Philippe De Deurwaerdère

Constitutive Activity of the Serotonin2C Receptor Inhibits In Vivo Dopamine Release in the Rat Striatum and Nucleus Accumbens

Numerous research has pointed out that serotonin2c (5-HT2C) receptor, a subtype of 5-HT receptors belonging to the G-protein-coupled receptor superfamily, modulates the activity of mesencephalic dopamine (DA) neurons, the dysfunction of which is involved in devastating diseases such as schizophrenia, Parkinsons disease, and drug addiction. In the present study, using in vivo intracerebral microdialysis and Chinese hamster ovary (CHO) cells expressing 5-HT2C receptors to identify appropriate 5-HT2C receptor ligands, we sought to determine whether the property of 5-HT2C receptors to spontaneously activate intracellular signaling pathways in vitro (constitutive activity) participates in the tonic inhibitory control that they exert on DA release in the rat striatum and nucleus accumbens in vivo. In CHO cells, the purported antagonist 5-methyl-1-(3-pyridylcarbamoyl)-1,2,3,5-tetrahydropyrrolo[2,3-f] indole hydrochloride (SB 206553), but not 6-chloro-5-methyl-1-[6-(2-methylpiridin-3-yloxy)pyridin-3-yl carbamoyl] indoline (SB 242084), decreased basal inositol phosphate accumulation, thus behaving as a 5-HT2C inverse agonist. Its effect was prevented by SB 242084. In vivo, SB 206553 (1–10 mg/kg) elicited a dose-dependent and clear-cut increase in accumbal and striatal DA release compared with SB 242084 (1–10 mg/kg), and the 5-HT2C agonist S-2-(6-chloro-5-fluoroindol-1-yl)-1-methylethylamine hydrochloride (Ro-60-0175) (0.3–3 mg/kg) inhibited DA release. Pretreatment by SB 242084 reversed the change in DA release elicited by Ro-60-0175 and SB 206553. Furthermore, SB 206553-stimulated DA release was insensitive to reduction of 5-HT neuronal function induced by the 5-HT1A agonist (±)-8-hydroxy-2-dipropylaminotetralin or intra-raphe injections of 5,7-dihydroxytryptamine neurotoxin. The obtained results provide the first in vivo evidence that constitutive activity of the 5-HT2C receptor tonically inhibits mesencephalic DA neurons and underscore the need for a better understanding of the pathophysiological role of constitutive receptor activity.

5-HT2A and 5-HT2C/2B receptor subtypes modulate dopamine release induced in vivo by amphetamine and morphine in both the rat nucleus accumbens and striatum.

In vivo microdialysis and single-cell extracellular recordings were used to assess the involvement of serotonin2A (5-HT2A) and serotonin2C/2B (5-HT2C/2B) receptors in the effects induced by amphetamine and morphine on dopaminergic (DA) activity within the mesoaccumbal and nigrostriatal pathways. The increase in DA release induced by amphetamine (2 mg/kg i.p.) in the nucleus accumbens and striatum was significantly reduced by the selective 5-HT2A antagonist SR 46349B (0.5 mg/kg s.c.), but not affected by the 5-HT2C/2B antagonist SB 206553 (5 mg/kg i.p.). In contrast, the enhancement of accumbal and striatal DA output induced by morphine (2.5 mg/kg s.c.), while insensitive to SR 46349B, was significantly increased by SB 206553. Furthermore, morphine (0.1–10 mg/kg i.v.)-induced increase in DA neuron firing rate in both the ventral tegmental area and the substantia nigra pars compacta was unaffected by SR 46349B (0.1 mg/kg i.v.) but significantly potentiated by SB 206553 (0.1 mg/kg i.v.). These results show that 5-HT2A and 5-HT2C receptors regulate specifically the activation of midbrain DA neurons induced by amphetamine and morphine, respectively. This differential contribution may be related to the specific mechanism of action of the drug considered and to the neuronal circuitry involved in their effect on DA neurons. Furthermore, these results suggest that 5-HT2C receptors selectively modulate the impulse flow–dependent release of DA.

Role of Serotonin2A and Serotonin2B/2C Receptor Subtypes in the Control of Accumbal and Striatal Dopamine Release Elicited In Vivo by Dorsal Raphe Nucleus Electrical Stimulation

