Abstract
Tyramine is a neuroactive compound that acts as neurotransmitter, neuromodulator, and neurohormone in insects. Three G protein-coupled receptors, TAR1-3, are responsible for mediating the intracellular pathway in the complex tyraminergic network. TAR1, the prominent player in this system, was initially classified as an octopamine receptor which can also be activated by tyramine, while it later appeared to be a true tyramine receptor. Even though TAR1 is currently considered as a well-defined tyramine receptor and several insect TAR1s have been characterized, a defined nomenclature is still inconsistent. In the last years, our knowledge on the structural, biochemical, and functional properties of TAR1 has substantially increased. This review summarizes the available information on TAR1 from different insect species in terms of basic structure, its regulation and signal transduction mechanisms, and its distribution and functions in the brain and the periphery. A special focus is given to the TAR1-mediated intracellular signaling pathways as well as to their physiological role in regulating behavioral traits. Therefore, this work aims to correlate, for the first time, the physiological relevance of TAR1 functions with the tyraminergic system in insects. In addition, pharmacological studies have shed light on compounds with insecticidal properties having TAR1 as a target and on the emerging trend in the development of novel strategies for pest control.
Highlights
The Tyraminergic and Octopaminergic Systems in InsectsIn insects, the main biogenic amines are dopamine (DA), serotonin (5-HT), tyramine (TA), and octopamine (OA)
TA, OA, and the catecholamines epinephrine and norepinephrine have many features in common: They are synthesized from the same precursor amino acid, share both structural and functional characteristics such as interaction with G proteincoupled receptors (GPCRs), and regulate similar behavioral and physiological traits [2]
Changing the normal function of these receptor classes by blocking or overstimulating them can lead to the death of an insect or interfere with physical fitness and reproductive capacity [12]. These receptors are classified into five main groups based on their primary structure: α1-adrenergic-like receptors (Octα1R, known as OAMB or OA1), α2-adrenergic-like receptors (Octα2R, known as OA3), β-adrenergic-like receptors (OctβR, known as OA2), tyramine type 1 receptors (TA/OA, Tyr1-R or TAR1), and tyramine type 2 and 3 receptors (Tyr2-R or TAR2 and Tyr3-R or TAR3) [13,14]
Summary
The main biogenic amines are dopamine (DA), serotonin (5-HT), tyramine (TA), and octopamine (OA). Changing the normal function of these receptor classes by blocking or overstimulating them can lead to the death of an insect or interfere with physical fitness and reproductive capacity [12] These receptors are classified into five main groups based on their primary structure: α1-adrenergic-like receptors (Octα1R, known as OAMB or OA1), α2-adrenergic-like receptors (Octα2R, known as OA3), β-adrenergic-like receptors (OctβR, known as OA2), tyramine type 1 receptors (TA/OA, Tyr1-R or TAR1), and tyramine type 2 and 3 receptors (Tyr2-R or TAR2 and Tyr3-R or TAR3) [13,14]. The five OARs and three TARs have been shown to be differentially expressed in Drosophila [32], therewith confirming their multiple and often unique roles in controlling physiology and behavior in insects. Since OARs and TARs play pivotal roles in insect physiology, they are possible targets for insecticides used in pest control [33]
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