Subthalamic neurons show increased firing to 5-HT2C receptor activation in 6-hydroxydopamine-lesioned rats

GABABR1 in DRN mediated GABA to regulate 5-HT expression in multiple brain regions in male rats with high and low aggressive behavior

The identity of the mechanism that controls aggressive behavior in rodents is unclear. Serotonin (5-HT) and GABA are associated with aggressive behavior in rodents. However, the regulatory relationship between these chemicals in the different brain regions of rats has not been fully defined. This study aimed to clarify the role of GABABR1 in DRN-mediated GABA to regulate 5-HT expression in multiple brain regions in male rats with high and low aggressive behavior. Rat models of highly and less aggressive behavior were established through social isolation plus resident intruder. On this basis, GABA content in the DRN and 5-HT contents in the PFC, hypothalamus, hippocampus and DRN were detected using ELISA. Co-expression of 5-HT and GB1 in the DRN was detected by immunofluorescence and immunoelectron microscopy at the tissue and subcellular levels, respectively. GB1-specific agonist baclofen and GB1-specific inhibitor CGP35348 were injected into the DRN by stereotaxic injection. Changes in 5-HT levels in the PFC, hypothalamus and hippocampus were detected afterward. After modeling, rats with highly aggressive behavior exhibited higher aggressive behavior scores, shorter latencies of aggression, and higher total distances in the open field test than rats with less aggressive behavior. The contents of 5-HT in the PFC, hypothalamus and hippocampus of rats with high and low aggressive behavior (no difference between the two groups) were significantly decreased, but the change in GABA content in the DRN was the opposite. GB1 granules could be found on synaptic membranes containing 5-HT granules, which indicated that 5-HT neurons in the DRN co-expressed with GB1, which also occurred in double immunofluorescence results. At the same time, we found that the expression of GB1 in the DRN of rats with high and low aggressive behavior was significantly increased, and the expression of GB1 in the DRN of rats with low aggressive behavior was significantly higher than that in rats with high aggressive behavior. Nevertheless, the expression of 5-HT in DRN was opposite in these two groups. After microinjection of baclofen into the DRN, the 5-HT contents in the PFC, hypothalamus and hippocampus of rats in each group decreased significantly. In contrast, the 5-HT contents in the PFC, hypothalamus and hippocampus of rats in each group increased significantly after injection with CGP35348. The significant increase in GABA in the DRN combined with the significant increase in GB1 in the DRN further mediated the synaptic inhibition effect, which reduced the 5-HT level of 5-HT neurons in the DRN, resulting in a significant decrease in 5-HT levels in the PFC, hypothalamus and hippocampus. Therefore, GB1-mediated GABA regulation of 5-HT levels in the PFC, hypothalamus and hippocampus is one of the mechanisms of highly and less aggressive behavior originating in the DRN. The increased GB1 level in the DRN of LA-behavior rats exhibited a greater degree of change than in the HA-group rats, which indicated that differently decreased 5-HT levels in the DRN may be the internal mechanisms of high and low aggression behaviors.

Serotonergic control of the glutamatergic neurons of the subthalamic nucleus

The subthalamic nucleus (STN) houses a dense cluster of glutamatergic neurons that play a central role in the functional dynamics of the basal ganglia, a group of subcortical structures involved in the control of motor behaviors. Numerous anatomical, electrophysiological, neurochemical and behavioral studies have reported that serotonergic neurons from the midbrain raphe nuclei modulate the activity of STN neurons. Here, we describe this serotonergic innervation and the nature of the regulation exerted by serotonin (5-hydroxytryptamine, 5-HT) on STN neuron activity. This regulation can occur either directly within the STN or at distal sites, including other structures of the basal ganglia or cortex. The effect of 5-HT on STN neuronal activity involves several 5-HT receptor subtypes, including 5-HT_1A, 5-HT_1B, 5-HT_2C and 5-HT₄ receptors, which have garnered the highest attention on this topic. The multiple regulatory effects exerted by 5-HT are thought to be modified under pathological conditions, altering the activity of the STN, or due to the benefits and side effects of treatments used for Parkinson's disease, notably the dopamine precursor l-DOPA and high-frequency STN stimulation. Originally understood as a motor center, the STN is also associated with decision making and participates in mood regulation and cognitive performance, two domains of personality that are also regulated by 5-HT. The literature concerning the link between 5-HT and STN is already important, and the functional overlap is evident, but this link is still not entirely understood. The understanding of this link between 5-HT and STN should be increased due to the possible importance of this regulation in the control of fronto-STN loops and inherent motor and non-motor behaviors.

