Effects of 5-HT2A and 5-HT2C receptor antagonists on acute and chronic dyskinetic effects induced by haloperidol in rats

Effects of (-)-MBP, a novel 5-HT2C agonist and 5-HT2A/2B antagonist/inverse agonist on brain activity: A phMRI study on awake mice

A novel serotonin ligand (-)-MBP was developed for the treatment of schizophrenia that has 5-HT2A/2B antagonist activity together with 5-HT2C agonist activity. The multi-functional activity of this novel drug candidate was characterized using pharmacological magnetic resonance imaging. It was hypothesized (-)-MBP would affect activity in brain areas associated with sensory perception. Adult male mice were given one of three doses of (-)-MBP (3.0, 10, 18 mg/kg) or vehicle while fully awake during the MRI scanning session and imaged for 15 min post I.P. injection. BOLD functional imaging was used to follow changes in global brain activity. Data for each treatment were registered to a 3D MRI mouse brain atlas providing site-specific information on 132 different brain areas. There was a dose-dependent decrease in positive BOLD signal in numerous brain regions, especially thalamus, cerebrum, and limbic cortex. The 3.0 mg/kg dose had the greatest effect on positive BOLD while the 18 mg/kg dose was less effective. Conversely, the 18 mg/kg dose showed the greatest negative BOLD response while the 3.0 mg/kg showed the least. The prominent activation of the thalamus and cerebrum included the neural circuitry associated with Papez circuit of emotional experience. When compared to vehicle, the 3.0 mg dose affected all sensory modalities, for example, olfactory, somatosensory, motor, and auditory except for the visual cortex. These findings show that (-)-MBP, a ligand with both 5-HT2A/2B antagonist and 5-HT2C agonist activities, interacts with thalamocortical circuitry and impacts areas involved in sensory perception.

SLC6A3, HTR2C and HTR6 Gene Polymorphisms and the Risk of Haloperidol-Induced Parkinsonism

Antipsychotic-induced parkinsonism (AIP) is the most common type of extrapyramidal side effect (EPS), caused by the blockage of dopamine receptors. Since dopamine availability might influence the AIP risk, the dopamine transporter (DAT) and serotonin receptors (5-HTRs), which modulate the dopamine release, may be also involved in the AIP development. As some of the individual differences in the susceptibility to AIP might be due to the genetic background, this study aimed to examine the associations of SLC6A3, HTR2C and HTR6 gene polymorphisms with AIP in haloperidol-treated schizophrenia patients. The Extrapyramidal Symptom Rating Scale (ESRS) was used to evaluate AIP as a separate entity. Genotyping was performed using a PCR, following the extraction of blood DNA. The results revealed significant associations between HTR6 rs1805054 polymorphism and haloperidol-induced tremor and rigidity. Additionally, the findings indicated a combined effect of HTR6 T and SLC6A3 9R alleles on AIP, with their combination associated with significantly lower scores of ESRS subscale II for parkinsonism, ESRS-based tremor or hyperkinesia and ESRS subscales VI and VIII. These genetic predictors of AIP could be helpful in better understanding its pathophysiology, recognizing the individuals at risk of developing AIP and offering personalized therapeutic strategies for the patients suffering from this EPS.

The role of neurotransmitter systems in mediating deep brain stimulation effects in Parkinson’s disease

Deep brain stimulation (DBS) is among the most successful paradigms in both translational and reverse translational neuroscience. DBS has developed into a standard treatment for movement disorders such as Parkinson’s disease (PD) in recent decades, however, specific mechanisms behind DBS’s efficacy and side effects remain unrevealed. Several hypotheses have been proposed, including neuronal firing rate and pattern theories that emphasize the impact of DBS on local circuitry but detail distant electrophysiological readouts to a lesser extent. Furthermore, ample preclinical and clinical evidence indicates that DBS influences neurotransmitter dynamics in PD, particularly the effects of subthalamic nucleus (STN) DBS on striatal dopaminergic and glutamatergic systems; pallidum DBS on striatal dopaminergic and GABAergic systems; pedunculopontine nucleus DBS on cholinergic systems; and STN-DBS on locus coeruleus (LC) noradrenergic system. DBS has additionally been associated with mood-related side effects within brainstem serotoninergic systems in response to STN-DBS. Still, addressing the mechanisms of DBS on neurotransmitters’ dynamics is commonly overlooked due to its practical difficulties in monitoring real-time changes in remote areas. Given that electrical stimulation alters neurotransmitter release in local and remote regions, it eventually exhibits changes in specific neuronal functions. Consequently, such changes lead to further modulation, synthesis, and release of neurotransmitters. This narrative review discusses the main neurotransmitter dynamics in PD and their role in mediating DBS effects from preclinical and clinical data.

High-frequency stimulation of the subthalamic nucleus induces a sustained inhibition of serotonergic system via loss of cell phenotype

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) has become a standard treatment for Parkinson's disease (PD). However, in a considerable number of patients debilitating psychiatric side-effects occur. Recent research has revealed that external stimuli can alter the neurotransmitters' homeostasis in neurons, which is known as "neurotransmitter respecification". Herein, we addressed if neurotransmitter respecification could be a mechanism by which DBS suppresses the serotonergic function in the dorsal raphe nucleus (DRN) leading to mood changes. We infused transgenic 5-HT-Cre (ePET-Cre) mice with AAV viruses to achieve targeted expression of eYFP and the genetically encoded calcium indicator GCaMP6s in the DRN prior to methyl-4phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment. Mice received bilateral DBS electrodes in the STN and an optic fiber in the DRN for calcium photometry. MPTP-treated mice demonstrated behavioral and histological PD phenotype, whereas all STN-DBS animals exhibited an increased immobility time in the forced swim test, reduced calcium activity, and loss of tryptophan hydroxylase-2 expression in the DRN. Given the prominent role of calcium transients in mediating neurotransmitter respecification, these results suggest a loss of serotonergic phenotype in the DRN following STN-DBS. These findings indicate that loss of serotonergic cell phenotype may underlie the unwanted depressive symptoms following STN-DBS.

