International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function.

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Spatiotemporal dynamics of 5-HT6 receptor ciliary localization during mouse brain development.

The serotonin 5-HT6 receptor (5-HT6R) is a promising target to improve cognitive symptoms of psychiatric diseases of neurodevelopmental origin, such as autism spectrum disorders and schizophrenia. However, its expression and localization at different stages of brain development remain largely unknown, due to the lack of specific antibodies to detect endogenous 5-HT6R. Here, we used transgenic mice expressing a GFP-tagged 5-HT6R under the control of its endogenous promoter (Knock-in) as well as embryonic stem cells expressing the GFP-tagged receptor to extensively characterize its expression at cellular and subcellular levels during development. We show that the receptor is already expressed at E13.5 in the cortex, the striatum, the ventricular zone, and to a lesser extent the subventricular zone. In adulthood, it is preferentially found in projection neurons of the hippocampus and cerebral cortex, in striatal medium-sized spiny neurons, as well as in a large proportion of astrocytes, while it is expressed in a minor population of interneurons. Whereas the receptor is almost exclusively detected in the primary cilia of neurons at embryonic and adult stages and in differentiated stem cells, it is located in the somatodendritic compartment of neurons from some brain regions at the neonatal stage and in the soma of undifferentiated stem cells. Finally, knocking-out the receptor induces a shortening of the primary cilium, suggesting that it plays a role in its function. This study provides the first global picture of 5-HT6R expression pattern in the mouse brain at different developmental stages. It reveals dynamic changes in receptor localization in neurons at the neonatal stage, which might underlie its key role in neuronal differentiation and psychiatric disorders of neurodevelopmental origin.

The Complex Formed by Group I Metabotropic Glutamate Receptor (mGluR) and Homer1a Plays a Central Role in Metaplasticity and Homeostatic Synaptic Scaling.

G-protein-coupled receptors can be constitutively activated following physical interaction with intracellular proteins. The first example described was the constitutive activation of Group I metabotropic glutamate receptors (mGluR: mGluR1,5) following their interaction with Homer1a, an activity-inducible early-termination variant of the scaffolding protein Homer that lacks dimerization capacity (Ango et al., 2001). Homer1a disrupts the links, maintained by the long form of Homer (cross-linking Homers), between mGluR1,5 and the Shank-GKAP-PSD-95-ionotropic glutamate receptor network. Two characteristics of the constitutive activation of the Group I mGluR-Homer1a complex are particularly interesting: (1) it affects a large number of synapses in which Homer1a is upregulated following enhanced, long-lasting neuronal activity; and (2) it mainly depends on Homer1a protein turnover. The constitutively active Group I mGluR-Homer1a complex is involved in the two main forms of non-Hebbian neuronal plasticity: "metaplasticity" and "homeostatic synaptic scaling," which are implicated in a large series of physiological and pathologic processes. Those include non-Hebbian plasticity observed in visual system, synapses modulated by addictive drugs (rewarded synapses), chronically overactivated synaptic networks, normal sleep, and sleep deprivation.

mTOR in Brain Physiology and Pathologies.

TOR (target of rapamycin) and its mammalian ortholog mTOR have been discovered in an effort to understand the mechanisms of action of the immunosuppressant drug rapamycin extracted from a bacterium of the Easter Island (Rapa Nui) soil. mTOR is a serine/threonine kinase found in two functionally distinct complexes, mTORC1 and mTORC2, which are differentially regulated by a great number of nutrients such as glucose and amino acids, energy (oxygen and ATP/AMP content), growth factors, hormones, and neurotransmitters. mTOR controls many basic cellular functions such as protein synthesis, energy metabolism, cell size, lipid metabolism, autophagy, mitochondria, and lysosome biogenesis. In addition, mTOR-controlled signaling pathways regulate many integrated physiological functions of the nervous system including neuronal development, synaptic plasticity, memory storage, and cognition. Thus it is not surprising that deregulation of mTOR signaling is associated with many neurological and psychiatric disorders. Preclinical and preliminary clinical studies indicate that inhibition of mTORC1 can be beneficial for some pathological conditions such as epilepsy, cognitive impairment, and brain tumors, whereas stimulation of mTORC1 (direct or indirect) can be beneficial for other pathologies such as depression or axonal growth and regeneration.

