Living-Cell Diffracted X-ray Tracking Analysis Confirmed Internal Salt Bridge Is Critical for Ligand-Induced Twisting Motion of Serotonin Receptors.

Serotonin receptors play important roles in neuronal excitation, emotion, platelet aggregation, and vasoconstriction. The serotonin receptor subtype 2A (5-HT2AR) is a Gq-coupled GPCR, which activate phospholipase C. Although the structures and functions of 5-HT2ARs have been well studied, little has been known about their real-time dynamics. In this study, we analyzed the intramolecular motion of the 5-HT2AR in living cells using the diffracted X-ray tracking (DXT) technique. The DXT is a very precise single-molecular analytical technique, which tracks diffraction spots from the gold nanocrystals labeled on the protein surface. Trajectory analysis provides insight into protein dynamics. The 5-HT2ARs were transiently expressed in HEK 293 cells, and the gold nanocrystals were attached to the N-terminal introduced FLAG-tag via anti-FLAG antibodies. The motions were recorded with a frame rate of 100 µs per frame. A lifetime filtering technique demonstrated that the unliganded receptors contain high mobility population with clockwise twisting. This rotation was, however, abolished by either a full agonist α-methylserotonin or an inverse agonist ketanserin. Mutation analysis revealed that the "ionic lock" between the DRY motif in the third transmembrane segment and a negatively charged residue of the sixth transmembrane segment is essential for the torsional motion at the N-terminus of the receptor.

Serotonergic signals enhanced hamster sperm hyperactivation.

In the present study, we investigated the regulatory mechanisms underlying sperm hyperactivation enhanced by 5-hydroxytryptamine (5-HT) in hamsters. First, we examined the types of 5-HT receptors that regulate hyperactivation. Hyperactivation was significantly enhanced by 5-HT2A and 5-HT4 receptor agonists. Moreover, the results of the motility assay revealed that 5-HT2A, 5-HT3, and 5-HT4 receptor agonists significantly decreased the velocity and/or amplitude of sperm. Under 5-HT2 receptor stimulation, hyperactivation was associated with phospholipase C (PLC), inositol 1,4,5-trisphosphate (IP3) receptor, soluble adenylate cyclase (sAC), and protein kinase A (PKA). In contrast, under 5-HT4 receptor stimulation, hyperactivation was associated with transmembrane adenylate cyclase (tmAC), sAC, PKA, and CatSper channels. Accordingly, under the condition that sperm are hyperactivated, 5-HT likely stimulates PLC/IP3 receptor signals via the 5-HT2A receptor and tmAC/PKA/CatSper channel signals via the 5-HT4 receptor. After sAC and PKA are activated by these stimulations, sperm hyperactivation is enhanced.

Eupnea and gasping in vivo are facilitated by the activation of 5-HT2A receptors.

Eupnea and gasping in infancy depend on central nervous system (CNS) serotonin (5-hydroxytryptamine; 5-HT). Although previous in vitro preparations have provided some evidence that 5-HT acts through type 2 A receptors (5-HT2A) to facilitate eupnea and gasping, here the hypothesis addressed is that 5-HT2A receptor activation is necessary for eupnea and the proper generation of gasping in vivo. To test this, we administered 2,5-dimethoxy-4-iodoamphetamine (DOI; 0.25 mg/kg i.p.), a 5-HT2A agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.25 mg/kg i.p.), a 5-HT1A agonist, or vehicle (saline) to 7-9-day-old tryptophan hydroxylase 2 knockout (TPH2-/-) mice. A second experiment assessed the effect of MDL-11,939 (MDL; 10 mg/kg i.p.), the specific 5-HT2A antagonist, or vehicle (DMSO) on the gasping of wild-type (TPH2+/+) animals. Drugs were given 15 min prior to five episodes of severe hypoxia that elicited gasping. TPH2-/- breathed more slowly but had the same V̇e and V̇e/V̇o2 compared with TPH2+/+. As previously reported, the gasping of TPH2-/- was significantly delayed (P < 0.001) and occurred at a significantly lower frequency compared with TPH2+/+ (P = 0.04). For both genotypes, DOI hastened eupneic frequency but had no effect on V̇e or V̇e/V̇o2. The gasping of TPH2-/-, although unaffected by 8-OH-DPAT, was indistinguishable from the gasping of TPH2+/+ following DOI. In TPH2+/+, application of MDL led to hypoventilation (P = 0.01), a delay in the appearance of gasping (P = 0.005), and reduced gasp frequency (P = 0.05). These data show that, in vivo, 5-HT2A receptors facilitate both eupnea and gasping. As has been shown in vitro, 5-HT2A probably promotes gasping by exciting hypoxia-resistant pacemaker neurons.NEW & NOTEWORTHY Previous in vitro studies suggest that 5-HT2A receptors contribute to eupnea and are necessary for fictive gasping. The current study shows that the impaired gasping displayed by neonatal TPH2-/- mice, deficient in CNS serotonin, is restored by 5-HT2A receptor activation. Following 5-HT2A blockade, wild-type mice hypoventilated and their gasping resembled that of TPH2-/- mice. This study shows that both eupnea and gasping in vivo rely on the activation of 5-HT2A receptors.