The Role of G Protein-Coupled Receptors (GPCRs) and Calcium Signaling in Schizophrenia. Focus on GPCRs Activated by Neurotransmitters and Chemokines

Schizophrenia is a common debilitating disease characterized by continuous or relapsing episodes of psychosis. Although the molecular mechanisms underlying this psychiatric illness remain incompletely understood, a growing body of clinical, pharmacological, and genetic evidence suggests that G protein-coupled receptors (GPCRs) play a critical role in disease development, progression, and treatment. This pivotal role is further highlighted by the fact that GPCRs are the most common targets for antipsychotic drugs. The GPCRs activation evokes slow synaptic transmission through several downstream pathways, many of them engaging intracellular Ca²⁺ mobilization. Dysfunctions of the neurotransmitter systems involving the action of GPCRs in the frontal and limbic-related regions are likely to underly the complex picture that includes the whole spectrum of positive and negative schizophrenia symptoms. Therefore, the progress in our understanding of GPCRs function in the control of brain cognitive functions is expected to open new avenues for selective drug development. In this paper, we review and synthesize the recent data regarding the contribution of neurotransmitter-GPCRs signaling to schizophrenia symptomology.

Effect of 5-HT2A receptor antagonism on levels of D2/3 receptor occupancy and adverse behavioral side-effects induced by haloperidol: a SPECT imaging study in the rat

Several studies suggested that 5-HT_2A receptor (5-HT_2AR) blockade may provide a more favorable efficacy and side-effect profile to antipsychotic treatment. We hypothesized that a combined haloperidol (a D_2/3 receptor (D_2/3R) antagonist) and MDL-100,907 (a 5-HT_2AR antagonist) treatment would reverse the side effects and the neurochemical alterations induced by haloperidol alone and would potentialize its efficacy. We thus chronically treated male Mdr1a knock-out rats with several doses of haloperidol alone or in combination with a saturating dose of a MDL-100,907. Receptor occupancy at clinically relevant levels was validated with a dual-radiotracer in-vivo SPECT imaging of D_2/3R and 5-HT_2AR occupancy. Experimental tests of efficacy (dizocilpine-disrupted prepulse inhibition (PPI) of the startle reflex) and side effects (catalepsy, vacuous chewing movements) were performed. Finally, a second dual-radiotracer in-vivo SPECT scan assessed the neurochemical changes induced by the chronic treatments. Chronic haloperidol failed to reverse PPI disruption induced by dizocilpine, whilst administration of MDL-100,907 along with haloperidol was associated with a reversal of the effect of dizocilpine. Haloperidol at 0.5 mg/kg/day and at 1 mg/kg/day induced catalepsy that was significantly alleviated (by ~50%) by co-treatment with MDL-100,907 but only at 0.5 mg/kg/day dose of haloperidol. Chronic haloperidol treatment, event at doses as low as 0.1 mg/kg/day induced a significant upregulation of the D_2/3R in the striatum (by over 40% in the nucleus accumbens and over 20% in the caudate-putamen nuclei), that was not reversed by MDL-100,907. Finally, an upregulation of 5-HT_2AR after chronic haloperidol treatment at a moderate dose only (0.25 mg/kg/day) was demonstrated in frontal cortical regions and the ventral tegmental area. Overall, a partial contribution of a 5-HT_2AR antagonism to the efficacy and side-effect profile of antipsychotic agents is suggested.

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.

The role of glutamate receptors and their interactions with dopamine and other neurotransmitters in the development of tardive dyskinesia: preclinical and clinical results

Tardive dyskinesia is a serious, disabling, movement disorder associated with the ongoing use of antipsychotic medication. Current evidence regarding the pathophysiology of tardive dyskinesia is mainly based on preclinical animal models and is still not completely understood. The leading preclinical hypothesis of tardive dyskinesia development includes dopaminergic imbalance in the direct and indirect pathways of the basal ganglia, cholinergic deficiency, serotonin receptor disturbances, neurotoxicity, oxidative stress, and changes in synaptic plasticity. Although, the role of the glutamatergic system has been confirmed in preclinical tardive dyskinesia models it seems to have been neglected in recent reviews. This review focuses on the role and interactions of glutamate receptors with dopamine, acetylcholine, and serotonin in the neuropathology of tardive dyskinesia development. Moreover, preclinical and clinical results of the differentiated effectiveness of N-methyl-D-aspartate (NMDA) receptor antagonists are discussed with a special focus on antagonists that bind with the GluN2B subunit of NMDA receptors. This review also presents new combinations of drugs that are worth considering in the treatment of tardive dyskinesia.

