Single amino acid of an octopamine receptor as a molecular switch for distinct G protein couplings

Characterization and functional analysis of an α-adrenergic-like octopamine receptor in the small brown planthopper Laodelphax striatellus

As an important biogenic amine in invertebrates and corresponding to the neurotransmitter norepinephrine in vertebrates, octopamine (OA) regulates diverse physiological and behavioral processes by binding to specific octopamine receptors (OARs) in invertebrates. At present, OARs have been identified and characterized in several insects. However, less is known about the OARs of Laodelphax striatellus, one of the most destructive pests in East Asian rice fields. In the present study, an α1-adrenergic-like OAR (LsOA1) from L. striatellus was cloned. LsOA1 has the typical characteristics of G-protein coupled receptors and is clustered with other insect homologs. The transcript level of LsOA1 varied in various stages and tissues, and was highly expressed at the egg stage and in the brain. Silencing of LsOA1 causes a reduction in vitellogenin (LsVg) and vitellogenin receptor (LsVgR) expression. Although LsOA1 interference did not affect the fecundity and survival of L. striatellus, the hatching rate of L. striatellus was significantly reduced, and the hatching period was prolonged. The decrease in the amount of honeydew excreted after silencing LsOA1 indicates that LsOA1 may be involved in regulating the feeding behavior of L. striatellus. In addition, the interference of LsOA1 significantly reduced the expression of capsid protein (CP) and viral RNA3 segment (RNA3) in rice stripe virus (RSV)-viruliferous L. striatellus, but did not affect the vertical transmission rate of RSV. The present study demonstrated that LsOA1 played a crucial role in the physiological and behavioral processes of L. striatellus, which will provide the basis for developing a new target gene for pest control.

A point mutation in the β-adrenergic-like octopamine receptor: possible association with amitraz resistance

BACKGROUND

Amitraz is a unique formamidine-class acaricide/insecticide that effectively controls ticks, mites, and insect pests. However, the recent emergence of amitraz-resistant cattle ticks is a serious problem that requires an urgent solution. A nonsynonymous single nucleotide polymorphism (A181T) leading to an amino acid substitution (I61F) in the β-adrenergic-like (β-AL) octopamine receptor (OAR) of amitraz-resistant southern cattle ticks (Rhipicephalus microplus) (RmβAOR) was proposed to be a cause of the amitraz resistance. However, it remains unclear whether this substitution exerts any functional effect on the action of amitraz. To make this clear, the functional role of this mutation was examined using an orthologous OAR (BmOAR2) from the silkworm (Bombyx mori).

RESULTS

Both amitraz and its metabolite N² -(2,4-dimethylphenyl)-N¹ -methyformamidine (DPMF) elevated intracellular cyclic AMP levels as orthosteric OAR agonists in HEK-293 cells stably expressing BmOAR2. The I45F mutant of BmOAR2 (equivalent to I61F in RmβAOR) was generated and tested for its sensitivity to amitraz and DPMF. The assay result showed that the I45F mutation reduces the potency of DPMF to a level similar to that of the endogenous agonist (R)-OA in wild-type BmOAR2.

CONCLUSION

The amino acid substitution found in the first transmembrane segment of RmβAOR most likely causes target-site insensitivity to DPMF, which might contribute to the resistance of R. microplus to amitraz. This needs to be further confirmed using RmβAOR. © 2020 Society of Chemical Industry.

Design, Synthesis, and Evaluation of X-ray Crystal Structure, Biological Activities, DFT Calculations, and Molecular Docking of Phenyl Imidazolidin-2-One Derivatives

Eight phenyl imidazolinone derivatives were synthesized from N2-(2,4-dimethylphenyl)-N1-methyformamidine (DPMF) via scaffold-hopping method using the ring-closure approach. The prepared compounds were verified using 1H and 13C NMR and HRMS spectroscopies. The structure of compound 3c was confirmed by single-crystal X-ray diffraction analysis. The mean plane of the phenyl and imidazolinone moieties was almost coplanar with an angle of 8.85(4)°. In the crystal, molecules were interlinked with intermolecular hydrogen bonds (N–H···O and C–H···O), generating a network structure. Additionally, compound 3f displayed the highest insecticidal activity (86.7%) against Plutella xylostella at 600 mg/L, which was significantly higher than the insecticidal activity (23.0%) of DPMF. Also, compound 3d displayed good fungicidal activities against Phytophthora capsici, Phytophthora sojae, and Phytophthora infestans. Density functional theory (DFT) calculations were performed to explain the insecticidal and fungicidal activities of phenyl imidazolidin-2-one derivatives, especially potent compounds 3f and 3d. Moreover, the binding modes of compounds 3a–h and DPMF against octopamine receptor of Plutella xylostella were studied by homology modeling and molecular docking. Therefore, a preliminary structure–activity relationship (SAR) was derived and discussed. These results encourage the exploration of novel insecticides and fungicides based on DPMF.

