Diabetic Neuropathic Pain and Serotonin: What Is New in the Last 15 Years?

International audienceThe neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) is involved in numerous physiological functions and plays a key role in pain modulation including neuropathic pain. Diabetic neuropathy is a common complication of diabetes mellitus often accompanied by chronic neuropathic pain. Animal models of diabetes offer relevant tools for studying the pathophysiological mechanisms and pharmacological sensitivity of diabetic neuropathic pain and for identifying new therapeutic targets. In this review, we report data from preclinical work published over the last 15 years on the analgesic activity of drugs acting on the serotonergic system, such as serotonin and noradrenaline reuptake inhibitor (SNRI) antidepressants, and on the involvement of certain serotonin receptors-in particular 5-HT1A, 5-HT2A/2c and 5-HT6 receptors-in rodent models of painful diabetic neuropathy

Heterodimers of serotonin receptor subtypes 2 are driven by 5-HT 2C protomers

International audienceThe serotonin receptor subtypes 2 comprise 5-HT2A, 5-HT2B, and 5-HT2C, which are Gαq-coupled receptors and display distinct pharmacological properties. Although co-expressed in some brain regions and involved in various neurological disorders, their functional interactions have not yet been studied. We report that 5-HT2 receptors can form homo- and heterodimers when expressed alone or co-expressed in transfected cells. Co-immunoprecipitation and bioluminescence resonance energy transfer studies confirmed that 5-HT2C receptors interact with either 5-HT2A or 5-HT2B receptors. Although heterodimerization with 5-HT2C receptors does not alter 5-HT2C Gαq-dependent inositol phosphate signaling, 5-HT2A or 5-HT2B receptor-mediated signaling was totally blunted. This feature can be explained by a dominance of 5-HT2C on 5-HT2A and 5-HT2B receptor binding; in 5-HT2C-containing heterodimers, ligands bind and activate the 5-HT2C protomer exclusively. This dominant effect on the associated protomer was also observed in neurons, supporting the physiological relevance of 5-HT2 receptor heterodimerization in vivo Accordingly, exogenous expression of an inactive form of the 5-HT2C receptor in the locus ceruleus is associated with decreased 5-HT2A-dependent noradrenergic transmission. These data demonstrate that 5-HT2 receptors can form functionally asymmetric heterodimers in vitro and in vivo that must be considered when analyzing the physiological or pathophysiological roles of serotonin in tissues where 5-HT2 receptors are co-expressed

Role of the N-terminal region in G protein-coupled receptor functions: negative modulation revealed by 5-HT2B receptor polymorphisms.

International audienceThe putative role of the N-terminal region of rhodopsin-like 7 transmembrane biogenic amine receptors in agonist-induced signaling has not yet been clarified despite recent advances in 7 transmembrane receptor structural biology. Given the existence of N-terminal nonsynonymous polymorphisms (R6G;E42G) within the HTR2B gene in a drug-abusing population, we assessed whether these polymorphisms affect 5-hydroxytryptamine 2B (5-HT2B) receptor in vitro pharmacologic and coupling properties in transfected COS-7 cells. Modification of the 5-HT2B receptor N terminus by the R6G;E42G polymorphisms increases such agonist signaling pathways as inositol phosphate accumulation as assessed by either classic or operational models. The N-terminal R6G;E42G mutations of the 5-HT2B receptor also increase cell proliferation and slow its desensitization kinetics compared with the wild-type receptor, further supporting a role for the N terminus in transduction efficacy. Furthermore, by coexpressing a tethered wild-type 5-HT2B receptor N terminus with a 5-HT2B receptor bearing a N-terminal deletion, we were able to restore original coupling. This reversion to normal activity of a truncated 5-HT2B receptor by coexpression of the membrane-tethered wild-type 5-HT2B receptor N terminus was not observed using a membrane-tethered 5-HT2B receptor R6G;E42G N terminus. These data suggest that the N terminus exerts a negative control over basal as well as agonist-stimulated receptor activity that is lost in the R6G;E42G mutant. Our findings reveal a new and unanticipated role of the 5-HT2B receptor N terminus as a negative modulator, affecting both constitutive and agonist-stimulated activity. Moreover, our data caution against excluding the N terminus and extracellular loops in structural studies of this 7 transmembrane receptor family

