Different physiological functions of two dopamine receptors in isolated salivary glands of blacklegged tick, Ixodes scapularis Say

Ticks are obligatory ectoparasites of many vertebrates and transmit various pathogens that cause numerous diseases, including Lyme disease, the most commonly reported vector-borne disease in the United States.  Tick salivary secretions are crucial for successful tick feeding, and include several bioactive components, which are responsible for compromising the host immune responses.  Salivation also functions in the removal of excess water and ions after the ingestion of large blood meals.  Control of the salivary glands involves dopamine, which is the most potent inducer of tick salivation.  Previously, two dopamine receptors were characterized in the salivary glands of the blacklegged tick (Ixodes scapularis Say): dopamine receptor (D1) and invertebrate specific D1-like dopamine receptor (InvD1L).  Here, we have identified receptor-specific antagonists and agonists via functional expression of the dopamine receptors in a mammalian cell line.  These pharmacological tools were further used for in vitro assays: monitoring changes in the size of salivary gland acini and quantifying the salivary secretions of isolated salivary glands.  Based on the results from the in vitro experiment using pharmacological tools, we propose that D1 receptor acts on the epithelial cells of the salivary gland acini, directing inward fluid transport, while InvD1L controls (or modulates) myoepithelial cell activities for pumping or gating, which facilitate the expelling of saliva out through the ducts of the salivary glands.  Understanding the mechanism of fluid secretion in tick salivary glands will lead us to the development of novel methods for the disruption of tick feeding, thus allowing for the prevention of pathogen transmission.