Stable expression of constitutively activated mutant h5HT6 and h5HT7 serotonin receptors: inverse agonist activity of antipsychotic drugs

β-arrestin biased signaling is not involved in the hypotensive actions of 5-HT7 receptor stimulation: use of Serodolin

The 5-hydroxytryptamine 7 receptor (5-HT7) is necessary for 5-HT to cause a concentration-dependent vascular relaxation and hypotension. 5-HT7 is recognized as having biased signaling, transduced through either Gs or β -arrestin. It is unknown whether 5-HT7 signals in a biased manner to cause vasorelaxation/hypotension. We used the recently described β-arrestin selective 5-HT7 receptor agonist serodolin to test the hypothesis that 5-HT7 activation does not cause vascular relaxation or hypotension via the β -arrestin pathway. Isolated abdominal aorta (no functional 5-HT7) and vena cava (functional 5-HT7) from male Sprague Dawley rats were used in isometric contractility studies. Serodolin (1 nM - 10 μM) did not change baseline tone of isolated tissues and did not relax the endothelin-1 (ET-1)-contracted vena cava or aorta. In the aorta, serodolin acted as a 5-HT2A receptor antagonist, evidenced by a rightward shift in 5-HT-induced concentration response curve [pEC50 5-HT [M]: Veh = 5.2 +/- 0.15; Ser (100 nM) = 4.49 +/- 0.08; p < 0.05]. In the vena cava, serodolin acted as a 5-HT7 receptor antagonist, shifting the concentration response curve to 5-HT left and upward (%10 μM NE contraction; Veh = 3.2 +/- 1.7; Ser (10 nM) = 58 +/- 11; p < 0.05) and blocking relaxation of pre-contracted tissue to the 5-HT1A/7 agonist 5-carboxamidotryptamine. In anesthetized rats, 5-HT or serodolin was infused at 5, 25 and 75 μg/kg/min, iv. Though 5-HT caused concentration-dependent depressor responses, serodolin caused an insignificant small depressor responses at all three infusion rates. With the final dose of serodolin on board, 5-HT was unable to reduce blood pressure. Collectively the data indicate that serodolin functions as a 5-HT7 antagonist with additional 5-HT2A blocking properties. 5-HT7 activation does not cause vascular relaxation or hypotension via the β -arrestin pathway.

Structure, function and drug discovery of GPCR signaling

G protein-coupled receptors (GPCRs) are versatile and vital proteins involved in a wide array of physiological processes and responses, such as sensory perception (e.g., vision, taste, and smell), immune response, hormone regulation, and neurotransmission. Their diverse and essential roles in the body make them a significant focus for pharmaceutical research and drug development. Currently, approximately 35% of marketed drugs directly target GPCRs, underscoring their prominence as therapeutic targets. Recent advances in structural biology have substantially deepened our understanding of GPCR activation mechanisms and interactions with G-protein and arrestin signaling pathways. This review offers an in-depth exploration of both traditional and recent methods in GPCR structure analysis. It presents structure-based insights into ligand recognition and receptor activation mechanisms and delves deeper into the mechanisms of canonical and noncanonical signaling pathways downstream of GPCRs. Furthermore, it highlights recent advancements in GPCR-related drug discovery and development. Particular emphasis is placed on GPCR selective drugs, allosteric and biased signaling, polyphamarcology, and antibody drugs. Our goal is to provide researchers with a thorough and updated understanding of GPCR structure determination, signaling pathway investigation, and drug development. This foundation aims to propel forward-thinking therapeutic approaches that target GPCRs, drawing upon the latest insights into GPCR ligand selectivity, activation, and biased signaling mechanisms.