5-HT2A receptors modulate dopamine D2-mediated maternal effects

Serotonin 5-HT_2A receptors are expressed throughout the mesolimbic and mesocortical dopamine pathways, and manipulation of this receptor system has a profound impact on dopamine functions and dopamine-mediated behaviors. It is highly likely that 5-HT_2A receptors may also modulate the D₂-mediated maternal effects. The present study investigated this issue and also explored the possible behavioral mechanisms. We tested the effects of two D₂ drugs (an agonist quinpirole: 0.5, 1.0 mg/kg, and a potent D₂ antagonist haloperidol: 0.05, 0.10 mg/kg, sc) and their combinations with two 5-HT_2A drugs (a selective 5-HT_2A agonist TCB-2: 2.5 mg/kg, and 5-HT_2A antagonist MDL100907, 1.0 mg/kg, sc) on maternal behavior in Sprague-Dawley postpartum females. Individually, TCB-2 (2.5 mg/kg, sc) and quinpirole (0.5 and 1.0 mg/kg, sc) reduced pup preference and disrupted home-cage maternal behavior. In contrast, haloperidol (0.10 mg/kg, sc) only disrupted home-cage maternal behavior, but did not suppress pup preference. MDL100907 (1.0 mg/kg, sc) by itself had no effect on either pup preference or maternal behavior. When administered in combination, pretreatment of TCB-2 did not alter quinpirole's disruption of pup preference and home-cage maternal behavior (possibly due to the floor effect), however, it did enhance haloperidol's disruption of pup retrieval in the home cage. MDL100907 had no effect both quinpirole's and haloperidol's disruption of pup preference and home-cage maternal behavior. Interestingly, haloperidol attenuated TCB-2's disruptive effect on pup preference. These findings suggest that activation of 5-HT_2A receptors tends to enhance D₂-mediated maternal disruption, whereas blockade of 5-HT_2A receptors is less effective. They also suggest that 5-HT_2A receptors may have a direct effect on maternal behavior independent of their interaction with D₂ receptors. The possible behavioral and neural mechanisms by which 5-HT_2A- and D2-mediated maternal effects and their interaction are discussed.

Potential Therapeutic Application for Nicotinic Receptor Drugs in Movement Disorders

Emerging studies indicate that striatal cholinergic interneurons play an important role in synaptic plasticity and motor control under normal physiological conditions, while their disruption may lead to movement disorders. Here we discuss the involvement of the cholinergic system in motor dysfunction, with a focus on the role of the nicotinic cholinergic system in Parkinson's disease and drug-induced dyskinesias. Evidence for a role for the striatal nicotinic cholinergic system stems from studies showing that administration of nicotine or nicotinic receptor drugs protects against nigrostriatal degeneration and decreases L-dopa-induced dyskinesias. In addition, nicotinic receptor drugs may ameliorate tardive dyskinesia, Tourette's syndrome and ataxia, although further study is required to understand their full potential in the treatment of these disorders. A role for the striatal muscarinic cholinergic system in movement disorders stems from studies showing that muscarinic receptor drugs acutely improve Parkinson's disease motor symptoms, and may reduce dyskinesias and dystonia. Selective stimulation or lesioning of striatal cholinergic interneurons suggests they are primary players in this regulation, although multiple central nervous systems appear to be involved.

IMPLICATIONS

Accumulating data from preclinical studies and clinical trials suggest that drugs targeting CNS cholinergic systems may be useful for symptomatic treatment of movement disorders. Nicotinic cholinergic drugs, including nicotine and selective nAChR receptor agonists, reduce L-dopa-induced dyskinesias, as well as antipsychotic-induced tardive dyskinesia, and may be useful in Tourette's syndrome and ataxia. Subtype selective muscarinic cholinergic drugs may also provide effective therapies for Parkinson's disease, dyskinesias and dystonia. Continued studies/trials will help address this important issue.

Identifying the In Vivo Cellular Correlates of Antipsychotic Drugs

GPCRs such as 5-HT_2A and D2 are implicated in the therapeutic and the side effects of antipsychotics. However, the pattern of brain activity that leads to the behavioral effects of antipsychotics is poorly understood. To address this question, we used the transgenic ‘FosTRAP’ mice (Mus musculus), where a fluorescent reporter marks the cells responsive to the stimulus of interest. Here, the stimulus was an administration of various antipsychotic drugs. In case of typical antipsychotics such as Haloperidol, the c-fos active cells were predominantly found in the striatum, whereas in case of the atypical antipsychotics (Clozapine and Olanzapine), c-fos-induced cells were more numerous in the cortical regions, e.g., orbital cortex, piriform cortex. Curiously, we also observed ependymal cells to be a novel cellular target of atypical antipsychotics. 5-HT_2A is considered to be a major target for atypical antipsychotics. Therefore, we bred ‘FosTRAP’ mice with 5-HT_2A knock-out (KO) mice and tested their response to the prototype of atypical antipsychotics, Clozapine. Interestingly, the absence of 5-HT_2A did not significantly affect the number of c-fos-induced cells in the cortical regions. However, the ependymal cells showed a dramatically reduced response to Clozapine in the absence of 5-HT_2A. In summary, the TRAP system has allowed us to identify various region-specific activity induced by antipsychotics and novel cellular targets of the antipsychotics. These results serve as a “proof of principle” study that can be extended to explore the biochemical and physiological changes brought about by antipsychotics and specifically identify antipsychotic-responsive cells in the live tissue.

Blockade of serotonin 5-HT2A receptors potentiates dopamine D2 activation-induced disruption of pup retrieval on an elevated plus maze, but has no effect on D2 blockade-induced one

Appetitive aspect of rat maternal behavior, such as pup retrieval, is motivationally driven and sensitive to dopamine disturbances. Activation or blockade of dopamine D₂ receptors causes a similar disruption of pup retrieval, which may also reflect an increase in maternal anxiety and/or a disruption of executive function. Recent work indicates that serotonin 5-HT_2A receptors also play an important role in rat maternal behavior. Given the well-known modulation of 5-HT_2A on the mesolimbic and mesocortical dopamine functions, the present study examined the extent to which blockade of 5-HT_2A receptors on dopamine D₂-mediated maternal effects using a pup retrieval on the elevated plus maze (EPM) test. Sprague-Dawley postpartum female rats were acutely injected with quinpirole (a D₂ agonist, 0.10 and 0.25 mg/kg, sc), or haloperidol (a D₂ antagonist, 0.1 or 0.2 mg/kg, sc), in combination of MDL100907 (a 5-HT_2A receptor antagonist, 1.0 mg/kg, sc, 30 min before quinpirole or haloperidol injection) or saline and tested at 30, 90 and 240 min after quinpirole or haloperidol injection on postpartum days 3 and 7. Quinpirole and haloperidol decreased the number of pup retrieved (an index of maternal motivation) and sequential retrieval score (an index of executive function), prolonged the pup retrieval latencies, reduced the percentage of time spent on the open arms (an index of maternal anxiety), and decreased the distance travelled on the maze in a dose-dependent and time-dependent fashion. MDL100907 treatment by itself had no effect on pup retrieval, but it exacerbated the quinpirole-induced disruption of pup retrieval, but had no effect on the haloperidol-induced one. These findings suggest a complex interactive effect between 5-HT_2A and D₂ receptors on one or several maternal processes (maternal motivation, anxiety and executive function), and support the idea that one molecular mechanism by which 5-HT_2A receptors mediate maternal behavior is through its modulation of D₂ receptors.