Blockade of Serotonin 5-HT6 Receptor Constitutive Activity Alleviates Cognitive Deficits in a Preclinical Model of Neurofibromatosis Type 1.

Neurofibromatosis type 1 (NF1) is a common inherited disorder caused by mutations of the NF1 gene that encodes the Ras-GTPase activating protein neurofibromin, leading to overactivation of Ras-dependent signaling pathways such as the mTOR pathway. It is often characterized by a broad range of cognitive symptoms that are currently untreated. The serotonin 5-HT6 receptor is a potentially relevant target in view of its ability to associate with neurofibromin and to engage the mTOR pathway to compromise cognition in several cognitive impairment paradigms. Here, we show that constitutively active 5-HT6 receptors contribute to increased mTOR activity in the brain of Nf1+/- mice, a preclinical model recapitulating some behavioral alterations of NF1. Correspondingly, peripheral administration of SB258585, a 5-HT6 receptor inverse agonist, or rapamycin, abolished deficits in long-term social and associative memories in Nf1+/- mice, whereas administration of CPPQ, a neutral antagonist, did not produce cognitive improvement. These results show a key influence of mTOR activation by constitutively active 5-HT6 receptors in NF1 cognitive symptoms. They provide a proof of concept that 5-HT6 receptor inverse agonists already in clinical development as symptomatic treatments to reduce cognitive decline in dementia and psychoses, might be repurposed as therapies alleviating cognitive deficits in NF1 patients.

5-HT2A receptor-dependent phosphorylation of mGlu2 receptor at Serine 843 promotes mGlu2 receptor-operated Gi/o signaling.

The serotonin 5-HT2A and glutamate mGlu2 receptors continue to attract particular attention, given their implication in psychosis associated with schizophrenia and the mechanism of action of atypical antipsychotics and a new class of antipsychotics, respectively. A large body of evidence indicates a functional crosstalk between both receptors in the brain, but the underlying mechanisms are not entirely elucidated. Here, we have explored the influence of 5-HT2A receptor upon the phosphorylation pattern of mGlu2 receptor in light of the importance of specific phosphorylation events in regulating G protein-coupled receptor signaling and physiological outcomes. Among the five mGlu2 receptor-phosphorylated residues identified in HEK-293 cells, the phosphorylation of Ser843 was enhanced upon mGlu2 receptor stimulation by the orthosteric agonist LY379268 only in cells co-expressing the 5-HT2A receptor. Likewise, administration of LY379268 increased mGlu2 receptor phosphorylation at Ser843 in prefrontal cortex of wild-type mice but not 5-HT2A-/- mice. Exposure of HEK-293 cells co-expressing mGlu2 and 5-HT2A receptors to 5-HT also increased Ser843 phosphorylation state to a magnitude similar to that measured in LY379268-treated cells. In both HEK-293 cells and prefrontal cortex, Ser843 phosphorylation elicited by 5-HT2A receptor stimulation was prevented by the mGlu2 receptor antagonist LY341495, while the LY379268-induced effect was abolished by the 5-HT2A receptor antagonist M100907. Mutation of Ser843 into alanine strongly reduced Gi/o signaling elicited by mGlu2 or 5-HT2A receptor stimulation in cells co-expressing both receptors. Collectively, these findings identify mGlu2 receptor phosphorylation at Ser843 as a key molecular event that underlies the functional crosstalk between both receptors.

Sustained Activation of Postsynaptic 5-HT2A Receptors Gates Plasticity at Prefrontal Cortex Synapses.

