Biogenic Amine Receptors Research Articles

Aripiprazole, a novel atypical antipsychotic drug with a unique and robust pharmacology.

Atypical antipsychotic drugs have revolutionized the treatment of schizophrenia and related disorders. The current clinically approved atypical antipsychotic drugs are characterized by having relatively low affinities for D(2)-dopamine receptors and relatively high affinities for 5-HT(2A) serotonin receptors (5-HT, 5-hydroxytryptamine (serotonin)). Aripiprazole (OPC-14597) is a novel atypical antipsychotic drug that is reported to be a high-affinity D(2)-dopamine receptor partial agonist. We now provide a comprehensive pharmacological profile of aripiprazole at a large number of cloned G protein-coupled receptors, transporters, and ion channels. These data reveal a number of interesting and potentially important molecular targets for which aripiprazole has affinity. Aripiprazole has highest affinity for h5-HT(2B)-, hD(2L)-, and hD(3)-dopamine receptors, but also has significant affinity (5-30 nM) for several other 5-HT receptors (5-HT(1A), 5-HT(2A), 5-HT(7)), as well as alpha(1A)-adrenergic and hH(1)-histamine receptors. Aripiprazole has less affinity (30-200 nM) for other G protein-coupled receptors, including the 5-HT(1D), 5-HT(2C), alpha(1B)-, alpha(2A)-, alpha(2B)-, alpha(2C)-, beta(1)-, and beta(2)-adrenergic, and H(3)-histamine receptors. Functionally, aripiprazole is an inverse agonist at 5-HT(2B) receptors and displays partial agonist actions at 5-HT(2A), 5-HT(2C), D(3), and D(4) receptors. Interestingly, we also discovered that the functional actions of aripiprazole at cloned human D(2)-dopamine receptors are cell-type selective, and that a range of actions (eg agonism, partial agonism, antagonism) at cloned D(2)-dopamine receptors are possible depending upon the cell type and function examined. This mixture of functional actions at D(2)-dopamine receptors is consistent with the hypothesis proposed by Lawler et al (1999) that aripiprazole has "functionally selective" actions. Taken together, our results support the hypothesis that the unique actions of aripiprazole in humans are likely a combination of "functionally selective" activation of D(2) (and possibly D(3))-dopamine receptors, coupled with important interactions with selected other biogenic amine receptors--particularly 5-HT receptor subtypes (5-HT(1A), 5-HT(2A)).

Peripheral interactions between dextromethorphan, ketamine and amitriptyline on formalin-evoked behaviors and paw edema in rats

The local, peripheral administration of antidepressants and excitatory amino acid receptor antagonists can cause analgesia in a number of conditions. The present study examined the effects of combinations of dextromethorphan and ketamine, two clinically used N-methyl- d-aspartate (NMDA) receptor antagonists, with amitriptyline on formalin-evoked behaviors and paw edema. Pretreatment with amitriptyline or dextromethorphan (10–300 nmol) resulted in suppression of flinching behaviors induced by 2.5% formalin, but ketamine had no intrinsic effect. Combination of an inactive dose of dextromethorphan with amitriptyline, and vice versa, resulted in an increase of analgesia so that previously inactive doses now caused significant analgesia. Combinations of multiple doses of ketamine with amitriptyline did not modify the response to amitriptyline. Both dextromethorphan and ketamine increased the paw edema induced by formalin, and this was blocked by low doses of amitriptyline. In the absence of formalin, amitriptyline (1–100 nmol) caused a dose-related suppression of the paw edema produced by dextromethorphan and ketamine. Amitriptyline also blocked paw edema produced by 5-hydroxytryptamine and compound 48/80. Each of the drugs used in this study exerts multiple pharmacological effects. Increased analgesia by drug combinations (amitriptyline/dextromethorphan) could show the involvement of a number of these mechanisms (e.g. NMDA receptor blockade, blockage of sodium channels, blockage of biogenic amine receptors), while a lack of intensification (amitriptyline/ketamine) could reflect occluded actions due to expression of similar actions by the other drug. Paw edema induced by dextromethorphan and ketamine involves inhibition of biogenic amine reuptake, and the ability of amitriptyline to block biogenic amine receptors likely accounts for its inhibiton of these actions. Combinations of these particular agents could represent a method for augmented analgesia and minimization of local adverse reactions.

