Role of quisqualic acid receptors in the hypermotility response produced by the injection of AMPA into the nucleus accumbens

Towards a reconceptualization of striatal interactions between glutamatergic and dopaminergic neurotransmission and their contribution to the production of movements

According to the current model of the basal ganglia organization, simultaneous activation of the striato-nigral direct pathway by glutamatergic and dopaminergic neurotransmission should lead to a synergistic facilitatory action on locomotor activity, while in contrast activation of the indirect pathway by these two neurotransmittions should lead to antagonistic effects on locomotor activity. Based on published data, as a break with the current thinking, we propose a reconceptualization of functional interactions between dopaminergic and glutamatergic neurotransmission. In this model, dopaminergic neurotransmission is seen as a motor pacemaker responsible for the basal and primary activation of striatal output neurons and glutamate as a driver providing a multiple combination of tonic, phasic, facilitatory and inhibitory influxes resulting from the processing of environmental, emotional and mnesic stimuli. Thus, in the model, glutamate-coded inputs would allow tuning the intrinsic motor-activating properties of dopamine to adjust the production of locomotor activity into goal-oriented movements.

Dopamine-glutamate reciprocal modulation of release and motor responses in the rat caudate-putamen and nucleus accumbens of "intact" animals

Functional interactions between dopaminergic neurotransmission and glutamatergic neurotransmission are well known to play a crucial integrative role in the striatum, the major input structure of the basal ganglia now widely recognized to contribute to the control of motor activity and movements but also to the processing of cognitive and limbic functions. However, the nature of these interactions is still a matter of debate and controversy. This review (1) summarizes anatomical data on the distribution of dopaminergic and glutamatergic receptors in the striatum-accumbens complex, (2) focuses on the dopamine-glutamate interactions in the modulation of each other's release in the striatum-accumbens complex, and (3) examines the dopamine-glutamate interactions in the entire striatum involved in the control of locomotor activity. The effects of dopaminergic and glutamatergic receptor selective agonists and antagonists on dopamine and glutamate release as well on motor responses are analyzed in the entire striatum, by reviewing both in vitro and in vivo data. Regarding in vivo data, only findings from focal injections studies in the nucleus accumbens or the caudate-putamen of "intact" animals are reviewed. Altogether, the available data demonstrate that dopamine and glutamate do not uniformly interact to modulate each others' release and postsynaptic modulation of striatal output neurons. Depending on the receptor subtypes involved, interactions between dopaminergic and glutamatergic transmission vary as a multiple and complex combination of tonic, phasic, facilitatory, and inhibitory properties.

Chronic hyperammonemia alters motor and neurochemical responses to activation of group I metabotropic glutamate receptors in the nucleus accumbens in rats in vivo

Hyperammonemia leads to altered cerebral function and neurological alterations in patients with hepatic encephalopathy. We studied the effects of hyperammonemia in rats on the modulation by group I metabotropic glutamate receptors (mGluR) of motor and neurochemical functions in vivo. Locomotion induced by injection of the mGluR agonist DHPG into nucleus accumbens was increased in hyperammonemic rats. In control rats DHPG increased extracellular dopamine (ca. 400%) but not glutamate. In contrast, in hyperammonemic rats DHPG increased extracellular glutamate (ca. 600%), while DHPG-induced dopamine increase was reduced. Blocking mGluR1 receptor with CPCCOEt prevented all DHPG effects, indicating that this receptor mediates its locomotor and neurochemical effects. Hyperammonemic rats showed increased (32%) mGluR1alpha, but not mGluR5 content in nucleus accumbens. These results show that modulation of locomotor and neurochemical functions by mGluRs in nucleus accumbens is strongly altered in hyperammonemia. These alterations may contribute to the neurological alterations in hyperammonemia and liver failure.

