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

The brain-derived neurotrophic factor, BDNF, was discovered more than 30 years ago and, like other members of the neurotrophin family, this neuropeptide is synthetized as a proneurotrophin, the pro-BDNF, which is further cleaved to yield mature BDNF. The myriad of actions of these two BDNF isoforms in the central nervous system is constantly increasing and requires the development of sophisticated tools and animal models to refine our understanding. This review is focused on BDNF isoforms, their participation in the process of neurogenesis taking place in the hippocampus of adult mammals, and the modulation of their expression by serotonergic agents. Interestingly, around this triumvirate of BDNF, serotonin, and neurogenesis, a series of recent research has emerged with apparently counterintuitive results. This calls for an exhaustive analysis of the data published so far and encourages thorough work in the quest for new hypotheses in the field.

The vesicular monoamine transporter type 2 gene (VMAT2) has 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 control of the serotonin transporter (slc6a4) promoter. VMAT2 sertÀcre mice showed a major (À95%) depletion of 5-HT levels in the brain with no major alterations in other monoamines. Raphe neurons contained no 5-HT immunoreactivity in VMAT2 sertÀcre mice but developed normal innervations, as assessed by both tryptophan hydroxylase 2 and 5-HT transporter labeling. Increased 5-HT 1A autoreceptor coupling to G protein, as assessed with agonist-stimulated [ 35 S]GTP-g-S binding, was observed in the raphe area, indicating an adaptive change to reduced 5-HT transmission. Behavioral evaluation in adult VMAT2 sertÀcre mice showed an increase in escape-like reactions in response to tail suspension and anxiolytic-like response in the novelty-suppressed feeding test. In an aversive ultrasoundinduced defense paradigm, VMAT2 sertÀcre mice displayed a major increase in 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, although in reduced amounts, in synaptosomes from VMAT2 sertÀcre mouse brain. These findings are coherent with the notion that 5-HT has an important role in anxiety, and provide new insights into the role of endogenous 5-HT in defense behaviors.

Pulmonary arterial hypertension (PAH) is IntroductionPulmonary arterial hypertension (PAH) is a rare but fatal disease, often of unknown origin, characterized by progressive increase in pulmonary vascular resistance and remodeling associated with vasoconstriction. 1 PAH is histologically characterized by a neomuscularization of small pulmonary arteries with intimal thickening, medial hypertrophy, adventitial proliferation, and abnormal extracellular matrix deposition. The progression of vascular remodeling results in vascular lumen narrowing, increased pulmonary artery resistance, hypoxia, and right heart hypertrophy, although the molecular pathways initiating this remodeling are not clearly established.On the one hand, stem cells, resident or not, may give rise to a significant proportion of differentiating/proliferating smooth muscle cells (SMCs) that contribute to intimal hyperplasia in lung vascular remodeling. 2 Moreover, genetic ablation of the transmembrane tyrosine kinase receptor for stem cell factor/c-kit pathway results in a marked reduction in intimal hyperplasia in animal models of vascular injury; conversely, wild-type (WT) bone marrow (BM) reconstitution in c-kit mutant mice led to intimal hyperplasia comparable with WT animals. 3 Pharmacologic antagonism of the c-kit pathway with STI-571 (imatinib mesylate; Gleevec) also results in a marked reduction in hyperplasia. 3 Mobilization of c-kit expressing cells from BM to blood circulation is a physiologic response to hypoxia. Increasing evidence supports the idea that these progenitor cells of BM origin may also contribute to vascular wall remodeling that is characteristic of PAH. [4][5][6][7][8] However, it is unclear, whether this entry of progenitors represents a protective or a worsening process in the development of PAH. 9 Other observations have also identified an association between PAH and BMrelated hematologic disorders 10 : in proliferative disorders of the hematopoietic stem cells such as myeloproliferative cancers, there is an unexplained high incidence of PAH. PAH is now a recognized complication of BM transplantation for leukemia, 11 chronic myeloproliferative disorders, 12 or in the treatment of malignant infantile osteopetrosis. 13 On the other hand, serotonin (5-hydroxytryptamine ) is associated with the pathogenesis of PAH. 14 Therapeutic drugs with PAH as a side effect, such as the amphetamine derivative and anorexigen dexfenfluramine, are potent 5-HT releasers acting at 5-HT transporter (SERT) and/or agonists at 5-HT receptors (5-HTRs). 15 An over-expression of 5-HT 2B Rs is observed in PAH. 16 Blockade of 5-HT 2B Rs using independent approaches, either genetic (5-HT 2B R knock-out mice; 5-HT 2B Ϫ/Ϫ ) or pharmacologic (5-HT 2B antagonist RS-12744) inactivation, completely prevented the development of hypoxia-induced pulmonary hypertension in mice. 16 By using the monocrotaline (MCT)-induced pulmonary hypertension rat model, recent studies confirmed that other 5-HT 2B antagonists (terguride, PRX-08066, or C-122) significantly reduc...