Serotonin 2A (5-HT2A) receptor affects cell-matrix adhesion and the formation and maintenance of stress fibers in HEK293 cells.

5-HT2A, a G-protein coupled receptor, is widely expressed in the human body, including in the gastrointestinal tract, platelets and the nervous system. It mediates various functions, for e.g. learning, memory, mood regulation, platelet aggregation and vasoconstriction, but its involvement in cell-adhesion remains largely unknown. Here we report a novel role for 5-HT2A in cell-matrix adhesion.In HEK293 cells, which are loosely adherent, expression and stimulation of human or rat 5-HT2A receptor by agonists such as serotonin or 2,5-dimethoxy-4-iodoamphetamine (DOI) led to a significant increase in adhesion, while inhibition of 5-HT2A by antipsychotics, such as risperidone, olanzapine or chlorpromazine prevented it. 5-HT2A activation gave rise to stress fibers in these cells and was also required for their maintenance. Mechanistically, the 5-HT2A-mediated adhesion was mediated by downstream PKC and Rho signaling. Since 5-HT2A is associated with many disorders such as dementia, depression and schizophrenia, its role in cell-matrix adhesion could have implications for neural circuits.

Identification of psychedelic new psychoactive substances (NPS) showing biased agonism at the 5-HT2AR through simultaneous use of ß-arrestin 2 and miniGαq bioassays.

Psychedelic new psychoactive substances (NPS), compounds exerting their main pharmacological effects through the activation of the serotonin 2A receptor (5-HT2AR), continuously comprise a substantial portion of the reported NPS. However, these substances and their exact mechanism of action, differentiating them from non-psychedelic 5-HT2AR agonists, require further characterization. One potentially relevant phenomenon is the occurrence of biased agonism, in which (a) certain signaling pathway(s) is preferentially activated over the other(s). To this end, a new bioassay was developed, monitoring the recruitment of an engineered miniGαq protein to the activated 5-HT2AR. The setup was designed to be analogous to that of a previously developed bioassay monitoring ß-arrestin 2 recruitment through the NanoBiT system, enabling estimation of the potential preference of a substance to trigger recruitment of one protein over the other. This approach yielded several statistically significantly biased agonists within the group of phenylalkylamine psychedelics, more specifically the N-benzyl substituted 25H analogues 25H-NBF, 25H-NBMD, 25H-NBOH and 25H-NBOMe. All four compounds show a statistically significant preference towards the recruitment of ß-arrestin 2 over miniGαq, as compared to the reference psychedelic substance LSD. We identified markedly different responses for Bromo-DragonFLY in the two bioassays, suggesting biased agonism, though the calculated bias factor equalled out to approximately 0. This demonstrates that the accurate assessment of biased agonism requires both the consideration of the observed trends in addition to the numerical value of the bias factor. A second panel of structural (I-substituted) analogues of the former group of phenylalkylamines showed a similar trend in the ranking order of the bias factors, resulting in one additional compound (25I-NBF) being statistically significantly biased.

