Regulation of 5-HT(1A) and 5-HT(1B) receptor systems by phospholipid signaling cascades

The role of brain serotonin signaling in excessive alcohol consumption and withdrawal: A call for more research in females

Alcohol Use Disorder (AUD) is a leading cause of death and disability worldwide, but current treatments are insufficient in fully addressing the symptoms that often lead to relapses in alcohol consumption. The brain's serotonin system has been implicated in AUD for decades and is a major regulator of stress-related behaviors associated with increased alcohol consumption. This review will discuss the current literature on the association between neurobiological adaptations in serotonin systems and AUD in humans as well as the effectiveness of serotonin receptor manipulations on alcohol-related behaviors like consumption and withdrawal. We will further discuss how these findings in humans relate to findings in animal models, including a comparison of systemic pharmacological manipulations modulating alcohol consumption. We next provide a detailed overview of brain region-specific roles for serotonin and serotonin receptor signaling in alcohol-related behaviors in preclinical animal models, highlighting the complexity of forming a cohesive model of serotonin function in AUD and providing possible avenues for more effective therapeutic intervention. Throughout the review, we discuss what is known about sex differences in the sequelae of AUD and the role of serotonin in these sequelae. We stress a critical need for additional studies in women and female animals so that we may build a clearer path to elucidating sex-specific serotonergic mechanisms and develop better treatments.

Increased coronary arteriolar contraction to serotonin in juvenile pigs with metabolic syndrome

Metabolic syndrome (MetS) is associated with alterations in coronary vascular smooth muscle and endothelial function. The current study examined the contractile response of the isolated coronary arterioles to serotonin in pigs with and without MetS and investigated the signaling pathways responsible for serotonin-induced vasomotor tone. The MetS pigs (8-weeks old) were fed with a hyper-caloric, fat/cholesterol diet and the control animals (lean) were fed with a regular diet for 12 weeks (n = 6/group). The coronary arterioles (90–180 μm in diameter) were dissected from the harvested pig myocardial tissues and the in vitro coronary arteriolar response to serotonin was measured in the presence of pharmacological inhibitors. The protein expressions of phospholipase A2 (PLA₂), TXA₂ synthase, and the thromboxane-prostanoid (TP) receptor in the pigs’ left ventricular tissue samples were measured using Western blotting. Serotonin (10⁻⁹–10⁻⁵ M) induced dose-dependent contractions of coronary-resistant arterioles in both non-MetS control (lean) and MetS pigs. This effect was more pronounced in the MetS vessels compared with those of non-MetS controls (lean, P < 0.05]. Serotonin-induced contraction of the MetS vessels was significantly inhibited in the presence of the selective PLA₂ inhibitor quinacrine (10⁻⁶ M), the COX inhibitor indomethacin (10⁻⁵ M), and the TP receptor antagonist SQ29548 (10⁻⁶ M), respectively (P < 0.05). MetS exhibited significant increases in tissue levels of TXA₂ synthase and TP receptors (P < 0.05 vs. lean), respectively. MetS is associated with increased contractile response of porcine coronary arterioles to serotonin, which is in part via upregulation/activation of PLA₂, COX, and subsequent TXA₂, suggesting that alteration of vasomotor function may occur at an early stage of MetS and juvenile obesity.

5-HT1B Receptor-Mediated Activation of ERK1/2 Requires Both Gαi/o and β-Arrestin Proteins

5-HT_1B receptors modulate synaptic serotonin (5-HT) levels and play a significant role in the regulation of emotional behaviors. These receptors are Gα_i/o-coupled and inhibit adenylyl cyclase but have also been reported to activate MAP kinases; however, the details of signaling cascades downstream of 5-HT_1B receptor activation remain unclear, particularly in neuronal cells. We generated a stable 5-HT_1B receptor-expressing Neuro2A (N2A-1B) neuronal cell line and demonstrate that activation of these receptors by the selective 5-HT_1B agonist CP-94253 results in activation of ERK1/2 but not of other closely related MAP kinases. Phosphoproteomics revealed four novel phosphorylation sites on the third intracellular loop of the 5-HT_1B receptor, and mutations of serine-256 and serine-291 to alanine led to reduced levels of ERK1/2 phosphorylation following receptor activation. Inhibition of Gα_i/o signaling with pertussis toxin, as well as MEK1/2 inhibition with U0126, also reduced 5-HT_1B-mediated ERK1/2 phosphorylation. Finally, we found that knockout of either β-arrestin 1 or β-arrestin 2 prevented 5-HT_1B-mediated phosphorylation of ERK1/2. Taken together, these results show that 5-HT_1B receptor activation selectively induces ERK1/2 activation through both the Gα_i subunit and β-arrestin proteins. This work elucidates the signal transduction pathway of 5-HT_1B receptors, as well as key phosphorylation sites within the receptor that modulate ERK1/2 activation, and further characterizes the intracellular mechanisms that underlie 5-HT_1B receptor function.