In Vitro and In Vivo Evaluation of Pellotine: A Hypnotic Lophophora Alkaloid

Quality of life is often reduced in patients with sleep-wake disorders. Insomnia is commonly treated with benzodiazepines, despite their well-known side effects. Pellotine (1), a Lophophora alkaloid, has been reported to have short-acting sleep-inducing properties in humans. In this study, we set out to evaluate various in vitro and in vivo properties of 1. We demonstrate that 1 undergoes slow metabolism; e.g. in mouse liver microsomes 65% remained, and in human liver microsomes virtually no metabolism was observed after 4 h. In mouse liver microsomes, two phase I metabolites were identified: 7-desmethylpellotine and pellotine-N-oxide. In mice, the two diastereomers of pellotine-O-glucuronide were additionally identified as phase II metabolites. Furthermore, we demonstrated by DESI-MSI that 1 readily enters the central nervous system of rodents. Furthermore, radioligand-displacement assays showed that 1 is selective for the serotonergic system and in particular the serotonin (5-HT)1D, 5-HT6, and 5-HT7 receptors, where it binds with affinities in the nanomolar range (117, 170, and 394 nM, respectively). Additionally, 1 was functionally characterized at 5-HT6 and 5-HT7, where it was found to be an agonist at the former (EC50 = 94 nM, Emax = 32%) and an inverse agonist at the latter (EC50 = 291 nM, Emax = -98.6). Finally, we demonstrated that 1 dose-dependently decreases locomotion in mice, inhibits REM sleep, and promotes sleep fragmentation. Thus, we suggest that pellotine itself, and not an active metabolite, is responsible for the hypnotic effects and that these effects are possibly mediated through modulation of serotonergic receptors.

Is the 5-hydroxytryptamine 7 Receptor Constitutively Active in the Vasculature? A Study in Veins/Vein

ABSTRACT

The 5-hydroxytryptamine 7 (5-HT 7 ) receptor is reported to have considerable constitutive activity when transfected into cells. Constitutive activity-receptor activity in the absence of known agonist-is important for understanding the contributions of a receptor to (patho)physiology. We test the hypothesis that the 5-HT 7 receptor possesses constitutive activity in a physiological situation. Isolated veins from male and female Sprague Dawley rats were used as models for measuring isometric force; the abdominal vena cava possesses a functional 5-HT 7 receptor that mediates relaxation, whereas the small mesenteric vein does not. Compounds reported to act as inverse agonists were investigated for their ability to cause contraction (moving a constitutively active relaxant receptor to an inactive state, removing relaxation). Compared with a vehicle control, clozapine, risperidone, ketanserin, and SB269970 caused no contraction in the isolated male abdominal vena cava. By contrast, methiothepin caused a concentration-dependent contraction of the male but not female abdominal vena cava, although with low potency (-log EC 50 [M] = 5.50 ± 0.45) and efficacy (∼12% of contraction to endothelin-1). Methiothepin-induced contraction was not reduced by the 5-HT 7 receptor antagonist (SB269970, 1 μM, not active in the vena cava). These same compounds showed little to no effect in the isolated mesenteric vein. We conclude that the 5-HT 7 receptor in the isolated veins of the Sprague Dawley rat does not possess constitutive activity. We raise the question of the physiological relevance of constitutive activity of this receptor important to such diverse physiological functions as sleep, circadian rhythm, temperature, and blood pressure regulation.

Dopamine and Dopamine-Related Ligands Can Bind Not Only to Dopamine Receptors

The dopaminergic system is one of the most important neurotransmitter systems in the central nervous system (CNS). It acts mainly by activation of the D₁-like receptor family at the target cell. Additionally, fine-tuning of the signal is achieved via pre-synaptic modulation by the D₂-like receptor family. Some dopamine drugs (both agonists and antagonists) bind in addition to DRs also to α₂-ARs and 5-HT receptors. Unfortunately, these compounds are often considered subtype(s) specific. Thus, it is important to consider the presence of these receptor subtypes in specific CNS areas as the function virtually elicited by one receptor type could be an effect of other—or the co-effect of multiple receptors. However, there are enough molecules with adequate specificity. In this review, we want to give an overview of the most common off-targets for established dopamine receptor ligands. To give an overall picture, we included a discussion on subtype selectivity. Molecules used as antipsychotic drugs are reviewed too. Therefore, we will summarize reported affinities and give an outline of molecules sufficiently specific for one or more subtypes (i.e., for subfamily), the presence of DR, α₂-ARs, and 5-HT receptors in CNS areas, which could help avoid ambiguous results.

