B. the OX1a structure to identify more potent Orco antagonists. Cqui\Orco+Cqui\Or21, an OR from (the Southern House Mosquito) that responds to 3-methylindole (skatole) and is thought to mediate oviposition behavior, was expressed in oocytes and receptor function assayed by two-electrode voltage clamp electrophysiology. 22 structural analogs of OX1a were screened for antagonism of OR activation by an Orco agonist. By varying the moieties decorating the phenyl and thiophene rings, and altering the distance between the rings, we were able to identify antagonists with improved potency. Detailed examination of three of Anisomycin these compounds (N-mesityl-2-thiophenecarboxamide, N-(4-methylbenzyl)-2-thiophenecarboxamide and N-(2-ethylphenyl)-3-(2-thienyl)-2-propenamide) demonstrated competitive inhibition of receptor activation by an Orco agonist and non-competitive inhibition of receptor activation by an odorant. The ability to inhibit OR activation by odorants may be a general house of this class of Orco antagonist, suggesting that odorant mediated behaviors can be manipulated through Orco antagonism. The high conservation of Orco across insect species and previous demonstrations that numerous Orco ligands are active at ORs derived from several different insect orders suggests that Orco antagonists may have broad applicability. Introduction The interactions of insects with humans can be beneficial (pollination of crops), as well as detrimental (disease transmission, crop destruction). Many insect behaviors, such as feeding, mating and oviposition, are driven by olfaction, making insect olfactory receptors appealing targets for insect control strategies . The OR class of insect olfactory receptors are a novel class of ligand (odorant) gated cation channel [2,3], located on the dendrites of olfactory sensory neurons in the antennae. ORs are composed of a common subunit (the odorant receptor co-receptor subunit, known as Orco ) that is highly conserved across species and a variable subunit that confers odorant specificity [5-12]. The specificity subunits are thought to mediate odorant acknowledgement because changing this subunit alters odorant preference [13-15] and mutations in a specificity subunit can alter odorant sensitivity [16,17]. Both Orco and the specificity subunit are thought to contribute to the structure of the ion channel pore [2,18,19]. However, the number Anisomycin and stoichiometry of these subunits in a functional OR is currently unknown. These receptors also may initiate, or be altered by, second messenger cascades [3,20,21]. Insect ORs are not related to the receptors and channels of humans and other tetrapods , suggesting that control of detrimental insect activity can be achieved, while minimizing environmental toxicity, through the development of insect OR selective compounds. One approach to developing these compounds involves the identification of particular specificity subunits that mediate IGFBP3 acknowledgement of behaviorally important odorants [13,15,22-24], followed by considerable ligand screening [25,26]. However, high diversity among the specificity subunit repertoires of different species, as well as variance in the odorants and specificity subunits that are important for species-specific behaviors, makes this approach exceptionally labor rigorous [1,27]. The development of compounds active at multiple ORs across different species would be more useful. The recent identification of VUAA1, an agonist of the Orco subunit , suggests Orco-directed compounds as a encouraging new direction for the development of insect repellants and additional agonists were subsequently recognized [28-30]. Orco agonists recognized to date are closely related to VUAA1, suggesting a restrictive set of structural requirements for Orco agonism. A larger, more diverse series of compounds can competitively antagonize Orco agonist activity [29,31]. Importantly, several of these Orco antagonists were shown to inhibit odorant activation of ORs through a non-competitive mechanism. Results and Discussion While many of the previously Anisomycin recognized Orco antagonists are large structures that are unlikely to be useful as repellants, our previous screen  recognized several Orco antagonists of smaller size, such as N-(4-ethylphenyl)-2-thiophenecarboxamide (Physique 1A), suggesting a encouraging starting point for the identification of new Orco ligands. We previously referred to this compound as OLC20.