Reduced serotonergic transmission alters sensitivity to cost and reward via 5-HT1A and 5-HT1B receptors in monkeys.

Serotonin (5-HT) deficiency is a core biological pathology underlying depression and other psychiatric disorders whose key symptoms include decreased motivation. However, the exact role of 5-HT in motivation remains controversial and elusive. Here, we pharmacologically manipulated the 5-HT system in macaque monkeys and quantified the effects on motivation for goal-directed actions in terms of incentives and costs. Reversible inhibition of 5-HT synthesis increased errors and reaction times on goal-directed tasks, indicating reduced motivation. Analysis found incentive-dependent and cost-dependent components of this reduction. To identify the receptor subtypes that mediate cost and incentive, we systemically administered antagonists specific to 4 major 5-HT receptor subtypes: 5-HT1A, 5-HT1B, 5-HT2A, and 5-HT4. Positron emission tomography (PET) visualized the unique distribution of each subtype in limbic brain regions and determined the systemic dosage for antagonists that would achieve approximately 30% occupancy. Only blockade of 5-HT1A decreased motivation through changes in both expected cost and incentive; sensitivity to future workload and time delay to reward increased (cost) and reward value decreased (incentive). Blocking the 5-HT1B receptor also reduced motivation through decreased incentive, although it did not affect expected cost. These results suggest that 5-HT deficiency disrupts 2 processes, the subjective valuation of costs and rewards, via 5-HT1A and 5-HT1B receptors, thus leading to reduced motivation.

5-HT modulation of a medial septal circuit tunes social memory stability.

Social memory-the ability to recognize and remember familiar conspecifics-is critical for the survival of an animal in its social group1,2. The dorsal CA2 (dCA2)3-5 and ventral CA1 (vCA1)6 subregions of the hippocampus, and their projection targets6,7, have important roles in social memory. However, the relevant extrahippocampal input regions remain poorly defined. Here we identify the medial septum (MS) as a dCA2 input region that is critical for social memory and reveal that modulation of the MS by serotonin (5-HT) bidirectionally controls social memory formation, thereby affecting memory stability. Novel social interactions increase activity in dCA2-projecting MS neurons and induce plasticity at glutamatergic synapses from MS neurons onto dCA2 pyramidal neurons. The activity of dCA2-projecting MS cells is enhanced by the neuromodulator 5-HT acting on 5-HT1B receptors. Moreover, optogenetic manipulation of median raphe 5-HT terminals in the MS bidirectionally regulates social memory stability. This work expands our understanding of the neural mechanisms by which social interactions lead to social memory and provides evidence that 5-HT has a critical role in promoting not only prosocial behaviours8,9, but also social memory, by influencing distinct target structures.

Serotonin Signaling in Hippocampus during Initial Cocaine Abstinence Drives Persistent Drug Seeking.

The initiation of abstinence after chronic drug self-administration is stressful. Cocaine-seeking behavior on the first day of the absence of the expected drug (Extinction Day 1, ED1) is reduced by blocking 5-HT signaling in dorsal hippocampal cornu ammonis 1 (CA1) in both male and female rats. We hypothesized that the experience of ED1 can substantially influence later relapse behavior and that dorsal raphe (DR) serotonin (5-HT) input to CA1 may be involved. We inhibited 5-HT1A/1B receptors (WAY-100635 plus GR-127935), or DR input (chemogenetics), in CA1 on ED1 to test the role of this pathway on cocaine-seeking persistence 2 weeks later. We also inhibited 5-HT1A or 5-HT1B receptors in CA1 during conditioned place preference (CPP) for cocaine, to examine mechanisms involved in the persistent effects of ED1 manipulations. Inhibition of DR inputs, or 5-HT1A/1B signaling, in CA1 decreased drug seeking on ED1 and decreased cocaine seeking 2 weeks later revealing that 5-HT signaling in CA1 during ED1 contributes to persistent drug seeking during abstinence. In addition, 5-HT1B antagonism alone transiently decreased drug-associated memory performance when given prior to a CPP test, whereas similar antagonism of 5-HT1A alone had no such effect but blocked CPP retrieval on a test 24 h later. These CPP findings are consistent with prior work showing that DR inputs to CA1 augment recall of the drug-associated context and drug seeking via 5-HT1B receptors and prevent consolidation of the updated nondrug context via 5-HT1A receptors. Thus, treatments that modulate 5-HT-dependent memory mechanisms in CA1 during initial abstinence may facilitate later maintenance of abstinence.