The association between HTR1B gene rs13212041 polymorphism and onset of alcohol abuse

BACKGROUND

Alcohol dependence displays a wide variety of clinical phenotypes. Various typology classifications of alcoholism include age of onset of alcohol abuse as one of the major phenotypic features. Serotonergic changes have been associated with alcoholism, while serotonin receptors type 1B (5-HT1B) play an important role in regulating serotonergic neurotransmission. The rs13212041 polymorphism modulates the expression of HTR1B gene coding for 5-HT1B receptor. This study examined the association of platelet serotonin (5-HT) and HTR1B gene with the onset of alcohol abuse in alcohol-dependent subjects.

MATERIALS AND METHODS

Determination of platelet 5-HT concentration and genotyping of rs13212041 HTR1B gene polymorphism were performed in 613 alcohol-dependent patients, subdivided according to early/late onset (before/after 25 years of age) of alcohol abuse.

RESULTS

Alcohol-dependent individuals with CC genotype were more frequent in the group with early onset of alcohol abuse compared to carriers of T allele. Besides HTR1B genotype, age and gender, but not platelet 5-HT, were major variables associated with the onset of alcohol abuse. Platelet 5-HT concentration was not significantly different between patients with early and late onset of alcohol abuse, or patients carrying various HTR1B genotypes. Although we observed no influence of co-variables such as age, gender, or somatic and psychiatric comorbidities, platelet 5-HT concentration was significantly affected by smoking.

CONCLUSION

These findings support potential involvement of 5-HT1B receptors in the onset of alcohol abuse and development of alcohol dependence. Additionally, the results of our study emphasize the importance of controlling for smoking status, as one of the significant confounding factors influencing platelet 5-HT concentration.

Decreased contractile response of peripheral arterioles to serotonin after CPB in patients with diabetes

BACKGROUND

Regulation of coronary vasomotor tone by serotonin is significantly changed after cardioplegic arrest and reperfusion. The current study investigates whether cardiopulmonary bypass may also affect peripheral arteriolar response to serotonin in patients with or without diabetes.

METHODS

Human peripheral microvessels (90-180 µm diameter) were dissected from harvested skeletal muscle tissues from diabetic and non-diabetic patients before and after cardiopulmonary bypass and cardiac surgery (n = 8/group). In vitro contractile response to serotonin was assessed by videomicroscopy in the presence or absence of serotonin alone (10^-9-10^-5M) or combined with the selective serotonin 1B receptor (5-HT1B) antagonist, SB224289 (10^-6M). 5-HT1A/1B protein expression in the skeletal muscle was measured by Western-blot and immunohistochemistry.

RESULTS

There were no significant differences in contractile response of peripheral arterioles to serotonin (10^-5M) pre-cardiopulmonary bypass between diabetic and non-diabetic patients. After cardiopulmonary bypass, contractile response to serotonin was significantly impaired in both diabetic and non-diabetic patients compared to their pre-cardiopulmonary bypass counterparts (P < .05). This effect was more pronounced in diabetic patients than non-diabetic patients (P < .05 versus non-diabetic). The contractile response to serotonin was significantly inhibited by the 5-HT1B antagonist in both diabetic and non-diabetic vessels (P < .05 versus serotonin alone). There were no significant differences in the expression/distribution of 5-HT1A/1B between non-diabetic and diabetic groups or between pre- versus post- cardiopulmonary bypass vessels.

CONCLUSIONS

Cardiopulmonary bypass is associated with decreased contractile response of peripheral arterioles to serotonin and this effect was exaggerated in the presence of diabetes. Serotonin-induced contractile response of the peripheral arterioles was via 5-HT1B in both diabetic and non-diabetic patients.