Role of AMPA and NMDA receptors in the nucleus accumbens shell in turning behaviour of rats: interaction with dopamine receptors

The role of AMPA and NMDA receptors in the shell of the nucleus accumbens in turning behaviour of rats was investigated. Unilateral injection of the AMPA receptor agonist, AMPA (0.25, 0.4, 0.5 and 1 microg), into the shell of the nucleus accumbens dose-dependently produced contraversive pivoting, namely tight head-to-tail turning marked by abnormal hindlimb backward stepping, while injection of AMPA (0.5 microg) into the core produced only a marginal effect. This shell-specific AMPA effect was dose-dependently inhibited by the AMPA receptor antagonist, NBQX (1 and 10 ng), which alone did not produce turning behaviour. The AMPA-induced pivoting was also dose-dependently inhibited by the non-competitive NMDA receptor antagonist, MK-801 (0.1 and 0.5 microg). Neither MK-801 (0.1, 0.5 and 5 microg) nor the NMDA receptor agonist, NMDA (0.5 and 1 microg), injected unilaterally into the shell, produced turning behaviour. Unilateral injection of a mixture of dopamine D(1) (SKF 38393, 5 microg) and D(2) (quinpirole, 10 microg) receptor agonists into the shell has been found to elicit contraversive pivoting. The dopamine D(1)/D(2) receptor antagonist, cis-(Z)-flupentixol (1 and 10 microg), injected into the shell, in doses known to block dopamine D(1)/D(2) receptor-mediated pivoting, also significantly inhibited AMPA (0.5 microg)-induced pivoting. Moreover, both NBQX (1 and 10 ng) and MK-801 (0.1 and 0.5 microg), injected into the shell, significantly inhibited dopamine D(1)/D(2) receptor-mediated pivoting. It is therefore concluded that unilateral stimulation of AMPA receptors in the shell of the nucleus accumbens can elicit contraversive pivoting, and that both AMPA and dopamine D(1)/D(2) receptors play a critical role in shell-specific pivoting in contrast to NMDA receptors that at best play only a modulatory role.

The involvement of the mediodorsal nucleus of the thalamus and the midbrain extrapyramidal area in locomotion elicited from the ventral pallidum

Motor activity is regulated by projections from the nucleus accumbens to the ventral pallidum, but it is unclear which efferents regulate behavioral output from the ventral pallidum. Motor activity was elicited pharmacologically by microinjecting either the mu opioid receptor agonist, Tyr-D-Ala-Gly-NmePhe-Gly-OH (DAMGO) or the glutamate receptor agonist, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) into the ventral pallidum. The involvement of efferent projections was determined by microinjecting the local anesthetic procaine into the mediodorsal nucleus of the thalamus (MD) or the midbrain extrapyramidal area (MEA) prior to administering DAMGO or AMPA into the ventral pallidum. The motor activity induced by DAMGO was blocked by procaine microinjected into either the MD or the MEA. In contrast, procaine microinjected into the MD did not block motor activity elicited by AMPA while procaine into the MEA abolished the behavioral activation. These data indicate that the involvement of efferent projections from the ventral pallidum to either the MD or MEA in motor activation depends upon the type of receptor stimulated in the ventral pallidum.

Dopaminergic agonists administered into the nucleus accumbens: effects on extracellular glutamate and on locomotor activity

The hypothesis to be tested was that increased dopaminergic transmission induced by amphetamine in the nucleus accumbens results in increased glutamatergic neurotransmission in this brain area and that the increase in level of this neurotransmitter contributes to behavioral effects of psychostimulant drugs. Amphetamine (1 mg/kg, i. p.) increased the amount of extracellular glutamate in the accumbens, as measured by in vivo dialysis, and stimulated locomotor activity. Amphetamine (10 mM) infused into the accumbens by reverse dialysis through the probe produced a similar stimulation of locomotor activity as systemic amphetamine but a greater increase in extracellular glutamate levels. Both of these responses to amphetamine were attenuated by either the selective D1 antagonist SCH23390 or the selective D2 antagonist eticlopride. The combination of a D1 and D2 agonist, SKF38393 (20 mM) and quinpirole (50 mM), administered into the accumbens by reverse dialysis also increased extracellular glutamate and stimulated locomotor activity. Administration of a glutamate uptake inhibitor, threo-beta-hydroxy-aspartate (50 mM), increased extracellular glutamate but did not stimulate locomotor activity. Systemic administration of caffeine (15 mg/kg, i.p.) increased locomotor activity but did not increase extracellular levels of glutamate. These data suggest that activation of dopaminergic receptors in the nucleus accumbens results in stimulation of locomotor activity and in activation of glutamatergic transmission in this brain region. However, an increase in glutamate levels in the nucleus accumbens is neither sufficient nor necessary to produce a stimulation of locomotor activity.