Further investigation of the effects of 5-hydroxytryptamine, 8-OH-DPAT and DOI to mediate contraction and relaxation responses in the intestine and emesis in Suncus murinus

5-HT receptors are implicated in many gastrointestinal disorders. However, the precise role of 5-HT in mediating GI responses in Suncus murnius is still unclear. Therefore in this study, the effects of 5-HT and its agonists were investigated in Suncus. The involvement of 5-HT_2C receptors in mediating emesis was also investigated. The ability of 5-HT and its agonists/antagonists at 5-HT_1A and 5-HT₂ to modify GI motility was investigated in vitro and in vivo. WAY100635 (a 5-HT_1A antagonist) inhibited the contraction response to 5-HT in the proximal segments without affecting the maximum response; whilst enhancing the contraction to 5-HT (>30.0nM) in the distal intestine. The selective 5-HT_2A and 5-HT_2B receptor antagonists MDL-100907 and RS-127445 attenuated 5-HT-induced contractions (<10.0µM) in the distal segments. RS-127445 also attenuated 5-HT-induced contractions in the central segments. The selective 5-HT_2C receptor antagonist SB-242084, attenuated the responses to 5-HT (> 3.0nM) in the proximal and central but not the distal regions. 8-OH-DPAT-induced relaxation was resistant to the antagonism by 5-HT_1A/7 antagonists. DOI in the presence of 5-HT_1A/2A/2B/2C antagonists induced greater contraction responses (>1.0µM) in most tissues, whilst RS-127445, or SB-242084, reduced the responses to DOI (< 1.0µM) in some tissues. SB-242084 also suppressed emesis-induced by motion and intragastric CuSO₄. In conclusion, within different regions of intestine, 5-HT₂ receptors are differently involved in contraction and emetic responses and that 8-OH-DPAT induces relaxation via non-5-HT_1A/7 receptors. Suncus could provide a model to investigate these diverse actions of 5-HT.

Genetics of tardive dyskinesia: Promising leads and ways forward

Tardive dyskinesia (TD) is a potentially irreversible and often debilitating movement disorder secondary to chronic use of dopamine receptor blocking medications. Genetic factors have been implicated in the etiology of TD. We therefore have reviewed the most promising genes associated with TD, including DRD2, DRD3, VMAT2, HSPG2, HTR2A, HTR2C, and SOD2. In addition, we present evidence supporting a role for these genes from preclinical models of TD. The current understanding of the etiogenesis of TD is discussed in the light of the recent approvals of valbenazine and deutetrabenazine, VMAT2 inhibitors, for treating TD.

Effects of subthalamic deep brain stimulation with duloxetine on mechanical and thermal thresholds in 6OHDA lesioned rats

Chronic pain is the most common non-motor symptom of Parkinson's disease (PD) and is often overlooked. Unilateral 6-hydroxydopamine (6-OHDA) medial forebrain bundle lesioned rats used as models for PD exhibit decreased sensory thresholds in the left hindpaw. Subthalamic deep brain stimulation (STN DBS) increases mechanical thresholds and offers improvements with chronic pain in PD patients. However, individual responses to STN high frequency stimulation (HFS) in parkinsonian rats vary with 58% showing over 100% improvement, 25% showing 30-55% improvement, and 17% showing no improvement. Here we augment STN DBS by supplementing with a serotonin-norepinephrine reuptake inhibitor commonly prescribed for pain, duloxetine. Duloxetine was administered intraperitoneally (30mg/kg) in 15 parkinsonian rats unilaterally implanted with STN stimulating electrodes in the lesioned right hemisphere. Sensory thresholds were tested using von Frey, Randall-Selitto and hot-plate tests with or without duloxetine, and stimulation to the STN at HFS (150Hz), low frequency (LFS, 50Hz), or off stimulation. With HFS or LFS alone (left paw; p=0.016; p=0.024, respectively), animals exhibited a higher mechanical thresholds stable in the three days of testing, but not with duloxetine alone (left paw; p=0.183). Interestingly, the combination of duloxetine and HFS produced significantly higher mechanical thresholds than duloxetine alone (left paw, p=0.002), HFS alone (left paw, p=0.028), or baseline levels (left paw; p<0.001). These findings show that duloxetine paired with STN HFS increases mechanical thresholds in 6-OHDA-lesioned animals more than either treatment alone. It is possible that duloxetine augments STN DBS with a central and peripheral additive effect, though a synergistic mechanism has not been excluded.

Pharmacogenetic study of antipsychotic induced acute extrapyramidal symptoms in a first episode psychosis cohort: role of dopamine, serotonin and glutamate candidate genes

This study investigated whether the risk of presenting antipsychotic (AP)-induced extrapyramidal symptoms (EPS) could be related to single-nucleotide polymorphisms (SNPs) in a naturalistic cohort of first episode psychosis (FEP) patients. Two hundred and two SNPs in 31 candidate genes (involved in dopamine, serotonin and glutamate pathways) were analyzed in the present study. One hundred and thirteen FEP patients (43 presenting EPS and 70 non-presenting EPS) treated with high-potency AP (amisulpride, paliperidone, risperidone and ziprasidone) were included in the analysis. The statistical analysis was adjusted by age, gender, AP dosage, AP combinations and concomitant treatments as covariates. Four SNPs in different genes (DRD2, SLC18A2, HTR2A and GRIK3) contributed significantly to the risk of EPS after correction for multiple testing (P<1 × 10(-4)). These findings support the involvement of dopamine, serotonin and glutamate pathways in AP-induced EPS.

