Excitability and Secretory Activity in the Salivary Gland Cells of Jawed Leeches (Hirudinea: Gnathobdellida)

Thousands of salivary cells fill the interstices throughout the anterior ends of jawed leeches. The somata are large (30-200 micron in diameter). They project single processes (ductules) into the three jaws, and were found to fire overshooting action potentials of 50-85 mV amplitude and 100-200 ms duration at low spontaneous rates. The action potentials were not detected in the presence of cobalt (10 mmol l-1), but could be recorded when sodium was absent from the Ringer, so they appear to be calcium-dependent. Salivary material is transported by the long processes of these unicellular glands and secreted into ducts which alternate with paired teeth on the jaws. Secretion is activated reliably by 10(-6) mol l-1 serotonin, but not by other neurotransmitters found in the leech nervous system. Each jaw secretes at an average rate of 230 nl min-1 in the presence of serotonin, and secretion is completely abolished by cobalt. Perfusion with serotonin excites the salivary gland cells into impulse activity, and often evokes bursting. Impulse activity of the peripherally projecting, serotonergic Retzius cells evokes both depolarizations and action potentials in the salivary gland cells. In jawed leeches, central neurones appear to control salivation by a peripheral release of serotonin. This neurotransmitter evokes calcium-dependent action potentials and calcium, in turn, stimulates secretion.