Glutamate transporter EAAC-1-deficient mice develop dicarboxylic aminoaciduria and behavioral abnormalities but no neurodegeneration

Four L-glutamate neurotransmitter transporters, the three Na(+)-dependent GLAST-1, GLT-1 and EAAC-1, and the Cl(-)-dependent EAAT-4, form a new family of structurally related integral plasma membrane proteins with different distribution in the central nervous system. They may have pivotal functions in the regulation of synaptic L-glutamate concentration during neurotransmission and are believed to prevent glutamate neurotoxicity. To investigate the specific physiological and pathophysiological role of the neuronal EAAC-1, which is also expressed in kidney and small intestine, we have generated two independent mouse lines lacking EAAC-1. eaac-1(-/-) mice develop dicarboxylic aminoaciduria. No neurodegeneration has been observed during a period of >12 months, but homozygous mutants display a significantly reduced spontaneous locomotor activity.

The behavioral effects of MK-801 injected into nucleus accumbens and caudate-putamen of rats

In this study, we investigated the behavioral effects of MK-801 (1-20 micrograms) injected into the posterior parts of nucleus accumbens (ACC) and caudate-putamen (CP) in rats. Interactions of diazepam (DZP, 10 micrograms), haloperidol (HPD, 2 micrograms), and scopolamine (SCOP, 10 micrograms) with 20 micrograms of MK-801 were also studied. All injections were done in 2 microliters. In ACC, MK-801 increased locomotion, rearing, and head shakes. The effect of MK-801 especially at 20 micrograms was accompanied by a motor syndrome: head weaves, circling, body rolls, and ataxia. DZP nonsignificantly reduced the locomotion but it significantly (p < 0.05) reduced head shakes, weaves, circling, and body rolls produced by MK-801. HPD reduced grooming and head shakes. SCOP potentiated MK-801 hyperlocomotion, whereas it decreased body rolls, head shakes, and weaves. In CP, MK-801 increased locomotion, but less than in ACC (p < 0.05). The effect of MK-801 was significantly increased by SCOP. MK-801 also increased grooming (reduced by HPD and increased by SCOP) and at 5-20 micrograms induced oral movements that were decreased by HPD. These results indicate that the posterior part of ACC is involved in MK-801 hyperlocomotion and motor syndromes, whereas CP is involved in mediating grooming and oral movements. Blockade of the muscarinic cholinergic receptors seems to facilitate hyperlocomotion and decrease head shakes produced by MK-801. Mechanisms influenced by DZP and HPD appear to be involved in motor syndrome and oral movement, respectively, induced by MK-801, but not in hyperlocomotion.

Glutamate-dopamine interactions in the ventral striatum: role in locomotor activity and responding with conditioned reinforcement

Previous evidence suggests that glutamatergic limbic afferents participate in the potentiation of responding with conditioned reinforcement produced by intra-accumbens d-amphetamine. The present experiments were designed to investigate glutamate-dopamine interactions in the ventral striatum in both conditioned reinforcement and locomotor activity. Glutamate receptor agonists and antagonists were infused into the nucleus accumbens both alone and in combination with 3 micrograms d-amphetamine, and the effects of these interactions on responding with conditioned reinforcement and locomotor activity were measured. The glutamate receptor agonists NMDA, AMPA and quisqualate (agonists at the NMDA, AMPA and metabotropic glutamate receptor subtypes, respectively) and the antagonists AP5 and CNQX, (antagonists at the NMDA and AMPA receptor subtypes, respectively) were used in these investigations. These compounds were used in a dose range of 0.3 to 3 nmol, except CNQX, which was used in 0.2 to 2 nmol doses. While all agonists and antagonists increased locomotor activity when administered alone, the antagonists attenuated the locomotor response to d-amphetamine. In contrast, the agonists AMPA and quisqualate enhanced d-amphetamine-induced locomotor activity, although NMDA interfered with the effects of d-amphetamine. In the conditioned reinforcement paradigm, both the agonists and the antagonists abolished amphetamine's potentiation of responding with conditioned reinforcement, suggesting that the glutamatergic transmission of information about the conditioned reinforcer could be blocked by glutamate receptor antagonists and disrupted by administration of the agonists. The dissociation between the effects of these excitatory amino acids on amphetamine-induced locomotor activity versus their effects on amphetamine's potentiation of responding with conditioned reinforcement provides insight into the nature of the reward enhancement by accumbens dopamine versus its locomotor stimulant effects.

