Michele Morari

Antagonism of metabotropic glutamate receptor type 5 attenuates l-DOPA-induced dyskinesia and its molecular and neurochemical correlates in a rat model of Parkinson’s disease

Metabotropic glutamate receptor type 5 (mGluR5) modulates dopamine and glutamate neurotransmission at central synapses. In this study, we addressed the role of mGluR5 in l‐DOPA‐induced dyskinesia, a movement disorder that is due to abnormal activation of both dopamine and glutamate receptors in the basal ganglia. A selective and potent mGluR5 antagonist, 3‐[(2‐methyl‐1,3‐thiazol‐4‐yl)ethynyl] pyridine, was tested for its ability to modulate molecular, behavioural and neurochemical correlates of dyskinesia in 6‐hydroxydopamine‐lesioned rats treated with l‐DOPA. The compound significantly attenuated the induction of abnormal involuntary movements (AIMs) by chronic l‐DOPA treatment at doses that did not interfere with the rat physiological motor activities. These effects were paralleled by an attenuation of molecular changes that are strongly associated with the dyskinesiogenic action of l‐DOPA (i.e. up‐regulation of prodynorphin mRNA in striatal neurons). Using in vivo microdialysis, we found a temporal correlation between the expression of l‐DOPA‐induced AIMs and an increased GABA outflow within the substantia nigra pars reticulata. When co‐administered with l‐DOPA, 3‐[(2‐methyl‐1,3‐thiazol‐4‐yl)ethynyl] pyridine greatly attenuated both the increase in nigral GABA levels and the expression of AIMs. These data demonstrate that mGluR5 antagonism produces strong anti‐dyskinetic effects in an animal model of Parkinson’s disease through central inhibition of the molecular and neurochemical underpinnings of l‐DOPA‐induced dyskinesia.

N‐methyl‐d‐Aspartic Acid Differentially Regulates Extracellular Dopamine, GABA, and Glutamate Levels in the Dorsolateral Neostriatum of the Halothane‐Anesthetized Rat: An In Vivo Microdialysis Study

Abstract: The effects of local perfusion with the glutamate receptor agonist NMDA and the noncompetitive NMDA receptor antagonist dizolcipine (MK‐801) on extracellular dopamine (DA), GABA, and glutamate (Glu) levels in the dorsolateral striatum were monitored using in vivo microdialysis in the halothane‐anesthetized rat. In addition, the sensitivity of both the basal and NMDA‐induced increases in levels of these neurotransmitter substances to perfusion with tetrodotoxin (TTX; 10−5 M) and a low Ca2+ concentration (0.1 mM) was studied. The results show that the local perfusion (10 min) with both the 10−3 and 10−4 M dose of NMDA increased striatal DA and GABA outflow, whereas only the (10−3 M) dose of NMDA was associated with a small and delayed increase in extracellular Glu levels. The NMDA‐induced effects were dose‐dependently counteracted by simultaneous perfusion with MK‐801 (10−6 and 10−5 M). Both the basal and NMDA (10−3 M)‐induced increase in extracellular striatal DA content was reduced in the presence of TTX and a low Ca2+ concentration, whereas both basal and NMDA‐stimulated GABA levels were unaffected by these treatments. Both the basal and NMDA‐stimulated Glu levels were enhanced following TTX treatment, whereas perfusion with a low Ca2+ concentration reduced basal Glu levels and enhanced and prolonged the NMDA‐induced stimulation. These data support the view that NMDA receptor stimulation plays a role in the regulation of extracellular DA, GABA, and Glu levels in the dorsolateral neostriatum and provide evidence for a differential effect of NMDA receptor stimulation on these three striatal neurotransmitter systems, possibly reflecting direct and indirect actions mediated via striatal NMDA receptors.

[Nphe1,Arg14,Lys15]Nociceptin‐NH2, a novel potent and selective antagonist of the nociceptin/orphanin FQ receptor

Nociceptin/orphanin FQ (N/OFQ) modulates several biological functions by activating a specific G‐protein coupled receptor (NOP). Few molecules are available that selectively activate or block the NOP receptor. Here we describe the in vitro and in vivo pharmacological profile of a novel NOP receptor ligand, [Nphe1,Arg14,Lys15]N/OFQ‐NH2 (UFP‐101). UFP‐101 binds to the human recombinant NOP receptor expressed in Chinese hamster ovary (CHO) cells with high affinity (pKi 10.2) and shows more than 3000 fold selectivity over classical opioid receptors. UFP‐101 competitively antagonizes the effects of N/OFQ on GTPγ35S binding in CHOhNOP cell membranes (pA2 9.1) and on cyclic AMP accumulation in CHOhNOP cells (pA2 7.1), being per se inactive at concentrations up to 10 μM. In isolated peripheral tissues of mice, rats and guinea‐pigs, and in rat cerebral cortex synaptosomes preloaded with [3H]‐5‐HT, UFP‐101 competitively antagonized the effects of N/OFQ with pA2 values in the range of 7.3–7.7. In the same preparations, the peptide was inactive alone and did not modify the effects of classical opioid receptor agonists. UFP‐101 is also active in vivo where it prevented the depressant action on locomotor activity and the pronociceptive effect induced by 1 nmol N/OFQ i.c.v. in the mouse. In the tail withdrawal assay, UFP‐101 at 10 nmol produces per se a robust and long lasting antinociceptive effect. UFP‐101 is a novel, potent and selective NOP receptor antagonist which appears to be a useful tool for future investigations of the N/OFQ‐NOP receptor system.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease.

