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.