The relationship between self-stimulation and sniffing in rats: does a common brain system mediate these behaviors?

The relationship between brain self-stimulation and brain-stimulation induced sniffing behavior was examined at three brain sites (frontal cortex, hypothalamus and lower brain stem). In the first experiment, sniffing was elicited in the prefrontal cortex and pontine reticular formation (PRF) of anesthetized rats. These sites corresponded to reported self-stimulation sites. In non-anesthetized animals (Expt. 2), all self-stimulation sites in the medial prefrontal cortex (MPC) and lateral hypothalamic-medial forebrain bundle (LH-MFB) also supported sniffing. In the PRF, this was also the case except for one subject which exhibited self-stimulation and jaw movements without sniffing. After unilateral lesions either in the MPC or PRF, stimulation-induced sniffing from the ipsilateral LH-MFB was not influenced. While MPC lesions did not affect self-stimulation either, medial PRF lesions disrupted ipsilateral self-stimulation. In summary, stimulation-induced sniffing and self-stimulation behavior appear to share strikingly similar anatomical loci, but the PRF appears to be differentially involved in these behaviors. The results were discussed from an appetitive motivational hypothesis of self-stimulation.

Glutamatergic mechanisms mediating stimulant and antipsychotic drug effects

This paper presents an hypothesis regarding the functions of striatal dopaminergic and glutamatergic neurotransmission. It is suggested that the principal functional role of dopaminergic neurotransmission is to regulate the efficacy of cortico-striatal and cortico-accumbens neurotransmission. Increased activity at dopamine-mediated synapses is suggested to interact with neurotransmission at adjacent cortically derived glutamate-mediated synapses, facilitating communication from the cerebral cortex, and thereby causing behavioral stimulation. Decreased activity at dopaminergic synapses, as produced by neuroleptic drugs, causes changes in the activation of cortically derived synapses in the corpus striatum and nucleus accumbens which result in behavioral sedation and decreased activity. This hypothesis suggests that activity at dopaminergic synapses produces behavioral effects only insofar as these changes modulate cortico-striatal (or cortico-accumbens) activity, and further, that the manifestations of activity in cortico-striatal systems are modulated by activity at dopaminergic synapses. It is further suggested that when neuroleptics are administered chronically, adjustments in the efficacy of cortico-striatal neurotransmission are responsible for the antipsychotic effect of neuroleptic drugs.

The role of dopamine and AMPA/kainate receptors in the nucleus accumbens in the hypermotility response to MK801

The purpose of this study was to evaluate the role of endogenous dopamine in the hypermotility response to MK801. The administration of MK801 (0.1 mg/kg, SC) to rats produced an intense stimulation of coordinated locomotor activity, which was not associated with stereotyped behavior. This stimulatory response was inhibited by pretreatment with either reserpine (5 mg/kg, IP) or alpha-methyl-p-tyrosine (2 doses of 250 mg/kg, IP). Similarly, pretreatment with the D2 antagonist eticlopride (0.03 mg/kg, SC) or the D1 antagonist SCH23390 (0.1 mg/kg, SC) produced a marked inhibition of MK801-stimulated hypermotility, and the combination of eticlopride (0.03 mg/kg, SC) and SCH23390 (0.03 mg/kg, SC) produced a greater inhibition of MK801-stimulated locomotion than either agent alone. The administration of SCH23390 or eticlopride directly into the nucleus accumbens inhibited the locomotor response to MK801, with the combination of both drugs producing a greater inhibition than either agent alone. The intra-accumbens administration of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate receptor antagonists DNQX or GAMS also inhibited the locomotor response produced by MK801. These data suggest that the activation of D1 and D2 dopaminergic receptors and AMPA/kainate excitatory amino acid receptors in the nucleus accumbens is required for the stimulation of locomotor activity produced by MK801.