The Unique Ligand-binding Pocket for the Human Prostacyclin Receptor: SITE-DIRECTED MUTAGENESIS AND MOLECULAR MODELING

The human prostacyclin receptor is a seven-transmembrane alpha-helical G-protein coupled receptor, which plays important roles in both vascular smooth muscle relaxation as well as prevention of blood coagulation. The position of the native ligand-binding pocket for prostacyclin as well as other derivatives of the 20-carbon eicosanoid, arachidonic acid, has yet to be determined. Through the use of prostanoid receptor sequence alignments, site-directed mutagenesis, and the 2.8-A x-ray crystallographic structure of bovine rhodopsin, we have developed a three-dimensional model of the agonist-binding pocket within the seven-transmembrane (TM) domains of the human prostacyclin receptor. Upon mutation to alanine, 11 of 29 candidate residues within TM domains II, III, IV, V, and VII exhibited a marked decrease in agonist binding. Of this group, four amino acids, Arg-279 (TMVII), Phe-278 (TMVII), Tyr-75 (TMII), and Phe-95 (TMIII), were identified (via receptor amino acid sequence alignment, ligand structural comparison, and computer-assisted homology modeling) as having direct molecular interactions with ligand side-chain constituents. This binding pocket is distinct from that of the biogenic amine receptors and rhodopsin where the native ligands (also composed of a carbon ring and a carbon chain) are accommodated in an opposing direction. These findings should assist in the development of novel and highly specific ligands including selective antagonists for further molecular pharmacogenetic studies of the human prostacyclin receptor.

Actions of Benzaldehyde Hydrazones and Semicarbazones on Biogenic Amine Receptors in the Silkworm Bombyx mori

Four hydrazones (HZs) and six semicarbazones (SCZs) of substituted benzaldehydes were synthesized and examined for their ability to control insect adenylate cyclase through their interaction with biogenic amine receptors. Among the compounds synthesized, two with a hydroxyl group at the 4-position of the phenyl moiety, HZ-01 and SCZ-03, were found to reduce the basal levels of cAMP in head membrane homogenates of fifth instar larvae of the silkworm Bombyx mori. The semicarbazone SCZ-03 dose-dependently attenuated not only basal but also forskolin-stimulated cAMP levels. Tyramine (TYR) and dopamine (DPM) also produced a dose-dependent reduction in basal cAMP levels. DPM and TYR receptor antagonists abolished the attenuating effects of SCZ-03. These findings suggest that SCZ-03 acts as a non-selective agonist for DPM and TYR receptors to inactivate adenylate cyclase in B. mori larvae.

Positive and negative modulation of Bombyx mori adenylate cyclase by 5-phenyloxazoles: identification of octopamine and tyramine receptor agonists.

Nineteen 5-phenyloxazoles (5POs) were examined for their ability to modulate adenylate cyclase by measuring cAMP produced in head membrane homogenates of fifth instar larvae of the silkworm Bombyx mori. Among the compounds tested, 5-(4-methoxyphenyl)oxazole (9) and the 2,6-dichlorophenyl congener showed the highest activation of adenylate cyclase; both compounds produced approximately half the level of cAMP produced by the action of octopamine (OCT). The OCT receptor antagonists chlorpromazine, mianserin, and metoclopramide attenuated 9-stimulated cAMP production. In contrast, 5-(4-hydroxyphenyl)oxazole (8) and the 4-cyanophenyl congener attenuated both OCT-stimulated and basal cAMP production. The tyramine (TYR) receptor antagonist yohimbine inhibited the negative effect of 8. These findings indicate that the 5PO class of compounds includes both positive and negative modulators of adenylate cyclase in the heads of B. mori larvae, and that 9 and 8 are OCT and TYR receptor agonists, respectively. These compounds might prove useful for a pharmacological dissection of biogenic amine receptors.