New therapeutic opportunities for 5-HT2C receptor ligands in neuropsychiatric disorders

The 5-HT2C receptor (R) displays a widespread distribution in the CNS and is involved in the action of 5-HT in all brain areas. Knowledge of its functional role in the CNS pathophysiology has been impaired for many years due to the lack of drugs capable of discriminating among 5-HT2R subtypes, and to a lesser extent to the 5-HT1B, 5-HT5, 5-HT6 and 5-HT7Rs. The situation has changed since the mid-90s due to the increased availability of new and selective synthesized compounds, the creation of 5-HT2C knock out mice, and the progress made in molecular biology. Many pharmacological classes of drugs including antipsychotics, antidepressants and anxiolytics display affinities toward 5-HT2CRs and new 5-HT2C ligands have been developed for various neuropsychiatric disorders. The 5-HT2CR is presumed to mediate tonic/constitutive and phasic controls on the activity of different central neurobiological networks. Preclinical data illustrate this complexity to a point that pharmaceutical companies developed either agonists or antagonists for the same disease. In order to better comprehend this complexity, this review will briefly describe the molecular pharmacology of 5-HT2CRs, as well as their cellular impacts in general, before addressing its central distribution in the mammalian brain. Thereafter, we review the preclinical efficacy of 5-HT2C ligands in numerous behavioral tests modeling human diseases, highlighting the multiple and competing actions of the 5-HT2CRs in neurobiological networks and monoaminergic systems. Notably, we will focus this evidence in the context of the physiopathology of psychiatric and neurological disorders including Parkinson's disease, levodopa-induced dyskinesia, and epilepsy.

Utilizing the concept of geste antagoniste for conservative management of oro-mandibular tardive dyskinesia: a case report and mini-review

OBJECTIVE

This case report highlights the implication of the concept of "geste antagoniste" in conservatively managing oromotor dysfunction and its complications.

CLINICAL PRESENTATION

A 66-year-old female with a 1-year history of tardive dyskinesia (TD) was referred to the Craniofacial Pain Department (CPC) at Tufts University School of Dental Medicine for management of sore labial/lingual mucosa secondary to excessive daytime involuntary activity of the tongue, lips, and mandible. A detailed head/neck examination revealed excessive involuntary movements of the tongue, lips, and mandible with generalized tenderness of her masticatory muscles. No TMJ or bone pathology was evident in a panoramic radiograph.

INTERVENTION

A lower daytime appliance with bilateral posterior contacts was fabricated to protect her oral mucosa. On reevaluation, excessive movement of the jaw/tongue was significantly reduced with the presence of the appliance in her mouth. Face/neck muscle tenderness was also greatly reduced.

CONCLUSION

The use of oral appliance therapy in TD patients plays an important role in protecting the teeth/oral mucosa. The subsequent inhibition of excessive motor activity is proposed and should be further investigated.

A dopamine D2 receptor-DISC1 protein complex may contribute to antipsychotic-like effects

Current antipsychotic drugs primarily target dopamine D2 receptors (D2Rs), in conjunction with other receptors such as those for serotonin. However, these drugs have serious side effects such as extrapyramidal symptoms (EPS) and diabetes. Identifying a specific D2R signaling pathway that could be targeted for antipsychotic effects, without inducing EPS, would be a significant improvement in the treatment of schizophrenia. We report here that the D2R forms a protein complex with Disrupted in Schizophrenia 1 (DISC1) that facilitates D2R-mediated glycogen synthase kinase (GSK)-3 signaling and inhibits agonist-induced D2R internalization. D2R-DISC1 complex levels are increased in conjunction with decreased GSK-3α/β (Ser21/9) phosphorylation in both postmortem brain tissue from schizophrenia patients and in Disc1-L100P mutant mice, an animal model with behavioral abnormalities related to schizophrenia. Administration of an interfering peptide that disrupts the D2R-DISC1 complex successfully reverses behaviors relevant to schizophrenia but does not induce catalepsy, a strong predictor of EPS in humans.

Electroconvulsive therapy as a treatment for protracted refractory delirium in the intensive care unit--five cases and a review

PURPOSE

Delirium in the intensive care unit (ICU) is conventionally treated pharmacologically but can progress into a protracted state refractory to medical treatment--a potentially life-threatening condition in itself.

METHODS

We treated 5 cases of severe protracted delirium in our ICU with electroconvulsive therapy (ECT) after failure of conventional medical therapy.

RESULTS

The delirious state of long standing agitation, anxiety, and discomfort was controlled in all patients. Electroconvulsive therapy was effective in controlling delirium in 4 patients. The last patient became calm, relieved of stress, and able to cooperate with the ventilator but remained in a state of posttraumatic amnesia after a head trauma.

CONCLUSION

Although controversial, ECT is nevertheless recognized as an efficient and safe treatment for various psychiatric illnesses including delirium. Considering the significantly increased mortality and severe cognitive decline associated with delirium in the ICU, we find ECT to be a valuable treatment option for this vulnerable patient population. It can be considered when agitation cannot be controlled with medical treatment, when agitation and delirium make weaning impossible, or prolonged deep sedation the only alternative.

