M), confirming that this compound, generally used as an AMPA antagonist, also blocks N-methyl-D-aspartate receptors. The AMPA (10(-3) M)-evoked release of [3H]dopamine was markedly potentiated by the combined application of N-methyl-D-aspartate (10(-3) M) and glycine (10(-6) M) in the presence of strychnine, indicating that the concomitant activation of AMPA receptors removes the voltage-dependent magnesium block of N-methyl-D-aspartate receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

GM1 ganglioside treatment promotes recovery of electrically-stimulated [3H]dopamine release in striatal slices from rats lesioned with kainic acid

The electrically-evoked release of [3H]dopamine ([3H]DA) from rat striatal slices was studied after a monolateral intrastriatal injection of kainic acid (KA). The release in the KA-lesioned striatum measured 4 days after the lesion was largely reduced (by 80%) with respect to the contralateral striatum. Administration of GM1 ganglioside (GM1) beginning on the day of the lesion resulted in restoration of the catecholamine release. Significant recovery was observed when GM1 was administered i.p. daily at the dose of 3 mg/kg for 6 days. The ganglioside given for 6 days at 30 mg/kg restored to near normal the electrically-evoked [3H]DA release. Similar recovery from the KA-induced injury occurred spontaneously but required 50 days.

Cellular basis for interactions between catecholaminergic afferents and neurons containing leu-enkephalin-like immunoreactivity in rat caudate-putamen nuclei

Dopaminergic afferents to the dorsal striatum, caudate-putamen nuclei, are known to modulate the levels and synthesis of endogenous opiate peptides (Leu5 and Met5-enkephalins). We examined the dual immunocytochemical localization of antisera raised against Leu5-enkephalin and the catecholamine-synthesizing enzyme, tyrosine hydroxylase (TH), to determine the cellular substrates for these and/or other functional interactions. The antisera were identified by combined immunogold-silver and immunoperoxidase labeling in single coronal sections through the caudate-putamen nuclei of adult rats. These animals were given intraventricular injections of colchicine, and the brains were fixed by acrolein perfusion prior to immunocytochemical labeling. By light microscopy, perikarya and processes containing enkephalin-like immunoreactivity (ELI) were seen in close proximity to varicose processes immunoreactive for TH. Electron microscopy further demonstrated that the ELI was localized to perikarya, dendrites, and axon terminals, whereas the TH was exclusively in axons and terminals. The dendrites containing ELI were postsynaptic to terminals that were either (1) without detectable immunoreactivity, or (2) immunoreactive for TH or enkephalin. Nonsynaptic portions of the dendrites containing ELI were covered with astrocytic processes or were in direct apposition to unlabeled dendrites. Terminals containing ELI were densely immunoreactive and were in direct contact with (1) unlabeled and occasionally enkephalin-labeled proximal dendrites, and (2) TH-labeled and unlabeled terminals. In comparison with the opiate terminals, most catecholaminergic terminals were lightly immunoreactive for TH and usually contacted more distal unlabeled dendrites or spines and, more rarely, dendrites containing ELI. In a few favorable planes of section, the terminals containing ELI and those containing TH (1) converged on common unlabeled dendrites, or (2) formed dual contacts on two different labeled or unlabeled targets. Junctions formed by terminals containing ELI and TH were sometimes characterized by symmetric synaptic densities. However, numerous other dendritic and all axonal appositions were without recognized membrane densities. The findings of the study provide anatomical substrates for multilevel interactions between catecholamines, mostly dopamine, and enkephalin in rat dorsal striatum. These include (1) monosynaptic input from dopaminergic terminals to neurons containing enkephalin, (2) presynaptic modulation of transmitter release through axonal appositions, and (3) dual regulation of common targets through convergent input. In addition, the findings suggest that both enkephalin and dopamine may have similar modulatory roles in synchronizing the activity of dual targets postsynaptic to individual axon terminals. Alterations in any one of these multiple types of interactions could account for noted motor or sensory symptoms in neurological disorders characterized by depletion of dopamine or endogenous opiate peptides, or both.

