A primordial dopamine D1-like adenylyl cyclase-linked receptor from Drosophila melanogaster displaying poor affinity for benzazepines

Imaging analysis of clock neurons reveals light buffers the wake-promoting effect of dopamine

How animals maintain proper amounts of sleep yet still be flexible to changes in the environmental conditions remains unknown. Here we showed that environmental light suppresses the wake-promoting effects of dopamine in fly brains. A subset of clock neurons, the 10 large lateral-ventral neurons (l-LNvs), are wake-promoting and respond to dopamine, octopamine as well as light. Behavioral and imaging analyses suggested that dopamine is a stronger arousal signal than octopamine. Surprisingly, light exposure not only suppressed the l-LNv responses but also synchronized responses of neighboring l-LNvs. This regulation occured by distinct mechanisms: light-mediated suppression of octopamine responses is regulated by the circadian clock, whereas light regulation of dopamine responses occurs by upregulation of inhibitory dopamine receptors. Plasticity therefore alters the relative importance of diverse cues based on the environmental mix of stimuli. The regulatory mechanisms described here may contribute to the control of sleep stability while still allowing behavioral flexibility.

Dopamine reveals neural circuit mechanisms of fly memory

A goal of memory research is to understand how changing the weight of specific synapses in neural circuits in the brain leads to an appropriate learned behavioral response. Finding the relevant synapses should allow investigators to probe the underlying physiological and molecular operations that encode memories and permit their retrieval. In this review I discuss recent work in Drosophila that implicates specific subsets of dopaminergic (DA) neurons in aversive reinforcement and appetitive motivation. The zonal architecture of these DA neurons is likely to reveal the functional organization of aversive and appetitive memory in the mushroom bodies. Combinations of fly DA neurons might code negative and positive value, consistent with a motivational systems role as proposed in mammals.

Pharmacology of signaling induced by dopamine D(1)-like receptor activation

Dopamine D(1)-like receptors consisting of D(1) and D(5) subtypes are intimately implicated in dopaminergic regulation of fundamental neurophysiologic processes such as mood, motivation, cognitive function, and motor activity. Upon stimulation, D(1)-like receptors initiate signal transduction cascades that are mediated through adenylyl cyclase or phosphoinositide metabolism, with subsequent enhancement of multiple downstream kinase cascades. The latter actions propagate and further amplify the receptor signals, thus predisposing D(1)-like receptors to multifaceted interactions with various other mediators and receptor systems. The adenylyl cyclase response to dopamine or selective D(1)-like receptor agonists is reliably associated with the D(1) subtype, while emerging evidence indicates that the phosphoinositide responses in native brain tissues may be preferentially mediated through stimulation of the D(5) receptor. Besides classic coupling of each receptor subtype to specific G proteins, additional biophysical models are advanced in attempts to account for differential subcellular distribution, heteromolecular oligomerization, and activity-dependent selectivity of the receptors. It is expected that significant advances in understanding of dopamine neurobiology will emerge from current and anticipated studies directed at uncovering the molecular mechanisms of D(5) coupling to phosphoinositide signaling, the structural features that might enhance pharmacological selectivity for D(5) versus D(1) subtypes, the mechanism by which dopamine may modulate phosphoinositide synthesis, the contributions of the various responsive signal mediators to D(1) or D(5) interactions with D(2)-like receptors, and the spectrum of dopaminergic functions that may be attributed to each receptor subtype and signaling pathway.

Two different forms of arousal in Drosophila are oppositely regulated by the dopamine D1 receptor ortholog DopR via distinct neural circuits

Arousal is fundamental to many behaviors, but whether it is unitary or whether there are different types of behavior-specific arousal has not been clear. In Drosophila, dopamine promotes sleep-wake arousal. However, there is conflicting evidence regarding its influence on environmentally stimulated arousal. Here we show that loss-of-function mutations in the D1 dopamine receptor DopR enhance repetitive startle-induced arousal while decreasing sleep-wake arousal (i.e., increasing sleep). These two types of arousal are also inversely influenced by cocaine, whose effects in each case are opposite to, and abrogated by, the DopR mutation. Selective restoration of DopR function in the central complex rescues the enhanced stimulated arousal but not the increased sleep phenotype of DopR mutants. These data provide evidence for at least two different forms of arousal, which are independently regulated by dopamine in opposite directions, via distinct neural circuits.

The role of dopamine in Drosophila larval classical olfactory conditioning

Learning and memory is not an attribute of higher animals. Even Drosophila larvae are able to form and recall an association of a given odor with an aversive or appetitive gustatory reinforcer. As the Drosophila larva has turned into a particularly simple model for studying odor processing, a detailed neuronal and functional map of the olfactory pathway is available up to the third order neurons in the mushroom bodies. At this point, a convergence of olfactory processing and gustatory reinforcement is suggested to underlie associative memory formation. The dopaminergic system was shown to be involved in mammalian and insect olfactory conditioning. To analyze the anatomy and function of the larval dopaminergic system, we first characterize dopaminergic neurons immunohistochemically up to the single cell level and subsequent test for the effects of distortions in the dopamine system upon aversive (odor-salt) as well as appetitive (odor-sugar) associative learning. Single cell analysis suggests that dopaminergic neurons do not directly connect gustatory input in the larval suboesophageal ganglion to olfactory information in the mushroom bodies. However, a number of dopaminergic neurons innervate different regions of the brain, including protocerebra, mushroom bodies and suboesophageal ganglion. We found that dopamine receptors are highly enriched in the mushroom bodies and that aversive and appetitive olfactory learning is strongly impaired in dopamine receptor mutants. Genetically interfering with dopaminergic signaling supports this finding, although our data do not exclude on naïve odor and sugar preferences of the larvae. Our data suggest that dopaminergic neurons provide input to different brain regions including protocerebra, suboesophageal ganglion and mushroom bodies by more than one route. We therefore propose that different types of dopaminergic neurons might be involved in different types of signaling necessary for aversive and appetitive olfactory memory formation respectively, or for the retrieval of these memory traces. Future studies of the dopaminergic system need to take into account such cellular dissociations in function in order to be meaningful.