Abstract : This study investigates, using in vivo microdialysis, the role of serotonin2A (5‐HT2A) and 5‐HT2B/2C receptors in the effect of dorsal raphe nucleus (DRN) electrical stimulation on dopamine (DA), 3,4‐dihydroxyphenylacetic acid (DOPAC), and 5‐hydroxyindoleacetic acid (5‐HIAA) extracellular levels monitored in the nucleus accumbens (NAC) and the striatum of halothane‐anesthetized rats. Following DRN stimulation (300 μA, 1 ms, 20 Hz, 15 min) DA release was enhanced in the NAC and reduced in the striatum. The 5‐HT2A antagonist SR 46349B (0.5 mg/kg) and the mixed 5‐HT2A/2B/2C antagonist ritanserin (0.63 mg/kg) significantly reduced the effect of DRN stimulation on DA release in the NAC but not in the striatum. DA responses to DRN stimulation were not affected by the 5‐HT2B/2C antagonist SB 206553 (5 mg/kg) in either region. None of these compounds was able to modify the enhancement of DOPAC and 5‐HIAA outflow induced by DRN stimulation in either the NAC or the striatum. Finally, in both brain regions basal DA release was significantly increased only by SB 206553. These results indicate that 5‐HT2A but not 5‐HT2B/2C receptors participate in the facilitatory control exerted by endogenous 5‐HT on accumbal DA release. Conversely, 5‐HT2B/2C receptors tonically inhibit basal DA release in both brain regions.

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.

Serotonergic neurons mediate ectopic release of dopamine induced by l-DOPA in a rat model of Parkinson's disease

Benefit and motor side effects of l-DOPA in Parkinsons disease have been related to dopamine transmission in the striatum. However, the putative involvement of serotonergic neurons in the dopaminergic effects of l-DOPA suggests that the striatum is not a preferential target of l-DOPA. By using microdialysis in a rat model of Parkinsons disease, we found that l-DOPA (3-100 mg/kg) increased dopamine extracellular levels monitored simultaneously in four brain regions receiving serotonergic innervation: striatum, substantia nigra, hippocampus, prefrontal cortex. The increase was regionally similar at the lowest dose and 2-3 times stronger in the striatum at higher doses. Citalopram, a serotonin reuptake blocker, or the destruction of serotonergic fibers by 5,7-dihydroxytryptamine impaired l-DOPA-induced dopamine release in all regions. These data demonstrate that l-DOPA induces an ectopic release of dopamine due to serotonergic neurons. The new pattern of dopamine transmission created by l-DOPA may contribute to the benefit and side effects of l-DOPA.

8-OH-DPAT, a 5-HT1A agonist and ritanserin, a 5-HT2A/C antagonist, reverse haloperidol-induced catalepsy in rats independently of striatal dopamine release

Abstract In this study, both catalepsy and changes in extracellular levels of striatal dopamine (DA) and dihydroxyphenyl acetic acid (DOPAC) induced by the typical neuroleptic haloperidol (HAL) were simultaneously assessed, using intracerebral microdialysis in freely moving rats, in the presence of either the 5-HT1A agonist 8-OH-DPAT or the 5-HT2A/C antagonist ritanserin. HAL (1 mg/kg, SC) elicited a strong cataleptic state, reaching its maximal intensity (about 240 s) 2 h after the drug administration. This effect was paralleled by a long-lasting enhancement of striatal DA and DOPAC extracellular levels, reaching 230 and 350% of basal values, respectively. 8-OH-DPAT (0.1 mg/kg, SC) given 2.5 h after, and ritanserin (0.63 and 1.25 mg/kg, IP), given 15 min prior to HAL, significantly reduced the neuroleptic-induced catalepsy. However, both 5-HT agents failed to modify basal DA and DOPAC striatal outflow as well as the stimulatory effect of HAL on these parameters. It can thus be concluded that the anticataleptic effect of these compounds is not related to an alteration of DA release within the striatum.

Noradrenaline and Parkinson's Disease

Parkinsons disease (PD) is characterized by the degeneration of dopamine (DA) neurons in the substantia nigra pars compacta, and motor symptoms including bradykinesia, rigidity, and tremor at rest. These symptoms are exhibited when striatal dopamine concentration has decreased by around 70%. In addition to motor deficits, PD is also characterized by the non-motor symptoms. However, depletion of DA alone in animal models has failed to simultaneously elicit both the motor and non-motor deficits of PD, possibly because the disease is a multi-system disorder that features a profound loss in other neurotransmitter systems. There is growing evidence that additional loss of noradrenaline (NA) neurons of the locus coeruleus, the principal source of NA in the brain, could be involved in the clinical expression of motor as well as in non-motor deficits. In the present review, we analyze the latest evidence for the implication of NA in the pathophysiology of PD obtained from animal models of parkinsonism and from parkinsonian patients. Recent studies have shown that NA depletion alone, or combined with DA depletion, results in motor as well as in non-motor dysfunctions. In addition, by using selective agonists and antagonists of noradrenaline alpha receptors we, and others, have shown that α2 receptors are implicated in the control of motor activity and that α2 receptor antagonists can improve PD motor symptoms as well as l-Dopa-induced dyskinesia. In this review we argue that the loss of NA neurons in PD has an impact on all PD symptoms and that the addition of NAergic agents to dopaminergic medication could be beneficial in the treatment of the disease.

In vivo evidence that 5-HT2C receptor antagonist but not agonist modulates cocaine-induced dopamine outflow in the rat nucleus accumbens and striatum.

