L-dopa stimulates the release of [3H]gamma-aminobutyric acid in the basal ganglia of 6-hydroxydopamine lesioned rats

Ophthalmate is a new regulator of motor functions via CaSR: implications for movement disorders

Dopamine's role as the principal neurotransmitter in motor functions has long been accepted. We broaden this conventional perspective by demonstrating the involvement of non-dopaminergic mechanisms. In mouse models of Parkinson's disease, we observed that L-DOPA elicited a substantial motor response even when its conversion to dopamine was blocked by inhibiting the enzyme aromatic amino acid decarboxylase (AADC). Remarkably, the motor activity response to L-DOPA in the presence of an AADC inhibitor (NSD1015) showed a delayed onset, yet greater intensity and longer duration, peaking at 7 h, compared to when L-DOPA was administered alone. This suggests an alternative pathway or mechanism, independent of dopamine signalling, mediating the motor functions. We sought to determine the metabolites associated with the pronounced hyperactivity observed, using comprehensive metabolomics analysis. Our results revealed that the peak in motor activity induced by NSD1015/L-DOPA in Parkinson's disease mice is associated with a surge (20-fold) in brain levels of the tripeptide ophthalmic acid (also known as ophthalmate in its anionic form). Interestingly, we found that administering ophthalmate directly to the brain rescued motor deficits in Parkinson's disease mice in a dose-dependent manner. We investigated the molecular mechanisms underlying ophthalmate's action and discovered, through radioligand binding and cAMP-luminescence assays, that ophthalmate binds to and activates the calcium-sensing receptor (CaSR). Additionally, our findings demonstrated that a CaSR antagonist inhibits the motor-enhancing effects of ophthalmate, further solidifying the evidence that ophthalmate modulates motor functions through the activation of the CaSR. The discovery of ophthalmate as a novel regulator of motor function presents significant potential to transform our understanding of brain mechanisms of movement control and the therapeutic management of related disorders.

Cerebral dopamine neurotrophic factor transfection in dopamine neurons using neurotensin-polyplex nanoparticles reverses 6-hydroxydopamine-induced nigrostriatal neurodegeneration

Overexpression of neurotrophic factors in nigral dopamine neurons is a promising approach to reverse neurodegeneration of the nigrostriatal dopamine system, a hallmark in Parkinson's disease. The human cerebral dopamine neurotrophic factor (hCDNF) has recently emerged as a strong candidate for Parkinson's disease therapy. This study shows that hCDNF expression in dopamine neurons using the neurotensin-polyplex nanoparticle system reverses 6-hydroxydopamine-induced morphological, biochemical, and behavioral alterations. Three independent electron microscopy techniques showed that the neurotensin-polyplex nanoparticles containing the hCDNF gene, ranging in size from 20 to 150 nm, enabled the expression of a secretable hCDNF in vitro. Their injection in the substantia nigra compacta on day 21 after the 6-hydroxydopamine lesion resulted in detectable hCDNF in dopamine neurons, whose levels remained constant throughout the study in the substantia nigra compacta and striatum. Compared with the lesioned group, tyrosine hydroxylase-positive (TH⁺) nigral cell population and TH⁺ fiber density rose in the substantia nigra compacta and striatum after hCDNF transfection. An increase in βIII-tubulin and growth-associated protein 43 phospho-S41 (GAP43p) followed TH⁺ cell recovery, as well as dopamine and its catabolite levels. Partial reversal (80%) of drug-activated circling behavior and full recovery of spontaneous motor and non-motor behavior were achieved. Brain-derived neurotrophic factor recovery in dopamine neurons that also occurred suggests its participation in the neurotrophic effects. These findings support the potential of nanoparticle-mediated hCDNF gene delivery to develop a disease-modifying treatment against Parkinson's disease. The Institutional Animal Care and Use Committee of Centro de Investigación y de Estudios Avanzados approved our experimental procedures for animal use (authorization No. 162-15) on June 9, 2019.

l-DOPA promotes striatal dopamine release through D1 receptors and reversal of dopamine transporter