Comparative pharmacology of two D1-like dopamine receptors cloned from the silkworm Bombyx mori

Dopamine (DA) is a physiologically important biogenic amine in insect peripheral and nervous tissues.We recently cloned two DA receptors (BmDopR1 and BmDopR2) from the silkworm Bombyx mori and identified them as D1-like receptors, which activate adenylate cyclase to increase intracellular cAMP levels. In this study, these two receptors were stably expressed in HEK-293 cells, and the dose-responsiveness to DA and their pharmacological properties were examined using cAMP assays. BmDopR1 showed a dose-dependent increase in cAMP levels at DA concentrations up to 10(-7) M with EC(50) of 3.30 nM, while BmDopR2 required 10(-6) M DA for activation. In BmDopR1-expressing cells, DA at 10(-6)-10(-4) M induced 30-50% lower cAMP production than 10(-7) MDA. BmDopR2-expressing cells showed a standard sigmoidal dose-response, with maximum cAMP levels attained with 10(-5)-10(-4) M DA and EC(50) of 1.30 microM. Both receptors had similar agonist profiles, and the typical vertebrate D1-like receptor agonist SKF-38393 was ineffective. Experiments with antagonists revealed that BmDopR1 exhibits D1-like features. However, the pharmacology of BmDopR2 was distinct from D1-like receptors; the typical vertebrate D1-like receptor antagonist SCH-23390 was less potent than the nonselective antagonist flupenthixol and the D2-like receptor antagonist chlorpromazine. The rank order of activities of several antagonists for BmDopR1 and BmDopR2 was more similar to that of Drosophila melanogaster DA receptors than Apis mellifera DA receptors. These data suggest that DA receptors could be potential targets for specific insecticides or insectistatics.

Interplay of JH, 20E and biogenic amines under normal and stress conditions and its effect on reproduction

Juvenile hormone (JH) and 20-hydroxyecdysone (20E) are well known to play a gonadotropic role in adult insects. In Drosophila the mechanism of reciprocal regulation of JH and 20E is shown to be responsible for their proper balance. Dopamine is a mediator in this JH and 20E interplay. A proper balance between JH and 20E is crucial for the normal progress of oogenesis. An imbalance of gonadotropins leads to reproductive defects: a rise in JH titre leads to oviposition arrest, a rise in 20E level, to the degradation of vitellogenic oocytes. Upon a change in the level of one of the gonadotropins, the balance is restored owing to the relative change in the titre of the other.

Neuroarchitecture of aminergic systems in the larval ventral ganglion of Drosophila melanogaster

Biogenic amines are important signaling molecules in the central nervous system of both vertebrates and invertebrates. In the fruit fly Drosophila melanogaster, biogenic amines take part in the regulation of various vital physiological processes such as feeding, learning/memory, locomotion, sexual behavior, and sleep/arousal. Consequently, several morphological studies have analyzed the distribution of aminergic neurons in the CNS. Previous descriptions, however, did not determine the exact spatial location of aminergic neurite arborizations within the neuropil. The release sites and pre-/postsynaptic compartments of aminergic neurons also remained largely unidentified. We here used gal4-driven marker gene expression and immunocytochemistry to map presumed serotonergic (5-HT), dopaminergic, and tyraminergic/octopaminergic neurons in the thoracic and abdominal neuromeres of the Drosophila larval ventral ganglion relying on Fasciclin2-immunoreactive tracts as three-dimensional landmarks. With tyrosine hydroxylase- (TH) or tyrosine decarboxylase 2 (TDC2)-specific gal4-drivers, we also analyzed the distribution of ectopically expressed neuronal compartment markers in presumptive dopaminergic TH and tyraminergic/octopaminergic TDC2 neurons, respectively. Our results suggest that thoracic and abdominal 5-HT and TH neurons are exclusively interneurons whereas most TDC2 neurons are efferent. 5-HT and TH neurons are ideally positioned to integrate sensory information and to modulate neuronal transmission within the ventral ganglion, while most TDC2 neurons appear to act peripherally. In contrast to 5-HT neurons, TH and TDC2 neurons each comprise morphologically different neuron subsets with separated in- and output compartments in specific neuropil regions. The three-dimensional mapping of aminergic neurons now facilitates the identification of neuronal network contacts and co-localized signaling molecules, as exemplified for DOPA decarboxylase-synthesizing neurons that co-express crustacean cardioactive peptide and myoinhibiting peptides.

Suppression of excitatory cholinergic synaptic transmission by Drosophila dopamine D1-like receptors

The physiological function of dopamine is mediated through its G-protein-coupled receptor family. In Drosophila, four dopamine receptors have been molecularly characterized so far. However, due largely to the absence of a suitable preparation, the role of Drosophila dopamine receptors in modulating central synaptic transmission has not been examined. The present study investigated mechanisms by which dopamine modulates excitatory cholinergic synaptic transmission in Drosophila using primary neuronal cultures. Whole-cell recordings demonstrated that cholinergic excitatory postsynaptic currents (EPSCs) were down-regulated by focally applied dopamine (10-500 microm). The vertebrate D1 specific agonists SKF38393 and 6-chloro-APB (10 microm) mimicked dopamine-mediated suppression of cholinergic synaptic transmission with higher potency. In contrast, the D2 agonists quinpirole and bromocriptine did not alter cholinergic EPSCs, demonstrating that dopamine-mediated suppression of cholinergic synaptic transmission is specifically through activation of Drosophila D1-like receptors. Biophysical analysis of miniature EPSCs indicated that cholinergic suppression by activation of D1-like receptors is presynaptic in origin. Dopamine modulation of cholinergic transmission is not mediated through the cAMP/protein kinase A signaling pathway as cholinergic suppression by dopamine occurred in the presence of the protein kinase A inhibitor H-89. In addition, an adenylate cyclase activator, forskolin, led to an increase, not a decrease, of cholinergic EPSC frequency. Finally, we showed that activation of D1-like receptors decreased the frequency of action potentials in cultured Drosophila neurons by inhibiting excitatory cholinergic transmission. All our data demonstrated that activation of D1-like receptors in Drosophila neurons negatively modulates excitatory cholinergic synaptic transmission and thus inhibits neuronal excitability.

A genome-wide inventory of neurohormone GPCRs in the red flour beetle Tribolium castaneum

Insect neurohormones (biogenic amines, neuropeptides, and protein hormones) and their G protein-coupled receptors (GPCRs) play a central role in the control of behavior, reproduction, development, feeding and many other physiological processes. The recent completion of several insect genome projects has enabled us to obtain a complete inventory of neurohormone GPCRs in these insects and, by a comparative genomics approach, to analyze the evolution of these proteins. The red flour beetle Tribolium castaneum is the latest addition to the list of insects with a sequenced genome and the first coleopteran (beetle) to be sequenced. Coleoptera is the largest insect order and about 30% of all animal species living on earth are coleopterans. Some coleopterans are severe agricultural pests, which is also true for T. castaneum, a global pest for stored grain and other dried commodities for human consumption. In addition, T. castaneum is a model for insect development. Here, we have investigated the presence of neurohormone GPCRs in Tribolium and compared them with those from the fruit fly Drosophila melanogaster (Diptera) and the honey bee Apis mellifera (Hymenoptera). We found 20 biogenic amine GPCRs in Tribolium (21 in Drosophila; 19 in the honey bee), 48 neuropeptide GPCRs (45 in Drosophila; 35 in the honey bee), and 4 protein hormone GPCRs (4 in Drosophila; 2 in the honey bee). Furthermore, we identified the likely ligands for 45 of these 72 Tribolium GPCRs. A highly interesting finding in Tribolium was the occurrence of a vasopressin GPCR and a vasopressin peptide. So far, the vasopressin/GPCR couple has not been detected in any other insect with a sequenced genome (D. melanogaster and six other Drosophila species, Anopheles gambiae, Aedes aegypti, Bombyx mori, and A. mellifera). Tribolium lives in very dry environments. Vasopressin in mammals is the major neurohormone steering water reabsorption in the kidneys. Its presence in Tribolium, therefore, might be related to the animal's need to effectively control water reabsorption. Other striking differences between Tribolium and the other two insects are the absence of the allatostatin-A, kinin, and corazonin neuropeptide/receptor couples and the duplications of other hormonal systems. Our survey of 340 million years of insect neurohormone GPCR evolution shows that neuropeptide/receptor couples can easily duplicate or disappear during insect evolution. It also shows that Drosophila is not a good representative of all insects, because several of the hormonal systems that we now find in Tribolium do not exist in Drosophila.