Acute serotonin 2A receptor activation impairs behavioral flexibility in mice.

Serotonin 2A (5-HT2A) receptors are the primary site of action of hallucinogenic drugs and the target of atypical antipsychotics. 5-HT2A receptors are also implicated in executive function, including behavioral flexibility. Previous studies showed that 5-HT2A receptor blockade improved behavioral flexibility in rodent models related to autism spectrum disorder and schizophrenia. The current study instead was conducted to examine the impact of acute 5-HT2A receptor activation on behavior flexibility in the control C57BL/6 J strain. Because of the therapeutic potential of serotonergic hallucinogens and the unknown impact of many of these compounds on cognition, the present study examined how the 5-HT2A/2C agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and the more selective 5-HT2A agonist 25CN-NBOH impacted behavioral flexibility in C57BL/6 J mice. Male mice were tested on a probabilistic spatial discrimination and reversal learning task after an intraperitoneal injection of vehicle, 2.5 mg/kg DOI, 1.0 mg/kg 25CN-NBOH, 1.0 mg/kg of the 5-HT2C receptor antagonist SER-082 or combined treatment with SER-082 (1.0 mg/kg) and 2.5 mg/kg DOI before testing of probabilistic reversal learning. All groups demonstrated comparable performance on the initial spatial discrimination, i.e. similar trials to criterion. DOI alone did not impair reversal learning, whereas 25CN-NBOH increased the number of trials to criterion during reversal learning. Because 5-HT2A and 5-HT2C receptors have been shown to functionally antagonize each other in several behavioral paradigms, we also tested whether blockade of 5-HT2C receptors would unmask 5-HT2A receptor activation by DOI and impair reversal learning. Mice treated with SER-082 in combination with DOI required significantly more trials to reach criterion. In an additional experiment, a dose response experiment with 25CN-NBOH revealed that the 1.0 mg/kg dose tested in reversal learning did not affect locomotor activity. Together, these findings indicate that activation of 5-HT2A receptors impairs probabilistic reversal learning and that 5-HT2A and 5-HT2C receptors exert opposing effects on behavioral flexibility in male mice.

Serotonin Regulates De Novo Lipogenesis in Adipose Tissues through Serotonin Receptor 2A.

BACKGROUND: Obesity is defined as excessive fat mass and is a major cause of many chronic diseases such as diabetes, cardiovascular disease, and cancer. Increasing energy expenditure and regulating adipose tissue metabolism are important targets for the treatment of obesity. Serotonin (5-hydroxytryptophan [5-HT]) is a monoamine metabolite of the essential amino acid tryptophan. Here, we demonstrated that 5-HT in mature adipocytes regulated energy expenditure and lipid metabolism. METHODS: Tryptophan hydroxylase 1 (TPH1) is the rate-limiting enzyme during 5-HT synthesis in non-neural peripheral tissues. We generated adipose tissue-specific Tph1 knockout (Tph1 FKO) mice and adipose tissue-specific serotonin receptor 2A KO (Htr2a FKO) mice and analyzed their phenotypes during high-fat diet (HFD) induced obesity. RESULTS: Tph1 FKO mice fed HFD exhibited reduced lipid accumulation, increased thermogenesis, and resistance to obesity. In addition, Htr2a FKO mice fed HFD showed reduced lipid accumulation in white adipose tissue and resistance to obesity. CONCLUSION: These data suggest that the inhibition of serotonin signaling might be an effective strategy in obesity.

Contribution of 5-HT2 Receptors to the Control of the Spinal Locomotor System in Intact Rats.