Previous studies have pointed out that l-DOPA can interact with D1 or D2 receptors independent of its conversion to endogenous dopamine. The present study was set to investigate whether l-DOPA modulates dopamine release from striatal nerve terminals, using a preparation of synaptosomes preloaded with [³H]DA. Levodopa (1 µM) doubled the K⁺-induced [³H]DA release whereas the D2/D3 receptor agonist pramipexole (100 nM) inhibited it. The l-DOPA-evoked facilitation was mimicked by the D1 receptor agonist SKF38393 (30-300 nM) and prevented by the D1/D5 antagonist SCH23390 (100 nM) but not the DA transporter inhibitor GBR12783 (300 nM) or the aromatic l-amino acid decarboxylase inhibitor benserazide (1 µM). Higher l-DOPA concentrations (10 and 100 µM) elevated spontaneous [³H]DA efflux. This effect was counteracted by GBR12783 but not SCH23390. Binding of [³H]SCH23390 in synaptosomes (in test tubes) revealed a dense population of D1 receptors (2105 fmol/mg protein). Both SCH23390 and SKF38393 fully inhibited [³H]SCH23390 binding (Ki 0.42 nM and 29 nM, respectively). l-DOPA displaced [³H]SCH23390 binding maximally by 44% at 1 mM. This effect was halved by addition of GBR12935 and benserazide. We conclude that l-DOPA facilitates exocytotic [³H]DA release through SCH23390-sensitive D1 receptors, independent of its conversion to DA. It also promotes non-exocytotic [³H]DA release, possibly via conversion to DA and reversal of DA transporter. These data confirm that l-DOPA can directly interact with dopamine D1 receptors and might extend our knowledge of the neurobiological mechanisms underlying l-DOPA clinical effects.

Expanding the repertoire of L-DOPA's actions: A comprehensive review of its functional neurochemistry

Though a multi-facetted disorder, Parkinson's disease is prototypically characterized by neurodegeneration of nigrostriatal dopaminergic neurons of the substantia nigra pars compacta, leading to a severe disruption of motor function. Accordingly, L-DOPA, the metabolic precursor of dopamine (DA), is well-established as a treatment for the motor deficits of Parkinson's disease despite long-term complications such as dyskinesia and psychiatric side-effects. Paradoxically, however, despite the traditional assumption that L-DOPA is transformed in residual striatal dopaminergic neurons into DA, the mechanism of action of L-DOPA is neither simple nor entirely clear. Herein, focussing on its influence upon extracellular DA and other neuromodulators in intact animals and experimental models of Parkinson's disease, we highlight effects other than striatal generation of DA in the functional profile of L-DOPA. While not excluding a minor role for glial cells, L-DOPA is principally transformed into DA in neurons yet, interestingly, with a more important role for serotonergic than dopaminergic projections. Moreover, in addition to the striatum, L-DOPA evokes marked increases in extracellular DA in frontal cortex, nucleus accumbens, the subthalamic nucleus and additional extra-striatal regions. In considering its functional profile, it is also important to bear in mind the marked (probably indirect) influence of L-DOPA upon cholinergic, GABAergic and glutamatergic neurons in the basal ganglia and/or cortex, while anomalous serotonergic transmission is incriminated in the emergence of L-DOPA elicited dyskinesia and psychosis. Finally, L-DOPA may exert intrinsic receptor-mediated actions independently of DA neurotransmission and can be processed into bioactive metabolites. In conclusion, L-DOPA exerts a surprisingly complex pattern of neurochemical effects of much greater scope that mere striatal transformation into DA in spared dopaminergic neurons. Their further experimental and clinical clarification should help improve both L-DOPA-based and novel strategies for controlling the motor and other symptoms of Parkinson's disease.

Synergistic Interactions of D1- and D2-Selective Dopamine Agonists in Animal Models for Parkinson’s Disease: Sites of Action and Implications for the Pathogenesis of Dyskinesias

The addition of a D2 agonist such as bromocriptine to L-Dopa therapy can often improve the response of patients with Parkinson’s disease dramatically. Simultaneous activation of D1 and D2 dopamine receptors can produce a synergistic effect on locomotion in rats and primates. However, despite the importance of this addition of a D2 agonist to the D1/D2 agonist L-Dopa, little is known of the sites of action of these agents. Recent work suggests that, in addition to D1 and D2 dopamine receptor sites in the striatum (caudate-putamen), L-Dopa and D1 agonists have important effects at D1 dopamine receptors in the substantia nigra. Animal experiments suggest that D1 and D2 dopamine receptor agonists probably also affect different outflow pathways from the striatum. An understanding of these pathways and how dopamine agonists affect them gives insight into some of the clinical problems experienced in treating Parkinson’s disease (the “on-off phenomenon, for example). D1/D2 dopamine receptors also differentially affect gene expression and regulation in the striatum. An understanding of the anatomical and biochemical location of the actions of dopamine receptor agonists will be important in maximizing the beneficial effects and minimizing the side-effects of both presently-used drugs and new treatments.

