Cloning and expression of a complementary DNA encoding a molluscan octopamine receptor that couples to chloride channels in HEK293 cells

Neuroendocrine functions of monoamines in invertebrates: Focus on bivalve molluscs

Monoamines (MA) such as serotonin, catecholamines (dopamine, norepinephrine, epinephrine), and trace amines (octopamine, tyramine), are neurotransmitters and neuroendocrine modulators in vertebrates, that contribute to adaptation to the environment. Although MA are conserved in evolution, information is still fragmentary in invertebrates, given the diversity of phyla and species. However, MA are crucial in homeostatic processes in these organisms, where the absence of canonical endocrine glands in many groups implies that the modulation of physiological functions is essentially neuroendocrine. In this review, we summarize available information on MA systems in invertebrates, with focus on bivalve molluscs, that are widespread in different aquatic environments, where they are subjected to a variety of environmental stimuli. Available data are reviewed on the presence of the different MA in bivalve tissues, their metabolism, target cells, signaling pathways, and the physiological functions modulated in larval and adult stages. Research gaps and perspectives are highlighted, in order to enrich the framework of knowledge on MA neuroendocrine functions, and on their role in adaptation to ongoing and future environmental changes.

Cyclic AMP-regulated opposing and parallel effects of serotonin and dopamine on phototaxis in the Marmorkrebs (marbled crayfish)

Phototactic behaviours are observed from prokaryotes to amphibians and are a basic form of orientation. We showed that the marbled crayfish displays phototaxis in which the behavioural response reversed from negative to positive depending on external light conditions. Animals reared in a 12-L/12-D light cycle showed negative phototaxis during daytime and positive phototaxis during night-time. Animals reared under constant light conditioning showed negative phototaxis during day- and night-time, while animals reared under constant dark conditioning showed positive phototaxis during day- and night-time. Injection of serotonin leads to a reversal of negative to positive phototaxis in both light/dark-reared and light/light-reared animals while injection of dopamine induced reversed negative phototaxis in dark/dark-reared animals. Four hours of dark adaptation were enough for light/dark-reared animals to reverse phototaxis from negative to positive. Injection of a serotonin 5HT₁ receptor antagonist blocked the reverse phototaxis while serotonin 5HT₂ receptor antagonists had no effects. Similarly, dark/dark-reared animals reversed to showing negative phototaxis after 4 h of light adaptation. Injection of a dopamine DA₁ receptor antagonist blocked this reverse phototaxis, while dopamine DA₂ receptor antagonists had no effects. Injection of a cAMP analogue into light/dark-reared animals blocked reverse phototaxis after dark adaptation, while adenylate cyclase inhibitor in dark/dark-reared animals blocked reverse phototaxis after light adaptation. These results strongly suggest that serotonin mediates positive phototaxis owing to decreased cAMP levels, while dopamine-mediated negative phototaxis occurs due to increased cAMP levels. Supporting this, the ratio of serotonin to dopamine in the brain was much higher in dark/dark-reared than light/dark-reared animals.

Molecular Characterization and Functional Analysis of a Putative Octopamine/Tyramine Receptor during the Developmental Stages of the Pacific Oyster, Crassostrea gigas

Octopamine (OA) and its precursor, tyramine (TA), participate in invertebrate development such as growth, maturation, and reproduction by activating their corresponding G protein-coupled receptors (GPCRs). Although OA was first discovered in mollusks (octopus), subsequent studies on OA, TA and related receptors have primarily been conducted in Ecdysozoa, especially in insects. Accordingly, only limited reports on OA/TA receptors in mollusks are available and their physiological roles remain unclear. Here, a full-length cDNA encoding a putative 524 amino acid OA/TA receptor (CgGPR1) was isolated from the Pacific oyster Crassostrea gigas. CgGPR1 was most closely related to the Lymnaea stagnalis OA receptor OAR2 in sequence. Phylogenetic analysis showed that CgGPR1 belongs to a poorly studied subfamily of invertebrate OA/TA receptors. The spatio-temporal expression of CgGPR1 in C. gigas larvae was examined by quantitative real-time PCR and Western blot analysis. CgGPR1 was expressed during all developmental stages of C. gigas with higher levels at mid-developmental stages, indicating its potential role in embryogenesis and tissue differentiation. Immunoreactive fluorescence of CgGPR1 was mainly observed in the velum, foot, gill and mantle of C. gigas larvae. CgGPR1 transcripts were detected in all the tested organs of adult C. gigas, with highest level in the mantle. Pharmacological analysis showed that cAMP and Ca²⁺ concentrations remained unchanged in HEK293 cells expressing CgGPR1 upon addition of OA, TA or related amines, suggesting that CgGPR1 modulates other unknown molecules rather than cAMP and Ca²⁺. Our study sheds light on CgGPR1 function in oysters.

Identification of G protein-coupled receptors in Schistosoma haematobium and S. mansoni by comparative genomics

BACKGROUND

Schistosomiasis is a parasitic disease affecting ~200 million people worldwide. Schistosoma haematobium and S. mansoni are two relatively closely related schistosomes (blood flukes), and the causative agents of urogenital and hepatointestinal schistosomiasis, respectively. The availability of genomic, transcriptomic and proteomic data sets for these two schistosomes now provides unprecedented opportunities to explore their biology, host interactions and schistosomiasis at the molecular level. A particularly important group of molecules involved in a range of biological and developmental processes in schistosomes and other parasites are the G protein-coupled receptors (GPCRs). Although GPCRs have been studied in schistosomes, there has been no detailed comparison of these receptors between closely related species. Here, using a genomic-bioinformatic approach, we identified and characterised key GPCRs in S. haematobium and S. mansoni (two closely related species of schistosome).

METHODS

Using a Hidden Markov Model (HMM) and Support Vector Machine (SVM)-based pipeline, we classified and sub-classified GPCRs of S. haematobium and S. mansoni, combined with phylogenetic and transcription analyses.

RESULTS

We identified and classified classes A, B, C and F as well as an unclassified group of GPCRs encoded in the genomes of S. haematobium and S. mansoni. In addition, we characterised ligand-specific subclasses (i.e. amine, peptide, opsin and orphan) within class A (rhodopsin-like).

