Pharmacological characterization of histamine-gated chloride channels from the housefly Musca domestica

Identification and pharmacological characterization of histamine-gated chloride channels in the fall armyworm, Spodoptera frugiperda

Histamine-gated chloride channels (HACls) mediate fast inhibitory neurotransmission in invertebrate nervous systems and have important roles in light reception, color processing, temperature preference and light-dark cycle. The fall armyworm, Spodoptera frugiperda is a main destructive pest of grain and row crops. However, the pharmacological characterization of HACls in S. frugiperda remain unknown. In this study, we identified two cDNAs encoding SfHACl1 and SfHACl2 in S. frugiperda. They had similar expression patterns and were most abundantly expressed in the head of larvae and at the egg stage. Electrophysiological analysis with the two-electrode voltage clamp method showed that histamine (HA) and γ-aminobutyric acid (GABA) activated inward currents when SfHACls were singly or collectively expressed with different ratios in Xenopus laevis oocytes. These channels were ≥2000-fold more sensitive to HA than to GABA. They were anion-selective channels, which were highly dependent on changes in external chloride concentrations, but insensitive to changes in external sodium concentrations. The insecticides abamectin (ABM) and emamectin benzoate (EB) also activated these channels with the EC₅₀ to SfHACl1 lower than that to SfHACl2. And the EC_50s of ABM and EB to the co-expressed channels gradually increased with increase in the injection ratio of SfHACl2 cRNA. Homology models and docking simulations revealed that HA bound to the large amino-terminal extracellular domain of SfHACl1 and SfHACl2 by forming 4 and 2 hydrogen bonds, respectively. The docking simulations of ABM and EB had similar binding sites in the transmembrane regions. Overall, these findings indicated that HACls act as targets for macrolide, and this study provides theoretical guidance for further derivatization of abamectin insecticides.

Automated phenotyping of mosquito larvae enables high-throughput screening for novel larvicides and offers potential for smartphone-based detection of larval insecticide resistance

Pyrethroid-impregnated nets have contributed significantly to halving the burden of malaria but resistance threatens their future efficacy and the pipeline of new insecticides is short. Here we report that an invertebrate automated phenotyping platform (INVAPP), combined with the algorithm Paragon, provides a robust system for measuring larval motility in Anopheles gambiae (and An. coluzzi) as well as Aedes aegypti with the capacity for high-throughput screening for new larvicides. By this means, we reliably quantified both time- and concentration-dependent actions of chemical insecticides faster than using the WHO standard larval assay. We illustrate the effectiveness of the system using an established larvicide (temephos) and demonstrate its capacity for library-scale chemical screening using the Medicines for Malaria Venture (MMV) Pathogen Box library. As a proof-of-principle, this library screen identified a compound, subsequently confirmed to be tolfenpyrad, as an effective larvicide. We have also used the INVAPP / Paragon system to compare responses in larvae derived from WHO classified deltamethrin resistant and sensitive mosquitoes. We show how this approach to monitoring larval response to insecticides can be adapted for use with a smartphone camera application and therefore has potential for further development as a simple portable field-assay with associated real-time, geo-located information to identify hotspots.

Evolutionary history of histamine receptors: Early vertebrate origin and expansion of the H3-H4 subtypes

Histamine receptors belonging to the superfamily of G protein-coupled receptors (GPCRs) mediate the diverse biological effects of biogenic histamine. They are classified into four phylogenetically distinct subtypes H₁-H₄, each with a different binding affinity for histamine and divergent downstream signaling pathways. Here we present the evolutionary history of the histamine receptors using a phylogenetic approach complemented with comparative genomics analyses of the sequences, gene structures, and synteny of gene neighborhoods. The data indicate the earliest emergence of histamine-mediated GPCR signaling by a H₂ in a prebilaterian ancestor. The analyses support a revised classification of the vertebrate H₃-H₄ receptor subtypes. We demonstrate the presence of the H₄ across vertebrates, contradicting the currently held notion that H₄ is restricted to mammals. These non-mammalian vertebrate H₄ orthologs have been mistaken for H₃. We also identify the presence of a new H₃ subtype (H₃B), distinct from the canonical H₃ (H₃A), and propose that the H₃A, H₃B, and H₄ likely emerged from a H₃ progenitor through the 1R/2R whole genome duplications in an ancestor of the vertebrates. It is apparent that the ability of the H₁, H₂, and H_3-4 to bind histamine was acquired convergently. We identified genomic signatures suggesting that the H₁ and H₃-H₄ shared a last common ancestor with the muscarinic receptor in a bilaterian predecessor whereas, the H₂ and the α-adrenoreceptor shared a progenitor in a prebilaterian ancestor. Furthermore, site-specific analysis of the vertebrate subtypes revealed potential residues that may account for the functional divergence between them.