Locomotor response to L-DOPA in reserpine-treated rats following central inhibition of aromatic L-amino acid decarboxylase: further evidence for non-dopaminergic actions of L-DOPA and its metabolites

L-DOPA is the most widely used treatment for Parkinson's disease. The anti-parkinsonian and pro-dyskinetic actions of L-DOPA are widely attributed to its conversion, by the enzyme aromatic L-amino acid decarboxylase (AADC), to dopamine. We investigated the hypothesis that exogenous L-DOPA can induce behavioural effects without being converted to dopamine in the reserpine-treated rat-model of Parkinson's disease. A parkinsonian state was induced with reserpine (3 mg/kg s.c.). Eighteen hours later, the rats were administered L-DOPA plus the peripherally acting AADC inhibitor benserazide (25 mg/kg), with or without the centrally acting AADC inhibitor NSD1015 (100 mg/kg). L-DOPA/benserazide alone reversed reserpine-induced akinesia (4158+/-1125 activity counts/6 h, cf vehicle 1327+/-227). Addition of NSD1015 elicited hyperactive behaviour that was approximately 7-fold higher than L-DOPA/benserazide (35755+/-5226, P<0.001). The hyperactivity induced by L-DOPA and NSD1015 was reduced by the alpha(2C) antagonist rauwolscine (1 mg/kg) and the 5-HT(2C) agonist MK212 (5 mg/kg), but not by the D2 dopamine receptor antagonist remoxipride (3 mg/kg) or the D1 dopamine receptor antagonist SCH23390 (1 mg/kg). These data suggest that L-DOPA, or metabolites produced via routes not involving AADC, might be responsible for the generation of at least some L-DOPA actions in reserpine-treated rats.

The nociceptin/orphanin FQ receptor antagonist J-113397 and L-DOPA additively attenuate experimental parkinsonism through overinhibition of the nigrothalamic pathway

By using a battery of behavioral tests, we showed that nociceptin/orphanin FQ receptor (NOP receptor) antagonists attenuated parkinsonian-like symptoms in 6-hydroxydopamine hemilesioned rats (Marti et al., 2005). We now present evidence that coadministration of the NOP receptor antagonist 1-[(3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H benzimidazol-2-one (J-113397) and L-DOPA to 6-hydroxydopamine hemilesioned rats produced an additive attenuation of parkinsonism. To investigate the neurobiological substrates underlying this interaction, in vivo microdialysis was used in combination with behavioral measurements (bar test). J-113397 and L-DOPA alone reduced the time on bars (i.e., attenuated akinesia) and elevated GABA release selectively in the lesioned substantia nigra reticulata. J-113397 also reduced nigral glutamate levels, whereas L-DOPA was ineffective. J-113397 and L-DOPA coadministration produced additive antiakinetic effect, which was associated with additive increase in nigral GABA release but no additional reductions in glutamate levels. To investigate whether the increase in nigral GABA release could translate to changes in nigrothalamic transmission, GABA release was monitored in the ventromedial thalamus (one of the main target areas of the nigrothalamic projections). J-113397 and L-DOPA decreased thalamic GABA release and attenuated akinesia, their combination resulting in a more profound effect. These actions were prevented by perfusing the voltage-dependent Na+ channel blocker tetrodotoxin or the GABA(A) receptor antagonist bicuculline in the substantia nigra reticulata. These data demonstrate that J-113397 and L-DOPA exert their antiparkinsonian action through overinhibition of nigrothalamic transmission and suggest that NOP receptor antagonists may be useful as an adjunct to L-DOPA therapy for Parkinson's disease.

Subchronic administration of l-DOPA to adult rats with a unilateral 6-hydroxydopamine lesion of dopamine neurons results in a sensitization of enhanced GABA release in the substantia nigra, pars reticulata

L-DOPA is the most effective pharmacological agent used for the symptomatic treatment of Parkinson's disease but long-term L-DOPA treatment induces involuntary abnormal movements such as dyskinesias. The present study, using in vivo microdialysis, investigated the effects of a single or subchronic administration of L-DOPA to adult rats with a unilateral 6-OHDA lesion of dopamine neurons on GABA release in the substantia nigra, pars reticulata (SNr). The results indicate that a challenge injection of L-DOPA (50 mg/kg, i.p.) significantly increases GABA levels in the SNr of rats treated with a daily repeated administration of L-DOPA (50 mg/kg, i.p.). Further statistical analysis between groups also showed that extracellular GABA levels were significantly higher in the subchronic L-DOPA group than in the group receiving only one injection of L-DOPA. These results show that the subchronic administration of L-DOPA results in a sensitization of enhanced extracellular GABA levels in the SNr.

L-DOPA inhibits depolarization-induced [3H]GABA release in the dopamine-denervated globus pallidus of the rat: the effect is dopamine independent and mediated by D2-like receptors

The effect of L-DOPA on [(3)H]GABA release in slices of globus pallidus from 6-OHDA-lesioned rats was studied. Release was evoked by high (15 mM) K(+). The lesion reduced dopamine content and dopamine synthesized from L-DOPA. The inhibition of DOPA decarboxylase blocked dopamine synthesis. Endogenous dopamine released by high K(+) inhibited [(3)H]GABA release in normal but not in lesioned slices. L-DOPA inhibited (IC(50) = 0.44 microM) evoked [(3)H]GABA release. The inhibition was via D2-like receptors but not mediated by dopamine. The turning behavior induced by L-DOPA methyl ester (25 mg/kg, i.p.) was not abolished by the DOPA decarboxylase inhibitor 3-hydroxybenzylhydrazine but in this condition it was abolished by sulpiride. Results suggest that L-DOPA acting as D2-like agonist inhibits GABA release in the rat globus pallidus and induces turning behavior in rats with unilateral lesions of the dopamine innervation. L-DOPA could control Parkinson's disease symptoms acting not only as dopamine precursor but also by itself.