The Trace Amine-Associated Receptor 1 Agonist 3-Iodothyronamine Induces Biased Signaling at the Serotonin 1b Receptor

Trace amine-associated receptors (TAARs) belong to the class A G-protein-coupled receptors (GPCR) and are evolutionary related to aminergic receptors. TAARs have been identified to mediate effects of trace amines. TAAR1 signaling is mainly mediated via activation of the G_s/adenylyl cyclase pathway. In addition to classical trace amines, TAAR1 can also be activated by the thyroid hormone derivative 3-iodothyronamine (3-T1AM). Pharmacological doses of 3-T1AM induced metabolic and anapyrexic effects, which might be centrally mediated in the hypothalamus in rodents. However, the observed anapyrexic effect of 3-T1AM persists in Taar1 knock-out mice which raises the question whether further GPCRs are potential targets for 3-T1AM and mediate the observed physiological effect. Anapyrexia has been observed to be related to action on aminergic receptors such as the serotonin receptor 1b (5-HT1b). This receptor primarily activates the G_i/o mediated pathway and PLC signaling through the G_βγ of G_i/o. Since the expression profiles of TAAR1 and 5-HT1b overlap, we questioned whether 3-T1AM may activate 5-HT1b. Finally, we also evaluated heteromerization between these two GPCRs and tested signaling under co-expressed conditions. In this study, we showed, that 3-T1AM can induce G_i/o signaling through 5-HT1b in a concentration of 10 μM. Strikingly, at 5-HT1b the ligand 3-T1AM only activates the G_i/o mediated reduction of cAMP accumulation, but not PLC activation. Co-stimulation of 5-HT1b by both ligands did not lead to additive or synergistic signaling effects. In addition, we confirmed the capacity for heteromerization between TAAR1 and 5-HT1b. Under co-expression of TAAR1 and HTR1b, 3-T1AM action is only mediated via TAAR1 and activation of 5-HT1b is abrogated. In conclusion, we found evidence for 5-HT1b as a new receptor target for 3-T1AM, albeit with a different signaling effect than the endogenous ligand. Altogether, this indicates a complex interrelation of signaling effects between the investigated GPCRs and respective ligands.

Genetic variation in the calcium/calmodulin-dependent protein kinase (CaMK) pathway is associated with antidepressant response in females

OBJECTIVE

Antidepressant effects on monoamine neurotransmission may be influenced by genetic variation in intracellular signal transduction pathways, such as the cyclic adenosine monophosphate (cAMP)--protein kinase A (PKA) pathway, Ras-mitogen activated protein kinase (MAPK) pathway and calcium/calmodulin-dependent protein kinase (CaMK) pathway. The aims of this study were to examine the association of polymorphisms in candidate genes of these three signal transduction pathways with response to antidepressant treatment, and to determine the effects of, and interactions with, environment factors.

METHODS

We recruited 412 patients who met diagnosis criteria for major depressive disorder (MDD) (DSM-IV Axis I). 284 patients completed 8 weeks treatment with selective serotonin reuptake inhibitors (SSRIs) or serotonin and noradrenergic reuptake inhibitors (SNRIs). Severity of depression was measured with the Hamilton Depression Rating Scale (HDRS) before and after 8 weeks antidepressant treatment. 209 patients completed the Childhood Trauma Questionnaire, 28 item Short Form (CTQ-SF) which was used to evaluate childhood adverse events. 218 patients completed the Life Events Scale (LES) which were used to evaluate life stress before onset. 155 SNPs in 66 candidate genes were genotyping by Illumina GoldenGate, including 28 SNPs in 15 genes of cAMP-PKA pathway, 37 SNPs in 17 genes of Ras-MAPK pathway and 90 SNPs in 34 genes of CaMK pathway. The remission criterion was HDRS score equal to or less than 7. Single SNP and haplotype associations were analyzed by UNPHASED 3.3.13. Gene-environment interactions were analyzed by binary logistic regression with SPSS 11.0 software.