CONCLUSIONS

Most GPCRs shared a high degree of similarity and conservation, except for members of a particular clade (designated SmGPR), which appear to have diverged between S. haematobium and S. mansoni and might explain, to some extent, some of the underlying biological differences between these two schistosomes. The present set of annotated GPCRs provides a basis for future functional genomic studies of cellular GPCR-mediated signal transduction and a resource for future drug discovery efforts in schistosomes.

Biphasic responses of the honeybee heart to nanomolar concentrations of amitraz

Amitraz is a pesticide targeting the octopaminergic receptors. In a previous study, octopamine, a biogenic amine, was found to induce a biphasic effect on the honeybee heart, inhibition at low concentrations and excitation at high concentrations. Furthermore, the honeybee heart was found to be far more sensitive to octopamine compared to other insect hearts. The objective of the present study was to investigate the effects of amitraz on the electrical and mechanical properties of the honeybee heart ex vivo and on the heart rate in vivo. In ex vivo conditions, amitraz at 10(-12) M caused a significant inhibition in the mechanical (p<0.05, n=4) and electrical properties (p<0.05, n=4). Higher concentrations such as 10(-9) and 10(-6) M induced a biphasic effect, with total inhibition for 7.86±1.26 min (n=7), followed by strong excitation of spontaneously-generated contractions (n=7). The initial elimination of heart activity was caused by strong hyperpolarization, while the subsequent excitation was caused by a depolarization in the membrane potential of pacemaker cells at 10(-9) M (n=8). In the in vivo experiments, abdominal injection or oral application of 0.20 ng of amitraz per bee induced a persistent increase of 134.28±4.07% (p<0.05, n=4) in the frequency of the cardiac action potentials. The above responses clearly show that the heart of the honeybee is extremely vulnerable to amitraz, which is nevertheless still used inside beehives, ostensibly to "protect" the honeybees against their main parasite, Varroa destructor.

Octopamine--a single modulator with double action on the heart of two insect species (Apis mellifera macedonica and Bactrocera oleae): Acceleration vs. inhibition

The effects of octopamine, the main cardioacceleratory transmitter in insects, were investigated, in the isolated hearts of the honeybee, Apis mellifera macedonica, and the olive fruit fly, Bactrocera oleae. Octopamine induced a biphasic effect on the frequency and force of cardiac contractions acting as an agonist, with a strong acceleratory effect, at concentrations higher than 10(-12)M for the honeybee and higher than 50×10(-9)M for the olive fruit fly. The heart of the honeybee is far more sensitive than the heart of olive fruit fly. This unusual sensitivity is extended to the blockers of octopaminergic receptors, where phentolamine at 10(-5)M stopped the spontaneous contractions of the honeybee heart completely and permanently, while the same blocker at the same concentration caused only 50% inhibition in the heart of the olive fruit fly. Phentolamine and mianserin at low concentrations of 10(-7)M also blocked the heart octopaminergic receptors, but for a short period of time, of less than 15.0 min, while a partial recovery in heart contraction started in spite of the presence of the antagonist. The unusual response of the honeybee heart in the presence of phentolamine and/or mianserin suggests excitatory effects of octopamine via two different receptor subtypes. At lower concentrations, 10(-14)M, the agonist octopamine was converted to an antagonist, inducing a hyperpolarization in the membrane potential of the honeybee cardiac pacemaker cells and inhibiting the firing rate of the heart. The inhibitory effects of octopamine on certain parameters of the rhythmic bursts of the heart of the honeybee, were similar to those of mianserin and phentolamine, typical blockers of octopaminergic receptors. The heart of the olive fruit fly was 10(5) times less sensitive to octopamine, since a persistent inhibition of heart contractions occurred at 10(-9)M. In conclusion, the acceleration of the insect heart is achieved by increasing the levels of octopamine, while there is a passive but also an active decrease in heart activity due to the minimization of octopamine.

Cloning and distribution of a putative octopamine/tyramine receptor in the central nervous system of the freshwater prawn Macrobrachium rosenbergii

There is ample evidence linking octopamine (OA) and tyramine (TA) to several neurophysiological functions in arthropods. In our laboratory we use the freshwater prawn Macrobrachium rosenbergii to study the neural basis of aggressive behavior. As a first step towards understanding the possible role of these amines and their receptors in the modulation of interactive behaviors, we have cloned a putative octopamine/tyramine receptor. The predicted sequence of the cloned OA/TA(Mac) receptor consists of 1,579 base pairs (bp), with an open reading frame of 1,350bp that encodes a 450 amino acid protein. This putative protein displays sequence identities of 70% to an Aedes aegypti mosquito TA receptor, followed by 60% to a Stegomyia aegypti mosquito OA receptor, 59% and 58% to the migratory locust TA-1 and -2 receptors respectively, and 57% with the silkworm OA receptor. We also mapped the OA/TA(Mac) receptor distribution by in-situ hybridization to the receptor's mRNA, and by immunohistochemistry to its protein. We observed stained cell bodies for the receptor's mRNA, mainly in the midline region of the thoracic and in the abdominal ganglia, as well as diffuse staining in the brain ganglia. For the receptor's protein, we observed extensive punctate staining within the neuropil and on the membrane of specific groups of neurons in all ganglia throughout the CNS, including the brain, the midline region and neuropiles of the thoracic ganglia, and ventral part and neuropiles of the abdominal ganglia. The same pattern of stained cells was observed on the thoracic and abdominal ganglia in both in-situ hybridization and immunohistochemistry experiments. Diffuse staining observed with in-situ hybridization also coincides with punctate staining observed in brain, SEG, thoracic, and abdominal ganglia in immunohistochemical preparations. This work provides the first step towards characterizing the neural networks that mediate octopaminergic signaling in prawn.

