Animals have chemosensory and mechanosensory systems that recognize a large array of environmental stimuli, generating behavioral responses. Many behavioral studies have been conducted to understand the genetics of these systems. The olfactory system plays a crucial role in the survival and maintenance of species, particularly in insects.
Biogenic amines serve a neurotransmitter or neuromodulator role in the olfactory system. The biogenic amine, octopamine, has a prominent role in insects and other invertebrates as it is involved in the regulation of multiple physiological events, for example, effects on muscular systems, sensory organs, endocrine tissues as well as learning and behavior. Octopamine (OA) occurs in large amounts in the nervous systems of species representing the phylum Arthropoda, including the classes Insecta and Crustacea. OA has a broad spectrum of biological roles in insects acting as a neurotransmitter, neurohormone and neuromodulator. OA exerts its effects through interaction with at least four classes of membrane bound receptors that belong to the family of G-protein coupled receptors (GPCRs). All members of GPCRs share the common motif of seven transmembrane (TM) domains.
When a GPCR is activated, depending on its type and the protein to which it binds, changes in intracellular concentrations of cAMP, Ca2+ or both often take place. Since changes in intracellular levels of cAMP or Ca2+ are the most commonly found cellular responses to biogenic amine treatments (e.g., serotonin, dopamine, octopamine, etc.), they are used to functionally classify receptor subtypes. As a result of GPCR activation, intracellular cAMP levels can either be elevated or reduced. The cellular response strictly relies on the specificity of interaction between the receptor and the G protein (See e.g., Gudermann T, Kalkbrenner F, Schultz G. 1996, “Diversity and selectivity of receptor-G protein interaction,” Annu Rev Pharmacol Toxicol 36: 429-459; and Gudermann T, Schoneberg T, Schultz G. 1997, “Functional and structural complexity of signal transduction via G-protein-coupled receptors,” Annu Rev Neurosci 20: 399-427, both of which are incorporated herein by this reference). When the receptor binds to Gs-type protein, the activated Gas subunit will interact with adenylyl cyclase (AC) in the plasma membrane. This leads to an increase of AC activity and production of cAMP from ATP.
Several biogenic amine receptors are also known to inhibit AC activity. This effect is mediated by interaction of the receptor with inhibitory G protein (Gi). Interaction of AC with activated Gαi subunits most likely competes with binding of activated Gas subunits and thereby interferes with AC activation.
Another pathway that is activated by several biogenic amine receptors results in a rise of intracellular Ca2+ levels. In such a scenario the amine-activated receptor binds to G proteins of the Gq/o family (See e.g., supra, Gudermann et al., 1996 and Gudermann et al., 1997). The activated Gαq/o subunits bind to and stimulate phospholipase C (PLC) activity. The enzyme hydrolyzes a membrane-bound substrate, phosphatidylinositol 4,5-bisphosphate which gives rise to two second messengers IP3 and DAG. After binding of IP3 to its receptors, the calcium channel pore is opened and Ca2+ is released into the cytoplasm. Ca2+ ions play a vital role in the regulation of many cellular functions by binding to members of large family of Ca2+-binding proteins and/or directly controlling enzymatic or ion channel activities.
Multiple insect species have been utilized to understand the biological functions and pharmacological characteristics of octopamine receptors. Studies with Periplaneta americana (American cockroach) have provided insight into the pharmacology and second messenger signaling of octopamine through octopamine receptors. For example, octopamine has been found to activate adenylate cyclase in certain cells in this species. Furthermore, octopamine has been found to increase inositol triphosphates in certain cells in this species.
As the octopaminergic system is believed to be unique to invertebrate physiology, this pathway has been proposed to offer a target for invertebrate pesticides with potential for low vertebrate toxicity. Formamidine-like chemicals have been found to be octopaminergic agonists and inhibit the uptake of sodium-sensitive octopamine in certain insects; for example, the formamidine pesticides chlordimeform and demethylchloridimeform were found to target the octopamine signaling pathway in certain invertebrates, including Periplaneta americana. To provide insight into the design of octopamine agonists that could be used as potential insecticides, structure function analyses have been performed with 2-(arylimino)oxazolidines and 2-(substituted benzylamino)-2-oxazolines in regard to activation of the octopamine sensitive adenylate cyclase in certain cells in Periplaneta Americana. More recently, it has been suggested that one site of action for the insecticidal activity of plant essential oils against Periplaneta americana is the octopaminergic system and that octopamine receptors may be targeted by these compounds, as described in Enan, E., 2001, “Insecticidal activity of essential oils: octopaminergic sites of action,” Comp. Biochem. Physiol. C Toxicol. Pharmacol. 130, 325-327, which is incorporated herein by this reference.
Identifying plant essential oils and combinations thereof, having insect-controlling activity is particularly desirable given that many such compounds do not produce unwanted or harmful affects on humans, other animal species, and certain plants. However, identifying the most effective plant essential oils and combinations thereof requires random selection and use of tedious screening methods, which, given the vast number of plant essential oils and possible combinations thereof, is a substantially impossible task.
As such, there is a need in the art for an improved method for screening compounds and compositions for insect control activity.