The prefrontal cortex (PFC) plays a key role in many high-level cognitive processes. It is densely innervated by serotonergic neurons originating from the dorsal and median raphe nuclei, which profoundly influence PFC activity. Among the 5-HT receptors abundantly expressed in PFC, 5-HT2A receptors located in dendrites of layer V pyramidal neurons control neuronal excitability and mediate the psychotropic effects of psychedelic hallucinogens, but their impact on glutamatergic transmission and synaptic plasticity remains poorly characterized. Here, we show that a 20-min exposure of mouse PFC slices to serotonin or the 5-HT2A receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) produces a long-lasting depression of evoked AMPA excitatory postsynaptic currents in layer V pyramidal neurons. DOI-elicited long-term depression (LTD) of synaptic transmission is absent in slices from 5-HT2A receptor-deficient mice, is rescued by viral expression of 5-HT2A receptor in pyramidal neurons and occludes electrically induced long-term depression. Furthermore, 5-HT2A receptor activation promotes phosphorylation of GluA2 AMPA receptor subunit at Ser880 and AMPA receptor internalization, indicating common mechanisms with electrically induced LTD. These findings provide one of the first examples of LTD gating under the control of a G protein-coupled receptor that might lead to imbalanced synaptic plasticity and memory impairment following a nonphysiological elevation of extracellular serotonin.

Presynaptic serotonin 2A receptors modulate thalamocortical plasticity and associative learning.

Higher-level cognitive processes strongly depend on a complex interplay between mediodorsal thalamus nuclei and the prefrontal cortex (PFC). Alteration of thalamofrontal connectivity has been involved in cognitive deficits of schizophrenia. Prefrontal serotonin (5-HT)2A receptors play an essential role in cortical network activity, but the mechanism underlying their modulation of glutamatergic transmission and plasticity at thalamocortical synapses remains largely unexplored. Here, we show that 5-HT2A receptor activation enhances NMDA transmission and gates the induction of temporal-dependent plasticity mediated by NMDA receptors at thalamocortical synapses in acute PFC slices. Expressing 5-HT2A receptors in the mediodorsal thalamus (presynaptic site) of 5-HT2A receptor-deficient mice, but not in the PFC (postsynaptic site), using a viral gene-delivery approach, rescued the otherwise absent potentiation of NMDA transmission, induction of temporal plasticity, and deficit in associative memory. These results provide, to our knowledge, the first physiological evidence of a role of presynaptic 5-HT2A receptors located at thalamocortical synapses in the control of thalamofrontal connectivity and the associated cognitive functions.

Physical and functional interactions between the serotonin transporter and the neutral amino acid transporter ASCT2.

The activity of serotonergic systems depends on the reuptake of extracellular serotonin via its plasma membrane serotonin [5-HT (5-hydroxytryptamine)] transporter (SERT), a member of the Na(+)/Cl(-)-dependent solute carrier 6 family. SERT is finely regulated by multiple molecular mechanisms including its physical interaction with intracellular proteins. The majority of previously identified SERT partners that control its functional activity are soluble proteins, which bind to its intracellular domains. SERT also interacts with transmembrane proteins, but its association with other plasma membrane transporters remains to be established. Using a proteomics strategy, we show that SERT associates with ASCT2 (alanine-serine-cysteine-threonine 2), a member of the solute carrier 1 family co-expressed with SERT in serotonergic neurons and involved in the transport of small neutral amino acids across the plasma membrane. Co-expression of ASCT2 with SERT in HEK (human embryonic kidney)-293 cells affects glycosylation and cell-surface localization of SERT with a concomitant reduction in its 5-HT uptake activity. Conversely, depletion of cellular ASCT2 by RNAi enhances 5-HT uptake in both HEK-293 cells and primary cultured mesencephalon neurons. Mimicking the effect of ASCT2 down-regulation, treatment of HEK-293 cells and neurons with the ASCT2 inhibitor D-threonine also increases 5-HT uptake. Moreover, D-threonine does not enhance further the maximal velocity of 5-HT uptake in cells depleted of ASCT2. Collectively, these findings provide evidence for a complex assembly involving SERT and a member of another solute carrier family, which strongly influences the subcellular distribution of SERT and the reuptake of 5-HT.