Cellular interactions in the striatum involving neuronal systems using ?classical? neurotransmitters: Possible functional implications

The neostriatum contains a wide variety of neuroactive substances associated with several well-defined functional neuronal systems. This structure, which is the seat of numerous neurological pathological disorders, is commonly used as a model for studying the basic mechanisms of neurotransmitter interactions in the brain and their putative involvement in striatal functions. Increasing interest has been focusing lately on the cellular interactions that may occur between the corticostriatal putatively glutamatergic system and the nigrostriatal dopaminergic input. Current evidence suggests that the activatory corticostriatal glutamatergic input may play a more crucial role in regulating striatal functions than was formerly assumed in comparison with the dopaminergic input. The key role of cholinergic interneurons in the striatum may therefore be attributable to the fact that they modulate the glutamatergic transmission to GABA striatal efferent neurons. Likewise, dopamine may actually act indirectly in the striatum by "tuning down" the cortical excitation of striatal neurons. Consequently, an impairment of the dopaminergic transmission such as that occurring in Parkinsonism may lead to an increase in the corticostriatal glutamatergic transmission, which may further contribute towards reinforcing the "imbalance" between subsets of striatal neuronal systems controlling the output of the basal ganglia.

The regulation of subcortical dopamine systems by the prefrontal cortex: interactions of central dopamine systems and the pathogenesis of schizophrenia

A recent hypothesis of the pathogenesis of schizophrenia posits a developmentally-specific dysfunction of the dopaminergic innervation of the prefrontal cortex (PFC; Weinberger, 1987; Berman and Weinberger, 1990). It has been difficult to reconcile this hypothesis with the observation that all clinically effective antipsychotic drugs used for the treatment of schizophrenia block dopamine D2 receptors (see Deutch et al., 1991a). A resolution between the suggestion of functional dopamine (DA) "depletion" in the PFC and enhanced subcortical DA function was offered by studies of Carter, Pycock, and associates (Carter and Pycock, 1980; Pycock et al., 1980a, b). These investigators reported that depletion of DA in the rat PFC enhanced DA utilization in subcortical sites such as the nucleus accumbens septi (NAS) and striatum. Thus, a functional deficit in DA neurotransmission in the PFC would increase subcortical DA turnover, and the D2 receptor blockade induced by antipsychotic drugs would counteract the increase in dopaminergic tone in subcortical sites. This hypothesis has been particularly influential because it incorporates both an explanation for negative symptoms, which are thought to reflect cortical dysfunction (a derangement in DA transmission in the PFC), and the efficacy of antipsychotic drugs in the treatment of positive symptoms (arising from increases in subcortical DA tone). As attractive as this hypothesis has been, the physiological underpinnings that subserve such system interactions have remained elusive. Pycock, Carter, and colleagues (Carter and Pycock, 1980; Pycock et al., 1980a, b) reported that 6-hydroxydopamine (6-OHDA) lesions of the PFC increase DA levels and DA turnover in the striatum; certain aspects of their findings have been confirmed (Martin-Iversen et al., 1986; Leccese and Lyness, 1987; Haroutounian et al., 1988). However, other groups have been unable to confirm either the biochemical or behavioral findings of Pycock and associates (Joyce et al., 1983; Oades et al., 1986; Deutch et al., 1990). Moreover, Pycock and colleagues did not observe consistent effects of PFC DA deafferentation on various indices of subcortical DA function (Carter and Pycock, 1980; Pycock et al., 1980a, b). In light of the importance that such DA system interactions may have in the pathogenesis of schizophrenia, we have reinvestigated the effects of cortical DA lesions on subcortical DA function.

Bellini, Carpaccio, and receptors in the central nervous system

With the convergence of science from the fields of neurobiology and immunology, many exciting and challenging surprises have emerged regarding cytokines, neuroendocrine hormones, neuropeptides, excitatory amino acids, and their receptors. For some time neurobiologists have known that subsets of neural cells had different receptors for the same ligand. Those subsets of cells could be as different as neurons and astrocytes and as closely related as astrocytes from different lineages or anatomical areas. The neurobiological puzzle has been to determine the functional meaning of these differences. Immunologists in contrast have long understood the clear cut differences between T and B lymphocytes or T helper/inducer and T cytotoxic/suppressor cells and their response to cytokines. However, it is only very recently that they have discovered preferential use by these cells of different receptors for an identical cytokine ligand. Indeed, identical cytokines in the central nervous system and immune response may induce their pleiotropic responses by utilizing different receptors in these two systems. Immunologic paradigms may help neurobiologists predict the existence of subsets of neural cells and their function. Likewise, neurobiology may enable immunologists to predict roles for receptors in gene families as well as the existence of as yet unidentified receptors.