Applying serotonergic (5-HT) agonists or grafting of fetal serotonergic cells into the spinal cord improves locomotion after spinal cord injury. Little is known about the role of 5-HT receptors in the control of voluntary locomotion, so we administered inverse agonists of 5-HT2 (Cyproheptadine; Cypr), 5-HT2A neutral antagonist (Volinanserin; Volin), 5-HT2C neutral antagonist (SB 242084), and 5-HT2B/2C inverse agonist (SB 206553) receptors intrathecally in intact rats and monitored their effects on unrestrained locomotion. An intrathecal cannula was introduced at the low thoracic level and pushed caudally until the tip reached the L2/L3 or L5/L6 spinal segments. Locomotor performance was evaluated using EMG activity of hindlimb muscles during locomotion on a 2 m long runway. Motoneuron excitability was estimated using EMG recordings during dorsi- and plantar flexion at the ankle. Locomotion was dramatically impaired after the blockage of 5-HT2A receptors. The effect of Cypr was more pronounced than that of Volin since in the L5/L6 rats Cypr (but not Volin) induced significant alteration of the strength of interlimb coordination followed by total paralysis. These agents significantly decreased locomotor EMG amplitude and abolished or substantially decreased stretch reflexes. Blocking 5-HT2B/2C receptors had no effect either on locomotion or reflexes. We suggest that in intact rats serotonin controls timing and amplitude of muscle activity by acting on 5-HT2A receptors on both CPG interneurons and motoneurons, while 5-HT2B/2C receptors are not involved in control of the locomotor pattern in lumbar spinal cord.

Platelet activation in experimental murine neonatal pulmonary hypertension.

Serotonin (5-HT) contributes to the pathogenesis of experimental neonatal pulmonary hypertension (PH) associated with bronchopulmonary dysplasia (BPD). Platelets are the primary source of circulating 5-HT and is released upon platelet activation. Platelet transfusions are associated with neonatal mortality and increased rates of BPD. As BPD is often complicated by PH, we tested the hypothesis that circulating platelets are activated and also increased in the lungs of neonatal mice with bleomycin-induced PH associated with BPD. Newborn wild-type mice received intraperitoneal bleomycin (3 units/kg) three times weekly for 3 weeks. Platelets from mice with experimental PH exhibited increased adhesion to collagen under flow (at 300 s-1 and 1,500 s-1 ) and increased expression of the αIIbß3 integrin and phosphatidylserine, markers of platelet activation. Platelet-derived factors 5-HT and platelet factor 4 were increased in plasma from mice with experimental PH. Pharmacologic blockade of the 5-HT 2A receptor (5-HT 2A R) prevents bleomycin-induced PH and pulmonary vascular remodeling. Here, platelets from mice with bleomycin-induced PH demonstrate increased 5-HT 2A R expression providing further evidence of both platelet activation and increased 5-HT signaling in this model. In addition, bleomycin treatment increased lung platelet accumulation. In summary, platelets are activated, granule factors are released, and are increased in numbers in the lungs of mice with experimental neonatal PH. These results suggest platelet activation and release of platelet-derived factors may increase vascular tone, promote aberrant angiogenesis, and contribute to the development of neonatal PH.

Effects of combined 5-HT2A and cannabinoid receptor modulation on a schizophrenia-related prepulse inhibition deficit in mice.

RATIONALE: Prepulse inhibition of the startle reflex (PPI) is disrupted in several psychiatric disorders including schizophrenia. Understanding PPI pharmacology may help elucidate the pathophysiology of these disorders and lead to better treatments. Given the advantages of multi-target approaches for complex mental illnesses treatment, we have investigated the interaction between receptors known to modulate PPI (5-HT1A and 5-HT2A) and the neuromodulatory endocannabinoid system. OBJECTIVES: To investigate serotonin and cannabinoid receptor (CBR) co-modulation in a model of PPI disruption relevant to schizophrenia METHODS: Male Swiss mice were pretreated with WIN 55,212-2 (CBR agonist), rimonabant (CB1R inverse agonist), 8-OH-DPAT (5-HT1A/7 agonist), and volinanserin (5-HT2A antagonist) or with a combination of a cannabinoid and a serotonergic drug. PPI disruption was induced by acute administration of MK-801. RESULTS: WIN 55,212-2 and rimonabant did not change PPI nor block MK-801-induced deficits. 8-OH-DPAT increased PPI in control mice and, in a higher dose, inhibited MK-801-induced impairments. Volinanserin also increased PPI in control and MK-801-treated mice, presenting an inverted U-shaped dose-response curve. Co-administration of either cannabinoid ligand with 8-OH-DPAT did not change PPI; however, the combination of volinanserin with rimonabant increased PPI in both control and MK-801-exposed mice. CONCLUSIONS: WIN 55,212-2 and rimonabant had similar effects in PPI. Moreover, serotonin and cannabinoid receptors interact to modulate PPI. While co-modulation of CBR and 5-HT1A receptors did not change PPI, a beneficial effect of 5-HT2A and CB1R antagonist combination was detected, possibly mediated through potentiation of 5-HT2A blockade effects by concomitant CB1R blockade.