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinsons disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

Review Article Reciprocaldopamine-glutamatemodulation of release in the basalganglia

Abstract Dopaminergic and glutamatergic transmissions have long been known to interactatmultiple levels in the basal ganglia to modulate motor and cognitive functions. Oneimportantaspect of their interactions is represented by the reciprocal modulation of release. Thistopic hasbeen the object of interest since the late 70s, particularly in the striatum and inmidbraindopaminergic areas (substantia nigra and ventral tegmental area). Analysisofglutamate-dopamine interactions in the control of each others release is complicated by thefactthat both glutamate and dopamine act on multiple receptor subtypes which can exertdifferenteffects. Therefore, glutamatergic modulation of dopamine release has been reviewed byanalyzingthe effects of glutamatergic selective receptor agonists and antagonists in the striatum (bothmotor and limbic portions) and in midbrain dopaminergic areas, as revealed by in vitro (slices,cell cultures, synaptosomes) and in vivo (push-pull, microdialysis and voltammetrytechniques)experimental approaches. The same approach has been followed for dopaminergicmodulation ofglutamate release. The facilitatory nature of glutamate modulating both presynapticand dendriticdopamine release has clearly emerged from in vitro studies. However, evidence ispresented that,at least in the striatum and in the nucleus accumbens of awake rats,glutamate-mediated inhibitoryeffects may also occur. In vitro and in vivo experiments in thestriatum and midbraindopaminergic areas mainly depict dopamine as an inhibitory modulator ofglutamate release.However, in vivo studies reporting dopamine D1 receptor mediated facilitatoryeffects are alsoconsidered. Therefore, the general notion that glutamate and dopamine actoppositely to regulateeach others release, is only partly supported by the available data.Conversely, the nature of theinteraction between the two neurotransmitters seems to varydepending on the experimentalapproach, the brain area considered and the subtype of receptorinvolved.

Glutamate antagonists prevent morphine withdrawal in mice and guinea pigs

The effects of excitatory amino acid antagonists on increased cortical acetylcholine release and behavioral hyperactivity induced by naloxone in morphine tolerant guinea pigs and mice were studied. The results show that the N-methyl-D-aspartic acid (NMDA) antagonist MK-801 (0.1-1 mg/kg, i.p.) injected 30 min before naloxone (3 mg/kg, s.c.) dose-dependently prevented the neurochemical and behavioral signs of morphine withdrawal in guinea pigs and mice. The non-selective antagonist glutamic acid diethylester only at 100 mg/kg i.p. reduced the naloxone-induced increase of cortical acetylcholine release without affecting the behavioral changes. These findings indicate that the activation of excitatory amino acid receptors, mainly the NMDA receptors, plays a relevant role in the expression of opiate abstinence.

Nociceptin/orphanin FQ and neurotransmitter release in the central nervous system

In this article, the effect of nociceptin (orphanin FQ) on transmitter release in the central nervous system in vitro and in vivo is reviewed. Nociceptin inhibits the electrically or K(+)-evoked noradrenaline, dopamine, serotonin, and glutamate release in brain slices from guinea-pig, rat, and mouse. This effect is usually naloxone-resistant but antagonized by OP(4) receptor antagonists like [Phe(1)psi(CH(2)-NH)Gly(2)]-nociceptin(1-13)NH(2). In the rat in vivo, nociceptin diminishes acetylcholine release in the striatum, reduces dopamine release, and prevents the stimulatory effect of morphine on this transmitter in the nucleus accumbens and also elevates extracellular glutamate and gamma-aminobutyric acid levels in mesencephalic dopaminergic areas. The effect of nociceptin on the mesencephalic dopaminergic system might explain its actions on motor behavior.