5-HT7 Receptor Restrains 5-HT-induced 5-HT2A Mediated Contraction in the Isolated Abdominal Vena Cava

ABSTRACT

Although discovered as a vasoconstrictor, 5-hydroxytryptamine (5-HT, serotonin) infused into man and rodent reduces blood pressure. This occurs primarily through activation of 5-HT7 receptors and, at least in part, venodilation. Vascular mechanisms by which this could occur include direct receptor activation leading to vasodilation and/or suppression of contractile 5-HT receptor activation. This study tests the hypothesis that the 5-HT7 receptor restrains activation of the 5-HT2A receptor. A subhypothesis is whether agonist-induced activation-independent of constitutive activity-of the 5-HT7 receptor is necessary for this restraint. The isolated abdominal aorta and vena cava from the normal male Sprague-Dawley rat was our model. Studies used real-time PCR and a pharmacological approach in the isolated tissue bath for measurement of isometric tone. Although 5-HT2A receptor mRNA expression in both aorta and vena cava was significantly larger than that of the 5-HT7 receptor mRNA, the 5-HT7/5-HT2A receptor mRNA ratio was greater in the vena cava (0.30) than in the aorta (0.067). 5-HT7 receptor antagonism by SB266970 and DR 4458 increased maximum contraction to 5-HT in the isolated vein by over 50% versus control. The 5-HT2A receptor agonists TCB-2 and NBOH were more potent in the aorta compared with 5-HT but less efficacious, serving as partial agonists. By contrast, these same three agonists caused no contraction in the vena cava isolated from the same rats up to 10 μM agonist. Antagonism of the 5-HT7 receptor by SB269970 did not increase either the potency or efficacy of TCB-2 or NBOH. These data support that the 5-HT7 receptor itself needs to be stimulated to reduce contraction and suggest there is little constitutive activity of the 5-HT7 receptor in the isolate abdominal vena cava.

Acute and repeated treatment with the 5-HT7 receptor antagonist SB 269970 induces functional desensitization of 5-HT7 receptors in rat hippocampus

BACKGROUND

SB 269970, a 5-HT(7) receptor antagonist may produce a faster antidepressant-like effect in animal models, than do antidepressant drugs, e.g., imipramine. The present work was aimed at examining the effect of single and repeated (14 days) administration of SB 269970 on the 5-HT(7) receptor in the hippocampus.

METHODS

The reactivity of 5-HT(7) receptors was determined using 5-carboxamidotryptamine (5-CT), which increased the bursting frequency of spontaneous epileptiform activity in hippocampal slices. Additionally, the effects of SB 269970 administration on the affinity and density of 5-HT(7) receptors were investigated using [(3)H]-SB 269970 and the influence of SB 269970 and imipramine on mRNA expression levels of Gα(s) and Gα(12) mRNA were studied using RT-qPCR.

RESULTS

Acute and repeated treatment with SB 269970 led to attenuation of the excitatory effects of activation of 5-HT(7) receptors. Neither single nor repeated administration of SB 269970 changed the mean affinity of 5-HT(7) receptors for [(3)H]-SB 269970. Repeated, but not single, administration of SB 269970 decreased the maximum density of [(3)H]-SB 269970 binding sites. While administration of imipramine did not change the expression of mRNAs for Gα(s) and Gα(12) proteins after both single and repeated administration of SB 269970, a reduction in Gα(s) and Gα(12) mRNA expression levels was evident.

CONCLUSIONS

These findings indicate that even single administration of SB269970 induces functional desensitization of the 5-HT(7) receptor system, which precedes changes in the receptor density. This mechanism may be responsible for the rapid antidepressant-like effect of the 5-HT(7) antagonist in animal models.