Characterization of a novel octopamine receptor expressed in the surf clam Spisula solidissima

We have cloned and sequenced a cDNA from the surf clam (Spisula solidissima, a pelecypod mollusc) that encodes an octopamine receptor which we have named Spi-OAR. The sequence of Spi-OAR shares many similarities with two Aplysia and three Drosophila octopamine receptors belonging to a sub-group of beta-adrenergic-like octopamine receptors. Using an expression vector and transient transfections of Spi-OAR into HEK 293 cells, we observed an increase of cAMP upon addition of octopamine and, to a lesser extent, of tyramine, but not after addition of dopamine, serotonin, or histamine. Using a battery of known agonists and antagonists for octopamine receptors, we observed a rather unique pharmacological profile for Spi-OAR through measurements of cAMP. Spi-OAR exhibited some constitutive activity in HEK 293 cells and no Ca(2+) responses could be detected following addition of octopamine to Spi-OAR-transfected cells. RT-PCR analysis revealed ubiquitous expression of Spi-OAR mRNA in all adult tissues, oocytes and early embryos examined. While addition of serotonin to isolated clam oocytes resulted in meiotic activation, similar additions of octopamine had no effect, suggesting that its potential role in clam reproductive physiology differs significantly from that of serotonin. This work identifies Spi-OAR as a novel mollusc octopamine receptor closely related to other invertebrate beta-adrenergic-like octopamine receptors, with possible reproductive and other physiological functions. This initial characterization of Spi-OAR makes possible further investigations and comparisons with more studied and familiar insect or gastropod mollusc octopamine receptors.

Octopamine promotes rhythmicity but not synchrony in a bilateral pair of bursting motor neurons in the feeding circuit of Aplysia

Octopamine-like immunoreactivity was localized to a limited number (<40) of neurons in the Aplysia central nervous system, including three neurons in the paired buccal ganglia (BG) that control feeding movements. Application of octopamine (OA) to the BG circuit produced concentration-dependent (10(-8)-10(-4) mol l(-1)) modulatory actions on the spontaneous burst activity of the bilaterally paired B67 pharyngeal motor neurons (MNs). OA increased B67's burst duration and the number of impulses per burst. These effects reflected actions of OA on the intrinsic tetrodotoxin-resistant driver potential (DP) that underlies B67 bursting. In addition to its effects on B67's burst parameters, OA also increased the rate and regularity of burst timing. Although the bilaterally paired B67 MNs both exhibited rhythmic bursting in the presence of OA, they did not become synchronized. In this respect, the response to OA differed from that of dopamine, another modulator of the feeding motor network, which produces both rhythmicity and synchrony of bursting in the paired B67 neurons. It is proposed that modulators can regulate burst synchrony of MNs by exerting a dual control over their intrinsic rhythmicity and their reciprocal capacity to generate membrane potential perturbations. In this simple system, dopaminergic and octopaminergic modulation could influence whether pharyngeal contractions occur in a bilaterally synchronous or asynchronous fashion.

Trace amine-associated receptor 1-Family archetype or iconoclast?

Interest has recently been rekindled in receptors that are activated by low molecular weight, noncatecholic, biogenic amines that are typically found as trace constituents of various vertebrate and invertebrate tissues and fluids. The timing of this resurgent focus on receptors activated by the "trace amines" (TA) beta-phenylethylamine (PEA), tyramine (TYR), octopamine (OCT), synephrine (SYN), and tryptamine (TRYP) is the direct result of 2 publications that appeared in 2001 describing the cloning of a novel G protein-coupled receptor (GPCR) referred to by their discoverers Borowsky et al. as TA1 and Bunzow et al. as TA receptor 1 (TAR1). When heterologously expressed in Xenopus laevis oocytes and various eukaryotic cell lines, recombinant rodent and human TAR dose-dependently couple to the stimulation of adenosine 3',5'-monophosphate (cAMP) production. Structure-activity profiling based on this functional response has revealed that in addition to the TA, other biologically active compounds containing a 2-carbon aliphatic side chain linking an amino group to at least 1 benzene ring are potent and efficacious TA receptor agonists with amphetamine (AMPH), methamphetamine, 3-iodothyronamine, thyronamine, and dopamine (DA) among the most notable. Almost 100 years after the search for TAR began, numerous TA1/TAR1-related sequences, now called TA-associated receptors (TAAR), have been identified in the genome of every species of vertebrate examined to date. Consequently, even though heterologously expressed TAAR1 fits the pharmacological criteria established for a bona fide TAR, a major challenge for those working in the field is to discern the in vivo pharmacology and physiology of each purported member of this extended family of GPCR. Only then will it be possible to establish whether TAAR1 is the family archetype or an iconoclast.

Are Caenorhabditis elegans receptors useful targets for drug discovery: pharmacological comparison of tyramine receptors with high identity from C. elegans (TYRA-2) and Brugia malayi (Bm4)

The biogenic amine, tyramine (TA), modulates a number of key processes in nematodes and a number of TA-specific receptors have been identified. In the present study, we have identified a putative TA receptor (Bm4) in the recently completed Brugia malayi genome and compared its pharmacology to its putative Caenorhabditis elegans orthologue, TYRA-2, under identical expression and assay conditions. TYRA-2 and Bm4 are the most closely related C. elegans and B. malayi BA receptors and differ by only 14aa in the TM regions directly involved in ligand binding. Membranes from HEK-293 cells stably expressing Bm4 exhibited specific, saturable, high affinity, [(3)H]LSD and [(3)H]TA binding with K(d)s of 18.1+/-0.93 and 15.1+/-0.2 nM, respectively. More importantly, both TYRA-2 and Bm4 TA exhibited similar rank orders of potencies for a number of potential tyraminergic ligands. However, some significant differences were noted. For example, chloropromazine exhibited an order of magnitude higher affinity for Bm4 than TYRA-2 (pK(i)s of 7.6+/-0.2 and 6.49+/-0.1, respectively). In contrast, TYRA-2 had significantly higher affinity for phentolamine than Bm4. These results highlight the utility of the nearly completed B. malayi genome and the importance of using receptors from individual parasitic nematodes for drug discovery.

Molecular cloning and heterologous expression of an alpha-adrenergic-like octopamine receptor from the silkworm Bombyx mori

A cDNA encoding an octopamine (OA) receptor (BmOAR1) was isolated from the nerve tissue of silkworm (Bombyx mori) larvae. Comparison of amino acid sequences showed that BmOAR1 is highly identical to OA receptors isolated from Periplaneta americana (Pa oa(1)), Apis mellifera (AmOA1), and Drosophila melanogaster (OAMB or DmOA1A). BmOAR1 was stably expressed in HEK-293 cells. OA above 1 microM led to an increase in intracellular cyclic AMP concentration ([cAMP](i)). The synthetic OA-receptor agonist demethylchlordimeform also elevated [cAMP](i) to the same maximal level (approximately 5-fold over the basal level) as that induced by OA. However, other biogenic amines, tyramine and dopamine, and chlordimeform were without effects. The [cAMP](i) level raised by OA was lowered by antagonists; the rank order of antagonist activity was chlorpromazine > mianserin = yohimbine. Cyproheptadine and metoclopramide had little effect. OA above 100 nM induced a transient or sustained increase in intracellular Ca(2+) concentration ([Ca(2+)](i)), depending on the concentration of OA. Sequence homology and functional analysis data indicate that BmOAR1 is an alpha-adrenergic-like OA receptor of B. mori.