Upregulation of dopamine D3, not D2, receptors correlates with tardive dyskinesia in a primate model

Tardive dyskinesia (TD) is a delayed and potentially irreversible motor complication arising in patients chronically exposed to centrally active dopamine D2 receptor antagonists, including antipsychotic drugs and metoclopramide. The classical dopamine D2 receptor supersensitivity hypothesis in TD, which stemmed from rodent studies, lacks strong support in humans. To investigate the neurochemical basis of TD, we chronically exposed adult capuchin monkeys to haloperidol (median, 18.5 months; n = 11) or clozapine (median, 6 months; n = 6). Six unmedicated animals were used as controls. Five haloperidol-treated animals developed mild TD movements, and no TD was observed in the clozapine group. Using receptor autoradiography, we measured striatal dopamine D1, D2, and D3 receptor levels. We also examined the D3 receptor/preprotachykinin messenger RNA (mRNA) co-expression, and quantified preproenkephalin mRNA levels, in striatal sections. Unlike clozapine, haloperidol strongly induced dopamine D3 receptor binding sites in the anterior caudate-putamen, particularly in TD animals, and binding levels positively correlated with TD intensity. Interestingly, the D3 receptor upregulation was observed in striatonigral neurons. In contrast, D2 receptor binding was comparable to controls, and dopamine D1 receptor binding was reduced in the anterior putamen. Enkephalin mRNA widely increased in all animals, but to a greater extent in TD-free animals. These results suggest for the first time that upregulated striatal D3 receptors correlate with TD in nonhuman primates, adding new insights to the dopamine receptor supersensitivity hypothesis. The D3 receptor could provide a novel target for drug intervention in human TD.

Role for the nicotinic cholinergic system in movement disorders; therapeutic implications

A large body of evidence using experimental animal models shows that the nicotinic cholinergic system is involved in the control of movement under physiological conditions. This work raised the question whether dysregulation of this system may contribute to motor dysfunction and whether drugs targeting nicotinic acetylcholine receptors (nAChRs) may be of therapeutic benefit in movement disorders. Accumulating preclinical studies now show that drugs acting at nAChRs improve drug-induced dyskinesias. The general nAChR agonist nicotine, as well as several nAChR agonists (varenicline, ABT-089 and ABT-894), reduces l-dopa-induced abnormal involuntary movements or dyskinesias up to 60% in parkinsonian nonhuman primates and rodents. These dyskinesias are potentially debilitating abnormal involuntary movements that arise as a complication of l-dopa therapy for Parkinson's disease. In addition, nicotine and varenicline decrease antipsychotic-induced abnormal involuntary movements in rodent models of tardive dyskinesia. Antipsychotic-induced dyskinesias frequently arise as a side effect of chronic drug treatment for schizophrenia, psychosis and other psychiatric disorders. Preclinical and clinical studies also show that the nAChR agonist varenicline improves balance and coordination in various ataxias. Lastly, nicotine has been reported to attenuate the dyskinetic symptoms of Tourette's disorder. Several nAChR subtypes appear to be involved in these beneficial effects of nicotine and nAChR drugs including α4β2*, α6β2* and α7 nAChRs (the asterisk indicates the possible presence of other subunits in the receptor). Overall, the above findings, coupled with nicotine's neuroprotective effects, suggest that nAChR drugs have potential for future drug development for movement disorders.

New and emerging treatments for symptomatic tardive dyskinesia

The aim of this review is to assess new, emerging, and experimental treatment options for tardive dyskinesia (TD). The methods to obtain relevant studies for review included a MEDLINE search and a review of studies in English, along with checking reference lists of articles. The leading explanatory models of TD development include dopamine receptor supersensitivity, GABA depletion, cholinergic deficiency, neurotoxicity, oxidative stress, changes in synaptic plasticity, and defective neuroadaptive signaling. As such, a wide range of treatment options are available. To provide a complete summary of choices we review atypical antipsychotics along with resveratrol, botulinum toxin, Ginkgo biloba, tetrabenazine, clonazepam, melatonin, essential fatty acids, zonisamide, levetiracetam, branched-chain amino acids, drug combinations, and invasive surgical treatments. There is currently no US Food and Drug Administration-approved treatment for TD; however, prudent use of atypical antipsychotics with routine monitoring remain the cornerstone of therapy, with experimental treatment options available for further management.

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.

Withdrawal symptoms and rebound syndromes associated with switching and discontinuing atypical antipsychotics: theoretical background and practical recommendations

With the widespread use of atypical or second-generation antipsychotics, switching treatment has become current practice and more complicated, as the pharmacological profiles of these agents differ substantially despite their similarity in being 'atypical'. All share the ability to block dopamine D₂ receptors, and most of them also block serotonin 5-HT2A receptors. Apart from these common features, some atypical antipsychotics are also able to block or stimulate other dopamine or serotonin receptors, as well as histaminergic, muscarinergic or adrenergic receptors. As a result of the varying receptor affinities, in switching or discontinuing compounds several possible pitfalls have to be considered, including the occurrence of withdrawal and rebound syndromes. This article reviews the pharmacological background of functional blockade or stimulation of receptors of interest in regard to atypical antipsychotics and the implicated potential withdrawal and rebound phenomena. A MEDLINE search was carried out to identify information on withdrawal or rebound syndromes occurring after discontinuation of atypical antipsychotics. Using the resulting literature, we first discuss the theoretical background to the functional consequences of atypical antipsychotic-induced blockade or stimulation of neurotransmitter receptors and, secondly, we highlight the clinical consequences of this. We then review the available clinical literature on switching between atypical antipsychotics, with respect to the occurrence of withdrawal or rebound symptoms. Finally, we offer practical recommendations based on the reviewed findings. The systematic evaluation of withdrawal or rebound phenomena using randomized controlled trials is still understudied. Knowledge of pharmacological receptor-binding profiles may help clinicians in choosing adequate switching or discontinuation strategies for each agent. Results from large switching trials indicate that switching atypical antipsychotics can be performed in a safe manner. Treatment-emergent adverse events during or after switching are not always considered to be, at least in part, associated with the pre-switch antipsychotic. Further studies are needed to substantiate the evidence gained so far on different switching strategies. The use of concomitant medication, e.g., benzodiazepines or anticholinergic drugs, may help to minimize symptoms arising from the discontinuation or switching of antipsychotic treatment.

