The acute and long-term L-DOPA effects are independent from changes in the activity of dorsal raphe serotonergic neurons in 6-OHDA lesioned rats.

BACKGROUND AND PURPOSE: L-DOPA is still the most efficacious pharmacological treatment for Parkinson's disease. However, in the majority of patients receiving long-term therapy with L-DOPA, its efficacy is compromised by motor complications, notably L-DOPA-induced dyskinesia. Evidence suggests that the serotonergic system is involved in the therapeutic and the side effects of L-DOPA. Here, we investigate if long-term L-DOPA treatment alters the activity of the dorsal raphe nucleus (DRN) and its responses to serotonergic drugs. EXPERIMENTAL APPROACH: We measured the responses of serotonergic neurons to acute and chronic L-DOPA treatment using in vivo electrophysiological single unit-extracellular recordings in the 6-OHDA-lesion rat model of Parkinson's disease. KEY RESULTS: The results showed that neither acute nor chronic L-DOPA administration (6 mg·kg(-1)  s.c.) altered the properties of serotonergic-like neurons. Furthermore, no correlation was found between the activity of these neurons and the magnitude of L-DOPA-induced dyskinesia. In dyskinetic rats, the inhibitory response induced by the 5-HT1A receptor agonist 8-OH-DPAT (0.0625-16 µg·kg(-1) , i.v.) was preserved. Nonetheless, L-DOPA impaired the ability of the serotonin reuptake inhibitor fluoxetine (0.125-8 mg·kg(-1) , i.v) to inhibit DRN neuron firing rate in dyskinetic animals. CONCLUSIONS AND IMPLICATIONS: Although serotonergic neurons are involved in the dopaminergic effects of L-DOPA, we provide evidence that the effect of L-DOPA is not related to changes of the activity of DRN neurons. Rather, L-DOPA might reduce the efficacy of drugs that normally enhance the extracellular levels of serotonin. LINKED ARTICLES: This article is part of a themed section on Updating Neuropathology and Neuropharmacology of Monoaminergic Systems. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.13/issuetoc.

Role of 5-HT2C receptors in the enhancement of c-Fos expression induced by a 5-HT2B/2C inverse agonist and 5-HT 2 agonists in the rat basal ganglia.

Some non-selective serotonin2C (5-HT2C) agonists or inverse agonists enhance the product of the proto-oncogene c-Fos within the basal ganglia, a group of brain regions involved in motor behavior and in the ability of these drugs to promote abnormal movements. The role of 5-HT2C receptors in these effects is unclear. The 5-HT2C antagonist SB243,213 (1 mg/kg), which enhanced Fos per se in the striatum and the subthalamic nucleus (STN) only, was used to study the implication of 5-HT2C receptors. The agonists Ro 60-0175 (3 mg/kg) and m-CPP (1 mg/kg) and the inverse agonist SB206,553 (10 mg/kg) enhanced Fos expression in the STN and faintly in the entopeduncular nucleus (EPN, the internal globus pallidus in primate). The effects of these drugs differed mainly in the striatum regarding the magnitude (m-CPP > Ro 60-0175> SB243,213 > SB206,553) or the striatal quadrants (faint to no labeling in lateral striatum) and in the substantia nigra. None of these compounds enhanced Fos expression by themselves in the globus pallidus or in the EPN when combined with SB243,213. Their Fos effect in the STN was reduced significantly by SB243,213 only in the case of m-CPP. In the ventromedial striatum, SB243,213 reduced the effects of m-CPP while SB206,553 reduced the effects of SB243,213. The results show that opposite pharmacological agents alter similarly Fos expression in the EPN or the STN. Although some of the effects of 5-HT agents are related to targets other than 5-HT2C receptors, the study confirms the existence of multiple 5-HT2C receptor-dependent controls recruited by these drugs upon basal ganglia activity.

Multiple controls exerted by 5-HT2C receptors upon basal ganglia function: from physiology to pathophysiology.