GABA signaling affects motor function in the honey bee

GABA is the most common inhibitory neurotransmitter in both vertebrate and invertebrate nervous systems. In insects, inhibition plays important roles at the neuromuscular junction, in the regulation of central pattern generators, and in the modulation of information in higher brain processing centers. Additionally, increasing our understanding of the functions of GABA is important since GABA_A receptors are the targets of several classes of pesticides. To investigate the role of GABA in motor function, honey bee foragers were injected with GABA or with agonists or antagonists specific for either GABA_A or GABA_B receptors. Compounds that activated either type of GABA receptor decreased activity levels. Bees injected with the GABA_A receptor antagonist picrotoxin lost the ability to right themselves, whereas blockade of GABA_B receptors led to increases in grooming. Injection with antagonists of either GABA_A or GABA_B receptors resulted in an increase in extended wing behavior, during which bees kept their wings out at right angles to their body rather than folded along their back. These data suggest that the GABA receptor types play distinct roles in behavior and that GABA may affect behavior at several different levels.

Physiological responses of ionotropic histamine receptors, PxHCLA and PxHCLB, to neurotransmitter candidates in a butterfly, Papilio xuthus

Histamine is the only known neurotransmitter released by arthropod photoreceptors. Synaptic transmission from photoreceptors to second order neurons is mediated by the activation of histamine-gated chloride channels (HCLs). These histaminergic synapses have been assumed to be conserved among insect visual systems. However, our understanding of the channels in question has thus far been based on studies in flies. In the butterfly Papilio xuthus, we have identified two candidate histamine-gated chloride channels, PxHCLA and PxHCLB, and studied their physiological properties using a whole-cell patch-clamp technique. We studied the responses of channels expressed in cultured cells to histamine as well as to other neurotransmitter candidates, namely GABA, tyramine, serotonin, D-/L- glutamate, and glycine. We found that histamine and GABA activated both PxHCLA and PxHCLB, while the other molecules did not. The sensitivity to histamine and GABA was consistently higher in PxHCLB than in PxHCLA. Interestingly, simultaneous application of histamine and GABA activated both PxHCLA and PxHCLB more strongly than either neurotansmitter individually; histamine and GABA may have synergistic effects on PxHCLs in the regions where they colocalize. Our results suggest that the physiological properties of the histamine receptors are basically conserved among insects, but that the response to GABA differs between butterflies and flies, implying variation in early visual processing among species.

GABA-, histamine-, and FMRFamide-immunoreactivity in the visual, vestibular and central nervous systems of Hermissenda crassicornis

Hermissenda crassicornis is a model for studying the molecular and cellular basis for classical conditioning, based on its ability to associate light with vestibular stimulation. We used confocal microscopy to map histamine (HA), FMRF-amide, and γ-aminobutyric acid (GABA) immunoreactivity in the central nervous system (CNS), eyes, optic ganglia and statocysts of the nudibranchs. For HA immunoreactivity, we documented both consistently and variably labeled CNS structures across individuals. We also noted minor differences in GABA immunoreactivity in the CNS compared to previous work on Hermissenda. Contrary to expectations, we found no evidence for GABA inside the visual or vestibular systems. Instead, we found only FMRFamide- and HA immunoreactivity (FMRFamide: 4 optic ganglion cells, 4-5 hair cells; HA: 3 optic ganglion cells, 8 hair cells). Overall, our results can act as basis for comparisons of nervous systems across nudibranchs, and suggest further exploration of intraspecific plasticity versus evolutionary changes in gastropod nervous systems.