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.

Clozapine-induced locomotor suppression is mediated by 5-HT2A receptors in the forebrain

The need for safer, more effective therapeutics for the treatment of schizophrenia is widely acknowledged. To optimally target novel pharmacotherapies, in addition to establishing the mechanisms responsible for the beneficial effects of antipsychotics, the pathways underlying the most severe side effects must also be elucidated. Here we investigate the role of serotonin 2A (5-HT(2A)), serotonin 2C (5-HT(2C)), and dopamine 2 receptors (D₂) in mediating adverse effects associated with canonical first- and second-generation antipsychotic drugs in mice. Wild-type (WT) and 5-HT(2A) knockout (KO) mice treated with haloperidol, clozapine, and risperidone were assessed for locomotor activity and catalepsy. WT mice showed a marked reduction in locomotor activity following acute administration of haloperidol and high-dose risperidone, which was most likely secondary to the severe catalepsy caused by these compounds. Clozapine also dramatically reduced locomotor activity, but in the absence of catalepsy. Interestingly, 5-HT(2A) KO mice were cataleptic following haloperidol and risperidone, but did not respond to clozapine's locomotor-suppressing effects. Restoration of 5-HT(2A) expression to cortical glutamatergic neurons re-instated the locomotor-suppressing effects of clozapine in the open field. In sum, we confirm that haloperidol and risperidone caused catalepsy in rodents, driven by strong antagonism of D₂. We also demonstrate that clozapine decreases locomotor activity in a 5-HT(2A)-dependent manner, in the absence of catalepsy. Moreover, we show that it is the cortical population of 5-HT(2A) that mediate the locomotor-suppressing effects of clozapine.

Contribution of decreased serotonin release to the antidyskinetic effects of deep brain stimulation in a rodent model of tardive dyskinesia: comparison of the subthalamic and entopeduncular nuclei

Mechanisms whereby deep brain stimulation (DBS) of the subthalamic nucleus (STN) or internal globus pallidus (GPi) reduces dyskinesias remain largely unknown. Using vacuous chewing movements (VCMs) induced by chronic haloperidol as a model of tardive dyskinesia (TD) in rats, we confirmed the antidyskinetic effects of DBS applied to the STN or entopeduncular nucleus (EPN, the rodent homolog of the GPi). We conducted a series of experiments to investigate the role of serotonin (5-HT) in these effects. We found that neurotoxic lesions of the dorsal raphe nuclei (DRN) significantly decreased HAL-induced VCMs. Acute 8-OH-DPAT administration, under conditions known to suppress raphe neuronal firing, also reduced VCMs. Immediate early gene mapping using zif268 in situ hybridization revealed that STN-DBS inhibited activity of DRN and MRN neurons. Microdialysis experiments indicated that STN-DBS decreased 5-HT release in the dorsolateral caudate-putamen, an area implicated in the etiology of HAL-induced VCMs. DBS applied to the EPN also suppressed VCMs but did not alter 5-HT release or raphe neuron activation. While these findings suggested a role for decreased 5-HT release in the mechanisms of STN DBS, further microdialysis experiments showed that when the 5-HT lowering effects of STN DBS were prevented by pretreatment with fluoxetine or fenfluramine, the ability of DBS to suppress VCMs remained unaltered. These results suggest that EPN- and STN-DBS have different effects on the 5-HT system. While decreasing 5-HT function is sufficient to suppress HAL-induced VCMs, 5-HT decrease is not necessary for the beneficial motor effects of DBS in this model.

Parallels between behavioral and neurochemical variability in the rat vacuous chewing movement model of tardive dyskinesia

The widely accepted rat vacuous chewing movement model for tardive dyskinesia could be more fully mined through greater focus on individual variability in vulnerability to this neuroleptic-induced behavior. We have examined parallels between behavioral and neurobiological variability within a cohort in order to evaluate the role that neurobiological factors might play in determining susceptibility to tardive dyskinesia. Inter-observer reliability and individual consistency across time, in both spontaneous and neuroleptic-induced vacuous chewing movements, were empirically demonstrated. While this behavior increased across 8 months of observation in both vehicle controls and haloperidol-treated rats, pre-treatment baselines were predictive of final levels across individuals only in the vehicle control group, not the haloperidol-treated group. Haloperidol-induced elevations in neostriatal D2 and GAD(67) mRNA were not correlated with individual variability in haloperidol-induced vacuous chewing movements. Ambient noise during the observations was found to exacerbate chronic haloperidol-induced, but not spontaneous vacuous chewing movements. Significant correlations were found among the haloperidol-treated rats between nigral and tegmental GAD(67) and tegmental α7 mRNA levels, measured by in situ hybridization histochemistry, and vacuous chewing movements, specifically in the noisy conditions. Variability in these secondary responses to primary striatal dopamine and GABA perturbations may play a role in determining vulnerability to vacuous chewing movements, and by analogy, tardive dyskinesia. Both the differential predictive value of baseline vacuous chewing movements and the differential effect of noise, between controls and haloperidol-treated rats, add to evidence that haloperidol-induced vacuous chewing movements are regulated, in part, by different mechanisms than those mediating spontaneous vacuous chewing movements.