D1 dopamine receptor dDA1 is required in the mushroom body neurons for aversive and appetitive learning in Drosophila

Drosophila has robust behavioral plasticity to avoid or prefer the odor that predicts punishment or food reward, respectively. Both types of plasticity are mediated by the mushroom body (MB) neurons in the brain, in which various signaling molecules play crucial roles. However, important yet unresolved molecules are the receptors that initiate aversive or appetitive learning cascades in the MB. We have shown previously that D1 dopamine receptor dDA1 is highly enriched in the MB neuropil. Here, we demonstrate that dDA1 is a key receptor that mediates both aversive and appetitive learning in pavlovian olfactory conditioning. We identified two mutants, dumb1 and dumb2, with abnormal dDA1 expression. When trained with the same conditioned stimuli, both dumb alleles showed negligible learning in electric shock-mediated conditioning while they exhibited moderately impaired learning in sugar-mediated conditioning. These phenotypes were not attributable to anomalous sensory modalities of dumb mutants because their olfactory acuity, shock reactivity, and sugar preference were comparable to those of control lines. Remarkably, the dumb mutant's impaired performance in both paradigms was fully rescued by reinstating dDA1 expression in the same subset of MB neurons, indicating the critical roles of the MB dDA1 in aversive as well as appetitive learning. Previous studies using dopamine receptor antagonists implicate the involvement of D1/D5 receptors in various pavlovian conditioning tasks in mammals; however, these have not been supported by the studies of D1- or D5-deficient animals. The findings described here unambiguously clarify the critical roles of D1 dopamine receptor in aversive and appetitive pavlovian conditioning.

Locomotor activity is regulated by D2-like receptors in Drosophila: an anatomic and functional analysis

In mammals, dopamine 2-like receptors are expressed in distinct pathways within the central nervous system, as well as in peripheral tissues. Selected neuronal D2-like receptors play a critical role in modulating locomotor activity and, as such, represent an important therapeutic target (e.g. in Parkinson's disease). Previous studies have established that proteins required for dopamine (DA) neurotransmission are highly conserved between mammals and the fruit fly Drosophila melanogaster. These include a fly dopamine 2-like receptor (DD2R; Hearn et al. PNAS 2002 99(22):14554) that has structural and pharmacologic similarity to the human D2-like (D2R). In the current study, we define the spatial expression pattern of DD2R, and functionally characterize flies with reduced DD2 receptor levels. We show that DD2R is expressed in the larval and adult nervous systems, in cell groups that include the Ap-let cohort of peptidergic neurons, as well as in peripheral tissues including the gut and Malpighian tubules. To examine DD2R function in vivo, we generated RNA-interference (RNAi) flies with reduced DD2R expression. Behavioral analysis revealed that these flies show significantly decreased locomotor activity, similar to the phenotype observed in mammals with reduced D2R expression. The fly RNAi phenotype can be rescued by administration of the DD2R synthetic agonist bromocriptine, indicating specificity for the RNAi effect. These results suggest Drosophila as a useful system for future studies aimed at identifying modifiers of dopaminergic signaling/locomotor function.

A review of neurohormone GPCRs present in the fruitfly Drosophila melanogaster and the honey bee Apis mellifera

G protein-coupled receptor (GPCR) genes are large gene families in every animal, sometimes making up to 1-2% of the animal's genome. Of all insect GPCRs, the neurohormone (neuropeptide, protein hormone, biogenic amine) GPCRs are especially important, because they, together with their ligands, occupy a high hierarchic position in the physiology of insects and steer crucial processes such as development, reproduction, and behavior. In this paper, we give a review of our current knowledge on Drosophila melanogaster GPCRs and use this information to annotate the neurohormone GPCR genes present in the recently sequenced genome from the honey bee Apis mellifera. We found 35 neuropeptide receptor genes in the honey bee (44 in Drosophila) and two genes, coding for leucine-rich repeats-containing protein hormone GPCRs (4 in Drosophila). In addition, the honey bee has 19 biogenic amine receptor genes (21 in Drosophila). The larger numbers of neurohormone receptors in Drosophila are probably due to gene duplications that occurred during recent evolution of the fly. Our analyses also yielded the likely ligands for 40 of the 56 honey bee neurohormone GPCRs identified in this study. In addition, we made some interesting observations on neurohormone GPCR evolution and the evolution and co-evolution of their ligands. For neuropeptide and protein hormone GPCRs, there appears to be a general co-evolution between receptors and their ligands. This is in contrast to biogenic amine GPCRs, where evolutionarily unrelated GPCRs often bind to the same biogenic amine, suggesting frequent ligand exchanges ("ligand hops") during GPCR evolution.

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.

Molecular cloning and characterization of crustacean type-one dopamine receptors: D1alphaPan and D1betaPan

Dopamine (DA) differentially modulates identified neurons in the crustacean stomatogastric nervous system (STNS). While the electrophysiological actions of DA have been well characterized, little is known about the dopaminergic transduction cascades operating in this system. As a first step toward illuminating the molecular underpinnings of dopaminergic signal transduction in the crustacean STNS, we have cloned and characterized two type-one DA receptors (DARs) from the spiny lobster (Panulirus interruptus): D(1alphaPan) and D(1betaPan). We found that the structure and function of these arthropod DARs are well conserved across species. Using a heterologous expression system, we determined that DA, but not serotonin, octopamine, tyramine or histamine activates these receptors. When stably expressed in HEK cells, the D(1alphaPan) receptor couples with Gs, and DA elicits an increase in [cAMP]. The D(1betaPan) receptor responds to DA with a net increase in [cAMP] that is mediated by Gs and Gz.

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.

Molecular biology of the invertebrate dopamine receptors

Dopamine is found in the nervous systems of both vertebrates and invertebrates. However, the specific actions of dopamine depend on the dopamine receptor type that is expressed in the target cell. As in mammals, different subtypes of dopamine receptors have been cloned and characterized from invertebrates, and these receptor subtypes have different structural and functional properties. Understanding how these receptors respond to dopamine and in which cells each receptor type is expressed is key to our understanding of the role of dopamine signaling. Comparison of the amino acid sequences and experimentally determined functional properties suggest that there are at least three distinct types of dopamine receptors in invertebrates. This review focuses on invertebrate dopamine receptors for which the genes have been isolated and identified, and examines our current knowledge of the functional and structural properties of these receptors, and their pharmacology and expression.