Altered Serotonin 1B Receptor Binding After Escitalopram for Depression Is Correlated With Treatment Effect.

BACKGROUND: Major depressive disorder (MDD) is commonly treated with selective serotonin reuptake inhibitors (SSRIs). SSRIs inhibit the serotonin transporter (5-HTT), but the downstream antidepressant mechanism of action of these drugs is poorly understood. The serotonin 1B (5-HT1B) receptor is functionally linked to 5-HTT and 5-HT1B receptor binding and 5-HT1B receptor mRNA is reduced in the raphe nuclei after SSRI administration in primates and rodents, respectively. The effect of SSRI treatment on 5-HT1B receptor binding in patients with MDD has not been examined previously. This positron emission tomography (PET) study aimed to quantify brain 5-HT1B receptor binding changes in vivo after SSRI treatment for MDD in relation to treatment effect. METHODS: Eight unmedicated patients with moderate to severe MDD underwent PET with the 5-HT1B receptor radioligand [11C]AZ10419369 before and after 3 to 4 weeks of treatment with the SSRI escitalopram 10 mg daily. Depression severity was assessed at time of PET and after 6 to 7 weeks of treatment with the Montgomery-Åsberg Depression Rating Scale. RESULTS: We observed a significant reduction in [11C]AZ10419369 binding in a dorsal brainstem (DBS) region containing the median and dorsal raphe nuclei after escitalopram treatment (P = .036). Change in DBS [11C]AZ10419369 binding correlated with Montgomery-Åsberg Depression Rating Scale reduction after 3-4 (r = 0.78, P = .021) and 6-7 (r = 0.94, P < .001) weeks' treatment. CONCLUSIONS: Our findings align with the previously reported reduction of 5-HT1B receptor binding in the raphe nuclei after SSRI administration and support future studies testing change in DBS 5-HT1B receptor binding as an SSRI treatment response marker.

The rs6296 polymorphism in the 5-HT1b receptor in Dutch men with lifelong premature ejaculation: a genetic case-control association study.

BACKGROUND: Lifelong premature ejaculation (LPE) is a rare sexual condition believed to be caused by genetic neurobiological disorders. AIM: In this study we sought to evaluate the genetic association between the rs6296 polymorphism of the 5-HT1b receptor and intravaginal ejaculation latency times (IELTs) in men with LPE compared with men in a control group. METHODS: This study was a prospective observational genetic case-control association study. The LPE definition of the International Society for Sexual Medicine (ISSM) 2013 was used. Patients were recruited in 2005-2009 while attending the department of Neurosexology, HagaZiekenhuis, the Netherlands. We obtained IELTs with the stopwatch method. Polymerase chain reaction (PCR) was used for genotyping rs6296. A randomly selected group of European Caucasian men from the 1000GENOMES project was used as a control group. OUTCOMES: Study outcomes included results of comparisons of analysis of variance (ANOVA) tests between genotypes and IELTs in study participants, genotypes of cases and controls determined with the chi-square test, and expressions of allelotype- and genotype-specific risks for LPE determined with odds ratios. RESULTS: In total, 67 men with LPE were included in this study. The geometric mean (SD) IELT was 32.0 (27.4) seconds and was non-normally distributed. Genotype frequencies consisted of 29 (43.3%) GG, 31 (46.3%) GC, and 7(10.4%) CC individuals in the LPE group. Log-transformed IELTs were not statistically significant (per ANOVA tests) in men with GG, GC, or CC genotypes (P = .54). Genotype frequencies consisted of 16 (6.6%) GG; 93 (38.8%) GC, and 131 (54.6%) CC individuals in the control group (n = 240). Significant differences were found when comparing allele (P = 1.02e-17) and genotype (P = 3.22e-16) frequencies in cases and controls using a chi-square test. A statistically significant increased risk for LPE was found for carriers of the G allele (OR 5.62; 95% CI 4.13-9.42). Statistically significant risks were also found for the CG genotype (OR 6.24; 95% CI 2.63-14.77) and the GG genotype (OR 33.92; 95% CI 12.79-89.93). CLINICAL IMPLICATIONS: By investigating polymorphisms in target genes the neuro-pathophysiology of LPE could be further elaborated, potentially leading to more effective treatment. STRENGTHS AND LIMITATIONS: This is to our knowledge the first study investigating rs6296 with regard to LPE. By using a strict definition for LPE (ISSM 2013) and using the stopwatch method for measuring IELTs, bias in selection of true LPE patients will be relatively low. This study is limited by a relatively small study population and the lack of IELT data in the control group. CONCLUSIONS: This study shows a genetic association in rs6296 in men with LPE compared with healthy controls. This result warrants attempted replication in future studies.

