Biosensing by insect olfactory receptor neurons

Action potentials of insect olfactory receptor neurons (ORN) were picked up with microelectrodes and amplified by a high-impedance amplifier connected to an oscilloscope, an audio amplifier, and an instrumentation amplifier. The amplified action potentials and odour exposure times were recorded on a DAT (digital audio tape) recorder for later analysis. This measurement of the stimulated action potential rates of the ORN of a blowfly (Calliphora vicina) made it possible to estimate odour concentrations to which the ORNs of the antenna were exposed. Odours of 1,4-diaminobutane (14DAB), 1-hexanol (HX), and butanoic acid (BA) were used in the experiments to stimulate various ORNs, which discharged action potentials. Spontaneous and stimulated action potential rates were analysed by means of a time-to-voltage converter connected to a signal analyser. timated exposure concentrations of the 14DAB odour were between a few ppb (parts per billion) and about 100 ppm (parts per million) in air, those of BA odour between 20 ppm and almost 200 ppm, and those of HX odour about 8 ppm and 500 ppm. The 14DAB-sensitive ORN generated action potentials between 100 and 275 pps (pulses per second), the BA-sensitive ORN between 70 and 165 pps, and the HX-sensitive ORN between 115 and 300 pps. Higher exposure concentrations of each odour caused the cessation of action potential production because of adaptation and overloading of the sensor. This could be seen predominantly as a saturation of the corresponding ORN. The spontaneous action potential rate before, and after, each stimulus exposure was between 20 and 25 pps. Clean-air exposures typically sent action potentials at the same rate as the spontaneous activity of the ORN.