Evolution and expression of D2 and D3 dopamine receptor genes in zebrafish

We mined the zebrafish genomic sequence database and identified contigs containing segments of several dopamine receptor genes. By using a polymerase chain reaction amplification strategy, we generated full-length cDNAs encoding a single dopamine D3 receptor and three distinct D2 receptor subtypes. Zebrafish dopamine receptor genes were mapped by using the T51 radiation hybrid panel. The D3 receptor gene (drd3) mapped to linkage group (LG) 24. The three D2 receptor genes were localized to LG 15 (drd2a), LG 16, (drd2b), and LG 5 (drd2c). With the exception of the drd2b gene, each of these map positions was syntenic with regions of human chromosomes containing orthologs of the zebrafish dopamine receptor genes. Whole-mount in situ hybridization was used to investigate expression of the D2 and D3 receptor genes. Expression of the drd3 gene was first detected at mid-somitogenesis and was particularly prominent in somites. Thereafter, the drd3 gene was expressed diffusely throughout the brain and spinal cord. The three D2 receptor genes were expressed throughout the central nervous system (CNS) in distinct but overlapping patterns. In early embryos, the drd2a gene was expressed exclusively in the epiphysis, whereas the drd2c gene was localized to the notochord. After 24 hpf, the drd2a, drd2b, and drd2c genes were differentially expressed throughout the CNS. The identification of dopamine receptor genes in zebrafish should allow us to use the power of zebrafish genetics to analyze the functional properties of this important class of neurotransmitter receptors.

Dopamine receptors in C. elegans

Dopamine regulates various physiological functions in the central nervous system and the periphery. Dysfunction of the dopamine system is implicated in a wide variety of disorders and behaviors including schizophrenia, addiction, and attention-deficit hyperactivity disorder. Medications that modulate dopamine signaling have therapeutic efficacy on the treatment of these disorders. However, the causes of these disorders and the role of dopamine are still unclear. Studying the dopamine system in a model organism, such as Caenorhabditis elegans, allows the genetic analysis in a simple and well-described nervous system, which may provide new insight into the molecular mechanisms of dopamine signaling. In this review, we summarize recent findings on pharmacological and biochemical properties of the C. elegans dopamine receptors and their physiological role in the control of behavior.

Dopamine stimulates snail albumen gland glycoprotein secretion through the activation of a D1-like receptor

The catecholamine dopamine is present in both the central nervous system and in the peripheral tissues of molluscs, where it is involved in regulating reproduction. Application of exogenous dopamine to the isolated albumen gland of the freshwater pulmonate snail Helisoma duryi (Wetherby) induces the secretion (release) of perivitelline fluid. The major protein component of the perivitelline fluid of Helisoma duryi is a native 288 kDa glycoprotein that is secreted around individual eggs and serves as an important source of nutrients for the developing embryos. The secretion of glycoprotein by the albumen gland is a highly regulated event that must be coordinated with the arrival of the fertilized ovum at the carrefour (the region where the eggs receive albumen gland secretory products). In order to elucidate the intracellular signalling pathway(s) mediating dopamine-induced glycoprotein secretion, albumen gland cAMP production and glycoprotein secretion were measured in the presence/absence of selected dopamine receptor agonists and antagonists. Dopamine D1-selective agonists dihydrexidine, 6,7-ADTN and SKF81297 stimulated cAMP production and glycoprotein secretion from isolated albumen glands whereas D1-selective antagonists SCH23390 and SKF83566 suppressed dopamine-stimulated cAMP production. Dopamine D2-selective agonists and antagonists generally had no effect on cAMP production or protein secretion. Based on the effects of these compounds, a pharmacological profile was obtained that strongly suggests the presence of a dopamine D1-like receptor in the albumen gland of Helisoma duryi. In addition, secretion of albumen gland glycoprotein was not inhibited by protein kinase A inhibitors, suggesting that dopamine-stimulated protein secretion might occur through a protein kinase A-independent pathway.

The effects of dopamine receptor agonists and antagonists on the secretory rate of cockroach (Periplaneta americana) salivary glands

The acinar salivary glands of the cockroach, Periplaneta americana, are innervated by dopaminergic and serotonergic nerve fibers. Serotonin stimulates the secretion of protein-rich saliva, whereas dopamine causes the production of protein-free saliva. This suggests that dopamine acts selectively on ion-transporting peripheral cells within the acini and the duct cells, and that serotonin acts on the protein-producing central cells of the acini. We have investigated the pharmacology of the dopamine-induced secretory activity of the salivary gland of Periplaneta americana by testing several dopamine receptor agonists and antagonists. The effects of dopamine can be mimicked by the non-selective dopamine receptor agonist 6,7-ADTN and, less effectively, by the vertebrate D1 receptor-selective agonist chloro-APB. The vertebrate D1 receptor-selective agonist SKF 38393 and vertebrate D2 receptor-selective agonist R(-)-TNPA were ineffective. R(+)-Lisuride induces a secretory response with a slower onset and a lower maximal response compared with dopamine-induced secretion. However, lisuride-stimulated glands continue secreting saliva, even after lisuride-washout. Dopamine-induced secretions can be blocked by the vertebrate dopamine receptor antagonists cis(Z)-flupenthixol, chlorpromazine, and S(+)-butaclamol. Our pharmacological data do not unequivocally indicate whether the dopamine receptors on the Periplaneta salivary glands belong to the D1 or D2 subfamily of dopamine receptors, but we can confirm that the pharmacology of invertebrate dopamine receptors is remarkably different from that of their vertebrate counterparts.

Octopamine receptors in the honeybee (Apis mellifera) brain and their disruption by RNA-mediated interference

Octopamine plays important neuromodulatory roles in the honeybee brain. Accordingly, mRNA from a recently identified honeybee octopamine receptor (AmOA1) is distributed throughout the brain. We have evaluated the occurrence of AmOA1 in the antennal lobe (AL) as well as rest of the brain (RB) by western blotting using an antiserum raised against a peptide selected from AmOA1 sequence. In addition to an expected band (78 kDa in the AL), one additional band (72 kDa) was identified from the AL and four bands (48, 60, 72 and 78 kDa) were observed in the RB. These bands were also recognized with antiserum against a different peptide segment from an octopamine receptor ortholog from the fruitfly (OAMB). Significant sequence identity with the peptide segment used to generate the antiserum was only found with OAMB and its splice variants in fruitfly; it was less conserved in other biogenic amine receptors from honeybee and other insects. Furthermore, western blot analysis performed on brains with dsRNA-treated antennal lobes showed a decrease in the intensity of all four bands. This suggests that AmOA1 antiserum specifically recognizes one or more types of AmOA1 receptors in the honeybee brain. We extend our earlier study of RNAi to quantify the rate of spread of dsRNA from a localized injection to other neuropils.