Cryo-EM structure of the serotonin 5-HT1B receptor coupled to heterotrimeric Go.

G-protein-coupled receptors (GPCRs) form the largest family of receptors encoded by the human genome (around 800 genes). They transduce signals by coupling to a small number of heterotrimeric G proteins (16 genes encoding different α-subunits). Each human cell contains several GPCRs and G proteins. The structural determinants of coupling of Gs to four different GPCRs have been elucidated1-4, but the molecular details of how the other G-protein classes couple to GPCRs are unknown. Here we present the cryo-electron microscopy structure of the serotonin 5-HT1B receptor (5-HT1BR) bound to the agonist donitriptan and coupled to an engineered Go heterotrimer. In this complex, 5-HT1BR is in an active state; the intracellular domain of the receptor is in a similar conformation to that observed for the ß2-adrenoceptor (ß2AR) 3 or the adenosine A2A receptor (A2AR) 1 in complex with Gs. In contrast to the complexes with Gs, the gap between the receptor and the Gß-subunit in the Go-5-HT1BR complex precludes molecular contacts, and the interface between the Gα-subunit of Go and the receptor is considerably smaller. These differences are likely to be caused by the differences in the interactions with the C terminus of the Go α-subunit. The molecular variations between the interfaces of Go and Gs in complex with GPCRs may contribute substantially to both the specificity of coupling and the kinetics of signalling.

Associations Between Cognition and Serotonin 1B Receptor Availability in Healthy Volunteers: A [11C]AZ10419369 Positron Emission Tomography Study.

BACKGROUND: The serotonin system has been implicated in several psychiatric disorders. All major psychiatric disorders are associated with cognitive impairment, but treatment improving cognitive deficits is lacking, partly due to limited understanding of the neurobiology of cognitive functioning. Several markers for the serotonin system have been associated with cognitive functions. Our research group previously has reported a positive correlation between serotonin (5-HT1B) receptor availability in the dorsal brainstem and visuospatial memory in a pilot study of healthy individuals. Here, we aim to replicate our previous finding in a larger group of healthy volunteers as well as to investigate putative associations between 5-HT1B receptor availability and other cognitive domains. METHODS: Forty-three healthy individuals were examined with positron emission tomography using the 5-HT1B receptor radioligand [11C]AZ10419369 and a visuospatial memory test to replicate our previous finding as well as tests of verbal fluency, cognitive flexibility, reaction time, and planning ability to explore other domains potentially associated with the serotonin system. RESULTS: Replication analysis revealed no statistically significant association between 5-HT1B receptor availability in the dorsal brainstem and visuospatial memory performance. Exploratory analyses showed age-adjusted correlations between 5-HT1B receptor availability in whole brain gray matter and specific brain regions, and number of commission errors, reaction time, and planning ability. CONCLUSIONS: Higher 5-HT1B receptor availability was associated with more false-positive responses and faster reaction time but lower performance in planning and problem-solving. These results corroborate previous research supporting an important role of the serotonin system in impulsive behavior and planning ability.

A midbrain-reticulotegmental circuit underlies exaggerated startle under fear emotions.

Exaggerated startle has been recognized as a core hyperarousal symptom of multiple fear-related anxiety disorders, such as post-traumatic stress disorder (PTSD) and panic disorder. However, the mechanisms driving this symptom are poorly understood. Here we reveal a neural projection from dorsal raphe nucleus (DRN) to a startle-controlling center reticulotegmental nucleus (RtTg) that mediates enhanced startle response under fear condition. Within RtTg, we identify an inhibitory microcircuit comprising GABAergic neurons in pericentral RtTg (RtTgP) and glutamatergic neurons in central RtTg (RtTgC). Inhibition of this RtTgP-RtTgC microcircuit leads to elevated startle amplitudes. Furthermore, we demonstrate that the conditioned fear-activated DRN 5-HTergic neurons send inhibitory projections to RtTgP GABAergic neurons, which in turn upregulate neuronal activities of RtTgC glutamatergic neurons. Chemogenetic activation of the DRN-RtTgP projections mimics the increased startle response under fear emotions. Moreover, conditional deletion of 5-HT1B receptor from RtTgP GABAergic neurons largely reverses the exaggeration of startle during conditioned fear. Thus, our study establishes the disinhibitory DRN-RtTgP-RtTgC circuit as a critical mechanism underlying exaggerated startle under fear emotions, and provides 5-HT1B receptor as a potential therapeutic target for treating hyperarousal symptom in fear-associated psychiatric disorders.