Molecular and pharmacological characterization of a β-adrenergic-like octopamine receptor from the green rice leafhopper Nephotettix cincticeps

As the counterparts of noradrenaline and adrenaline in vertebrates, octopamine (OA) regulates multiple physiological and behavioral processes in invertebrate. OA mediates its effects via binding to specific octopamine receptors (OARs). Functional and pharmacological characterization of OARs have been reported in several insects. However, little work was documented in hemipteran insects. We cloned a β-adrenergic-like OAR (NcOA2B2) from Nephotettix cincticeps. NcOA2B2 shares high similarity with members of the OA2B2 receptor class. Transcript level of NcOA2B2 varied in various tissues and was highly expressed in the leg. After heterologous expression in CHO-K1 cells, NcOA2B2 was dose-dependently activated by OA (EC₅₀ = 2.56 nM) and tyramine (TA) (EC₅₀ = 149 nM). Besides putative octopaminergic agonists, dopaminergic agonists and amitraz and DPMF potently activated NcOA2B2 in a dose-dependent manner. Receptor activity was blocked by potential antagonists and was most efficiently antagonized by asenapine. Phentolamine showed both antagonist and agonist effects on NcOA2B2. Our results offer the important information about molecular and pharmacological characterization of an OAR from N. cincticeps that will provide the basis for forthcoming studies on its roles in physiological processes and behaviors, and facilitate the design of novel insecticides for pest control.

Ligand-Induced Conformational Dynamics of A Tyramine Receptor from Sitophilus oryzae

Tyramine receptor (TyrR) is a biogenic amine G protein-coupled receptor (GPCR) associated with many important physiological functions in insect locomotion, reproduction, and pheromone response. Binding of specific ligands to the TyrR triggers conformational changes, relays the signal to G proteins, and initiates an appropriate cellular response. Here, we monitor the binding effect of agonist compounds, tyramine and amitraz, to a Sitophilus oryzae tyramine receptor (SoTyrR) homology model and their elicited conformational changes. All-atom molecular dynamics (MD) simulations of SoTyrR-ligand complexes have shown varying dynamic behavior, especially at the intracellular loop 3 (IL3) region. Moreover, in contrast to SoTyrR-tyramine, SoTyrR-amitraz and non-liganded SoTyrR shows greater flexibility at IL3 residues and were found to be coupled to the most dominant motion in the receptor. Our results suggest that the conformational changes induced by amitraz are different from the natural ligand tyramine, albeit being both agonists of SoTyrR. This is the first attempt to understand the biophysical implication of amitraz and tyramine binding to the intracellular domains of TyrR. Our data may provide insights into the early effects of ligand binding to the activation process of SoTyrR.

Molecular features and expression profiles of octopamine receptors in the brown planthopper, Nilaparvata lugens

BACKGROUND

Octopamine, the invertebrate counterpart of adrenaline and noradrenaline, regulates and modulates many physiological and behavioral processes in insects. It mediates its effects by binding to specific octopamine receptors, which belong to the superfamily of G-protein coupled receptors (GPCRs). The expression profiles of octopamine receptor genes have been well documented in different developmental stages and multiple tissue types in several different insect orders. However, little work has addressed this issue in Hemiptera.

RESULTS

In this study, we cloned four octopamine receptor genes from brown planthopper. The deduced amino acid sequences share high identity with other insect homologues and have the characteristic GPCRs domain architecture: seven transmembrane domains. These genes were expressed in all developmental stages and examined tissues. The expression of NlOA2B3 and NlOA3 was relatively higher in egg and first instar nymph stage than in other stages and other receptor genes. All of these receptor genes were more highly expressed in brain than other tissues.

CONCLUSION

The identification of octopamine receptor genes in this study will provide a foundation for investigating the diverse roles played by NlOARs and for exploring specific target sites for chemicals that control agricultural pests. © 2019 Society of Chemical Industry.