Arthropod 5-HT2 receptors: a neurohormonal receptor in decapod crustaceans that displays agonist independent activity resulting from an evolutionary alteration to the DRY motif

The stomatogastric nervous system (STNS) is a premiere model for studying modulation of motor pattern generation. Whereas the cellular and network responses to monoamines have been particularly well characterized electrophysiologically, the transduction mechanisms that link the different monoaminergic signals to specific intracellular responses are presently unknown in this system. To begin to elucidate monoaminergic signal transduction in pyloric neurons, we used a bioinformatics approach to predict the existence of 18 monoamine receptors in arthropods, 9 of which have been previously cloned in Drosophila and other insects. We then went on to use the two existing insect databases to clone and characterize the 10th putative arthropod receptor from the spiny lobster, Panulirus interruptus. This receptor is most homologous to the 5-HT2 subtype and shows a dose-dependent response to 5-HT but not to any of the other monoamines present in the STNS. Through a series of pharmacological experiments, we demonstrate that this newly described receptor, 5-HT2betaPan, couples with the traditional G(q) pathway when expressed in HEK293 cells, but not to G(s) or G(i/o). Moreover, it is constitutively active, because the highly conserved DRY motif in transmembrane region 3 has evolved into DRF. Site-directed mutagenesis that reverts the motif back to DRY abolishes this agonist-independent activity. We further demonstrate that this receptor most likely participates in the modulation of stomatogastric motor output, because it is found in neurites in the synaptic neuropil of the stomatogastric ganglion as well as in the axon terminals at identified pyloric neuromuscular junctions.

The effects of dopamine agonists and antagonists on the secretory responses in the salivary glands of the locust (Locusta migratoria)

A study has been made on the effect of dopamine on salivary gland secretion rates from isolated locust salivary glands. Application of dopamine induced a concentration-dependent secretion with an IC(50) of approximately 0.3 microM. We investigated the pharmacological profile of this receptor using dopaminergic agonists and antagonists. The effects of dopamine could be mimicked by the selective D1 agonist SKF82958, but not by the D2 agonist TNPA-HCl. The receptor also showed selectively towards certain D1 agonists. SKF82958 was more potent at inducing secretion than SKF81297. We found that dopamine-induced salivary secretions were blocked by the selective D1 antagonist SCH23390, whereas the D2 antagonist sulpiride was relatively ineffective. The cAMP analogue 8-Bromo cAMP also increased secretion rates from isolated salivary glands. These data and the rank order of potency of the agonists and antagonists in this screen suggest that this receptor is a D1-type receptor.

Developmental changes in expression patterns of two dopamine receptor genes in mushroom bodies of the honeybee,Apis mellifera

The expression patterns of two dopamine receptor genes, Amdop1 and Amdop2, in the developing mushroom bodies of the honeybee brain were determined by using in situ hybridisation. Both genes were expressed throughout pupal development, but their patterns of expression in the three major divisions of mushroom body intrinsic neurons (outer compact cells, noncompact cells, and inner compact cells) were quite distinct. Amdop1 expression could be detected in all three mushroom body cell groups throughout development. Staining for Amdop1 mRNA was particularly intense in newly born Kenyon cells, suggesting that levels of Amdop1 expression are higher in newborn cells than in more mature mushroom body neurons. This was not the case for Amdop2. Amdop2 expression in the mushroom bodies was restricted to inner and outer compact cells during most of pupal development, appearing in noncompact cells only late in metamorphosis or at adult eclosion. In contrast to the case with Amdop1, staining for Amdop2 mRNA was observed in glial cells. Expression of Amdop2 in glial cells was detected only at early stages of glial cell development, when the cells are reported to be actively dividing. This study not only implicates dopamine in the development of honeybee mushroom bodies but also suggests different roles for the two dopamine receptors investigated.

Modulation of early olfactory processing by an octopaminergic reinforcement pathway in the honeybee

Processing of olfactory information in the antennal lobes of insects and olfactory bulbs of vertebrates is modulated by centrifugal inputs that represent reinforcing events. Octopamine release by one such pathway in the honeybee antennal lobe modulates olfactory processing in relation to nectar (sucrose) reinforcement. To test more specifically what role octopamine plays in the antennal lobe, we used two treatments to disrupt an octopamine receptor from Apis mellifera brain (AmOAR) function: (1) an OAR antagonist, mianserin, was used to block receptor function, and (2) AmOAR double-stranded RNA was used to silence receptor expression. Both treatments inhibited olfactory acquisition and recall, but they did not disrupt odor discrimination. These results suggest that octopamine mediates consolidation of a component of olfactory memory at this early processing stage in the antennal lobe. Furthermore, after consolidation, octopamine release becomes essential for recall, which suggests that the modulatory circuits become incorporated as essential components of neural representations that activate odor memory.

Cloning and characterization of a Caenorhabditis elegans D2-like dopamine receptor

The neurotransmitter dopamine plays an important role in the regulation of behavior in both vertebrates and invertebrates. In mammals, dopamine binds and activates two classes of dopamine receptors, D1-like and D2-like receptors. However, D2-like dopamine receptors in Caenorhabditis elegans have not yet been characterized. We have cloned a cDNA encoding a putative C. elegans D2-like dopamine receptor. The deduced amino acid sequence of the cloned cDNA shows higher sequence similarities to vertebrate D2-like dopamine receptors than to D1-like receptors. Two splice variants that differ in the length of their predicted third intracellular loops were identified. The receptor heterologously expressed in cultured cells showed high affinity binding to [125I]iodo-lysergic acid diethylamide. Dopamine showed the highest affinity for this receptor among several amine neurotransmitters tested. Activation of the heterologously expressed receptor led to the inhibition of cyclic AMP production, confirming that this receptor has the functional property of a D2-like receptor. We have also analyzed the expression pattern of this receptor and found that the receptor is expressed in several neurons including all the dopaminergic neurons in C. elegans.

Invertebrate D2 type dopamine receptor exhibits age-based plasticity of expression in the mushroom bodies of the honeybee brain

We have isolated a cDNA clone from the honeybee brain encoding a dopamine receptor, AmDop2, which is positively coupled to adenylyl cyclase. The transmembrane domains of this receptor are 88% identical to the orthologous Drosophila D2 dopamine receptor, DmDop2, though phylogenetic analysis and sequence homology both indicate that invertebrate and vertebrate D2 receptors are quite distinct. In situ hybridization to mRNA in whole-mount preparations of honeybee brains reveals gene expression in the mushroom bodies, a primary site of associative learning. Furthermore, two anatomically distinct cell types in the mushroom bodies exhibit differential regulation of AmDop2 expression. In all nonreproductive females (worker caste) and reproductive males (drones) the receptor gene is strongly and constitutively expressed in all mushroom body interneurons with small cell bodies. In contrast, the large cell-bodied interneurons exhibit dramatic plasticity of AmDop2 gene expression. In newly emerged worker bees (cell-cleaning specialists) and newly emerged drones, no AmDop2 transcript is observed in the large interneurons whereas this transcript is abundant in these cells in the oldest worker bees (resource foragers) and older drones. Differentiation of the mushroom body interneurons into two distinct classes (i.e., plastic or nonplastic with respect to AmDop2 gene expression) indicates that this receptor contributes to the differential regulation of distinct neural circuits. Moreover, the plasticity of expression observed in the large cells implicates this receptor in the behavioral maturation of the bee.