Inverse agonism and its therapeutic significance

A large number of G-protein-coupled receptors (GPCRs) show varying degrees of basal or constitutive activity. This constitutive activity is usually minimal in natural receptors but is markedly observed in wild type and mutated (naturally or induced) receptors. According to conventional two-state drug receptor interaction model, binding of a ligand may initiate activity (agonist with varying degrees of positive intrinsic activity) or prevent the effect of an agonist (antagonist with zero intrinsic activity). Inverse agonists bind with the constitutively active receptors, stabilize them, and thus reduce the activity (negative intrinsic activity). Receptors of many classes (α-and β-adrenergic, histaminergic, GABAergic, serotoninergic, opiate, and angiotensin receptors) have shown basal activity in suitable in vitro models. Several drugs that have been conventionally classified as antagonists (β-blockers, antihistaminics) have shown inverse agonist effects on corresponding constitutively active receptors. Nearly all H₁ and H₂ antihistaminics (antagonists) have been shown to be inverse agonists. Among the β-blockers, carvedilol and bucindolol demonstrate low level of inverse agonism as compared to propranolol and nadolol. Several antipsychotic drugs (D₂ receptors antagonist), antihypertensive (AT₁ receptor antagonists), antiserotoninergic drugs and opioid antagonists have significant inverse agonistic activity that contributes partly or wholly to their therapeutic value. Inverse agonism may also help explain the underlying mechanism of beneficial effects of carvedilol in congestive failure, naloxone-induced withdrawal syndrome in opioid dependence, clozapine in psychosis, and candesartan in cardiac hypertrophy. Understanding inverse agonisms has paved a way for newer drug development. It is now possible to develop agents, which have only desired therapeutic value and are devoid of unwanted adverse effect. Pimavanserin (ACP-103), a highly selective 5-HT_2A inverse agonist, attenuates psychosis in patients with Parkinson's disease with psychosis and is devoid of extrapyramidal side effects. This dissociation is also evident from the development of anxioselective benzodiazepines devoid of habit-forming potential. Hemopressin is a peptide ligand that acts as an antagonist as well as inverse agonist. This agent acts as an antinociceptive agent in different in vivo models of pain. Treatment of obesity by drugs having inverse agonist activity at CB_1/2 receptors is also underway. An exciting development is evaluation of β-blockers in chronic bronchial asthma—a condition akin to congestive heart failure where β-blockade has become the standard mode of therapy. Synthesis and evaluation of selective agents is underway. Therefore, inverse agonism is an important aspect of drug–receptor interaction and has immense untapped therapeutic potential.

Antagonist interaction with the human 5-HT(7) receptor mediates the rapid and potent inhibition of non-G-protein-stimulated adenylate cyclase activity: a novel GPCR effect

BACKGROUND AND PURPOSE

The human 5-hydroxytryptamine(7) (h5-HT(7)) receptor is G(s) -coupled and stimulates the production of the intracellular signalling molecule cAMP. Previously, we reported a novel property of the h5-HT(7) receptor: pseudo-irreversible antagonists irreversibly inhibit forskolin-stimulated (non-receptor-mediated) cAMP production. Herein, we sought to determine if competitive antagonists also affect forskolin-stimulated activity and if this effect is common among other G(s) -coupled receptors.

EXPERIMENTAL APPROACH

Recombinant cell lines expressing h5-HT(7) receptors or other receptors of interest were briefly exposed to antagonists; cAMP production was then stimulated by forskolin and quantified by an immunocompetitive assay.

KEY RESULTS

In human embryonic kidney 293 cells stably expressing h5-HT(7) receptors, all competitive antagonists inhibited nearly 100% of forskolin-stimulated cAMP production. This effect was insensitive to pertussis toxin, that is, not G(i/o) -mediated. Potency to inhibit forskolin-stimulated activity strongly correlated with h5-HT(7) binding affinity (r(2) = 0.91), indicating that the antagonists acted through h5-HT(7) receptors to inhibit forskolin. Potency and maximal effects of clozapine, a prototypical competitive h5-HT(7) antagonist, were unaffected by varying forskolin concentration. Antagonist interaction with h5-HT(6), human β(1), β(2), and β(3) adrenoceptors did not inhibit forskolin's activity.

CONCLUSIONS AND IMPLICATIONS

The inhibition of adenylate cyclase, as measured by forskolin's activity, is an underlying property of antagonist interaction with h5-HT(7) receptors; however, this is not a common property of other G(s) -coupled receptors. This phenomenon may be involved in the roles played by h5-HT(7) receptors in human physiology. Development of h5-HT(7) antagonists that do not elicit this effect would aid in the elucidation of its mechanisms and shed light on its possible physiological relevance.

5-hydroxtryptamine receptors in systemic hypertension: an arterial focus

BACKGROUND

Serotonin (5-hydroxytryptamine [5-HT]) was named for its isolation from blood serum (sero-) and ability to contract smooth muscle (-tonin). Thus, its relationship with the cardiovascular system began with its discovery.

AIMS

This review will focus on the effects of 5-HT and its receptors in the vasculature, with a focus on their involvement in high blood pressure (hypertension). Two seemingly contradictory bodies of evidence exist that make it difficult to assign any one function to 5-HT in vascular control of blood pressure.

RESULTS

 In vitro, 5-HT is an established vasoconstrictor, the effects of which are amplified in hypertension. By contrast, 5-HT (or its precursor 5-hydroxytryptophan) lowers blood pressure when given chronically in vivo. We will discuss ideas that might help us understand these differences, discuss relatively new pharmacology parameters (e.g. biased, inverse agonism) as they pertain to 5-HT receptors, and pose questions that are vital to answer so as to understand the role played by 5-HT in control of blood pressure, especially as it pertains to vascular function.