Correlation between the potency of hallucinogens in the mouse head-twitch response assay and their behavioral and subjective effects in other species.

Serotonergic hallucinogens such as lysergic acid diethylamide (LSD) induce head twitches in rodents via 5-HT2A receptor activation. The goal of the present investigation was to determine whether a correlation exists between the potency of hallucinogens in the mouse head-twitch response (HTR) paradigm and their reported potencies in other species, specifically rats and humans. Dose-response experiments were conducted with phenylalkylamine and tryptamine hallucinogens in C57BL/6J mice, enlarging the available pool of HTR potency data to 41 total compounds. For agents where human data are available (n = 36), a strong positive correlation (r = 0.9448) was found between HTR potencies in mice and reported hallucinogenic potencies in humans. HTR potencies were also found to be correlated with published drug discrimination ED50 values for substitution in rats trained with either LSD (r = 0.9484, n = 16) or 2,5-dimethoxy-4-methylamphetamine (r = 0.9564, n = 21). All three of these behavioral effects (HTR in mice, hallucinogen discriminative stimulus effects in rats, and psychedelic effects in humans) have been linked to 5-HT2A receptor activation. We present evidence that hallucinogens induce these three effects with remarkably consistent potencies. In addition to having high construct validity, the HTR assay also appears to show significant predictive validity, confirming its translational relevance for predicting subjective potency of hallucinogens in humans. These findings support the use of the HTR paradigm as a preclinical model of hallucinogen psychopharmacology and in structure-activity relationship studies of hallucinogens. Future investigations with a larger number of test agents will evaluate whether the HTR assay can be used to predict the hallucinogenic potency of 5-HT2A agonists in humans. "This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Modulation of the Tryptophan Hydroxylase 1/Monoamine Oxidase-A/5-Hydroxytryptamine/5-Hydroxytryptamine Receptor 2A/2B/2C Axis Regulates Biliary Proliferation and Liver Fibrosis During Cholestasis.

BACKGROUND AND AIMS: Serotonin (5HT) is a neuroendocrine hormone synthetized in the central nervous system (CNS) as well as enterochromaffin cells of the gastrointestinal tract. Tryptophan hydroxylase (TPH1) and monoamine oxidase (MAO-A) are the key enzymes for the synthesis and catabolism of 5HT, respectively. Previous studies demonstrated that 5-hydroxytryptamine receptor (5HTR)1A/1B receptor agonists inhibit biliary hyperplasia in bile-duct ligated (BDL) rats, whereas 5HTR2B receptor antagonists attenuate liver fibrosis (LF) in mice. Our aim was to evaluate the role of 5HTR2A/2B/2C agonists/antagonists in cholestatic models. APPROACH AND RESULTS: While in vivo studies were performed in BDL rats and the multidrug resistance gene 2 knockout (Mdr2-/- ) mouse model of PSC, in vitro studies were performed in cell lines of cholangiocytes and hepatic stellate cells (HSCs). 5HTR2A/2B/2C and MAO-A/TPH1 are expressed in cholangiocytes and HSCs from BDL rats and Mdr2-/- - mice. Ductular reaction, LF, as well as the mRNA expression of proinflammatory genes increased in normal, BDL rats, and Mdr2-/- - mice following treatment 5HTR2A/2B/2C agonists, but decreased when BDL rats and Mdr2-/- mice were treated with 5HTR2A/2B/2C antagonists compared to BDL rats and Mdr2-/- mice, respectively. 5HT levels increase in Mdr2-/- mice and in PSC human patients compared to their controls and decrease in serum of Mdr2-/- mice treated with 5HTR2A/2B/2C antagonists compared to untreated Mdr2-/- mice. In vitro, cell lines of murine cholangiocytes and human HSCs express 5HTR2A/2B/2C and MAO-A/TPH1; treatment of these cell lines with 5HTR2A/2B/2C antagonists or TPH1 inhibitor decreased 5HT levels as well as expression of fibrosis and inflammation genes compared to controls. CONCLUSIONS: Modulation of the TPH1/MAO-A/5HT/5HTR2A/2B/2C axis may represent a therapeutic approach for management of cholangiopathies, including PSC.