Analysis of two D1-like dopamine receptors from the honey bee Apis mellifera reveals agonist-independent activity

Dopamine is found in many invertebrate organisms, including insects, however, the mechanisms through which this amine operates remain unclear. We have expressed two dopamine receptors cloned from honey bee (AmDOP1 and AmDOP2) in insect cells (Spodoptera frugiperda), and compared their pharmacology directly using production of cAMP as a functional assay. In each assay, AmDOP1 receptors required lower concentrations of dopamine and 6,7-ADTN for maximal activation than AmDOP2 receptors. Conversely, butaclamol and cis(Z)-flupentixol were more potent at blocking the cAMP response mediated through AmDOP2 than AmDOP1 receptors. Expression of AmDOP1, but not AmDOP2, receptors significantly increased levels of cAMP even in the absence of ligand. This constitutive activity was blocked by cis(Z)-flupentixol. This work provides the first evidence of a constitutively activated dopamine receptor in invertebrates and suggests that although AmDOP1 and AmDOP2 share much less homology than their vertebrate counterparts, they display a number of functional parallels with the mammalian D1-like dopamine receptors.

Expression of a D1 dopamine receptor dDA1/DmDOP1 in the central nervous system of Drosophila melanogaster

The diverse physiological effects of dopamine are mediated by multiple receptor systems. The dDA1 represents one of the Drosophila dopamine receptors that activate the cAMP cascade. To gain insight into the role of dDA1, we generated a polyclonal antibody against the unique sequence in dDA1 and investigated dDA1 distribution in the central nervous system (CNS) of Drosophila melanogaster. In both larval and adult CNS pronounced dDA1 immunoreactivity was present in the neuropil of the mushroom bodies, a brain structure crucial for learning and memory in insects, and four unpaired neurons in each thoracic segment. In addition, the larval abdominal ganglion contained two dDA1 cells in each segment. This expression pattern appeared to be maintained in the condensed adult abdominal ganglion although the precise number and the intensity of staining were somewhat variable. The adult CNS also exhibited intense dDA1 immunoreactivity in the central complex, a structure controlling higher-order motor function, moderate expression in several neurosecretory cells, and weak staining in two unpaired neurons in the mesothoracic neuromere. The dDA1 expression in these areas was only detected in adult, but not in third instar larval CNS.

Pharmacological characterization of mammalian D1 and D2 dopamine receptors expressed in Drosophila Schneider-2 cells

Mammalian D1 and D2 dopamine receptors were stably expressed in Drosophila Schneider-2 (S2) cells and screened for their pharmacological properties. Saturable, dose-dependent, high affinity binding of the D1-selective antagonist [3H]SCH-23390 was detected only in membranes from S2 cells induced to express rat dopamine D1 receptors, while saturable, dose-dependent, high affinity binding of the D2-selective antagonist [3H]methylspiperone was detected only in membranes from S2 cells induced to express rat dopamine D2 receptors. No specific binding of either radioligand could be detected in membranes isolated from uninduced or untransfected S2 cells. Both dopamine D1 and D2 receptor subtypes displayed the appropriate stereoselective binding of enantiomers of the nonselective antagonist butaclamol. Each receptor subtype also displayed the appropriate agonist stereoselectivities. The dopamine D1 receptor bound the (+)-enantiomer of the D1-selective agonist SKF38393 with higher affinity than the (-)-enantiomer, while the dopamine D2 receptor bound the (-)-enantiomer of the D2-selective agonist norpropylapomorphine with higher affinity than the (+)-enantiomer. At both receptor subtypes, dopamine binding was best characterized as occurring to a single low affinity site. In addition, the low affinity dopamine binding was also found to be insensitive to GTPgammaS and magnesium ions. Overall, the pharmacological profiles of mammalian dopamine D1 and D2 receptors expressed in Drosophila S2 cells is comparable to those observed for these same receptors when they are expressed in mammalian cell lines. A notable distinction is that there is no evidence for the coupling of insect G proteins to mammalian dopamine receptors. These results suggest that the S2 cell insect G system may provide a convenient source of pharmacologically active mammalian D1 and D2 dopamine receptors free of promiscuous G protein contaminants.

A Drosophila dopamine 2-like receptor: Molecular characterization and identification of multiple alternatively spliced variants

Dopamine is an important neurotransmitter in the central nervous system of both Drosophila and mammals. Despite the evolutionary distance, functional parallels exist between the fly and mammalian dopaminergic systems, with both playing roles in modulating locomotor activity, sexual function, and the response to drugs of abuse. In mammals, dopamine exerts its effects through either dopamine 1-like (D1-like) or D2-like G protein-coupled receptors. Although pharmacologic data suggest the presence of both receptor subtypes in insects, only cDNAs encoding D1-like proteins have been isolated previously. Here we report the cloning and characterization of a newly discovered Drosophila dopamine receptor. Sequence analysis reveals that this putative protein shares highest homology with known mammalian dopamine 2-like receptors. Eight isoforms of the Drosophila D2-like receptor (DD2R) transcript have been identified, each the result of alternative splicing. The encoded heptahelical receptors range in size from 461 to 606 aa, with variability in the length and sequence of the third intracellular loop. Pharmacologic assessment of three DD2R isoforms, DD2R-606, DD2R-506, and DD2R-461, revealed that among the endogenous biogenic amines, dopamine is most potent at each receptor. As established for mammalian D2-like receptors, stimulation of the Drosophila homologs with dopamine triggers pertussis toxin-sensitive Gi/o-mediated signaling. The D2-like receptor agonist, bromocriptine, has nanomolar potency at DD2R-606, -506, and -461, whereas multiple D2-like receptor antagonists (as established with mammalian receptors) have markedly reduced if any affinity when assessed at the fly receptor isoforms. The isolation of cDNAs encoding Drosophila D2-like receptors extends the range of apparent parallels between the dopaminergic system in flies and mammals. Pharmacologic and genetic manipulation of the DD2Rs will provide the opportunity to better define the physiologic role of these proteins in vivo and further explore the utility of invertebrates as a model system for understanding dopaminergic function in higher organisms.