Serotonergic mechanisms as targets for existing and novel antipsychotics

A variety of serotonin (5-HT) receptors, especially 5-HT(2A), 5-HT(1A), 5-HT(6), 5-HT(7), and 5-HT(2C), have been postulated to contribute to the mechanism of action of atypical antipsychotic drugs (APDs), i.e., APDs which cause fewer extrapyramidal side effects (EPS) at clinically optimal doses, in contrast with typical APDs, which are more likely to cause EPS. This advantage, rarely disputed, has made such drugs the preferred treatment for schizophrenia and other indications for APDs. These 5-HT receptors are still of interest as components of novel multireceptor or stand-alone APDs, and potentially to remediate cognitive deficits in schizophrenia. Almost all currently available atypical APDs are 5-HT(2A) receptor inverse agonists, as well as dopamine (DA) D(2) receptor antagonists or partial agonists. Amisulpride, an exceptional atypical APD, has 5-HT(7) antagonism to complement its DA D(2/3) antagonism. Some atypical APDs are also 5-HT(1A) partial agonists, 5-HT(6), or 5-HT(7) antagonists, or some combination of the above. 5-HT(2C) antagonism has been found to contribute to the metabolic side effects of some atypical APDs, whereas 5-HT(2C) agonists have potential as stand-alone APDs and/or cognitive enhancers. This review will provide an update of current preclinical and clinical evidence for the role of these five 5-HT receptors in the actions of current APDs and for the development of novel psychotropic drugs.

Receptor mechanisms of antipsychotic drug action in bipolar disorder - focus on asenapine

The atypical antipsychotic drugs are considered a first-line treatment for mania in bipolar disorder with many having a proven superiority to the classical mood stabilisers. This review addresses the pharmacological mechanisms underlying this therapeutic efficacy, as well as those mechanisms considered responsible for the adverse effects of antipsychotic drugs, with a particular focus on the recently introduced asenapine. The high efficacy in bipolar mania of haloperidol, a relatively selective dopamine D2-like receptor antagonist, indicates that the one common receptor mechanism underlying antipsychotic effects on mania is antagonism at the D2 receptor. Serotonin receptors are implicated in antidepressant response, and relief of depressed mood in mixed states is likely to involve drug effects at one, or more likely several interacting, serotonin receptors. Asenapine shows a unique breadth of action at these sites, with potential effects at clinical doses at 5HT1A, 1B, 2A, 2C, 6 and 7 receptors. Antagonism at alpha2 adrenoceptors may also be involved. Adverse effects include those classically associated with dopamine D2 receptor blockade, the extrapyramidal side effects (EPS), and which are relatively diminished in the atypical (in comparison with the conventional) antipsychotics. A variety of protective mechanisms against EPS associated with different drugs include low D2 affinity, D2 partial agonism, high 5-HT2A and 2C antagonism. Similar effects at the D2 and 5-HT2C receptors may underlie the low propensity for hyperprolactinaemia of the atypicals, although the strong prolactin-elevating effect of risperidone reflects its relatively high blood/brain concentration ratio, a consequence of it being a substrate for the p-glycoprotein pump. Weight gain is a further concern of antipsychotic treatment of bipolar disorder which is particularly severe with olanzapine. Histamine H1, alpha1 adrenergic and particularly 5-HT2C receptors are implicated in this effect, although the lower propensity for weight gain shown by asenapine which, like olanzapine, binds to these receptors, indicates that other protective receptor mechanisms, or subtle differences in the 5-HT2C receptor-mediated effects, may be important. Of other peripheral and central effects, the pharmacological basis of sedation (H1 receptors) and postural hypotension (alpha1 adrenoceptors) are rather better understood. The relative benefits of atypical antipsychotics like asenapine can be understood from their receptor pharmacology, and this understanding is key to the future development of improved treatment for bipolar disorder.

The role of serotonin receptors in the action of atypical antipsychotic drugs

The main class of atypical antipsychotic drugs (APDs) in current use includes the protypical atypical APD, clozapine, as well as aripiprazole, asenapine, iloperidone, lurasidone, olanzapine, quetiapine, risperidone, and ziprasidone. At clinically effective doses, these agents produce extensive blockade of serotonin (5-HT)(2A) receptors, direct or indirect stimulation of 5-HT(1A) receptors, and to a lesser extent, reduction in dopamine (DA) D(2) receptor-mediated neurotransmission. This contrasts with typical APDs, for example haloperidol and perphenazine, which are mainly DA D(2/)D(3) receptor antagonists and have weaker, if any, potency as 5-HT(2A) receptor antagonists. Some, but not all, atypical APDs are also effective 5-HT(2C) receptor inverse agonists or neutral antagonists, 5-HT(6) or 5-HT(7) receptor antagonists. This diverse action on 5-HT receptors may contribute to significant differences in efficacy and tolerability among the atypical APDs. There is considerable preclinical and some clinical evidence that effects on 5-HT receptors contribute to the low risk of producing extrapyramidal side effects, which is the defining characteristic of an atypical APD, the lack of elevation in plasma prolactin levels (with risperidone and 9-hydroxyrisperidone being exceptions), antipsychotic action, and ability to improve some domains of cognition in patients with schizophrenia. The serotonergic actions of the atypical APDs, especially 5-HT(2A) receptor antagonism, are particularly important to the differential effects of typical and atypical APDs to overcome the effects of acute or subchronic administration of N-methyl-d-aspartate (NMDA) receptor antagonists, such as phencyclidine, ketamine, and dizocipline (MK-801). 5-HT(1A) receptor stimulation and 5-HT(6) and 5-HT(7) receptor antagonism may contribute to beneficial effects of these agents on cognition. In particular, 5-HT(7) receptor antagonism may be the basis for the pro-cognitive effects of the atypical APD, amisulpride, a D(2)/D(3) receptor antagonist, which has no effect on other 5-HT receptor. 5-HT(2C) receptor antagonism appears to contribute to the weight gain produced by some atypical APDs and may also affect cognition and psychosis via its influence on cortical and limbic dopaminergic activity.