Cdk5 induces constitutive activation of 5-HT6 receptors to promote neurite growth.

The serotonin6 receptor (5-HT6R) is a promising target for treating cognitive deficits of schizophrenia often linked to alterations of neuronal development. This receptor controls neurodevelopmental processes, but the signaling mechanisms involved remain poorly understood. Using a proteomic strategy, we show that 5-HT6Rs constitutively interact with cyclin-dependent kinase 5 (Cdk5). Expression of 5-HT6Rs in NG108-15 cells induced neurite growth and expression of voltage-gated Ca(2+) channels, two hallmarks of neuronal differentiation. 5-HT6R-elicited neurite growth was agonist independent and prevented by the 5-HT6R antagonist SB258585, which behaved as an inverse agonist. Moreover, it required receptor phosphorylation at Ser350 by Cdk5 and Cdc42 activity. Supporting a role of native 5-HT6Rs in neuronal differentiation, neurite growth of primary neurons was reduced by SB258585, by silencing 5-HT6R expression or by mutating Ser350 into alanine. These results reveal a functional interplay between Cdk5 and a G protein-coupled receptor to control neuronal differentiation.

Quantitative phosphoproteomics unravels biased phosphorylation of serotonin 2A receptor at Ser280 by hallucinogenic versus nonhallucinogenic agonists.

The serotonin 5-HT(2A) receptor is a primary target of psychedelic hallucinogens such as lysergic acid diethylamine, mescaline, and psilocybin, which reproduce some of the core symptoms of schizophrenia. An incompletely resolved paradox is that only some 5-HT(2A) receptor agonists exhibit hallucinogenic activity, whereas structurally related agonists with comparable affinity and activity lack such a psychoactive activity. Using a strategy combining stable isotope labeling by amino acids in cell culture with enrichment in phosphorylated peptides by means of hydrophilic interaction liquid chromatography followed by immobilized metal affinity chromatography, we compared the phosphoproteome in HEK-293 cells transiently expressing the 5-HT(2A) receptor and exposed to either vehicle or the synthetic hallucinogen 1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane (DOI) or the nonhallucinogenic 5-HT(2A) agonist lisuride. Among the 5995 identified phosphorylated peptides, 16 sites were differentially phosphorylated upon exposure of cells to DOI versus lisuride. These include a serine (Ser(280)) located in the third intracellular loop of the 5-HT(2A) receptor, a region important for its desensitization. The specific phosphorylation of Ser(280) by hallucinogens was further validated by quantitative mass spectrometry analysis of immunopurified receptor digests and by Western blotting using a phosphosite specific antibody. The administration of DOI, but not of lisuride, to mice, enhanced the phosphorylation of 5-HT(2A) receptors at Ser(280) in the prefrontal cortex. Moreover, hallucinogens induced a less pronounced desensitization of receptor-operated signaling in HEK-293 cells and neurons than did nonhallucinogenic agonists. The mutation of Ser(280) to aspartic acid (to mimic phosphorylation) reduced receptor desensitization by nonhallucinogenic agonists, whereas its mutation to alanine increased the ability of hallucinogens to desensitize the receptor. This study reveals a biased phosphorylation of the 5-HT(2A) receptor in response to hallucinogenic versus nonhallucinogenic agonists, which underlies their distinct capacity to desensitize the receptor.

Calcineurin interacts with the serotonin transporter C-terminus to modulate its plasma membrane expression and serotonin uptake.