Heterozygous and homozygous gene knockout of the 5-HT1B receptor have different effects on methamphetamine-induced behavioral sensitization.

The psychostimulant drug methamphetamine (METH) causes euphoria in humans and locomotor hyperactivity in rodents by acting on the mesolimbic dopamine (DA) pathway and has severe abuse and addiction liability. Behavioral sensitization, an increased behavioral response to a drug with repeated administration, can persist for many months after the last administration. Research has shown that the serotonin 1B (5-HT1B) receptor plays a critical role in the development and maintenance of drug addiction, as well as other addictive behaviors. This study examined the role of 5-HT1B receptors in METH-induced locomotor sensitization using 5-HT1B knockout (KO) mice. To clarify the action of METH in 5-HT1B KO mice the effects of METH on extracellular levels of DA (DAec) and 5-HT (5-HTec) in the caudate putamen (CPu) and the nucleus accumbens (NAc) were examined. Locomotor sensitization and extracellular monoamine levels were determined in wild-type mice (5-HT1B +/+), heterozygous 5-HT1B receptor KO (5-HT1B +/-) mice and homozygous 5-HT1B receptor KO mice (5-HT1B -/-). Behavioral sensitization to METH was enhanced in 5-HT1B -/- mice compared to 5-HT1B +/+ mice but was attenuated in 5-HT1B +/- mice compared to 5-HT1B +/+ and 5-HT1B -/- mice. In vivo, microdialysis demonstrated that acute administration of METH increases DAec levels in the CPu and NAc of 5-HT1B KO mice compared to saline groups. In 5-HT1B +/- mice, METH increased 5-HTec levels in the CPu, and DAec levels in the NAc were higher than in others.5-HT1B receptors play an important role in regulating METH-induced behavioral sensitization.

Lrp5 controls bone formation by inhibiting serotonin synthesis in the duodenum.

Loss- and gain-of-function mutations in the broadly expressed gene Lrp5 affect bone formation, causing osteoporosis and high bone mass, respectively. Although Lrp5 is viewed as a Wnt coreceptor, osteoblast-specific disruption of beta-Catenin does not affect bone formation. Instead, we show here that Lrp5 inhibits expression of Tph1, the rate-limiting biosynthetic enzyme for serotonin in enterochromaffin cells of the duodenum. Accordingly, decreasing serotonin blood levels normalizes bone formation and bone mass in Lrp5-deficient mice, and gut- but not osteoblast-specific Lrp5 inactivation decreases bone formation in a beta-Catenin-independent manner. Moreover, gut-specific activation of Lrp5, or inactivation of Tph1, increases bone mass and prevents ovariectomy-induced bone loss. Serotonin acts on osteoblasts through the Htr1b receptor and CREB to inhibit their proliferation. By identifying duodenum-derived serotonin as a hormone inhibiting bone formation in an Lrp5-dependent manner, this study broadens our understanding of bone remodeling and suggests potential therapies to increase bone mass.

Serotonergic modulation of visual neurons in Drosophila melanogaster.

Sensory systems rely on neuromodulators, such as serotonin, to provide flexibility for information processing as stimuli vary, such as light intensity throughout the day. Serotonergic neurons broadly innervate the optic ganglia of Drosophila melanogaster, a widely used model for studying vision. It remains unclear whether serotonin modulates the physiology of interneurons in the optic ganglia. To address this question, we first mapped the expression patterns of serotonin receptors in the visual system, focusing on a subset of cells with processes in the first optic ganglion, the lamina. Serotonin receptor expression was found in several types of columnar cells in the lamina including 5-HT2B in lamina monopolar cell L2, required for spatiotemporal luminance contrast, and both 5-HT1A and 5-HT1B in T1 cells, whose function is unknown. Subcellular mapping with GFP-tagged 5-HT2B and 5-HT1A constructs indicated that these receptors localize to layer M2 of the medulla, proximal to serotonergic boutons, suggesting that the medulla neuropil is the primary site of serotonergic regulation for these neurons. Exogenous serotonin increased basal intracellular calcium in L2 terminals in layer M2 and modestly decreased the duration of visually induced calcium transients in L2 neurons following repeated dark flashes, but otherwise did not alter the calcium transients. Flies without functional 5-HT2B failed to show an increase in basal calcium in response to serotonin. 5-HT2B mutants also failed to show a change in amplitude in their response to repeated light flashes but other calcium transient parameters were relatively unaffected. While we did not detect serotonin receptor expression in L1 neurons, they, like L2, underwent serotonin-induced changes in basal calcium, presumably via interactions with other cells. These data demonstrate that serotonin modulates the physiology of interneurons involved in early visual processing in Drosophila.