Modulation of formalin-induced behaviors and edema by local and systemic administration of dextromethorphan, memantine and ketamine

The present study examined the effects of local peripheral and systemic administration of three clinically used excitatory amino acid receptor antagonists (dextromethorphan, memantine, ketamine) on pain behaviors and edema produced by formalin (1.5% and 5%) in rats. Peripheral administration of dextromethorphan produced a locally mediated suppression of flinching behaviors induced by 1.5% and 5% formalin, but biting/licking behaviors were not affected. Memantine and ketamine had no effect on either of these behaviors. All three agents augmented edema produced by 1.5% and 5% formalin. When administered alone, dextromethorphan, memantine and ketamine produced an intrinsic paw swelling response, and this was blocked by the biogenic amine receptor antagonists mepyramine, phentolamine, methysergide and ketanserin. Following systemic administration, all three agents suppressed biting/licking behaviors, had no effect on flinching behaviors, and suppressed paw swelling induced by 5% formalin to varying degrees. These results provide evidence for a peripherally mediated antinociceptive action of dextromethorphan in the rat formalin test, but this may not necessarily be due to block of excitatory amino acid receptors as it is not observed with memantine or ketamine. All three agents produce a peripherally mediated paw swelling, which is likely due to blockade of biogenic amine reuptake. Systemic administration of all three agents produces antinociceptive and anti-inflammatory actions that may be due to block of excitatory amino acid receptors in the spinal cord.

In vivo genetics of anaesthetic action

In vivo genetics of anaesthetic action

Optimizing antidepressant treatment: efficacy and tolerability.

It has been suggested that dual-action antidepressants acting on both serotonin and noradrenaline pathways in the brain may offer superior therapeutic benefit over classical antidepressants, particularly in severe depression. Directly acting dual-action antidepressant drugs include venlafaxine and milnacipran. In addition, mirtazapine and nefazodone, may indirectly potentiate serotonergic and noradrenergic transmission, although evidence that they do indeed do so in vivo is limited. Meta-analysis of clinical trials suggests that venlafaxine has a more rapid onset of action than selective serotonin reuptake inhibitors (SSRIs), and the same may also be true for milnacipran and mirtazapine. Efficacy, both in terms of extent of antidepressant effect and in the proportion of patients responding, is probably superior to that of SSRIs, at least for venlafaxine and milnacipran. In terms of tolerability, all dual-action drugs clearly appear to be better tolerated than tricyclic antidepressants. Mirtazapine and nefazodone have specific side-effect profiles as a result of their antagonist action at biogenic amine and other receptors. Milnacipran, and to a lesser extent venlafaxine, are slightly better tolerated than SSRIs.

In vitro receptor screening of pure constituents of St. John's wort reveals novel interactions with a number of GPCRs.

Hypericum perforatum L. (St. John's wort; SJW) is one of the leading psychotherapeutic phytomedicines and great effort has been devoted to clarifying its mechanism of action. We have undertaken a comprehensive analysis of several pure compounds isolated from the crude extract to gain further insight into the molecular actions of various substituents of SJW. We characterized the in vitro pharmacology of the naphthodianthrones hypericin and pseudohypericin, the phloroglucinol derivative hyperforin, and several flavonoids at 42 biogenic amine receptors and transporters using the resources of the National Institute of Mental Health Psychoactive Drug Screening Program. The biflavonoid amentoflavone significantly inhibited binding at serotonin (5-HT(1D), 5-HT(2C)), D(3)-dopamine, delta-opiate, and benzodiazepine receptors. The naphthodianthrone hypericin had significant activity at D(3)- and D(4)-dopamine receptors and beta-adrenergic receptors. With the exception of the D(1)-dopamine receptor, the phloroglucinol derivative hyperforin was less active than other SJW constituents tested on all screened receptors. Our present in vitro data clearly show that several pure substances in SJW are potential CNS psychoactive agents and may contribute to the antidepressant efficacy of the plant in a complex manner. Our data also reveal novel and heretofore unexpected interactions of pure compounds in SJW at a number of GPCRs, transporters, and ion channels. We hypothesize that additive or synergistic actions of different single compounds may be responsible for the antidepressant efficacy of SJW. These results and this general approach may impact our understanding of phytomedicines in general and H. perforatum specifically.

Biochemistry and molecular biology of receptors for biogenic amines in locusts.

The biochemistry and molecular biology of biogenic amines and their metabotropic receptors in insects, with a focus on locusts, is reviewed. These compounds are known to be responsible for the control of a huge variety of different behaviours. Receptors for these amines usually belong to the class of G-protein coupled receptors (GPCR) and transmit all known functions of these compounds. The physiological significance of biogenic amine neurotransmission in insects, especially in locusts is briefly summarised. Regarding the corresponding receptors, their pharmacological features and the molecular properties are described in detail.