Amitraz and its metabolite differentially activate α- and β-adrenergic-like octopamine receptors

BACKGROUND

Amitraz is a formamidine acaricide and insecticide used to control ticks, mites and fleas. N² -(2,4-Dimethylphenyl)-N¹ -methyformamidine (DPMF), a metabolite of amitraz, is thought to be an active agent that exerts acaricidal and insecticidal effects by acting as an agonist on octopamine receptors. The emergence of cattle ticks resistant to amitraz is a serious problem that requires urgent attention. The objective of this research was to determine which type of octopamine receptor is the primary target of amitraz and thereby understand the molecular mechanisms of action and resistance to amitraz.

RESULTS

Amitraz and DPMF potently activated Bombyx mori α- and β-adrenergic-like octopamine receptors (α- and β-AL OARs) that were stably expressed in HEK-293 cells. Notably, DPMF elevated intracellular cAMP levels, with an EC₅₀ of 79.6 pm in β-AL OARs, the transcripts of which were prevalently and widely localised in B. mori body parts. Furthermore, DPMF elevated the intracellular Ca²⁺ levels, with an EC₅₀ of 1.17 nm in α-AL OARs.

CONCLUSION

Although both amitraz and DPMF acted as OAR agonists, the metabolite DPMF was more potent than amitraz and differentially activated α- and β-AL OARs. The present findings provide a basis for studies to examine the mechanism of amitraz resistance and to develop novel acaricides and insecticides. © 2016 Society of Chemical Industry.

De novo assembly and characterization of central nervous system transcriptome reveals neurotransmitter signaling systems in the rice striped stem borer, Chilo suppressalis

Background

Neurotransmitter signaling systems play crucial roles in multiple physiological and behavioral processes in insects. Genome wide analyses of de novo transcriptome sequencing and gene specific expression profiling provide rich resources for studying neurotransmitter signaling pathways. The rice striped stem borer, Chilo suppressalis is a destructive rice pest in China and other Asian countries. The characterization of genes involved in neurotransmitter biosynthesis and transport could identify potential targets for disruption of the neurochemical communication and for crop protection.

Results

Here we report de novo sequencing of the C. suppressalis central nervous system transcriptome, identification and expression profiles of genes putatively involved in neurotransmitter biosynthesis, packaging, and recycling/degradation. A total of 54,411 unigenes were obtained from the transcriptome analysis. Among these unigenes, we have identified 32 unigenes (31 are full length genes), which encode 21 enzymes and 11 transporters putatively associated with biogenic aminergic signaling, acetylcholinergic signaling, glutamatergic signaling and GABAergic signaling. RT-PCR and qRT-PCR results indicated that 12 enzymes were highly expressed in the central nervous system and all the transporters were expressed at significantly high levels in the central nervous system. In addition, the transcript abundances of enzymes and transporters in the central nervous system were validated by qRT-PCR. The high expression levels of these genes suggest their important roles in the central nervous system.

Conclusions

Our study identified genes potentially involved in neurotransmitter biosynthesis and transport in C. suppressalis and these genes could serve as targets to interfere with neurotransmitter production. This study presents an opportunity for the development of specific and environmentally safe insecticides for pest control.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1742-7) contains supplementary material, which is available to authorized users.

Characterization of a tyramine receptor type 2 from hemocytes of rice stem borer, Chilo suppressalis

Calcium acts as a second messenger in many cell types, including insect hemocytes. Intracellular calcium level has a definite role in innate and adaptive immune signaling. Biogenic amines such as octopamine (OA), tyramine (TA), dopamine (DA) and serotonin (5-HT) play various important physiological roles in insects by activating distinct G-protein-coupled receptors (GPCRs) that share a putative seven transmembrane domain structure. OA and 5-HT have been shown that can mediate insect hemocytic immune reactions to infections and invasions. Here, we showed that TA increase hemocyte spreading in the rice stem borer, Chilo suppressalis. Furthermore, we cloned a cDNA encoding a tyramine receptor type 2 from the hemocytes in the C. suppressalis, viz., CsTA2, which shares high sequence similarity to members of the invertebrate tyramine receptor family. The CsTA2 receptor was stably expressed in human embryonic kidney (HEK) 293 cells, and its ligand response has been examined. Receptor activation with TA induced a dose-dependent increase in intracellular Ca(2+) concentration ([Ca(2+)]i) in cells, with an EC50 value of 18.7±5.3 nM, whereas OA, DA, 5-HT and other potential agonists did not have this response. The mRNA is present in various tissues including nerve cord, hemocytes, fat body, midgut, Malpighian tubules, and epidermis in the larval stage. Western blot analysis and immunohistochemistry assay displayed that CsTA2 was detected and presented on hemocytes. We also showed that TA induced Ca(2+) release from the hemocytes of C. suppressalis.