On Associations between Fear-Induced Aggression, Bdnf Transcripts, and Serotonin Receptors in the Brains of Norway Rats: An Influence of Antiaggressive Drug TC-2153.

The Bdnf (brain-derived neurotrophic factor) gene contains eight regulatory exons (I-VIII) alternatively spliced to the protein-coding exon IX. Only exons I, II, IV, and VI are relatively well studied. The BDNF system and brain serotonergic system are tightly interconnected and associated with aggression. The benzopentathiepine TC-2153 affects both systems and exerts antiaggressive action. Our aim was to evaluate the effects of TC-2153 on the Bdnf exons I-IX's expressions and serotonin receptors' mRNA levels in the brain of rats featuring high aggression toward humans (aggressive) or its absence (tame). Aggressive and tame adult male rats were treated once with vehicle or 10 or 20 mg/kg of TC-2153. mRNA was quantified in the cortex, hippocampus, hypothalamus, and midbrain with real-time PCR. Selective breeding for high aggression or its absence affected the serotonin receptors' and Bdnf exons' transcripts differentially, depending on the genotype (strain) and brain region. TC-2153 had comprehensive effects on the Bdnf exons' expressions. The main trend was downregulation in the hypothalamus and midbrain. TC-2153 increased 5-HT1B receptor hypothalamusc mRNA expression. For the first time, an influence of TC-2153 on the expressions of Bdnf regulatory exons and the 5-HT1B receptor was shown, as was an association between Bdnf regulatory exons and fear-induced aggression involving genetic predisposition.

The Effect of Serotonin Receptor 5-HT1B on Lateral Inhibition between Spiny Projection Neurons in the Mouse Striatum.

The principal neurons of the striatum, the spiny projection neurons (SPNs), make inhibitory synaptic connections with each other via collaterals of their main axon, forming a local lateral inhibition network. Serotonin, acting via the 5-HT1B receptor, modulates neurotransmitter release from SPN terminals in striatal output nuclei, but the role of 5-HT1B receptors in lateral inhibition among SPNs in the striatum is unknown. Here, we report the effects of 5-HT1B receptor activation on lateral inhibition in the mouse striatum. Whole-cell recordings were made from SPNs in acute brain slices of either sex, while optogenetically activating presynaptic SPNs or fast-spiking interneurons (FSIs). Activation of 5-HT1B receptors significantly reduced the amplitude of IPSCs evoked by optical stimulation of both direct and indirect pathway SPNs. This reduction was blocked by application of a 5-HT1B receptor antagonist. Activation of 5-HT1B receptors did not reduce the amplitude of IPSCs evoked from FSIs. These results suggest a new role for serotonin as a modulator of lateral inhibition among striatal SPNs. The 5-HT1B receptor may, therefore, be a suitable target for future behavioral experiments investigating the currently unknown role of lateral inhibition in the function of the striatum.SIGNIFICANCE STATEMENT We show that stimulation of serotonin receptors reduces the efficacy of lateral inhibition between spiny projection neurons (SPNs), one of the biggest GABAergic sources in the striatum, by activation of the serotonin 5-HT1B receptor. The striatum receives serotonergic input from the dorsal raphe nuclei and is important in behavioral brain functions like learning and action selection. Our findings suggest a new role for serotonin in modulating the dynamics of neural interactions in the striatum, which extends current knowledge of the mechanisms of the behavioral effects of serotonin.

Serotonin 5-HT1A and 5-HT1B receptor-mediated inhibition of glutamatergic transmission onto rat basal forebrain cholinergic neurones.