Serotonin2C (5-HT2C) receptors are expressed in the basal ganglia, a group of subcortical structures involved in the control of motor behaviour, mood and cognition. These receptors are mediating the effects of 5-HT throughout different brain areas via projections originating from midbrain raphe nuclei. A growing interest has been focusing on the function of 5-HT2C receptors in the basal ganglia because they may be involved in various diseases of basal ganglia function notably those associated with chronic impairment of dopaminergic transmission. 5-HT2C receptors act on numerous types of neurons in the basal ganglia, including dopaminergic, GABAergic, glutamatergic or cholinergic cells. Perhaps inherent to their peculiar molecular properties, the modality of controls exerted by 5-HT2C receptors over these cell populations can be phasic, tonic (dependent on the 5-HT tone) or constitutive (a spontaneous activity without the presence of the ligand). These controls are functionally organized in the basal ganglia: they are mainly localized in the input structures and preferentially distributed in the limbic/associative territories of the basal ganglia. The nature of these controls is modified in neuropsychiatric conditions such as Parkinson's disease, tardive dyskinesia or addiction. Most of the available data indicate that the function of 5-HT2C receptor is enhanced in cases of chronic alterations of dopamine neurotransmission. The review illustrates that 5-HT2C receptors play a role in maintaining continuous controls over the basal ganglia via multiple diverse actions. We will discuss their interest for treatments aimed at ameliorating current pharmacotherapies in schizophrenia, Parkinson's disease or drugs abuse.

Multisite intracerebral microdialysis to study the mechanism of L-DOPA induced dopamine and serotonin release in the parkinsonian brain.

L-DOPA is currently one of the best medications for Parkinson's disease. It was assumed for several years that its benefits and side effects were related to the enhancement of dopamine release in the dopamine-depleted striatum. The use of intracerebral microdialysis combined with a pharmacological approach has led to the discovery that serotonergic neurons are responsible for dopamine release induced by L-DOPA. The subsequent use of multisite microdialysis has further revealed that L-DOPA-stimulated dopamine release is widespread and related to the serotonergic innervation. The present Review emphasizes the functional impact of extrastriatal release of dopamine induced by L-DOPA in both the therapeutic and side effects of L-DOPA.

The enhanced oral response to the 5-HT2 agonist Ro 60-0175 in parkinsonian rats involves the entopeduncular nucleus: electrophysiological correlates.

Lesions of nigrostriatal dopaminergic neurons as seen in Parkinson's disease (PD) increase orofacial responses to serotonergic (5-HT) agonists in rodents. Although this response to 5-HT agonists has been related to aberrant signalling in the basal ganglia, a group a subcortical structures involved in the control of motor behaviours, it deserves additional studies with respect to the specific loci involved. Using measurements of orofacial activity, as well as single-cell recordings in vivo, we have studied the role of the entopeduncular nucleus (EPN; equivalent to the internal globus pallidus of primates), an output structure of basal ganglia, in the hypersensitized responses to a 5-HT agonist in sham- or unilaterally dopamine-depleted rats. Intra-EPN injections of Ro 60-0175 (0.3 and 1 µg/100 nl) promoted robust oral movements in 6-OHDA rats without affecting oral activity in sham-depleted rats. Peripheral administration of Ro 60-0175 (3 mg/kg ip) decreased EPN neuronal firing rate in 6-OHDA rats compared to sham-depleted rats. Such an effect was also observed when the agonist (0.2 µg/20 nl) was locally applied onto EPN neurons. These data demonstrate the contribution of EPN to hypersensitized responses to 5-HT agonists in a rat model of PD.

What can we expect from the serotonergic side of L-DOPA?

Parkinson's disease has long been associated with neurodegeneration of the dopaminergic neurons located in the substantia nigra. The metabolic precursor L-DOPA, administered exogenously to patients, has proven its superiority over other medications. Yet, its effectiveness is altered after long-term use by diverse motor and non-motor symptoms. Knowledge of its mechanism of action would be necessary to better apprehend the side effects, but do we really know where and how it works? The connexion between L-DOPA and the serotonergic system, after a sort of crusade lasting for more than 40 years, has been acknowledged recently. The purpose of this review, mainly based on preclinical data, is to present the pharmacological and biochemical evidence demonstrating that serotonergic neurons are mainly involved in the enhancement of dopamine transmission induced by L-DOPA. We are addressing thereafter the two main expectations coming from this mechanism that are fundamental and clinical. The fundamental part will focus on the conceptual framework imposed by such a mechanism, questioning notably the notion that the benefit of L-DOPA is associated with a restoration of dopamine levels in the caudate-putamen. The clinical part will discuss serotonergic strategies to ameliorate the benefit of L-DOPA treatment in line with past and current clinical trials.