5-HT2A and 5-HT3 receptors contribute to the exacerbation of targeted and non-targeted effects of ionizing radiation-induced cell death in human colon carcinoma cells.

Purpose: Serotonin (5-HT) is implicated in the underlying mechanisms which mediate cell death following ionizing radiation exposure, however, effects appear to be cell type-dependent. We sought to further characterize the role of 5-HT and 5-HT receptors (5-HTRs) in the exacerbation of cell death following ionizing radiation exposure in human colon carcinoma cells.Materials and methods: We examined the clonogenic survival of colon carcinoma HCT116 cells treated with 5-HT and the selective 5-HTR antagonists ketanserin (5-HT2A) and ondansetron (5-HT3), following exposure to direct ionizing radiation and irradiated cell-conditioned medium (ICCM). The relative expression of these target receptors was measured using western blotting.Results: Western blotting results revealed that relative protein levels of the 5-HT2A and 5-HT3 receptors were similar. 5-HT concentration-dependent increases in cell death that occurred following direct ionizing radiation exposure were abolished by both 5-HTR antagonists. Death of nonirradiated cells recipient of ICCM was increased in a concentration-dependent manner by 5-HT when present during donor cell irradiation. Both 5-HTR antagonists completely abolished the increases in bystander-induced cell death generated by 5-HT. Finally, we show that exposure of cells to 5-HT prior to receipt of ICCM can also dictate the degree of bystander-induced cell death.Conclusions: Our findings demonstrate a definitive role for 5-HT in the exacerbation of cell death following ionizing radiation exposure in colon carcinoma cells and highlight 5-HTRs as potential markers for predicting cellular radiosensitivity.

Serotonin 5-HT2A and 5-HT2C receptors regulate rat maternal behavior through distinct behavioral and neural mechanisms.

Serotonin 5-HT2A and 5-HT2C receptors play important yet distinctive roles in the regulation of rat maternal behavior. The present study investigated their neural substrates and explored the possible behavioral mechanisms (i.e., behavioral organization or maternal motivation). Sprague-Dawley postpartum females were microinjected with either a selective 5-HT2A agonist (TCB-2, 0.4 or 4.0 µg/side) or a 5-HT2C agonist (MK212, 2.5 or 5.0 µg/side) into the medial prefrontal cortex (mPFC) or ventral tegmental area (VTA). Ten and 60 min later, their maternal activities were observed in the home cage; and their motivational responses towards pups were examined in a pup preference test and pup retrieval test throughout the first week of postpartum. In the mPFC, TCB-2 microinjection disrupted major components of maternal behavior (e.g., pup retrieval, pup crouching), as well as the sequential pup retrieval score (a measure of behavioral organization). In contrast, MK212 microinjection had a minimal disruption of maternal behavior. In the VTA, TCB-2 microinjection impaired pup retrieval, nest building, and pup crouching, whereas MK212 microinjection severely impaired pup retrieval, nest building and pup crouching. Moreover, only intra-VTA injection of MK212 significantly suppressed pup preference. Together, our data suggest that 5-HT2A receptors in the mPFC and VTA may play an important role in the behavioral organization or executive control of maternal activities, but not in the motivational processing of the rewarding value of pups (maternal motivation). In contrast, 5-HT2C receptors in the VTA play a critical role in maternal motivation, but not in the organization of maternal responses.

Allergic sensitization increases contractile responses to 5-HT in guinea pig aorta.