Cloning, Expression, and Regulation of a Glucocorticoid-Induced Receptor in Rat Brain: Effect of Repetitive Amphetamine

Behavioral sensitization to psychostimulants involves neuroadaptation of stress-responsive systems. We have identified and sequenced a glucocorticoid-induced receptor (GIR) cDNA from rat prefrontal cortex. The full-length GIR cDNA encodes a 422 amino acid protein belonging to G-protein-coupled receptor superfamily. Although the ligand for GIR is still unknown, the dendrogram construction indicates that GIR may belong to peptide receptor subfamily (e.g., substance P receptor), with more distant relationship to subfamilies of glycoprotein hormone receptors (e.g., thyrotropin receptor) and biogenic amine receptors (e.g., dopamine receptor). GIR shares 31-34% amino acid identity to the tachykinin receptors (substance P receptor, neurokinin A receptor, and neurokinin B receptor). GIR mRNA is expressed preferentially in brain, and its neuronal expression is relegated to limbic brain regions, particularly in forebrain. GIR transcript levels are increased significantly and persistently in prefrontal cortex for 7 d after discontinuation of chronic amphetamine exposure. The induction of GIR expression by amphetamine is associated with augmented behavioral activation. These findings suggest that modulation of GIR expression may be involved in behavioral sensitization, and GIR may play a role at the interface between stress and neuroadaptation to psychostimulants.

Molecular and pharmacological properties of insect biogenic amine receptors: lessons from Drosophila melanogaster and Apis mellifera.

In the central nervous system (CNS) of both vertebrates and invertebrates, biogenic amines are important neuroactive molecules. Physiologically, they can act as neurotransmitters, neuromodulators, or neurohormones. Biogenic amines control and regulate various vital functions including circadian rhythms, endocrine secretion, cardiovascular control, emotions, as well as learning and memory. In insects, amines like dopamine, tyramine, octopamine, serotonin, and histamine exert their effects by binding to specific membrane proteins that primarily belong to the superfamily of G protein-coupled receptors. Especially in Drosophila melanogaster and Apis mellifera considerable progress has been achieved during the last few years towards the understanding of the functional role of these receptors and their intracellular signaling systems. In this review, the present knowledge on the biochemical, molecular, and pharmacological properties of biogenic amine receptors from Drosophila and Apis will be summarized. Arch.

The in vitro pharmacology of the β-adrenergic receptor pet ligand ( s )-fluorocarazolol reveals high affinity for cloned β-adrenergic receptors and moderate affinity for the human 5-HT 1A receptor

s-Fluorocarazolol [(S)-FCZ] is the major positron emission tomography (PET) ligand currently used to visualize central beta-adrenergic receptors in vivo, although its pharmacology is incompletely known. Our objective was to comprehensively characterize the in vitro pharmacology of (S)- and (R)-FCZ to determine its suitability for study of central and peripheral beta-adrenergic receptors. We characterized the in vitro pharmacology of (S)-FCZ at 42 biogenic amine receptors and transporters in vitro using the resources of the National Institute of Mental Health Psychoactive Drug Screening Program. As expected (R)- and (S)-FCZ had high affinities for beta-adrenergic receptors (Ki values=0.08-0.45 nM) and negligible affinities (Ki values>100 nM) for nearly all other tested receptors and transporters with the exception of the h5-HT1A receptor for which (S)-FCZ had high affinity (Ki=34 nM). Interestingly, (R)-FCZ had low affinity for the h5-HT1A receptor (Ki=342 nM). The high affinity of (S)-FCZ for the h5-HT1A receptor is not likely to interfere with studies of peripheral beta-adrenergic receptors, since 5-HT1A receptors are expressed at very low levels outside the central nervous system. Indeed, computer simulations predict that even at low ligand concentrations, 5-HT1A binding in brain regions like hippocampus are likely to be substantial. Thus, (S)-FCZ may not be a suitable PET ligand for studies of central nervous system beta-adrenergic receptors unless the contribution by 5-HT1A sites can be shown to be negligible.

