Matteo Marti

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.

[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.

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.

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.

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 GABAA 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 Parkinsons disease.

Striatal dopamine-NMDA receptor interactions in the modulation of glutamate release in the substantia nigra pars reticulata in vivo: opposite role for D1 and D2 receptors.

Dual probe microdialysis was employed in conscious rats to investigate whether endogenous dopamine is involved in the stimulation of glutamate release in the substantia nigra pars reticulata following striatal NMDA receptor activation. Intrastriatal perfusion with NMDA (1 and 10 µm) facilitated nigral glutamate release (dizocilpine‐ and tetrodotoxin‐sensitive). The D2 dopamine receptor antagonist raclopride increased spontaneous nigral glutamate release and caused a leftward shift in the NMDA sensitivity, lowering NMDA effective concentrations to submicromolar levels. Conversely, the D1 antagonist SCH23390 prevented the effect of NMDA (1 µm) and caused a rightward shift in the NMDA sensitivity. It was tested whether the antagonist effects were due to dopamine receptor blockade or increased tone on D1/D2 receptors. SCH23390 prevented the raclopride‐induced enhancement of spontaneous but not NMDA‐evoked glutamate release while raclopride left unchanged the SCH23390‐induced inhibition. The physiopathological relevance of the dopaminergic modulation was strengthened by perfusing NMDA in the dopamine‐depleted striatum of hemiparkinsonian rats. Nigral glutamate responsiveness to NMDA was enhanced as with raclopride. We conclude that endogenous striatal dopamine regulates both spontaneous and NMDA‐induced nigral glutamate release via an opposite control mediated by D1 facilitatory and D2 inhibitory receptors. Alterations of this control may subserve the motor symptoms of Parkinsons disease.

Nociceptin/orphanin FQ receptor blockade attenuates MPTP-induced parkinsonism

Endogenous nociceptin/orphanin FQ (N/OFQ) inhibits the activity of dopamine neurons in the substantia nigra and affects motor behavior. In this study we investigated whether a N/OFQ receptor (NOP) antagonist, J-113397, can modify movement in naive mice and nonhuman primates and attenuate motor deficits in MPTP-treated parkinsonian animals. J-113397 facilitated motor activity in naïve mice at low doses (0.1-1 mg/kg) and inhibited it at higher ones (10 mg/kg). Likewise, in MPTP-treated mice, J-113397 reversed motor deficit at 0.01 mg/kg but worsened hypokinesia at higher doses (1 mg/kg). In naïve nonhuman primates, J-113397, ineffective up to 1 mg/kg, produced inconsistent motor improvements at 3 mg/kg. Conversely, in parkinsonian primates J-113397 (0.01 mg/kg) reversed parkinsonism, being most effective against hypokinesia. We conclude that endogenous N/OFQ modulates motor activity in mice and nonhuman primates and contributes to parkinsonian symptoms in MPTP-treated animals. NOP receptor antagonists may represent a novel approach to Parkinsons disease.

Neuronal vulnerability following inhibition of mitochondrial complex II: a possible ionic mechanism for Huntington's disease

An impaired complex II (succinate dehydrogenase, SD) striatal mitochondrial activity is one of the prominent metabolic alterations in Huntingtons disease (HD), and intoxication with 3-nitropropionic acid (3-NP), an inhibitor of mitochondrial complex II, mimics the motor abnormalities and the pathology of HD. We found that striatal spiny neurons responded to this toxin with an irreversible membrane depolarization/inward current, while cholinergic interneurons showed a hyperpolarization/outward current. Both these currents were sensitive to intracellular concentration of ATP. The 3-NP-induced depolarization was associated with an increased release of endogenous GABA, while acetylcholine levels were reduced. Moreover, 3-NP induced a higher depolarization in presymptomatic R6/2 HD transgenic mice compared to wild-type (WT) mice, showing an increased susceptibility to SD inhibition. Conversely, the hyperpolarization did not significantly differ from the one recorded in WT mice. The diverse membrane changes induced by SD inhibition may contribute to the cell-type-specific neuronal death in HD.