Comparative effects of acute or chronic administration of levodopa to 6-hydroxydopamine-lesioned rats on the expression of glutamic acid decarboxylase in the neostriatum and GABAA receptors subunits in the substantia nigra, pars reticulata

Current evidence suggests that behavioral sensitization to the chronic administration of levodopa (L-DOPA) to dopamine-depleted animals involves a plasticity of GABA-mediated signaling in output regions of the basal ganglia. The purpose of this study was to compare in adult rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion the effects of an acute or chronic (for 3 or 7 days) injection of L-DOPA on mRNA levels encoding for glutamic acid decarboxylase (GAD65 and GAD67) in the striatum and GABA(A) receptor alpha1, beta2 and gamma2 subunits in the substantia nigra, pars reticulata (SNr), by in situ hybridization histochemistry. In addition, immunostaining levels for the alpha1 subunit were examined in the SNr. In agreement with previous studies, we found that L-DOPA administration increased GAD mRNA levels in the striatum of 6-OHDA-lesioned rats. However, the magnitude of this effect increased with the number of injections of L-DOPA. On the other hand, we found that 6-OHDA lesions resulted in increases in alpha1, beta2 and gamma2 mRNA levels in the ipsilateral SNr, which were normalized or decreased compared with the contralateral side by the acute or chronic administration of L-DOPA. In addition, alpha1 immunostaining in the SNr was significantly decreased in rats injected for 7 days but not for 3 days or acutely with L-DOPA. Our results demonstrate that a chronic administration of L-DOPA results in a progressive increase in GAD and decrease in GABA(A) receptor expression in the striatum and SNr, respectively. They provide further evidence that behavioral sensitization and dyskinesia induced by a chronic administration of L-DOPA in an experimental model of Parkinson's disease is paralleled by a plasticity of GABA-mediated signaling in the SNr.

Intrapallidal dopamine restores motor deficits induced by 6-hydroxydopamine in the rat

To explore whether dopamine deficits in the globus pallidus have a role in generating the motor symptoms of Parkinson's disease, we examined the effects of selective intrapallidal administration of dopamine or its antagonists in rats unilaterally lesioned with 6-hydroxydopamine into the medial forebrain bundle. Either the turning behavior induced by apomorphine or the deficit in the performance of a skilled forelimb-reaching task was used as assay for drug action. Microinjection of either the D2 receptor antagonist, sulpiride, or the D1 receptor antagonist, SCH-23390, into the dopamine-denervated pallidum significantly reduced apomorphine induced turning. In animals trained to perform a skilled forelimb-reaching task, 6-OHDA lesions caused a marked motor deficit in the contralateral forelimb. Intrapallidal dopamine applied either intermittently or continuously, restored up to 50% of the motor performance. Continuous application promoted a motor recovery that outlasted dopamine administration. These results show that lack of dopamine in the GP plays an important role in generating the motor symptoms caused by lesion of dopaminergic pathways. Moreover, motor recovery was produced by selectively injecting dopamine into the globus pallidus.

The central aromatic amino acid DOPA decarboxylase inhibitor, NSD-1015, does not inhibit L-DOPA-induced circling in unilateral 6-OHDA-lesioned-rats

The centrally acting aromatic amino acid dopa decarboxylase (AADC) inhibitor, 3-hydroxybenzyl hydrazine (NSD-1015), is widely used to study the neurotransmitter-like actions of L-DOPA. However, the effects of NSD-1015 on L-DOPA-induced motor activity are unclear as both increases and decreases have been reported. We now investigate the effects of NSD-1015 on L-DOPA-induced contralateral circling behaviour in 6-OHDA-lesioned rats and on striatal levels of L-DOPA, 3-O-methyl-DOPA (3-OMD), dopamine, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) using microdialysis techniques. NSD-1015 (50-200 mg/kg i.p.) inhibited AADC activity both in the liver and striatum of normal rats. Administration of NSD-1015 (50-200 mg/kg i.p.), delayed the onset of circling produced by administration of L-DOPA (25 mg/kg i.p.) and carbidopa (12.5 mg/kg i. p.), suggesting blockade of central AADC activity. However, the duration of the L-DOPA-induced circling was prolonged and overall no inhibition of circling behaviour occurred. L-DOPA (25 mg/kg i.p.) plus carbidopa (12.5 mg/kg i.p.) increased extracellular levels of L-DOPA, 3-OMD, dopamine, DOPAC and HVA in the 6-OHDA-lesioned striatum. Pretreatment of rats with the central AADC inhibitor, NSD-1015 (100 mg/kg i.p.), potentiated the increase in dialysate levels of L-DOPA and 3-OMD. However, it did not reduce striatal dopamine levels in the 6-OHDA-lesioned hemisphere, which were elevated following L-DOPA administration. The increases in DOPAC and HVA levels were abolished by NSD-1015 pretreatment. These results suggest that, while NSD-1015 blocks central AADC activity, it also acts as a monoamine oxidase inhibitor so maintaining striatal dopamine concentration by reducing dopamine metabolism. NSD-1015, therefore, may not be an appropriate tool for the study of brain AADC activity and for assessing the neuromodulatory role of L-DOPA.