AMPA/kainate antagonists in the nucleus accumbens inhibit locomotor stimulatory response to cocaine and dopamine agonists

The purpose of this study was to determine whether AMPA/kainate excitatory amino acid receptors in the nucleus accumbens (NAc) play a role in the locomotor stimulation produced by cocaine and dopamine receptor agonists. The stimulation of locomotor activity produced by the systemic administration of cocaine was markedly attenuated by either the D1 receptor antagonist SCH23390 or the D2 receptor antagonist eticlopride administered directly into the NAc. This indicates that both dopaminergic receptor subtypes in the NAc are involved in the motor stimulant response to cocaine. The intra-accumbens administration of DNOX or GAMS, which have been shown to inhibit the locomotor stimulation produced by the excitatory amino acid agonist AMPA, antagonized the locomotor stimulant response to cocaine administered either systemically or directly into the NAc. DNOX and GAMS also inhibited the stimulation of locomotor activity produced by the coinjection of the D1 agonist SKF38393 and the D2 agonist quinpirole injected into the NAc of normal animals and of animals pretreated with reserpine. These results suggest that the activation of AMPA/kainate receptors in the NAc plays an important role in the locomotor stimulation produced by cocaine and directly acting dopaminergic receptor agonists. The effects produced by the activation of these receptors is independent of endogenous dopamine stores, suggesting that these receptors are located postsynaptic to the dopaminergic nerve terminals.

Effects of the AMPA/kainate receptor antagonist DNQX in the nucleus accumbens on drug-induced conditioned place preference

Activation of AMPA/kainate glutamatergic receptors in the nucleus accumbens may be a component of the mechanism of drug induced reward. To test this hypothesis, 6,7-dinitroquinoxaline-2,3-dione (DNQX), an alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA)/kainate glutamatergic receptor anatagonist, was injected into the nucleus accumbens before the administration of amphetamine or morphine during the training phase (acquisition) of a conditioned place preference paradigm. Rats were then tested for place preference in the absence of drugs. In other experiments, DNQX was given before testing for place preference (expression) but not during the training phase. Bilateral injection of DNQX (1 microgram/0.5 microliters/side) inhibited acquisition of place preference to amphetamine (1 mg/kg) but not morphine (10 mg/kg). During acquisition, DNQX marginally attenuated the locomotor stimulation elicited by amphetamine during the first but not subsequent training sessions, while the combination of morphine plus DNQX produced marked akinesia during each training session. When given prior to testing, DNQX inhibited the expression of place preference induced by amphetamine and morphine but did not affect locomotor activity. The results suggest that activation of AMPA/kainate receptors is involved in the primary reward stimulation (acquisition of place preference) of amphetamine but not morphine and in behaviors elicited by memory of primary reward stimulation (expression of place preference) for both drugs. Furthermore, locomotor activity during conditioning is not necessary for acquisition of place preference.

Opposite effects of NMDA and AMPA receptor blockade on catalepsy induced by dopamine receptor antagonists

Excitatory amino acid antagonists have been proposed as novel therapeutic agents for Parkinson's disease due to their ability to reverse akinesia in animal models of this disorder. To further evaluate this therapeutic potential, we examined the effects of a N-methyl-D-aspartate (NMDA) and an alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor antagonist on catalepsy produced by dopamine D1 or D2 receptor antagonists in rats. Male Sprague-Dawley rats were injected with dizocilpine (MK-801 0.025, 0.05 or 0.1 mg/kg i.p.), 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (NBQX 12.5 mg/kg i.p.) or saline prior to administration of either raclopride (2.5 mg/kg i.p.) or SCH 23390 (0.5 mg/kg i.p.). Catalepsy was evaluated with both grid and bar tests every 20 min for 2.7 h. MK-801 (0.1 mg/kg) reversed the catalepsy produced by either raclopride or SCH 23390 but did not stimulate locomotion when given alone at this dose. At 0.05 mg/kg, MK-801 markedly decreased SCH 23390-induced catalepsy, but did not affect the catalepsy produced by raclopride. In contrast, NBQX increased raclopride-induced catalepsy, but had no effect on catalepsy elicited by SCH 23390. These findings suggest that blockade of NMDA receptors, but not non-NMDA receptors, may reverse the catalepsy produced by dopamine receptor antagonists.

Differential effects of quinpirole in the nucleus accumbens depending on the initial level of locomotor activity

Effects of dopamine D1 and D2 receptor agonists (SKF 38393 and quinpirole, respectively) on locomotion were studied in two behavioural situations characterized by low and high level of exploratory locomotor activity. Administration of quinpirole bilaterally into the nucleus accumbens increased locomotor activity at the low initial level of activity and decreased locomotor activity at the high activity level, while the administration of SKF 38393 increased locomotor activity in both behavioural situations. It was concluded that quinpirole has differential effects on locomotion, depending on the initial level of activity.

