Different physiological roles of two dopamine receptors in isolated salivary glands of the blacklegged tick, Ixodes scapularis

Ticks are obligatory ectoparasites of many vertebrate transmitting pathogens causing diseases including the Lyme disease.  Tick salivary secretion is crucial not only for successful feedings for injection of bioactive salivary components into the host but also for osmoregulation after the ingestion of large amount of blood.  Control of the salivary glands involves dopamine that is the most potent salivation inducer.  We previously have identified two dopamine receptors expressed in the salivary glands and named as dopamine receptor (D1) and dopamine-like receptor (D1L) based on their sequence similarities to the vertebrate dopamine receptors.  We established heterologous functional assays of each receptor for pharmacological characterizations.  We have identified one D1 specific agonist (SKF82958) and two D1L specific antagonists (acepromazine and fluphenazine) that pharmacologically discriminate the two receptors.  Isolated tick salivary glands were examined for measuring the secretory activities after pharmacological treatments. Increases in the volume of individual acinus of the salivary glands were measured over time after the treatment.  Based on the results from our studies using receptor-specific antagonists and agonists, dopamine triggers two different physiological actions through two respective receptors.  We propose that high-sensitive receptor D1 triggers fluid transport in the epithelial cells of the acini and low-sensitive receptor D1L is involved in pumping actions of the acini.  Understanding the mechanisms of tick salivary gland control will allow us to develop the novel method for disruption of tick feeding.