Lehrstuhl Tierphysiologie, Universität Bayreuth, 95440 Bayreuth, Germany.
The termite Schedorhinotermes lamanianus probes food sources by antennal taste receptors. On each antennal segment three types of taste sensilla can morphologically be distinguished, one of which - type TP I - contains five taste receptor cells. Using the tip recording technique, we focus on the physiology of one of the five cells, Cell 1. For this cell we provide strong evidence for two distinct receptor sites: a water site and a sodium site. Cell 1 responded to pure water with a mean frequency of 15 Imp/200 ms. Typically for water receptor cells in insects, the response dropped to zero at stimulation with increasing concentrations of KCl and CaCl2. Half-maximal response for CaCl2 was at 0.01mM and for KCl at 1mM. Thus, CaCl2 was two orders of magnitude more effective than KCl. However, at stimulation with rising NaCl concentrations, the response rate of Cell 1 increased. This suggests an additional site in Cell 1 responding specifically to sodium. To investigate this sodium site independently from the water site, the water site was specifically inhibited by low CaCl2 concentrations. Then, Cell 1 showed a logarithmic dose response curve typical for salt receptors in insects with a remarkable low threshold for NaCl at about 1mM. The unexpected response characteristics of Cell 1 to NaCl, showing a response to pure water and elevated response levels at stimulation with high NaCl concentrations, was due to a simultaneous response of both sites in a ratio depending on the NaCl concentration.