AMPA glutamate receptor activation in the posterior zona incerta inhibits amphetamine- and apomorphine-induced stereotypy

Previous work demonstrated that alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) glutamate receptor antagonism in the zona incerta (ZI) dorsal to the subthalamic nucleus inhibits stereotypy in rats. The current investigation was undertaken to determine if AMPA receptors in a more caudal portion of the ZI have a role in the expression of stereotyped behavior. Rats were injected bilaterally with AMPA into the posterior ZI dorsal to the substantia nigra, and immediately given a systemic injection of d-amphetamine (10 mg/kg, s.c.) or apomorphine (1 mg/kg s.c.). AMPA produced a dose-dependent inhibition of stereotypy induced by both drugs which was prevented by the coadministration of the AMPA/kainic acid antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX) (0.5 microgram/0.5 microliter). A dose of AMPA as low as 62.5 ng completely abolished the oral component of stereotypy induced by both apomorphine and amphetamine. This dose of AMPA alone had no significant effect on spontaneous locomotor activity but enhanced the locomotor response stimulated by amphetamine (10 mg/kg, s.c.) due to an inhibition of stereotypy. The finding that activation of AMPA receptors in the posterior ZI inhibits stereotypy shows a contrast to results in the neighboring medial ZI dorsal to the subthalamic nucleus, where blockade of AMPA/kainic acid glutamate receptors with DNQX inhibits stereotypy.

Injections of the NMDA-antagonist D-2-amino-7-phosphonoheptanoic acid (AP-7) into the nucleus accumbens of rats enhance switching between cue-directed behaviours in a swimming test procedure

The present study explores the behavioural effects of intra-accumbens injections of D-2-amino-7-phosphonoheptanoic acid (AP-7), a selective competitive N-methyl-D-aspartate (NMDA) receptor antagonist, using a swimming test procedure, in which rats were forced to swim for 6 min. The behaviour of the rats was analysed in terms of cue-directed (CDBs) and non-cue-directed behaviours (NCDBs). AP-7 (100-500 ng/0.5 microliter) dose-dependently enhanced the number of switches to CDBs, without affecting the number of switches to NCDBs. Further analysis of the data showed that the number of switches between CDBs was enhanced, while no effect was found on the number of switches from NCDBs to CDBs, from CDBs to NCDBs or between NCDBs. These data suggest that the NMDA-receptor in the nucleus accumbens is involved in the ability to switch between cue-directed behaviours.

Stimulation of locomotor activity by intra-accumbens AMPA is not inhibited by neonatal 6-hydroxydopamine-induced lesions

The involvement of dopamine in the hypermotility responses to amphetamine s.c. or AMPA injected into the nucleus accumbens was evaluated in adult rats depleted of dopamine as neonates with 6-hydroxydopamine. The hypermotility response to amphetamine was markedly inhibited in the lesioned animals, while that to AMPA was enhanced. In addition, the hypermotility produced by AMPA in these rats was not inhibited by sulpiride+SCH-23390; however, it was inhibited completely by alpha-methyl-p-tyrosine. These results suggest that the hypermotility produced by AMPA requires endogenous dopamine, but is mediated by a different mechanism than that produced by amphetamine.

Activation of AMPA/kainic acid glutamate receptors in the zona incerta stimulates locomotor activity

Direct injections of DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), an AMPA/quisqualic acid receptor agonist, into the medial or posterior zona incerta (ZI) produced a marked stimulation of locomotor activity accompanied by a postural change. Similar responses were obtained by injection of kainic acid (KA) into the same areas. The behavioral effects of AMPA and KA were antagonized by coinjection of 6,7-dinitroquinoxaline-2,3-dione (DNQX), and non-N-methyl-D-aspartate (NMDA) glutamate receptor antagonist. In contrast, injections of NMDA or ibotenic acid failed to significantly stimulate locomotor activity. These results suggest that the AMPA/kainate glutamate receptor subtypes in the zona incerta may have a functional role in regulating locomotor activity.