Endogenous dopaminergic tone and dopamine agonist action

Dopamine receptor agonists provide symptomatic relief in the early stages of Parkinson's disease, but with disease progression, their efficacy decreases. The reason behind this decrease in effectiveness is unknown, but maximal efficacy may be dependent on endogenous dopaminergic tone to provide stimulation of D1 and D2 receptor subtypes. Therefore, we have investigated the effects of the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine (AMPT) on the actions of D1, D2, and D1/D2 agonists and levodopa (L-dopa) in common marmosets treated with 1 -methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Administration of AMPT alone further increased motor disability and decreased locomotor activity. Administration of L-dopa reversed motor disability and increased locomotor activity, and this reversal was not affected by previous AMPT treatment. The D1 agonist A-77636 and the D2 agonist quinpirole reversed motor deficits, but these effects were markedly inhibited by previous AMPT treatment. Administration of quinpirole with A-77636 produced a reversal of motor deficits that was more resistant to AMPT pretreatment than was the effect produced by quinpirole or A-77636 alone. These data suggest that D1 and D2 receptor stimulation are required for dopamine receptor agonists to produce a maximal antiparkinsonian response. The reversal of motor deficits produced by the mixed D1/D2 agonist apomorphine was more resistant to AMPT treatment than that produced by quinpirole or A-77636. However, the motor effects of A-77636 plus quinpirole and of apomorphine were still affected by AMPT treatment. This suggests that loss of tyrosine hydroxylase activity may also alter motor activity through inhibition of endogenous L-dopa or norepinephrine synthesis, because both are also involved in the genesis of motor activity.

Substantia nigra pars reticulata lesion induces preconvulsive behavior and changes in glutamate receptor gene expression in the rat brain

The substantia nigra pars reticulata (SNpr) has been proposed to play an important role in the control of the propagation and/or the generation of epileptic seizures. Earlier studies have shown differential effects of the lesion of the SNpr on seizure genesis that demonstrated a regional difference in the anterior and posterior parts of the SNpr in preconvulsive behavior induced by unilateral reticulata injection of dopamine (DA). This study was aimed to investigate some of the underlying mechanisms of the preconvulsive behavior elicited by unilateral SNpr DA injection by the study of changes in the gene expression of glutamate receptor subunits (GluR1, GluR2 and NMDAR1) and of changes in animal behavior following coinfusion of DA and a DA D1 antagonist SCH 23390 into the SNpr. Unilateral injection of exogenous DA into the anterior region of the SNpr induced rapid and short lasting preconvulsive behavior up to wet dog shakes stage and a significant reduction of gene expression for GluR1, GluR2 and NMDAR1 subunits in rat hippocampal subfields including CA1 through CA4 and dentate gyrus (DG) at 1 day after nigral DA injection. The effect was long lasting and persisted for at least 3 weeks. Both preconvulsive behavior and downregulation of glutamate receptor subunit genes were completely blocked by simultaneous coinfusion of DA and SCH 23390. The results suggest, for the first time, that DA D1 receptor in the SNpr may mediate the nigral-involved seizure development. Glutamate desensitization, and/or selective early neuronal damage might be responsible for the downregulation of glutamate receptor subunits by transient preconvulsive activity.

