Electrophysiological identification of antennal pH receptors in the ground beetle Pterostichus oblongopunctatus

Electrophysiological responses of antennal taste bristles to 100 mM acetate and phosphate buffers were tested at pH 3-11 in the ground beetle Pterostichus oblongopunctatus (F.) (Coleoptera, Carabidae). Additionally, responses of these sensilla to 10 and 100 mM phosphate buffers were compared with each other. Generally, in response to these stimulating solutions, two sensory cells, classified as a salt cell (cation cell) and a pH cell, respectively, showed action potentials distinguished by differences in their amplitudes and polarity of spikes. The firing rate of the cation cell increased with increasing buffer concentration, and was influenced by buffer pH in a complicated way. The best stimulus for the second cell (pH cell) was pH of the stimulating buffer solution. As the pH of the stimulus solution increased, higher rates of firing were produced by the pH cell. For example, the number of action potentials elicited by 100 mM phosphate buffer at pH 11.1 was approximately 16-fold higher compared with that at pH 8.1, and firing rates during the first second of the response were 27.9 and 1.7 imp/s, respectively. The pH cell did not fire or fired at very low frequency (first second response below 5 imp/s) at pH 3-6. This level of acidity probably represents the pH preferences of this ground beetle in its forest habitat and hibernating sites. By contrast to the cation cell, the pH cell responded to increases in buffer concentration by decreasing its firing rate.