Voltage-activated outward currents in adult and nymphal locust photoreceptors

Abstract

The membrane properties of dark-adapted green sensitive photoreceptors of adult and nymphal desert locust (Schistocerca gregaria) were investigated in situ with single electrode (discontinuous) current and voltage clamp techniques. The photoreceptors had a resting potential of about -70 mV. The membrane rectified strongly in response to depolarizing current pulses at and above the resting potential. Two outward currents could be distinguished on the basis of different kinetics. The first, a transient current, was inactivating between-40 mV and -100 mV, and activated above -90 mV. The second, a sustained current had a similar activation range. The inactivating current could be blocked by application of 50 mM TEA into the retinal extracellular space. Both currents were expressed in nymphal photoreceptors already in the 1st instar larva and have similar properties as in the adults, although the behaviour of adults and nymphs is different. The conductances underlying these currents could be shown to modify the frequency response of the photoreceptor membrane.