Salivary glands in ixodid ticks: control and mechanism of secretion

The salivary glands are vital to the biological success of ixodid ticks and the major route for pathogen transmission. Important functions include the absorption of water vapor from unsaturated air by free-living ticks, excretion of excess fluid for blood meal concentration, and the secretion of bioactive protein and lipid compounds during tick feeding. Fluid secretion is controlled by nerves. Dopamine is the neurotransmitter at the neuroeffector junction regulating secretion via adenylate cyclase and an increase in cellular cAMP. Dopamine also affects the release of arachidonic acid which is subsequently converted to prostaglandins. Prostaglandin E2 (PGE2) is secreted at extremely high levels into tick saliva for export to the host where it impacts the host physiology. Additionally, PGE2 has an autocrine or paracrine role within the salivary gland itself where it interacts with a PGE2 receptor to induce secretion (exocytosis) of bioactive saliva proteins via a phosphoinositide signalling pathway and an increase in cellular Ca2+. Regulation of fluid secretion has been extensively studied, but little is known about the mechanism of fluid secretion. Continuing advances in tick salivary gland physiology will be made as key regulatory and secretory gland proteins are purified and/or their genes cloned and sequenced.