CONCLUSIONS

Our goal is to understand if the actions of 5-HT in hypertension are physiologically and clinically relevant. The community understands 5-HT has complex cardiovascular effects, and clinical studies have proven equivocal in terms of the involvement of 5-HT. This article provides a balanced view of evidence/literature that illustrates involvement of 5-HT in hypertension as controversial. It contributes new pharmacological knowledge of 5-HT compounds, and poses timely questions as to how this field can move forward. The take home message is that the cardiovascular effects of 5-HT are markedly complex such that we have not yet answered the question of whether 5-HT is beneficial or detrimental to hypertension.

Clozapine and other competitive antagonists reactivate risperidone-inactivated h5-HT7 receptors: radioligand binding and functional evidence for GPCR homodimer protomer interactions

RATIONALE

The h5-HT(7) receptor is subject to inactivation by risperidone and 9-OH-risperidone, apparently through a pseudo-irreversible complex formed between these drugs and the receptor. Although risperidone and 9-OH-risperidone ("inactivating antagonists") completely inactivate the receptor, only 50% of the receptors form a pseudo-irreversible complex with these drugs.

OBJECTIVES

This study aims to more fully determine the mechanism(s) responsible for the novel effects of risperidone and 9-OH-risperidone and to determine if the inactivation can be reversed (reactivation).

METHODS

The ability of non-inactivating drugs (competitive antagonists) to dissociate wash-resistant [(3)H]risperidone binding from h5-HT(7) receptors was investigated. Also, the ability of non-inactivating drugs to reactivate inactivated h5-HT(7) receptors was investigated, using cAMP accumulation as a functional endpoint.

RESULTS

The competitive (non-inactivating) antagonists clozapine and mesulergine released the wash-resistant [(3)H]risperidone binding to the h5-HT(7) receptor. The competitive antagonists clozapine, SB269970, mianserin, cyproheptadine, mesulergine, and ICI169369 reactivated the risperidone-inactivated h5-HT(7) receptors in a concentration-dependent manner. The potencies for reactivation closely match the affinities of these drugs for the h5-HT(7) receptor (r(2) = 0.95), indicating that the reactivating antagonists are binding to and producing their effects through the orthosteric binding site of the h5-HT(7) receptor. Bioluminescence resonance energy transfer analyses indicate that the h5-HT(7) receptor forms homodimers.

CONCLUSIONS

The ability of the non-inactivating drugs to bind h5-HT(7) orthosteric sites and reverse the wash-resistant effects of risperidone or 9-OH-risperidone, also bound to h5-HT(7) orthosteric sites, is evidence for protomer-protomer interactions between h5-HT(7) homodimers. This is the first demonstration of a non-mutated G-protein-coupled receptor homodimer engaging in protomer-protomer interactions in an intact cell preparation.

Pharmacological analysis of the novel, rapid, and potent inactivation of the human 5-Hydroxytryptamine7 receptor by risperidone, 9-OH-Risperidone, and other inactivating antagonists

In a previous publication, using human 5-hydroxytryptamine(7) (h5-HT(7)) receptor-expressing human embryonic kidney (HEK) 293 cells, we reported the rapid, potent inactivation of the h5-HT(7) receptor stimulation of cAMP production by three antagonists: risperidone, 9-OH-risperidone, and methiothepin (Smith et al., 2006). To better understand the drug-receptor interaction producing the inactivation, we 1) expanded the list of inactivating drugs, 2) determined the inactivating potencies and efficacies by performing concentration-response experiments, and 3) determined the potencies and efficacies of the inactivators as irreversible binding site inhibitors. Three new drugs were found to fully inactivate the h5-HT(7) receptor: lisuride, bromocryptine, and metergoline. As inactivators, these drugs displayed potencies of 1, 80, and 321 nM, respectively. Pretreatment of 5-HT(7)-expressing HEK cells with increasing concentrations of the inactivating drugs risperidone, 9-OH-risperidone, methiothepin, lisuride, bromocriptine, and metergoline potently inhibited radiolabeling of the h5-HT(7) receptor, with IC(50) values of 9, 5.5, 152, 3, 73, and 10 nM, respectively. We were surprised to find that maximal concentrations of risperidone and 9-OH-risperidone inhibited only 50% of the radiolabeling of h5-HT(7) receptors. These results indicate that risperidone and 9-OH risperidone may be producing 5-HT(7) receptor inactivation by different mechanisms than lisuride, bromocryptine, metergoline, and methiothepin. These results are not interpretable using the conventional model of G-protein-coupled receptor function. The complex seems capable of assuming a stable inactive conformation as a result of the interaction of certain antagonists. The rapid, potent inactivation of the receptor-G-protein complex by antagonists implies a constitutive, pre-existing complex between the h5-HT(7) receptor and a G-protein.