Larvae of the small white butterfly, Pieris rapae, express a novel serotonin receptor

The biogenic amine serotonin (5-hydroxytryptamine, 5-HT) is a neurotransmitter in vertebrates and invertebrates. It acts in regulation and modulation of many physiological and behavioral processes through G-protein-coupled receptors. Five 5-HT receptor subtypes have been reported in Drosophila that share high similarity with mammalian 5-HT1A, 5-HT1B, 5-HT2A, 5-HT2B, and 5-HT7 receptors. We isolated a cDNA (Pr5-HT8 ) from larval Pieris rapae, which shares relatively low similarity to the known 5-HT receptor classes. After heterologous expression in HEK293 cells, Pr5-HT8 mediated increased [Ca(2+)]i in response to low concentrations (< 10 nM) of 5-HT. The receptor did not affect [cAMP]i even at high concentrations (> 10 μM) of 5-HT. Dopamine, octopamine, and tyramine did not influence receptor signaling. Pr5-HT8 was also activated by various 5-HT receptor agonists including 5-methoxytryptamine, (±)-8-Hydroxy-2-(dipropylamino) tetralin, and 5-carboxamidotryptamine. Methiothepin, a non-selective 5-HT receptor antagonist, activated Pr5-HT8 . WAY 10635, a 5-HT1A antagonist, but not SB-269970, SB-216641, or RS-127445, inhibited 5-HT-induced [Ca(2+)]i increases. We infer that Pr5-HT8 represents the first recognized member of a novel 5-HT receptor class with a unique pharmacological profile. We found orthologs of Pr5-HT8 in some insect pests and vectors such as beetles and mosquitoes, but not in the genomes of honeybee or parasitoid wasps. This is likely to be an invertebrate-specific receptor because there were no similar receptors in mammals. We isolated a cDNA (Pr5-HT8) from larval Pieris rapae, which shares relatively low similarity to the known GPCRs. After heterologous expression in HEK293 cells, Pr5-HT8 mediated increased [Ca(2+)]i in response to low concentrations (< 10 nM) of 5-HT and various 5-HT receptor agonists. We found orthologs of Pr5-HT8 in some insect pests and vectors such as beetles and mosquitoes, but not in the genomes of honeybee, parasitoid wasps, or mammals.

Molecular, pharmacological, and signaling properties of octopamine receptors from honeybee (Apis mellifera) brain

G protein-coupled receptors are important regulators of cellular signaling processes. Within the large family of rhodopsin-like receptors, those binding to biogenic amines form a discrete subgroup. Activation of biogenic amine receptors leads to transient changes of intracellular Ca²⁺-([Ca²⁺](i)) or 3',5'-cyclic adenosine monophosphate ([cAMP](i)) concentrations. Both second messengers modulate cellular signaling processes and thereby contribute to long-lasting behavioral effects in an organism. In vivo pharmacology has helped to reveal the functional effects of different biogenic amines in honeybees. The phenolamine octopamine is an important modulator of behavior. Binding of octopamine to its receptors causes elevation of [Ca²⁺](i) or [cAMP](i). To date, only one honeybee octopamine receptor that induces Ca²⁺ signals has been molecularly and pharmacologically characterized. Here, we examined the pharmacological properties of four additional honeybee octopamine receptors. When heterologously expressed, all receptors induced cAMP production after binding to octopamine with EC₅₀(s) in the nanomolar range. Receptor activity was most efficiently blocked by mianserin, a substance with antidepressant activity in vertebrates. The rank order of inhibitory potency for potential receptor antagonists was very similar on all four honeybee receptors with mianserin >> cyproheptadine > metoclopramide > chlorpromazine > phentolamine. The subroot of octopamine receptors activating adenylyl cyclases is the largest that has so far been characterized in arthropods, and it should now be possible to unravel the contribution of individual receptors to the physiology and behavior of honeybees.