The effects of central aromatic amino acid DOPA decarboxylase inhibition on the motor actions of L-DOPA and dopamine agonists in MPTP-treated primates

1. Endogenous L-DOPA may act as a neuromodulator contributing to the production of motor activity. We now investigate the effects of the centrally acting aromatic amino acid dopa decarboxylase (AADC) inhibitor NSD-1015 (3-hydroxybenzyl hydrazine) on the motor actions of L-DOPA and dopamine agonist drugs in MPTP treated common marmosets. 2. Pretreatment with NSD-1015 (10 - 50 mg kg(-1); i.p.) worsened baseline motor deficits in MPTP-treated common marmosets. Similarly, it abolished L-DOPA (5 - 18 mg kg(-1) s.c.) induced locomotor activity and reversal of disability. NSD-1015 pretreatment inhibited dopamine formation and elevated L-DOPA levels in plasma. 3. The increase in locomotor activity and improvement in disability produced by the administration of the D-1 agonist A-86929 (0.03 - 0. 04 mg kg(-1) s.c.) or the D-2 agonist quinpirole (0.05 - 0.3 mg kg(-1) i.p.) was abolished by NSD-1015 (25 mg kg(-1) i.p.) pretreatment. While the effects of a low dose combination of A-86929 (0.04 mg kg(-1) s.c.) and quinpirole (0.05 mg kg(-1) i.p.) were inhibited by NSD-1015 (25 mg kg(-1) i.p.), there was little effect on the action of a high dose combination of these drugs (0.08 mg kg(-1) A-86929 and 0.1 mg kg(-1) quinpirole). 4. Following central AADC inhibition with NSD-1015 (25 mg kg(-1) i.p.), locomotor behaviour induced by administration of high dose combinations of A-86929 (0.08 mg kg(-1) s.c.) and quinpirole (0.1 mg kg(-1) i.p.) was unaffected by L-DOPA (5 mg kg(-1) s.c.) pretreatment. 5. These results do not support a role for endogenous L-DOPA in spontaneous or drug induced locomotor activity. Rather, they strengthen the argument for the importance of endogenous dopaminergic tone in the motor actions of dopamine agonists.

Tremulous jaw movements in rats: a model of parkinsonian tremor

Several pharmacological and neurochemical conditions in rats induce 'vacuous' or 'tremulous' jaw movements. Although the clinical significance of these movements has been a subject of some debate, considerable evidence indicates that the non-directed, chewing-like movements induced by cholinomimetics, dopamine antagonists and dopamine depletions have many of the characteristics of parkinsonian tremor. These movements occur within the 3-7 Hz peak frequency range that is characteristic of parkinsonian tremor. Tremulous jaw movements are induced by many of the conditions that are associated with parkinsonism, and suppressed by several different antiparkinsonian drugs, including scopolamine, benztropine, L-DOPA, apomorphine, bromocriptine, amantadine and clozapine. Striatal cholinergic and dopaminergic mechanisms are involved in the generation of tremulous jaw movements, and substantia nigra pars reticulata appears to be a major basal ganglia output region through which the jaw movements are regulated. Future research on the neurochemical and anatomical characteristics of tremulous jaw movements could yield important insights into the brain mechanisms that generate tremulous movements.

Glutamate decarboxylase (GAD65) gene expression is increased by dopamine receptor agonists in a subpopulation of rat striatal neurons

The mRNA levels encoding for the two isoforms of glutamate decarboxylase (GAD65 and GAD67) were measured in the adult rat striatum following systemic administration of dopamine receptor agonists. Double-labeling in situ hybridization histochemistry was used to measure GAD65 or GAD67 mRNA levels in neurons labeled or not with a preproenkephalin (PPE) cRNA probe. Chronic treatment with the D1/D2 dopamine receptor agonist apomorphine or with the D1 dopamine receptor agonist SKF-38393 induced an increase in GAD65 but not GAD67 mRNA levels in different sectors of the striatum. These effects were abolished by pre-administration of the D1 dopamine receptor antagonist SCH-23390. On double-labeled sections, GAD65 mRNA labeling was distributed in neurons labeled and unlabeled with the PPE cRNA probe. About half of all neuronal profiles labeled with the GAD65 cRNA probe were also labeled with the PPE cRNA probe. Quantification of labeling at cellular level demonstrated a significant increase of GAD65 mRNA levels in PPE-unlabeled neurons. On the other hand, no significant changes of GAD65 mRNA levels were detected in PPE-labeled neurons. Our results demonstrate a differential effect of dopamine receptor agonists on striatal GAD65 and GAD67 gene expression. In particular, we show that GAD65 mRNA levels are selectively increased in presumed striato-nigral neurons following treatments with dopamine receptor agonists. These data provide evidence that the GAD65 isoform is preferentially involved in the regulation of GABAergic neurotransmission in striato-nigral neurons.

The regulation of motor control: an evaluation of the role of dopamine receptors in the substantia nigra

The importance of the nigrostriatal dopaminergic pathway in motor control is widely accepted and it is generally believed that the motor symptoms of Parkinson's disease result solely from reduced release of dopamine from terminals in the striatum. Over recent years there has been a growing body of evidence which suggests that dendritic dopamine release in the substantia nigra is of importance in the regulation of neuronal activity and behaviour. This evidence is reviewed together with a description of our recent findings that show nigral dopamine receptors are essential for the maintenance of normal muscle tone. It is concluded that current views of the basal ganglia circuitry involved in motor control need to be re-evaluated to take into account these recent reports. A scheme is suggested to explain how dopamine mechanisms in the substantia nigra regulate motor activity.