Epidemiological and clinical studies suggest that asthma is associated with adverse cardiovascular outcomes, but its mechanism is uncertain. 5-Hydroxytryptamine (5-HT) is a mediator involved in asthma and in cardiovascular functioning. Thus, in the present study, we explored whether allergic sensitization in guinea pigs modifies 5-HT-induced contractile responses and 5-HT2A receptor expression in thoracic aorta rings. We found that sensitization produced a significant increase of 100 microM 5-HT-induced contractions of aorta rings (~27 % greater contraction than in non-sensitized animals, p<0.05). Preincubation with 10 nM ketanserin (a 5-HT2A receptor antagonist) reduced by ~30 % (p=0.003) and ~36 % (p=0.005) the area under the curve of 5-HT-induced contractions in aortas from non-sensitized and sensitized animals, respectively. There were no differences between sensitized and non-sensitized animals with respect to mRNA (qPCR) and protein (Western blot) expression of 5-HT2A receptor in thoracic aortas. We concluded that in this guinea pig model of asthma, allergic sensitization is not confined to airways, but also affects arterial contractile responses to 5-HT; changes in the expression of the 5-HT2A receptor appear not to be involved in this phenomenon.

A Hallucinogenic Serotonin-2A Receptor Agonist Reduces Visual Response Gain and Alters Temporal Dynamics in Mouse V1.

Sensory perception arises from the integration of externally and internally driven representations of the world. Disrupted balance of these representations can lead to perceptual deficits and hallucinations. The serotonin-2A receptor (5-HT2AR) is associated with such perceptual alterations, both in its role in schizophrenia and in the action of hallucinogenic drugs. Despite this powerful influence on perception, relatively little is known about how serotonergic hallucinogens influence sensory processing in the neocortex. Using widefield and two-photon calcium imaging and single-unit electrophysiology in awake mice, we find that administration of the hallucinogenic selective 5-HT2AR agonist DOI (2,5-dimethoxy-4-iodoamphetamine) leads to a net reduction in visual response amplitude and surround suppression in primary visual cortex, as well as disrupted temporal dynamics. However, basic retinotopic organization, tuning properties, and receptive field structure remain intact. Our results provide support for models of hallucinations in which reduced bottom-up sensory drive is a key factor leading to altered perception.

Serotonin 2A receptors are a stress response system: implications for post-traumatic stress disorder.

Serotonin, one of the first neurotransmitters to be identified, is an evolutionarily old molecule that is highly conserved across the animal kingdom, and widely used throughout the brain. Despite this, ascribing a specific set of functions to brain serotonin and its receptors has been difficult and controversial. The 2A subtype of serotonin receptors (5-HT2A receptor) is the major excitatory serotonin receptor in the brain and has been linked to the effects of drugs that produce profound sensory and cognitive changes. Numerous studies have shown that this receptor is upregulated by a broad variety of stressors, and have related 5-HT2A receptor function to associative learning. This review proposes that stress, particularly stress related to danger and existential threats, increases the expression and function of 5-HT2A receptors. It is argued that this is a neurobiological adaptation to promote learning and avoidance of danger in the future. Upregulation of 5-HT2A receptors during stressful events forms associations that tune the brain to environmental cues that signal danger. It is speculated that life-threatening situations may activate this system and contribute to the symptoms associated with post-traumatic stress disorder (PTSD). 3,4-Methylenedioxymethamphetamine, which activates 5-HT2A receptors, has been successful in the treatment of PTSD and has recently achieved status as a breakthrough therapy. An argument is presented that 3,4-methylenedioxymethamphetamine may paradoxically act through these same 5-HT2A receptors to ameliorate the symptoms of PTSD. The central thematic contention is that a key role of serotonin may be to function as a stress detection and response system.

The test retest model of anxiety: An appraisal of findings to explain benzodiazepine tolerance.

A test retest protocol in animal model of anxiety induces an increase of anxious behavior and a loss of benzodiazepine-induced effect. This effect, known as the "one trial tolerance", is mainly observed in the elevated plus maze, an ethological model of anxiety in mice, but also in the four plate test, a model based on punishment. A review of some hypotheses based on behavioral, pharmacological and neurochemical approaches are proposed here to explain this benzodiazepines tolerance phenomenon.

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.

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.