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.

Phenolamine-dependent adenylyl cyclase activation in Drosophila Schneider 2 cells

Drosophila Schneider 2 (S 2) cells are often employed as host cells for non-lytic, stable expression and functional characterization of mammalian and insect G-protein-coupled receptors (GPCRs), such as biogenic amine receptors. In order to avoid cross-reactions, it is extremely important to know which endogenous receptors are already present in the non-transfected S 2 cells. Therefore, we analyzed cellular levels of cyclic AMP and Ca 2+, important second messengers for intracellular signal transduction via GPCRs, in response to a variety of naturally occurring biogenic amines, such as octopamine, tyramine, serotonin, histamine, dopamine and melatonin. None of these amines (up to 0.1 mM) was able to reduce forskolin-stimulated cyclic AMP production in S 2 cells. Furthermore, no agonist-induced calcium responses were observed. Nevertheless, the phenolamines octopamine (OA) and tyramine (TA) induced a dose-dependent increase of cyclic adenosine monophosphate (AMP) production in S 2 cells, while serotonin, histamine, dopamine and melatonin (up to 0.1 mM) did not. The pharmacology of this response was similar to that of the octopamine-2 (OA 2) receptor type. In addition, this paper provides evidence for the presence of an endogenous mRNA encoding an octopamine receptor type in these cells, which is identical or very similar to OAMB. This receptor was previously shown to be positively coupled to adenylyl cyclase.

Benzophenone derivatives and related compounds as potent histamine H3-receptor antagonists and potential PET/SPECT ligands.

Para-substituted aromatic ethers with benzophenone or related structural elements and a 3-(1H-imidazol-4-yl)propyloxy moiety were prepared by Mitsunobu-type ether synthesis or SNAr reaction. Most of the title compounds possess high antagonist potency in histamine H3-receptor assays in vitro as well as in vivo in mouse CNS following oral administration. After defining 4-(3-(1H-imidazol-4-yl)propyloxy)phenyl phenyl methanone as a new lead, structure-activity relationships were investigated for this new class of compounds. Substitution of the meta'-position of the benzophenone moiety with halogen atoms (e.g., iodine, fluorine) led to compounds with high antagonist potency in vitro as well as in vivo (Ki = 9.3 and 4.3 nM, ED50 = 0.7 and 0.47 mg/kg p.o., 18 and 12, respectively). A receptor profile of several functional in vitro assays for several biogenic amine receptors for the meta'-iodinated derivative demonstrated high selectivity toward the histamine H3 receptor.

A Novel G-Protein-Coupled Receptor Gene Expressed in Striatum

Differential display screening for region-specific transcripts in rat brain revealed a novel striatum-specific transcript encoding an orphan G-protein-coupled receptor (GPCR) designated Strg/Gpr88 for striatum-specific GPCR. We isolated its homologues from human (HGMW-approved symbol GPR88) and mouse and mapped them to chromosomes 1p21.3 and 3G1, respectively. These loci are syntenic to each other, thereby suggesting their orthology. The predicted primary sequences of Strg/Gpr88 proteins are highly conserved between human and rodents and show the highest level of homology to receptors for biogenic amines. However, Strg/Gpr88 lacks some residues conserved in all known biogenic amine receptors and hence may represent a novel subtype of GPCR. Northern blot and in situ hybridization analyses revealed that Strg/Gpr88 transcripts are expressed almost exclusively in striatum in both human and rodents. Remarkable conservation in primary structure and a unique expression pattern may indicate a role for Strg/Gpr88 in the fundamental functions of striatum such as the control of motor behavior.

Identification of the Binding Site for a Novel Class of CCR2b Chemokine Receptor Antagonists: BINDING TO A COMMON CHEMOKINE RECEPTOR MOTIF WITHIN THE HELICAL BUNDLE

