Properties of a Set of Internal Receptors in the Medicinal Leech: The Nephridial Nerve Cells Monitor Extracellular Chloride Concentration

In the leech, Hirudo medicinalis, the functional properties of the peripheral nephridial nerve cells (NNCs) were investigated using long-term extracellular recordings from both intact and isolated preparations. The NNCs respond selectively to changes in external Cl− concentration: their electrical activity is highest in fluid containing normal (i.e. low) extracellular [Cl−]. Their sensitivity to Cl− is confined to the dendritic tree. Extracellular [Cl−] is a control factor for ion homeostasis. It increases drastically after a blood meal, and after the animal has left the water or invaded brackish water. The NNCs continuously monitor the extracellular Cl− concentration: their burst rate changes, without adapting, by a factor of 4 in the physiological range between 40 and 90 mmoll−1. Intracellular recordings from the NNC in isolated nephridial complexes suggest that a high Cl− conductance in combination with active transport of Cl− could be responsible for the observed Cl− sensitivity.