Identification of a dopamine receptor from Caenorhabditis elegans

The neurotransmitter dopamine regulates locomotion and egg laying in the nematode Caenorhabditis elegans. We have cloned a cDNA encoding the C. elegans G protein-coupled receptor (CeDOP1). The deduced amino acid sequence of the cloned cDNA shows high sequence similarities with D1-like dopamine receptors from other species. Three splice variants that differ in the length of the predicted third intracellular loop and C-terminal tail were identified. COS-7 cells transiently transfected with CeDOP1 showed high affinity binding to [(125)I]iodo-lysergic acid diethylamide (K(D)=3.43 +/- 0.83 nM). Dopamine showed the highest affinity (K(i)=0.186 microM) for this receptor among several vertebrate and invertebrate amine neurotransmitters tested, suggesting that the natural ligand for this receptor is dopamine.

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.

Dopamine induces hyperpolarization of locust salivary gland acinar cells via D(1)-like receptors

The effect of dopamine on the salivary gland acinar cells of the locust was examined using conventional intracellular recording techniques. Application of dopamine induced a reversible, dose-dependent hyperpolarization of the acinar cells, with an EC(50) of 0.1 &mgr;M dopamine. We investigated the pharmacology of the dopamine receptor mediating hyperpolarization of the acinar cells using a range of dopaminergic agonists and antagonists. The effect of dopamine could be mimicked by the selective D(1) receptor agonist SKF82958, whilst the D(2) receptor agonists PPHT-HCl and TNPA-HBr were far less potent at inducing hyperpolarization. The receptor also showed selectivity to certain synthetic D(1)-like agonists. SKF82958 was much more effective at inducing a hyperpolarization than SKF81297. The dopamine-induced hyperpolarization of locust acinar cells could be blocked using the selective D(1) receptor antagonist SCH23390 whilst the D(2) receptor antagonists sulpiride and spiperone were inactive. The rank order of potency of several dopaminergic agonists and antagonists was obtained and suggests that the dopamine receptor mediating the hyperpolarization in locust salivary gland acinar cells is similar to a mammalian D(1) receptor. Stimulation of the salivary nerve mimicked the effect of dopamine on the acinar cells, inducing a rapid reversible hyperpolarization. This neurally-evoked hyperpolarization of the locust acinar cells was suppressed using 1.0 &mgr;M SCH23390, whilst 10 &mgr;M sulpiride was inactive. This demonstrated that both exogenously applied dopamine and endogenously released dopamine are probably acting on the same receptor.

Genetic analysis of the Drosophila Gs(alpha) gene

One of the best understood signal transduction pathways activated by receptors containing seven transmembrane domains involves activation of heterotrimeric G-protein complexes containing Gs(alpha), the subsequent stimulation of adenylyl cyclase, production of cAMP, activation of protein kinase A (PKA), and the phosphorylation of substrates that control a wide variety of cellular responses. Here, we report the identification of "loss-of-function" mutations in the Drosophila Gs(alpha) gene (dgs). Seven mutants have been identified that are either complemented by transgenes representing the wild-type dgs gene or contain nucleotide sequence changes resulting in the production of altered Gs(alpha) protein. Examination of mutant alleles representing loss-of-Gs(alpha) function indicates that the phenotypes generated do not mimic those created by mutational elimination of PKA. These results are consistent with the conclusion reached in previous studies that activation of PKA, at least in these developmental contexts, does not depend on receptor-mediated increases in intracellular cAMP, in contrast to the predictions of models developed primarily on the basis of studies in cultured cells.

Pharmacological characterization of dopamine receptors in the corpus allatum of Manduca sexta larvae

Dopamine receptors previously identified in corpora allata (CA) of Manduca sexta last instars on the basis of dopamine effects on JH (juvenile hormone)/JH acid biosynthesis and cyclic AMP (cAMP) accumulation, were characterized pharmacologically. For this study, a broad spectrum of agonists or antagonists of D1, D2, D3 or D4 dopamine receptors, together with the dopamine metabolite N-acetyl-dopamine, other neurotransmitters and their agonists/antagonists, were tested for their effects on gland activity and cAMP production. The lack of effect of other neurotransmitters supports the specificity of the effect of dopamine and the dopamine specificity of the receptors. Only the D2 receptor antagonist spiperone had a potent effect on JH biosynthesis and cAMP formation by CA taken on day 0 of the last stadium, when dopamine stimulates both activities and thus appears to be acting via a D1-like receptor. Several other D2 receptor antagonists, and D1, D2/D1 and D4,3/D2 receptor antagonists were less effective. Thus, the D1-like receptor of the Manduca CA appears to be distinct pharmacologically from vertebrate D1 receptors. By contrast, a number of D2 agonists/antagonists had a significant effect on JH acid biosynthesis and cAMP production by the CA from day 6 of the last stadium, when dopamine inhibits both activities and thus appears to be acting via a D2-like receptor. Certain D1-specific agonists/antagonists were equally effective. The Manduca D2-like receptor therefore bears some pharmacological resemblance to vertebrate D2 receptors. N-acetyl dopamine acted as a dopamine agonist with day 6 CA, the first identified function for an N-acetylated biogenic amine in insects. Dopamine was found to have the same differential affect on the formation of cAMP in homogenates of day 0 and day 6 brains as it did with CA, and in the same concentration range. Dopamine receptor agonists/antagonists affecting cAMP formation by day 0 and day 6 CA homogenates had similar effects with brain homogenates. By contrast, dopamine only stimulated cAMP formation by homogenates of day 0 and day 6 abdominal or ventral nerve cord. These results suggest that D1- and D2-like dopamine receptors of Manduca are regionally as well as temporally localized.

The pharmacology of a dopamine receptor in the locust nervous tissue

A dopamine receptor in the nervous tissue of the desert locust (Schistocerca gregaria Forskâl) was studied using ¿3Hlysergic acid diethylamide (LSD) as the radioligand. Its expression is almost entirely restricted to the mushroom bodies, centres for learning and memory in the insect brain. This G-protein coupled receptor is present in relatively low concentrations in the locust brain (35 fmol/mg protein). The pharmacological characterisation reveals high affinity for the putative natural agonist dopamine (K(i)=28 nM). Substances with high subtype specificity for vertebrate dopamine receptors such as SCH 23390 (K(i)=639 nM) and sulpiride (K(i)=21,200 nM) have low affinity for the locust neuronal dopamine receptor. In opposite, substances with a broad pharmacological profile such as LSD, spiperone (K(i)=7.26 nM), and chlorpromazine (K(i)=9.52 nM) have high affinity properties. Comparison of the pharmacological data reveals no significant homology to any vertebrate dopamine receptor class characterised so far. This uncertainty about the pharmacological relatedness of insect dopamine receptors mirrors the available molecular data. It is almost impossible to classify cloned insect dopamine receptors into vertebrate dopamine receptor schemes. This lack of pharmacological relatedness opens the opportunity to develop highly specific insecticides against insect dopamine receptors.