Homeostasis of serotonergic transmission critically depends on the rate of serotonin reuptake via its plasma membrane transporter (SERT). SERT activity is tightly regulated by multiple mechanisms, including physical association with intracellular proteins and post-translational modifications, such as phosphorylation, but these mechanisms remain partially understood. Here, we show that SERT C-terminal domain recruits both the catalytic and regulatory subunits of the Ca(2+)-activated protein phosphatase calcineurin (CaN) and that the physical association of SERT with CaN is promoted by CaN activity. Coexpression of constitutively active CaN with SERT increases SERT cell surface expression and 5-HT uptake in HEK-293 cells. It also prevents the reduction of 5-HT uptake induced by an acute treatment of cells with the protein kinase C activator ß-PMA and concomitantly decreases PMA-elicited SERT phosphorylation. In addition, constitutive activation of CaN in vivo favors 5-HT uptake in the adult mouse brain, whereas CaN inhibition reduces cerebral 5-HT uptake. Constitutive activation of CaN also decreases immobility in the forced swim test, indicative of an antidepressant-like effect of CaN. These results identify CaN as an important regulator of SERT activity in the adult brain and provide a novel molecular substrate of clinical interest for the understanding of increased risk of mood disorders in transplanted patients treated with immunosuppressive CaN inhibitors.

Shank3-Rich2 interaction regulates AMPA receptor recycling and synaptic long-term potentiation.

Synaptic long-term potentiation (LTP) is a key mechanism involved in learning and memory, and its alteration is associated with mental disorders. Shank3 is a major postsynaptic scaffolding protein that orchestrates dendritic spine morphogenesis, and mutations of this protein lead to mental retardation and autism spectrum disorders. In the present study we investigated the role of a new Shank3-associated protein in LTP. We identified the Rho-GAP interacting CIP4 homolog 2 (Rich2) as a new Shank3 partner by proteomic screen. Using single-cell bioluminescence resonance energy transfer microscopy, we found that Rich2-Shank3 interaction is increased in dendritic spines of mouse cultured hippocampal neurons during LTP. We further characterized Rich2 as an endosomal recycling protein that controls AMPA receptor GluA1 subunit exocytosis and spine morphology. Knock-down of Rich2 with siRNA, or disruption of the Rich2-Shank3 complex using an interfering mimetic peptide, inhibited the dendritic spine enlargement and the increase in GluA1 subunit exocytosis typical of LTP. These results identify Rich2-Shank3 as a new postsynaptic protein complex involved in synaptic plasticity.

ß-Arrestin2 plays a key role in the modulation of the pancreatic beta cell mass in mice.

AIMS/HYPOTHESIS: Beta cell failure due to progressive secretory dysfunction and limited expansion of beta cell mass is a key feature of type 2 diabetes. Beta cell function and mass are controlled by glucose and hormones/neurotransmitters that activate G protein-coupled receptors or receptor tyrosine kinases. We have investigated the role of ß-arrestin (ARRB)2, a scaffold protein known to modulate such receptor signalling, in the modulation of beta cell function and mass, with a specific interest in glucagon-like peptide-1 (GLP-1), muscarinic and insulin receptors. METHODS: ß-arrestin2-knockout mice and their wild-type littermates were fed a normal or a high-fat diet (HFD). Glucose tolerance, insulin sensitivity and insulin secretion were assessed in vivo. Beta cell mass was evaluated in pancreatic sections. Free cytosolic [Ca(2+)] and insulin secretion were determined using perifused islets. The insulin signalling pathway was evaluated by western blotting. RESULTS: Arrb2-knockout mice exhibited impaired glucose tolerance and insulin secretion in vivo, but normal insulin sensitivity compared with wild type. Surprisingly, the absence of ARRB2 did not affect glucose-stimulated insulin secretion or GLP-1- and acetylcholine-mediated amplifications from perifused islets, but it decreased the islet insulin content and beta cell mass. Additionally, there was no compensatory beta cell mass expansion through proliferation in response to the HFD. Furthermore, Arrb2 deletion altered the islet insulin signalling pathway. CONCLUSIONS/INTERPRETATION: ARRB2 is unlikely to be involved in the regulation of insulin secretion, but it is required for beta cell mass plasticity. Additionally, we provide new insights into the mechanisms involved in insulin signalling in beta cells.