Differential modulation of Beta-adrenergic receptor signaling by trace amine-associated receptor 1 agonists

Trace amine-associated receptors (TAAR) are rhodopsin-like G-protein-coupled receptors (GPCR). TAAR are involved in modulation of neuronal, cardiac and vascular functions and they are potentially linked with neurological disorders like schizophrenia and Parkinson's disease. Subtype TAAR1, the best characterized TAAR so far, is promiscuous for a wide set of ligands and is activated by trace amines tyramine (TYR), phenylethylamine (PEA), octopamine (OA), but also by thyronamines, dopamine, and psycho-active drugs. Unfortunately, effects of trace amines on signaling of the two homologous β-adrenergic receptors 1 (ADRB1) and 2 (ADRB2) have not been clarified yet in detail. We, therefore, tested TAAR1 agonists TYR, PEA and OA regarding their effects on ADRB1/2 signaling by co-stimulation studies. Surprisingly, trace amines TYR and PEA are partial allosteric antagonists at ADRB1/2, whereas OA is a partial orthosteric ADRB2-antagonist and ADRB1-agonist. To specify molecular reasons for TAAR1 ligand promiscuity and for observed differences in signaling effects on particular aminergic receptors we compared TAAR, tyramine (TAR) octopamine (OAR), ADRB1/2 and dopamine receptors at the structural level. We found especially for TAAR1 that the remarkable ligand promiscuity is likely based on high amino acid similarity in the ligand-binding region compared with further aminergic receptors. On the other hand few TAAR specific properties in the ligand-binding site might determine differences in ligand-induced effects compared to ADRB1/2. Taken together, this study points to molecular details of TAAR1-ligand promiscuity and identified specific trace amines as allosteric or orthosteric ligands of particular β-adrenergic receptor subtypes.

Chemogenomic analysis of G-protein coupled receptors and their ligands deciphers locks and keys governing diverse aspects of signalling

Understanding the molecular mechanism of signalling in the important super-family of G-protein-coupled receptors (GPCRs) is causally related to questions of how and where these receptors can be activated or inhibited. In this context, it is of great interest to unravel the common molecular features of GPCRs as well as those related to an active or inactive state or to subtype specific G-protein coupling. In our underlying chemogenomics study, we analyse for the first time the statistical link between the properties of G-protein-coupled receptors and GPCR ligands. The technique of mutual information (MI) is able to reveal statistical inter-dependence between variations in amino acid residues on the one hand and variations in ligand molecular descriptors on the other. Although this MI analysis uses novel information that differs from the results of known site-directed mutagenesis studies or published GPCR crystal structures, the method is capable of identifying the well-known common ligand binding region of GPCRs between the upper part of the seven transmembrane helices and the second extracellular loop. The analysis shows amino acid positions that are sensitive to either stimulating (agonistic) or inhibitory (antagonistic) ligand effects or both. It appears that amino acid positions for antagonistic and agonistic effects are both concentrated around the extracellular region, but selective agonistic effects are cumulated between transmembrane helices (TMHs) 2, 3, and ECL2, while selective residues for antagonistic effects are located at the top of helices 5 and 6. Above all, the MI analysis provides detailed indications about amino acids located in the transmembrane region of these receptors that determine G-protein signalling pathway preferences.

Pheromone transduction in moths

Calling female moths attract their mates late at night with intermittent release of a species-specific sex-pheromone blend. Mean frequency of pheromone filaments encodes distance to the calling female. In their zig-zagging upwind search male moths encounter turbulent pheromone blend filaments at highly variable concentrations and frequencies. The male moth antennae are delicately designed to detect and distinguish even traces of these sex pheromones amongst the abundance of other odors. Its olfactory receptor neurons sense even single pheromone molecules and track intermittent pheromone filaments of highly variable frequencies up to about 30 Hz over a wide concentration range. In the hawkmoth Manduca sexta brief, weak pheromone stimuli as encountered during flight are detected via a metabotropic PLCβ-dependent signal transduction cascade which leads to transient changes in intracellular Ca²⁺ concentrations. Strong or long pheromone stimuli, which are possibly perceived in direct contact with the female, activate receptor-guanylyl cyclases causing long-term adaptation. In addition, depending on endogenous rhythms of the moth's physiological state, hormones such as the stress hormone octopamine modulate second messenger levels in sensory neurons. High octopamine levels during the activity phase maximize temporal resolution cAMP-dependently as a prerequisite to mate location. Thus, I suggest that sliding adjustment of odor response threshold and kinetics is based upon relative concentration ratios of intracellular Ca²⁺ and cyclic nucleotide levels which gate different ion channels synergistically. In addition, I propose a new hypothesis for the cyclic nucleotide-dependent ion channel formed by insect olfactory receptor/coreceptor complexes. Instead of being employed for an ionotropic mechanism of odor detection it is proposed to control subthreshold membrane potential oscillation of sensory neurons, as a basis for temporal encoding of odors.