Conservation of the ligand recognition site of metabotropic glutamate receptors during evolution

Mammalian metabotropic glutamate receptors (mGluRs) are classified into 3 groups based on their sequence similarity and ligand recognition selectivity. Recently, we identified a Drosophila mGluR (DmGlu(A)R) which is about equidistant, phylogenetically, from the 3 mGluR groups. However, both the G-protein coupling selectivity and the pharmacological profile of DmGlu(A)R, as analysed with mutated G-proteins and a few compounds, look similar to those of mammalian group-II mGluRs. In the present study we carefully examined the pharmacological profile of DmGlu(A)R, and compared it to those of the rat mGlu(1a), mGlu(2) and mGlu(4a) receptors, representative of group-I, II and III respectively. The pharmacological profile of DmGlu(A)R was found to be similar to that of mGlu(2)R, and only very small differences could be identified at the level of their pharmacophore models. These data strongly suggest that the binding sites of these two receptors are similar. To further document this idea, a 3D model of the mGlu(2) binding domain was constructed based on the low sequence similarity with periplasmic amino acid binding proteins, and was used to identify the residues that possibly constitute the ligand recognition pocket. Interestingly, this putative binding pocket was found to be very well conserved between DmGlu(A)R and the mammalian group-II receptors. These data indicate that there has been a strong selective pressure during evolution to maintain the ligand recognition selectivity of mGluRs.

Atypical SCH23390 binding sites are present on bovine adrenal medullary membranes

D1-selective dopamine receptor agonists inhibit secretagogue-stimulated catecholamine secretion from bovine adrenal chromaffin cells. The purpose of the studies reported here was to use the radiolabeled D1-selective dopamine receptor antagonist, SCH23390, to characterize putative D1-like dopamine receptors responsible for this effect. Characterization of SCH23390 binding sites demonstrated an unusual pharmacological profile inconsistent with classical D1-like receptors. [125I]SCH23390 bound to adrenal medullary membranes was competed for by nonradioactive iodo-SCH23390 (Kd = 490 +/- 50 nM), but not by (+)butaclamol. Other classical D1 antagonists had little, if any, effect. Competition with dopamine receptor agonists demonstrated a relative rank order of potency profile characteristic of D1-like dopamine receptors, however, K(i)s were higher than those found in other tissues. The K(i)s for competition of [125I]SCH23390 binding by Cl-APB and SKF38393 (16 and 118 microM, respectively) are nearly identical to the IC(50)s previously observed for inhibition of secretion (9 and 100 microM, respectively). Combined these data suggest that adrenal medullary membranes contain a novel SCH23390 binding site involved in the inhibition of secretion by D1-selective agonists.

Circadian modulation of dopamine receptor responsiveness in Drosophila melanogaster

We investigated the circadian function of Drosophila dopamine receptors by using a behaviorally active decapitated preparation that allows for direct application of drugs to the nerve cord. Quinpirole, a D2-like dopamine receptor agonist, induces reflexive locomotion in decapitated flies. We show that the amount of locomotion induced changes as a function of the time of day, with the highest responsiveness to quinpirole during the subjective night. Furthermore, dopamine receptor responsiveness is under circadian control and depends on the normal function of the period gene. The head pacemaker is at least partly dispensable for the circadian modulation of quinpirole-induced locomotion, because changes in agonist responsiveness persist in decapitated flies that are aged for 12 h. This finding suggests a role for the period-dependent molecular oscillators in the body in the modulation of amine receptor responsiveness.

The influence of endogenous dopamine levels on the density of [3H]SCH23390-binding sites in the brain of the honey bee, Apis mellifera L

This paper examines the relationship between endogenous dopamine (DA) levels and the density of [3H]SCH23390-binding sites in the brain of the adult worker honey bee. DA levels were reduced pharmacologically using a single 10 microl injection of either alpha-methyl-DL-p-tyrosine (AMT; 250 microg or 500 microg) or alpha-methyl-DL-tryptophan (AMTP; 250 or 500 microg) into the haemolymph of the bee. In all cases, maximum depletion of DA was observed 3 h after treatment, but in bees treated with AMTP (250 or 500 microg) or with 250 microg AMT, DA levels returned to normal within 24 h of treatment. Neither AMT nor AMTP was selective for DA: both drugs also reduced serotonin (5-hydroxytryptamine, 5HT) levels in the brain. However, AMTP was more effective than AMT at depleting 5HT, whereas for DA, the reverse was true. Depletion of DA levels, using 250 microg AMT, led to a dramatic decline in the levels of specific binding of [3H]SCH23390, defined in this study as binding in the presence of 5x10(-6) M cis-(Z)-flupentixol (see Ref. [28] ). In contrast, naturally occurring diel fluctuations in DA levels, identified in the optic lobes of the brain, and changes in brain DA levels resulting from queenlessness, had no significant effect on the density of [3H]SCH23390-binding sites in the brain of the bee. Overall, these results indicate that under normal physiological conditions, there is no direct link in honey bees between changes in endogenous brain DA levels and the density of D(1)-like receptors labelled by [3H]SCH23390.

Modulation of the light response by cAMP in Drosophila photoreceptors

Phototransduction in Drosophila is mediated by a G-protein-coupled phospholipase C transduction cascade in which each absorbed photon generates a discrete electrical event, the quantum bump. In whole-cell voltage-clamp recordings, cAMP, as well as its nonhydrolyzable and membrane-permeant analogs 8-bromo-cAMP (8-Br-cAMP) and dibutyryl-cAMP, slowed down the macroscopic light response by increasing quantum bump latency, without changes in bump amplitude or duration. In contrast, cGMP or 8-Br-cGMP had no effect on light response amplitude or kinetics. None of the cyclic nucleotides activated any channels in the plasma membrane. The effects of cAMP were mimicked by application of the non-specific phosphodiesterase inhibitor IBMX and the adenylyl cyclase activator forskolin; zaprinast, a specific cGMP-phosphodiesterase inhibitor, was ineffective. Bump latency was also increased by targeted expression of either an activated G(s) alpha subunit, which increased endogenous adenylyl cyclase activity, or an activated catalytic protein kinase A (PKA) subunit. The action of IBMX was blocked by pretreatment with the PKA inhibitor H-89. The effects of cAMP were abolished in mutants of the ninaC gene, suggesting this nonconventional myosin as a possible target for PKA-mediated phosphorylation. Dopamine (10 microM) and octopamine (100 microM) mimicked the effects of cAMP. These results indicate the existence of a G-protein-coupled adenylyl cyclase pathway in Drosophila photoreceptors, which modulates the phospholipase C-based phototransduction cascade.

Cocaethylene synthesis in Drosophila

Cocaethylene is an active cocaine metabolite that targets mammalian neural reward pathways and thus contributes to the reinforcing and addictive properties of ethanol and cocaine. Using gas chromatography-mass spectrometry, we find that fruit flies (Drosophila melanogaster) possess a cellular mechanism through which cocaine can be converted to cocaethylene, presumably via ethanol-sensitive enzymes. These findings illustrate the striking similarity of gene products in humans and flies, which might reflect a homologous role in the metabolic inactivation of cocaine. Further, this conservation of metabolic steps suggests that Drosophila can be used to study cellular, molecular and biochemical processes leading to polydrug abuse and addiction.

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.