The role of endogenous dopamine in the hypermotility response to intra-accumbens AMPA

The present study was designed to investigate the role of dopamine in the locomotor stimulant response produced by the bilateral administration of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) into the nucleus accumbens. The hypermotility produced by lower doses of AMPA (up to 0.25 microgram) was inhibited by either SCH23390 or sulpiride, a D1 and D2 receptor antagonist, respectively. The locomotor response to a higher dose of AMPA (0.5 microgram) was greater than the maximum response to intra-accumbal injection of amphetamine and was significantly inhibited only when both the D1 and D2 antagonists were administered together. Alpha-Methyl-p-tyrosine inhibited the locomotor response to AMPA (0.5 microgram), and this inhibition was reversed by the co-injection of AMPA with either SKF38393, a D1 agonist, or quinpirole, a D2 agonist, at doses which were ineffective in the absence of AMPA. AMPA when infused into the nucleus accumbens produced an increase in extracellular dopamine, suggesting that AMPA can enhance dopamine efflux. The injection of AMPA into the nucleus accumbens significantly increased the DOPAC/dopamine ratio, which is different from the decrease in ratio reported for amphetamine. These data suggest that the stimulation of locomotor activity by intra-accumbal AMPA may be the result of an enhancement in dopamine efflux as well as a change in the response to dopaminergic receptor activation.

Effect of the NMDA receptor antagonist, MK-801, on locomotor activity and on the metabolism of dopamine in various brain areas of mice

Various doses (0.1-0.5 mg/kg i.p.) of the N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801, produced a dose-dependent increase in well-coordinated locomotor activity of NMRI mice. Higher doses (greater than 0.5 mg/kg) produced a typical motor syndrome characterized by head weaving, body rolling, ataxia and salivation. MK-801, 0.2 mg/kg i.p., a dose which produced marked locomotor stimulation, increased the rate of disappearance of dopamine in the striatum and in the limbic forebrain of the animals, whereas the rate of disappearance of noradrenaline remained unchanged in the limbic forebrain and in the hippocampus. MK-801 increased the rate of tyrosine hydroxylation (measured as the accumulation of 3,4-dihydroxyphenylalanine (DOPA) after inhibition of DOPA decarboxylase) in the striatum with no change in DOPA formation in the limbic forebrain. The levels of 3,4-dihydroxyphenylacetic acid (DOPAC) remained unchanged both in the striatum and in the limbic forebrain following the administration of MK-801. It is concluded that MK-801 may facilitate the activation of dopaminergic mechanisms through an indirect (perhaps by reducing glutamatergic activity) rather than a direct effect on dopamine neurons.

Dopamine release in the nucleus caudatus and in the nucleus accumbens is under glutamatergic control through non-NMDA receptors: a study in freely-moving rats

Perfusion with quisqualate (5 x 10(-6) M) and kainate (5 x 10(-7) M), selective agonists of glutamate receptors, enhanced the release of dopamine in both caudate and accumbens nuclei of freely-moving rats, measured by the transcerebral microdialysis technique. In contrast, N-methyl-D-aspartate (NMDA) did not affect dopamine release, except at very high concentrations (10(-2) M). The quisqualate-kainate antagonist, FG 9041 (DNQX), antagonized the elevation of dopamine release induced by quisqualate and, furthermore, reduced that of kainate. CPP, a selective NMDA antagonist, did not counteract the quisqualate- or kainate-induced stimulation of dopamine release. The enhancement of dopamine release after quisqualate and kainate was accompanied by behavioural stimulation characterized by grooming, rearing, hypermotility with sniffing and confined sniffing. This behavioural syndrome could be blocked by haloperidol. Conversely, perfusion with NMDA did not activate behaviour even at high concentrations. These results indicate that the dopaminergic system, within the caudate and the accumbens nuclei, is under glutamatergic control through kainate and quisqualate receptors, while the NMDA receptors do not appear to be involved.