Selective 5-HT6 receptor ligands: progress in the development of a novel pharmacological approach to the treatment of obesity and related metabolic disorders

The increasing global prevalence of obesity unequivocally demonstrates that neither behavioural (diet and exercise) nor pharmacological approaches to this health problem are working. In this area of high unmet clinical need, the 5-HT6 receptor has generated enormous interest amongst academic and pharmaceutical industry scientists as a molecular target for the development of a new generation of safe and more effective anti-obesity drugs. In this review, we have described the major developments that have occurred in the fields of the medicinal chemistry and pharmacology of 5-HT6 ligands, with particular emphasis on their potential application as novel anti-obesity drugs. The last 5 years have witnessed an increasing understanding of the 5-HT6 receptor and its structural requirements that has produced an explosion in the number and diversity of novel, highly selective 5-HT6 receptor agonists, partial agonists and antagonists that have been designed and synthesized. In animal models, 5-HT6 receptor ligands of all functional types have been shown to decrease food intake when given acutely and chronically, to evoke profound and sustained weight-loss in obese animals, and concomitantly to improve a number of cardio-metabolic risk factors. Comparator studies in obese animal models, which are highly predictive of clinical outcomes, indicate that 5-HT6 ligands may have the potential to be more efficacious in the treatment of obesity than the current generation of anti-obesity drugs.

Differential profile of typical, atypical and third generation antipsychotics at human 5-HT7a receptors coupled to adenylyl cyclase: detection of agonist and inverse agonist properties

5-HT(7) receptors are present in thalamus and limbic structures, and a possible role of these receptors in the pathology of schizophrenia has been evoked. In this study, we examined binding affinity and agonist/antagonist/inverse agonist properties at these receptors of a large series of antipsychotics, i.e., typical, atypical, and third generation compounds preferentially targeting D(2) and 5-HT(1A) sites. Adenylyl cyclase (AC) activity was measured in HEK293 cells stably expressing the human (h) 5-HT(7a) receptor isoform. 5-HT and 5-CT increased cyclic adenosine monophosphate level by about 20-fold whereas (+)-8-OH-DPAT, the antidyskinetic agent sarizotan, and the novel antipsychotic compound bifeprunox exhibited partial agonist properties at h5-HT(7a) receptors stimulating AC. Other compounds antagonized 5-HT-induced AC activity with pK (B) values which correlated with their pK (i) as determined by competition binding vs [(3)H]5-CT. The selective 5-HT(7) receptor ligand, SB269970, was the most potent antagonist. For antipsychotic compounds, the following rank order of antagonism potency (pK (B)) was ziprasidone > tiospirone > SSR181507 > or = clozapine > or = olanzapine > SLV-314 > SLV-313 > or = aripiprazole > or = chlorpromazine > nemonapride > haloperidol. Interestingly, pretreatment of HEK293-h5-HT(7a) cells with forskolin enhanced basal AC activity and revealed inverse agonist properties for both typical and atypical antipsychotics as well as for aripiprazole. In contrast, other novel antipsychotics exhibited diverse 5-HT(7a) properties; SLV-313 and SLV-314 behaved as quasi-neutral antagonists, SSR181507 acted as an inverse agonist, and bifeprunox as a partial agonist, as mentioned above. In conclusion, the differential properties of third generation antipsychotics at 5-HT(7) receptors may influence their antipsychotic profile.