Interactions between dopamine and GABA in the control of ambulatory activity

Ambulatory activity of male rats was quantified in an open field. The subjects were treated with DL-amphetamine and amfonelic acid alone or combined with the GABA transaminase inhibitors gamma-acetylen GABA (GAG) and sodium valproate as well as with the GABAA agonist THIP and the GABAB agonist baclofen. Subeffective doses of the GABAergic drugs did not modify the effects of moderate doses of the dopaminergic stimulants whereas effective doses continued to reduce ambulatory activity just as in the absence of dopaminergic activation. When DL-amphetamine or amfonelic acid were administered in doses that strongly enhanced ambulatory activity, doses of the GABAergic drugs that were inhibitory in the absence of dopaminergic stimulation were no longer effective. The mixed D1/D2 dopamine antagonist pimozide, the D1 antagonist SCH 23390 and the D2 antagonist sulpiride were then combined with subeffective doses of the GABA agonists. GAG, sodium valproate and baclofen were potentiated by pimozide and SCH 23390 but not by sulpiride. THIP was ineffective. These data show that GABAergic drugs had a reduced effect after stimulation of dopaminergic neurotransmission. On the other hand, when dopamine D1 receptors were blocked, nonselective GABA agonists and the GABAB agonist baclofen were potentiated. This was not the case for the GABAA agonist THIP, suggesting that the GABAA receptor is of slight importance for the interactions between GABA and dopamine in the control of ambulatory activity. No potentiation of GABAergic agonists was obtained after treatment with a dopamine D2 antagonist.

D-1 receptor mediated modulation of the release of gamma-aminobutyric acid by endogenous dopamine in the basal ganglia of the rat

1. Presynaptic D1 receptors are present on GABAergic terminals of neostriatal projections. 2. By activating these receptors, exogenous dopamine enhances the release of GABA. 3. Here the authors have explored whether endogenous dopamine was also able to activate the receptors, thus enhancing GABA release. 4. The effect of methamphetamine, a dopamine releaser, on the release of tritiated GABA was studied in slices of substantia nigra pars reticulata, entopeduncular nucleus and caudate-putamen, targets of the striatal projections. 5. Methamphetamine enhanced the release of the label. However the enhancement required an intact dopaminergic innervation, since it was lost in slices isolated from rats with 6-hydroxydopamine-induced lesions of the dopaminergic nigrostriatal system. 6. The activation of the receptors by endogenous dopamine was also judged by the effect of the selective D1 antagonist SCH 23390 in potassium depolarized slices. By preventing activation of the receptors by dopamine released as result of depolarization, the antagonist reduced GABA release. In 6-OHDA lesioned slices, no reduction was observed, even though the slices were also depolarized. 7. The results indicate that endogenous dopamine enhances GABA release from striatal terminals in the pars reticulata of the substantia nigra, entopeduncular nucleus and caudate-putamen. This would facilitate GABAergic neurotransmission. 8. The study suggests that the function of DA in the basal ganglia is widespread, modulating not only the firing of the striatal efferent neurons but also the transmission of the fired impulses across synapses in the target nuclei of these neurons.

D1/D2 actions of dopaminergic drugs studied with [14C]-2-deoxyglucose autoradiography

1. To define the neural circuits activated by dopaminergic stimulation in rat models of parkinsonism, the author studied the effects of L-dopa and selective D1 and D2 agonists on RCGU in rats with unilateral 6-OHDA substantia nigra lesions. 2. Systemic administration of L-dopa markedly increased RCGU in the EP and SNr ipsilateral to the nigral lesions; it is suggested that this represents metabolic activity primarily in axon terminals of GABAergic striatal projection neurons. These effects were reproduced by selective D1, but not D2, dopamine agonists, and were blocked completely by a D1 antagonist, indicating their critical dependence on D1 stimulation. L-dopa moderately increased RCGU in the STN; this effect was reproduced by D1 and D2 agonists and was blocked completely only by combined D1 and D2 antagonist pretreatment. 3. The RCGU data support a direct stimulatory action of dopamine, formed from L-dopa, on D1 receptor-bearing striatal GABAergic neurons projecting to the EP and SNr as well as a net stimulatory action on the GP output to the STN. 4. The marked D1-mediated RCGU increase in the SNr ipsilateral to the dopamine depletion contrasts with the modest increase seen on the contralateral side and in naive rats, suggesting that the enhanced RCGU response to D1 stimulation is an index of dopaminergic supersensitivity. The stimulatory effect of the D1 agonist SKF 38393 on RCGU in the SNr is enhanced 6-12 hours after acute dopamine depletion with reserpine/AMPT indicating that supersensitive responses develop within this rapid time frame. 5. The RCGU data indicate that D1 receptor stimulation contributes importantly, in an anatomically selective manner, to the effects of L-dopa on basal ganglia circuits and that the response to D1 stimulation is rapidly modifiable by dopamine depletion.