Excitatory amino acid pathways in brain-stimulation reward

A range of agonists and antagonists active at different glutamate/aspartate (Glu/Asp) receptor subtypes were injected into rat ventral tegmental (VTA) sites downstream from self-stimulation electrodes in the medial forebrain bundle. Control injections were made into the contralateral tegmentum. Variable-interval (VI 10 s) self-stimulation was not significantly affected by a specific antagonist of N-methyl-D-aspartate (NMDA)-type receptors (D,L-2-amino-5-phosphonovaleric acid (2-AP5), 10 and 50 nmol). Broad-spectrum excitatory amino acid (EAA) antagonists viz cis-2,3-piperidine dicarboxylate (cPDA) (10 and 50 nmol), gamma-D-glutamylaminomethyl sulphonic acid (GAMS) (10 nmol) and p-chlorobenzoyl-2,3-piperazine dicarboxylic acid (pCB PzDA) (2.0 and 10 nmol), active at kainate, quisqualate, as well as NMDA receptors, all produced significant depression of responding when injected into the ipsilateral, but not the contralateral, tegmentum. Compounds inhibiting Glu/Asp reuptake had variable effects: strong depression with dihydrokainic acid (7.5 nmol), or no significant effect (L-threo-3-hydroxyaspartic acid, 2.0 and 10 nmol). The receptor agonist, NMDA (10 nmol), depressed responding regardless of injection side; kainic and responding regardless of injection side; kainic and quisqualic acid elicited myoclonic and other non-specific responses in preliminary tests, and were not examined further; enhanced responding was not seen. The side-specific blockade of responding by non-NMDA antagonists indicates the existence of non-NMDA EAA terminals in the VTA, signalling the receipt of hypothalamic brain-stimulation reward. Caudally directed EAA projections terminating on A10 dopamine cell bodies may account for depression of self-stimulation by EAA antagonists.

A possible role of AA2 excitatory amino acid receptors in the expression of stimulant drug effects

GDEE, an antagonist of the AA2 or quisqualic acid category of excitatory amino acid receptor, decreases behavioral activity and locomotor stimulation induced by cocaine and amphetamine when locally injected into the nucleus accumbens. The present experiment was intended to examine the effects of systemic GDEE and other excitatory amino acid antagonists on stimulant-induced locomotor activity. GDEE markedly attenuated the stimulant effect of amphetamine, and partially blocked the effects of phencyclidine (PCP). Apomorphine-induced cage climbing behavior was partially decreased by lower dosages of GDEE, but was almost completely blocked by the highest dosage tested. Amphetamine-induced stimulation of locomotor activity was not decreased by any of the other excitatory amino acid antagonists that were tested, including MK-801, 2-amino-7-phosphonoheptanoic acid (APH), or CNQX. APH decreased stereotypy only at a high dosage (250 mg/kg), which also produces ataxia. Several other compounds, including L-glutamic acid gamma ethyl ester (GMEE), L-glutamic acid, glycine, and L-glutamine did not block amphetamine-induced stimulation in molar dosages equivalent to the highest dosage of GDEE (8 mmol/kg). It is concluded that the AA2 excitatory amino acid receptor is important in the expression of activating effects of stimulant drugs.

AMPA, kainic acid, and N-methyl-D-aspartic acid stimulate locomotor activity after injection into the substantia innominata/lateral preoptic area

The substantia innominata/lateral preoptic area (SI/LPO) is a subpallidal region which has been shown to regulate the hypermotility produced by drugs acting in the nucleus accumbens. Evidence has been presented that the SI/LPO contains glutamatergic nerve terminals and receptors for excitatory amino acids. The purpose of this study was to determine the effects of the activation of excitatory amino acid receptors in the SI/LPO on locomotor activity following the direct injection of excitatory amino acids into this brain site. It was found that the bilateral injection of alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA), kainic acid, and N-methyl-D-aspartic acid into the SI/LPO produced marked dose-dependent stimulations of locomotor activity which resembled the effects of these agents after their injection into the nucleus accumbens. The effect, however, was bell-shaped in that at high doses, the locomotor activity values decreased from their peak values. The coinjection of gamma-glutamylaminomethylsulfonate (GAMS) with AMPA into the SI/LPO was found to inhibit the hypermotility response to AMPA at doses that were unable to produce a significant inhibition of the hypermotility responses to kainic acid or N-methyl-D-aspartic acid. The injection of 6,7-dinitroquinoxaline-2,3-dione (DNQX) into the SI/LPO inhibited the hypermotility responses to AMPA or kainic acid while having no significant inhibitory effect on N-methyl-D-aspartic acid stimulated locomotor activity. The injection of D-alpha-aminoadipic acid into the SI/LPO produced a significant inhibition of the hypermotility response produced by N-methyl-D-aspartic acid at a dose that did not produce a significant inhibition of the hypermotility response produced by AMPA.(ABSTRACT TRUNCATED AT 250 WORDS)