As the purveyors of unpleasant diseases, ticks have a justly deserved bad reputation and Yoonseong Park, from Kansas State University, USA, explains that salivation is one of the keys to the disagreeable arachnid's success. The tick's complex saliva not only helps it to extract water from the air between feeds and concentrates blood meals by extracting water, but it also contains compounds that defeat the immune system of the host. ‘Salivary secretion is crucial for successful feeding and is the mediator of pathogen transmission’ says Park. So understanding how this process is regulated by the endocrine (hormone) system is essential if we are to outwit the pernicious pest. ‘Dopamine has long been known as the as the most potent inducer of tick salivation,’ explains Park, adding that his lab had already discovered two protein receptors (D1 and InvD1L) in the tick salivary gland that are activated by the hormone to regulate saliva production. So he and his colleagues, Donghun Kim and Ladislav Šimo decided to investigate which aspects of the salivation process the two receptors regulate (p. 3656).
‘We hypothesised that there are two different dopamine actions mediated through the receptors in different cell types of salivary glands’, says Park. However, tracking down the receptors' functions had not been easy as it was not possible to use conventional methods of receptor inactivation to find out which processes they regulated. Instead, the team had to first identify chemicals that independently activate (agonists) and inactivate (antagonists) the receptors in a bid to learn more about their functions.
Inserting the genes for the receptors into animal cells and testing how various chemicals affected the receptors, Kim successfully identified an agonist for the D1 receptor (SKF82958), an antagonist for the InvD1L receptor (fluphenazine) and an agonist that worked on both receptors (SCH23390) that they could use to tease apart their functions in intact salivary glands.
‘Dissecting out the entire salivary gland from these tiny animals without damage was challenging,’ admits Park, adding that measuring the minute volumes of saliva produced by the minuscule organs was also tricky. However, once Kim had mastered the fiddly techniques, he found that salivary glands that had been stimulated with 100 μmol l−1 dopamine produce 1.2 μl of saliva over 30 min. And when he applied the D1 agonist to the salivary glands at the same concentration, they also produced saliva, but at a slower rate.
Next, the team focused on how the receptors functioned in each of the salivary gland's three different types of lobe – known as acini. After finding that the acini expand and contract in response to dopamine (expanding as fluid flows into the acini lumen and contracting as the acinus expels saliva into the acinar duct), Kim filmed the acini as he soaked the gland in dopamine, the two agonists or the antagonist and calculated that dopamine exposure caused the lumen of each acinus to expand by 4 nl.
Finally, the team turned their attention to the type III acinus which produces the concentrated saliva that reduces the volume of blood meals. Analysing how the acinus changed volume as they systematically applied the dopamine receptor agonists and antagonist, the team realised that the D1 receptor regulates uptake of fluid by the acnius while the InvD1L receptor regulates the pumping and gating mechanisms that eject saliva from the acinus into the salivary duct. ‘Dopamine acts on the D1 and the InvD1L receptors and leads different physiological actions to orchestrate tick salivary secretion,’ concludes Park and his team.