Identification and characterisation of the dopamine receptor II from the cat flea Ctenocephalides felis (CfDopRII)

G protein-coupled receptors (GPCRs) represent a protein family with a wide range of functions. Approximately 30% of human drug targets are GPCRs, illustrating their pharmaceutical relevance. In contrast, the knowledge about invertebrate GPCRs is limited and is mainly restricted to model organisms like Drosophila melanogaster and Caenorhabditis elegans. Especially in ectoparasites like ticks and fleas, only few GPCRs are characterised. From the cat flea Ctenocephalides felis, a relevant parasite of cats and dogs, no GPCRs are known so far. Thus, we performed a bioinformatic analysis of available insect GPCR sequences from the honeybee Apis mellifera, the mosquito Anopheles gambiae, the fruit fly Drosophila melanogaster and genomic sequences from insect species. Aim of this analysis was the identification of highly conserved GPCRs in order to clone orthologs of these candidates from Ctenocephalides felis. It was found that the dopamine receptor family revealed highest conservation levels and thus was chosen for further characterisation. In this work, the identification, full-length cloning and functional expression of the first GPCR from Ctenocephalides felis, the dopamine receptor II (CfDopRII), are described.

Stimulation of cyclic AMP formation and nerve electrical activity by octopamine in the terminal abdominal ganglion of the female gypsy moth Lymantria dispar

The biogenic amine octopamine is known to be present in the abdominal ganglia of some insects, but the expression of functional octopamine receptors in these neuronal structures has not yet been characterized. In the present study, we describe the presence in the female gypsy moth terminal abdominal ganglion (TAG), a key structure in the control of the insect reproductive behavior, of an octopamine receptor coupled to stimulation of adenylyl cyclase through the GTP-binding protein G(s). The rank order of potency of different antagonists, which discriminate between the different classes of octopamine receptors, indicated the involvement of the neuronal type 3 receptor. The octopamine-stimulated adenylyl cyclase activity was inhibited by Ca(2+) in the low micromolar range and by activation of either protein kinase A or protein kinase C. In the isolated TAG, bath application of octopamine caused an increase of the spontaneous bursting activity of the emerging nerve of the 5th pair (V), whereas the antagonist mianserin reduced the nerve spiking activity and blocked the stimulatory effect of octopamine. These data demonstrate that the gypsy moth TAG expresses functional octopamine receptors, which may participate in the neuronal control of the insect reproductive behavior.

Spider peripheral mechanosensory neurons are directly innervated and modulated by octopaminergic efferents

Octopamine is a chemical relative of noradrenaline providing analogous neurohumoral control of diverse invertebrate physiological processes. There is also evidence for direct octopaminergic innervation of some insect peripheral tissues. Here, we show that spider peripheral mechanoreceptors are innervated by octopamine-containing efferents. The mechanosensory neurons have octopamine receptors colocalized with synapsin labeling in the efferent fibers. In addition, octopamine enhances the electrical response of the sensory neurons to mechanical stimulation. Spider peripheral mechanosensilla receive extensive efferent innervation. Many efferent fibers in the legs of Cupiennius salei are GABAergic, providing inhibitory control of sensory neurons, but there is also evidence for other neurotransmitters. We used antibody labeling to show that some efferents contain octopamine and that octopamine receptors are concentrated on the axon hillocks and proximal soma regions of all mechanosensory neurons in the spider leg. Synaptic vesicles in efferent neurons were concentrated in similar areas. Octopamine, or its precursor tyramine, increased responses of mechanically stimulated filiform (trichobothria) leg hairs. This effect was blocked by the octopamine antagonist phentolamine. The octopamine-induced modulation was mimicked by 8-Br-cAMP, a cAMP analog, and blocked by Rp-cAMPS, a protein kinase A inhibitor, indicating that spider octopamine receptors activate adenylate cyclase and increase cAMP concentration. Frequency response analysis showed that octopamine increased the sensitivity of the trichobothria neurons over a broad frequency range. Thus, the major effect of octopamine is to increase its overall sensitivity to wind-borne signals from sources such as flying insect prey or predators.

TYRA-2 (F01E11.5): a Caenorhabditis elegans tyramine receptor expressed in the MC and NSM pharyngeal neurons

Tyramine appears to regulate key processes in nematodes, such as pharyngeal pumping, and more complex behaviors, such as foraging. Recently, a Caenorhabditis elegans tyramine receptor, SER-2, was identified that is involved in the TA-dependent regulation of these processes. In the present study, we have identified a second C. elegans gene, tyra-2 (F01E11.5) that encodes a tyramine receptor. This is the first identification of multiple tyramine receptor genes in any invertebrate. Membranes from COS-7 cells expressing TYRA-2 bind [(3)H]tyramine with high affinity with a K(d) of 20 +/- 5 nM. Other physiologically relevant biogenic amines, such as octopamine and dopamine, inhibit [(3)H]tyramine binding with much lower affinity (K(i)s of 1.55 +/- 0.5 and 1.78 +/- 0.6 microM, respectively), supporting the identification of TYRA-2 as a tyramine receptor. Indeed, tyramine also dramatically increases GTPgammaS binding to membranes from cells expressing TYRA-2 (EC(50) of 50 +/- 13 nM) and the TA-dependent GTPgammaS binding is PTX-sensitive suggesting that TYRA-2 may couple to Galpha(i/o). Based on fluorescence from tyra::gfp fusion constructs, TYRA-2 expression appears to be exclusively neuronal in the MC and NSM pharyngeal neurons, the AS family of amphid neurons and neurons in the nerve ring, body and tail. Taken together, these results suggest that TYRA-2 encodes a second Galpha(i/o)-coupled tyramine receptor and suggests that TA-dependent neuromodulation may be mediated by multiple receptors and more complex than previously appreciated.