Intracellularly recorded response of rat striatal neurons in vitro to fenoldopam and SKF 38393 following lesions of midbrain dopamine cells

The effect of long-term (6-19 weeks) 6-hydroxydopamine-induced (6-OHDA) lesions of midbrain dopamine cells on dopamine D1-like agonist-induced changes in the excitability of rat striatal neurons was investigated in vitro using tissue slices and intracellular recording techniques. Fenoldopam and (+/-)-SKF 38393 predominantly decreased excitability in control preparations including striatal neurons located contralateral to 6-OHDA injection sites and neurons obtained from rats receiving sham injections or no treatment. Fenoldopam also inhibited neurons ipsilateral to lesions of midbrain dopamine cells. (+/-)-SKF 38393, unlike fenoldopam, produced predominantly increases in the excitability of ipsilateral striatal neurons. Superfusion of the D1 receptor antagonist, SCH 23390, blocked fenoldopam-induced decreases in excitability but not the (+/-)-SKF 38393-induced excitation of neurons ipsilateral to the lesion. Sequential application of fenoldopam and quinpirole, a D2/D3 receptor agonist, produced responses to both drugs in a majority of neurons. The results demonstrate that inhibitory responses to fenoldopam are mediated by D1 receptors, while excitatory effects of (+/-)-SKF 38393 in the striatum ipsilateral to the lesion are apparently not dependent on D1 receptor activation. These findings also suggest that dopamine D1 and D2/D3 receptors are able to concurrently influence the excitability of striatal neurons in the dopamine deafferentated striatum. Similar regulation of striatal neurons in vivo may contribute to dopaminergic regulation of basal ganglia output and the ability of dopaminomimetic agents to ameliorate symptoms of dopaminergic deficiency in Parkinson's disease.

The aromatic amino acid decarboxylase inhibitor, NSD-1015, increases release of dopamine: response characteristics

Addition of the aromatic amino acid decarboxylase inhibitor, NSD-1015 (10 microM), to Krebs'-Ringer phosphate (KRP) superfusion medium, significantly increased the release of dopamine in vitro from superfused corpus striatum tissue fragments of male rats. A dose-dependent increase in release of dopamine was obtained in response to increasing concentrations of NSD-1015, with 1.0 microM being the minimally effective dose. In addition to the striatum, NSD-1015 also increased the release of dopamine from superfused hypothalamic tissue fragments. This capacity of NSD-1015 to increase release of dopamine was calcium-independent, appeared to be somewhat specific and could apparently increase the release of dopamine in vivo, as indicated by increases in the release of the metabolite of dopamine, DOPAC, under conditions of push-pull perfusion. Although the putative role of NSD-1015 is as an aromatic amino acid decarboxylase inhibitor, the present results demonstrate that, either as a result of this function and/or in addition to this role, NSD-1015 is a potent activator of the release of dopamine.

Activation of D1 receptors stimulates accumulation of gamma-aminobutyric acid in slices of the pars reticulata of 6-hydroxydopamine-lesioned rats

D1 dopamine receptors are present on terminals of striatal neurons to the pars reticulata of the substantia nigra in the rat. Here we have studied the effect of the activation of these receptors on the synthesis of gamma-aminobutyric acid (GABA) in slices of the pars reticulata of the substantia nigra isolated from 6-hydroxydopamine-lesioned rats. The synthesis was judged by the accumulation of GABA after inhibiting GABA transaminase with aminooxyacetic acid. Both dopamine and SCH 23390, a D1 agonist, stimulated the synthesis. The effect of both compounds was blocked by SCH 23390, a D1 antagonist, but not by sulpiride, a D2 antagonist. In the absence of receptor activation, the synthesis was very slow. The results suggest a trophic influence of dopamine upon the synthesis of GABA via D1 receptors.

Dopamine receptor interactions: some implications for the treatment of Parkinson's disease

Since the discovery that L-DOPA could alleviate the symptoms of Parkinson's disease, it has been assumed that the striatum is the site of action of the dopamine formed from L-DOPA. However, for the past 15 years, evidence has accumulated to suggest that dopamine is also released by the dendrites of dopamine neurons in the substantia nigra and D1 dopamine receptors in this region of the brain appear to play an important role in the actions of L-DOPA. Activation of D1 receptors in the substantia nigra may, in part, explain some of the synergistic effects of D1 and D2 agonists in animal models for Parkinson's disease. These effects are discussed in light of recent studies suggesting that dopamine, acting on D1 and D2 dopamine receptor subtypes, activates distinct efferent pathways from the striatum. Clinical studies suggest that these findings may have important implications for the treatment of Parkinson's disease.