A Subset of Purposeless Oral Movements Triggered by Dopaminergic Agonists Is Modulated by 5-HT2C Receptors in Rats: Implication of the Subthalamic Nucleus

Dopaminergic medication for Parkinson’s disease is associated with troubling dystonia and dyskinesia and, in rodents, dopaminergic agonists likewise induce a variety of orofacial motor responses, certain of which are mimicked by serotonin2C (5-HT_2C) receptor agonists. However, the neural substrates underlying these communalities and their interrelationship remain unclear. In Sprague-Dawley rats, the dopaminergic agonist, apomorphine (0.03–0.3 mg/kg) and the preferential D2/3 receptor agonist quinpirole (0.2–0.5 mg/kg), induced purposeless oral movements (chewing, jaw tremor, tongue darting). The 5-HT_2C receptor antagonist 5-methyl-1-[[2-[(2-methyl-3-pyridyl)oxyl]-5-pyridyl]carbamoyl]-6-trifluoromethylindone (SB 243213) (1 mg/kg) reduced the oral responses elicited by specific doses of both agonists (0.1 mg/kg apomorphine; 0.5 mg/kg quinpirole). After having confirmed that the oral bouts induced by quinpirole 0.5 mg/kg were blocked by another 5-HT_2C antagonist (6-chloro-5-methyl-1-[6-(2-methylpiridin-3-yloxy)pyridine-3-yl carbamoyl] indoline (SB 242084), 1 mg/kg), we mapped the changes in neuronal activity in numerous sub-territories of the basal ganglia using c-Fos expression. We found a marked increase of c-Fos expression in the subthalamic nucleus (STN) in combining quinpirole (0.5 mg/kg) with either SB 243213 or SB 242084. In a parallel set of electrophysiological experiments, the same combination of SB 243213/quinpirole produced an irregular pattern of discharge and an increase in the firing rate of STN neurons. Finally, it was shown that upon the electrical stimulation of the anterior cingulate cortex, quinpirole (0.5 mg/kg) increased the response of substantia nigra pars reticulata neurons corresponding to activation of the “hyperdirect” (cortico-subthalamonigral) pathway. This effect of quinpirole was abolished by the two 5-HT_2C antagonists. Collectively, these results suggest that induction of orofacial motor responses by D2/3 receptor stimulation involves 5-HT_2C receptor-mediated activation of the STN by recruitment of the hyperdirect (cortico-subthalamonigral) pathway.

Purposeless oral activity induced by meta-chlorophenylpiperazine (m-CPP): Undefined tic-like behaviors?

BACKGROUND

The pathophysiological hypothesis underlying tic disorders in Tourette syndrome (TS) is that basal ganglia are not capable of properly filtering cortical information, leading patients with difficulties in inhibiting unwanted behaviors or impulses. One of the main challenges for furthering such a hypothesis is to find appropriate animal models summarizing some aspects of the disease.

METHODS

It has been established for more than 25 years in rodents that the prototypical serotonin (5-HT) agonist meta-chlorophenylpiperazine (m-CPP) elicits purposeless oral movements including chewing behavior. These bouts of oral movements, originally thought to mimic human oral dyskinesia consequent to long-term administration of antipsychotic drugs or parkinsonian tremor, could correspond to an undefined form of tics. Here, we describe the nature of the purposeless oral movements triggered by m-CPP and other agonists which could be associated with obsessive compulsive disorders. We report the pharmacology of this response with a focus on the 5-HT_2C receptor subtype and the degree to which the dopaminergic and cholinergic systems are involved. The orofacial dyskinetic effects are related to the action of these compounds in associative/limbic territories of the basal ganglia, rather than sensorimotor ones, as expected from the human disease.

CONCLUSION

In spite of the low translational value of these oral movements, the neurobiological analysis of these oral movements could help to a better understanding of the pathophysiology of tics and compulsive disorders often cormorbid with TS.

Increased extracellular dopamine and 5-hydroxytryptamine levels contribute to enhanced subthalamic nucleus neural activity during exhausting exercise

The purpose of the study was to explore the mechanism underlying the enhanced subthalamic nucleus (STN) neural activity during exhausting exercise from the perspective of monoamine neurotransmitters and changes of their corresponding receptors. Rats were randomly divided into microdialysis and immunohistochemistry study groups. For microdialysis study, extracellular fluid of the STN was continuously collected with a microdialysis probe before, during and 90 min after one bout of exhausting exercise. Dopamine (DA) and 5-hydroxytryptamine (5-HT) levels were subsequently detected with high-performance liquid chromatography (HPLC). For immunohistochemistry study, the expression of DRD2 and HT2C receptors in the STN, before, immediately after and 90 min after exhaustion was detected through immunohistochemistry technique. Microdialysis study results showed that the extracellular DA and 5-HT neurotransmitters increased significantly throughout the procedure of exhausting exercise and the recovery period (P<0.05 or P<0.01). Immunohistochemistry study results showed that the expression levels of DRD2 and HT2C in the rat STN immediately after exhausting exercise and at the time point of 90 min after exhaustion were both higher than those of the rest condition, but the difference was not significant (P>0.05). Our results suggest that the increased extracellular DA and 5-HT in the STN might be one important factor leading to the enhanced STN neural activity and the development of fatigue during exhausting exercise. This study may essentially offer useful evidence for better understanding of the mechanism of the central type of exercise-induced fatigue.