Cholinergic neurones in the basal forebrain (BF) project into various brain regions and receive excitatory inputs from the cortex and brain stem. These cholinergic neurones receive serotonergic fibres from the dorsal raphe nuclei. This study was aimed to elucidate serotonin (5-HT)-induced modulation of glutamatergic transmission onto rat BF cholinergic neurones identified with Cy3-192IgG. Excitatory postsynaptic currents (EPSCs) were evoked by focal stimulation. Bath application of either 5-HT, the 5-HT1A receptor agonist 8-OH-DPAT (DPAT), or the 5-HT1B receptor agonist CP93129 (CP), inhibited the amplitude of EPSCs. In the presence of both 5-HT1A and 5-HT1B receptor antagonists, the 5-HT-induced effect disappeared. The paired-pulse ratio (PPR) and coefficient of variation (CV) of the EPSCs were increased by CP, whereas DPAT had no effect on PPR or CV. DPAT inhibited the inward currents induced by puff application of l-glutamate, which were unaffected by CP. DPAT suppressed the amplitude of miniature EPSCs (mEPSCs) without affecting their frequency. CP decreased the frequency of mEPSCs in more than half of the neurones examined, whereas the amplitude was unaffected. DPAT or CP alone inhibited the NMDA receptor-mediated currents. 5-HT-induced inhibition of EPSCs was reduced in the presence of ω-agatoxin TK (Aga). Furthermore, CP-induced inhibition of EPSCs was eliminated in the presence of Aga. DPAT-induced inhibition of EPSCs was unchanged in the presence of Aga. These results suggest that activation of 5-HT1A receptors reduces the sensitivity of postsynaptic glutamate receptors to glutamate, whereas presynaptic activation of 5-HT1B receptors inhibits glutamate release by blocking P/Q-type calcium channels. KEY POINTS: We performed a patch-clamp study to investigate serotonin (5-HT)-induced modulation of glutamatergic transmission onto cholinergic neurones in the rat basal forebrain slices. Excitatory postsynaptic currents (EPSCs) were inhibited by 5-HT as well as agonists of 5-HT1A or 5-HT1B receptors. 5-HT-induced inhibition was antagonized by co-application of 5-HT1A and 5-HT1B receptor antagonists. The effects of 5-HT receptor agonists on the paired-pulse ratio, coefficient of variation of EPSCs, inward currents induced by puff application of l-glutamate as well as miniature EPSCs suggest that activation of 5-HT1A receptors decreases the sensitivity of postsynaptic glutamate receptors to glutamate, whereas 5-HT1B receptors presynaptically inhibit glutamate release. The 5-HT1B agonist-induced inhibition was eliminated in the presence of a P/Q-type calcium channel blocker, whereas the 5-HT1A agonist still inhibited the EPSCs even in the presence of the blocker. The present study reveals different pre- and postsynaptic mechanisms underlying 5-HT1A and 5-HT1B receptor-mediated modulation of excitatory transmission.

5-HT and α-m-5-HT attenuate excitatory synaptic transmissions onto the lateral amygdala principal neurons via presynaptic 5-HT1B receptors.

Accumulating evidence suggests that the serotonergic (5-HT) system in the amygdala has significant effects on affective states. Dysregulation of the 5-HT system in the basolateral amygdaloid complex causes affective disorders. To search for therapeutic targets, subtype specification of 5-HT receptors is crucial. The present study was undertaken to identify the 5-HT receptor subtype responsible for the 5-HT-mediated suppression of excitatory transmission to principal neurons (PNs) in the lateral amygdala (LA). Whole-cell recordings were performed to record excitatory post synaptic currents (EPSCs) in acute rat brain slices. We confirmed that 5-HT and α-m-5-HT, a broad 5-HT2 receptor agonist, attenuated EPSCs in LA PNs. The extent of suppressions by 5-HT and α-m-5-HT remained unchanged in the presence of ritanserin, a broad 5-HT2 receptor antagonist. In the presence of NAS-181, a selective 5-HT1B receptor antagonist, the extent of EPSC suppressions by 5-HT and α-m-5-HT was diminished. CP93129, a selective 5-HT1B receptor agonist, attenuated EPSCs in LA PNs, and this effect was abolished in the presence of NAS-181. Additionally, the paired-pulse ratio of EPSCs was increased by CP93129. Thus, our results indicate that 5-HT and α-m-5-HT attenuate excitatory transmissions to LA PNs via presynaptic 5-HT1B receptors.