Serotonin 5-HT2B receptors are required for bone-marrow contribution to pulmonary arterial hypertension

Free to read Pulmonary arterial hypertension (PAH) is a progressive disease characterized by lung endothelial dysfunction and vascular remodeling. Recently, bone marrow progenitor cells have been localized to PAH lungs, raising the question of their role in disease progression. Independently, serotonin (5-HT) and its receptors have been identified as contributors to the PAH pathogenesis. We hypothesized that 1 of these receptors, 5-HT(2B), is involved in bone marrow stem cell mobilization that participates in the development of PAH and pulmonary vascular remodeling. A first study revealed expression of 5-HT(2B) receptors by circulating c-kit(+) precursor cells, whereas mice lacking 5-HT(2B) receptors showed alterations in platelets and monocyte-macrophage numbers, and in myeloid lineages of bone marrow. Strikingly, mice with restricted expression of 5-HT(2B) receptors in bone marrow cells developed hypoxia or monocrotaline-induced increase in pulmonary pressure and vascular remodeling, whereas restricted elimination of 5-HT(2B) receptors on bone marrow cells confers a complete resistance. Moreover, ex vivo culture of human CD34(+) or mice c-kit(+) progenitor cells in the presence of a 5-HT(2B) receptor antagonist resulted in altered myeloid differentiation potential. Thus, we demonstrate that activation of 5-HT(2B) receptors on bone marrow lineage progenitors is critical for the development of PAH

A population-specific HTR2B stop codon predisposes to severe impulsivity

Impulsivity, describing action without foresight, is an important feature of several psychiatric diseases, suicidality and violent behaviour. The complex origins of impulsivity hinder identification of the genes influencing it and the diseases with which it is associated. Here we perform exon-focused sequencing of impulsive individuals in a founder population, targeting fourteen genes belonging to the serotonin and dopamine domain. A stop codon in HTR2B was identified that is common (minor allele frequency > 1%) but exclusive to Finnish people. Expression of the gene in the human brain was assessed, as well as the molecular functionality of the stop codon, which was associated with psychiatric diseases marked by impulsivity in both population and family-based analyses. Knockout of Htr2b increased impulsive behaviours in mice, indicative of predictive validity. Our study shows the potential for identifying and tracing effects of rare alleles in complex behavioural phenotypes using founder populations, and indicates a role for HTR2B in impulsivity

Severe serotonin depletion after conditional deletion of the vesicular monoamine transporter 2 gene in serotonin neurons: neural and behavioral consequences

International audienceThe vesicular monoamine transporter type 2 gene (VMAT2) plays a crucial role in the storage and synaptic release of all monoamines, including serotonin (5-HT). To evaluate the specific role of VMAT2 in 5-HT neurons, we produced a conditional ablation of VMAT2 under the control of the serotonin transporter (slc6a4) promoter. VMAT2sert-cre mice showed a major (-95%) depletion of 5-HT levels in the brain with no major alterations of the other monoamines. Raphe neurons contained no 5-HT immunoreactivity in VMAT2sert-cre mice but developed normal innervations, as assessed by both tryptophan hydroxylase 2 and 5-HT transporter labeling. Increased 5-HT1A autoreceptor coupling to G protein, as assessed with agonist stimulated [35S]GTP-γ-S binding, was observed in the raphe area, indicating an adaptive change to the reduced 5-HT transmission. Behavioral evaluation in adult VMAT2sert-cre mice showed an increase of escape-like reactions in response to tail suspension, and anxiolytic-like response in the novelty suppressed feeding test. In an aversive ultrasound-induced defense paradigm, VMAT2sert-cre mice displayed a major increase of escape-like behaviors. Wild-type-like defense phenotype could be rescued by replenishing intracellular 5-HT stores with chronic pargyline (a monoamine oxidase inhibitor) treatment. Pargyline also allowed some form of 5-HT release, albeit in reduced amount, in synaptosomes from VMAT2sert-cre mice brain. These findings are coherent with the notion that 5-HT plays an important role in anxiety, and provide new insights on the role of endogenous 5-HT in defense behaviors