Characterization of a D2-like dopamine receptor (AmDOP3) in honey bee, Apis mellifera

Dopamine is an important neurotransmitter in vertebrate and invertebrate nervous systems and is widely distributed in the brain of the honey bee, Apis mellifera. We report here the functional characterization and cellular localization of the putative dopamine receptor gene, Amdop3, a cDNA clone isolated and identified in previous studies as AmBAR3 (Apis mellifera Biogenic Amine Receptor 3). The Amdop3 cDNA encodes a 694 amino acid protein, AmDOP3. Comparison of AmDOP3 to Drosophila melanogaster sequences indicates that it is orthologous to the D2-like dopamine receptor, DD2R. Using AmDOP3 receptors expressed in HEK293 cells we show that of the endogenous biogenic amines, dopamine is the most potent AmDOP3 agonist, and that activation of AmDOP3 receptors results in down regulation of intracellular levels of cAMP, a property characteristic of D2-like dopamine receptors. In situ hybridization reveals that Amdop3 is widely expressed in the brain but shows a pattern of expression that differs from that of either Amdop1 or Amdop2, both of which encode D1-like dopamine receptors. Nonetheless, overlaps in the distribution of cells expressing Amdop1, Amdop2 and Amdop3 mRNAs suggest the likelihood of D1:D2 receptor interactions in some cells, including subpopulations of mushroom body neurons.

Isolation, cloning, and tissue expression of a putative octopamine/tyramine receptor from locust visceral muscle tissues

Octopamine has been shown to play major roles in invertebrate nervous systems as a neurotransmitter, neuromodulator, and neurohormone. Tyramine is the biochemical precursor of octopamine and its neuromodulatory role is now being investigated and clarified in invertebrates, particularly in insects. Both octopamine and tyramine mediate their actions via G protein-coupled receptors (GPCRs) and are believed to play important functions in the regulation of physiological processes in locust oviduct. Here we report the isolation, cloning, and tissue expression of a putative octopamine/tyramine receptor from the locust, Locusta migratoria. Degenerate oligonucleotides in PCR reactions were first used to obtain partial cDNA sequences and then these partial sequences were used in screens to obtain a full-length cDNA. The cloned cDNA is about 3.1 kb long and encodes a protein of 484 amino acid residues with typical characteristics of GPCRs including seven transmembrane domains and many signature residues. The amino acid sequence of the cloned cDNA displays sequence similarities with known GPCRs, particularly octopamine/tyramine receptors. Screening of the locust genomic DNA library resulted in isolation of a genomic DNA with the same size as the cDNA, indicating that the gene is intron-less. RT-PCR and Northern blot analyses revealed the expression of the receptor mRNA in brain, ventral nerve cord, oviduct, and midgut tissues. Southern blot analyses using EcoRI and HindIII restriction endonucleases recognized at least two distinct gene bands.

Molecular and pharmacological analysis of an octopamine receptor from American cockroach and fruit fly in response to plant essential oils

Octopamine receptors from American cockroach, Periplaneta americana (Pa oa1), and fruit fly, Drosophila melanogaster (OAMB), were cloned and permanently expressed in HEK-293 cells, and found to activate adenylate cyclase activity and increase [Ca2+]i levels through G-protein coupled receptor signaling pathways. Sequencing information (GenBank accession number AY333178) and functional data of Pa oa1 were recently published. Saturation binding analysis with 3H-yohimbine was performed with Pa oa(1) and OAMB expressed in COS-7 cells. The K(d) values were determined to be 28.4 and 43.0 nM, respectively. B(max) was determined to be 11.8 and 8.04 pmol receptor/mg protein, respectively. Competitive binding data using cell membranes expressing either OAMB or Pa oa1 demonstrated significantly decreased binding activity in binding assays performed in the presence of plant essential oils, eugenol, cinnamic alcohol, and trans-anethole. Eugenol decreased cAMP level in HEK-293 cells expressing Pa oa1, but trans-anethole increased cAMP in HEK-293 cells expressing OAMB. All three chemicals increased [Ca2+]i level in both cell models. Toxicity data against fruit flies and American cockroaches demonstrated species differences in response to treatment with tested plant essential oils. The toxicity of tested chemicals against wild type and octopamine mutant (iav) fly strains suggested that an octopamine receptor mediates the toxicity of cinnamic alcohol, eugenol, trans-antehole, and 2-phenethyl propionate against fruit flies. Collectively, the data suggest a correlation between cellular changes induced by tested plant essential oils and their toxicity against fruit fly and American cockroach.

A family of octopamine [corrected] receptors that specifically induce cyclic AMP production or Ca2+ release in Drosophila melanogaster

In invertebrates, the biogenic-amine octopamine is an important physiological regulator. It controls and modulates neuronal development, circadian rhythm, locomotion, 'fight or flight' responses, as well as learning and memory. Octopamine mediates its effects by activation of different GTP-binding protein (G protein)-coupled receptor types, which induce either cAMP production or Ca(2+) release. Here we describe the functional characterization of two genes from Drosophila melanogaster that encode three octopamine receptors. The first gene (Dmoa1) codes for two polypeptides that are generated by alternative splicing. When heterologously expressed, both receptors cause oscillatory increases of the intracellular Ca(2+) concentration in response to applying nanomolar concentrations of octopamine. The second gene (Dmoa2) codes for a receptor that specifically activates adenylate cyclase and causes a rise of intracellular cAMP with an EC(50) of approximately 3 x 10(-8) m octopamine. Tyramine, the precursor of octopamine biosynthesis, activates all three receptors at > or = 100-fold higher concentrations, whereas dopamine and serotonin are non-effective. Developmental expression of Dmoa genes was assessed by RT-PCR. Overlapping but not identical expression patterns were observed for the individual transcripts. The genes characterized in this report encode unique receptors that display signature properties of native octopamine receptors.