Increased adult neurogenesis is a major neurobiological correlate of the beneficial effects of antidepressants. Indeed, selective serotonin (5-HT) re-uptake inhibitors, which increase 5-HT transmission, enhance adult neurogenesis in the dentate gyrus (DG) of the hippocampus. However, the consequences of 5-HT depletion are still unclear as studies using neurotoxins that target serotonergic neurons reached contradictory conclusions on the role of 5-HT on DG cell proliferation. Here, we analysed two genetic models of 5-HT depletion, the Pet1(-/-) and the VMAT2(f/f) ; SERT(cre/+) mice, which have, respectively, 80 and 95% reductions in hippocampal 5-HT. In both models, we found unchanged cell proliferation of the neural precursors in the DG subgranular zone, whereas a significant increase in the survival of newborn neurons was noted 1 and 4 weeks after BrdU injections. This pro-survival trait was phenocopied pharmacologically with 5-HT synthesis inhibitor PCPA treatment in adults, indicating that this effect was not developmental. Furthermore, a 1-week administration of the 5-HT1A receptor agonist 8-OH-DPAT in Pet1(-/-) and PCPA-treated mice normalised hippocampal cell survival. Overall, our results indicate that constitutive 5-HT depletion does not alter the proliferation of neural precursors in the DG but promotes the survival of newborn cells, an effect which involves activation of postsynaptic 5-HT1A receptors. The role of 5-HT in selective neuronal elimination points to a new facet in its multiple effects in controlling neural circuit maturation.

Since nitric oxide has been found to control the function of many organs of the body by the non-adrenergic, non-cholinergic branch of the autonomic nervous system, we hypothesized that it might play a role in salivary secretion. Therefore, we investigated the distribution of nitric oxide synthase (NOS) throughout the submaxillary gland and also studied the ability of inhibitors of NOS to interfere with salivation induced by a cholinergic agonist, metacholine, and by a polypeptide, substance P. The secretory responses were determined in rats anesthetized with chlorolose following intravenous injection of the various pharmacological agents. There was no basal flow of saliva and dose-response curves were obtained by sequential intravenous injection of increasing doses of the drugs. Then, in the same animal, the same dose-response curves were performed in the presence of NOS inhibitors. L-Nitro-arginine-methyl-ester (L-NAME; 20 mg/kg) produced an over 50% inhibition of the dose-related salivation induced by metacholine. Similar results were produced with L-N^G-monomethyl-L-arginine (L-NMMA; 5 mg/kg). The salivation induced by much lower molar doses of substance P was dramatically greater than that obtained with metacholine. The response to substance P was almost completely inhibited by L-NMMA at the lowest dose (0.3 mg/kg), but at higher doses (1 mg/kg), the inhibition was only around 60% and at the highest dose (3 mg/kg) only about 20%. In control rats, there were roughly equal amounts of calcium-dependent and calcium-independent NOS in the gland at this time. At the end of the experiment, the effect of the inhibitor of NOS, L-NMMA, on the NOS activity in the submandibular gland was determined. At this time, the Ca²⁺-dependent NOS was decreased and the Ca²⁺-independent NO was increased. The prior injection of L-NMMA reduced calcium-dependent NOS activity by approximately 70% but calcium-independent activity by only 30%. These results indicate that, at least at the end of the experiment, the blockade of NOS imposed by NMMA was incomplete. This could account in part for the failure of the inhibitors to block completely the stimulatory effect of the two secretagogues. Analysis of the distribution of NOS in the salivary gland revealed that it was not present in the acinar cells, but in neural terminals within the gland and also in the ductile system which contained neural (n) NOS in the apical membrane of the excretory and striated ducts, the cytoplasm of granular convoluted tubules and, to a lesser extent, in the cytoplasm of excretory and striated ducts. Macrophage (inducible) NOS was also found not only in the macrophages, but also in the tubules and ducts. Since drugs were used that would act on the receptors in the gland, the role of NO in our conditions is probably mediated by nNOS and iNOS in the ductile and tubular structures. Since iNOS would already be active, it is unlikely to play a role in this acute secretory activity. Rather the nNOS in these non-neural cells is probably ac...