During recent years, much attention has been devoted at investigating the modulatory role of central 5-HT2C receptors on dopamine (DA) neuron activity, and it has been proposed that these receptors modulate selectively DA exocytosis associated with increased firing of DA neurons. In the present study, using in vivo microdialysis in the nucleus accumbens (NAc) and the striatum of halothane-anesthetized rats, we addressed this hypothesis by assessing the ability of 5-HT2C agents to modulate the increase in DA outflow induced by haloperidol and cocaine, of which the effects on DA outflow are associated or not with an increase in DA neuron firing, respectively. The intraperitoneal administration of cocaine (10–30 mg/kg) induced a dose-dependent increase in DA extracellular levels in the NAc and the striatum. The effect of 15 mg/kg cocaine was potentiated by the mixed 5-HT2C/2B antagonist SB 206553 (5 mg/kg i.p.) and the selective 5-HT2C antagonist SB 242084 (1 mg/kg i.p.) in both brain regions. The mixed 5-HT2C/2B agonist, Ro 60-0175 (1 mg/kg i.p.), failed to affect cocaine-induced DA outflow, but reduced significantly the increase in DA outflow induced by the subcutaneous administration of 0.1 mg/kg haloperidol. The obtained results provide evidence that 5-HT2C receptors exert similar effects in both the NAc and the striatum, and they modulate DA exocytosis also when its increase occurs independently from an increase in DA neuron impulse activity. Furthermore, they show that 5-HT2C agonists, at variance with 5-HT2C antagonists, exert a preferential control on the impulse-stimulated release of DA.

Chronic L-DOPA therapy alters central serotonergic function and L-DOPA-induced dopamine release in a region-dependent manner in a rat model of Parkinson's disease

The therapeutic benefit of L-DOPA is commonly attributed to restoration of dopamine (DA) extracellular levels in the striatum of Parkinsonian patients. However, the loss of efficacy of L-DOPA after chronic use is paradoxically associated with a similar or enhanced striatal DA response. Release of L-DOPA-derived DA depends on the widespread serotonergic (5-HT) innervation in the brain. Chronic exposure of 5-HT neurons to L-DOPA could lead to aberrant neurochemical responses beyond the striatum. Using multi-site intracerebral microdialysis in a rat model of Parkinsons disease, we showed that chronic L-DOPA treatment at a therapeutic dose (12 mg/kg/day for 10 days) homogeneously reduced basal 5-HT release and metabolism. These effects were paralleled by a decrease in tissue content of 5-HT and its metabolite. Chronic L-DOPA treatment severely altered the brain pattern of 5-HT and DA release responses to L-DOPA (3-12 mg/kg) with an overall loss of efficacy of L-DOPA to increase DA release. Our data demonstrate for the first time in vivo that the impairment of 5-HT neuronal function induced by chronic L-DOPA alters in a region-dependent manner L-DOPA-induced DA release. Changes in neurochemical pattern of L-DOPA in the brain may favour the occurrence of both motor and non-motor side effects.

High-Frequency Stimulation of the Subthalamic Nucleus and l-3,4-Dihydroxyphenylalanine Inhibit In Vivo Serotonin Release in the Prefrontal Cortex and Hippocampus in a Rat Model of Parkinson's Disease

High-frequency stimulation of the subthalamic nucleus (STN-HFS) and l-3,4-dihydroxyphenylalanine (l-DOPA) medication are the most used therapeutic approaches in Parkinsons disease (PD), but their beneficial motor effects are burdened by the emergence of cognitive and depressive disorders. Although a reduced serotonergic function has been linked to the psychiatric effects of antiparkinsonian treatments, biochemical evidence supporting this hypothesis is still lacking. By using a microdialysis approach in anesthetized rats, we investigated the ability of STN-HFS (130 Hz, 30 μA, 20 min) and l-DOPA (6–12 mg/kg) to change extracellular levels of serotonin (5-HT) monitored simultaneously in the prefrontal cortex (PFC) and hippocampus (HIPP), two brain regions involved in the regulation of mood and cognition that receive a distinct 5-HT innervation. The results show that STN-HFS inhibited 5-HT levels in the PFC and HIPP of sham-lesioned and 6-hydroxydopamine (6-OHDA)-lesioned rats. The effect elicited by STN-HFS was blocked by the administration of the 5-HT1A agonist 8-hydroxy-N,N-dipropyl-2-aminotetralin. l-DOPA (6 and 12 mg/kg) reduced 5-HT levels in the PFC and HIPP of 6-OHDA rats. STN-HFS did not further decrease 5-HT levels induced by l-DOPA, but attenuated l-DOPA-induced dopamine release in the PFC and HIPP. These neurochemical data show that STN-HFS inhibits 5-HT release by modulating serotonergic neuron activity, while the decrease in 5-HT levels induced by l-DOPA may include its direct action inside serotonergic neurons. These results support the premise that antiparkinsonian treatments reduce central serotonergic transmission, which may favor the development of nonmotor side effects in PD.