Antidepressant-like effects of psychedelics in a chronic despair mouse model: is the 5-HT2A receptor the unique player?

Major depressive disorder (MDD) is one of the most disabling psychiatric disorders in the world. First-line treatments such as selective serotonin reuptake inhibitors (SSRIs) still have many limitations, including a resistance to treatment in 30% of patients and a delayed clinical benefit that is observed only after several weeks of treatment. Increasing clinical evidence indicates that the acute administration of psychedelic agonists of the serotonin 5-HT2A receptor (5-HT2AR), such as psilocybin, to patients with MDD induce fast antidepressant effects, which persist up to five weeks after the treatment. However, the involvement of the 5-HT2AR in these antidepressant effects remains controversial. Furthermore, whether the hallucinogenic properties of 5-HT2AR agonists are mandatory to their antidepressant activity is still an open question. Here, we addressed these issues by investigating the effect of two psychedelics of different chemical families, DOI and psilocybin, and a non-hallucinogenic 5-HT2AR agonist, lisuride, in a chronic despair mouse model exhibiting a robust depressive-like phenotype. We show that a single injection of each drug to wild type mice induces anxiolytic- and antidepressant-like effects in the novelty-suppressed feeding, sucrose preference and forced swim tests, which last up to 15 days. DOI and lisuride administration did not produce antidepressant-like effects in 5-HT2A-/- mice, whereas psilocybin was still effective. Moreover, neither 5-HT1AR blockade nor dopamine D1 or D2 receptor blockade affected the antidepressant-like effects of psilocybin in 5-HT2A-/- mice. Collectively, these findings indicate that 5-HT2AR agonists can produce antidepressant-like effects independently of hallucinogenic properties through mechanisms involving or not involving the receptor.

GPCR interacting proteins (GIPs) in the nervous system: Roles in physiology and pathologies.

G protein-coupled receptors (GPCRs) are key transmembrane recognition molecules for regulatory signals such as light, odors, taste hormones, and neurotransmitters. In addition to activating guanine nucleotide binding proteins (G proteins), GPCRs associate with a variety of GPCR-interacting proteins (GIPs). GIPs contain structural interacting domains that allow the formation of large functional complexes involved in G protein-dependent and -independent signaling. At the cellular level, other functions of GIPs include targeting of GPCRs to subcellular compartments and their trafficking to and from the plasma membrane. Recently, roles of GPCR-GIP interactions in central nervous system physiology and pathologies have been revealed. Here, we highlight the role of GIPs in some important neurological and psychiatric disorders, as well as their potential for the future development of therapeutic drugs.

Beta-arrestin1 phosphorylation by GRK5 regulates G protein-independent 5-HT4 receptor signalling.

G protein-coupled receptors (GPCRs) have been found to trigger G protein-independent signalling. However, the regulation of G protein-independent pathways, especially their desensitization, is poorly characterized. Here, we show that the G protein-independent 5-HT(4) receptor (5-HT(4)R)-operated Src/ERK (extracellular signal-regulated kinase) pathway, but not the G(s) pathway, is inhibited by GPCR kinase 5 (GRK5), physically associated with the proximal region of receptor' C-terminus in both human embryonic kidney (HEK)-293 cells and colliculi neurons. This inhibition required two sequences of events: the association of beta-arrestin1 to a phosphorylated serine/threonine cluster located within the receptor C-t domain and the phosphorylation, by GRK5, of beta-arrestin1 (at Ser(412)) bound to the receptor. Phosphorylated beta-arrestin1 in turn prevented activation of Src constitutively bound to 5-HT(4)Rs, a necessary step in receptor-stimulated ERK signalling. This is the first demonstration that beta-arrestin1 phosphorylation by GRK5 regulates G protein-independent signalling.

Impact of 5-HT6 Receptor Subcellular Localization on Its Signaling and Its Pathophysiological Roles.