Primate trace amine receptor 1 modulation by the dopamine transporter

Recently identified trace amine receptors are potential direct targets for drugs of abuse, including amphetamine and 3,4-methylenedioxymethamphetamine (MDMA). We cloned full-length rhesus monkey trace amine receptor 1 (rhTA(1)) that was 96% homologous to human TA(1). The trace amines tyramine and beta-phenylethylamine (PEA) and the monoamine transporter substrates (+/-)-amphetamine and (+/-)-MDMA stimulated cAMP accumulation in rhTA(1)-expressing cell lines, as measured by a cAMP response element-luciferase assay. Cocaine did not stimulate cAMP accumulation in rhTA(1) cells, but it blocked [(3)H]PEA transport mediated by the dopamine transporter. Cotransfection with the human dopamine transporter enhanced PEA-, amphetamine-, and MDMA-mediated rhTA(1) receptor activation, but it diminished tyramine activation of rhTA(1). Because TA(1) (EGFP-rhTA(1) chimera) was largely intracellular, conceivably the dopamine transporter can facilitate access of specific agonists to intracellular TA(1). rhTA(1) mRNA expression was detected in rhesus monkey substantia nigra, implying that TA(1) may be colocalized with the dopamine transporter in dopamine neurons. In summary, primate TA(1) receptors are direct targets of trace amines, amphetamine, and MDMA. These receptors could also be indirect targets of amphetamine, MDMA, and cocaine through modification of monoamine transporter function. Conceivably, rhTA(1) receptors may be located on pre- or postsynaptic membranes. Interference with the carrier function of monoamine transporters with a consequent rise of extracellular levels of trace amines could activate these receptors. The cloning of a highly homologous TA(1) from rhesus monkey and demonstration that rhTA(1) receptors are activated by drugs of abuse, indicate that nonhuman primates may serve to model physiological and pharmacological TA(1)-mediated responses in humans.

Second messengers of octopamine receptors in the snail Lymnaea

We describe octopamine responses of 3 large buccal neurons of Lymnaea and test the hypothesis that these are cAMP-dependent. The B1 neuron is excited by octopamine and the depolarisation is significantly enlarged (P < 0.05) by application of the blocker of cAMP breakdown, 3-isobutyl-1-methylxanthine (IBMX). The B1 neuron is also depolarised by forskolin, an activator of adenylyl cyclase. The B2 and B3 neurons are inhibited by octopamine, and the response is not affected by IBMX. Both cells are excited by forskolin. We conclude that the B1 neuron response to octopamine is likely to be mediated by cAMP, while the B2 and B3 responses are cAMP-independent.

Tyramine and octopamine: ruling behavior and metabolism

Octopamine (OA) and tyramine (TA) are the invertebrate counterparts of the vertebrate adrenergic transmitters. They are decarboxylation products of the amino acid tyrosine, with TA as the biological precursor of OA. Nevertheless, both compounds are independent neurotransmitters that act through G protein-coupled receptors. OA modulates a plethora of behaviors and peripheral and sense organs, enabling the insect to respond correctly to external stimuli. Because these two phenolamines are the only biogenic amines whose physiological significance is presumably restricted to invertebrates, pharmacologists have focused their attention on the corresponding receptors, which are still believed to represent promising targets for new insecticides. Recent progress made on all levels of OA and TA research has enabled researchers to understand better the molecular events underlying the control of complex behaviors.

Cloning, expression and functional analysis of an octopamine receptor from Periplaneta americana

Octopamine regulates multiple physiological functions in invertebrates. The biological effects of octopamine and the pharmacology of octopamine receptors have been extensively studied in the American cockroach, Periplaneta americana. This paper reports the cloning of the first octopamine receptor from Periplaneta americana. A cDNA encoding a putative 7 transmembrane receptor was isolated from the head of Periplaneta americana. The encoded protein contains 628 amino acids and has sequence similarity to other biogenic amine receptors. This protein was expressed in COS-7 cells for radioligand binding studies using the antagonist 3H-yohimbine. Competitive binding comparing biogenic amines that could potentially function as endogenous ligands demonstrated this receptor had the highest affinity for octopamine (Ki = 13.3 microM) followed by tyramine, dopamine, serotonin and histamine. Octopamine increased both cAMP levels (EC50 = 1.62 microM) and intracellular concentrations of calcium through the receptor expressed in HEK-293 cells. Tyramine increased levels of both of these second messengers but only at significantly higher concentrations than octopamine. The cAMP increase by octopamine was independent of the increase in calcium. Competitive binding with antagonists revealed this receptor is similar to Lym oa1 from Lymnaea stagnalis. The data indicate that this cDNA is the first octopamine receptor cloned from Periplaneta americana and therefore has been named Pa oa1.

Tyramine and octopamine: antagonistic modulators of behavior and metabolism

The phenolamines tyramine and octopamine are decarboxylation products of the amino acid tyrosine. Although tyramine is the biological precursor of octopamine, both compounds are independent neurotransmitters, acting through various G-protein coupled receptors. Especially, octopamine modulates a plethora of behaviors, peripheral and sense organs. Both compounds are believed to be homologues of their vertebrate counterparts adrenaline and noradrenaline. They modulate behaviors and organs in a coordinated way, which allows the insects to respond to external stimuli with a fine tuned adequate response. As these two phenolamines are the only biogenic amines whose physiological significance is restricted to invertebrates, the attention of pharmacologists was focused on the corresponding receptors, which are still believed to represent promising targets for new insecticides. Recent progress made on all levels of octopamine/tyramine research enabled us to better understand the molecular events underlying the control of complex behaviors.

Molecular and functional characterization of an octopamine receptor from honeybee (Apis mellifera) brain

Biogenic amines and their receptors regulate and modulate many physiological and behavioural processes in animals. In vertebrates, octopamine is only found in trace amounts and its function as a true neurotransmitter is unclear. In protostomes, however, octopamine can act as neurotransmitter, neuromodulator and neurohormone. In the honeybee, octopamine acts as a neuromodulator and is involved in learning and memory formation. The identification of potential octopamine receptors is decisive for an understanding of the cellular pathways involved in mediating the effects of octopamine. Here we report the cloning and functional characterization of the first octopamine receptor from the honeybee, Apis mellifera. The gene was isolated from a brain-specific cDNA library. It encodes a protein most closely related to octopamine receptors from Drosophila melanogaster and Lymnea stagnalis. Signalling properties of the cloned receptor were studied in transiently transfected human embryonic kidney (HEK) 293 cells. Nanomolar to micromolar concentrations of octopamine induced oscillatory increases in the intracellular Ca2+ concentration. In contrast to octopamine, tyramine only elicited Ca2+ responses at micromolar concentrations. The gene is abundantly expressed in many somata of the honeybee brain, suggesting that this octopamine receptor is involved in the processing of sensory inputs, antennal motor outputs and higher-order brain functions.