RESULTS

The rs2230372 SNP in ITPR2, rs2280272 in PRKCZ, rs17109671, and rs17109674 in PLCE1 were significant associated with remission, as were haplotypes in PRKCZ and PLCE1. All these positive associations were found in genes of the CaMK pathway, but not the cAMP-PKA or Ras-MAPK pathways. There were no significant differences in CTQ scores and LES scores between remitters and non-remitters. No significantly interactions between candidate genes and environment effects were observed.

CONCLUSION

The CaMK pathway may be important in determining antidepressant response. But recent adverse life events, childhood adversity, and interactions between candidate genes and environment factors appear not to influence short term antidepressant outcome.

Reduced signal transduction by 5-HT4 receptors after long-term venlafaxine treatment in rats

BACKGROUND AND PURPOSE

The 5-HT(4) receptor may be a target for antidepressant drugs. Here we have examined the effects of the dual antidepressant, venlafaxine, on 5-HT(4) receptor-mediated signalling events.

EXPERIMENTAL APPROACH

The effects of 21 days treatment (p.o.) with high (40 mg·kg(-1)) and low (10 mg·kg(-1)) doses of venlafaxine, were evaluated at different levels of 5-HT(4) receptor-mediated neurotransmission by using in situ hybridization, receptor autoradiography, adenylate cyclase assays and electrophysiological recordings in rat brain. The selective noradrenaline reuptake inhibitor, reboxetine (10 mg·kg(-1), 21 days) was also evaluated on 5-HT(4) receptor density.

KEY RESULTS

Treatment with a high dose (40 mg·kg(-1)) of venlafaxine did not alter 5-HT(4) mRNA expression, but decreased the density of 5-HT(4) receptors in caudate-putamen (% reduction = 26 ± 6), hippocampus (% reduction = 39 ± 7 and 39 ± 8 for CA1 and CA3 respectively) and substantia nigra (% reduction = 49 ± 5). Zacopride-stimulated adenylate cyclase activation was unaltered following low-dose treatment (10 mg·kg(-1)) while it was attenuated in rats treated with 40 mg·kg(-1) of venlafaxine (% reduction = 51 ± 2). Furthermore, the amplitude of population spike in pyramidal cells of CA1 of hippocampus induced by zacopride was significantly attenuated in rats receiving either dose of venlafaxine. Chronic reboxetine did not modify 5-HT(4) receptor density.

CONCLUSIONS AND IMPLICATIONS

Our data indicate a functional desensitization of 5-HT(4) receptors after chronic venlafaxine, similar to that observed after treatment with the classical selective inhibitors of 5-HT reuptake.

Altered coronary microvascular serotonin receptor expression after coronary artery bypass grafting with cardiopulmonary bypass

OBJECTIVE

We evaluated roles of serotonin 1B and 2A receptors, thromboxane synthase and receptor, and phospholipases A(2) and C in response to cardiopulmonary bypass.

METHODS

Patients' atrial tissues were harvested before and after cardiopulmonary bypass with cardioplegia (n = 13). Coronary microvessels were assessed for vasoactive response to serotonin with and without inhibitors of serotonin 1B and 2A receptors and phospholipases A(2) and C. Expressions of serotonin receptor messenger RNA were determined with reverse transcriptase polymerase chain reaction. Expressions of serotonin receptors and thromboxane A(2) receptor and synthase proteins were determined with immunoblotting and immunohistochemistry.

RESULTS

Microvessel exposure to serotonin elicited 7.3% +/- 2% relaxation before bypass, changing to contraction of -19.2% +/- 2% after bypass (P <.001). Additions of specific serotonin 1B receptor antagonist and inhibitor of phospholipase A(2) resulted in significantly decreased contraction, -8.6% +/- 1% (P < .001) and 2.8% +/- 3% (P = .001), respectively. Serotonin 1B receptor messenger RNA expression increased 1.82 +/- 0.34-fold after bypass (p = .044); serotonin 2A receptor messenger RNA expression did not change. Serotonin 1B but not 2A receptor protein expression increased after bypass by 1.35 +/- 0.7-fold (P = .0413). Thromboxane synthase and receptor expressions were unchanged after bypass. Serotonin 1B receptor increased mainly in arterial smooth muscle. There were no appreciable differences in arterial expressions of thromboxane synthase or receptor.

