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

Ticks are obligatory ectoparasites that transmit pathogens causing human diseases, such as Lyme disease.  Salivary secretion in ticks is crucial, not only for the injection of bioactive salivary components into the host, but also for osmoregulation after the ingestion of large amounts of blood.  Paracrine or autocrine dopamine in the salivary glands induces salivary secretion.  Two dopamine receptors have recently been characterized in the salivary glands and named as the dopamine receptor (D1) and D1-like receptor (D1L), based upon their sequence similarities to vertebrate dopamine receptors.  We were able to identify a D1-specific agonist (SKF82958) and three D1L-specific antagonists (acepromazine, fluphenazine, and clozapine) in the heterologous functional assay of these receptors.  These compounds served as pharmacological discriminators of the two receptors in vivo.  Isolated tick salivary glands were examined for the measurement of secretory activities following exposure to pharmacological discriminators.  This semi- in vivo study using receptor specific antagonists and agonists provides evidence that the two receptors, D1 and D1L, are leading to two different physiological actions in salivary secretion.  We propose that D1 triggers fluid transport in the epithelial cells of the acini, and that D1L is involved in gating/pumping actions of the salivary acini.  Understanding the mechanisms of tick salivary gland control will lead us to the development of novel methods for the disruption of tick feeding.