Targeting the dopamine D2 receptor in schizophrenia

After a 12-year search for the antipsychotic receptor, the binding site was discovered and labelled by [3H]haloperidol in 1975. Of the various neurotransmitters, dopamine was the most potent in inhibiting the binding of [3H]haloperidol, indicating that the antipsychotic receptor was a dopamine receptor, now named the dopamine D2 receptor, a major targeting site in schizophrenia. All antipsychotic drugs, including traditional and newer antipsychotics, either bind to D2 in direct relation to their clinical potencies or hinder normal dopamine neurotransmission, as in the case of partial dopamine agonists. In fact, the antipsychotic concentrations found in the plasma water of treated patients closely match the predicted therapeutic absolute concentrations, adjusted for the 60-75% D2 occupancy needed for clinical efficacy. Antipsychotics that elicit low or no Parkinsonism or prolactinaemia are loosely attached to D2 and rapidly dissociate from D2, whereas those eliciting Parkinsonism stay tightly attached to D2 for many hours. Because animal models of psychosis (amfetamine sensitisation, brain lesions) all show a marked elevation in the number of high-affinity states of D2, the antipsychotics are thought to specifically target these D2High states in psychosis in general and schizophrenia in particular.

Reanalysis of constitutively active rat and human 5-HT7(a) receptors in HEK-293F cells demonstrates lack of silent properties for reported neutral antagonists

The present study reinvestigated a series of 5-HT receptor antagonists at both constitutively active rat and human 5-HT7(a) receptors in HEK-293F cells using the cAMP signalling pathway as a functional read-out. Both rat and human 5-HT7(a) receptors were expressed in similar amounts ([3H]-LSD binding: 1.0 to 1.1 pmol/mg protein). Attenuation of basal cAMP formation by the inverse agonist SB-691673 (1 microM) was slightly larger by the human 5-HT7(a) (-73+/-3 %) than rat 5-HT7(a) receptor (-62+/-3 %). The 5-HT receptor antagonists investigated here displayed systematically inverse agonism. While methiothepin and SB-269970 displayed similar negative intrinsic activity to SB-691673 at the rat 5-HT7(a) receptor, the compounds SB-258719, mesulergine and metergoline displayed some lower negative intrinsic activity (between -38 and -49%). Inverse agonist properties were observed with potencies fitting with their respective binding pIC50 values and pKB values as estimated from antagonist studies with 5-HT. With the exception of SB-258719 and mesulergine, which remained a partial inverse agonist at the human 5-HT7(a) receptor, the other compounds behaved with a similar Emax value to the full inverse agonist SB-691673. In conclusion, none of the 5-HT receptor antagonists investigated displayed silent properties at the rat or human 5-HT7(a) receptor, when these are expressed in a system allowing detection of constitutive activity. They appear to be partial to full inverse agonists, further illustrating that an antagonist is preferentially an inverse agonist when investigated under constitutively active receptor conditions.

Risperidone irreversibly binds to and inactivates the h5-HT7 serotonin receptor

Risperidone displays a novel mechanism of antagonism of the h5-HT7 receptor. Pretreatment of the cells with 5 or 20 nM risperidone, followed by removal of the drug from the media, renders the 5-HT7 receptors unresponsive to 10 microM 5-HT for at least 24 h. Thus, risperidone seems to be producing a rapid, long-lasting inactivation of the h5-HT7 receptor. Whole-cell radioligand binding studies indicate that risperidone interacts in an irreversible or pseudo-irreversible manner with the h5-HT7 receptor, thus producing the inactivation. Internalization of the h5-HT7 receptor was not detected by monitoring green fluorescent protein-labeled fluorescent forms of the h5-HT7 receptor exposed to 20 nM risperidone. Ten other antagonists were tested for h5-HT7-inactivating properties, and only 9-OH-risperidone and methiothepin were found to demonstrate the same anomalous properties as risperidone. These results indicate that the h5-HT7 receptor may possess unique structural features that allow certain drugs to induce a conformation resulting in an irreversible interaction in the intact membrane environment. This may indicate that the h5-HT7 receptor is part of a subfamily of G-protein-coupled receptors (GPCRs) possessing this property or that many GPCRs have the potential to be irreversibly blocked, but only select drugs can induce this effect. At the very least, the possibility that highly prescribed drugs, such as risperidone, are irreversibly antagonizing GPCR function in vivo is noteworthy.