CONCLUSIONS

Serotonin-induced vascular dysfunction after cardiopulmonary bypass with cardioplegic arrest may be mediated by increased expression of serotonin 1B receptor and subsequent phospholipase A(2) activation in myocardial coronary smooth muscle.

Current status of inverse agonism at serotonin2A (5-HT2A) and 5-HT2C receptors

Contemporary receptor theory was developed to account for the existence of constitutive activity, as defined by the presence of receptor signaling in the absence of any ligand. Thus, ligands acting at a constitutively active receptor, can act as agonists, antagonists, and inverse agonists. In vitro studies have also revealed the complexity of ligand/receptor interactions including agonist-directed stimulus trafficking, a finding that has led to multi-active state models of receptor function. Studies with a variety of cell types have established that the serotonin 5-HT(2A) and 5-HT(2C) receptors also demonstrate constitutive activity and inverse agonism. However, until recently, there has been no evidence to suggest that these receptors also demonstrate constitutive activity and hence reveal inverse agonist properties of ligands in vivo. This paper describes our current knowledge of constitutive activity in vitro and then examines the evidence for constitutive activity in vivo. Both the serotonin 5-HT(2A) and 5-HT(2C) receptors are involved in a number of physiological and behavioral functions and are the targets for treatment of schizophrenia, anxiety, weight control, Parkinsonism, and other disorders. The existence of constitutive activity at these receptors in vivo, along with the possibility of inverse agonism, provides new avenues for drug development.

5-HT1B receptors play a prominent role in the proliferation of T-lymphocytes

Serotonin plays a role in T cell activation, but there is no clear consensus of which of the 14 serotonergic receptors control this activations pathway. We have used a broad range of serotonergic receptor antagonists to define the functional involvement of these receptors governing the proliferation of primary T cells as well as in T cell lines. Our data shows that antagonism of the 5-HT(1B) receptor inhibits the proliferation of both human and murine primary helper T cells and of human helper T cell lines. As a whole, our data suggest that other serotonergic receptors may contribute to the proliferative signals, but the 5-HT(1B) receptor plays the most dominant role.

Multiple receptors mediate the trophic effects of serotonin on ventroposterior thalamic neurons in vitro

Serotonin (5-HT) exerts prominent morphogenetic roles during development. For example, somatosensory cortical barrel formation is altered in mouse models characterized by excessive extracellular 5-HT, suggesting that 5-HT affects development of thalamic afferents and/or neocortical target regions. The present study assessed 5-HT effects in primary cultures of fetal ventroposterior thalamic (VPT) neurons. 5-HT produces concentration-dependent trophic effects, with impressive 59% and 106% peak increases in total neurite length and number of branching points, respectively, at a dose of 30 microM 5-HT. The exposure of VPT neurons to specific 5-HT receptor agonists 8-OH-DPAT (5-HT(1A)), CGS-12066A (5-HT(1B)), DOI (5-HT(2A/2C)), and m-CPBG (5-HT(3)), enhances primary neurite length and number of branching points with rank-order potency 5-HT(1B) > 5-HT(2A/2C) = 5-HT(3) > 5-HT(1A) = vehicle. Trophic 5-HT effects on embryonic VPT neurons are thus much more prominent than previously reported, and can be mediated by multiple 5-HT receptor subtypes.

Transmembrane signaling in the brain by serotonin, a key regulator of physiology and emotion

Serotonin (5-HT) is an ancient chemical that plays a crucial functional role in almost every living organism. It regulates platelet aggregation, activation of immune cells, and contraction of stomach and intestinal muscles. In addition, serotonin acts as a neurotransmitter in the brain and the peripheral nervous system. These activities are initiated by the binding of serotonin to 15 or more receptors that are pharmacologically classified into seven groups, 5-HT1 through 5-HT7. Each group is further divided into subgroups of receptors that are homologous but are encoded by discrete genes. With the exception of the 5-HT3 receptor--a cation channel--all of the others are G protein-coupled receptors that potentially activate or inhibit a large number of biochemical cascades. This review will endeavor to compare and contrast such signaling pathways with special attention to their tissue-specific occurrence, their possible role in immediate effects on covalent modification of other proteins, and relatively slower effects on gene expression, physiology and behavior.