Serotonin 1B receptor effects on response inhibition are independent of inhibitory learning.

Impulsivity is defined in terms of deficits in instrumental response inhibition, when the inability to withhold an action produces a negative outcome. However, there are many behavioral and cognitive constructs which theoretically could contribute to disordered impulsivity, including Pavlovian responding, which few studies have considered in this context. In the present set of studies, we examine Pavlovian inhibitory learning and excitatory responding in a mouse model for dysregulated impulsivity, specifically, mice lacking the serotonin 1B receptor (5-HT1BR). Consistent with previous results, we show that these mice display increased impulsivity as measured by premature responding in the operant 5-choice serial reaction time test. In a Pavlovian conditioned inhibition paradigm, they also show a decreased ability to withhold responding, but importantly have an intact ability to learn inhibitory associations. In a Pavlovian appetitive conditioning experiment, 5-HT1BR knockout mice show normal responding under a positive contingency schedule, however, they display increased responding to cues presented on an independent schedule from reinforcement in a zero contingency schedule. Interestingly this difference does not occur when the cues are explicitly unpaired in a negative contingency schedule, nor during a 25% reinforcement schedule. Overall, while our results show that the deficits in operant response inhibition in mice lacking 5-HT1BR are likely not due to Pavlovian inhibitory or excitatory learning, it is relevant to consider associative learning in the context of dysregulated impulsive behavior.

Minimally modified low-density lipoprotein upregulates mouse mesenteric arterial 5-HT1B receptor in vivo via activation of the JAK2/STAT3 pathway.

OBJECTIVES: To explore whether minimally modified low-density lipoprotein (mmLDL) upregulates mesenteric arterial 5-hydroxytryptamine 1B (5-HT1B) receptor expression by activating the JAK2/STAT3 signaling pathway. METHODS: Mice were randomly divided into the following groups: the normal saline (NS), LDL, mmLDL, mmLDL+galiellactone (GL, a JAK2/STAT3 pathway inhibitor), and mmLDL+DMSO groups. The dose-response curve of mesenteric arterial ring constriction after administration of 5-carboxamidotryptamine (5-CT), an agonist of 5-HT1B, was recorded with a microvascular tensiometer. JAK2, p-JAK2, STAT3, p-STAT3, and 5-HT1B receptor protein expression levels were determined by Western blotting. 5-HT1B receptor mRNA levels were measured by RT-PCR. 5-HT1B receptor protein expression was determined by immunofluorescence. RESULTS: Injection of mmLDL into the tail vein significantly increased the contractile dose-response curve after 5-CT stimulation, as the Emax was 82.15 ±â€¯6.15% in the NS group and 171.88 ±â€¯5.78% in the mmLDL group (P < 0.01); significantly elevated 5-HT1B receptor mRNA and protein expression levels; and significantly increased p-JAK2 and p-STAT3 protein expression levels. After intraperitoneal injection of GL, the vasoconstrictive response was significantly reduced compared with that in the mmLDL group, as the Emax was decreased to 97.14 ±â€¯1.20% (P < 0.01); 5-HT1B receptor mRNA and protein expression levels were significantly reduced; STAT3 phosphorylation and p-JAK2 and p-STAT3 protein expression were not significantly changed; and 5-HT1B receptor expression was altered via inhibition of p-STAT3 binding to DNA, which suppressed transcription. CONCLUSIONS: mmLDL can upregulate 5-HT1B receptor expression in mouse mesenteric arteries by activating the JAK2/STAT3 signaling pathway.

Serotonin functions as a bidirectional guidance molecule regulating growth cone motility.

The neurotransmitter serotonin has been implicated in a range of complex neurological disorders linked to alterations of neuronal circuitry. Serotonin is synthesized in the developing brain before most neuronal circuits become fully functional, suggesting that serotonin might play a distinct regulatory role in shaping circuits prior to its function as a classical neurotransmitter. In this study, we asked if serotonin acts as a guidance cue by examining how serotonin alters growth cone motility of rodent sensory neurons in vitro. Using a growth cone motility assay, we found that serotonin acted as both an attractive and repulsive guidance cue through a narrow concentration range. Extracellular gradients of 50 µM serotonin elicited attraction, mediated by the serotonin 5-HT2a receptor while 100 µM serotonin elicited repulsion mediated by the 5-HT1b receptor. Importantly, high resolution imaging of growth cones indicated that these receptors signalled through their canonical pathways of endoplasmic reticulum-mediated calcium release and cAMP depletion, respectively. This novel characterisation of growth cone motility in response to serotonin gradients provides compelling evidence that secreted serotonin acts at the molecular level as an axon guidance cue to shape neuronal circuit formation during development.