Efficacy of selective 5-HT6 receptor ligands determined by monitoring 5-HT6 receptor-mediated cAMP signaling pathways

1. Two novel selective 5-HT6 receptor ligands E-6801 (6-chloro-N-(3-(2-(dimethylamino)ethyl)-1H-indol-5-yl)imidazo[2,1-b]thiazole-5-sulfonamide) and E-6837 (5-chloro-N-(3-(2-(dimethylamino)ethyl)-1H-indol-5-yl)naphthalene-2-sulfonamide) were investigated and compared to the putative 5-HT6 receptor antagonists SB-271046 (5-chloro-N-(4-methoxy-3-(piperazin-1-yl)phenyl)-3-methylbenzo[b]thiophene-2-sulfonamide) and Ro 04-06790 (N-(2,6-bis(methylamino)pyrimidin-4-yl)-4-aminobenzenesulfonamide) using a cAMP-mediated pathway. 2. Forskolin stimulation, to increase the magnitude of agonist cAMP responses, and site-directed mutagenesis of the 5-HT6 receptor, in order to yield constitutively active receptor, were applied. 3. 5-HT (E(max), % over basal: 200), E-6801 (120) and E-6837 (23) induced cAMP formation at the rat 5-HT6 receptor. In the copresence of forskolin, cAMP responses were more potent and enhanced to 294 (5-HT, % over forskolin), 250 (E-6801) and 207 (E-6837), respectively. 5-HT-mediated cAMP formation was dose-dependently blocked by SB-271046 (pA(2): 8.76+/-0.22) and Ro 04-6790 (pA(2): 7.89+/-0.10) and not affected by the copresence of forskolin. Both E-6801 and E-6837 yielded partial antagonism of the 5-HT response in the absence of forskolin, whereas antagonism was either completely absent (E-6801) or attenuated (E-6837) in the copresence of forskolin. Intrinsic activity of these 5-HT6 receptor ligands at a constitutively active human S267K 5-HT6 receptor in Cos-7 cells indicated similar efficacy (E(max), % over basal) for 5-HT (97), E-6801 (91) and E-6837 (100), while Ro 04-6790 (-33) and SB-271046 (-39) were equi-efficacious inverse agonists. 4. The use of either forskolin or a constitutively active S267K 5-HT6 receptor enhances the resolution for monitoring the efficacy of 5-HT6 receptor ligands. E-6801 and E-6837 are potent partial agonists at the 5-HT6 receptor. Ro 04-6790 and SB-271046 appear to act as inverse agonists/antagonists.

Whole spectrum analysis of ligand efficacy at constitutively active human wild-type and S267K 5-HT6 receptors in HEK-293F cells

INTRODUCTION

Modulation of constitutive activity by the recombinant wild-type human 5-HT6 receptor was investigated with a series of 5-HT6 receptor ligands by monitoring the cAMP signalling pathway. The impact of the mutation S267K near the B(261)BXXB(265) CIII-loop motif was analyzed on the magnitude of constitutive receptor activity as previously conflicting results have been reported.

METHODS

The wild-type 5-HT6 receptor plasmid was obtained by PCR and the mutant S267K5-HT6 receptor was constructed by site-directed mutagenesis and stably transfected in HEK-293F cells by electroporation. The cAMP signalling pathway was monitored as a functional read-out to investigate ligands' responses using homogeneous time resolved fluorescence.

RESULTS

Constitutive activity was present both at wild-type and mutant S267K 5-HT6 receptors. Negative efficacy (E(max), % versus basal) as observed at nanomolar concentrations with SB-271046 was larger for mutant (-92+/-1%) than wild-type 5-HT6 receptor (-45+/-1%). Ro 04-6790 also demonstrated negative efficacy at the wild-type 5-HT6 receptor with a magnitude similar to SB-271046 but with a 36-fold lower potency. MS-245 demonstrated at nanomolar concentrations intermediate negative efficacy; -48+/-3% and -16+/-2% at mutant and wild-type 5-HT6 receptor, respectively. The 5-HT-mediated cAMP response was blocked by SB-271046, MS-245 and Ro 04-6790 to their respective level of negative efficacy with pKB values fitting with their binding pK(i) values. E-6801 was a highly potent (pEC50: 10.17 to 10.19) and efficacious agonist (+98 to +102% versus 5-HT) at both wild-type and mutant 5-HT6 receptors.

DISCUSSION

The recombinant wild-type human 5-HT6 receptor is constitutively active in HEK-293F cells and displays a high resolution to monitor efficacy properties of 5-HT6 receptor ligands. The resolution capacity to differentiate between efficacy properties of 5-HT6 receptor ligands, in particular for negative efficacy, can be further enhanced by monitoring the mutant S267K 5-HT6 receptor.