Delayed vasodilatory response to methylnicotinate in patients with unipolar depressive disorder

BACKGROUND

Recent evidence has suggested an important role for lipids in the etiology and treatment of depression. Methylnicotinate-induced vasodilation can be used to investigate lipid-dependent signalling mechanisms involving the phospholipase A2 (PLA2)/cyclooxygenase pathway, an important signalling system involved in the action of several neurotransmitters including serotonin. To investigate whether abnormalities in this signalling system may occur in depressive illness, we undertook a study of methylnicotinate response in unipolar depression (UD).

METHODS

Methylnicotinate was applied to the forearm of 20 patients with depression and 38 age and sex-matched healthy volunteers (HV). The resulting erythema was assessed over a 15-min period.

RESULTS

Methylnicotinate-induced erythema was reduced in subjects with depression compared to HV at 5 min after application, it returned to normal after 15 min. Thus, although the maximal response to methylnicotinate appears normal, patients with UD exhibit an apparently delayed response.

LIMITATIONS

The major limitation is that all unipolar patients were medicated at the time of testing.

CONCLUSIONS

Our results support the hypothesis that UD may be associated with abnormalities in lipid-associated signalling systems, and may provide insight into how lipid intake may modulate depressive symptoms.

Contribution of 5-hydroxytryptamine1B receptors and 20-hydroxyeiscosatetraenoic acid to fall in cerebral blood flow after subarachnoid hemorrhage

BACKGROUND AND PURPOSE

This study examined the interaction between 5-hydroxytryptamine1B (5-HT1B) receptors and 20-hydroxyeiscosatetraenoic acid (20-HETE) in contributing to the acute fall in regional cerebral blood flow (rCBF) after subarachnoid hemorrhage (SAH) in rats.

METHODS

The effects of intracisternal injection of 0.3 mL of arterial blood, artificial cerebrospinal fluid, and 5-HT on rCBF and the levels of 20-HETE and 5-HT in cerebrospinal fluid were measured in rats pretreated with vehicle, a 5-HT1B receptor antagonist (isamoltane hemifumarate), or an inhibitor of the synthesis of 20-HETE (HET0016). The effects of HET0016 and isamoltane on the vasoconstrictor response and changes in [Ca2+]i to 5-HT were also studied in middle cerebral arteries and vascular smooth muscle cells isolated from these vessels.

RESULTS

20-HETE and 5-HT levels in cerebrospinal fluid rose from 172+/-10 to 629+/-44 ng/mL and from 6+/-4 to 1163+/-200 nmol/mL, respectively, after SAH. rCBF fell by 30% 10 minutes after SAH, and it remained at this level for the next 2 hours. Blockade of 5-HT1B receptors prevented the sustained fall in rCBF seen after SAH. Intracisternal injection of 5-HT mimicked SAH by increasing 20-HETE levels in cerebrospinal fluid to 475+/-94 ng/mL and reducing rCBF by 30%. Blockade of the synthesis of 20-HETE with HET0016 prevented the fall in rCBF produced by 5-HT. Isamoltane and HET0016 reduced the vasoconstrictor response of isolated MCA to 5-HT by >60% and diminished the rise in [Ca2+]i produced by 5-HT in vascular smooth muscle cells isolated from these arteries.

CONCLUSIONS

These results suggest that the release of 5-HT after SAH activates 5-HT1B receptors and the synthesis of 20-HETE and that 20-HETE contributes to the acute fall in rCBF by potentiating the vasoconstrictor response of cerebral vessels to 5-HT.

Serotonergic regulation of somatosensory cortical development: lessons from genetic mouse models

Monoaminergic neurotransmitter systems appear early during embryogenesis, suggesting that they could play important roles in brain development. Accumulated evidence indicates that serotonin (5-hydroxytryptamine, 5-HT) regulates neural as well as nonneural development, including early aspects of embryonic development, differentiation of neuronal progenitors, and morphogenesis of the craniofacial region, heart and limb. Recent studies using monoamine oxidase-A (MAO-A), 5-HT transporter, vesicular monoamine transporter-2 (VMAT2) and 5-HT1B receptor single, double and triple knockout mice have provided evidence that the serotonergic system plays important roles in barrel field formation in the developing somatosensory cortex. Here we review evidence from these genetic mouse models and, based on the accumulated evidence, propose a testable model for future studies of mechanisms underlying serotonergic regulation of cortical development.