A trace amine, tyramine, functions as a neuromodulator in Drosophila melanogaster

The tyramine receptor (TyrR) is a G protein-coupled receptor for trace amines, cloned in Drosophila melanogaster, and claimed to be either an octopamine receptor or a tyramine receptor. We previously reported that in the larval neuromuscular junctions, the modulatory effect on the excitatory junction potentials of tyramine is distinctly different from that of octopamine. The effect of tyramine but not of octopamine was selectively abolished in the TyrR mutant hono, suggesting that this gene encodes a receptor for tyramine, and not for octopamine. We examined whether there was a gene-dosage effect of this tyramine modulation using combinations of hono, deficiency (Df) and wild-type alleles. The tyramine effect was observed in hono heterozygotes (+/hono), which showed intermediate levels of response, but was not seen in +/Df or hono/Df hemizygotes. While these further suggest that tyramine is the true ligand, it is possible that the gene-dosage effect is only evident above some threshold of gene expression levels. Immunohistochemical staining using an anti-tyramine antibody identified tyramine-containing neurons in the larval central nervous system, some of which were distinct from the octopamine-containing neurons. Taken together, these results strongly suggest that tyramine functions as a neuromodulator.

Characterization of a tyramine receptor from Caenorhabditis elegans

Octopamine (OA) plays an important role in the regulation of a number of key processes in nematodes, including pharyngeal pumping, locomotion and egg-laying. However, while putative OA receptors can be tentatively identified in the Caenorhabditis elegans database, no OA receptors have been functionally characterized from any nematode. We have isolated two cDNAs, ser-2 and ser-2a, encoding putative C.elegans serotonin/OA receptors (C02D4.2, ser-2). The sequences of these cDNAs differ from that predicted by GeneFinder and lack 42 bp of exon 2. In addition, ser-2a appears to be alternatively spliced and lacks a predicted 23 amino acids in the third intracellular loop. COS-7 cells expressing SER-2 bind [3H]LSD in the low nM range and exhibit Kis for tyramine, octopamine and serotonin of 0.07, 2, and 13.7 micro m, respectively. Significantly, tyramine reduces forskolin-stimulated cAMP levels in HEK293 cells stably expressing SER-2 with an IC50 of about 360 nm, suggesting that SER-2 is a tyramine receptor.

Functional expression of a locust tyramine receptor in murine erythroleukaemia cells

The LCR/MEL system (Locus Control Region/Murine Erythroleukaemia cells) was employed to express and characterize the Locusta migratoria tyramine receptor (TyrLoc), an insect G protein-coupled receptor. Functional agonist-dependent responses were recorded in stable, tyramine receptor expressing cell clones (MEL-TyrLoc). Tyramine elicited a dose-dependent increase of cytosolic Ca2+-ions and an attenuation of forskolin-induced cyclic adenosine monophosphate (AMP) production. Octopamine was shown to be a weak agonist for both responses. In addition, yohimbine proved to be a potent tyramine receptor antagonist. This study reports the first application of the LCR/MEL expression system in functional assays for G protein-coupled receptors and therefore expands the capabilities of this system by exploiting the functionality of the signal transduction pathways.

Trace amines: identification of a family of mammalian G protein-coupled receptors

Tyramine, beta-phenylethylamine, tryptamine, and octopamine are biogenic amines present in trace levels in mammalian nervous systems. Although some "trace amines" have clearly defined roles as neurotransmitters in invertebrates, the extent to which they function as true neurotransmitters in vertebrates has remained speculative. Using a degenerate PCR approach, we have identified 15 G protein-coupled receptors (GPCR) from human and rodent tissues. Together with the orphan receptor PNR, these receptors form a subfamily of rhodopsin GPCRs distinct from, but related to the classical biogenic amine receptors. We have demonstrated that two of these receptors bind and/or are activated by trace amines. The cloning of mammalian GPCRs for trace amines supports a role for trace amines as neurotransmitters in vertebrates. Three of the four human receptors from this family are present in the amygdala, possibly linking trace amine receptors to affective disorders. The identification of this family of receptors should rekindle the investigation of the roles of trace amines in mammalian nervous systems and may potentially lead to the development of novel therapeutics for a variety of indications.

Octopamine is the synaptic transmitter between identified neurons in the buccal feeding network of the pond snail lymnaea stagnalis

We report the pharmacological properties of synaptic connections from the three octopamine-containing OC interneurons to identified buccal feeding neurons in the pond snail, Lymnaea stagnalis. Intracellular stimulation of an OC interneuron evokes inhibitory postsynaptic potentials in the B3 motoneurons and N2 (d) interneurons, while the synapse between OC and N3 (phasic) interneurons has two components: an initial electrical excitation followed by chemical inhibition. All synaptic responses persist in a saline with elevated calcium and magnesium suggesting that the connections are monosynaptic. Local perfusion of 10(-4) M octopamine produces the same inhibitory membrane responses from these buccal neurons as OC stimulation. These responses also persist in high Mg(2+)/Ca(2+) saline indicating direct membrane effects. The similarities in reversal potentials for the synaptic hyperpolarization evoked on B3 neurons after OC stimulation (-89.0 mV, S.E.M.=14.1, n=10) and the octopamine response of the B3 neurons (-84.7 mV, S.E.M.=6.6, n=6) indicate that increased K(+)-conductance underlies both responses. Bath application of the octopaminergic drugs phentolamine (10(-6) M), epinastine (10(-6) M) or DCDM (10(-4) M) blocks the inhibitory synapse onto B3 or N2 neurons and the chemical component of the N3 response. They also block the octopamine-evoked inhibition of B3, N2 and N3 neurons. NC-7 (2x10(-5) M) has a hyperpolarizing agonist effect (like octopamine) on these neurons and also blocks their chemical synaptic input from the OC interneurons. These results provide pharmacological evidence that the neurotransmitter between the octopamine-immunopositive OC interneurons and its followers is octopamine. This is the first example of identified octopaminergic synaptic connections within the snail CNS.

Octopamine receptors in the honey bee and locust nervous system: pharmacological similarities between homologous receptors of distantly related species

Honey bees are perhaps the most versatile models to study the cellular and pharmacological basis underlying behaviours ranging from learning and memory to sociobiology. For both aspects octopamine (OA) is known to play a vital role. The neuronal octopamine receptor of the honey bee shares pharmacological similarities with the neuronal octopamine receptor of the locust. Both, agonists and antagonists known to have high affinities for the locust neuronal octopamine receptor have also high affinities for the bee neuronal octopamine receptor. The distribution of receptors is more or less congruent between locusts and bees. Optic lobes and especially the mushroom bodies are areas of greatest octopamine receptor expression in both species, which mirrors the physiological significance of octopamine in the insect nervous system. The neuronal octopamine receptor of insects served as a model to study the pharmacological similarity of homologous receptors from distantly related species, because bees and locusts are separated by at least 330 million years of evolution.

