Functional characterization of a putative octopamine/tyramine receptor from the southern cattle tick, Rhipicephalus microplus

Octopamine and tyramine are biogenic monoamines found in a variety of invertebrates, including insects and ticks.  The southern cattle tick, Rhipicephalus (Boophilus) microplus, vectors the causative agents (Babesia spp.) that result in Texas Cattle Fever.  The tick was officially eradicated from the United States with the exception of a permanent quarantine zone along the Mexico-Texas border.  Chemical control of R. microplus has been achieved with a variety of chemical acaricides including amitraz.  It is believed that octopamine receptors are the target of amitraz and other formamidine insecticides.  Previously, a putative octopamine receptor was reported from susceptible and resistant strains of R. microplus.  However, functional characterization of this putative octopamine receptor has not previously been performed.  Further, it has been suggested that this putative octopamine receptor may preferentially respond to tyramine. The putative receptor was expressed in Chinese hamster ovary cells along with chimeric G-proteins to perform calcium mobilization assays.  Functional characterization of this putative receptor resulted in tyramine being the preferred ligand.  Furthermore, the biochemical signalling pathway, in this heterologous expression system, indicates that it is similar to a Type-1 tyramine receptor signalling through a G_αi pathway.  Formamidine insecticides, Chlordimeform and Amitraz, were shown to interact with the tyramine receptor along with several tyraminergic compounds in a calcium-liberation assay.