The impact of HTR1A and HTR1B methylation combined with stress/genotype on early antidepressant efficacy.

AIMS: Antidepressants are effective in the treatment of major depressive disorder (MDD), while many patients fail to respond to antidepressants. Both 5-HT1A (HTR1A) and 5-HT1B (HTR1B) receptors play an important role in antidepressant activity. Meanwhile, DNA methylation is associated with MDD and antidepressant efficacy. In this study we investigate the influence of HTR1A and HTR1B methylation combined with stress/genotype on antidepressant efficacy. METHODS: A total of 291 MDD patients and 100 healthy controls received the Life Events Scale (LES) and the Childhood Trauma Questionnaire (CTQ) as stress assessment. Eight single nucleotide polymorphisms (SNPs) of HTR1A and HTR1B involved in antidepressant mechanisms were tested. Methylation status in 181 cytosine-phosphate-guanine (CpG) sites of HTR1A and HTR1B were assessed. All MDD patients were divided into response (RES) and non-response (NRES) after 2 weeks of antidepressant treatment. Logistic regression was conducted for interactions between methylation, NLES/CTQ score and genotype. RESULTS: Low HTR1A-2-143 methylation is connected with better antidepressant efficacy in subgroup. Low HTR1A-2-143 methylation combined with low CTQ score is related to better antidepressant efficacy. The interaction between high HTR1B methylation with the rs6298 AA/AG genotype affects better antidepressant efficacy. CONCLUSIONS: HTR1A and HTR1B methylation combined with stress/genotype is associated with antidepressant efficacy.

Lasmiditan and 5-Hydroxytryptamine in the rat trigeminal system; expression, release and interactions with 5-HT1 receptors.

BACKGROUND: 5-Hydroxytryptamine (5-HT) receptors 1B, 1D and 1F have key roles in migraine pharmacotherapy. Selective agonists targeting these receptors, such as triptans and ditans, are effective in aborting acute migraine attacks and inhibit the in vivo release of calcitonin gene-related peptide (CGRP) in human and animal models. The study aimed to examine the localization, genetic expression and functional aspects of 5- HT1B/1D/1F receptors in the trigeminal system in order to further understand the molecular sites of action of triptans (5-HT1B/1D) and ditans (5-HT1F). METHODS: Utilizing immunohistochemistry, the localization of 5-HT and of 5-HT1B/1D/1F receptors was examined in rat trigeminal ganglion (TG) and combined with quantitative polymerase chain reaction to quantify the level of expression for 5-HT1B/1D/1F receptors in the TG. The functional role of these receptors was examined ex vivo with a capsaicin/potassium induced 5-HT and CGRP release. RESULTS: 5-HT immunoreactivity (ir) was observed in a minority of CGRP negative C-fibres, most neuron somas and faintly in A-fibres and Schwann cell neurolemma. 5-HT1B/1D receptors were expressed in the TG, while the 5-HT1F receptor displayed a weak ir. The 5-HT1D receptor co-localized with receptor activity-modifying protein 1 (RAMP1) in Aδ-fibres in the TG, while 5-HT1B-ir was weakly expressed and 5-HT1F-ir was not detected in these fibres. None of the 5-HT1 receptors co-localized with CGRP-ir in C-fibres. 5-HT1D receptor mRNA was the most prominently expressed, followed by the 5-HT1B receptor and lastly the 5-HT1F receptor. The 5-HT1B and 5-HT1D receptor antagonist, GR127935, could reverse the inhibitory effect of Lasmiditan (a selective 5-HT1F receptor agonist) on CGRP release in the soma-rich TG but not in soma-poor TG or dura mater. 5-HT release in the soma-rich TG, and 5-HT content in the baseline samples, negatively correlated with CGRP levels, showing for the first time a physiological role for 5-HT induced inhibition. CONCLUSION: This study reveals the presence of a subgroup of C-fibres that store 5-HT. The data shows high expression of 5-HT1B/1D receptors and suggests that the 5-HT1F receptor is a relatively unlikely target in the rat TG. Furthermore, Lasmiditan works as a partial agonist on 5-HT1B/1D receptors in clinically relevant dose regiments.