Isolation of a cDNA for an octopamine-like, G-protein coupled receptor from the cattle tick, Boophilus microplus

Octopamine is a biogenic amine neurotransmitter of invertebrates that binds to a G-protein coupled receptor that has seven transmembrane domains. Formamidine pesticides like amitraz are highly specific agonists of the octopamine receptor. Amitraz is used extensively to control the cattle tick, Boophilus microplus, and many other ticks but now there are strains of ticks that are resistant to amitraz. We have isolated a cDNA from the cattle tick, B. microplus, that belongs to the biogenic amine family of receptors. The predicted amino acid sequence from this cDNA is most similar to octopamine receptors from insects. The nucleotide sequence of this gene from amitraz-resistant and amitraz-susceptible cattle ticks was identical. Thus, a point mutation/s did not confer resistance to amitraz in the strains we studied. Alternative explanations for resistance to amitraz in B. microplus are discussed.

Distribution of dopamine receptors and dopamine receptor homologs in the brain of the honey bee, Apis mellifera L

In the brain of the honey bee, Apis mellifera, the radioligands [3H]-SCH23390 and [3H]-spiperone recognise D1- and D2-like receptors, respectively. In addition to being pharmacologically distinct and exhibiting significantly different expression profiles during the lifetime of the bee, [3H]-SCH23390- and [3H]-spiperone-binding sites differ markedly in their distribution within the brain. Estimates of [3H]-SCH23390-binding site density are highest in the somatal rind, whereas [3H]-spiperone-binding sites are most concentrated in the beta lobe neuropil of the mushroom bodies. Molecular cloning techniques have been used to identify two honey bee genes encoding dopamine receptor homologs. The first is the honey bee counterpart of a Drosophila D1-like dopamine receptor and is expressed in the mushroom bodies of both workers and drones. The second is related to D2-like dopamine receptors from vertebrates and is expressed in the brain of the bee, but the precise distribution of expression is not yet known.

Cytoarchitecture of histamine-, dopamine-, serotonin- and octopamine-containing neurons in the cricket ventral nerve cord

The present article provides a comparative neuroanatomical description of the cellular localization of the biogenic amines histamine, dopamine, serotonin and octopamine in the ventral nerve cord of an insect, namely the cricket, Gryllus bimaculatus. Generally, different immunocytochemical staining techniques reveal a small number of segmentally distributed immunoreactive (-IR) amine-containing neurons allowing single cell reconstruction of prominent elements. Aminergic neurons share common morphological features in that they innervate large portions of neurophil and often connect different neuromeres by intersegmental 'wide-field' projections of varicose appearance. In many cases aminergic terminals are also found on the surface of peripheral nerves suggesting additional neurohemal release sites. Despite such morphological similarities histological analysis demonstrates for any given amine functionally distinct neuron types with specific innervation patterns establishing discrete pathways. Histamine-IR interneurons are characterized by both ascending and descending projections forming central and peripheral terminals. The descending branches from dopamine-IR cells mainly converge within the terminal ganglion, whereas serotonin-IR interneurons with ascending projections often terminate within the brain. Serotonin is also present in sensory and motor neurons. In contrast to other aminergic neurons, most octopamine-IR cells represent unpaired neurons projecting through motor nerves of the soma-containing neuromere. Octopamine-IR cells with intersegmental branches are only rarely found. Based on these findings, a colocalization of different amines within the same neuron seems to be unlikely to occur in the cricket ventral nerve cord. With respect to the neuroanatomical description of amine-containing neurons known physiological effects of biogenic amines and their possible neuromodulatory functions in insects are discussed.

An octopaminergic system in the CNS of the snails, Lymnaea stagnalis and Helix pomatia

Octopamine (OA) levels in each ganglion of the terrestrial snail, Helix pomatia, and the pond snail, Lymnaea stagnalis, were measured by using the HPLC technique. In both species an inhomogeneous distribution of OA was found in the central nervous system. The buccal ganglia contained a concentration of OA (12.6 pmol mg^-1 and 18.8 pmol mg^-1) that was two to three times higher than the pedal (4.93 pmol mg^-1 and 9.2 pmol mg^-1) or cerebral (4.46 pmol mg^-1 and 4.9 pmol mg^-1) ganglia of Helix and Lymnaea, respectively, whereas no detectable amount of OA could be assayed in the visceroparietal complex. In Lymnaea ganglia, the OA uptake into the synaptosomal fraction had a high (K_m1 = 4.07 ± 0.51 μM, V_max1 = 0.56 ± 0.11 pmol mg^-1 per 20 min), and a low (K_m2 = 47.6 ± 5.2 μM, V_max2 = 4.2 ± 0.27 pmol mg^-1 per 20 min), affinity component. A specific and dissociable ³H-OA binding to the membrane pellet prepared from the CNS of both Helix and Lymnaea was demonstrated. The Scatchard analysis of the ligand binding data showed a one-binding site, representing a single receptor site. The K_d and B_max values were found to be 33.7 ± 5.95 nM and 1678 ± 179 fmol g^-1 tissue in Helix and 84.9 ± 17.4 nM and 3803 ± 515 fmol g^-1 tissue in Lymnaea preparation. The pharmacological properties of the putative molluscan OA receptor were characterized in both species and it was demonstrated that the receptor resembled the insect OA₂ rather than to the cloned Lymnaea OA receptor. Immunocytochemical labelling demonstrated the presence of OA-immunoreactive neurons and fibres in the buccal, cerebral and pedal ganglia in the central nervous system of both species investigated. Electrophysiological experiments also suggested that the Lymnaea brain possessed specific receptors for OA. Local application of OA onto the identified buccal B2 neuron evoked a hyperpolarization which could selectively be inhibited by the OAergic agents phentolamine, demethylchlordimeform and 2-chloro-4-methyl-2-(phenylimino)-imidazolidine. Among the dopamine antagonists, ergotamine reversibly inhibited the OA response, whereas sulpiride had no effect. Based on our findings, a neurotransmitter-modulator role of OA is suggested in the gastropod CNS.