The G protein-coupled receptors in the silkworm, Bombyx mori

G protein-coupled receptors (GPCRs) are the largest and most versatile family of transmembrane receptors in the cell, occupying the highest hierarchical positions in the regulation of many physiological processes. Although they have been extensively studied in a number of model insects, there have been few investigations of GPCRs in large Lepidopterans, such as Bombyx mori, an organism that provides a means to perform detailed tissue expression analyses, which may help to characterize GPCRs and their ligands. In addition, B. mori, also known as the silkworm, is an insect of substantial economic importance, due to its use in silk production and traditional medicines. In this work, we computationally identified 90 putative GPCRs in B. mori, 33 of which represent novel proteins. These GPCRs were annotated and compared with their homologs in Drosophila melanogaster and Anopheles gambiae. Phylogenetics analyses of the GPCRs from these three insects showed that GPCRs may easily duplicate or disappear during insect evolution, especially in the neuropeptide and protein hormone receptor subfamily. Interestingly, we observed a decrease in the quantity and diversity of the stress-tolerance gene, Methuselah, in B. mori, which may be related to its long history of domestication. Moreover, the presence of many Bombyx-specific GPCRs suggests that neither Drosophila nor Anopheles is good representatives for the GPCRs in the Class Insecta.

Pharmacological characterization of a Bombyx mori alpha-adrenergic-like octopamine receptor stably expressed in a mammalian cell line

Series of agonists and antagonists were examined for their actions on a Bombyx morialpha-adrenergic-like octopamine receptor (OAR) stably expressed in HEK-293 cells. The rank order of potency of the agonists was clonidine>naphazoline>tolazoline in Ca(2+) mobilization assays, and that of the antagonists was chlorpromazine>yohimbine. These findings suggest that the B. mori OAR is more closely related to the class-1 OAR in the intact tissue than to the other classes. N'-(4-Chloro-o-tolyl)-N-methylformamidine (DMCDM) and 2-(2,6-diethylphenylimino)imidazolidine (NC-5) elevated the intracellular calcium concentration ([Ca(2+)](i)) with EC(50)s of 92.8 microM and 15.2 nM, respectively. DMCDM and NC-5 led to increases in intracellular cAMP concentration ([cAMP](i)) with EC(50)s of 234 nM and 125 nM, respectively. The difference in DMCDM potencies between the cAMP and Ca(2+) assays might be due to "functional selectivity." The Ca(2+) and cAMP assay results for DMCDM suggest that the elevation of [cAMP](i), but not that of [Ca(2+)](i), might account for the insecticidal effect of formamidine insecticides.

Molecular cloning and pharmacological characterization of a Bombyx mori tyramine receptor selectively coupled to intracellular calcium mobilization

Tyramine (TA) is a biogenic amine in invertebrates. cDNA encoding the TA receptor (TAR) BmTAR2 was cloned from the nerve tissue of the silkworm Bombyx mori. The receptor's functional and pharmacological properties were examined in BmTAR2-transfected HEK-293 cells. In [(3)H]TA binding assays, BmTAR2 showed considerably higher affinity for TA than for other biogenic amines, with an IC(50) value of 57.5 nM. Moreover, TA induced a dose-dependent increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) in cells, with an EC(50) value of 11.6 nM, whereas octopamine and dopamine increased [Ca(2+)](i) only at concentrations above 100 microM. A few antagonists were found to inhibit the TA-induced increases in [Ca(2+)](i); the rank order of potency was yohimbine > chlorpromazine > mianserin. TA showed no effect on intracellular cAMP concentration. The data indicate that BmTAR2 belongs to the second class of TARs, which are selectively coupled to intracellular Ca(2+) mobilization. RT-PCR analysis revealed that BmTAR2 was expressed predominantly in the nervous tissue of B. mori larvae, suggesting that TA has neurotransmitter and neuromodulatory roles that are mediated by BmTAR2.