First Analysis of the Electroretinogram in the Madagascar Hissing Cockroach (Gromphadorhina portentosa)

The electroretinogram (ERG) is a sub-corneal extracellular recording revealing the summed electrical responses of compound eye (i.e., optic lobe) photoreceptors to discrete light pulses. Previous studies in other species of cockroaches have shown the ERG to be a robust technique to analyze changes in photoreceptor sensitivity and activity under various ambient conditions. However, no data has been published on the ERG of the unique, apterous cockroach, Gromphadorhina portentosa. Our current research reveals that this insect’s ERG consists of four distinct components: transient and sustained ON (electrical) potentials elicited by light stimulus onset, and transient and sustained OFF potentials elicited by stimulus offset; the transient and sustained potentials are distinguished by their differences in repolarization times. Specifically, increasing light pulse (stimulus) duration from 100-6500 ms caused an exponential decrease in repolarization rate of the sustained ON and the latency to the maximum amplitude of the sustained OFF potential. In addition, increased stimulus duration (up to 2500 ms) caused an exponential increase in the maximum amplitude of the sustained OFF at which point the response saturated. Changes in light intensity affected the amplitudes (but not shapes) of the four component waveforms. Additionally, ERGs recorded every 15 minutes over 72 hours under constant light conditions revealed rhythmic oscillatory changes with periods of approximately 24 hours in the latencies to the maximum amplitudes of both the transient ON and OFF potentials. This first analysis of ERGs in G. portentosa provides a novel source of data regarding the cellular and physiological changes in visual systems.