Effects of noradrenaline and serotonin depletions on the neuronal activity of globus pallidus and substantia nigra pars reticulata in experimental parkinsonism.

Parkinson's disease (PD) is characterized by a degeneration of dopaminergic neurons and also by a degradation of noradrenergic neurons from the locus coeruleus and serotonergic neurons from the dorsal raphe. However, the effect of these depletions on the neuronal activity of basal ganglia nuclei is still unknown. By using extracellular single-unit recordings, we have addressed this question by testing the effects of selective depletions of noradrenaline (NA) (with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP-4)) and serotonin (5-HT) (with 4-chloro-l-phenylalanine (pCPA)) on the neuronal activity of globus pallidus (GP) and substantia nigra pars reticulata (SNr) neurons in the 6-hydroxydopamine (6-OHDA) rat model of PD and sham-lesioned rats. We showed that 6-OHDA-induced dopamine (DA) depletion resulted in an increased number of GP and SNr neurons discharging in a bursty and irregular manner, confirming previous studies. These pattern changes were region-dependently influenced by additional monoamine depletion. Although the number of irregular and bursty neurons in 6-OHDA rats tended to decrease in the GP after NA depletion, it did not change after pCPA treatment in both GP and SNr. Furthermore, a significant interaction between DA and 5-HT depletions was observed on the firing rate of SNr neurons. By themselves, NA depletion did not change GP or SNr neuronal activity, whereas 5-HT depletion decreased the firing rate and increased the proportion of bursty and irregular neurons in both brain regions, suggesting that 5-HT, but not NA, plays a major role in the modulation of both the firing rate and patterns of GP and SNr neurons. Finally, our data suggest that, in addition to the primary role of DA in the control of basal ganglia activity, NA and 5-HT depletion also contribute to the dysregulation of the basal ganglia in PD by changes to neuronal firing patterns.

Stimulation of serotonin2C receptors elicits abnormal oral movements by acting on pathways other than the sensorimotor one in the rat basal ganglia.

Serotonin2C (5-HT(2C)) receptors act in the basal ganglia, a group of sub-cortical structures involved in motor behavior, where they are thought to modulate oral activity and participate in iatrogenic motor side-effects in Parkinson's disease and Schizophrenia. Whether abnormal movements initiated by 5-HT(2C) receptors are directly consequent to dysfunctions of the motor circuit is uncertain. In the present study, we combined behavioral, immunohistochemical and extracellular single-cell recordings approaches in rats to investigate the effect of the 5-HT(2C) agonist Ro-60-0175 respectively on orofacial dyskinesia, the expression of the marker of neuronal activity c-Fos in basal ganglia and the electrophysiological activity of substantia nigra pars reticulata (SNr) neuron connected to the orofacial motor cortex (OfMC) or the medial prefrontal cortex (mPFC). The results show that Ro-60-0175 (1 mg/kg) caused bouts of orofacial movements that were suppressed by the 5-HT(2C) antagonist SB-243213 (1 mg/kg). Ro-60-0175 (0.3, 1, 3 mg/kg) dose-dependently enhanced Fos expression in the striatum and the nucleus accumbens. At the highest dose, it enhanced Fos expression in the subthalamic nucleus, the SNr and the entopeduncular nucleus but not in the external globus pallidus. However, the effect of Ro-60-0175 was mainly associated with associative/limbic regions of basal ganglia whereas subregions of basal ganglia corresponding to sensorimotor territories were devoid of Fos labeling. Ro-60-0175 (1-3 mg/kg) did not affect the electrophysiological activity of SNr neurons connected to the OfMC nor their excitatory-inhibitory-excitatory responses to the OfMC electrical stimulation. Conversely, Ro-60-0175 (1 mg/kg) enhanced the late excitatory response of SNr neurons evoked by the mPFC electrical stimulation. These results suggest that oral dyskinesia induced by 5-HT(2C) agonists are not restricted to aberrant signalling in the orofacial motor circuit and demonstrate discrete modifications in associative territories.