The serotonin (5-HT)6 receptor still raises particular interest given its unique spatio-temporal pattern of expression among the serotonin receptor subtypes. It is the only serotonin receptor specifically expressed in the central nervous system, where it is detected very early in embryonic life and modulates key neurodevelopmental processes, from neuronal migration to brain circuit refinement. Its predominant localization in the primary cilium of neurons and astrocytes is also unique among the serotonin receptor subtypes. Consistent with the high expression levels of the 5-HT6 receptor in brain regions involved in the control of cognitive processes, it is now well-established that the pharmacological inhibition of the receptor induces pro-cognitive effects in several paradigms of cognitive impairment in rodents, including models of neurodevelopmental psychiatric disorders and neurodegenerative diseases. The 5-HT6 receptor can engage several signaling pathways in addition to the canonical Gs signaling, but there is still uncertainty surrounding the signaling pathways that underly its modulation of cognition, as well as how the receptor's coupling is dependent on its cellular compartmentation. Here, we describe recent findings showing how the proper subcellular localization of the receptor is achieved, how this peculiar localization determines signaling pathways engaged by the receptor, and their pathophysiological influence.

Epileptiform activity induces vascular remodeling and zonula occludens 1 downregulation in organotypic hippocampal cultures: role of VEGF signaling pathways.

Recent studies suggest that blood-brain barrier (BBB) permeability contributes to epileptogenesis in symptomatic epilepsies. We have previously described angiogenesis, aberrant vascularization, and BBB alteration in drug-refractory temporal lobe epilepsy. Here, we investigated the role of vascular endothelial growth factor (VEGF) in an in vitro integrative model of vascular remodeling induced by epileptiform activity in rat organotypic hippocampal cultures. After kainate-induced seizure-like events (SLEs), we observed an overexpression of VEGF and VEGF receptor-2 (VEGFR-2) as well as receptor activation. Vascular density and branching were significantly increased, whereas zonula occludens 1 (ZO-1), a key protein of tight junctions (TJs), was downregulated. These effects were fully prevented by VEGF neutralization. Using selective inhibitors of VEGFR-2 signaling pathways, we found that phosphatidylinositol 3-kinase is involved in cell survival, protein kinase C (PKC) in vascularization, and Src in ZO-1 regulation. Recombinant VEGF reproduced the kainate-induced vascular changes. As in the kainate model, VEGFR-2 and Src were involved in ZO-1 downregulation. These results showed that VEGF/VEGFR-2 initiates the vascular remodeling induced by SLEs and pointed out the roles of PKC in vascularization and Src in TJ dysfunction, respectively. This suggests that Src pathway could be a therapeutic target for BBB protection in epilepsies.

G protein activation by serotonin type 4 receptor dimers: evidence that turning on two protomers is more efficient.

The discovery that class C G protein-coupled receptors (GPCRs) function as obligatory dimeric entities has generated major interest in GPCR oligomerization. Oligomerization now appears to be a common feature among all GPCR classes. However, the functional significance of this process remains unclear because, in vitro, some monomeric GPCRs, such as rhodopsin and ß(2)-adrenergic receptors, activate G proteins. By using wild type and mutant serotonin type 4 receptors (5-HT(4)Rs) (including a 5-HT(4)-RASSL) expressed in COS-7 cells as models of class A GPCRs, we show that activation of one protomer in a dimer was sufficient to stimulate G proteins. However, coupling efficiency was 2 times higher when both protomers were activated. Expression of combinations of 5-HT(4), in which both protomers were able to bind to agonists but only one could couple to G proteins, suggested that upon agonist occupancy, protomers did not independently couple to G proteins but rather that only one G protein was activated. Coupling of a single heterotrimeric G(s) protein to a receptor dimer was further confirmed in vitro, using the purified recombinant WT RASSL 5-HT(4)R obligatory heterodimer. These results, together with previous findings, demonstrate that, differently from class C GPCR dimers, class A GPCR dimers have pleiotropic activation mechanisms.