Modulation of the subthalamic nucleus activity by serotonergic agents and fluoxetine administration

RATIONALE

Within the basal ganglia, the subthalamic nucleus (STN) is the only glutamatergic structure and occupies a central position in the indirect pathway. In rat, the STN receives serotonergic input from the dorsal raphe nucleus and expresses serotonergic receptors.

OBJECTIVE

This study examined the consequences of serotonergic neurotransmission modulation on STN neuron activity.

METHODS

In vivo single-unit extracellular recordings, HPLC determination, and rotarod and bar test were performed in control, 4-chloro-DL-phenylalanine methyl ester hydrochloride- (pCPA, a serotonin synthesis inhibitor) and chronically fluoxetine-treated rats.

RESULTS

The pCPA treatment and the administration of serotonin (5-HT) receptor antagonists increased number of bursting neurons in the STN. The systemic administration of the 5-HT(1A) agonist, 8-OH-DPAT, decreased the firing rate and increased the coefficient of variation of STN neurons in pCPA-treated rats but not in control animals. Additionally, microinjection of 8-OH-DPAT into the STN reduced the firing rate of STN neurons, while microinjection of the 5-HT(2C) agonist, Ro 60-0175, increased the firing rate in both control and fluoxetine-treated animals. Finally, the fluoxetine challenge increased the firing rate of STN neurons in fluoxetine-treated rats and induced catalepsy.

CONCLUSIONS

Our results indicate that the depletion and the blockage of 5-HT modify STN neuron firing pattern. STN neuron activity is under the control of 5-HT(1A) and 5-HT(2C) receptors located both inside and outside the STN. Finally, fluoxetine increases STN neuron activity in chronically fluoxetine-treated rats, which may explain the role of this nucleus in fluoxetine-induced extrapyramidal side effects.

Serotonin2C Receptors and the Motor Control of Oral Activity

Data from many experiments has shown that serotonin2C (5-HT2C) receptor plays a role in the control of orofacial activity in rodents. Purposeless oral movements can be elicited either by agonists or inverse agonists implying a tight control exerted by the receptor upon oral activity. The effects of agonists has been related to an action of these drugs in the subthalamic nucleus and the striatum, the two input structures for cortical efferents to the basal ganglia, a group of subcortical structures involved in the control of motor behaviors. The oral effects of agonists are dramatically enhanced in case of chronic blockade of central dopaminergic transmission induced by neuroleptics or massive destruction of dopamine neurons. The mechanisms involved in the hypersensitized oral responses to 5-HT2C agonists are not clear and deserve additional studies. Indeed, while the oral behavior triggered by 5-HT2C drugs would barely correspond to the dyskinesia observed in humans, the clinical data have consistently postulated that 5-HT2C receptors could be involved in these aberrant motor manifestations.

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.

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.

Subthalamic neuronal firing in obsessive-compulsive disorder and Parkinson disease

OBJECTIVE

Although electrophysiologic dysfunction of the subthalamic nucleus is putative, deep brain stimulation of this structure has recently been reported to improve obsessions and compulsions. In Parkinson disease, sensorimotor subthalamic neurons display high-frequency burst firing, which is considered as an electrophysiologic signature of motor loop dysfunction. We addressed whether such neuronal dysfunction of the subthalamic nucleus also exists in the nonmotor loops involved in patients with obsessive-compulsive disorder.

METHODS

We compared the neuronal activity of the subthalamic nucleus recorded in 9 patients with obsessive-compulsive disorder with that of 11 patients with Parkinson disease measured during intraoperative exploration for deep brain stimulation.

RESULTS

The mean subthalamic neuron discharge rate was statistically lower in patients with obsessive-compulsive disorder than in patients with Parkinson disease (20.5 ± 11.0 Hz, n = 100 and 30.8 ± 15.6 Hz, n = 93, respectively, p < 0.001). The relative proportion of burst neurons did not differ significantly between the 2 diseases (75% vs 73%). Interestingly, burst neurons were predominantly left-sided in obsessive-compulsive disorder.

INTERPRETATION

The recording of burst neurons within the nonmotor subthalamic nucleus in patients with obsessive-compulsive disorder is a novel finding that suggests the existence of deregulation of the nonmotor basal ganglia loop, possibly left-sided. Potentially, burst activity might interfere with normal processes occurring within nonmotor loops.

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.