Monocyte chemoattracant-1 (MCP-1) stimulates leukocyte chemotaxis to inflammatory sites, such as rheumatoid arthritis, atherosclerosis, and asthma, by use of the MCP-1 receptor, CCR2, a member of the G-protein-coupled seven-transmembrane receptor superfamily. These studies identified a family of antagonists, spiropiperidines. One of the more potent compounds blocks MCP-1 binding to CCR2 with a K(d) of 60 nm, but it is unable to block binding to CXCR1, CCR1, or CCR3. These compounds were effective inhibitors of chemotaxis toward MCP-1 but were very poor inhibitors of CCR1-mediated chemotaxis. The compounds are effective blockers of MCP-1-driven inhibition of adenylate cyclase and MCP-1- and MCP-3-driven cytosolic calcium influx; the compounds are not agonists for these pathways. We showed that glutamate 291 (Glu(291)) of CCR2 is a critical residue for high affinity binding and that this residue contributes little to MCP-1 binding to CCR2. The basic nitrogen present in the spiropiperidine compounds may be the interaction partner for Glu(291), because the basicity of this nitrogen was essential for affinity; furthermore, a different class of antagonists, a class that does not have a basic nitrogen (2-carboxypyrroles), were not affected by mutations of Glu(291). In addition to the CCR2 receptor, spiropiperidine compounds have affinity for several biogenic amine receptors. Receptor models indicate that the acidic residue, Glu(291), from transmembrane-7 of CCR2 is in a position similar to the acidic residue contributed from transmembrane-3 of biogenic amine receptors, which may account for the shared affinity of spiropiperidines for these two receptor classes. The models suggest that the acid-base pair, Glu(291) to piperidine nitrogen, anchors the spiropiperidine compound within the transmembrane ovoid bundle. This binding site may overlap with the space required by MCP-1 during binding and signaling; thus the small molecule ligands act as antagonists. An acidic residue in transmembrane region 7 is found in most chemokine receptors and is rare in other serpentine receptors. The model of the binding site may suggest ways to make new small molecule chemokine receptor antagonists, and it may rationalize the design of more potent and selective antagonists.

No Behavioral Control over Mating Frequency in Queen Honey Bees (Apis melliferaL.): Implications for the Evolution of Extreme Polyandry

Ever since the seminal papers of Hamilton (1964), a common focus of sociobiology has been the genetic relationships among individuals of insect societies. At issue are agents that reduce the average relatedness among colony members since they are difficult to interpret in light of the evolution of sociality by kin selection. One such factor is polyandry or female multiple mating. Low levels of polyandry are common among the social Hymenoptera, occurring across a broad range of taxa and social systems (Page and Metcalf 1982; Page 1986; Crozier and Pamilo 1996). In spite of that, high mating frequencies (‚2 mates/ female) are relatively rare among the highly eusocial Hymenoptera, limited to only one or a few genera in each of the ants, bees, and wasps (Boomsma and Ratnieks 1996). These extraordinary levels of polyandry have been of particular interest among students of social insect research since their adaptive significance is still in question and highly debated (Kraus and Page 1998; Sherman et al. 1998). The genus Apis has been studied in depth for both queen reproductive behavior and mating frequency, facts that allow insight into the evolution of extreme polyandry in these species. The reproductive behavior of honey bee queens (Apis mellifera) occurs early in their lifetimes. Virgin queens initiate mating behavior when they are about 1 wk old by briefly exiting the colony and orienting to local landmarks (Ruttner 1956). After such orientation * To whom correspondence should be addressed; e-mail: drtarpy@

Amtyr1: characterization of a gene from honeybee (Apis mellifera) brain encoding a functional tyramine receptor.

Biogenic amine receptors are involved in the regulation and modulation of various physiological and behavioral processes in both vertebrates and invertebrates. We have cloned a member of this gene family from the CNS of the honeybee, Apis mellifera. The deduced amino acid sequence is homologous to tyramine receptors cloned from Locusta migratoria and Drosophila melanogaster as well as to an octopamine receptor cloned from Heliothis virescens. Functional properties of the honeybee receptor were studied in stably transfected human embryonic kidney 293 cells. Tyramine reduced forskolin-induced cyclic AMP production in a dose-dependent manner with an EC50 of approximately 130 nM. A similar effect of tyramine was observed in membrane homogenates of honeybee brains. Octopamine also reduced cyclic AMP production in the transfected cell line but was both less potent (EC50 of approximately 3 microM) and less efficacious than tyramine. Receptor-encoding mRNA has a wide-spread distribution in the brain and subesophageal ganglion of the honeybee, suggesting that this tyramine receptor is involved in sensory signal processing as well as in higher-order brain functions.