In previous studies we have demonstrated a possible interaction between the GABAergic and opioid systems involved in the antinociceptive effect of the GABA(B) agonist, baclofen (BAC). On the other hand, sex differences have been observed for the antinociceptive effect of morphine (MOR). In the present study, we analyzed sex-related differences in the MOR abstinence syndrome and its prevention with BAC. Prepubertal male and female Swiss-Webster albino mice (27-33 g) were rendered dependent by intraperitoneal (i.p.) injection of MOR (2, 4 and 8 mg/kg), twice daily for 9 days. On the tenth day the dependent animals were divided into two groups: one received naloxone (NAL) (6 mg/kg, i.p.) 60 min after the last dose of MOR, to precipitate the abstinence syndrome; the other group received BAC (2 mg/kg, i.p.) followed by NAL (6 mg/kg, i.p.), injected 30 and 60 min after the last dose of MOR, respectively. Behavioral signs were recorded in the open field for 30 min. Although there were sex differences in the MOR withdrawal syndrome, we found a lack of sex differences in the prevention of the MOR abstinence syndrome by BAC.

Serotonin is a neurotransmitter involved in many psychiatric diseases. In humans, a lack of 5-HT receptors is associated with serotonin-dependent phenotypes, including impulsivity and suicidality. A lack of 5-HT receptors in mice eliminates the effects of molecules that directly target serotonergic neurons including amphetamine derivative serotonin releasers, and selective serotonin reuptake inhibitor antidepressants. In this work, we tested the hypothesis that 5-HT receptors directly and positively regulate raphe serotonin neuron activity. By ex vivo electrophysiological recordings, we report that stimulation by the 5-HT receptor agonist, BW723C86, increased the firing frequency of serotonin Pet1-positive neurons. Viral overexpression of 5-HT receptors in these neurons increased their excitability. Furthermore, in vivo 5-HT-receptor stimulation by BW723C86 counteracted 5-HT autoreceptor-dependent reduction in firing rate and hypothermic response in wild-type mice. By a conditional genetic ablation that eliminates 5-HT receptor expression specifically and exclusively from Pet1-positive serotonin neurons (Htr2b mice), we demonstrated that behavioral and sensitizing effects of MDMA (3,4-methylenedioxy-methamphetamine), as well as acute behavioral and chronic neurogenic effects of the antidepressant fluoxetine, require 5-HT receptor expression in serotonergic neurons. In Htr2b mice, dorsal raphe serotonin neurons displayed a lower firing frequency compared to control Htr2b mice as assessed by in vivo extracellular recordings and a stronger hypothermic effect of 5-HT-autoreceptor stimulation was observed. The increase in head-twitch response to DOI (2,5-dimethoxy-4-iodoamphetamine) further confirmed the lower serotonergic tone resulting from the absence of 5-HT receptors in serotonin neurons. Together, these observations indicate that the 5-HT receptor acts as a direct positive modulator of serotonin Pet1-positive neurons in an opposite way as the known 5-HT-negative autoreceptor.