Amplification of small signals by voltage-gated sodium channels in drone photoreceptors

Photoreceptor cells of the drone, Apis mellifera male, have a voltage-gated Na+ membrane conductance that can be blocked by tetrodotoxin (TTX) and generates an action potential on abrupt depolarization: an action potential is triggered by the rising phase of a receptor potential evoked by an intense light flash (Autrum and von Zwehl 1964; Baumann 1968). We measured the intracellular voltage response to a small (9%), brief (30 ms) decrease in light intensity from a background, and found that its amplitude was decreased by 1 microM TTX. The response amplitude was maximal when the background intensity depolarized the cell to -38 mV. With intensities depolarizing the cell membrane to -45 to -33 mV the average response amplitude was decreased by TTX from 1.2 mV to 0.5 mV. TTX is also known to decrease the voltage noise during steady illumination (Ferraro et al. 1983) but, despite this, the ratio of peak-to-peak signal to noise was, on average, decreased by TTX. The results suggest that drone photoreceptors use voltage-gated Na+ channels for graded amplification of responses to small, rapid changes in light intensity.