Blockade of Nociceptin/Orphanin FQ Receptor Signaling in Rat Substantia Nigra Pars Reticulata Stimulates Nigrostriatal Dopaminergic Transmission and Motor Behavior

A multidisciplinary approach was followed to investigate whether the opioid-like peptide nociceptin/orphanin FQ (N/OFQ) regulates the nigrostriatal dopaminergic pathway and motor behavior. Nigrostriatal dopaminergic cells, which express N/OFQ peptide (NOP) receptors, are located in the substantia nigra pars compacta and extend their dendrites in the substantia nigra pars reticulata, thereby modulating the basal ganglia output neurons. In vitro electrophysiological recordings demonstrated that N/OFQ hyperpolarized the dopaminergic cells of the substantia nigra pars compacta and inhibited their firing activity. In vivo dual-probe microdialysis showed that N/OFQ perfused in the substantia nigra pars reticulata reduced dopamine release in the ipsilateral striatum, whereas UFP-101 ([Nphe1,Arg14,Lys15]N/OFQ(1-13)-NH2) (a selective NOP receptor peptide antagonist) stimulated it. N/OFQ microinjected in the substantia nigra pars reticulata impaired rat performance on a rotarod apparatus, whereas UFP-101 enhanced it. Electromyography revealed that N/OFQ and UFP-101 oppositely affected muscle tone, inducing relaxation and contraction of triceps, respectively. The selective NOP receptor nonpeptide antagonist J-113397 (1-[3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H benzimidazol-2-one), either injected intranigrally or given systemically, also elevated striatal dopamine release and facilitated motor activity, confirming that these effects were caused by blockade of endogenous N/OFQ signaling. The inhibitory role played by endogenous N/OFQ on motor activity was additionally strengthened by the finding that mice lacking the NOP receptor gene outperformed wild-type mice on the rotarod. We conclude that NOP receptors in the substantia nigra pars reticulata, activated by endogenous N/OFQ, drive a physiologically inhibitory control on motor behavior, possibly via modulation of the nigrostriatal dopaminergic pathway.

Mechanisms underlying the impairment of hippocampal long-term potentiation and memory in experimental Parkinson's disease.

Although patients with Parkinsons disease show impairments in cognitive performance even at the early stage of the disease, the synaptic mechanisms underlying cognitive impairment in this pathology are unknown. Hippocampal long-term potentiation represents the major experimental model for the synaptic changes underlying learning and memory and is controlled by endogenous dopamine. We found that hippocampal long-term potentiation is altered in both a neurotoxic and transgenic model of Parkinsons disease and this plastic alteration is associated with an impaired dopaminergic transmission and a decrease of NR2A/NR2B subunit ratio in synaptic N-methyl-d-aspartic acid receptors. Deficits in hippocampal-dependent learning were also found in hemiparkinsonian and mutant animals. Interestingly, the dopamine precursor l-DOPA was able to restore hippocampal synaptic potentiation via D1/D5 receptors and to ameliorate the cognitive deficit in parkinsonian animals suggesting that dopamine-dependent impairment of hippocampal long-term potentiation may contribute to cognitive deficits in patients with Parkinsons disease.

Modulation of 5-hydroxytryptamine efflux from rat cortical synaptosomes by opioids and nociceptin

The modulation of [3H]‐5‐hydroxytryptamine ([3H]‐5‐HT) efflux from superfused rat cortical synaptosomes by delta, kappa, mu and ORL1 opioid receptor agonists and antagonists was studied. Spontaneous [3H]‐5‐HT efflux was reduced (20% inhibition) by either 0.5 μM tetrodotoxin or Ca2+‐omission. Ten mM K+‐evoked [3H]‐5‐HT overflow was largely Ca2+‐dependent (90%) and tetrodotoxin‐sensitive (50%). The delta receptor agonist, deltorphin‐I, failed to modulate the K+‐evoked neurotransmitter efflux up to 0.3 μM. The kappa and the mu receptor agonists, U‐50,488 and endomorphin‐1, inhibited K+‐evoked [3H]‐5‐HT overflow (EC50=112 and 7 nM, respectively; Emax=28 and 29% inhibition, respectively) in a norBinaltorphimine‐ (0.3 μM) and naloxone‐ (1 μM) sensitive manner, respectively. None of these agonists significantly affected spontaneous [3H]‐5‐HT efflux. The ORL1 receptor agonist nociceptin inhibited both spontaneous (EC50=67 nM) and K+‐evoked (EC50=13 nM; Emax=52% inhibition) [3H]‐5‐HT efflux. The effect of NC was insensitive to naloxone (up to 10 μM), but was antagonized by [Nphe1]nociceptin(1‐13)NH2 (a novel selective ORL1 receptor antagonist; pA2=6.7) and by naloxone benzoylhydrazone (pA2=6.3). The ORL1 ligand [Phe1ψ(CH2‐NH)Gly2]nociceptin(1‐13)NH2 also inhibited K+ stimulated [3H]‐5‐HT overflow (EC50=64 nM; Emax=31% inhibition), but its effect was partially antagonized by 10 μM naloxone. It is concluded that the ORL1 receptor is the most important presynaptic modulator of neocortical 5‐HT release within the opioid receptor family. This suggests that the ORL1/nociceptin system may have a powerful role in the control of cerebral 5‐HT‐mediated biological functions.